Vanadium adsorption effects

A discussion of diffusion phenomena is not complete without mention of the importance of adsorption effects. Physical adsorption has a direct impact on the observed diffusion process. For example, diffusion of an organovanadium complex may be influenced by competitive adsorption with other vanadium species as well as by nickel compounds or other impurities in the oil. This competitive adsorption likely contributes to the large differences observed in the diffusion-limited reaction rates of nickel and vanadium compounds (Sato et al., 1971). 

Nitrogen adsorption/desorption isotherms on Zeolite and V-Mo-zeolite are very similar and close to a type I characteristic of microporous materials, although the V-Mo-catalysts show small hysterisis loop at higher partial pressures, which reveals some intergranular mesoporosity. Table 1 shows that BET surface area, microporous and porous volumes, decrease after the introduction of Molybdenum and vanadium in zeolite indicating a textural alteration probably because of pore blocking by vanadium or molybdenum species either dispersed in the channels or deposited at the outer surface of the zeolite. The effect is far less important for the catalysts issued from ZSM-5. 

Cathodic stripping voltammetry has been used [807] to determine lead, cadmium, copper, zinc, uranium, vanadium, molybdenum, nickel, and cobalt in water, with great sensitivity and specificity, allowing study of metal specia-tion directly in the unaltered sample. The technique used preconcentration of the metal at a higher oxidation state by adsorption of certain surface-active complexes, after which its concentration was determined by reduction. The reaction mechanisms, effect of variation of the adsorption potential, maximal adsorption capacity of the hanging mercury drop electrode, and possible interferences are discussed. 

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... 

Information relating to the diffusion of metal-bearing compounds in catalytic materials at reaction conditions has been obtained indirectly through classic diffusion and reaction theory. Shah and Paraskos (1975) calculated effective diffusitivities of 7 x 10-8 and 3 x 10-8 cm2/sec for V and Ni compounds in reduced Kuwait crude at 760°F. These low values may be indicative of a small-pore HDS catalyst. In contrast, Sato et al. (1971) report that the effective diffusivity of vanadium compounds was one-tenth that of the nickel compounds on the basis of metal deposition profiles in aged catalysts. This large difference may be influenced by relative adsorption strengths not explicitly considered in their analysis. 

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... 

A number of dissimilar surface compositions and structures develop at the NiAl ordered bulk alloy by preparation dependent effects. Completely different chemical behavior against oxygen adsorption and dissociation has been found for two CujAu surfaces, the (100) and (110) plane, consisting of the same surface composition. The (100) surface with Cu atoms surrounded by Au atoms turns up non-reactive alike Au. On the other hand, the (110) surface with Cu chains in the first layer acts similar to a Cu(l 10) surface. Intermixing or phase separation can be manipulated at CujAu too. Upon vanadium deposition on the bare alloy surface, strong intermixing and alloy formation towards a V3AU... 

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... 

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. 

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. 

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