Pulsed laser induced excitation of metal surfaces: Application as a probe of surface reaction kinetics of methanol on Ni

Abstract

Abstract We have studied the decomposition of methanol on Ni surfaces using a new technique, temperature programmed pulsed-laser-induced-desorption. The pulsed laser induces a temperature jump in a spatially well defined zone of the surface itself; the heat remains at the surface for less than 0.1 μs. We find that the usual decomposition reactions do not occur on this time scale. At T surface = 100 K, methanol is desorbed intact and the decomposition step as well as the appearance of the stable products, H 2 and CO, can be followed in real time as the surface is warmed. Unlike conventional temperature programmed reaction spectroscopy, this technique can be used to follow the course of surface reactions below the temperatures required for product desorption. With a sample heating rate of 6 K s −1 , the maximum CH 3 OH decomposition rate occurs at 240 K. This indicates that the energy barrier to decomposition is approximately 14 kcal mol −1 , assuming a pre-exponential factor of 1 × 10 13 s −1 . The appearance of both CO and H 2 follows similar kinetics suggesting that the initial decomposition step is rate limiting under these conditions. At slower sample heating rates, individual reaction steps can be resolved. A quantitative model of the laser induced desorption process is developed from separate studies of CO desorption from Ni. The spatial properties of the laser beam must be included in order to interpret the results accurately. The model is used to evaluate the kinetics of more complex laser induced surface reactions such as the recombinatory desorption of hydrogen and the decomposition of methanol. The kinetic variables obtained for the desorption of hydrogen are ν = 0.4 cm 2 s −1 and E d = 23 kcal mol −1 . Implications on the kinetics and mechanism of methanol decomposition are discussed.