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The reactions of benzene on Mo(110) have been investigated under ultrahigh vacuum conditions using temperature programmed reaction spectroscopy (TPRS), x-ray photoelectron spectroscopy (XPS), and near-edge x-ray absorption fine structure (NEXAFS) measurements. Benzene undergoes competing decomposition and desorption processes during temperature programmed reaction; saturation exposures of C6H6 made at a crystal temperature of 120 K yield a small amount of benzene desorption at 360 K as well as substantial H2 evolution at 400 and 530 K. Additionally, a trio of molecular benzene desorptions attributed to weakly bound and multilayer states is observed below 200 K. Isotopic exchange experiments demonstrate that the 360 K molecular desorption arises from intact chemisorbed benzene. For surface temperatures up to 300 K, carbon 1s photoelectron spectra show only a single photoemission peak at a binding energy of 284.0 eV, attributed to molecular benzene. Above 300 K, increasing amounts of atomic carbon are observed at a binding energy of 282.8 eV, indicating that the kinetics for C–C and C–H bond activation in this system are similar. Decomposition to surface C and gaseous H2 is complete by 700 K. At 200 K, near-edge x-ray absorption fine structure measurements confirm the presence of molecular benzene in an orientation with the C6 plane parallel to the surface. After annealing to 450 K, the NEXAFS is complicated by the admixture of absorption structures from a surface intermediate and atomic carbon. Comparison of this NEXAFS with that for surface benzyne (C6H4) produced by annealing benzenethiol (C6H5SH) adsorbed on Mo(110) to 600 K suggests that the intermediate formed from benzene is C6H4 as well. [ABSTRACT FROM AUTHOR]

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 SO2 and O2 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 SO2 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. [ABSTRACT FROM AUTHOR]

Gold (Au) is an interesting catalytic material because of its ability to catalyze reactions, such as partial oxidations, with high selectivities at low temperatures; but limitations arise from the low O2 dissociation probability on Au. This problem can be overcome by using Au nanoparticles supported on suitable oxides which, however, are prone to sintering. Nanoporous Au, prepared by the dealloying of AuAg alloys, is a new catalyst with a stable structure that is active without any support. It catalyzes the selective oxidative coupling of methanol to methyl formate with selectivities above 97% and high turnover frequencies at temperatures below 80°C. Because the overall catalytic characteristics of nanoporous Au are in agreement with studies on Au single crystals, we deduced that the selective surface chemistry of Au is unaltered but that O2 can be readily activated with this material. Residual silver is shown to regulate the availability of reactive oxygen. [ABSTRACT FROM AUTHOR]