Chromia amorphous

Surprisingly, the direct evidence of Cr(II) has not ended the controversy, Several groups have since proposed Cr(III) as the active valence. The most direct evidence comes from Kazansky (45) and more recently from Lunsford and co-workers (46,47). Trivalent chromium salts, notably CrCl3, impregnated onto silica and calcined under vacuum, were found to polymerize ethylene. In fact, similar catalysts made from the divalent salts were not active. Unsupported chromia has also been reported to polymerize ethylene weakly (48). However, the attachment of amorphous Cr203 to the silica (30, 46,49) probably does not resemble that of the Cr(VI) and Cr(II) species discussed in Section II, C. It may have little relevance to the commercial catalyst, even if Cr(III) is active. 

There appear to be no extrinsic field studies on the familiar amorphous chromia gel catalyst although it is known (24) to have only a slight indication of the zero field anomaly shown by a-Cr203 at TN. A gel catalyst aged in hydrogen has a small positive field effect at 323 K and a small negative effect at 273 K. 

The catalytic activity of chromia for the NO + NH3 reaction in the presence of oxygen and activity/morphology relations has been investigated (511, 512). The activities of amorphous and of crystalline a-chromia in various reaction mixtures (NH3 + NO + O2, NH3 + O2, and NH3 + NO) have been compared. The specific activity and selectivity of each system were reviewed (476). Amorphous chromia was found to be more active than crystalline chromia in the typical temperature range (423-473 K) because of its higher density of labile oxygen sites. 

Concurrently with the discovery and development in this country of the catalytic conversion of paraffins to aromatics (131) three different groups in the U.S.S.R. discovered this reaction independently of each other. Moldavskil and co-workers (238,239) showed that paraffins with six or more carbon atoms form aromatics by closure of a six-membered ring. For example, n-octane gives xylene and some ethylbenzene over amorphous chromia at about 470°C. Olefins also undergo this reaction. In subsequent publications, the group headed by Moldavskil demonstrated that molybdenum sulfide, titanium dioxide, and other oxides as well as activated carbon also may be used for dehydrocyclization (237,239). 

To summarize the qualitative findings, the methanol synthesis activity in the binary Cu/ZnO catalysts appears to be linked to sites that also irreversibly chemisorb CO and not to sites that adsorb CO reversibly. Since irreversible adsorption of CO follows linearly the concentration of amorphous copper in zinc oxide, these sites are likely to be that part of the copper solute that is present on the zinc oxide surface. No correlation of the catalyst activity and the copper metal surface area, titrated by reversible form of CO or by oxygen, could be found in the binary Cu/ZnO catalysts (43). In contrast with this result, it has been claimed that the synthesis activity is proportional to copper metal area in copper-chromia (47), copper-zinc aluminate (27), and copper-zinc oxide-alumina (46) catalysts. In these latter communications (27,46,47), the amount of amorphous copper has not been determined, and obviously there is much room for further research to confirm one or another set of results and interpretations. However, in view of the lack of activity of pure copper metal quoted earlier, it is unlikely that the synthesis activity is simply proportional to the copper metal surface area in any of the low-temperature methanol-synthesis catalysts. 

It is clear that we cannot give such a specific discussion of the generation of surface coordinative unsaturation on amorphous chromia. However, similar considerations must be involved and we may suspect that various types of site pairs of coordinatively unsaturated Cr + and 0 -can be formed. 

Nitrogen adsorption isotherms at —195° were determined gravi-metrically on chromia gel activated at 126, 200, 275, 350, 425, and 450°. The amorphous chromias all exhibited Type-I isotherms whereas the microcrystalline a-Cr203 prepared by activation in hydrogen at 450° gave a Type-IV isotherm with a hysteresis loop (27). The surface... 

A very large fraction of the porosity of amorphous chromias is in inicropores (27, 28). From the slope of the Lippens and de Boer t plot beyond the relative pressure at which micropores are filled, we estimate the external surface area to be only about 2 m /gm. As the activation temperature is increased, the adsorption of nitrogen at low P/Po decreases relatively, particularly after activation in helium at 425°. Thus, the average diameter of the micropores appears to increase somewhat as the activation temperature is increased from 200 to 425°. 

Fig. 7. Chemisorption of oxygen and of carbon monoxide on amorphous chromias as a function of the temperature of activation squares, carbon monoxide solid circles, carbon monoxide after helium flush at 25 open circles, oxygen x, carbon monoxide on microcrystalline rt Cr203, axis at the right triangles, oxygen on microcrystalline a-Cr203, axis at the right.

Fio. 10. Catalytic activity for hydrogenation of 1-hexene on amorphous chromia a.s a function of the temperature of activation. The curve for oxygen chemisorption is repeated from Fig. 7 using the axis at the left. Crosses give rate in molecules hydrogenated per second initially using the axis at the right. 

Both on the microcrystalline chromia mentioned above and on its amorphous predecessor, the rate of exchange of benzene at 110° was several times faster than the rate of exchange of toluene. From the data of a few runs at large conversions using deuterium and toluene on a catalyst activated in nitrogen at 470°, we earlier thought that toluene probably exchanged more rapidly than benzene (12). However, we had no runs with toluene and benzene on the same sample of catalyst. 

Figure 10 shows catalytic activity for hydrogenation as a function of activation temperature under standard conditions of activation. Activations were in hydrogen except for that of the amorphous chromia at 402°, in which hydrogen was replaced by helium at 300°. Activity is negligible after activation at 200°, becomes significant at about 275°, and then climbs rapidly. An amorphous catalyst activated at 400° hydrogenated 0.15 molecule of 1-hexene per second per 100 A a microcrystalline a-CroOs, 0.7. 

Most of the work of Burwell et al. (21) involved catalysts activated in nitrogen at 470°. They were initially amorphous but after long use and many regenerations they had become microcrystalline a-Cr203-The differences to be expected between exchange on amorphous and microcrystalline chromia were not clear from this work. We have, therefore, examined the exchange of cyclopentane on amorphous and on crystalline catalysts activated at 400°. 

We have examined activations at temperatures between 110 and 420° under conditions chosen to give high-area amorphous chromias and at 400 450° in hydrogen to obtain microcrystalline a-Cr203 of a surface area of about 80 m /gm. 

The (001) plane of a-Cr203 (Fig. 2) contains 9.8 Cr3+ per 100 56, 59), or let us say 10. We shall use this value as a surface average for a-CrgOa and for amorphous chromia even though the value should... 

It is difficult to imagine any band theory for an amorphous chromia activated at 250°. In terms of semiconductor theory, it is difficult to imagine such large surface coverages as we observe at —78°. The close correlations which are developing between homogeneous catalysis and catalysis on chromia cannot be understood at all on the basis of semiconductor theory. 

Fio. 11. Infrared absorption spectra of amorphous chromias and crystalline chromic oxides in region of 500 cm" . (a) Dried at 110° in air. (b) Amorphous chromia activated to 405° in helium, (o) Microcrystalline a-Cr203 activated to 402° in hydrogen, (d) Chromia of curve a calcined at 1100°. 

The reaction of NH3 and NO produces mainly N2O over both types of catalysts. The reaction rate of NH3, NO, and O2 over amorphous chromia appeared to be four times that over crystalline ehromia. The amorphous chromia catalyst is very selective for N2, whereas the crystalline chromia produces both N2 and N2O. 

Schraml-Marth et al. [89] studied the two types of chromia by means of diffuse reflectance FTIR and Raman spectroscopy. Amorphous chromia has a higher density of labile oxygen sites than that of crystalline chromia in the temperature range 423-473 K and is accordingly more active. 

The X-Ray Diffraction results shows the presence of only AI2O3, Zr02 and Th02 and in none of the cases Cr203 is observed for the chromia loaded sample. This probably indicates that the chromia deposited is well dispersed and probably in an amorphous form. 

Chromia gel dried at about 100° is normally amorphous (9, 10), but a crystalline form is readily precipitated near room temperature (11), and this decomposes to an amorphous material at 60°. The x-ray pattern of this crystalline form is very similar to that of alumina bayerite with about 4 % linear expansion of the lattice spacings, so that the compound may be called chromia bayerite. 

In an earlier work Lazier and Vaughen (11) reported that amorphous chromia obtained by precipitation promoted the hydrogenation of olefin hydrocarbons. No details of conditions or yields were given in this work. Ipatieff, Corson, and Kurbatov (12) found no hydrogenation ability for pure chromia on either isopentene or benzene at atmospheric pressure and no hydrogenation of benzene at high pressures. 

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