Supports/catalysts after calcination

Infrared spectra of VjO, supported catalysts after calcination at 550°C showed the V=0 stretching vibration band at 923 cm. This indicates that surface vanadium species is located on the basic sites within the AIPO,-5 surface. According to the diffused reflectance spectra of the samples with low VjO, contents, the vanadium species supported on AIPO4-5 surface was mainly in a distorted tetrahedral environment. However, upon increasing the V Oj coment, crystalline VjOj was formed on the AIPO -S surface prior to the formation of approximately a monolayer of the surface vanadium species. The vanadium species measured by EPR and DRS and catalytic properties of vanadium loaded VjOg/AIPO -S were similar to those of vanadium substituted VAPO -S. 

BET Surface Area, Pore Volume, and Average Pore Diameter of the Supports and Catalysts after Calcination at 623 K... 

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]).

The reduced Co-Zn supported catalysts are air unstable. It is the matter why only the surface of the catalyst precursors (catalysts after calcination at 320 C) are studied by the XPS method. The comparison of XPS data of catalyst samples 3 and 5 (active and inactive catalysts in relation to 2,3-DHF formation, notation as in Table 1) shows that the binding energies (BE) of Si and A1 in both cases are similar to those in the pure oxides (Si2p 103.3 and 103 eV, A12p 74.7 and 74.6 eV, respectively). It points out the lack of strong interaction between Si and other elements as well as the absence (or few) of typical aluminosilicate network. The surface of active sample 3 contains more of Si than the inactive catalyst 5 (Si/Al 2.6 and 2.1, respectively). 

Table 1. BET surface area, mean pore diameter, and pore volume for the supports and catalysts after calcination. For the nitrogen sorption data, the experimental error ( 2ff) is 5 m /g for the surface areas, 0.2 nm for the mean pore diameters, and 0.02 cm /g for the pore volumes.

The OSCl are much greater for unsupported and su rted HMn04 based catalysts after calcination at 300 and 600 0. The main parameter which explains tibus difference is the surface area for the unsuppmted samples and die manganese oxide dispmsion for the supported ones. The transformation into cz-Mn203 induces a su lementary drop of activity which is clearly evidenced after calcination at 600° C for both series of catalysts. 

It is thus likely that the carbon coating on the alumina support lessened the interaction between AI2O3 and P and inhibited the formation of nickel phosphate after calcination and also lowered the reduction temperature. Structural models of the supported Ni2P catalysts are shown in Figure 4. 

The Ni and S contents on the catalyst series were determined after calcination at 600°C. As shown in Table 1, sulfate was only retained on the silica support when Ni was present. Infrared studies have previously shown that sulfate groups impregnated on pure silica are thermally unstable [13], Therefore, the S04/Ni molar ratios, close to unity, together with the colors resulting after calcining the silica-supported samples made us conclude that Ni was in the form of NiS04 On zirconia, the S04/Ni ratios were larger than one because the sulfate can be associated with both, Ni and the support. 

The viabiUty of using site-isolated Ta(V) centers for cyclohexene epoxi-dation was explored by grafting ( PrO)2Ta[OSi(O Bu)3]3 onto a mesoporous silica material [83]. After calcinations, the material formed is less active and selective in the oxidation of cyclohexene than the surface-supported Ti(IV) catalysts using organic peroxides however, the site-isolated Ta(V) catalysts are more active under aqueous conditions. 

The result of the calculations is that the calcined catalysts obtained from nitrate have dispersions between 5 and 15% only, whereas the Zr02 catalyst prepared from ethoxide has a favorable dispersion of 75 15% after calcination at 700 °C. The equivalent layer thickness for this system is 0.42 nm and the support coverage about 27%. Table 3.3 summarizes all results. 

Even more remarkable differences arise after calcination at 775 K. The sharp Raman spectra of the calcined Mo03/Si02 catalysts are those of crystalline Mo03. No crystalline Mo03 is present in the calcined Mo03/Al203 catalysts, however. Here the Raman spectra show similarities with those of the polymolybdates, which indicates that these species have a sufficiently strong interaction with the alumina support to withstand calcination at 775 K. 

Figure 8.12 Raman spectra of alumina- and silica-supported molybdena catalysts after impregnation of the supports with solutions of ammonium heptamolybdate, (NH4)6Mo7024 4 H20 of different pH values, and after calcination in air at 775 K. See Table 8.3 for a list of characteristic Raman frequencies of molybdate species. The sharp peaks in the spectra of the calcined MoOySiOj catalyst are those of crystalline Mo03 (from Kim el at. [43J).

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