WO_x/ZrO_2 Catalysts Prepared by Anionic Exchange:In Situ Raman Investigation from the Precursor Solutions to the Calcined Catalysts

摘要

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 700degC.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_2centre dot n(H_2O) 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 H_2WO_4-H_2O_2 precursor solutions evidenced the presence of (W_2O_3(O_2)_4(H_2O)_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 (WO_4)~(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 700degC 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.