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