The interaction of size-selected Ru 3 clusters with RF-deposited TiO 2 : probing Ru-CO binding sites with CO-temperature programmed desorption.

Small Ru clusters are efficient catalysts for chemical reactions such as CO hydrogenation. In this study 3-atom Ru 3 clusters were deposited onto radio frequency (RF)-deposited TiO 2 which is an inexpensive, nanoparticulate form of TiO 2 . TiO 2 substrates 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 Ru 3 deposited using both an ultra-high vacuum cluster source and chemical vapour deposition of Ru 3 (CO) 12 . The TiO 2 was treated with different Ar + sputter doses prior to cluster depositions, and SiO 2 was also used as a comparison substrate. For cluster source-deposited Ru 3 , heating to 800 K caused cluster agglomeration on SiO 2 and oxidation on non-sputtered TiO 2 . For cluster source-deposited Ru 3 on sputtered TiO 2 substrates, 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 TiO 2 before cluster deposition cannot be applied. Conversely, for Ru 3 (CO) 12 on sputtered TiO 2 the 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 TiO 2 could 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 TiO x layer reduced through sputtering and also partial oxidation of the Ru clusters.