In Situ FTIR Spectroscopic Study of 2-Propanol Adsorptive and Catalytic Interactions on Metal-Modified Aluminas

Pure alumina was modified by additives of Ni, Rh, and Pt metal particles by impregnation from aqueous solutions of corresponding precursor compounds and subsequent reduction in a stream of hydrogen at 400 °C for 2 h. The catalysts thus obtained were subjected to N2, H2, and O2 sorptometry, high-resolution electron microscopy, and X-ray photoelectron spectroscopy. Accordingly, the metal reducibility, surface area, dispersion, and particle size were determined. Thin wafers of the catalysts, pretreated in situ in a specially designed IR reactor/cell, were exposed to 2-propanol vapor at various temperatures (room temperature to 400 °C) for 10 min, and IR spectra of the gas-phase and adsorbed species were measured. The results indicated that the metal additives provided the surface with dehydrogenation/hydrogenation and cracking (hydrogenolysis) sites. Consequently, the initial alcohol dehydration selectivity of alumina (to give pure propene at 300−400 °C) was successfully challenged by a strong activity toward formation of major products of acetone, propane, and methane in the gas phase. A range of relevant adsorbed species were characterized and found to result predominantly from adsorptive interactions of 2-propanol and acetone molecules. Surface reaction pathways were suggested, assuming concerted and sequential interactions of the alcohol (and product) molecules with acid−base and metal sites. It was believed that gas-phase migration of alkene molecules and surface diffusion of hydrogen adatoms facilitate the sequential interactions.