Steady‐state rate of decomposition of formaldehyde on Ru(001)

Abstract

The steady‐state, absolute rate of decomposition of formaldehyde on the Ru(001) surface has been measured as a function of surface temperature from 310 to 750 K and for reactant pressures between 9.7×10^(−8) and 1.6×10^(−6) Torr. The only observed reaction products were carbon monoxide and hydrogen. Postreaction Auger spectroscopic analysis revealed a clean surface. Two distinct kinetic regimes are observed. At relatively low surface temperatures (and/or high formaldehyde pressures), the rate is zero order in formaldehyde pressure with an apparent activation energy of 23±0.5 kcal/mol and an apparent preexponential factor of 4×10^(11) s^(−1). The rate becomes flux limited and first order in formaldehyde pressure with an apparent activation energy of zero at relatively high surface temperatures (and/or low formaldehyde pressures). Perdeutero‐formaldehyde decomposes on Ru(00l) with the same kinetic parameters as HCHO. The lack of a kinetic isotope effect indicates that C–H bond cleavage is not the rate‐limiting step under these experimental conditions. In the low‐temperature regime, the surface is saturated with CO, and the rate is CO desorption limited. Dissociative chemisorption of formaldehyde is irreversible since no isotopic exchange products were observed by reacting a mixture of HCHO and D2 with a molar ratio of 1:4 at a total pressure of 1×10^(−6) Torr.