Ruthenium alkali promoted catalyst

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. 

In the original work on catalytic ammonia synthesis, Haber [41] had used an osmium catalyst, but this metal was much too expensive to be the basis of the large-scale industrial plants. In the long search for alternatives to the Mittasch catalyst, alkali-promoted ruthenium was found to exhibit specific activity, which is even superior to the iron catalyst [42] and which was subsequently developed to an industrial catalyst [43]. The Mittasch catalyst is cheap and the alumina promoter provides a high specific surface area. This situation is different with Ru catalysts that are prepared as small particles on a suitable support. Figure 1.1 was a t)q)ical electron microscopic picture from such a catalyst particle on MgAl204 (spinel) support [44]. 

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... 

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. 

The platinum group metals demonstrate clearly the impact of the key parameters that influence the efficiency of ammonia synthesis catalysts structure sensitivity, the heat of adsorption of reactants and products, and the roles of promoters and supports. The key requirement to minimize the activation energy for nitrogen dissociation limits the active metals to alkali-promoted ruthenium and osmium. Similar activities are then obtained, irrespective of the genesis of the alkali metal promoter (salt or metal), indicating that both convert to a similar... 

The essential requirement for ruthenium catalysts to be active in homologation reactions of oxygenated substrates is the presence of an iodide promoter which may be I2, HI, an alkyl or metal iodide, or a quaternary ammonium or phosphonium iodide (3). With alkali iodides as promoters, ion-pairs of the [fac-Ru(CO)3l3] anion are formed in the catalytic solution of the homologation reactions starting from different precursors Ru(Acac>3, Ru3(CO)] 2> Ru(CO)4l2 etc. ( ). ... 

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 elTiciency of cobalt and ruthenium catalysis is not very sensitive to the presence of promoters )21]. With cobalt, the addition of thorium and alkali promoters increases wax production and supports were incorporated to increase the active metal surface area. On the other hand, promoters and supports are essentia) for iron catalysts. 

In this paper, we postulate that the primary role of an alkali promoter is to reduce the mobility of the chemisorbed hydrogen on the ruthenium surface based on the data obtained from NMR spectroscopy. We will examine the active hydrogen adsorption states (a and P) in supported ruthenium catalysts and report the effects of the alkali promoters on the population and the mobility of the adsorbed states. 

It is interesting to note that the addition of ruthenium to iron produces effects on selectivity similar to those observed when alkali promoters are added to iron that is, methane production is reduced and olefin formation is enhanced. However, alkali metals also tend to. enhance the formation of oxygenated compounds which results in a less favorable selectivity and in product separation problems. Under our reaction conditions, we see no evidence for the formation of any oxygenated compounds. It will be interesting to see if this olefin. paraflBn ratio over Ru-Fe catalysts can be enhanced at higher pressures, as is presently achieved with typical, promoted Fischer-Tropsch catalysts, without the concomitant production of less desirable oxygenated compounds. It appears from this study that alloy catalysts can provide favorable shifts in selectivity, and future studies should provide further evidence of this capability. 

Research with an alkali-promoted (potassium or K2O) ruthenium catalyst has demonstrated that ammonia synthesis can be effected at lower temperatures and pressures than those required by the Haber process. As the price of energy increases, ruthenium catalysis might become increasingly important, because the energy-expensive compression process could be avoided. Another advantage of ruthenium if its diminished susceptibility to poisoning by H2O and CO. Ruthenium catalysts can carry out the direct synthesis of ammonia from N2, CO, and H2O ... 

In ammonia synthesis reaction, the dissociative adsorption of dinitrogen is the rate determining step. The electron donor type promoter, such as alkali metals, which can provide electrons to the Ru, is more propitious to activate the N=N bond in N2 molecule and thus enhance the dissociative adsorption of dinitrogen and increase the reaction rate of ammonia synthesis. The acidity and basicity of supports have important influence on the activity of the ruthenium catalysts without promoter. Acidic supports can easily accept electrons and decrease the activity of catalysts. Basic supports can easily provide electrons and enhance the dissociative... 

It is generally considered that the structural factor of ruthenium catalysts is not as important as those of iron-based catalysts, while the electronic factor is the most important one. The catalytic activity of Ru/AC catalyst without promoter is very low. The promoters which can donate electron can remarkably increase the activity of catalyst. The promoters of ruthenium catalysts mainly include alkali metals, alkaline earth metals, rare earth metal oxides and actinide metal compounds. 

The ruthenium catalyst without promoter almost is inactive, and the outlet ammonia concentration of 4% Ru/C catalyst Is only 0.13% under the conditions of 430°C, 10.0MPa and 10,000h For single promoter of Ru/C catalyst with the alkali metal, alkaline earth metal nitrate acts as the precursors of promoter respectively, the alkali metal and alkaline earth metal nitrate can Increase the activity of the ruthenium catalyst In different extent. The order of the Influence, alkali metal nitrates are CsNOs > RbNOa > KNO3 > NaNOa and that of alkali earth metal nitrate is Ba(N03)2 > Sr(N03)2 > Mg(N03)2 > Ca(N03)2. In the selected eight kinds of promoters, Ba(N03)2 and CsNOa are the most effective and the outlet ammonia concentration are 13.38% and 9.89% at 430°C, 10.0MPa and 10,000h respectively. 

There is no consistent understanding on the role of mechanism of alkali metals and alkaline earth metals on ruthenium catalysts and their state under the operating conditions. The current studies show that the dynamics of the ammonia synthesis reaction are different using the alkali metals and alkaline earth metals as the promoters, respectively. Therefore, using combination of promoters is more favorable to increase the catalytic activity. 

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 that because there are the competitive adsorption of H2 and N2 on the surface of ruthenium catalyst and the adsorption heat of H2 is larger than N2, so the strong adsorption of H2 occupies the active site of the catalyst surface, and is likely to generate hydrogen bridged compounds of sub-layer, especially on the system in which alkali metal hydroxide is used as promoter. Many of the activity sites on ruthenium catalyst are firmly occupied by H2, which hindered the determining reaction step of N2 dissociative adsorption and the reaction rate of ammonia formation is decreased. 

This has been examined in detail in an earlier chapter but will be briefly reviewed here. As indicated above there is still no definitive evidence for the chemical state of the alkali on magnetite-derived catalysts. However, in the case of supported ruthenium catalysts it is reasonably clear that the alkali metal is responsible for the observed enhancement in performance. Such promotion proceeds via electron transfer from the alkali to the transition metal. The extent of charge transfer can be monitored by the changes in the work function of the transition metal/alkali metal complex. 

Alkali promoters can also inhibit ammonia adsorption. In the case of magnetite catalysts there is indirect evidence that this is caused by neutralization of the acidic alumina phase. A similar effect is also observed on alumina-supported transition metals. ESCA evaluation of ruthenium catalysts demonstrated a significant increase in the ammonia bonding on alumina supports that could be reversed in the presence of alkali promoters. This is also apparent from the marked changes in the reaction order in ammonia for ruthenium on a variety of acidic and basic supports (Table 9.7). However, recent surface-science studies (discussed in detail in Chapter 5)... 

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... 

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