Chromic Oxide Catalysts For Room Temperature NO Catalytic Oxidation

With the accelerating process of urbanization and consequent increase in vehicle numbers,automobile exhaust emissions progressively increased.Among these polluted gases,there existed lots of NO_x,which aggravated air deterioration and threatened human health.Thus the removal of NO_x becomes urgent.Presently the common techniques to reduce NOx were SCR,plasma technology and adsorption method at room temperature.However,there were many disadvantages for these technologies,such as the required high temperature for SCR,high energy consumption for plasma technology and limited adsorbing capacity for adsorption method.Given the fact that more than 90% percentage of the NOx was NO and NO₂ was much easier to absorb than NO,it will be an effective strategy to catalyze NO to NO₂ and subsequently absorb NO₂.So it is scientifically significant to explore such a catalyst with high performance for NO oxidation reaction at room temperature.In the thesis we focused on the studies on fabrication of various chromium-based catalysts using different methods,the properties characterization by many technologies and the activity test in the NO oxidation reaction at room temperature.The significant results were as follows:（1）The chromium-based catalysts doped with Fe,Cu,Mn and Ni were prepared by coprecipitation method using NH₃·H₂O as the precipitant.The activity results showed that bimetallic catalysts exhibited no synergistic effect in NO oxidation reaction at room temperature while the catalytic performance of chromium oxide catalyst was superior to the ones doped with Fe,Cu,Mn and Ni,respectively.The best activity was presented in the chromium oxide derived from NH₃·H₂O precipitant.The excellent performance of chromium oxide was attributed to the weaker crystallization degree and more active sites with high valence state on the surface of the catalyst.（2）The chromium-based catalysts prepared by hydrothermal treatment under the conditions of temperature=100 ^oC and hold time=20 h gave a removal ratio of 90%and 5 day-long stability for NO oxidation when NO concentration was 1ppm and SV was equal to 60000 ml/（g·h）at room temperature,superior to that from coprecipitation method.The surface Cr⁶⁺content had an important influence on the catalytic performance.XPS and activity results indicated that the activity increased with the incremental order of surface Cr⁶⁺/Cr³⁺ratio while the used catalyst got the decreased surface Cr⁶⁺/Cr³⁺ratio.The catalyst deactivation was mainly due to the active site coverage caused by nitrate accumulation on the surface.Furthermore,the activity test in different conditions demonstrated that low speed velocity and NO concentration was favored to slow down the nitrate accumulation rate and prolong the lifetime.（3）The monolithic catalysts were obtained by coating Cr-100uniformly on the cordierite substrates,and exhibited better catalytic performance for low concentration of NO oxidation reaction.The effects of additional CO on the catalytic performance of monolithic catalysts were also investigated,and the results demonstrated that the CO inhibited the NO oxidation due to the competitive adsorption for both gases on the catalyst surface.The monolithic catalyst presented the excellent performance in the NO purification test of practical underground parking,giving the NO removal ratio of more than 80%after 14 day-long time.