Iron pentacarbonyl decomposition over grafoil: II. Effect of sample outgassing on decomposition kinetics

Abstract

Mössbauer spectroscopy and volumetric gas phase measurements have been used to study the thermal decomposition (at 383 K) of Fe(CO) 5 on Grafoil subjected to different outgassing pre-treatments. Mössbauer spectroscopy showed that only two iron-containing surface species exist during the entire decomposition process: Fe(CO) 5 and metallic iron particles. Furthermore, the Mössbauer spectra suggest that the metallic iron particles on the surface have a high surface to volume ratio, consistent with the presence of thin metallic rafts. These findings are supported by volumetric gas phase measurements which show that following complete decomposition of Fe(CO) 5, one molecule of CO is adsorbed on the sample for every two atoms of metallic iron deposited on the Grafoil. Kinetic measurements indicate that after the initial stages of reaction, the decomposition is a first-order process. This is interpreted in terms of the nucleation and growth of metallic iron particles at edges, steps or other defects on the Grafoil surface. The rate of decomposition increased as the outgassing temperature was increased up to 773 K. Concomitant consideration of Mössbauer and kinetic results suggests that outgassing increases the reactivity of the graphite surface not by significantly increasing the number of nucleation and growth sites but rather by altering the reactivity of existing sites and/or the mobility of Fe(CO) 5 on Grafoil.