The interaction of size-selected Ru₃ clusters with RF-deposited TiO₂: 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 ₃clusters were deposited onto radio frequency (RF)-deposited TiO ₂which is an inexpensive, nanoparticulate form of TiO ₂. TiO ₂substrates 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 ₃deposited using both an ultra-high vacuum cluster source and chemical vapour deposition of Ru ₃(CO) ₁₂. The TiO ₂was treated with different Ar ⁺sputter doses prior to cluster depositions, and SiO ₂was also used as a comparison substrate. For cluster source-deposited Ru ₃, heating to 800 K caused cluster agglomeration on SiO ₂and oxidation on non-sputtered TiO ₂. For cluster source-deposited Ru ₃on sputtered TiO ₂substrates, 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 ₂before cluster deposition cannot be applied. Conversely, for Ru ₃(CO) ₁₂on sputtered TiO ₂the 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 ₂could 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 _xlayer reduced through sputtering and also partial oxidation of the Ru clusters.