Insight into the Correlation between Cu Species Evolution and Ethanol Selectivity in the Direct Ethanol Synthesis from CO Hydrogenation

Cu/SiO2 catalyst was prepared by the ammonia evaporation method for the direct synthesis of ethanol from CO hydrogenation. The catalyst exhibited the initial ethanol selectivity as high as 40.0 wt%, which dramatically decreased from 40.0 to 9.6 wt% on the stream of 50 h. XRD, XPS, TEM and N2O titration techniques were employed to elucidate the ethanol selectivity change and catalyst structure evolution during reaction process. The experiment and characterization results indicated that both Cu+/(Cu++Cu-0) value and copper crystallite size had great effects on the ethanol selectivity. During the initial 38 h, the ethanol selectivity obviously decreased from 40.0 to 18.2 wt%, and Cu+/(Cu++Cu-0) value on the catalyst surface rapidly dropped from 0.67 to 0.39, while the copper crystallite size remained almost unchanged. However, during the reaction period of 38-50 h, the Cu+/(Cu++Cu-0) value possessed no distinct change, but a further decrease in ethanol selectivity and a rapid aggregation in Cu particles were observed simultaneously. The present systematic investigation demonstrated that the decrease of Cu+/(Cu++Cu-0) value was the main factor for the loss of ethanol selectivity during the initial 38 h, whereas the rapid growth of Cu particles during the reaction period of 38-50 h were mainly contributed to the further decline of ethanol selectivity.