Role of Support in the Selective, Aerobic Methane Oxidation to Formaldehyde over Pt/TiO2

The direct aerobic oxidation of methane in the presence of liquid water over platinum-based catalysts at 220 degrees C offers an interesting opportunity to selectively produce formaldehyde. The turnover frequency over Pt/TiO2-P25 for methane conversion under trickle bed conditions is ca. 40-50% lower than the TOF over Pt/TiO2-Rutile, which is attributed to the difference in the hydrophobicity of the support material. The product selectivity is even more strongly influenced by the support material: Pt/TiO2-Rutile shows a high selectivity for the formation of formaldehyde (similar to 90%), whereas changing the support to TiO2-P25 resulted in the formation of some methanol and methoxymethanol with CO2 as the major product. Methanol can be formed over basic sites on the support. The formation of CO2 is attributed to the conversion of primarily formed formaldehyde or methanediol over acid sites on the support. Furthermore, strong catalyst deactivation is observed, which is attributed to the buildup of polymeric species on the surface catalyzed by the presence of basic sites on the catalyst.