Catalytic Performance, Characterization, and Mechanism Study of Fe2

ARTICLE pubs.acs.org/JPCC

Catalytic Performance, Characterization, and Mechanism Study of Fe2(SO4)3/TiO2 Catalyst for Selective Catalytic Reduction of NOx by Ammonia Lei Ma,† Junhua Li,*,†,‡ Rui Ke,† and Lixin Fu†,‡ † ‡

Department of Environmental Science and Engineering and State Key Joint Laboratory of Environment Simulation and Pollution Control, Tsinghua University, Beijing 100084, China ABSTRACT: Iron oxides supported on TiO2 catalysts show good activity in selective catalytic reduction (SCR) of NOx by ammonia. In this work, Fe2(SO4)3/TiO2 and other catalysts containing iron and sulfates were prepared by the impregnation method, and their activities for SCR of NOx by ammonia were investigated. On Fe2(SO4)3/TiO2 catalyst, NOx conversion reached 98.0% in the temperature range of 350
450 °C, while yielding little N2O. Characterization results showed that R-Fe2O3 and sulfates were mainly formed on the Fe2(SO4)3/ TiO2 catalyst. Iron oxide catalysts with sulfates [e.g., Fe2(SO4)3/TiO2] caused more dispersed iron phase, and the sulfation effect might have mostly occurred on iron oxides but not TiO2. In contrast, Fe(NO3)3 precursor supported on TiO2 without sulfates led to the Fe2O3 particle products, which physically connected to the surface of TiO2 and resulted in some agglomerations or heterogeneous distribution of Fe2O3 particles. In addition, sulfates played an important role in the SCR reactions, which were responsible for strong Br€onsted acid sites. The SCR reaction mechanism on Fe2(SO4)3/TiO2 might have taken place as follows: NH3 was adsorbed onto the surface of Fe2(SO4)3/TiO2 and mainly formed NH4þ on Br€onsted acid sites. NO was oxidized to NO2 by the oxygen in the reaction gas or on the iron oxide sites of the catalyst. Then, nitrates originated from NO2 were generated on Fe2(SO4)3/TiO2 catalyst for NH3
SCR reaction. Finally, an active intermediate species like ammonium nitrite was formed, which eventually decomposed to gaseous nitrogen.

1. INTRODUCTION Nitrogen oxides (NO, NO2) in the flue gas from combustion of fossil fuels are a major cause for photochemical smog, acid rain, and ozone depletion. Selective catalytic reduction (SCR) of NOx by NH3 is the most efficient technology for the removal of nitrogen oxides from the stationary sources.1
3 Although the SCR technology based on V2O5
WO3/TiO2 catalysts has been commercialized,4
7 some problems still exist, such as high activity for the oxidation of SO2 to SO3,8 formation of N2O at high temperatures (HTs),9 and toxicity of vanadium.10 Hence, continuing efforts are being made to develop new catalysts in both the academic and the industrial fields. Iron-based catalysts have attracted attention in the SCR of NOx by NH3. They are particularly attractive due to their high activity, low cost, and lack of toxicity as compared to V2O5/TiO2 catalysts. It has been reported that SCR activities on Fe2O3pillared clays were even higher than those obtained on the commercial V2O5
WO3/TiO2 catalyst.11,12 Iron supported on zeolite catalysts are promising alternative SCR catalysts for practical application, such as Fe-exchanged zeolite10,13
15 and Fe-containing oxide.16
18 Especially, Fe-ZSM-5 was studied extensively in the past decade and recently reviewed by Brandenberger et al.19 Besides iron-based catalysts, a large number of other catalysts, such as sulfates, were reported to be active for SCR reaction. r 2011 American Chemical Society

Chen et al.20 reported that SO42
/TiO2 exhibited a considerable activity, exceeding that of V2O5/TiO2 at temperatures above 400 °C, and reached a peak activity at 500
525 °C. Jung et al.21
23 intensively studied the physicochemical properties and NH3
SCR reaction mechanism on sulfated TiO2 and suggested that strong Lewis sites generated by doping TiO2 with SO42
were responsible for the higher reactivity of sulfated TiO2 at HTs. Recently, Pietrogiacomi24 and Ke25 reported that cobalt sulfate was probably the active site for SCR reaction, and the existence of sulfates enhanced the surface acidity of the catalysts. Guo et al.26 found that sulfation could enhance NOx reduction activity by increasing the number of active sites on V2O5/TiO2 catalyst without changing the activation energy or site acid strength. The iron-based oxide and sulfate catalysts for SCR of NOx by NH3 have been investigated in recent years. However, few studies have been performed on the Fe2(SO4)3/TiO2 catalyst for NH3
SCR reaction, which uses Fe2(SO4)3 as a precursor to obtain iron oxides and sulfates directly. In this study, the Fe2(SO4)3/TiO2 catalyst was prepared, and its catalytic activity and mechanism were studied in detail. In comparison, we also prepared other catalysts including sulfated TiO2, sulfated TiO2 Received: January 17, 2011 Revised: March 11, 2011 Published: March 24, 2011 7603

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doped with Fe2O3, Fe2O3/TiO2, and sulfated Fe2O3/TiO2 catalysts and investigated the respective catalytic activities and physicochemical characteristics. The results showed that a clear difference was obtained between sulfated and unsulfated catalysts. The high activity of Fe2(SO4)3/TiO2 catalyst was also explained on the basis of the characterization results. The detailed mechanism of NH3
SCR on Fe2(SO4)3/TiO2 catalyst was studied by in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS).

2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. Fe2(SO4)3/TiO2 catalyst was prepared by a wet impregnation method with Fe2(SO4)3 (AR) precursors and denoted as Fe
S/Ti. An amount of 100 mL of distilled water was added to a 200 mL beaker containing 2.0 g of Degussa AEROSIL TiO2 P25 support with stirring. Consecutively, 8 wt % Fe2(SO4)3 (AR) precursor was added, and the mixture was stirred for 2 h. The paste obtained was dried overnight at 110 °C and then calcined at 500 °C for 4 h in static air. Finally, the catalyst was palletized and crushed to 40
60 mesh for evaluation. For comparison, other catalysts were also prepared to investigate the effect of different elements on the catalytic activity. Fe(NO3)3 (AR) precursor was used to impregnate TiO2 (P25) denoted as Fe
N/Ti. (NH4)2SO4 (AR) was used to impregnate TiO2 (P25) denoted as S/Ti. Then, some Fe
N/Ti catalysts (60 mesh) were impregnated with (NH4)2SO4 (AR) for introducing the SO42
ions and denoted as S/Fe
Ti. Some S/Ti catalysts (60 mesh) were impregnated with Fe(NO3)3 (AR) and denoted as Fe/S
Ti. The above catalysts were prepared in the same procedure as Fe
S/Ti: drying followed by calcinations and crush. They similarly went through characterization and catalytic activity tests. In this work, the theoretical loadings of Fe and SO42
ions of all samples are the same as that on Fe
S/Ti catalyst. 2.2. Activity Test of Catalyst. Catalytic activity tests were performed in a fixed-bed quartz tube reactor of 9 mm internal diameter containing 200 mg of catalyst (40
60 mesh). The concentration of NH3 and NOx (NO, NO2, and N2O) in the inlet and outlet gas was measured by a FTIR spectrometer (Gasmet FTIR DX4000) made in Finland. At steady state, a gas N2 mixture containing 5% H2O (when used), 1000 ppm SO2 (when used), 500 ppm NO, 500 ppm NH3, and 5% O2 was introduced into the reactor. Water vapor was generated by passing N2 through a heated gas-wash bottle containing deionized water. In the tests, the total flow rate was fixed at 500 mL/ min, which corresponded to a GHSV (gas hourly space velocity) of 80000 h
1. The performance of the catalysts is presented in terms of conversion of NOx [X(NOx)] and selectivity of N2O [S(N2O)] as defined by eqs 1 and 2.

XðNOxÞ ¼

½NOxinlet
½NOxoutlet ½NOxinlet

100% with ½NOx ¼ ½NO þ ½NO2  SðN2 OÞ ¼

2½N2 O  100% ½NOxinlet
½NOxoutlet

ð1Þ ð2Þ

Catalyst activity tests were carried out at the temperature range of 250
500 °C. To avoid the impact of gas adsorption on

the catalyst samples, the test data were recorded after the reactions had maintained stable states for 30 min. 2.3. Characterization of Catalyst. Thermal analysis was performed in a TGA/DSC 1 system from Mettler-Toledo. The samples were heated from 100 to 900 °C at a rate of 10 °C/min as 50 mL/min N2 as reactive gas and 20 mL/min N2 as protective gas flowed through the reactor. The gas outlet was analyzed by a mass spectrometer (MS) of Omnistar from Pfeiffer of Germany. A Quantachrome Nova Automated Gas Sorption System was used to measure the N2 adsorption isotherms of the samples at liquid N2 temperature (
196 °C). The specific surface area was determined from the linear portion of the BET plot. The pore size distribution was calculated from the desorption branch of the N2 adsorption isotherm using the Barrett
Joyner
Halenda (BJH) method. Prior to taking the surface area and pore size distribution measurements, the samples were degassed in a vacuum at 300 °C for 4 h. UV
vis diffuse reflectance spectra (UV
vis DRS) were recorded in air in the wavelength range 200
800 nm on a UV
vis 2100S (Shimadzu). The X-ray photoelectron spectroscopy (XPS) experiment was carried out on a PHI Quantera SXM system at room temperature under 3.1  10
8 Pa, using Alþ radiation. The binding energy of all of the elements was calibrated relative to the carbon impurity with a C1s at 284.8 eV. Hydrogen temperature programmed reduction (H2-TPR) experiments were performed on ChemiSorb 2720 (Micromeritics). In each experiment, 100 mg of sample was loaded into a quartz reactor and then pretreated in N2 (50 mL/min) at 500 °C for 1 h. The sample was then cooled down to room temperature in a flow of N2. The reduction of the sample was carried out from room temperature to 1000 °C in a flow of 10% H2/Ar (50 mL/min) at 10 °C/min. The consumption of H2 was monitored continuously with a thermal conductivity detector. The water produced during reduction was trapped in a U-tube immersed in a cold trap. The powder X-ray diffraction (XRD) measurements were carried out with a D8 advance system with Cu KR (λ = 0.1543 nm) radiation. The samples were loaded on a sample holder with a depth of 1 mm. The in situ DRIFTS experiment was recorded with a Nicolet Nexus spectrometer equipped with a liquid nitrogen-cooled MCT detector. Prior to the experiment, the samples were purged in a flow of N2 at 450 °C for 60 min and then cooled to desired temperatures, that is, 350, 300, 250, 200, 100, and 25 °C. At each temperature, the background spectrum was recorded in flowing N2 and was subtracted from the sample spectrum that was obtained at the same temperature. Thus, the IR absorption features that originated from the structural vibrations of the catalyst were eliminated from the sample spectra. In the experiment, the IR spectra were recorded by accumulating 100 or 32 scans at a spectral resolution of 4 cm
1. The gas mixtures (i.e., NH3/N2, NO þ O2/N2, NO2/N2, and NO þ NH3 þ O2/N2) had the same concentrations as those used in the activity measurements, that is, 500 ppm NO (when used), 500 ppm NO2 (when used), 500 ppm NH3 (when used), 5% O2 (when used), and balance of N2. The total gas flow rate was 100 mL/min (ambient conditions).

3. RESULTS AND DISCUSSION 3.1. SCR Performance for Different Catalysts. 3.1.1. Catalytic Activities. The NH3
SCR activities of all of the catalysts are

shown in Figure 1. TiO2 showed moderate activity. The NOx 7604

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Figure 2. Effect of O2 concentration on catalytic activity for Fe
S/Ti catalyst. Reaction conditions: 0.2 g of catalyst, 500 ppm NO, 500 ppm NH3, 0
8% O2, balance N2, and GHSV = 80000 h
1.

Figure 1. Catalytic performance for SCR of NOx by ammonia on different catalysts. Reaction conditions: 0.2 g of catalyst, 500 ppm NO, 500 ppm NH3, 5% O2, balance N2, and GHSV = 80000 h
1. (a) NOx conversion on different catalysts and (b) N2O selectivity on different catalysts.

conversion increased from 4.8 to 31.1% when the temperature was increased from 250 to 500 °C. This was in good agreement with the previous result stating that TiO2 was active for the SCR reaction at HT.20 As compared to TiO2, the S/Ti catalyst showed a relatively higher catalytic activity at HT (>350 °C), and the highest activity achieved 90.7% at 450 °C, but a little decrease occurred as the temperature was gradually increased. On Fe
N/ Ti catalyst, NOx conversion improved at low temperature (LT), and the highest NOx conversion was 73.6% at 400 °C. As compared to S/Ti and Fe
N/Ti catalysts, catalytic activities were relatively enhanced on Fe/S
Ti and S/Fe
Ti. However, when the Fe2(SO4)3 precursor was loaded on the TiO2, relatively higher NOx conversions were obtained, which also gave a broader temperature window. About 98.0% NOx conversion was obtained at 350
450 °C on the Fe
S/Ti catalyst and decreased to 79.1% at 500 °C. The above results clearly indicate that both iron oxides and sulfates on the catalyst play important roles in the SCR reaction. It has been reported that ammonia is adsorbed on the Br€onsted or Lewis acid sites to form NH4þ or coordinated NH3 in the SCR reaction, and then, gaseous or adsorbed nitric oxides react with NH4þ or coordinated NH3 to form N2 and H2O.27 Hence, the surface acidity of catalyst is critical for the SCR reaction of NOx by NH3. Because S/Ti showed a little increase in activity in the SCR reaction, the increase of the SCR activity might be attributed to the increase of acid sites on iron oxides. This conclusion will be further discussed in section 3.2.1. Because the catalytic reaction is related to acid sites and sulfates,

it is easier to understand why the Fe
S/Ti catalyst yields better SCR activity. Figure 1b shows that the lowest N2O was yielded on the impregnated catalyst of Fe
S/Ti, which is one of the most important aspects in catalyst evaluation. On the other hand, the curve of NH3 oxidation (figure not given) was almost the same as NOx conversion (Figure 1a). Fe
S/Ti was the most effective catalyst for ammonia oxidization, probably due to the same mechanism as SCR reaction.28 In addition to a more convenient preparation method, Fe
S/ Ti catalysts have other advantages over the other catalysts, such as highest activity, highest selectivity, and widest reaction temperature window. According to the above results, Fe
S/Ti was the most effective catalyst in both theory and application. For the practical application, we also analyzed the O2, H2O, and SO2 effect on the Fe
S/Ti catalyst. 3.1.2. O2 Effect. The effects of oxygen concentration on SCR activity are shown in Figure 2. In the absence of oxygen, NOx conversions were low at 250 °C for Fe
S/Ti catalyst but were relatively higher at 350 °C. However, when a small amount of oxygen was added to the reactants, NOx conversions increased sharply, indicating an oxygen-promoting role in the SCR reaction. At 250 °C, NOx conversion increased with oxygen concentration increasing from 0 to 8%. However, at 350 °C, nearly 95% of NOx conversions were obtained in the presence of oxygen. The importance of O2 was also verified by another experiment. As shown in Figure 3, after O2 was shut off from the reactions at 350 °C, the NOx conversion declined gradually over a period of time. About 3 h later, the NOx conversion declined to about 46.5%. This is similar to the behavior on supported vanadium catalysts.1 The lattice oxygen may play an important role for the Fe
S/Ti catalyst. 3.1.3. Oxidation of NO to NO2. The comparison of oxidation activity for NO to NO2 with O2 over selected samples is shown in Figure 4. The NO conversion on Fe
N/Ti catalyst increased from 150 to 450 °C and then decreased somewhat from 450 to 500 °C. The reason that the highest NO conversion was achieved at 400 and 450 °C is due to NO oxidation being kinetically limited at lower temperature and thermodynamically limited at higher temperature. For Fe
S/Ti, the NO conversion increased in the whole temperature range but was still lower than that of Fe
N/Ti. Because NO oxidized to NO2 was usually considered to take place on iron sites, we suggested that sulfates inhibited the 7605

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Table 1. ICP Results of Different Catalysts Fe
S/Ti

Fe
N/Ti

Fe (wt %)

1.9

2.0

S (wt %)

2.3

Ti (wt %)

51.2

56.7

S/Fe
Ti

Fe/S
Ti

S/Ti

1.5

2.2

1.7

0.9

1.0

50.2

53.7

59.5

Figure 3. Transient response on Fe
S/Ti catalyst upon switching off and on O2 at 350 °C. Reaction conditions: 0.2 g of catalyst, 500 ppm NO, 500 ppm NH3, 5% O2 (when used), balance N2, and GHSV = 80000 h
1.

Figure 5. TG and DTA profiles of different catalysts: (a) Fe
S/Ti and (b) S/Ti.

Figure 4. Oxidation activity of NO to NO2 by O2 on different catalysts. Reaction conditions: 0.1 g of catalyst, 500 ppm NO, 5% O2, balance N2, and GHSV = 160000 h
1.

oxidability of iron oxide.29 As compared to iron-containing samples, TiO2 and S/Ti showed relatively lower activity for NO oxidation. 3.1.4. Water and Sulfur Tolerance. The water and sulfur tolerance of catalysts plays a vitally important role in SCR reaction, so we studied the effect of water and sulfur on Fe
S/ Ti catalyst. The simulated gas consisted of 5% H2O, 1000 ppm SO2, 500 ppm NO, 500 ppm NH3, 5% O2, and balance gas N2. The total flow rate was 500 mL/min, and the reaction was kept at 400 °C for 48 h. The results showed that Fe
S/Ti catalyst had good stability for 2 days in the test. The activity performance of the catalyst was kept above 98% and even increased a little when SO2 was injected into the reactor for longer period of time (figure not given). A variety of techniques have been reported in the literature for introducing SO42
ions to the surfaces of metal oxides to increase the acidity of the metal oxides. One of them is that catalysts react with SO3, SO2, or H2S followed by oxidation. As suggested from the results of sulfation of titania, sulfation increased both Lewis and Br€onsted acidity when the catalysts were not completely dehydroxylated.20 When iron oxide was loaded with sulfates, the system developed a Br€onsted acidity that was never present in the pure oxide.29 When we injected 1000 ppm SO2 into the quartz tube reactor, SO2 would be oxidized to SO3 on Fe
S/Ti

catalyst.30 Much stronger acidity was gotten on the catalyst surface and would be conducive to adsorb NH3. NH3 adsorption on acid sites is usually considered to be the first step of NH3
SCR reaction. Therefore, the injection of high concentration of SO2 did not inhibit the activity of Fe
S/Ti in the stability test. 3.2. Characterization of Catalysts. 3.2.1. ICP and TG/DTA-MS Analysis of Catalysts. All of the catalysts prepared were analyzed by ICP, and the results are shown in Table 1. The Fe content shows the theoretical content, but the S contents of S/Ti and Fe/ S
Ti catalysts were lower than the other catalysts containing S species. It has been reported31 that the increase of surface acidity on Fe-TiO2
PILC by the SO2 treatment is attributed to the increase of acidity sites on iron ions or oxides, which also indicates that iron ions and oxides are more easily sulfated by SO2/O2 than TiO2 in Fe-TiO2
PILC samples. He et al.32 have fully investigated the influence of sulfation on the structure of the FeTiOx catalyst. The XRD results show that the sulfate species might form on iron sites as ferric sulfate, which has been proven by the XAFS (X-ray absorption fine structure spectroscopy) results. Fe
K and Ti
K EXAFS shows that Fe
O
S bond, but not Ti
O
S bond, was formed in the FeTiOx catalyst after sulfation. According to the above research, we similarly concluded that this phenomenon also occurred on the TiO2 catalyst, which was hardly sulfated as compared to iron oxide species. When the S/Ti catalysts were calcined at 500 °C, some of the sulfate species were unstable and easily decomposed, resulting in lower S contents as compared to the other catalysts, such as Fe
S/Ti. To demonstrate this viewpoint, the S/Ti and Fe
S/Ti catalysts were used in a TG/DTA-MS analysis. Figure 5 shows that the weight of S/Ti catalyst decreased to 92.8% at 800 °C, while the weight of Fe
S/Ti only decreased to 95.2% at 800 °C. According to MS results, SO2 (m/e = 64) was mainly formed between 300 and 700 °C, and H2O (m/e = 18) formed below 300 °C on both of the catalysts. SO2 can be attributed to the 7606

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Table 2. BET Surface Area and Pore Structure Results of Different Catalysts sample

SBET (m2/g)

pore volume (cc/g)

pore diameter (nm)

Fe
S/Ti

58

0.32

15.4

Fe
N/Ti S/Fe
Ti

54 51

0.31 0.29

15.1 15.7

Fe/S
Ti

44

0.26

15.6

S/Ti

53

0.33

15.6

TiO2

60

0.33

15.7

Figure 6. UV
vis DRS spectra of different catalysts.

decomposition of sulfates on S/Ti, and H2O can be attributed to desorption of physical adsorbed water or surface hydroxide. 3.2.2. BET Analysis of Catalysts. The BET surface area, total pore volume, and average pore size of the investigated catalysts are listed in Table 2. Fe
S/Ti catalyst, Fe
N/Ti, and S/Ti catalyst reduced the TiO2 surface area from 60 to 58, 54, and 53 m2/g, respectively. The differences might be due to the introduction of iron and sulfates. In addition, the surface area of S/ Fe
Ti and Fe/S
Ti catalysts decreased to 51 and 44 m2/g, respectively, and the total pore volume and average pore size of the catalysts also slightly reduced as compared with the support. Thus, it is suggested that iron oxides dispersed well on the surface of Fe
S/Ti catalyst and diffused into the pores of TiO2 structure. Our results were consistent with Wu et al., who reported that sulfate-containing catalysts exhibited a higher surface area and more dispersed iron phase.33 3.2.3. UV
vis DRS. The UV
vis DRS results are shown in Figure 6. Schwidder et al. ascribed the bands at 220 and 285 nm to isolated Fe3þ, the band at 350 nm to oligomeric clusters, and the band at >400 nm to large Fe2O3 particles.34 It can be seen that all of the bands of Fe
N/Ti, S/Fe
Ti, Fe/S
Ti, and Fe
S/Ti catalyst assigned to Fe2O3 particles had long tails, and the band >400 nm followed the sequence of absorbance intensity. This represents that Fe2O3 particles were physically connected to the external surface of TiO2 and formed agglomerations on these catalysts but that Fe
N/Ti might have formed to a much greater extent or have had a more heterogeneous distribution of Fe2O3 particles than the others because of the strongest band absorbance. Because S/Fe
Ti and Fe/S
Ti showed intermediate tail broadening between Fe
S/Ti and Fe
N/Ti, we speculated that there was some interaction between the sulfates and the iron oxides, and the sulfates might have promoted the dispersion of iron oxides. We would also point out the strong phase

dependence of the UV
vis spectra of iron oxides. R- and γFe2O3 exhibit very different absorption spectra between 400 and 600 nm. The former does contribute above 400 nm, whereas the latter does not.34 According to the XPS results (see the XPS section) and previous research,34 R-Fe2O3 might be mainly formed on the catalysts containing iron oxides. As ref 34 shows, highly dispersed oligomeric Fex3þOy clusters should give a very sharp absorbance centered at 300 nm, while isolated sites (which would identify extreme dispersion and contact with TiO2 interface) give a sharp peak below 250 nm. Both of these features are absent in this study, suggesting that the catalysts containing iron oxides did not produce a highly dispersed surface iron ions. 3.2.4. XPS. To investigate the chemical states of samples prepared, Fe2p (only Fe
S/Ti and Fe
N/Ti), S2p, O1s, and Ti2p were measured by XPS. The results are deconvoluted to obtain detailed information of the surface species and are summarized in Table 3. The binding energy value of 710.6 eV for Fe2p3/2 of Fe
S/Ti and 710.3 eV for Fe
N/Ti catalyst indicates that iron is present mainly as Fe3þ.35 Because the Fe content was relatively lower than the other elements and the interference peaks were high, Fe was difficult to detect, and we did not test other catalysts for elemental Fe. The S2p XPS spectrum of the sulfated catalysts exhibited a main peak at 168.8 or 168.9 eV with a shoulder at 170.0 or 170.1 eV. This value is consistent with S6þ such as sulfur in SO42
.20,35,36 The assignment of peak at 170.0 or 170.1 eV was controversial. The binding energy near 169 eV was observed for all samples containing sulfur, indicating one sulfate species on the surface. No XPS peaks at 161
162 eV for sulfide or 164 eV for elemental sulfur were observed,35 indicating that S is in the S6þ oxidation state on the catalyst. The O1s XPS data showed a peak around 530.4 eV with a shoulder around 532.3 eV. The common main peaks at 530.4 eV might belong to lattice oxygen, and the peak at 532.3 eV could be attributed to sulfate oxygen or surface chemisorbed oxygen.26,37 From the data observed, S/Ti, Fe/S
Ti, S/Fe
Ti, and Fe
S/ Ti mainly exhibited lattice oxygen and some amount of sulfate oxygen or surface chemisorbed oxygen, but Fe
N/Ti showed a relatively lower sulfate oxygen or surface chemisorbed oxygen. The binding energy at about 459.2 eV of Ti2p3/2 photoelectrons was identical for all of the samples with sulfates. It should be noted that the Ti2p3/2 energy values shift slightly in all cases toward higher values with respect to the reference TiO2,38 indicating an interaction between the sulfate anion and the titanium cation with increased positive polarity. The same effect is observed for the O1s contribution associated to lattice oxygen, with averaged binding energies higher than 530 eV in all catalysts with sulfates.37 3.2.5. H2-TPR. The H2-TPR profiles are presented in Figure 7, and the integration of profiles are summarized in Table 4. In the temperature region 100
900 °C, TiO2 did not show any reduction peak, but S/Ti showed an intense peak at 544 °C, which could be attributed to sulfate reduction peak. As previous work has shown that SO2 and S2
ions are preferred reduction products of sulfate groups, the results are rationalized by assuming that no other reduction products are formed.39 This leads to the result that sulfates in S/Ti was reduced by ∼40% to sulfur dioxide and 60% to sulfide ions. Fe
N/Ti catalyst showed a LT reduction peak centered at 330 °C and a relatively HT (a broad peak) between 410 and 7607

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Table 3. XPS Results of Different Catalystsa Fe2p sample

Fe2p3/2

S2p

O1s

Fe2p1/2

SRb

Sβb

168.8 (59.9)

170.0 (40.1)

Fe
S/Ti

710.6

723.9

Fe
N/Ti

710.3

723.6

O Rb

Ti2p Oβb

Ti2p3/2

Ti2p1/2

530.4 (78.6)

532.2 (21.4)

459.2

464.9

529.8 (94.2)

531.9 (5.8)

458.5

464.2

S/Fe
Ti

168.9 (64.6)

170.1 (35.4)

530.3 (76.9)

532.3 (23.1)

459.1

464.8

Fe/S
Ti

168.9 (52.7)

170.0 (47.3)

530.5 (77.5)

532.2 (22.5)

459.2

464.9

S/Ti

168.9 (67.8)

170.1 (32.2)

530.5 (82.2)

532.2 (17.8)

459.2

464.9

OR, the lattice oxygen; Oβ, the sulfate oxygen or chemisorbed oxygen. b The data in parentheses stand for percentage of XR or Xβ to XR þ Xβ (%); X = S or O. a

Figure 7. H2-TPR profiles of different catalysts.

Table 4. Hydrogen Consumption in H2-TPR Experiments over Different Catalysts total H2

H2 consumption

consumption a

of Fe b

S6þ to

(μmol)

4þ

sample

(μmol)

Fe
S/Ti

H2/S S

(%) S2
(%)

187.1

50.9

Fe
N/Ti S/Fe
Ti

130.3

53.6 40.2

1.7

77

23

Fe/S
Ti

112.1

58.9

1.9

70

30

2.8

40

60

S/Ti

88.5

1.9

S6þ to

70

30

a

Actual amount of hydrogen consumption by integrating peaks of different catalysts. b Theoretical amount of hydrogen consumption based on ICP results and molar ratio of H2/Fe = 1.5.

780 °C. These results confirm literature data that prove the reduction of Fe2O3 by H2 to be a two-step mechanism according to the sequence Fe2O3 f Fe3O4 f Fe.40,41 It is recognized that pure R-Fe2O3 sample indicates a TPR pattern with a LT/HT peak ratio close to 1:8, and the total consumption of H2 is expressed as a molar ratio of H2/Fe amounts to 1.5. This seems to be in contradiction to our result, as a molar ratio of H2 consumption of the LT peak to the HT one is ca. 1:3. According to the results of UV
vis DRS, we speculate only some Fe2O3 are reduced in the reduction process because of some agglomerations or heterogeneous distribution of Fe2O3 particles on Fe
N/Ti. In addition, the actual H2 consumption of Fe
N/Ti is not listed in Table 4, because the H2 consumption peak area of Fe
N/Ti is much lower than the other catalysts and cannot be calculated very accurately. The catalysts containing both sulfates and iron oxides (e.g., Fe
S/Ti, Fe/S
Ti, and S/Fe
Ti) caused the sulfate reduction

Figure 8. DRIFTS spectra (100 scans) at 25 °C of chemisorbed NH3 on different catalysts pretreated by N2 at 450 °C.

peak at a temperature (440, 375, and 410 °C) lower than that of S/Ti sample (544 °C), which indicated that Fe strongly catalyzed the reduction of sulfate groups. These results were consistent with Sachtler et al.,39 who reported that the reduction peak of sulfate would be shifted to a lower temperature in the presence of Fe. The hydrogen consumption in Fe
S/Ti, Fe/S
Ti, and S/ Fe
Ti is much higher, suggesting that the reduction involved both iron and sulfates. 3.2.6. XRD. All of the catalysts prepared were analyzed by XRD, only anatase peaks mainly appeared without R-Fe2O3 or γ-Fe2O3 peaks in the XRD pattern of the catalysts (figure not given). It illustrated that iron oxides were too scarce to be detected or highly dispersed on the support. In addition, iron oxides and sulfates did not alter the TiO2 (anatase) structure. 3.2.7. DRIFTS of Ammonia Adsorption. The surface acidity and the strength of acid sites can be determined by using ammonia as a probe molecule. Such information was obtained by studying the DRIFTS spectra of the adsorbed NH3. The spectra of adsorbed ammonia over all of the catalysts are illustrated in Figure 8. After Fe
S/Ti was treated in flowing NH3/N2 for 60 min and then purged with N2 for 20 min at 25 °C, a strong band at 1445 cm
1 and three weaker bands at 1673, 1596, and 1245 cm
1 were observed. The bands at 1673 and 1445 cm
1 are due to the symmetric and asymmetric bending vibration of NH4þ formatting on Br€onsted acid sites, while the band at 1596 cm
1 can be assigned to asymmetric bending vibrations of the N
H bonds in NH3 coordinately linked to Lewis acid sites.42
44 Because the decrease of sulfate coordination to the catalyst surface induced by NH3 and/or NOx adsorption (the negative peak at about 1300
1400 cm
1) caused a broad positive absorption at lower 7608

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The Journal of Physical Chemistry C

Figure 9. DRIFTS spectra (100 scans) of chemisorbed 500 ppm NH3/ N2 on Fe
S/Ti catalyst at 25 °C followed by purge in N2 at different temperatures.

Figure 10. DRIFTS spectra (100 scans) of chemisorbed 500 ppm NO þ 5% O2/N2 on Fe
S/Ti catalyst at 25 °C followed by purge in N2 at different temperatures.

Figure 11. DRIFTS spectra (32 scans) taken at 250 °C upon passing 500 ppm NO þ 5% O2/N2 over Fe
S/Ti catalyst with preadsorbed NH3.

wavenumbers (1245 cm
1 band in Figure 8, 1258 cm
1 bands in Figures 9 and 11, 1320 cm
1 band in Figure 10, and 1307 cm
1 band in Figure 13), all of these bands were assigned to less coordinated sulfates.29,45 The present results indicated that there were more Br€onsted acid sites than Lewis acid sites on Fe
S/Ti at 25 °C. In contrast, Fe
N/Ti showed weaker peaks at 1596 and 1160 cm
1. It has been reported that Fe
N/Ti is a medium

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Figure 12. Consumption of NH4þ ions (indicated by band area at 1445 cm
1) at 250 °C upon passing 500 ppm NO/N2, 500 ppm NO þ 5% O2/N2, and 500 ppm NO2/N2 over Fe
S/Ti catalyst with preadsorbed NH3.

Figure 13. DRIFTS spectra (32 scans) taken at 250 °C upon passing 500 ppm NH3/N2 over Fe
S/Ti catalyst with preadsorbed NO þ O2.

strength Lewis acid solid on the basis of ammonia and pyridine adsorption FT-IR experiment,46 so we concluded 1596 and 1160 cm
1 were assigned to symmetric and asymmetric bending vibration of the N
H bonds in NH3 coordinated to Lewis acid sites.44 Because S/Fe
Ti mainly formed Br€onsted acid sites in the same way as Fe
S/Ti, it could be speculated that the acid sites were altered when the sulfates were introduced to Fe
N/Ti. Two negative bands (at 1630 and 1345 cm
1) were also seen on these catalysts. Because of bands of surface OH absorbing well above 3500 cm
1, the 1630 cm
1 band might not be the hydroxyl groups on the surface (that was displaced) but was the little amount of adsorbed water (bending of H2O at 1630 cm
1) on the surface before adsorption. The 1345 cm
1 band was due to the displacement of the sulfate species on the surface. Clearly, NH3 was more strongly adsorbed on the surface than some of the hydroxyls and sulfate species.47 According to the above analysis of the peak results, it can be seen that Lewis acid sites are mainly located on TiO2 and Fe
N/Ti, but Br€onsted acid sites are formed on S/Ti, Fe
S/Ti, S/Fe
Ti, and Fe/S
Ti. It has been reported that TiO2 and FeOx/TiO2 were Lewis acid solid, but the acid characteristic would be changed when the sulfate was introduced. We conclude that sulfation has great influence on the formation of Br€onsted acid sites for the SCR reaction. 7609

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The Journal of Physical Chemistry C Because Fe
S/Ti could be conveniently prepared by the impregnation method and showed relatively higher catalytic activity of SCR reaction, it should be one of the most promising catalysts put into practical application. We studied its detailed mechanism by DRIFTS. 3.3. DRIFTS Study. 3.3.1. IR Spectra of Ammonia Adsorption. The IR spectra of ammonia adsorbed on Fe
S/Ti at different temperatures are shown in Figure 9. After the sample was treated in flowing 500 ppm NH3/N2 for 60 min and then purged with N2 for 20 min at 25 °C, some bands similar to that in Figure 8, a strong band at 1445 cm
1 and weaker bands at 1680, 1613, 1258, and 1055 cm
1 were observed. In the NH stretching region, bands near 3256 cm
1 and at 3012 and 2830 cm
1 can be assigned to coordinated ammonia and ammonium ions, respectively.48 The intensities of all the bands decreased with increasing temperature indicating desorption of NH3. The IR bands at 1680 and 1613 cm
1 due to NH4þ and coordinated NH3 almost disappeared at 250 °C, whereas those due to NH4þ ions (1445 cm
1) were still detected at 350 °C in N2. This result indicates that NH4þ ions on Br€onsted acid sites are more stable at HTs than the other ammonia adspecies. The sulfate ions bonded to iron might show a characteristic IR absorption band at near 1375 cm
1, so the negative peak at about 1400 cm
1 was ascribed to SdO band.49,50 IR band at 1315 cm
1 was observed at 250 °C and even observed at a higher temperature of 300 and 350 °C and not detected at room temperature. However, amide species might not be observed at HT, due to its thermal instability.41 So, we concluded that the band at 1315 cm
1 in Figure 9 was not due to amide but was probably due to nitrate species formed by the oxidation of ammonia at higher temperatures (>250 °C).47,51 3.3.2. IR Spectra of Nitrogen Oxides Adsorption. Fe
S/Ti was treated in flowing 500 ppm NO þ 5% O2/N2, and the IR spectra were shown in Figure 10. At 25 °C, two bands due to weakly adsorbed NO2 (1622 cm
1) and nitrate (1577 cm
1) adspecies were detected.42 When the temperature was increased to 200 °C, the bands at 1622 and 1577 cm
1 either disappeared or decreased sharply. As the temperature was further increased to 300 °C, all of the bands vanished. 3.3.3. IR Spectra of the Reaction between Nitrogen Oxides and Ammonia Adspecies. The IR spectra of the reaction between NH3 adsorbed species and NO þ O2 are shown in Figure 11. After 500 ppm NO þ 5% O2 were passed over the ammonia-adsorbed Fe
S/Ti, the band attributed to NH4þ ions (1445 cm
1) decreased significantly in 4 min. New IR bands were seen at 1620 cm
1 after 4 min, indicating the formation of NO2 adsorbed species. We speculated that SCR might mainly take place on the NH4þ ions (1445 cm
1). To demonstrate oxygen's effect on the reaction rate between NO and NH4þ ions (1445 cm
1), the IR spectra of the reactions between NH4þ ions (1445 cm
1) and NO or NO2 were also studied. The integrated area of 1445 cm
1 band of the reaction between NH4þ ions (1445 cm
1) and NO, NO þ O2, or NO2 at 250 °C are summarized in Figure 12. After 500 ppm NO was passed over the NH3-treated Fe
S/Ti, the IR band due to NH4þ ions decreased slightly and diminished until 15 min. This further proves that NO was quite inactive in reacting with NH4þ ions. By comparison, when 500 ppm NO þ 5% O2 was passed over the sample, the NH4þ band decreased slightly and vanished in 5 min, and then, the catalyst surface was gradually dominated by the adsorbed NO2 and nitrate species. When 500 ppm NO2 was passed over the sample, the NH4þ band decreased significantly

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Figure 14. DRIFTS spectra (100 scans) of Fe
S/Ti catalyst in a flow of 500 ppm NO þ 500 ppm NH3 þ 5% O2/N2 at 250
450 °C.

and vanished in 5 min. The IR band due to NO2 [1620 cm
1] appeared after 1 min (figure not given). The Fe
S/Ti catalyst was also used to test NOx conversion under 500 ppm NO þ 500 ppm NH3/N2, 500 ppm NO þ 500 ppm NH3 þ 5% O2/N2, and 500 ppm NO2 þ 500 ppm NH3/ N2 at 250 °C for 60 min. The results showed that NOx conversions reached 14, 60, and 97%, respectively. The present results further verify that the reactivities with NH4þ ions decrease in the following sequence: NO2 > NO þ O2 > NO at 250 °C. It obviously showed that O2 increased the reaction rate, and the transformation of NO to NO2 might be one of the main steps of the NH3
SCR reaction on Fe
S/Ti catalyst. 3.3.4. IR Spectra of the Reaction between Ammonia and Nitrogen Oxides Adspecies. Fe
S/Ti was first treated with NO þ O2/N2 for 60 min followed by N2 purged at 250 °C for 20 min. NH3/N2 was then introduced into the IR cell, and IR spectra were recorded as a function of time and shown in Figure 13. As noted already, nitrate and NO2 species were formed on Fe
S/Ti upon treatment with NO þ O2/N2. After NH3/N2 was passed over the sample for 0
10 min, the band attributed to both nitrate and NO2 adspecies decreased. At the same time, two new weak bands were observed at 1445 and 1307 cm
1. The band at 1445 cm
1 might come from asymmetric bending vibration of NH4þ that is chemisorbed on the Br€onsted acid sites, and the band at 1307 cm
1 was also attributed to less coordinated sulfates.29,45 These results showed that a reaction between N-containing adspecies (mainly nitrate and NO2) and ammonia occurred. After 1 min, all of the N-containing adspecies bands diminished (on the basis of the diminished band at 1622 and 1577 cm
1). The band intensity at 1445 cm
1 increased gradually. 3.3.5. IR Spectra in a Flow of NO þ NH3 þ O2/N2. To identify the species present on the catalyst under reaction conditions, IR spectra were recorded when Fe
S/Ti was heated from 250 to 450 °C in a flow of NO þ NH3 þ O2/N2. As shown in Figure 14, the bands due to nitrite (1604 cm
1) and NH4þ (1676 and 1440 cm
1) species were observed at 250 °C. Raising the temperature resulted in a decrease in the band intensities of nitrite and NH4þ. With an increase in temperature to 450 °C, the nitrite almost diminished, and the bands at 1440 and 1300 cm
1, which were attributed to NH4þ and nitrate or ammonia oxidation species, decreased with temperature.43,47,51 This showed that the rate of NO consume was faster than its formation. 7610

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The Journal of Physical Chemistry C Ramis and Busca52 supposed the NH3
SCR mechanism over Fe2O3 that ammonia is first coordinated over Lewis sites and later undergoes hydrogen abstraction giving rise either to NH2 amide species or to its dimeric form N2H4, hydrazine. In the presence of NO, coordinated ammonia rapidly disappears, and water is produced, showing that SCR reaction occurred. Kureti16 almost agreed with this mechanism of NH3
SCR reaction over Fe2O3/WO3/ZrO2 catalyst. In our results, even Lewis acid sites were observed on Fe
N/Ti catalyst, Br€onsted acid sites were mainly obtained when sulfates were introduced to Fe
N/Ti. Thus, the NH3
SCR reaction mechanism might be different from the study of Ramis, Busca, and Kureti. On the other hand, with the extensive study of Yang27 and Eng,53 an intermediate of NO2(NH4þ)2 originated from NH4þ and NO2 was supposed and identified in the mechanism study of NH3
SCR on Fe/ Zeolite. Recently, Tronconi54
56 indentified the role of nitrates as key intermediates in the SCR reaction and proposed a reaction mechanism that NO2 first reacted to form nitrate species, which were consecutively reduced by NO to nitrites. In the presence of ammonia, this leads to ammonium nitrites, which eventually decompose to gaseous nitrogen. With the present DRIFTS results, we carefully deduced the mechanism of the NH3
SCR reaction similar to the study of Tronconi et al. NH4þ was mainly adsorbed on Br€onsted acid sites of Fe
S/Ti catalyst, which were considered to be the first step of NH3
SCR reaction. NO was oxidized to NO2 by the oxygen in the reaction gas or on the iron oxide sites of the catalyst. Then, nitrates originated from NO2 were generated on Fe
S/Ti catalyst for the NH3
SCR reaction. Finally, an active intermediate species like ammonium nitrite was formed, which eventually decomposed to gaseous nitrogen.

4. CONCLUSIONS The present work has shown that Fe2(SO4)3/TiO2 catalysts were relatively active in the NH3
SCR reaction. About 98.0% NOx conversion on Fe2(SO4)3/TiO2 catalyst was obtained at 350
450 °C, and lower N2O selectivity was observed on it. According to the above results and the stability test, Fe2(SO4)3/ TiO2 might be a promising catalyst for practical application in NH3
SCR reactions in stationary sources. From XPS and UV
vis DRS results, we concluded that RFe2O3 might be mainly formed on the catalysts containing iron oxides. Some interaction might exist between iron oxides and sulfates, and sulfation might occur most on iron oxides but not TiO2. It was speculated that the acid sites would be changed from Lewis acid to Br€onsted acid, when the sulfates were introduced to Fe
N/Ti. On the other hand, when sulfates were introduced into the catalysts containing iron oxides, different dispersed states would be observed on the catalyst surface. BET, UV
vis DRS, and H2-TPR analyses of these catalysts demonstrated that the Fe(NO3)3 precursor led to the product of Fe2O3 particles, which physically connected to the surface of TiO2 and resulted in some agglomerations or heterogeneous distribution of Fe2O3 particles. In contrast, catalysts containing sulfates resulted in a relatively higher dispersed surface Fe2O3. In addition, sulfates played an important role in the SCR reactions, which was responsible for the strong Br€onsted acid sites. Ammonia molecules were mainly adsorbed on the Br€onsted acid sites of catalyst to generate NH4þ ions species, which could react with NO2 at HTs. DRIFTS spectra showed that NO molecules were oxidized by O2 to form adsorbed NO2 and nitrate species on the Fe2(SO4)3/TiO2

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catalyst. NO2 adspecies were the dominant species and could be reduced by ammonia at HTs. A possible reaction scheme for the SCR reaction on Fe2(SO4)3/TiO2 was proposed. First, NH3 is adsorbed on the surface of Fe2(SO4)3/TiO2 and mainly forms NH4þ on Br€onsted acid sites. NO was oxidized to NO2 by the oxygen in the reaction gas or on the iron oxide sites of the catalyst. Then, nitrates originated from NO2 were generated on Fe2(SO4)3/TiO2 catalyst for NH3
SCR reaction. Finally, an active intermediate species like ammonium nitrite was formed, which eventually decomposed to gaseous nitrogen.

’ AUTHOR INFORMATION Corresponding Author

*Tel: þ86-10-62771093. E-mail: [email protected].

’ ACKNOWLEDGMENT We thank Dr. Lan Ma of Department of Chemistry of Tsinghua University and Mr. Hamid Reza Arandiyan for their encouragement and discussions. This work was financially supported by the National Natural Science Foundation of China (Grant No. 51078203) and the National High-Tech Research and Development (863) Program of China (Grant No. 2010AA065002). ’ REFERENCES (1) Bosch, H.; Janssen, F. Catal. Today 1988, 2, v. (2) Busca, G.; Lietti, L.; Ramis, G.; Berti, F. Appl. Catal., B 1998, 18, 1. (3) Liu, Z. M.; Woo, S. I. Catal. Rev. - Sci. Eng. 2006, 48, 43. (4) Mutin, P. H.; Popa, A. F.; Vioux, A.; Delahay, G.; Coq, B. Appl. Catal., B 2006, 69, 49. (5) Phil, H. H.; Reddy, M. P.; Kumar, P. A.; Ju, L. K.; Hyo, J. S. Appl. Catal., B 2008, 78, 301. (6) Zhang, X.; Li, X. G.; Wu, J. S.; Yang, R. C.; Zhang, Z. H. Catal. Lett. 2009, 130, 235. (7) Heck, R. M. Catal. Today 1999, 53, 519. (8) Dunn, J. P.; Koppula, P. R.; G. Stenger, H.; Wachs, I. E. Appl. Catal., B 1998, 19, 103. (9) Qi, G.; Yang, R. T. Appl. Catal., B 2005, 60, 13. (10) Balle, P.; Geiger, B.; Kureti, S. Appl. Catal., B 2009, 85, 109. (11) Chen, J. P.; Hausladen, M. C.; Yang, R. T. J. Catal. 1995, 151, 135. (12) Cheng, L. S.; Yang, R. T.; Chen, N. J. Catal. 1996, 164, 70. (13) Long, R. Q.; Yang, R. T. J. Am. Chem. Soc. 1999, 121, 5595. (14) Ma, A. Z.; Grunert, W. Chem. Commun. 1999, 71. (15) Grossale, A.; Nova, I.; Tronconi, E. Catal. Today 2008, 136, 18. (16) Apostolescu, N.; Geiger, B.; Hizbullah, K.; Jan, M. T.; Kureti, S.; Reichert, D.; Schott, F.; Weisweiler, W. Appl. Catal., B 2006, 62, 104. (17) Jan, M. T.; Kureti, S.; Hizbullah, K.; Jan, N. Chem. Eng. Technol. 2007, 30, 1440. (18) Liu, F. D.; He, H.; Ding, Y.; Zhang, C. B. Appl. Catal., B 2009, 93, 194. (19) Brandenberger, S.; Krocher, O.; Tissler, A.; Althoff, R. Catal. Rev. - Sci. Eng. 2008, 50, 492. (20) Chen, J. P.; Yang, R. T. J. Catal. 1993, 139, 277. (21) Jung, S. M.; Grange, P. Catal. Today 2000, 59, 305. (22) Jung, S. M.; Grange, P. Appl. Catal., B 2000, 27, L11. (23) Jung, S. M.; Grange, P. Catal. Lett. 2001, 76, 27. (24) Pietrogiacomi, D.; Magliano, A.; Ciambelli, P.; Sannino, D.; Campa, M. C.; Indovina, V. Appl. Catal., B 2009, 89, 33. (25) Ke, R.; Li, J. H.; Liang, X.; Hao, J. M. Catal. Commun. 2007, 8, 2096. 7611

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