Electron microscopy study of gold nanoparticles deposited on transition metal oxides

Selective catalytic oxidative-dehydrogenation of carboxylic acids-acrylate and crotonate formation at the Au/TiO2 interface

The oxidative-dehydrogenation of carboxylic acids to selectively produce unsaturated acids at the second and third carbons regardless of alkyl chain length was found to occur on a Au/TiO2 catalyst. Using transmission infrared spectroscopy (IR) and density functional theory (DFT), unsaturated acrylate (H2C═CHCOO) and crotonate (CH3CH═CHCOO) were observed to form from propionic acid (H3CCH2COOH) and butyric acid (H3CCH2CH2COOH), respectively, on a catalyst with ∼3 nm diameter Au particles on TiO2 at 400 K. Desorption experiments also show gas phase acrylic acid is produced. Isotopically labeled (13)C and (12)C propionic acid experiments along with DFT calculated frequency shifts confirm the formation of acrylate and crotonate. Experiments on pure TiO2 confirmed that the unsaturated acids were not produced on the TiO2 support alone, providing evidence that the sites for catalytic activity are at the dual Au-Ti(4+) sites at the nanometer Au particles' perimeter. The DFT calculated energy barriers between 0.3 and 0.5 eV for the reaction pathway are consistent with the reaction occurring at 400 K on Au/TiO2.

Insights into catalytic oxidation at the Au/TiO(2) dual perimeter sites

Gold (Au) nanoparticles supported on reducible oxides such as TiO2 demonstrate exceptional catalytic activity for a wide range of gas phase oxidation reactions such as CO oxidation, olefin epoxidation, and water gas shift catalysis. Scientists have recently shifted their hypotheses on the origin of the reactivity of these materials from the unique electronic properties and under-coordinated Au sites on nanometer-sized particles to bifunctional sites at the Au-support interface. In this Account, we summarize our recent experimental and theoretical results to provide insights into the active sites and pathways that control oxidation over Au/TiO2 catalysts. We provide transmission IR spectroscopic data that show the direct involvement of the Au-Ti(4+) dual perimeter sites, and density functional theory results that connect the electronic properties at these sites to their reactivity and to plausible reaction mechanisms. We also show the importance of interfacial Au-Ti(4+) sites in adsorbing and activating O2 as a result of charge transfer from the Au into antibonding states on O2 causing di-σ interactions with interfacial Au-Ti(4+) sites. This results in apparent activation energies for O2 activation of 0.16-0.60 eV thus allowing these materials to operate over a wide range of temperatures (110-420 K) and offering the ability also to control H-H, C-H, and C-O bond scission. At low temperatures (100-130 K), adsorbed O2 directly reacts with co-adsorbed CO or H2. In addition, we observe the specific consumption of CO adsorbed on TiO2. The more strongly held CO/Au species do not react at ∼120 K due to high diffusion barriers that prevent them from reaching active interfacial sites. At higher temperatures, O2 directly dissociates to form active oxygen adatoms (O*) on Au and TiO2. These readily react with bound hydrocarbon intermediates via base-catalyzed nucleophilic attack on unsaturated C═O and C═C bonds or via activation of weakly acidic C-H or O-H bonds. We demonstrate that when the active Au-Ti(4+) sites are pre-occupied by O*, the low temperature CO oxidation rate is reduced by a factor 22. We observe similar site blocking for H2 oxidation by O2, where the reaction at 210 K is quenched by ice formation. At higher temperatures (400-420 K), the O* generated at the perimeter sites is able to diffuse onto the Au particles, which then activate weakly acidic C-H bonds and assist in C-O bond scission. These sites allow for active conversion of adsorbed acetate intermediates on TiO2 (CH3COO/TiO2) to a gold ketenylidene species (Au2═C═C═O). The consecutive C-H bond scission steps appear to proceed by the reaction with basic O* or OH* on the Au sites and C-O bond activation occurs at the Au-Ti(4+) dual perimeter sites. There is a bound-intermediate transfer from the TiO2 support to the Au sites during the course of reaction as the reactant (CH3COO/TiO2) and the product (Au2═C═C═O) are bound to different sites. We demonstrate that IR spectroscopy is a powerful tool to follow surface catalytic reactions and provide kinetic information, while theory provides atomic scale insights into the mechanisms and the active sites that control catalytic oxidation.

Atomic Details of Interfacial Interaction in Gold Nanoparticles Supported on MgO(001)

Atomic-scale imaging using aberration-corrected scanning transmission electron microscopy reveals direct evidence for semicoherent interfacial epitaxy and coordinate-dependent surface contraction for the fcc (001) oriented Au nanoparticles (2-3 nm in diameter), suggesting that their interaction with the substrate is weaker than previously assumed. A significant change in interfacial separation distance from 2.47 ± 0.12 Å for the fcc (001) oriented Au nanoparticles to 3.07 ± 0.11 Å for the fcc (111) oriented Au nanoparticles has also been observed. These results are used to verify the atomistic models generated by the global optimization calculations, which shed further light on the intricate relation between the interfacial energy and the atomic structure of the nanoparticle and their combined effect on the inhomogeneous surface structural relaxation of supported nanoparticles.

Engineering nanointerfaces for nanocatalysis

We focus on recent advances in the delicate design of well-defined nanointerfaces to promote nanocatalysis towards renewable energy.