Electrocatalytic urea synthesis from NO and CO₂ on In₁Pd single atom alloys

Abstract

Electrocatalytic urea synthesis from CO₂/NO co-electrolysis (EUCN) has emerged as a promising strategy for sustainable urea production, while simultaneously mitigating greenhouse gas emissions and NO pollutants. Herein, we have developed single-atom In₁ alloyed Pd (In₁Pd) as a high-performance EUCN catalyst, delivering a remarkable FE_urea of 45.9% and a urea yield rate of 55.2 mmol h⁻¹ g⁻¹ in a membrane electrode assembly electrolyzer. The combination of in situ spectroscopic measurements and theoretical calculations reveals the synergy of In₁ and Pd, which enables the co-activation of CO₂/NO and their C–N coupling while hampering the competing reactions, leading to greatly enhanced EUCN activity and selectivity.

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Urea is a vital nitrogen fertilizer that is widely used in global agriculture.^1–3 Currently, the Bosch–Meiser process serves as the primary industrial route for urea synthesis, accounting for over 2% of global energy consumption and generating substantial CO₂ emissions.^4–8 Electrocatalytic urea synthesis from co-reduction of CO₂ and NO (EUCN) has emerged as a promising strategy for sustainable urea production, while simultaneously mitigating greenhouse gas emissions and NO pollutants.⁹ However, the EUCN process involves a complex multi-step reaction pathway requiring the activation of inert CO₂ and NO molecules and enhanced C–N coupling kinetics, posing significant challenges for catalyst design.¹⁰ Current catalysts still suffer from low urea faradaic efficiency (FE_urea) and poor selectivity due possibly to the competitive side reactions (i.e., the hydrogen evolution reaction (HER)) and independent reduction pathways.⁹ Therefore, the development of high-performance catalysts with high activity, selectivity and durability is crucial to advance the practical application of EUCN.

Single-atom alloys (SAAs), which combine the merits of single-atom catalysts and alloy catalysts, have garnered significant attention in various electrocatalytic reactions involving both carbon and nitrogen cycles,^11–13 thus demonstrating their considerable potential for electrocatalytic urea synthesis from CO₂/NO co-electrolysis (EUCN). Among them, Pd-based materials have shown particular efficacy attributed to their unfilled d-electron orbitals (4d¹⁰) that facilitate optimized adsorption/desorption behavior of key intermediates.^14–16 Nevertheless, Pd-based catalysts often suffer from insufficient co-activation capability toward both nitrogen and carbon sources. Additionally, Pd exhibits a strong tendency for H adsorption,¹⁷ which competes with the C–N coupling reaction and diminishes both EUCN activity and selectivity for urea synthesis. Notably, these limitations may be effectively mitigated by incorporating p-block metals such as indium (In). By virtue of its partially occupied p-orbitals, In can intrinsically hinder H binding,¹⁸ while promoting CO₂ reduction to generate and stabilize C-intermediates beneficial for coupling with N-intermediates towards urea generation.^19–21 Motivated by these insights, we propose that single-atom In alloyed with Pd (In₁Pd) may serve as a highly promising catalyst, enabling enhanced activity and selectivity for efficient EUCN.

In this study, we developed In₁Pd as a highly active and selective EUCN catalyst for urea electrosynthesis. Notably, the In₁Pd catalyst exhibits exceptional performance in a membrane electrode assembly (MEA) electrolyzer, delivering a remarkable FE_urea of 45.9% and a urea yield rate of 55.2 mmol h⁻¹ g⁻¹ at −0.7 V. The catalytic EUCN mechanism of the In₁Pd catalyst is further elucidated through combined in situ spectroscopic measurements and theoretical calculations, revealing that the enhanced EUCN efficiency of In₁Pd stems from the synergistic effect of In₁ and Pd, which promotes the co-activation of CO₂ and NO to *CO/*NH₂ and their C–N coupling.

In₁Pd was synthesized via a one-pot wet chemistry method. The XRD patterns (Fig. 1a) reveal that In₁Pd retains the crystalline structure of pristine Pd (JCPDS No. 65-2867), suggesting that In₁ incorporation does not change the crystalline phase of pristine Pd. The TEM image (Fig. 1b) shows that In₁Pd exhibits a typical graphene-like morphology. Elemental mapping images of In₁Pd (Fig. 1c) illustrate a uniform distribution of In₁ atoms on Pd. The coordination structure and electronic characteristics of In₁Pd are systematically characterized via X-ray absorption spectroscopy. The XANES analysis at the In K-edge (Fig. 1d) reveals that the absorption edge of In₁Pd lies between In foil and In₂O₃ reference samples, indicating an In oxidation state between 0 and +3, which arises from electron transfer from In₁ to Pd due to the lower electronegativity of In (1.78) relative to Pd (2.20). The EXAFS spectra (Fig. 1e) show that In₁Pd exhibits a prominent In–Pd coordination peak at 2.75 Å, with no observable In–In or In–Pd coordination bonds, confirming the atomic dispersion of In on the Pd substrate.^22–24 The corresponding wavelet transform (WT) analysis (Fig. 1f) displays a single In–Pd coordination peak at 6.5 Å⁻¹, further verifying the monoatomic In dispersion within the Pd matrix.

Fig. 1 Characterization of In₁Pd: (a) XRD patterns, (b) TEM image, (c) elemental mapping images, (d) In K-edge XANES, (e) EXAFS spectra and (f) WT analyses of In₁Pd and reference samples.

The electronic structure of In₁Pd is systematically investigated using DFT calculations. Electron density difference analysis (Fig. S1a) reveals a distinct electron transfer from In₁ to Pd, in line with the XANES result (Fig. 1d). This interfacial In₁–Pd electron interaction guarantees the robust bonding between In₁ and the Pd substrate, which endows In₁Pd with exceptional structural stability. The corresponding PDOS analysis (Fig. S1b) presents favorable orbital overlapping between the 5p orbital of In₁ and the 4d orbital of Pd, offering a theoretical foundation for the efficient charge transfer and robust structural stability.²⁵ Additionally, ab initio molecular dynamics (AIMD) simulation results (Fig. S2) indicate that In₁Pd maintains stable energy and temperature profiles throughout the simulation, further validating its excellent thermal stability.^26–30

The EUCN performance of In₁Pd is assessed in a MEA cell containing 0.1 M KHCO₃ catholyte saturated with humidified NO and CO₂. Gas and liquid products are quantified by gas chromatography and colorimetry, respectively (Fig. S3). LSV curves (Fig. 2a) show a relatively low current density of In₁Pd in sole CO₂-saturated electrolyte. Strikingly, the current density is significantly enhanced in the presence of both NO and CO₂, suggesting the high catalytic EUCN activity of In₁Pd toward urea synthesis. The EUCN performance of In₁Pd is quantitatively evaluated after 1 h of electrolysis. Remarkably, In₁Pd achieves the highest FE_urea of 45.9% at −0.7 V, with a corresponding urea yield rate of 55.2 mmol h⁻¹ g⁻¹ (Fig. 2b), surpassing most recently reported catalysts for urea electrosynthesis (Fig. S4 and Table S1). Control experiments are conducted to validate the nitrogen and carbon sources (Fig. S5). No urea formation is observed under conditions lacking NO or CO₂, or at the open-circuit potential (OCP), effectively ruling out the possibility of system contamination as C/N sources of urea. Further ¹³C and ¹⁵N isotope tracing via nuclear magnetic resonance (NMR) spectroscopy, using ¹³CO₂ (Fig. 2c) and ¹⁵NO (Fig. 2d) as tracers, reveals the characteristic signals of ¹³CO(NH₂)₂ and CO(¹⁵NH₂)₂, respectively, further confirming that the synthesized urea originates exclusively from the EUNC process catalyzed by In₁Pd.

Fig. 2 (a) LSV curves of In₁Pd under different conditions. (b) Urea yield rates and FE_urea of In₁Pd at various potentials. (c) ¹³C NMR spectra of the ¹³CO(NH₂)₂ standard sample and those fed by ¹³CO₂ after electrolysis at −0.7 V. (d) ¹H NMR spectra of the CO(¹⁵NH₂)₂ standard sample and those electrolyzed in ¹⁵NO electrolyte at −0.7 V. (e) Urea yield rates and FE_urea during eight cycling tests at −0.7 V. (f) Comparison of urea yield rates and FE_urea between pristine Pd and In₁Pd at −0.7 V.

The selectivity of In₁Pd toward ECNU is evaluated by quantifying the FEs of other byproducts (CO, H₂ and NH₄⁺). At the optimal applied potential of −0.7 V, FE_urea remains significantly higher than the FEs of all byproducts (Fig. S6), confirming the exceptional ECNU selectivity of In₁Pd for urea synthesis. For stability evaluation, we conducted an eight-cycle test, which shows that both urea yield rate and FE_urea exhibit minimal fluctuations (Fig. 2e), confirming the excellent catalysis durability of In₁Pd.^30–32 Comparative analysis shows that the pristine Pd catalyst (Fig. 2f) exhibits a much inferior ECNU performance relative to In₁Pd, highlighting the critical synergistic interaction between In₁ and the Pd substrate in boosting the ECNU activity.

To elucidate the fundamental understanding of the significantly improved EUCN performance of In₁Pd, we employed in situ FTIR and online differential electrochemical mass spectrometry (DEMS) to identify the reaction intermediates. First, in situ FTIR measurements are conducted on In₁Pd over the potential range from the OCP to −0.7 V. As shown in Fig. 3a–c, the characteristic peak at 1396 cm⁻¹ is assigned to the symmetric stretching vibration of the *COOH intermediate, while the C=O stretching peak at 2017 cm⁻¹ corresponds to the generated *CO intermediate³³ (Fig. 3b). Notably, additional peaks observed at 1695 cm⁻¹ and 1433 cm⁻¹ are attributed to *CONH₂ intermediates and C–N bonds,^34–36 respectively (Fig. 3c), indicating that EUCN proceeds via *NH₂ + *CO → *NH₂CO. Meanwhile, the enhanced peak intensity of urea (1610/1195 cm⁻¹) with increasing potentials indicates that the generated *NH₂CO intermediates are readily converted into urea.³⁵ In addition, online DEMS measurements (Fig. 3d) show the prominent m/z signals corresponding to key intermediates and products, including *CONH₂ (m/z = 44), *CO(NH₂)₂ (m/z = 60), *NO (m/z = 30), *NH₂ (m/z = 16), *CO (m/z = 28) and *COOH (m/z = 45). These DEMS results are in line with in situ FTIR data, collectively providing compelling evidence that In₁Pd facilitates the efficient co-reduction of CO₂ and NO to urea through a sequential relay catalysis mechanism (Fig. S7), where In₁Pd first promotes the co-activation of CO₂ and NO, forming critical *CO and *NH₂ intermediates. These *CO/*NH₂ intermediates then undergo C–N coupling to generate *CONH₂, which is ultimately converted into urea.

Fig. 3 (a–c) In situ FTIR spectra of In₁Pd during the EUNC electrolysis at different potentials from the OCP to −0.7 V. (d) Online DEMS spectra of In₁Pd at −0.7 V.

DFT calculations are utilized to unravel the atomic-level EUCN mechanism of In₁Pd. Given that the adsorption and activation of NO/CO₂ represent the initial step of the catalytic EUCN process, our analysis first focused on NO/CO₂ adsorption behaviors on both In₁ and Pd sites of In₁Pd. As illustrated in Fig. S8, the adsorption free energy calculations reveal that In₁ sites are more favorable for CO₂ adsorption (−0.26 eV), while Pd sites are more favorable for NO adsorption (−0.48 eV), suggesting that during the EUCN process, In₁ sites primarily facilitate CO₂ reduction while Pd sites dominate NO reduction.

We then constructed the free energy diagram for the conversion of CO₂ to *CO on In₁ sites (Fig. 4a), with the corresponding intermediate configurations shown in Fig. S9. Calculation results reveal that the rate-determining step (RDS) for CO₂ → *CO reduction is the first CO₂ hydrogenation (*CO₂ + *H → *COOH), with an energy barrier of 0.38 eV. Notably, the *CO desorption energy barrier on In₁ sites is significantly lower (0.19 eV) than that required for further hydrogenation (*CO + *H → *COH, 0.54 eV), indicating that *CO generated on In₁ sites is more inclined to desorb and spontaneously migrate closer to Pd sites and participate in the C–N coupling reaction.

Fig. 4 (a) Free energy diagram of the CO₂ → CO pathway on In₁ sites. (b) Free energy diagram of the entire EUNC pathway on Pd sites and possible competing side reactions. (c) PDOS profile of *CONH₂ on In₁Pd. (d) Adsorption free energies of *H/*CO₂ on In₁ sites and *H/*NO on Pd sites.

Subsequently, we constructed the free energy diagram for the entire EUCN process on Pd sites, with the corresponding intermediate configurations shown in Fig. S10. As illustrated in Fig. 4b, the RDS of this pathway is also the initial NO hydrogenation (*NO + *H → *NHO, 0.29 eV). Further analysis of the energy barriers for the competing reactions, such as *NO + *CO → *NOCO and *NO + *CO₂ → *NOCO₂, reveals that compared to direct C–N coupling with *CO or *CO₂, *NO tends to undergo hydrogenation reduction via the NHO pathway. Significantly, after stepwise *NOH reduction to *NH₂, the generated *NH₂ intermediate is more likely to undergo C–N coupling with *CO to form *NH₂CO relative to other competing reactions (*NH₂ + H → *NH₃, *NH₂ + CO₂ → *NH₂CO₂). The generated *NH₂CO can be spontaneously converted into urea. These findings correlate closely with the above in situ FTIR and DEMS results (Fig. 3).

For the critical *NH₂CO intermediate on both pristine Pd and In₁Pd surfaces (Fig. S11), the charge density difference map displays that In₁Pd provides more electrons to *NH₂CO than pristine Pd does, suggesting the significantly enhanced *NH₂CO stabilization on In₁Pd. Further PDOS analysis (Fig. 4c and Fig. S12) reveals that compared to pristine Pd, the overlap region of electron orbitals between In₁Pd and *NH₂CO is much expanded, further verifying the stronger *NH₂CO activation capability of In₁Pd. These results indicate that the introduced In₁ not only promotes the reduction of CO₂ to *CO but also modulates the electronic structure of the Pd substrate and enhances *NH₂CO stabilization and activation towards urea conversion. Given the HER as the major competing reaction for the EUCN,⁴ we examined the adsorption characteristics of *H on In₁Pd. Fig. 4d shows that In₁ sites are more favorable for adsorbing *CO₂ over *H, while Pd sites exhibit a stronger tendency to adsorb *NO over *H. Molecular dynamics (MD) simulations (Fig. S13) further reveal a more pronounced enrichment effect of *NO on the In₁Pd surface, and the corresponding radial distribution function (RDF, Fig. S14) shows that the NO/In₁Pd interaction is stronger than the H/In₁Pd interaction. These results demonstrate that In₁Pd can well hamper the competing HER towards the selective conversion of CO₂/NO to urea.

In summary, In₁Pd is demonstrated as a high-performance EUCN catalyst for urea electrosynthesis. Combined in situ spectroscopic measurements and theoretical calculations reveal that the enhanced EUCN performance of In₁Pd stems from the synergy of In₁ and Pd, which enables the co-activation of CO₂/NO and their C–N coupling, while hampering the competing reactions. Impressively, In₁Pd exhibits an unprecedented urea synthesis performance with urea yield rate up to 55.2 mmol h⁻¹ g⁻¹ and FE_urea of 45.9% in a MEA cell. This work provides in-depth insights into the EUCN mechanism and opens up a new avenue for developing efficient and robust catalysts.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The data supporting this article have been included as part of the SI. Supplementary information is available. See DOI: https://doi.org/10.1039/d5dt02018b.

Acknowledgements

This work is supported by Hebei North University 2025 College Students Innovation Training Program (no. 202510092022).

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