The interaction of size-selected Ru3clusters with RF-deposited TiO2: Probing Ru-CO binding sites with CO-temperature programmed desorption

Abstract

Small Ru clusters are efficient catalysts for chemical reactions such as CO hydrogenation. In this study 3-atom Ru3clusters were deposited onto radio frequency (RF)-deposited TiO2which is an inexpensive, nanoparticulate form of TiO2. TiO2substrates are notable in that they form strong metal-substrate interactions with clusters. Using temperature programmed desorption to probe Ru-CO binding sites, and X-ray photoelectron spectroscopy to provide chemical information on clusters, differences in cluster-support interactions were studied for Ru3deposited using both an ultra-high vacuum cluster source and chemical vapour deposition of Ru3(CO)12. The TiO2was treated with different Ar+sputter doses prior to cluster depositions, and SiO2was also used as a comparison substrate. For cluster source-deposited Ru3, heating to 800 K caused cluster agglomeration on SiO2and oxidation on non-sputtered TiO2. For cluster source-deposited Ru3on sputtered TiO2substrates, all Ru-CO binding sites were blocked as-deposited and it was concluded that for the binding sites to be preserved for potential catalytic benefit, sputtering of TiO2before cluster deposition cannot be applied. Conversely, for Ru3(CO)12on sputtered TiO2the clusters were protected by their ligands and Ru-CO binding sites were only blocked once the sample was heated to 723 K. The mechanism for complete blocking of CO sites on sputtered TiO2could not be directly determined; however, comparisons to the literature indicate that the likely reasons for blocking of the CO adsorption sites are encapsulation into the TiOxlayer reduced through sputtering and also partial oxidation of the Ru clusters.