Effect of nature of ceria supports on the growth and sintering behavior of Au nanoparticles

Growth, sintering, and chemical states of Co supported on reducible CeO2(111) thin films: The effects of the metal coverage and the nature of the support

The growth, sintering, and interaction of cobalt with ceria were studied under ultrahigh vacuum conditions by vapor-deposition of Co onto well-defined CeO_x(111) (1.5 < x < 2) thin films grown on Ru(0001). Charge transfer from Co to ceria occurs upon deposition of Co on CeO_1.96 and partially reduced CeO_1.83 at 300 K. X-ray photoelectron spectroscopy studies show that Co is oxidized to Co²⁺ species at the cost of the reduction of Ce⁴⁺ to Ce³⁺, at a lesser extent on reduced ceria. Co²⁺ is the predominant species on CeO_1.96 at low Co coverages (e.g., ≤0.20 ML). The ratio of metallic Co/Co²⁺ increases with the increase in the Co coverage. However, both metallic Co and Co²⁺ species are present on CeO_1.83 even at low Co coverages with metallic Co as the major species. Scanning tunneling microscopy results demonstrate that Co tends to wet the CeO_1.96 surface at very low Co coverages at room temperature forming one-atomic layer high structures of Co-O-Ce. The increase in the Co coverage can cause the particle growth into three-dimensional structures. The formation of slightly flatter Co particles was observed on reduced CeO_1.83. In comparison with other transition metals including Ni, Rh, Pt, and Au, our studies demonstrate that Co on ceria exhibits a smaller particle size and higher thermal stability, likely arising from strong metal-support interactions. The formed particles upon Co deposition at 300 K are present on the ceria surface after heating to 1000 K. The Co-ceria interface can be tuned by varying the Co metal coverage, the annealing temperature, and the nature of the ceria surface.

TOF-SIMS Analysis Using Bi3 + as Primary Ions on Au Nanoparticles Supported by SiO2/Si: Providing Insight into Metal-Support Interactions

Au nanoparticles with a mean diameter of 20 nm with a coverage of ∼20% of the surface were distributed on a Si wafer surface and studied both before and after being annealed (at 100 and 300 °C). The two types of samples were analyzed using secondary ion mass spectroscopy (SIMS) with Bi3 + clusters as the primary ions combined with surface etching using Ar1000 + clusters. We observed a substantial difference in the SIMS spectra combined with a relatively short sputtering time of Ar1000 +. In the nonannealed samples, bare Au cluster cations and Si+ were observed in the SIMS spectra; AuSi+ clusters were also observed in the annealed samples. These results indicate Au-silicide formation at a part of the periphery of the Au nanoparticles upon annealing. We suggest that SIMS experiments using cluster ions such as Bi3 + can not only be used for surface elemental analyses but also provide information on local chemical environments of elements on the surface. This is an important issue in heterogeneous catalysis (e.g., strong metal-support interactions). We also advise that one should be careful interpreting the SIMS data combined with a longer Ar1000 + sputtering time because this can deteriorate the surfaces from their original structures.

Recent Advances in Design of Gold-Based Catalysts for H2 Clean-Up Reactions

Over the past three decades, supported gold nanoparticles have demonstrated outstanding properties and continue to attract the interest of the scientific community. Several books and comprehensive reviews as well as numerous papers cover a variety of fundamental and applied aspects specific to gold-based catalyst synthesis, characterization by different techniques, relationship among catalyst support features, electronic and structural properties of gold particles, and catalytic activity, reaction mechanism, and theoretical modeling. Among the Au-catalyzed reactions targeting environmental protection and sustainable energy applications, particular attention is paid to pure hydrogen production. The increasing demands for high-purity hydrogen for fuel cell systems caused a renewed interest in the water-gas shift reaction. This well-known industrial process provides an attractive way for hydrogen generation and additional increase of its concentration in the gas mixtures obtained by processes utilizing coal, petroleum, or biomass resources. An effective step for further elimination of CO traces from the reformate stream after water-gas shift unit is the preferential CO oxidation. Developing highly active, stable, and selective catalysts for these two reactions is of primary importance for efficient upgrading of hydrogen purity in fuel cell applications. This review aims to extend the existing knowledge and understanding of the properties of gold-based catalysts for H2 clean-up reactions. In particular, new approaches and strategies for design of high-performing and cost-effective formulations are addressed. Emphasis is placed on efforts to explore appropriate and economically viable supports with complex composition prepared by various synthesis procedures. Relevance of ceria application as a support for new-generation WGS catalysts is pointed out. The role of the nature of support in catalyst behavior and specifically the existence of an active gold-support interface is highlighted. Long-term stability and tolerance toward start-up/shutdown cycling are discussed. Very recent advances in catalyst design are described focusing on structured catalysts and microchannel reactors. The latest mechanistic aspects of the water-gas shift reaction and preferential CO oxidation over gold-based catalysts from density functional theory calculations are noted because of their essential role in discovering novel highly efficient catalysts.

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.

Low Temperature CO oxidation over Iron Oxide Nanoparticles Decorating Internal Structures of a Mesoporous Alumina

Using a chemical vapor deposition method with regulated sample temperatures under ambient pressure conditions, we were able to fully decorate the internal structure of a mesoporous Al₂O₃ bead (~1 mm in particle diameter) with iron oxide nanoparticles (with a mean lateral size of less than 1 nm). The iron oxide-decorated Al₂O₃ showed a high CO oxidation reactivity, even at room temperature. Very little deactivation of the CO oxidation activity was observed with increasing reaction time at ~100 °C. Additionally, this catalyst showed high CO oxidation activity, even after annealing at ~900 °C under atmospheric conditions (i.e., the structure of the catalysts could be maintained under very harsh treatment conditions). We show that our catalysts have potential for application as oxidation catalysts in industrial processes due to the simplicity of their fabrication process as well as the high and stable catalytic performance.

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.