Investigation Of Ceria And Chromium Composite Oxides For Selective Catalytic Reduction Of NOx By NH₃

Nitrogen oxides are main atmospheric pollutants, which can cause acid rain and photochemical smog. The NOx control technology has been a research focus in the international environmental protection field. Among the present technologies of NOx control, the selective catalytic reduction (SCR) with NH3 is the most widely used treatment technology. The development of an SCR catalyst with high activity has been the key of SCR technology. Because of the unique crystal structure and high redox properties, the composite metal oxides are paid more and more attention, which have broad application in the domain of deNOx.The wide application of cerium oxide either as a promoter or as an active catalyst is due to its unique redox and acid–base properties. The doping of undersized lower valence ions to form a ceria-based solid solution can enhance the thermal stability, oxygen storage capacity, and catalytic activities. Cr is an elements with variable valences and has excellent redox ability, therefore Cr doped CeO₂ for deNOx is intriguing.In this work, Ce-Cr composite oxides were prepared by co-precipitation method using ammonia as the precipitation agent and characterized by N2 adsorption/desorption, X-ray diffraction(XRD), transmission electron microscopy (TEM), raman spectra (Raman), X-ray photoelectron spectra (XPS). Temperature programmed reduction of H2 (H2-TPR) tests were employed to determine the redox properties. The effects of Cr contents and calcination temperatures on crystal phase and surface properties were investigated. The NH3-SCR was performed on a fixed bed reactor to study the catalytic activity.Before calcinations, the Ce-Cr composites had a larger specific surface area than that of CeO₂. TEM results show that the samples with 1-5 (molar content) of Cr doping show nanorod morphologies, which coexists with some nanoscaled particles. The length of the nanorods is about 60-150 nm and the width is 15-20 nm. With the increase in Cr, the yield of nanorods reduced gradually. When Cr content is 20 at%, no nanorods were observed. After calcinations, the samples with the Cr content less that 3 at% show improved anti-sintering ability than that of CeO₂. Further increase in Cr results in the decrease in the surface area compared with CeO₂. The Cr2O3 phase was observed for the 20 at.% Cr doped sample. Ce is mainly in + 4 valence, but there are a number of Ce³⁺ simultaneously. However, Cr is mainly segregated to the surface in two forms (Cr6+ and Cr³⁺). The presence of Ce³⁺, Cr³⁺ and Cr6+ in mixed oxides results in an increased in concentrations of oxygen vacancies compared with pure CeO₂.The H2-TPR results show that the redox properties are enhanced by the doping of Cr. There may be two kinds of Ce6+ species in the mixed oxides. Hydrogen consumption peak at the lower temperature can be attributed to the reduction of soluble Ce6+, and the peak at the higher temperature is caused by the grafted Cr6+, which is formed by the interaction between Ce6+ and Ce4+. The results of catalytic activities for NH3-SCR show that the addition of Cr improved the catalytic performance of the composite oxides. Ce90Cr10 shows the highest activity. Although the samples calcined at 400℃had a higher surface area than that at 650℃, the activity shows little difference.The specific surface area is an important factor for the NH3-SCR reaction system, but it is not the only one. SCR reaction is essentially a gas-solid reaction, the diffusion and adsorption of gases will be affected by pore sizes. The larger pore size can often results in the higher activity.