Transition metals for catalytic oxidation reactions often form oxides under environmental reaction conditions. One example is palladium, where the PdO(101) facet recently has been suggested to be the active phase for methane oxidation [1]. In this study [2], we have explored the mechanism for oxidation of methane to carbon dioxide and water over the PdO(101) surface. This was done using a first-principles kinetics approach, where the kinetic parameters are obtained by applying density functional and transition state theory. The HSE06 functional was used to describe the electronic exchange and correlation, as hybrid functionals were found to be more accurate than gradient-corrected functionals. The kinetic model was constructed on the basis of circa 80 elementary reactions. Due to the presence of strong attractive lateral interactions, spatial correlations between adsorbates needed to be taken into account beyond the mean-field approximation. The results of the kinetic modeling confirm the common assumption that the first dissociation reaction is the main (though not sole) rate limiting step, where both energetic and entropic contributions create a large free energy barrier for this reaction. The resulting CH3 species is converted to CO2 through a series of surface intermediates such as CH2, CH2O, CHO and CO (see Figure 1). Incorporation of oxygen into the molecule mainly occurs via a Mars – van Krevelen mechanism. The reduced oxide is then reoxidized by O2 adsorption and dissociation. Another finding is that, at low temperatures and in the presence of water vapor, the process is hindered by adsorbed H2O. This is in agreement with the negative reaction order in water pressure observed experimentally under such conditions. [1] Hellman A., et al., J. Phys. Chem. Lett. 2012, 3, 678. [2] Van den Bossche M., Grönbeck H. J. Am. Chem. Soc. 2015, 137, 37.

First-principles microkinetic modeling of methane oxidation over PdO(101)

251st National Meeting (2016)

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