Computational Screening of Near-Surface Alloys for CO2 Electroreduction

Nonbonding Metal-Metal Interaction in Metal-Nitrogen-Carbon Single-Atom Catalysts Boosts CO Electroreduction

Metal-nitrogen-carbon single-atom catalysts (SACs) have recently emerged as selective electrocatalysts for the reduction of CO2 to CO, but their ability to further electroreduce CO is poor. Here, based on constant-potential density functional theory simulations, we predict that Co-N-M (M = Fe, Co) SACs with nonbonding metallic centers bridged by a common nitrogen atom can catalyze four-electron reduction of CO to methanol at an ultralow overpotential of 220-310 mV. We show that the metal atoms in the SACs are terminated by H species which prevent the formation of σ bonding between CO and the metal atoms. Thanks to the nonbonding electrostatic repulsion between Co and its adjacent M atom, the Co dxz band is broadened and shifted toward the Fermi level, leading to enhanced dxz - 2π* interaction that gives rise to stable CO adsorption and promotes its active and selective reduction. This work offers an alternative strategy to tackle the challenge of CO electroreduction on SACs and highlights the role of nonbonding metal-metal interactions in modulating adsorption properties of SACs.

Breaking the Linear Scaling Relationship by Alloying Micro Sn to a Cu Surface toward CO2 Electrochemical Reduction

The electrochemical CO2 reduction reaction (CO2RR) to HCOOH provides an avenue for reducing global accelerated CO2 emissions and producing high-value-added chemicals. Nevertheless, the presence of an inherent linear scaling relationship (LSR) between *OCHO and *HCOOH leads to the electrosynthesis of HCOOH being achieved at high cathodic potentials. In this work, by adjusting the different Cu:Sn ratio of SnxCu(1-x) alloys, we comprehensively explored the electrocatalytic 2e- CO2RR performance toward the production of HCOOH. Combining density functional theory calculations with the constant-potential implicit solvent model, the Sn0.03Cu0.97 surface alloy was posited to be a promising electrocatalyst with superior HCOOH selectivity and an ultralow limiting potential of -0.20 V in an environment of pH = 7.2. The high performance was found to originate from the breaking of the LSR, which is a result of an extraordinary electronic property of the active Cu site. This work not only advances a global-searched strategy for the rational design of efficient catalysts toward HCOOH production but also provides in-depth insights into the underlying mechanism for the enhanced performance of microalloy electrocatalysts.

Regulation Strategy of Nanostructured Engineering on Indium-Based Materials for Electrocatalytic Conversion of CO2

Electrochemical carbon dioxide reduction (CO2 RR), as an emerging technology, can combine with sustainable energies to convert CO2 into high value-added products, providing an effective pathway to realize carbon neutrality. However, the high activation energy of CO2 , low mass transfer, and competitive hydrogen evolution reaction (HER) leads to the unsatisfied catalytic activity. Recently, Indium (In)-based materials have attracted significant attention in CO2 RR and a series of regulation strategies of nanostructured engineering are exploited to rationally design various advanced In-based electrocatalysts, which forces the necessary of a comprehensive and fundamental summary, but there is still a scarcity. Herein, this review provides a systematic discussion of the nanostructure engineering of In-based materials for the efficient electrocatalytic conversion of CO2 to fuels. These efficient regulation strategies including morphology, size, composition, defects, surface modification, interfacial structure, alloying, and single-atom structure, are summarized for exploring the internal relationship between the CO2 RR performance and the physicochemical properties of In-based catalysts. The correlation of electronic structure and adsorption behavior of reaction intermediates are highlighted to gain in-depth understanding of catalytic reaction kinetics for CO2 RR. Moreover, the challenges and opportunities of In-based materials are proposed, which is expected to inspire the development of other effective catalysts for CO2 RR.

One-dimensional single atom arrays on ferroelectric nanosheets for enhanced CO2 photoreduction

Single-atom catalysts show excellent catalytic performance because of their coordination environments and electronic configurations. However, controllable regulation of single-atom permutations still faces challenges. Herein, we demonstrate that a polarization electric field regulates single atom permutations and forms periodic one-dimensional Au single-atom arrays on ferroelectric Bi4Ti3O12 nanosheets. The Au single-atom arrays greatly lower the Gibbs free energy for CO2 conversion via Au-O=C=O-Au dual-site adsorption compared to that for Au-O=C=O single-site adsorption on Au isolated single atoms. Additionally, the Au single-atom arrays suppress the depolarization of Bi4Ti3O12, so it maintains a stronger driving force for separation and transfer of photogenerated charges. Thus, Bi4Ti3O12 with Au single-atom arrays exhibit an efficient CO production rate of 34.15 µmol·g-1·h-1, ∼18 times higher than that of pristine Bi4Ti3O12. More importantly, the polarization electric field proves to be a general tactic for the syntheses of one-dimensional Pt, Ag, Fe, Co and Ni single-atom arrays on the Bi4Ti3O12 surface.

Single-Atom Catalysts and Dual-Atom Catalysts for CO2 Electroreduction: Competition or Cooperation?

Developing highly active and selective electrocatalysts for electrochemical reduction of CO2 can reduce environmental pollution and mitigation of greenhouse gas emission. Owing to maximal atomic utilization, the atomically dispersed catalysts are broadly adopted in CO2 reduction reaction (CO2 RR). Dual-atom catalysts (DACs), with more flexible active sites, distinct electronic structures, and synergetic interatomic interactions compared to single-atom catalysts (SACs), may have great potential to enhance catalytic performance. Nevertheless, most of the existing electrocatalysts have low activity and selectivity due to their high energy barrier. Herein, 15 electrocatalysts are explored with noble metallic (Cu, Ag, and Au) active sites embedded in metal-organic hybrids (MOHs) for high-performance CO2 RR and studied the relationship between SACs and DACs by first-principles calculation. The results indicated that the DACs have excellent electrocatalytic performance, and the moderate interaction between the single- and dual-atomic center can improve catalytic activity in CO2 RR. Four among the 15 catalysts, including (CuAu), (CuCu), Cu(CuCu), and Cu(CuAu) MOHs inherited a capability of suppressing the competitive hydrogen evolution reaction with favorable CO overpotential. This work not only reveals outstanding candidates for MOHs-based dual-atom CO2 RR electrocatalysts but also provides new theoretical insights into rationally designing 2D metallic electrocatalysts.

Computational screening of high activity and selectivity of CO2 reduction via transition metal single-atom catalysts on triazine-based graphite carbon nitride

Single-atom catalysts (SACs) are emerging as promising catalysts in the field of the electrocatalytic CO2 reduction reaction (CO2RR). Herein, a series of 3d to 5d transition metal atoms supported on triazine-based graphite carbon nitride (TM@TGCN) as a CO2 reduction catalyst are studied via density functional theory computations. Eventually, four TM@TGCN catalysts (TM = Ni, Rh, Os, and Ir) are selected using a five-step screening method, in which Rh@TGCN and Ni@TGCN show a low limiting potential of -0.48 and -0.58 V, respectively, for reducing CO2 to CH4. The activity mechanism shows that the catalysts with a negative d-band center and optimal positive charge can improve the CO2RR performance. Our study provides theoretical guidance for the rational design of highly active and selective catalysts.

Precise electronic structure modulation on MXene-based single atom catalysts for high-performance electrocatalytic CO2 reduction reaction: A first-principle study

Electrocatalytic CO₂ reduction reaction (CO₂RR) to CO is a logical approach to achieve a carbon-neutral cycle. In this work, a series of Ti₂CO₂ and O vacancy containing Ti₂CO₂ MXene-based transition metal (TM) single atom catalysts (SACs), including TM-Ti₂CO₂ and TM-O_v-Ti₂CO₂, are explored for high-performance CO₂RR. Sc/Ti/V/Cr-Ti₂CO₂ and Ni-O_v-Ti₂CO₂ are screened out with limiting potential (U_L) more positive than -0.50 V. Ni-O_v-Ti₂CO₂ is a candidate catalyst for CO₂RR to CO, considering its activity with U_L of -0.27 eV, and the selectivity relevant to hydrogen evolution reaction and HCOOH production. Meanwhile, a novel activity descriptor of TM-Ti-O group valence state is proposed according to that TMs work in synergy with coordinated Ti and O atoms and a level of around 0.64 e^- benefits to CO₂RR. This work highlights oxygen vacancy containing Ti₂CO₂-based Ni SAC as a promising catalyst for CO₂RR, and provides a feasible electronic structure design principle for guiding the design of MXene-based SACs for CO₂RR.

High Throughput Preparation of Ag-Zn Alloy Thin Films for the Electrocatalytic Reduction of CO2 to CO

Ag-Zn alloys are identified as highly active and selective electrocatalysts for CO₂ reduction reaction (CO₂RR), while how the phase composition of the alloy affects the catalytic performances has not been systematically studied yet. In this study, we fabricated a series of Ag-Zn alloy catalysts by magnetron co-sputtering and further explored their activity and selectivity towards CO₂ electroreduction in an aqueous KHCO₃ electrolyte. The different Ag-Zn alloys involve one or more phases of Ag, AgZn, Ag₅Zn₈, AgZn_3, and Zn. For all the catalysts, CO is the main product, likely due to the weak CO binding energy on the catalyst surface. The Ag₅Zn₈ and AgZn₃ catalysts show a higher CO selectivity than that of pure Zn due to the synergistic effect of Ag and Zn, while the pure Ag catalyst exhibits the highest CO selectivity. Zn alloying improves the catalytic activity and reaction kinetics of CO₂RR, and the AgZn₃ catalyst shows the highest apparent electrocatalytic activity. This work found that the activity and selectivity of CO₂RR are highly dependent on the element concentrations and phase compositions, which is inspiring to explore Ag-Zn alloy catalysts with promising CO₂RR properties.

Synergistic Cr2 O3 @Ag Heterostructure Enhanced Electrocatalytic CO2 Reduction to CO

The electrocatalytic CO₂ RR to produce value-added chemicals and fuels has been recognized as a promising means to reduce the reliance on fossil resources; it is, however, hindered due to the lack of high-performance electrocatalysts. The effectiveness of sculpturing metal/metal oxides (MMO) heterostructures to enhance electrocatalytic performance toward CO₂ RR has been well documented, nonetheless, the precise synergistic mechanism of MMO remains elusive. Herein, an in operando electrochemically synthesized Cr₂ O₃ -Ag heterostructure electrocatalyst (Cr₂ O₃ @Ag) is reported for efficient electrocatalytic reduction of CO₂ to CO. The obtained Cr₂ O₃ @Ag can readily achieve a superb FE_CO of 99.6% at -0.8 V (vs RHE) with a high J_CO of 19.0 mA cm^-2 . These studies also confirm that the operando synthesized Cr₂ O₃ @Ag possesses high operational stability. Notably, operando Raman spectroscopy studies reveal that the markedly enhanced performance is attributable to the synergistic Cr₂ O₃ -Ag heterostructure induced stabilization of CO₂ ^•- /*COOH intermediates. DFT calculations unveil that the metallic-Ag-catalyzed CO₂ reduction to CO requires a 1.45 eV energy input to proceed, which is 0.93 eV higher than that of the MMO-structured Cr₂ O₃ @Ag. The exemplified approaches in this work would be adoptable for design and development of high-performance electrocatalysts for other important reactions.

A Universal Descriptor for Complicated Interfacial Effects on Electrochemical Reduction Reactions

Supported catalysts have exhibited excellent performance in various reactions. However, the rational design of supported catalysts with high activity and certain selectivity remains a great challenge because of the complicated interfacial effects. Using recently emerged two-dimensional materials supported dual-atom catalysts (DACs@2D) as a prototype, we propose a simple and universal descriptor based on inherent atomic properties (electronegativity, electron type, and number), which can well evaluate the complicated interfacial effects on the electrochemical reduction reactions (i.e., CO₂, O₂, and N₂ reduction reactions). Based on this descriptor, activity and selectivity trends in CO₂ reduction reaction are successfully elucidated, in good agreement with available experimental data. Moreover, several potential catalysts with superior activity and selectivity for target products are predicted, such as CuCr/g-C₃N₄ for CH₄ and CuSn/N-BN for HCOOH. More importantly, this descriptor can also be extended to evaluate the activity of DACs@2D for O₂ and N₂ reduction reactions, with very small errors between the prediction and reported experimental/computational results. This work provides feasible principles for the rational design of advanced electrocatalysts and the construction of universal descriptors based on inherent atomic properties.

Applications of Machine Learning in Alloy Catalysts: Rational Selection and Future Development of Descriptors

At present, alloys have broad application prospects in heterogeneous catalysis, due to their various catalytic active sites produced by their vast element combinations and complex geometric structures. However, it is the diverse variables of alloys that lead to the difficulty in understanding the structure-property relationship for conventional experimental and theoretical methods. Fortunately, machine learning methods are helpful to address the issue. Machine learning can not only deal with a large number of data rapidly, but also help establish the physical picture of reactions in multidimensional heterogeneous catalysis. The key challenge in machine learning is the exploration of suitable general descriptors to accurately describe various types of alloy catalysts, which help reasonably design catalysts and efficiently screen candidates. In this review, several kinds of machine learning methods commonly used in the design of alloy catalysts is introduced, and the applications of various reactivity descriptors corresponding to different alloy systems is summarized. Importantly, this work clarifies the existing understanding of physical picture of heterogeneous catalysis, and emphasize the significance of rational selection of universal descriptors. Finally, the development of heterogeneous catalytic descriptors for machine learning are presented.

Circumventing the scaling relationship on bimetallic monolayer electrocatalysts for selective CO2 reduction

Dual-functional active sites are designed to circumvent the scaling relationship between the HER and CO2RR on bimetallic monolayer electrocatalysts.

The role of site coordination on the CO2 electroreduction pathway on stepped and defective copper surfaces

Changes in adsorption site coordination on stepped and defective Cu surfaces affect reaction pathways and potential-determining steps for CO₂ electroreduction.

Selective electroreduction of CO2 to ethanol over a highly stable catalyst derived from polyaniline/CuBi2O4

We highlights the importance of surface evolution by electrochemical pre-treatment while stabilizing the main body of the catalyst. The PANi/CuBi2O4via electro-chemical activation process shows high faraday efficiency to ethanol.

Dynamic Restructuring Induced Oxygen Activation on AgCu Near-Surface Alloys

Recent studies have shown that the addition of Cu to Ag catalysts improves their epoxidation performance by increasing the overall selectivity of the bimetallic catalyst. We have prepared AgCu near-surface alloys and used scanning tunneling microscopy to gain an atomistic picture of O₂ dissociation on the bimetallic system. These data reveal a higher dissociative sticking probability for O₂ on AgCu than on Ag(111), and density functional theory (DFT) confirms that the O₂ dissociation barrier is 0.17 eV lower on the alloy. Surprisingly, we find that, after a slow initial uptake of O₂, the sticking probability increases exponentially. Further DFT calculations indicate that surface oxygen reverses the segregation energy for AgCu, stabilizing Cu atoms in the Ag layer. These single Cu atoms in the Ag surface are found to significantly lower the O₂ dissociation barrier. Together, these results explain nonlinear effects in the activation of O₂ on this catalytically relevant surface alloy.

Hybrid Cu0 and Cux + as Atomic Interfaces Promote High-Selectivity Conversion of CO2 to C2 H5 OH at Low Potential

The mixing of charge states of metal copper catalysts may lead to a much improved reactivity and selectivity toward multicarbon products for CO₂ reduction. Here, an electrocatalyst model composed of copper clusters supported on graphitic carbon nitride (g-C₃ N₄ ) is proposed; the connecting Cu atoms with g-C₃ N₄ can be oxidized to Cu^{x +} due to substantial charge transfer from Cu to N atoms, while others stay as Cu⁰ . It is revealed that CO₂ can be captured and reduced into *CO on the Cu_t ⁰ site, owing to its zero oxidation state. More importantly, C-C coupling reaction of two *CHO species on the Cu_t ⁰ -Cu_b ^{x +} atomic interface can occur with a rather low kinetic barrier of 0.57 eV, leading to the formation of the final C₂ product, namely, C₂ H₅ OH. During the whole process, the limiting potential is just 0.68 V. These findings may open a new avenue for CO₂ reduction into high-value fuels and chemicals.

Anchoring of black phosphorus quantum dots onto WO3 nanowires to boost photocatalytic CO2 conversion into solar fuels

A 0D–1D direct Z-scheme heterojunction consisting of black phosphorus quantum dots (BPQDs) anchored onto WO₃ nanowires was well designed.

Colloidal silver diphosphide (AgP2) nanocrystals as low overpotential catalysts for CO2 reduction to tunable syngas

Production of syngas with tunable CO/H₂ ratio from renewable resources is an ideal way to provide a carbon-neutral feedstock for liquid fuel production. Ag is a benchmark electrocatalysts for CO₂-to-CO conversion but high overpotential limits the efficiency. We synthesize AgP₂ nanocrystals (NCs) with a greater than 3-fold reduction in overpotential for electrochemical CO₂-to-CO reduction compared to Ag and greatly enhanced stability. Density functional theory calculations reveal a significant energy barrier decrease in the formate intermediate formation step. In situ X-ray absorption spectroscopy (XAS) shows that a maximum Faradaic efficiency is achieved at an average silver valence state of +1.08 in AgP₂ NCs. A photocathode consisting of a n⁺p-Si wafer coated with ultrathin Al₂O₃ and AgP₂ NCs achieves an onset potential of 0.2 V vs. RHE for CO production and a partial photocurrent density for CO at −0.11 V vs. RHE (j_{−0.11, CO}) of −3.2 mA cm⁻².

Conversion of CO₂ into value-added chemicals by use of renewable energy is promising to achieve a carbon-neutral energy cycle. Here, the authors show that AgP₂ is a stable, selective and efficient syngas catalyst for solar-to-fuel conversion with a 3-fold lower overpotential compared to the benchmark Ag catalyst.

Electrochemical CO2 Reduction to C1 Products on Single Nickel/Cobalt/Iron-Doped Graphitic Carbon Nitride: A DFT Study

Electrocatalytic CO₂ reduction reaction (CRR) is one of the most promising strategies to convert greenhouse gases to energy sources. Herein, the CRR was applied towards making C₁ products (CO, HCOOH, CH₃ OH, and CH₄ ) on g-C₃ N₄ frameworks with single Ni, Co, and Fe introduction; this process was investigated by density functional theory. The structures of the electrocatalysts, CO₂ adsorption configurations, and CO₂ reduction mechanisms were systematically studied. Results showed that the single Ni, Co, and Fe located from the corner of the g-C₃ N₄ cavity to the center. Analyses of the adsorption configurations and electronic structures suggested that CO₂ could be chemically adsorbed on Co-C₃ N₄ and Fe-C₃ N₄ , but physically adsorbed on Ni-C₃ N₄ . The H₂ evolution reaction (HER), as a suppression of CRR, was investigated, and results showed that Ni-C₃ N₄ , Co-C₃ N₄ , and Fe-C₃ N₄ exhibited more CRR selectivity than HER. CRR proceeded via COOH and OCHO as initial protonation intermediates on Ni-C₃ N₄ and Co/Fe-C₃ N₄ , respectively, which resulted in different C₁ products along quite different reaction pathways. Compared with Ni-C₃ N₄ and Fe-C₃ N₄ , Co-C₃ N₄ had more favorable CRR activity and selectivity for CH₃ OH production with unique rate-limiting steps and lower limiting potential.

Influence of Van der Waals Interactions on the Solvation Energies of Adsorbates at Pt-Based Electrocatalysts

Solvation can significantly modify the adsorption energy of species at surfaces, thereby influencing the performance of electrocatalysts and liquid-phase catalysts. Thus, it is important to understand adsorbate solvation at the nanoscale. Here we evaluate the effect of van der Waals (vdW) interactions described by different approaches on the solvation energy of *OH adsorbed on near-surface alloys (NSAs) of Pt. Our results show that the studied functionals can be divided into two groups, each with rather similar average *OH solvation energies: (1) PBE and PW91; and (2) vdW functionals, RPBE, PBE-D3 and RPBE-D3. On average, *OH solvation energies are less negative by ∼0.14 eV in group (2) compared to (1), and the values for a given alloy can be extrapolated from one functional to another within the same group. Depending on the desired level of accuracy, these concrete observations and our tabulated values can be used to rapidly incorporate solvation into models for electrocatalysis and liquid-phase catalysis.

Pd-Ag Alloy Electrocatalysts for CO2 Reduction: Composition Tuning to Break the Scaling Relationship

Constructing solid-solution-alloy electrocatalysts with tunable surface electronic configurations is the key to optimize intermediate bindings and thereby to promote the activity and selectivity of the CO₂ reduction reaction (CO₂RR). Herein, Pd_1-xAg_x alloy electrocatalysts are investigated as a platform to uncover the electronic effects on the CO₂RR. The optimal Pd_0.75Ag_0.25/C affords a superior CO Faradaic efficiency of 95.3% at -0.6 V (vs RHE) in 0.5 M KHCO₃, performing at a high level among recently reported electrocatalysts. Experimental and theoretical analysis further evidence that varying the composition of Pd_1-xAg_x alloys can effectively alter the electronic configurations and consequently break the inherent scaling relationship of the binding energy of different intermediates (*COOH and *CO). Among Pd_1-xAg_x, Pd_0.75Ag_0.25 gains the obviously weakened *CO and *H bindings but retained well the binding with *COOH, contributing to the facilitated kinetics toward CO product. Elucidating a feasible way to break the scaling relationship and further uncover the underlying mechanism, this work will inspire new design strategies toward active and selective electrocatalysts.

Theoretical insight into the electrocatalytic reduction of CO2 with different metal ratios and reaction mechanisms on palladium-copper alloys

Environmental impacts of continued CO2 production have led to an increased need for new methods of CO2 removal and energy development. Electrochemical reduction of CO2 has been shown to be a good method through recent studies. Alloys are of special interest for these applications, because of their unique chemical and physical properties that allow for highly active surfaces. Here, PdnCum (m + n = 15 and n > m) bimetallic electrocatalysts were used for systematic studies to understand the effect of the composition of Pd and Cu on the electrochemical reduction of CO2 to CO. In particular, the Pd-Cu alloy with the Pd/Cu = 2/1 atomic ratio (i.e., Pd10Cu5) has the best catalytic effect, particularly true at the step of the hydrogenation of CO2 to COOH, and the Pd10Cu5 catalyst is better than most known electrodes. With the energetic analysis of the proposed reaction pathways over the Pd10Cu5 catalyst, the limiting voltages for CO2 reduction to CH3OH, CH4, and CH3CH2O have been compared. Most importantly, the kinetic model analysis showed that the rate constant values indicate that the probability of generating C2H5OH on the Pd10Cu5 catalyst is greater than that of CH3OH or CH4. The findings revealed in this study may shed some light on the design of cost-effective and efficient electrocatalysts for CO2 conversion to CO or to other useful hydrocarbons.