Manganese Oxide as a Promoter for Copper Catalysts in CO 2 and CO Hydrogenation.

In this work, we discuss the role of manganese oxide as a promoter in Cu catalysts supported on graphitic carbon during hydrogenation of CO 2 and CO. MnO x is a selectivity modifier in an H 2 /CO 2 feed and is a highly effective activity promoter in an H 2 /CO feed. Interestingly, the presence of MnO x suppresses the methanol formation from CO 2 (TOF of 0.7 ⋅ 10 -3  s -1 at 533 K and 40 bar) and enhances the low-temperature reverse water-gas shift reaction (TOF of 5.7 ⋅ 10 -3  s -1 ) with a selectivity to CO of 87 % C . Using time-resolved XAS at high temperatures and pressures, we find significant absorption of CO 2 to the MnO, which is reversed if CO 2 is removed from the feed. This work reveals fundamental differences in the promoting effect of MnO x and ZnO x and contributes to a better understanding of the role of reducible oxide promoters in Cu-based hydrogenation catalysts.