In situ characterization of adsorbed species on methanol synthesis catalysts by FT-IR spectroscopy
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Transmission infrared spectroscopy was used to characterize adsorbed species on methanol synthesis catalysts during reaction conditions. A copper carbonyl, bidentate formate, and methoxy species were identified as stable surface groups. An adsorbed formaldehyde species was unstable at the reaction temperature, but could be observed on the catalyst surface at the beginning of the reaction. Surface species were very similar for feed mixtures of (1) carbon monoxide and hydrogen, (2) carbon monoxide, carbon dioxide, and hydrogen, and (3) formic acid and hydrogen. The role of copper in methanol synthesis catalysts was to increase the adsorption of carbon monoxide to form a linear carbonyl species. This carbonyl promoted the hydrogenation of formate groups. The formate species was adsorbed on a zinc site (Zn(,(beta))) different from the zinc site (Zn(,(gamma))) on which formaldehyde and methoxy groups were adsorbed. The rate-determining step in methanol synthesis was determined to be the reaction of hydrogen from a hydroxyl species adsorbed on another zinc site (Zn(,(alpha))) with a methoxy group to yield methanol. It was established that at the experimental conditions;used in this study, the methanol synthesis reaction was far from equilibrium while the water-gas shift reaction was near equilibrium; ('1)DOE Report IS-T-1121. This work was performed under Contract No. W-7405-Eng-82 with the U.S. Department of Energy.