Lewis band

M0O3) has been investigated for samples in both original and rehydroxylated form. The spectra are shown in Figure 3. It appears that Brdnsted acid sites, characterized by the 1636 and 1540 cm l bands are observed only, when the discs are rehydroxylated in wet air before the calcination under high vacuum in the IR cell takes place (Figure 3b). By consequence all spectra have been recorded for such rehydroxylated samples. Only one Lewis band is observed for the molybdenum-alumina sample, opposite to the observations of Kiviat and Petrakis (JL9), who have observed two Lewis bands for their samples. 

However, the molybdenum-alumina and the high calcined cobalt-molybdenum-alumina samples still show an important difference. The pyridine spectra of MoCo-124 indicate a second Lewis acid site, characterized by the 1612 cm-1 band. This band differs from the weak Lewis acid sites of the alumina support (1614 cm- ) because the position is significantly different. It also appears that the strength of the bond between pyridine and the catalyst is stronger, for the 1612 cm-1 band is still present after evacuation at 250°C, while the weak Lewis band (1614 cm-1) of the alumina has disappeared at this desorption temperature. Obviously the second Lewis band for the MoCo-124 catalyst is introduced by the interaction of cobalt with the surface molybdate layer. This interaction is... 

Spectra of pyridine, adsorbed on this CoMo-124 sample are shown in Figure 6. The second Lewis band (1612 cm 1) is present, indicating that the interaction between the cobalt ions and the surface molybdate layer is present too. 

The spectra of adsorbed pyridine are shown in Figure 8. The spectrum of the sample NiMo-124 480/480, which has been calcined twice at the lowest calcination temperature shows a clear 1612 cm"l Lewis band. This band has a far weaker intensity for the 650/650 catalyst. The 480/650 sample shows a more intense 1612 cm-1 band than the 650/650 and 650/480 samples. This might indicate that the... 

The spectra of adsorbed pyridine for MoCo-153 and MoNi-153 are compared in Figure 9. Two final calcination temperatures have been applied, 480 and 650°C. The spectra of the 480°C samples (Figure 9a and 9b) are nearly identical. The BrtSnsted acid bands are weak, while the 1612 cm l Lewis bands are strong. The intensity of the Brdnsted acid bands increases for both 650°C calcined samples (Figure 9c and 9d). The Lewis acid bands show a marked difference now. The 1612 band remains high in intensity for the MoCo-153 catalyst, but this Lewis band decreases appreciably in intensity for the MoNi-153 catalyst. 

The molybdate surface layer in the molybdenum-alumina samples is characterized by the presence of BrGnsted acid sites ( 1545 cm- ) and one type of strong Lewis acid sites (1622 cm l). Cobalt or nickel ions are brought on this surface on impregnation of the promotor. The absence of BrtSnsted acid sites is observed for both cobalt and nickel impregnated catalysts, calcined at the lower temperatures (400-500°C). Also a second Lewis band is observed at 1612 cnrl.The reflection spectra of these catalysts indicate that no cobalt or nickel aluminate phase has been formed at these temperatures. This indicates that the cobalt and nickel ions are still present on the catalyst surface and neutralize the Brdnsted acid sites of the molybdate layer. These configurations will be called "cobalt molybdate" and "nickel molybdate" and are shown schematically in Figure 11a. 

The optimal activity for a cobalt-molybdenum-alumina catalyst is obtained by calcination at the higher temperatures. This means that the cobalt ions, present as a cobalt aluminate phase according to the reflectance spectra and the magnetic susceptibility measurements, still have a pronounced promoting action after this calcination. The assumption of cobalt present in the surface layer of the alumina lattice explains both the high activity due to the cobalt promotion as well as the presence of the second Lewis band. This configuration is shown schematically in Figure lib. 

The reappearance of Brdnsted acid sites has been observed for the high calcined nickel-molybdenum-alumina catalysts. The presence of a nickel aluminate phase has been concluded from the reflectance spectra. The second Lewis band (1612 cm l) has a very low intensity, in comparison with the cobalt containing catalysts of a same composition and after the same calcination conditions. 

It has been found by measuring spectra of adsorbed pyridine on various C0O-M0O3-AI2O3 systems that there is a clear interaction of the cobalt ions with the Mo03 Al203 surface layer. This conclusion is based upon the occurence of a second Lewis band in the spectrum of adsorbed pyridine, which has been observed in the whole region of the calcination temperatures applied. 

A picture has been formed of the way in which the promotor ions are built in the M0O3-AI2O3 system. The neutralization of the Brdnsted acid sites, as originally present in M0O3-AI2O3 systems by the cobalt ions for the catalysts calcined at low temperatures ( 500°C) indicates that the cobalt ions are present on the catalyst surface. The liberation of these sites in catalysts calcined at high temperatures ( 650°C) and the observation of the characteristic reflectance spectrum of C0AI2O4 show that the cobalt ions enter the alumina lattice. However the interaction between cobalt and molybdenum, as indicated by the second Lewis band remains present. This leads to the conclusion that the cobalt ions are present in the surface layers of the alumina lattice. 

The integrated areas of the deformation band of NH/ are given in Table 2. The population of Bronsted sites increases with the nominal sulfate content up to 7.5 wt%, and decreases for the sample with the highest sulfate content. No reliable figures could be estimated for the Lewis bands at 1610 and 1200-1300 cm (6as and 6s NH3) due to the overlapping with the deformation band of HOH or HsO species, and the sulfate bands with ionic character, respectively. 

Spectra of chemisorbed pyridine show, as expected, bands characteristic of Bronsted (B, at 1542 cm ) and of Lewis (L, at 1453 cm ) acid. Ghos h, and Curthoys (13) reported an additional Lewis band at 1462 cm in dealuminatetf lnordenites degassed above 400°C this band was not observed in any of the siliceous H-mordenites (containing Al(VI)) examined in this study, lending some support to the correlation between the 1462 cm band and the existence of hydroxyl nests in the crystal lattice proposed by these authors (13). 

Besides identifying the sites, it is possible to calculate their concentrations from the integration of the area of characteristic Bronsted and Lewis bands and the knowledge of the molar absorptivity of the sample and the self-supporting area of the pellet, by applying Lambert-Beer law. 

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