Vanadium oxides, adsorption

To investigate the effect of the synthesis method on the structure-reactivity relationship of the supported metal oxide catalysts, a series of V205/Ti02 catalysts were synthesized by equilibrium adsorption, vanadium oxalate, vanadium alkoxides and vanadium oxychloride grafting [14]. The dehydrated Raman spectra of all these catalysts exhibit a sharp band at 1030 cm characteristic of the isolated surface vanadium oxide species described previously. Reactivity studies with... 

The data on the rate of reaction of o-, m-, and p-xylene over vanadium oxide catalyst and of m-xylene over mixed vanadium oxide catalysts (chromium-vanadium and antimony-vanadium) were correlated with the reaction scheme below by the following rate expressions, which are based on the Langmuir-Hinshelwood mechanisms where the adsorption of m-xylene is strong. 

When the vanadium oxide catalyst is used, is much smaller than kx, while the specific rate constant of the reaction of m-tolunitrile in the absence of xylene is larger than that of xylene. This seems to be caused by a strong adsorption of xylene on the catalyst surface. 

Krylov (62) studied the adsorption of oxygen and propylene on vanadium oxide/MgO and molybdenum oxide/MgO catalysts by ESR and IR at 25°C. He observed the formation of Qr radicals and ir-allyl complexes during the simultaneous adsorption of 02 and C3H . The data indicated that an electron transfer took place from the olefin to the oxygen through the transition metal ion forming the following complex ... 

Reduction of Nitric Oxide with Ammonia. - Control of the emission of NO from stationary sources is possible by selective catalytic reduction, for which up to now NH3 is the only effective reductant in the presence of excess 02. Beside noble metal catalysts Bauerle etal.101 109 and Wu and Nobe108 studied Al2 03-supported vanadium oxide and found this to be highly effective in NO removal which is considerably enhanced by the presence of 02. Alkali metal compounds which are usually added as promoters for S02 oxidation completely inactivate the catalysts for NO reduction. Adsorption kinetic studies indicated first-order dependence on NH3 adsorption. Similar results were obtained for NO on reduced vanadium oxide, but its adsorption on... 

The adsorption of sulfur dioxide of the catalysts rapidly decreased with the vanadium content of the samples. As sulfur dioxide is not chemisorbed on bulk vanadium pentoxide and also appears not to chemisorb on vanadate species, on V20s/Y-Al203 catalysts this probe can be considered to be selective for the titration of the basic sites of vanadium-free alumina. Thus, by selectively probing uncovered alumina, SO2 further allows the distinction between the acid-base features of the vanadium oxide layer and those of uncovered alumina. 

This review deals with the applications of photolurainescence techniques to the study of solid surfaces in relation to their properties in adsorption, catalysis, and photocatalysis, After a short introduction, the review presents the basic principles of photolumines-cence spectrosajpy in relation to the definitions of fluorescence and phosphorescence. Next, we discuss the practical aspects of static and dynamic photoluminescence with emphasis on the spectral parameters used to identify the photoluminescent sites. In Section IV, which is the core of the review, we discuss the identification of the surface sites and the following coordination chemistry of ions at the surface of alkaline-earth and zirconium oxides, energy and electron transfer processes, photoluminesccncc and local structure of grafted vanadium oxide, and photoluniinescence of various oxide-... 

UV irradiation of anchored vanadium oxide/Si02 catalysts at 280 K in the presence of CO leads to the photoformation of CO2 (i.e., the photoreduction of the catalyst by CO) accompanied by small amounts of a photoin-duced adsorption (photouptake) of CO (56). After photoreduction of the anchored catalysts and further evacuation of excess CO, O2 was introduced onto the catalysts at pressures less than 530 Pa and at 280 K for a few... 

Adsorption of lUO on Highly Dispersed Anchored Vanadium Oxide Catalysts... 

Atomic and molecular adsorption at vanadium oxide surfaces have been studied theoretically using both periodic slab and cluster models where so far studies are restricted to the pentoxide, V2O5, as a substrate due to its possible importance in catalytic applications as mentioned before. Further, adsorbate species include in all cases atoms (H [122-123, 126, 136-142], O (see below)) or rather small molecules (O2 (see below), H2O [143-144], NH3 [145-147], NO [146, 148], C2H4 [149], propene (CsHg) [140], toluene (CeHsCHj) [140]) that are of catalytic interest but also small enough to make meaningful calculations feasible. 

Performance improvement in 1102 is achieved by doping the titania with platinum or vanadium oxide and copper. In order to improve the adsorption characteristics of Ti02, attempts have been made to support fine Ti02 on the porous adsorbent materials such as silica, alumina, activated carbon, clay, and zeolites. Ti02-supported adsorbent provides higher specific surface area and introduces more effective adsorption sites than bare Ti02. Decomposition rates of the substrates are found to increase due to one or... 

It may be seen from comparison of results on ethylene oxidation over silver and vanadium pentoxide that with both catalysts the oxidation of unsaturated hydrocarbons will proceed by the same mechanism. C02 generation is not accelerated in the presence of aldehydes and these cannot be intermediates in ethylene combustion. When aldehydes are introduced into the reactant mixture, the ratio of ethylene oxide to C02 formation rates undergoes a change, due to strong adsorption of aldehydes on the catalyst surface. Ethylene oxide will form on silver and is in fact absent on vanadium oxides. It was shown experimentally that the absence of acetaldehyde and formaldehyde in the products of oxidation over silver, and the low absolute content of these substances for vanadium oxides is due to the fact that they are not formed at all, or formed at a low rate, and not to their oxidation or decomposition. 

Comparing hydrogenation yields vs. temperature curve with hydrogen adsorption vs. temperature curve (Fig. 1) on vanadium oxide, a close similarity can be noticed. This parallelism led to a study of the effect of temperature on catalytic structure. 

Fig. 1. Comparison of hydrogenation curve for isobutylene and hydrogen adsorption curve vanadium oxide catalyst.

A possible mechanism for the effect of vanadia is suggested by the observation that vanadium oxide severely reduces the propensity of a Pt/alumina catalyst to store sulfate. It seems reasonable to speculate that SO2 oxidation on Pt/alumina involves adsorption of SO2 onto the alumina surface, with migration ( spillover ) of a sulfite species onto neighboring Pt crystallites and/or of oxygen atoms from the Pt onto the support. Coverage of alumina with high-valent vanadia can, because of its acidic nature, reduce affinity for SO2 and thereby disrupt the mechanism. 

Theory has been used predominantly to probe the nature of the sites on vanadium clusters and model vanadium oxide surfaces. Cluster and p>eriodic DFT calculations [68,69] have been carried out in order to imderstand the electronic and structural properties of the exposed (100) surface of (VO)2P207. Both cluster and slab calculations reveal that surface vanadium sites can act as both local acid and base sites, thus enhancing the selective activation of n-butane as well as the adsorption of 1-butene. Vanadium accepts electron density from methylene carbon atoms and, thus aids in the subsequent activation of other C-H bonds. Calculations reveal that that the terminal P=0 bonds lie close to the Fermi level and thus present the most nucleophihc oxygen species present at the surface for both the stoichiometric as well as phosphate-terminated surfaces. These sites may be involved in the nucleophilic activation of subsequent CCH bonds necessary in the selective oxidative conversion of butane into maleic anhydride. Full relaxation of the surface, however, tends to lead to a contraction of the terminal P=0 bonds and a lengthening of the P V bonds. This pushes the P V states, initially centered on the oxygen atoms, higher in energy and thus increases their tendency to be involved in nucleophilic attack . 

Abd El-Salaam, K.M., et al.. Physical adsorption studies on mixed vanadium oxide catalysts, Adsorpt. Sci. Technol., 1(2), 169-176(1984). 

Sambeth, J. Gambaro, L., and Thomas. II. Study of the adsorption. oxidation of methanol over vanadium pentoxide, Adsorpt. Sci. Technol, 12(3). 171-180(1995). 

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