The structures and dynamics of atomic and molecular adsorbates on metal surfaces by scanning tunneling microscopy and low energy electron diffraction

Abstract

The structures and dynamics of atomic and molecular adsorbates on metal surfaces by scanning tunneling microscopy and low energy electron diffraction adsorbate and adsorbate-substrate interactions through surface studies of coverage dependency and coadsorption using both scanning tunneling microscopy (STM) and low energy electron diffraction (LEED).The effect of adsorbate coverage on the surface structures of sulfur on Pt( 1 1 1) and Rh( 1 11) was examined. On Pt(l1 l), sulfur forms ~( 2 x 2 ) at 0.25 ML of sulfur, which transforms into a more compressed (d3xd3)R3Oo at 0.33 ML. On both structures, it was found that sulfur adsorbs only in fcc sites. When the coverage of sulfur exceeds 0.33 ML, it formed more complex c(d3x7)rect structure with 3 sulfur atoms per unit cell. In this structure, two different adsorption sites for sulfur atoms were observed -two on fcc sites and one on hcp site within the unit cell.In contrast to Pt(ll1) the lowest coverage of ordered sulfur formed on Rh( 1 11) was the (d3xd3)R3Oo overlayer at 0.33 ML, in which the adsorption site of sulfur was determined to be fcc site from LEED I-V analysis. When the sulfur overlayer was compressed by adsorbing additional sulfur atoms on the surface, it forms a ~(4x2)' overlayer at 0.5 ML. STM experiments clearly show that there are two different adsorption sites for sulfur in this structure, notably one fcc and one hcp site in the unit cell. A (4x4) overlayer can also be formed at the same sulfur coverage as the ~ (4x2) structure as determined by Auger electron spectroscopy, but at a higher annealing temperature. STM images of the (4x4) overlayer show that all sulfur atoms have same adsorption site, presumably fcc site, but the distances between sulfur atoms are much shorter than those for the ~(4x2) structure, suggesting the cluster formation. This is more evident in a (7x7) structure, which it is postulated to form a surface sulfide with sulfur dissolved into the subsurface. In order to accommodate the large number of sulfur atoms on the surface, the substrate metal atoms may have gone through reconstruction to form a more energetically stable overlayer.STM was also used to study the structures of carbon monoxide on Rh(ll1) surface at different partial pressures of CO. Carbon monoxide forms two different (2x2) ordered overlayers depending on the background pressure of CO. At low pressure, % ML structure was observed, while at the high partial pressure of CO, the coverage increased to % ML and the formation of CO cluster were observed.The coadsorption of benzene and carbon monoxide was used as a model system to elucidate the interaction between different molecular species on Rh( 1 1 1) surface. Benzene 2 molecules do not form a long range ordered overlayer on Rh( 1 1 1) at saturation coverage, but form well ordered overlayers when they were coadsorbed with carbon monoxide. When the coverage of benzene reaches approximately 0.5 IvfL, only benzene molecules at the top side of the step edges were imaged, while those on the terraces could not be ima...

Preparation of sulfated zirconia catalyst loaded by copper in nano-scale Green application to synthesis of biolubricant

“…These observations were also reported by other researchers [23-25, 27, 28]. Additional peaks are observed at 2θ of 36 and 38 corresponding to CuO monoclinic structure (JCPDS:80-0076 card ICSD≠:067850) [29][30][31] In the 2θ region from 34 to 37, the peak includes tetragonal phase of zirconia and the monoclinic structure of CuO and we can observe that there is a small split in the peak of Cu/SZ (Imp) catalyst. This splitting becomes clearer in the Cu/SZ (Sol) catalyst.…”