Catalysts, general vanadium reduced

Analysis of structure-activity relationships shows that various species characterized by different reactivities exist on the surface of vanadium oxide-based catalysts.339 The redox cycle between V5+ and V4+ is generally accepted to play a key role in the reaction mechanism, although opposite relationships between activity and selectivity, and reducibility were established. More recent studies with zirconia-supported vanadium oxide catalysts showed that vanadium is present in the form of isolated vanadyl species or oligomeric vanadates depending on the loading.345,346 The maximum catalytic activity was observed for catalysts with vanadia content of 3-5 mol% for which highly dispersed polyvanadate species are dominant. 

These observations on the sulfuric acid catalyst arc full in line with the general thermodynamic behaviour of fused catalyst systems. The mctastablc solid in Figure 2 has to be replaced in this case by a cascade of the partly reduced vanadium ternary sulfates. The processes sketched above occur under thermodynamic control in a quaternary phase diagram, vanadium-oxygen-sulfur-alkali, as illustrated by the reversibility of the exsolution of the partly reduced vanadium compounds under suitable partial pressures of oxygen... 

Table 6.2). In general, XPS of fresh catalysts shows mainly oxidized vanadium (V ). After reduction treatments, a decrease in oxidation state is observed. However, the extent of reduction depends on a number of factors, such as reduction temperature, reducing agent and partial pressure of the reducing gas, as well as the method used to transfer the reduced sample to the measurement chamber. Ideally, contact with air should be minimized, or excluded if possible, to prevent re-oxidation of the catalyst when the sample is transferred from a reactor to the UHV measurement chamber. This problem can be circumvented by in situ instrumentation as discussed later.

Both the early data and the more precise values given in Table 20A differ significantly from published estimates based on monomer and polymer composition (e.g. rj r2 = 0.60 in Table 19). As all the data relate to the, in general, more consistent soluble vanadium systems, this work reinforces doubts concerning the accuracy of much of the published information. A complication is that since C2 and C4 sequences are observed in ethylene/propene copolymers inverted head to head prop-ene units must be present and this will reduce the accuracy of analyses of EP sequences. In copolymers prepared by VC /AlEtj 4% of head to head propene sequences have been reported with the catalyst VCl4/AlEt2Cl which is syndiospecific for polypropene 8% of head to head sequences was found [295]. ... 

Vanadium interacts with nickel in a manner which inhibits the deactivation behavior of nickel. Metals-resistant catalysts must therefore be evaluated in the presence of both nickel and vanadium. Also, the mobility of vanadium is reduced in the presence of nickel. In general, cyclic deactivation will be the preferred deactivation method in order to simulate the actual metal distribution and interactions on the catalyst and the correct metal age distribution. 

Ethylbenzene dehydrogenation is generally catalyzed by a potassium-promoted iron oxide catalyst. The most widely used catalysts are composed of iron oxide, potassium carbonate, and various metal oxide promoters. Examples of metal oxide promoters include chromium oxide, cerium oxide, molybdenum oxide, and vanadium oxide. " The potassium component substantially increases catalyst activity relative to an unpromoted iron oxide catalyst. Potassium has been shown to provide other benefits. In particular, it reduces the formation of carbonaceous deposits on the catalyst surface, which prolongs catalyst life. 

The selective oxidation of methanol to give formaldehyde is in practice performed in two different processes, one using metallic silver, the other using iron molybdate as catalyst. Vanadium oxide has been shown to be a good selective catalyst in a variety of oxidation processes (refs. 1-2) and we have previously shown that it is also selective for methanol oxidation (refs. 3-5) when the V Og is applied as a very thin layer (monolayer) on different supports the support can have a significant influence on the activity and selectivity of these monolayer catalysts, as was shown by Roozeboom (ref. 6). In a previous paper (ref. 5), it was shown that both the type of support (A Og or TiC ) and the crystal structure of the TiO have an influence on the selectivity of the catalyst for the production of formaldehyde in general, production of the formaldehyde increases with a decrease in the reducibility of the vanadia. 

Many industrially important selective oxidation reactions are catalyzed by transition metal oxides. The activity of such catalysts is related to the reducibility of the transition metal ion, which enables the bulk oxide lattice to participate actively in the redox processes present in the Mars van Krevelen mechanism. Unfortunately, NMR spectroscopic investigations are severely limited by the occurrence of paramagnetic oxidation states. As a general rule, NMR signals from atoms bearing unpaired electron spins cannot be detected by conventional methtxls, and the spectra of atoms nearby are often severely broadened. For this reason, most of the work published in this area has dealt with diamagnetic vanadium(V) oxide-based catalysts. 

The amount of hydrogen in relation of the estequiometric quantities of Ni to Ni and reduced to V is very interesting. These results are presented on Table 1. For 1-Ni catalyst this quantity is twice that required by the estequiometric. The amount of hydrogen consumption progressively decrease with the increase of vanadium concentration. This effect generally supports the idea of the nickel vanadium interaction. 

The addition of residual fractions to gas oil feed results in an increase in the impurity content of the equilibrium FCC catalyst and causes a decrease in activity. Metal impurities exist as porphyrin complexes which crack and deposit metal residues on the catalyst surface, causing catalyst deactivation. The most serious effects on catalyst performance result from nickel and vanadium compounds. Sodium can also deactivate acid sites on the catalyst, but the effect is generally reduced by desalting erode oils and by absorption of small amounts of sodium on the matrix. Sulfur compoimds in the feed contaminate products and regenerator flue gas. 

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