Alkali promoted carbon, ruthenium

Ru-based catalysts have been identified as highly active catalysts for ammonia synthesis [156]. The catalytic activity of an Ru-based catalyst can be improved by the addition of alkali or earth-alkali promoters [157, 158]. Many kinetic and theoretic studies have been carried out in order to understand the role of these promoters in ammonia synthesis. Guraya et al. [159] investigated the alkali- and earth-alkali-promoted ruthenium eatalysts supported on graphitized carbon by... 

Figure 9.1. Comparison of the performance characteristics of alkali-promoted ruthenium on carbon and triple promoted magnetite a. Pressure and H/N ratio response (data at 6vol% ammonia, 673 K) b. Ammonia inhibition (data at 13,790 kPa, 703 K).

Promoter Location. It might be expected that promotion of supported catalysts would be difficult because the promoter localizes on the support. However, ESCA studies have shown that, in the case of silica-supported ruthenium promoted with alkali carbonates, the promoter preferentially associates with the ruthenium. Conversely, the high heat of adsorption of the alkali on carbon supports (up to 450 kJ moP leads to preferential association of the alkali with the carbon. 

This rate expression provides an excellent description of the operating characteristics of alkali-promoted ruthenium on carbon catalysts over a wide range of pressure, temperature, and gas composition based on kinetic constants derived directly from pilot plant studies. 

Of the platinum group metals only ruthenium and osmium show significant ammonia synthesis activity, though only in the presence of alkali promoters. This can be seen from the performance of the potassium metal promoted carbon-supported catalysts in Table Although osmium was one of the earliest and... 

The results in Refs. 91 and 127 (Table 9.7) also demonstrate that some alkali salts (nitrate, carbonate, hydroxide) give rise to efficiencies similar to the alkali metals as promoters, while others (chlorides) are almost totally inactive, which is in marked contrast to alumina>supported catalysts where the addition of alkali salts has little promoting effect. The active state of the promoter on ruthenium/carbon catalysts is unlikely to be metallic, as the high vapor pressure of the alkali metals would give rise to substantial losses under synthesis conditions. The more probable state is a charge transfer complex with the graphite... 

Figure 9.10. Effect of alkali metals on the activity of ruthenium-carbon catalysts (a) Activity vs. alkali loading for Cs, K, and Na promoted 2.5 %wt Ru/C catalysts (b) Activity vs. ionization potential for alkali promoted 2.5 %Ru/C catalysts (data from Ref. 47).

The per cent of dicyclohexylamine formed in hydrogenation of aniline increases with catalyst in the order ruthenium < rhodium platinum, an order anticipated from the relative tendency of these metals to promote double bond migration and hydrogenolysis (30). Small amounts of alkali in unsupported rhodium and ruthenium catalysts completely eliminate coupling reactions, presumably through inhibition of hydrogenolysis and/or isomerization. Alkali was without effect on ruthenium or rhodium catalysts supported on carbon, possibly because the alkali is adsorbed on carbon rather than metal (22). 

The early development of catalysts for ammonia synthesis was based on iron catalysts prepared by fusion of magnetite with small amounts of promoters. However, Ozaki et al. [52] showed several years ago that carbon-supported alkali metal-promoted ruthenium catalysts exhibited a 10-fold increase in catalytic activity over conventional iron catalysts under the same conditions. In this way, great effort has been devoted during recent years to the development of a commercially suitable ruthenium-based catalyst, for which carbon support seems to be most promising. The characteristics of the carbon surface, the type of carbon material, and the presence of promoters are the variables that have been studied most extensively. 

The effect of alkali metal salt as promoter can be fully shown after it is well reduced under the conditions of ammonia s3mthesis. The study by Forni et showed that cesium can prevent metal sintering and increase the dispersion of active components. When activated carbon is used as support, Kowalczyk Z et al. considered that the promoting role mainly occur in contact points between ruthenium and cesium adsorbed on activated carbon surface because parts of cesium salt are reduced to metal cesium. As alkali metals are unstable in ruthenium catalysts, (Cs - - O) groups also exist, which mainly are distributed on the surface of ruthenium particles. The promotional effect of Cs + O groups is relatively lower when activated carbon is the support, while they play a major role when MgO is used as support, although with a lower extent than that in Cs-Ru/AC. ... 

It is seen from Fig. 6.24 that the alkali metal hydroxide can be easily absorbed by carbon support due to its low melting point and its large fluidity. Therefore, only with enough quantity, the alkali metal can be accreted on the interface between ruthenium and carbon support and then plays the promotional roles effectively. As alkaline earth metal oxides have high melting point and poor fluidity, small amounts of them can be accreted on the interface between ruthenium and carbon support, which can produce effective active sites. The excessive promoters might cover the active sites of catalyst smface, which can influence the effective contact between active sites of ruthenimn smface and reactant gases and therefore decrease the catalytic activity. 

Figure 9.11. Comparison of alkali metal (Ref. 47) and alkali salt promoted (Ref. 129) ruthenium-carbon catalysts.

---