Experimental and theoretical evidence suggests that certain active oxygen species can facilitate methane (CH4) activation and lower the required reaction temperature, but the nature of these species remains ambiguous.1,2. Here, we present insight into the nature of these active oxygen species that we obtained from periodic DFT calculations used to study the role of the oxidizing agent during CH4 activation on selected transition metal surfaces. We compared the activation energies of CH4 dissociation with and without assistance of O, OH and OOH on Ni(211), Pd(211) and Au(211). Multiple co-adsorption conformations were considered to identify the most favored pathway for each reaction. The DFT results indicated that the oxidizing agents are most significant on noble metals and lower the activation energy of CH4 activation on Au(211). This conclusion is confirmed by a double-optimal volcano plot based on microkinetic modeling and linear scaling relations, in which the second volcano peak is located in the nobel metal region. The main volcano peak remains close to traditional nickel catalyst, which favors methane direct dissociation.
The reaction pathways towards different products is investigated in more detail on Au(211), which was considered as potential catalyst candidate for selective CH4 oxidation based on our initial considerations. The selectivity toward dimethyl ether and methanol is explained, and the resistance of formation of CO2 is discussed. These detailed investigations on the active oxygen species involved in CH4 activation can be used to guide the design of new catalysts and processes that can convert CH4 selectively into higher value products under mild reaction conditions.
(1) Lunsford, J. H.; Angew. Chem. Int. Ed. 1995, 34, 970-980.
(2) Bradford, M. C. J.; Vannice, M. A.; Catal. Rev. Sci. Eng. 1999, 41, 1-42.