Structure and reactivity of model thin film catalysts

The purpose of this project is to characterize the structure and reactivity of model bimetallic catalysts, e.g., ultrathin films of Pt deposited on single crystal surface of tungsten. Studies during this reporting period (January 15, 1989 to the present) have involved a comparison between the atomically smooth W(110) surface and the atomically rough W(111) surface using several ultrahigh vacuum surface science methods (low energy electron diffraction, LEED, Auger electron spectroscopy, AES, and thermal desorption spectroscopy, TDS). In addition, we have used scanning tunneling microscopy (STM) to provide a microscopic view of Pt induced surface structures with Angstrom level resolution. On W(110), a single Pt monolayer is stable upon heating to >1800K. It grows in a layer-by-layer mode at 300K, but Pt coverages in excess of one monolayer will cluster into 3-dimensional bulk Pt crystallites upon annealing to approx-gt 800K. When W(111) is covered by approx-gt 1 x 1015 Pt atoms/cm2 and heated in the range 800--1600 K, the surface undergoes a massive restructuring to form microscopic facets. At 1200 K, the average facet dimensions are ∼100 angstrom, and the dominant facet orientations is W(211). The faceting appears to be driven by a Pt-enhanced anisotropy in the surface free energy. To the best of our knowledge, the only other reports of faceting induced by a single monolayer of metal on a metal surface are based on field emission microscopy. The PI joined Rutgers in Fall 1988, and initiated this project shortly afterwards. It is an outgrowth of work performed by the PI and his colleagues at the National Institute of Standards and Technology (formerly NBS). 8 refs., 2 figs