Effect of pretreatment atmosphere on CuO/TiO2 activities in NO plus CO reaction

Using TiO2 as carrier, CuO/TiO2 catalysts with different CuO loading were prepared by the impregnation method. The catalytic activities in NO+CO reaction were examined with a micro-reactor gas chromatography reaction system and the methods of TPR, XPS and NO-TPD. It was found that the catalytic activities were affected by pretreatment atmosphere, i.e. H-2 atmosphere > reduction-reoxidation > 10%CO/He > reaction gas (fresh sample). NO decomposition was better by low-valence Cu species than by high-valence Cu species, i.e. Cu-0 > Cu+> Cu-2. The XPS results indicated that Cu species on CuO/TiO2 were Cu-0, Cu+, normal Cu2+(Cu2+(I)) and chain-structured Cu2+(Cu2+(II)) as -Cu-O-Ti-O-. The activities of Cu2+(II) were much higher than that of Cu2+(I), but both species were very unstable in the reaction atmosphere and easily reduced by CO, which accounted for the variable activities of fresh catalysts with increasing reaction temperature. In NO+CO reaction, the redox process was a cycle of Cu+-Cu2+(I) at low reaction temperature but was a cycle of Cu-0-Cu+ at high reaction temperature. As shown by NO-TPD, high catalytic activities could be attributed to the following factors, e.g. oxygen caves on the catalyst's surface after pretreatment with H-2 and reduction-reoxidation, formation of Cu(0)after pretreatment with H-2, and increment of Cu species dispersion and formation of Cu2+(II) after pretreatment with reduction-reoxidation.