Dario J. Stacchiola, PhD, Michigan Technological University
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/RuOx 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 RuOx structures were formed on top of the TiO2 substrate. In contrast to the high stability of bulk RuO2, the 1D structures of RuOx in direct contact with the reducible oxide substrate became very reactive. It is possible to reduce the RuOx 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.