Electron Microscopy Study of Gold Nanoparticles Deposited on Transition Metal Oxides

Many researchers have investigated the catalytic performance ofgold nanoparticles (GNPs) supported on metal oxides for various catalytic reactions of industrial importance. These studies have consistently shown that the catalytic activity and selectivity depend on the size of GNPs, the kind of metal oxide supports, and the gold/metal oxide interface structure. Although researchers have proposed several structural models for the catalytically active sites and have identified the specific electronic structures of GNPs induced by the quantum effect, recent experimental and theoretical studies indicate that the perimeter around GNPs in contact with the metal oxide supports acts as an active site in many reactions. Thus, it is of immense importance to investigate the detailed structures of the perimeters and the contact interfaces of gold/metal oxide systems by using electron microscopy at an atomic scale.This Account describes our investigation, at the atomic scale using electron microscopy, of GNPs deposited on metal oxides. In particular, high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) are valuable tools to observe local atomic structures, as has been successfully demonstrated for various nanoparticles, surfaces, and material interfaces. TEM can be applied to real powder catalysts as received without making special specimens, in contrast to what is typically necessary to observe bulk materials. For precise structure analyses at an atomic scale, model catalysts prepared by using well-defined single-crystalline substrates are also adopted for TEM observations. Moreover, aberration-corrected TEM, which has high spatial resolution under 0.1 nm, is a promising tool to observe the interface structure between GNPs and metal oxide supports including oxygen atoms at the interfaces. The oxygen atoms in particular play an important role in the behavior of gold/metal oxide interfaces, because they may participate in catalytic reaction steps. Detailed information about the interfacial structures between GNPs and metal oxides provides valuable structure models for theoretical calculations which can elucidate the local electronic structure effective for activating a reactant molecule. Based on our observations with HRTEM and HAADF-STEM, we report the detailed structure of gold/metal oxide interfaces

One-Pot Synthesis of Au₁₁(PPh₂Py)₇Br₃ for the Highly Chemoselective Hydrogenation of Nitrobenzaldehyde

In this study, the gold clusters Au₁₁(PPh₃)₇Cl₃ and Au₁₁(PPh₂Py)₇Br₃ (PPh₂Py = diphenyl-2-pyridylphosphine) are synthesized via a one-pot procedure based on the wet chemical reduction method. The Au₁₁(PPh₃)₇Cl₃ cluster is found to be active in the chemoselective hydrogenation of 4-nitrobenzaldehyde in the presence of hydrogen (H₂) and a base (e.g., pyridine). Interestingly, the cluster with the functional ligand PPh₂Py shows similar activity without losing catalytic efficiency in the absence of the base. The structure of the gold clusters and reaction pathway of the catalytic hydrogenation are investigated at the atomic/molecular level via UV–vis spectroscopy, electrospray ionization (ESI) mass spectrometry, and density functional theory (DFT) calculations. It is found that one ligand (PPh₃ or PPh₂Py) removal is the first step to expose the core of the gold clusters to reactants, providing an active site for the catalytic reaction. Then, the H–H bond of the H₂ molecule becomes activated with the aid of either free amine (base) or ligand PPh₂Py which is attached to the gold clusters. This work demonstrates the promise of the functional ligand PPh₂Py in the catalytic hydrogenation to reduce the amount of materials (free base: e.g., pyridine) that ultimately enter the waste stream, thereby providing a more environmentally friendly reaction medium

Stepwise Displacement of Catalytically Active Gold Nanoparticles on Cerium Oxide

Aberration-corrected environmental transmission electron microscopy (ETEM) proved that catalytically active gold nanoparticles (AuNPs) move reversibly and stepwise by approximately 0.09 nm on a cerium oxide (CeO₂) support surface at room temperature and in a reaction environment. The lateral displacements and rotations occur back and forth between equivalent sites, indicating that AuNPs are loosely bound to oxygen-terminated CeO₂ and may migrate on the surface with low activation energy. The AuNPs are likely anchored to oxygen-deficient sites. Observations indicate that the most probable activation sites in gold nanoparticulate catalysts, which are the perimeter interfaces between an AuNP and a support, are not structurally rigid

Stepwise Displacement of Catalytically Active Gold Nanoparticles on Cerium Oxide

Aberration-corrected environmental transmission electron microscopy (ETEM) proved that catalytically active gold nanoparticles (AuNPs) move reversibly and stepwise by approximately 0.09 nm on a cerium oxide (CeO₂) support surface at room temperature and in a reaction environment. The lateral displacements and rotations occur back and forth between equivalent sites, indicating that AuNPs are loosely bound to oxygen-terminated CeO₂ and may migrate on the surface with low activation energy. The AuNPs are likely anchored to oxygen-deficient sites. Observations indicate that the most probable activation sites in gold nanoparticulate catalysts, which are the perimeter interfaces between an AuNP and a support, are not structurally rigid

Low-Temperature CO Oxidation over Combustion Made Fe- and Cr-Doped Co₃O₄ Catalysts: Role of Dopant’s Nature toward Achieving Superior Catalytic Activity and Stability

Co₃O₄ with a spinel structure shows unique activity for CO oxidation at low temperature under dry conditions; however the active surface is not very stable. In this study, two series of Fe- and Cr-doped Co₃O₄ catalysts were prepared by a single-step solution combustion technique. Fe was chosen because of its redox activity corresponding to the Fe²⁺/Fe³⁺ redox couple and compared to Cr, which is mainly stable in the Cr³⁺ state. The catalytic activity of new materials for low-temperature CO oxidation was correlated to the nature of the dopant. As a function of dopant concentration, the temperature corresponding to the 50% CO conversion (<i>T</i>₅₀) demonstrated significant differences. The maximal activity was achieved for 15% Fe-doped Co₃O₄ with <i>T</i>₅₀ of −85 °C and remained almost constant up to 25% Fe. In the case of Cr, the activity was observed to be maximum for 7% of Cr with <i>T</i>₅₀ of −42 °C and significantly decreased for higher Cr loadings. Similarly, there was a contrasting behavior in catalyst stability too. 100% CO conversion was achieved below −60 °C for 15% Fe/Co₃O₄ catalyst and remained unchanged even after calcination at 600 °C. In contrast, Co₃O₄ or 15% Cr/Co₃O₄ catalysts strongly deactivated after the same treatment. These differences were correlated to the oxidation states, coordination numbers, the nature of surface planes, and the redox properties. We observed that both Cr and Fe were typically present in the +3 oxidation state, occupying octahedral sites in the spinel structure. The catalysts were mainly exposed to (111) and (220) planes on the surface. H₂-TPR indicated clear differences in the redox activity of materials due to Fe and Cr substitutions. The reducibility of surface Co³⁺ species remained similar in all Fe-doped Co₃O₄ catalysts in contrast to nonreducible Cr-doped analogs, which shifted the reduction temperature to the higher values. As the Fe³⁺/Fe²⁺ redox couple partly substituted the Co³⁺/Co²⁺ redox couple in the spinel structure, similar bond strength of Fe–O keep redox activity of Co³⁺ species almost unchanged leading to higher activity and stability of Fe/Co₃O₄ catalysts for low-temperature CO oxidation. In contrast, nonreducible Cr³⁺ species characterized by strong Cr–O bond substituting active Co³⁺ sites can make the Cr/Co₃O₄ surface less active for CO oxidation