Green Electrosynthesis of Formamide: C—N Bond Construction From Plastic Waste and Ammonia

Abstract

Formamide is an essential chemical intermediate, yet its industrial synthesis remains fossil‐dependent and energy‐intensive. Here, we report an electrocatalytic route for directly synthesizing formamide from ammonia and ethylene glycol (EG) using a Pd/NiCo‐LDH catalyst, achieving a Faradaic efficiency of 58.4% and a yield rate of 1.28 mmol cm⁻² h⁻¹. Mechanistic studies reveal that Pd and NiCo‐LDH act synergistically to promote reactant adsorption and activation, yielding *CHO‐CH₂OH and *NH₂ intermediates for C—N coupling to form formamide. Furthermore, this system can be extended to the transformation of real‐life polyethylene terephthalate (PET) plastic‐derived EG, achieving formamide yield rates of 0.84–1.12 mmol cm⁻² h⁻¹ and Faradaic efficiencies of 42.7%−50.6%. This work establishes a mild and sustainable electrocatalytic route for formamide synthesis using PET waste as feedstock.

An effective electrocatalytic strategy was reported for sustainable formamide synthesis from alcohol and ammonia. Specifically, Pd/NiCo‐LDH delivers a high formamide production rate of ∼1.28 mmol  cm⁻² h⁻¹ with a Faradaic efficiency of 58.4%.

1. Introduction

Formamide (HCONH₂), the simplest amide compound, serves as a key intermediate in the synthesis of various chemicals, fertilizers, and pharmaceuticals [1, 2, 3]. It is projected that the global formamide market will reach USD 2.45 billion by 2035. Industrially, formamide is primarily synthesized through the reaction of carbon monoxide (CO) and ammonia (NH₃) under elevated temperatures and pressures (313–373 K, 0.1–30 MPa) (Scheme 1) [4, 5]. However, such conventional synthesis processes not only rely on nonrenewable fossil resources but also contribute substantially to greenhouse gas emissions. Therefore, there is an urgent need to develop green, efficient, and sustainable synthetic alternatives.

SCHEME 1.

Schematic comparison of thermocatalytic and the proposed electrocatalytic route for formamide synthesis.

Electrochemistry represents a promising avenue for the green synthesis of organonitrogen compounds via C—N bond construction [6, 7, 8, 9]. This method utilizes environmental waste or easily accessible small molecules, including polyethylene terephthalate (PET) and polylactic acid (PLA) plastics, biomass, and CH₃OH/HCOOH/CO₂/CO as carbon sources, along with nitrogen sources like N₂/NH₃/NO _xx /NO₃ ⁻/NO₂ ⁻ [10, 11, 12, 13, 14, 15]. Powered by renewable electricity under mild conditions, this strategy substantially reduces the carbon footprint relative to traditional thermochemical routes. Recent pioneering studies have validated the feasibility of electro‐synthesizing formamide from such feedstocks. For example, Zhang and colleagues reported an electrochemical process for converting HCOOH and NO₃ ⁻ into formamide over a Cu catalyst, achieving a Faradaic efficiency (FE) of 29.7% [12]. Further advancing this field, Duan et al. developed a WO₃ electrocatalyst that produces formamide with an FE of 43.2% using PET‐derived ethylene glycol (EG) and biomass‐derived polyols as feedstocks [10]. Despite these advancements, the development of more efficient catalyst systems to achieve higher formamide selectivity and FE remains a major challenge.

In this work, we report a green electrochemical strategy for converting PET‐derived EG and NH₃ into formamide using a Pd/NiCo‐LDH cooperative catalyst under ambient conditions. The Pd/NiCo‐LDH catalyst achieves a high formamide yield rate of 1.28 mmol cm⁻² h⁻¹ with an FE of 58.4% at 2.1 V vs. RHE using commercial EG and NH₃. Mechanistic investigations reveal that Pd and NiCo‐LDH act in concert to enhance the adsorption and activation of EG and NH₃, generating key *CHO‐CH₂OH and *NH₂ intermediates that undergo C—N coupling to form formamide. When using real‐life PET plastic‐derived EG as raw materials, this system achieves formamide yield rates of 0.84–1.12 mmol cm⁻² h⁻¹ and FEs of 42.7%−50.6%. This work not only establishes a mild and sustainable electrocatalytic pathway for formamide synthesis from PET waste but also provides fundamental insights into catalyst design principles for efficient electrochemical C—N bond formation.

2. Results and Discussion

2.1. Synthesis and Characterization

The preparation procedure of Pd/NiCo‐LDH is depicted in Figure 1a. Initially, NiCo‐LDH precursors were grown on nickel foam (NF) by a hydrothermal method, followed by the synthesis of the Pd/NiCo‐LDH catalyst via a spontaneous galvanic replacement reaction. Powder X‐ray diffraction (XRD) patterns of Pd/NiCo‐LDH exhibit characteristic diffraction peaks corresponding to both cubic Pd (JCPDS 87‐0638) and NiCo‐LDH (JCPDS 48–0083) (Figure 1b) [16, 17]. Specifically, peaks at 39.8°, 46.4° and 67.8° can be assigned to the (111), (200), and (220) planes of Pd, whereas those at 19.1°, 32.9°, 38.5°, and 59.2° are attributed to the (006), (101), (015), and (110) facets of NiCo‐LDH, respectively (Figure 1b). Additionally, two peaks at 44.5° and 51.8° originate from the metallic NF substrate (JCPDS 04‐0850). Scanning/transmission electron microscopy (SEM and TEM) images reveal that the catalyst possesses a well‐defined nanosheet morphology, with Pd nanoparticles dispersed on the NiCo‐LDH nanosheets (Figures 1c and S1–S2). Due to the surfactant‐free synthesis via spontaneous galvanic displacement, the Pd nanoparticles exhibit a nonuniform spatial distribution (Figures 1c and S2), with sizes ranging from several nanometers to hundreds of nanometers (5–100 nm). High‐resolution TEM (HRTEM) image shows clear lattice fringes with spacings of 0.225 and 0.204 nm, corresponding to the (111) planes of metallic Pd and the (101) planes of NiCo‐LDH, respectively (Figure 1d). Energy‐dispersive X‐ray spectroscopy (EDS) and corresponding mapping (Figures 1e and S3) of Pd/NiCo‐LDH confirm the homogeneous distribution and coexistence of Pd, Ni, and Co elements throughout the nanosheet. These results demonstrate the successful synthesis of Pd/NiCo‐LDH.

FIGURE 1.

(a) Schematic diagram of the synthesis of Pd/NiCo‐LDH. (b) XRD patterns. (c) SEM image, (d) HRTEM image, and (e) elemental mapping images of Pd/NiCo‐LDH. (f) XPS survey spectrum of Pd/NiCo‐LDH. (g) Pd 3d XPS spectra of Pd/NiCo‐LDH and Pd.

X‐ray photoelectron spectroscopy (XPS) was employed to investigate the surface composition and chemical states. The XPS survey spectrum corroborates the presence of Pd, Ni, and Co, consistent with the EDS results (Figure 1f). For the high‐resolution Pd 3d spectra, pure Pd exhibits binding energies of 335.4 eV (3d _5/2) and 340.7 eV (3d _3/2), while for Pd/NiCo‐LDH, these peaks shift positively to 335.5 and 340.8 eV, respectively. This 0.1 eV shift indicates electron transfer from Pd to the NiCo‐LDH support and affirms a strong interfacial electronic interaction (Figure 1g). Such electronic modulation is known to optimize the adsorption behavior of reactants, thereby enhancing the electrocatalytic performance [18, 19, 20].

2.2. Electrocatalytic Performance

The electrocatalytic performance of the catalyst was evaluated using commercially available EG and NH₃ as model substrates. As shown in Figure 2a, the current density in the presence of both NH₃ and EG is significantly enhanced compared with EG alone, suggesting the contribution of NH₃ participation in EG transformation. Notably, Pd/NiCo‐LDH exhibits superior catalytic performance compared to individual Pd and NiCo‐LDH (Figure 2b). This enhancement is corroborated by the lower charge transfer resistance of Pd/NiCo‐LDH, indicative of accelerated interfacial electron‐transfer kinetics by the synergistic effect of Pd and NiCo‐LDH (Figure S4). The electrolyte was periodically extracted for nuclear magnetic resonance (NMR) analysis to qualitatively and quantitatively identify oxidation products. As depicted in Figure 2c, formamide (δ = 7.91 ppm) is identified as the primary product, with only trace amounts of formic acid detected (Figures S5 and S6). The amino protons (–NH₂) of formamide undergo rapid H/D exchange with D₂O, and consequently, they are typically not observed in the ¹H NMR spectrum. Reaction conditions were systematically optimized by varying the volume ratio of EG/NH₃, the applied potential, as well as the Ni/Co ratio using chronopotentiometry measurements (Figures 2d,e and S7–S9). The optimal conditions for formamide production are identified as an EG:NH₃ volume ratio of 2:1 and the Ni:Co ratio of 1:1. As shown in Figure 2e, Pd/NiCo‐LDH exhibits FE exceeding 45% for formamide over a wide potential range from 2.0 to 2.4 V vs. RHE. A maximum formamide yield rate of 1.28 mmol cm⁻² h⁻¹ with an FE of 58.4% is attained at 2.1 V vs. RHE. Benefiting from the synergistic effect between Pd and NiCo‐LDH, the Pd/NiCo‐LDH composite displays superior activity and FE for the electrocatalytic EG/NH₃‐to‐formamide compared to its individual components (Figure 2f). Furthermore, when benchmarked against previously reported electrocatalysts for formamide synthesis, Pd/NiCo‐LDH delivers a higher formamide yield rate at comparable current densities, highlighting its competitive advantage (Figure 2g and Table S1).

FIGURE 2.

(a) LSV curves in NH₃ solution with EG. (b) LSV curves of Pd/NiCo‐LDH, Pd, and NiCo‐LDH. (c) ¹H NMR spectrum of the electrolyte after reaction. (d) Formamide FEs at various EG:NH₃ volume ratios. (e) Formamide FEs at various potentials. (f) Formamide FEs and yield rate of Pd/NiCo‐LDH, Pd, and NiCo‐LDH. (g) Comparison of this work with previous reports for formamide electrosynthesis. (h) Formamide FEs and yield rate over 10 successive cycles.

Catalyst stability is a critical indicator for practical applicability. Over 10 successive electrolysis cycles, no obvious decay could be observed in formamide yield and FE, highlighting the robust cycling durability of the Pd/NiCo‐LDH catalyst (Figures 2h and S10). Post‐reaction characterizations via XRD and SEM confirm that no significant structural and morphological changes occur in Pd/NiCo‐LDH (Figures S11 and S12). Additionally, ICP‐MS analysis of the electrolyte reveals that the amount of dissolved Pd is negligible, corresponding to a leaching ratio below 0.5% of the total Pd loading (Table S2). Taken together, these electrochemical and structural analyses demonstrate that Pd/NiCo‐LDH possesses excellent catalytic activity, high selectivity, and robust stability.

2.3. Electrocatalytic Mechanistic Study

The adsorption of EG and NH₃ species constitutes the initial step in their electrooxidation. The open‐circuit potential (OCP), which reflects adsorbates in the inner Helmholtz layer, was measured to evaluate EG/NH₃ adsorption [21, 22, 23]. When EG or NH₃ is added individually, Pd/NiCo‐LDH exhibits a more obvious OCP shift, indicating excellent adsorption for a single reactant species (Figure S13). Upon the introduction of EG/NH₃, the OCP shift for Pd/NiCo‐LDH (68.2 mV) is more pronounced than that for Pd (42.5 mV) and NiCo‐LDH (11.6 mV), indicating its superior co‐adsorption capacity for both reactants (Figure 3a). To quantify the adsorption capacity of EG and NH₃, electrochemical adsorbate stripping experiments and NH₃ temperature programmed desorption (NH₃‐TPD) were performed (Figures S14 and S15). Both experiments consistently show that Pd/NiCo‐LDH possesses significantly enhanced adsorption capacity toward EG (by ∼1.45 times) and NH₃ (by ∼3.84 times) compared to pristine NiCo‐LDH. This superior co‐adsorption capability establishes a localized, reactant‐rich microenvironment on the catalyst surface, shortening the diffusion distance to the active sites and favoring C—N coupling.

FIGURE 3.

(a) OCP of the catalysts before and after EG/NH₃ was injected. (b) Time‐dependent FEs of different products. (c) EPR spectra. (d) In situ ATR‐FTIR spectra of Pd/NiCo‐LDH. (e) Proposed reaction pathway for formamide production from the co‐oxidation of EG and NH₃.

Electrochemical in situ Raman spectroscopy was carried out to elucidate the potential‐driven structural evolution of catalysts. As shown in Figure S16, when the applied potential exceeded 1.4 V versus RHE, two new bands emerged at ∼863.8 and ∼1047.2 cm⁻¹, corresponding to the Ni—O and Co—O stretching modes of NiCoOOH, respectively. These bands persist up to 2.2 V versus RHE without attenuation, indicating that the catalyst surface undergoes reconstruction to form a NiCoOOH phase. The control experiment of LSV shows that Pd and NiCo act as adsorption active sites for NH₃ and EG, respectively (Figures S17 and S18). In this collaborative mechanism, Pd activates NH₃, while NiCo‐LDH activates EG, and the Pd‐NiCo interface serves as the locus for C—N coupling.

To elucidate the reaction pathway from EG and NH₃ to formamide, control experiments and in situ characterizations were conducted. Control tests reveal that formamide is not produced in the absence of EG, NH₃, or applied potential (Figure S19), confirming its origin from coupling oxidative intermediates derived from EG and NH₃. When NH₃ is omitted, oxidation products are primarily C₂ species, mainly glycolaldehyde (Figure S20). In contrast, with NH₃ present, formamide becomes the dominant product, and C₁ products increase significantly. These results indicate that NH₃ promotes C—C cleavage of EG, facilitating subsequent C—N coupling to form formamide. To identify key intermediates, comparative tests were conducted using potential N⁻ and N⁻‐based species (Table S3). When glycolaldehyde or formaldehyde serves as a substrate, formamide is generated but with a much lower FE than with EG, implying that the aldehydes generated in situ during EG oxidation are more reactive toward C—N coupling. Conversely, substituting NH₃ with N‐containing intermediates such as hydroxylamine, NO₃ ⁻, or NO₂ ⁻ yields no formamide, excluding them as direct nitrogen sources. Instead, reactive nitrogen radicals (e.g., *NH₂) from NH₃ oxidation likely couple with in situ‐generated aldehyde intermediates. Time‐dependent product distribution analysis reveals a progressive increase in formamide FE, accompanied by a decrease in formaldehyde, suggesting that formaldehyde may be an important aldehyde intermediate in the C—N coupling process (Figure 3b). At the same time, the FE of glycolic acid is also decreasing, indicating that C₂ intermediates are being consumed through C—C bond cleavage in the presence of NH₃.

Electron paramagnetic resonance (EPR) measurements were performed to identify the oxidized reactive species using 5,5‐dimethyl‐1‐pyrroline N‐oxide (DMPO) as a trapping agent. Using the Pd/NiCo‐LDH catalyst, amino radicals (•NH₂) and carbon‐centered radicals (•R) were detected when using bare NH₃ and EG alone, respectively, with the •NH₂ signal being significantly stronger than that of •R (Figure 3c). When both NH₃ and EG were present in the electrolyte, the dominant signal corresponded to •NH₂ rather than •R. This suggests that dehydrogenative oxidation of NH₃ is considerably more favored than EG oxidation, thereby dictating the C—N coupling reaction pathway. Moreover, in situ electrochemical attenuated total reflection Fourier‐transform infrared (ATR‐FTIR) spectroscopy was employed to monitor reaction intermediates. A characteristic absorption band at ∼1491 cm⁻¹ was observed, attributable to the C—N stretching vibration of formamide (Figure 3d and Table S4) [24, 25, 26, 27]. Simultaneously, adsorbed aldehyde (*CHO) intermediates were also detected at ∼1723 cm⁻¹. The NO₂ ⁻ signal detected at ∼1227 cm⁻¹ arises from electrochemical overoxidation of NH₃ as a minor side product at high potentials [12]. Based on the identification of reactive intermediates, a plausible reaction pathway for formamide production from the electrocatalytic co‐oxidation of EG and NH₃ over Pd/NiCo‐LDH is proposed (Figure 3e). EG is first oxidized to reactive aldehydic (*CHO‐CH₂OH) intermediates, which are subsequently attacked nucleophilically by *NH₂, inducing sequential C—C bond cleavage and C—N bond formation.

2.4. Electrocatalytic Upgrading of Real‐World PET Waste

To assess the practical feasibility of this electrocatalytic system, a custom‐made two‐electrode flow electrolyzer (25 cm²) was constructed for the electrosynthesis of formamide using real‐world PET plastics and NH₃ as feedstocks (Figure 4a). Following common practice, alkaline hydrolysis was first conducted to obtain EG and terephthalate salt, followed by acid neutralization to separate EG from the terephthalic acid (TPA) precipitate. The resulting EG aqueous solution was used for electrocatalytic conversion. A low cell voltage of ∼2.5 V is sufficient to achieve the simultaneous production of formamide and H₂ (Figure 4b). We have performed formamide electrosynthesis using authentic PET‐derived EG solutions and evaluated the performance of the Pd/NiCo‐LDH catalyst. The results, summarized in Figure 4c and Table S5, confirm that formamide is successfully produced from all tested PET hydrolysates, achieving yield rates of 0.84–1.12 mmol cm⁻² h⁻¹ and FEs of 42.7%−50.6%. The moderate activity decline relative to commercial EG is primarily attributed to residual TPA in the hydrolysate. TPA acts as an anodic hole scavenger, competing with EG oxidation, and simultaneously blocks active sites on the catalyst surface [28]. Additionally, additives and pigments present in real‐world PET plastics may further contribute to the observed performance differences.

FIGURE 4.

(a) Schematic illustration and photograph of a two‐electrode flow electrolyzer for electrosynthesis of formamide. (b) LSV curves of overall water splitting and EG/NH₃ co‐oxidation. (c) FEs of formamide from the co‐oxidation of different PET plastics.

3. Conclusion

In summary, we have developed a sustainable electrochemical strategy for the conversion of PET‐derived EG and NH₃ into formamide using a Pd/NiCo‐LDH cooperative catalyst under ambient conditions. The Pd/NiCo‐LDH catalyst achieves a high formamide yield rate of 1.28 mmol cm⁻² h⁻¹ with FE of 58.4% at 2.1 V vs. RHE. Moreover, when applied to real‐world PET hydrolysates, the catalyst successfully produces formamide from all tested samples, achieving yield rates of 0.84–1.12 mmol cm⁻² h⁻¹ and FEs of 42.7%−50.6%, confirming its practical feasibility. Mechanistic investigations reveal that Pd and NiCo‐LDH synergistically promote EG/NH₃ adsorption and activation, generating key *CHO‐CH₂OH and *NH₂ intermediates that undergo C—N coupling to form formamide. This work presents a sustainable and efficient electrochemical strategy for upgrading plastic‐derived polyols into value‐added amides, highlighting a promising route for waste valorization and the green production of organonitrogen compounds.

Funding

This work was supported by the Natural Science Foundation of Zhejiang Province (No. LQN26B030014) and the Quzhou Science and Technology Bureau Project (No. 2023K232).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supplementary Material

Liu Kesheng, Li Yue, Zhang Hugang, Zhou Jingtao, Liu Fulai, Zhao Junhua, Green Electrosynthesis of Formamide: C—N Bond Construction From Plastic Waste and Ammonia, ChemSusChem 2026, 19, e70806. 10.1002/cssc.70806

Data Availability Statement

The data that support the findings of this study are available in the Supporting Information of this article.