Variations of Cobalt Catalytic Activity and Selectivity in Ethylene Oxidation against Stepwise Oxidation of Cobalt Surface

Catalytic activity of Co foil in ethylene oxidation was studied against oxidation degree of Co surface at stepwise foil oxidation. Experiments were conducted at temperatures of 500–800°C by a pulse method using alternative pulses of 0.2% C₂H₄–0.25% O₂–1% Ar–He testing mixture and 1% O₂–1% Ar–He oxidative mixture. Oxidation degree of Co foil varied from a totally reduced surface to an oxidation depth about a hundred of cobalt oxide “monolayers”. Using XRD, SEM and EDS, it was shown that CoO phase formed during a first stage of the stepwise oxidation (from 0 to ~60 oxide “monolayers”) at the all tested temperatures and modifications of surface morphology could be observed. At this stage the samples had a relatively high activity in both partial and total oxidation of ethylene at 500–600°C. On the contrary, at 700–800°C total oxidation was practically absent and the rate of partial oxidation was much lower than that at 500–600°C. During a second stage of Co surface oxidation (from ~60 to ~120 oxide “monolayers”) at 500–600°C also Co₃O₄ phase was found as well as a gradual ordering of the oxide crystals. In that state, the samples demonstrated a stationary (at 500°C) or an extremal (at 600°C) activity in total oxidation of ethylene. On the contrary, a temperature increase up to 800°C led to a sharp decrease of catalytic activity of the Co foil in this interval of oxidation degree.