Mechanism of Adsorption-Catalytic Activity at the Nanostructured Surface of Silicon Doped with Clusters of Transition Metals and Their Oxides
Mechanisms of adsorption-catalytic activation of composites fabricated on the basis of porous silicon with incorporated nanoparticles of transition metals (Pd, W, Cu) and their oxides have been analyzed theoretically. The influence of adsorbed atoms of acceptor elements (O, S, F, Cl) on the catalytic activity of transition metals during the formation of surface nanoclusters of transition metal oxides is revealed. The enhancement of the catalytic activity of transition metals with the completely filled d-band may consist in a change of the filling of d-states with electrons (the appearance of holes above the Fermi level) at the formation of surface nanoclusters of transition metal oxides. The results of experimental researches of the adsorption-electric effect in gas-sensitive structures with Schottky barriers obtained within the method of high-frequency volt-farad characteristics are presented. The experimental adsorption isotherms of hydrogen and hydrogen sulfide on the surface of nanostructured silicon composites with copper, tungsten, palladium, and their oxides in the pores are analyzed. An increased adsorption sensitivity of those composites to various gases (H2, H2S, H2O) in comparison with an ordinary porous silicon layer is found. It is established that the mechanism of physical adsorption is realized at low gas pressures (≤ 25 ppm) and/or short times of the adsorbate-substrate interaction, and the chemisorption mechanism at higher pressures and in the course of long-term processes. This conclusion agrees with the theoretical data calculated for the adsorption heat from experimental isotherms (0.3–0.5 eV).
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