Oxide catalysts, promoter effect

It is well known that the presence of water on an oxide catalyst surface effectively reduces the catalytic activity versus a wide variety of reactions. On the other hand, promotional effects have also been observed, so that criterion h (Section II.C.l) is invalidated. Water can be held by incompletely coordinated... 

V2O5 promoted by P or Mo is widely used for selective oxidation. V P oxide catalysts are effective for the selective oxidation of n-butane to maleic anhydride. The reaction is a 14-electron oxidation involving the abstraction of eight hydrogen atoms and insertion of three oxygen atoms (equation 9). 

There is a limited effect of ceria on Pt and Pd catalysts, rather negative in alkane oxidation. On the contrary, there is a beneficial effect of ceria on rhodium catalysts, particularly marked in CO oxidation. This promoter effect of ceria on Rh in CO oxidation has been the object of several investigations [81-83]. However, most authors found a promoter effect more pronounced around the stoichiometry than in O2 excess. The results obtained by Oh and Eickel [82] clearly illustrate this tendency (Fig. 7.14). 

In Figure 5, however, it is seen that the pre-treatment atmosphere has a significant effect on the low temperature activity of Pd/Co/AbOa. The effect of catalyst pre-treatment is most pronounced for the cobalt oxide catalyst promoted with Pd or Pt (see Table 2). In lean reactant gas, pre-reduced Pd/Co/AbOs has light-off temperatures at 169°C and 177°C for CO and HC, respectively, whereas the light-off temperatures over the same catalyst, but pre-oxidised, are 246°C for both CO and HC. A clear effect of pre-reduction is also seen for Pt/Ce/Ab03, whereas no obvious effect of the pre-treatment atmosphere on the oxidation activities for CO or HC is seen for Pd/Ce/AbOs. 

Nicotinonitrile is a precursor to nicotinic acid (Niacin) and nicotinamide, both of which are important B-complex vitamins. High selectivities to nicotinonitrile by ammoxidation of 3-picoline are possible with a variety of supported and promoted vanadium oxide catalysts. Particularly effective is V2O5 supported on titania. Merck Co., Inc., U.S.A. disclosed the use of V205/Ti02 catalysts promoted with M0O3, P2O5, and MnS04 for 3-picoline ammoxidation. The reaction was conducted at a relatively low temperature for ammoxidation in order to... 

Sakurai, Y., Suzaki,T., Nakagawa, K., et al. (2002). Dehydrogenation of ethylbenzene over vanadium oxide-loaded MgO Catalyst Promoting effect of carbon dioxide, J. Catal., 209, pp. 16-24. 

A derivative of the Claus process is the Recycle Selectox process, developed by Parsons and Unocal and Hcensed through UOP. Once-Thm Selectox is suitable for very lean acid gas streams (1—5 mol % hydrogen sulfide), which cannot be effectively processed in a Claus unit. As shown in Figure 9, the process is similar to a standard Claus plant, except that the thermal combustor and waste heat boiler have been replaced with a catalytic reactor. The Selectox catalyst promotes the selective oxidation of hydrogen sulfide to sulfur dioxide, ie, hydrocarbons in the feed are not oxidized. These plants typically employ two Claus catalytic stages downstream of the Selectox reactor, to achieve an overall sulfur recovery of 90—95%. 

An effect which is frequently encountered in oxide catalysts is that of promoters on the activity. An example of this is the small addition of lidrium oxide, Li20 which promotes, or increases, the catalytic activity of dre alkaline earth oxide BaO. Although little is known about the exact role of lithium on the surface structure of BaO, it would seem plausible that this effect is due to the introduction of more oxygen vacancies on the surface. This effect is well known in the chemistry of solid oxides. For example, the addition of lithium oxide to nickel oxide, in which a solid solution is formed, causes an increase in the concentration of dre major point defect which is the Ni + ion. Since the valency of dre cation in dre alkaline earth oxides can only take the value two the incorporation of lithium oxide in solid solution can only lead to oxygen vacaircy formation. Schematic equations for the two processes are... 

Hall and Hassell (50) continued these studies with the intention of proving that possible traces of oxide dissolved in the metal play no significant role in the poisoning or promoting effects arising from hydrogen which had been presorbed during the pretreatment procedure. The catalysts were prepared in essentially the same manner as before. The kinetics... 

C. Pliangos, I.V. Yentekakis, V.G. Papadakis, C.G. Vayenas, and X.E. Verykios, Support-induced promotional effects on the activity of automotive exhaust catalysts 1. The case of oxidation of light hydrocarbons, Appl. Catal. B 14, 161-173 (1997). 

It is worth mentioning that both the carboxylation of epoxides and anilines are acid-base reactions, which do not entail redox processes. Therefore a catalyst active in these reactions must provide acid-base functionality. In this perspective, positively charged gold could be the real player, although a co-catalytic or promotion effect of ze-rovalent gold could also be important. Therefore the catalysts for the oxidative carbonylation of aniline, supported on Merck Ion-exchanger IV, could be actually bifunctional. On one side, Au could catalyze the oxidation of CO with O2 to CO2, a reaction for which it is... 

It should be mentioned here that Sn sites are not considered to be the solitary source for OHad, which could be adsorbed on Pt sites owing to the influence of adjunct Sn atoms [Stamenkovic et al., 2005], The promotional effect of Sn was later confirmed on a PtSn/C nanocatalyst [Arenz et al., 2005], which exhibits similar behavior that was assigned primarily to the formation of reactive OH species at much lower potential than on pure Pt catalysts. Based on these findings, the bifunctional effect was unambiguously confirmed for Pt-Sn surfaces, where Sn sites serve as a source of oxygenated species that boost CO oxidation at low potentials and allow these surfaces to be employed as CO-tolerant catalysts. 

Within the inverse model catalyst approach, the y/7-V309-Rh(l 11) nanostructures have been used to visualize surface processes in the STM with atomic-level precision [104]. The promoting effect of the V-oxide boundary regions on the oxidation of CO on Rh(l 1 1) has been established by STM and XPS by comparing the reaction on two differently prepared y/7-V309-Rh(l 11) inverse catalyst surfaces, which consist of large and small two-dimensional oxide islands and bare Rh areas in between [105]. A reduction of the V-oxide islands at their perimeter by CO has been observed, which has been suggested to be the reason for the promotion of the CO oxidation near the metal-oxide phase boundary. 

Bezemer, G. L., Radstake, P. B., Falke, U., Oosterbeek, H., Kuipers, H. P. C. E., van Dillen, A., and de Jong, K. P. 2006. Investigation of promoter effects of manganese oxide on carbon nanofiber-supported cobalt catalysts for Fischer-Tropsch synthesis. Journal of Catalysis 237 152-61. 

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