Following Molecules through Reactive Networks: Surface Catalyzed Decomposition of Methanol on Pd(111), Pt(111), and Ni(111)

We present a model of the surface kinetics of the dehydrogenation reaction of methanol on the Pd(111), Pt(111), and Ni(111) metal surfaces. The mechanism consists of 10 reversible dehydrogenation reactions that lead to the final products of CO and H-2. The rate coefficients for each step are calculated using ab initio transition state theory that employs a new approach to obtain the symmetry factors. The potential energies and frequencies of the reagents and transition states are computed using plane wave DFT with the PW91 exchange correlation functional. The mechanism is investigated for low coverages using a global sensitivity analysis that monitors the response of a target function of the kinetics to the value of the rate coefficients. On Pd(111) and Ni(111), the reaction COH --> CO + H is found to be rate limiting, and overall rates are highly dependent upon the decomposition time of the COH intermediate. Reactions at branches in the reaction network are also particularly important in the kinetics. A stochastic atom-following approach to pathway analysis is used to elucidate both the pathway probabilities in the kinetics and the dependence of the pathways on the values of the key rate coefficients of the mechanisms. On Pd(111) and Ni(111) there exists significant competition between the pathway containing the slow step and faster pathways that bypass the slow step. A discussion is given of the dependence of the model target's probability density function on the chemical pathways.