DFT and In Situ EXAFS Investigation of Gold/Ceria−Zirconia Low-Temperature Water Gas Shift Catalysts: Identification of the Nature of the Active Form of Gold

Local structure of solid and liquid gold probed by reverse Monte Carlo analysis of X-ray absorption data

The increase in computational capabilities of modern computers has allowed us to develop new accurate methods for EXAFS (extended X-ray absorption fine structure) data-analysis. In particular, the RMC-GNXAS package provides models of the three-dimensional structure applying the reverse Monte Carlo (RMC) method to EXAFS of condensed or molecular systems. Simulations of the EXAFS signals within RMC-GNXAS are based on accurate multiple-scattering (MS) simulations accounting for relativistic effects. The method is implemented to include MS signals related to pair and triplet distributions, using different data sets and long-range constraints provided by complementary techniques (diffraction). In this work we have validated this approach studying the local structure of solid and liquid Au, providing also a benchmark for analysis of functional materials based on heavy noble atoms such as Au. An accurate reconstruction of the pair and triplet distribution functions in solid and liquid Au was obtained and compared with literature data. We show that the inclusion of the contribution of MS signals related to three-body configurations is especially important in solid Au at low temperatures. In liquid Au, we obtained the pair distribution function and analyzed the bond-angle distribution. We applied common-neighbor analysis to identify the local symmetry of the atomic configurations. Liquid Au is found to follow the trend of other close-packing liquids for which the dominant structures are distorted or defective icosahedral configurations.

A minireview on the synthesis of single atom catalysts

Single atom catalysis is a prosperous and rapidly growing research field, owing to the remarkable advantages of single atom catalysts (SACs), such as maximized atom utilization efficiency, tailorable catalytic activities as well as supremely high catalytic selectivity. Synthesis approaches play crucial roles in determining the properties and performance of SACs. Over the past few years, versatile methods have been adopted to synthesize SACs. Herein, we give a thorough and up-to-date review on the progress of approaches for the synthesis of SACs, outline the general principles and list the advantages and disadvantages of each synthesis approach, with the aim to give the readers a clear picture and inspire more studies to exploit novel approaches to synthesize SACs effectively.

Cation-doping strategies for tuning of zirconia acid-base properties

The role of Y-, Ca- and Ce-doping of cubic zirconia (c-ZrO₂) (111) surface on its acidity, basicity and the interplay between surface acid-base pairs is investigated by computational methods. The most stable surface structures for this investigation were initially determined based on previous studies of Y-doped c-ZrO₂ (111) and by a detailed exploration of the most stable configuration for Ca-doped c-ZrO₂ (111) and Ce-doped c-ZrO₂ (111). Next, surface mapping by basic probe molecules (NH₃ and pyridine) revealed a general reduction of the acidity of the surface sites, although a few exceptions were observed for zirconium ions at next nearest neighbour (NNN) positions to the oxygen vacancy and at the nearest neighbour (NN) position to the dopants. Adsorption of CO₂ over basic sites revealed a cooperative interplay between acid-base groups. In this case, the overall effect observed was the decrease of the calculated adsorption energies when compared with the pristine surface. Moreover, spontaneous formation of η ³-CO₂ systems from initial η ²-CO₂ configurations indicates a decrease in the required energy for forming oxygen vacancies in the doped ZrO₂ systems at NNN positions or further away from the existing vacancy site.

Probing Active Sites in CuxPdy Cluster Catalysts by Machine-Learning-Assisted X-ray Absorption Spectroscopy

Size-selected clusters are important model catalysts because of their narrow size and compositional distributions, as well as enhanced activity and selectivity in many reactions. Still, their structure-activity relationships are, in general, elusive. The main reason is the difficulty in identifying and quantitatively characterizing the catalytic active site in the clusters when it is confined within subnanometric dimensions and under the continuous structural changes the clusters can undergo in reaction conditions. Using machine learning approaches for analysis of the operando X-ray absorption near-edge structure spectra, we obtained accurate speciation of the Cu_xPd_y cluster types during the propane oxidation reaction and the structural information about each type. As a result, we elucidated the information about active species and relative roles of Cu and Pd in the clusters.

Comparative study of single-atom gold and iridium on CeO2{111}

Oxide-supported single-atom catalysts have shown promise for a variety of heterogeneous processes. In addition to their inherent activity and selectivity, these materials come at much lower financial cost, avoiding the use of full-bodied precious-metal catalysts, but at the conceptual expense that more complex structural and electronic considerations need to be understood if we are to exploit their full potential. Here, we focus on the adsorption of single-atom iridium at both stoichiometric and defective CeO₂{111} surfaces, by means of first-principles density functional theory. Reference calculations for the adsorption of single-atom gold, on the same set of substrates, provide a valuable set of benchmarks against which to interpret our iridium results.

Temperature reversible synergistic formation of cerium oxyhydride and Au hydride: a combined XAS and XPDF study

In situ studies on the physical and chemical properties of Au in inverse ceria alumina supported catalysts have been conducted between 295 and 623 K using high energy resolved fluorescence detection X-ray absorption near edge spectroscopy and X-ray total scattering. Precise structural information is extracted on the metallic Au phase present in a 0.85 wt% Au containing inverse ceria alumina catalyst (ceria/Au/alumina). Herein evidence for the formation of an Au hydride species at elevated temperature is presented. Through modelling of total scattering data to extract the thermal properties of Au using Grüneisen theory of volumetric thermal expansion it proposed that the Au Hydride formation occurs synergistally with the formation of a cerium oxyhydride. The temperature reversible nature, whilst remaining in a reducing atmosphere, demonstrates the activation of hydrogen without consumption of oxygen from the supporting ceria lattice.

A DFT and KMC based study on the mechanism of the water gas shift reaction on the Pd(100) surface

We present a combined density functional theory (DFT) and Kinetic Monte Carlo (KMC) study of the water gas shift (WGS) reaction on the Pd(100) surface. We propose a mechanism comprising both the redox and the associative pathways for the WGS within a single framework, which consists of seven core elementary steps, which in turn involve splitting of a water molecule followed by the production of an H-atom and an OH-species on the Pd(100) surface. In the following steps, these intermediates then recombine with each other and with CO leading to the evolution of CO₂, and H₂. Seven other elementary steps, involving the diffusion and adsorption of the surface intermediate species are also considered for a complete description of the mechanism. The geometrical and electronic properties of each of the reactants, products, and the transition states of the core elementary steps are presented. We also discuss the analysis of Bader charges and spin densities for the reactants, transition states and the products of these elementary steps. Our study indicates that the WGS reaction progresses simultaneously via the direct oxidation and the carboxyl paths on the Pd(100) surface.

Ultrasmall Au clusters supported on pristine and defected CeO2: Structure and stability

The atomistic simulation of supported metal catalysts has long been challenging due to the increased complexity of dual components. In order to determine the metal/support interface, efficient theoretical tools to map out the potential energy surface (PES) are generally required. This work represents the first attempt to apply the recently developed SSW-NN method, stochastic surface walking (SSW) global optimization based on global neural network potential (G-NN), to explore the PES of a highly controversial supported metal catalyst, Au/CeO₂, system. By establishing the ternary Au-Ce-O G-NN potential based on first principles global dataset, we have searched for the global minima for a series of Au/CeO₂ systems. The segregation and diffusion pathway for Au clusters on CeO₂(111) are then explored by using enhanced molecular dynamics. Our results show that the ultrasmall cationic Au clusters, e.g., Au₄O₂, attaching to surface structural defects are the only stable structural pattern and the other clusters on different CeO₂ surfaces all have a strong energy preference to grow into a bulky Au metal. Despite the thermodynamics tendency of sintering, Au clusters on CeO₂ have a high kinetics barrier (>1.4 eV) in segregation and diffusion. The high thermodynamics stability of ultrasmall cationic Au clusters and the high kinetics stability for Au clusters on CeO₂ are thus the origin for the high activity of Au/CeO₂ catalysts in a range of low temperature catalytic reactions. We demonstrate that the global PES exploration is critical for understanding the morphology and kinetics of metal clusters on oxide support, which now can be realized via the SSW-NN method.

Enhanced Activity and Stability of Gold/Ceria-Titania for the Low-Temperature Water-Gas Shift Reaction

Gold supported on ceria-zirconia is one of the most active low temperature water-gas shift catalysts reported to date but rapid deactivation occurs under reaction conditions. In this study, ceria-titania was evaluated as an alternative catalyst support. Materials of different Ce:Ti compositions were synthesized using a sol-gel methodology and gold was supported onto these using a deposition-precipitation method. They were then investigated as catalysts for the low-temperature water-gas shift reaction. Au/Ce_0.2Ti_0.8O₂ exhibited superior activity and stability to a highly active, previously reported gold catalyst supported on ceria-zirconia. High activity and stability was found to be related to the support comprising a high number of oxygen defect sites and a high specific surface area. These properties were conducive to forming a highly active catalyst with well-dispersed Au species.

Operando X-ray absorption spectroscopy study of the Fischer-Tropsch reaction with a Co catalyst

This article describes the setting up of a facility on the energy-scanning EXAFS beamline (BL-09) at RRCAT, Indore, India, for operando studies of structure-activity correlation during a catalytic reaction. The setup was tested by operando X-ray absorption spectroscopy (XAS) studies performed on a Co-based catalyst during the Fischer-Tropsch reaction to obtain information regarding structural changes in the catalyst during the reaction. Simultaneous gas chromatography (GC) measurements during the reaction facilitate monitoring of the product gases, which in turn gives information regarding the activity of the catalyst. The combination of XAS and GC techniques was used to correlate the structural changes with the activity of the catalyst at different reaction temperatures. The oxide catalyst was reduced to the metallic phase by heating at 400°C for 5 h under H₂ at ambient pressure and subsequently the catalytic reaction was studied at four different temperatures of 240, 260, 280 and 320°C. The catalyst was studied for 10 h at 320°C and an attempt has been made to understand the process of its deactivation from the XANES and EXAFS results.

On the nature of active sites for formic acid decomposition on gold catalysts

Atomic scale size-sensitivity of the catalytic properties of sub-nanometer gold clusters for HCOOH decomposition.

Recent Advances in the Gold-Catalysed Low-Temperature Water–Gas Shift Reaction

The low-temperature water–gas shift reaction (LTS: CO + H2O ⇌ CO2 + H2) is a key step in the purification of H2 reformate streams that feed H2 fuel cells. Supported gold catalysts were originally identified as being active for this reaction twenty years ago, and since then, considerable advances have been made in the synthesis and characterisation of these catalysts. In this review, we identify and evaluate the progress towards solving the most important challenge in this research area: the development of robust, highly active catalysts that do not deactivate on-stream under realistic reaction conditions.

Tuning and Probing the Distribution of Cu+ and Cu2+ Trap States Responsible for Broad-Band Photoluminescence in CuInS2 Nanocrystals

The processes that govern radiative recombination in ternary CuInS₂ (CIS) nanocrystals (NCs) have been heavily debated, but recently, several research groups have come to the same conclusion that a photoexcited electron recombines with a localized hole on a Cu-related trap state. Furthermore, it has been observed that single CIS NCs display narrower photoluminescence (PL) line widths than the ensemble, which led to the conclusion that within the ensemble there is a distribution of Cu-related trap states responsible for PL. In this work, we probe this trap-state distribution with in situ photoluminescence spectroelectrochemistry. We find that Cu²⁺ states result in individual "dark" nanocrystals, whereas Cu⁺ states result in "bright" NCs. Furthermore, we show that we can tune the PL position, intensity, and line width in a cyclic fashion by injecting or removing electrons from the trap-state distribution, thereby converting a subset of "dark" Cu²⁺ containing NCs into "bright" Cu⁺ containing NCs and vice versa. The electrochemical injection of electrons results in brightening, broadening, and a red shift of the PL, in line with the activation of a broad distribution of "dark" NCs (Cu²⁺ states) into "bright" NCs (Cu⁺ states) and a rise of the Fermi level within the ensemble trap-state distribution. The opposite trend is observed for electrochemical oxidation of Cu⁺ states into Cu²⁺. Our work shows that there is a direct correlation between the line width of the ensemble Cu⁺/Cu²⁺ trap-state distribution and the characteristic broad-band PL feature of CIS NCs and between Cu²⁺ cations in the photoexcited state (bright) and in the electrochemically oxidized ground state (dark).

Development of Low-ppm CO Sensors Using Pristine CeO2 Nanospheres with High Surface Area

Mesoporous CeO₂ nanospheres with appreciably high surface area are prepared using reversed micelles by a water-in-oil microemulsion method. The structural morphology and semiconducting properties of the nanoparticles are thoroughly investigated using X-ray diffraction, field effect scanning electron microscopy, transmission electron microscopy, and UV-visible spectroscopic techniques. Even after high-temperature calcination, the morphological retention of the material is apparent by electron microscopy. The deployment of undoped CeO₂ nanospheres for the detection of low-ppm CO yields superior performances in terms of sensitivity, response-recovery times, and selectivity compared to those of other sensors of the same genre. These CO sensors exhibit ∼ 52% sensitivity with a response time of only 13 s. The sensor parameters are analyzed as a function of both temperature and gas concentration. In addition to that on the cost-effective and scalable synthesis of CeO₂ nanospheres, this article also reports on the fabrication of packaged CO sensors, which can be potentially utilized for industrial and environmental monitoring purposes.

Activation and Deactivation of Gold/Ceria-Zirconia in the Low-Temperature Water-Gas Shift Reaction

Gold (Au) on ceria-zirconia is one of the most active catalysts for the low-temperature water-gas shift reaction (LTS), a key stage of upgrading H₂ reformate streams for fuel cells. However, this catalyst rapidly deactivates on-stream and the deactivation mechanism remains unclear. Using stop-start scanning transmission electron microscopy to follow the exact same area of the sample at different stages of the LTS reaction, as well as complementary X-ray photoelectron spectroscopy, we observed the activation and deactivation of the catalyst at various stages. During the heating of the catalyst to reaction temperature, we observed the formation of small Au nanoparticles (NPs; 1-2 nm) from subnanometer Au species. These NPs were then seen to agglomerate further over 48 h on-stream, and most rapidly in the first 5 h when the highest rate of deactivation was observed. These findings suggest that the primary deactivation process consists of the loss of active sites through the agglomeration and possible dewetting of Au NPs.

Non-Thermal Plasma Activation of Gold-Based Catalysts for Low-Temperature Water-Gas Shift Catalysis

Non-thermal plasma activation has been used to enable low-temperature water-gas shift over a Au/CeZrO4 catalyst. The activity obtained was comparable with that attained by heating the catalyst to 180 °C providing an opportunity for the hydrogen production to be obtained under conditions where the thermodynamic limitations are minimal. Using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), structural changes associated with the gold nanoparticles in the catalyst have been observed which are not found under thermal activation indicating a weakening of the Au-CO bond and a change in the mechanism of deactivation.

Water-gas-shift reaction on reduced gold-substituted Ce1-xO2(111) surfaces: the role of Au charge

Density functional theory is employed to investigate the role of Au charge in the water-gas-shift (WGS) reaction on a CeO₂(111) surface with a cerium atom replaced by a gold atom. The oxidation state of the gold atom, varied between +3 and -1, is controlled by altering the number and configuration of oxygen vacancies. The findings indicate that Au³⁺ and Au^- are not catalytically active for the WGS reaction because of a high energy barrier of +1.54 eV required to dissociate water and +1.40 eV to produce H₂ and CO₂, respectively. However, when Au is in a modest oxidation state of +1, the overall reaction barrier for the WGS reaction via the carboxyl mechanism is reduced to 0.79-0.98 eV. It therefore appears that Au species with an oxidation state of +1 play a significant role in the WGS reaction at low temperatures (T < ∼550 K).

Deep insight into Zr/Fe combination for successful Pt/CeO2/Al2O3 WGS catalyst doping

Efficient promotion of the Pt/CeO₂/Al₂O₃ catalytic system was achieved by the addition of two different ceria promoters, Zr and Fe.

Synergy and Anti-Synergy between Palladium and Gold in Nanoparticles Dispersed on a Reducible Support

Highly active and stable bimetallic Au-Pd catalysts have been extensively studied for several liquid-phase oxidation reactions in recent years, but there are far fewer reports on the use of these catalysts for low-temperature gas-phase reactions. Here we initially established the presence of a synergistic effect in a range of bimetallic Au-Pd/CeZrO₄ catalysts, by measuring their activity for selective oxidation of benzyl alcohol. The catalysts were then evaluated for low-temperature WGS, CO oxidation, and formic acid decomposition, all of which are believed to be mechanistically related. A strong anti-synergy between Au and Pd was observed for these reactions, whereby the introduction of Pd to a monometallic Au catalyst resulted in a significant decrease in catalytic activity. Furthermore, monometallic Pd was more active than Pd-rich bimetallic catalysts. The nature of the anti-synergy was probed by several ex situ techniques, which all indicated a growth in metal nanoparticle size with Pd addition. However, the most definitive information was provided by in situ CO-DRIFTS, in which CO adsorption associated with interfacial sites was found to vary with the molar ratio of the metals and could be correlated with the catalytic activity of each reaction. As a similar correlation was observed between activity and the presence of Au⁰* (as detected by XPS), it is proposed that peripheral Au⁰* species form part of the active centers in the most active catalysts for the three gas-phase reactions. In contrast, the active sites for the selective oxidation of benzyl alcohol are generally thought to be electronically modified gold atoms at the surface of the nanoparticles.

Kinetic analysis of low concentration CO detection by Au-loaded cerium oxide sensors

Gold nanoparticle-loaded cerium oxide sensors detect 10–30 ppm of carbon monoxide in air with very fast response. The response and recovery transients of conductance have been modelled using two-site Langmuir adsorption kinetics.

The effect of an Fe promoter on Cu/SiO2 catalysts for improving their catalytic activity and stability in the water-gas shift reaction

By adding a small amount of iron, the catalytic activity and stability of Cu/SiO₂ are effectively improved.

The charge states of Au on gold-substituted Ce1−xO2(111) surfaces with multiple oxygen vacancies

Not only does the Au charge have a trend of decreasing when the number of oxygen vacancies increases, but this charge also can be significantly changed by the oxygen vacancy configuration.

Visualisation of single atom dynamics in water gas shift reaction for hydrogen generation

In situ real time single atom resolution observations of dynamic water gas shift catalysts in CO + water (WGS) environments.

Characterisation of gold catalysts

Au-based catalysts have established a new important field of catalysis, revealing specific properties in terms of both high activity and selectivity for many reactions.

Activity of faujasite supported gold monomer towards water gas shift reaction: hybrid density functional theory/molecular mechanics approach

Neutral gold monomer supported on faujasite (Au⁰/FAU) exhibits superior catalytic activity towards water gas shift reaction compared to cationic monomer.

Gold atoms stabilized on various supports catalyze the water-gas shift reaction

For important chemical reactions that are catalyzed by single-site metal centers, such as the water-gas shift (WGS) reaction that converts carbon monoxide and water to hydrogen and carbon dioxide, atomically dispersed supported metal catalysts offer maximum atom efficiency. Researchers have found that for platinum metal supported on ceria and doped ceria in the automobile exhaust catalyst, atomic Pt-Ox-Ce species are the active WGS reaction sites. More recently, preparations of gold at the nanoscale have shown that this relatively "new material" is an active and often more selective catalyst than platinum for a variety of reactions, including the WGS reaction. The activity of gold is typically attributed to a size effect, while the interface of gold with the support has also been reported as important for oxidation reactions, but exactly how this comes about has not been probed satisfactorily. Typical supported metal catalysts prepared by traditional techniques have a heterogeneous population of particles, nanoclusters, subnanometer species, and isolated atoms/ions on the support surfaces, making the identification of the active sites difficult. Both we and other researchers have clearly shown that gold nanoparticles are spectator species in the WGS reaction. Evidence has now amassed that the gold active site for the WGS reaction is atomic, that is, Au-Ox species catalyze the reaction, similar to Pt-Ox. In this Account, we review the relevant literature to conclude that the intrinsic activity of the Au-Ox(OH)-S site, where S is a support, is the same for any S. The support effect is indirect, through its carrying (or binding) capacity for the active sites. Destabilization of the gold under reducing conditions through the formation of clusters and nanoparticles is accompanied by a measurable activity loss. Therefore, it is necessary to investigate the destabilizing effect of different reaction gas mixtures on the gold atom sites and to consider regeneration methods that effectively redisperse the gold clusters into atoms. For gold catalysts, we can remove weakly bound clusters and nanoparticles from certain supports by leaching techniques. Because of this, we can prepare a uniform dispersion of gold atoms/ions strongly bound to the support surface by this two-step (loading followed by leaching) approach. Presently, one-step preparation methods to maximize the number of the single atom sites on various supports need to be developed, specific to the type of the selected support. Often, it will be beneficial to alter the surface properties of the support to enhance metal ion anchoring, for example, by shape and size control of the support or by the use of light-assisted deposition and anchoring of the metal on photoresponsive supports. Because of their importance for practical catalyst development, synthesis methods are discussed at some length in this Account.

Water-gas shift on gold catalysts: catalyst systems and fundamental studies

Since the pioneering finding by Haruta et al. that small gold nanoparticles on reducible supports can be highly active for low-temperature CO oxidation, the synthesis, characterization, and application of supported gold catalysts have attracted much attention. The water-gas shift reaction (WGSR: CO + H2O = CO2 + H2) is important for removing CO and upgrading the purity of H2 for fuel cell applications, ammonia synthesis, and selective hydrogenation processes. In recent years, much attention has been paid to exploration the possibility of using supported gold nanocatalysts for WGSR and understanding the fundamental aspects related to catalyst deactivation mechanisms, nature of active sites, and reaction mechanisms. Here we summarize recent advances in the development of supported gold catalysts for this reaction and fundamental insights that can be gained, and furnish our assessment on the status of research progress.