TiO2 Nanocatalysts for

Feb 16, 2017 - Facet–Activity Relationship of TiO2 in Fe2O3/TiO2 Nanocatalysts for Selective Catalytic Reduction of NO with NH3: In Situ DRIFTs and ...
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Facet−Activity Relationship of TiO2 in Fe2O3/TiO2 Nanocatalysts for Selective Catalytic Reduction of NO with NH3: In Situ DRIFTs and DFT Studies Jie Liu,† Jittima Meeprasert,‡ Supawadee Namuangruk,‡ Kaiwen Zha,† Hongrui Li,† Lei Huang,† Phornphimon Maitarad,† Liyi Shi,† and Dengsong Zhang*,† †

Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, China National Nanotechnology Center, NSTDA, 111 Thailand Science Park, Pahonyothin Road, Klong Luang, Pathum Thani 12120, Thailand

‡

S Supporting Information *

ABSTRACT: Anatase TiO2 nanosheets (TiO2-NS) and nanospindles (TiO2-NSP) have been successfully prepared with F− and glacial acetic acid as structure-directing agents, respectively. The Fe2O3/TiO2-NS and Fe2O3/TiO2-NSP nanocatalysts were prepared by a wet incipient impregnation method with a monolayer amount of Fe2O3. All the catalysts were employed for the selective catalytic reduction of NO with NH3 (NH3-SCR) in order to understand the morphologydependent effects. It is interesting that the Fe2O3/TiO2-NS nanocatalyst exhibited better removal efficiency of NOx in the temperature range of 100−450 °C, which was attributed to more oxygen defects and active oxygen, acid sites, as well as adsorbed nitrate species based on Raman spectra, XPS, NH3-TPD, NO+O2-TPD, and in situ DRIFTS. The density functional theory (DFT) method was used to clarify the NO and NH3 adsorption abilities over the catalyst models of Fe2O3/TiO2{001} and Fe2O3/TiO2{101}. The results showed that the NH3 adsorption energy over the TiO2{001} (−2.00 eV) was lower than that over TiO2{101} (−1.21 eV), and the NO adsorption energy over TiO2{001} (−1.62 eV) was also lower than that over TiO2{101} (−0.29 eV), which agreed well with the experimental results that Fe2O3/TiO2-NS achieved higher catalytic activity than Fe2O3/ TiO2-NSP for NH3-SCR of NO. In addition, the rapid electron transfer and regeneration of Fe3+ on the {001} facet of Fe2O3/ TiO2-NS also promoted the NH3-SCR reaction efficiency. This work paves a way for understanding the facet−activity relationship of Fe2O3/TiO2 nanocatalysts in the NH3-SCR reaction.

1. INTRODUCTION The selective catalytic reduction of NO with NH3 (NH3-SCR) is an efficient and economic technique for the removal of nitrogen oxides (NOx).1,2 Recently, the reaction processes and kinetics of NH3-SCR with transition metals catalyzed have been intensively studied by scientific researchers.3−5 The V-based catalysts, such as V2O5−WO3 (MoO3)/TiO2, are commonly used to eliminate NOx in stationary sources which exhibit an excellent catalytic activity and selectivity.6,7 Despite these achievements, the narrow operation temperature window, as well as the volatility and toxicity of VOx, inhibit its enormous application.8 Therefore, other environmentally friendly transition metal components such as Fe, Ce, Cu, and Mn instead of the vanadium species have recently attracted intense attention.9−15 In recent years, the facet-dependent catalysis has exerted a tremendous scientific and technological fascination for denitrification (De-NOx).16−21 Shen et al. prepared VOx/ TiO2 nanosheets and nanospindles used for NH3-SCR and found that the octahedral vanadia species on TiO2 nanosheets showed a significantly higher activity than that of TiO2 nanospindles.16 Shen et al. also prepared a kind of rod-shaped © XXXX American Chemical Society

Fe2O3 and achieved 80% NO conversion ranging from 200 to 400 °C with 98% N2 selectivity.17 Fan et al. prepared MnOx/ TiO2 nanosheets and nanoparticles used for NH3-SCR and found that MnOx/TiO2 nanosheets achieved higher NO conversion and N2 selectivity at 80−280 °C.18 In addition, we prepared MnOx/ZrO2−CeO2 nanorods, nanocubes, and nanopolyhedra used for NH3-SCR and found that the MnOx/ ZrO2−CeO2 nanorods achieved a significantly higher rate constant with respect to NO and higher NO conversion than that of nanocubes and nanopolyhedra.19 We also prepared Fe2O3/CeO2 nanorods and nanopolyhedra used for NH3-SCR and found that the Fe2O3/CeO2 nanorods achieved higher catalytic activity than that of Fe2O3/CeO2 nanopolyhedra, which was attributed to the adsorbed surface oxygen, oxygen defects, and atomic concentration of Fe on Fe2O3/CeO2 nanorods.20 In general, anatase TiO2(A-TiO2) is the most widely investigated polymorph among rutile, anatase, and brookite because of its high activity in catalyzing many chemical Received: November 7, 2016 Revised: February 16, 2017 Published: February 16, 2017 A

DOI: 10.1021/acs.jpcc.6b11175 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C reactions.18,21,22 A-TiO2 is surrounded by the crystal facets {001}, {100}, and {101} with the average surface energies of 0.90 J/m2, 0.53 J/m2, and 0.44 J/m2, respectively.22,23 It is generally recognized that the higher-surface-energy facet has much higher chemical activities. However, the A-TiO2 with {101} facet exposure is mostly reported because it contains the lowest surface energy with high chemical stability.24,25 Therefore, the selection of A-TiO2 with a highly active facet is one of the research directions to improve the catalytic performance in NH3-SCR reaction.16,21,25 This study is mainly focused on investigating the influence of {001} and {101} facets of A-TiO2 for the NH3-SCR reaction in order to understand the facet−activity relationship of Fe2O3/ TiO2 nanocatalysts in terms of surface coordination structure, physicochemical character, surface acidity, and electronic properties.

The X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAS-RB X-ray diffractometer, using Cu−Kα radiation operated at 40 kV and 40 mA. The Raman spectra were recorded on a spectrometer (JY H800UV) equipped with an optical microscope at room temperature. The X-ray photoelectron spectroscopy (XPS) experiments were carried out on an RBD upgraded PHI-5000C ESCA system with Mg Kα radiation. The binding energies were calibrated using the containment carbon (C 1s = 284.6 eV) as a reference. H2 temperature-programmed reduction (H2-TPR) was carried out on a Tianjin XQ tp5080 autoadsorption apparatus. In a typical run, 60 mg of catalyst was used and heated to 300 °C under N2 (30 mL/min) to remove any adsorbed species for 30 min with a ramp rate of 10 °C/min. After cooling to 25 °C, the catalyst was exposed to 5% H2/N2 (30 mL/min), and the temperature was subsequently raised from 25 to 900 °C with a ramp rate of 10 °C/min. NH3 temperature-programmed desorption (NH3-TPD) and NO+O2 temperature-programmed desorption (NOx-TPD) were carried out on a Tianjin XQ tp5080 autoadsorption apparatus. For NH3-TPD, 100 mg of catalyst was used and heated to 300 °C under He (30 mL/min) to remove any adsorbed species for 30 min with a ramp rate of 10 °C/min. After cooling to 100 °C, the catalyst was exposed to 500 ppm of NH3 (30 mL/min) for 1 h, followed by He purging for 0.5 h to remove physisorbed NH3, and the temperature was subsequently raised from 100 to 800 °C with a ramp rate of 10 °C/ min. For NOx-TPD, the adsorption process was carried out at 25 °C, and the catalyst was exposed to 500 ppm of NO+5% O2 for 1 h, followed by He purging for 0.5 h to remove physisorbed NOx, and the temperature was raised subsequently from 100 to 800 °C with a ramp rate of 10 °C/min. 2.3. In Situ DRIFTs. The in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTs) experiments were carried out on a Nicolet 6700 spectrometer equipped with a Harrick Scientific DRIFT cell and a mercury−cadmium− telluride (MCT) detector. Prior to every experiment, the catalyst was pretreated at 400 °C in a flow of gaseous N2 for 1.5 h and cooled to a target temperature. Background spectra were subtracted from the sample spectra. The DRIFTs were collected at 4 cm−1 resolution and accumulated 64 scans. 2.4. DFT Calculations. Based on our catalyst preparations and characterizations, nanosheet and nanospindle morphologies of A-TiO2 contain {001} and {101} as the dominant facets, respectively. The A-TiO2 supporting models with a neutral stoichiometric facet of {001} or {101} were created and setted as the supercell for periodic calculations. The Fe2O3 molecular group was created over the optimized two A-TiO2 facets. In the boundary conditions, a surface was represented by a thin slab, which was separated from its images in the direction perpendicular to the surface by a vacuum gap, in this case 15 Å. All calculations were performed using the DMol3 module of Material Studio Program package version 7. DFT of doublenumerical plus polarization function and GGA with PBE were used. The effective core potentials (ECPs) and spin polarizations were applied. The Brillouin zone of the Monkhorst− Pack grid was set at 2 × 2 × 1, and the cut-off radius was assigned at 4.2 Å. The SCF convergence energy was set as the default at 1 × 10−5 Ha. 2.5. Catalytic Activity Test. The NH3-SCR activity was tested in a fixed-bed quartz reactor (8 mm i.d.) using 400 mg of the catalysts (20−40 mesh). The feed gas mixture in a N2

2. EXPERIMENTAL SECTION 2.1. Materials Preparation. All the chemicals were supplied by the Sinopharm Chemical Regent Company, and they were used without further purification. The TiO2-NS was synthesized by a hydrothermal method with F− as a structure-directing agent.26,27 In a typical synthesis, 25 mL of Ti(OBu)4 was measured in a 100 mL glass beaker, and 4 mL (40%) of HF was added with stirring. Subsequently, the glass beaker was transferred to a 100 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 24 h. After the reaction, the autoclave was cooled to room temperature naturally. The product was treated with centrifugation and washed by deionized water several times until the pH of solution was neutral. The last sample was dried at 80 °C overnight. The TiO2-NSP was synthesized by a hydrothermal method with glacial acetic acid as a structure-directing agent.28 In a typical synthesis, 10 mL of Ti(OBu)4 was put into a 100 mL glass beaker, and 40 mL of glacial acetic acid was interfused with stirring. Then the glass beaker was transferred to a 100 mL Teflon-lined stainless steel autoclave and heated at 200 °C for 24 h. After the reaction, the autoclave was cooled to room temperature naturally. The product was treated with centrifugation and washed by deionized water for several times until the pH of solution was neutral. The last sample was dried at 80 °C overnight. The Fe2O3/TiO2 nanocatalysts were prepared by a wet incipient impregnation method. In a typical synthesis, 1.00 g of TiO2-NS or TiO2-NSP was mixed with a small amount of aqueous solution containing 0.2663 g of Fe(NO3)3·9H2O and stirred under an 80 °C water bath. The loading amount of Fe2O3 was 5 wt % calculated from the feeding amounts of various catalyst precursors. The prepared nanocatalysts were dried at 80 °C for 6 h and calcined at 450 °C for 2 h in a temperature-programmed muffle furnace. 2.2. Materials Characterizations. The morphology and structure of the nanocatalysts were characterized by transmission electron microscopy (TEM, JEOL JEM-200CX) and high-resolution transmission electron microscopy (HRTEM, JEOL LEM-2100F). The specific surface areas were calculated from the Brunauer−Emmett−Teller (BET) analysis of the nitrogen adsorption−desorption isotherm. Before measurement, the nanocatalysts were degassed by vacuum at 300 °C for 4 h. B

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Figure 1. (a) NOx conversion versus reaction temperature over the TiO2-NS, TiO2-NSP, Fe2O3/TiO2-NS, and Fe2O3/TiO2-NSP nanocatalysts. (b) Arrhenius plots of NO oxidation over Fe2O3/TiO2-NS and Fe2O3/TiO2-NSP nanocatalysts. (c) TOF profiles as a function of temperature over Fe2O3/TiO2-NS and Fe2O3/TiO2-NSP nanocatalysts. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 3 vol %, N2 balance, and GHSV = 25 000 h−1.

where P is the standard atmospheric pressure (1.01 × 105 Pa); ν is the flow rate of NO (0.13 mL/min); R is the proportional constant (8.314 J/mol·K); T is the temperature (K); XNO is the NO conversion of the catalysts (%); mcat is the mass of the catalysts (0.4 g); βFe is the Fe2O3 loading calculated from the XPS results (%); and MFe is the molar mass of Fe (55.85 g/ mol).

stream contained 500 ppm of NO, 500 ppm of NH3, and 3 vol % O2. The total flow rate of feed gases was 260 mL/min with the gas hourly space velocity (GHSV) of 25 000 h−1. The reaction temperature was selected from 100 to 450 °C. The concentration of NOx in the inlet and outlet gases was measured continuously by a 4000 VM analyzer. NO x conversion was calculated according to the following equation NOx conversion (%) =

[NOx ]in − [NOx ]out 100% [NOx ]in

3. RESULTS AND DISCUSSION 3.1. Catalytic Performance. The NH3-SCR reactions over TiO2-NS, TiO2-NSP, Fe2O3/TiO2-NS, and Fe2O3/TiO2-NSP nanocatalysts were performed during 100−450 °C (Figure 1a). The TiO2-NS nanocatalysts showed a higher catalytic activity for a whole temperature window than the TiO2-NSP. Furthermore, in pairwise comparison of Fe2O3/TiO2-NS and Fe2O3/TiO2-NSP nanocatalysts, the Fe2O3/TiO2-NS nanocatalyst greatly promoted the NOx conversions in the temperature range of 100−450 °C. It is noted that the Fe2O3/TiO2-NS and Fe2O3/TiO2-NSP nanocatalysts present high N2 selectivity (Figure S1) and high water resistance (Figure S2). In order to study the reaction rate of NO and clearly elucidate the catalytic capacity of each catalytic active site over Fe2O3/TiO2-NS and Fe2O3/TiO2-NSP, the Arrhenius plots and the NOx turnover frequency (TOF) over the nanocatalysts were shown in Figure 1b and 1c. As we know, the adsorption of reaction gas mainly occurred on the catalyst carrier and then transferred to the reaction site. After the addition of iron, the reaction activity was increased obviously. This proves that iron species were active sites. The Fe2O3 was loaded onto the surface of titanium dioxide using the method of impregnation. The iron species is highly dispersed. In this sense, it could reflect the accuracy of iron species using the Fe% calculated from the XPS results. The slope of the plot determined the activation energy for NO reaction. The activation energy (11.1 kJ/mol) of Fe2O3/TiO2-NS was lower than that of Fe2O3/ TiO2-NSP (15.7 kJ/mol). Meanwhile, the TOF of NOx over Fe2O3/TiO2-NS nanocatalyst was higher than that of Fe2O3/ TiO2-NSP. These results further revealed that the active sites on the Fe2O3/TiO2-NS nanocatalyst were more efficient in NH3-SCR reaction, which could be attributed to more charge around the Fe elements and rapid regeneration of Fe3+ over Fe2O3/TiO2-NS nanocatalyst. 3.2. Characteristics of Nanocatalysts. The TEM and HR-TEM images of the TiO2-NS and TiO2-NSP were shown in Figure 2. All the catalysts had been calcined at 450 °C for 2 h

N2 selectivity (%) ⎛ ⎞ 2[N2O]out = ⎜1 − ⎟100% [NOx ]in + [NH3]in − [NOx ]out − [NH3]out ⎠ ⎝

where the [NOx]in, [NOx]out, [N2O]out, and [NH3]in indicate the inlet and outlet concentration at the steady state, respectively. The expression of the GHSV was followed to qv GHSV = πhr 2 where the qv represents the total flow rate; h corresponds to the height of the catalyst in the reactor; and r means the inner radius of the reactor. By assuming that the reaction components were free of diffusion limitations, the NO reaction rates normalized by the specific surface area of the catalyst can be calculated according to the following equation rate (mmol/m 2·h) =

XNOQCf VmWSBET

where XNO is the NO → N2 conversion; Q is the volumetric flow rate (mL/h); and Cf is the feeding concentration of NO (500 ppm). Vm is the molar volume of gas (22.4 mL/mmol); W is the catalyst weight (g); and SBET is the specific area of the catalyst (m2/g). The Ea for SCR reaction was calculated based on the slope of the NO reaction rates. The relative turnover frequency (TOF) value was conducted to compare the activities of the different catalysts. The relative TOF (s−1) of NO over each Fe atom at different temperature was calculated by the following equation TOF =

(Pν /RT )XNO mcat βFe /MFe C

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catalysts, which confirmed that the iron oxide species are highly dispersed on the facet of TiO2-NS and TiO2-NSP. The EDS mapping images of Fe2O3/TiO2-NS and Fe2O3/TiO2-NSP nanocatalysts also proved that the iron species are highly dispersed on the catalyst surface (Figure S3). Further, all catalysts were subjected to the N2 adsorption− desorption measurements as depicted in Figure 3b. The corresponding results, including specific surface area, pore size, and pore volume, were summarized in Table 1. It could be

Figure 2. (a, b, c) TEM and HRTEM images of TiO2-NS nanocatalysts; (d) TEM images of Fe2O3/TiO2-NS nanocatalysts; (e, f, g) TEM and HRTEM images of TiO2-NSP nanocatalysts; (h) TEM images of Fe2O3/TiO2-NSP nanocatalysts.

Table 1. Physical Properties of Various Nanocatalysts

before using the electron microscope. The TiO2-NS exhibited a uniform sheet-like morphology with about 50 nm in length. The lattice fringes of the main exposure surfaces with spacing of 0.19 nm as viewed from the top side corresponded to the {001} facet. The TiO2-NSP possessed a homogeneous spindle-like structure with the average 50 nm in length. The lattice fringes of the main exposure surfaces with spacing of 0.36 nm were observed, corresponding to the {101} facet. The XRD patterns of TiO2-NS, TiO2-NSP, Fe2O3/TiO2-NS, and Fe2O3/TiO2-NSP nanocrystals were shown in Figure 3a.

catalysts

surface area (m2 g−1)

pore volume (cm3 g−1)

pore size (nm)

Fe2O3/TiO2-NS Fe2O3/TiO2-NSP TiO2-NS TiO2-NSP

66.9 89.6 60.6 83.3

0.29 0.27 0.30 0.26

3.8 9.6 1.9 12.4

noted that the surface area of TiO2-NS (60.6 m2/g) was lower than that of TiO2-NSP (83.3 m2/g). In addition, when loading Fe2O3 on the A-TiO2 nanocrystals, the surface areas were increased to some extent. This was attributed to that the surface roughness of the catalysts was increased after the introduction of Fe2O3 onto TiO2. However, the average pore diameters of Fe2O3/TiO2 were decreased, which agreed well with the previous reports.29 The Raman spectra were applied to characterize the vibration of molecular bonds of TiO2-NS, TiO2-NSP, Fe2O3/TiO2-NS, and Fe2O3/TiO2-NSP (Figure 4). The peaks at the 145 cm−1,

Figure 4. Raman spectra of TiO2-NS, TiO2-NSP, Fe2O3/TiO2-NS, and Fe2O3/TiO2-NSP nanocatalysts.

395 cm−1, 516 cm−1, and 639 cm−1 suggested the presence of A-TiO2 in the four catalysts. The Eg peaks represented symmetric stretching vibration of O−Ti−O. The B1g peak was referred to symmetric bending vibration of O−Ti−O, and the antisymmetric bending vibration of O−Ti−O was represented by the A1g peak.30 Generally, the binding mode on the {101} facet of A-TiO2 was mainly unsaturated 5c-Ti and 2c-O as well as saturated 6c-Ti and 3c-O; however, only unsaturated 5c-Ti and 2c-O were involved in the highly active {001} facet of A-TiO2. It indicated that there was a lot of oxygen defect on the {001} facet of A-TiO2. Besides, TiO2-NS presented a smaller intensity rate of Eg/A1g than TiO2-NSP, which indicated the reduced symmetric stretching vibration modes of O−Ti−O, as well as the intensified symmetric bending vibration and the antisymmetric bending vibration of O−Ti−O on the surfaces of TiO2-NS. It could be concluded that a higher proportion of the exposed {001} facet exists on

Figure 3. (a) XRD patterns of TiO2-NS, TiO2-NSP, Fe2O3/TiO2-NS, and Fe2O3/TiO2-NSP nanocatalysts. (b) N2 adsorption−desorption isotherms of TiO2-NS, TiO2-NSP, Fe2O3/TiO2-NS, and Fe2O3/TiO2NSP nanocatalysts.

The characteristic peaks in the XRD patterns mainly corresponded to A-TiO2 (JCPDs: 21-1272). It is worth mentioning that the intensity ratio of I(400)/I(200) of the TiO2-NS pattern was weaker than that of TiO2-NSP. Further, it was visually observable that the full width at half-maximum of the (400) diffraction peaks in the pattern of TiO2-NS was broader than that of TiO2-NSP. These results apparently elucidated that the TiO2-NS and TiO2-NSP grew mainly along (010) and (001) directions, respectively. Besides, it should be noted that there were not obvious characteristic peaks of iron oxide species for both Fe2O3/TiO2-NS and Fe2O3/TiO2-NSP D

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with a hydroxyl group. However, in the Fe 2p spectra of Fe2O3/ TiO2-NS, the binding energy lower shifted to 709.9, 711.8, and 714.2 eV, correspondingly.34−36 This meant that the electrons were more biased toward Fe3+ in the Fe2O3/TiO2-NS. At the same time, combined with Ti 2p spectra, the binding energies at 459.6 and 465 eV were ascribed to Ti4+ 2p, indicating that the titanium was mainly presented as Ti4+.16,37 The Ti 2p binding energy of Fe2O3/TiO2-NS had slightly shifted to higher binding energy at 459.8 and 465.2 eV compared with that in Fe2O3/TiO2-NSP. It meant that the electrons of Ti4+ migrated away from Ti4+ in the Fe2O3/TiO2-NS. The above results suggested that Fe−Ti existed in both Fe2O3/TiO2-NS and Fe2O3/TiO2-NSP, but the electron redistribution between Fe3+ species and Ti 4+ was a little different. The electron redistribution was also confirmed by DFT calculations (Figure S5). Electron density difference plot showed that electron density was depleted from the TiO2 surface and accumulated on te Fe2O3 fragment upon Fe2O3 adsorption. Charge distribution on atoms of the two systems revealed that Fe and O atoms of the Fe2O3/TiO2{001} catalyst tended to have more charges than those of Fe2O3/TiO2{101} indicating higher catalytic activity of Fe2O3/TiO2{001} which confirmed the experimental observation. In conclusion, there was more charge around the Fe element, which was beneficial to the regeneration of Fe3+ and thus promoted NH3-SCR reaction. The redox properties of the catalysts are crucial to the NH3SCR reaction. Therefore, we applied H2-TPR to understand the redox process of Fe2O3/TiO2 nanocatalysts with exposing different facets in the NH3-SCR reaction as shown in Figure 6.

the surface of TiO2-NS, while a higher proportion of the exposed {101} facet exists in TiO2-NSP. The Eg peaks became higher, extraordinary to the TiO2-NS after loading Fe on the surfaces of A-TiO2, which indicated Fe species doped into the surface lattices of TiO2-NS and made some changes on the surface element composition. The O 1s, Ti 2p, and Fe 2p photoelectron spectra of the Fe2O3/TiO2 serial nanocatalysts were shown in Figure 5. The

Figure 5. (a) O 1s spectra of XPS for Fe2O3/TiO2-NS and Fe2O3/ TiO2-NSP nanocatalysts. (b) Ti 2p spectra of XPS for Fe2O3/TiO2-NS and Fe2O3/TiO2-NSP nanocatalysts. (c) Fe 2p spectra of XPS for Fe2O3/TiO2-NS and Fe2O3/TiO2-NSP nanocatalysts.

XPS peak deconvolutions of C, Ti, O, and Fe species were derived as listed in Table 2. The F 1s peak at 684.5 eV corresponding to fluorine was not observed in the full scan XPS spectra of TiO2-NS and TiO2-NSP (Figure S4), which indicated that F ions had been completely removed. The high-resolution O 1s spectra of all catalysts could be deconvoluted into two peaks located at about 530.9 and 532.6 eV, corresponding to the lattice oxygen O2− of metal oxides (denoted as Oβ) and the surface-adsorbed oxygen (denoted as Oα), such as defect oxide including O22− or O−.13 In general, Oα was highly active in oxidation reactions due to its faster mobility than Oβ, which was beneficial to promote the oxidation of NO to NO2 and result in the “fast SCR” process.31−33 Further, it was observed that the ratio Oα/(Oα + Oβ) of Fe2O3/TiO2-NS (51.74%) was higher than that of Fe2O3/TiO2-NSP (44.42%), indicating more active oxygen on Fe2O3/TiO2-NS. This could be attributed to the structural differentiation of TiO2 supports, which implied that there was more active oxygen on Fe2O3/TiO2-NS. In the Fe 2p spectra, two characteristic peaks of Fe3+ 2p3/2 and Fe3+ 2p1/2 at around 710 and 725 eV appeared in both Fe2O3/TiO2-NS and Fe2O3/TiO2-NSP. It indicated that the Fe3+ was the main existing form of Fe species in the two nanocatalysts. The relative contents of Fe3+ species of the Fe2O3/TiO2-NS and Fe2O3/TiO2-NSP were 0.44% and 0.35%, respectively, which could be attributed to the smaller specific surface areas of Fe2O3/TiO2-NS. Furthermore, for Fe 2p spectra of Fe2O3/TiO2-NSP, the binding energy at 711.2 and 712.7 eV might be ascribed to Fe3+ in the spinel structure, and the binding energy at 714.5 eV may be ascribed to Fe3+ bonded

Figure 6. H2-TPR profiles of TiO2-NS, TiO2-NSP, Fe2O3/TiO2-NS, and Fe2O3/TiO2-NSP nanocatalysts.

For the A-TiO2 supports, the specific reduction peak of the TiO2-NS and TiO2-NSP was 604 and 651 °C, respectively, suggesting that the sheet-like A-TiO2 was prone to be reduced compared to the spindle-like one.38 After loading Fe species in the different morphologies of A-TiO2 supports, the main reduction of adsorbed oxygen on the surfaces of the catalysts could be the following: Fe2O3 → Fe3O4, Fe3O4 → FeO, FeO → Fe, and titania supports. These reduction steps on the Fe2O3/ TiO2-NS corresponded to the peaks at 279 °C, 374 °C, 548 °C, and 661 °C, while the Fe2O3/TiO2-NSP resulted in the peaks at

Table 2. Surface (XPS) Compositions of Various Nanocatalysts surface atomic concentration/%

relative concentration ratios/%

catalysts

C

Ti

O

Fe

Oα/(Oα + Oβ)

Oβ/(Oα + Oβ)

Fe2O3/TiO2-NSP Fe2O3/TiO2-NS

59.90 53.99

7.72 10.14

32.03 35.43

0.35 0.44

44.42 51.74

55.58 48.26

E

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The Journal of Physical Chemistry C 336 °C, 454 °C, 571 °C, and 657 °C.9,34,39 The peak positions of these Fe species on the Fe2O3/TiO2-NSP obviously shifted to lower temperatures as compared with the Fe2O3/TiO2-NSP. It cound be demonstrated that there were stronger interactions between Fe species and A-TiO2 on Fe2O3/TiO2-NS than that on Fe2O3/TiO2-NSP, which promoted the reduction of Fe species. 3.3. In Situ DRIFTs. 3.3.1. Adsorption Behavior of NH3 and NO + O2. The in situ DRIFT spectra of NH3 desorption on Fe2O3/TiO2-NS versus temperatures were shown in Figure 7a.

NH3 species were particularly difficult to decompose from the surfaces of Fe2O3/TiO2NSP, which could affect the subsequent catalytic reactions. The NH3-TPD measurements were used to evidence the adsorption behavior of NH3 on the catalysts as shown in Figure 7c, and the TiO2-NS had a fairly wide desorption window between 150 and 650 °C. However, TiO2-NSP only had one strong desorption peak at around 326 °C, confirming the superior ammonia storage capacity onto the surfaces of TiO2NS. After loading Fe2O3, the desorption peak of ammonia was produced at 213 and 331 °C for Fe2O3/TiO2-NSP and at 222 and 415 °C for Fe2O3/TiO2-NS. The bands below 250 °C were attributed to weakly adsorbed ammonia. Other peaks between 250 and 450 °C could be elaborated by the moderately strong acid sites, and the peaks above 450 °C could be ascribed to the strong acid sites.46 Furthermore, due to the moderately strong and strong acid sites in the TiO2-NS and Fe2O3/TiO2-NS, they displayed stronger adsorption properties of ammonia. On the basis of previous reports, the absorption of NO + O2 onto the catalyst also plays a significant role in the SCR reaction.31 In addition, the in situ DRIFT spectra of NO + O2 desorption from Fe2O3/TiO2-NS and Fe2O3/TiO2-NSP nanocatalysts with temperature were studied as shown in Figure 8a and Figure 8b, respectively. The NO + O2 absorption of Fe2O3/TiO2-NS was at 30 °C, and the bands at 1204 cm−1, 1245 cm−1, 1568 cm−1, 1581 cm−1, and 1602 cm−1 were assigned to monodentate nitrite (1204 and 1245 cm−1), bidentate nitrate, monodentate nitrate, and gaseous NO2, respectively.42,47,48 After thermal treatment, the monodentate nitrite and gaseous NO2 quickly disappeared, while the monodentate nitrite, bidentate nitrate, and monodentate nitrate showed a regular decrease until they almost completely vanished at 400 °C. The oxidation of nitric oxide to nitrogen dioxide was an important step in the NH3-SCR reaction, and the adsorbed gaseous NO2 obviously promoted the lowtemperature DeNOx.31 At the same time, the adsorbed bidentate- and monodentate-nitrate were frequently used as reactive species in high-temperature NH3-SCR reaction. For Fe2O3/TiO2-NSP, the NO + O2 absorption was at 30 °C, and there are only two bands containing monodentate nitrite and gaseous NO2 at 1188 and 1621 cm−1, respectively. The single nature of the adsorbed nitrogen oxide species reduced the transmission diversity of electrons in catalytic reactions, which was not conducive to the NH3-SCR reaction. The NO + O2-TPD was illustrated in Figure 8c. The TiO2NS possessed two desorption peaks at around 75 and 177 °C; meanwhile, two desorption peaks at around 103 and 221 °C were presented by the TiO2-NSP. After loading Fe2O3, a fairly strong desorption peak at 91 °C was demonstrated for Fe2O3/ TiO2-NS, while at 199 °C for Fe2O3/TiO2-NSP.49 The peaks below 100 °C were attributed to the decomposition of surface nitrite desorption, while the decomposition of nitrite and nitrate species accounted for the peaks between 150 and 300 °C. Obviously, the TiO2-NS and Fe2O3/TiO2-NS showed easier desorption of nitrite and nitrate species than TiO2-NSP and Fe2O3/TiO2-NSP, which agreed with the results of in situ DRIFT spectra on NO + O2 desorption. 3.3.2. Transient Reaction. The in situ DRIFT spectra of the transient reaction between NO + O2 and preadsorbed NH3 on Fe2O3/TiO2-NS were shown in Figure 9a. After the adsorption of NH3 at 250 °C, the bands at 1194 and 1230 cm−1 were assigned to the symmetric bending vibrations of N−H bonds, and the band at 1607 cm−1 corresponded to the asymmetric

Figure 7. (a) In situ DRIFT spectra of NH3 desorption on the Fe2O3/ TiO2-NS nanocatalyst. (b) In situ DRIFT spectra of NH3 desorption on the Fe2O3/TiO2-NSP nanocatalyst. (c) NH3-TPD profiles of TiO2NS, TiO2-NSP, Fe2O3/TiO2-NS, and Fe2O3/TiO2-NSP nanocatalysts.

NH3 adsorption on Fe2O3/TiO2-NS was tested at 30 °C, and the bands at 1167 and 1603 cm−1 were assigned to the symmetric and asymmetric bending vibrations of N−H bonds in NH3-coordinated bonding to Lewis acid sites. For Fe2O3/ TiO2-NSP (Figure 7b), the bands at 1163 and 1599 cm−1 were also assigned to the symmetric and the asymmetric bending vibrations of N−H bonds in NH3-coordinated bonding to Lewis acid sites.40−45 After heating nanocatalysts, the intensity of two bands at 1167 and 1603 cm−1 of Fe2O3/TiO2-NS showed a regular decrease until they almost completely disappeared at 300 °C. However, for Fe2O3/TiO2-NSP, the intensity of two bands decreased slowly and could not completely disappear until at 400 °C. It indicates that the F

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One min later, the trans-(NO)2 signal was quickly detected at 1715 cm−1. This procedure revealed that the Langmuir− Hinshelwood reaction mechanism was followed in the catalytic reaction of Fe2O3/TiO2-NSP nanocatalyst.31 The in situ DRIFT spectra of NO + O2 on Fe2O3/TiO2-NS were also probed, as shown in Figure 9c. After the adsorption of NO + O2 at 250 °C, the band at 1560 cm−1 was assigned to one of the split ν3 vibrations of bidentate nitrates. After NH3 was introduced, the unique band corresponding to adsorbed bidentate nitrates species diminished over time, and then the bands of NH3 species were presented including 1194, 1230, and 1580 cm−1, corresponding to coordinated NH3 (at 1194 cm−1, 1230 cm−1), and the deformation or oxidation species of adsorbed NH3 (at 1580 cm−1). The in situ DRIFT spetra of NO + O2 on Fe2O3/TiO2-NSP were shown in Figure 9d. After the adsorption of NO + O2 at 250 °C, the band at 1354 cm−1, 1560 cm−1, and 1715 cm−1 was formed and assigned to the ν3 stretch vibration of monodentate nitrite, one of the split ν3 vibrations of bidentate nitrates, and trans-(NO)2. After NH3 was imported, the bands corresponding to monodentate nitrite and adsorbed bidentate nitrates species quickly diminished over time, and then the bands of NH3 species displaying at 1187 and 1578 cm−1 corresponded to coordinated NH3 and the deformation or oxidation species of adsorbed NH3, respectively. It was worth noting that the adsorbed trans-(NO)2 on the surfaces of Fe2O3/TiO2-NSP still presented after NH3 was introduced for 20 min, and a large number of active sites were covered by the inactive trans-(NO)2. Then the reactive species, including monodentate nitrite and bidentate nitrates species, were hard to be adsorbed on the Fe2O3/TiO2-NSP surfaces. It definitely affected the efficiency of the reaction. This may be one of the reasons that the Fe2O3/TiO2-NSP had a poor reaction efficiency compared to Fe2O3/TiO2-NS. 3.4. DFT Calculations. The NO and NH3 adsorption abilities over the catalyst models of Fe2O3/TiO2{001} and Fe2O3/TiO2{101} were theoretically studied using the DFT calculations. On the basis of theoretical investigation in terms of NO and NH3 adsorption energy over the surface models of the catalysts was calculated. The adsorption energy (Ed) is defined as Figure 8. (a) In situ DRIFT spectra of NO + O2 desorption on the Fe2O3/TiO2-NS nanocatalyst. (b) In situ DRIFT spectra of NO + O2 desorption on Fe2O3/TiO2-NSP nanocatalyst. (c) NO + O2-TPD profiles of TiO2-NS, TiO2-NSP, Fe2O3/TiO2-NS, and Fe2O3/TiO2NSP nanocatalysts.

Ed = Esurface + gas − Esurface − Egas

where Esurface+gas is the total energy of the complex system, and Esurface and Egas are the total energies of the surface and gas, respectively. Therefore, the negative Ed value means more attractive adsorption energy, whereas the positive value means more repulsive adsorption energy. The doping groups of Fe2O3 over A-TiO2{001} and ATiO2{101} resulted in −26.21 eV and −26.76 eV, respectively, in the pairwise relative energy comparison with the pure TiO2{001} and TiO2{101}, respectively. Thus, the Fe2O3 molecular groups over those A-TiO2 facets showed a similar stability energy. For the optimized structures of NO and NH3, adsorption over the Fe2O3/TiO2{001} and Fe2O3/TiO2{101} model catalysts has been shown in Figure 10. As is known, the NO was favorable to adsorbing over the surface oxygen in the form of the NOO*, where O* is a topmost surface oxygen, while the NH3 molecule would be more favorable to adsorbing on the metal Lewis site. The NO adsorption energies over TiO2{001} and TiO2{101} are found to be −1.62 eV and −0.29 eV, respectively. The NH3 adsorption energies are −2.00 eV and −1.21 eV for that adsorbed over Fe2O3/TiO2{001} and Fe2O3/TiO2{101} model systems, respectively. The NH3

bending vibrations of N−H bonds.43,48 These results suggested that coordinated NH3 was the primary existing form in the NH3 adsorption on Fe2O3/TiO2-NS. After NO + O2 was introduced, these bands corresponding to adsorbed NH3 species gradually diminished over time. After 15 min, the NH3 species were completely depleted, and the signal of gaseous NO2 appeared at 1630 cm−1 subsequently. This process reflected the catalytic reaction on the surfaces of Fe2O3/TiO2-NS nanocatalyst followed by the Eley−Rideal reaction mechanism.50 As the control, the in situ DRIFT spectra of NH3 on Fe2O3/TiO2-NSP was shown in Figure 9b. After the adsorption of NH3 at 250 °C, the band at 1181 cm−1 represented the symmetric bending vibrations of N−H bonds. The NH3 adsorbed on the surfaces of Fe2O3/TiO2-NSP nanocatalyst existed mainly in the form of coordinated NH3. However, after the introduction of NO + O2, these bands corresponding to adsorbed NH3 species at 1181 cm−1 barely diminished over time but shifted to 1190 cm−1. G

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Figure 9. (a) and (b) In situ DRIFT spectra of the transient reactions at 250 °C between NO + O2 species and preadsorbed NH3 species over Fe2O3/TiO2-NS and Fe2O3/TiO2-NSP nanocatalysts as a function of time, respectively. (c) and (d) In situ DRIFT spectra of the transient reactions at 250 °C between NH3 and preadsorbed NO + O2 species over Fe2O3/TiO2-NS and Fe2O3/TiO2-NSP nanocatalysts as a function of time, respectively.

supported the experimental TPD and in situ DRIFTs results well. 3.5. Promoted Reaction Mechanism. Based on the results and discussion mentioned above, the possible reaction mechanism of Fe2O3/TiO2-NS and Fe2O3/TiO2-NSP nanocatalysts for the NH3-SCR reaction was proposed in Scheme 1. We had demonstrated that the Fe2O3/TiO2-NS and Fe2O3/ TiO2-NSP were mainly exposing {001} and {101} planes by HRTEM, XRD, and Raman spectra analyses, respectively. In addition, the Raman spectra and XPS results also indicated that there were more oxygen defects and active oxygen on Fe2O3/ TiO2-NS, which were crucial factors to promote the NH3-SCR reaction. The DFT calculations clarified the NO and NH3 adsorption abilities over the catalyst models of Fe2O3/ TiO2{001} and Fe2O3/TiO2{101}. The results showed that the NH3 adsorption energy over the TiO2{001} (−2.00 eV) was lower than that over TiO2{101} (−1.21 eV), and the NO adsorption energy over TiO2{001} (−1.62 eV) was also lower than that over TiO2{101} (−0.29 eV). Moreover, the adsorption species monitoring showed that the surfaces of nanocatalysts adsorbed with similar coordinated NH3 species bonding to Lewis acid sites. Furthermore, there were more

Figure 10. NO and NH3 adsorption energy (eV) over the Fe2O3/ TiO2{001} and Fe2O3/TiO2{101} nanocatalyst surface models.

adsorption energies are found to have the same trend as those NO adsorption energies. These obtained calculations strongly

Scheme 1. Possible Reaction Mechanism of Fe2O3/TiO2-NS and Fe2O3/TiO2-NSP Nanocatalysts

H

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moderately strong and strong acid sites in the Fe2O3/TiO2-NS, which displayed stronger adsorption properties of ammonia. Meanwhile, there were more abundant nitrogen oxide species adsorbed on the surfaces of Fe2O3/TiO2-NS, such as monodentate nitrite, bidentate nitrate, monodentate nitrate, and gaseous NO2, which enriched the transmission diversity of electrons in the NH3-SCR reaction. Meanwhile, the transient reaction studies indicated that a large number of active sites were covered by the inactive trans-(NO)2, which inhibited the occurrence of the NH3-SCR reaction seriously.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b11175. N2 selectivity over the Fe2O3/TiO2-NS and Fe2O3/TiO2NSP nanocatalysts; NOx conversion on stream in the presence of 8 vol % H2O; STEM and mapping images of Fe2O3/TiO2-NSP and Fe2O3/TiO2-NS nanocatalysts; full scan XPS spectra of TiO2-NS and TiO2-NSP; charge on atoms and electron density difference plot of Fe2O3/ TiO2{001} and Fe2O3/TiO2{101} (PDF)

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REFERENCES

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4. CONCLUSIONS In this study, Fe2O3/TiO2-NS or Fe2O3/TiO2-NSP nanocatalysts were successfully prepared for selective catalytic reduction of NO with NH3. We found that the morphology and the Fe species had a significant impact on NH3-SCR reaction, and the Fe2O3/TiO2-NS nanocatalyst showed more excellent catalytic activity than Fe2O3/TiO2-NSP. It was attributed to more vacancy defects, active oxygen, acid sites, and easier desorption of nitrite and nitrate species on the {001} facet of Fe2O3/TiO2-NS. The experimental results revealed that desorbed gaseous NO2 on the surfaces of Fe2O3/TiO2-NS obviously promoted the low-temperature DeNOx process, and the adsorbed nitrates species on the surfaces of Fe2O3/TiO2-NS were used as reactive species in high-temperature NH3-SCR reaction. In addition, Fe species possessed stronger interaction with A-TiO2 on the surfaces of Fe2O3/TiO2-NS nanocatalyst than Fe2O3/TiO2-NSP. The rapid electron transfer and regeneration of Fe3+ on the surfaces of Fe2O3/TiO2-NS also promoted the NH3-SCR reaction efficiency. It would pave the way for understanding the facet−activity relationship of Fe2O3/ TiO2 nanocatalysts in the NH3-SCR reaction.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-21-66137152. E-mail: [email protected]. ORCID

Dengsong Zhang: 0000-0003-4280-0068 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support of the National Natural Science Foundation of China (U1462110). All calculations were done at Nanoscale simulation laboratory, National Nanotechnology Center (NANOTEC), Thailand. P.M. would like to thank the National Natural Science Foundation of China (NSFC) Research Fund for International Young Scientists FY 2016 (21650110450). I

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