Preparation And Its NH₃-SCR Performance Investigation Of Ceria Catalyst Modified By Solid Superacid

Nitrogen oxides?NO_x?,emitted mainly from the combustion of fossil fuel and automobile exhausts,induce various atmospheric pollution problems.Currently,selective catalytic reduction of NO with ammonia?NH₃-SCR?has been regarded as one of the most promising techniques to eliminate harmful NO_x from diesel exhausts.However,an inevitable problem of this application is that commercially available V₂O₅-WO₃?or MoO₃?/TiO₂ catalysts for the NH₃-SCR cannot effectively control NO_x emission beyond 300-400°?.Moreover,other shortcomings of this commercial catalyst,including toxicity and volatility of vanadium,the phase transformation of TiO₂ between anatase and rutile at elevated temperature,also restrict the further application.Therefore,in order to find replaceable catalysts,we search for environmentally benign and structurally stable catalysts that could achieve high NO conversion and N₂ selectivity,simultaneously.Recently,nontoxic ceria-containing NH₃-SCR catalysts have been widely researched due to the excellent reducibility and remarkable oxygen storage capacity?OSC?.However,pure ceria exhibits rather poor NH₃-SCR activity.The high surface active oxygen of ceria catalyst contributes to ammonia oxidation,especially at high temperature,resulting in dramatic decline of NO_x conversion and N₂ selectivity.It is well known that the acidic components are essential to enhance the NH₃-SCR performance of pure ceria.The strong surface acidity can facilitate the adsorption of NH₃ and inhibit the oxidation of NH₃,resulting in an increase of NH₃-SCR activity.Therefore,in the present work,two solid acids with different acid strength were used to modify CeO₂,and then we explore their influence to the surface acidity,redox property and NH₃-SCR reaction routes of CeO₂.The main results could be illustrated as followed:?1?A series of ceria modified zirconium phosphate catalysts were synthesized for selective catalytic reduction of NO with ammonia?NH₃-SCR?.Over 98%NO_x conversion and 98%N₂ selectivity were obtained by the CeO₂/ZrP catalyst with 20wt.%CeO₂ loading at 250-425°C.The interaction between CeO₂ and zirconium phosphate enhanced the redox abilities and surface acidities of the catalysts,resulting in the improvement of NH₃-SCR activity.The in situ DRIFTS results indicated that the NH₃-SCR reaction over the catalysts followed both Eley-Rideal and Langmuir-Hinshelwood mechanisms.The amide?-NH₂?groups and the NH₄⁺bonded to Br?nsted acid sites were the important intermediates of Eley-Rideal mechanism.?2?The solid superacid TiO₂-ZrO₂-SO₄^2--supported 20 wt.%CeO₂ catalyst?20CeO₂/Ti-Zr-S?was synthesized for selective catalytic reduction of NO with NH₃?NH₃-SCR?.The NH₃-SCR performance was significantly enhanced by the construction of strong acidic sites on the surface of 20CeO₂/Ti-Zr-S and over 96%NO conversion was obtained at 225-425°?.Meanwhile,the strong interaction between solid superacid and CeO₂ resulted in excellent redox property and abundant surface oxygen species.Furthermore,the NH₃-SCR reaction over 20CeO₂/Ti-Zr-S catalyst mainly followed the Langmuir-Hinshelwood mechanism at low-temperature?250°??.The M-NO₂?M=Ce,Ti,Zr?nitrate compounds,monodentate and bridging nitrates were the crucial intermediates in Langmuir-Hinshelwood mechanism.In addition,amide?-NH₂?species were available at 350°?over 20CeO₂/Ti-Zr-S catalyst,which facilitated the high-temperature NH₃-SCR activity via Eley-Rideal pathway.