57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

Heterogeneous catalysts are preferred in industrial applications because of their robustness, but homogeneous and enzymatic catalysts are better for complex catalysis where product selectivity is required. The crossroad of organometallic chemistry with material science has recently allowed the rational development of catalysts that incorporate some of the basic elements found in homogeneous catalysts. This area of research has been coined as “surface organometallic” chemistry.

During the first year of the grant we focus on setting up an ultrahigh vacuum system coupled with a reactor cell where reactions could be follow in-situ by infrared reflection-absorption spectroscopy (IRRAS). To test the system, we studied the interaction of simple organic molecules, such as carboxylates and carbon monoxide with surfaces. The IRAS system is now working and this effort has resulted in a publication.

As a first step towards the research on hybrid organometallic/oxide systems, we initiated studies on the interaction of organometallic precursors with oxide surfaces. A Ru₃(CO)₁₂ precursor was deposited on TiO₂(110) surfaces by chemical vapor deposition (CVD). Due to its photo and chemical stability, TiO₂ based catalysts have drawn attention to produce hydrogen photocatalically from water or organic compounds. However, owing to the wide band gap of TiO₂ and sluggish charge transfer kinetics to adsorbed species on the surface, it is necessary to modify the TiO₂ based photo-catalyst either by reducing band gap or by facilitating the charge transfer in order to reduce electron-hole recombination. Several previous studies have shown that the TiO₂ decorated with RuO₂ particles or films could catalyze photo-reactions by improving the efficiency of charge separation at the metal oxide/semiconductor interface. Furthermore, it has been shown that RuO₂ can work also as a good catalyst for hydrogen evolution. Others have found that the Ru-doped TiO₂ exhibits photo catalytic activity under visible light due to the formation of an impurity energy level which then modifies the band gap of TiO₂. It has also been reported that ruthenium oxide caused the anatase to rutile transformation to occur at lower temperatures resulting in a decrease in the band-gap. Thus the literature survey indicates that there is controversy on the role of Ru/RuO_x in enhancing the photocatlytic activity of TiO₂ and moreover there is a clear need to understand the reaction pathways on these mixed-metal oxide surfaces.

Upon oxidation of the organometallic precursor, 1D RuO_x structures were formed on top of the TiO₂ substrate. In contrast to the high stability of bulk RuO₂, the 1D structures of RuO_x in direct contact with the reducible oxide substrate became very reactive. It is possible to reduce the RuO_x structures to Ru⁰ by exposure under moderate conditions to CO and re-oxidize them buck to RuO₂. In catalytic tests, this structures show high activity for CO oxidation, while zero reactivity is obtained from either TiO₂ or RuO₂ samples alone. Ethanol has been proposed as a good renewable energy source. We have studied the reaction of ethanol over the ruthenium metal and metal-oxides deposited on TiO₂(110) single crystal surfaces under dark and photo conditions in the presence and the absence of molecular oxygen. Gas phase reaction on these surfaces under dark and UV condition was monitored by an online mass spectrometer. Furthermore, the photo stability of the surface ethoxide species and the possible formation of reaction intermediates on these surfaces were studied in details. Presence of Ru metal and metal oxides imposes changes in the electronic structure of TiO₂; under UV light exposure, the mobility of electrons in the conduction band is enhanced significantly. Ethoxide is formed on both, rutile TiO₂(110) and ruthenium modified TiO₂(110) surfaces spontaneously when exposed to ethanol at room temperature. A fraction of the ethoxide species adsorbed on the TiO₂ surface form acetaldehyde by dehydrogenation starting at about 350K, indicating that it’s an endothermic reaction. In contrast, when RuO₂/TiO₂ surfaces are exposed to ethanol, ethoxide species immediately form surface acetate by taking one of the bridging oxygen from five-fold Ru⁴⁺ sites, thus partially reducing the ruthenium oxide on the TiO₂ surface. Upon UV excitation and in the presence of O₂, the main reaction product observed at 300 K is acetaldehyde on both bare TiO₂(110) and on Ru metal/metal oxide modified TiO₂ surfaces. This product, formed via a two electron transfer process, requires the presence of molecular O₂ to trap the excited electrons from the conduction band, and thus decrease the rate of electron–hole recombination. It is found that the Ru/TiO₂ system is more active for the photo-oxidation of ethanol to acetaldehyde than TiO₂. A linear trend of the rate of acetaldehyde and carbon dioxide production from Ru/TiO₂ with O₂ pressures indicates that more surface sites are available for the adsorption of O₂ than on plain TiO₂ surfaces, possibly at the interface of the Ru metal nanoparticles and TiO₂ surfaces, which facilitates the photo-oxidation. A second publication is been submitted with the results from the photoreactivity of the model catalysts based on Ru organometallic precursors deposited on TiO₂.

A post-doc working in this project was fully funded from the PRF grant. The post-doc obtained a permanent position in fundamental petroleum research at a different institution at the beginning of the summer of 2012. An undergraduate student funded by a different fellowship also worked during the summer of 2011 in the project interacting with the post-doc. A graduate student funded from the PRF grant started to work during the summer of 2012.