The rate of CO oxidation to CO2 depends strongly on the reaction temperature and characteristics of the oxygen overlayer on Au(111). The factors that contribute to the temperature dependence in the oxidation rate are (1) the residence time of CO on the surface, (2) the island size containing Au-O complexes, and (3) the local properties, including the degree of order of the oxygen layer. Three different types of oxygen--defined as chemisorbed oxygen, a surface oxide, and a bulk oxide--are identified and shown to have different reactivity. The relative populations of the various oxygen species depend on the preparation temperature and the oxygen coverage. The highest rate of CO oxidation was observed for an initial oxygen coverage of 0.5 monolayers that was deposited at 200 K where the density of chemisorbed oxygen is maximized. The rate decreases when two-dimensional islands of the surface oxide are populated and further decreases when three-dimensional bulk gold oxide forms. Our results are significant for designing catalytic processes that use Au for CO oxidation, because they suggest that the most efficient oxidation of CO occurs at low temperature--even below room temperature--as long as oxygen could be adsorbed on the surface.

The reaction of sulfur and oxygen with the gold surface is important in many technological applications, including heterogeneous catalysis, corrosion, and chemical sensors. We have studied reactions on Au(111) using scanning tunneling microscopy (STM) in order to better understand the surface structure and the origin of gold's catalytic activity. We find that the Au(111) surface dynamically restructures during deposition of sulfur and oxygen and that these changes in structure promote the reactivity of Au with respect to SO 2 and O 2 dissociation. Specifically, the Au(111) herringbone reconstruction lifts when either S or O is deposited on the surface. We attribute this structural change to the reduction of tensile surface stress via charge redistribution by these electronegative adsorbates. This lifting of the reconstruction was accompanied by the release of gold atoms from the herringbone structure. At high coverage, clusters of gold sulfides or gold oxides form by abstraction of gold atoms from regular terrace sites of the surface. Concomitant with the restructuring is the release of gold atoms from the herringbone structure to produce a higher density of low-coordinated Au sites by forming serrated step edges or small gold islands. These undercoordinated Au atoms may play an essential role in the enhancement of catalytic activity of gold in reactions such as oxygen dissociation or SO 2 decomposition. Our results further elucidate the interaction between sulfur and oxygen and the Au(111) surface and indicate that the reactivity of Au nanoclusters on reducible metal oxides is probably related to the facile release of Au from the edges of these small islands. Our results provide insight into the sintering mechanism which leads to deactivation of Au nanoclusters and into the fundamental limitation in the edge definition in soft lithography using thiol-based self-assembled monolayers (SAMs) on Au. Furthermore, the enhanced reactivity of Au after release of undercoordinated atoms from the surface indicate a relatively insignificant role of an oxide support for high reactivity.

On Cu(110)-p(2×1)-O at 300 K SO2(g) reacts stoichiometrically with O(a) to form a surface covered with both c(4×2)-SO3 and p(2×2)-SO3 structures. With heating SO2(g) evolves from the surface in distinct reaction-limited states at 384 K, 425 K, and 470 K, and the surface reverts to its initially oxidized state. On Cu(110), SO2(g) adsorbs molecularly below 300 K; upon annealing to 300 K, the sulfur dioxide disproportionates according to 3SO2(a)→S(a)+2SO3(a) with concomitant desorption of excess SO2(a). The surface formed in this manner exhibits large c(2×2)-S domains which encompass scattered c(4×2)-SO3 and p(2×2)-SO3 structures in a 1:2 coverage ratio. After being annealed to 400 K, the surface exhibits large p(2×2)-SO3 domains surrounding smaller c(4×2)-SO3 and c(2×2)-S islands. Continued heating past 400 K results in decomposition of sulfite according to SO3(a)→SO2(g)+O(a), evolving sulfur dioxide at 470 K and leaving the surface covered with atomic sulfur and oxygen. Real-time STM images show the mobility of oxygen at island boundaries and the mobility of sulfite amid the p(2×1)-O structures. STM measurements suggest that the sulfite occupy four-fold hollow sites.

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The adsorption of formate and acetate on the Ni(110) surface has been investigated with STM, LEED, TPRS, and XPS. At saturation coverage formate and acetate form a c(2 × 2) structure amid a pitted surface, which is suggestive of the incorporation of "added" Ni atoms into the c(2 × 2) structure. Heating the surface to 350 K to decompose the formate produces islands of step height, indicative of the release of Ni from the c(2 × 2) structure during HCOO decomposition. Upon further heating to 370 K the islands dissipate and p(4 × 1)-CO domains appear. Heating the acetate-covered surface affects a restructuring of the c(2 × 2) to scattered domains of a p(8 × 1) structure, which is attributed to acetate. Concomitant with this restructuring is the release of Ni to form islands of step height. Upon further heating the acetate decomposes to yield a p(4 × 5)-C covered surface. The present investigation adds further insight into the phenomena underlying the autocatalytic decomposition of formate and acetate. It is suggested that the added Ni in the formate structure facilitates the decomposition of formate into CO 2 , H 2 , and scattered Ni x nuclei, which act as sites for further formate decomposition.