The Construction And Application Of Steady-state Kinetic Study Over Several Typical NH₃-SCR Catalysts

Most of traditional NH₃-SCR reaction kinetic models were 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 reaction?including NSCR reaction and C-O reaction?on the SCR process.It is difficult to meet the increasingly needs of the research on the mechanism of NH₃-SCR reaction.In this paper,a novel NH₃-SCR reaction kinetic model was constructed,which included the contribution of the Eley-Rideal mechanism,the Langmuir-Hinshelwood mechanism,the NSCR reaction and the C-O reaction.According to the model,kinetic constants of the SCR reaction through the Eley-Rideal mechanism and the Langmuir-Hinshelwood mechanism and kinetic constants of the NSCR reaction were obtained.By comparing the kinetic constants,the effects of different reaction conditions on the SCR process can be analyzed by semi quantitative analysis.Moreover,the effects of various reaction conditions and influencing factors on the SCR process were deeply studied by combining with in situ DRIFT,transient reaction,XPS,TPD and TPR.Specific research results were as follows:?1?The N₂ formation?SCR reaction?rate of NO reduction over Mn-Fe spinel was linearly correlated with the concentration of gaseous NO.Meanwhile,the N₂O formation?NSCR reaction?rate was independent of the gaseous NO concentration.Therefore,N₂O selectivity of NO reduction over Mn-Fe spinel decreased with the increase of gaseous NO concentration.Moreover,N₂O formation rate and N₂O selectivity of NO reduction over Mn-Fe spinel increased with the increase of the concentration of gaseous NH₃,and N₂O selectivity decreased with the decrease of the gas hourly space velocity.NO reduction and N₂O formation over Mn-Fe spinel through the Eley-Rideal mechanism were notably restrained by H₂O due to the decrease in the oxidation ability,the inhibition of NH₃ adsorption and the inhibition of the interface reaction.Furthermore,N₂O formation over Mn-Fe spinel through the Langmuir-Hinshelwood mechanism was completely suppressed by H₂O due to the suppression of the formation and/or decomposition of NH4NO₃.As a result,H₂O showed a notable inhibition on NO reduction and N₂O formation over Mn-Fe spinel.?2?The oxidation ability of MnOx-CeO₂ obviously decreased in the presence of H₂O.Moreover,the interface reaction of NH with gaseous NO was restrained by H₂O.Therefore,H₂O showed a marked inhibition of N₂O formation during NO reduction over MnOx-CeO₂,resulting in a novel promotion on the SCR reaction.Potassium showed a negative effect on the low temperature SCR reaction over MnOx-CeO₂.The decrease of the SCR activity of MnOx-CeO₂ due to the doping of potassium was mainly attributed to the decrease of acid site and Mn4+ concentration on the surface.However,the increase of N₂O selectivity of NO reduction over MnOx-CeO₂ due to the doping of potassium was mainly assigned to the occurrence of N₂O formation over K-MnOx-CeO₂ through the Langmuir-Hinshelwood mechanism.?3?The NSCR reaction rate of NO reduction over V₂O₅-WO₃/TiO₂ was nearly independent of gaseous NO concentration,and the reaction order of the C-O reaction with respect to gaseous NO concentration was less than zero.Because of the competition adsorption between NH₃ and NOx,the SCR reaction,the NSCR reaction,and the C-O reaction all promoted with the increase of gaseous NH₃ concentration.