NiMo Dual‐Atom Dimers on Pd Nanosheets for Selective C─H and C─C Bond Cleavage of Ethylene Glycol From Waste Plastics

ABSTRACT

Herein, we report the CO gas–mediated synthesis of palladium nanosheets (PdNS) as the first palladene support for atomically dispersed dimer catalysts. We further present the first report of NiMo dual‐atom dimer immobilization on PdNS (NiMo‐DA/PdNS) using rapid CO₂ laser irradiation within 10 min. The NiMo‐DA/PdNS platform exhibits enhanced ethylene glycol (EG) oxidation reaction (EGOR) activity with potential‐dependent product selectivity. At 1.0 V vs. RHE, glycolate (C2 product from selective C─H bond cleavage) was obtained with a Faradaic efficiency (FE) of 81% and a yield rate (YR) of 1.14 mmol h⁻¹ cm⁻². At 1.6 V vs. RHE, formate (C1 product from C─C bond cleavage) was selectively produced with an FE of 77.9% and a YR of 0.39 mmol h⁻¹ cm⁻². Theoretical calculations further corroborate these performance trends and elucidate the mechanism that C─C bond scission is facilitated by electron transfer to the catalyst surface at higher potentials, lowering the reaction free energy and enhancing C1 formation. This study demonstrates concurrent high selectivity toward both glycolate and formate while efficiently recovering high‐purity terephthalic acid (TPA) monomers from polyethylene terephthalate (PET) hydrolysate electrolysis over 50 h, highlighting the practical potential of atomically engineered catalysts with real‐world PET upcycling.

CO₂ laser–assisted synthesis enables rapid immobilization of NiMo dual‐atom dimers on Pd nanosheets (NiMo‐DA/PdNS) for electro‐reforming of PET‐derived ethylene glycol. The catalyst achieves potential‐tunable selectivity toward glycolate (C2) and formate (C1) with high Faradaic efficiency, while simultaneously recovering high‐purity terephthalic acid and demonstrating promising techno‐economic viability for plastic upcycling.

1. Introduction

Synthetic plastics are widely used in modern consumer and industrial products, playing an essential role in daily life. Polyethylene terephthalate (PET) is one of the most commonly used polyester polymers, with a global production of over 70 million tons per year owing to its extensive applications in automotive components, packaging, and textiles [1, 2, 3]. Despite this widespread utilization, large quantities of PET are discarded or landfilled because of unclear end‐of‐life disposal routes and its intrinsic resistance to degradation, resulting in wasted carbon resources and posing significant ecological and human health risks. Recycling PET plastics and upcycling them into high‐value chemicals is therefore crucial for both economic and environmental sustainability. Alkaline hydrolysis of PET plastics produces its monomers, terephthalic acid (TPA) and ethylene glycol (EG), collectively referred to as PET hydrolysate. The electrochemical EG oxidation reaction (EGOR) presents a promising strategy for transforming PET waste into high‐value chemicals, such as formic acid (FA) and glycolic acid (GA), thereby improving the overall profitability of PET recycling compared to conventional methods [4, 5, 6].

Electro‐reforming of PET plastic has emerged as a promising approach, offering a clean, cost‐effective, and controllable reaction pathway for converting EG into selective key products: FA, a C1 product from C–C bond cleavage involving a six‐electron process, and GA, a C2 product from C─H bond cleavage involving a four‐electron process [7]. While FA has been widely studied [8, 9, 10, 11], GA remains underexplored despite its market value being 8–16 times higher than that of EG, along with its diverse applications in food processing, cosmetics, adhesives, leather dyeing, and biodegradable materials [5, 12, 13]. The limited focus on GA is largely attributed to the dominance of C─C bond cleavage pathways and the poor GA selectivity of existing catalysts. Therefore, the development of catalysts that are both highly selective and stable at practical operating current densities is crucial.

Single‐atom catalysts (SACs) have attracted significant attention in electrocatalysis, offering superior activity and selectivity compared to traditional heterogeneous and homogeneous catalysts. This advantage arises from their maximum metal atom utilization, well‐defined active sites, tunable electronic structures, and excellent stability and reusability [14, 15, 16]. In the EGOR, SACs facilitate efficient cleavage of C─H and C─C bonds, leading to the selective formation of high‐value chemicals [2, 17]. However, the structural simplicity and isolated active sites of SACs limit their effectiveness in complex multistep oxidation processes involving various reaction intermediates. To overcome these limitations, the introduction of dual active sites, forming dual‐atom catalysts (DACs) or dimers stabilized on supports, has emerged as a promising strategy [18, 19, 20]. Such configurations enable synergistic interactions between adjacent metal centers, leading to optimized adsorption energies and enhanced reaction kinetics. For example, Moges et al. reported that Pd‒N₄/Cu‒N₄ DACs exhibit synergistic electronic interactions and tailored adsorption properties, resulting in improved EGOR kinetics and enhanced selectivity toward the desired C2 product, GA [21]. Similarly, Yue et al. demonstrated that Ni–Fe DACs can effectively upgrade PET hydrolysate–derived EG to GA, achieving a high Faradaic efficiency (FE) of 96.1% and a GA selectivity of 90% [22]. Despite these advances, the synthesis of SACs and DACs typically relies on high‐temperature programmed reactions, wet chemical adsorption, or atomic layer deposition [21]. However, these traditional approaches are often costly, time‐consuming, complex, and prone to issues such as uncontrolled metal adsorption, high energy consumption, and active site agglomeration. Thus, synthesizing dual‐atom dimers (DA) with precise atomic control and elucidating the synergistic interaction between two metal‐atom sites remains a significant challenge.

Herein, we present an important new insight into the potential‐dependent EGOR mechanism and selectivity along C1 and C2 electro‐oxidation pathways for the production of formate and glycolate using NiMo dual‐atom dimers supported on Pd nanosheets (NiMo‐DA/PdNS). To the best of our knowledge, no prior research has explored potential‐dependent EGOR pathway for the highly selective formation of both C1 and C2 products. Beyond mechanistic understanding, which was established by combining various in situ/ex situ spectro‐electrochemical experiments, the stabilization of NiMo‐DA on PdNS can be achieved using a simple, rapid, and cost‐effective CO₂ laser irradiation method (wavelength ≈ 10.6 µm, power ≈ 7 W) within 10 min. This approach enables precise atomic dispersion, prevents surface sintering, and strengthens interactions between NiMo‐DAs and the PdNS support, thereby enhancing electrocatalytic performance. We also complement these experiments with DFT calculations using the constant electrode potential (CEP) framework to explicitly regulate the applied potential and identify the key descriptors governing the C1–C2 product branching [23]. PdNS serves as an ideal support by providing a flat, highly conductive metallic surface with intrinsic EGOR activity and a known preference for glycolate (C2) formation via C─H activation. This establishes a well‐defined baseline that enables potential‐dependent switching toward C─C bond cleavage upon incorporation of NiMo‐DA sites. Moreover, the metallic PdNS framework acts as an electron reservoir, facilitating rapid charge transfer and strong electronic coupling with the NiMo‐DA, while robust metal–metal interactions enable stable immobilization of atomically dispersed DA sites compared with conventional carbon or oxide supports. Furthermore, a technoeconomic analysis (TEA) was conducted to evaluate the economic feasibility of PET plastic upcycling. Overall, this study highlights the critical role of CO₂ laser–assisted synthesis in advancing DAC design and underscores its potential for scalable, selective conversion of plastic waste into high‐value chemicals.

2. Results and Discussion

2.1. NiMo‐DA/PdNS Synthesis Mechanism and Structural Analyses

NiMo‐DA/PdNS synthesis was conducted via a two‐step process. In the first step, PdNS were prepared as the support, followed by the stabilization of NiMo‐DAs on PdNS using a simple, rapid, and cost‐effective CO₂ laser irradiation method (λ ≈ 10.6 µm, power ≈ 7 W) within 10 min. A detailed schematic illustration of NiMo‐DA/PdNS synthesis is shown in Figure 1a. In detail, the CO gas‐mediated solution‐phase chemical reduction synthesis of PdNS proceeds via a surface ligand–assisted anisotropic growth mechanism. Upon heating Pd(acac)₂ in acetic acid at 90°C, partial ligand exchange occurs between acetylacetonate and acetate ions from the solvent, increasing the solubility and reactivity of the Pd precursor. Introduced CO gas coordinates strongly with Pd²⁺, serving both as a mild reductant, gradually reducing Pd²⁺ to Pd⁰, and as a surface passivator: the adsorbed CO preferentially binds to specific Pd crystal facets (predominantly low‐index Pd(111) facets) while leaving lateral growth directions accessible to precursor diffusion and subsequent atomic assembly [24, 25]. This selective surface adsorption suppresses vertical stacking of PdNS and drives their two‐dimensional (2D) lateral extension, while the acetate environment further stabilizes the nanosheet morphology of PdNS with high crystallinity. The optical image in Figure S1 shows the transformation of the yellow precursors into the black PdNS product. Subsequently, the stabilization of NiMo‐DAs on PdNS was achieved via the CO₂ laser irradiation process, which induces localized thermal activation of Ni and Mo precursor species on the PdNS surface [26, 27, 28]. Figure S2 shows real photographs of the side, front, and top views of the experimental setup for the CO₂ laser irradiation technique used in the synthesis of NiMo‐DA/PdNS. Initially, PdNS were dispersed in a mixture of DI water and ethanol solvents containing Ni(OCOCH₃)₂·4H₂O and (NH₃)₂MoO₄·4H₂O under alkaline conditions, adjusted with NH₄OH. In this environment, Ni²⁺ readily interacts with the Pd surface owing to favorable metal–metal orbital overlaps and a strong thermodynamic driving force for Ni–Pd coordination, while Mo⁶⁺ species remain weakly solvated in the bulk solution. Upon CO₂ laser irradiation (λ ≈ 10.6 µm, power ≈ 7 W) for 10 min, rapid and localized heating promotes the reduction of Ni²⁺ to near‐zero‐valent Ni⁰ atoms and partial reduction of Mo⁶⁺ to Mo⁴⁺. Ethanol acts as a mild reducing agent, while the high surface reactivity of Pd facilitates interfacial electron transfer to the adsorbed species. The reduced Ni atoms strongly anchor to the Pd surface via Pd‐lattice‐site substitution or defect‐site coordination. Simultaneously, Mo species migrate on the heated Pd surface, where they preferentially bind with adjacent Ni atoms owing to strong d‐orbital hybridization and the favorable thermodynamic stability of the Ni–Mo pair, leading to the formation of a heteronuclear Ni–Mo DA dimer configuration anchored on the PdNS support. Based on XAS and XPS analyses (discussed below), Ni exists predominantly in the near‐zero‐valent state, while Mo remains mainly present in the Mo⁴⁺ state, suggesting interatomic charge redistribution from Ni to Mo (Ni⁰ → Mo⁴⁺). PdNS serves not only as a structural support for atomic dispersion but also enhances electronic coupling and spatial confinement of Ni and Mo atoms, thereby preventing their aggregation into nanoparticles.

FIGURE 1.

(a) Detailed schematic illustration of NiMo‐DA/PdNS synthesis via the CO₂ laser irradiation process, (b, c) XRD and Raman spectra of pure PdNS, Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS samples, (d, e) low‐ and high‐magnification HRTEM images of NiMo‐DA/PdNS, (f, g) SAED pattern and AFM topography of NiMo‐DA/PdNS, and (h, i) HAADF‐STEM images, where Ni and Mo atoms are marked by green and blue circles, respectively (inset shows the contour plot with highlighted DAs), and STEM–EDS elemental distribution mappings of NiMo‐DA/PdNS.

The XRD pattern of the as‐prepared PdNS exhibits diffraction peaks at 2θ = 40.3°, 46.7°, 68.4°, 82.0°, and 86.6°, corresponding to the (111), (200), (220), (311), and (222) planes of the cubic Pd crystalline phase (JCPDS No. # 46‐1043). No additional peaks attributable to PdO are detected, confirming the phase purity of PdNS synthesized via the CO gas–mediated solution‐phase reduction method (Figure 1b). For Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS, no reflections corresponding to Ni, Mo, NiMo alloy, or their oxides are observed, indicating that Ni and Mo are atomically dispersed on the PdNS surface (Figure 1b). However, these samples exhibit noticeable diffraction peak broadening and slight intensity reduction compared to pristine PdNS, which can be ascribed to a decrease in crystallite size (D) and an increase in lattice strain (ε) induced by the CO₂ laser process and strong metal (Ni/Mo)–PdNS support interactions (Table S1). The crystallographic parameters, including D, ε, and interplanar distance (d‐spacing), were determined using the Scherrer, Bragg, and Williamson–Hall equations [29] and are summarized in Table S1. The calculated d‐spacing of 2.23 Å for the Pd(111) plane is consistent with the standard cubic Pd phase (JCPDS No. 46‐1043), while the observed peak broadening aligns with localized lattice distortion caused by heteroatom incorporation into the Pd lattice. These structural modifications confirm the atomic‐level dispersion of Ni and Mo on PdNS and suggest that the formation of defect‐rich, strained Pd surfaces modulate the electronic structure (d‐band center) of Pd, optimizing the adsorption energies of EGOR intermediates. This facilitates efficient charge transfer and improves the stability of active sites in the NiMo‐DA/PdNS catalyst during EGOR catalysis.

The Raman spectrum of pristine PdNS exhibits distinct peaks at ∼270 cm^{− 1} (ν₁), ∼426 cm^{− 1} (ν₂), and ∼626 cm^{− 1} (ν₃), assigned to Pd–Pd lattice vibrations (Figure 1c). By contrast, bulk Pd and conventional nanoparticles are typically Raman‐inactive owing to their high crystal symmetry and metallic screening [30]. The ultrathin 2D nanosheet morphology of pristine PdNS obtained via CO‐mediated synthesis breaks translational symmetry of the Pd lattice and introduces edge sites, defects, and lattice distortions, thereby relaxing Raman selection rules and enhancing phonon–photon coupling, which activates these vibrational modes in pristine PdNS. Incorporation of Ni‐SA, Mo‐SA, or NiMo‐DA onto the PdNS preserves the fundamental PdNS modes but induces slight peak shifts, broadening, and intensity changes, indicative of charge transfer and local strain from heteroatom coordination with PdNS. The enhancement of the ν₃ band along with suppression and broadening of ν₁ and ν₂ suggests increased electron–phonon coupling and local symmetry perturbations consistent with the coordination effects of Ni‐SA, Mo‐SA, or NiMo‐DA on the 2D PdNS lattice. These changes signify substantial structural and electronic modulation into the PdNS structure, which is expected to enhance catalytic EGOR performance. FTIR spectra of PdNS, Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS exhibit two characteristic modes at ∼559 and ∼603 cm⁻¹, attributable to Pd–Pd lattice vibrations and surface Pd–O stretching (Pd–O_surf), respectively (Figure S3). Incorporation of SA and DA leads to slight intensity enhancements in both modes and alters the coordination environment and electronic structure of PdNS, consistent with the Raman analysis results.

The surface structure and elemental distribution of the synthesized catalysts were investigated using FESEM and EDS. As shown in Figure S4, the pristine PdNS exhibits a well‐defined nanosheet structure obtained via CO gas–mediated chemical reduction. EDS elemental mapping confirms that the PdNS are primarily composed of Pd, with minor carbon content, likely arising from residual carbonaceous species generated by CO dissociation on the Pd surface and from the precursor, present as surface‐deposited amorphous carbon. A negligible amount of oxygen was also detected, attributable to surface‐adsorbed oxygen. FESEM images of Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS reveal a nanosheet morphology similar to that of pristine PdNS. No distinct Ni‐SA, Mo‐SA, or NiMo‐DA phases were observed, suggesting that Ni, Mo, and NiMo species are atomically dispersed on the PdNS support (Figures S5–S7). EDS elemental mapping confirms the presence of Ni in Ni‐SA/PdNS, Mo in Mo‐SA/PdNS, and both Ni and Mo in NiMo‐DA/PdNS, indicating homogeneous distribution over the PdNS support. The Pd, Ni, and Mo contents (wt.%) determined from EDS analysis (Figure S8 and Table S2) were further validated via ICP‐OES measurements, as summarized in Table S3, confirming the reliability of the compositional analysis. Furthermore, the effects of CO₂ laser parameters—including power (7, 14, 21, and 25 W) and irradiation time (10 and 30 min)—on the structural evolution of NiMo‐DA/PdNS were systematically investigated (Figures S7 and S9). FESEM and EDS analyses (Figure S7) show that irradiation at 7 W for 10 min preserves the nanosheet morphology and produces a homogeneous distribution of NiMo‐DA, corresponding to the optimized NiMo‐DA/PdNS structure. In contrast, prolonged irradiation (30 min) or higher laser power (≥14 W) induces significant morphological changes, including partial loss of nanosheet features, increased particle aggregation, and the formation of Ni/Mo clusters (Figure S9). These results indicate that precise control of CO₂ laser parameters (λ ≈ 10.6 µm, power ≈ 7 W, and irradiation time ≈ 10 min) is critical for simultaneously preserving the PdNS morphology and achieving stable atomic dispersion of NiMo‐DA sites.

The HRTEM image of NiMo‐DA/PdNS also confirms the formation of a nanosheet structure (Figure 1d), with measured lattice spacings of 2.02 and 2.31 Å corresponding to the (200) and (111) planes of cubic Pd, respectively (Figure 1e). These results validate that the surface morphology and lattice structure of pristine PdNS remain unchanged after NiMo‐DA incorporation. The corresponding selected area electron diffraction (SAED) pattern (Figure 1f) exhibits distinct bright circular rings with well‐defined spots, indicating the highly crystalline nature of the synthesized NiMo‐DA/PdNS samples [31]. The lattice spacings derived from the diffraction rings were indexed to the (111), (200), (220), (311), (222), and (331) planes of cubic Pd, in good agreement with the XRD results. AFM imaging of an individual NiMo‐DA/PdNS (Figure 1g) reveals a thickness of approximately 31 nm, with a characteristic height profile showing a central depression. This thickness suggests a stacked nanosheet assembly rather than a single compact Pd layer. The NiMo‐DA sites are predominantly exposed on the PdNS surfaces, while the interlayer nanochannels enable electrolyte penetration and efficient reactant diffusion, thereby minimizing mass‐transport limitations. Furthermore, the continuous Pd framework provides high electrical conductivity and structural stability during prolonged electrolysis, which collectively contribute to the enhanced EGOR performance of the NiMo‐DA/PdNS catalyst.

Aberration‐corrected HAADF‐STEM analysis was conducted to verify the formation of NiMo‐DA. As shown in Figure 1h, the HAADF‐STEM image clearly reveals paired atomic species, with Ni and Mo atoms highlighted by green and blue circles, respectively. Owing to the Z‐contrast nature of HAADF‐STEM imaging, the heavier Mo atoms appear significantly brighter than the lighter Ni atoms under identical imaging conditions. The corresponding contour plots of regions A and B (inset of Figure 1h) further resolve the Ni–Mo atomic pairs, providing direct evidence of DA configuration. Furthermore, HAADF‐STEM–EDS elemental mapping (Figure 1i) confirms the uniform spatial distribution of Ni and Mo across the PdNS support, consistent with the EDS spectra (Figure S10). To further validate the atomic‐level dispersion, statistical analysis was performed over multiple HAADF‐STEM regions (Figure S11a,b). NiMo‐DA sites were identified based on intensity contrast and 3D surface mapping, and their spatial distribution was quantified. The analysis reveals the presence of Ni‐SA (15.35%), Mo‐SA (14.38%), and a dominant fraction of NiMo‐DA sites (70.27%). The calculated dimer number density confirms a high population of isolated DA sites, while the measured center‐to‐center distances exhibit a broad spatial distribution without evidence of clustering or nanoparticle formation. These results collectively confirm the successful stabilization and atomic‐level dispersion of NiMo‐DA sites on the PdNS support. For comparison, analogous HRTEM, SAED, HAADF‐STEM, and EDS analyses were also conducted for Ni‐SA/PdNS and Mo‐SA/PdNS samples (Figures S12–S15). Similar to NiMo‐DA/PdNS, both samples retain the pristine Pd nanosheet structure. In Ni‐SA/PdNS, isolated Ni atoms are uniformly anchored on PdNS, whereas Mo‐SA/PdNS displays atomically dispersed Mo species. The elemental ratios (atomic%) obtained from STEM‐EDS analysis (Table S4) are consistent with the results from ICP‐OES analysis. These results further confirm the successful stabilization of Ni‐SA and Mo‐SA species on PdNS, consistent with the observations for NiMo‐DA/PdNS.

The surface coordination environment and electronic structure of PdNS, Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS were analyzed using XPS. The full survey spectrum of pristine PdNS (Figure S16) confirmed the presence of Pd, residual carbon species originating from CO dissociation on the Pd surface and from the precursor, and surface‐adsorbed oxygen, consistent with the elemental composition determined via EDS. For Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS, the survey spectra additionally revealed the presence of Ni, Mo, and both Ni and Mo, respectively, confirming the successful atomic incorporation of Ni and Mo within the PdNS framework. The XPS binding energies (BEs) and their correlations for all samples are summarized in Table S5. High‐resolution Pd 3d spectra (Figure 2a) for all samples exhibited well‐defined spin–orbit doublets (Pd 3d _5/2 and Pd 3d _3/2). In pristine PdNS, deconvolution revealed two Pd⁰ 3d _5/2 components at 334.47 eV and 334.97 eV, corresponding to surface and bulk metallic Pd, respectively [25, 32], indicating that Pd exists mainly in the metallic state. A minor Pd²⁺ contribution was attributed to surface‐adsorbed oxygen species. Upon incorporation of Ni‐SA, Mo‐SA, or NiMo‐DA, the Pd⁰ peaks shifted slightly toward higher binding energies relative to pure PdNS, indicating electron transfer from Pd to the incorporated atoms and a corresponding downshift of the Pd d‐band center. This electronic (d‐band center) modulation is known to optimize the adsorption and activation of EG reactants [33], thereby enhancing the intrinsic EGOR activity of the catalysts.

FIGURE 2.

(a–c) XPS core‐level spectra of Pd 3d, Ni 2p, and Mo 3d for PdNS, Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS, (d–f) normalized XANES spectra at the Pd K‐edge, Ni K‐edge, and Mo K‐edge for PdNS, Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS, compared with reference materials (Pd foil, PdO, Ni foil, NiO, Mo foil, and MoO₃), (g) k ²‐weighted extended X‐ray absorption fine structure (EXAFS) oscillation functions (χ(k)) at the Pd, Ni, and Mo K‐edges, along with the corresponding FT‐EXAFS spectra in radial distance (R‐space) for Pd, Ni, and Mo in PdNS, Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS, (h) schematic illustration of the proposed atomic configuration of NiMo‐DA sites stabilized on PdNS, and (j) Wavelet transform EXAFS (WT‐EXAFS) contour plots at the Pd, Ni, and Mo K‐edges for NiMo‐DA/PdNS.

The Ni 2p spectra (Figure 2b) further confirm the incorporation of Ni into the PdNS framework. While pristine PdNS and Mo‐SA/PdNS show no Ni signal, both Ni‐SA/PdNS and NiMo‐DA/PdNS exhibit well‐defined Ni 2p _3/2 and Ni 2p _1/2 doublets. Deconvolution of the Ni 2p spectrum of Ni‐SA/PdNS reveals contributions from Ni⁰ (∼852.72 eV) and Ni²⁺ (∼853.96 eV), together with satellite features. The Ni²⁺ arises from surface oxidation, whereas Ni⁰ signifies atomically dispersed Ni stabilized by the PdNS lattice. In NiMo‐DA/PdNS, the Ni 2p features persist but show slight positive BE shifts relative to Ni‐SA/PdNS, indicating electronic redistribution through Ni–Mo coordination. This electronic modulation is consistent with the observed Pd d‐band downshift, which optimizes EGOR intermediate adsorption on NiMo‐DA/PdNS and enhances its EGOR activity. The Mo 3d spectra (Figure 2c) exhibit spin–orbit doublets (Mo 3d _5/2 and Mo 2d _3/2), with Mo⁴⁺ as the major state and minor Mo⁶⁺ contributions in Mo‐SA/PdNS and NiMo‐DA/PdNS, which is absent in PdNS or Ni‐SA/PdNS. The stabilization of Mo⁴⁺ indicates atomically dispersed Mo sites on the PdNS support, confirming Mo‐SA formation. In NiMo‐DA/PdNS, the observed BE shifts of both Ni and Mo evidence the formation of NiMo‐DA. The coordination preference reflects the stronger affinity of Ni–Pd interactions compared to weaker Mo–Pd bonding, which drives Mo to couple preferentially with Ni, thereby stabilizing Ni–Mo dimers. Mechanistically, the CO₂ laser synthesis process enables rapid reduction of Ni²⁺ to Ni⁰, favoring SA anchoring on PdNS, whereas Mo undergoes partial reduction (Mo⁴⁺ → Mo⁶⁺) and some residual oxygen coordination. The resulting atomic dispersion of Ni‐SA, Mo‐SA, and NiMo‐DA on PdNS stabilizes the catalytic sites and modulates the PdNS electronic structure, thereby optimizing intermediate adsorption and significantly enhancing EGOR performance.

UPS reveals that the incorporation of Ni‐SA, Mo‐SA, and NiMo‐DA systematically modulates the electronic structure of PdNS, providing direct insight into their structure–property relationships. The secondary electron cutoff energies (ESE_cut‐off) from UPS spectra of PdNS, Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS are determined to be 14.26, 13.91, 14.48, and 14.96 eV, respectively (Figure S17), reflecting progressive Φ modulation. The Φ, defined as the minimum energy required for an electron to escape from the solid (electrode) surface, is a key descriptor of the ability of an electrocatalyst to donate electrons to adsorbed species (reactants and intermediates) [34]. Using the relation Φ = hν − ESE_cut‐off, where hν is the incident photon energy (21.22 eV for He I radiation), the calculated Φ values for PdNS, Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS are 6.96, 7.31, 6.74, and 6.20 eV, respectively. These results demonstrate that NiMo‐DA/PdNS possesses the lowest Φ, reducing the energy barrier for electron transfer from the catalyst surface to the adsorbed species [35]. This reduced barrier suggests enhanced charge redistribution around NiMo‐DA and PdNS, strengthening chemical interactions between the catalyst and reactants during the EGOR. By contrast, the slightly higher Φ for Ni‐SA/PdNS, along with its lower ESE_cut‐off, indicates that isolated Ni‐SA anchored on PdNS lattice sites introduces localized electronic states with limited charge redistribution. This results in reduced electron donation to adsorbed species and, consequently, lower expected electrocatalytic activity toward the EGOR compared with Mo‐SA/PdNS and NiMo‐DA/PdNS. Furthermore, REELS show a progressive decrease in elastic peak intensity from PdNS to NiMo‐DA/PdNS (Figure S18), indicative of enhanced inelastic scattering arising from increased electronic interactions between incident electrons and the modified electronic structure of NiMo‐DA/PdNS, as well as increased surface disorder. These observations are consistent with the optimized EGOR catalytic activity of NiMo‐DA/PdNS.

XAS was employed to investigate the electronic and coordination structures of PdNS, Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS catalysts. Measurements were performed at the Core‐Facility Center for Photochemistry & Nanomaterials, Gyeongsang National University, using an in situ cryogenic X‐ray absorption spectrometer, IC‐XAS (NFEC‐2025‐07‐307201). Figure 2d–f shows the normalized XANES spectra at the Pd K‐edge, Ni K‐edge, and Mo K‐edge, respectively, in comparison with reference materials including Pd foil, PdO, Ni foil, NiO, Mo foil, and MoO₃. As shown in Figure 2d, the Pd K‐edge XANES spectrum of PdNS closely resembles that of metallic Pd foil. In contrast, Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS exhibit edge positions at half‐height intensity between those of Pd foil and PdO, with a clear shift toward higher energies relative to pristine PdNS (inset of Figure 2d). This behavior indicates modulation of the Pd electronic structure due to strong interactions with neighboring Ni and Mo atoms. Quantitative analysis of the edge position reveals variations in Pd oxidation state following the order Mo‐SA/PdNS (∼1.07⁺) > NiMo‐DA/PdNS (∼0.92⁺) > Ni‐SA/PdNS (∼0.81⁺) > PdNS (∼0) (Figure S19a). For Ni‐SA/PdNS (Figure 2e), the Ni K‐edge closely matches that of metallic Ni, confirming the presence of Ni⁰ SAs coordinated with Pd sites. Upon incorporation of Mo to form NiMo‐DA/PdNS, the Ni K‐edge shifts to higher energy with an enhanced white‐line intensity, consistent with partial oxidation of Ni⁰ SAs (∼Ni^0.52+) owing to bonding with both Pd and Mo (Figure S19b). This modification in the Ni electronic structure is reflected in the behavior of Mo in NiMo‐DA/PdNS: the Ni K‐edge in NiMo‐DA/PdNS shifts slightly to lower energy relative to Ni‐SA/PdNS, indicating a modest reduction owing to electron back‐donation from Ni, which confirming the electronic coupling between Ni and Mo within the dual‐atom configuration. By contrast, the Mo K‐edge XANES of Mo‐SA/PdNS and NiMo‐DA/PdNS (Figure 2f) lies between those of Mo foil and MoO₃, consistent with a high oxidation state of ∼Mo^5.55+ coordinated to Pd and O (Figure S19c). Collectively, these spectral features demonstrate charge redistribution across the Ni–Mo atomic pair: Ni tends to be oxidized by Mo, while Mo is partially reduced by Ni, with Pd serving as an electron reservoir that donates electron density to stabilize the heterobimetallic Ni–Mo active sites.

Furthermore, EXAFS analysis was performed to elucidate the local coordination environments of Pd in PdNS, Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS, as well as Ni and Mo in the corresponding samples. The Fourier‐transformed EXAFS (FT‐EXAFS) spectra at the Pd K‐edge (Figure 2g) exhibit two characteristic features. The dominant peak at ∼2.5 Å corresponds to the Pd─Pd coordination, confirming the preservation of the metallic Pd framework across all samples. Compared with Pd foil, PdNS exhibits a reduced Pd─Pd peak intensity along with the appearance of a weak peak at ∼1.5 Å, attributed to surface Pd─O coordination (Pd─O_surf). Upon incorporation of Ni and/or Mo species (Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS), the Pd─Pd peak intensity further decreases, accompanied by an enhancement of the short‐range peak, indicating the formation of Pd─Ni and/or Pd─Mo coordination in addition to Pd─Pd bonding. Notably, NiMo‐DA/PdNS exhibits the most pronounced short‐range Pd─O_surf contribution (∼1.5 Å), suggesting strong structural and electronic coupling between the NiMo‐DA sites and the PdNS support.

The FT‐EXAFS spectra at the Ni K‐edge (Figure 2g) reveal that Ni‐containing samples exhibit a dominant Ni─M (M = Pd or Mo) coordination peak, confirming that Ni atoms are coordinated with neighboring Pd and/or Mo atoms. In contrast, Ni foil shows a sharp Ni─Ni peak at ∼2.15 Å, characteristic of metallic Ni. For Ni‐SA/PdNS and NiMo‐DA/PdNS, the Ni–M peak appears at ∼2.20 Å, slightly shifted relative to the Ni─Ni bond in Ni foil, consistent with Ni─Pd and Ni─Mo interactions and in agreement with the XANES results. The considerably reduced amplitudes in these samples indicate low coordination numbers and increased structural disorder, characteristic of atomically dispersed Ni species anchored on the PdNS surface (Figure 2h). The FT‐EXAFS spectra at the Mo K‐edge (Figure 2g) reveal two distinct peaks at ∼1.19 and ∼2.17 Å for both Mo‐SA/PdNS and NiMo‐DA/PdNS, corresponding to Mo─O and Mo─Pd coordination, respectively. In NiMo‐DA/PdNS, an additional feature at ∼2.68 Å is assigned to Mo─Ni coordination, confirming the formation of heterobimetallic Ni─Mo sites. These Mo species are coordinated with Pd, Ni, and O atoms, forming a stable and electronically coupled DA configuration on the conductive PdNS support. Importantly, the Mo–O peak is clearly distinguishable from the Mo─O scattering in MoO₃ (∼1.50 Å) and the Mo─Mo metallic bond in Mo foil (∼2.60 Å), confirming the absence of MoO₃ phases and metallic Mo nanoparticles. Overall, the combined EXAFS results provide strong evidence for the formation of atomically dispersed NiMo‐DA sites stabilized on the PdNS support, with well‐defined coordination environments and strong interfacial coupling.

Wavelet transform EXAFS (WT‐EXAFS) analysis was further performed to resolve backscattering contributions in both R‐space and k‐space [36]. The Pd K‐edge WT‐EXAFS contour plots of NiMo‐DA/PdNS (Figure 2j) display three major intensity maxima. By comparison with reference samples (Pd foil, PdO, PdNS, Ni‐SA/PdNS, and Mo‐SA/PdNS; Figure S20), these features are assigned to Pd─Pd, Pd─Ni/Pd─Mo (partially overlapping with Pd─Pd), and Pd─O scattering paths, in good agreement with the FT‐EXAFS results. The Ni K‐edge WT‐EXAFS contour plots of NiMo‐DA/PdNS (Figure 2j), in comparison with Ni foil, NiO, and Ni‐SA/PdNS (Figure S21), reveal a dominant Ni─Pd/Ni─Mo scattering contribution, confirming the coordination of Ni atoms with neighboring Pd and Mo species. Similarly, the Mo K‐edge WT‐EXAFS contour plots (Figure 2j), compared with Mo foil, MoO₃, and Mo‐SA/PdNS (Figure S22), exhibit characteristic signals corresponding to Mo─Pd/Mo─Ni and Mo─O scattering path. These results confirm that atomically dispersed Mo species are anchored through Mo─Pd, Mo─Ni, and Mo─O─Pd coordination environments.

For quantitative structural analysis, EXAFS curve fitting was performed at the Pd, Ni, and Mo K‐edges in both k‐space and R‐space for PdNS, Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS, along with corresponding reference materials (Figures S23–S26). The extracted structural parameters are summarized in Table S6. The fitting results indicate that the Pd framework retains a Pd–Pd coordination number close to 12, comparable to metallic Pd foil, demonstrating that incorporation of Ni‐SA, Mo‐SA, and NiMo‐DA sites preserves the PdNS structure while introducing subtle electronic and structural modifications. Ni K‐edge EXAFS fitting reveals that Ni is primarily coordinated with Pd atoms in both Ni‐SA/PdNS and NiMo‐DA/PdNS, forming a Ni─Pd₈ coordination environment without detectable Ni–Ni metallic bonding. In NiMo‐DA/PdNS, an additional Ni–Mo₁ coordination is observed, providing direct evidence for the formation of heterobimetallic Ni─Mo sites. Similarly, Mo K‐edge EXAFS fitting reveals that Mo is predominantly coordinated with Pd and O atoms, forming Mo─Pd₁ and Mo─O₂ environments in both Mo‐SA/PdNS and NiMo‐DA/PdNS, with no evidence of Mo─Mo metallic bonding. The presence of Mo─Ni₁ coordination in NiMo‐DA/PdNS further confirms the formation of atomically dispersed NiMo‐DA sites. Collectively, the Pd, Ni, and Mo XANES results reveal oxidation states modulated by interfacial charge transfer: Pd sites are electron‐depleted at the edges, while both Ni and Mo exist in partially oxidized states. Specifically, in NiMo‐DA/PdNS, Ni is directly bonded to both Mo and Pd, whereas Mo is coordinated to Ni, Pd, and O atoms, forming a stable heterobimetallic structure anchored on conductive PdNS. This unique atomic configuration enables electron‐deficient Ni sites to activate alkoxide and oxo intermediates, while Mo fine‐tunes BEs and stabilizes reaction transition states. Meanwhile, PdNS acts as both an electron reservoir and a conductive scaffold, facilitating charge transport during EGOR catalysis. Overall, these synergistic effects directly account for the significantly improved electrochemical EGOR performance of NiMo‐DA/PdNS.

2.2. Electrocatalysis of EGOR and PETHOR Experiments

The electrocatalytic reforming of EG and PET hydrolysate was evaluated over PdNS, Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS catalysts in a conventional H‐type electrochemical cell (Figure S27). Before the electrochemical reactions, the electrocatalyst‐coated CC substrates were pre‐activated by 200 CV cycles in 1.0 M KOH to remove surface impurities and stabilize the catalyst–electrolyte interface, thereby ensuring reliable and reproducible EGOR and PETHOR measurements (Figure S28). LSV curves of PdNS, Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS catalysts were first recorded in 1.0 M KOH without EG (Figure 3a and Figure S29a), representing the oxygen evolution reaction (OER) activity. Among the catalysts, NiMo‐DA/PdNS exhibited the lowest overpotentials of 341 and 484 mV at current densities of 10 and 50 mA cm⁻², respectively (Figure S29b), and the smallest Tafel slope of 174.7 mV dec⁻¹ (Figure S29c), confirming its superior intrinsic OER kinetics compared to PdNS, Ni‐SA/PdNS, and Mo‐SA/PdNS. Upon the addition of 0.5 M EG, the anodic polarization shifted significantly to lower potentials, and the current density increased markedly relative to the OER background, indicating the onset of the EGOR, which proceeds through a kinetically more favorable pathway than the OER (Figure S30). The LSV profiles recorded in the presence of EG display two distinct anodic oxidation regions (Figure 3a). The first set of oxidation peaks emerges in the low potential range of 0.5–1.3 V vs. RHE, whereas a second oxidation feature appears at high potentials between 1.5 and 2.1 V vs. RHE. This potential‐dependent behavior indicates selective pathways for EG electro‐oxidation: the low potential region is predominantly associated with partial oxidation via C2 pathways involving C─H bond cleavage, while the high potential region corresponds to extensive oxidation through C─C bond cleavage, leading to C1 products. Li et al. [37] reported that the low‐potential electro‐oxidation of EG over a PtAg catalyst primarily generates glycolate. Similarly, Si et al. [38] demonstrated selective formation of glycolate during the EGOR at remarkably low potentials using PdAg supported on nickel foam. By contrast, Li et al. [39] showed that the EGOR on NiSe₂ heterostructures at high potentials yields formate as the dominant product, reflecting superior C─C bond cleavage activity under these conditions. These findings highlight that EG, owing to its two hydroxyl groups, is a versatile small organic molecule that can be electrochemically converted into various C1 (formate and carbonate) and C2 (glycolate, oxalate, and glyoxal) products, depending on both the applied potential and catalyst composition.

FIGURE 3.

Electrochemical EGOR studies. (a) LSV curves of PdNS, Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS in 1.0 M KOH with and without 0.5 M EG, (b) FE of glycolate, formate, glycolaldehyde, and CO₂ produced during 1 h of EGOR at different applied potentials using NiMo‐DA/PdNS, (c) YR of glycolate and formate produced during 1 h of the EGOR at various applied potentials using NiMo‐DA/PdNS, (d) ¹H NMR spectra of the anolyte after 1 h of EGOR at 1.0 and 1.6 V vs. RHE using NiMo‐DA/PdNS, (e) YR of glycolate and formate produced during 1 h of the EGOR at fixed potentials (1.0 and 1.6 V vs. RHE) using different catalysts, (f) FE of glycolate, formate, glycolaldehyde, and CO₂ produced during 1 h of EGOR at fixed potentials (1.0 and 1.6 V vs. RHE) using different catalysts, and (g, h) cyclic stability tests of EGOR electrolysis using NiMo‐DA/PdNS at 1.6 and 1.0 V vs. RHE, along with the corresponding FE and YR of formate and glycolate products (the anolyte was freshly replaced after each cycle).

Figure 3a reveals the superior OER and EGOR activity of NiMo‐DA/PdNS compared to PdNS, Ni‐SA/PdNS, and Mo‐SA/PdNS. This enhanced performance is further supported by electrochemically active surface area (ECSA), accessible active sites (N), turnover frequency (TOF), and charge‐transfer kinetics. The ECSA (ECSA = C _dl/C _s, where C _s is the specific capacitance of the CC substrate, ∼88 mF cm⁻²) of the electrocatalysts was determined from the double‐layer capacitance (C _dl) obtained via scan rate–dependent CV measurements [40]. CV curves of PdNS, Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS, recorded in the non‐Faradaic potential window at different scan rates in 1.0 M KOH, both without (OER) and with 0.5 M EG (EGOR), are shown in Figures S31 and S32. The C _dl plots, derived from Δj/2 versus scan rate (Δj = j _a − j _c, where j _a and j _c are the anodic and cathodic current densities), reveal the highest C _dl values for NiMo‐DA/PdNS under both conditions (20.4 mF cm⁻² without EG and 21.7 mF cm⁻² with EG), compared to PdNS (8.4 and 7.8 mF cm⁻²), Ni‐SA/PdNS (3.7 and 6.8 mF cm⁻²), and Mo‐SA/PdNS (11.9 and 13.9 mF cm⁻²) (Figure S33). These large C _dl values correspond to high ECSA for NiMo‐DA/PdNS, reaching 0.26 and 0.28 cm² under OER and EGOR conditions, respectively (Figure S34), indicating a high number of accessible catalytic sites. The N values of PdNS, Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS were further estimated from the surface charge (Q _s) obtained by integrating CV curves recorded over a wide potential window (−0.07 to 1.12 V vs. RHE) at a scan rate of 50 mV s⁻¹ using the relation N = Q _s/F (F = Faraday constant) (Figures S35 and S36). NiMo‐DA/PdNS exhibited the highest N values in 1.0 M KOH, both without and with 0.5 M EG (1.63 × 10⁻³ and 4.51 × 10^‒4 mol cm⁻², respectively), significantly surpassing those of PdNS, Ni‐SA/PdNS, and Mo‐SA/PdNS. The superior C _dl, ECSA, and N values of NiMo‐DA/PdNS arise from the synergistic interplay of electronic and geometric effects unique to its DA configuration. The heteroatomic NiMo‐DA sites anchored on PdNS promote strong electronic coupling, redistributing charge density and optimizing the adsorption–desorption energetics of hydroxyl and carbonaceous intermediates, as supported by the UPS analysis. In contrast to Ni‐SA and Mo‐SA sites, the NiMo‐DA configuration provides bifunctional catalytic centers, where Ni and Mo synergistically stabilize different reaction intermediates, thereby lowering activation barriers for both C─H and C─C bond cleavage during the EGOR. Furthermore, incorporation of NiMo‐DA induces local lattice strain and defect‐rich regions in PdNS, exposing a high fraction of electrochemically active edge and corner sites. This combined structural and electronic synergy enhances active‐site utilization and catalyst stability, leading to the highest ECSA and N among the studied catalysts.

Figure S37 illustrates that NiMo‐DA/PdNS exhibits the highest intrinsic activity compared with PdNS, Ni‐SA/PdNS, and Mo‐SA/PdNS for both the OER and the EGOR across the investigated wide potential range. For the OER (Figure S37a), the TOF increases with potential for all catalysts, with NiMo‐DA/PdNS consistently outperforming the others owing to its optimized electronic structure and high N. By contrast, the EGOR (Figure S37b) shows a volcano‐type behavior, where the TOF increases at low potentials from 0.9 to 1.1 V vs. RHE and then decreases at higher potentials (>1.1 V vs. RHE). This decline likely originates from the competing OER and the blocking of N by strongly adsorbed EGOR intermediates (carbonaceous fragments) and oxygenated adlayers, which impede EG adsorption, reduce turnover, and hinder product desorption.

Electrochemical impedance spectroscopy was conducted to further investigate the charge‐transfer kinetics of the catalysts in 1.0 MM KOH, both without and with 0.5 M EG, at fixed potentials. For the EGOR, two representative potentials were selected from the LSV profiles: one corresponding to the low oxidation region (0.526 V vs. RHE) and the other to the high oxidation region (1.326 V vs. RHE). The Nyquist plots (Figure S38) show that NiMo‐DA/PdNS consistently exhibits the smallest charge‐transfer resistance (R _ct) under all tested conditions, highlighting its superior charge‐transfer capability compared with PdNS, Ni‐SA/PdNS, and Mo‐SA/PdNS. In 1.0 M KOH + 0.5 M EG at 0.526 V vs. RHE, NiMo‐DA/PdNS shows the most efficient charge transfer (R _ct = 34.68 Ω), significantly lower than that of PdNS (R _ct = 87.35 Ω), Ni‐SA/PdNS (R _ct = 105.66 Ω), and Mo‐SA/PdNS (R _ct = 36.67 Ω). At 1.326 V vs. RHE, R _ct decreases further for all catalysts owing to the enhanced driving force, with NiMo‐DA/PdNS retaining the lowest value (12.13 Ω). These results confirm that the NiMo‐DA configuration accelerates charge‐transfer kinetics and ensures efficient electron transport across a wide potential window, accounting for its superior EGOR activity. At higher potential (1.326 V vs. RHE), the increased driving force not only reduces R _ct by accelerating electron transfer but also promotes C─C bond cleavage, favoring C1 product formation. By contrast, at low potentials, the limited driving force primarily enables C─H bond cleavage, directing the reaction toward C2 products. Overall, the gradual decrease in R _ct with increasing potential (Figures S39–S41) reflects the enhanced driving force for interfacial electron transfer between the electrode and electrolyte, while the stable architecture and strong adhesion of NiMo‐DA on PdNS enable efficient charge transport without structural degradation, supporting long‐term practical operation in both the OER and the EGOR.

The C1 and C2 products and the selectivity of the EGOR were monitored using ¹H NMR measurements. Initially, long‐term chronoamperometry tests of the EGOR were conducted using the NiMo‐DA/PdNS electrocatalyst at different potentials (0.7–1.7 V vs. RHE). The resulting 1 h EGOR electrolysis curves are shown in Figure S42. After 1 h of electrolysis, the electrolyte was collected and analyzed using ¹H NMR to quantify the products. The resulting ¹H NMR spectra (Figure S43) revealed two prominent peaks corresponding to products at ∼3.89 and ∼8.4 ppm, which are identified as glycolate (C2 product) at low oxidation potentials and formate (C1 product) at high oxidation potentials, respectively. The EG substrate peak appeared at ∼3.59 ppm. The ¹H NMR spectra clearly showed that the peak intensity of glycolate was dominant at low potentials (0.7–1.3 V vs. RHE), with the maximum intensity observed at 1.1 V vs. RHE. This confirms that the first oxidation peak in the low potential region of the EGOR LSV curves corresponds to the selective C2 pathway via C─H bond cleavage. Conversely, formate appeared predominantly at high potentials (1.4–1.7 V vs. RHE), reaching maximum intensity at 1.5 V vs. RHE, confirming that the second oxidation peak in the high potential region of the EGOR LSV curves corresponds to the selective C1 pathway via C─C bond cleavage. FE and yield rate (YR) of glycolate and formate products were quantitatively analyzed using standard solutions of GA (1, 2, 5, and 10 mM in 1.0 M KOH) and potassium formate (0.5, 1.0, 3.0, and 5.0 mm in 1.0 M KOH), as shown in Figures S44 and S45. In addition to the primary products, glycolaldehyde (liquid intermediate) and CO₂ (gaseous byproduct) were quantified using ¹³C NMR and gas chromatography (GC), respectively. The glycolaldehyde concentration was determined using calibration curves derived from standard ¹³C NMR spectra of glycolaldehyde dimer solutions (Figure S46). The CO₂ amount was quantified using calibration curves obtained by correlating known CO₂ concentrations with their corresponding integrated GC peak areas (Figure S47). Figure S48 presents the ¹³C NMR spectra of the anolyte collected after 1 h of EGOR over NiMo‐DA/PdNS at different applied potentials (0.7–1.7 V vs. RHE) for glycolaldehyde quantification. Two characteristic ¹³C NMR resonances at ∼87.1 and ∼61.5 ppm are assigned to the monomeric hydrated (gem‐diol) form of glycolaldehyde. The corresponding GC chromatograms used for CO₂ analysis are shown in Figure S49, where a distinct CO₂ peak is observed at a retention time of ∼12.5 min.

Figure 3b,c shows the estimated FE and YR of glycolate and formate produced during 1 h of EGOR electrolysis using the NiMo‐DA/PdNS electrocatalyst at various potentials (0.7–1.7 V vs. RHE). The FE of glycolate gradually increased at low potentials, where it was the predominant product, with only a trace amount of formate, reaching a maximum FE of ∼92.8% at 1.1 V vs. RHE. At high potentials, the FE of formate gradually increased from ∼52 % at 1.4 V vs. RHE, reaching a maximum of ∼78.4% at 1.5 V vs. RHE. Regarding YR, glycolate reached its highest value of 1.14 mmol h⁻¹ cm⁻² at 1.0 V vs. RHE with a FE of 81%, while formate reached its maximum YR of 0.39 mmol h⁻¹ cm⁻² at 1.6 V vs. RHE with a FE of 77.9%. Therefore, 1.0 and 1.6 V vs. RHE were selected as the optimal potentials for the selective production of glycolate and formate, respectively, via EG oxidation. At low potentials, the limited driving force predominantly enables C─H bond cleavage, favoring the C2 product (glycolate), while at high potentials, the strong driving force promotes C─C bond cleavage, favoring the C1 product (formate). However, at potentials above 1.6 V vs. RHE, the YR of formate decreases due to the increasing contribution of the competing OER. The calculated FE and YR for glycolaldehyde and CO₂ are summarized in Figures 3b,c and Figure S50. CO₂ formation is negligible at low potentials but gradually increases with increasing applied potential, reaching a maximum FE of ∼5.9% at 1.7 V vs. RHE with a corresponding YR of 0.40 mmol h⁻¹ cm⁻². In contrast, glycolaldehyde is predominantly formed at lower potentials and is progressively oxidized into downstream products, namely glycolate and formate, as the potential increases. This behavior indicates that EG is initially oxidized to glycolaldehyde via a two‐electron process, which serves as a key intermediate for both C2 (glycolate) and C1 (formate) product pathways [41]. At the optimal potential of 1.0 V vs. RHE, the FE and YR of glycolaldehyde are 11.5% and 0.334 mmol h⁻¹ cm⁻², respectively, while CO₂ formation remains minimal (FE ≈ 2.0%, YR ≈ 0.01 mmol h⁻¹ cm⁻²). At a higher potential of 1.6 V vs. RHE, the FE and YR of glycolaldehyde decrease sharply to 0.07% and 0.001 mmol h⁻¹ cm⁻², respectively, whereas CO₂ formation increases (FE ≈ 5.8%, YR ≈ 0.07 mmol h⁻¹ cm⁻²), reflecting enhanced overoxidation at elevated potentials. Overall, the total FE approaches ∼100%, with only trace amounts of glycolaldehyde and CO₂ detected, while glycolate and formate remain the dominant products over the NiMo‐DA/PdNS electrocatalyst. Figure 3d shows the ¹H NMR spectra of the anolyte collected after EGOR electrolysis using NiMo‐DA/PdNS at 1.0 and 1.6 V vs. RHE, confirming the presence of four major peaks corresponding to glycolate (∼3.89 ppm), formate (∼8.4 ppm), EG (∼3.59 ppm), and water (∼4.8 ppm), with no detectable impurities. The reliability and reproducibility of the EGOR performance were verified through repeated experiments (Figures S51–S54), with the results falling within acceptable error limits, as indicated by the error bars in Figure 3b,c and Figure S50.

To elucidate the advantages of NiMo‐DA/PdNS for enhancing EGOR efficiency toward glycolate and formate production, a systematic comparative study was conducted. EGOR electrolysis was performed at the optimal oxidation potentials of 1.0 and 1.6 V vs. RHE for selective C1 and C2 electro‐oxidation pathways using PdNS, Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS electrocatalysts. The 1 h EGOR electrolysis profiles performed in 1.0 M KOH + 0.5 M EG at a fixed oxidation potential of 1.0 and 1.6 V vs. RHE for the various electrocatalysts are shown in Figure S55. Reproducibility and reliability were assessed through repeated EGOR electrolysis tests using PdNS, Ni‐SA/PdNS, Mo‐SA/PdNS, and NiMo‐DA/PdNS under the same conditions at 1.0 and 1.6 V vs. RHE, with acceptable error margins, as shown in Figures S56 and S57. The anolytes collected after EGOR electrolysis at 1.0 and 1.6 V vs. RHE were analyzed using ¹H NMR and ¹³C NMR spectroscopy to quantify glycolate, formate, and glycolaldehyde, while gaseous CO₂ was quantified using GC. The corresponding spectra are shown in Figures S58–S60. The FE and YR of glycolate and formate at 1.0 and 1.6 V vs. RHE for each electrocatalyst are summarized in Figure 3e,f. NiMo‐DA/PdNS exhibited substantially boosted EGOR performance and selectivity toward glycolate and formate as the C1 and C2 products, respectively. It achieved a glycolate YR of 1.14 mmol h⁻¹ cm⁻² with a FE of 81% and a formate YR of 0.39 mmol h⁻¹ cm⁻² with a FE of 77.9%. These results outperform those of PdNS (glycolate: YR = 0.84 mmol h⁻¹ cm⁻², FE = 88.4%; formate: YR = 0.14 mmol h⁻¹ cm⁻², FE = 77.3%), Ni‐SA/PdNS PdNS (glycolate: 0.12 mmol h⁻¹ cm⁻², FE = 72%; formate: 0.05 mmol h⁻¹ cm⁻², FE = 54.5%), and Mo‐SA/PdNS PdNS (glycolate: 1.06 mmol h⁻¹ cm⁻², FE = 57.1%; formate: 0.32 mmol h⁻¹ cm⁻², FE = 85.2%). Furthermore, a comparative evaluation of the FE, YR, and synthetic advantages for selective glycolate and formate production using the NiMo‐DA/PdNS electrocatalyst (Table S7) demonstrates its superior performance over previously reported methods, both in terms of catalytic EGOR selectivity and structural integrity.

The superior EGOR performance and product selectivity of NiMo‐DA/PdNS compared to PdNS, Ni‐SA/PdNS, and Mo‐SA/PdNS can be attributed to the synergistic interaction between Ni and Mo atoms within its dimeric configuration. At low oxidation potentials, the NiMo‐DA sites efficiently activates C─H bond while suppressing extensive C─C bond cleavage, thereby favoring glycolate formation. By contrast, at high potentials, the synergy of Ni and Mo provides adjacent adsorption sites that stabilize transition states associated with C─C bond cleavage, driving EGOR selectively toward formate. The PdNS support further acts as both an electron reservoir and as a conductive support, ensuring efficient electron transport during EGOR catalysis. Moreover, the atomic‐level dispersion of Ni and Mo induces defect‐rich, strained PdNS surfaces and induces a downshift of the Pd d‐band center, as evidenced by XPS (discussed above. Together, these effects optimize the adsorption energies of EGOR intermediates and enhance the intrinsic EGOR activity of the NiMo‐DA/PdNS electrocatalyst. These structural and electronic modulations are further corroborated by the observed increases in C _dl, ECSA, and N sites, along with the reduced R _ct, thereby providing a mechanistic rationale for the significantly improved EGOR performance of NiMo‐DA/PdNS compared to PdNS, Ni‐SA/PdNS, and Mo‐SA/PdNS. Notably, the EGOR activity of Ni‐SA/PdNS was lower than that of bare PdNS, likely owing to limited charge redistribution upon Ni incorporation, which reduces electron donation to adsorbed species and, consequently, lower EGOR activity relative to Mo‐SA/PdNS and NiMo‐DA/PdNS—consistent with the higher Φ of Ni‐SA/PdNS obtained from UPS analysis (discussed above).

The electrochemical durability and structural stability of a catalyst are key parameters for evaluating its practical applicability. The long‐term EGOR stability of the NiMo‐DA/PdNS catalyst was assessed through 10 successive cyclic tests at optimized potentials of 1.0 and 1.6 V vs. RHE in 1.0 M KOH + 0.5 M EG electrolyte, corresponding to selective C2 and C1 product formation, respectively (Figure 3g,h). Each cycle consisted of 1 h of continuous electrolysis, with the electrolyte replaced after each cycle to maintain EG concentration owing to its consumption during the reaction. Following each cycle, the YR and FE of glycolate (the major C2 product at 1.0 V vs. RHE) and formate (the major C1 product at 1.6 V vs. RHE) were quantified via ¹H NMR spectroscopy (Figures S61 and S62), further confirming the durability and reproducibility of the catalyst. The negligible variations in YR and FE for both products across all cycles reinforce the sustained EGOR performance of NiMo‐DA/PdNS. In addition to its stability and reproducibility, the catalyst maintained distinct selectivity toward C2 and C1 products at the respective applied potentials. Furthermore, the stability evaluation was extended to 100 h of long‐term cyclic testing at two different potentials (1.0 and 1.6 V vs. RHE) to assess the practical applicability of the NiMo‐DA/PdNS catalyst. The results confirmed its excellent electrochemical durability and stability, supporting its potential for practical and economic viability (Figures S63 and S64). At 1.0 V vs. RHE, the estimated FE and YR of glycolate remained nearly constant throughout the 100‐h cyclic test, showing only minor fluctuations. By contrast, at 1.6 V vs. RHE, both the current density and FE of formate were mostly maintained with slight variations, whereas the YR of formate decreased significantly. This decline may be attributed to the accumulation of KOH and carbonate species (K₂CO₃ and KHCO₃) on the electrode surface during prolonged electrolysis, as evidenced by post‐characterization analyses.

The structural stability of NiMo‐DA/PdNS after 100 h of EGOR cycling was further examined using multiple post‐characterization techniques. As shown in Figure S65, the XRD pattern of NiMo‐DA/PdNS supported on CC (NiMo‐DA/PdNS/CC) before electrolysis exhibited diffraction peaks at 2θ = 40.1° corresponding to the (111) plane of cubic PdNS (JCPDS No. 46‐1043), along with peaks at 2θ = 25.2° and 44.4° characteristic of the graphitic structure of CC. After 100 h of the EGOR, these characteristic peaks of NiMo‐DA/PdNS and the CC substrate were preserved; however, several new peaks appeared, consistent with the deposition of KOH and carbonate phases (K₂CO₃ and KHCO₃) on the electrode surface during long‐term operation. FESEM images (Figure S66) further confirmed that the overall surface morphology of NiMo‐DA/PdNS/CC was largely retained after 100 h of cycling, showing only slight agglomeration, indicating strong catalyst adhesion to the CC substrate. In addition, EDS elemental mapping demonstrated uniform retention of Pd, Ni, Mo, O, N, and C after 100 h of electrolysis at both 1.0 and 1.6 V vs. RHE (Figures S66 and S67), confirming the excellent compositional and structural stability of the catalyst. Notably, post‐electrolysis EDS also revealed the presence of K, further supporting the XRD evidence of surface deposition of KOH and carbonate species.

XPS analysis of NiMo‐DA/PdNS/CC before and after 100 h of EGOR cyclic testing at two different potentials (1.0 and 1.6 V vs. RHE) was performed to further confirm the structural stability of NiMo‐DA/PdNS/CC and to validate the post‐characterization results from XRD and FESEM with EDS mapping. The XPS survey spectra of NiMo‐DA/PdNS/CC before EGOR testing (Figure S68a) revealed the consistent presence of Pd, Ni, Mo, and C, along with surface O species, as well as F and S originating from the Nafion binder used to coat the NiMo‐DA/PdNS catalyst on the CC substrate. After EGOR testing at both 1.0 and 1.6 V vs. RHE, NiMo‐DA/PdNS/CC retained all these elements. Additionally, K species (K₂CO₃ and KHCO₃) deposited from the KOH electrolyte during EGOR electrolysis were observed. The increased O and C signals may be attributed to adsorbed EGOR intermediates and the formation of glycolate (C2) and formate (C1) products. These results confirm the structural and elemental integrity of NiMo‐DA/PdNS/CC after prolonged EGOR operation. Further insights into the chemical states were obtained via deconvolution of the XPS core‐level spectra of Pd 3d, Ni 2p, and Mo 2p (Figure S68b–d), with the corresponding peak assignments and binding energies summarized in Table S8. Owing to the overlap of O 1s and Pd 3d signals, detailed information on surface reconstruction of NiMo‐DA/PdNS during EGOR electrolysis could not be extracted. The Pd 3d spectra exhibited characteristic spin–orbit splitting into Pd 3d _5/2 and Pd 3d _5/2, further deconvoluted into bulk and surface Pd⁰ and Pd²⁺ oxidation states, with bulk Pd⁰ as the dominant species both before and after EGOR stability tests (Figure S68b). The Ni 2p spectra revealed multiple components, including Ni⁰, Ni²⁺, and satellite peaks (Figure S68c), with an additional F 1s contribution from the Nafion binder. Similarly, the Mo 3d spectra exhibited spin–orbit splitting into Mo 3d _5/2 and Mo 3d _3/2, with overlapping S 2s signals from the Nafion binder. Deconvolution revealed the coexistence of Mo⁴⁺ and Mo⁶⁺ states, with Mo⁴⁺ as the dominant species both before and after EGOR stability tests (Figure S68d). Slight shifts toward higher binding energies after EGOR stability tests indicate interactions between the NiMo‐DA/PdNS/CC surface and adsorbed oxygen species from the electrolyte, as well as EGOR intermediates (glycolaldehyde) or products (glycolate and formate). These interactions, reflected in the Pd 3d, Ni 2p, and Mo 2p spectra (Table S8), suggest electronic polarization and changes in the local electronic environment of the NiMo‐DA/PdNS catalyst surface. Overall, these findings highlight the adaptability and dynamic surface chemistry of NiMo‐DA/PdNS/CC under EGOR conditions.

To elucidate the EGOR mechanism and rationalize the NiMo‐DA/PdNS performance toward selective C1 and C2 electro‐oxidation pathways of EG for glycolate and formate production, respectively, in situ Raman spectroelectrochemistry was conducted to identify EGOR intermediates under applied oxidation potentials spanning a wide range from low to high values. NiMo‐DA/PdNS/CC served as the working electrode, a graphite rod served as the counter electrode, and Hg/HgO as the reference electrode in a specially designed Teflon flow cell for electrochemical measurements. A Raman microscope with a 532 nm excitation laser was connected to the cell. In situ Raman spectra of NiMo‐DA/PdNS recorded in 1.0 M KOH + 0.5 M EG electrolyte from 0.7 to 1.7 V vs. RHE at 0.1 V increments are shown in Figure 4a. At open‐circuit potential (OCP), the in situ Raman spectra display a distinct vibrational band at 951 cm⁻¹, corresponding to the characteristic EG modes (ν(C─C/C─O) and δ(CH₂)) along with the D and G bands of the CC substrate observed at 1346 and 1590 cm⁻¹, respectively. Upon applying potentials in the low region (0.7–1.0 V vs. RHE), new vibrational features emerges at 863, 1061–1087, and 1456 cm⁻¹, which can be assigned to glycolate‐related ν(C─C), ν(C─O), and δ(CH₂) modes, respectively. The coexistence of these signals with the EG peaks indicates that oxidation proceeds predominantly toward the C2 glycolate product at low potentials. Notably, the intensities of the D and G bands decrease progressively as the molecular δ(CH₂) vibrations of glycolate become prominent, suggesting increasing coverage of oxidation intermediates on the catalyst surface. At high potentials (1.5–1.7 V vs. RHE), the ν(C─O) bands of formate dominate, accompanied by near‐complete suppression of glycolate features. This confirms a stepwise oxidation pathway of EG: EG → glycolate (C2) at low potentials and EG → formate (C1) at high potentials, highlighting the potential‐dependent selectivity of NiMo‐DA/PdNS catalysts toward C2 and C1 mechanistic pathway of the EGOR.

FIGURE 4.

(a) In situ Raman spectra recorded during EGOR over NiMo‐DA/PdNS at different potentials, (b, c) Free energy diagram for EGOR on PdNS and NiMo‐DA/PdNS at different applied potentials (from 0 to 1.5 V vs. RHE), computed with CEP approach. The key steps and potentials where the free energy changes from positive to negative are labeled in black text, (d) Schematic representation of proposed C─C bond cleavage/oxidation mechanism for EG to FA, (e) Free energy profile for EG C─C bond dissociation as a function of applied potential for PdNS (black) and NiMo‐DA/PdNS (blue), (f) Side and top view of the optimized structures before and after EG cleavage on PdNS and NiMo‐DA/PdNS, and (g) Schematic of electron transfer during the cleavage process.

Guided by the preceding XPS, EDS, and EXAFS analyses, we constructed DFT slab models that explicitly incorporate partially oxidized surface states for pristine PdNS and NiMo‐DA/PdNS. The optimized top and side view geometries shown in Figure S69 indicate that in NiMo‐DA/PdNS, Ni exhibits higher coordination with Pd, yielding an average Pd‐Ni bond length of ∼2.66 Å. By contrast, Mo interacts less strongly with Pd but couples more strongly with Ni, forming a Ni‐Mo dimer with a bond length is 2.59 Å. The projected density of states (PDOS) for NiMo‐DA/PdNS (Figure S70a–c) further reveals pronounced Ni‐Mo signal overlap near the Fermi‐level, consistent with strong electronic coupling. These structural motifs persist throughout subsequent optimizations of adsorbates and intermediates. We further calculated the d‐band centers of the two systems. NiMo‐DA/PdNS exhibits a d‐band center at ‒2.45 eV, much lower than that of PdNS (‒2.37 eV). This downward shift is consistent with moderated adsorbate binding and more efficient interfacial charge transfer, offering a plausible electronic‐structure rationale for the experimentally observed enhancement in oxidation performance.

The EGOR network was first treated within the computational hydrogen electrode (CHE) framework, evaluating two proton‐coupled electron transfer (PCET) manifolds: an Eley‐Rideal (E‐R) route in which H atoms are abstracted by solution OH^‒, and a Langmuir‐Hinshelwood (L‐H) route in which H transfers to a neighboring surface oxygen site (Figure S71) [42, 43, 44, 45]. Within these mechanisms, EG undergoes four sequential PCET steps to yield GA (HOCH₂COOH), which subsequently desorbs. In addition, the *HOCH₂OH intermediate can couple with an adjacent oxygen to directly form GA.

The free energy landscape for EGOR on PdNS structure (Figure S72), along with optimized structures of intermediates (Figure S73), indicates that EG binds weakly with only 0.05 eV of free energy and preferentially at residual metallic Pd sites embedded in the partially oxidized surface. From this state, the first E‐R PCET step requires 0.38 eV to form *HOCH₂CHOH, for which the distance between Pd to the unsaturated carbon is 2.19 Å. The L–H mechanism of this step is thermodynamically favorable; however, it necessitates an additional PCET to re‐oxidize (recover) the neighboring site, incurring 1.08 eV of free energy cost. The next PCET step to produce *HOCH₂COH demands an energy comparable to the first step (0.43 eV), and its L–H mechanism also remains favorable, underscoring the high hydrogen affinity of proximal oxygen sites.

We further scanned a direct O‐coupling pathway from *HOCH₂COH in which a neighboring oxygen directly couples with the intermediate and furnish a GA intermediate. However, this process is strongly endothermic and therefore implausible under operating conditions; instead, an exergonic PCET step forms *HOCH₂CHOOH on the surface. The terminal PCET to generate *HOCH₂COOH intermediate on the surface requires 0.75 eV. From the optimized product geometry (Figure S73), we can see the carbonyl group orients toward a Pd site, while the hydroxyl group forms a hydrogen bond with a neighboring surface oxygen, yielding a desorption free energy of 0.58 eV. Within the calculations, the energetics translate into a limiting potential of 0.75 V vs. RHE for the E‐R pathway, set by the final PCET step. When the L‐H mechanism is operative, the necessity of the surface‐recovery PCET raises the limiting potential to 1.14 V vs. RHE. Collectively, the calculations highlight how partially oxidized Pd ensembles and adjacent oxygen sites modulate PCET thermodynamics and steer the mechanistic preference for EG oxidation toward GA on Pd‐based nanosheet catalysts.

Extending the analysis to NiMo‐DA/PdNS, we computed the free energy profile for EGOR and summarized the results in Figure S74a. EG binds appreciably more strongly on NiMo‐DA/PdNS than on PdNS, with an adsorption free energy of ‒0.20 eV (0.05 eV on PdNS). The optimized adsorption geometry (Figure S74b) reveals that EG adopts a cis conformation that enables both O atoms to interact with the Mo center; the Mo‐O distances to the two oxygens are 2.15 and 2.27 Å, respectively. This bidentate interaction underscores the electronic and geometric advantage of the NiMo dimer and rationalizes the enhanced adsorption strength. To elucidate the bonding interaction, we analyzed the PDOS for Mo and the O atoms of EG before and after adsorption (Figures S70b,c). Prior to adsorption, the O‐2p states are dominated by localized lone‐pair features, while the Mo‐4d manifold exhibits appreciable density near the corresponding energies. Upon adsorption, the O‐2p peaks broaden and decrease in intensity, and pronounced overlap with Mo‐4d states emerges between ‒5 and ‒2 eV, providing clear evidence of orbital hybridization and interfacial charge redistribution. From this state, the first PCET step to form *HOCH₂CHOH requires 0.78 eV, which is larger than on PdNS, indicating that the stronger initial binding partially elevates the barrier to the first deprotonation/oxidation. In contrast, the next two PCET steps are thermodynamically favorable, and the terminal PCET to furnish GA requires 0.57 eV. The resulting product binds through both its carbonyl and hydroxyl oxygens to the Mo site, which yields a larger desorption free energy (1.12 eV) relative to PdNS. Nevertheless, under strongly alkaline operating conditions, product deprotonation to glycolate is expected to mitigate surface retention, suggesting that product release will not be rate‐limiting on NiMo‐DA/PdNS despite the intrinsically stronger Mo–O interactions.

In addition, the EGOR reaction pathways were calculated for Ni‐SA/PdNS and Mo‐SA/PdNS and compared with those of PdNS and NiMo‐DA/PdNS (Figure S75). The results show that Ni‐SA/PdNS follows a reaction pathway similar to that of PdNS; however, it exhibits a higher free‐energy barrier for the potential‐determining step (PDS), with a value of 0.83 eV compared to 0.75 eV for PdNS. This increased PDS energy is consistent with the experimentally observed lower current density of Ni‐SA/PdNS (Figure 3a). In contrast, Mo‐SA/PdNS exhibits a reaction pathway more similar to that of NiMo‐DA/PdNS. Despite demonstrating stronger adsorption of EG intermediates, Mo‐SA/PdNS shows an even higher PDS free‐energy barrier (0.87 eV), indicating suboptimal reaction energetics. These results suggest that excessively strong adsorption leads to over‐stabilization of intermediates, thereby hindering subsequent reaction steps and reducing overall catalytic efficiency.

To further interrogate potential effects, we adopted the CEP approach [46], wherein the system's Fermi level is adjusted to emulate a controlled applied potential. This method enables a more faithful assessment of how electrode potential modulates reaction energetics. We compiled free energy profiles for EGOR on PdNS and NiMo‐DA/PdNS at 0.0, 0.5, 1.0, 1.25, and 1.5 V vs. RHE (Figure 4b,c). On PdNS, EG adsorption strengthens slightly with increasing potential (red to purple to blue dashed lines), indicating a subtle but non‐negligible degree of electron transfer accompanying adsorption, which is an effect that CEP captures but is often neglected in conventional CHE analyses. For the subsequent PCET steps, greater charge transfer produces pronounced potential‐dependent shifts. On PdNS, the first deprotonation (*EG to *HOCH₂CHOH) is uphill at 0.0 V (red dashed line) and becomes favorable only at potentials >1.0 V. The second step (*HOCH₂CHOH to *HOCH₂COH) shows a similar trend: increasing the potential from 1.0 to 1.25 V shifts the free‐energy change from 0.06 eV to ‒0.28 eV. The third step (*HOCH₂COH to *HOCH₂CHOOH) is exergonic at all surveyed potentials. The fourth PCET becomes favorable only above 1.0 V. Nevertheless, consistent with the weak and only subtly charge transfer‐assisted adsorption of GA, desorption exhibits minimal potential dependence.

For NiMo‐DA/PdNS (Figure 4c), EG adsorption is favorable at all potentials examined, and the second PCET step is spontaneously downhill across the full potential range. By contrast, the third PCET step requires intermediate potentials (0.5–1.0 V) to proceed favorably. The first and fourth PCET steps are then emerged as the potential‐determining steps (PDS), requiring potentials >1.0 V to drive the reaction. Overall, within the CEP framework, both PdNS and NiMo‐DA/PdNS exhibit PDS in the 1.0‐1.25 V vs. RHE range, aligning with the LSV responses in Figure 3a, where the peak current density for glycolate formation appears between 1.0 and 1.25 V. The higher current densities observed for NiMo‐DA/PdNS can plausibly be attributed, at least in part, to its superior EG adsorption arising from the bidentate Mo‐O binding motif.

Having established the EG‐to‐glycolic‐acid pathway, we next analyze the high‐potential route leading to FA formation. Figure 4d summarizes the proposed mechanism from EG to FA. Upon EG adsorption, in addition to the PCET sequence described above, C‒C bond cleavage can occur at this early stage to yield two surface *HOCH₂ species. Each *HOCH₂ can couple with a neighboring surface oxygen to form *OCH₂OH, followed by a PCET step to generate *HCOOH and ultimately, desorption. Alternatively, *HOCH₂ may first undergo deprotonation to *HOCH₂OH, which can then evolve to *OCH₂OH or *HOCHOH route to FA. A third branch involves deprotonation to *HOCH, the fragment expected from C‒C scission of *HOCH₂CHOH; *HOCH can then undergo two PCET steps to *HOCHOH and *HCOOH, or directly couple with a neighboring surface oxygen to form FA. We considered all of these possibilities in the proposed network.

We then computed the corresponding free energy profile for EG on PdNS within this framework in Figure S76 and demonstrate optimized structures in Figure S77. On PdNS, direct C─C cleavage of EG (blue dashed line) requires 0.55 eV to form two *HOCH₂ fragments. In contrast, if EG first executes the initial PCET to *HOCH₂CHOH and then undergoes C─C scission, the step is highly endothermic (2.10 eV) and therefore implausible. We therefore focus on the downstream chemistry of the *HOCH₂ fragment produced by EG decomposition. In this state, the Pd─C bond length is 2.17 Å. PCET to form *HOCH or *HOCH₂OH is endothermic (0.68 and 0.21 eV, respectively), whereas coupling with a neighboring surface oxygen to produce *OCH₂OH is favorable. However, this O‐coupling branch demands additional free energy to re‐oxidize the surface (recover *O_vac to *OH). By comparison, *HOCH can proceed through two PCET steps to *HOCHOH and then *HCOOH; in the optimized *HCOOH structure, the carbonyl oxygen approaches a Pd site with a Pd‐O distance of 2.33 Å. O‐coupling from *HOCH is likewise thermodynamically favorable, but again entails a sizable energetic cost for surface recovery. The *HOCH₂OH intermediate can undergo two distinct deprotonation steps to form *OCH₂OH and *HOCHOH, both favorable, while *OCH₂OH binds at a Pd bridge site with Pd─O bond distances of 2.12 and 2.14 Å. At the end of the reaction profile, FA desorption is favorable. In sum, along the formic‐acid pathway, even the PDS does not require prohibitively large free energies. Instead, at high potentials, the dominant factor governing FA production appears to be the potential dependence of the initial C─C bond cleavage.

Motivated by these observations, we finally employed the CEP framework to investigate how the applied potential modulates C─C bond cleavage in EG. We scanned potentials from 0.0 to 2.0 V vs. RHE and compared the dissociation energetics on PdNS and NiMo‐DA/PdNS. The resulting trends are compiled in Figure 4e, while representative optimized geometries before and after cleavage are shown in Figure 4f. At 0.0 V, the dissociation free energy is similar for both catalysts, approximately 1.20 eV for PdNS (black line) and NiMo‐DA/PdNS (blue line). As the potential increases, the dissociation energy decreases sharply. At 2.0 V, it drops to 0.39 eV on PdNS and to only 0.17 eV on NiMo‐DA/PdNS, consistent with enhanced formation of C1 products at high potentials. Notably, NiMo‐DA/PdNS exhibits a stronger potential sensitivity, manifested as a steeper slope in Figure 4e. To rationalize this behavior, we conducted a Bader charge analysis. During C─C bond dissociation, EG transfers an electron to the surface (Figure 4g); the net transfer amounts to 0.48|e| on PdNS and a more substantial 0.57|e| on NiMo‐DA/PdNS. These results indicate an electronically driven mechanism in which the applied potential modulates interfacial charge distribution, thereby tuning adsorption‐induced charge transfer and stabilizing the transition state associated with C─C bond scission. Consistently, for NiMo‐DA/PdNS, the surface charge density evolves from −18.02 µC cm^‒2 at 0 V to +13.58 µC cm^‒2 at 1.0 V, corresponding to an interfacial capacitance of 31.6 µF cm^‒2. Because the C2 intermediate donates electrons to the surface during C─C cleavage, a more positively polarized surface at higher potentials enhances electron withdrawal and redistribution, thereby lowering ΔG for C─C bond scission. The larger electron transfer on NiMo‐DA/PdNS naturally yields a stronger potential dependence, consistent with the steeper decrease observed in Figure 4e.

For comparison, the potential‐dependent C─C cleavage energetics on Ni‐SA/PdNS and Mo‐SA/PdNS were also evaluated (Figure S78). Ni‐SA/PdNS exhibits the highest C─C cleavage barriers across the entire potential range, indicating that isolated Ni sites are ineffective in promoting C─C bond scission. In contrast, Mo‐SA/PdNS shows relatively low barriers at low potentials; however, its potential sensitivity is limited, and the barrier increases with increasing potential, eventually approaching or exceeding that of PdNS. Overall, these DFT results elucidate the origin of the superior catalytic performance of NiMo‐DA/PdNS. The DA configuration provides an optimal energetic landscape by (i) avoiding the high PDS penalties observed for SA systems and (ii) maintaining strong, potential‐responsive C─C cleavage energetics. These synergistic effects promote efficient EGOR and selective formation of C1 products.

Although noble‐metal‐based catalysts generally exhibit high activity and selectivity toward EG oxidation, their high cost and scarcity limit large‐scale applications. In contrast, the NiMo‐DA/PdNS catalyst, which incorporates atomically dispersed Ni and Mo atoms with PdNS, not only exhibits superior intrinsic EGOR performance but also achieves efficient and highly selective conversion to both C1 and C2 products, thereby reducing noble‐metal loading and enhancing the feasibility of sustainable EG valorization. In situ Raman spectroelectrochemistry coupled with DFT elucidates the reaction network: at lower potentials, the EGOR proceeds predominantly along a C2 pathway via a four‐electron process to produce glycolate as the major product without C─C bond cleavage; at higher potentials, a C1 pathway becomes operative, initiated by C─C cleavage of EG and followed by successive three‐electron steps for each intermediate to yield FA. Within the CEP framework, electron transfer analysis reveals that NiMo‐DA/PdNS exhibits greater potential sensitivity for EG bond cleavage than pristine PdNS, thereby lowering the energetic cost of C─C dissociation at elevated potentials and promoting the formation of FA under high potential conditions.

Based on the outstanding EGOR performance of NiMo‐DA/PdNS, we extended our investigation to real‐world PET‐derived hydrolysate for the selective production of value‐added C2 and C1 products, glycolate and formate, respectively. The PET hydrolysate was prepared via alkaline hydrolysis of 35 g of crushed PET bottles in 200 mL of 6.0 m KOH at 80°C under continuous stirring for 72 h, yielding a solution containing TPA‐K and EG as the primary PET monomers (Figure 5a). Quantification via ¹H NMR (Figure S79) revealed an EG concentration of 0.84 M in 6.0 M KOH. For subsequent PETHOR studies, the hydrolysate was diluted to 1.0  KOH, resulting in an effective EG concentration of ∼0.15 m. Compared to the standard electrolyte containing 0.5 M EG in 1.0 M KOH, the PET hydrolysate exhibited significantly lower oxidation peak intensities. To establish a reference, EGOR LSVs were recorded in electrolytes with controlled EG concentrations (0.25, 0.20, and 0.15 M in 1.0 M KOH) alongside PET hydrolysate (Figure S80). The LSV profile of PET hydrolysate closely matched that of 0.15 M EG + 0.02 M TPA, confirming the coexistence of EG and TPA‐K in the hydrolysate. During prolonged PETHOR for 50 h at fixed potentials (Figure S81), EG in the hydrolysate underwent selective oxidation, yielding glycolate (C2) at 1.0 V vs. RHE and formate (C1) at 1.6 V vs. RHE, while TPA‐K remained unreacted in the electrolyte. A slight decline in current density was observed over time, consistent with the gradual depletion of EG monomers. Post‐electrolysis ¹H NMR spectra (Figures S82 and S83) confirmed the product distribution: at 1.0 V vs. RHE, glycolate was the dominant product (δ ≈ 3.85 ppm) with a FE of 76.5% and a YR of 0.04 mmol h⁻¹ cm⁻², along with residual EG (δ ≈ 3.56 ppm), TPA (δ ≈ 7.79 ppm), and H₂O (δ ≈ 4.80 ppm). At 1.6 V vs. RHE, formate became the major product (δ ≈ 8.36 ppm) with a FE of 43.8% and a YR of 0.03 mmol h⁻¹ cm⁻², while signals from EG, TPA, and H₂O were also observed. These results confirm the potential‐dependent selectivity of PETHOR on NiMo‐DA/PdNS catalysts, consistent with the EGOR pathway, and highlight the feasibility of converting PET hydrolysates into value‐added C2 and C1 products.

FIGURE 5.

(a) PET hydrolysis in 6.0 M KOH to prepare PET hydrolysate, followed by 100 h of the PETHOR process using NiMo‐DA/PdNS at 1.0 and 1.6 V vs. RHE, leading to the recovery of TPA and the formation of value‐added products (KDF and PGGA), along with recrystallization yields. Techno‐economic analysis: (b) breakdown of total production costs under different operational scenarios, (c) total production cost (red color) and net profit at various value‐added products for high‐ and low‐potential PETHOR branches, and (d) comparison of treatment cost and net profit cost (USD per tonne of PET) for the present study versus literature‐reported catalysts.

To capitalize on the economic value of waste PET upcycling, the PET hydrolysate obtained after 50 h of PETHOR was subjected to the separation and purification of the selective C2 and C1 products, along with recovery of the TPA monomer from PET plastics. Following PETHOR at both low and high potentials, the electrolyte pH was adjusted to below 3.7 using GA for the 1.0 V vs. RHE electrolyte and FA for the 1.6 V vs. RHE electrolyte to isolate TPA as a crystalline powder. The detailed recrystallization process is provided in the Experimental Section (Supporting Information). The yield of recrystallized TPA was 95.92% and 96.78% for post‐electrolysis at 1.0 and 1.6 V vs. RHE, respectively (Figure 5a). The purity of the recovered TPA was confirmed via powder XRD and FTIR analyses (Figure S84), which matched well with previous reports [47, 48]. Its XRD diffraction pattern corresponded closely to the standard JCPDS card of TPA (#03‐0127). After isolating TPA, the residual post‐electrolytes were further processed to recover PGGA from the 1.0 V vs. RHE sample and KDF from the 1.6 V vs. RHE sample. This was achieved by concentrating the electrolytes under reduced pressure using a rotary evaporator at 80°C until supersaturation, followed by aging for 2 days. Detailed procedures for the crystallization of PGGA and KDF are provided in the Experimental Section (Supporting Information). The purity of the recovered PGGA and KDF powders was validated via XRD and FTIR analyses (Figures S85 and S86), both of which matched the corresponding standard patterns and were consistent with previous literature reports [41, 49]. Overall, these results demonstrate that electrochemical PET upcycling via PETHOR not only enables the selective production of high‐value glycolate (C2) and formate (C1) products but also allows efficient recovery of the TPA monomer. This integrated process maximizes the economic profitability of PET recycling and underscores its real‐world practicality, with the recovered TPA monomer offering additional value for re‐polymerization into new PET plastics.

2.3. Techno‐Economic Analysis and Cost–Benefit Evaluation

A comprehensive TEA was performed to evaluate the economic feasibility of electro‐reforming PET hydrolysate (obtained from the alkaline hydrolysis of waste PET plastics) into glycolate (C2 pathway) and formate (C1 pathway), with simultaneous recovery of high‐purity TPA monomers. PETHOR was conducted for 50 h at 1.0 and 1.6 V vs. RHE using the NiMo‐DA/PdNS catalyst, after which the electrolyzed products were separated and purified to obtain PGGA, KDF, and TPA. TEA was conducted using models proposed in the literature [8, 50], with a detailed step‐by‐step procedure for cost–benefit evaluation provided in the Supporting Information. The total production costs were estimated by summing the reagent cost, electricity‐related cost (electrolyzer capital expenditure (CapEx) and electricity operational expenditure OPEX), balance‐of‐plant (BoP) costs, other OPEX, and installation costs. As shown in Figure 5b, the total production cost for the PGGA (C2) pathway was calculated to be USD 2,125.25 t⁻¹, whereas the KDF (C1) pathway exhibited a higher cost of USD 4,196.17 t⁻¹. Reagent costs contributed the largest fraction of the total production cost, followed by electricity‐related costs and BoP expenses. The lower cost of the C2 pathway mainly arises from its reduced energy demand (1.0 V vs. RHE) compared with the formate route (1.6 V vs. RHE), highlighting the energy‐efficiency advantage of glycolate production.

Profitability assessment (Figure 5c) shows that the gross profit per ton of product is substantially higher for PGGA (USD 87,874.74 t^{− 1}) and KDF (USD 75,803.82 t^{− 1}), with gross margins of 97.64% and 94.75%, respectively. By contrast, the TPA chemcial recovered from both pathways show lower gross margins (42.5%–71.0%) owing to their comparatively lower market value than PGGA and KDF. When normalized per tonne of PET processed, the NiMo‐DA/PdNS catalyst developed in this study exhibits the highest economic benefit, with net profit estimated at USD 1,785.08 tPET⁻¹ for GA, PGGA, and TPA) and USD 1,168.25 tPET⁻¹ for FA, KDF, and TPA) (Figure 5d). These values substantially outperform previously reported systems based on NiMo‐W, NiMoO₄·xH₂O, Pd/Ni(OH)₂, Fe₂O₃/Ni(OH) _x , and CuCo₂O₄ [51, 52, 53, 54, 55], as well as conventional PET treatment methods. Compared to conventional PET disposal and recycling processes—landfilling (USD 80 tPET^{− 1}), incineration (USD 295 tPET^{− 1}), and mechanical recycling (USD 125 tPET^{− 1})—the present electro‐reforming approach offers not only an environmentally benign route but also superior economic returns. The dual production of GA and TPA or FA and TPA establishes a closed‐loop, value‐added PET upcycling strategy, where waste plastics are transformed into high‐value fine chemicals and recyclable monomers with significant profit margins. Overall, TEA clearly demonstrates that the proposed NiMo‐DA/PdNS‐catalyzed electro‐reforming system provides a scalable, economically viable, and sustainable approach for PET upcycling into high‐value selective products.

3. Conclusion

We report a rapid and scalable strategy for immobilizing heteronuclear NiMo‐DA sites on PdNS via CO₂ laser irradiation (λ ≈ 10.6 µm, power ≈ 7 W) within 10 min. This CO₂ laser–assisted approach establishes a generalizable route for constructing atomically precise SAC and DAC catalysts, overcoming the high‐temperature requirements, prolonged synthesis times, and metal aggregation commonly associated with conventional methods. PdNS (palladene), synthesized via CO gas‐mediated solution‐phase reduction, serves as a highly conductive and structurally robust support for stabilizing DA sites. Leveraging this platform, NiMo‐DA/PdNS exhibits pronounced potential‐dependent product selectivity in EGOR compared to Ni‐SA/PdNS and Ni‐SA/PdNS, producing glycolate (C2 product) at 1.0 V vs. RHE with a YR of 1.14 mmol h⁻¹ cm⁻² and a FE of 81% via C─H bond activation, and formate (C1 product) at 1.6 V vs. RHE with a YR of 0.39 mmol h⁻¹ cm⁻² and a FE of 77.9% via C─C bond cleavage. At low potentials, glycolate formation is favored, whereas high potentials drive formate production. The PdNS support also enhances electron transport by serving as a conductive reservoir. DFT calculations employing the CEP framework resolve the EGOR mechanism and reveal that C─C scission in EG is accompanied by electron transfer to the catalyst surface. This effect lowers the reaction energy at elevated potentials, particularly on NiMo‐DA/PdNS, thereby accounting for the enhanced C1 productivity at higher potential. Importantly, this strategy enables the selective production of glycolate and formate from PET hydrolysate electrolysis, while also allowing the recovery of high‐purity TPA over 50 h of continuous operation—extending beyond prior studies that focused on single‐product pathways. Mechanistic insights from in situ Raman spectroelectrochemistry and DFT calculations validate the C2 and C1 pathways, and TEA confirms the scalability and cost‐effectiveness of the system. Overall, this study establishes CO₂ laser–assisted synthesis of PdNS‐supported heteronuclear dimers as a powerful design paradigm and demonstrates potential‐tunable electrocatalysis as a practical approach for the sustainable upcycling of PET waste into high‐value chemicals and monomers.

4. Experimental Section

4.1. Synthesis of Palladium Nanosheets via CO Gas–Mediated Solution‐Phase Reduction Method

PdNS were synthesized using a CO gas–mediated solution‐phase chemical reduction process. In a typical procedure, Pd(acac)₂ (10 mg, 0.03 mmol) and glacial acetic acid (10 mL) were placed in a 25 mL two‐neck round‐bottom flask equipped with a reflux condenser, a CO gas inlet, and a septum‐sealed outlet. The suspension was heated to 90°C on a hot plate. Once the temperature stabilized, CO gas was purged through the reaction mixture for 20 min at a constant flow rate of 10 mL min⁻¹, establishing a CO‐rich atmosphere within the flask. The CO inlet was then withdrawn while maintaining a slight CO gas atmosphere in the reaction flask, and the reaction was aged at 90°C for an additional 2 h. During this period, the mixture gradually darkened, indicating the formation of ultrathin PdNS crystals. After completion, the reaction mixture was cooled to ambient temperature, and the solid product was collected via centrifugation (10000 rpm, 10 min). The precipitate was washed three times each with deionized (DI) water and ethanol to remove residual acetic acid and unreacted precursor, followed by vacuum drying at ambient temperature for 6 h. The resulting product consisted of high‐purity PdNS.

4.2. Synthesis of NiMo‐DA/PdNS via the CO₂ Laser Irradiation Method

PdNS (5.0 mg) were ultrasonically dispersed in a mixture of DI water (2.0 mL) and ethanol (5.0 mL) in a 20 mL vial. Ni(OCOCH₃)₂·4H₂O (0.5 mg/mL, 6.0 mg) and (NH₃)₂MoO₄·4H₂O (0.5 mg/mL, 6.0 mg) were added sequentially, followed by the dropwise addition of NH₄OH solution (5.0 mL) under continuous sonication for 10 min. The reaction mixture was then irradiated with a continuous‐wave CO₂ laser (λ ≈ 10.6 µm, power ≈ 7 W) for 10 min under sonication in an ice bath. The resulting product was collected via centrifugation (10 000 rpm, 10 min), washed three times each with DI water and ethanol, and vacuum‐dried at ambient temperature for 6 h, yielding NiMo‐DA/PdNS as a black powder.

For comparison, Ni single‐atoms on PdNS (Ni‐SA/PdNS) and Mo single‐atoms on PdNS (Mo‐SA/PdNS) were synthesized following the same procedure as NiMo‐DA/PdNS, except without the Mo precursor for Ni‐SA/PdNS and without the Ni precursor for Mo‐SA/PdNS.

4.3. EGOR and PET Hydrolysate Oxidation Experiments

All electrochemical experiments were performed using a CHI708E electrochemical analyzer in a three‐electrode H‐cell configuration, comprising a Hg/HgO reference electrode, a graphite rod counter electrode, and the synthesized catalyst (via CO₂ laser irradiation) coated onto a CC substrate as the working electrode (WE). The H‐cell was separated by a pre‐treated Nafion‐118 proton exchange membrane (PEM). For EGOR experiments, 1.0 M KOH was used as the catholyte, and 0.5 M EG in 1.0 M KOH was used as the anolyte. The PEM was pre‐treated by sequential soaking in 3% H₂O₂ (1 h), DI water (2 h), and 0.5 M H₂SO₄ at 80°C (1 h), followed by thorough rinsing with DI water. The CC substrate was pre‐treated by soaking in 0.5 Mm H₂SO₄ for 24 h to enhance wettability, then air‐dried at 50°C overnight. The WE was prepared by ultrasonically dispersing 1 mg of the synthesized catalyst in a mixture of 95 µL of DI water, 95 µL of ethanol, and 10 µL of 5 wt.% Nafion solution (binder) for 30 min to obtain a homogeneous ink. The ink was drop‐cast onto the pre‐treated CC substrate (1 cm × 1 cm) and dried on a hot plate at 75°C, followed by overnight drying in a desiccator.

Prior to EGOR measurements, the WE was electrochemically activated via 200 cyclic voltammetry (CV) cycles in 1.0 M KOH over a potential window of 0.92–1.52 V vs. RHE at a scan rate of 50 mV s⁻¹. EGOR activity was evaluated using linear sweep voltammetry (LSV) at a scan rate of 5 mV s⁻¹, with and without EG in the anolyte. Chronoamperometry was conducted for 1 h at various potentials to quantify glycolate and formate as the selective C2 and C1 products, respectively. Unless otherwise specified, all potentials are reported versus the RHE scale using the following relation:

$$E_{\mathrm{RHE}}=E_{\mathrm{Hg}/\mathrm{HgO}}^{\circ }+E_{\mathrm{Hg}/\mathrm{HgO}}+0.059\times \mathrm{pH}$$

where E° _Hg/HgO and E_Hg/HgO denote the standard and experimental potentials of the Hg/HgO reference electrode, respectively, and pH ≈ 14 corresponds to 1.0 M KOH.

The PET upcycling reaction was performed using PET hydrolysate prepared via alkaline hydrolysis of 35 g of crushed PET bottles in 200 mL of 6.0 M KOH at 80°C under continuous stirring for 72 h. The resulting hydrolysate contained potassium terephthalate (TPA‐K) and EG as the primary PET monomers, with an estimated EG concentration of 0.84 M in 6 M KOH. Residual PET solids were removed via vacuum filtration using Whatman filter paper.

The PET hydrolysate oxidation reaction (PETHOR) was conducted following the same procedure as the EGOR because EG derived from PET hydrolysate undergoes the same oxidation pathway to produce glycolate and formate, while TPA‐K remains unreacted in the electrolyte. For PETHOR, the PET hydrolysate in 6.0 M KOH was diluted to 1.0 M KOH, thereby reducing the EG concentration to 0.16 M. The practical feasibility of PET upcycling was demonstrated by recrystallizing TPA, potassium diformate (KDF), and potassium glycolate–glycolic acid (PGGA) as real selective products after 50 h of PETHOR electrolysis at fixed potentials.

Supporting Information: Experimental section (Section S1.1–S1.7): Chemicals; Real product recrystallization (TPA, KDF, PGGA recrystallization); Instrumentation techniques; Quantification of glycolate and formate via ¹H NMR spectroscopy; Faradaic efficiency and yield rate calculation; DFT calculation details; Techno‐economic analysis; Figures S1–S86; Tables S1–S8.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: adma73526‐sup‐0001‐SuppMat.docx.

Data Availability Statement

Data will be made available on request.