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| - Study of the Elementary Processes Involved in the Selective Oxidation of Methane overMoOx/SiO2
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| - Isolated molybdate species supported on silica are reported to have the highest specific activity and selectivityfor the direct oxidation of methane to formaldehyde. The present investigation was undertaken to understandthe elementary redox processes involved in the formation of formaldehyde over such species. A MoOx/SiO2catalyst was prepared with a Mo loading of 0.44 Mo/nm2. On the basis of evidence from extended X-rayabsorption fine structure (EXAFS) and Raman spectroscopy, the Mo atoms in this catalyst are present asisolated, pentacoordinated molybdate species containing a single MoO bond. Isotopic labeling experimentsin combination with in-situ Raman spectroscopy were used to examine the reducibility of the dispersedmolybdate species and the exchange of O atoms between the gas phase and the catalyst. It was establishedthat treatment of MoOx/SiO2 at 873 K under pure methane reduces the dispersed molybdate species to onlya limited extent and results mainly in the deposition of amorphous carbon. During CH4 oxidation toformaldehyde, the catalyst undergoes only a very small degree of reduction and typically only ∼50−500ppm of MoVI is reduced to MoIV. Reactions carried out using CH4 and 18O2 show that there is extensivescrambling of O atoms between the species in the gas phase and the catalyst. Additional experiments revealedthat H2O formed in the reaction is the principal species responsible for the exchange of O atoms between thegas 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 wasformulated in light of the observations made here and is discussed in the light of previous studies. It isproposed that peroxides are produced by the reaction of O2 with a small concentration of reduced molybdatespecies and that the reaction of CH4 with these peroxide species leads to the formation of formaldehyde. Theproposed mechanism also accounts for the positive effects of low concentrations of H2O on the rate offormaldehyde formation.
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