Poisons promotional effects

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... 

This paper identifies alumina, rare earths, platinum, and magnesia as important SOx capture materials. Alumina is either incorporated directly into the matrix of a cracking catalyst or added as a separate particle. Cerium is shown to promote the capture of SO2 on high alumina cracking catalyst, alumina, and magnesia. Other rare earths are ranked by their effectiveness. The promotional effect of platinum is shown between 1200 and 1400 F for SO2 capture on alumina. Silica, from free silica or silica-alumina in the matrix of cracking catalyst, acts as a poison by migrating to the additive. 

Cheekatamarla and Lane [62, 63] studied the effect of the presence of Ni or Pd in addition to Pt in the formulation of catalysts for the ATR of synthetic diesel. For both metals, a promotional effect with respect to catalytic activity and sulfur poisoning resistance was found when either alumina or ceria was used as the support. Surface analysis of these formulations suggests that the enhanced stability is due to strong metal-metal and metal-support interactions in the catalyst. 

Unlike boron fluoride, titanium tetrachloride does not catalyze the liquid phase polymerization of isobutylene under anhydrous conditions (Plesch et al., 83). The addition of titanium tetrachloride to a solution of the olefin in hexane at —80° failed to cause any reaction. Instantaneous polymerization occurred when moist air was added. Oxygen, nitrogen, carbon dioxide, and hydrogen chloride had no promoting effect. Ammonia and sulfur dioxide combined with the catalyst if these were added in small quantity only, subsequent addition of moist air permitted the polymerization to occur. Ethyl alcohol and ethyl ether, on the other hand, prevented the polymerization even on subsequent addition of moist air. They may be regarded as true poisons. 

The modern methanol synthesis catalyst consists of copper, zinc oxide, and alumina. Copper metal is seen as the catalytically active phase, and ZnO as the promoter. It is well known that the interaction between the two components is essential for achieving a high activity, but the nature of the promoting effect is still a matter of debate. Loss of activity is caused by sintering of the Cu crystallites, and, if the feed gas contains impurities such as chlorine and sulfur, by poisoning. 

The promoting effect of the addition of alkali on the catalytic performance of many transition-metal-based catalysts is experimentally well known, but there is no general agreement on its theoretical explanation. The same holds for the opposite effect the poisoning of catalysts by, e.g., the adsorption of sulphur. 

The promoting effect of the third component is also compared with the case of Pt and Pt-Ru catalysts dispersed in PAni. During the oxidation of methanol, the production of carbon dioxide (final product) is observed at a potential as low as 350 mV versus RHE on PAni/Pt-Ru-Mo. Concerning the case of CO adsorption from gaseous CO, formation of CO2 is observed at 250 mV versus RHE, indicating clearly that Pt-Ru-Mo is less poisoned by CO ads in comparison with Pt-Ru and Pt (the formation of CO2 occurring, respectively, at 400 mV and 750 mV versus RHE). 

Thus, the reaction is poisoned by the formation of Pt-(CO)ads after complete methanol dehydrogenation. There have been intensive searches for other active materials, which can provide oxygen in its active form from water at low overpotentials, to increase the oxidation rate of chemisorbed CO. Pt-Ru alloys are reported as having excellent promotional effects, " and PtSn,[ l PtMo,t ° PtRuCo, and PtRuIrOs have also been studied for the MOR reaction of DMFC. 

Ceria is also a very good promoter of the water-gas shift reaction (WGSR) [90,91], which can be linked to the fact that hydroxyl groups are extremely mobile on this oxide [63]. Barbier Jr. and Duprez showed there was a beneficial effect of ceria both on CO oxidation and on WGSR, more marked over Rh than over Pt catalysts [92]. Moreover, a promoter effect of H2O can be observed in CO oxidation while O2 was rather a poison of WGSR. 

Surface reaction (sections 6-8). This is the largest section, concerned with the reaction event itself occurring between adsorbed species. It will include a brief description of reaction kinetics at surfaces, together with a classification of the kind of catalytic reactions which are important and a consideration of work mainly carried out on well-defined surfaces where the surface structure is well-characterised. It includes further sections on the effect of atomic number (electronic structure) on reactions on metals in the transition series, on the effects of surface structure on reaction rates, and on important aspects of catalysis, namely poisoning and promoter effects. 

Catalytic activity of MCM-41 with rhodium oxide nanoparticles prepared by addition of RhClj 3H2O in sol-gel mixture were studied in the high-tempera-ture NO-CO catalytic reaction [22]. Catalyst containing RhO nanoparticles with diameters less than 3 run exhibited a novel promotional effect in the amount of N2 and N2O formation with excess O2, while most of Rh catalysts become poisoned with the O2 excess. The authors also claim that catalysts with 6-8 nm RhO nanoparticles in the MCM-41 had drastically retarded the formation of target molecules. Since the Rh precursor is added to the sol-gel mixture and particles can be located in the siHca body, the comparison of solely particle size in determination of catalytic properties can be misleading, as particle location (accessible for gases or not) may be a crucial feature. 

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