I. CHEMICAL CHARACTERIZATION OF THE CLEAN AND OXIDIZED MOLYBDENUM (100) SURFACE BY THE ADSORPTION OF GASEOUS ACIDS AND BASES IN ULTRAHIGH VACUUM AND AT ATMOSPHERIC PRESSURES. II. HIGH TEMPERATURE OXIDATION OF THE ALUMINUM(111) SURFACE.

摘要

Changes in the Lewis acid-base character of the Mo(100) surface with oxidation have been investigated by a variety of adsorption experiments using LEED, Auger and temperature programmed reaction spectroscopy (TPRS). The clean Mo(100) surface is found to be an exceedingly strong Lewis base (electron donor). Submonolayer amounts of oxygen change the surface into a moderate to strong Lewis acid which shows a marked preference towards the adsorption of a (sigma) orbital Lewis base, over a (pi)-orbital Lewis base. The qualitative ideas of hard and soft acids and bases (HSAB) is shown to be useful in interpreting the observed adsorption selectivity.;Exposure of the P(1 x 2) oxidized Mo(100) surface to ammonia at atmospheric pressures using a novel high pressure cup shows the formation of a binding state not observably in UHV. High pressure adsorption experiments were also performed on the P(1 x 2) surface using a wide variety of Lewis acids and bases. These results are consistent with the interpretation of the P(1 x 2) surface as a hard acid.;The oxidation of the Al(111) surface at 5 x 10('-8) torr at room temperature and at 575(DEGREES)C was also investigated. The oxidation of the Al(111) surface at 850K under clean, ultrahigh vacuum conditions produces a sapphire blue coloration of the surface. The blue color is observable for oxygen exposures in the range 400L to 3200L. XPS, ELS and LEED measurements indicate that a crystalline aluminum oxide forms on Al(111) upon initial exposure to oxgyen and then subsequently grows as islands on the surface. Previously reported soft x-ray absorption measurements of the Al 2p core level conduction band energy separation suggests that the blue color arises from an optically excited electron transfer between the aluminum metal Fermi energy and the aluminum oxide conduction band.;The adsorption properties of a series of methylamines was investigated on clean and oxidized Mo(100) surfaces and in particular the mechanism of thermal decomposition of trimethylamine to methane and hydrogen cyanide was studied in detail. Trimethylamine adsorbs molecularly on the oxidized surface ((theta)(,O) = 0.8 up to MoO(,2)) at room temperature. The mechanism is independent of surface oxygen concentration. The initial step in the decomposition of trimethylamine is C-H bond breakage followed by C-N bond breakage. The rate limiting step is C-N bond cleavage. A dramatic reduction in the temperature for N(,2) desorption due to the presence of adsorbed carbon is also observed.