Insights into catalytic roles of noble-metal-free catalysts CoxSy for reduction of 4-nitrophenol.

To develop noble-metal-free catalysts, three CoxSy samples, i.e. Co9S8, CoS and CoS2, have been prepared and investigated for the reduction of 4-nitrophenol. Co9S8 has the best surface electron immigration efficiency and the highest H2-adsorption capability, which make it the most promising catalyst. On the other hand, CoS has higher electron transfer efficiency than CoS2, leading to stronger H absorption capability. As a result, the catalytic activity of the three catalysts shows the following trend: Co9S8 > CoS > CoS2 under the same reaction conditions. Finally, a plausible catalytic mechanism has also been proposed, which is helpful in acquiring an in-depth insight into the catalytic role of CoxSy.