Low-temperature activity and mechanism of WO_3-modified CeO_2-TiO_2 catalyst under NH_3-NO/NO_2 SCR conditions

The CeO2‐TiO2(CeTi)and CeO2/WO3‐TiO2(CeWTi)catalysts were prepared by a sol‐gel precipitation method and their NH3‐NO/NO2 selective catalytic reduction(SCR)performance was studied.N2O formation and effect of oxygen concentration on SCR performance over CeWTi catalyst were also investigated while varying the NO2/NOx ratio.Results indicate that fast SCR behavior of CeWTi catalyst has the best NH3‐NO/NO2 SCR performance due to the catalyst reoxidation rate by NO2 higher than by O2.Compared with CeTi catalyst,CeWTi catalyst exhibits higher de‐NOx performance under NH3‐NO/NO2 SCR conditions.As the CeTi and CeWTi catalysts exhibit similar redox property,addition of WO3 provides more acid sites which accelerate the reaction between NH4NO3 and NO to get a superior low‐temperature activity.Amount of N2O formation shows a peak at 250 oC mainly derived from NH4NO3 decomposition.

A time-saving method to assess the efficiency of corrosion inhibitors by electro osmosis

The corrosion inhibitor is one of the most important technologies to enhance the durability of steel-reinforced concrete. A kind of time-saving method was developed to assess the inhibitor efficiency by using a 32 V electric field to accelerate chloride ion migration in concrete. Potentiodynamic polarization scanning test was used to evaluate the corrosion states. The comprehensive efficiency of an inhibitor should be assessed in two aspects： resistance to chloride ion permeability and inhibiting efficiency. The specimens with different mixing amount of sodium nitrite and migration corrosion inhibitors were used to verify the accuracy and reliability of this method. The results show the differences in inhibiting efficiency of the inhibitors clearly, indicating the reliability of this time-saving method.

Application of silica-alumina as hydrothermally stable supports for Pt catalysts for acid-assisted soot oxidation

In this work,silica-alumina mixed oxides with different SiO_(2)contents(5%and 30%)were adopted as acidic supports for platinum catalysts for soot oxidation.The obtained catalysts were hydrothermally aged in 10%H_(2)0/air at 750℃for 20 h.The catalysts were characterized by X-ray diffraction(XRD),N_(2)adsorption,inductively coupled plasma(ICP),CO chemisorption,NH3temperature-programmed desorption(TPD),infrared(IR)spectroscopy of CO adsorption,temperature-programmed oxidation(TPO)of NO,and TPD of NO_(x).The surface acidity of catalyst was positive correlated with the content of SiO_(2),which kept platinum in metallic and partially oxidized states in an oxidizing atmosphere.Compared with sulfation treatment on the alumina support,the application of SiO_(2)-Al_(2)O_(3)mixed oxides does not result in the coverage of Pt active sites and the prepared catalysts exhibit excellent activity for NO oxidation.They promote NOxpreferential adsorption on soot and decomposition of surface oxygenated compounds(SOCs)as the sulfated Pt/Al_(2)O_(3)catalyst does.

Progress on metal-support interactions in Pd-based catalysts for automobile emission control

The interactions between metals and oxide supports,so-called metal-support interactions(MSI),are of great importance in heterogeneous catalysis.Pd-based automotive exhaust control catalysts,especially Pd-based three-way catalysts (TWCs),have received considerable research attention owing to its prominent oxidation activity of HCs/CO,as well as excellent thermal stability.For Pd-based TWCs,the dispersion,chemical state and thermal stability of Pd species,which are crucial to the catalytic performance,are closely associated with interactions between metal nanoparticles and their supporting matrix.Progress on the research about MSI and utilization of MSI in advanced Pd-based three-way catalysts are reviewed here.Along with the development of advanced synthesis approaches and engine control technology,the study on MSI would play a notable role in further development of catalysts for automobile exhaust control.

Effects of WOx modification on the activity, adsorption and redox properties of CeO_2 catalyst for NO_x reduction with ammonia

A series of WO3/CeO2 （WOx/CeO2） catalysts were synthesized by wet impregnation of ammonium metatungstate on a CeO2 support. The resulting solid acid catalysts were characterized by X-ray diffraction （XRD）, UV-Vis spectroscopy （UV-Vis）, Raman spectroscopy （Raman）, in-situ Fourier transform infrared spectroscopy （in-situ FT-IR） of ammonia adsorption, NH3-TPD, H2 temperature- programmed reduction （H2-TPR）, NH3/NO oxidation and activity measurements for NOx reduction by NH3 （NH3-SCR）. The results show that polytungstate （WOx） species are the main species of tungsten oxide on the surface of ceria. The addition of tungsten oxide enhances the BriSnsted acidity of ceria catalysts remarkably and decreases the amount of surface oxygen on celia, with strong interaction between CeO2 and WOx. As a result, the N2 selectivity of NH3 oxidation and NH3-SCR at high temperatures （〉 300℃） is enhanced. Therefore, a wide working temperature window in which NOx conversion exceeds 80% （NOx conversion 〉 80%） from 200 to 450℃, is achieved over 10 wt.% WOx/CeO2 catalyst. A tentative model of the NH3-SCR reaction route on WOx/CeO2 catalysts is presented.

Hydrothermal and sulfur aging of CeTi/CeWTi catalysts for selective catalytic reduction of NOx with NH3

The CeO2-TiO2(CeTi)and CeO2/WO3-TiO2(CeWTi)catalysts were prepared by sol-gel method.The asprepared catalysts were hydrothermally treated at 760℃for 48 h in air containing 10 vol%H2O to obtain the hydrothermal aged catalysts.The sulfur aged catalysts were treated at 400℃with 100 ppm SO2,10%water vapor,air balance for 48 h and catalysts.The powder X-ray diffraction(XRD)and Raman results indicate that the crystallization of hydrothermal aged catalysts is more serious than sulfur aged catalysts.In addition,tungsten species can stabilize the CeTi catalyst from grain growth.According to the results of in situ diffuse reflectance infrared Fourier transform spectra(DRIFTS),temperatureprogrammed desorption of ammonia(NH3-TPD),H2 temperature-programmed reduction(H2-TPR)and ammonia oxidation,the aging process leads to loss of surface area,redox properties,surface acidities and surface ceria concentration,especially for the hyd rothermal aging.The NH3-NO/NO2 SCR perfo rmances of sulfur aged catalysts are better than that of hydrothermal aged catalysts.Compared with CeTi catalyst,the addition of tungsten inhibits the crystallization of catalyst.So that more acid sites and active sites are retained.This is also the reason why tungsten addition improves the NH3-NO/NO2 SCR performance of CeTi catalyst.

Chemical deactivation of V2Os-WO3/Ti02 SCR catalyst by combined effect of potassium and chloride

V2O5-WO3/WiO2 catalyst was poisoned by impregnation with NHaC1, KOH and KC1 solution, respectively. The catalysts were characterized by X-ray diffraction （XRD）, inductively coupled plasma （ICP）, N2 physisorption, Raman, UV-vis, NH3 adsorption, temperature-programmed reduction of hydrogen （H2- TPR）, temperature-programmed oxidation of ammonia （NH3-TPO） and selective catalytic reduction of NOx with ammonia （NH3-SCR）. The deactivation effects of poison- ing agents follow the sequence of KC1 〉 KOH 〉〉 NH4CI. The addition of ammonia chloride enlarges the pore size of the titania support, and promotes the formation of highly dispersed V = O vanadyl which improves the oxidation of ammonia and the high-temperature SCR activity. K~ ions are suggested to interact with vanadium and tungsten species chemically, resulting in a poor redox property of catalyst. More importantly, potassium can reduce the Bronsted acidity of catalysts and decrease the stability of Bronsted acid sites significantly. The more severe deactivation of the KCl-treated catalyst can be mainly ascribed to the higher amount of potassium resided on catalyst.

A novel insight into enhanced propane combustion performance on PtUSY catalyst

Platinum was supported on c-AlOand ultrastable Y zeolite(USY) by an incipient wetness impregnation method. The catalysts were characterized by nitrogen physisorption, transmission electron microscopy(TEM),CO/CHisothermal oxidations, NHtemperature-programmed desorption(NH–TPD) and infrared(IR) spectroscopy of adsorbed probe molecules(CO, CHand CH+ O). Compared with Pt/AlO, Pt USY catalyst shows obviously higher activity for the combustion of propane. After estimating the size effect of Pt particles and propane adsorption capacity of USY, the excellent activity of Pt USY is also attributed to the strong interactions between the precious metal and the acidic zeolite. It inhibits the oxidation of Pt in an oxygen-rich atmosphere at high temperatures, which facilitate the initial oxidation step involving the C–H bond activation on metallic Pt as reflected by in situ diffuse reflectance infrared Fouriertransformed(DRIFT) spectra.

VxMn(4-x)Mo3Ce3/Ti catalysts for selective catalytic reduction of NO by NH3

A series of vanadium based catalysts(VxMn(4-x)Mo3Ce3/Ti) with different vanadium(x wt.%) and manganese((4-x) wt.%) contents have been prepared by the wet impregnation method and investigated for selective catalytic reduction(SCR) of NOx by NH3 in the presence of 8 vol.% H2O and 500 ppm V SO2.The physicochemical characteristics of the catalysts were thoroughly characterized.The SCR of NOx by NH3(NH3-SCR) activity, especially the low-temperature activity, significantly increased with increasing V2O5 content in the catalyst until the V2O5 content reached 1.5 wt.%, which corresponds well with the redox properties of the catalyst.All of the metal oxides were well dispersed and strongly interacted with each other on the catalyst surface.V mainly exists in the V^5+state in the catalysts.The strong synergistic effect between the vanadium and cerium species led to formation of more Ce^3+species, and that between the vanadium and manganese species contributed to formation of more manganese species with low valences.All of the catalysts exhibited strong acidity, while the redox properties determined the NH3-SCR activity, especially the low-temperature activity.H2O and SO2 had severe inhibiting effects on the activity of V1.5Mn2.5Mo3Ce3/Ti.However, good H2O and SO2 resistance and high NOx conversion by V1.5Mn2.5Mo3Ce3/Ti could be achieved in the presence of SO2 and almost no decline was observed in a long-term test at 275℃ for 168 hr in the presence of SO2 and H2O, which can be attributed to the sulfate species formed on the catalyst surface.

Controlled pore size of Pt/KIT-6 used for propane total oxidation

Mesoporous silica KIT-6 with different pore sizes was controllably synthesized by hydrothermal meth- ods. The same contents of Pt were loaded on KIT-6 sup- ports to apply for propane total oxidation. Low-angle X-ray diffraction （XRD）, nitrogen adsorption-desorption, CO chemisorption and transmission electron microscopy （TEM） were carried out to investigate the physicochemical properties of the catalysts. The results reveal that different pore sizes of KIT-6 supports could affect the Pt particle sizes on KIT-6. The mesopores on KIT-6-80 and KIT-6- 130 effectively confine the size of the inside Pt nanopar- ticles during calcining. Pt/KIT-6-80 with the appropriate pore size as well as the Pt particle size exhibits the best catalytic performance with T50 （the temperature at which hydrocarbon （HC） conversion reaches 50%） of only 237 ℃. However, Pt particles prefer dispersing on the external surface of KIT-6-40 due to those too small pores.

Effect of manganese and/or ceria loading on V_2O_5-MoO_3/TiO_2 NH_3 selective catalytic reduction catalyst

The effect of manganese and/or ceria loading of V2 O5-MoO3/TiO2 catalysts was investigated for selective catalytic reduction(SCR) of NOx by NH3.The manganese and/or ceria loaded V2 O5-MoO3/TiO2 catalysts we re prepared by the wetness impregnation method.The physicochemical characteristics of the catalysts were thoroughly characterized.The catalytic performance of 1.5 wt% V2 O5-3 wt% MoO3/TiO2(V1.5 Mo3/Ti) is greatly enhanced by addition of 2.5 wt% MnOx and 3.0 wt% CeO2(V1.5 Mo3 Mn2.5 Ce3/Ti) below450℃.Compared with the V1.5 Mo3/Ti catalyst with NOx conversion of 75% at 275 ℃,V1.5 Mo3 Mn2.5 Ce3/Ti exhibits higher NOx conversion of 84% with good resistance to SO2 and H2 O at a gas hourly space velocity value of 150000 h-1.The active manganese,cerium,molybdenum,and vanadium oxide species are highly dispersed on the catalyst surface and some synergistic effects exist among these species.Addition of MnOx significantly enhances the redox ability of the cerium,vanadium,and molybdenum species.Addition of Ce increases the acidity of the catalyst.More active oxygen species,including surface chemisorbed oxygen,form with addition of Mn and/or Ce.Because of the synergistic effects,appropriate proportions of manganese in different valence states exist in the catalysts.In summary,the good redox ability and the strong acidity contribute to the high NH3-SCR activity and N2 selectivity of the V1.5 Mo3 Mn2.5 Ce3/Ti catalyst in a wide temperature range.And the V1.5 Mo3 Mn2.5 Ce3/Ti catalyst shows good resistance to H2 O and SO2 in long-time catalytic testing,which can be ascribed to the highly sulfated species adsorbed on the catalyst.

Effect of barium sulfate modification on the SO2 tolerance of V2O5/TiO2 catalyst for NH3-SCR reaction

Sulfur poisoning of V_2O_5/BaSO_4–TiO_2（VBT）,V_2O_5/WO_3–TiO_2（VWT） and V_2O_5/BaSO_4–WO_3–TiO_2（VBWT） catalysts was performed in wet air at 350℃ for 3 hr,and activities for the selective catalytic reduction of NO_x with NH_3 were evaluated for 200–500℃.The VBT catalyst showed higher NO_x conversions after sulfur poisoning than the other two catalysts.The introduction of barium sulfate contributed to strong acid sites for the as-received catalyst,and eliminated the redox cycle of active vanadium oxide to some extent,which resulted in a certain loss of activity.Readily decomposable sulfate species formed on VBT-S instead of inactive sulfates on VWT-S.These decomposable sulfates increased the number of strong acid sites significantly.Some sulfate species escaped during catalyst preparation and barium sulfate was reproduced during sulfur poisoning,which protects vanadia from sulfur oxide attachment to a great extent.Consequently,the VBT catalyst exhibited the best resistance to sulfur poisoning.

Effects of tungsten oxide on the activity and thermal stability of a sulfate-derived titania supported platinum catalyst for propane oxidation

A Pt/WO3/TiO2 catalyst for propane oxidation was prepared by a stepwise wet impregnation method, and was aged at 800°C for 5 hr. Compared to the sulfate-derived titania supported catalyst, the introduction of tungsten oxide as stable Br nsted acid sites led to the formation of more metallic platinum active sites at the Pt/WO3 interface. The dissociation of surface intermediates for propane oxidation was promoted on the WO3-modified catalyst. This, as well as the inhibition effects of tungsten oxide on the sintering of anatase and the phase transformation to rutile, resulting in a high activity and thermal stability for the Pt/WO3/TiO2 catalyst.

An isolation strategy to anchor atomic Ni or Co cocatalysts on TiO_(2)(A)for photocatalytic hydrogen production

TiO_(2) has been considered as an ideal photocatalyst for water splitting.However,narrow light absorbance,low charge separation efficiency,and rare surface active sites lead to the low photocatalytic efficiency of TiO_(2).Although extensive research attempted to improve the situation,there is still lack of method for constructing high active and noble-metal-free TiO_(2) photocatalyst for H_(2) evolution reactions(HER).In this work,we loaded single atomic(SA)Ni(or Co)on the surface of anatase TiO_(2)(TiO_(2)(A))nanosheets by an isolation strategy.Ethylene diamine tetraacetic acid and ethylene glycol(EDTA-EG)compounds were used to chelate metal ions in solution and form carbon quantum dots in the following thermal treatment to isolate the metal ions on surface of TiO_(2)(A).The prepared Ni SA/TiO_(2)(A)catalyst owned a“skin wrapped body”structure with in-situ formed twodimensional(2D)heterojunction facilitating the fast electron transfer.As a result,the Ni SA/TiO_(2)(A)catalyst showed a high H_(2) evolution rate of 2,900μmol·g−1·h−1.This work provides an isolation strategy for constructing promising single-atom metal catalyst for photocatalysis and beyond.

Improved activity and durability of Rh-based three-way catalyst under diverse aging atmospheres by ZrO2 support

The catalytic activity and durability of Rh/ZrO2 catalyst were investigated compared with Rh/Al2O3 catalyst under diverse aging atmospheres, including lean, rich and lean–rich cyclic aging atmospheres, to simulate the real working conditions of three-way catalyst.Oxidation states and microstructures of rhodium species were investigated to correlate with the catalytic performance of the catalysts. The catalytic performance and durability of the Rh catalyst under diverse aging atmospheres were drastically enhanced by ZrO2 support. ZrO2 support was confirmed to be able to effectively inhibit rhodium sintering even under diverse aging conditions. It can also successfully keep Rh species in an active low-valence state on the surface of the catalyst. The superiority of ZrO2 support compared to Al2O3 was verified by the Rh-based monolith catalyst.

Effect of water vapor on NH_3–NO/NO_2 SCR performance of fresh and aged MnO_x–NbO_x–CeO_2 catalysts

A MnOx-NbOx-CeO2 catalyst for low temperature selective catalytic reduction（SCR） of NOx with NH3 was prepared by a sol-gel method, and characterized by NH3-NO/NO2 SCR catalytic activity, NO/NH3 oxidation activity, NOx/NH3 TPD, XRD, BET, H2-TPR and in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy（DRIFTS）. The results indicate that the Mn Ox-Nb Ox-CeO2 catalyst shows excellent low temperature NH3-SCR activity in the temperature range of 150-300℃. Water vapor inhibits the low temperature activity of the catalyst in standard SCR due to the inhibition of NOx adsorption. As the NO2 content increases in the feed, water vapor does not affect the activity in NO2 SCR. Meanwhile, water vapor significantly enhances the N2 selectivity of the fresh and the aged catalysts due to its inhibition of the decomposition of NH4NO3 into N2O.

Optimizing the crystallinity and acidity of H-SAPO-34 by fluoride for synthesizing Cu/SAPO-34 NH_3-SCR catalyst

A series of H-SAPO-34 zeolites were synthesized by a hydrothermal method in fluoride media.The as-synthesized H-SAPO-34 zeolites were characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,N_2 physisorption,temperature-programmed desorption of NH_3（NH_3-TPD） and nuclear magnetic resonance（NMR） measurements.The results showed that a certain concentration of F- anions promoted the nucleation and crystallization of H-SAPO-34.The H-SAPO-34 synthesized in the fluoride media showed high crystallinity,uniform particle size distribution,large specific surface area and pore volume,and enhanced acidity.Therefore,Cu/SAPO-34 based on the fluoride-assisted zeolite showed a broadened temperature window for the selective catalytic reduction of NO by NH_3（NH_3-SCR） reaction due to the enhanced acidity of the zeolite and the improved dispersion of copper species.

Deactivation of Cu-SAPO-34 by urea-related deposits at low temperatures and the regeneration

Selective catalytic reduction(SCR) with urea catalyzed by Cu-SAPO-34 is an effective method to eliminate NO_x from diesel exhaust. However, urea-related deposits may form during cold-start and urban driving due to low exhaust temperatures. The activity of CuSAPO-34 at 175°C is significantly degraded by urea exposure, and 300°C is required for regeneration. Through in-situ diffuse reflectance infrared Fourier transform spectroscopy(DRIFTS) and temperature-programmed hydrolysis studies, the dominant stable deposit at 175°C is identified as biuret, which can be eliminated at 300°C. The urea-derived deactivation and regeneration mechanisms of Cu-SAPO-34 were compared with those of anatase-supported catalysts.

Controllable synthesis of argentum decorated CuO_(x)@CeO_(2) catalyst and its highly efficient performance for soot oxidation

In this paper,CuO_(x)@Ag/CeO_(2) catalysts were synthesized by simple wet-chemical method and equal volume impregnation method.The obtained catalysts were subjected to soot temperature programmed oxidation(soot-TPO)activity tests and were further characterized by various techniques such as X-ray diffraction(XRD),transmission electron microscopy/high-resolution transmission electron microscopy(TEM/HR-TEM),N_(2) physisorption,X-ray photoelectron spectroscopy(XPS)and H_(2)-temperature programmed reduction(H_(2)-TPR).The results show that CuO_(x)@Ag/CeO_(2) synthesized presents well controlled core-shell structures,with nano-cube like Cu_(2)O as the core and Ag decorated polycrystalline CeO_(2) grafting layers as the shell.Such core-shell structured CuO_(x)@Ag/CeO_(2) can successfully construct a secondary oxygen delivery channel(CuO_(x)→CeO_(2)→Ag)to effectively transfer bulk oxygen of the catalyst to the soot,resulting in its excellent soot oxidation activity compared to CuO_(x)@CeO_(2).The potential benefiting effect by Ag introduction over Cu@Ag/Ce can be concluded as:(ⅰ)pumping lattice oxygen and accelerating gaseous O_(2) dissociation to generate significantly increased active surface oxygen content;(ⅱ)modulating a moderate surface oxygen vacancies concentration to maintain more highly active O_(2) species.

Size effect of Pt nanoparticles in acid-assisted soot oxidation in the presence of NO

Pt/Al2O3 catalysts with mean Pt particle size ranged from 2.7 to 7.1 nm were synthesized by chemical reduction method,and the sulfated counterparts were prepared by impregnation of sulfuric acid.The turnover frequency of platinum for soot oxidation under loose contact conditions in a feed flow containing NO and O2 are positively correlated with the size of platinum.The sulfated Pt/Al2O3 exhibits higher catalytic activity for soot oxidation in the presence of NO despite their reduced ability for NO2 production.Such a contradiction is more significant for those catalysts with smaller platinum particles.Herein,the catalysts were characterized by X-ray diffraction(XRD),Brunauer-Emmett-Teller(BET),transmission electron microscopy(TEM),inductive coupled plasma(ICP)emission spectrometry,CO chemisorption,thermogravimetric analysis(TGA),NH3 temperature-programmed desorption(NH 3-TPD),NO temperature-programmed oxidation(TPO)and NO x temperatureprogrammed desorption(TPD).Possible effect of Pt particle size for the catalytic oxidation of soot in the presence of NO was presented based primarily on the promoted NO2 transfer efficiency onto the soot pushed by the acidic catalysts.