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°C by the pulse method using alternating pulses of a 0.2%C₂H₄–0.25%O₂–1%Ar–He test mixture and a 1%O₂–1%Ar–He oxidative mixture. The degree of oxidation of the Co foil surface varied from total reduction to oxidation of ~100 cobalt oxide “monolayers.” The XRD, SEM, and EDS studies showed that CoO formed at the first stage of stepwise oxidation (from 0 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°C. At 700–800°C, however, there was no total oxidation, and the rate of partial oxidation was much lower than at 500–600°C. At the second stage of Co surface oxidation (from ~60 to ~120 oxide “monolayers”) at 500–600°C, Co₃O₄ formed, and gradual ordering of oxide crystals was observed. In this state, the samples exhibited constant (at 500°C) or extreme (at 600°C) activity in the total oxidation of ethylene. In contrast, an increase in the temperature to 800°C led to a drastic decrease in the catalytic activity of Co foil in the given range of the degrees of oxidation.