Cluster Ion Beam Studies of Fundamental Dynamics Related to Physical Deposition; Kinematic Sample Mounting System for Accurate Positioning of Transferrable Samples; Sintering, Oxidation, and Chemical Properties of Size- Selected Nickel Clusters on TiO2 (1

摘要

This final report and the three journal articles following it support the complementary scientific goals of both AFOSR and DOE: stable catalysts for monopropellant decomposition, and the effects of cluster size and support defects on catalytic activity and selectivity, respectively. With the agreement of AFOSR, the original DOE effort was refocused on cluster deposition as a means to prepare and study model catalysts composed of size-selected clusters on oxide supports. The objectives were to probe the effects of cluster size, substrate structure, and deposition conditions on catalytic activity. The focus was on catalysts for monopropellant decomposition in spacecraft thrusters. During the first half of the grant period instrument upgrades were conducted to support the catalysis experiments. These uprades included improvements in the cluster deposition beamline, addition of low energy ion scattering spectroscopy (ISS), addition of Auger electron spectroscopy (AES), and the redesign of sample holders and TPD station. The first experiment focused on the use of ISS to characterize size-selected deposited clusters by studying Ni(n)/TiO2 samples. The effects of deposition energy and support preparation conditions on the oxidation side of the clusters were examined by x-ray photoelectron spectroscopy (XPS). ISS was used to characterize the dispersion of Ni on the support and provide insight into binding morphology. The second study used iridium as the catalyst of choice for monopropellant decomposition with rutile TiO2 as the initial substrate. Experiments also were conducted on XPS, ISS, C(superscript 16)O and C(superscript 18)O adsorption and desorption behavior to characterize deposited iridium clusters, the adsorption of adventitious CO on Ir(n)/TiO2 samples, and CO TPD from Ir2 and Ir10 following different CO exposure sequences. (10 figures, 36 refs.).