Morphology oriented ZrO2 supported vanadium oxide for NH3-SCR

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Morphology oriented ZrO2 supported vanadium oxide for NH3SCR process: importance of structural and textural properties Shanshan Liu, Hao Wang, Ying Wei, Runduo Zhang, and Sebastien Royer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03429 • Publication Date (Web): 24 May 2019 Downloaded from http://pubs.acs.org on May 24, 2019

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ACS Applied Materials & Interfaces

Morphology oriented ZrO2 supported vanadium oxide for NH3-SCR process: importance of structural and textural properties Shanshan Liu a, Hao Wang a, Ying Wei a, Runduo Zhang a,*, Sebastien Royer b,* a

State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of

Energy Environmental Catalysis, Beijing University of Chemical Technology, 100029, P. R. China. Email: [email protected] b

Univ. Lille, CNRS, ENSCL, Centrale Lille, Univ. Artois, UMR 8181 - UCCS - Unité

de Catalyse et de Chimie du Solide, F-59000 Lille, France. Email: [email protected] ABSTRACT ZrO2 supports, with diverse morphologies (hollow sphere, star, rod, mesoporous) were produced using hydrothermal and evaporation-induced self-assembly (EISA) methods. Zirconia supported vanadium oxide catalysts were prepared by wet impregnation, and used for the low-temperature selective catalytic reduction (SCR) of NO with ammonia. Characterization of catalysts includes N2 physisorption, elementary analysis, X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, temperature programmed reduction by H2, Temperature programmed desorption of NH3. Significant differences in terms of activity are measured. V/MZ (3 wt.% V2O5 supported on mesoporous ZrO2) presents excellent N2 yields (>90%, in the 200400

1

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ºC interval), with a wide operating temperature window (NO conversion > 95%, in the 225425 ºC interval), and less interesting performances were obtained when vanadium oxide is supported over stars, hollow spheres, rods. Surface characterization showed a content of tetravalent vanadium ion, when supported, decreasing in the order mesoporous > hollowsphere > star > rod. This order is in perfect agreement which order of performance of catalyst in the NH3-SCR reaction. The impact of tetravalent ion presence on the surface is confirmed by DRIFTS analysis, Brønsted acid sites generated on surface, V4+-OH, species being involved in the reaction. The more important production of nitrite species over tetragonal supported vanadium oxide catalyst could be another reason for the excellent NH3-SCR performance displayed by the V/MZ catalyst. When supported over monoclinic zirconia, like vanadium oxide over star-type morphology, the adsorbed NH3 species (NH4+ and coordinated NH3) reacted with NOx adsorption species (nitrate) to form ammonium nitrate. Ammonium nitrate can be decomposed to N2 and N2O (or NO2). Thus, NO conversion curves and N2 yield curves over tetragonal zirconia (MZ) at lower temperature were ahead of over V/SZ because of the higher V4+ surface content and more active B acid sites associated with an easy formation of nitrito intermediate.

Keywords: vanadium oxide; zirconia; morphology; tetragonal; monoclinic; NH3-SCR; NO

2


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1. Introduction Nitrogen oxide (NOx) emissions from stationary (e.g. thermal power stations) and mobile sources (e.g. vehicle exhaust) have significant impact on the environment, being at the origin of acid rains, photochemical smog, and ozone depletion. Different approaches to control NOx emission exist. Direct decomposition of NOx into N2 and O2 is quite simple, but the oxygen atoms formed at the catalyst surface strongly adsorb and cause its rapid deactivation.1 A second approach is the NOx Storage Reduction (NSR), also named Lean NOx Trap (LNT).2 NSR runs the engine under cyclic lean-fuel/rich-fuel conditions. During the lean period, NOx are stored over alkali/alkali-earth oxide surface sites, in the form of nitrate species.3 During the rich period, the stored NOx are released and reduced to N2 over noble metal active sites, with formation of NH3 and N2O occurring at different levels depending on the reaction conditions and catalysts used. NSR catalysts work with different kinds of reductant (H2,4 CO5 and various hydrocarbons6), with bifunctional metalbasic catalysts as Pt/BaO2/Al2O3. With aforementioned catalyst, barium oxide acts as the storage phase and platinum acts as active phase for oxidation (of NO) and reduction (of stored species).7 The last efficient approach for NOx emission control is the selective catalytic reduction (SCR), including HC, CO and H2-SCR for automotive depollution processes.8 The SCR of NOx by NH3 has been widely studied for stationary sources applications and mobile lean-burn diesel engines emission control.9 Some drawbacks are associated to NH3-SCR, such as ammonia slip, ash odor, airheaters fouling and high cost.10 However, the NOx conversion, N2 yield, and stability reached with NH3-SCR technology are superior to those of HC/CO/H2-SCR processes.11,

12

Commercial NH3-SCR catalysts are of V2O5-WO3/TiO2 or V2O5-

MoO3/TiO2 basis.13 These catalysts present several advantages including low cost and good sulfur resistance. Despite high NOx removal efficiencies can be achieved, the narrow temperature window of efficiency,14 as well as the significant decrease in N2 selectivity when temperature exceeds 350 ºC,15 are two aspects of performances that cannot be ignored. In addition, in these formulations, TiO2 phase transition from 3



anatase to rutile can occur. Metal (Fe, Cu) exchanged zeolites [ZSM-5 (MFI topology), mordenite (MOR topology), and Beta (BEA topology)] have also been proposed. Unfortunately, the presence of hydrocarbons in exhaust gases induces their deactivations, as evidenced by Ma et al.16 Among other metal oxides, performances of V2O5 dispersed over TiO217, Al2O318, CeO219 , and SiO2, as well as of ZrO2-supported active WO320, MoOx21, MnO222, CeO220 and other 23 were evaluated. Smirniotis24 then pointed out the potential of manganese-based catalyst, with the highest performance being obtained for MnOx/TiO2 (anatase phase) and an activity correlated to the redox properties of manganese oxide. Among the supports, ZrO2 remains attractive due to the presence of weak acid and basic sites on its surface.25 Boningari et al.26 proposed V2O5-MeOx/ZrO2 (Me = W, Mo) formulations and a series of V2O5/ZrO2 (various V/Zr atomic ratios) for the lowtemperature NH3-SCR under excess of oxygen. Authors evidenced the formation of ZrV2O7 solid solution when vanadium content increases, a phase already observed.27 This phase is evidenced to negatively impact the catalytic performances due to decreases in: (i) oxygen release capacity and (ii) reducibility of VOx surface clusters. Rasmussen et al.28 reported excellent properties for V2O5-ZrO2-SO42--sepiolite materials, with an NH3-SCR activity which is directly related to the active phase surface concentration while no formation of ZrV2O7 phase was evidenced. ZrO2 however adopts monoclinic,29 tetragonal,29 and cubic30 crystal symmetry, which can explain why it has been used for several different applications. 31 During the last decade, solvent-mediated synthesis approaches were developed to produce morphology oriented ZrO2 nanostructures (nanotube,32 mesoporous,33 3DOM,34 nanowire,35 nanorod,36 flower-like,37 and sphere38), in addition to nanocasting for mesoporous zirconia synthesis.32 In this work, ZrO2 supports with diverse morphologies were synthesized and used for the preparation of vanadium containing catalysts in view of the NH3-SCR of NO application. Physicochemical properties of catalysts were evaluated by N2 physisorption, XRD, SEM, HRTEM, XPS, H2-TPR, NH3-TPD. Importance of support morphology and phase nature is highlighted. 4


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Mechanism of reaction was investigated by in situ DRIFT and a reaction pathway proposed. 2. Experimental 2.1. Supports and catalyst preparation Mesoporous ZrO2 (MZ): MZ is obtained by Evaporation Induced Self Assembly (EISA) using non ionic triblock copolymer as template.39 0.9 g of Pluronic F127 (poly(ethyleneoxide)-block-poly(propyleneoxide)-block-poly(ethyleneoxide)-block, MW: 12.6 kDa, BASF Corp.) was dissolved in 10 mL of absolute ethanol at 30 ºC. 1.61 g of ZrOCl2·8H2O (98%, Aladdin) was thereafter added under stirring and the mixture was aged for 2 h at room temperature. The whole solution was transferred into 30 mm × 60 mm porcelain crucibles for solvent evaporation step (48 h, 40 ºC, and 50% relative humidity). The obtained gel was dried at 100 ºC for 12 h and decomposed at 450 ºC (temperature increasing rate of 1 ºC min-1; isothermal step time of 4 h). Hollow Sphere ZrO2 (HSZ): HSZ is obtained using solvothermal approach.40 1.0 g of ZrOCl2·8H2O (98%, Aladdin) and 0.6 g of urea (AR, Sinopharm) were dissolved in 40 mL of absolute ethanol. 10 mL of 36.5 wt.% HCl solution was added under stirring. The solution was transferred into a 75 mL Teflon-lined autoclave and was thermally treated at 160 ºC for 24 h. The precipitate was recovered by filtration, washing with distilled water (Vtot = 800 mL), and drying under vacuum at 60 ºC for 24 h. Before use, the material was calcined at 450 ºC (identical conditions than for MZ). Star ZrO2 (SZ): SZ is obtained using solvothermal approach.41 0.644 g of ZrOCl2·8H2O (98%, Aladdin) and 0.082 g of CH3COONa (99%, Sinopharm) were dissolved in 12.0 mL of deionized water under magnetic stirring. The solution was transferred to a Teflon-lined autoclave, which was heated to 240 ºC for 6 h. After cooling down to room temperature, the product was collected by centrifugation, and washed with distilled water for 3 times, before being dried at 90 ºC during 6 h. Finally, the material was calcined at 450 ºC (identical conditions than for MZ). 5



Rod ZrO2 (RZ): RZ was produced by solvothermal approach.42 1.28 g of ZrOCl2·8H2O (98%, Aladdin) was dissolved in 40 mL of water under stirring. NH3·H2O (35%, Sinopharm) was dropwise added until pH = 9.5 was reached. 4.045 g of NaOH (AR, Sinopharm) dissolved in 12 mL of H2O was thereafter added dropwise. The mixture was transferred into a Teflon-lined stainless steel autoclave and heated at 250 ºC during 48 h. Product is recovered by centrifugation, washed with distilled water for 3 times and finally dried at 70 ºC in a vacuum oven for 12 h. Before use, the material was calcined at 450 ºC (identical conditions than for MZ). Preparation of the zirconia supported V2O5 catalysts: supported V2O5 catalysts (with the supports being the HSZ, MZ, SZ, and RZ) are obtained by wet impregnation. V2O5 loading in the catalyst is fixed at 3 wt.%. For this purpose, 0.0710 g of NH4VO3 (AR, Sinopharm) was dissolved in 20 mL of distilled water. Then, 1.0 g of selected ZrO2 support (among HSZ, MZ, SZ, RZ) is immersed in the solution. The suspension was stirred during 8 h. The solvent was eliminated under vacuum drying at 80 ºC. The solid was finally dried at 100 ºC during 6 h, before being calcined at 450 ºC during 4 h (temperature increasing rate = 5 ºC min-1). The calcined catalysts are named: V/“support acronym”. 2.2. Characterization methods N2 physisorption: Specific surface area (SSA), as well as total pore volume (Vp) are obtained from N2 physisorption isotherms recorded at -196 ºC on a Micromeritics ASAP 2420 analyzer. Before analysis, samples are outgassed at 300 ºC under vacuum during 3 h. SSA was calculated on the linear part of the B.E.T. plot (0.05 < P/P0 < 0.35). The pore volume was measured on the desorption branch of the isotherm, at a relative pressure (P/P0) of 0.99. X-ray diffraction (XRD): diffractograms of supports and catalysts were recorded on a Bruker D8 diffractometer equipped with a copper anticathode and a nickel filter (λ = 0.15406 nm). Small angle diffraction patterns were recorded from 2 = 0.5ºto 10º, with step of 0.05ºeach 0.5 s. Wide angle XRD diffraction patterns were collected from 2 = 10ºto 80º, with step of 0.1ºand time of 1 s per step. 6


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Microscopy: Morphology of support was determined using scanning electron microscopy (SEM, Philips-FEI Quanta 200) and transmission electron microscopy (TEM, JEM-3010). Some high resolution TEM (HRTEM) observations were also performed on a Tecnai G2 F20 S-Twin equipment. Chemical composition: compositions of catalysts were determined by X-ray Fluorescence (XRF) spectroscopy (Rigaku Industrial Corporation 3271E). Surface analysis: material surface composition was determined by X-ray photoelectron spectroscopy (XPS). Experiments were carried out on a Thermofisher ESCALAB 250 system with Al Kα radiation as the X-ray source. Material reducibility: temperature programmed reduction by hydrogen (H2-TPR) was carried out on a home-made setup composed of a heated fixed-bed reactor connected to a gas chromatograph (4000A model from EWAI) equipped with a thermal conductivity detector (TCD). Reactor was loaded with 100.0 mg of freshly calcined catalyst. Reduction was performed from 100 ºC to 900 ºC at a temperature increasing rate of 10 ºC min-1 and under a flow of 5.0 vol.% H2/Ar (35 mL min-1). H2 consumption is recorded with temperature on an on line TCD positioned after water trapping zone. NH3 adsorption capacity: temperature programmed desorption of NH3 (NH3TPD) was performed on a Micromeritics AutoChem II 2920 instrument equipped with a TCD. Approximately 100.0 mg of sample was first pretreated at 600 ºC under a 5.0 vol.% O2 in He flow (20 mL min-1) for 30 min. When the catalyst temperature was stabilized at 100 ºC, saturation step with NH3 flow was performed (4.0 vol.% in He, 20 mL min-1 for 1 h). Loosely bounded ammonia was further removed from the catalyst surface by flowing the material under 20 mL min-1 of He (> 99.999%) for 1 h. Desorption step was carried out from 100 ºC to 550 ºC at a heating rate of 10 ºC min-1 under a He flow (> 99.999%, 20 mL min-1). Quantification on TCD signal of NH3 desorption was conducted by injecting 4% NH3/He pulses through six-point valve. 2.3. Catalytic activity measurement Selective catalytic reduction of NO by NH3 was carried out in a quartz fixed-bed 7



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reactor. The reactor (I.D. = 6 mm, L = 400 mm) was positioned in an electrical furnace, equipped with a temperature controller and a thermocouple located in close contact of the catalytic bed. The catalyst (100.0 mg) was pelletized, crushed, sieved at 40-60 mesh, and loaded into the reactor between two quartz wool plugs. The composition of the reaction flow was 1000 ppm NO : 1500 ppm NH3 : 2.0 vol.% O2 in Ar, passing through the catalytic bet at a total flow rate of 100 mL min-1 (given a GHSV of 30,000 h-1). Reaction was studied for temperatures from 100 ºC to 450 ºC. The composition of effluent gases (NO, NH3, N2O, and NO2 concentrations) was analyzed under stabilized conditions with a multigas FTIR spectrometer (NEXUS 470, Nicolet) equipped with an ultra-mini long path gas cell (model 2.4-H, IR analysis). N2 concentration was determined using a gas chromatograph (HP5890) equipped with a TCD for quantification and 5A and TDX-01 combined columns for separation. The NO conversion, NO2 yield, and N2O yield are calculated according to the following equations: NO Conversion = N2 Yield = [NO ]

[NO ]inlet −[NO ]outlet [NO ]inlet

2[N 2 ]outlet

inlet

NO2 Yield = [NO ]

+[NH 3 ]inlet

× 100%

[NO 2 ]outlet inlet

+[NH 3 ]inlet

2×[N 2 O]outlet

N2 O Yield = [NO ]

inlet

× 100%

+[NH 3 ]inlet

(1) (2)

× 100%

(3)

× 100%

(4)

Where [NO]inlet is the inlet NO concentration, and [NO]outlet is outlet NO concentration, [N2]outlet is the outlet N2 concentration, [NH3]inlet is the inlet NH3 concentration. 2.4. In situ DRIFT experiments In situ diffuse reflectance Fourier transform infrared spectroscopy was conducted using a TENSOR 27 spectrometer from BRUKER, equipped with a high sensitivity MCT detector. A high temperature reaction chamber (Praying Mantis, Harrick) is used for in situ experiments. The ground catalyst ( 50 mg) is placed in the diffuse reflecting cell. After signal stabilization, the catalyst is flowed under He (20 mL min-1) for 10 min, then the catalyst is exposed to the reacting gases. The infrared spectra 8


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were recorded as 32 cumulative scans with a spectral resolution of 4 cm-1. Two experiments were performed: ① exposure to NH3 containing flow (1500 ppm NH3 in He; total flow rate of 20 mL min-1) for 30 min, and then exposure to NO + O2 containing flow (1000 ppm NO, 2 vol.% O2 in He; total flow rate of 20 mL min-1) for 30 min; ② exposure to NO + O2 containing flow (1000 ppm NO, 2 vol.% O2 in He; total flow rate of 20 mL min-1) for 30 min, and then exposure to NH3 containing flow (1500 ppm NH3 in He; total flow rate of 20 mL min-1) for 30 min. 3. Results and discussion 3.1. Physicochemical properties 3.1.1. XRD

V/HSZ HSZ

V/SZ SZ

Intensity/ a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


V/MZ MZ

V/RZ RZ

Tetragonal ZrO2 Monoclinic ZrO2 10

20

30

40

50

2/

60

70

80

o

Figure 1. Powder X-ray diffraction patterns recorded for the ZrO2 supports and derived catalysts. Figure 1 presents the X-ray diffraction patterns of supports and derived catalysts. Among the supports, HSZ, SZ and RZ are presenting pure monoclinic ZrO2 phase, of 9



space group P21/c (JCPDS reference file no. 86-1451). Main differences between the three supports are visible on the reflection widths, with FWHM of the reflections evolving as follow: HSZ > SZ >> RZ. The evolution is characterizing significant differences in terms of crystal domain size of ZrO2, with the smallest crystals obtained over HSZ (mean crystal domain size = 2.8 nm, applying the Scherrer equation after Warren’s correction for instrumental broadening) and the largest obtained for RZ (not accurate value since > 50 nm, using the Scherrer equation). Completely different diffractogram is obtained for the MZ support, with only four main reflections characteristic of tetragonal zirconia phase (JCPDS file no. 88-1007) visible, with no additional reflection ascribed to other phases. Over MZ, for which formation of periodic pore structure is awaited, low angle analysis was additionally performed. Diffractogram, presented in Figure S1 in Supporting Information (SI), shows one reflection located at 2 = 0.96ºwhich is ascribed to the (100) plan of the pore network. The absence of additional low angle reflections, as classically obtained for mesoporous zirconia,39,

43

suggests the formation of mesoporous periodic pore

structure with limited long range ordering as observed for wormhole-like type pore topologies. All vanadium containing materials are exhibiting comparable diffractograms than the parent supports with no modifications of the identified phases. The absence of supplementary reflections (no detection of crystalline vanadium oxide containing phases) is an indication of small nanoparticles and/or amorphous V2O5 phase formation. The only difference observed when compared with the parent supports is a decrease of the FWHM of the reflections. This indicates that the impregnation cycle, and probably the second calcination cycle to produce the V2O5 supported phase, is inducing ZrO2 crystal growth in a limited extend. This last phenomenon is easily observed over HSZ, SZ and RZ supports, while it remains limited over MZ support. Miciukiewicz et al.44 pointed out the formation ZrV2O7 over zirconia, originating from a strong interaction between surface-dispersed VOx species and zirconia support. However, reflections for this phase locates at comparable position than those of 10


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monoclinic zirconia. Therefore, the reflections from the ZrV2O7 phase can partially contribute to the change in crystal size in the case of HSZ, SZ and RZ supported catalysts. For tetragonal ZrO2 (MZ) after loading with vanadium, some outermost layers lattice zirconium atoms may be substituted by vanadium atoms of smaller atomic radii.45 This lattice substitution results in a slight decrease in crystallinity of the tetragonal zirconia.

3.1.2. Morphology of support and catalysts Information on the large scale morphology of the supports is obtained by the observation using SEM and low magnification TEM. Drastically different morphologies are observed for the supports on the micrometer scale, as presented in Figure S2 (SEM) in Supporting Information (SI) and Figure 2 (TEM). The targeted particle shapes are easily observed, with: 

For HSZ support, the observation of complete and broken hollow spheres of size from < 1 µm to 2 µm (Figure S2(a) in SI and Figure 2(a)). Hollow spheres are aggregated and linked together, while they are displaying a rough surface (characteristic of an assembly of NPs).



For SZ support, microscopy allows the observation of star-like assemblies of ZrO2 nanoparticles, with sizes ranging from 50 nm to 150 nm; the elementary star-like assemblies forms aggregates of several micrometers in size (Figure S2(b) in SI and Figure 2(d)). From high magnification TEM images, star-like assemblies are formed from primary quadrilateral shape crystals (Figure S3(b)).



For MZ support, the formation of dense aggregates of elongated elementary particles is observed (Figure S2(c) (SI), Figure 2(g)). At the observable scale of SEM, the ordered pore network is not visible, while the regular elongated particles observed are typical of morphology observed for classical ordered mesostructured particles. TEM images, at higher magnification, evidenced the presence of tubular-type pore morphology inside these particles.



For RZ support, large elongated particles of various length and width are 11



observed, also fitting with the awaited rod-like morphology (Figure S2(d) in SI and Figure 2(j)). TEM images of RZ clearly demonstrated the formation of nanorods of 100-300 nm in width and ~1 µm in length.

Figure 2. HRTEM images obtained for: (a) HSZ and (b, c) V/HSZ; (d) SZ and (e, f) V/SZ; (g) MZ and (h, i) V/MZ; (j) RZ and (k, l) V/RZ. HRTEM obtained for HSZ and V/HSZ shows planar distances of 0.284 nm, 12


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0.316 nm and 0.369 nm that are matching with the (111), (-111) and (110) planes of monoclinic ZrO2 phase (Figure S3(a) in SI and Figure 2(c)), according to XRD identified phase. HRTEM also confirmed formation of monoclinic ZrO2 phase in the case of: 

SZ and V/SZ, with interplanar distances ascribed to the (100) and (200) planes of the monoclinic structure observed in Figure S3(b) in SI and Figure 2(f);



RZ and V/RZ, with interplanar distance fitting with (001), (110) and (200) planes measured in Figure S3(d) in SI and Figure 2(l). Due to the large cavity of the inner structure, the collapse of a few hollow

spheres and the thickening of the shell can be observed after impregnation of the vanadium phase (Figure 2(b), V/HSZ). From TEM analysis, coupled with EDX spectroscopy, vanadium is observed to mostly cover the outside shell of HSZ, while large bulk V2O5 particles cannot be detected using TEM analysis. In the case of V/SZ and V/RZ, no significant modification of the morphology of the particles can be observed after impregnation-calcination for vanadium oxide formation, even if some agglomeration of the elementary stars seems to occur for V/SZ (Figure 2(e) and Figure 2(k)), while no bulk V2O5 particle can be observed. TEM analysis then evidenced, as concluded from XRD analysis, the formation of small size particles or amorphous phase of vanadium oxide on the HSZ, SZ and RZ surfaces. Finally, TEM analysis (Figure 2(h)) shows that the channel-type morphology of the MZ is partially retained over V/MZ catalyst. Parallel channel of different length and tortuosity are observed, with the zirconia walls easily distinguished from the empty channels. Support pore ordering observed for V/MZ seems to be decreased as compared with MZ pristine support (shorter channels, not perfectly straight), as was also concluded by the slightly less defined pattern recorded at low 2 analysis (Figure S1 in SI). This highlights the alteration of the pore network upon impregnation. High resolution TEM analysis evidenced lattice fringes, confirming the crystalline character of the material (as deduced from XRD analysis), and with lattices spacing of 0.2910.296 nm, agreeing with the value for the (101) planes of tetragonal ZrO2. 46 13



160 140 120 100 80 60 40 20 0

Vads / cm g

3 -1

160 140 120 100 80 60 40 20 0

Vads / cm g

3 -1

3.1.3. Textural properties

HSZ

V/HSZ

3 -1

50

MZ

40 30 20

V/MZ

10 0

SZ V/SZ

60

Vads / cm g

3 -1

60

Vads / cm g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50 40 30 20

RZ

10 0.0

0.2

0.4

0.6

0.8

0

1.0

V/RZ 0.0

0.2

0.4

0.6

0.8

1.0

P/P0

P/P0

Figure 3. N2 adsorption–desorption isotherms obtained for ZrO2 supports and V/ZrO2 catalysts. N2 adsorption/desorption isotherms of supports and catalysts are shown in Figure 3. The isotherm recorded for HSZ is of type II (IUPAC classification), with a progressive increase in adsorbed volume with P/P0, and sharp adsorption at high P/P0 with small hysteresis between adsorption and desorption branches. Such isotherm is characteristic of a multimolecular adsorption on the surface of a porous support. HSZ is presenting high surface area, reaching 196 m2 g-1 (Table 1). Deposition of the active phase induces significant decrease of the surface area (-52%) and pore volume (-73%) showing that the support porosity is significantly modified upon impregnation procedure, which can be explained by some hollow sphere collapse and thickening of the shell. SZ support is showing an intermediate Type II/IV isotherm with most of the adsorbed volume being measured at P/P0 higher than 0.7. The obtained shape fits well with the adsorption in the porosity generated between aggregated particles, morphology observed by TEM. Contrarily to the HSZ support, the material is 14


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presenting a better stability toward impregnation with a more limited loss of surface area (-34%) and pore volume (< 10%) upon impregnation. MZ support is presenting a type IV isotherm, as can be awaited for mesoporous support, with a capillary condensation step. However, the positioning of this step, at low P/P0, leads to a small hysteresis. Again, the impregnation of the active phase is inducing significant surface area (-47%) and pore volume (>70%) decreases, indicating a significant modification of the porosity as awaited from the TEM observation and the pore ordering quality decrease and the absence of bulky vanadium oxide particle formation by microscopy. Finally, RZ support is presenting a type II isotherm with no visible hysteresis and most of the N2 adsorbed at P/P0 > 0.8. Such isotherm characterizes a non-porous support, with adsorption over the external surface of large particles, which is confirmed by very low surface area (22 m2 g-1) and pore volume (< 0.1 cm3 g-1) measured (Table 1). As for the other solids, significant surface area and pore volume decreases are measured after deposition of the vanadium oxide phase. 3.1.4. Vanadium surface oxidation state V 2p 516.2 517.4

(a) O 1s 530.2 532.1

(b)

V/HSZ

Intensity/ a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


V/SZ

V/MZ

V/RZ 514 515 516 517 518 519 520

528

530

532

534

Binding Energy/ eV Figure 4. V 2p3/2 (a) and O 1s (b) peak obtained for the different catalysts. 15



Page 16 of 42

Figure 4 presents V 2p3/2 and O1s XPS signals recorded for the different catalysts. V 2p3/2 peaks (Fig.4a) can be decomposed into two main contributions ascribed to V4+ and V5+ oxidation states, located at 516.2 eV and 517.4 eV, respectively.47 Fraction of surface V4+ [V4+/(V4++V5+)] are listed in Table 1. The fraction of surface tetravalent vanadium ion in the materials ranks as: V/MZ > V/HSZ > V/SZ > V/RZ. Boningari and coll.48 described the surface of vanadium oxide in V2O5/ZrO2 catalysts as composed of V5+, V4+, and even V3+ surface species, in different proportions, with V5+ species being the most abundant species. Significant differences in V4+ species surface proportion are obtained in our work, from 0.22 for V/RZ to 0.45 for V/MZ, but values are comparable with those reported by Boningari and coll. (proportions ranging in the 0.18-0.39 interval). However, in our study, no V3+ species formation can be observed, since no component at 515.8 eV is needed to obtain a satisfying decomposition of the global signal. In the tetragonal zirconia phase, Zr4+ cation is octacoordinated and O2- anion is tetracoordinated. In the monoclinic phase, Zr4+ cation is heptacoordinated and O2- anion is tri-or tetracoordinated.49 Extra electron charge of the tetragonal zirconia lattice can be transferred to vanadium phase in the catalyst, an effect that can be at the origin of the slightly higher V4+ content in V/MZ catalyst (who has tetragonal phase support) than in monoclinic ZrO2 supported vanadium oxide catalysts (V/HSZ, V/SZ, and V/RZ).50 Finally,

XPS

analysis

evidenced

vanadium

surface

proportion

[Vsurf/(Vsurf+Zrsurf+Osurf)] at 2.4-2.8 at.% for V/HSZ and V/SZ catalysts. The obtained values suggests a homogeneous distribution by the vanadium phase on these catalysts (both on the surface and inside), since larger differences between surface and bulk vanadium contents (calculated around 5.42 ± 0.05 at.% in Table 1). The V 2p3/2 peak area of the V/HSZ was much lower than those of other catalysts, implying some vanadium species, confined inside the hollow-spheric structure, are not visible by XPS. Considering the very close values obtained by XRF and XPS, the formation of surface monomeric VOx species and small nanocrystalline clusters species over HSZ and SZ surfaces is expected. Less difference between bulk and surface vanadium 16


Page 17 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


contents are measured for V/MZ and V/RZ. Over MZ and RZ, then formation of polymeric vanadium species as well as bulk vanadium oxide particles are expected to form despite being not visible by X-ray diffraction. O 1s spectra (Figure 4b) were decomposed into two contributions: adsorbed oxygen (Osurf) at 532.1 eV; lattice oxygen (Obulk) at 530.2 eV. The fraction of adsorbed oxygen (Table 1) decreased as: V/MZ > V/HSZ > V/SZ > V/RZ, which was in agreement with V4+ fraction evolution. Adsorbed surface oxygen species were already identified as important species for the SCR process.51 These species seem more abundant on V/MZ surface.

17


ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Page 18 of 42

Table 1. Selected textural and structural properties of ZrO2 and V2O5/ZrO2 materials.

Sample

XRD phase

SBETb/ m2 g-1

Vp / cm g

HSZ SZ MZ RZ

M M T M

196 94 92 22

0.45 0.21 0.22 0.05

a

b

3

-1

Sample

SBETb/ m2 g-1

Vp / cm g

V load.a/ wt.% (at.%)

V/HSZ V/SZ V/MZ V/RZ

93 62 49 6

0.12 0.19 0.06 0.02

2.69 (5.42) 2.74 (5.50) 2.78 (5.57) 2.74 (5.42)

b

3

-1

Surface proportionc/ at.% V Zr O 2.4 30.6 67 2.8 32.2 65 4.3 28.7 67 4.1 29.9 66

(V4+/Vn+)c/ Osurf/(Osurf+Obulk)d/ at.% % 40 31 45 22

25.0 24.2 27.9 14.3

, Value obtained by XRF analysis; b, N2 physisorption extracted values; c, element surface proportion and V4+ relative amounts being calculated using

[V4+]/([V4+]+[V5+]) formulae (concentrations being expressed in at.%); d, fraction of surface adsorbed oxygen issued from O 1s spectrum decomposition.

Table 2. Vanadium reducibility and acidic properties measured for the different V2O5/ZrO2 catalysts.

Sample HSZ SZ MZ RZ a

Total NH3 uptake/ mmol g-1

Sample

0.22 0.13 0.16 0.01

V/HSZ V/SZ V/MZ V/RZ

Total NH3 uptake/ mmol g-1

Theoretical H2 consumptiona/ 10-5 mmol g-1

Experimental H2 consumption/ 10-5 mmol g-1

Vanadium reduction degreea/ %

0.42 0.31 0.26 0.06

52.8 53.8 54.6 53.8

39.0 39.5 41.1 34.1

73.9 73.5 75.2 63.4

H2 consump./ 10-5 mmol g-1 (Temperature max./ ºC) peak I 14.9 (451) 16.3 (445) 14.0 (488) 6.9 (447)

peak II

8.6 (511)

Peak III 24.1 (565) 23.2 (575) 27.1 (584) 18.6 (586)

, theoretical H2 consumption calculated to reduce all V(+V) to V(+III) and vanadium reduction degree calculated using experimental consumption, assuming an

initial oxidation number of vanadium at +V ; b, acidic characteristics of supports and catalysts as obtained by NH3-TPD (expressed in NH3 uptake).

18


Page 19 of 42

3.2. Reducibility of Vanadium catalyst 40

40

30

TCD Signal/ a.u.

TCD Signal/ a.u.

(a) 451

20

565 10 0

(b) 30

445

20

573 10 0

100 200 300 400 500 600 700 800 900

100 200 300 400 500 600 700 800 900

o

o

Temperature/ C

40

TCD Signal/ a.u.

30

Temperature/ C

40

(c)

TCD Signal/ a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


584 488

20 10 0

(d) 30

511 590

20 10

447

0

100 200 300 400 500 600 700 800 900

100 200 300 400 500 600 700 800 900

o

o

Temperature/ C

Temperature/ C

Figure 5. H2-TPR profiles recorded over: (a) V/HSZ, (b) V/SZ, (c) V/MZ, and (d) V/RZ. H2-TPR was performed to assess the redox ability of the vanadium oxide species. Figure 5 shows the reduction profiles obtained for the different catalysts. The reduction peak positions (Tmax), H2 consumption values, and reduction degrees of V2O5/ZrO2 catalysts are gathered in Table 2. In previous works, unsupported V2O5 was described to reduce in the temperature range of 500 to 900 ºC.50 In addition, reduction of unsupported vanadium oxide is described to occur in successive steps, with following steps identified: -

Reduction of V2O5 into V6O13, followed at only slightly higher temperature to the reduction up to V2O4 formation; reduction of vanadium from +V to +IV oxidation number

-

Reduction at higher temperature of V2O4 into V2O3; reduction of vanadium from +IV to +III oxidation number The reduction of supported vanadium catalysts, synthesized in this study, occurs

19



at lower temperatures than for the bulk materials, and temperatures observed, from 300 ºC to 800 ºC, are in accordance with those reported for ZrO2 supported V2O5.26, 52, 53

Reduction profiles, presented in Figure 5, are composed by two main hydrogen

consumption steps. The lowest reduction temperature peak is observed at 450 ºC (temperature of maximum H2 consumption) for V/HSZ and V/SZ catalysts, while it located at higher temperature for V/MZ (around 490 ºC). This first hydrogen consumption is assigned to the reduction of a significant part of V5+ species into V4+ species, as described in the literature, 26, 52, 53 but did not allow to obtain a V2O4 phase considering the incomplete conversion of V5+ species into V4+ species if we refer to the H2 consumptions in Table 2.53 During the second peak, occurring for all catalysts above 560 ºC, a 1.2 to 1.9 times higher hydrogen consumption than during the first peak is recorded. This means that V5+ species are finishing to be reduced in this temperature range, and that V3+ forms. For V/RZ, three overlapping peaks are observed, with the sum of the two first hydrogen consumptions being comparable with the first hydrogen consumption measured for the other three materials. Reduction can be described as successive processes: (i) V2O5 to V6O13 (at 447 ºC), (ii) V6O13 to V2O4 (at 511 ºC), and V2O4 to V2O3 (at 590 ºC).54 In the case of the other materials, considering the smaller particle sizes and higher surface areas, the two first steps occur at comparable temperature, with maximum consumption observed at around 450480 ºC. At the end of the reduction cycle, the total amount of hydrogen consumed showed that only 63.4-75.2% of the V5+ is reduced to V3+ if we consider that all vanadium atoms are at the +V oxidation number initially, meaning that: -

Oxygen deficiency exists in the initial phase, or a phase less oxidized, globally represented by V2O5-δ, was initially obtained;

-

At the end of the reduction, a phase more oxidized than V2O3 or mixed phase remains. XPS analysis however showed that a significant fraction of V4+ species formed

on the surface of the oxidized solids, as often reported for this kind of supported 20


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materials, and then, the reduction starting from a phase less oxidized than V2O5 is a realistic hypothesis. For all analyzed materials, a comparable reduction degree is measured (the differences being in the experimental error interval of the experiment) and they are not correlating with the evolution of the V4+/V5+ surface proportions. In addition, no significant difference in starting reduction temperature (visible deviation of TCD signal baseline) can be observed between the materials, being all located at 340350 ºC even if three catalysts are prepared over monoclinic zirconia (V/SZ and V/HSZ, and V/RZ) and one is prepared over tetragonal zirconia (V/MZ). This result differs from the literature for which supported phase was reported more reducible over monoclinic zirconia than over tetragonal zirconia.50 This indicates that the TPR analysis is probably not enough sensible to probe the reducibility of extreme surface V5+/V4+ sites, sites which are awaited to have a role on the NO oxidation rate as well as on the N2 selectivity during NH3-SCR reaction.55 3.3. Acidic Property of catalysts Acidic properties of supports and derived catalysts were probed by NH3 temperature programmed desorption. Desorption profiles are plotted in Figure 6, and values of ammonia desorbed are given in Table 2. Over supports, broad and poorly defined desorption peaks of ammonia are observed. Desorption started few above 100 ºC, with maximum of desorption at 170175 ºC for SZ and MZ, while it locates at higher temperature for RZ (209 ºC) and HSZ (243 ºC). Quantification of ammonia desorbed evidenced a quantity evolving as follow over the supports: HSZ > MZ ~ SZ >> RZ. The highest quantity recorded for the supports, 0.22 mmol of NH3 g-1 in the case of HSZ, reflects a limited acidity. This value is however comparable with values reported for zirconia in the literature (Chary et al. for example reported a value of 0.36 mmol of NH3 g-1 for a monoclinic ZrO2 support synthesized by sol-gel method and presenting a surface area of 44 m2 g-1).50 The quantity desorbed is roughly dependent on the support surface area, as can be observed in Figure 6(e), an indication that the synthesis procedure as well as the nature of the phase formed, has a limited impact of the density of the 21



acidic sites on the surface of the material. For supported vanadium catalysts, the desorption profiles shows three desorption zones, more or less visible depending on the catalyst. The first desorption, the most intense of the three desorption peaks, is located at a temperature below 200 ºC. The second component is located in the interval of temperature 220280 ºC, and the last component is located above 290 ºC. While for monoclinic zirconia supported vanadium catalysts, the three contributions overlapped given in fine a broad NH3 massif starting below 200 ºC and ending at 400500 ºC, the three contributions are more visible for V/MZ (MZ being tetragonal zirconia). According to the literature, the desorption at low temperature can be assigned to NH3 desorbing from weak acid sites; the second peak (220280 ºC) was ascribed to the desorption from acid sites of medium strength, and the peak above 290 ºC was ascribed to the desorption from strong acid sites.56 Compared with the acidity of zirconia alone, the acidity of zirconia supported V2O5 is significantly higher (Table 2)57. From the data, the order of total NH3 uptake follows: V/HSZ > V/SZ > V/MZ >> V/RZ, with a factor 7 between the more acidic and the less acidic catalyst. As in the case of the supports, there is a roughly linear relationship between the surface area and the amount of ammonia desorbed. According to the literature, the acidic sites over zirconia are of Brønsted and of Lewis types, which are derived from coordinatively unsaturated Zrn+ sites. After deposition of vanadium oxide on the surface, the increase of acidity is related to the formation of Brønsted acid sites located over vanadium species, associated with surface hydroxyl groups. According to the NH3-TPD results, the introduction of vanadium increased the number of surface Brønsted and Lewis acid sites, and the desorption temperature shifted towards lower temperature, which is in agreement with the results of Kustov.58 Both effects should consequently be positive to the catalytic properties in SCR process. Indeed, Zhang et al.59 had reported that weak acid sites mainly contribute to the reaction only at relatively low temperatures due to their lower thermal stability, whereas the medium and strong acid sites, which have better thermal stability, contribute to the SCR activity of catalysts at intermediate to high 22


Page 22 of 42

Page 23 of 42

0.016 0.014 0.012 0.010 0.008

160

338

V/HSZ

HSZ

100

TCD Signal / a. u.

217 243

0.006 0.004 0.002 0.000

TCD Signal / a. u.

(a)

200

300

400 o

500

Temperature/ C

0.016 0.014 0.012 0.010 0.008

(c)

179 279

V/MZ 170

0.006 0.004 0.002 0.000

351

MZ

100

200

300

400

0.016 0.014 0.012 0.010 0.008

500

0.006 0.004 0.002 0.000

175 241

200

-1

V/HSZ

300

400

o

500

Temperature/ C (d)

134 V/RZ

100

226 209 200

293 300

RZ 400

500 o

Temperature/ C 0.40

V/SZ

345

SZ

o

0.45

219 331

0.006 0.004 0.002 0.000

0.016 0.014 0.012 0.010 0.008

(b)

161

100

TCD Signal / a. u.

TCD Signal / a. u.

temperatures.

NH3 uptake / mmol g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Temperature/ C (e)

0.35 0.30

V/MZ

0.25

V/SZ

0.20

HSZ

MZ

0.15 0.10

SZ

V/RZ RZ

0.05 0.00 0

20 40 60 80 100 120 140 160 180 200 2

-1

SBET / m g

Figure 6. NH3-TPD profiles of (a) V/HSZ and HSZ, (b) V/SZ and SZ, (c) V/MZ and MZ, (d) V/RZ and RZ; (e) Evolution of ammonia uptake with support / catalyst surface area. 3.4. Catalytic Property Catalytic activities obtained for V2O5/ZrO2 samples in the NH3-SCR of NO are presented in Figure 7. Figure describes NO conversion, N2 yield, N2O yield, and NO2 yield evolution with the reaction temperature. In Table 3 are gathered the characteristic values (interval of >95% NO conversion, interval of >90% N2 yield) 23



obtained during catalytic tests. Results over pure ZrO2 support are presented but will not be detailed considering the poor performances obtained. Indeed, whatever the morphology and porosity, the NO conversion remains very low ( 250 ºC, significant formation of N2O occurs and the N2 yield decreases; T > 350 ºC, additionally, NO conversion decreases. This result is consistent with the literature since Feng et al.

60

reported that the active phase of

vanadia for SCR reaction consists of monolayer/submonolayer coverages of isolated and/or polymerized VO4 units in direct contact with the support. Thus, the V2O5 phase alone is not able to provide such active vanadium species on its surface. Among the different zirconia supported vanadia catalysts, significant differences in terms of activity for NO conversion are observed. The obtained activity order is V/MZ >> V/HSZ > V/RZ  V/SZ. V/RZ and V/SZ are presenting poor performances with limited NO conversion (< 20%) below 200 ºC. Complete NO conversion is reached at 300 ºC. However, since 350 ºC, a decrease in NO conversion is observed as for the bulk V2O5 sample. The N2 yield starts to decrease when the NO conversion decreases. Also, consequent production of N2O is measured over V/RZ when temperature exceeds 250 ºC, while low N2O and NO2 yields are measured for V/SZ when temperature remains below 400 ºC (both staying below 10%). To conclude, these two catalysts are not interesting due to limited performances, especially when temperature exceeds 350 ºC: decrease in NO conversion and N2 yield. This is reflected in Table 3 by activity temperature window and N2 yield temperature window of limited widths. V/HSZ presents better performance. Indeed, NO conversion curve is shifted toward lower temperature by 25 ºC compared to those of V/SZ and V/RZ. However, NO conversion occurs at temperatures always above those recorded over bulk vanadia (Figure 7(a)). For V/HSZ, a 100% N2 yield is obtained when complete NO conversion is achieved, since T = 300 ºC. At higher temperature, the N2 yield is progressively declining. A yield into N2 of 96% is obtained at 350 ºC, and it drops progressively down to 70% at 450 ºC. The N2 yield decrease is accompanied by an increase in NO2 production, as observed in Figure 7(d). Interestingly, N2O production remains very 25



limited over the whole reaction temperature range (always below 10%). Consequently, V/HSZ is exhibiting good properties when the temperature of reaction exceeds 300 ºC but a declining of the performances is observed when temperature of reaction exceeds 350 ºC. Over this catalyst, a conversion higher than 95% is maintained in a 150 ºC interval, while N2 yield higher than 90% is maintained in a 125 ºC interval (Table 3). Stability test performed at 300 ºC during 10 days confirmed the stability of the catalyst (Figure S4 in SI). Table 3. Catalyst activity temperature window for NH3-SCR Samples V/HSZ V/SZ V/MZ V/RZ

Activity temperature window, X >95% / oC 150 (300-450) 125 (300-425) 200 (225-425) 50 (300-350)

N2 yield temperature window, YN2> 90% / oC 125 (275-400) 50 (300-350) 200 (200-400) 0 (300-300)

Finally, the V/MZ is the only supported vanadia catalyst presenting NO conversion at lower temperatures than the bulk V2O5. Over this catalyst, complete NO conversion is achieved at T = 225 ºC, while 250 ºC is needed over V2O5. In addition a 100% NO conversion is maintained until the temperature reaches 425 ºC, temperature at which the conversion starts to decrease. The yield to N2 remains at 100% in the temperature range from 225 ºC to 350 ºC, given an interval of temperature at which N2 yield is higher than 90% of 200 ºC (Table 3). When the temperature exceeds 350 ºC, the N2 yield starts to decrease with a comparable slope than observed for V/HSZ. The decrease in N2 yield is accompanied again by an increase in N2O and NO2 productions. These performances are summarized in Table 3 in the form of interval of temperature for NO conversion above 95% and temperature window for N2 yield above 90%. The performances obtained for V/MZ are far better than those reported for the over supported catalysts since: (i)

the >95% NO conversion is obtained at 225 ºC, and the temperature window for 95% NO conversion is 200 ºC, which is far above the values obtained for the 26


Page 26 of 42

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other catalysts (ii) the temperature window for >90% N2 yield is of 200 ºC, which is again far above the other obtained values Low temperature of NO conversion, and large interval of temperature for selective NO conversion to N2 are the main characteristics of the excellent performances recorded for the V/MZ catalyst. From these results, it seems that the main parameter governing the catalyst performances is the support phase which will induce modification in V4+ surface proportion (Table 1). Parameters such as total acidic, pore volume, surface area seems rather less important than this V4+ surface proportion which will correlate with catalyst performance: V/MZ > V/HSZ > V/SZ > V/RZ. 3.5 DRIFT Studies Table 4. Observed species of adsorbed NH3 and adsorbed NOx, as observed by IR Wavenumber (cm-1) 1019

Species

Formula

hydrogen atom of NH3 bonding to H2NH-O-Zr the surface oxygen atom of ZrO2 1215/1286/1620 Coordinated NH3 NH3 1452/ NH4+ NH4+ 1640-1850 1315 Deformation species of adsorbed / ammonia 1540-1565 Amide NH2 1025 cis-N2O22O-N-N-O Zr O 1230/1605 Bridging nitrate N

1286/1520

Monodentate nitrate

Zr

O

Zr

O

Reference 61 61,62 62 63

O O

N

63 64 64 62

O

1625 1341/1260

Gas-phase or weakly adsorbed NO2 Nitrito compounds

O-N-O Zr-O-N-O

62, 64 61, 63

Figure 8 and 9 shows the DRIFT spectra on MZ, V/MZ, SZ, and V/SZ catalysts, obtained after reactants exposure. In Figure 8, the catalysts were first exposed to NH3 containing He flow for 30 min. Flow of (NO + O2)/He was then introduced into the IR 27



cell at 100 ºC, and spectra were recorded as a function of time. In Figure 9, the reactants were introduced in the reversed order: initial exposure to NO+O2 containing flow, followed to exposure to NH3 containing flow. When exposing first to NH3 and then to NO+O2: It is seen in Figure 8(a) that ammonia adsorbed on zirconia surface is mainly observed as coordinated ammonia and NH4+ with bands in the 1200-1620 cm-1 (Table 4). For the V containing catalysts, the intensities of the bands ascribed to NH4+ species at 1452 cm-1 is significantly increased compared to over the zirconia alone (Figure 8(e)). This evolution is coherent with the observation of the band at 3665 cm-1 in Figure 8(e) and that can be ascribed to OH groups65 which can act as Brønsted acid sites in NH3-SCR reaction. Consequently, the initial step exposure to NH3 also leads to the formation of coordinated NH3 and NH4+ surface species. An additional band, at 1010-1030 cm-1 is observed over V-containing catalysts, not observed over zirconia alone, and can be ascribed to oxygen atom of the surface bonding with hydrogen from NH3 (Table 4). When V/MZ and V/SZ are compared, significant differences are noticed, especially in the intensity ratio between band at 1452 cm-1 (NH4+ surface species) and 1210 cm-1 (coordinated NH3), showing a preferential formation of NH4+ surface species over V/SZ than over V/MZ. When the V/MZ was purged by NO + O2 for 30 min, bands ascribed to monodentate nitrate (bands at 1280, 1525 cm-1) and bridging nitrate species (1255 and 1605 cm-1) appeared. In addition, a low intensity band at 1340 cm-1 appears (Figure 8(b)) which can be attributed to nitrito compounds species, a band that did not appeared on MZ support. Evolution of spectra suggested that the presence of V2O5 would favor the formation of bridging nitrate as initial step of reactants adsorption. Introduction of NO + O2 for 30 min leads to comparable observations for SZ-based materials: presence of vanadia seems to promote formation of asymmetric vibration bridging nitrate surface species at the expense of the monodentate species. However, intense band at 1630 cm-1 is noticed over SZ and V/SZ (gas-phase/weakly adsorbed NO2, Table 4) while such band is of very low intensity for MZ and V/MZ materials. In addition, no band at 28


Page 28 of 42

Page 29 of 42

1341 cm-1 (nitrito compounds species) can be observed in the case of V/SZ. 1286

1604 1625

NO+O2 30 min 10 min 6 min 5 NH3 30 min

1503

1569

4000 3500 3000

1012

1451

(c)

1279

NO+O2 30 min 10 min 5 6 min N-H

1605 1624

1540 1341

+

NH4 +B

1455

1023

1213

1019

NH3 30 min

1217

1800 1600 1400 1200 1000

Wavenumber/ cm

1255

(b)

1254

Kubelka-Munk

Kubelka-Munk

(a)

4000 3500 3000

-1

1800 1600 1400 1200 1000

Wavenumber/ cm (d)

1632

-1

1629

1598 1523

1287 1218

NO+O2 30 min 10 min 6 min

1024

1300 1204

1447

NH3 30 min 4000 3500 3000

Kubelka-Munk

Kubelka-Munk

1601

1622 1800 1600 1400 1200 1000

Wavenumber/ cm

1279 1248 1516 1041

NO+O2 30 min 10 min 6 min

1452 NH3 30 min N-H 4000 3500 3000

-1

+

NH4 +B

1613

1032 1330 1209

1800 1600 1400 1200 1000

Wavenumber/ cm

-1

(e)

Kubelka-Munk

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1452 -OH

V/SZ

1622

1030 1330 1204 1300

SZ

1012

V/MZ

1213

1452

MZ

4000 3500

1600

1400

1200

Wavenumber/ cm

1000

-1

Figure 8. DRIFT spectra of (a) MZ, (b) V/MZ, (c) SZ, (d) V/SZ pretreated by exposure to 1500 ppm NH3 flow, followed by exposure to 1000 ppm NO + 2 vol.% O2 flow for various times at 100 ºC; (e) DRIFT spectra of NH3 adsorbed for 30 min at 100 ºC, comparison between catalysts.

29



NH3

1470 1286 1237

30 min +

N-H

NH4 +B 1604 1503 1626 1349

10 min 6 min

1349

1012

N-H 30 min 10 min 6 min NO+O2 30 min

Wavenumber/ cm

-1

1226

1621 1629

10 min 6 min NO+O2 30 min 3000

1600

+

30 min 10 min 6 min

N-H

Wavenumber/ cm (e)

1200

1000

4000 3500 3000

-1

NH4 +B

1026

1630

NO+O2 30 min 1400

-1

1613

NH3

1594 1800

1800 1600 1400 1200 1000

1461

1288 4000

1625

1320

Kubelka-Munk

NH3

1026

1519

(d)

1463 1523

30 min

+

NH4 +B 1604

Wavenumber/ cm

1298

(c)

Kubelka-Munk

4000 3500 3000

1800 1600 1400 1200 1000

1282 1245

1445

NH3

NO+O2 30 min 4000 3500 3000

1297

(b)

1294

Kubelka-Munk

Kubelka-Munk

(a)

1279

1232

1516 1599 1800 1600 1400 1200 1000

Wavenumber/ cm

-1

1237

1286 1626

Kubelka-Munk

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 42

1604 1012

1503 1349 MZ V/MZ 2000

1800

1600

1400

1200

Wavenumber/ cm

1000

-1

Figure 9. DRIFT spectra of (a) MZ, (b) V/MZ, (c) SZ, (d) V/SZ pretreated by exposure to 1000 ppm NO + 2 vol.% O2 flow, followed by exposure to 1500 ppm NH3 flow for various times at 100 ºC; (e) DRIFT spectra of NOx adsorbed for 30 min at 100 ºC, comparison between MZ and V/MZ. When exposing first to NO+O2 and then to NH3: over supports and catalysts, a band at 1630 cm-1 is observed (gas-phase/weakly adsorbed NO2, Table 4). This band is far more intense for supports and for V/SZ than for V/MZ. Bands ascribed to 30


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monodentate nitrate and bridging nitrate species appeared. Over V/MZ, a band located at 1350 cm-1 appears, being of very low intensity over MZ and not visible for SZ and V/SZ. This band is assigned to the nitrito compounds species (Table 4). For the V containing catalysts, the intensities of the bands ascribed to NOx species are significantly decreased compared to over the zirconia (Figure 9(e)). This indicated that NOx adsorption species are mainly adsorbed on supports. When NH3 flow is introduced, the bands assigned to gas/weakly adsorbed NO2 (1630 cm-1), monodentate and bridging nitrates as well as nitrito compounds decreased dramatically over catalysts. New bands appears, located at: 1290 cm-1 ascribed to coordinated NH3 species, and 1450-1470 cm-1 ascribed to NH4+ species (Figure 9(b) for V/MZ), 1320 cm-1 deformation species of adsorbed NH3 (Figure 9(d) for V/SZ). Reaction mechanism: according to the literature about NH3-SCR of NO61-64 and results presented in this work, the mechanism of reaction can be depicted as presented in Scheme 1. The initial steps are consisting in the reactants (NOx, NH3) adsorption steps. Experimental results evidenced the formation of NH4+ species (band at 1452 cm-1 in DRIFT spectra, Figure 8) as well as coordinated NH3 species (band at 1215 cm-1, Figure 8), while capacity of NH3 storage of catalyst is confirmed by NH3-TPD experiment (Figure 6). The two following reactions can represent the initial steps of NH3 adsorption on V2O5 surface, Eqs. (5) and (6): V − O − NH4+ + e−

V − OH + NH3(g) V = O + NH3(g)

V − O − NH3

(5) (6)

Initial steps are also involving NOx adsorption. For V/SZ, the formation of NO2 (gas or weakly adsorbed) is supported by DRIFT experiments (bands at 1630 cm-1, Figure 8(d) and Figure 9(d)), and the detection of produced NO2(g) during catalytic test (Figure 7(d)). Then, and as evidenced by DRIFT experiments, nitrate species, monodentate and bridging nitrates, will forms on the surface, as represented in Eq. (7): NO(g) + O2(g)

Zr −□

Zr … NO2(gas ,

ads )

31



Page 32 of 42

O Zr

monodentate nitrate (

O2

N

O

O)

(7) Zr

O

bridging nitrate (Zr

O

N

O

)

The presence of nitrito compounds bands (1349 cm-1 in DRIFT spectra) and the low intensity peak for NO2(g) at 1625 cm-1 (Figure 8(b) and Figure 9(b)), in the case of V/MZ, suggests the formation of nitrite species. A possible formation can occurs as presented in Eqs. (8): NO(g)

Zr −□

Zr … NO(ads ) O O2

Zr − O − N − O nitrito

monodentate nitrate (

O2

Zr

Zr

O

bridging nitrate (Zr

O

O

N

N

O)

O

) (8)

Next step of the process consists in the surface reaction between adsorbed species of NH3 and NOx intermediates. This reaction induces the consumption of monodentate/bridging nitrate and of nitrito compounds species. Consequently, the reactions can be formulated as Eqs. (9), (10), (11), and (12): Zr

V − O − NH4+ +

Zr

O

O N

O

O

or

Zr

O

N O

V = O + Zr − □ + +NH4 NO3 (9)

V − O − NH4+ + Zr − O − N − O + e−

V = O + Zr − □ + NH4 NO2 (10)

Zr

V − O − NH3 +

Zr

O O

O

N

O

or

Zr

O

N

O + H2 O

V − OH + Zr − O +

NH4 NO3

(11)

V − O − NH3 + Zr − O − N − O

V − OH + Zr − O + NH2 NO (12)

The significant decrease in intensity of bands associated to gas phase/weakly adsorbed NO2 (at 1630 cm-1, Figure 9(c, d)) shows that NO2(g) is progressively 32


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consumed, with NH4NO3 as possible intermediate. Such intermediate is reported in the literature to be at the origin of N2/N2O/NO2 production,62 according to decomposition equations depicted in Eqs. (13), (14), (15), and (16). NH4 NO3

N2 O + 2H2 O

NH4 NO3 +NO

(13)

N2 + NO2 + 2H2 O

(14)

NH4 NO2

N2 + H2 O

(15)

NH2 NO

N2 + H2 O

(16)

Scheme 1. Proposed Reaction Pathway for NH3-SCR of NO over 3wt.%V2O5/ZrO2

Consequently, nitrito compound was formed on V/MZ compared to V/SZ, and NH3 adsorbed species could react with nitrito to form ammonium nitrite [Eqs. (10)] or nitrosamine [Eqs. (12)]. Both ammonium nitrite and nitrosamine were unstable and would decompose into N2 and H2O. This may be one of the reasons of higher activity for NO reduction over V/MZ catalysts. On the other hand, the tetragonal zirconia could leads to the formation of low-valent vanadium species (the higher ratio of V4+ on V/MZ than on V/SZ in Fig. 4a). The same findings were also reported by Shi.66

33



Thus, the V/MZ catalyst has higher V4+ surface content and more active B acid sites to promote the formation of ammonium nitrite may give some contribution to the improvement of catalytic activity. 4. Conclusion V2O5/ZrO2 catalysts were studied in the NH3-SCR of NO process. Supports of different morphologies (hollow sphere, star, rod, mesoporous) and structures (tetragonal, monoclinic) were synthesized using solvothermal routes and evaporation induced assembly. Morphology obtained induces significant differences in textural properties such as surface area and pore volume. Among the different properties of the catalysts, zirconia phase, surface area, V4+ surface contents and reducibility, active oxygen, as well as acid site density are the key factors affecting performances in NH3SCR. Consequently, when the support is of monoclinic structure, excellent activities are obtained with vanadium oxide dispersed over hollow sphere type support, this morphology allowing to develop high surface area and relatively high V4+ surface density. Better performances, in terms of NO conversion temperature and operating temperature window for a selective reduction to N2, are obtained using mesoporous zirconia having tetragonal-phase structure, intermediate surface area and high V4+ and adsorbed oxygen surface contents. Catalyst acidity contribute to the global performances, with contribution of weak acidity at low reaction temperature, while efficiency of SCR process is maintained at high temperature due to the presence of medium to strong acidity on the catalysts. On the basis of the results obtained, it is expected that NH3 adsorption will occurs on V4+-OH, leading to the NH4+ species reacting with N-species (nitrate and nitrito). For V/SZ, reaction on catalyst surface (nitrate) with ammonia-derived intermediates leads to the production of ammonium nitrate, which could after decompose to N2O (or NO2). However, the NH3 adsorption species on V/MZ react with nitrito compound to form NH4NO2 (ammonium nitrite) or NH2NO (nitrosamine). Ammonium nitrite (or nitrosamine) is relatively unstable compared with ammonium nitrate, and easy to decompose to N2, which greatly promote the SCR activity. 34


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ASSOCIATED CONTENT Supporting Information Small-angle XRD pattern, SEM images, HRTEM images for supports (PDF), and the long-period activity test. The supporting information file is available free of charge. AUTHOR INFORMATION Corresponding Author * R. Zhang, [email protected], Tel & Fax: 86-10-64419619, * S. Royer, [email protected], Tel. 03-20-43-69-54, Fax. 03-20-43-65-61

Funding Sources Financial supports of the National Natural Science Foundation of China (Nos. U1862102), the Fundamental Research Funds for the Central Universities (XK1802-1, JD1903). ACKNOWLEDGMENT Financial supports of the National Natural Science Foundation of China (Nos. U1862102), the Fundamental Research Funds for the Central Universities (XK1802-1, JD1903) are gratefully acknowledged. Chevreul Institute (FR 2638), Ministère de l’Enseignement Supérieur et de la Recherche et de l’Innovation, Région Hauts-deFrance and FEDER are acknowledged for supporting and funding partially this work. ABBREVIATIONS 35



HSZ, Hollowsphere zirconia; SZ, Star-like zirconia; MZ, Mesoporous zirconia; RZ, Rod-like zirconia; V/HSZ, 3wt.%V2O5 supported on hollowsphere zirconia; V/SZ, 3wt.%V2O5 supported on star-like zirconia; V/MZ, 3wt.%V2O5 supported on mesoporous zirconia; V/RZ, 3wt.%V2O5 supported on rod-like zirconia.

36


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Morphology oriented ZrO2 supported vanadium oxide for NH3-SCR

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