Selective Catalytic Reduction (SCR) Of N0_x On Cerium-based Catalysts

Selective catalytic reduction of NO_xwith NH₃(NH₃-SCR) is an effective method for abatingthe NO_x, both for stationary sources, such as power plants and vehicles, such as diesel engines.The current commercial catalyst for this reaction is V₂O5-WO₃/TiO₂(V-W/Ti). However, thiscatalyst is not without disadvantages, such as the toxicity of vanadium species, narrow activitywindow, high activity for the oxidation of SO₂to SO₃, and the alkali metal poisoning effect.Recently, CeO₂-WO₃(CeW) catalysts were found to yield good SCR activity and N₂selectivityby investigating the recipes of catalysts and the technologies of preparations. In order to putthese catalysts in reality, based on the co-precipitation method, we first studied the lowtemperature SCR of CeW catalysts by doping with transition metal, and selected NH₃moleculesas a probe to investigate the acidity and oxidation ability of transition metal (Fe, Mn) andrare-earth metal (La, Y) doped to CeO₂, and finally studied the SO₂and alkali metal poisoningeffects of CeW and CeW/Ti. BET, XRD and XPS were used to study the physical structuralproperties of catalysts. A combination of experimental and theoretical methods (TPD, TPR,DRIFTS and DFT calculations) were utilized to study the chemical or catalytic properties andobtained some stable adsorption configurations.(1) Based on DFT calculations, we first investigate the NH₃and NO_xadsorptions on α-andβ-MnO₂(001) surface models. The results indicated that no matter NH₃or NO_xadsorptions, theadsorption energies on α-MnO₂(001) were greater than those on β-MnO₂(001). Moreover, weinvestigated the NH₃adorptions on graphene and graphene oxides, and obtained that OH couldprovide more stable adsorptions than O does, and NH₃can forming Br nsted and Lewis acid siteadsorptions on both sides of graphene and graphene oxides.(2) The CeO₂-WO₃(CeW) and Mn-doped CeO₂-WO₃(MnCeW) catalysts were prepared byco-precipitation method. The MnCeW catalyst exhibited more activity for NO_xconversion thanthe CeW catalyst did below200°C. The results showed that the MnCeW catalyst provided moreBr nsted acid sites and higher reducibility than the CeW catalyst. Theoretical studies showed that oxygen vacancies can easily form on the MnCeW (110) surface, resulting in more facileNH₃adsorption and higher activity. The reaction mechanism mainly followed the L-Hmechanism at150°C over the MnCeW catalyst: both adsorbed NH₃and NH₄⁺react with nitritespecies.(3) Four kinds of metals (Fe, Mn, La and Y) as dopants to modify the ceria were prepared andthe representative slab models based on the structural characterization (XRD, EDS and XPS)coupled with the experimental approaches (TPD, TPR) were used to investigate the differencesof the acidity and reducibility of the catalysts. The higher reducibility, more amounts of activelattice oxygen and greater extent surface distortion can be responsible for the more NH₃adsorption on the Fe-Ce and Mn-Ce catalysts than the La and Y doped catalysts. Moreover, thedistribution of Lewis and Br nsted acidity and the specific NH₃adsorption configurations wereobtained by a combination of DRIFTS spectra and DFT calculations. The Fe-Ce and Mn-Cecatalysts can provide more NH₃adsorption sites on the Lewis acid sites than the La-Ce and Y-Cecatalysts, and the Mn dopant can provide tightly bonded interaction with NH₄⁺on the Br nstedacid sites. The charges of NH₃were also in agreement with the results above and indicated thatthe lone pair electrons of nitrogen can transfer to the iron or manganese ions but less favorable tothe rare-earth ions. Finally, two reaction for NH₃and NO oxidation were carried out, and theresults are followed the reducibility order of the M-Ce catalysts: Fe-Ce～Mn-Ce＞La-Ce～Y-Ce.(4) The alkali metal-induced deactivation of a novel CeO₂-WO₃(CeW) catalyst used forselective catalytic reduction (SCR) was investigated. The CeW catalyst could resist greateramounts of alkali metals than V₂O5-WO₃/TiO₂. At the same molar concentration, the K-poisonedcatalyst exhibited a greater loss in activity compared with the Na-poisoned catalyst below200°C.Experiments results indicated that decreases in the reduction activity and the quantity ofBr nsted acid sites rather than the acid strength were responsible for the catalyst deactivation.The DFT calculations revealed that Na and K could easily adsorb on the CeW (110) surface andthat the surface oxygen could migrate to cover the active tungsten, and then inhibit the SCR ofNO_xwith ammonia. We also tested the SO₂resistance of the fresh and poisoned CeW catalystsand there is no evidence of a synergistic inhibition effect from the SO₂and alkali metals on the SCR activity. At low temperature, the activity was significantly affected by SO₂; however, athigh temperature the affection is not apparent. The hot water washing is a convenient andeffective method to regenerate alkali metal poisoned CeW catalysts, and the catalytic activitycould be recovered80%activity of the fresh one.