Adsorption and Oxidation of n-Butane on the Stoichiometric RuO2(110) Surface

We investigated the adsorption and oxidation of n-butane on a stoichiometric RuO2(110) surface using temperature-programmed reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. At low coverage, molecularly adsorbed n-butane achieves a binding energy of similar to 100 kJ/mol on RuO2(110), consistent with a strongly bound sigma-complex that forms through dative bonding interactions between the n-butane molecule and coordinatively unsaturated (cus) Ru atoms. We find that a fraction of the n-butane reacts with the RuO2 surface during TPRS to produce CO, CO2, and H2O that desorb above similar to 400 K and present evidence that adsorbed sigma-complexes serve as precursors to the initial C-H bond cleavage and ultimately the oxidation of n-butane on RuO2(110). From measurements of the product yields as a function of surface temperature we estimate that the initial reaction probability of n-butane on RuO2(110) decreases from 9% to similar to 4% with increasing surface temperature from 280 to 300 K and show that this temperature dependence is accurately described by a precursor-mediated mechanism. From kinetic analysis of the data we estimate a negative, apparent activation energy of -35.1 kJ/mol for n-butane dissociation on RuO2(110) and an apparent reaction prefactor of 6 x 10(-8). The low value of the apparent reaction prefactor suggests that motions of the adsorbed n-butane precursor are highly restricted on the RuO2(110) surface. DFT calculations confirm that n-butane forms strongly bound sigma-complexes on RuO2(110) and predict that C-H bond cleavage is strongly favored energetically. The n-butane binding energies and energy barrier for C-H bond cleavage predicted by DFT agree quantitatively with our experimental estimates.