Supported Metal Oxide Catalysts Research Articles

Relations between active phase characteristics and catalytic properties in supported metal oxide catalysts: the dehydrogenation of cyclohexane on molybdena catalysts supported on γ-Al 2O 3 doped with alkali cations

The dehydrogenation of cyclohexane to benzene on molybdena catalysts supported on γ-Al 2O 3 modified by various amounts of alkali cations has been studied. Kinetic experiments were carried out on the reduced form of these catalysts using a flow bed reactor. These allowed the determination of the fractional conversions in the temperature range 410-490°. It was found that the conversion increased with time of reduction while one “plateau” of constant catalytic activity was attained after 5 h. Moreover, it was found that the modification of the y-A1203 causes a decrease in the catalytic activity. Conductivity measurements obtained at various temperatures allowed the determination of the activation energy of conduction for the catalysts studied. No relation exists between this parameter and kind or content of the modifier. D.R.S. spectra of some catalysts obtained after catalytic tests for 12 h allowed the estimation of the concentration of the supported Mo ions with valence lower than six. Modification by Li+ cations does not practically affect this concentration, whereas doping by the remaining alkali cations inhibits the transformation of Mo(VI) into Mo with valence lower than six. Using Arrhenius law as a criterion the mechanistic scheme of the reaction has been established. On the base of this scheme apparent activation energies were determined for the catalytically active specimens. In our system, it was demonstrated that the apparent activation energy is not an appropriate measure of the catalytic activity. The mechanism of the reaction was found to be independent of the geometrical and structural characteristics of the molybdenum phase as well as of the semi-conducting properties of the specimens. The catalytic activity of the catalysts examined depends almost exclusively on the valence of the supported molybdenum. The dispersion of the molybdenum phase does not relate to the catalytic activity. Moreover, no relation exists between the semiconducting properties of the specimens as estimated by the activation energy of conduction and the catalytic activity.

Selective gas phase oxidation of toluene by vanadium oxide/TiO 2 catalysts

Several supported metal oxide catalysts were studied qualitatively in the selective gas phase oxidation of toluene. Of these catalysts the combination of vanadium oxide and TiO 2 was most effective and was therefore studied more thoroughly. Two types of TiO 2 were used as support. For catalysts supported on Degussa TiO 2 the catalytic properties improved with increasing vanadium content up to monolayer coverage becoming constant at higher loadings, indicating that the vanadium oxide present as multilayers and/or crystallites does not affect the catalytic reaction. The activity and selectivity of catalysts supported on Tioxide TiO 2 were much lower and increased continuously with increasing vanadium content (up to 4 x monolayer coverage). This different catalytic behaviour is attributed to the presence of impurities (phosphorus and potassium) in the Tioxide material, which had a large, negative effect on the catalytic properties.

ChemInform Abstract: METATHESIS OF PROPENE ON METAL OXIDE SUPPORTED CATALYSTS

Chemischer InformationsdienstVolume 14, Issue 11 Preparative Organic Chemistry ChemInform Abstract: METATHESIS OF PROPENE ON METAL OXIDE SUPPORTED CATALYSTS K. ANDERS, K. ANDERSSearch for more papers by this authorR. FELDHAUS, R. FELDHAUSSearch for more papers by this authorS. NOWAK, S. NOWAKSearch for more papers by this authorJ. S. CHODAKOV, J. S. CHODAKOVSearch for more papers by this authorC. M. MINACHEV, C. M. MINACHEVSearch for more papers by this author K. ANDERS, K. ANDERSSearch for more papers by this authorR. FELDHAUS, R. FELDHAUSSearch for more papers by this authorS. NOWAK, S. NOWAKSearch for more papers by this authorJ. S. CHODAKOV, J. S. CHODAKOVSearch for more papers by this authorC. M. MINACHEV, C. M. MINACHEVSearch for more papers by this author First published: March 15, 1983 https://doi.org/10.1002/chin.198311058AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume14, Issue11March 15, 1983 RelatedInformation

Recent developments in the vibrational spectroscopies (infrared, Raman, electron energy loss etc.) as applied to the structural analysis of species chemisorbed on metal surfaces

An account is given of the methods of vibrational spectroscopy that are nowadays used to obtain structural information about species adsorbed on metal catalyst surfaces. The discussion deals separately with experiments involving a) oxide-supported metal catalysts in the finely divided form, and b) specific surfaces of metal single crystals. A discussion is also given of the advantages and limitations of each method, e.g. infrared transmission and Raman spectroscopy in the case of finely-divided metals, and electron energy loss spectroscopy and reflection-absorption infrared spectroscopy as used for metal single crystals. Infrared and Raman spectroscopic studies of ligands attached to multi-atom metal clusters are shown to lead to convincing identifications of species adsorbed on single crystal surfaces. The latter results in turn contribute to the understanding of the overlapping spectra from several different adsorbed species that commonly occur on finely-divided metal catalysts.

A study of high temperature treated supported metal oxide catalysts

The oxidation of CO and C 2H 4 at 200–400 °C over two groups of supported metal oxide catalysts has been studied. The catalysts in group A were mechanical mixtures of presintered metal oxides (CuO, Co 3O 4, NiO, and CuCr 2O 4, 0.1–10% by wt) with presintered catalyst supports (α-Al 2O 3, γ-Al 2O 3, and ZrO 2) and those in group B were prepared by the conventional method of impregnation of the presintered supports followed by 500 °C calcination. With the exception of Cr 2O 3, the catalytic activity of group A supported on α-Al 2O 3 or ZrO 2 increased 10–200-fold after being heated at 700 °C and above for 10–30 min and reached a maximum at ~900 °C, while the catalytic activity of group B remained constant or decreased slightly with increasing heat treatment temperature. The maximum activity per total surface area of group A approached that of the corresponding pair of group B, and was dependent on the composition of the catalyst. The activity of the supported catalyst per total surface area was found to be in all cases less than the corresponding specific activity of the unsupported metal oxide. This phenomenon is discussed in terms of increased dispersion induced by the high temperature treatment. When γ-Al 2O 3 was used as the support the thermally induced activity was much less and usually short-lived, presumably because the rate of chemical interaction and sintering overtook the rate of thermal dispersion.

Catalytic Reductions of NO with H<sub>2</sub>, CO or NH<sub>3</sub> over Supported Metal Oxide Catalysts

The roles of reducing agents such as H2, CO and NH3 in catalytic reduction of NO over supported metal oxide catalysts were studied from the viewpoint of reaction mechanisms. The reaction of NO with H2 or CO was proved to proceed through the redox cycles of the catalyst, that is, catalyst was oxidized by NO and then was reduced, to complete the catalytic sequence by the reducing agents. On the other hand, it was confirmed that NH3 or fragments of NH3 directly reacted with the adsorbed NO.These differences of the roles of reducing agents in the reaction mechanism satisfactorily explain some features of catalytic removal of NO in the practical process containing oxygen.

The heterogeneous oxidation of hydrogen sulfide at concentrations below 1000 ppm in nitrogen/air mixtures over supported metal oxide catalysts

The oxidation of hydrogen sulfide in concentrations below 1000 ppm in nitrogen/ air mixtures has been studied from 180° to 320°C over mixed oxide catalysts of cobalt/ molybdenum, cobalt/nickel and nickel/molybdenum supported on α-alumina. The change in the degree of conversion of hydrogen sulfide to sulfur dioxide with temperature has been determined and the temperature regions of maximum efficiency of each catalyst defined. The cobalt/molybdenum catalyst was the most effective and a possible method for its regeneration is described.

Disproportionation of Propylene over Supported Metal Oxide Catalysts

Disproportionation of propylene on several metal oxide catalysts was studied by means of a flow method. The effects of pretreatment of catalyst with various gases and of various supports on the reaction were investigated in order to clearify the formation of active centers of catalyst. The reaction was carried out at the temperature range from 100°C to 200°C on each catalyst activated 'previously with dry nitrogen gas, dry oxygen or dry hydrogen at 500°C.It was found that WO3-γ-Al2O3 was the most active catalyst for this reaction, but selectivity was rather small. On the other hand, the activity of MoO8-γ-Al2O3 was very high at the beginning of the reaction, but the activity decreased gradually with the reaction time. When the catalyst was pretreated with nitrogen, the activity was greatly different with the various supports, that is, MoO3-SiO2.Al2O3 was most active, and both MoO2-SiO, and MoO3-α-Al2O3 had no activity for this reaction. From these results, it seems that the activity of several supported MoO3 was correlated with the acidity of these carriers.By reducing all these catalysts with hydrogen before the reaction, the activity prominently increased, especially the activity of both MoO3-SiO2 and MoO3-α-Al2O3 became almost equivalent to that of MoO3-γ-Al2O3.From these phenomena, it is concluded that the active center of these catalyst is slightly reduced molybdenum oxide, and the acidic carrier plays a role in forming the active center during the pretreatment of catalyst, without directly participating in the reaction.

Reduction studies on supported metal oxide catalysts

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.