(85d) Theoretical Insights Into the Catalytic Oxidation of Methane Over Pt and Rh Surfaces
The catalytic conversion of natural gas into energy, liquid fuels and chemicals via reforming, partial oxidation and combustion processes occur through similar elementary C-H and O2 bond activation steps which differ only in the nature of the active sites, whether or not oxygen is present and the local surface coverage of oxygen that result under operating conditions. The prevailing chemistry is ultimately controlled by the surface coverage and reactivity of chemisorbed oxygen. Experimental results for the partial oxidation of methane over supported Pt clusters, for example, reveal the presence of four different kinetic regimes which can be described by unique rate expressions that result for methane over the bare, low oxygen-covered, high-covered, and fully-covered metal surfaces. First-principle density functional theoretical calculations are used here to elucidate the elementary C-H and O2 activation steps at different sites over both model Pt and Rh surfaces to establish the influence of surface coverage on the activity to form CO and CO2 over different transition metal surfaces. The results are used to understand the diverse experimental kinetic regimes in methane partial oxidation and to elucidate the reactivity of other C1 and C2 intermediates, such as those derived from methanol or dimethyl ether over Pt as well as other transition metals.
The results reveal that oxygen can take on oxygen can influence the reactivity of methane in various different ways. The specific metal used to carry out the reaction as well as the thermodynamic processing conditions ultimately dictate the surface coverage that exists under working conditions which thus controls the nature of the active sites that form and their resulting reactivity.
The calculations results found here show that in the absence of oxygen, the rate is governed by the initial activation of the C-H bond of methane which involves a late three-center transition state characteristic of oxidative addition of C-H bonds to metals. The activation barriers for methane, as well as for reactions of subsequent CHx products that form, were linearly correlated with the carbon binding energies, as expected from bond-order conservation principles. Oxygen ultimately reacts with CHX surface intermediates to form CO. Comparisons between theory and experiment suggests that the calculated barriers for methane activation over Ru and Rh are typically smaller than those measured, apparently because of structural changes in the metal during reaction or the possible formation of unreactive sub-surface carbon on Rh and Ru.
At low oxygen surface coverages, the C-H activation barrier for methane and for other CHx intermediates increase slightly as the result of repulsive interactions between the CHX--H moiety and chemisorbed oxygen in the transition state. The C-H activation barrier, however, decreases significantly, however, as oxygen coverage increases. The adsorbed oxygen becomes more basic as the surface coverage increases; this effectively decreases the barrier for C-H activation. The decrease in the barrier with increasing oxygen coverage coincides with the increase in the rate of methane activation found experimentally at intermediate oxygen coverages which balances the presence of reactive oxygen and available metal sites. At higher oxygen coverages, the O*-O* pair becomes the most abundant surface species. Methane activation barriers are higher on O*-O* pairs than on *-O* site pairs, consistent with the lower C-H activation over stoichiometric transition metal oxides. selectivity The reaction proceeds by a characteristically different transition state.. The rate of reaction of the CO that forms on the surface with oxygen is found to be orders of magnitude higher than the rate of CO desorption as such any CO that forms is rapidly converted to CO2 in the presence of oxygen.
The results for Pt and Rh are characteristically different in that Rh tends to form oxides. The barriers to activate over the O*-O* pairs on Rh are significantly smaller than on Pt. The results between the two metals are comparied with one another.