Reactions and reaction intermediates on iron surfaces: I. Methanol, ethanol, and isopropanol on Fe(100)

Abstract

The reactions of methanol, ethanol, and isopropanol on an Fe(100) surface were studied using temperature-programmed reaction spectroscopy (TPRS) and X-ray photoelectron spectroscopy (XPS). Methanol and ethanol readily formed alkoxy intermediates at or below room temperature by loss of the hydroxyl hydrogens. These alkoxys reacted above 400 K in three ways: (i) complete decomposition to CO and hydrogen, (ii) rehydrogenation to the alcohol, and (iii) scission of the C C or CO bonds with hydrogenation of the hydrocarbon fragment. All these reactions appeared to occur simultaneously and proceeded with first-order kinetics. The first-order rate constants for reactions of the methoxy and ethoxy species were determined to be k CH 3O = (4 × 10 12) exp(−105 kJ/mole/RT)s −1 k CH 3CH 2O = (8 × 10 13) exp(−111 kJ/mole/RT)s −1, respectively. Isopropanol reacted differently from either methanol or ethanol; it did not readily form a stable alkoxy intermediate. The difference in the reactions of primary and secondary alcohols was suggested to arise from steric interference of the methyl groups with the surface. These results were consistent with the results of Kummer and Emmett, which showed alcohol-related intermediates were important in Fischer-Tropsch synthesis. These observations also showed that alkoxy intermediates must be considered as possible routes in the Fischer-Tropsch synthesis.