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JOURNAL OF RARE EARTHS, Vol. 34, No. 3, Mar. 2016, P. 259

Foundation item: Project supported by the Grant from the China Huadian Science and Technology Institute (CHDI.KJ-20) and the National High-Tech Research and Development Program of China (863, 2011AA03A405)

* Corresponding author: WANG Jun, ZHAI Yanping ( E-mail: [email protected]; [email protected]; Tel.: +86-22-27892301; +86-10-59216261)DOI: 10.1016/S1002-0721(16)60023-6

Effect of synthesis methods on activity of V 2O 5/CeO 2/WO 3-TiO 2

catalyst for selective catalytic reduction of NO x with NH 3 SHEN Meiqing ( )1,2,3 , XU Lili ( )1, WANG Jianqiang ( )1, LI Chenxu ( )1,WANG Wulin ( )1, WANG Jun ( )1,*, ZHAI Yanping ( )4,* (1. Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering & Technology, Tianjin University, Tianjin300072, China; 2. Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China; 3. State Key Laboratory of Engines,

Tianjin University, Tianjin 300072, China; 4. China Huadian Science and Technology Institute, Beijing 100070, China)

Received 21 August 2015; revised 12 November 2015

Abstract: The effect of synthesis methods on the activity of V/Ce/WTi catalysts was investigated for the selective catalytic reduction(SCR) of NO x by NH 3. V/Ce/WTi-DP (deposition precipitation) catalyst showed excellent NH 3-SCR performance, especially the bet-ter medium-temperature activity and the less N 2O formation than V/Ce/WTi-IMP (impregnation). These catalysts were characterized

by X-ray diffraction (XRD), Brumauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), temperature-programmedreduction (H 2-TPR), and in situ DRIFTS techniques. The XPS and H 2-TPR results revealed that V/Ce/WTi-DP exhibited more sur-face Ce species, higher level of O α and higher reducibility of Ce species. Reflected by in situ DRIFTS results, the deposition precipi-tation method (DP) contributed to a greater amount of weakly adsorbed NO 2, monodentate nitrate and NH 3 species with better reac-tive activity. Meanwhile, the cis-N 2O2

2– species, an intermediate for N 2O formation, was very limited. As a result, these advantages brought about the superior SCR activity and N 2 selectivity for V/Ce/WTi-DP.

Keywords: V/WTi; Ce-modified; synthesis method; Ce species; NH 3-SCR; rare earths

The removal of NO x from the stationary and mobile

sources has been regarded as a major environmentalconcern [1]. At present, the selective catalytic reduction(SCR) is one of the most favored De-NO x technologies.Generally, V 2O5-WO 3-TiO 2 catalyst has been the mostwidely employed commercial catalyst with good activityand resistance to SO 2. Nevertheless, there are some dis-advantages. For example, V 2O5-WO 3-TiO 2 catalyst is onlyhighly efficient in the temperature range of 300–400ºC[2–4] , and it is circumscribed since the temperatures offlue and exhaust gas exceed the applicable temperaturerange [5]. On the other hand, N 2O generation is foundabove 400 ºC [2]. Therefore, it is urgent to develop a novel,efficient SCR catalyst for a wide temperature range andlow N 2O formation.

Recently, Ce-based oxides have attracted much atten-tion for being used as a promoter or an active catalyst,due to their high oxygen storage capacity and superiorredox properties [6,7] . As a SCR catalyst, ceria supportedon titania seems to be promising for improving mediumtemperature activities [8–10] . Researchers have made greatefforts to introduce Ce to traditional V 2O5-WO 3-TiO 2 catalyst. In our precious research [11] , the addition of Cegreatly enhanced the low temperature activity of the

fresh V 2O5-ZrO 2/WO 3-TiO 2 catalyst, and the promo-

tional effect of CeO 2 was attributed to the enrichment ofCe3+, the increased redox properties and more active ad-sorbed nitrates. Chen et al. [12] improved SCR perform-ance for V 2O5-WO 3/TiO 2 catalyst in the broad tempera-ture range of 200–500 ºC by wet impregnation method,using CeO x as a promoter. Lee et al.

[13] investigated thatthe addition of 10% ceria to Sb-V 2O5/TiO 2 catalyst by

precipitation method significantly enhanced the NO x conversion and showed high N 2 selectivity. Moreover, Liet al. [14] prepared the V 2O5-CeO x/TiO 2-carbon nanotube

by sol-gel method and found that the appearance of Ce 3+ increased chemisorbed oxygen thus facilitated the SCRactivity. Although the promotional effect of Ce forV2O5-WO 3-TiO 2 catalyst in NH 3-SCR reaction is gener-ally accepted, there is not any work dealing with the ef-fect of preparation methods on the structure and activityof V/Ce/WTi catalyst. In this regard, this research wouldcontribute to developing novel Ce-modified catalyst.

In this work, we systematically investigated Ce-modi-fied V/WTi catalysts, focusing on the effect of synthesismethods on the catalyst structure and activity in NH 3-SCR reaction. The V/Ce/WTi catalysts were prepared bydeposition-precipitation method (DP) which has been

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260 JOURNAL OF RARE EARTHS, Vol. 34, No. 3, Mar. 2016

slightly studied and the conventional impregnationmethod (IMP). By means of a series of characterizationmethods, it was found that V/Ce/WTi catalyst prepared

by deposition-precipitation method showed higher NO x conversion and less N 2O production. And this result wasmainly attributed to the status of Ce species influenced

by preparation method.

1 Experimental

1.1 Catalysts preparation

Impregnation and deposition precipitation methodswere applied to prepare the CeO 2/WO 3-TiO 2 catalyst first.The commercial support, 10 wt.%WO 3-90 wt.%TiO 2 (WTi),was obtained from Millennium Inorganic Chemicals Inc.In the impregnation method, the catalyst was prepared byincipient wetness impregnation. The calculated quantityof WTi powder was added slowly into the calculatedamount of cerium nitrate solution, and kept stirring for 2h. Then the sample was statically dried overnight at 110ºC, followed by calcination in air at 500 ºC for 4 h. Forthe deposition precipitation method, the catalyst wassynthesized by hydrolysis with ammonium hydroxide.The calculated amount of cerium nitrate (Ce(NO 3)3·6H2O)was dissolved in a beaker containing 300 mL de-ionizedwater, and the desired quantity of WTi powder wasadded into the solution with stirring at the room tem-

perature for 30 min. Subsequently, the dilute aqueous

ammonia as a precipitating agent was added dropwise tothe suspension until the PH value reached about 9.0. Af-ter being continuously stirred for 2 h, the resultant pre-cipitate was filtered, dried overnight at 110 ºC and thencalcined at 500 ºC for 4 h in air. The CeO 2/WO 3-TiO 2 catalysts obtained via the two methods were denoted asCe/WTi-IMP and Ce/WTi-DP respectively.

The V 2O5/CeO 2/WO 3-TiO 2 catalysts with 1% V 2O5 loading were prepared by incipient wetness impregnationof V on prepared Ce/WTi-IMP and Ce/WTi-DP catalysts,which were denoted as V/Ce/WTi-IMP and V/Ce/WTi-

DP. The complex of VO(CO 2)2 was prepared by reactingV2O5 powder with oxalic acid with continuous stirring at70 ºC for 20 min. Subsequently, the desired quantity ofCe/WTi powder was added into the mixed solution andstirred for 2 h. The as-synthesized sample was driedovernight at 110 ºC and then calcined at 500 ºC for 4 h inair. Synthesis methods and chemical composition of allthe catalysts are shown in Table 1.

1.2 Catalytic activity measurement

The activity tests were performed in a quartz reactor atatmospheric pressure, using 200 mg sample (60–80 mesh)sufficiently mixed with 800 mg quartz (60–80 mesh).The concentrations of NO, NO 2, N2O, H 2O and NH 3 were monitored by a Fourier transform infrared (FTIR)

Table 1 Synthesis methods and chemical composition of allthe catalysts

Composition/

wt.% * Samples Preparation process

V2O5 CeO 2

Ce/WTi-IMP Ce impregnation – 3.610Ce/WTi-DP Ce deposition precipitation – 3.623

V/Ce/WTi-IMP Ce impregnation/V impregnation 1.015 3.553

V/Ce/WTi-DP Ce deposition precipitation/V impregnation 1.015 3.573

V/WTi V impregnation 1.025 –

* The chemical composition were obtained by ICP-AES

spectrometer (MKS-2030). The gas flow rates in all experiments were controlled at 500 mL/min by mass flowcontrollers. Prior to the experiments, the catalysts were

pre-treated at 500 ºC for 30 min under 5% O 2/N2. The NH 3-SCR activity tests were performed using the gascomposition of 500 ppm NO, 500 ppm NH 3, 5% O 2 and4% H 2O with N 2 as the balance. The testing temperaturewas from 80 ºC to 550 ºC at a ramping rate of 10 ºC/min.The NO x conversion was calculated using the followingequation:

inlet outlet

inlet

×100% NO NO

NO conversion = NO

x x x

x

− (1)

NO x=NO+NO 2 (2)

1.3 Catalysts characterization

The Ce and V contents were performed by inductivelycoupled plasma atomic emission spectroscopy (ICP-AES). BET-surface areas were measured by N 2 adsorp-tion at 77 K using an F-Sorb 3400 volumetric adsorp-tion/desorption apparatus. The X-ray powder diffraction

patterns were collected with a step size of 0.02º from 20º

to 80º (XRD, Bruker D8 Advance TXS, Cu K α radia-tion). X-ray photoelectron spectroscopy (XPS) experi-ments were obtained with a PHI-1600 ESCA system.The base pressure was about 3×10 −9 mbar. The bindingenergy was calibrated internally by the carbon deposit C1s binding energy (BE) at 284.8 eV.

Temperature-programmed reduction by hydrogen experiments (H 2-TPR) was performed to characterize thereducibility of V species and Ce species in the V/Ce/WTisamples. Prior to the reduction, the samples (100 mg)were pre-treated at 500 ºC under 2% O 2/N2 (30 mL/min)for 1 h. Then after cooling down to 30 ºC in N 2, the sam-

ples were heated at a ramping rate of 10 ºC/min from 30to 900 ºC under a flow of 5% H 2/N2 (15 mL/min). Theconsumption of H 2 was monitored by a thermal conduc-tivity detector (TCD).

In situ diffuse reflectance infrared Fourier transformspectra ( in situ DRIFTS) of adsorption species were per-formed on Nicolet 6700 FTIR equipped with a MCT de-tector at a resolution of 4 cm –1, averaging 10 scans foreach spectrum. The samples were initially treated with

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SHEN Meiqing et al. , Effect of synthesis methods on activity of V 2O5 /CeO 2 /WO 3-TiO 2 catalyst for selective … 261

10% O 2/N2 (50 mL/min) at 500 ºC for 30 min, and thenthe samples were cooled down to the target temperature.Prior to reactant gas (NH 3 or NO) chemisorption, the

background spectra were collected. Then the gas con-taining 3000 ppm NH 3 or 3000 ppm NO+5% O 2 in N 2 (50 ml/min) passed through the sample at the target tem-

perature for 45 min and the in situ DRIFTS spectra wererecorded after purging the weakly adsorbed gas molecules.

2 Results and discussion

2.1 NH 3-SCR catalytic activity

Fig. 1 shows the NO x conversion and N 2O formationcurves in SCR activity evaluation. The V/Ce/WTi cata-lysts prepared by two different methods exhibit better ac-tivity than V/WTi catalyst especially below 400 ºC (Fig.1(a)). Compared with the V/Ce/WTi-IMP, V/Ce/WTi-DP catalyst shows excellent NH 3-SCR activity, with over90% NO x conversion in a wide temperature range from275 to 500 ºC. As seen from Fig. 1(b), V/Ce/WTi-DP

produces little N 2O, which is also superior to V/Ce/WTi-IMP and V/WTi. Moreover, Ce/WTi catalysts wereevaluated as a comparison to confirm this result. As dis-

played in Fig. 1(c), below 400 ºC, V/Ce/WTi-DP andCe/WTi-DP exhibit almost the same NO x conversion,and that is remarkably higher than the NO x conversion ofV/WTi. In contrast, as illustrated in Fig. 1(d), the NO x

conversion over V/Ce/WTi-IMP is obviously lower thanCe/WTi-IMP and hence slightly higher than that ofV/WTi. Above 400 ºC, both V/Ce/WTi catalysts show aclose activity to V/WTi without dramatic decrease asCe/WTi presented. These comparison results suggest thatsynthesis methods affect the catalytic activity, especiallyin the relatively low temperature range, which should beclosely related to the status of Ce species.

2.2 Phase composition and surface area of catalysts

Fig. 2 shows the XRD patterns for the Ce/WTi, V/WTiand V/Ce/WTi catalysts. The diffraction peaks of allsamples are ascribed to standard anatase-phase TiO 2

peaks (PDF-ICDD 65 −5714), but no peaks due to CeO 2 and V 2O5 are observed. That could be because ceriumand vanadium are well dispersed on the surface of thecommercial support as highly amorphous states or the

formed crystallites are too small to be detected by XRD.It has been reported that the highly dispersed ceria should

be responsible for the excellent performance of Ce-Ti based catalysts [10] .

The BET surface area and average crystallite size ofTiO 2 calculated by Scherrer’s equation are shown in Ta-

ble 2. No significant difference has been figured outamong all these catalysts, except for V/Ce/WTi-IMP withthe obviously decreased BET surface area. It is clear thatthe deposit precipitation method is in favor of improving

Fig. 1 NH 3-SCR performance over various catalysts as a function of temperature(a), (c), (d) NO x conversion; (b) N 2O concentration

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Fig. 2 XRD profiles of different catalysts

Table 2 BET surface area and average TiO 2 crystallite sizeof samples

Sample BET surface area/(m 2/g) TiO 2 crystallite size/nm*

Ce/WTi-IMP 80.33 18.32

Ce/WTi-DP 83.42 18.23

V/Ce/WTi-IMP 74.83 18.60

V/Ce/WTi-DP 80.81 18.42

V/WTi 80.55 18.10

* Crystallite size was calculated by Scherrer equation from XRD results

the BET surface area of catalysts, which could offermore active sites for reaction and thus be beneficial to

NH 3-SCR activity.

2.3 Valence state and distribution of Ce species

To understand the chemical states of Ce and O on thecatalysts surface, Fig. 3 shows the XPS spectra of Ce 3d,and O 1s, together with a summary of the atomic surfaceconcentrations of Ce, O, W and Ti in Table 3.

As presented in Fig. 3, the complex Ce 3d peaks arefitted into eight peaks. The bands labeled u ′ and v ′ repre-sent the 3d 104f 1 initial electronic state corresponding toCe3+, while the peaks labeled u, u ′′, u′′′, v, v ′′ and v ′′′ rep-resent the 3d 104f 0 state of Ce 4+ ions [15,16] . The surface

mole percentages of Ce3+

in Ce is calculated from thenormalized peak areas of Ce 3+ and Ce 4+ core level spectra.It is evident from Table 3 that V/Ce/WTi-DP shows ahigher Ce 3+ ratio than V/Ce/WTi-IMP (53.22% vs44.43%). Furthermore, V/Ce/WTi catalysts both exhibitlower surface Ce content (at.%) than the correspondingCe/WTi, because of the addition of V. And more notably,the surface Ce concentration over V/Ce/WTi-DP changeslittle from 0.91 to 0.75 (down 17.6%), while V/Ce/WTi-IMP catalyst displays a significant decrease from 1.19 to0.70 (down 41.2%). Two possible explanations could besuggested. Firstly, more Ce species are covered by theadditive V over V/Ce/WTi-IMP than V/Ce/WTi-DP.Secondly, surface Ce species may migrate into the bulk

phase more easily during the additional calcination proc-ess for V impregnated on Ce/WTi-IMP catalyst. Theabove results indicate that the status of surface Ce spe-cies may be affected by preparation method, and thedeposition precipitation method contributes to more sur-face Ce species insusceptible to the impregnated V spe-cies.

The O 1s peak was fitted into two kinds of peaks. Thesub-bands at 529.5–530.0 eV could be attributed to the

lattice oxygen O2−

(denoted as O β). The shouldersub-bands at 531.0–531.6 eV are assigned to the surfaceadsorbed oxygen (denoted as O α) such as O 2

2− or O − be-longing to defect oxide or hydroxyl-like species [13,16] . Asshown in Table 3, the O α ratio of Ce/WTi-DP is 66.50%which is evidently higher than the 55.22% of Ce/WTi-

Fig. 3 XPS spectra of Ce 3d of V/Ce/WTi catalysts (a), O 1s of Ce/WTi catalysts (b) and O 1s of V/Ce/WTi catalysts (c)

Table 3 XPS surface compositional analysis

Surface atomic concentration/at.% Surface mole ratio/%Samples

Ti W Ce O Ce 3+/(Ce 3++Ce4+) Oα/(Oα+Oβ)

Ce/WTi-IMP 18.62 3.58 1.19 76.61 – 55.22

Ce/WTi-DP 17.52 3.63 0.91 77.94 – 66.50V/Ce/WTi-IMP 22.16 4.01 0.70 73.12 44.43 40.44

V/Ce/WTi-DP 21.68 4.11 0.75 73.46 53.22 45.84

V/WTi 22.24 4.09 – 73.67 – 36.52

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IMP. Likewise, through comparing V/Ce/WTi catalystswith V/WTi, the O α ratio follows the sequence:V/Ce/WTi- DP>V/Ce/WTi-IMP>V/WTi, which is ingood accordance with the activity order below 400 ºC. Itis well- known that surface adsorbed oxygen (O α) ismore reactive in the oxidation reactions due to its highermobility than lattice oxygen (O β)

[17] . In other words, O α can react easily with NO to produce NO 2 or nitrate spe-cies [18,19] , then participating further in the consequent re-actions with NH 3. Hence, the V/Ce/WTi-DP catalystwith a higher ratio of O α presents better SCR activitythan V/Ce/WTi-IMP catalyst.

2.4 Reducibility of Ce species

The H 2-TPR experiment was used to investigate the presence of reducible species in the prepared catalysts.Reduction curves of the different catalysts were com-

pared as is shown in Fig. 4. Moreover, the signals have been fitted with Gaussian lines to assess the H 2 con-sumptions for the individual species, and the results ofquantitative analysis are displayed in Table 4.

As shown in Fig. 4(a), the commercial WTi support(reference) exhibits two major reduction peaks at 562and 686 ºC. They can be assigned to the reduction ofsome sulfate species existing on the surface of sup-

port [14,20] , well dispersed tungsten species and TiO 2 sup-

Fig. 4 H 2-TPR profiles on the catalysts(a) Impregnation method; (b) Deposition precipitation method

Table 4 H 2-temperature programmed reduction

Reduction peaks temperature/ºC

Samples Ce4+-Ce3+ and

V5+-V4+ sulfate species W 6+-W4+

Total H 2

consumption of

Ce and V (a.u.)

V/Ce/WTi-IMP 489 553 630 1.75

Ce/WTi-IMP 466 533 609 1.81

V/Ce/WTi-DP 480 540 623 1.80

Ce/WTi-DP 484 595 697 1.20

V/WTi 483 537 620 1.00

WTi – 562 686 –

port [20,21] . Both Ce/WTi-IMP and V/WTi-IMP show threeoverlapped reduction peaks: the low temperature peaksaround 400–500 ºC are attributed to the reduction of sur-face Ce 4+ to Ce 3+ [16,22] and the V 5+ to V 3+ [13] , the othertwo peaks are attributed to the reduction of the support.

The addition of V over the Ce/WTi-IMP catalyst causessome changes of the TPR profiles. The first peak forV/Ce/WTi-IMP is ascribed to the coreduction of surfaceCe4+ to Ce 3+ and V 5+ to V 3+, which is difficult to be dis-tinguished due to the similar reduction temperatures.Compared with the reduction peak of surface Ce specieson Ce/WTi-IMP catalyst, it shifts to higher temperaturethat is close to the reduction temperature of V species onV/WTi catalyst. The coreduction of surface Ce 4+ and V 5+ consumes less H 2 than the reduction of Ce species onCe/WTi-IMP catalyst by itself. These results indicate thatthe changed reduction peak over V/Ce/WTi-IMP ismainly caused by the changed Ce species rather than Vspecies, which is also consistent with XPS results. Andthe reduction of surface Ce species is inhibited by theadditive V, resulting in the lower activity overV/Ce/WTi-IMP than that over Ce/WTi-IMP.

Compared with Ce/WTi-IMP, Ce/WTi-DP showsthree distinct peaks with the corresponding reduction ofsurface Ce species and the support (Fig. 4(b)). While thehigher reduction temperature and smaller amount of H 2 consumption from the reduction of surface Ce 4+ to Ce 3+

are related with the presence of more Ce 3+, according to

XPS results. After adding V on Ce/WTi-DP, the obtainedV/Ce/WTi-DP catalyst presents an overlapped reduction

peak and the first peak is also ascribed to the coreductionof surface Ce 4+ to Ce 3+ and V 5+ to V 3+. More importantly,the reduction temperature of the first peak overV/Ce/WTi-DP catalyst is similar with that of Ce specieson Ce/WTi and V species on V/WTi. The H 2 consump-tion of the coreduction is more than that of each reduc-tion alone. Thereby the constant reduction temperature ofsurface Ce species may contribute to the almost same ac-tivity with Ce/WTi-DP below 400 ºC.

In addition, the distribution of the active componentsand the interaction of metal-support may vary with the

preparation methods. Comparing with commercial WTisupport, the reduction peaks of the support in all im-

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pregnated samples shift to the similar position at thelower temperature. It is most likely to be a V-O-Ti orCe-O-Ti interaction, when the V species or Ce speciesare impregnated on TiO 2 surface

[23] . While, for Ce/WTi-DP catalyst, these reduction peaks shift slightly to highertemperature, implying the different interaction betweenCe species and the WTi support, and hence different dis-tribution of Ce species. Subsequently, after V impregna-tion (V/Ce/WTi-DP), these reduction peaks shift to thesimilar temperature if it is compared to impregnatedsamples, suggesting that the V-O-Ti species are formedat this moment. Consequently, the preparation methodsmake a critical effect on the distribution of active species.Then Ce species and V species may be in the similarsituation due to the same process of addition.

2.5 Surface adsorbed species of catalysts

2.5.1 NH 3 adsorption In situ DRIFTS experiments were conducted to inves-

tigate the effect of synthesis methods on the variety andquantity of Lewis acid sites and Brønsted acid sites. Asshown in Fig. 5(a), after NH 3 adsorption on differentV/Ce/WTi catalysts at 50 ºC, several bands are detectedin the range of 1000–1800 cm –1. The bands observed at1670 and 1440 are assigned to symmetric and asymmet-

Fig. 5 DRIFTS spectra of V/Ce/WTi (a) and Ce/WTi (b) cata-lysts during NH 3 desorption at different temperatures(100, 200 and 300 ºC) after NH 3 adsorption at 50 ºC

Table 4 Assignments of DRIFTS bands observed during theNH 3 adsorption

Wavenumber/cm –1 Assignments Ref.

1440 and 1672

Asymmetric and symmetric vibrations of

N–H bonds in NH 4+ coordinately linked to

Brønsted acid sites

[24]

1207, 1250 and 1600

Asymmetric and symmetric vibrations of

the coordinated NH 3 linked to Lewis acid

sites

[24]

ric bending vibrations of the N–H band in NH 4

+ linked toBrønsted acid sites [24] , while the bands at 1600 and 1250,1207 cm –1 are assigned to asymmetric and symmetric

bending vibrations of the coordinated NH 3 linked toLewis acid sites [24] . The assignments of in situ DRIFTS

bands are listed in Table 4. It is worth noting that the twocatalysts exhibit a significant difference on the propor-

tion between both acid sites (L/B). The V/Ce/WTi-DPcatalyst shows less Brønsted acid sites but more Lewisacid sites especially for that corresponding to the band at1207 cm –1. Parallelly, these differences on acid sites arealso observed between Ce/WTi-DP and Ce/WTi-IMP(shown in Fig. 5(b)). Therefore, it is most likely that thedistinctions between the two acid sites are mainly caused

by the changed status of Ce species on WTi, as a result ofdifferent synthesis methods. For Ce/WTi catalyst, theLewis acid sites are mainly provided by wolframyl spe-cies and Ce species and the Brønsted acid sites are as-signed to WOH species [25,26] . Thereby, more Lewis acidsites over Ce/WTi-DP and V/Ce/WTi-DP can be attrib-uted to more Ce species induced by deposition precipita-tion method, which contributes to more adsorbed NH 3 species especially for that at 1207 cm –1. Reflected by the

better SCR activity on V/Ce/WTi-DP and Ce/WTi-DP,the absorbed NH 3 species at 1207 cm

–1 may play themost crucial role.2.5.2 Reaction between pre-adsorbed NH 3 species and

NO+O 2To investigate the reactivity of adsorbed NH 3 species

linked to Lewis acid sites, in situ DRIFTS spectra of the

reaction between pre-adsorbed NH 3 species and NO+O 2 are shown in Fig. 6. For the two V/Ce/WTi samples, af-ter NH 3 are pre-adsorbed and N 2 purging for 30 min, the

NH 3 species linked to Lewis acid sites (1600 and 1250,1210 cm –1) and Brønsted acid sites (1670 and 1440 cm –1)form on the surface. When NO+O 2 are introduced, theintensities of the bands due to NH 3 species decrease.Meanwhile, some new bands attributed to adsorbed ni-trate species (1620, 1572 and 1540 cm –1) appear. As foradsorbed NH 3 species on V/Ce/WTi-IMP, the intensityof dominant band at 1250 cm –1 decreases slowly anddose not vanish until 30 min, whereas the inferior band at1210 cm –1 almost disappears after 7 min. As compared toV/Ce/WTi-IMP, the dominant adsorbed NH 3 species onV/Ce/WTi-DP is corresponding to the band at 1210 cm –1.

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Fig. 6 DRIFTS spectra of V/Ce/WTi-IMP (a) and V/Ce/WTi-DP (b) catalysts pretreated with 3000 ppm NH 3, followed by being ex- posed to 3000 ppm NO+4% O 2 as a function of time at 200 ºC

After the catalyst is purged with NO+O 2, the band at1210 cm –1 decreases quickly and totally vanishes at 7min, accompanied by the appearance of nitrate species(1280 and 1241 cm –1). Thereby, the comparison resultsindicate that the adsorbed NH 3 species at the band of1210 cm –1 shows higher reactive activity than that at1250 cm –1 band. Then the abundant reactive NH 3 speciescorresponding to 1210 cm –1 bring about better activityfor V/Ce/WTi-DP.2.5.3 NO x adsorption

The DRIFTS spectra of NO+O 2 over Ce/WTi and

V/Ce/WTi catalysts at different temperatures are shownin Fig. 7. Several distinct bands appear at 1625, 1572,1540, 1280 and 1241 cm –1, which are assigned to thegas-phase or weakly adsorbed NO 2 species (1625cm –1)[27,28] , bidentate nitrate (1572 cm –1)[29] , monodentatenitrate (1540 and 1280 cm −1)[30] and bridged nitrate (1241cm –1)[31] respectively. Compared with V/Ce/WTi-IMP,the V/Ce/WTi-DP catalyst exhibits more adsorbed nitratespecies. As reported in many studies [10,25] , the weakly ad-sorbed NO 2 and monodentate nitrate are the major active

nitrate species in SCR reaction. Therefore, more activenitrate species on V/Ce/WTi-DP catalyst especiallyabove 200 ºC could strongly illuminate the better SCR

performance at above 200 ºC.Interestingly, a new band at 1372 cm –1 appears above

200 ºC over V/Ce/WTi-IMP catalyst as shown in Fig 7(a).The DRIFTS spectra from 200–350 ºC of Ce/WTi cata-lysts are provided to further explain the presence of the

band (Fig. 7(b)). The same band is observed on theCe/WTi-IMP and the intensity of the band increases withthe increasing temperature, which is consistent with the

changes on V/Ce/WTi-IMP. Based on the results, wecould conclude that the band at 1372 cm –1 ofV/Ce/WTi-IMP stems from the Ce/WTi-IMP. The bandcan be assigned to cis-N 2O2

2– species, which are able todissociate easily the N–O bond to give N 2O

[32,33] . The process was described by precious studies on CeO 2. Theyhave emphasized that an initial disproportionation of NOinto NO 2

– (with participation of surface O 2– ) and N 2O22–

occurs, then the generated N 2O22- further evolves into

NO 3 – and gaseous N 2O at high temperature, respect-

Fig. 7 DRIFTS spectra of V/Ce/WTi (a) and Ce/WTi (b) catalysts during NO x desorption at different temperatures after NO and O 2 co-adsorption at 50 ºC

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