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 Cu_1.5Mn_1.5O₄ 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)₆·H₂O 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.