Quantification of multilayer-induced dissociative chemisorption: cyclobutane on Ru(001)

Multilayer-induced dissociative chemisorption of cyclobutane on Ru(001) has been quantified using temperature programmed desorption (TPD). Monolayer and submonolayer coverages of cyclobutane desorb essentially completely, with negligible dissociative chemisorption upon heating. The adsorption of cyclobutane multilayers, however, results in significant dissociative chemisorption. By varying the initial coverage and the TPD ramp rate, we are able to change the maximum temperature at which multiple layers remain on the surface, thus altering the extent of reaction. From these measurements we are able to quantify the kinetics of the reaction. We find that the reaction kinetics are well described by a rate expression which is first order in the coverage of unreacted cyclobutane in the first layer, and whose rate coefficient can be expressed in Polanyi-Wigner form with a pre-exponential factor of 5.0 x 10(10) s(-1) and an activation energy of 7510+/-500 cal/mol. The latter represents a significant decrease from the barrier of 10 090 +/- 180 cal/mol measured previously for the trapping-mediated dissociative chemisorption of cyclobutane in the submonolayer regime. This reduction in activation barrier has potential implications to the study of catalytic reactions at the solid-liquid interface, which are discussed. (C) 1999 Published by Elsevier Science B.V. All rights reserved.