Establishing reaction networks in the 16-electron sulfur reduction reaction.

The sulfur reduction reaction (SRR) plays a central role in high-capacity lithium sulfur (Li-S) batteries. The SRR involves an intricate, 16-electron conversion process featuring multiple lithium polysulfide intermediates and reaction branches1-3. Establishing the complex reaction network is essential for rational tailoring of the SRR for improved Li-S batteries, but represents a daunting challenge4-6. Herein we systematically investigate the electrocatalytic SRR to decipher its network using the nitrogen, sulfur, dual-doped holey graphene framework as a model electrode to understand the role of electrocatalysts in acceleration of conversion kinetics. Combining cyclic voltammetry, in situ Raman spectroscopy and density functional theory calculations, we identify and directly profile the key intermediates (S8, Li2S8, Li2S6, Li2S4 and Li2S) at varying potentials and elucidate their conversion pathways. Li2S4 and Li2S6 were predominantly observed, in which Li2S4 represents the key electrochemical intermediate dictating the overall SRR kinetics. Li2S6, generated (consumed) through a comproportionation (disproportionation) reaction, does not directly participate in electrochemical reactions but significantly contributes to the polysulfide shuttling process. We found that the nitrogen, sulfur dual-doped holey graphene framework catalyst could help accelerate polysulfide conversion kinetics, leading to faster depletion of soluble lithium polysulfides at higher potential and hence mitigating the polysulfide shuttling effect and boosting output potential. These results highlight the electrocatalytic approach as a promising strategy for tackling the fundamental challenges regarding Li-S batteries.

Tuning the Hydrogenation Selectivity of an Unsaturated Aldehyde via Single-Atom Alloy Catalysts.

Selective hydrogenation of α,ß-unsaturated aldehydes to produce unsaturated alcohols remains a challenge in catalysis. Here, we explore, on the basis of first-principles simulations, single-atom alloy (SAA) catalysts on copper as a class of catalytic materials to enhance the selectivity for C═O bond hydrogenation in unsaturated aldehydes by controlling the binding strength of the C═C and C═O bonds. We show that on SAA of early transition metals such as Ti, Zr, and Hf, the C═O binding mode of acrolein is favored but the strong binding renders subsequent hydrogenation and desorption impossible. On SAA of late-transition metals, on the other hand, the C═C binding mode is favored and C═C bond hydrogenation follows, resulting in the production of undesired saturated aldehydes. Mid-transition metals (Cr and Mn) in Cu(111) appear as the optimal systems, since they favor acrolein adsorption via the C═O bond but with a moderate binding strength, compatible with catalysis. Additionally, acrolein migration from the C═O to the C═C binding mode, which would open the low energy path for C═C bond hydrogenation, is prevented by a large barrier for this process. SAA of Cr in Cu appears as an optimal candidate, and kinetic simulations show that the selectivity for propenol formation is controlled by preventing the acrolein migration from the more stable C═O to the less stable C═C binding mode and subsequent H-migration and by the formation of the O-H bond from the monohydrogenated intermediate. Dilute alloy catalysts therefore enable tuning the binding strength of intermediates and transition states, opening control of catalytic activity and selectivity.

H and CO Co-Induced Roughening of Cu Surface in CO2 Electroreduction Conditions.

The dynamic restructuring of Cu has been observed under electrochemical conditions, and it has been hypothesized to underlie the unique reactivity of Cu toward CO2 electroreduction. Roughening is one of the key surface phenomena for Cu activation, whereby numerous atomic vacancies and adatoms form. However, the atomic structure of such surface motifs in the presence of relevant adsorbates has remained elusive. Here, we explore the chemical space of Cu surface restructuring under coverage of CO and H in realistic electroreduction conditions, by combining grand canonical DFT and global optimization techniques, from which we construct a potential-dependent grand canonical ensemble representation. The regime of intermediate and mixed CO and H coverage─where structures exhibit some elevated surface Cu─is thermodynamically unfavorable yet kinetically inevitable. Therefore, we develop a quasi-kinetic Monte Carlo simulation to track the system's evolution during a simulated cathodic scan. We reveal the evolution path of the system across coverage space and identify the accessible metastable structures formed along the way. Chemical bonding analysis is performed on the metastable structures with elevated Cu*CO species to understand their formation mechanism. By molecular dynamics simulations and free energy calculations, the surface chemistry of the Cu*CO species is explored, and we identify plausible mechanisms via which the Cu*CO species may diffuse or dimerize. This work provides rich atomistic insights into the phenomenon of surface roughening and the structure of involved species. It also features generalizable methods to explore the chemical space of restructuring surfaces with mixed adsorbates and their nonequilibrium evolution.

Selective Reduction of Nitroarenes via Noncontact Hydrogenation.

In traditional hydrogenation, where H2 and substrates with unsaturated bonds are activated on the same catalyst (contact mode), competitive hydrogenation of multiple reducible groups often occurs. We employ an unbiased H-cell for selective hydrogenation of the nitro group when multiple reducible groups are present. The setup spatially separates H2 and nitroarenes into two chambers connected by a proton-exchange membrane, thus adding barriers for a Langmuir-Hinshelwood-type mechanism that is common in thermocatalytic hydrogenation. Through a unique proton/electron transfer pathway that is specific to nitro functional group reduction to hydroxylamine, side reactions like C═C, C═O, and C≡C bond hydrogenation are fully avoided. Using Pd/C for H2 activation, and CNT for selective proton/electron transfer to -NO2 groups while being inert to C≡C, C═C, and C═O hydrogenation, the system effectively eliminates the competitive hydrogenation, achieving 100% nitro-group reduction selectivity in the hydrogenation of various nitroarenes, in sharp contrast to negligible selectivity over the same catalysts in a batch reactor under contact mode. This device enables selectivity control in hydrogenation reactions, moving beyond the traditional focus on catalyst active site engineering.

Electroreduction of Captured CO2 on Silver Catalysts: Influence of the Capture Agent and Proton Source.

In the context of carbon reutilization, the direct electroreduction of captured CO2 (c-CO2RR) appears as an appealing approach since it avoids the energetically costly separation of CO2 from the capture agent. In this process, CO2 is directly reduced from its captured form. Here, we investigate the influence of the capture agent and proton source on that reaction from a combination of theory and experiment. Specifically, we consider methoxide-captured CO2, NH3-captured CO2, and bicarbonate on silver electrocatalysts. We show that the proton source plays a key role in the interplay of the chemistries for the electroreduction of protons, free CO2, and captured CO2. Our density functional theory calculations, including the influence of the potential, demonstrate that a proton source with smaller pKa improves the reactivity for c-CO2RR, but also increases the selectivity toward the hydrogen evolution reaction (HER) on silver surfaces. Since c-CO2RR requires an additional chemical protonation step, the influence of the proton source is stronger than that of the HER. However, c-CO2RR cannot compete with the HER on Ag, Experimentally, the dominant product observed is H2 with low amounts of CO being produced. Through a rotating cylinder electrode cell of well-defined mass-transport properties, we conclude that although methanol solvent exhibits a lower HER activity, HER remains dominant over c-CO2RR. Our work suggests that methoxide is a potential alternative capture agent to NH3 for direct reduction of captured CO2, though challenges in catalyst design, particularly in reducing the onset potential of c-CO2RR to surpass the HER, remain to be addressed.

Dilute Alloys Based on Au, Ag, or Cu for Efficient Catalysis: From Synthesis to Active Sites.

The development of new catalyst materials for energy-efficient chemical synthesis is critical as over 80% of industrial processes rely on catalysts, with many of the most energy-intensive processes specifically using heterogeneous catalysis. Catalytic performance is a complex interplay of phenomena involving temperature, pressure, gas composition, surface composition, and structure over multiple length and time scales. In response to this complexity, the integrated approach to heterogeneous dilute alloy catalysis reviewed here brings together materials synthesis, mechanistic surface chemistry, reaction kinetics, in situ and operando characterization, and theoretical calculations in a coordinated effort to develop design principles to predict and improve catalytic selectivity. Dilute alloy catalysts─in which isolated atoms or small ensembles of the minority metal on the host metal lead to enhanced reactivity while retaining selectivity─are particularly promising as selective catalysts. Several dilute alloy materials using Au, Ag, and Cu as the majority host element, including more recently introduced support-free nanoporous metals and oxide-supported nanoparticle "raspberry colloid templated (RCT)" materials, are reviewed for selective oxidation and hydrogenation reactions. Progress in understanding how such dilute alloy catalysts can be used to enhance selectivity of key synthetic reactions is reviewed, including quantitative scaling from model studies to catalytic conditions. The dynamic evolution of catalyst structure and composition studied in surface science and catalytic conditions and their relationship to catalytic function are also discussed, followed by advanced characterization and theoretical modeling that have been developed to determine the distribution of minority metal atoms at or near the surface. The integrated approach demonstrates the success of bridging the divide between fundamental knowledge and design of catalytic processes in complex catalytic systems, which can accelerate the development of new and efficient catalytic processes.

Metallic Impurities in Electrolysis: Catalytic Effect of Pb Traces in Reductive Amination and Acetone Reduction.

The electrochemical hydrogenation (e-hydrogenation) of unsaturated compounds like imines or carbonyls presents a benign reduction method. It enables direct use of electrons as reducing agent, water as proton source, while bypassing the need for elevated temperatures or pressures. In this contribution, we discuss the active species in electrocatalytic reductive amination with the transformation of acetone and methylamine as model reaction. Surprisingly, lead impurities in the ppm-range proved to possess a significant effect in e-hydrogenation. Accordingly, the influence of applied potential and cathode material in presence of 1 ppm Pb was investigated. Finally, we transferred the insights to the reduction of acetone manifesting comparable observations as for imine reduction. The results suggest that previous studies on electrochemical reduction in the presence of lead electrodes should be re-evaluated.

Stabilizing *CO2 Intermediates at the Acidic Interface using Molecularly Dispersed Cobalt Phthalocyanine as Catalysts for CO2 Reduction.

CO2 electroreduction (CO2 R) operating in acidic media circumvents the problems of carbonate formation and CO2 crossover in neutral/alkaline electrolyzers. Alkali cations have been universally recognized as indispensable components for acidic CO2 R, while they cause the inevitable issue of salt precipitation. It is therefore desirable to realize alkali-cation-free CO2 R in pure acid. However, without alkali cations, stabilizing *CO2 intermediates by catalyst itself at the acidic interface poses as a challenge. Herein, we first demonstrate that a carbon nanotube-supported molecularly dispersed cobalt phthalocyanine (CoPc@CNT) catalyst provides the Co single-atom active site with energetically localized d states to strengthen the adsorbate-surface interactions, which stabilizes *CO2 intermediates at the acidic interface (pH=1). As a result, we realize CO2 conversion to CO in pure acid with a faradaic efficiency of 60 % at pH=2 in flow cell. Furthermore, CO2 is successfully converted in cation exchanged membrane-based electrode assembly with a faradaic efficiency of 73 %. For CoPc@CNT, acidic conditions also promote the intrinsic activity of CO2 R compared to alkaline conditions, since the potential-limiting step, *CO2 to *COOH, is pH-dependent. This work provides a new understanding for the stabilization of reaction intermediates and facilitates the designs of catalysts and devices for acidic CO2 R.

Nature of Zirconia on a Copper Inverse Catalyst Under CO2 Hydrogenation Conditions.

The growing concern over the escalating levels of anthropogenic CO2 emissions necessitates effective strategies for its conversion to valuable chemicals and fuels. In this research, we embark on a comprehensive investigation of the nature of zirconia on a copper inverse catalyst under the conditions of CO2 hydrogenation to methanol. We employ density functional theory calculations in combination with the Grand Canonical Basin Hopping method, enabling an exploration of the free energy surface including a variable amount of adsorbates within the relevant reaction conditions. Our focus centers on a model three-atom Zr cluster on a Cu(111) surface decorated with various OH, O, and formate ligands, noted Zr3Ox (OH)y (HCOO)z/Cu(111), revealing major changes in the active site induced by various reaction parameters such as the gas pressure, temperature, conversion levels, and CO2/H2 feed ratios. Through our analysis, we have unveiled insights into the dynamic behavior of the catalyst. Specifically, under reaction conditions, we observe a large number of composition and structures with similar free energy for the catalyst, with respect to changing the type, number, and binding sites of adsorbates, suggesting that the active site should be regarded as a statistical ensemble of diverse structures that interconvert.

Atomic-Scale Mechanism of Platinum Catalyst Restructuring under a Pressure of Reactant Gas.

Heterogeneous catalysis is key for chemical transformations. Understanding how catalysts' active sites dynamically evolve at the atomic scale under reaction conditions is a prerequisite for accurately determining catalytic mechanisms and predictably developing catalysts. We combine in situ time-dependent scanning tunneling microscopy observations and machine-learning-accelerated first-principles atomistic simulations to uncover the mechanism of restructuring of Pt catalysts under a pressure of carbon monoxide (CO). We show that a high CO coverage at a Pt step edge triggers the formation of atomic protrusions of low-coordination Pt atoms, which then detach from the step edge to create sub-nano-islands on the terraces, where under-coordinated sites are stabilized by the CO adsorbates. The fast and accurate machine-learning potential is key to enabling the exploration of tens of thousands of configurations for the CO-covered restructuring catalyst. These studies open an avenue to achieve an atomic-scale understanding of the structural dynamics of more complex metal nanoparticle catalysts under reaction conditions.

Electrochemical Oxidation of Methane to Methanol on Electrodeposited Transition Metal Oxides.

Electrochemical partial oxidation of methane to methanol is a promising approach to the transformation of stranded methane resources into a high-value, easy-to-transport fuel or chemical. Transition metal oxides are potential electrocatalysts for this transformation. However, a comprehensive and systematic study of the dependence of methane activation rates and methanol selectivity on catalyst morphology and experimental operating parameters has not been realized. Here, we describe an electrochemical method for the deposition of a family of thin-film transition metal (oxy)hydroxides as catalysts for the partial oxidation of methane. CoOx, NiOx, MnOx, and CuOx are discovered to be active for the partial oxidation of methane to methanol. Taking CoOx as a prototypical methane partial oxidation electrocatalyst, we systematically study the dependence of activity and methanol selectivity on catalyst film thickness, overpotential, temperature, and electrochemical cell hydrodynamics. Optimal conditions of low catalyst film thickness, intermediate overpotentials, intermediate temperatures, and fast methanol transport are identified to favor methanol selectivity. Through a combination of control experiments and DFT calculations, we show that the oxidized form of the as-deposited (oxy)hydroxide catalyst films are active for the thermal oxidation of methane to methanol even without the application of bias potential, demonstrating that high valence transition metal oxides are intrinsically active for the activation and oxidation of methane to methanol at ambient temperatures. Calculations uncover that electrocatalytic oxidation enables reaching an optimum potential window in which methane activation forming methanol and methanol desorption are both thermodynamically favorable, methanol desorption being favored by competitive adsorption with hydroxide anion.

Electrocatalytic Hydrogen Evolution at Full Atomic Utilization over ITO-Supported Sub-nano-Ptn Clusters: High, Size-Dependent Activity Controlled by Fluxional Pt Hydride Species.

A combination of density functional theory (DFT) and experiments with atomically size-selected Ptn clusters deposited on indium-tin oxide (ITO) electrodes was used to examine the effects of applied potential and Ptn size on the electrocatalytic activity of Ptn (n = 1, 4, 7, and 8) for the hydrogen evolution reaction (HER). Activity is found to be negligible for isolated Pt atoms on ITO, increasing rapidly with Ptn size such that Pt7/ITO and Pt8/ITO have roughly double the activity per Pt atom compared to atoms in the surface layer of polycrystalline Pt. Both the DFT and experiment find that hydrogen under-potential deposition (Hupd) results in Ptn/ITO (n = 4, 7, and 8) adsorbing ∼2H atoms/Pt atom at the HER threshold potential, equal to ca. double the Hupd observed for Pt bulk or nanoparticles. The cluster catalysts under electrocatalytic conditions are hence best described as a Pt hydride compound, significantly departing from a metallic Pt cluster. The exception is Pt1/ITO, where H adsorption at the HER threshold potential is energetically unfavorable. The theory combines global optimization with grand canonical approaches for the influence of potential, uncovering the fact that several metastable structures contribute to the HER, changing with the applied potential. It is hence critical to include reactions of the ensemble of energetically accessible PtnHx/ITO structures to correctly predict the activity vs Ptn size and applied potential. For the small clusters, spillover of Hads from the clusters to the ITO support is significant, resulting in a competing channel for loss of Hads, particularly at slow potential scan rates.

Achieving Ultra-High Selectivity to Hydrogen Production from Formic Acid on Pd-Ag Alloys.

Palladium-silver-based alloy catalysts have a great potential for CO-free hydrogen production from formic acid for fuel cell applications. However, the structural factors affecting the selectivity of formic acid decomposition are still debated. Herein, the decomposition pathways of formic acid on Pd-Ag alloys with different atomic configurations have been investigated to identify the alloy structures yielding high H2 selectively. Several PdxAg1-x surface alloys with various compositions were generated on a Pd(111) single crystal; their atomic distribution and electronic structure were determined by a combination of infrared reflection absorption spectroscopy (IRAS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). It was established that the Ag atoms with Pd neighbors are electronically altered, and the degree of alteration correlates with the number of nearest Pd. Temperature-programmed reaction spectroscopy (TPRS) and DFT demonstrated that the electronically altered Ag domains create a new reaction pathway that selectively dehydrogenates formic acid. In contrast, Pd monomers surrounded by Ag are demonstrated to have a similar reactivity compared to pristine Pd(111), yielding CO and H2O in addition to the dehydrogenation products. However, they bind to the produced CO weaker than pristine Pd, demonstrating an enhancement in resistance to CO poisoning. This work therefore shows that surface Ag domains modified by interaction with subsurface Pd are the key active sites for selective decomposition of formic acid, while surface Pd atoms are detrimental to selectivity. Hence, the decomposition pathways can be tailored for CO-free H2 production on Pd-Ag alloy systems.

Toward more accurate surface properties of ceria using many-body perturbation theory.

Despite the wide applications, the ab initio modeling of the ceria based catalyst is challenging. The partial occupation in the 4f orbitals creates a fundamental challenge for commonly used density functional theory (DFT) methods, including semilocal functionals with Hubbard U correction to force localization and hybrid functionals. In this work, we benchmark the random phase approximation (RPA) for ceria surface properties, including surface energy and hydrogenation energy, compared to the results utilizing the DFT + U approach or hybrid functionals. We show that, for the latter approaches, different surface properties require opposite directions of parameter tuning. This forms a dilemma for the parameter based DFT methods, as the improvement of a certain property by tuning parameters will inevitably lead to the worsening of other properties. Our results suggest that the parameter-free many-body perturbation theory methods exemplified by RPA are a promising strategy to escape the dilemma and provide highly accurate descriptions, which will allow us to better understand the catalytic reactions in ceria related systems.

Facilitating hydrogen atom migration via a dense phase on palladium islands to a surrounding silver surface.

The migration of species across interfaces can crucially affect the performance of heterogeneous catalysts. A key concept in using bimetallic catalysts for hydrogenation is that the active metal supplies hydrogen atoms to the host metal, where selective hydrogenation can then occur. Herein, we demonstrate that, following dihydrogen dissociation on palladium islands, hydrogen atoms migrate from palladium to silver, to which they are generally less strongly bound. This migration is driven by the population of weakly bound states on the palladium at high hydrogen atom coverages which are nearly isoenergetic with binding sites on the silver. The rate of hydrogen atom migration depends on the palladium-silver interface length, with smaller palladium islands more efficiently supplying hydrogen atoms to the silver. This study demonstrates that hydrogen atoms can migrate from a more strongly binding metal to a more weakly binding surface under special conditions, such as high dihydrogen pressure.

Restructuring and Activation of Cu(111) under Electrocatalytic Reduction Conditions.

The dynamic restructuring of Cu surfaces in electroreduction conditions is of fundamental interest in electrocatalysis. We decode the structural dynamics of a Cu(111) electrode under reduction conditions by joint first-principles calculations and operando electrochemical scanning tunneling microscopy (ECSTM) experiments. Combining global optimization and grand canonical density functional theory, we unravel the potential- and pH-dependent restructuring of Cu(111) in acidic electrolyte. At reductive potential, Cu(111) is covered by a high density of H atoms and, below a threshold potential, Cu adatoms are formed on the surface in a (4×4) superstructure, a restructuring unfavorable in vacuum. The strong H adsorption is the driving force for the restructuring, itself induced by the electrode potential. On the restructured surface, barriers for hydrogen evolution reaction steps are low. Restructuring in electroreduction conditions creates highly active Cu adatom sites not present on Cu(111).

Hydrogen Evolution on Electrode-Supported Ptn Clusters: Ensemble of Hydride States Governs the Size Dependent Reactivity.

We report the size-dependent activity and stability of supported Pt1,4,7,8 for electrocatalytic hydrogen evolution reaction, and show that clusters outperform polycrystalline Pt in activity, with size-dependent stability. To understand the size effects, we use DFT calculations to study the structural fluxionality under varying potentials. We show that the clusters can reshape under H coverage and populate an ensemble of states with diverse stoichiometry, structure, and thus reactivity. Both experiment and theory suggest that electrocatalytic species are hydridic states of the clusters (≈2 H/Pt). An ensemble-based kinetic model reproduces the experimental activity trend and reveals the role of metastable states. The stability trend is rationalized by chemical bonding analysis. Our joint study demonstrates the potential- and adsorbate-coverage-dependent fluxionality of subnano clusters of different sizes and offers a systematic modeling strategy to tackle the complexities.

Hydrogen-Induced Restructuring of a Cu(100) Electrode in Electroreduction Conditions.

The rearrangement of Cu surfaces under electrochemical conditions is known to play a key role in the surface activation for major electrocatalytic reactions. Despite the extensive experimental insights into such rearrangements, from surface-sensitive spectroscopy and microscopy, the spatial and temporal resolution of these methods is insufficient to provide an atomistic picture of the electrochemical interface. Theoretical characterization has also been challenged by the diversity of restructuring configurations, surface stoichiometry, adsorbate configurations, and the effect of the electrode potential. Here, atomistic insight into the restructuring of the electrochemical interface is gained from first principles. Cu(100) restructuring under varying applied potentials and adsorbate coverages is studied by grand canonical density functional theory and global optimization techniques, as well as ab initio molecular dynamics and mechanistic calculations. We show that electroreduction conditions cause the formation of a shifted-row reconstruction on Cu(100), induced by hydrogen adsorption. The reconstruction is initiated at 1/6 ML H coverage, when the Cu-H bonding sufficiently weakens the Cu-Cu bonds between the top- and sublayer, and further stabilized at 1/3 ML when H adsorbates fill all the created 3-fold hollow sites. The simulated scanning tunneling microscopy (STM) images of the calculated reconstructed interfaces agree with experimental in situ STM. However, compared to the thermodynamic prediction, the onsets of reconstruction events in the experiment occur at more negative applied voltages. This is attributed to kinetic effects in restructuring, which we describe via different statistical models, to produce the potential- and pH-dependent surface stability diagram. This manuscript provides rich atomistic insight into surface restructuring in electroreduction conditions, which is required for the understanding and design of Cu-based materials for electrocatalytic processes. It also offers the methodology to study the problem of in situ electrode reconstruction.

Direct Observation of Solvent-Reaction Intermediate Interactions in Heterogeneously Catalyzed Alcohol Coupling.

The relative stability of reactive intermediates and reactants on a surface, which dictates the rate and selectivity of catalytic reactions in both gas and liquid phases, is dependent on numerous factors. One well-established example is secondary interactions, such as van der Waals interactions between the catalyst surface and the pendant group of the intermediate, which can govern reaction selectivity for coupling reactions. Herein, we directly show that interactions between adsorbed reaction intermediates and reactant molecules increase the binding energy and affects the geometrical arrangement of coadsorbed reactant/solvent molecules. Evidence for this effect is demonstrated for the oxidative coupling reaction of methanol on a single crystal gold (Au(110)) surface. The rate-limiting reaction intermediate for methanol self-coupling, methoxy, stabilizes excess adsorbed methanol, which desorbs as a result of beta-hydride decomposition of the adsorbed methoxy. Direct molecular-scale imaging by scanning tunneling microscopy, supplemented by density functional theory, revealed interactive structures formed by methoxy and coadsorbed methanol. Interactions between the methoxy intermediate and coadsorbed methanol stabilizes a hydrogen-bonded network comprising methoxy and methanol by a minimum of 0.13 eV per methanol molecule. Inclusion of such interactions between reaction intermediates and coadsorbed reactants and solvents in kinetic models is important for microkinetic analysis of the rates and selectivities of catalytic reactions in both the gas and liquid phases whenever appreciable coverages of species from the ambient phase exist.

Dynamical Study of Adsorbate-Induced Restructuring Kinetics in Bimetallic Catalysts Using the PdAu(111) Model System.

Dynamic restructuring of bimetallic catalysts plays a crucial role in their catalytic activity and selectivity. In particular, catalyst pretreatment with species such as carbon monoxide and oxygen has been shown to be an effective strategy for tuning the surface composition and morphology. Mechanistic and kinetic understanding of such restructuring is fundamental to the chemistry and engineering of surface active sites but has remained challenging due to the large structural, chemical, and temporal degrees of freedom. Here, we combine time-resolved temperature-programmed infrared reflection absorption spectroscopy, ab initio thermodynamics, and machine-learning molecular dynamics to uncover previously unidentified timescale and kinetic parameters of in situ restructuring in Pd/Au(111), a highly relevant model system for dilute Pd-in-Au nanoparticle catalysts. The key innovation lies in utilizing CO not only as a chemically sensitive probe of surface Pd but also as an agent that induces restructuring of the surface. Upon annealing in vacuum, as-deposited Pd islands became encapsulated by Au and partially dissolved into the subsurface, leaving behind isolated Pd monomers on the surface. Subsequent exposure to 0.1 mbar CO enabled Pd monomers to repopulate the surface up to 373 K, above which complete Pd dissolution occurred by 473 K, with apparent activation energies of 0.14 and 0.48 eV, respectively. These restructuring processes occurred over the span of ∼1000 s at a given temperature. Such a minute-timescale dynamics not only elucidates the fluxional nature of alloy catalysts but also presents an opportunity to fine-tune the surface under moderate temperature and pressure conditions.