Ag/a-Al2O3 catalyst

Investigation of the Origins of Selectivity in Ethylene Epoxidation on Promoted and Unpromoted Ag/a-Al2O3 Catalysts A Detailed Kinetic, Mechanistic and Adsorptive Study... 

The Detailed Kinetics of the Adsorption and Desorption of Ethylene on an Oxidised Ag/a-Al2O3 Catalyst Subtending the... 

FIGURE 7.1 The temperature-programmed desorption (TPD) spectra of O2 from an Ag/a-Al2O3 catalyst O2 dosing, O2 101 kPa, 25 cmVmin, I h, 513 K. 

Origins of Selectivity in Ethylene Epoxidation on Ag/a-Al2O3 Catalysts... 

FIGURE 7.4 The temperature-programmed desorption (TPD) spectrum of O2 from the Ag/a-Al2O3 catalyst subtending the / -O state principally on the surface. 

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 effect of that Cs loading was studied by O2 TPD and by ethylene temperature-programmed reaction [3]. The O2 desorption spectrum obtained by Atkins and co-workers for the Cs/Ag/v-AhOa catalyst is shown in Fig. 7.9, lower curve. The upper curve with two peak maxima at 523 K (250 °C) and 573 K (300 °C) is that shown previously for O2 desorption from a fresh, unpromoted Ag/a-Al2O3 catalyst (Fig. 7.1). 

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