Preparation And Its Mechanisms Of CeO₂ Catalyst Modified By Solid Acid

Selective catalytic reduction of NO_x with NH₃?NH₃-SCR?is the most widely used technology for the removal of nitrogen oxides.Nowadays,V2O₂-WO₃/TiO₂ is the well-known NH₃-SCR catalyst in the practical application.Although the V-based catalyst exhibits high NO_x reduction efficiencies,some inevitable disadvantages,such as the narrow working temperature window?300-400 ??,the toxicity of V species and the poor N2 selectivity at high temperatures,have limited its further industrial application.Thereby,the CeO₂-based catalysts are intensively investigated in the NH₃-SCR reaction.However,pure CeO₂ catalysts possess the poor catalytic activity,low N2 selectivity and inferior resistance to H₂O +SO₂.A series of CeO₂ catalysts modified by solid acids were prepared,and the relationships among the coordination mechanism,surface acidity and the reaction pathway over the solid acids modification CeO₂ were explored.The effects of different CeO₂ supports,precipitants,pH values,preparation methods and hydrothermal temperatures on the CeO₂ catalysts modified by solid acids were systematically studied in the process of NH₃-SCR reaction.The main results can be concluded as follows:?1?A series of CeO₂ catalysts modified by solid acids?phosphotungstic acid,silicotungstic acid,phosphomolybdic acid and ammonium sulfate?were prepared by the impregnation method and used for selective catalytic reduction?SCR?of NO_x with NH₃.The results indicated that CeO₂ catalysts modified by phosphotungstic acid or silicotungstic acid possessed better catalytic activity and superior resistance to H₂O + SO₂.Furthermore,the effects of different CeO₂ supports on the catalytic performance over CeO₂ catalyst modified by phosphotungstic acid?P-W/CeO₂?were investigated.An increase in the oxidation ability of NH₃ and NO was attributed to its higher Ce3+ concentrations,abundance of oxygen vacancies and larger amounts of surface acid sites,resulting in the improvement of SCR activity.Besides,the NH₃-SCR reaction over P-W/CeO₂ followed both the Langmuir-Hinshelwood mechanism and the Eley-Rideal mechanism.The reaction between NH4+ absorbed on the B acid sites and bidentate nitrate?NO₃-?produced NO₂,H₂O and N2,which improved the low-temperature SCR activity following the L-H mechanism.NH₃ species adsorbed on the L acid sites reacted with NO₂ and NO,generating N2 and H₂O,contributing to broadening the working temperature window,which followed the Eley-Rideal?E-R?mechanism.?2?The influences of different precipitants and pH values on micro-structure and surface acidity over silicotungstic acid-modified CeO₂ catalysts were studied.Silicotungstic acid-doped CeO₂ prepared by NH₃·H₂O or an appropriate pH value?pH = 9?was mainly enclosed by?111?plane,which could be conductive to the interaction between WO₃ and CeO₂,and then the enhancement of the amount of surface unsaturated W=O species,Ce3+ concentrations and oxygen vacancies,causing an increase in the surface acidity and redox property;however,the catalyst prepared with?NH4?2CO₃ predominantly exposed to?220?and?200?surfaces,and the catalyst prepared with the mixture of NH₃ H₂O and?NH4?2CO₃ chiefly exhibited the?200?planes.Furthermore,when the pH values were adjusted from 9 to 8,10 and 11,the exposed CeO₂ surface?111?was turned to?220?and?200?,which promoted the agglomeration of silicotungstic acid on CeO₂ surfaces,resulting in a decrease in the dose and types of surface acidity,as well as redox ability.In addition,the NH₃-SCR reaction pathway was described as follows:NH4-?a?+ NO₃-?a??NH4NO₃?a?NH4NO₃?a??NH₃?a?+HNO₃?a?NO?g?+NH4NO₃?a??N2+2H₂O+NO₂?a?2NH₃?a?+NO₂?a?+NO?g??2N2+3H₂OThe NH₃-SCR reaction followed the Langmuir-Hinshelwood?L-H?and the E-R mechanisms,and the reaction of B acid sites and NO₃-was in favored of the low-temperature catalytic activity.?3?The CeO₂-ZrO₂-WO₃?CZW?catalysts were prepared by hydrothermal synthesis,co-precipitation,incipient impregnation and sol-gel methods.The results implied that the CZW catalyst prepared by hydrothermal method showed the best SCR activity and the excellent resistance to H₂O + SO₂.The superior SCR activity of CZW catalyst was assigned to its large BET surface area,excellent redox property,and higher content of Ce3+ and chemisorbed oxygen species.Furthermore,the co-existence of L and B acid sites also enhanced the SCR activity.Besides,the decrease in the SCR activity over CZW by hydrothermal treatment was due to the loss of surface acid sites.?4?The different WO₃ states?i.e.,the amorphous WO₃ state and Ce2?WO4?3 compounds?over CeO₂-ZrO₂-WO₃ were prepared by hydrothermal method and used for the selective catalytic reduction of NO_x by NH₃?NH₃-SCR?.The amorphous W03 state could not only enhance the amount of L and B acid sites on the surface of catalyst,but also promote the oxidation of NO into bridging nitrate and monodentate nitrate species.Only bridging nitrate species and L acid sites were observed over the CeO₂-ZrO₂-WO₃ catalysts with the presence of Ce2?WO4?3 compounds.The reaction between monodentate nitrate and adsorbed ammonia species contributed to improvement of the catalytic activity,especially for the low-temperature SCR activity.The NH₃-SCR reaction pathway of CeO₂-ZrO₂-WO₃ mainly followed the L-H mechanism.