Variation of the Catalytic Activity and Selectivity of Cobalt in Ethylene Oxidation during Stepwise Oxidation of the Cobalt Surface

The dependence of the catalytic activity of Co foil in ethylene oxidation on the degree of oxidation of the Co surface during stepwise oxidation of foil has been studied. The experiments were performed at 500-800 degrees C by the pulse method using alternating pulses of a 0.2%C2H4-0.25%O-2-1%Ar-He test mixture and a 1%O-2-1%Ar-He oxidative mixture. The degree of oxidation of the Co foil surface varied from total reduction to oxidation of similar to 100 cobalt oxide "monolayers." The XRD, SEM, and EDS studies showed that CoO formed at the first stage of stepwise oxidation (from 0 to similar to 60 oxide "monolayers") at all temperatures under study, and changes in the surface morphology could be observed. In this state the samples had a relatively high activity in both partial and total oxidation of ethylene at 500-600 degrees C. At 700-800 degrees C, however, there was no total oxidation, and the rate of partial oxidation was much lower than at 500-600 degrees C. At the second stage of Co surface oxidation (from similar to 60 to similar to 120 oxide "monolayers") at 500-600 degrees C, Co3O4 formed, and gradual ordering of oxide crystals was observed. In this state, the samples exhibited constant (at 500 degrees C) or extreme (at 600 degrees C) activity in the total oxidation of ethylene. In contrast, an increase in the temperature to 800 degrees C led to a drastic decrease in the catalytic activity of Co foil in the given range of the degrees of oxidation.