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Abstract:
The reactivity of SiO2-supported MoOx, for the selective oxidation of methane was investigated, with the objectives of understanding the processes limiting the single-pass formaldehyde yield obtainable with this catalyst, determining the reducibility of MoOx/SiO2 and the mechanism by which isolated MoOx on SiO2 interacts with CH4/O2 mixtures, and understanding the effects of various operating conditions on the catalytic structure and performance. The kinetics of CH4 oxidation and of CH2O decomposition were measured independently for the gas phase, the exposed SiO2 support, and the MoOx/SiO2 catalyst to isolate the contributions of the individual reactive phases to the observed rates of CH2O formation and consumption. It was determined that CH2O formation occurs almost exclusively on the supported MoOx moieties, whereas the decomposition of CH2O occurs to an appreciable extent over both the MoOx moieties and the exposed SiO2 of MoO x/SiO2, and results from both oxidative and non-oxidative decomposition processes. A kinetic model assembled from the individual rates of reaction of each of the loci of reaction was utilized to interpret the effects of the operating conditions on the catalytic performance. The reducibility of MoOx/SiO2 by H2 and CH4 and the level of reduction during steady-state oxidation of H2 and CH4 were investigated using in-situ Raman spectroscopy, in-situ XANES, and pulsed O 2 chemisorption. Under H2 at 920 K, the level of reduction is 2 electrons per Mo (MoVIOx → MoIV Ox-1), but under CH4 the level of reduction is << 2 electrons per Mo, and amorphous carbon is deposited on the catalyst without significantly reducing the supported MoOx. 18O is incorporated directly into the surface oxygen of the exposed SiO2 support during re-oxidation of H2-reduced MoOx/SiO 2 by 18O2. A mechanism involving formation of the peroxide MoVIOx+1 and its propagation across the SiO2 surface was developed to explain the re-oxidation of two SiO2-isolated MoIVOx-1 by O2. Kinetic analysis of 18O exchange rates reveals that this propagation mechanism does not occur during steady-state oxidation of H2 or CH4, and it is concluded that the peroxide MoVIO x+1 is responsible for the oxidation of CH4 to CH2O. Co-feeding of small feed fractions of H2O (1%) increased the rate of reaction, suggesting that the reaction of surface hydroxide with surface methoxide is a rate-limiting step in CH4 oxidation over MoO x/SiO2. However, co-feeding of large fractions of H 2O (50%) was found to result in catalyst instability and poorer CH 2O selectivity at a fixed conversion.
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