Alkali-promoted surfaces

Adsorption of Gases on Surfaces Modified by Alkali Promoters... 

The last point is confirmed by measuring the work function changes upon CO chemisorption on clean and alkali-promoted metal surfaces. Figures 2.16 and 2.17 show the work function changes induced by CO adsorption on a K/Pt(lll) and on a Na/Ru(1010) surface respectively, for various alkali... 

Besides the effect of the presence of alkali on CO adsorption, there is also a stabilizing effect of adsorbed CO on the adsorption state of alkali. Within the high alkali coverage range the number of CO molecules adsorbed on promoted surface sites becomes practically equal to the number of alkali metal species and their properties are not dependent on the CO coverage. In this region CO adsorption causes also stabilization of the adsorbed alkali, as indicated by the observed high temperature shift of the onset of alkali desorption. 

The alkali promotion of CO dissociation is substrate-specific, in the sense that it has been observed only for a restricted number of substrates where CO does not dissociate on the clean surface, specifically on Na, K, Cs/Ni( 100),38,47,48 Na/Rh49 and K, Na/Al(100).43 This implies that the reactivity of the clean metal surface for CO dissociation plays a dominant role. The alkali induced increase in the heat of CO adsorption (not higher than 60 kJ/mol)50 and the decrease in the activation energy for dissociation of the molecular state (on the order of 30 kJ/mol)51 are usually not sufficient to induce dissociative adsorption of CO on surfaces which strongly favor molecular adsorption (e. g. Pd or Pt). 

The presence of alkali promoters on the substrate surface can affect both the rate of chemisorption, (e.g. on K/Rh(100))55 and the adsorptive capacity... 

Alkali promoted NO dissociation is clearly illustrated in the case of NO adsorption on K/Pt(lll), as NO is not adsorbed dissociatively on the alkali-clean surface. The dissociative adsorption of NO on K/Pt(l 11) takes place at temperatures higher than 300 K and the number of dissociated NO molecules... 

The effect of alkali presence on the adsorption of oxygen on metal surfaces has been extensively studied in the literature, as alkali promoters are used in catalytic reactions of technological interest where oxygen participates either directly as a reactant (e.g. ethylene epoxidation on silver) or as an intermediate (e.g. NO+CO reaction in automotive exhaust catalytic converters). A large number of model studies has addressed the oxygen interaction with alkali modified single crystal surfaces of Ag, Cu, Pt, Pd, Ni, Ru, Fe, Mo, W and Au.6... 

The effect of the presence of alkali promoters on ethylene adsorption on single crystal metal surfaces has been studied in the case ofPt (111).74 77 The same effect has been also studied for C6H6 and C4H8 on K-covered Pt(l 11).78,79 As ethylene and other unsaturated hydrocarbon molecules show net n- or o-donor behavior it is expected that alkalis will inhibit their adsorption on metal surfaces. The requirement of two free neighboring Pt atoms for adsorption of ethylene in the di-o state is also expected to allow for geometric (steric) hindrance of ethylene adsorption at high alkali coverages. 

In this sense subsurface oxygen is also acting as a promoter. The role of the alkali promoter is then to stabilize Cl and anionically bonded O (or nitrate ions) on the catalyst surface, so they can exert their promotional action. Thus alkalis in this system, which requires electronegative promoters according to the mles of section 2.5, are not really promoters but rather promoter stabilizers. This is proven by their inability to promote selectivity in absence of Cl. 

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... 

J. Paul, and F.M. Hoffmann, Alkali promoted CO bond weakening on aluminum A comparison with transition metal surfaces,/. Chem Phys. 86(9), 5188-5195 (1987). 

The rate constants in table 4 for Ru/AlaOs should be considered as initial rate constants since it was not possible to achieve a higher coverage of N— than 0.25. Furthennorc, it was not possible to detect TPA peaks for Ru/AlaOs within the experimental detection limit of about 20 ppm. Ru/MgO is a heterogeneous system with respect to the adsorption and desorption of Na due to the presence of promoted active sites which dominate under NH3 synthesis conditions. The rate constant of desorption given in table 4 for Ru/MgO refers to the unpromoted sites [19]. The Na TPD, Na TPA and lER results thus demonstrate the enhancing influence of the alkali promoter on the rate of N3 dissociation and recombination as expected based on the principle of microscopic reversibility. Adding alkali renders the Ru metal surfaces more uniform towards the interaction with Na. 

Wesner, D. A., Linden, G., and Bonzel, H. P. 1986. Alkali promotion on cobalt Surface analysis of the effects of potassium on carbon monoxide adsorption and Fischer-Tropsch reaction. Appl. Surf. Sci. 26 335-56. 

Supported Rhodium Catalysts Alkali Promoters on Metal Surfaces Cobalt-Molybdenum Sulfide Hydrodesulfurization Catalysts Chromium Oxide Polymerization Catalysts... 

Promoters are generally divided in two classes. Structural promoters help to stabilize certain surface structures of the catalyst, or to prevent sintering. Structural promoters are not involved in the catalytic reaction itself and have no interaction with the reacting species. Chemical promoters, on the other hand, directly influence the reacting species on the surface of the catalyst. Obviously, alkali promoters fall into the latter category. 

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