A Study of Catalytic Reactions on Semiconductors: Hydrogen Deuterium Exchange and Formic Acid Decomposition on Chemically-Doped Germanium

Summary: Rates and activation energies for the hydrogen-deuterium exchange and formic acid vapor decomposition were measured on a series of chemically-doped germanium catalyst over the temperature range 100° to 400°. The germanium catalyst were intrinsic, and n or p-type extrinsic semiconductors; the position of the Fermi level of the solid was located suitably on the forbidden energy gap.

Kinetic parameters of the exchange and decomposition reactions were related to the Fermi level (electronic chemical potential) of the solid. Their dependence suggested that the rate-limiting processes involved electronic charge shifts between adsorbate and semiconductor.

Two different rate processes appeared to limit the hydrogen-deuterium exchange. A process involving an electron shift from adsorbate to solid appeared to control the rate in the region of n-type semiconductivity. In the region of p-type semiconductivity, the rate-determining process appeared to be an electron shift in the opposite direction (from solid to absorbate).

Dehydrogenation and dehydration of formic acid on germanium were observed. Dehydrogenation was the predominant method of decomposition on p-type germanium; however, dehydrogenation on n-type germanium could not be detected. A process involving an electron shift from adsorbate to solid appeared to control the dehydrogenation reaction.

Dehydration of formic acid occurred on all germanium catalyst, and appeared independent of the Fermi level of the solid.

Both dehydrogenation and dehydration of formic acid appeared to be primary decomposition processes at the lower temperatures. There was evidence, however, that the water gas equilibrium affected the distribution of products at some of the higher temperatures that were used.

Nonstoichimetry of dehydrogenation products in the gas phase over p-type germanium was detected, and seemed to be best explained by the removal of hydrogen atoms by the germanium during the process of formic acid decomposition.