Grafting nanometer metal/oxide interface towards enhanced low-temperature acetylene semi-hydrogenation

Abstract

Metal/oxide interface is of fundamental significance to heterogeneous catalysis because the seemingly “inert” oxide support can modulate the morphology, atomic and electronic structures of the metal catalyst through the interface. The interfacial effects are well studied over a bulk oxide support but remain elusive for nanometer-sized systems like clusters, arising from the challenges associated with chemical synthesis and structural elucidation of such hybrid clusters. We hereby demonstrate the essential catalytic roles of a nanometer metal/oxide interface constructed by a hybrid Pd/Bi₂O₃ cluster ensemble, which is fabricated by a facile stepwise photochemical method. The Pd/Bi₂O₃ cluster, of which the hybrid structure is elucidated by combined electron microscopy and microanalysis, features a small Pd-Pd coordination number and more importantly a Pd-Bi spatial correlation ascribed to the heterografting between Pd and Bi terminated Bi₂O₃ clusters. The intra-cluster electron transfer towards Pd across the as-formed nanometer metal/oxide interface significantly weakens the ethylene adsorption without compromising the hydrogen activation. As a result, a 91% selectivity of ethylene and 90% conversion of acetylene can be achieved in a front-end hydrogenation process with a temperature as low as 44 °C.

Metal/oxide interface is of fundamental significance to heterogeneous catalysis. Here, the authors construct a nanometer Pd/Bi₂O₃ interface by grafting Pd clusters onto Bi₂O₃ clusters and demonstrate its essential roles in the low-temperature semi-hydrogenation of acetylene.

Introduction

Metal/oxide interface is of great fundamental and practical interest to heterogeneous catalysis, because it raises essential questions regarding the strong metal–support interaction^1,2 and plays a pivotal role in several catalytic processes^3,4. From a structural perspective, a metal/oxide interface is constructed by components that significantly differ from each other in terms of chemical compositions, bonding characters, lattice parameters, and electric and mechanical properties^5,6, of which the adhesion structure and chemistry turn out to be a compelling research topic, while from a functional perspective, the chemical bonding and associated charge transfer⁷ at the metal/oxide interface allow the modulations of morphology, size, and electronic structures of metals to optimize the bonding strength of reaction intermediates for better catalytic performances³. During the past few decades, considerable progress has been achieved in the structural elucidation and tuning of well-defined metal/oxide interfaces that usually adopt a bulk oxide support to facilitate the nucleation, adsorption, or deposition of metals⁸. It is expected that a nanometer metal/oxide interface, perhaps formed by heterografting between metal and oxide clusters, would reinforce the structural and electronic effect to achieve better catalytic performance. However, due to the great challenges in the chemical synthesis and structural elucidation of such hybrid clusters, there are limited insights into the nanometer metal/oxide interface.

As a representative reaction where oxides supported metal catalysts are frequently used, the selective hydrogenation of acetylene to ethylene is subjected to an inherent trade-off between two essential requirements for both high catalytic activity and selectivity: the facile activation of hydrogen and the weak binding of ethylene⁹. Despite the significant progress achieved by Pd-based catalysts¹⁰, a simultaneous optimization of these two parameters is still challenging, especially in the front-end process where H₂ and C₂H₄ are in large excess. To reach this goal requires a sophisticated tuning of the geometric and electronic structure of Pd, which motivates people to engineer the metal/oxide interface. In most Pd/oxide catalysts, only Pd nanoparticles or isolated Pd atoms are loaded. Unfortunately, Pd nanoparticles are efficient to activate hydrogen at low temperatures, but their strong binding with ethylene favors sequential hydrogenation of ethylene to ethane^10,11. Isolated Pd site catalysts including Pd single-atom catalysts^12–15 and Pd-based intermetallic compounds^16–22 feature weak π-bonding with ethylene and thus good selectivity in the acetylene hydrogenation reaction, but their concomitant weakened hydrogen activation requires a relatively high reaction temperature (>100 °C) to achieve high conversion of acetylene, which potentially leads to a safety concern in the reactor beds²¹. Decreasing the size of oxide supports to nanocluster scale would remarkably change their coordination number (CN)²³, surface termination²⁴, and d-band character²⁵, which allow a strong chemical and electronic interaction with Pd to continuously regulate the size and electronic structure of Pd. Among them, ligand-free Pd clusters stabilized by a nanometer metal/oxide interface are expected to bridge the size and performance gaps between Pd nanoparticles and single atoms with maximized interfacial effects.

Herein we propose a facile stepwise photochemical strategy to fabricate hybrid Pd/Bi₂O₃ clusters where the Pd clusters are stabilized by the ~1 nm ordered Bi₂O₃ cluster through the nanometer interfaces. These hybrid clusters are well and stably dispersed on TiO₂ substrate with a high Pd loading up to 2.3 wt.%. They exhibit a low Pd–Pd CN of 4.7 and more importantly a Pd–Bi spatial correlation ascribed to the heterografting between Pd and Bi-terminated Bi₂O₃ clusters. Interestingly, these hybrid clusters feature an intra-cluster electron transfer toward Pd and result in a deeper d-band center compared with other Pd metals, which enables much weaker ethylene binding without compromising the hydrogen activation activity. As a result, a 90% conversion of acetylene together with a 91% selectivity to ethylene is achieved in excess of ethylene and at a temperature as low as 44 °C.

Results and discussion

Synthesis and structural characterizations

Pd_1.0/Bi₂O₃/TiO₂ catalysts were prepared by a stepwise photo-deposition of Bi and Pd on TiO₂ with a molar ratio of Bi/Pd kept at 1.0 (please see details in “Methods”). In brief, Bi³⁺ was reduced by the photogenerated electrons to produce Bi⁰ clusters on TiO₂. Subsequently, Pd was deposited onto Bi/TiO₂ to form Pd/Bi/TiO₂. When Bi/TiO₂ and Pd/Bi/TiO₂ were exposed to the air, Bi was spontaneously oxidized into Bi₂O₃ because the Gibbs free energy of the reaction is minus²⁶. The synthetic procedure is schematically illustrated in Fig. 1a. Inductively coupled plasma–atomic emission spectroscopy (ICP-AES) suggests that the mass loading of Bi and Pd are 4.9 wt.% and 2.3 wt.%, respectively, close to the feeding ratio. Our previous study has proved that TiO₂ has a strong interaction with Bi³⁺, characterized by an unprecedented 1.5 eV upshift of Ti 2p²⁷. Such interaction ensures the formation of highly dispersed Bi on TiO₂ during the following reduction of Bi^3+ 28. As shown in Supplementary Fig. 1a, b, Bi species are uniformly dispersed on TiO₂ as ~1 nm clusters with a Bi loading up to 5 wt.%. Interestingly, close inspection of individual Bi cluster on the TiO₂ support by using aberration-corrected annular dark-field scanning transmission electron microscopy (ADF-STEM) indicates that the Bi cluster has an ordered α-Bi₂O₃ structure with a highly distorted lattice, which exhibits relatively weak diffuse peaks in the fast Fourier transform (FFT; Fig. 1b). This is consistent with the literature result²⁶, which indicates that the mild oxidation of Bi⁰ in the air is thermodynamically spontaneous and usually forms monoclinic α-Bi₂O₃ (monoclinic, P2₁/c(14)). In addition to the intrinsic lattice distortion that blurs the image contrast, the beam-induced structural dynamics of the highly beam-sensitive Bi₂O₃ clusters may further introduce image blurring, mainly arising from the remarkable knock-on displacements and radiolysis. Despite these beam-induced effects, careful inspection on these ADF-STEM images allows the identification of local contrasts that closely resemble the projected structures and simulated ADF-STEM image of Bi₂O₃ along [100] (Fig. 1b). Specifically, the structural projection and simulated ADF-STEM image of ordered Bi₂O₃ feature a wave-shaped arrangement of Bi atomic columns from the [100] projection, while the experimental ADF-STEM image exhibits a similar contrast but with more lattice distortions (Fig. 1b). Accordingly, the FFT pattern of the experimental image exhibits rather diffuse spots compared with those in the FFT pattern of simulated image.

Fig. 1. Microstructure of Pd_1.0/Bi₂O₃/TiO₂.

a Schematic illustration of the synthetic procedures. b–d STEM images of Bi₂O₃/TiO₂ (b) and Pd_1.0/Bi₂O₃/TiO₂ (c, d). The insets from upper to lower and left to right: HRSTEM images, projected structural models, simulated ADF-STEM images, FFTs from dashed circular regions in HRSTEM images, and FFTs from simulated ADF-STEM images, respectively. e Elemental mapping of Pd_1.0/Bi₂O₃/TiO₂.

Followed by the secondary deposition of Pd, the clusters of Bi₂O₃ intergrown with Pd particles can be found according to the low-magnification ADF-STEM images (Supplementary Fig. 1). Besides, Pd_1.0/Bi₂O₃/TiO₂ exhibits no signals for Pd in the X-ray diffraction (XRD) patterns (Fig. 2a) while photo-deposited Pd/TiO₂ with an identical Pd loading (2.5 wt.%) shows a characteristic Pd(111) diffraction peak²⁹ at 40.1^o, suggesting that pre-deposited Bi₂O₃ clusters assist in the high dispersion of Pd species in Pd_1.0/Bi₂O₃/TiO₂. According to the literatures^30,31, pre-deposited component with high work function can serve as a sink of photo-induced electrons, which preferentially reduce the second metal on the surface of the pre-deposited metal. By fixing the loading of Bi at 5.0 wt.%, we can also fabricate Pd_0.5/Bi₂O₃/TiO₂ (Supplementary Fig. 2), Pd_0.2/Bi₂O₃/TiO₂, and Pd_0.1/Bi₂O₃/TiO₂ with high dispersion of Pd (Pd_x/Bi₂O₃/TiO₂, x represents Pd-to-Bi molar ratio). In contrast, when we increase Pd/Bi ratio to 3.0, the excess Pd also deposits directly on TiO₂. As a result, Pd nanoparticles are obtained (~6.7 nm, Supplementary Fig. 3). Taking all above results together, Pd species are most likely deposited on the existing Bi₂O₃ clusters although the discrimination between Pd and Bi₂O₃ components is not straightforward by STEM imaging due to their small size and irradiation vulnerability. The nanoscopic elemental distribution can be mapped by a Super-X energy dispersive X-ray spectroscope (EDS) system with superior sensitivity. As shown in Fig. 1e and Supplementary Fig. 4, elemental mappings of Pd and Bi components suggest that their spatial distribution in most cases are correlated. In other words, these clusters are bicomponent with segregated Pd- and Bi-containing hemi-clusters, which unambiguously confirm the Pd-grafted Bi₂O₃ hybrid cluster structure. Careful inspection on several representative ADF-STEM images of Pd_1.0/Bi₂O₃/TiO₂ (Fig. 1c, d and Supplementary Fig. 5) allows the discrimination of Pd- and Bi-containing hemi-clusters from their distinct contrasts for adjacent clusters. The hemi-clusters featuring less bright contrast (marked by circle) are directly attached to brighter ones assigned to Bi₂O₃ hemi-clusters, further confirming that Pd is grafted onto the surface of Bi₂O₃ clusters. Notably, most such Pd clusters are identified to bond and hybridize with Bi₂O₃ clusters without any fixed orientation relationship or facet preference, probably due to the ultra-small size and large strain of the clusters. Two representative atomic-resolution ADF-STEM images of Pd-Bi₂O₃ nanoclusters with well-resolved Pd- and Bi-containing hemi-clusters are shown in Fig. 1c, d, of which the image contrasts and FFT patterns closely resemble the simulated ones of artificially constructed hybrid cluster models with different orientation relationships between two types of hemi-clusters made of Pd and α-Bi₂O₃ structures, respectively. The above observations unambiguously validate the proposed Pd–Bi₂O₃ hybrid structural model. Quite rarely, such Pd clusters are observed to directly nucleate onto the TiO₂ substrate.

Fig. 2. Characterization of Pd_1.0/Bi₂O₃/TiO₂.

a XRD patterns of TiO₂, Pd/TiO₂, Bi₂O₃/TiO₂, and Pd_1.0/Bi₂O₃/TiO₂; b Fourier transform spectra of Pd K-edge EXAFS for Pd/TiO₂, Pd_1.0/Bi₂O₃/TiO₂, and oxidized Pd_1.0/Bi₂O₃/TiO₂ (Pd_1.0/Bi₂O₃/TiO₂-ox); c Fourier transform spectra of Bi L3-edge EXAFS; d Bi L3-edge XANES spectra for Bi₂O₃/TiO₂, Pd_1.0/Bi₂O₃/TiO₂, and Pd_1.0/Bi₂O₃/TiO₂-ox. Bi and Bi₂O₃ powder were used as references. e Pd K-edge XANES spectra for Pd/TiO₂, Pd_1.0/Bi₂O₃/TiO₂, and Pd_1.0/Bi₂O₃/TiO₂-ox. Pd foil was used as a reference. f CO-adsorbed FT-IR spectra for various samples. Source data are provided in a Source data file.

To confirm the coordination environment of Pd clusters, X-ray absorption fine structure (XAFS) of Pd K-edge and Bi L3-edge were performed, as shown in Fig. 2b–e. In the Fourier transformed extended X-ray absorption fine structure (FT-EXAFS) data of Bi L3-edge, it is important to note that a new coordination peak in R-space at about 2.6 Å is observed on Pd_1.0/Bi₂O₃/TiO₂ (Fig. 2c), but it is totally absent on other samples containing Bi₂O₃ (i.e., Bi₂O₃ and Bi₂O₃/TiO₂). Considering that the only difference between Pd_1.0/Bi₂O₃/TiO₂ and Bi₂O₃/TiO₂ is the secondary deposition of 2.3 wt.% Pd on Bi₂O₃/TiO₂, the peak at 2.6 Å should be contributed by the Bi–Pd interaction. Interestingly, the peak can be well fitted by a single Bi–Pd shell (Supplementary Fig. 6 and Supplementary Table 1), suggesting it is a characteristic peak of Bi–Pd coordination. To further prove this idea, the Pd_1.0/Bi₂O₃/TiO₂ is mildly treated under air at 150 °C (denoted as Pd_1.0/Bi₂O₃/TiO₂-ox) to oxidize Pd species while maintaining the structural integrity of Bi₂O₃ clusters³². In Fig. 2e, Pd_1.0/Bi₂O₃/TiO₂-ox shows clearly blue shift of the absorption edge position comparing with Pd_1.0/Bi₂O₃/TiO₂, indicating the oxidation of Pd after the treatment. Meanwhile, the peak at ~2.6 Å of Bi L3-edge EXAFS diminishes significantly upon oxidation of Pd_1.0/Bi₂O₃/TiO₂ (Fig. 2c). The fitting results reveal that the Bi–Pd CN is decreased from 2.9 for Pd_1.0/Bi₂O₃/TiO₂ to 1.1 for Pd_1.0/Bi₂O₃/TiO₂-ox (Supplementary Table 1). Accordingly, this experiment identifies that the peak at 2.6 Å is from Bi–Pd bond and in turn proves the Pd–Bi pairs across the interfaces of Pd/Bi₂O₃ hybrid clusters observed by ADF-STEM in Fig. 1c, d. The Bi–Pd interaction eventually changes the coordination environment of Pd in Pd_1.0/Bi₂O₃/TiO₂. As compared to Pd foil and Pd/TiO₂, the significantly weaker and slightly broader coordination peak in the Pd K-edge EXAFS of Pd_1.0/Bi₂O₃/TiO₂ (Fig. 2b) implies a decreased CN of Pd–Pd pairs and distorted structure for the Pd clusters similarly as observed by STEM imaging³³. The fitting of Pd K-edge EAXFS (Supplementary Fig. 7 and Supplementary Table 2) further confirms that the presence of Pd–Bi coordination (CN = 4.6) decreases the Pd–Pd CN from 10.0 for Pd/TiO₂ to 4.7 for Pd_1.0/Bi₂O₃/TiO₂, which is consistent with Pd cluster structure observed by ADF-STEM. The Pd–Bi bond length obtained from the fitting of Pd K and Bi L3 edge is 2.79 ± 0.03 Å, suggesting the observed spatial correlation of Pd–Bi pairs arises from the direct bonding between Pd- and Bi-terminated clusters instead of the Pd–O–Bi moieties that attain a larger interatomic distance (~3.5 Å). Photo-deposition procedure is critical to ensure the formation of direct Pd–Bi bonding in Pd/Bi₂O₃ clusters as depicted in Fig. 1a. Theoretically, Bi₂O₃ favors an oxygen termination. Pd supported on Bi₂O₃ would be in direct contact with O rather than Bi. We prepared a Pd/Bi₂O₃ sample by directly depositing Pd onto commercial Bi₂O₃ support. Interestingly, no characteristic peak at ~2.6 Å was observed in the Bi L3 EXAFS of Pd/Bi₂O₃, excluding the presence of Pd–Bi bond (Fig. 2c). In this study, photo-deposition procedure ensures the Pd cluster deposited on reduced Bi⁰ clusters and allows the formation of Pd–Bi bonding during the synthesis as depicted in Fig. 1a. The Pd–Bi bond preserves during the mild oxidation of Bi to Bi₂O₃ at room temperature (RT) that occurred after the deposition of Pd onto Bi/TiO₂, as evidenced by the characteristic Bi–Pd peak in Bi L3 EXAFS (Fig. 2c). More interestingly, 36% of this peak preserves even after oxidation in air at a higher temperature of 150 °C (Pd_1.0/Bi₂O₃/TiO₂-ox). These results clearly indicate the good stability of Pd–Bi bond in Pd_1.0/Bi₂O₃/TiO₂ and suggest that Pd supported on Bi-terminated Bi₂O₃ as the structural model for Pd_1.0/Bi₂O₃/TiO₂ in the hydrogenation reaction conditions. Taken together, the above-mentioned results unambiguously confirmed that the Pd species are predominantly in the form of nanometer-sized Pd clusters embedded in the Pd/Bi₂O₃ hybrid clusters through chemical adhesion.

In order to identify the electronic structure of the as-synthesized Pd/Bi₂O₃ hybrid clusters in reaction conditions, in situ X-ray photoelectron spectra (XPS) was collected at 100 °C under H₂ atmosphere (Supplementary Fig. 8). As shown in Supplementary Fig. 8b, there are symmetric Bi 4f peaks at 158.8/164.1 eV, confirming that the majority of Bi are in the form of Bi₂O₃³⁴. Both Bi₂O₃/TiO₂ and Pd_1.0/Bi₂O₃/TiO₂ exhibit similar Bi L3-edge structures with Bi₂O₃ by inspecting the X-ray absorption near edge structure (XANES), indicating similar valence states among them (Fig. 2d). It is frequently reported that intermetallic compounds (IMC) could be possibly formed by a so-called reactive metal–support interaction³⁵. However, according to the density functional theory (DFT) calculations (Supplementary Table 3), the Pd–Bi distance in PdBi IMC ranges from 2.85 to 3.03 Å, which is obviously larger than that of Pd_1.0Bi/TiO₂ (2.79 Å, Supplementary Table 2). This result excludes PdBi IMC as the main phase of Pd_1.0/Bi₂O₃/TiO₂. To better illustrate the difference between Pd–Bi₂O₃ nanoclusters and PdBi intermetallic compounds, we synthesized a PdBi IMC (denoted as PdBi/TiO₂) for comparison (Supplementary Fig. 9). XRD patterns (Supplementary Fig. 9a) of PdBi/TiO₂ exhibit characteristic peaks at 39.9 and 42.8°, corresponding to (102) and (110) of hexagonal PdBi IMC (sobolevskite, P6₃/mmc(194), a = b = 4.22 Å, c = 5.709 Å, α = β = 90°, and γ = 120°). Pd–M (M = Pd or Bi) coordination peaks in R space of PdBi/TiO₂ obviously shift (~0.06 Å) from that of Pd_1.0/Bi₂O₃/TiO₂, confirming that PdBi IMC have longer Pd–Bi distances than Pd–Bi₂O₃ nanoclusters (Supplementary Fig. 9b). More importantly, a massive Bi⁰ peak at 156.6/162.0 eV (Supplementary Fig. 9d) is observed in the Bi 4f of PdBi/TiO₂ but is absent in that of Pd_1.0/Bi₂O₃/TiO₂ (Supplementary Fig. 8b). These results clearly indicate that Pd_1.0/Bi₂O₃/TiO₂ is composed of Pd–Bi₂O₃ hybrid clusters rather than PdBi IMC. It is reasonable because the reaction temperature in this study is too low to reduce Bi₂O₃ to Bi, which is a prerequisite for the formation of PdBi IMC. On the other side, the Pd 3d patterns of Pd/TiO₂ and Pd_1.0/Bi₂O₃/TiO₂ exhibit a slightly asymmetric lineshape (Supplementary Fig. 8a), which is most likely due to the many-body screening response of conduction electrons to the photoemission of a core electron³⁶. The predominant Pd 3d signals of Pd/TiO₂ and Pd_1.0/Bi₂O₃/TiO₂ locate at ~334.9/340.0 eV, which are characteristic of Pd⁰. It is important to note that, due to the differences in the extra-atomic relaxation of metal particles of different sizes, decreasing the particle size of Pd generally upshifts Pd 3d to higher binding energy (BE)³⁷. In this study, the Pd 3d and 3p of Pd_1.0/Bi₂O₃/TiO₂ downshift slightly to lower BE although the particle size of Pd_1.0/Bi₂O₃/TiO₂ is significantly smaller than that of Pd/TiO₂ (Supplementary Figs. 8a, c). These results, opposite to the particle-size-induced BE shift, indicate a charge transfer from Bi to Pd. Similar result was also reported in Au/TiO₂ system³⁸.

The more important feature associated with the electronic structure of Pd clusters is observed in the Pd K-edge and L3-edge structures as shown in Fig. 2e and Supplementary Fig. 10. Specifically, the Pd K-edge XANES profile of Pd/TiO₂ is very similar to that of Pd foil. In contrast, Pd_1.0/Bi₂O₃/TiO₂ shows a marked red-shift of the absorption edge energy and decrease in the white line intensity. This indicates the electron-richness of Pd atoms in Pd_1.0Bi/TiO₂ compared to those in Pd foil and Pd/TiO₂, which most likely arises from the Bi₂O₃-to-Pd intra-cluster electron transfer. Moreover, Pd_1.0/Bi₂O₃/TiO₂ shows a much weaker Pd L3 white line intensity at ~3173 eV than Pd/TiO₂ (Supplementary Fig. 10), which indicates an enhanced d-band filling. Such phenomenon is similarly predicted by the Bader charge analysis over Pd₈ clusters supported by the Bi₂O₃ cluster as shown in Supplementary Fig. 11. It is quite surprising to observe such an enhanced d-band filling and associated negative shift of d-band center away from the Fermi level for nanometer-sized Pd clusters, because the strong “size effect” of most supported metals usually leads to a decreased d-band filling and thus positive shift of d-band center toward Fermi level for smaller nanoparticles³⁹. Actually, this has become a limiting factor for applying supported metal nanoparticles in the selective acetylene hydrogenation reaction due to the strong adsorption of ethylene molecules on electron-deficient metals and results in over-hydrogenation. The metal termination of Bi₂O₃ observed here at the Pd–Bi interface of the hybrid cluster could lead to a strong downshift of its surface conduction band. Similar results were also reported in metal–metal interface of Pd/MgO, Pd/ZnO, Ru/MgO, and Au/TiO₂^38,40–42. Electrons transferred from terminated magnesium or zinc to adsorbed Pd can also result in negatively charged Pd. This charge transfer was ascribed to the band filling modification and the orbital hybridization between substrate and metal atoms⁴⁰. Specifically, for Pd deposited on magnesium termination, the surface Mg conduction band is shifted toward higher energy and is emptied, while the Pd d band is shifted in the opposite direction and becomes filled. Similar band filling modification and orbital hybridization might also apply to Pd–Bi₂O₃ system. This allows the Bi₂O₃-to-Pd intra-cluster electron transfer through direct Pd–Bi bonding across the interface, which leads to a greater filling of high-lying d bands in Pd clusters and largely circumvented over-hydrogenation problem. It is important to note that the charge transfer between Pd and Bi is localized in the Pd–Bi interface. Considering the relatively higher concentration of Bi₂O₃ comparing with Pd, the signal of electron-deficient Bi is likely overwhelmed by the signal of unaffected Bi₂O₃ and is therefore not observed by XPS and XANES.

The unique atomic and electronic structures of heterografted Pd cluster further result in its largely modulated gas adsorption behaviors compared with other supported Pd metals, which can be investigated by the in situ diffuse reflectance infrared Fourier transform spectroscopy using CO as the probe molecule. As shown in Fig. 2f, signals at 2089 and 2059 cm⁻¹ for linear-bonded CO⁴³ over Pd/TiO₂ redshift to 2076 and 2055 cm⁻¹ for those over Pd_1.0/Bi₂O₃/TiO₂, suggesting a strengthened π-back donation of metal d-electrons to π* orbitals of CO over Pd_1.0/Bi₂O₃/TiO₂ and a downshift of d-band center⁴⁴. This result is well consistent with the XPS and XANES results. It is noteworthy that signals for bridge-bonded CO (1996 and 1948 cm⁻¹) are observed over Pd/TiO₂ but invisible over Pd_1.0/Bi₂O₃/TiO₂, likely arising from combined size and electronic effects. Specifically, the downshifted d-band center weakens the adsorption strength of CO on Pd⁴⁴. More importantly, the small size and large structural distortion of Pd clusters disfavor the bridge adsorption mode of CO molecules due to the low average CN and broad distance distribution of Pd–Pd pairs.

Catalytic performance in acetylene hydrogenation

With the unique hybrid cluster structure and Bi₂O₃-to-Pd intra-cluster electron transfer, Pd_1.0/Bi₂O₃/TiO₂ readily serves as a model catalyst to demonstrate the essential catalytic role of a nanometer metal/oxide interface. The catalytic properties were evaluated in selective hydrogenation of acetylene with excess ethylene, mimicking the front-end condition. In this condition, the thermodynamically favored over-hydrogenation of ethylene with the large excess hydrogen generally leads to an unsatisfied C₂H₄ selectivity at high C₂H₂ conversion, accompanied by a thermal runaway. To overcome this problem, a small amount of CO is usually added in the feed stream to reduce the reaction rate so as to improve the C₂H₄ selectivity¹⁰. At very low CO levels, high C₂H₄ selectivity and C₂H₂ conversion are difficult to achieve simultaneously. In this study, no CO is added in the feed stream. Both Pd/TiO₂ and well-established Pd₁Ag₃/Al₂O₃ catalysts^30,45 were evaluated for comparison with Pd_1.0/Bi₂O₃/TiO₂. The composition and synthesis procedure of the PdAg₃/Al₂O₃ catalyst is the same as OleMax@251, a widely used industrial catalyst for acetylene hydrogenation⁴⁵. The carbon balances are all >99%. Negligible oligomers were formed during the hydrogenation process, likely due to the short contact time and high concentration of hydrogen. According to the literatures, the large excess of hydrogen would change the adsorption modes of C₂H₂ and C₂H₄ from C-CH₂ vinylidene and C-CH₃ ethylidyne to weak π-bonded C₂H₂ and C₂H₄^46,47. As a result, the possible reaction between vinylidene and acetylene to form C₄ species as well as the hydrocarbon isomerization and decomposition are suppressed.

Figure 3a plots the C₂H₄ selectivity as a function of C₂H₂ conversion on various catalysts. Consistent with the literature results³⁰, the C₂H₄ selectivity drops rapidly on well-established Pd₁Ag₃/Al₂O₃ catalyst once the C₂H₂ conversion exceeds 40%. On the contrary, Pd_1.0/Bi₂O₃/TiO₂ and Pd_0.2/Bi₂O₃/TiO₂ catalysts preserve very high selectivity toward C₂H₄ at much higher C₂H₂ conversions. Figure 3b compares the C₂H₄ selectivity at 95% C₂H₂ conversion. Interestingly, all catalysts except Pd_1.0/Bi₂O₃/TiO₂ and Pd_0.2/Bi₂O₃/TiO₂ exhibit negative selectivity toward C₂H₄ due to the over-hydrogenation of ethylene to ethane. Specifically, Pd_1.0/Bi₂O₃/TiO₂ exhibits much higher selectivity than 2.3 wt.% Pd/TiO₂ regardless of the same Pd loading. In addition, we also synthesized Bi_x/Pd/TiO₂ (x = 0.5, 1, x is the molar ratio of Bi to Pd) by photo-depositing Bi onto 2.3 wt% Pd/TiO₂. Interestingly, both Bi_0.5/Pd/TiO₂ and Bi_1.0/Pd/TiO₂ exhibit 100% conversion of C₂H₂ and negative selectivity toward ethylene (−123% for Bi_0.5/Pd/TiO₂ and −143% for Bi_0.5/Pd/TiO₂) at RT. These results suggest that a simple site blocking mechanism cannot explain the beneficial effect of Bi in this study. PdBi/TiO₂ composed of PdBi IMC (Supplementary Fig. 9) shows 100% conversion of C₂H₂ and negative selectivity toward ethylene (−1319%) at RT. The strong exothermic effect of unselective acetylene hydrogenation eventually leads to a runaway temperature up to 58.5 °C. These results exclude PdBi IMC as the active site for Pd_1.0/Bi₂O₃/TiO₂ and further indicates the critical role of the nanometer Pd/Bi₂O₃ interface in the catalytic selectivity of Pd. It is important to highlight that 91% selectivity of C₂H₄ with 90% conversion of C₂H₂ is achieved by Pd_1.0/Bi₂O₃/TiO₂ at a temperature as low as 44 °C (Fig. 3c). Such an excellent low-temperature performance of acetylene hydrogenation has never been reported previously, suggesting the unique structure and catalytic properties of Pd_1.0/Bi₂O₃/TiO₂. Moreover, the C₂H₂ conversion and C₂H₄ selectivity remain almost constant over 24 h operating at 40 °C (Fig. 3d). XRD and XAFS also confirm that Pd/Bi₂O₃ hybrid cluster structure is still maintained (Fig. 2), demonstrating a good long-term stability of Pd_1.0/Bi₂O₃/TiO₂.

Fig. 3. Catalytic performances of Pd_1.0/Bi₂O₃/TiO₂ in acetylene hydrogenation.

a Selectivity as a function of acetylene conversion over Pd_1.0/Bi₂O₃/TiO₂, Pd_0.2/Bi₂O₃/TiO₂, and PdAg₃/Al₂O₃. b The selectivity to C₂H₄ for 95% acetylene conversion over different catalysts. c Reaction temperature (T₉₀) and C₂H₄ selectivity for 90% acetylene conversion. *For Pd_3.0/Bi₂O₃/TiO₂, hydrogen dissociation easily takes place at room temperature. The strong exothermic effect of unselective acetylene hydrogenation eventually leads to a runaway temperature up to 63.5 °C. d C₂H₂ conversion and C₂H₄ selectivity with time on stream over Pd_1.0/Bi₂O₃/TiO₂ at 40 °C. e H₂-TPR profiles for Pd/TiO₂, Bi₂O₃/TiO₂, and Pd_1.0/Bi₂O₃/TiO₂. f Microcalorimetric studies of C₂H₄ pulse adsorption over Pd/TiO₂ and Pd_1.0/Bi₂O₃/TiO₂. Source data are provided in a Source data file.

The variation of Pd-to-Bi ratio leads to an evolution of Pd/Bi₂O₃ hybrid clusters and major alteration of their catalytic performances in selective acetylene hydrogenation. Usually, the high Pd-to-Bi ratio results in the formation of Pd nanoparticles along with decreased Pd/Bi₂O₃ clusters, while low Pd-to-Bi ratio results in Pd single atoms. Here the reaction temperature (T₉₀) and C₂H₄ selectivity at 90% conversion of acetylene are utilized to evaluate the catalytic performance of the samples. As shown in Fig. 3c, T₉₀ increases along with the decrease of Pd-to-Bi ratio, implying that small Pd size is unfavorable to hydrogenation. Under an extreme condition when Pd is atomically dispersed (Pd-to-Bi ratio ≤0.1), a very high T₉₀ (>90 °C) will be obtained, likely due to its poor ability of hydrogen activation²¹. In contrast, the moderate size of Pd in the hybrid clusters allows the efficient activation of hydrogen at much lower temperatures. All these catalysts exhibit quite high C₂H₄ selectivity. The Pd_3.0/Bi₂O₃/TiO₂ with an increased Pd-to-Bi ratio generates Pd nanoparticles and can even convert >90% of acetylene at RT. However, it suffers from the negative C₂H₄ selectivity at 90% conversion of acetylene. The above results experimentally confirm the trade-off between the conversion of acetylene and the selectivity of C₂H₄ upon the size effect of Pd, while an optimal catalytic performance is achieved over the Pd clusters stabilized by a nanometer metal–oxide interface.

The origin of such inherent trade-off in the catalytic performance of Pd lies in two aspects: the hydrogen activation and ethylene adsorption. It is generally accepted that the facile dissociative activation of hydrogen on Pd nanoparticles produces too much active H species, which migrate into the Pd lattice and generate β-hydride phase that leads to over-hydrogenation of ethylene⁴³. Accordingly, a negative peak (55–75 °C, Fig. 3e) characteristic for the decomposition of β-hydride phase is observed from the temperature programmed reduction (TPR) profile of Pd/TiO₂, which is, however, not observed from that of Pd_1.0/Bi₂O₃/TiO₂. These results are consistent with literatures that report high-coordinated Pd ensembles (the case of Pd/TiO₂) as active sites for the formation of β-hydride⁴³. As for Pd_1.0/Bi₂O₃/TiO₂, the Pd/Bi₂O₃ hybrid clusters feature a small Pd–Pd CN and nanometer Pd–Bi interface, which prevent the formation of β-hydride. The lack of β-hydride suppresses the over-hydrogenation of ethylene to ethane and therefore contribute to the high C₂H₄ selectivity⁴⁸. It is important to highlight that the reduced size of Pd cluster does not compromise the hydrogen activation and thus the catalytic activity. As a result, a low T₉₀ could be achieved on Pd_1.0/Bi₂O₃/TiO₂ (Fig. 3c).

In addition to the formation of β-hydride, the adsorption behavior of ethylene is also strongly influenced by the Pd structures, which leads to different microcalorimetric profiles⁴⁹. Figure 3f shows the heat flow during the C₂H₄ pulse adsorption process as a function of time for Pd_1.0/Bi₂O₃/TiO₂, Pd/TiO₂, and Bi₂O₃/TiO₂ at 40 °C. Obvious heat flow signals are observed on Pd/TiO₂ and Pd_1.0/Bi₂O₃/TiO₂ but are absent on Bi₂O₃/TiO₂ and blank test, demonstrating that C₂H₄ is adsorbed on Pd instead of on Bi₂O₃. It is interesting to note that the amplitude of the heat flow signal of Pd_1.0/Bi₂O₃/TiO₂ is much smaller than that of Pd/TiO₂.The calculated adsorption heat of Pd_1.0/Bi₂O₃/TiO₂ (~5.9 kJ mol^-1) is significantly lower than that of Pd/TiO₂ (~234.5 kJ mol⁻¹), clearly indicating a much weaker ethylene adsorption on Pd_1.0/Bi₂O₃/TiO₂. These observations can be attributed to the unique geometric and electronic structure of hybrid cluster, similar to the results reported in the alloying of Pd^50,51. The low Pd–Pd coordination and the Bi₂O₃-to-Pd intra-cluster electron transfer likely change the adsorption configuration of C₂H₄ from stable ethylidyne to weak π-bonded C₂H₄ and promote the desorption of ethylene as the desired product. To confirm this hypothesis, we further performed the temperature programmed desorption of ethylene by monitoring the mass signal of m/e = 27 (Supplementary Fig. 12). According to the literature, the peak at ~65 °C could be assigned to weak π-bonded ethylene species, which readily desorb without decomposition⁵². The peak centered at ~115 °C originates from di-σ-bonded ethylene, which undergoes decomposition followed by the recombination of surface hydrocarbon species and hydrogen to produce ethylene and ethane⁵². Compared with Pd/TiO₂, Pd_1.0/Bi₂O₃/TiO₂ exhibits a much weaker peak at ~115 °C but a significantly larger peak at 65 °C. These results confirm that the adsorption configuration of C₂H₄ is changed from the strong σ-bonding for Pd/TiO₂ to weak π-bonding for Pd_1.0/Bi₂O₃/TiO₂.

Reaction mechanism investigated by DFT calculations

DFT calculations were performed to further provide insights into the molecular-level mechanisms of acetylene hydrogenation on experimental Pd_1.0/Bi₂O₃/TiO₂ catalyst. Model of Bi₂O₃-supported Pd₈ cluster catalyst was built according to the experimental characterization results and the structure of which is shown in Fig. 4a (please see the details of model development in Supplementary Information). In this model, the size of Pd cluster on Bi₂O₃(100) is around 1.6 nm × 1.5 nm. In addition, this Pd cluster shows an average Pd–Pd CN of 4.0, which is close to the experimental values measured, i.e., 4.7 ± 0.5 (Supplementary Table 2). The Pd–Bi pair distribution function of the Pd₈ cluster structure is shown in Supplementary Fig. 13. In this figure, the dominant peak appears at ~2.75 Å, which is smaller than that in the PdBi intermetallic model (~2.91 Å, Supplementary Table 3). In addition, Bader charge analysis suggests that Pd atoms in PdBi IMC (average charge of −0.36 e) model are more electron-rich than those in Pd-Bi₂O₃ hybrid clusters model (−0.21 e). These results are well consistent with Pd K-edge XANES and Pd 3d XPS shown in Supplementary Fig. 9, which therefore validate the reliability of the Pd₈ cluster model. The possibility of hydride formation over the Pd cluster was studied, and it was found that hydrogen atoms prefer to adsorb at surface Pd sites after optimization (Supplementary Fig. 14), suggesting that formation of Pd hydride from this cluster is difficult, which again agrees well with the results shown in Fig. 3e.

Fig. 4. Reaction mechanism revealed by DFT calculations.

a Optimized Pd cluster structure for DFT calculation (Pd: cyan, Bi: purple, O: red) and b energy profile of acetylene hydrogenation to ethane on Pd(111) and Pd₈ cluster supported on Bi₂O₃(100). c DOS projected onto d electrons over Pd atom of Pd(111) and Pd₈ cluster structures. A surface Pd atom of Pd(111), and the most active Pd atom of Pd₈ cluster structure (on which C₂H₄ adsorbs most strongly) are chosen to plot the DOS. The position of d-band center (ε_d) is highlighted with a red bar. d E_ads of C₂H₄ as a function of ε_d over different Pd atom on Pd cluster surface (black squares). The most stable adsorption configuration is shown as solid square, while the other less stable adsorption structures are denoted by hollow squares. A surface Pd atom of Pd(111) is also shown as red solid square for comparison. The blue fitted line is a guide for the eyes. It shows that a more negative ε_d corresponds to a more positive E_ads of C₂H₄. Source data are provided in a Source data file.

It was reported that C-CH₂ vinylidene and C-CH₃ ethylidyne are energetic stable and important spectator species in acetylene hydrogenation^53–55. However, in the front-end condition where H₂ is in large excess, vinylidene and ethylidyne are readily hydrogenated and are insignificant at steady-state conditions¹⁰. Besides, the adsorption configurations of C₂H₂ and C₂H₄ also depend on the catalyst structure. For the Pd cluster structure studied in this work, it is found that the hydrogenation of CH≡CH to CHCH₂ has the lowest activation energy (E_a) of 0.74 eV compared with the dehydrogenation or hydrogen shift of CH≡CH (Supplementary Table 4). This strongly suggests that formation of CCH₂ would be unfavorable on the Pd cluster structure. Similarly, the hydrogenation of CHCH₂ to CH₂CH₂ has the lowest E_a among all reactions starting from CHCH₂, again indicating that the spectator species CCH₃ is hard to form on Pd cluster model (please see Supplementary Information for details). To this end, the effects of these species are not discussed here. The energy profile of acetylene hydrogenation to ethane over this Pd cluster is shown in Fig. 4b, and the corresponding optimized configurations of surface intermediates and transition states are shown in Supplementary Fig. 15. Meanwhile, the energy profile on Pd(111) representing Pd foil is also shown in Fig. 4b for comparison. The calculated barriers are generally consistent with the reported values (Supplementary Table 5), demonstrating that our calculated results are reliable. In addition, we further calculated the vibrational frequency based on the transition state structures on Pd(111). All the transition states were characterized to possess only one imaginary frequency along the bond formation of hydrogenation reactions. These results further demonstrate the reliability of the constrained minimization method that we used in this work. As can be seen from Fig. 4b, the adsorption of C₂H₂ over Pd(111) and Pd₈ cluster is exothermic, and the transition state energies of C₂H₂ hydrogenation to C₂H₄ on both surfaces are below the energy of gaseous C₂H₂, suggesting that the hydrogenation processes should be facile. The semi-hydrogenation product C₂H₄ would either desorb from the surface or undergo further hydrogenation to ethane (C₂H₆). In Fig. 4b, the transition state energy of C₂H₄ hydrogenation on Pd₈ cluster is above the gaseous C₂H₄ energy, suggesting that desorption of C₂H₄ from the Pd₈ cluster may be favored compared with its further hydrogenation to C₂H₅. Herein the difference between the barriers for further hydrogenation and desorption of ethylene can be used to estimate the possibility of selective C₂H₄ formation^17,56–60. Within this framework, a more positive value of E_a,hydro − |E_ads,C2H4|, where E_a,hydro is the effective hydrogenation barrier of C₂H₄ to C₂H₆ and |E_ads,C2H4| is the absolute value of C₂H₄ adsorption energy, corresponds to higher C₂H₄ selectivity. The calculated values of E_a,hydro − |E_ads,C2H4| over Pd₈ cluster and Pd(111) are 0.53 and 0.31 eV, respectively, demonstrating higher C₂H₄ selectivity over the Pd₈ cluster than Pd(111), which is consistent with the experimental results.

We further carried out electronic structure analysis to understand the weaker adsorption of reaction intermediates over the Pd cluster than Pd(111). In Fig. 4c, we plotted the density of states projected onto d-electrons of the surface Pd atom where C₂H₄ adsorbs over Pd(111) and Pd cluster, and the d-band center (ε_d) were calculated to be −1.53 and −1.79 eV, respectively. According to the d-band center theory⁶¹, the more negative ε_d indicates weaker binding to adsorbates, which is in agreement with the trend of adsorption energies calculated for the reaction intermediates over Pd(111) and Pd cluster. In addition, the ε_d of each Pd atom on Pd cluster was found to be correlated with E_ads of C₂H₄ as shown in Fig. 4d, and a more negative ε_d generally corresponds to weaker adsorption of C₂H₄. The most stable adsorption of ethylene, corresponding to the black solid square in Fig. 4d and *C₂H₄ in Fig. 4b, gives rise to the E_ads of −0.54 eV, which is slightly weaker than that over Pd(111), i.e., −0.57 eV. However, one can also see that C₂H₄ adsorbs much weaker at other Pd sites over the Pd₈ cluster (shown as black hollow squares), therefore Pd₈ cluster shows weaker adsorption on average compared with those sites over Pd(111) where all the surface sites are identical. C₂H₄ would be more likely to desorb from the Pd cluster, resulting in the high C₂H₄ selectivity observed.

Our findings demonstrate the essential catalytic roles of a nanometer metal/oxide interface in selective hydrogenation of acetylene. Pd–Bi₂O₃ hybrid clusters feature a small Pd–Pd CN and intra-cluster electron transfer, which enables a weak C₂H₄ adsorption without compromising the H₂ activation activity. The superior low-temperature catalytic performance of Pd–Bi₂O₃ nanocluster ensembles over Pd single atom and nanoparticles might open new opportunities for fundamental research of hybrid nanoclusters. Besides, the demonstrated stepwise photochemical strategy also provides a new path for fabricating hybrid nanoclusters and nanometer metal/oxide interface.

Methods

Synthesis of Pd_x/Bi₂O₃/TiO₂

Pd_x/Bi₂O₃/TiO₂ (x is the nominal molar ratio of Pd to Bi) catalysts were prepared by a two-step photo-deposition method using a high-pressure Xe lamp (300 W) as the light source. Typically, 100 mg of TiO₂ and 11.6 mg of Bi(NO₃)₃·5H₂O (Sinopharm chemicals, 99%) were dispersed in 4 mL of ethylene glycol in a Pyrex glass reactor. Prior to ultraviolet irradiation for 1 h, the suspension was bubbled with Ar for 30 min to eliminate dissolved O₂. Subsequently, 8 mL of PdCl₂ aqueous solution (the concentration is determined by x) was added into the suspension. After irradiation in Ar for another 1 h, the precipitates were collected by centrifugation, washed twice by water and ethanol, and then dried in an oven at 40 °C. When the sample was exposed in the air, Bi was oxidized to Bi₂O₃ spontaneously. The catalyst was activated by H₂ at 100 °C for 1 h and then cooled to RT in N₂ prior to the catalytic reaction and characterizations. The synthetic procedures of Bi₂O₃/TiO₂, Pd/TiO₂, Pd_1.0/Bi₂O₃/TiO₂-ox, PdBi/TiO₂, and Pd/Bi₂O₃ are presented in Supplementary Information.

Characterization

Powder XRD patterns were recorded on a Rigaku Ultimate IV diffractometer using Cu Kα radiation operated at 40 mA and 40 kV (scan rate: 5^o min⁻¹). XPS measurements were performed in a VG Scientific ESCALAB Mark II spectrometer. All BEs were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon to correct the shift caused by charge effect. In situ near ambient pressure XPS (NAP-XPS) was conducted using a Specs NAP-XPS system with a PHOIBOS NAP hemispherical energy analyzer in Vacuum Interconnected Nanotech Workstation, Suzhou Institute of Nano-Tech and Nano-Bionics. The NAP operation was conducted under 1 mbar H₂ atmosphere. The actual loading of Pd and Bi were analyzed by ICP-AES using a Profile Spec ICP-AES spectrometer (Leeman, USA). Structural characterizations and chemical composition distribution were determined by a probe Cs-corrected electron microscope (FEI Titan) equipped with a Super-X EDS. Microcalorimetric measurements of C₂H₄ adsorption were carried out on a home-designed adsorption microcalorimetry system consisting of a chemisorption apparatus (Micromeritics Autochem II 2920) and a microcalorimeter (Setaram Sensys EVO 600).

The XAFS spectra at Pd K-edge and Bi L3-edge of the samples were measured at beamline 14W of Shanghai Synchrotron Radiation Facility in China⁶². The output beam was filtered by Si(311) monochromator. Pd foil was used to calibrate the energy.

In situ Fourier transform infrared (FT-IR) adsorption spectroscopy of CO experiments were recorded on a Nicolet iS50 instrument. Prior to CO chemisorption, the catalysts were activated by H₂ at 100 °C for 1 h and then cooled down to RT in Ar. The FT-IR spectrum of Ar at RT was taken as the background spectrum and subtracted automatically from subsequent spectra. Then the catalysts were exposed to a CO flow for 10 min and degassed by Ar for 10 min to desorb the physical adsorbed CO, and IR spectra were recorded.

TPR measurements were conducted on Micromeritics ChemiSorb 2920, equipped with a thermal conductivity detector. Prior to TPR measurements, 100 mg of the catalyst was activated by H₂ at 100 °C for 1 h and then cooled down to RT in Ar. Afterwards, the catalyst was subjected to 10 vol% H₂/Ar at a flow rate of 30 mL min⁻¹ and heated to 150 °C at 10 °C min⁻¹.

Catalytic tests

Selective hydrogenation of acetylene in excess ethylene was carried out in a fixed bed vertical quartz reactor, with a space velocity of 120,000 mL h^–1 g^–1. The reaction gas consisting of 1.0 vol% C₂H₂, 20.0 vol% C₂H₄, 20.0 vol.% H₂, and 59 vol.% N₂ was fed at a flow rate of 60 mL min⁻¹, simulating the front-end hydrogenation conditions. Typically, 30 mg of the catalyst (diluted by 400 mg of quartz sand) was activated by H₂ (20 mL min^–1) at 100 °C for 1 h and then cooled to RT in N₂ prior to the catalytic reaction. The gas components from the microreactor outlet were analyzed by online gas chromatography (GC; Shimadzu GC-2010) equipped with a flame ionization detector. It is important to note that the pretreatment in H₂ is important as the long-term exposure of Pd_1.0/Bi₂O₃/TiO₂ in air might oxidize Pd and demolish Pd–Bi bonding. Control experiment also suggests that untreated Pd_1.0/Bi₂O₃/TiO₂ has poor selectivity toward ethylene. To this end, all catalysts were pretreated in H₂ before catalytic measurements.

C₂H₄ and C₂H₆ were the only products detected by GC. Negligible oligomers were formed during the hydrogenation process, likely due to the short contact time and high concentration of hydrogen¹⁰. C₂H₂ conversion and C₂H₄ selectivity were calculated as:

$${\mathrm{C}}_2{\mathrm{H}}_2\mathrm{conversion}=\frac{{\left[{\mathrm{C}}_2{\mathrm{H}}_2\right]}_{\mathrm{inlet}}-{\left[{\mathrm{C}}_2{\mathrm{H}}_2\right]}_{\mathrm{outlet}}}{{\left[{\mathrm{C}}_2{\mathrm{H}}_2\right]}_{\mathrm{inlet}}}\times 100\%$$ 1

$${\mathrm{C}}_2{\mathrm{H}}_4\mathrm{selectivity}=\frac{{\left[{\mathrm{C}}_2{\mathrm{H}}_2\right]}_{\mathrm{inlet}}-{\left[{\mathrm{C}}_2{\mathrm{H}}_2\right]}_{\mathrm{outlet}}-{\left[{\mathrm{C}}_2{\mathrm{H}}_6\right]}_{\mathrm{outlet}}}{{\left[{\mathrm{C}}_2{\mathrm{H}}_2\right]}_{\mathrm{inlet}}-{\left[{\mathrm{C}}_2{\mathrm{H}}_2\right]}_{\mathrm{outlet}}}\times 100\%$$ 2

Computational details

In this work, Vienna Ab initio Simulation Package^63–66 was used to conduct density functional calculations within the generalized gradient approximation of RPBE functional developed by the Nørskov group⁶⁷. Ionic cores and electrons were described by the projector augmented wave method^68,69. The energy cutoff was set to be 500 eV and the force threshold was 0.05 eV Å⁻¹. We used constrained minimization method^59,70–72 to locate the transition state structures. The adsorption energies of C₂H_x (x = 2, 4, and 6) were calculated as: E_ads = E_C2Hx+slab − (E_C2Hx + E_slab), where E_C2Hx+slab is the energy of the system after adsorption of C₂H_x species, E_C2Hx is the energy of the gas-phase C₂H_x adsorbate, and E_slab is the energy of the slab.

The optimized lattice parameters of Bi₂O₃ were a = 5.981 Å, b = 8.340 Å, and c = 7.591 Å, which are close to experimental values (monoclinic, P2₁/c(14), a = 5.849 Å, b = 8.166 Å, and c = 7.510 Å). A Bi₂O₃(100) slab was built with a 2 × 2 supercell with nine atomic layers, and the Pd cluster structure was built by adding 8 Pd atoms onto the Bi₂O₃(100) surface, followed by structure optimization. To build a valid catalyst model of Pd cluster, we followed these rules: (i) Pd would disperse on the Bi₂O₃ surface with a size <2 nm; (ii) the structure should give rise to similar CNs listed in Supplementary Table 2; (iii) the model should be stable and would not deform under reaction condition, which has been widely discussed. During the optimizations involving Pd cluster structure, the bottom two layers of atoms of Bi₂O₃(100) component were fixed to simulate bulk structure of Bi₂O₃, while the other atoms were fully relaxed. The vacuum layer was set higher than 12 Å to avoid spurious interaction between adjacent slabs. The k-point grid used in the surface Brillouin zone was 1 × 1 × 1 for all the calculations. More details about the structure development are provided in Supplementary Information (Supplementary Figs. 16 and 17).

Source data

Source Data ^{(427.4KB, xlsx)}

Author contributions

J.F. and S.Z. designed the study. S.Z. and B.L. performed most of the experiments. W.Y., C.Z., Y.Z., and Y.W. performed the electron microscopy characterization. K.Y. and B.Y. finished the DFT calculations. L.M., Y.D., J.M., and Z.J. carried out the X-ray structure characterization and analysis. W.H. carried out the microcalorimetric measurements. Z.G. and Y.C. performed the in situ NAP-XPS studies. L.L., J.L., and L.X. performed the TPR and TPD measurements. S.Z., K.Y., J.F., B.Y., and Y.Z. wrote the paper. All authors interpreted the data and contributed to the preparation of the manuscript.

Data availability

All the data supporting the findings of this study are available within the article and its Supplementary Information files or from the corresponding authors upon reasonable request. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-021-25984-8.