DRIFTS of Probe Molecules Adsorbed on Magnesia, Zirconia, and Hydroxyapatite Catalysts

Acid sites, base sites, and acid–base site pairs on zirconia, magnesia, and hydroxyapatite were investigated using diffuse reflectance FT-IR spectroscopy (DRIFTS) to evaluate the interaction of various adsorbed probe molecules with their surfaces. The DRIFTS spectra were recorded under continuous flow conditions at atmospheric total pressure during a temperature-programmed thermal ramp. Lewis acidity was assessed by observing the various pyridine ring mode conformations and the peak shift associated with adsorption of CO. Basicity was probed by the adsorption of CO2 to form carbonates and bicarbonates on the samples. The acid–base bifunctional nature of the oxides was explored by adsorption of acetylene and glycine. As expected, zirconia exposed the strongest Lewis acid sites of the three samples, whereas magnesia exhibited the strongest basic sites. In contrast, hydroxyapatite had a poor affinity for all probe molecules used in this study based on temperature-programmed desorption experiments, indicating the presence of only weak acid and base sites on the surface, which might account for its high catalytic activity and unique selectivity in the Guerbet coupling of ethanol to butanol.