In Situ X-ray Absorption Near-edge Structure (xanes) spectroscopic Investigation Of The Pre-reduction Of Iron-based catalysts For Non-oxidative Alkane Dehydrogenation

摘要

The reduction in a methane atmosphere of two as-prepared ferric oxide catalysts for the non-oxidative dehydrogenation of alkanes has been investigated by in situ X-ray absorption near-edge structure (XANES) spectroscopy using a novel X-ray transmission reaction cell. The two catalysts were prepared by different synthesis methods (incipient wetness and nanoparticle impregnation) and were supported on Al-substituted magnesium oxide obtained by decomposition of a synthetic hydrotalcite. The reduction of the ferric oxides by methane was followed by iron XANES spectroscopy at temperatures up to 650 ℃ complemented by a residual gas analyzer (RGA) used to track changes in the product gas. Results showed that the ferric oxides in the two catalysts underwent a stepwise reduction to first ferrous oxide, releasing mainly H_2O in the case of the nanoparticle catalyst but H_2 and CO in the case of the incipient wetness formulation at temperatures between 200 and 550 ℃, and then more slowly to metallic iron at higher temperatures. Reaction of the ferrous oxide with the support to form magnesiowiistite also occurred in conjunction with the reduction. This in situ investigation confirms that metallic iron is the active catalytic phase for alkane dehydrogenation and that observations of ferric iron in samples investigated at room temperature after reduction and reaction are most likely due to re-oxidation of the iron in the catalyst upon exposure to air rather than incomplete reduction of the original ferric iron in the catalyst.