Sequential Saturating Reactions of ZrCl4 and H2O Vapors in the Modification of Silica and γ-Alumina with ZrO2

Abstract

Zirconia-modified silica and alumina samples were prepared by the atomic layer epitaxy technique using successive saturating reactions of ZrCl4 and H2O vapors. The growth of ZrO2 was followed by X-ray diffraction, diffuse reflectance IR Fourier transform spectroscopy, and 1H magic angle spinning NMR measurements. In addition, coverage of the silica surface was estimated by low-energy ion scattering (LEIS). At the reaction temperature of 300 °C the main reaction of ZrCl4, on both supports, occurred with surface OH groups, leading to amorphous ZrClx species. In addition, part of the molecules reacted to form ZrO2 agglomerates while simultaneously chlorinating OH groups of the supports. Water treatment removed Cl from the ZrClx surface species, leaving behind Zr−OH groups, and released the directly chlorinated OH groups. Both the reappearing OH groups of the support and the Zr−OH groups were then again available during the next ZrCl4 treatment for both main reactions of ZrCl4. As the reaction cycles were repeated the concentration of amorphous Zr in the samples increased together with the amount of crystalline ZrO2 and surface coverage. On silica, after five cycles of repeated reactions when an average surface density of 5 Zr/nm2 had been achieved, 25% of the original accessible OH groups were still present on the surface. At that point, LEIS measurements showed 50% coverage. On the surface of both silica and alumina, ZrOx was present in the form of tetragonal agglomerates and an amorphous network phase. Two types of Zr−OH groups were identified.