Fourier-transform laser Raman spectroscopy of adsorbed pyridine and nature of acid sites on calcined phosphate/Zr(OH) 4

Abstract

Two phosphated zirconias were prepared by calcination at 600°C of (NH 4) 2HPO 4-impregnated zirconium hydroxide [Zr(OH) 4] and monoclinic zirconia (m-ZrO 2). X-ray powder diffractometry, infrared spectroscopy and laser Raman spectroscopy showed the calcination product of the former precursor [(NH 4) 2HPO 4/Zr(OH) 4] to assume a bulk structure consisting of face-centred cubic zirconia (c-ZrO 2), and exhibit large surfaces (102 m 2/g) enriched with pyrophosphate-like polymeric species. In contrast, the calcination product of the latter precursor [(NH 4) 2HPO 4/m-ZrO 2] was found to retain the bulk structure of the oxide (m-ZrO 2), and expose moderate surfaces (33 m 2/g) enriched with orthophosphate-like monomeric species. Raman spectroscopy of adsorbed pyridine probed on the phosphated c-ZrO 2 hydrogen-bond donor (ZrOH), Lewis (coordinatively unsaturated Zr 4+) and Brønsted (POH) acid sites. On the phosphated m-ZrO 2, hydrogen-bond donor and Lewis acid sites were detectable, whereas Brønsted acid sites were not. It is concluded that the generation of Brønsted acid sites on phosphated zirconia necessitates the employment of Zr(OH) 4 as a precursor compound for the zirconia.