Promotional Effects on Chemisorption

Strength of oxygen dissociatively chemisorbed on Pt supported on YSZ. Increasing eV R and e0 by 0.6 eV causes a 150°C decrease in Tp and a 0.6-eV decrease in E. The latter is computed by varying the heating rate p via the modified Redhead equation of Falconer and Madix  

It is important to notice that E decreases linearly with eO with a slope of 1, in excellent agreement with the observed decrease in activation energy E with e P in the Pt-catalyzed oxidation of C2H4 and CH4 (Fig. 21). 

The effect of and on the kinetics of oxygen adsorption and desorption on Ag deposited on YSZ has also been investigated recently. It was found that decreasing e P causes a fivefold increase in the rate of atomic oxygen adsorption, a sixfold decrease in the rate of atomic oxygen desorption, and a twofold increase in the equilibrium atomic oxygen uptake.  

These results establish that increasing/decreasing e0 causes a de-crease/increase in the chemi sorptive bond strength of electron acceptor adsorbates such as chemisorbed atomic oxygen and an increase/decrease in the chemisorptive bond strength of electron donor adsorbates such as benzene or dissociatively chemisorbed hydrogen. This is a key observation for rationalizing the in situ controlled promotional phenomena. 

ELECTROCHEMICAL PROMOTION WITH 0 -CONDUCTING SOLID ELECTROLYTES 

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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... 

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

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. 

The last explanation for methanol formation, which was proposed by Ponec et al., 26), seems to be well supported by experimental and theoretical results. They established a correlation between the gfiethanol activity and the concentration of Pd , most probably Pd. Furthermore, Anikin et al. (27) performed ab initio calculations and found that a positive charge on the palladium effectively stabilizes formyl species. Metals in a non-zero valent state were also proposed by Klier et al. (28) on Cu/ZnO/Al O, by Apai (29) on Cu/Cr O and by Somorjai for rhodium catalyts (30). Recently results were obtained with different rhodium based catalysts which showed the metal was oxidized by an interaction with the support (Rh-0) (on Rh/Al 0 ) by EXAFS ( -32) and by FT-IR ( ) and on Rh/MgO by EXAFS ( ). The oxidation of the rhodium was promoted by the chemisorption of carbon monoxide (, ). ... 

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]. 

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. 

The Cs promoter has no effect on the bonding of ethylene to the a-Al20s support. It is located, at submonolayer amoimts, on the stepped Ag surface where it blocks the chemisorption of the less selective, more strongly held, O atom. 

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. 

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