Stability, structure, and oxidation state of Mo/H-ZSM-5 catalysts during reactions of CH 4 and CH 4–CO 2 mixtures

Abstract

Mo 2 O 5 2 + -ZSM-5 (Mo/Al f=0.4, Si/Al f=20) samples prepared by sublimation of MoO 3 were carburized in CH 4 to form MoC x clusters active in CH 4 pyrolysis and then exposed to different CO 2/CH 4 mixtures. CO 2/CH 4 reactant ratios between 0 and 0.1 increased catalyst stability but decreased pyrolysis rates, and ratios above 0.1 led to a sudden loss of activity that was reversed after removal of CO 2. Below CO 2/CH 4 ratios of 0.1, the catalyst bed can be described as a CO 2-reforming and pyrolysis reactor in series. In the first segment of the bed, where CO 2 is present, pyrolysis is completely suppressed by reverse Boudouard reactions; pyrolysis reactions begin after CO 2 is completely consumed. CO 2 cannot directly influence rates or deactivation for pyrolysis reactions. Rather, the greater stability observed with CO 2-containing reactants arises solely from the presence of H 2, formed in the CO 2-reforming section, in the pyrolysis regions within the catalyst bed. The evolution of catalyst structure and composition in CO 2/CH 4 reactants was also probed by mass spectrometric analysis of effluent streams and by in situ X-ray absorption spectroscopy to determine the underlying processes responsible for reversible deactivation at CO 2/CH 4 ratios greater than 0.1. MoC x -ZSM-5 samples exposed to CO 2/CH 4 streams with 0.022 and 0.055 ratios at 950 K acquire 0.3 ± 0.01 and 1.75 ± 0.03 O-atoms/Mo, respectively. X-ray absorption edge energies in MoC x -ZSM-5 increased from 0.2 to 1.9 eV (relative to Mo 0) after contact with 0.025 CO 2/CH 4 mixtures at 950 K for 1 h, indicating that oxidation of some Mo centers occurs. These spectral changes occurred concurrently with the detection of pre-edge features typical of MoO x structures. Radial structure functions resemble those for samples exposed to pure CH 4, which consist of 0.6-nm MoC x clusters, but show an additional Mo O coordination shell also detected in bulk β-Mo 2C exposed to ambient air. These data suggest that the inhibition and ultimate suppression of catalytic pyrolysis reactions with CO 2 addition reflect the oxidation of active MoC x structures, the extent of which increases with increasing CO 2/CH 4 reactant ratios. CO 2/CH 4 reactant ratios above 0.1 lead to conversion of MoC x to MoO x structures, which are inactive for both reforming and pyrolysis reactions of CH 4, but which reform active MoC x after an induction period when exposed to pure CH 4 reactants at reaction conditions.