Alkali-metal chemisorption

Prior to the early 1990s, all structural studies of alkali-metal chemisorption found the adatom located at high coordination sites at which the alkali-metal atom is bound in three- or four-fold hollow sites. A comprehensive survey of alkali-metal adsorption studies prior to 1988 may be found in the book edited by Bonzel (Bonzel et al., 1989). Several more recent LEED, SEXAFS and X-ray studies have implicated low coordination (top) sites, as in the case of Cu(lll)p(2x2)-Cs, or substitutional behavior. These results may signal that the current understanding of the alkali-metal bonding at surfaces is incomplete. 

NakatsujI H, Kuwano R, Merita H and Nakal H 1993 Dipped adcluster model and SAC-CI method applied to harpooning, chemical luminescence and electron emission in halogen chemisorption on alkali metal surface J. Mol. Catal. 82 211-28... 

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 significance of the sign and the magnitude of the S.P. for the chemisorption of alkali metals and gases will be dealt with in Sec. VIII. 

Thermionic and photoelectric methods have been successfully employed in the measurement of the mobility of alkali metals on W surfaces. The migration of adsorbed gases like H2, O2, and CO over a metal surface may be followed in the F.E.M., and data concerning the mobility of these adsorbates are of particular interest, since they are frequently involved in surface reactions and other chemisorption phenomena. 

Sato and Akamatu (139) report that alkali metals enhance the chemisorption of oxygen on carbon and weaken the carbon-carbon bonds at the surface so as to accelerate combustion. On the other hand, they report that phosphorus, while catalyzing the adsorption of oxygen on carbon, has a retarding effect on the release of the surface oxide. 

We propose that the alkali metal neutralizes the hydroxyl groups on the catalyst surface and prevents the chemisorption of substrates on the acidic center of the support. 

The mutual depolarization of the surface dipoles may be responsible for a third contribution. This effect is of importance in the chemisorption of atoms of alkali metals on metal surfaces. It causes, also in the chemisorption of other gases a minimum or a maximum in the surface potential as a function of 0. 

Alkali-metals are frequently used in heterogeneous catalysis to modify adsorption of diatomic molecules over transition metals through the alteration of relative surface coverages and dissociation probabilities of these molecules.21 Alkali-metals are electropositive promoters for red-ox reactions they are electron donors due to the presence of a weakly bonded s electron, and thus they enhance the chemisorption of electron acceptor adsorbates and weaken chemisorption of electron donor adsorbates.22 The effect of alkali-metal promotion over transition metal surfaces was observed as the facilitation of dissociation of diatomic molecules, originating from alkali mediated electron enrichment of the metal phase and increased basic strength of the surface.23 The increased electron density on the transition metal results in enhanced back-donation of electrons from Pd-3d orbitals to the antibonding jr-molecular orbitals of adsorbed CO, and this effect has been observed as a downward shift in the IR spectra of CO adsorbed on Na-promoted Pd catalysts.24 Alkali-metal-promotion has previously been applied to a number of supported transition metal systems, and it was observed to facilitate the weakening of C-0 and N-0 bonds, upon the chemisorption of these diatomic molecules over alkali-metal promoted surfaces.25,26... 

L. F. Liotta, G. A. Martin, and G. Deganello, The influence of alkali metal ions in the chemisorption of CO and C02 on supported palladium catalysts, a Fourier transform infrared spectroscopic study, J. Catal. 164, 322-333 (1996). 

This section reports a series of examples of application of the cluster model approach to problems in chemisorption and catalysis. The first examples concern rather simple surface science systems such as the interaction of CO on metallic and bimetallic surfaces. The mechanism of H2 dissociation on bimetallic PdCu catalysts is discussed to illustrate the cluster model approach to a simple catalytic system. Next, we show how the cluster model can be used to gain insight into the understanding of promotion in catalysis using the activation of CO2 promoted by alkali metals as a key example. The oxidation of methanol to formaldehyde and the catalytic coupling of prop)me to benzene on copper surfaces constitute examples of more complex catalytic reactions. 

The alkali metals have a role to play in ammonia synthesis. The K20 promoter in the conventional NH3 synthesis catalyst enhances the chemisorption of nitrogen and causes a hydrogen-promoted dissociation of the N2... 

The alkaline earth metal addition to the Pd catalyst improved the hydrocarbon oxidation activity. Similar phenomena have been observed on Pd/Ba and Pd/La catalysts, and it is concluded that the suppression of hydrocarbon chemisorption on Pd by the addition of Ba or La allows the catalytic reaction to proceed smoothly under reducing conditions( 16,20). On the other hand, the alkali metal addition, especially K or Cs, to the Pd catalyst deteriorated the hydrocarbon oxidation activity. 

From the partial reaction orders in the CjH -Oj reaction system and characterization by XPS and TPR on the catalysts, it was concluded that the alkaline addition to the Pd three-way catalyst weakened the adsorption strength of hydrocarbons on Pd. The addition of alkaline earth metal suppressed the hydrocarbon chemisorption on the Pd catalyst and therefore allowed the catalytic reaction to proceed smoothly. On the other hand, the addition of alkali metals, in particular K or Cs, caused such a strong oxygen adsorption on Pd that rejected the hydrocarbon adsorption and therefore suppressed the reaction. It was considered that the effect of the alkaline addition to the strength of adsorbed hydrocarbons on Pd was caused by the increase of electron density of Pd. 

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. 

The opposite situation from weak interaction of inert gases with the surface space charge is surface ionization, when the adsorbate is ionized by the substrate. This typically occurs in alkali-metal adsorption on transition-metal surfaces. In the more usual situation with chemisorbed molecules, only partial charge transfer occurs to or from the substrate to the molecule. If the negative pole of the molecule points toward the vacuum, the induced electric fields cause an increase in the work function. Table 5.4 lists the work-function changes obtained by the chemisorption of several molecules on rhodium. 

Alkali metals are often used as additives during catalytic reactions. They are bonding modifiers that is, they influence the bonding and thus the reactivity of the coadsorbed molecules. Potassium is a promoter in CO hydrogenation reactions where CO dissociation is desired and is one of the elementary reaction steps. The alkali metal also reduces the hydrogen chemisorption capacity of the transition metal. Potassium is a promoter in ammonia synthesis for the opposite reason, because it weakens the NH3 product molecule bonding to the metal, thereby reducing its sur-... 

Halogen species can also be important bonding modifiers, because they are powerful electron acceptors. Indeed, they are used as promoters in several catalytic processes (for example, ethylene oxidation to ethylene oxide over silver, or during partial oxidation of methane). Nevertheless, their molecular and atomic chemisorption behavior has been studied less and therefore is not as well understood as the role of coadsorbed alkali-metal ions. 

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