WO_x/ZrO₂ Catalysts Prepared by Anionic Exchange:  In Situ Raman Investigation from the Precursor Solutions to the Calcined Catalysts

Abstract

W/ZrO(2) catalysts were prepared using anionic exchange of peroxotungstate species with hydroxyl groups of zirconium hydroxide at low pH. The solids were dried and calcined under air at 700 degrees C. Each step of this novel method of preparation was investigated by Raman spectroscopy. A reference sample was also prepared by incipient wetness impregnation of ZrO(2).n(H2O) with an ammonium tungstate solution and characterized throughout its preparation process. Complementary data were collected from X-ray diffraction, chemical analysis, surface area measurements, and thermal analysis. The Raman spectra of the H2WO4-H2O2 precursor solutions evidenced the presence of (W2O3(O2)4(H2O)2)(2-) dimers. These low-nuclearity species were exchanged with zirconium hydroxide at low pH. The Raman spectra of the dried solids did not reveal peroxotungstate species but were typical of tetrahedral (WO4)(2-) species. A slight agglomeration of W species was observed with an increase in the W content. However, for an equivalent W loading, a higher W dispersion was obtained by anionic exchange, compared to the impregnation method. Furthermore, a remarkable homogeneity of the exchanged samples was evidenced by the micro-Raman spectra. The in situ Raman spectra recorded during calcination characterized both crystalline phases and supported tungsten species. Significant modifications were observed during the calcination process. The exchanged and the impregnated samples, with the same W loading, evidenced a similar type of tungsten species with one W=O bond. However, their behavior during calcination up to 700 degrees C was different. This was attributed to different strengths of interaction with the support. Moreover, the spectra recorded after calcination on various points of the exchanged sample with a high W content revealed a better spatial homogeneity than the impregnated one.

Monitoring Hydrotreating Catalysts Synthesis and Deactivation using Raman Spectrometry

“…As a consequence, an increase of the frequency reveals a structural modification: the cluster size of tungsten surface species probably increases with temperature. This phenomena have been already observed for WO x /ZrO 2 [19]. At 240…”

Supported Heteropoly Acids and Multicomponent Polyoxometalates as Eco-Friendly Solid Catalysts for Bulk and Fine Chemicals Synthesis

“…The position of ν (W═O) band was shifted from 973 to 992 cm −1 with an increase in calcination temperature from 120 to 750 °C, indicating an increase in W═O bond order. The increase in ν (W═O) wave number with calcination temperature could be due to an increase in the agglomeration of silicotungstate species [ 25 ]. This was also evidenced from the increase in tungsten surface density with calcination temperature (Table 5.1 ).…”

Spectroscopic characterization of tungstated zirconia prepared by equilibrium adsorption from hydrogen peroxide solutions of tungsten(VI) precursors

“…Loridant et al [22] did not observe WO 3 on their calcined samples prepared by ion exchange from the same peroxo precursor. The reason for this difference could be the short adsorption time employed in their synthesis (15 min).…”

“…Compared to the non-peroxide systems, the adsorption of vanadium(V) peroxo ions resulted in approximately 2-fold increase of the loading and better dispersion of the VO x species deposited on zirconia. Recently, Loridant et al [22] used similar scheme in the preparation of tungstated zirconia by anionic exchange of peroxotungstate species with hydroxyl groups of ZrO x (OH) 4-2x at low pH.…”