Date of Award

1992

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical & Biochemical Engineering

First Advisor

Bruce E. Koel

Second Advisor

John L. Falconer

Abstract

The growth and structure of ultrathin Pd films deposited on Mo(IOO) and the chemisorption properties of ultrathin Pd films on Mo(IO0) and Ta(l 10) have been studied. Pd films deposited on Mo(l00) at 150 K grow by a layer-by-layer mechanism. Deposition at higher temperatures leads to layer plus crystallite growth. Chemisorption studies on the Pd monolayer on both Mo(l00) and Ta(l 10) indicate that it has a chemical reactivity similar to Ag. CO is weakly bound to the Pd monolayer on Mo( l 00) with an adsorption energy of 20 kcal/mole. In contrast, the adsorption energy of CO on bulk Pd( 100) is 36.5 kcal/mo!. The sticking probability of CO on the Pd mono layer on Mo( 100) is also reduced and the propensity for population of atop sites is increased. C2H4 is also weakly bound on the Pd monolayer on Mo( 100) and the propensity for reversible adsorption is increased relative to thick Pd films. The adsorption of H2 was studied on the Pd monolayer on both Mo( 100) and Ta( 110). In both cases a reduced sticking coefficient and a reduction in the hydrogen chemisorption bond strength was observed. A low temperature desorption state attributed to a hydride species was also detected. The amount of H2 desorbing from the hydride state increased with increasing Pd film thickness. NO adsorption on the Pd monolayer on Ta(l 10) is molecular at 95 K. Heating the surf ace causes desorption and decomposition of NO and the reaction . products N2O and N2 are observed to desorb. A similar reactivity is seen for Ag(l 11) surfaces. An N2O3 species is also formed for very large NO exposures. Acetylene cyclotrimerization to benzene has been observed to occur on the Pd monolayer on Ta(l 10). A flat lying benzene species is formed which desorbs at 400 K. This is the same temperature at which benzene desorption occurs following benzene exposures suggesting that the desorption is not reaction rate limited. Studies of thicker Pd films show that there is some ability to "tune" the chemistry of the surface by controlling the Pd film thickness. Bulk Pd properties are observed for Pd films greater than five layers thick.

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