Zirconium ethoxides

Figure Bl.25.9(a) shows the positive SIMS spectrum of a silica-supported zirconium oxide catalyst precursor, freshly prepared by a condensation reaction between zirconium ethoxide and the hydroxyl groups of the support [17]. Note the simultaneous occurrence of single ions (Ff, Si, Zr and molecular ions (SiO, SiOFf, ZrO, ZrOFf, ZrtK. Also, the isotope pattern of zirconium is clearly visible. Isotopes are important in the identification of peaks, because all peak intensity ratios must agree with the natural abundance. In addition to the peaks expected from zirconia on silica mounted on an indium foil, the spectrum in figure Bl. 25.9(a)...

Zirconium alkoxides behave similarly in regard to thermal stability. The series zirconium methoxide [28469-78-5] zirconium ethoxide [18267-08-8] zirconium isopropoxide [2171 -98 ] zirconium /r /f-butoxide [2081 -12-1] shows decreasing thermal stability. 

This information is interpreted by comparing the spectra of the catalysts with reference spectra of Zr02 and zirconium ethoxide. 

In nonpolar solvents, for example alcohols, the hydroxyls of the support can also be used to anchor alkoxy compounds to the surface in a condensation reaction, in which one alkoxy ligand reacts with the proton of the surface OH to give the corresponding alcohol, and the complex binds to the support. An example is the anchoring of zirconium ethoxide, Zr(OC2H5)4, to silica by means of the reaction... 

Figure 3.11 shows the Zr/Si intensity ratio as a function of calcination temperature. The three conventionally impregnated catalysts show a clear decrease of the /Zr//Si ratio at relatively low temperatures, indicating loss of dispersion, whereas the bJIs, ratio of the catalyst from zirconium ethoxide decreases only slightly over a temperature range up to 700 °C. 

Figure 3.10 XPS spectra in the range from 150 to 200 eV, showing the Zr 3d and Si 2s peaks of the 7.r02/Si02 catalysts after calcination at 700 °C. All XPS spectra have been corrected for electrical charging by positioning the Si 2s peak at 154 eV. The spectra labeled nitrate correspond to the catalysts prepared by incipient wetness impregnation with an aqueous solution of zirconium nitrate, and the spectrum labeled ethoxide to that prepared by contacting the support with a solution of zirconium ethoxide and acetic acid in ethanol. The latter preparation leads to a better Zr02 dispersion over the Si02 than the standard incipient wetness preparation does, as is evidenced by the high Zr 3d intensity of the bottom spectrum (adapted from Meijers et at, [33]).

Figure 3.12 Schematic representation of the dispersion of Zr02 particles and the extent to which the silica support is covered in three Zr02/Si02 catalysts prepared by impregnation with an aqueous zirconium nitrate solution, and one prepared via an exchange reaction of the support with zirconium ethoxide. The rectangles represent 100 nm2 of silica support area, and the circles represent a half-spherical particle of Zr02 seen from above. See Table 3.3 for corresponding numbers (adapted from Meijers et al. [33]).

Another study on the preparation of supported oxides illustrates how SIMS can be used to follow the decomposition of catalyst precursors during calcination. We discuss the formation of zirconium dioxide from zirconium ethoxide on a silica support [15], Zr02 is catalytically active for a number of reactions such as isosynthesis, methanol synthesis, and catalytic cracking, but is also of considerable interest as a barrier against diffusion of catalytically active metals such as rhodium or cobalt into alumina supports at elevated temperatures. 

Figure 4.6 Positive SIMS spectrum of a 9 wt% Zr02/Si02 catalyst prepared from zirconium ethoxide, after impregnation and drying, and after calcination in air at 400°C (from Meijers et al. [151).

Figure 4.7 SIMS ZrOW and ZrO.W intensity ratios as a function of calcination temperature for the same catalyst as in Fig. 4.6, along with ratios measured from zirconium ethoxide and oxide reference compounds (data from Meijers et al. [151).

Interpretation of the spectra in Fig. 4.6 is best done by comparison with those of zirconium ethoxide and zirconium oxide reference compounds. Figure 4.7 contains the ZrO+/Zr+ and ZrO/Zr+ ratios from the SIMS spectra of the reference compounds, and of the catalysts as a function of the calcination temperature. The figure clearly shows that catalysts calcined at temperatures up to 200 °C have ZrO+/Zr+ and Zr02+ /Zr+ ratios about equal to those measured from a zirconium ethoxide reference compound. However, samples calcined above 300 °C have intensity ratios close to that of Zr02. 

Thus the SIMS intensity ratios sensitively reflect the transition from zirconium ethoxide to zirconium oxide, and indicate that this reaction takes place at tempera-... 

Thus, the SIMS intensity ratios sensitively reflect the transition from zirconium ethoxide to zirconium oxide, and indicate that this reaction takes place at temperatures between 300 and 400 °C. Infrared spectroscopy measured in transmission confirms the disappearance of ethoxide groups at the same temperatures at which the SIMS ZrO+/Zr+ and ZrC>2+/Zr+ intensity ratios change to those characteristic of Zr02. Whilst the infrared spectra might also be due to free ethoxide ligands on the support, the inherent advantage of SIMS is that it confirms -albeit indirectly - that the ethoxide ligands are connected to zirconium [17]. 

Fig. 10.8. Left XPS spectra of calcined ZrOz/SiCh catalysts right Zr/Si intensity ratios indicate that the catalyst prepared from zirconium ethoxide is much better dispersed than the ones from...

In spite of quantisation problems, SIMS can give useful information if one uses appropriate reference materials. The spectra in Fig. 10.11b and c indicate that the relative intensities of the ZrC>2, ZrO+ and Zr+, contain information on the chemical environment of the zirconium Spectrum lib of the zirconium ethoxide (0 Zr = 4 1) shows higher intensities of the ZrC>2 and ZrO+ signals than the calcined Zr02 (0 Zr = 2 1) does. The way to interpret this information is to compare the spectra of the catalysts with reference spectra of ZrC>2 and zirconium ethoxide [21]. 

Figure 1.6. Positive SIMS spectrum of a Si02 + 9 wt% ZrOj catalyst prepared from zirconium ethoxide. a) original sample b) Zr-Zr-ZrOj region for a sample dried at 40 C c) ibid, calcined at 400°C. (courtesy of J.W. Niemantsverdrlet. See A.C.M.Q. Meyers. A.M. de Jong, L.M.P. van Gruijthuijsen and J.W. Niemantsverdriet. Appl. Catal. 70 (1991) 53.)...

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