Temperature programmed desorption (TPD) and X-Ray photoelectron spectroscopy (XPS) studies on clean polycrystalline graphite under Ultra High Vacuum conditions are described. The same three strongly bound oxygenated species are formed after O 2 , CO 2 and H 2 0 adsorption. They decompose to give CO at 973, 1093 and 1253 K. Small amounts of CO 2 are also produced after adsorption of these gases, with desorption temperatures at 463, 573, 693, 793 and 793 K. Attempts are made to ascribe these TPD features more precisely. After H 2 0 adsorption, some H2 is evolved at ca. 1300 K. Hydrocarbons (C l -C 6 ) are also produced, but is smaller amounts. A general mechanism is proposed for the gasification reactions of graphite with O 2 , CO 2 and H 2 0. Physical wetting of the clean graphite surface leads to a H 2 0 molecule re\'ersibly bound to the carbon surface. According to XPS data, a hydrate type of bond is proposed. Considerations on the non-catalytic as well as on the catalytic steam gasification of graphite are made. It is suggested that in both cases the reaction is not only controlled by the desorption of the products, i. e. the decomposition of the surface intermediates, but also by the sticking probability of the H 2 0 on the graphite edges.

Temperature programmed desorption (TPD) results after chemisorption of carbon monoxide (CO) and carbon dioxide (C0 2 ) on polycrystalline graphite are presented. CO adsorbs onto graphite 'with a very low sticking coefficient « 10-12 ).After CO chemisorption, CO (mass 28 amu) desorbs in two temperature regions, between 400 and 700 K, and between 1000 and 1300 K, and CO 2 (mass 44 amu) des orbs below 950 K. The intensity of the CO 2 signal is less than one order of magnitude lower than the CO intensity. After CO 2 adsorption the major desorption product is CO at high temperatures (lOOO

The rate and product distribution of chemical reactions catalyzed by metals can depend on the particle size of supported catalysts. This can be explained by the interaction between chemisorbed species and surface sites of different configuration in the metal associated with atoms in terrace, steps, and kinks. Single-crystal catalysts are excellent models for studying these interactions. In this review, the contribution of single-crystal studies to the understanding of structure sensitivity in catalytic reaction is discussed. Reactions are classified in three types: structure-sensitive reactions, structure-insensitive reactions, and reactions that show both behaviors, depending on the experimental conditions. Several examples are discussed in each of the three categories.

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