SCR of NO by NH_3 over TiO_2-supported V_2O_5-MoO_3 catalysts:reactivity and redox behavior

摘要

The reactivity in the selective catalytic reduction (SCR) reaction and the redox behavior of V_2O_5-MoO_3/TiO_2 catalysts was investigated by means of the temperature programmed reduction (TPR)/reaction technique, and compared with that of binary V_2O_5/TiO_2 catalysts having the same metal oxide loading. It was found that the ternary catalysts are more active in the SCR reaction at low temperatures compared to the corresponding binary samples: hence the 'temperature window' of the reaction is widened and shifted towards lower temperatures. Transient reactivity data provide clear evidence in favor of the hypothesis of a redox mechanism for the SCR reaction and point out that the ternary catalysts are more easily reduced and reoxidized than the corresponding binary samples: this indicates that the simultaneous presence of V and Mo enhances the catalyst redox properties, and thus its reactivity. Such conclusions are also in line with the results of the characterization studies pointing out the existence of electronic interactions involving the V and Mo surface oxide species. The overall picture closely resembles the one obtained in the case of the analogous V_2O_5-WO_3/TiO_2 system and indicates that the effects of the addition of WO_3 and MoO_3 to V_2O_5/TiO_2 are similar, both oxides acting as 'chemical' promoters besides playing a 'structural' function as well.