Structural Rearrangement of Au-Pd Nanoparticles under Reaction Conditions: An ab Initio Molecular Dynamics Study

Dynamics of Dilute Nanoalloy Catalysts

Capturing the dynamic character of metal nanoparticles under the reaction conditions is one of the major challenges within heterogeneous catalysis. The role of nanoparticle dynamics is particularly important for metal alloys as the surface composition responds sensitively to the gas environment. Here, a first-principles-based kinetic Monte Carlo method is developed to compare the dynamics of dilute PdAu alloy nanoparticles in inert and CO-rich atmospheres, corresponding to reaction conditions for catalyst deactivation and activation. CO influences the dynamics of the activation by facilitating the formation of vacancies and mobile Au-CO complexes, which are needed to obtain CO-stabilized Pd monomers on the surface. The structure of the catalyst and the location of the Pd monomers determine the rate of deactivation. The rate of catalyst deactivation is slow at low temperatures, which suggests that metastable structures determine the catalyst activity at typical operating conditions. The developed method is general and can be applied to a range of metal catalysts and reactions.

Tailoring the catalytic activity and selectivity on CO2 to C1 products by the synergistic effect of reactive molecules: A DFT study

The conversion of CO2 to CO is one of the crucial pathways in the carbon dioxide reduction reaction (CO2RR). Iron and nitrogen co-doped carbon matrix (FeN4) is a promising catalyst for converting CO2to CO with excellent activity and selectivity. However, the reactive mechanism of CO2RR on the FeN4 catalyst is not fully unveiled. For example, it is still evasive that the obtained C1 product is methanol and/or methane instead of CO in some cases. Herein, DFT calculation is conducted to unravel the effect from both solvent molecules and intermediates as axial groups on the selectivity of C1 products in CO2RR using FeN4 catalyts. Calculation results demonstrate that the FeN4(H), FeN4(OH), FeN4(COOH), and FeN4(CO) configurations are not only beneficial to the removal of CO, but also effectively suppress the hydrogen evolution reaction, whereas the FeN4, FeN4(CO2) and FeN4(H2O) configurations are inclined to produce CH3OH and/or CH4. The mechanism studied in this work provides an inspiration of optimizing the selectivity of C1 products in CO2RR from the perspective of regulating solvent molecules and intermediates as axial groups on FeN4.

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.

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.

Realistic Modelling of Dynamics at Nanostructured Interfaces Relevant to Heterogeneous Catalysis

The focus of this short review is directed towards investigations of the dynamics of nanostructured metallic heterogeneous catalysts and the evolution of interfaces during reaction—namely, the metal–gas, metal–liquid, and metal–support interfaces. Indeed, it is of considerable interest to know how a metal catalyst surface responds to gas or liquid adsorption under reaction conditions, and how its structure and catalytic properties evolve as a function of its interaction with the support. This short review aims to offer the reader a birds-eye view of state-of-the-art methods that enable more realistic simulation of dynamical phenomena at nanostructured interfaces by exploiting resource-efficient methods and/or the development of computational hardware and software.

Ab initio molecular dynamics with enhanced sampling in heterogeneous catalysis

Enhanced sampling ab initio simulations enable to study chemical phenomena in catalytic systems including thermal effects & anharmonicity, & collective dynamics describing enthalpic & entropic contributions, which can significantly impact on reaction free energy landscapes.

Bimetallic Au-Pd nanoparticles supported on silica with a tunable core@shell structure: enhanced catalytic activity of Pd(core)-Au(shell) over Au(core)-Pd(shell)

A facile ligand-assisted approach of synthesizing bimetallic Au-Pd nanoparticles supported on silica with a tunable core@shell structure is presented. Maneuvering the addition sequence of metal salts, both Aucore-Pdshell (Au@Pd-SiO2) and Pdcore-Aushell (Pd@Au-SiO2) nanoparticles were synthesized. The structures and compositions of the core-shell materials were confirmed by probe-corrected HRTEM, TEM-EDX mapping, EDS line scanning, XPS, PXRD, BET, FE-SEM-EDX and ICP analysis. The synergistic potentials of the core-shell materials were evaluated for two important reactions viz. hydrogenation of nitroarenes to anilines and hydration of nitriles to amides. In fact, in both the reactions, the Au-Pd materials exhibited superior performance over monometallic Au or Pd counterparts. Notably, among the two bimetallic materials, the one with Pdcore-Aushell structure displayed superior activity over the Aucore-Pdshell structure which could be attributed to the higher stability and uniform Au-Pd bimetallic interfaces in the former compared to the latter. Apart from enhanced synergism, high chemoselectivity in hydrogenation, wide functional group tolerance, high recyclability, etc. are other advantages of our system. A kinetic study has also been performed for the nitrile hydration reaction which demonstrates first order kinetics. Evaluation of rate constants along with a brief investigation on the Hammett parameters has also been presented.

Maximizing the Catalytic Performance of Pd@Aux Pd1-x Nanocubes in H2 O2 Production by Reducing Shell Thickness to Increase Compositional Stability

We report a simple route based upon seed-mediated growth to the synthesis of Pd@Au_x Pd_1-x (0.8≤x≤1) core-shell nanocubes. Benefiting from the well-defined {100} facets and an optimal Au/Pd ratio for the surface, the nanocubes bearing a shell made of Au_0.95 Pd_0.05 work as an efficient electrocatalyst toward H₂ O₂ production, with high selectivity of 93-100 % in the low-overpotential region of 0.4-0.7 V. When the Au_0.95 Pd_0.05 alloy is confined to a shell of only three atomic layers in thickness, the electrocatalyst is able to maintain its surface structure and elemental composition, endowing continuous and stable production of H₂ O₂ during oxygen reduction at a high rate of 1.62 mol g_(Pd+Au) ^-1  h^-1 . This work demonstrates a versatile route to the rational development of active and durable electrocatalysts based upon alloy nanocrystals.

Spectroscopic Probe Molecule Selection Using Quantum Theory, First-Principles Calculations, and Machine Learning

Probe molecule vibrational spectra have a long history of being used to characterize materials including metals, oxides, metal-organic frameworks, and even human proteins. Furthermore, recent advances in machine learning have enabled computationally generated spectra to aid in detailed characterization of complex surfaces with probe molecules. Despite widespread use of probe molecules, the science of probe molecule selection is underdeveloped. Here, we develop physical concepts, including orbital interaction energy and the energy overlap integral, to explain and predict the ability of probe molecules to discriminate structural descriptors. We resolve the crystal orbital overlap population (COOP) to specific molecular orbitals and quantify their bonding character, which directly influences vibrational frequencies. Using only a single adsorbate calculation from density function theory (DFT), we compute the interaction energy of individual adsorbate molecular orbitals with adsorption site atomic orbitals across many different sites. Combining the molecular orbital resolved COOP and changes in orbital interaction energy enables probe molecule selection for improved discrimination of various sites. We demonstrate these concepts by comparing the predicted effectiveness of carbon monoxide (CO), nitric oxide (NO), and ethylene (C₂H₄) to probe Pt adsorption sites. Finally, using a previously developed machine learning framework, we show that models trained on hundreds of thousands of C₂H₄ spectra, computed from DFT, which regress surface binding-type and generalized coordination number, outperform those trained using CO and NO spectra. A python package, pDOS_overlap, for implementing the electron density-based analysis on any combination of adsorbates and materials, is also made available.

Prediction of Structures and Atomization Energies of Coinage Metals, (M)n, n < 20: Extrapolation of Normalized Clustering Energies to Predict the Cohesive Energy

The geometries of the group 11 coinage metals (n = 2-20) were optimized to determine the lowest energy isomers for each cluster size, singlets for even numbers and doublets for odd numbers. For copper and silver, 2-D (planar) geometries were favored up to n = 6. For gold, 2D (planar) geometries were favored up to n = 13. Normalized clustering energies were plotted as a function of cluster size (n^-1/3, for n = 4-20) with various DFT functionals and the CCSD(T)-F12b method and were extrapolated to predict the bulk cohesive energy. In the case of copper and silver, there is excellent agreement between the cohesive energies predicted at the CCSD(T)-F12b level of theory and the experimental values. For gold, the CCSD(T)-F12b values needed to be corrected for spin-orbit relativistic effects to obtain good agreement with experiment. Electronic properties including the HOMO-LUMO gaps for the even clusters and the spin densities for the odd clusters were calculated. The lowest gap is predicted to occur for n = 16 where the HOMO and LUMO are very similar in shape.

Dynamic vs static behaviour of a supported nanoparticle with reaction-induced catalytic sites in a lattice model

Modern literature shows a rapidly growing interest to the supported nanocatalysts with dynamic behaviour under reaction conditions. This new frontier of heterogeneous catalysis is recognized as one of the most challenging and worthy of consideration from all possible angles. In this context, a previously suggested lattice model is used to get an insight, by means of kinetic Monte Carlo, into the influence of the mobility of reaction-induced catalytic sites of a two-dimensional supported nanoparticle on the system behaviour. The results speak in favour of feasibility of dynamic nanocatalysts with self-organized structures capable of robust functioning. This approach, from the macroscopic end, is believed to be a useful complement to ever developing experimental and first principle approaches.

Surface reconstruction of AgPd nanoalloy particles during the electrocatalytic formate oxidation reaction

Formate is a kind of carbon-neutral fuel that can be synthesized by electrochemical conversion of CO₂, however, the generated aqueous formate electrolyte is still short of potential application. Here, formate solution is proposed to be utilized as anode fuels of direct formate fuel cells through the formate oxidation reaction (FOR), and graphene supported AgPd nanoalloys (AgPd/rGO) are prepared to catalyze the FOR. Specifically, the mass activity of the as-prepared Ag₄₉Pd₅₁/rGO catalyst is 4.21 A mg^-1_Pd and the retention activity of Ag₄₉Pd₅₁/rGO is 49.1% of initial activity after successive 500 cycles, which is 2.48 and 3.03 times higher than that of unsupported Ag₅₁Pd₄₉ nanoalloys. When increasing the positive scan limit from 0.0 to 0.8 V, the mass activity of the Ag₄₉Pd₅₁/rGO catalyst increases from 2.32 to 6.03 A mg^-1_Pd and Pd surface coverage increases from 51.87% to 62.42%, indicating the occurrence of surface reconstruction where Pd atoms migrate to the surface of AgPd nanoalloys, and less intensive reconstruction is observed in unsupported Ag₅₁Pd₄₉ nanoalloys, whose mass activity increases from 1.35 to 2.49 A mg^-1_Pd. The driving force and kinetic path are calculated for the surface reconstruction induced by the adsorption of H, O and C atoms, in the case of C atoms on graphene, the segregation energy of surface Pd atoms in the AgPd nanoalloy is -1.16 eV, and the activation energy for the migration of subsurface Pd atoms to the surface is 0.54 eV, which are lower than the segregation (0.03 eV) and activation (2.06 eV) energy on a clean alloy surface.

Cinnamyl Alcohol Oxidation Using Supported Bimetallic Au–Pd Nanoparticles: An Optimization of Metal Ratio and Investigation of the Deactivation Mechanism Under Autoxidation Conditions

The aerobic oxidation of cinnamyl alcohol in toluene under autoxidation conditions has been studied using a range of 1 wt% Au–Pd/TiO2 catalysts. The catalysts have been studied to determine the effect of preparation method (impregnation and sol immobilisation) and metal ratio on the conversion of cinnamyl alcohol and the selectivity to cinnamaldehyde. The catalysts prepared by sol-immobilisation demonstrate higher selectivity to the desired aldehyde than the analogous impregnation materials. The most active catalyst was found to be 0.75 wt% Au–0.25 wt% Pd/TiO2 prepared by sol-immobilisation and this demonstrates the importance of metal ratio optimisation in this catalytic process. Furthermore, this metal ratio was found to be most stable under the reactions conditions with little change observed over multiple uses.

Atomic-Scale Observation of Bimetallic Au-CuOx Nanoparticles and Their Interfaces for Activation of CO Molecules

Supported gold nanoparticles with sizes below 5 nm display attractive catalytic activities for heterogeneous reactions, particularly those promoted by secondary metal (e.g., Cu) because of the well-defined synergy between metal compositions. However, the specific atomic structure at interfaces is less interpreted systematically. In this work, various bimetallic Au-CuO_x catalysts with specific surface structures were synthesized and explored by aberration-corrected scanning transmission electron microscopy (AC-STEM), temperature-programmed experiments and in situ DRIFT experiments. Results suggest that the atomic structure and interfaces between gold and CuO_x are determined by the nucleation behaviors of the nanoparticles and result in subsequently the distinctive ability for CO activation. Bimetallic CuO*/Au sample formatted by gold particles surrounded with CuO_x nanoclusters have rough surface with prominently exposed low-coordinated Au step defects. Whereas the bimetallic Au@CuO sample formatted by copper precursor in the presence of gold nanoparticles have core-shell structure with relatively smooth surface. The former structure of CuO*/Au displays much accelerated properties for CO adsorption and activation with 90% CO converted to CO₂ at 90 °C and nice stability with time on stream. The results clearly determine from atomic scale the significance of exposed gold step sites and intrinsic formation of defected surface by different nucleation. The above properties are directly responsible for the induced variation in chemical composition and the catalytic activity.

Catalytically Active Au Layers Grown on Pd Nanoparticles for Direct Synthesis of H2O2: Lattice Strain and Charge-Transfer Perspective Analyses

Despite its effectiveness in improving the properties of materials, strain engineering has not yet been employed to endow catalytic characteristics to apparently nonactive metals. This limitation can be overcome by controlling simultaneously lattice strains and charge transfer originated from the epitaxially prepared bimetallic core-shell structure. Here, we report the experimental results of the direct H₂O₂ synthesis enabled by a strained Au layer grown on Pd nanoparticles. This system can benefit the individual catalytic properties of each involved material, and the heterostructured catalyst displays an improved productivity for the direct synthesis of H₂O₂ by ∼100% relative to existing Pd catalysts. This is explained here by exploring the individual effects of lattice strain and charge transfer on the alteration of the electronic structure of ultrathin Au layers grown on Pd nanoparticles. The approach used in this study can be viewed as a means of designing catalysts with multiple catalytic functions.

Secondary phosphine oxides stabilized Au/Pd nanoalloys: metal components-controlled regioselective hydrogenation toward phosphinyl (Z)-[3]dendralenes

A series of gold/palladium nanoalloys stabilized by secondary phosphine oxides have been prepared and applied in selective hydrogenation for the first time.

General Synthetic Route toward Highly Dispersed Ultrafine Pd-Au Alloy Nanoparticles Enabled by Imidazolium-Based Organic Polymers

Bimetallic Pd-Au nanoparticles (NPs) usually show superior catalytic performances over their single-component counterparts, the general and facile synthesis of subnanometer-scaled Pd-Au NPs still remains a great challenge, especially for electronegative ultrafine bimetallic NPs. Here, we develop an anion-exchange strategy for the synthesis of ultrafine Pd-Au alloy NPs. Simple treatment of main-chain imidazolium-based organic polymer (IOP) with HAuCl₄ and Na₂PdCl₄, followed by reduction with NaBH₄ generated Pd-Au alloy NPs (Pd-Au/IOP). These NPs possess an unprecedented tiny size of 1.50 ± 0.20 nm and are uniformly dispersed over IOP. The electronic structure of the surface Pd and Au atoms is optimized via electron exchange during alloying, a net charge flowing resulting from counteranions is injected into Au and Pd to form a strong ensemble effect, which is responsible for a remarkably higher catalytic activity of Pd-Au/IOP in the hydrolytic dehydrogenation of ammonia borane than those of monometallic counterparts.

Understanding the atomic-level process of CO-adsorption-driven surface segregation of Pd in (AuPd)147 bimetallic nanoparticles

When the elements that compose bimetallic catalysts interact asymmetrically with reaction feedstock, the surface concentration of the bimetallic catalysts and the morphology of the reaction center evolve dynamically as a function of environmental factors such as the partial pressure of the triggering molecule. Relevant experimental and theoretical findings of the dynamic structural evolution of bimetallic catalysts under the reaction conditions are emerging, thus enabling the design of more consistent, reliable, and efficient bimetallic catalysts. In an initial attempt to provide an atomic-level understanding of the adsorption-induced structural evolution of bimetallic nanoparticles (NPs) under CO oxidation conditions, we used density functional theory to study the details of CO-adsorption-driven Pd surface segregation in (AuPd)₁₄₇ bimetallic NPs. The strong CO affinity of Pd provides a driving force for Pd surface segregation. We found that the vertex site of the NP becomes a gateway for the initial Pd-Au swapping and the subsequent formation of an internal vacancy. This self-generated internal vacancy easily diffuses inside the NP and activates Pd-Au swapping pathways in the (100) NP facet. Our results reveal how the surface and internal concentrations of bimetallic NPs respond immediately to changes in the reaction conditions. Our findings should aid in the rational design of highly active and versatile bimetallic catalysts by considering the environmental factors that systematically affect the structure of bimetallic catalysts under the reaction conditions.

Quantitative and Atomic-Scale View of CO-Induced Pt Nanoparticle Surface Reconstruction at Saturation Coverage via DFT Calculations Coupled with in Situ TEM and IR

Atomic-scale insights into how supported metal nanoparticles catalyze chemical reactions are critical for the optimization of chemical conversion processes. It is well-known that different geometric configurations of surface atoms on supported metal nanoparticles have different catalytic reactivity and that the adsorption of reactive species can cause reconstruction of metal surfaces. Thus, characterizing metallic surface structures under reaction conditions at atomic scale is critical for understanding reactivity. Elucidation of such insights on high surface area oxide supported metal nanoparticles has been limited by less than atomic resolution typically achieved by environmental transmission electron microscopy (TEM) when operated under realistic conditions and a lack of correlated experimental measurements providing quantitative information about the distribution of exposed surface atoms under relevant reaction conditions. We overcome these limitations by correlating density functional theory predictions of adsorbate-induced surface reconstruction visually with atom-resolved imaging by in situ TEM and quantitatively with sample-averaged measurements of surface atom configurations by in situ infrared spectroscopy all at identical saturation adsorbate coverage. This is demonstrated for platinum (Pt) nanoparticle surface reconstruction induced by CO adsorption at saturation coverage and elevated (>400 K) temperature, which is relevant for the CO oxidation reaction under cold-start conditions in the catalytic convertor. Through our correlated approach, it is observed that the truncated octahedron shape adopted by bare Pt nanoparticles undergoes a reversible, facet selective reconstruction due to saturation CO coverage, where {100} facets roughen into vicinal stepped high Miller index facets, while {111} facets remain intact.