Selective Catalytic Reduction of NO x with NH 3 over Mn, Ce Substitution Ti 0.9 V 0.1 O 2−δ...

Published: May 24, 2011

r 2011 American Chemical Society 12850 dx.doi.org/10.1021/jp112283g | J. Phys. Chem. C 2011, 115, 12850–12863

ARTICLE

pubs.acs.org/JPCC

Selective Catalytic Reduction of NOx with NH3 over Mn, CeSubstitution Ti0.9V0.1O2�δ Nanocomposites Catalysts Prepared bySelf-Propagating High-Temperature Synthesis MethodBin Guan, He Lin,* Lin Zhu, and Zhen Huang*

Key Laboratory for PowerMachinery and Engineering ofMinistry of Education, Shanghai Jiao TongUniversity, Shanghai 200240, China

1. INTRODUCTION

The selective catalytic reduction of NOx (NH3�SCR) underthe lean condition has been well-proven to be an effective andeconomical technique for the removal of NOx in the exhaustgases from stationary and mobile sources.1�4 For the marineengine, the International Maritime Organization (IMO) issuedthe stringent Tier III standards by 2016 that the NOx emissionsfrom marine engines entering an Emission Control Area (ECA)must be lower than 3.49 g/kW h, which is 80% less than that ofTier I in 2000. Among lots of techniques for the marine engineNOx abating, SCR is the most promising one for Titer III.5 Itshould be pointed out that the low speed marine diesel engineexhaust is low temperature in the range of 200�280 �C andusually contains several hundreds ppm of SO2 due to thecombustion of the fuel with more than 4.5% sulfur. Therefore,it is urgent to study and develop the catalysts which possess highactivity and robust SO2 resistance at low temperatures.

In recent years, the manganese oxide (MnOx) has become anattractive interest due to its high SCR activity at low tem-peratures.6�9 The manganese oxides contain various types oflabile oxygen which are necessary for completing a catalytic cyclein SCR.10�17 The activity of pure manganese oxides in SCR hasbeen intensively investigated by Kapteijin et al., and 90% of NO

conversion was obtained at 170 �C.18 Pe~na et al. reported that thelow-temperature SCR of NO with NH3 in the presence of excessoxygen on the oxides of first row transition metals as V, Cr, Mn,Fe, Co, Ni, and Cu supported on anatase TiO2 was studied. Theyfound thatMn/TiO2 supported onHombikat TiO2 provided thebest performance with 100% N2 selectivity and complete NOconversion at temperatures as low as 393 K under numerousconditions8 and that a new highly active, time-stable, and waterresistant Hombikat TiO2 supported Mn catalyst had beendeveloped for the selective reduction of NO by NH3 andobtained 100% yield of N2 operating at much lower reactiontemperatures (e120 �C).9,19�21 Additive of ceria could enhancethe oxidation of NO to NO2, thereby increasing the catalyticactivity of SCR of NOx with NH3.

22�28 Qi et al. found that themanganese�cerium catalyst yielded nearly 100%NO conversionat 120 �C.29�32 Tang et al. also reported that the manganese�cerium mixed oxide catalyst showed excellent denitrificationactivity at 80 �C.33 It was found that the addition of CeO2

significantly improved the dispersion of the active component

Received: December 27, 2010Revised: May 20, 2011

ABSTRACT: This study focuses on promoting the low tem-perature performance of vanadium-based catalysts; for this, theSHS method was applied to synthesize a series of Ti0.9-VxM0.1�xO2�δ catalysts. The performances of the catalysts(Ti0.9V0.1O2�δ, Ti0.9Mn0.05V0.05O2�δ, and Ti0.9Ce0.05V0.05O2�δ)were fully investigated with the temperature-programmed-reaction, which proved that these SHS catalysts with nanometersize had high activity over a broad temperature window of150�400 �C. Compared with Ti0.9V0.1O2�δ, the substitutedcatalysts, Ti0.9Mn0.05V0.05O2�δ and Ti0.9Ce0.05V0.05O2�δ,showed higher N2 selectivity at high temperatures. The Ce substituted catalyst exhibited good resistance to H2O and SO2

poisoning at low temperatures. The structural and physical-chemical properties of catalysts were characterized comprehensively byBET, XRD, FTIR, TEM, EDX, XPS, and TPD. The XRD results indicated that the active components of V, Mn, and Ce were highlydispersed over the catalysts. The Mn substitution could enhance the Brønsted acid sites on the catalyst surface and accelerate theSCR reaction at low temperatures. The XPS shows that the Ce substitution led to high concentration of chemisorbed oxygen, whichdiminished the unselective oxidation of NH3 by O2 to N2O, NO, or NO2 and resulted in superior N2 selectivity. The activecomponents of the catalysts, such as V, Mn, and Ce, mostly existed in the form of mixed-valence which was beneficial for theoxidation of NO to NO2. Furthermore, the SCR reaction mechanism over Ti0.9Ce0.05V0.05O2�δ catalyst was also examined usingin situ DRIFTS. The results revealed that high active monodentate nitrate and bridging nitrate species as well as abundant ionicNH4

þ (Brønsted acid sites) were the key intermediates in the SCR reaction since the ad-monodentate nitrate and bridging nitratespecies disappeared quickly in the presence of NH3.

12851 dx.doi.org/10.1021/jp112283g |J. Phys. Chem. C 2011, 115, 12850–12863

The Journal of Physical Chemistry C ARTICLE

over the surface of the catalyst and enhanced active sites per unitarea.34,35 However, because of the strong oxidizing ability, themanganese oxide series catalysts could oxidize NH3 to NO andN2O easily as the temperatures increased, which decreased theN2 selectivity.

25,36 Pe~na et al. also found that the N2 selectivityover 20% Mn/TiO2 decreased sharply from 95% at 100 �C to52% at 250 �C.9 For the SO2 resistance of Mn, Kijlstra and Wuet al. reported that SO2 was combined with Mn to producesulfate easily, making the Mn�Ce catalysts deactivate at lowtemperatures.37�39 Jin et al. reported that the Mn/TiO2 catalystprepared by sol�gel method was very sensitive to SO2 and thatthe NO conversion decreased from 93% to only about 30% after6 h test in the presence of 100 ppm SO2 at 150 �C.40�42

Recently, Roy et al. have synthesized Ti0.9M0.1O2�δ (M = Cr,Mn, Fe, Co, and Cu) serial catalysts prepared by the self-propagating high-temperature synthesis (SHS) method.43�45 Itwas reported that Ti0.9Mn0.1O2�δ was the most active catalyst atlow temperatures, but its selectivity for N2 was poor. On thecontrary, Ti0.9Fe0.1O2�δ ranked first in N2 selectivity but was notactive at low temperatures.43 Roy found that the optimizedcatalyst of Ti0.9Mn0.05Fe0.05O2�δ had good activity in the rangeof 150�500 �C andmore than 80%N2 selectivity even at 450 �C.However, the H2O and SO2 tolerance performance of thiscatalyst has not yet been reported.43

The most widely accepted commercial vanadium-based SCRcatalysts (such as V2O5�WO3 (MoO3)/TiO2) are active withina relatively narrow operation temperature window of 300�400 �C;however, they are less active when the temperature is lower than300 �C.46�49 It is known that the vanadium-based catalystspossess strong SO2 tolerance, whichmay be a potential candidatefor the treatment of the marine engine emissions with high SO2

content. Considering the high SO2 resistance of vanadium-basedcatalysts, there is great interest in modifying this kind of catalystsand developing highly active SCR catalysts within a low tem-perature window.

In the present research, we intend to develop a new vanadium-(V) based SCR catalyst that can meet with the demand of themarine engine NOx reduction. A series of Ti0.9V0.1O2�δ basedcatalysts were tested with a partial substitution of vanadium withmanganese and cerium, with respect to their high activity at lowtemperatures. The catalysts were prepared with the SHSmethod,which can produce homogeneously dispersed nanocompositeswith unique properties. In this way, we obtained a perfect catalystof Ti0.9Ce0.05V0.05O2�δ, which showed high activity within thetemperature window of 150�400 �C as well as strong H2O andSO2 resistance capability. The physical structural and physical-chemical properties of the catalysts were characterized compre-hensively by N2 physisorption, XRD, SEM, TEM, and XPS. Inaddition, theNH3�SCRmechanism over the Ti0.9Ce0.05V0.05O2�δ

catalyst was investigated by in situ DRIFTS.

2. EXPERIMENTAL SECTION

2.1. Catalyst Synthesis. The catalysts in this study wereprepared by a single step solution combustion method, namelythe SHS method that has been well addressed in the literature.43

The precursor mixture used in this study is an aqueous solutionof stoichiometric amount of titanyl nitrate (TiO(NO3)2) andrespective metal nitrates (vanadium nitrate V(NO3)3, manga-nous nitrate Mn(NO3)2 3 4H2O, and cerium nitrate Ce(NO3)3 36H2O), where TiO(NO3)2 was prepared by dissolving TiO(OH)2,the product of tetra-n-butyl titanate Ti{OCH3(CH2)3}4 hydrolysis,

into the nitric acid HNO3 (68%, w/w solution). Before combus-tion, stoichiometric glycine (CH2NH2COOH) was added intothe precursor mixture as a fuel. After a few minutes’ stirring on aheating plate to ensure proper homogeneity, the so-preparedsolution was transferred into a corundum crucible, which wassubsequently introduced into a muffle furnace kept at a constanttemperature of 350 �C. At the initial stage, the aqueous solutionboiled with frothing and foaming with concomitant dehydra-tion. At the point of its complete dehydration, the mixtureignited yielding a voluminous finely dispersed solid productin high volume. The solid product was calcined at the sametemperature (350 �C) for another 1 h to get a clean productwithout any impurity. Under the equilibrium conditions, theglobal reactions of catalyst synthesis can be summarized asfollows:

162TiOðNO3Þ2 þ 18VðNO3Þ3 þ 208C2H5NO2

f 180Ti0:9V0:1O2�δ þ 520H2Oþ 293N2 þ 416CO2

324TiOðNO3Þ2 þ 18VðNO3Þ3 þ 18MnðNO3Þ2 þ 404C2H5NO2

f 360Ti0:9Mn0:05V0:05O2�δ þ 1010H2Oþ 571N2 þ 808CO2

162TiOðNO3Þ2 þ 9VðNO3Þ3 þ 9CeðNO3Þ3 3 6H2O

þ 208C2H5NO2 f 180Ti0:9Ce0:05V0:05O2�δ þ 574H2O

þ 293N2 þ 416CO2

For comparison, 3% V2O5/TiO2 was prepared by theconventional wet impregnation method. The NH4VO3 wasdissolved in oxalic acid solution, followed by stirring themixture solution on a heating plate for a few minutes toobtain a dark blue solution of (NH4)2[VO(C2O4)2]. Stoi-chiometric pretreatment nano-TiO2 was added to the ob-tained solution, which was subsequently desiccated at 200 �Cfor 2 h and then calcinated at 500 �C for 3 h in air. Allcatalysts prepared were ground and sieved to 40�80 meshfor evaluation.2.2. Catalytic Activity, Selectivity, and H2O and SO2 Tol-

erance Studies. The performances of various catalysts inNH3�SCR were investigated in a temperature programmedreaction (TPR) system equipped with a FTIR (Thermo NICO-LET 6700). In each TPR run, 0.5 mL of fresh catalyst sample waspacked in the center of a fixed-bed quartz reactor with an innerdiameter of 9 mm. The catalyst sample was plugged with silicawool to prevent the catalyst from being blen away. The reactorwas installed in a tubular electric resistance furnace whosetemperature was measured by a chromel-alumel thermocoupledipped in the catalyst bed and regulated by a proportionalintegral derivative (PID) controller. The furnace temperatureranging from the room temperature to 450 �C was ramped at5 �C/min. The synthetic feed gas mixture consisted of 600ppm NO, 600 ppm NH3, 400 ppm SO2 (when used), 7% H2O(when used), and 5 vol %O2 (balanced with N2) was adjusted bymass flower controllers. Water vapor was generated by passingN2 through a heated gas-wash bottle containing deionized water.The effect of SO2 on the NOx removal efficiency was investigatedby adding 400 ppm SO2 to the feed gas mixture. The typical totalgas flow rate was 100 mL/min at the standard temperature andpressure (STP), and the corresponding gas hourly space velocity(GHSV) was 12 000 h�1. The effluent gas including NO, NO2,N2O, and unreacted NH3 was continuously quantified with anonline high resolution FTIR spectrometer (Thermo NICOLET



6700, OMNICQuantpad software) equipped with a gas-measur-ing cell. Because the reaction may occur at low temperatures, itmust be confirmed that the decrease of NOwas not caused by thelow temperature adsorption. For this reason, the catalyst samplewas purged with reactant gas at the room temperature at thebeginning of each TPR run until no difference of NO concentra-tion between the inlet and the outlet gas mixtures was detected.The NOx conversion (ηNOx

) and the N2O selectivity (SN2O)were calculated as follows:

ηNOxð%Þ ¼ ½NOx�in � ½NOx�out

½NOx�in� 100% ð1Þ

where NOx is the sum of NO and NO2 concentrations

SN2Oð%Þ ¼ 2½N2O�out½NO�in þ ½NH3�in � ½NO�out � ½NH3�out

�100%

ð2ÞDuring the NH3�SCR reaction process, no other nitrogen-

containing component was detected in the effluent gas exceptNO, NO2, N2O, and NH3, so we define the N2 selectivity (SN2

)as formula 3

SN2ð%Þ¼ ½NO�in þ ½NH3�in � ½NO�out � ½NH3�out � ½NO2�out � 2½N2O�out

½NO�in þ ½NH3�in � ½NO�out � ½NH3�out�100%

ð3ÞThe NH3 conversion (ηNH3

) and the selectivities for theformation of N2 (SN2

), NOx (SNO2), and N2O (SN2O) during

separate NH3 oxidation were defined by the following equations:

ηNH3ð%Þ ¼ ½NH3�in � ½NH3�out

½NH3�in� 100% ð4Þ

sNOxð%Þ ¼ ½NO�out þ ½NO2�out½NH3�in � ½NH3�out

� 100% ð5Þ

sN2Oð%Þ ¼ 2½N2O�out½NH3�in � ½NH3�out

� 100% ð6Þ

sN2ð%Þ ¼ 100%� ðsNOx þ sN2OÞ ð7Þ

2.3. Characterization of Catalysts. N2 adsorption�desorptionisotherms were obtained at �196 �C using a QuantachromeNOVA 1000a Automated Gas Sorption Instrument. Prior to N2

adsorption, all samples were degassed at 100, 150, 200, and250 �C for 10 min and at 50 and 300 �C for 30 min undervacuum, respectively. The specific surface areas of the catalystswere determined from these isotherms by applying the Brunauer�Emmett�Teller (BET) method in the 0.05�0.3 partial pressurerange. The total pore volume (at a relative N2 pressure (P/P0) of0.987), average pore diameters, and pore size distributions weremeasured from the N2 desorption branches of the isothermsusing the Barret�Joyner�Halenda (BJH) method.The XRD patterns of catalysts were checked using a compu-

terized Rigaku D/max-2200/PC X-ray diffractometer equippedwith Cu KR (λ = 0.15418 nm) radiation source. The sampleswere located on a sample holder with a depth of 1 mm. XRDpatterns were recorded in the 2θ range of 20 to 90� at a scan rateof 2θ= 5�/min with a scan step size of 0.02�. The identification of

crystalline phase was made by matching the Powder DiffractionFile-International Centre for Diffraction Data (PDF-ICDD).The surface morphology and microstructures of the catalysts

prepared and the catalysts after SO2 tolerance reaction wereobserved by scanning electron microscopy (SEM) in a HitachiS-2150 system at an accelerating voltage of 20 kV.Transmission electron microscopy (TEM) was performed on

a JEOL JEM-2010 analytical electron microscope operating at anaccelerating voltage of 200 kV to obtain indications about themicroscopic structure of the catalysts. Meanwhile, the dispersionof the elemental composition and semiquantitative determina-tion of the ratio of Ti, V, Mn, Ce, and O in the catalysts wereverified by the energy dispersive X-ray spectrometer (EDX)analysis in the Oxford INCA EDAX Detecting Unit. Before theTEM test, the catalyst powder was ultrasonically suspended inalcohol for 10 min, and the obtained suspension was depositedon copper-grid-supported amorphous carbon films. Then theimpregnated mesh was dried in air for TEM analysis. Meanwhile,10 sites of EDX data were collected to get an average value of theelemental composition.To identify the surface nature and concentration of the active

species, the catalysts were analyzed using the X-ray photoelec-tron spectroscopy (XPS). XPS was collected on a RBD upgradedPHI 5000C ESCA scientific electron spectrometer (Perkin-Elmer) with Mg KR radiation (hν = 1253.6 eV) as the excitationsource. The operation conditions were kept constant at a highvoltage of 14.0 kV and 250.0W. The base pressure of the analyzerwas about 5 � 10�8 Pa. To subtract the surface charging effect,the binding energies of V (2p), Mn (2p), Ce (3d), Ti (2p), andO(1s) were calibrated using the C-(C�H) component of thecontaminated carbon (C1s = 284.6 eV) as the standard.2.4. NH3-TPD Studies.Temperature programmed desorption

of NH3 (NH3-TPD) experiments were carried out in a fixed-bedquartz reactor. During the experiments, 0.3 mL of catalyst samplewas first pretreated in pure N2 at 500 �C for 2 h to yield a clearsurface and then cooled to the room temperature. The cooledsample was saturated with 0.6 vol % NH3 in N2 at a flow rate of100 mL/min for 1 h and then purged by N2 for another 1 h untilno NH3 could be detected by FTIR at the outlet. In thedesorption process, the carried gas N2 was kept at a flow rateof 100 mL/min while the temperature increased from roomtemperature to 500 �C with a heating rate of 5 �C/min. Theeffluent composition was analyzed continuously with a FTIRspectrometer.2.5. In Situ DRIFTS Studies. The in situ diffuse reflectance

infrared Fourier transform spectroscopy (in situ DRIFTS)experiments were used to investigate the adsorption and thereaction of NH3, NO, and O2 on the catalyst surface. The in situDRIFTS was performed on a Thermo NICOLET 6700 FTIRspectrometer equipped with an in situ diffuse reflection chamberwith a high sensitive mercury�cadmium�telluride (MCT-A)detector. An approximately 70 mg sample was finely ground andpressed into a self-supported wafer. The wafer was placed in asample holder assembly in a Harrick praying mantis DRIFTShigh-temperature reaction chamber which was directly con-nected to a purging and adsorption gas control system. Thetemperature of the reaction chamber was controlled precisely byan Omega temperature controller. Prior to each experiment, thesample was treated at 450 �C in flowing N2 for 2 h and thencooled to the desired temperature. At each temperature, thebackground spectrum of the catalyst was recorded at a gas flow



rate of 50 mL/min N2 and was automatically subtracted from thesample spectrum taken at the same temperature.

3. RESULTS AND DISCUSSION

3.1. Catalytic Activity and Selectivity Studies. 3.1.1.NH3�SCR activity of 3% V2O5/TiO2 and Ti0.9V0.1O2�δ Catalysts.Roy reported that the Ti0.9Mn0.05Fe0.05O2�δ catalyst preparedwith the SHS method was superior to the catalyst of 5% V2O5/TiO2 prepared by the wet impregnation method.43 Figure 1shows that the NOx conversion over Ti0.9V0.1O2�δ preparedby the SHS method and 3% V2O5/TiO2 prepared by the wetimpregnation method. It can be seen that the NOx conversionover 3% V2O5/TiO2 is very low (no more than 30%) below200 �C. However, the SHS catalyst of Ti0.9V0.1O2�δ showssuperior catalytic activity within a wide temperature window of170 to 390 �C, within which more than 90% NOx is reduced.Particularly, at the extremely low temperature of 75 �C, thiscatalyst removes more than 50% NOx. The decrease of NOx

conversion above 380 �C can be ascribed to the unselective NH3

oxidation by O2, which usually takes place at high temperatures.The active reaction temperature window (75�450 �C, in whichmore than 50% NOx is reduced) of Ti0.9V0.1O2�δ is much widerthan that of the conventional V2O5/TiO2 catalyst. This may beattributed to the fact that V and Ti in the catalyst prepared by theSHS method interact strongly and lead to high dispersion ofactive phases and excellent structural characteristics that benefitSCR reactions.50�53

3.1.2. Effect of Partial Substitution of V with Mn, Ce on theActivity and Selectivity of Ti0.9V0.1O2�δ. In order to furtherimprove the activity of Ti0.9V0.1O2�δ at low temperatures, wepartially substitute V with Mn and Ce. Figure 2a presents theeffect of Mn and Ce on the NOx conversion and N2O formation.It can be seen that by substituting half of V with equivalent Mn(Ti0.9Mn0.05V0.05O2�δ), the NOx conversion in 50�80 �C isfurther improved. At 55 �C, the NOx conversion over Ti0.9-Mn0.05V0.05O2�δ is greatly improved from 14.5% to 54.2%. It isworth noting that, when the V content is partially substitutedwith Mn and Ce, the catalysts show almost the same activity inthe range of 100�450 �C. As shown in Figure 2b, the selectivitiesof N2 and N2O of Ti0.9Mn0.05V0.05O2�δ and Ti0.9Ce0.05V0.05O2�δ

are obviously higher than that of Ti0.9V0.1O2�δ at high temperatures(250�450 �C). In Figure 2a, N2O appears in high concentration(approximate 70 ppm) above 400 �Cover Ti0.9V0.1O2�δ catalyst.The formation of N2O over Ti0.9Mn0.05V0.05O2�δ increases with

elevating temperatures and reaches a maximum of 42 ppm at310 �C, and then declines to 16 ppm at 450 �C. It is noticedthat there is only a few ppm of N2O detected on Ti0.9Ce0.05-V0.05O2�δ at any temperature. This indicates that the Cesubstitution catalyst displays excellent N2 selectivity. In aword, we conclude that among the three catalysts in this studyTi0.9Ce0.05V0.05O2�δ catalyst has the best NOx conversionand N2 selectivity.3.1.3. Effect of GHSV. GHSV is a very important parameter to

be considered in the catalyst design. Therefore, the influence ofGHSV on the NOx removal efficiency over Ti0.9Ce0.05V0.05O2�δ

is investigated. The activities of Ti0.9Ce0.05V0.05O2�δ are testedover a wide range of GHSV from 12 000 to 60 000 h�1, and theresults are shown in Figure 3. It can be seen that theNOx removalefficiency decreases dramatically with the increasing of GHSV,especially in the low temperature range below 250 �C. However,

Figure 1. NOx conversion as a function of reaction temperature in theNH3�SCR reaction over Ti0.9V0.1O2�δ and 3% V2O5/TiO2 preparedby the impregnation method. Reaction conditions: [NO] = [NH3] =600 ppm, [O2] = 5 vol %, N2 as balance gas, total gas flow rate 100 mL/min, and GHSV = 12 000 h�1.

Figure 2. NOx conversion (a) and N2 selectivity and N2O selectivity(b) over Ti0.9V0.1O2�δ, Ti0.9Mn0.05V0.05O2�δ, and Ti0.9Ce0.05V0.05-O2�δ. Reaction conditions: [NO] = [NH3] = 600 ppm, [O2] = 5vol %, N2 as balance gas, total gas flow rate 100 mL/min, and GHSV =12 000 h�1.

Figure 3. NOx conversion as a function of GHSV in the NH3�SCRreaction over Ti0.9Ce0.05V0.05O2�δ catalyst. Reaction conditions:[NO] = [NH3] = 600 ppm, [O2] = 5 vol %, and N2 as balance gas.



it should be noted that the NOx removal efficiency still couldmaintain above 85% in a range of 310�390 �C even though theGHSV reaches 60 000 h�1, although the active reaction tem-perature window is much narrower than that in the low GHSVcondition. Furthermore, the influence of GHSV on the NOx

removal efficiency in the high temperature range above 350 �C isnot obvious. On the contrary, the NOx removal efficiency at highGHSV is relatively higher than at low GHSV at the temperatureabove 400 �C, which is explained by the inhibition effect on NH3

oxidation to NOxwith increasing GHSV at high temperatures. Insummary, the NH3�SCR reaction over Ti0.9Ce0.05V0.05O2�δ

catalyst is very sensitive to the contact time. GHSV is animportant factor worth considering to ensure the appropriatecatalytic efficiency in designing the performed catalyst forindustrial applications.3.2. NH3 Oxidation. In the SCR reaction, the oxidation of

NH3 gives rise to NO and N2O, which will decrease theselectivity of N2. Therefore, the selectivity of SCR is closelycorrelated with the oxidation degree and the oxidation productsof NH3.

55,56 As shown in eqs 8�11, there are competitivereactions in NH3�SCR, in which NH3 is consumed by NOand O2 in reaction 8 as well as is oxidized to N2, NOx, and N2Odirectly in reactions 9�11.25,36

4NOþ 4NH3 þO2 f 4N2 þ 6H2O ð8Þ

4NH3 þ 3O2 f N2 þ 6H2O ð9Þ

4NH3 þ 5O2 f 4NOþ 6H2O ð10Þ

2NH3 þ 2O2 f N2Oþ 3H2O ð11ÞTo evaluate the performance of the catalysts on the NH3

oxidation, the NH3�O2 mixture was led over the catalysts at theroom temperature. After a saturated adsorption of NH3, the

temperature is risen to 450 �C at a ramp of 5 �C/min. Figure 4apresents the oxidation rate of NH3 over Ti0.9V0.1O2�δ,Ti0.9Mn0.05V0.05O2�δ, and Ti0.9Ce0.05V0.05O2�δ as a functionof the reactor temperature. The activity of the NH3 catalyticoxidation based on T50 (the temperature at which 50% NH3

being oxidized) follows this order: Ti0.9V0.1O2�δ < Ti0.9-Ce0.05V0.05O2�δ < Ti0.9Mn0.05V0.05O2�δ. The products of theNH3 oxidation include N2, NOx, N2O, and H2O. Figure 4b�dpresents the selectivities of N2, NOx, and N2O, respectively. Theselectivity to NOx over three catalysts is almost the same in thewhole range of reaction temperatures. It is shown in Figure 4bthat most NH3 is oxidized into N2 over three catalysts, in whichTi0.9Ce0.05V0.05O2�δ shows the maximum selectivity for N2. Thecatalyst of Ti0.9Mn0.05V0.05O2�δ shows the minimum selectivitytoward N2 and the maximum selectivity to N2O within160�360 �C. When the temperature exceeds 270 �C, the N2Ogenerated from Ti0.9Mn0.05V0.05O2�δ decreases markedly,whereas the N2O from Ti0.9V0.1O2�δ increases sharply. As aresult, the N2 selectivity of Ti0.9Mn0.05V0.05O2�δ is superior tothat of Ti0.9V0.1O2�δ over 360 �C. We propose that theselectivity in NH3 oxidation is correlated with the SCR reactionover the catalysts, and that the high N2 selectivity in NH3

oxidation is in favor of the total N2 selectivity in SCR reactions.3.3. Catalyst H2O and SO2 Tolerance Durability Test. The

diesel engine exhaust gas usually contains both H2O and SO2,which are likely to poison the catalysts and depress theiractivities. Therefore, the resistance of the catalysts to thedeactivation by H2O and SO2 is very important for industrialapplications in the DeNOx process. For this purpose, Weinvestigated the H2O and SO2 tolerance of Ti0.9Ce0.05V0.05O2-δ

that is the best catalyst in activity and N2 selectivity under thecircumstance of containing 400 ppm SO2 alone, 7% H2O alone,and both 7% H2O and 400 ppm SO2, respectively.As illustrated in Figure 5, in the absence of SO2, a steady-state

NOx conversion of 83.1% at 150 �C is obtained in the first 2 h.

Figure 4. Separate oxidation of NH3 over Ti0.9V0.1O2�δ, Ti0.9Mn0.05V0.05O2�δ, and Ti0.9Ce0.05V0.05O2�δ. (a) NH3 oxidation efficiency, (b) selectivityfor N2, (c) selectivity for NOx, and (d) selectivity for N2O. Reaction conditions: [NH3] = 600 ppm, [O2] = 5 vol %, N2 as balance gas, total gas flow rate100 mL/min, and GHSV = 12 000 h�1.



When 400 ppm SO2 is introduced into the feed gas mixture, theNOx conversion first increases slightly to 83.4% (shown in theinsert graph in Figure 4) and then gradually decreases to 79%after 26 h. When SO2 is eliminated from the feed gas stream, theNOx conversion gradually recovers to 80.5% and then remainsalmost unchanged during the next 2 h. The result demonstratesthat Ti0.9Ce0.05V0.05O2�-δ shows good resistance to SO2 duringNOx reduction at low temperatures. The promoted effect of SO2

in the initial state may be attributed to the formation of SO42- on

the catalyst surface, which can increase the catalyst surface acidityand promote the NH3 absorption and thus the reactions betweenadsorbed NH3 and NOx.

57 The deactivation of the catalyst mayresult from the chemiadsorption competition between SO2 andthe reactants (NH3 and NOx) on the activity sites of the catalystsurface.58�60 Besides, SO2 can easily combine with NH3 to forminstable sulphite on the catalyst surface at low temperatures.40�42

The sulphite would react with chemisorbed oxygen to formsulfate that covers the active site and decreases the activity of thecatalyst.40,41,61

The effect of H2O on the NOx conversion over theTi0.9Ce0.05V0.05O2�δ catalyst was monitored as a function oftime on stream at 150 �C. As indicated in Figure 6, before H2O isadded, the SCR reaction is stabilized at a steady-state NOx

conversion of 83.1% at 150 �C for 2 h. When only 7% H2O isintroduced into the feed gas mixture, the NOx conversiondecreases slowly to about 80.2% after 26 h. After the removalof H2O from the feed gas stream, the NOx removal efficiencyrecovers to the original level before poisoning, suggesting thatthe poisoning effect of H2O on the Ti0.9Ce0.05V0.05O2�δ catalyst

is reversible, and that the catalyst possesses strong resistance toH2O under our test conditions.The synergetic effect of H2O and SO2 on the SCR activity over

the Ti0.9Ce0.05V0.05O2�δ catalyst at 150 �C is also illustrated inFigure 6. When 7%H2O and 400 ppm of SO2 are simultaneouslyadded to the reaction gas mixture, the NOx removal efficiencyshows a relatively significant decrease from 83.1% to 70.1%during the following 26 h and then returns quickly to 74.4% inthe next 4 h after both H2O and SO2 are eliminated from thereaction gas. The results clearly indicate that the deactivation ofthe Ti0.9Ce0.05V0.05O2�δ catalyst is due to the synergistic poi-soning effect of SO2 and H2O. In the presence of both H2O andSO2, SO2 can be oxidized to SO3 by O2 and eventually formsammonium bisulfate (NH4HSO4) and ammonium sulfate((NH4)2SO4) by reacting with NH3 and H2O.

59 Meanwhile,the formation and deposition rate of ammonium bisulfate orammonium sulfate salts is much greater than the one in thepresence of H2O alone and the one in the presence of SO2 alone,respectively. On the one hand, the SOx compounds (SO2 þSO3) are adsorbed on the active metal oxides, and they aredifficult to be adsorbed at the low temperatures; on the otherhand, the easy formation of the ammonium bisulfate andammonium sulfate species results in blocking the catalyst poresand occupying the catalytic active sites, which then deactivatesthe catalyst. It is also noted that when the catalyst is heat treatedin N2 at 400 �C for 2 h the activity could be rapidly restored toalmost its original level in the absence of H2O and SO2. Therecovery of the catalytic activity after the heating treatment at thehigh temperature is due to the fact that the SOx compounds maybe desorbed and that the sulfate species formed on the surface ofcatalysts have been decomposed completely,57,59 which can beverified by the FTIR spectra results about the various conditionsof the Ti0.9Ce0.05V0.05O2�δ catalyst shown in Figure 8. Theresults mentioned above indicate that the Ti0.9Ce0.05V0.05O2�δ

catalyst is robustly tolerant to H2O and SO2.In order to investigate the species formed on the surface of

Ti0.9Ce0.05V0.05O2�δ catalyst, the morphologies of the fresh andthe poisoned catalysts are examined respectively by the scanning

Figure 6. Effect of H2O and SO2 on the NOx conversion over Ti0.9-Ce0.05V0.05O2�δ at 150 �C. Reaction conditions: [NO] = [NH3] = 600ppm, [O2] = 5 vol %, 7%H2O, 400 ppm SO2, N2 as balance gas, total gasflow rate 100 mL/min, and GHSV = 12 000 h�1.

Figure 7. SEM micrographs of various conditions of Ti0.9Ce0.05-V0.05O2�δ surface (a) the fresh, (b) after SO2 tolerance test in thepresence of 400 pmmSO2 at 150 �C for 26 h, (c) after calcined at 200 �Cfor 2 h, and (d) after calcined at 400 �C for 2 h. SEM operationconditions: accelerating voltage: 20 kV, magnification times: 1000, andmarker length: 50 μm.

Figure 5. Effect of SO2 on the NOx conversion over Ti0.9Ce0.05-V0.05O2�δ at 150 �C. Reaction conditions: [NO] = [NH3] = 600ppm, [O2] = 5 vol %, 400 ppm SO2, N2 as balance gas, total gas flow rate100 mL/min, and GHSV = 12 000 h�1.



electron microscope (SEM). As presented in Figure 7a, thesurface of the fresh catalyst is smooth and uniform. After we feed400 ppm SO2 for 26 h at 150 �C, a significant agglomeration andbulk deposition with a size of 30�50 μm could be observedobviously in the SEM images as shown in Figure 7b. Thedeposition on the catalyst surface may be attributed to theformation of ammonium nitrate (NH4NO3) and ammoniumsulfate ((NH4)2SO4). For comparison, the poisoned catalyst iscalcined and its morphology is checked with SEM again. Whenthe poisoning catalyst is calcined at 200 �C for 2 h, the depositionbecomes thin (with the size of 10�20 μm) as shown in Figure 7c.As the decomposition temperature of NH4NO3 is about 170 �C,most of NH4NO3 has been decomposed at 200 �C, leaving onlythe residue of (NH4)2SO4, whose decomposition temperature isnear 300 �C. When the poisoned catalyst is calcined further at400 �C for 2 h, the deposition vanishes and the surface of thecatalyst returns to sublimate, as illustrated in Figure 7d.The transmission IR analysis is performed to qualitatively

analyze whether the ammonium nitrate and ammonium sulfateare actually formed and deposited onto the catalyst to clarify thetrace of the deposition during calcination. Before the test, thesamples are diluted by KBr and pressed into wafers and thenplaced in the Harrick Praying Mantis Sampling Kit. As shown inFigure 8, the spectrum of the fresh catalyst exhibits threecharacteristic peaks at 932 and 3677 cm�1 and a large band atabout 3232 cm�1. The catalyst after SO2 exposure exhibits notonly all of the characteristic IR bands of the fresh catalyst but alsosome new IR bands. The new bands at 2939, 3132, and1443 cm�1 can be assigned to the absorbance peaks of ammo-nium nitrate,62�64 and the IR bands at 1280 and 1040 cm�1 aredue to the absorbance peak of ammonium sulfate.41,59,61,65�67

The IR spectrum of the poisoned catalyst after 2 h calcination at200 �C shows that the ammonium nitrate IR bands disappear,whereas the IR bands of ammonium sulfate survive and becomeweaker. The IR spectrum from the poisoned catalyst after 2 hcalcination at 400 �C shows no difference to the spectrum of thefresh catalyst except that a weak band of 1040 cm�1 is residual.The IR results are consistent with those of SEM micrographs. Itshould be pointed out that, because of the high resolution of theFTIR spectrometer, traces of ammonium sulfate (at band of1040 cm�1) can still be detected by FTIR but cannot be depictedby the SEM due to the lower magnification times.

It is known that the NH3�SCR system of the low speedmarine diesel engine exhaust is in the low temperature range of150�280 �Cwith relatively low GHSV from 6000 to 10 000 h�1,and usually contains several hundreds ppm of SO2 due to thecombustion of the fuel with more than 4.5% sulfur. The resultsmentioned above indicate that the Ti0.9Ce0.05V0.05O2�δ catalystprepared by the SHS method has the superior performance withmore than 83% NOx removal efficiency and higher than 95% N2

selectivity over a broad temperature window of 150�400 �C.Besides, according to the H2O and SO2 tolerance and durabilitytest results, the Ti0.9Ce0.05V0.05O2�δ catalyst is also robustlytolerant H2O and SO2. Therefore, the catalyst is especiallysuitable for the NOx removal in the SCR system of marine dieselengines.3.4. Catalyst Characterization. 3.4.1. BET. Table 1 sum-

marizes the specific surface area, the BJH desorption porevolume, and the average pore diameter of Ti0.9V0.1O2�δ, Ti0.9-Mn0.05V0.05O2�δ, and Ti0.9Ce0.05V0.05O2�δ. As can be noticedfrom this table, the SHS catalysts prepared under appropriateconditions exhibit high specific surface areas. The BET surfaceareas of the three catalysts are almost identical and achieve morethan 140 m2/g. The large specific surface areas may be attributedto the good dispersion of active components on the titaniasupport.53,68 Among these catalysts, Ti0.9V0.1O2�δ has thelargest pore volume and the largest average pore diameter. Largepore volume might be beneficial to the catalytic activity, but alarge average pore diameter might be one of the reasons for theactivity decline. It is also noticeable that the Ti0.9Mn0.05-V0.05O2�δ and Ti0.9Ce0.05V0.05O2�δ catalysts possess moreabundant micropores (<2 nm) than Ti0.9V0.1O2�δ, providingmore inner surface area and active sites for the occurrence of SCRreaction.Figure 9a presents the N2 adsorption�desorption isotherms

of Ti0.9V0.1O2�δ, Ti0.9Mn0.05V0.05O2�δ, and Ti0.9Ce0.05V0.05-

O2�δ measured at the liquid nitrogen temperature. Accordingto the BDDT classification, the adsorption isotherms of thecatalysts resemble closely with type IV isotherms, which aretypically associated with the capillary condensation in mesopor-ous (2�50 nm).50,52,69 The hysteresis loops correlated with thepore shape exhibit typical type E, indicating the presence of an“ink-bottle-shaped” pores structure.50,69 Besides, the closurepoints of the hysteresis loops of these samples occur at lowP/P0 (P/P0 = 0.35), indicating the abundance of micropores(<2 nm) due to the interaction of vanadium, manganese, cerium,and titanium species.50 The BJH desorption pore size distribu-tions derived from the desorption branches of N2 adsorption�desorption isotherms are shown in Figure 9b, revealing that theactive crystallite phases formed in the above catalysts mainlypossess micropores and mesopores and show distribution peaksnear 3.5 nm. The desorption pore size distributions of Ti0.9Mn0.05-V0.05O2�δ andTi0.9Ce0.05V0.05O2�δ gather in the range of 2�7 nm,

Figure 8. Comparison of FTIR spectra of various conditions ofTi0.9Ce0.05V0.05O2�δ (a) the fresh, (b) after SO2 tolerance test in thepresence of 400 ppm SO2 at 150 �C for 26 h, (c) after calcined at 200 �Cfor 2 h, and (d) after calcined at 400 h for 2 h. FTIR operationconditions: spectral resolution: 1 cm�1 and 64 scans per spectrum.

Table 1. Physical and Morphological Properties ofTi0.9V0.1O2�δ, Ti0.9Mn0.05V0.05O2�δ, andTi0.9Ce0.05V0.05O2�δ

catalysts

BET surface

area (m2/g)

pore volume

(cm3/g)

average pore

diameter (nm)

Ti0.9V0.1O2�δ 146.4 0.2049 5.596

Ti0.9Mn0.05V0.05O2�δ 144.0 0.1438 3.993

Ti0.9Ce0.05V0.05O2�δ 146.2 0.1657 4.535



whereas those of Ti0.9V0.1O2�δ are relatively broader (2�10 nm).It is worth noting that plenty of vanadium species in Ti0.9V0.1-O2�δ might be in the form of aggregated V2O5 particles, whichresult in the bimodal mesopore size distributions with the peak at3.3 and 7.7 nm. Additionally, in the diameter range below 3.5 nm,the N2 adsorbed volumes (cm3 g�1 nm�1) of Ti0.9Mn0.05V0.05-O2�δ and Ti0.9Ce0.05V0.05O2�δ are much larger than those ofTi0.9V0.1O2�δ. This means that the Ti0.9Mn0.05V0.05O2�δ andTi0.9Ce0.05V0.05O2�δ catalysts have more abundant microporesand mesopores, which can provide more inner surface area andactive sites for SCR reactions.50,53 Therefore, the high N2

desorption differential pore volume and an appropriate ratio ofmicropores and mesopores are important for the SCR catalystwith high activity, especially at lower temperatures.3.4.2. XRD. The power XRD patterns of the catalyst samples

are shown in Figure 10. As can be seen from this figure,crystalline phases are identified by being compared withICDD files (TIO2 anatase phase: PDF # 21-1272; TiO2 rutilephase: PDF # 21-1276). For all titania supported catalysts,the typical anatase phase with well-defined diffraction peaks isthe main phase. No V2O5, Mn2O3, or CeO2 crystallites aredetected, suggesting that the V, Mn, and Ce oxides are presentin a highly dispersed state and in an amorphous or poorlycrystalline state on titania. The coexistence of vanadium,manganese, cerium, and titanium species results in the forma-tion of an amorphous or poorly crystallite phase, whichexhibits large broad bump diffraction patterns and is helpfulto enhance the surface areas, the pore volumes, and thus theSCR activity.53,70,71

3.4.3. TEM and EDX. TEM is also employed to observe themorphologies and sizes of the catalyst samples. Figure 11a�c

presents the TEM images of Ti0.9V0.1O2�δ, Ti0.9Mn0.05-V0.05O2�δ, and Ti0.9Ce0.05V0.05O2�δ, respectively. We can seethat there are manymicropores dispersed on theses catalysts, andno obvious pore agglomeration appears. The TEM image ofTi0.9V0.1O2�δ, shows that the typical nanoparticles are welldispersed with the sizes in the range of 5�15 nm.Ti0.9Mn0.05V0.05O2�δ shows slightly smaller crystalline sizes of5�10 nm. The panel c indicates that Ti0.9Ce0.05V0.05O2�δ hasexcellent dispersion and the smallest particle size around3�4 nm, which might be due to the strongest interactionbetween vanadium, cerium, and titanium species.50,68

The energy dispersive X-ray analysis (EDX) is carried out todetermine the ratios of O, Ti, V, Mn, and Ce semiquantitatively.The EDX shows that the composition of Ti�Mn�V oxide isatomicMn = 5.60%, V = 5.3%, Ti = 89.10% (shown in Figure 12a),and that the corresponding composition of Ti�Ce�V oxide isatomicCe = 5.23%, V= 5.30%,Ti = 89.47% (shown in Figure 12b).The EDX results of Ti, Mn (or Ce), and V element contents in theprepared catalysts confirm the stoichiometries of the moleculeformula of Ti0.9Mn0.05V0.05O2�δ and Ti0.9Ce0.05V0.05O2�δ.3.4.4. XPS. The surface atomic concentrations and valence

states of all elements in Ti0.9V0.1O2�δ, Ti0.9Mn0.05V0.05O2�δ,and Ti0.9Ce0.05V0.05O2�δ are characterized by XPS. The surfaceatomic concentrations of Ti, V, Mn, Ce, and O are measured andsummarized in Table 2, and the corresponding XPS spectra of Ti,V, Mn, Ce, and O are presented in Figure 13.As listed in Table 2, the surface concentration of Ti in the

complex metal oxide increases slightly after the partial substitu-tion of V with Mn and decreases to a certain extent in the partialsubstitution of V with Ce. The changes of surface species statewill be discussed seriatim in the following text.As shown in Figure 13a, the Mn (2p) core level region

photoelectron spectrum of Ti0.9Mn0.05V0.05O2�δ contains twopeaks, Mn (2p1/2) and Mn (2p3/2). The core level bindingenergies of Mn (2p1/2) and Mn (2p3/2) peaks in pure MnO2 are653.9 and 641.5 eV and are 653.2 and 641.0 eV in pure Mn2O3,respectively.20,21,53,72,97 The binding energies of the spin�orbitdoublet Mn (2p1/2) and Mn (2p3/2) peaks in Ti0.9Mn0.05-V0.05O2�δ can be found at 653.6 and 641.2 eV, indicating thepresence of the mixed-valence of Mn3þ and Mn4þ, which isbeneficial to promoting the low temperature SCR activity byoxidizing NO to NO2.

10,51,52,98

The assignment of Ce (3d) photoelectron peaks is ambiguousdue to the complex nature of the spectra arising from themultipleoxidation states as well as the mixing of the Ce (4f) levels andO (2p) states during the primary photoemission process.

Figure 9. (a) N2 adsorption�desorption isotherms and (b) BJH desorption pore size distributions of Ti0.9V0.1O2�δ, Ti0.9Mn0.05V0.05O2�δ, andTi0.9Ce0.05V0.05O2�δ based on the N2 adsorption�desorption isotherms.

Figure 10. Power X-ray diffraction patterns of anatase TiO2, Ti0.9V0.1-O2�δ, Ti0.9Mn0.05V0.05O2�δ, and Ti0.9Ce0.05V0.05O2�δ.



This hybridization leads to the multiplet splitting of the peaksinto doublets, with each doublet showing a further structure that

is due to the final-state effects.76 Two main characteristic peaksdue to Ce (3d3/2) and Ce (3d5/2) with the binding energies of901.0 and 883.0 eV are observed. The Ce (3d3/2) and Ce (3d5/2)peaks are broad, which is attributed to the mixed-valence ceriumoxidation (intermediate between the Ce3þ and Ce4þ oxidationstates) over Ti0.9Ce0.05V0.05O2�δ.

75 The Ce (3d) spectrum isvery complicated. In order to identify the specific valence of Ce,i.e., the condition of Ce3þ and Ce4þ, we label the spectra of Ce(3d) in Figure 13b. Two sets of spin orbital multiplets, whichcorrespond to the split 3d3/2 and 3d5/2 core hole states con-tributions, are labeled u and v, respectively. Based on the previousstudies, the peaks labeled v2 and v could be assigned to a mixingof the Ce 3d9 4f2 (O 2P4) and Ce 3d9 4f1 (O 2p5) initialelectronic states corresponding to the CeO2 (Ce

4þ) final states,and the peaks denoted v3 correspond to the Ce 3d9 4f0 (O 2p6)CeO2 (Ce4þ) irons. On the other hand, lines v1 and v0 are

Figure 11. Transmission electron micrograph images of (a) Ti0.9V0.1O2�δ, (b) Ti0.9Mn0.05V0.05O2�δ, and (c) Ti0.9Ce0.05V0.05O2�δ.

Figure 12. EDX results of (a) Ti0.9Mn0.05V0.05O2�δ and (b) Ti0.9-Ce0.05V0.05O2�δ.

Table 2. Semiquantitative Analysis of Surface AtomicConcentrations (% in Molar Ratio) over the Ti0.9V0.1O2�δ,Ti0.9Mn0.05V0.05O2�δ, and Ti0.9Ce0.05V0.05O2�δ Obtainedwith XPS Data

surface atomic concentration (%)

O

catalysts Ti V Mn Ce OA OL

Ti0.9V0.1O2�δ 22.7 4.8 10.2 62.3

Ti0.9Mn0.05V0.05O2�δ 23.5 2.3 2.0 14.0 58.2

Ti0.9Ce0.05V0.05O2�δ 20.9 3.6 3.7 17.0 54.8



attributed to the Ce 3d9 4f1 (O 2p6) and Ce 3d9 4f2 (O 2p5) finalstates of Ce3þ. The bands labeled u3, u2, and u are assigned to the3d10 4f0 state of the CeO2 (Ce

4þ) ions, whereas the peaks labeledu1 and u0 represent the 3d104f1 initial electronic states corre-sponding to Ce3þ.76 As seen in Figure 13b, the XPS spectrum ofCe (3d) exhibits peaks due to the presence of both Ce4þ andCe3þ, thus implying that cerium is present at the surface in both4þ and 3þ oxidation states. The intensity of the Ce3þ desig-nated peaks is relatively strong, which can be attributed primarilyto the formation of the CeVO4 compound.73,74 It was reportedthat the presence of the Ce3þ species could create a chargeimbalance, the vacancies and unsaturated chemical bands on thecatalysts surface, which will lead to the change of the O (1s)chemical state as well as the increase of the chemisorbed oxygenon the catalyst surface.80 It is widely accepted that the migrationof oxygen in ceria takes place mainly via vacancy hopping.Therefore, the migration of oxygen from bulk to the surfacewould become much easier due to the formation of Ce3þ.100

Moreover, the increase of the oxygen vacancies could lead tomore gaseous oxygen being replenished on the sample. Bothcould facilitate the activation and transportation of the active

oxygen species in the SCR reactions, causing a significantincrease in the SCR activity and N2 selectivity for the Ti0.9-Ce0.05V0.05O2�δ catalyst over a wide temperature window.Figure 13c shows the V (2p) core level photoelectron spectra

of Ti0.9V0.1O2�δ, Ti0.9Mn0.05V0.05O2�δ, and Ti0.9Ce0.05V0.05-O2�δ. For Ti0.9V0.1O2�δ, the binding energies of V (2p1/2)and V (2p3/2) peaks are 521.2 and 516.4 eV, respectively,implying that V exists in the V5þ oxidation state in Ti0.9V0.1-O2�δ.

77,78 A decrease in intensity and a broadening of the V (2p)line are observed in the substitution of V with Mn and Ce. Thebroadening of the XPS peakmay be explained by various reasons,including (1) the presence of more than one type of V5þ specieswith different chemical characteristics, which cannot be dis-cerned by XPS, and (2) the electron transfer between the activecomponent Mn (or Ce), V, and the support Ti (metal oxide-support oxide interaction).19,56,75,79 The binding energies of V(2p1/2) and V (2p3/2) peaks decrease to 521.0 and 515.8 eVwhen V is partially substituted by Mn and decrease to 521.2 and516.2 eV in the substitution of V with Ce. According to thehandbook of the elements and native oxides for XPS,99 thebinding energies of V (2p3/2) reported for V

5þ range from 517.4

Figure 13. X-ray photoelectron spectra of (a) Mn (2p) of Ti0.9Mn0.05V0.05O2�δ; (b) Ce (3d) of Ti0.9Ce0.05V0.05O2�δ; (c) V (2p) of Ti0.9V0.1O2�δ,Ti0.9Mn0.05V0.05O2�δ, and Ti0.9Ce0.05V0.05O2�δ; (d) Ti (2p) of Ti0.9V0.1O2�δ, Ti0.9Mn0.05V0.05O2�δ, and Ti0.9Ce0.05V0.05O2�δ; and (e) O(1s) ofTi0.9V0.1O2�δ, Ti0.9Mn0.05V0.05O2�δ, and Ti0.9Ce0.05V0.05O2�δ.



to 516.4 eV and the state of V4þ shows values in the range of515.7 to 515.4 eV. This implies coexistence of V5þ and V4þ on thesurface of Ti0.9Mn0.05V0.05O2�δ and Ti0.9Ce0.05V0.05O2�δ.

80,81

Figure 13d displays the Ti (2p1/2) and Ti (2p3/2) bindingenergy spectra of the catalyst samples. For Ti0.9V0.1O2�δ, thebinding energies of Ti (2p1/2) and Ti (2p3/2) peaks are 464.4 and458.6 eV, respectively, indicating that Ti exists in the Ti4þ

oxidation state.19,20,22,50 It should be pointed out that the bindingenergies of Ti (2p1/2) andTi (2p3/2) in Ti0.9V0.1O2�δ are smallerthan those in pure TiO2 (in which the binding energies of Ti(2p1/2) and Ti (2p3/2) peaks are 464.7 and 458.9 eV, re-spectively) due to the deviation of electronic cloud from V5þ

to Ti4þ since Ti4þ shows stronger affinity of electrons comparedwith V5þ.22,50,53,56 The partial substitution of V with Mn (or Ce)leads to an enhancement in the electron affinity of Ti and causeslower binding energies of 464.0 and 458.4 eV (or 463.8 and458.2 eV). The results reveal that Mn and Ce have stronginteraction with the TiO2 support, and affect the activity of thecatalysts. Interestingly, the intensity of the Ti (2p) core levelspectra is found to decrease in the following sequence:Ti0.9V0.1O2�δ > Ti0.9Mn0.05V0.05O2�δ > Ti0.9Ce0.05V0.05O2�δ.This is mainly due to the decrease of well-defined titania speciesand the increase of V and Mn (or Ce) on the catalyst surface.6,22,52

The O (1s) peaks could be fitted into two peaks, in which theone at lower binding energies (530.2�529.7 eV) is correspond-ing to the lattice oxygen O2- (denoted as OL), and the subpeak athigher binding energies (532.5�531.1 eV) is ascribed to thesurface chemisorbed oxygen, such as O2

2- or O� (hereafter,denoted as OA).

6,9,20,82 As shown in Figure 13e and Table 2, afterV is partially substituted by Mn, the concentration of OA

increases from 10.2% to 14.0%, and the OA/(OA þ OL) ratiohas an obvious increase from 14.0% to 19.4%. When V is partiallysubstituted by Ce, the concentration of OA increases further,reaching 17.0%, and the correspondingOA/(OAþOL) ratio alsoincreases greatly to 23.7%, which is much higher than that ofTi0.9V0.1O2�δ. The surface chemisorbed oxygen OA has beenreported to be the most active oxygen and highly active in theoxidation reaction due to its higher mobility than the latticeoxygen OL.

6,82 Moreover, the high relative concentration ratio ofOA/(OA þ OL) on the catalyst surface could be correlated withthe high SCR activity. The doping of the Ce and Mn inTi0.9V0.1O2�δ could create a charge imbalance, the vacancies,and the unsaturated chemical bonds on the catalyst surface, andlead to the increase of chemisorbed oxygen on the surface.6,100

Surface chemisorbed oxygen has been reported to be the mostactive oxygen and plays an important role in oxidation reactions.The doped catalysts have more oxide defects or hydroxyl-likegroups acting as Brønsted acid sites and benefiting the NH3

adsorption in the form of NH4þ.82�84 The formed NH4

þ canreact with adsorbed NO2 and nitrate species to produce activeintermediate species, and then further react with gaseous NO toproduce N2 and H2O. This means that the high OA/(OA þOL)ratio of Ti0.9Ce0.05V0.05O2�δ might diminish the unselectiveoxidation of NH3 by O2 to N2O, NO, or NO2 and thus giverise to its superior N2 selectivity. The results of the activity andselectivity studies in section 3.1.2 are well consistent with theXPS findings.According to the results of BET, it is observed that the BET

surface areas of Ti0.9V0.1O2�δ, Ti0.9Mn0.05V0.05O2�δ, andTi0.9Ce0.05V0.05O2�δ catalysts are almost identical, indicatingthat the BET surface area may not be the main factor causing thedifferent catalytic performances of the three catalysts. It is also

noticeable that the Ti0.9Mn0.05V0.05O2�δ and Ti0.9Ce0.05-V0.05O2�δ catalysts possess more abundant micropores(<2 nm) than Ti0.9V0.1O2�δ, thus providing more inner surfacearea and active sites for the occurrence of SCR reactions. TheXRD results show that V, Mn, and Ce oxides are present in ahighly dispersed state and in an amorphous or poorly crystallinestate on titania, which may be helpful to enhance the surfaceareas, the pore volumes, and thus the SCR activity. The TEMresults indicate that Ti0.9Ce0.05V0.05O2�δ has excellent disper-sion and the smallest particle size, which might be attributed tothe strongest interaction between vanadium, cerium, and tita-nium species. The XPS results show that the Ti0.9Ce0.05-V0.05O2�δ catalyst possesses the highest OA/(OA þ OL) ratio,which diminishes the unselective oxidation of NH3 by O2 toN2O, NO, or NO2 and thus give rise to its superior N2 selectivity.The Ti0.9Ce0.05V0.05O2�δ catalyst possesses the smallest particlesize and the highest OA/(OA þ OL) ratio; however, the NOx

removal efficiency of Ti0.9Ce0.05V0.05O2�δ at low temperatures isnot the highest, although the N2 selectivity is the highest. TheTi0.9Mn0.05V0.05O2�δ catalyst has the highest NOx removalefficiency at low temperatures, which might be beneficial to theenhancement of the NO oxidation to NO2 by the presence of themixed-valence of Mn3þ and Mn4þ and thus the low temperatureSCR activity. Therefore, there might be other factors that affectthe activity of the catalysts, especially at low temperatures. It isknown that the apparent SCR activity of catalysts usuallycorrelates with the adsorption amount of NOx and NH3,especially in the low temperature range where the SCR reactionmainly follows a Langmuir�Hinshelwood (LH) mechanismbetween the NOx and NH3 adsorbed species. Therefore, thecorrelation between the catalytic performance and the reactantadsorption capability/conformation over the catalysts is exam-ined in detail using the NH3-TPD method to measure the acidsites distribution and changes of the surface acidity as well as thein situ DRIFTS method to detect the surface species.3.5. NH3-TPD Studies. The acid site distribution and changes

of the surface acidity caused by the partial substitution of Mn andCe investigated by NH3-TPD and the TPD profiles are illu-strated in Figure 14. All catalysts show two NH3 broad desorp-tion peaks: one spans in the temperature range of 45�120 �Cand the other occurs within 210�370 �C. It has been reportedthat the ammonia adsorbed on Brønsted acid sites desorbs atlower temperatures than that of Lewis-coordinated ammonia.43,85

Therefore, we can assign the peak at the low temperature to thedesorption of physicsorbed NH3 and the partial ionic NH4

þ

bound to the weak Brønsted acid sites and ascribe the hightemperature peak to the desorption of ionic NH4

þ bound to

Figure 14. Comparisons of TPD profiles of NH3 over theTi0.9V0.1O2�δ, Ti0.9Mn0.05V0.05O2�δ, and Ti0.9Ce0.05V0.05O2�δ.



strong Brønsted acid sites and coordinated NH3 bound to theLewis acid sites. It is observed from the NH3-TPD profiles thatthe intensity and the area of the desorption band of Ti0.9-Mn0.05V0.05O2�δ are remarkably larger than those of Ti0.9V0.1-O2�δ at low temperatures and that the initial desorptiontemperature (37 �C) of the former is lower than that (45 �C)of the latter. The improvement in the NH3 desorption canelucidate the promotion of Mn on the activity of Ti0.9Mn0.05-V0.05O2�δ at low temperatures. By comparing the NH3 TPDprofiles of Ti0.9V0.1O2�δ and Ti0.9Ce0.05V0.05O2�δ, we find thatthe NH3 desorption amount from the latter is larger at lowtemperatures. The NH3 desorption of the two catalysts is ofapproximate amount at high temperatures. Among the threecatalysts, Ti0.9Mn0.05V0.05O2�δ shows the highest Brønstedacidity, indicating that the Brønsted acid sites might be theimportant active sites for the SCR reaction, especially at lowtrmperatures. If we consider the total acidity (irrespective ofLewis or Brønsted acidity) of the catalysts, Ti0.9Ce0.05V0.05O2�δ

shows the highest acidity, which may be responsible for itsexcellent N2 selectivity. In addition, it should be noted that thereis a possibility of oxidation of the chemisorbed NH3 by the latticeoxygen O2-; thence, a small amount of N2O and NO is also de-sorbed in our tests, but the concentrations are only a few ppm.43

3.6. In Situ DRIFTS Studies. The above-mentioned resultsindicate that Ti0.9Ce0.05V0.05O2�δ provides the satisfactory cat-alytic activity and N2 selectivity. In the following section, thereaction mechanism of NO reduction by NH3 over theTi0.9Ce0.05V0.05O2�δ surface will be investigated with in situDRIFTS.3.6.1. NO þ O2 Co-Adsorption. The DRIFTS spectra for the

coadsorption of NO andO2 at various temperatures are shown inFigure 15. The bands at 1438 and 1544 cm�1 are assigned to bethe monodentate nitrate,86�88 and the bands at 1595 cm�1 couldbe attributed to the chelating bidentate nitrate,29,30,87,89,90 whilethe bands at 1183, 1216, 1245, and 1637 cm�1 are typicalbridging nitrate due to the disproportionation of NO.87,89�92

The intensity of bands at 1200�1640 cm�1 describes theamount of nitrate on the catalyst surface.93 It is obvious thatthe intensity of the bands at 1183, 1216, and 1245 cm�1 increasesgradually with the temperature, reaches maximum at 300 �C, andthen decreases sharply when the temperature increases further.These bands completely disappear when the temperature reaches400 �C. The band at 1637 cm�1 has the highest intensity at 50 �Cand becomes weaker as the temperature increases. The intensityof the band at 1438 cm�1 increases as the temperature rises from50 to 100 �C, then decreases gradually with the temperature, and

almost vanishes at 400 �C. The intensity of the band at1544 cm�1 increases continually with the temperature. Uponan increase in the temperature, the intensity of the band at1595 cm�1 increases gradually. The results suggest that theactivity of monodentate nitrate and bridging nitrates over theTi0.9Ce0.05V0.05O2-δ catalyst is much higher than that of biden-tate nitrate.3.6.2. NH3 Adsorption. Figure 16 displays the in situ DRIFTS

spectra of the Ti0.9Ce0.05V0.05O2�δ catalyst after the adsorptionof NH3. After being exposed to NH3 at various temperatures,many bands attributed to the intermediate species of NH3 appearat 1188, 1441, 1595, 1680, 3159, 3238, 3334, 3580, and3658 cm�1. The bands at 1188, 1595, 3159, and 3334 cm�1

can be assigned to the asymmetric and symmetric deformationvibrations of N�H bonds in the coordinated NH3 bound toLewis acid sites.29,30,87,88 The bands at 1441 and 1680 cm�1 areattributed to the asymmetric and symmetric bending vibrationsof ionic NH4

þ which are chemisorbed on the Brønsted acidsites.29,30,54,87�92 In the NH stretching region, bands are found at3238 and 3580 cm�1.94 Some negative bands around 3658 cm�1

are also observed, which could be assigned to the surface O�Hstretching.6,86,95 With the increase of the temperature, the bandsat 1441 and 1595 cm�1 shift to the low wave band whereas theband at 1188 cm�1 shifts to the high wave band for thedesorption of NH3 at high temperatures. The intensity of the1441 cm�1 band decreases remarkably with the increasingtemperature and almost disappears at 450 �C, whereas the bandat 1188 cm�1 only shows slightly decrease even at 450 �C. Thisreveals that the Lewis acid sites aremuchmore stable and inactivethan the Brønsted acid sites, indicating that the Brønsted acidsites are much more important than the Lewis acid sites in theNH3�SCR process over Ti0.9Ce0.05V0.05O2�δ, which is consis-tent with the NH3-TPD results. This is also in accordance withthe NH3�SCR mechanism over the vanadium-titantia catalystsstudied by Topsøe, who claimed that the key intermediates in theSCR process were the ionic NH4

þ species, which are generatedby the NH3 adsorbed on the Brønsted acid sites.96

3.6.3. NH3 Adsorption after Nitric Oxide.To trace the behaviorof the species on the catalyst surface, the transient processexperiment is carried out. The catalyst sample is first treatedwith NOþO2/N2 at 150 �C for 30 min, followed by N2 purgingfor 30 min, and then by the NH3 introduction. The celltemperature is kept at 150 �C during the whole test. As shownin Figure 17, monodentate nitrate (1438 cm�1), bridging nitrate(1183, 1245, and 1637 cm�1), and bidentate nitrate (1595 cm�1)are formed before NH3 enters into the cell. When NH3 is passed

Figure 15. In situ DRIFTS spectra of Ti0.9Ce0.05V0.05O2�δ during NOand O2 adsorption in a steady state at various temperatures. Reactionconditions: [NO] = 600 ppm, [O2] = 5 vol %, and N2 as balance.

Figure 16. In situ DRIFTS spectra of Ti0.9Ce0.05V0.05O2�δ during NH3

adsorption in a steady state at various temperatures. Reaction condi-tions: [NH3] = 600 ppm, and N2 as balance.



over the NOx absorbed catalyst, the IR spectra are recorded as afunction of the reaction time. It is obvious that after NH3/N2 isintroduced into the cell, the intensities of the bands of mono-dentate nitrate at 1438 cm�1 and bridging nitrate at 1183, 1245,and 1637 cm�1 decrease rapidly and vanish quickly after a 5-minreaction, implying that these two kinds of nitrate species arereducible or highly reactive at low temperatures. Oppositely, theintensity of the bands assigned to bidentate nitrate (1595 cm�1)remains unchanged even after 30 min, showing that this specie isinactive. After the monodenate nitrate and bridging nitrate areconsumed, a new band at about 1212 cm�1 attributed to thecoordinated NH3 bound to Lewis acid sites appears.In summary, the bands transition in Figure 17 suggests that the

formation of the reactive monodenate nitrate and bridgingnitrate species, and the abundant amount of ionic NH4

þ boundto Brønsted acid sites make important contributions to thesuperior SCR performance on Ti0.9Ce0.05V0.05O2�δ.Thus, the short conclusion can be drawn that the strong

interaction between activemetal species and titanium species, ad-monodentate nitrate, bridging nitrate species, Brønsted acidsites, the redox properties of the active metal oxides, and theirhigh surface concentration on the catalysts surface are the maincontributors to the superior SCR performance over a broadtemperature window of 150�400 �C.

4. CONCLUSION

Nanocomposites of Ti0.9V0.1O2�δ, Ti0.9Mn0.05V0.05O2�δ, andTi0.9Ce0.05V0.05O2�δ are prepared by the SHS method. All ofthese catalysts show excellent SCR activity in a broad tempera-ture window of 150�400 �C, in which more than 80% NOx isreduced. The N2 selectivities of Ti0.9Mn0.05V0.05O2�δ andTi0.9Ce0.05V0.05O2�δ are superior to that of Ti0.9V0.1O2�δ, andTi0.9Ce0.05V0.05O2�δ ranks first in N2 selectivity. TheTi0.9Ce0.05V0.05O2�δ catalyst exhibits high H2O and SO2 toler-ance in the test with the presence of both 7% H2O and 400ppm SO2 at 150 �C.

The catalysts are characterized by BET, XRD, TEM, EDAX, andXPS. The nanocomposites catalysts prepared by the SHS methodexhibit reasonablyhigh specific surface areas, appropriate pore volumesand average pore diameters, with the typical nanoparticle sizes in therange of 5�15 nm. The active components of vanadium, manganese,and ceriumoxides are highly dispersed, and thesemetals supported ontitania exhibit synergic interaction. InTi0.9Ce0.05V0.05O2�δ, Cemainlyexists in the form of the intermediate between Ce3þ and Ce4þ

oxidation state, which could create defects, such as the chargeimbalance, vacancies and unsaturated chemical bonds on thecatalyst surface. The defects improve the concentration ofthe chemisorbed oxygen and the ratio of OA/(OA þ OL), whichare beneficial for the SCR activity and the N2 selectivity.

The NH3-TPD studies show that Ti0.9Mn0.05V0.05O2�δ has thehighest Brønsted acidity, which is responsible for the higher activityat low temperatures, whereas the highest total acidity includingBrønsted and Lewis acidity on Ti0.9Ce0.05V0.05O2�δ may accountfor the excellent N2 selectivity. Furthermore, the NH3�SCRreaction mechanism over Ti0.9Ce0.05V0.05O2�-δ is fully investigatedusing in situ DRIFTS methods. The DRIFTS results indicate thatthe active monodentate nitrate and bridging nitrate species comefrom the NOx adsorption, and that the ionic NH4

þ bound toBrønsted acid sites are essential for the SCR process. The reactionof the adsorbed NH3 with the ad-monodentate nitrate andbridging nitrate species contributes to the higher SCR reaction atlow temperatures. Studies concerning the mechanism for SO2/H2O poisoning over these catalysts are under way.

’AUTHOR INFORMATION

Corresponding Author*Tel.: þ86 21 34206540. Fax: þ86 21 34205553. E-mail:[email protected] (H.L). Tel.: þ86 21 34206379. Fax: þ86 2134205553. E-mail: [email protected] (Z.H.)

’ACKNOWLEDGMENT

This work is supported by the Natural Science Foundation ofShanghai (09ZR1413800), the Joint Research Foundation ofScholars fromOverseas andHongKong andMacao (51028601),and the Visiting Scholar Foundation of Key Lab. in ZhejiangUniversity (ZJUCEU 2010018).

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Figure 17. In situ DRIFTS spectra of SCR reaction overTi0.9Ce0.05V0.05O2�δ at 150 �C during which NO þ O2 was let in firstandNH3 was let in subsequently. Reaction conditions: [NO] = [NH3] =600 ppm, [O2] = 5 vol %, and N2 as balance.



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Selective Catalytic Reduction of NO x with NH 3 over Mn, Ce Substitution Ti 0.9 V 0.1 O 2−δ...

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