Assignment and identification of reaction sites on solid surfaces

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A comparison of product desorption dynamics between thermally and photolytically CO oxidation was performed on two stepped platinum surfaces, i.e., pt (113) consisting of two-atom wide terraces with a (111) structure and one-atom high stpes with a (001) structure, and Pt (112) with three-atom wide terraces of a (111) structure. No difference is in the CO_2 desorption dynamics in the usage of 193 nm and 308 nm irradiation. The oxygen orientation was analyzed on Pt (113) and also on Pt (133).1. Oxygen orientation, It was examined by NEXAFS.The admolecule lies along the trough on Pt (113), whereas its orientation on Pt (133) is shifted from the trough parallel at high coverages.2. Photolytic oxidation, The CO_2 desorption with high velocity is collimated in two directional ways in a plane along the trough on Pt (113) at low O_2 and CO covergaes. Oxygen atoms emitted along the trough interact with CO,yielding the sites different from the thermal reaction. The velocity analysis also confirms the presence of this fast component The CO_2 desorption collimated along the terrace (29.5ﾟ) and step (-25.2ﾟ) normal is enhanced at high O_2 coverages. The terrace desorption is also observed on Pt (112).3. Thermal oxidation, The spatial and velocity distributions of desorbing product yield the structural information of the sites even in the thermal oxidation, when the product is repulsively desorbed. The site is moved from the terrace to the step at higher CO coverage. The CO_2 desorption is characterized as (1) the collimation is shifted about 30% from the site normal, (2) anisotropic angular distribution is correlated to the site symmetry, (3) high translational temperature indicates repulsive desorption but about half of the value in the photolytic reaction, and (4) no significant difference in the translational temperature of CO_2 between the terrace and step.