Solid-electrolyte-aided study of the ethylene oxidation on polycrystalline silver

Abstract

The oxidation of ethylene on porous polycrystalline Ag films supported on stabilized zirconia was studied in a CSTR at temperatures between 250 and 450 °C and atmospheric pressure. The technique of solid-electrolyte potentiometry (SEP) was used to monitor the chemical potential of oxygen adsorbed on the catalyst. The rates r 1, r 2 of ethylene epoxidation and combustion were found to satisfy r 1 = K 1K ET P ET (1 + K ET P ET ), r 2 = K 2K ET P ET /(1 + k et p et ) , respectively, with K ET = 8.7 · 10 −4 exp( 5800 T ) bar −1 , K 1 = 0.28 exp( −7300 T ) mole/s , and K 2 = 2. · 10 2 exp( − 11100 T ) mole/s on a porous film that could adsorb a total of 2 · 10 −6 moles O 2. The steady-state atomic oxygen activity a O satisfies the equation P O2 1 2 a O = 1 + KP ET P O 2 with K = 3.4 · 10 −5 exp( 7800 T ) . The effect of carbon dioxide on the catalytic oxidation of ethylene was also studied at temperatures between 250 and 400 °C and atmospheric total pressure. Over the range of conditions investigated CO 2 was found to inhibit ethylene epoxidation only, without measurably affecting the rate of complete oxidation to CO 2 or the potentiometrically measured atomic oxygen activity on the Ag catalyst. The inhibiting effect of CO 2 on the rate of ethylene epoxidation r 1 can be described by r 1 r 1 O = P O 2 (P O 2 + K'P CO 2) , where r 1O is the rate of epoxidation for vanishingly low PCO 2 and K′ = 1.2 · 10 −5 · exp( 6600 T ) . A simple mechanism is proposed which explains all the experimental observations.