Advances in hydraulic fracturing have led to a surplus of natural gas. Methane is the main component of natural gas and, according the Environmental Protection Agency, is a potent greenhouse gas and the second most prevalent emitted . With the expected emergence of more methane regulations in the coming years, a renewed interest in understanding methane activation for conversion to value added products is underway. Partial oxidation of methane (POM) is an attractive alternative to methane steam reforming due to it being mildly exothermic (ΔHreaction = -35.9 kJ mol-1) and producing syngas as reactants for the Fischer-Tropsch and methanol synthesis reactions [2,3]. POM has been extensively investigated on noble metals such as Rh, Pt, and Pd or cheaper alternative such as Ni . There is considerable debate as to whether CO and H2 are formed directly or indirectly through combustion to CO2 and H2 then reforming [2,4]. The literature suggests the mechanism depends on the nature of the active sites [5,6].
In this study, to gain further insight on these active sites, the rate of reaction was related to Pd crystallite size, geometry and ease of oxygen vacancy formation, to determine where on the catalyst methane was activated. Pd supported on Al2O3 and rutile TiO2 were investigated. Pd/Al2O3 initially shows a decrease in the reaction rate with increasing crystallite size but then after about 4 nm, an increase in rate is noted with further particle size increase. This can be associated with the lack of oxygen vacancies on Pd caused by stronger interactions with oxygen for smaller particles . A decline in the rate is seen on Pd/TiO2 with increasing crystallite size. Since TiO2 is easily reducible, the Pd-O bond is weaker allowing more oxygen vacancies at the edge of the Pd particle. With oxygen vacancies predominately at the Pd-TiO2 interface, the rate of methane consumption tends to scale with the particles perimeter as suspected. The availability of these oxygen vacancies plays the critical role in allowing methane to be activated on Pd.
(1) Methane and Nitrous Oxide Emissions from Natural Sources; U.S. Environmental Protection Agency; Washington, DC, 2010.
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