No abstract

Oxide-supported Au nanostructures are promising low-temperature oxidation catalysts. It is generally observed that Au supported on reducible oxides is more active than Au supported on irreducible oxides. Recent studies also suggest that cationic Au(delta+) is responsible for the unique Au/oxide catalytic activity, contrary to the conventional perception that oxide supports donate electronic charge to Au. We have utilized density functional calculations and ab initio thermodynamic studies to investigate the oxidation state of Au nanostructures deposited on reducible and irreducible supports. We find that there are fundamental differences in the electronic structure of Au deposited on the different oxides. We propose a simple model, grounded in the first principles calculations, which can explain the oxide-specific catalytic activity of Au nanostructures and which can account for the presence and the role of cationic Au(delta+).

The nature of the active sites involved in the gold catalyzed Sonogashira cross-coupling reaction between iodobenzene and phenylacetylene, and in the competitive homocoupling reactions, has been investigated by means of DFT calculations, kinetic measurements, and synthesis of catalysts with different gold surface species. Several catalyst models have been theoretically investigated to simulate gold nanoparticles of different size either isolated, supported on inert materials, or supported on CeO 2 . The mechanistic studies show that IB dissociation occurs on low coordinated Au 0 atoms present in small gold nanoparticles, either isolated or supported, while PA is preferentially adsorbed and activated on Au δ+ species existing at the metal−support interface. When this occurs, the activation energy of the rate-determining step of the Sonogashira reaction, which has been found experimentally to be the bimolecular coupling, is minimized. The product distribution obtained with Au/CeO 2 catalysts containing different ratios of Au 0 /Au δ+ sites confirms the positive role played by cationic gold in the Sonogashira cross-coupling reaction. Importantly, only metallic Au 0 atoms present in gold nanoparticles are required to perform the homocoupling of iodobenzene.

The effect of off-stoichiometric point defects (oxygen vacancies or extra oxygen atoms at the metal-oxide interface) on oxide surfaces on the chemical activity of Au nanostructures supported on the oxide was studied using the density functional theory and ab initio thermodynamic calculations. We find that highly active Au sites are present on the Au nanostructures, when Au is anchored to the defects at the Au-oxide interface. Depending on the nature of the defect, we create electron-rich or electron-deficient Au. We show that the presence of the highly active Au sites does not correlate with the nominal charge on Au, and that the offstoichiometric defects in the oxide play a critical role not only in anchoring metal particles, but also in inducing chemical activity on the particles.

Quantum chemical and ab initio thermodynamic calculations were used to investigate the mechanism of CO oxidation on Au/TiO(2) and the geometric and electronic character of active sites. We show that CO oxidation over Au/TiO(2) might proceed via a two site mechanism with oxygen adsorbing and dissociating at the Au/oxide interface or the perimeter of Au particles and CO adsorbing on Au sites away from the interface. The electronic fingerprint of active Au is a function of external conditions, and it is likely that most Au atoms are populated by CO and electronically neutral. Under highly oxidizing conditions, the Au/oxide interface can accommodate oxygen adsorbates, and these Au atoms will have a cationic fingerprint due to their interaction with oxygen. The choice of precursors used to synthesize catalysts as well as the catalyst preparation and pretreatment procedures significantly affect the electronic characteristics and catalytic activity of Au nano-structures. Based on our first-principles analysis we propose a hypothesis that might help us understand these experimental observations.

Synthesis of kinetically trapped, alumina-supported Ni3Ga nanoparticles with particle surface composition partially controlled by off-stoichiometric Ni:Ga loading ratios enabled active, highly selective, and stable catalysts to be developed for the direct dehydrogenation of ethane to ethylene. Experimental studies indicated a direct correlation between superstoichiometric Ga loading and ethylene selectivity yet an inverse correlation with ethane conversion. A catalyst with a 1:1 Ni:Ga loading ratio exhibited a good balance between activity (TOF 5.0 × 10–2 s–1), selectivity (94%), and stability (94 to 90% over the 32-h test). The catalyst could also be easily regenerated using an oxidation and reduction cycle. DFT calculations and in situ DRIFTS CO adsorption confirm that surface reactivity is attenuated as Ga concentration at the particle surface increased.

Photocatalytic reduction of CO 2 for the production of synthesis gas, hydrocarbons, alcohols, or aldehydes from only CO 2 , H 2 O, and solar radiation may be a promising route to recycle CO 2 waste at the source of its production, reduce atmospheric CO 2 through direct removal from Earth's atmosphere, and/or produce CH 4 rocket fuel on Mars. In this study, we present the first studies of high-temperature (350 °C) photocatalytic reduction of CO 2 in the gas phase over a suite of semiconductors with a range of surface chemical reactivity and bulk electronic properties, namely, SiC, Si, Pt/TiO 2 , and GaN. The results indicate that catalysts that exhibit high Debye temperatures and elevated surface reactivity may enable the production of new reaction intermediates that facilitate C−O cleavage and full CO 2 reduction to CH 4 . Elevated photocatalyst surface reactivity also correlated with dramatically improved selectivity between hydrogenation (CH 4 production) and H 2 evolution, for example, 1:8, 1:71, 1:400, and 1:1000+ for SiC, Si, Pt/TiO 2 , and GaN, respectively. Tests at low (250 °C) and high (350 °C) temperatures over top-performing catalysts, SiC and Si, show that both C−O cleavage and H 2 O dissociation may be thermally promoted, leading to enhanced CH 4 , CO, and H 2 production, further indicating surfacechemical and thermal-input contributions.

Catalyst design: First principles calculations predict that phosgene can be substituted by dimethyl carbonate (DMC) in the carbamoylation of aromatic amines when using a CeO2 catalyst preferentially exposing the low energy {111} facet. Experimental results confirm the theoretical predictions, and CeO2 nano‐octahedra terminated by low energy {111} facets show high activity and selectivity toward the desired dicarbamoylated product.