Mechanism of dehydration decomposition of formic acid over alumina and silica. Adsorption measurements during the catalytic reaction

Abstract

The adsorption and the reaction rate were studied during the course of the decomposition of formic acid on alumina and silica catalysts and the types of chemisorbed species, their surface concentrations and also the partial pressures of each gases were determined. Accordingly, the dependence of the overall rate upon the activities of each chemical species was examined. The decomposition of formic acid on alumina took place in two stages; first, carbon monoxide is evolved with a considerable rate, probably at the Lewis acid site on the surface; the site was readily destroyed by the strong adsorption of water produced. The rate r of carbon monoxide evolution in the first stage was given by r=kP_HCOOH(1–a[H₂O]_ads). Secondly, in the stationary state of the reaction, the decomposition mainly took place between formic acid molecule and the surface protons, which were supplied from the dissociative adsorption of formic acid and the rate is expressed by r=k′[H⁺]P_HCOOH/(1+b[H₂O]_ads). The decomposition of formic acid on a silica gel was also studied; the reaction takes place between formic acid molecule and the surface OH group, originally retained on the silica gel.