Cs promotion

C.T. Campbell, Cs promoted Ag(lll) Model studies of selective ethylene oxidation catalysts,/. Phys. Chem. 89(26), 5789-5795 (1985). 

From the data shown in table 3, it is evident that the effect of Cs promotion on the power law kinetics is twofold First, the reaction order for NH3 is changed to essentially zero, and secondly, the apparent activation energy is higher by more than 20 kJ/mol in the presence of Cs. Contrary to the results obtained by Aika et al. [5], the reaction order for H2 was negative for all catalysts investigated. The positive reaction order for H2 reported by Aika et al. [5] for... 

Studying the kinetics of the interaction of N2 with the Ru catalysts revealed that the Cs promoter enhances both the rate of dissociative chemisorption and the rate of recombinative desorption. Ru catalysts were found to be rather inactive for NH3 synthesis without alkali... 

The C02 chemisorption results reveal that the equivalent of only 1.6% of the Cs present on the catalyst is an active basic site, accounting for the low reaction conversions observed. C02-TPD shows that the basic sites that do exist are weak, with desorption peaks observed at 373 and 437 K. An investigation into Cs-promoted a-NiM04 catalysts by Madeira et al. [10] demonstrated with C02-... 

Nakamura J, Campbell JM, Campbell CT. Kinetics and mechanism of the water-gas shift reaction catalyzed by the clean and Cs-promoted Cu(l 10) surface - a comparison with Cu (111). J Chem Soc Faraday Trans. 1990 86( 15) 2725—34. 

J. W. Medlin, J. R. Monnier, M. A. Barteau, Deuterium kinetic isotope effects in butadiene epoxidation over unpromoted and Cs-promoted silver catalysts, ]. Catal. 204 (2001) 71. 

Key Words Ethylene oxide, Ethylene, Epoxidation, Silver, Cl promotion, Cs promotion. Promotion, Selectivity, Oxametallacycle, Adsorption, Desorption, Chemisorption, Activation energy, Ag-O bond. Reaction mechanism, Oxidation, Cyclisation, Heterogeneous catalysis, Selective oxidation, Eletrophilic oxygen. Nucleophilic oxygen. Subsurface O atoms, Ag/a-A Oj catalyst. 2008 Elsevier B.V. 

In the absence of additives, Ag/a-Al2O3 catalysts typically produce selectivities in the conversion of ethylene to EO of 50%. The addition of the promoters, Cs and Cl in combination, increases the selectivity to 80-85%. The use of Cl as a promoter alone increases the selectivity from 50 to < 75% while their combined use raises the overall selectivity to 85%. The Cl promoter is added continuously from the gas phase. The Cs promoter is added during the catalyst preparation. We shall deal first with Cs promotion because of its simplicity and because it allows for an unambiguous definition of the role of the Cl promoter. 

The Cs-promoted catalyst studied by Atkins and co-workers was prepared by adding CsOH solution to the Ag/ethylene diamine liquid. The v-ATOa support was added to the Cs/Ag/ethylene diamine liquid and the material was dried overnight at 373 K. The catalyst so produced was 10 wt% Ag on AhCT, with a Cs loading of 300 ppm w/w [3]. 

FIGURE 7.9 The O2 temperature-programmed desorption (TPD) spectrum from a Cs-promoted Ag/a-Al203 catalyst and from a Cs-free Ag/a-Al203 catalyst. 

ETHYLENE DESORPTION FROM AN UNOXIDISED AND OXIDISED Cs-PROMOTED Ag/ct-AljOj CATALYST... 

The desorption spectrum of ethylene from the imoxidised Cs-promoted Ag/a-Al203 catalyst is identical to that shovm in Fig. 7.13 for ethylene desorption from an unoxidised, unpromoted Ag/a-Al20s catalyst. The ethylene desorption spectrum from an oxidised Cs/Ag/a-Al203 catalyst is identical to that from an... 

The Cs promoter has no effect on the bonding of ethylene to the a-Al20s support. It is located, at submonolayer amoimts, on the stepped Ag surface where it blocks the chemisorption of the less selective, more strongly held, O atom. 

In the industrial Ag/a-Al203 catalyst, the Cs promoter is located on the stepped Ag surface at submonolayer (probably sub-optimal) levels. It is not held on the a-A Oa support. 

The preparation of Ru supported catalysts by sol-gel method, indeed, was extended to obtain new formulations by changing the type of support. Alkali-promoted Ru/MgO systems were prepared starting from magnesium ethoxide, Ru3(CO)i2 and a cesium compound [9]. The gels were subjected to an activation/reduction procedure to substantially obtain Ru-CsOH/MgO and then tested as catalysts in the ammonia synthesis at atmospheric pressure. It was evidenced that the sol-gel prepared Cs-promoted Ru/MgO catalysts are much more active, under similar reaction comlitions, than the analogous catalysts prepared by the impregnation procedures reported in literature [10]. 

For improving our knowledge of the factors determining the kinetics and mechanism of n-butane ODH, and due to the small amount of available Information for the Ni-Mo-0 system, a systematic kinetic study with unpromoted and 3% Cs promoted NiMo04 was undertaken. 

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