Cesium, metal catalyst promotion

Silver alone on a support does not give rise to a good catalyst (150). However, addition of minor amounts of promoter enhance the activity and the selectivity of the catalyst, and improve its long-term stabiHty. Excess addition lowers the catalyst performance (151,152). Promoter formulations have been studied extensively in the chemical industry. The most commonly used promoters are alkaline-earth metals, such as calcium or barium, and alkaH metals such as cesium, mbidium, or potassium (153). Using these metals in conjunction with various counter anions, selectivities as high as 82—87% were reported. Precise information on commercial catalyst promoter formulations is proprietary (154—156). 

Ru/Rh/Promoter/HOAc Catalyst Compositions. The behavior described in the previous section is however by no means limited to cesium ions as promoters, nor is it confined to cations of the alkali metal group. 

The selectivity decreased for catalysts doped with cesium, palladium, ruthenium, zinc, and zirconium. The influence of these metals was thought to be an indication of the role of these metals in promoting the overoxidation of MA to carbon oxides. However, molybdenum was found to poison the overoxidation reaction. 

As far as finding improved catalysts or operating conditions is concerned, a variety of experimental studies have been conducted. Promotion by alkali metal salts [11], especially cesium [12,13], has been shown to increase the EO selectivity. Similarly, it is observed that addition of chlorine to the catalyst results in higher EO selectivity [14-18]. Atmospheric pressure experiments in our laboratory have indicated that Cu-Ag bimetallic catalysts are more effective than pure Ag [19]. Furthermore, it is observed that after addition of Cs and Cl to the Cu-Ag bimetallic catalyst, it outperforms the corresponding promoted silver catalyst [20]. Published experimental studies have t)q)ically focused on a limited number of catalyst components, whereas one could potentially explore a wide variety of catalyst combinations using theoretical tools. 

Rather than survey all of the possible modifications that can be made to an alumina surface, we will focus on a subset involved in two different types of surface-catalyzed chemical reactions, namely, the partial oxidation of ethylene to ethylene oxide (EO) and hydrodesulfurization (HDS) processes. Both of these catalytic systems have functional points in common, in that alumina serves as a support (a-alumina for the EO process and 7-alumina for the HDS process) and alkali-metal salts serve as promoters for both reactions. To illustrate this commonality, this section will be divided into three parts (1) the adsorption of alkali-metal salts to 7-alumina, as reflected in the Rb and Cs solid-state NMR spectroscopy of these systems (2) the absorption of ethylene to silver supported on aluminas in the presence and absence of cesium salts, as followed by C NMR spectroscopy, and (3) the solid-state Mo NMR of fresh and reduced/ sulfided molybdena-alumina catalysts. 

The selective oxidation of ethylene to ethylene oxide (EO) is performed on supported silver catalysts at temperatures of250—280 °C, and a pressure of roughly 20 bar. In this process, it is necessary to avoid secondary reactions of EO. Typical industrial catalysts may contain 8—15 wt% silver dispersed on low surface area (X-AI2O3 (0.5-1.3 m g ) with a porosity of about 0.2—0.7 cm g In addition, the catalyst may contain several promoters in varying amounts (ppm by weight) 500—1200 ppm alkali metal (mostly cesium), 5-300 ppm of sulfur as cesium or ammonium sulfate, 10-300 ppm offluorine as ammonium fluoride, or alkaft metal fluoride (427). 

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. ... 

Since the early 1980s there have been several catalyst developments, including the use of cobalt oxide with magnetite to increase activity. The most significant, however, is the successful use of a mthenium catalyst supported on a special carbon and promoted with cesium and barium. Although still expensive, cost and availability should not restrict the use of mthenium in the way that osmium was excluded by Bosch, provided that the metal is recycled. 

Further work has shown that cesium is a better promoter than potassium and that the alkali hydroxides are, surprisingly, reduced to a metallic state during catalyst activation. This seems thermodynamically improbable, but may result from the high heat of adsorption of the alkali metal hydroxides on carbon, which leads to charge transfer to the carbon, and could drive the reduction of the hydroxide to the metal. The alkali promoter may neutralize residual chloride ions and develop catalyst activity dttring reduction as mthenimn ions diffuse from the lattice to form crystallites on the edges of graphite crystals. 

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