Characterization Of CuO/Ce_xTi_1-xO₂ Structure And Catalytic Activity In NO+CO Reaction

Nitric oxide (NO_X) is one of major pollutants in the atmosphere and causes great harm to natural environment and human life. The abatement of NO_X has become one of the most urgent tasks in environmental management nowadays. Precious metal catalyst has been widely used in the non selective catalytic reduction using CO and H₂ as reducer, as reported by many papers. Precious metal catalyst has many defects, such as high price, resource shortage and instability at high temperatures. In recent years, non-precious metal catalyst as the substitute of precious metal catalyst has gained the global attention.As carrier, TiO₂ has large surface area, good selectivity of N₂, high conversion and good ability against SO₂, which inhibits the form of sulfate and reduces active surface area. The properties of TiO₂ make it become a very common carrier used to remove NO_X. As a structural and electronic promoter, CeO₂ can improve the catalytic activity, the selectivity and thermal stability. CeO₂ is also a good agent for storing oxygen because of the redox cycle between its two oxidation states. Researches have shown that thermal stability of TiO₂ can be increased by adding CeO₂ to TiO₂, which can also inhibit the sintering of particles and the collapse of structure. The catalyst loaded with CuO on Ce-Ti complex oxides has good catalytic activity in NO+CO reaction because of its special reduction properties.In this study, a series of catalysts were made by the impregnation method using Ce_xTi_1-xO₂ complex oxide as carrier. The structure and activity of CuO/Ce_xTi_1-xO₂catalysts in NO+CO reaction were examined by BET, TG-DTA, H₂-TPR, XRD and Raman technologies.1. CuO loading of Ce_xTi_1-xO₂ complex oxide and catalytic activity in NO+CO reactionCe_xTi_1-xO₂ complex oxides were prepared by sol-gel method at different volume ratios of H₂O to TiCl₄ and different molar ratios of CeO₂ to TiO₂. The catalysts were made by loading CuO on Ce_xTi_1-xO₂ complex oxides. The results showed that when the volume ratio of H₂O to TiCl₄ was 0 (i.e. no water added) and the molar ratio of CeO₂ to TiO₂ was 2:8, the catalysts had the best activity.This study also examined the effect of different loading of CuO and different calcination temperature. It was found that catalysts with 12%CuO loading calcined at 500℃had the highest catalytic activity. Its XRD spectrum showed crystal phase peaks of TiO₂ and CuO only, when CuO loading and calcined temperature were low.2. The structure of CuO/Ce_0.5Ti_0.5O₂ and catalytic activity in NO+CO reactionThe activity of CuO/Ce_0.5Ti_0.5O₂ prepared by coprecipitation increased with the increase of CuO loading and became best at 22%CuO loading. 14%CuO/Ce_0.5Ti_0.5O₂ calcined at 700℃had the highest activity with 100% NO conversion at 275℃.As shown by H₂-TPR analysis, different loadings of CuO on Ce_0.5Ti_0.5O₂ had five reductive species. There were four reduction peaks of CuO. Theαandβpeaks were probably due to the reduction of hightly dispersed CuO species. Theγpeak was ascribed to the bulk CuO, which had strongly interaction with Ce_0.5Ti_0.5O₂, and theδpeak was likely due to CuO crystallites. Theηpeak lied at 450℃was the reduction of oxygen on the surface of CeO₂ catalyzed by CuO. The XRD results showed that the CeTi₂O₆ phase occurred in the catalyst calcined at 800℃. With increasing calcined temperature, this phase become more obvious, indicating that high-temperature was beneficial to generating CeTi₂O₆. As CuO loading was lowered, CuO dispersed highly on the surface of the catalyst. As CuO loading increased, the CuO phase was detectedat 2θ35.5°and 38.7°, and intensified with further increase in CuO loading. Raman profile also showed that after calcination, the carrier formed not only the complex of TiO₂ and CeO₂ but also new phases.3. The structure of CuO/Ce_0.2Ti_0.8O₂ and catalytic activity in NO+CO reactionThe activity of CuO/Ce_0.2Ti_0.8O₂ catalysts prepared by coprecipitation increased gradually with the increase of CuO loading, and became the best when CuO loading was 12% or 18%. The activity of 12%CuO/Ce_0.2Ti_0.8O₂ became maximal when it was calcined at 700℃. The conversion of NO was 100% at reaction temperature of 275℃. When calcined temperature was below 700℃, the catalytic activity increased with increment of calcined temperature. When calcined temperature was above 700℃, the activity of catalysts was lowered with increment of calcined temperature, i.e. catalysts calcined at 900℃had no activity in NO+CO reaction basically. The results of specific surface area indicated that it was calcined temperature not CuO loading which caused the decrease in specific surface area.The H₂-TPR results showed that there were two active centers on the surface of the catalysts calcined at 500℃, i.e. highly dispersed CuO and crystallize CuO. The CuO species on the surface of 12%CuO/Ce_0.2Ti_0.8O₂ became more with the increment of calcined temperature, i.e. four reduction peaks (α,δ,θandγ). The XRD profile indicated that the catalysts existed in anatase and amorphism mainly and CuO species dispersed highly on the surface of catalysts calcined at 500℃, and crystalline CuO appeared clearly when the catalysts were calcined at high temperatures. Consistent with the Raman profile, the CuO phase began to appear at 600℃, and many new phases occurred at 700℃, but disappeared at 800℃. Crystalline phase reaction took place slowly between Ce, Ti and O, and the middle product Ce₄Ti₉O₂₄ formed at 700℃. It was at 800℃that CeTiO₄ finished Crystalline phase transformation.