Mechanism of formic acid decomposition over dehydrogenation catalysts

Abstract

The mechanism of dehydrogenation of formic acid over zinc oxide and magesium oxide was studied by adsorption measurements and the infra-red technique during the course of the decomposition. The acid was absorbed into the bulk of the oxides to form metal formate and water, and the subsequent decomposition proceeds over the formate surface. Consequently, no differences in the reaction order and the catalytic activity were observed in the decomposition over oxide and formate catalysts. The decomposition of, and the exchange reaction with, DCOOD over Zn(HCOO)₂ revealed that it is only the formate ion on the surface which participates in the decomposition and exchange reaction. Also, the hydrogen produced from the decomposition of HCOOD was solely HD, no H₂ nor D₂ being involved. The rate does not depend upon the pressure of formic acid, and if formic acid is condensed out of the ambient gas, the evolution of hydrogen ceased, but the rate of evolution of carbon dioxide remained constant. It was concluded that the decomposition proceeds via the surface formate ion; carbon dioxide is evolved via the decomposition of formate ion on the surface, and hydrogen atom remaining on the surface is then attacked by a formic acid molecule to produce molecular hydrogen and leaves the formate ion on the surface.