Effect of Alkali Promoters

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 

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Fig. 2. Effect of alkali promotion on the average STY over Fe UFP catalysts. Reaction conditions temperature, 220°C pressure, 30 atm H2/CO, 1 mol/mol ...

The Effects Of Alkali Promoters On The Dynamics Of Hydrogen Chemisorption And Sjmgas Reaction Kinetics On Ru/SiOj Surfaces... 

Alkali promoters are frequently employed in syngas reaction catalysts for their effects on improving the selectivity for higher molecular weight hydrocarbons. However, their mechanism of interaction is not clearly understood due to the complexity of the syngas reactions and the diversity of effects caused by the alkali promoters. The effects of alkali promoters on syngas reactions are reported as (i) increased selectivity towards higher hydrocarbons, (ii) decreased rates of the overall reaction, (iii) increased CO2 production and... 

The effect of alkali promoters on dynamics of chemisorbed hydrogen was determined in terms of an exchange parameter which was determined from lineshape simulations as described in [4]. The exchange parameters determined for 0 and 66 at. % K promoted catalysts at a hydrogen coverage of 0.8 were listed in Table 1. 

The site blocking effects of alkali promoters as monitored via microcalorimetry indicated that K promoters significantly reduced the adsorbed hydrogen amounts and eliminated the intermediate (50 kJ/mol) and weakly (10 kJ/mol) bound states [13]. The authors concluded that K promoter selectively populated the defect like sites where the weakly bound hydrogen states are present. 

Alkali promoters are often used for altering the catalytic activity and selectivity in Fischer-Tropsch synthesis and the water-gas shift reaction, where C02 adsorption plays a significant role. Numerous studies have investigated the effect of alkalis on C02 adsorption and dissociation on Cu, Fe, Rh, Pd, A1 and Ag6,52 As expected, C02 always behaves as an electron acceptor. 

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 alkali additives on N2 chemisorption has important implications for ammonia synthesis on iron, where alkali promoters (in the form of K or K20) are used in order to increase the activity of the iron catalyst. 

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. 

The readsorption and incorporation of reaction products such as 1-alkenes, alcohols, and aldehydes followed by subsequent chain growth is a remarkable property of Fischer-Tropsch (FT) synthesis. Therefore, a large number of co-feeding experiments are discussed in detail in order to contribute to the elucidation of the reaction mechanism. Great interest was focused on co-feeding CH2N2, which on the catalyst surface dissociates to CH2 and dinitrogen. Furthermore, interest was focused on the selectivity of branched hydrocarbons and on the promoter effect of alkali on product distribution. All these effects are discussed in detail on the basis... 

Alkalization of iron catalysts causes two different effects. The selectivities of 1-alkenes are raised and both the growth probability a2 and the fraction f2 are markedly increased, as already shown in Figure 11.2. Detailed studies on the promoter effect of alkali have revealed the effect on 1-alkene selectivity to saturates at 1 mass% of K2C03, while the effect on f2 already begins at 0.2 mass% of K2C03.1213 This difference points to specific active sites in Fischer-Tropsch syn-... 

Chapter 11 Studies on the Reaction Mechanism of the Fischer-Tropsch Synthesis Co-Feeding Experiments and the Promoter Effect of Alkali.199... 

Figure 9.10 Up work function of alkali-promoted metals as a function of alkali coverage (see also Table 9.2). Down electrostatic potential around a single alkali atom adsorbed on jellium. The effective local work function at each position is the sum of the substrate work function and the value of the electrostatic potential in the figure (from Lang el at. [39]).

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