Chromium oxide, adsorption

The adsorption of gases and vapors on mesoporous materials is generally characterized by multilayer adsorption followed by a distinct vertical step (capillary condensation) in the isotherm accompanied by a hysteresis loop. Studies of adsorption on MCM-41 have also demonstrated the absence of hysteresis for materials having pore size below a critical value. While this has been reported for silica gel and chromium oxide containing some mesopores, no consistent explanation has been offered [1], However, conventional porous materials, having interconnected pores with a broader size distribution, are generally known to display a hysteresis loop with a point of closure which is characteristic of the adsorptive. These materials have an independent method of estimating the pore size from XRD and TEM, that allows comparison with theoretical results. Consequently, we have chosen these materials to test the proposed model. 

The source of this discrepancy is unknown to us. Equation (349) is, undoubtedly, adequate for the description of the reaction kinetics on an iron-chromium oxide catalyst. The fact that in one of the works (124) magnetite without the addition of chromium oxide served as a catalyst can hardly be of consequence since a study of adsorption-chemical equilibrium (344) on an iron-chromium oxide catalyst (7% Cr203) (52) led to the value of the average energy of liberation of a surface oxygen atom that practically coincides with that found earlier (50) for an iron oxide catalyst with no chromium oxide. It may be suspected that in the first work (124) the catalyst was poisoned with sulfur of H2S that possibly was contained in unpurified C02... 

It is often useful to consider that sites for chemisorption result from surface coordinative unsaturation, i.e., that atoms at the surface have a lower coordination number than those in bulk. Thus, for example a chromium ion at the surface of chromium oxide has a coordination number less than that of a chromium ion in the bulk. The chromium ion will tend to bind a suitable adsorptive so as to restore its coordination number. An atom in the (100) surface of a face-centered cubic metal has a coordination number of 8 vs 12 for an atom in bulk this, too, represents surface coordinative unsaturation. However, of course, there are sites to which the concept of surface coordinative unsaturation does not apply, for example, Br nsted acid sites. 

The adsorption of formic acid and acetic acid leads to the formation of car-boxylate groups on aluminas (194, 295-299), titanium dioxides, (134, 135b, 176, 194, 300, 301), chromium oxide (134, 302, 303), zinc oxide (298, 304-306), and magnesium oxide (299, 304, 306). The corresponding dissociative chemisorption step most probably takes place on acid-base pair sites of the type... 

It has been known for many years that the adsorptive and catalytic properties of chromium oxide gels are very sensitive to the conditions of preparation, storage and heat treatment (Burwell et al., 1960 Deren et al., 1963 Carruthers and Sing, 1967 Baker etal., 1970,1971). 

Many oxidants, especially the older and stronger chromium oxidizing agents, may have their reactivity and selectivity modified by adsorption on to inert supports.Reactions utilizing supported oxidants have the advantage that the residual chromium salts remain bound to the support and thus work-up often becomes reduced to a mere filtration. Many of these systems are discussed in detail later (Volume 7, Chapter 7.3). 

This concept was first mathematically developed by Clark and Bailey according to an exponential distribution of active centres with respect to adsorption energy. The Langmuir-Hinshelwood mechanism for adsorption and reaction was found to fit experimental results (ethylene polymerization over chromium oxide-silica-alumina catalyst) more closely than the Rideal mechanism. 

Heterogeneity of the surfaces of oxide catalysts such as chromium oxide, zinc oxide, and zinc-chromium oxide has been postulated by H. S. Taylor (75) on the basis of adsorption studies. In the author s view, Taylor s experimental observations may be also explained without assuming a heterogeneous character of the oxide surfaces. 

Winter 10) relates isotopic exchange of molecular oxygen with magnesium oxide, zinc oxide, chromium oxide, nickel oxide, and iron oxide. He also compares the rates of isotopic exchange of these oxides with oxygen and the rates of adsorption and catalytic activity relating to the oxidation of CO and the decomposition of NgO. 

One treatment has been worked out by Clark and Bailey (34), who considered the polymerization of ethylene at a solid surface from the point of view of variable adsorption sites. Their conclusions were compared with the results obtained with supported chromium oxide... 

Hubaut et has studied the liquid phase hydrogenation of polyunsaturated hydrocarbons and carbonyl compounds over mixed copper-chromium oxides. The selectivity of monohydrogenation was almost 100 % for conjugated dienes but much lower for a,p-unsaturated carbonyls. This was due to the adsorption competition between the unsaturated carbonyls and alcohols as primary products. It was suggested that the hydrogenation site was an octahed-rally coordinated Cu ion with two anionic vacancies, and an attached hydride ion. The Cr ion in the same environment was probably the active site for side reactions (hydrodehydroxylation, nucleophilic substitution, bimolecular elimination). 

When, however, desorption does occur in the ascending portion of the curve BC, there should, on cooling down from C, no longer be a horizontal section CE but continuously increasing values of the adsorption CG, measuring, in excess of the values over the horizontal section, the quantity of gas which was desorbed during the measurements made in passing upwards in temperature from B to C. It is this behavior which is shown by zinc oxide, chromium oxide gel, and zinc-chromium oxide. 

In a study of the effect of gas pressure on the chemisorption of hydrogen on chromium oxide gel, Taylor and Burwell (6f) found it necessary to resort to an arbitrary process of subtraction from experimentally observed amounts of adsorbed gas in order to display a relatively uniform area of gel for hydrogen chemisorption. Within close limits, at temperatures of 457 and 491° K. and at pressures of 1, 0.5 and 0.25 atmospheres, the energy of activation for the adsorption process was nearly independent of the amounts of adsorption (0-35 cc.) and the activation energy was about 21 kcal. In the temperature range 383 to 457° K. no such uniformity was observed. For example, if the amounts adsorbed with time at 405 and 427° K. be used to calculate activation energies the values received increase from 0 to 18.5 kcal. as the amount of gas adsorbed on the surface increases from 1.6 to 8.5 cc. Similar results can be drawn from the earlier measurements of Kohlschutter (17). 

Beebe and Dowden (18) measured the heat of adsorption of hydrogen on chromium oxide gel at liquid air temperatures. They found a heat of adsorption of 5 kcal./mole which is larger than that to be expected for van der Waals adsorption. This gas, however, with the low energy of activation allowable at liquid air temperature cannot be held to the surface in the temperature range 457 to 491° K. where chemisorption with an activation energy of 21.7 kcal. on an area relatively uniform and large in extent is occurring. 

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