Chemisorption promotional effects

For alkali modified noble and sp-metals (e.g. Cu, Al, Ag and Au), where the CO adsorption bond is rather weak, due to negligible backdonation of electronic density from the metal, the presence of an alkali metal has a weaker effect on CO adsorption. A promotional effect in CO adsorption (increase in the initial sticking coefficient and strengthening of the chemisorptive CO bond) has been observed for K- or Cs-modified Cu surfaces as well as for the CO-K(or Na)/Al(100) system.6,43 In the latter system dissociative adsorption of CO is induced in the presence of alkali species.43... 

The effects of precious metals on ln/H-ZSM-5 was found not only to simply catalyze NO oxidation but also to enhance NOx chemisorption. It is noted that NO conversion on the lr/ln/H-ZSM-5 exceeded NO2 conversion in NO2-CH4-O2 reaction on in/H-ZSM-5, when the concentration of NOx was decreased [14]. This study shows the catalytic activities of ln/H-ZSM-5 promoted by precious metals for the removal of low concentration NOx and the promotive effects of the precious metal will be discussed. 

There is evidence of a promoting action of chromium on nickel catalysts for the reaction of hydrogenation of valeronitrile in our conditions. Introduction of chromium increased the initial specific activity and the selectivity. The promoting effect of chromium on activity could be correlated to the increase of the metallic surface. Another explanation could be that the Cr+ segregated at the surface of the catalyst may play the role of a Lewis acid center and may be responsible for a better chemisorption of valeronitrile on the catalysts, through nitrogen lone pair electrons or the n orbital of the CN bond. However, further examination of the results obtained (see Fig. 3)... 

This appendix begins with a brief introduction to the physics of metal surfaces. We limit ourselves to those properties of surfaces that play a role in catalysis or in catalyst characterization. The second part includes an introduction to the theory of chemisorption, and is intended to serve as a theoretical background for the chapters on vibrational spectroscopy, photoemission, and the case study on promoter effects. General textbooks on the physics and chemistry of surfaces are listed in [1-8]. 

The final section is concerned with the NMR of supported metal particles, predominantly Pt NMR. The data and their interpretation are given in relation to a number of concepts in phenomenological (NMR spectrum and dispersion NMR spectrum and chemisorption) or theoretical (electron deficiency promoting effect) catalysis. 

In most of the catalysts composed of copper zinc and copper-pyrochlore a correlation between the copper surface area, the amount of formates located on the copper sites after CO2 or methanol chemisorption, and the catalytic activity can be found, but in most cases the relation is not strictly linear. The promoting effect of ZnO on Cu-LaZr catalysts cannot be ascribed to an enhancement of copper coverage or formate formation. Zinc plays therefore rather a positive role in the formate hydrogenation than its formation. 

Alkali metal promoters are known to control acidity in supported metal catalysts. Our studies on alkali promoted Pt/Al203 catalysts through H2-O2 chemisorption. Temperature Programmed Reduction and ammonia TPD techniques have shown that besides the attenuation of acidity, added alkali affects the binding of Pt species on the support, thereby influencing its reducibility and dispersion. Based on the studies above, several aspects of promoter effects in supported platinum catalysts are discussed. 

Using the concept that two sites are responsible for the promotional effects observed, it is possible to correlate quantitatively C02 production activity with the surface concentration of both metals provided by the characterization results. The contribution of the Pt sites can be calculated by multi piing the number of Pt sites Npt, measured by CO chemisorption on each catalyst, with the turnover number of the Pt sites, T0Npt. 

Low activity and selectivity toward carbonylation products, methyl acetate, and acetic acid are obtained in methanol carbonylation on Ni/C at normal pressure (Table 11). However, by using Sn-Ni/C catalysts a higher activity is reached (34). The promoting effect of Sn on Ni/C for carbonylation is clearly observed in Table 11. For example, with the addition of 1 wt% Sn to Ni/C, the Sn-Ni/C catalysts produces a 24% increase in methanol conversion (34). The increase is in proportion to the Sn loading within the experimental range. This result can be interpreted as Sn being capable of increasing the number of active sites for carbonylation. This interpretation was supported by the CO chemisorption data. An increase in CO adsorption indicated by these data to occur as Sn is added to Ni/C. 

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