Effect of precipitator on the texture and activity of copper-manganese mixed oxide catalysts for the water gas shift reaction

Copper-manganese mixed oxides were synthesized by co-precipitation with either KOH or NaOH as precipitators from CuSO4·5H2O and MnSO4·H2O as starting materials. The as-synthesized samples were tested for the water gas shift reaction and characterized by XRD, low temperature N2 absorption/desorption, TG and TPR. The obtained end precipitate consists mainly of Mn3O4 and Cu2+1O for samples prepared using NaOH as the precipitator whereas Cu4SO4(OH)6·H2O is obtained for the samples using KOH as the precipitator. Both the end precipitates prepared with NaOH and KOH as precipitators are converted to Cu1.5Mn1.5O4 after calcination, and reduced to Cu and MnO after the water gas shift reaction. Drastic differences are observed in their catalytic properties for the water gas shift reaction. The dry samples are composed of Mn3O4 and Cu2+1O when using NaOH as a precipitator and these samples maintain a relatively high texture stability and composition uniformity which ensure higher activity and thermal stability. However, for the layered structure of Cu4SO4(OH)6ċH2O and the amorphous manganese oxide, the composition of the dry samples prepared with KOH as a precipitator shows that the samples undergo a complex evolution during calcination resulting in a weakening of the synergistic effect between copper and manganese. Therefore, under the water gas shift reaction conditions the stability of the texture and composition uniformity of the KOH precipitated sample decrease greatly, which leads to a decrease in activity and thermal stability. The present comprehensive study results show that the Cu-Mn mixed oxide prepared using NaOH as a precipitator has significantly higher texture stability and catalytic activity than that prepared using KOH as a precipitator, and present excellent thermal stability.