Highly Efficient Spinel Catalysts For The Selective Catalytic Reduction Of NO With H₂ At Low Temperature

Nitrogen oxides （NO_x） are among those atmospheric pollutants that contribute to the global environmental problems, and they also seriously affect the human survival environment and health. Thus strict mitigation of NO_x emissions is necessary. In this paper, we use H2-selective catalytic reduction （H2-SCR）, that is a low-cost, environment friendly and can reduce NOX emissions at low temperature. And the H2-SCR performance of a series of spinels and Pd-doped spinels were studied in accordance to their physical and chemical characteristics. The effects of divalent A-site cations, trivalent B-site cations, and the doping of palladium in the spinels on the SCR activity were also considered. Moreover, the influences of the preparation method, H2/NO ratios, temperature, gas hourly space velocity （GHSV）, O2 feed concentration, SO2 and H2O were also considered on the activity of the SCR reaction.Firstly, we compared the SCR performance of three different kinds of nickel ferrite spinels （i.e. NiFe₂O₄,0.5% Pd/NiFe₂O₄ and NiFe1.95Pdo.05O₄） and chose the better one, i.e. a mixed crystal spinel of NiFe1.95Pdo.05O₄, for the follow-up study. We also compared the perparation methods of hydrosolvent hydrothermal method, PEG-400 hydrothermal method and sol-gel auto-ignition technique over NiFe1.95Pdo.05O₄ for H2-SCR reaction, and found that the catalyst which prepared by sol-gel auto-ignition technique contained higher SCR performance. Therefore, this method would be adopted for the catalyst preparation discribed as below.Through the study of NiFe₂-xPdx04 （x= 0,0.03,0.05,0.07 and 0.1）, we found that the doping of noble metal Pd can effectively improve the SCR performance on the nickel ferrite catalysts. A decrease in the H2/NO ratio from 10:1 to 5:1 and an increase in the O2 concentration from 0% to 6% produced only a slight decrease in the NO conversion over NiFe1.95Pdo.05O₄, indicating that H2 was more favorably to react with NO in the presence of O2. Furthermore, this catalyst possessed excellent water tolerance, and the NO conversion only showed a slight decrease after additing water to the reacting gases.Nickel ferrite spinels have exhibited high activity in the H2-SCR reaction, so in order to prove the feasibility of other ferrite spinels, we used the metals of Co, Cu and Zn as A-site cations. From the experimental results, we can conclude that these three catalysts also contained high efficiency, but were affected by the A-site metals in the ferrite spinels. The catalytic activities ranked as CoFe1.95Pdo.05O₄> ZnFe1.95Pd0.05O₄> CuFe1.95Pdo.05O₄. The SCR performance was affected by the cations at the spinel octahedral sites which are exposed on the outermost surface layer. CoFe1.95Pdo.05O₄ catalyst had excellent water tolerance. SO2 exhibited both a reversible and an irreversible effect on the SCR reaction over CoFe1.95Pdo.05O₄.Both the pure and Pd-doped aluminate spinel exhibited the same sequences with respect to activity:CoAl2O₄> ZnAl2O₄> CuAl2O₄ and CoAl1.95Pdo.05O₄> ZnAl1.95Pdo.05O₄> CuAl1.95Pdo.05O₄, also indicating that SCR activity was affected by the A-site metals. A slight positive influence of SCR activity was observed in the presence of H2O on CoAl1.95Pdo.05O₄, because of the formation of Hads species which can facilitate the dissociation of NOads species, and thus increase the NO conversion. The SCR activity of CoAl1.95Pdo.05O₄ can be affected by SO2, but with no further decline of NO conversion after increasing the SO2 concentration.To further elucidate the effect of B-site cations in the spinels on SCR performance, we cited manganate spinels as catalysts. Comparing the SCR results of ferrite and aluminate spinels, we found that manganate spinels exhibited inferior physical-chemical characteristics and H2-SCR performance. However, their SCR activity was also affected by the A-site cations, and the activities ranked as follows:NiMn1.95Pdo.05O₄> CuMn1.95Pdo.05O₄> CoMn1.95Pdo.05O₄. The GHSV was a critical factor for H2-SCR, and a decrease in the GHSV resulted in an increase in the SCR activity. NiMn1.95Pdo.05O₄ catalyst contained excellent water tolerance. Due to the competitive adsorption of SO2 with other reactant gases on the active sites, the presence of 200 ppm SO2 can significantly decrease the SCR activity but without affact the overall trend of SCR activity.In general, the addition of palladium cations to the spinels can significantly improve the H2-SCR performance, and different A-site and B-site cations in the spinel produce distinct effects on the SCR activity, so choosing proper cations as A-site and B-site ions is very important. The performance of spinel catalysts can be affected by the preparation methods. The appropriate adjustments of influencing parameter, such as H2/NO ratios and O2 concentration, produce only a slight decrease in the NO conversion. All the catalysts studied in this paper contain excellent water tolerance. The SCR activity was affected by SO2, but no further decline was observed with the increasing of SO2 concentration.