Vernon C F Holm

Reduction studies on supported metal oxide catalysts

Abstract A procedure is described for determining the reduction characteristics of metal oxidepromoted catalysts. The method involves circulating a measured volume of hydrogen in a closed system that includes a suitable tube for the catalyst, an absorption tube for removing the water formed, and a manometer for measuring the consumption of hydrogen. Reductions may be conducted either at a definite temperature or the temperature may be increased on a fixed time schedule. In the latter case, regular temperature and pressure readings provide information necessary to plot “reduction profiles” in which the reduction rate is plotted versus temperature (or time). Characteristic curves are obtained for various promoter-support combinations and the areas below the “reduction profile” curves are proportional to the amount of reduction. However, the latter is obtained more precisely by accurate measurement of the hydrogen consumption. The procedure was used for studying the reduction characteristics of supported nickel oxide and chromium oxide catalysts as affected by methods of preparation, kinds of support, and the effects of heat treatment, temperature, and promoter concentrations. The results showed that promoter oxide-support oxide interaction increased the difficulty of reducing the promoter metal oxide. This effect was most pronounced for nickel oxide when supported on alumina and for chromium oxide supported on silica. Coprecipitated catalysts were more difficult to reduce than those prepared by impregnation, apparently because of better distribution and therefore better opportunity for interaction. For the same reason, higher temperatures of catalyst heat treatment resulted in increased difficulty of reduction. The interaction between the metal oxide promoter and the support oxide appears to have an important effect on the catalytic properties of metal oxidepromoted catalysts. This is illustrated by results obtained with chromium oxide—silicaalumina catalysts.

The nature of silica-alumina surfaces: I. Thermodynamics of adsorption of ammonia

Abstract The adsorption of ammonia on eleven silica-alumina gels ranging in composition from pure alumina to pure silica has been studied. Isosteric heats and differential surface entropies have been determined from adsorption isotherms at 50-degree intervals in the range 100–400 °C. The adsorption data are correlated satisfactorily by means of the Freundlich equation. Isosteric heats drop off sharply with increasing coverage for all the gels. Compositions in the range of 55–90% silica have the largest fraction of weak adsorption sites. This is the region which exhibits high catalytic activity for various acid-type reactions. The magnitude of the differential surface entropies indicates that in this range most of the adsorbed molecules are in a highly mobile state.

The nature of silica-alumina surfaces II. Cracking of n-octane, polymerization of propylene, o-xylene isomerization, hydrogen transfer, and HD exchange

Abstract In Part I the thermodynamics of ammonia adsorption on eleven silica-alumina gels ranging from pure alumina to pure silica was presented. In this paper, thermodynamic quantities are shown to correlate qualitatively with catalytic reactions occurring on these materials. The catalyst compositions showing highest activity for acid-type reactions are those possessing the highest differential surface entropies, indicating mobile adsorption.

The nature of silica-alumina surfaces III. Statistical interpretation of the adsorption of ammonia on alumina

Abstract Statistical calculations have been made for the adsorption of ammonia on alumina in the region of fixed adsorption. There is a broad distribution of adsorption energies, and an expression for this distribution has been derived. Adsorption data fit the integral form of the Langmuir equation. Configurational entropies have been shown to be several-fold less than those for uniform sites. Evidence is given which indicates that the changes in vibrational entropies of adsorbent and adsorbate as a result of perturbations caused by adsorption are small. Below a coverage of 0.36 × 10 14 molecules/cm 2 , adsorbed ammonia molecules have lost all translation and rotation. Above this value, it is possible that one degree of rotational freedom may have been regained.