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Vanadyl , adsorption

Wehrli, B. and W. Stumm, 1989, Vanadyl in natural waters Adsorption and hydrolysis promote oxygenation. Geochimica et Cosmochimica Acta 53, 69-77. [Pg.533]

Wehrli, B S. Ibric, and W. Stumm (1990), "Adsorption Kinetics of Vanadyl(IV) and Chromium(III) to Aluminum Oxide Evidence for a Two-step Mechanism , Colloids and Surfaces 51,77-88. [Pg.416]

Hydrodemetallation reactions require the diffusion of multiringed aromatic molecules into the pore structure of the catalyst prior to initiation of the sequential conversion mechanism. The observed diffusion rate may be influenced by adsorption interactions with the surface and a contribution from surface diffusion. Experiments with nickel and vanadyl porphyrins at typical hydroprocessing conditions have shown that the reaction rates are independent of particle diameter only for catalysts on the order of 100 /im and smaller (R < 50/im). Thus the kinetic-controlled regime, that is, where the diffusion rate DeU/R2 is larger than the intrinsic reaction rate k, is limited to small particles. This necessitates an understanding of the molecular diffusion process in porous material to interpret the diffusion-disguised kinetics observed with full-size (i -in.) commercial catalysts. [Pg.173]

Diffusion and adsorption studies with Boscan VO-porphyrin extracts and pure VO-TPP in C0M0/AI2O3 catalysts have been reported by Morales and co-workers (Galiasso and Morales, 1983 Morales and Galiasso, 1982 Andreu et al., 1981 Morales et al., 1984). The Boscan extract contained up to 30 wt. % vanadyl DPEP and vanadyl etioporphy-rin. Effective diffusion coefficients at 300°C in a catalyst with an average pore diameter of 150 A (A < 0.1) are on the order of 10-5 cm2/sec. Configurational effects are minor for this system. [Pg.174]

Vanadyl porphyrin interaction with the surface is a function of the catalyst. Adsorption through electron acceptor sites dominates on the oxide surface, whereas the sulfided catalyst interacts through electron donor sites (see Section IV,B,5). Heats of adsorption have been estimated to be 8 to 12 kcal/mole. Values in this range are indicative of weak adsorption interactions that are of reduced importance at hydroprocessing conditions. [Pg.174]

Limited experimental data for nickel porphyrins indicate weaker adsorption than their vanadyl analogs, which is consistent with the oxygen ligand on vanadium playing an active role. With this difference it stands to reason that competitive adsorption contributes to the inhibition observed in nickel demetallation rates in the presence of vanadyl porphyrins. Therefore, the higher vanadium reactivity relative to nickel can be rationalized on the... [Pg.174]

Vanadyl and nickel reactivity differences resulting from the chemistry of the oxygen ligand on vanadium were discussed in Section IV,A,l,c. Enhanced V reactivity could also arise from molecular size constraints. Beuther and co-workers (Beuther and Schmid, 1963 Larson and Beuther, 1966) speculate that nickel concentrates in the interior of asphaltene micelles while vanadium concentrates on the exterior. Thus a combination of stronger adsorption due to the oxygen ligand and inhibition of Ni reaction, coupled with the exposed position at the periphery of the asphaltene, may all contribute to the enhanced vanadium reactivity relative to nickel. [Pg.193]

Mazur et al. described the organization of vanadyl oxide 2,9,16,23-tetraphenoxy-29H, 31//-phthalocyanine (V02+PcPhO) on highly oriented pyrolytic graphite (HOPG) [47], They focused on the fact that the adsorption geometry of nonplanar Pc complexes of titanyl and vanadyl (TiOPc and VOPc) is not well understood... [Pg.142]

Wehrii, B., and Stumm, W. (1989) Vanadyl in Natural Waters Adsorption and Hydrolysis Promote Oxygenation, Geochim. Cosmochim. Acta 53, 69-77. [Pg.972]

Moreover, no adsorption band near 600 nm typical of VO ions is observed in the spectra of the two as-made samples, although vanadyl cations in a square pyramidal geometry have been detected by ESR in V-MCM-41. This is probably due to the fact that d-d transitions in VO are generally 10-30 times weaker than those of CT transitions and therefore are undetectable by diffuse reflectance UV-vis [34]. However, a small band at ca 605 nm has been detected by Chao et al. [24] in the as-synthesized V-MCM-41 samples having Si/V= 30, This band is responsible for the pale violet colour of the as-made sample. Nevertheless, our as-made samples are white in colour, suggesting that VO species should occur, if any, in very small concentration... [Pg.285]

The adsorption of CO on pristine V=O terminated V2O3 was limited (Fig. 17.10a). The small TPD desorption peak at 105 K was most likely due to the adsorption of CO on a small number of defects (sites where the vanadyl oxygen was missing... [Pg.384]

Wehrli, B., Ibric, S., and Stumm, W., Adsorption kinetics of vanadyl(IV) and chromium(III) to aluminum oxide Evidence for a two-step mechanism, Colloids Surf., 51,11, 1990. [Pg.950]

To favour the simultaneous adsorption and conversion of the substrate the catalysts should contain centres which are neighboured closely enough to match the distance between the n -complex centre and the methyl carbon atom of the toluene molecule which can be estimated to be 3 A from the respective C-Cmg (14 A) and C-CH3 bond length (1.52 A) [15]. In addition, the above mentioned redox process at the surface vanadyl ions leads to a fluctuation of their electron density. When they are incorporated into units of exchange-coupled centres the alteration of the electron density can be rapidly delocalized over a certain range of the structure. We assume that in this case the electronic distortion at a discrete surface vanadyl centre is diminished and the activation energy is lowered. Thus, effective exchange pathways within the catalyst structure appear to be another requirement for effective catalytic performance. [Pg.926]

Vanadyl distances must be close enough to allow simultaneously the adsorption of the n-system and the ammoxidation of the methyl group in neighbouring sites. [Pg.928]

Catalyst structures with isolated sin e chains of V06 octahedra (e. g. crystalline V0(P03)2) are markedly less active than those with layers or double chains of vanadyl centres (e. g. (VO)2P207). The reason is probably that the latter materials enable both the adsorption of the aromatic n -system and the ammoxi tion of the methyl group. In contrast, catalysts with single chains do support the adsorption but not the ammoxidation due to the absence of a second closely neighbouring vanadyl site. [Pg.928]

Figure 2. Adsorption of vanadyl and Fe(II) to Ti02 (anatase). Both cations adsorb specifically to the positively charged surface. Conditions 25°C, / = 0.1 (NaC104) [VO2 + ] = 50gM, 2gL-1 Ti02 [Fe2 + ] = 100 jiM, 10 gL 1 Ti02. Figure 2. Adsorption of vanadyl and Fe(II) to Ti02 (anatase). Both cations adsorb specifically to the positively charged surface. Conditions 25°C, / = 0.1 (NaC104) [VO2 + ] = 50gM, 2gL-1 Ti02 [Fe2 + ] = 100 jiM, 10 gL 1 Ti02.
Vanadyl (V02 +) is an ideal cation for the study of heterogeneous oxidations for several reasons (1) the electron-transfer behavior of VO2 + is in many aspects similar to that of Fe2+ (Rosseinsky, 1972, Wehrli et al., in press), (2) the experimental conditions can be chosen so that vanadyl is completely adsorbed at pH >4 (Fig, 2), (3) the adsorbed V(IV) species have been characterized by ENDOR spectroscopy as inner-sphere surface complexes (=MO)xVO (Motschi and Rudin, 1984). Adsorption experiments are compatible with x = 2. The oxygenation rates of V02+ adsorbed to anatase and <5-Al203 follow the empirical rate law... [Pg.328]

Adsorption of vanadium and chromium are of direct significance in geo-chemical cycles. Hydroxide surfaces largely determine their transformation and complex formation. Such surfaces are additionally built up by a thin adherent water film. The adsorption kinetics of vanadyl (IV) and chromium (III) depend on the surface OH ligands (Wehrli Stumm 1988). For the interpretation of the results the relation between the adsorption rate of different ions and the rate of water exchange is based on the data obtained by Hachiya et al. (1984). [Pg.95]


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