Electropositive Promoters Alkali Metals

Alkalis are the most important electropositive promoters of metal and metal oxide catalysts. They are used in many important industrial catalysts but are also quite suitable for fundamental studies since they can be easily introduced under vacuum conditions on well-characterized model metal surfaces. 

Alkali metals are strongly electropositive elements with low (2-3 eV) work function and low ionization potential. Upon adsorption on other metal surfaces they cause a severe (up to 3 eV) lowering of the metal work function, as already established by Langmuir in the early 1920 s. 

The adsorption of alkali metals on single crystal surfaces can result in the formation of ordered structures (commensurate or incommensurate super- 

Using the Helmholtz equation (2.21) and the initial AO vs 9aik slopes of Fig. 2.4 one computes alkali initial dipole moments P as high as 15 D. 

On the basis of the dipole moment, Paik, values computed from the Helmholtz equation (2.21) and the alkali ion radius one can estimate the effective positive charge, q, on the alkali adatom, provided its coordination on the surface is known. Such calculations give q values between 0.4 and 0.9 e (e.g. 0.86e for K on Pt(lll) at low coverages) which indicate that even at very low coverages the alkali adatoms are not fully ionized.6 This is confirmed by rigorous quantum mechanical calculations.27,28 

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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... 

In nonreactive molten salts, on the other hand, flux components are not incorporated into the product phase. Here, the molten salt acts more in the classical sense as a reagent to promote the reaction at a lower temperature than would be required by the ceramic, or direct, route (Section 5.2). This is accomplished by two attributes of molten salts an acid-base equilibrium that enables the general dissolution-recrystallization of metal oxides and a highly electropositive (oxidizing) environment that stabilizes the highest oxidation state of many transition metals (Gopalakrishnan, 1995), which can lead to mixed valency. A plethora of complex transition metal oxides have been synthesized in nonreactive molten alkali metal hydroxides, carbonates, and hypochlorites. Examples of such molten salt routes to mixed transition metal oxides include (Rao and Raveau, 1998) ... 

It is well known from catalysis that electropositive (e.g. Na, Cs, K) and electronegative (e.g. S, O, C, Cl) adatoms decrease or increase the reaction rate and thus poison or promote the reaction, respectively [153-155]. Alkali-metal influenced adsorption on transition metals was reviewed by Bonzel [154]. Coadsorption of alkali metals and H, or D, on Al(lOO) revealed that the sticking coefficient and dissociation rate are extremely weak ( 10 at all alkali coverages [156]). Upon exposing alkali-covered metal substrates to a beam of atomic H or D, alkali hydride formation was observed. 

Extraordinarily effective promotion by electropositive modifiers (alkalies or alkaline earths) of Pt-group metals (Pd, Pt, and Rh) during reactions related to TWCs has been firstly reported by Yentekakis, Lambert, and coworkers at the end of 1990s [39-44]. Indeed, it has been shown that the CO and hydrocarbons oxidation activity, but more importantly the de-NO activity and N2-selectivity, of noble metals can be substantially enhanced by alkali or alkaline earths. For example Pt, the noble metal mostly studied in this turn, demonstrated an enhancement on NO reduction rate by up to two orders of magnitude accompanied by a notable increase of N2-selectivity upon optimal promotion by Li, Na, K,... 

CSNO3 was also proved to be an excellent promoter for Raney Ru catalyst. The activity was compared with other alkali metal nitrate-promoted catalysts and is shown in Fig. 3.8. CSNO3 was suggested to remain mainly as Cs (Cs20, CsOH) and partly as Cs metal because water vapor decreased the activity. The activity sequence is in the order of electropositivity (Cs > Rb > K > Na). Since Raney Ru contains aluminium (mainly AI2O3, some Al), CsN03-Raney Ru quite resembles the Fe-Al203 K20 catalyst with respect to the composition. 

When atoms or molecules (e.g., promoters or reactant adsorbates) adsorb on a metal surface, they change its work function. Electronegative (electron acceptor) adsorbates such as O or Cl can increase the O of a metal surface up to 1 eV. Electropositive (electron donor) adsorbates such as H or, particularly, alkalis can decrease the O of a metal surface up to 3 eV. 

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