Surface photochemistry on confined systems: UV-laser-induced photodesorption of NO from Pd-nanostructures on Al2O3

Abstract

UV-laser induced desorption of NO from nanostructured palladium aggregates on an epitaxial alumina support has been studied by means of resonance enhanced multiphoton ionisation (REMPI) to detect desorbing molecules quantum state resolved by Fourier-transform infrared reflection absorption spectroscopy (FT-IRAS), X-ray photoemission spectroscopy (XPS) and thermal desorption spectroscopy (TPD). The size of the Pd-aggregates was systematically varied between 5 Å and 80 Å. Different morphologies were chosen depending on the growth conditions of the aggregates by Pd-atom deposition on the support. The aggregates were either amorphous (deposition at 100 K) or ordered (deposition at 300 K) with the aggregates having a cubooctahedral shape with dominating (111) terraces and a minority of (100) terraces. Adsorption is only similar to single crystal data for aggregate sizes beyond 75 Å. For smaller aggregates NO is bound on on-top sites of palladium. On small amorphous aggregates a substantial amount of NO is weakly bound, which has only been observed for stepped single Pd-crystals. Dissociation of NO occurs at elevated temperatures above 350 K. The system was excited with nanosecond laser pulses at 6.4 eV. In contrast to single crystals, desorption of intact NO molecules has been observed for small aggregates with increasing efficiencies with decreasing aggregate size for aggregate sizes of 80 Å and below. Desorption cross sections vary by at least one order of magnitude. Dominantly the weakly bound species desorbs. REMPI data do not show a strong size dependence. Different models are discussed to explain the data, including the role of local effects of the adsorption site, spill-over to the alumina support or formation of oscillations in electron densities.