Gold Catalysts For CO Oxidation Research Articles

Operando X‐ray Powder Diffraction Study of Mechanochemical Activation Tested for the CO Oxidation over Au@Fe2O3 as Model Reaction

AbstractMechanochemistry has proven to be an excellent green synthesis method for preparing organic, pharmaceutical, and inorganic materials. Mechanocatalysis, inducing a catalytic reaction by mechanical forces, is an emerging field because neither external temperature nor pressure inputs are required. Previous studies reported enhanced catalytic activity during the mechanical treatment of supported gold catalysts for CO oxidation. So far, the processes inside the milling vessel during mechanocatalysis could not be monitored. In this work, the results of high‐energy operando X‐ray powder diffraction experiments and online gas analysis will be reported. A specific milling setup with a custom‐made vessel and gas dosing system was developed. To prove the feasibility of the experimental setup for operando diffraction studies during mechanocatalysis, the CO oxidation with Au@Fe2O3 as a catalyst was selected as a well‐known model reaction. The operando studies enabled monitoring morphology changes of the support as well as changes in the crystallite size of the gold catalyst. The change of the crystal size is directly correlated to changes in the active surface area and thus to the CO2 yield. The studies confirm the successful implementation of the operando setup, and its potential to be applied to other catalytic reactions.

Spiers Memorial Lecture: Understanding reaction mechanisms in heterogeneously catalysed reactions.

Heterogeneous catalysis lies at the heart of the chemical and fuel manufacturing industries and hence is a cornerstone of many economies. Many of the commercially operated heterogeneous catalysts have remained basically unchanged for decades, undergoing small but important optimisation of their formulations. Yet we all acknowledge that there is a continuous drive towards improved catalysts or designing new ones. At the heart of these studies has been the need to gain an improved understanding of the reaction mechanism for these important reactions since this can unlock new ways to improve catalyst design and, of course, the ultimate aim is to design catalysts based on the detailed understanding of the reaction mechanism. These advanced studies have been aided in the last decade by two key factors, namely: (a) access to advanced characterisation techniques based on synchrotron methods and aberration-corrected microscopy that can probe the nature of the active site, and (b) the application of high-level computational methods to understand how the reactants and products interact at the active site. In this paper this theme will be explored using two examples to bring out the complexity in gaining an understanding of a reaction mechanism. Using the zeolite H-ZSM-5 as an example of a single site catalyst, the mechanism of the conversion of methanol to the first hydrocarbon carbon-carbon bond will be discussed. In this section the use of model reactants and reaction probes will be used to try to differentiate between different mechanistic proposals. The second example explores the use of gold catalysts for CO oxidation and acetylene hydrochlorination. In both these examples the importance of advanced characterisation and theory will be highlighted.

Effect of Nickel Oxide Doping to Ceria-Supported Gold Catalyst for CO Oxidation and Water-Gas Shift Reactions

Ceria-supported gold catalyst has drew much research interest owing to its high reactivity on CO oxidation and water-gas shift (WGS) reactions. However, till now, there were relatively limited studies on the effect of secondary metal/metal oxide component into gold-ceria system to enhance its catalytic performance. In this work, we synthetized the ceria supported gold-nickel samples via a deposition-precipitation method with the base of NaHCO3 to adjust final pH value of 8~9. We found that the addition of nickel oxide drove off the gold species from the stock solution during synthesis, and thus resulted in a dramatical decrease on doped Au concentration. No crystallized phases of gold and nickel were observed on the surface of ceria nanorods in both X-ray diffraction (XRD) and transmission electron microscopy (TEM). The valence of nickel was maintained as Ni2+ for all the measured samples by X-ray photoelectron spectroscopy (XPS), while gold was oxidized with the increased nickel amount after analysis of X-ray absorption near edge spectroscopy (XANES). The corresponding catalytic tests showed that with the introduction of nickel oxide, the activity of gold-ceria catalyst was promoted for the WGS reaction, but inhibited for the CO oxidation reaction.

Nanoceria Supported Gold Catalysts for CO Oxidation

AbstractTwo types of CeO2 nanocubes (average size of 5 and 20 nm, respectively) prepared via the hydrothermal process were selected to load gold species via a deposition‐precipitation (DP) method. Various measurements, including X‐ray diffraction (XRD), Raman spectra, high resolution transmission electron microscopy (HRTEM), in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), and temperature‐programmed reduction by hydrogen (H2‐TPR), were applied to characterize the catalysts. It is found that the sample with ceria size of 20 nm (Au/CeO2‐20) was covered by well dispersed both Au3+ and Auδ+ (0 < δ < 1). For the other sample with ceria size of 5 nm (Au/CeO2‐5), Au3+ is the dominant gold species. Au/CeO2‐20 performed better catalytic activity for CO oxidation because of the strong CO adsorption of Auδ+ in the catalysts. The catalytic activity of Au/CeO2‐5 was improved due to the transformation of Au3+ to Auδ+. Based on the CO oxidation and in situ DRIFTS results, Auδ+ is likely to play a more important role in catalyzing CO oxidation reaction.

Optimum Particle Size for Gold-Catalyzed CO Oxidation.

The structure sensitivity of gold-catalyzed CO oxidation is presented by analyzing in detail the dependence of CO oxidation rate on particle size. Clusters with less than 14 gold atoms adopt a planar structure, whereas larger ones adopt a three-dimensional structure. The CO and O2 adsorption properties depend strongly on particle structure and size. All of the reaction barriers relevant to CO oxidation display linear scaling relationships with CO and O2 binding strengths as main reactivity descriptors. Planar and three-dimensional gold clusters exhibit different linear scaling relationship due to different surface topologies and different coordination numbers of the surface atoms. On the basis of these linear scaling relationships, first-principles microkinetics simulations were conducted to determine CO oxidation rates and possible rate-determining step of Au particles. Planar Au9 and three-dimensional Au79 clusters present the highest CO oxidation rates for planar and three-dimensional clusters, respectively. The planar Au9 cluster is much more active than the optimum Au79 cluster. A common feature of optimum CO oxidation performance is the intermediate binding strengths of CO and O2, resulting in intermediate coverages of CO, O2, and O. Both these optimum particles present lower performance than maximum Sabatier performance, indicating that there is sufficient room for improvement of gold catalysts for CO oxidation.

Remarkable enhancement of Fe–V–Ox composite metal oxide to gold catalyst for CO oxidation in the simulated atmosphere of CO2 laser

A novel Au/Fe–V–Ox/Al2O3 displays remarkable activity and good stability for CO oxidation in simulated atmosphere of CO2 laser. The higher Au dispersion and more labile lattice oxygen after modification contributed to its outstanding performance.

Gold on Different Manganese Oxides: Ultra-Low-Temperature CO Oxidation over Colloidal Gold Supported on Bulk-MnO2 Nanomaterials.

Nanoscopic gold particles have gained very high interest because of their promising catalytic activity for various chemicals reactions. Among these reactions, low-temperature CO oxidation is the most extensively studied one due to its practical relevance in environmental applications and the fundamental problems associated with its very high activity at low temperatures. Gold nanoparticles supported on manganese oxide belong to the most active gold catalysts for CO oxidation. Among a variety of manganese oxides, Mn2O3 is considered to be the most favorable support for gold nanoparticles with respect to catalytic activity. Gold on MnO2 has been shown to be significantly less active than gold on Mn2O3 in previous work. In contrast to these previous studies, in a comprehensive study of gold nanoparticles on different manganese oxides, we developed a gold catalyst on MnO2 nanostructures with extremely high activity. Nanosized gold particles (2-3 nm) were supported on α-MnO2 nanowires and mesoporous β-MnO2 nanowire arrays. The materials were extremely active at very low temperature (-80 °C) and also highly stable at 25 °C (70 h) under normal conditions for CO oxidation. The specific reaction rate of 2.8 molCO·h(-1)·gAu(-1) at a temperature as low as -85 °C is almost 30 times higher than that of the most active Au/Mn2O3 catalyst.

Titania nanotubes decorated with anatase nanocrystals as support for active and stable gold catalysts for CO oxidation

Gold nanoparticles supported on titania nanostructured materials were prepared, characterized and tested in CO oxidation. Two nanostructured supports were used: titania nanotubes (TNT) synthesized by the Kasuga method and titania nanotubes decorated with TiO2 anatase nanocrystals prepared by post-synthetic treatment of TNT with 0.05M HNO3 solution at 70°C for 48h (TNT-T). Gold catalysts with 4wt.% of Au nominal loading were prepared by deposition-precipitation with urea. A reference Au catalyst supported on TiO2 Degussa P-25 was also prepared. Supports and catalysts were characterized by N2 physisorption, XRD, UV–vis and Raman spectroscopy, TPR, SEM-EDX and HRTEM. HRTEM characterization showed that gold nanoparticles with an average size between 2 and 7nm were obtained in all catalysts making them suitable for CO oxidation. Catalytic tests showed differences in activity and stability among the prepared catalysts. The Au/TNT catalyst presented poor activity for CO oxidation, meanwhile the Au/TNT-T catalyst showed higher activity than the Au/TiO2 Degussa P-25, especially at a low temperature interval (−10 to 100°C). Stability tests performed for Au/TNT-T and Au/TiO2 Degussa P-25 catalysts at 25°C revealed higher stability of the Au/TNT-T catalyst, attributed to low sintering of gold nanoparticles.

Pillared clay-supported gold catalysts for CO oxidation

Gold supported on various bentonites was prepared using hydrothermal and co-precipitation methods.

CO oxidation over Au supported on Mn–ZSM5 and Mn–MOR

Active gold catalysts for CO oxidation were obtained by the deposition–precipitation of gold on ZSM-5 and mordenite, both of them ion-exchanged with manganese. A strong promotion of Mn upon the activity of bimetallic Au/Mn catalysts was observed when compared with the monometallic Au–zeolites. In turn, when an in-situ reduction of the bimetallic catalysts was performed, a further increase in activity was observed. Characterizations suggested that smaller gold nanoparticles were stabilized in the bimetallic solids and that the reduction process caused a rearrangement of Au and Mn species on the catalyst surface.

Influence of Au Particle Size on Au/TiO2 Catalysts for CO Oxidation

A series of Au/TiO2 catalysts for CO oxidation with same Au loading but different Au nanoparticles (NPs) sizes were prepared by varying the calcination temperatures and biomass concentration via a biosynthetic approach. The resulting catalysts were characterized by DRUV–vis, TEM, and TG techniques. The experimental results showed that the activity of the gold catalysts for CO oxidation was very sensitive to the particle size. Among the tested catalysts, the one with mean size of 3.8 nm was the most active. As determined by TEM, the contact boundary between the Au NPs and the TiO2 support was related to the size of the Au NPs. For the most active catalyst, hemispherical Au NPs (3.8 ± 0.6 nm) had the best contact boundary with the TiO2 support, yielding the longest perimeter interface, suggesting that the contact boundary was the most critical factor for the CO oxidation. The in-situ FTIR study of CO adsorption on the catalysts showed that CO was not adsorbed on the Au surface. This might be due to the modification of the Au/TiO2 catalysts by the residual biomass. The intensity of the peak at 2185 cm–1 for the Au/TiO2 catalysts with the longest perimeter interface was highest, demonstrating that the Au–TiO2 contact boundary played an important role in the adsorption of CO.

Non-thermal plasma-treated gold catalyst for CO oxidation

Plasma under oxygen atmosphere is available for promoting the microporosity, redox properties, and the catalytic performance of gold nanoparticles.

Clay-based SiO2 as active support of gold nanoparticles for CO oxidation catalyst: Pivotal role of residual Al

SiO2 is generally regarded as an “inert” support of gold catalysts for CO oxidation. We find that silica obtained from a natural clay mineral can be used as excellent gold-support for efficient CO oxidation at relatively low temperature. Fine Au particles are found to locate around the Al sites, rather than on pure SiO2 area. Accordingly, high Au loading and catalytic activity for the clay-based SiO2 supports are ascribed to the residual aluminum originally from the clay.

The Influence of Different Preparation Methods on the Activity for CO Oxidation on the LDHs-Supported Gold Catalysts

Au nanoparticles supported on MgAl-LDHs has been synthesized by coprecipitation method, gold colloid routs as well as intercalation and assembly method respectively. The influence of the above different preparation methods on the activity of the gold catalysts for CO oxidation was investigated, and the phase and gold particle size distribution of the catalysts were analyzed by XRD and TEM respectively. The results show that 100% conversion of CO on the fresh prepared gold catalysts synthesized by gold colloid routs and intercalation assembly method can be achieved at room temperature when the catalysts were activated at 473K for 2h in the air. The gold particle size distribution was range from 2nm to 7nm.

Insights into support effects on Ce-Zr-O mixed oxide-supported gold catalysts in CO oxidation

Au/Ce1–xZrxO2 catalysts (x = 0–0.8) were prepared by a deposition-precipitation method using Ce1–xZrxO2 nanoparticles as supports with variable Ce and Zr contents. Their structures were characterized by complimentary means such as X-ray diffraction, Raman, scanning transmission electron microscopy and X-ray photoelectron spectroscopy (XPS). These Au catalysts possessed similar sizes and crystalline phases of Ce1–xZrxO2 supports as well as similar sizes and oxidation states of Au nanoparticles. The oxidation state of Au nanoparticles was dominated by Au0 especially in CO oxidation. Their activities were examined in CO oxidation at different temperatures in the range of 303–333 K. The CO oxidation rates normalized per Au atoms increased with the increasing Ce contents, and reached the maximum value over Au/CeO2. Such change was in parallel with the change in the oxygen storage capacity values, i.e. the amounts of active oxygen species on Au/Ce1–xZrxO2 catalysts. The excellent correlation between the two properties of the catalysts suggests that the intrinsic support effects on the CO oxidation rates is related to the effects on the adsorption and activation of O2 on Au/Ce1–xZrxO2 catalysts. Such understanding on the support effects may be useful for designing more active Au catalysts, for example, by tuning the redox properties of oxide supports.

Novel Method of Controlling Formation of Hot-Spot over Gold Catalysts for CO Oxidation

Au catalysts supported on Al2O3 and MOx-Al2O3(M=Fe and Zn) were prepared by the deposition-precipitation method.Their catalytic activities for CO oxidation in the absence and presence of an H2-rich steam at room temperature were investigated in detail.Catalyst bed temperatures were determined directly by a thermocouple.The catalyst surface temperature depended on the volume ratio of O2/CO,and the concentrations of CO and H2.The temperature on the Au/Al2O3 surface can reach 170 ℃ during CO oxidation,and is decreased to 55 ℃ by addition of FeOx.These results indicate that formation of hot-spots on γ-alumina-supported gold catalysts could be controlled by adding an appropriate dopant.The structure of the catalysts was characterized by techniques such as X-ray powder diffraction,X-ray photoelectron spectroscopy and transmission electron microscopy.Addition of a dopant could transfer the active center from Au to AuIII,which resulted in different reaction mechanisms of preferential oxidation of CO in the presence of H2.

In situ growth synthesis of heterostructured LnPO4–SiO2 (Ln = La, Ce, and Eu) mesoporous materials as supports for small gold particles used in catalytic CO oxidation

A general in situ growth method was successfully employed to prepare lanthanide phosphate–SiO2 mesostructured cellular foams (MCFs) (LnPO4–MCFs; Ln = La, Ce, and Eu; MCFs = SiO2). These heterostructured MCFs (LnPO4–MCFs) feature binary interpenetrating LnPO4 and silica frameworks, large surface areas, and uniform mesopore diameters. They were characterized by small-angle X-ray scattering, X-ray diffraction, nitrogen sorption, and transmission electron microscopy. The essence of this in situ growth synthesis lies in the controlled heterogeneous reaction of highly dispersed lanthanide oxides embedded in MCFs with phosphate ions in solution, leading to the formation of highly dispersed crystalline phosphate nanorods (nanocrystalline LnPO4) on the walls of MCFs. The resultant heterostructured LnPO4–MCFs were used as a novel support system for gold catalysts in CO oxidation at low temperatures. Gold precursor species can be readily introduced on LnPO4 nanophases of LnPO4–MCFs via a simple deposition–precipitation method. The resulting Au–LnPO4–MCF (2 wt% Au) catalysts exhibited high catalytic activities even below room temperatures. Because of the alteration of surface properties engineered by the in situ growth methodology and the strong interaction of metallic gold species with LnPO4, these catalysts are highly sinter-resistant. Although some cationic Au species are also present on the LnPO4–MCF surfaces, the metallic gold species are shown to be the key catalytic active sites for CO oxidation via in situ infrared spectroscopy.

CO oxidation at 20 °C over Au/SBA-15 catalysts decorated by Fe2O3 nanoparticles

Abstract This contribution describes the effect of SBA-15 substrate modification with variable amounts of Fe2O3 (5, 10, 15 and 20 wt.%) on the catalytic response of supported gold catalysts in CO oxidation at 20 °C. Catalytic activity was found to increase with the Fe2O3 loading even though this increase was not linear: the highest catalytic activity was observed for the catalyst loaded with 15 wt.% Fe2O3. For the most active Au/S15–15Fe catalyst, this behavior is explained in terms of the largest Fe2O3 cluster dispersion on the surface of the SBA-15 substrate (by XRD), the highest surface exposure of the Au0 species (by XPS) and its large stability during on-stream reaction.

Effect of Water Addition on Catalytic Activity of Nanosized Gold Catalysts for CO Oxidation

The effect of water in the reactant gas on the catalytic activity for CO oxidation over Au/Co3O4 catalysts has been investigated. Water in the reactant gas had a negative effect on the catalytic activity for CO oxidation in Au/Co3O4 catalyst. Furthermore, it was observed that the average particle size of gold on Au/Co3O4 catalysts increased after stability testing both in dry and wet conditions.

Highly Active Iron Oxide Supported Gold Catalysts for CO Oxidation: How Small Must the Gold Nanoparticles Be?

The shape of gold: The title catalyst has been prepared through a colloidal deposition method. Scanning transmission electron microscopy studies confirmed that for the catalyst, gold clusters with a bilayer structure and a diameter of about 0.5 nm are not mandatory to achieve the high activity (see image).