Study of the elementary processes involved in the selective oxidation of methane over MoOx/SiO2

Isolated molybdate species supported on silica are reported to have the highest specific activity and selectivity for the direct oxidation of methane to formaldehyde. The present investigation was undertaken to understand the elementary redox processes involved in the formation of formaldehyde over such species. A MoOx/SiO2 catalyst was prepared with a Mo loading of 0.44 Mo/nm(2). On the basis of evidence from extended X-ray absorption fine structure (EXAFS) and. Raman spectroscopy, the Mo atoms in this catalyst are present as isolated, pentacoordinated molybdate species containing a single Mo=O bond. Isotopic labeling experiments in combination with in-situ Raman spectroscopy were used to examine the reducibility of the dispersed molybdate species and the exchange of O atoms between the gas phase and the catalyst. It was established that treatment of MoOx/SiO2 at 873 K under pure methane reduces the dispersed molybdate species to only a limited extent and results mainly in the deposition of amorphous carbon. During CH4 oxidation to formaldehyde, the catalyst undergoes only a very small degree of reduction and typically only similar to 50-500 ppm of Mo-VI is reduced to Mo-IV. Reactions carried out using CH4 and O-18(2) show that there is extensive scrambling of O atoms between the species in the gas phase and the catalyst. Additional experiments revealed that H2O formed in the reaction is the principal species responsible for the exchange of O atoms between the gas phase and the SiO2 support. Low concentrations of H2O were observed to enhance the activity of MoOx/SiO2 for CH4 oxidation to formaldehyde. A mechanism for the oxidation of CH4 over MoOx/SiO2 was formulated in light of the observations made here and is discussed in the light of previous studies. It is proposed that peroxides are produced by the reaction of O-2 with a small concentration of reduced molybdate species and that the reaction of CH4 with these peroxide species leads to the formation of formaldehyde. The proposed mechanism also accounts for the positive effects of low concentrations of H2O on the rate of formaldehyde formation.