Catalytic decomposition of formic acid on Ru(001): Transient measurements

Abstract

The kinetics of decomposition of formic acid on Ru(001) have been investigated with thermal desorption mass spectrometry following the adsorption of DCOOH and HCOOH at both 130 and 305 K. Formic acid chemisorbs dissociatively on the perfect (001) surface via O–H bond cleavage to form a formate and a hydrogen adatom, and at defect (step) sites on the surface via C–O bond cleavage to form CO, a hydrogen adatom and a hydroxyl. The saturation fractional coverage of the dissociatively chemisorbed formic acid is approximately 0.33 following adsorption at 130 K, and approximately 0.49 following adsorption at 305 K. Thermal desorption spectra of CO2 are a direct probe of the kinetics of C–H bond cleavage of the formate and are characterized by a kinetic isotope effect (C–H vs C–D bond cleavage), a progressively narrowing peak width, and an upward shift in peak temperature with increasing surface coverage of the formate. The latter indicates an increasing apparent activation energy for the C–H bond cleavage reaction with increasing formate coverage, which results in self-accelerating decomposition kinetics that cause the observed sharp peak in the production of CO2. Although the activation energy for the C–H bond cleavage reaction varies strongly with the formate coverage, the relative yields of CO2 and CO are independent of the formate coverage and are in a ratio of 1:1. This observation, together with the observation that the formation of HDO and D2O, which results from C–O bond cleavage of the formate, appears at approximately the same temperature as that of CO2 formation following saturation adsorption of DCOOH at both 130 and 305 K, prove that cleavage of the C–D and C–O bonds of the formate are not two independent reaction pathways. A novel mechanism for the reaction is postulated which involves the hydrogen from the C–H bond cleavage reaction in one formate (producing carbon dioxide) reacting with an adjacent formate to form a hydroxyl and formyl. The latter react to form the other observed products: carbon monoxide, and a mixture of water and molecular hydrogen, with some chemisorbed oxygen remaining on the surface. The increase in the activation energy for decomposition of the formate with increasing formate coverage is due to the fact that a chemisorbed formate is stabilized by the presence of another nearby formate.