The kinetics and mechanism of the autocatalytic decomposition of HCOOH on clean Ni(110)

Abstract

The flash decomposition of DCOOH was studied on a clean nickel (110) surface following adsorption at 37°C. The reaction proceeded by a two-dimensional autocatalytic mechanism to form D 2, CO 2 and CO products. The results indicated DCOOH adsorbed dissociatively at 37°C by splitting off H 2O and forming an adsorbed molecule composed of DCO and DCOO. Above ten percent of saturation coverage these molecules formed a condensed phase or island structure. The decomposition of the molecules was rate determining for the formation of CO 2 and D 2 products. Theoretical calculations for branched chain mechanisms and coadsorption experiments with CO and H 2 separately with DCOOH indicated the intermediate involved in the explosion was not associated with the observed product molecules. The intermediate in the explosive decomposition was shown by interrupted flashes to be stable at 37°C. The autocatalytic flash decomposition curves were explained by reaction occurring at bare metal sites within the islands, and as product molecules desorbed the number of sites increased, causing the rate to accelerate. The rate of decomposition was well described by the equation Rate = −k( c c I )(c I −c + fc I) , where c is the surface concentration, c I is the initial surface concentration, and f is the density of initiation sites. The activation energy of 26.6kcal/mol was determined from heating rate variation. The narrow flash curves were fit with a first order pre-exponential factor of 1.6 × 10 15 sec −1 with a density of initiation sites of 0.004.