Effect Of Various Factors On The Selective Catalytic Reduction Of NO With NH₃ Over TiO₂-Supported Catalysts:Mechanism And Kinetic Study

Among the technologies for the control of NOx emission,the selective catalytic reduction(NH3-SCR)technology is mature,reliable and easy to maintain.In the near future,NH3-SCR technology will be the preferred technology for the control of NOx emissions in thermal power plants of China.The research show that factors such as flue gas composition and catalyst composition could affect the SCR performance of the catalyst.Meanwhile,catalyst is the core component of the SCR process,and its performance has important effect on the overall performance of the SCR system.So,it is of great practical significance to study the influence of various factors on the performance of the SCR catalyst.Most of the studies on the effect of influence factors on the performance of SCR catalyst are carried out under black box models.Generally,the research under this kind of models can reflect the causal relationship between the factors and the consequences of those on the catalyst,but can not reflect the law of the action in the SCR reaction.It is still far to explore clearly the essential reasons for the influence of various factors on the SCR performance.Therefore,it is necessary to study the influence of factors on the performance of the catalyst at the mechanism level.The traditional NH3-SCR reaction kinetic models based on power function are simple in form and difficult to reasonably describe the reaction mechanism.The kinetic models derived from the mechanism are constructed based on the theory of the pseudo first order reaction,without considering the effects of reaction mechanism(including the Eley-Rideal mechanism and the Langmuir-Hinshelwood mechanism)and side reactions(including the non selective catalytic reduction and the catalytic oxidation of NH3,i.e.NSCR reaction and C-O reaction)in the SCR process.It is difficult to meet the increasingly needs of the research on the mechanism of NH3-SCR reaction.In the previous work,our group has constructed a novel NH3-SCR reaction kinetic model which included the contribution of the Eley-Rideal mechanism,the Langmuir-Hinshelwood mechanism,the NSCR reaction and the C-O reaction.The model is in good agreement with the experimental results of steady-state kinetics and can predict the NOx conversion well under various conditions.Based on the previous studies,the kinetic models of NH3-SCR reaction of corresponding catalysts was established.By comparing the kinetic parameters under different reaction conditions,the influence of various influencing factors on the SCR reaction was analyzed semi-quantitatively.Moreover,combined with the results of transient reaction and characterization,we explored the role of WO3 in NO Reduction with NH3 over V2O5-WO3/TiO2 catalyst and the effect of H2O and SO2 on NO reduction with NH3 over CeO2/TiO2 catalyst.The specific research results are as follows:(1)The steady-state kinetic study demonstrated that the SCR reaction over V2O5/TiO2 was promoted after WO3 incorporation due to the promotion of NH3 adsorption and the increase of oxidation ability.Meanwhile,the C-O reaction was restrained due to the decrease of V5+ concentration.As the SCR reaction contributed to NO reduction while the C-O reaction contributed to NO formation,NOx conversion over V2O5/TiO2 obviously increased after WO3 incorporation.Furthermore,the NSCR reaction was restrained due to the decrease of V5+ concentration,resulting in an obvious increase of N2 selectivity.(2)Both SO2 and H2O showed a promotion on NO reduction over CeO2/TiO2 at higher temperatures.However,their mechanisms were quite different.The promotion of NOx conversion at higher temperatures due to the presence of H2O was mainly related to the inhibition of the C-O reaction while the SCR reaction over CeO2/TiO2 was still restrained.However,the SCR reaction over CeO2/TiO2 at higher temperatures was promoted after the introduction of SO2 due to the promotion of NH3 adsorption.Therefore,the promotion of NOx conversion at higher temperatures due to the introduction of SO2 was related to not only the inhibition of the C-O reaction but also the promotion of the SCR reaction.