Demetallized surfaces

Several additional conclusions concerning the nature of the chemisorbed layer can be drawn from the Hall effect measurements (33, 34) The chemisorbed species, together with the surface metal atoms, represent complexes analogical to the ordinary chemical compounds and, consequently, one might expect that the metal atoms involved in these complexes will contribute to lesser extent or not at all to the bulk properties of the metal. Then we should speak about the demetallized surface layer (41). When the Hall voltage was measured as a function of the evaporated film thickness... 

Vijh AK (1972) Electrode reactions on demetallized surfaces. J Electrochem Soc 119 1498-1502... 

A final issue that faces this class of catalysts is stability in the fuel cell environment. Deactivation of materials in a fuel cell environment has been shown to be minimal in some studies,31,137 and severe in others.128,142 More active catalysts seem more susceptible to deactivation. Deactivation has been linked to the formation of peroxide and the loss of metal from the catalyst.128 On the other hand, demetallization has also been observed in pyrolyzed samples that did not lose activity with time.84 Another possible mode of deactivation could be due to the oxidation of the carbon surface. However, it seems reasonable that a complete understanding of the deactivation mechanism would first require a well-developed understanding of the active site. 

The extreme acidity at the surface of air-dried montmorillonite at room temperature is demonstrated by the demetallation of Sn(IV)-tetrapyridylporphyrin complexes. In contrast, in homogeneous... 

Webster (1984) reported an optimum Co/(Mo + Co) atomic ratio of 0.65 for HDM of Ni-porphyrins in model compound studies. A similar optimum ratio has been reported for HDS reactions (Wivel et al., 1981). However, evidence supporting this optimum Co/Mo ratio for demetallation of real feedstocks is lacking. It is likely that this optimum is important only with clean catalysts in the initial stages of oil processing. Once Ni and V begin to accumulate on the surface, the original composition becomes less important. 

Metals accumulate more slowly on the catalyst surfaces because the inlet concentrations of metals are lower than for coke precursors. The accumulation of metals can be even greater than coke, for example the vanadium concentration can reach 30-50 wt% of the catalyst on a fresh catalyst basis (Thakur and Thomas, 1985). Demetallization reactions can be considered autocatalytic in the sense that once the surface of the catalyst is covered with metal sulfides the catalyst remains quite active and continues to accumulate metal sulfides. The final loss of catalyst activity is usually associated with the filling of pore mouths in the catalyst by metal sulfide deposits. 

The mechanisms for electrocatalyst surface area loss are by a) crystallite migration or b) atom or ion dissolution and reprecipitation, either to the electrolyte or over the carbon surface. It is well known that surface diffusion of atoms on the individual crystallites can provide for mobility (much like the treads on a military tank). In either case, small crystallite become annihilated and fewer but larger crystallites are produced.18 In either event, these processes lead to demetallization of the less noble components in the alloy. 

The Z-isomer arises from a consecutive induction of active metallic titanium surface to the polydented pinacolic intermediate formed by homolytic coupling of a radical anion species generated from reduction of two carbonyl compounds that is followed by subsequent demetallation and deoxygenation reactions. In this regard, the phenoxy-Ti-sulfone induction plays the key role for Z-stereoselection by forcing the phenoxy and sulfone moieties to be positioned on the same side. 

Removal of Ni and V from residual oils is diffusionally limited [8], and therefore Ni and V are deposited in the catalyst pores in a characteristic deposition profile [7, 11, 131 which is either U- or M-shaped, i.e. the maximum metal deposition is either located at the catalyst pellet surface or inside the pellet. These phenomena have been investigated for both V and Ni using various V/Ni porphyrins as model compounds [1, 2, 3, 4, 5], It has been concluded that the removal of V and Ni porphyrins proceeds via a sequential reaction network, where the porphyrins are hydrogenated in the first step and deposited in a subsequent one. These studies have also indicated that the degree of complexity of reaction pattern depends on the types of porphyrins used. In these studies, no HjS was present In a recent study [6] of demetallization of Ni and V porphyrins, the reaction path of HDV and HDNi reactions has been investigated in the presence of H S. 

Graph 6 shows the equilibrium catalyst surface area and activity plotted against Ni + V PPM. This graph shows there is no significant difference in the deactivation rate of the equilibrium catalyst when part of the fresh catalyst was replaced with recycled demetallized catalyst. 

One further difference exists between HDS and HDM. Bridge [37] has shown, very clearly, that HDS is not limited by diffusion while HDM is. Using a nickel-molybdate based catalyst with a unimodal microporous size distribution, the demetalation of Arabian heavy atmospheric residuum was found to be affected by catalyst particle size, while HDS was not. As the diameter of the pore was decreased, the maximum in the metals deposition profile moved closer to the external surface of the pellet, agmn indicating difiusional limitations for FIDM. 

The HDM activity is aflfected in a different way by accumulated Ni and V on the catalyst. Demetallation is autocatalytic in nature, i.e. deposited Ni and V have a significant HDM activity [6]. Deposited Ni and V do not deactivate the HDM activity by coverage of the active surface but rather by restricting the access to the catalyst particle. 

The "HDS loading" has the lowest SOR temperature of the three. This is to be expected since TK-771, the active HDS catalyst, occupies more than 90% ofthe reactor volume. The "HDS" catalyst system is rapidly deactivated since the pore system of the small pore (low Qv) HDS catalyst is readily plugged. This is because the layer of TK-711 is too small to offer protection for the subsequent TK-771 catalyst. The "HDM loading" has a higher SOR temperature because a large fraction of the catalyst bed consists of TK-711, a large pore and low surface area demetallation catalyst. 

RM-430 demetallation catalyst is designed for maximum metal deposition capacity and metal removal activity and is made from Group IVB metals highly dispersed over a high surface area support. RM-430 catalyst has an optimal pore size distribution with very large pore volume, which provides it with a sizeable capacity for deposition of metals. The increase in pore volume, without the concomitant loss in catalyst pellet strength or surface area, has been accomplished by the use of specialized support materials and distinctive support preparation techniques. 

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