Selective Catalytic Reduction of NOx with NH3 over Mn, Ce

ARTICLE pubs.acs.org/JPCC

Selective Catalytic Reduction of NOx with NH3 over Mn, Ce Substitution Ti0.9V0.1O2
δ Nanocomposites Catalysts Prepared by Self-Propagating High-Temperature Synthesis Method Bin Guan, He Lin,* Lin Zhu, and Zhen Huang* Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China ABSTRACT: This study focuses on promoting the low temperature performance of vanadium-based catalysts; for this, the SHS method was applied to synthesize a series of Ti0.9VxM0.1
xO2
δ catalysts. The performances of the catalysts (Ti0.9V0.1O2
δ, Ti0.9Mn0.05V0.05O2
δ, and Ti0.9Ce0.05V0.05O2
δ) were fully investigated with the temperature-programmedreaction, which proved that these SHS catalysts with nanometer size had high activity over a broad temperature window of 150
400 °C. Compared with Ti0.9V0.1O2
δ, the substituted catalysts, Ti0.9Mn0.05V0.05O2
δ and Ti0.9Ce0.05V0.05O2
δ, showed higher N2 selectivity at high temperatures. The Ce substituted catalyst exhibited good resistance to H2O and SO2 poisoning at low temperatures. The structural and physical-chemical properties of catalysts were characterized comprehensively by BET, XRD, FTIR, TEM, EDX, XPS, and TPD. The XRD results indicated that the active components of V, Mn, and Ce were highly dispersed over the catalysts. The Mn substitution could enhance the Brønsted acid sites on the catalyst surface and accelerate the SCR reaction at low temperatures. The XPS shows that the Ce substitution led to high concentration of chemisorbed oxygen, which diminished the unselective oxidation of NH3 by O2 to N2O, NO, or NO2 and resulted in superior N2 selectivity. The active components of the catalysts, such as V, Mn, and Ce, mostly existed in the form of mixed-valence which was beneficial for the oxidation of NO to NO2. Furthermore, the SCR reaction mechanism over Ti0.9Ce0.05V0.05O2
δ catalyst was also examined using in situ DRIFTS. The results revealed that high active monodentate nitrate and bridging nitrate species as well as abundant ionic NH4þ (Brønsted acid sites) were the key intermediates in the SCR reaction since the ad-monodentate nitrate and bridging nitrate species disappeared quickly in the presence of NH3.

1. INTRODUCTION The selective catalytic reduction of NOx (NH3
SCR) under the lean condition has been well-proven to be an effective and economical technique for the removal of NOx in the exhaust gases from stationary and mobile sources.1
4 For the marine engine, the International Maritime Organization (IMO) issued the stringent Tier III standards by 2016 that the NOx emissions from marine engines entering an Emission Control Area (ECA) must be lower than 3.49 g/kW h, which is 80% less than that of Tier I in 2000. Among lots of techniques for the marine engine NOx abating, SCR is the most promising one for Titer III.5 It should be pointed out that the low speed marine diesel engine exhaust is low temperature in the range of 200
280 °C and usually contains several hundreds ppm of SO2 due to the combustion of the fuel with more than 4.5% sulfur. Therefore, it is urgent to study and develop the catalysts which possess high activity and robust SO2 resistance at low temperatures. In recent years, the manganese oxide (MnOx) has become an attractive interest due to its high SCR activity at low temperatures.6
9 The manganese oxides contain various types of labile oxygen which are necessary for completing a catalytic cycle in SCR.10
17 The activity of pure manganese oxides in SCR has been intensively investigated by Kapteijin et al., and 90% of NO r 2011 American Chemical Society

conversion was obtained at 170 °C.18 Pe~na et al. reported that the low-temperature SCR of NO with NH3 in the presence of excess oxygen on the oxides of first row transition metals as V, Cr, Mn, Fe, Co, Ni, and Cu supported on anatase TiO2 was studied. They found that Mn/TiO2 supported on Hombikat TiO2 provided the best performance with 100% N2 selectivity and complete NO conversion at temperatures as low as 393 K under numerous conditions8 and that a new highly active, time-stable, and water resistant Hombikat TiO2 supported Mn catalyst had been developed for the selective reduction of NO by NH3 and obtained 100% yield of N2 operating at much lower reaction temperatures (e120 °C).9,19
21 Additive of ceria could enhance the oxidation of NO to NO2, thereby increasing the catalytic activity of SCR of NOx with NH3.22
28 Qi et al. found that the manganese
cerium catalyst yielded nearly 100% NO conversion at 120 °C.29
32 Tang et al. also reported that the manganese
cerium mixed oxide catalyst showed excellent denitrification activity at 80 °C.33 It was found that the addition of CeO2 significantly improved the dispersion of the active component Received: December 27, 2010 Revised: May 20, 2011 Published: May 24, 2011 12850

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The Journal of Physical Chemistry C over the surface of the catalyst and enhanced active sites per unit area.34,35 However, because of the strong oxidizing ability, the manganese oxide series catalysts could oxidize NH3 to NO and N2O easily as the temperatures increased, which decreased the na et al. also found that the N2 selectivity N2 selectivity.25,36 Pe~ over 20% Mn/TiO2 decreased sharply from 95% at 100 °C to 52% at 250 °C.9 For the SO2 resistance of Mn, Kijlstra and Wu et al. reported that SO2 was combined with Mn to produce sulfate easily, making the Mn
Ce catalysts deactivate at low temperatures.37
39 Jin et al. reported that the Mn/TiO2 catalyst prepared by sol
gel method was very sensitive to SO2 and that the NO conversion decreased from 93% to only about 30% after 6 h test in the presence of 100 ppm SO2 at 150 °C.40
42 Recently, Roy et al. have synthesized Ti0.9M0.1O2
δ (M = Cr, Mn, Fe, Co, and Cu) serial catalysts prepared by the selfpropagating high-temperature synthesis (SHS) method.43
45 It was reported that Ti0.9Mn0.1O2
δ was the most active catalyst at low temperatures, but its selectivity for N2 was poor. On the contrary, Ti0.9Fe0.1O2
δ ranked first in N2 selectivity but was not active at low temperatures.43 Roy found that the optimized catalyst of Ti0.9Mn0.05Fe0.05O2
δ had good activity in the range of 150
500 °C and more than 80% N2 selectivity even at 450 °C. However, the H2O and SO2 tolerance performance of this catalyst has not yet been reported.43 The most widely accepted commercial vanadium-based SCR catalysts (such as V2O5
WO3 (MoO3)/TiO2) are active within a relatively narrow operation temperature window of 300
400 °C; however, they are less active when the temperature is lower than 300 °C.46
49 It is known that the vanadium-based catalysts possess strong SO2 tolerance, which may be a potential candidate for the treatment of the marine engine emissions with high SO2 content. Considering the high SO2 resistance of vanadium-based catalysts, there is great interest in modifying this kind of catalysts and developing highly active SCR catalysts within a low temperature window. In the present research, we intend to develop a new vanadium(V) based SCR catalyst that can meet with the demand of the marine engine NOx reduction. A series of Ti0.9V0.1O2
δ based catalysts were tested with a partial substitution of vanadium with manganese and cerium, with respect to their high activity at low temperatures. The catalysts were prepared with the SHS method, which can produce homogeneously dispersed nanocomposites with unique properties. In this way, we obtained a perfect catalyst of Ti0.9Ce0.05V0.05O2
δ, which showed high activity within the temperature window of 150
400 °C as well as strong H2O and SO2 resistance capability. The physical structural and physicalchemical properties of the catalysts were characterized comprehensively by N2 physisorption, XRD, SEM, TEM, and XPS. In addition, the NH3
SCR mechanism over the Ti0.9Ce0.05V0.05O2
δ catalyst was investigated by in situ DRIFTS.

2. EXPERIMENTAL SECTION 2.1. Catalyst Synthesis. The catalysts in this study were prepared by a single step solution combustion method, namely the SHS method that has been well addressed in the literature.43 The precursor mixture used in this study is an aqueous solution of stoichiometric amount of titanyl nitrate (TiO(NO3)2) and respective metal nitrates (vanadium nitrate V(NO3)3, manganous nitrate Mn(NO3)2 3 4H2O, and cerium nitrate Ce(NO3)3 3 6H2O), where TiO(NO3)2 was prepared by dissolving TiO(OH)2, the product of tetra-n-butyl titanate Ti{OCH3(CH2)3}4 hydrolysis,

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into the nitric acid HNO3 (68%, w/w solution). Before combustion, stoichiometric glycine (CH2NH2COOH) was added into the precursor mixture as a fuel. After a few minutes’ stirring on a heating plate to ensure proper homogeneity, the so-prepared solution was transferred into a corundum crucible, which was subsequently introduced into a muffle furnace kept at a constant temperature of 350 °C. At the initial stage, the aqueous solution boiled with frothing and foaming with concomitant dehydration. At the point of its complete dehydration, the mixture ignited yielding a voluminous finely dispersed solid product in high volume. The solid product was calcined at the same temperature (350 °C) for another 1 h to get a clean product without any impurity. Under the equilibrium conditions, the global reactions of catalyst synthesis can be summarized as follows: 162TiOðNO3 Þ2 þ 18VðNO3 Þ3 þ 208C2 H5 NO2 f 180Ti0:9 V 0:1 O2
δ þ 520H2 O þ 293N2 þ 416CO2 324TiOðNO3 Þ2 þ 18VðNO3 Þ3 þ 18MnðNO3 Þ2 þ 404C2 H5 NO2

f 360Ti0:9 Mn0:05 V 0:05 O2
δ þ 1010H2 O þ 571N2 þ 808CO2 162TiOðNO3 Þ2 þ 9VðNO3 Þ3 þ 9CeðNO3 Þ3 3 6H2 O

þ 208C2 H5 NO2 f 180Ti0:9 Ce0:05 V 0:05 O2
δ þ 574H2 O þ 293N2 þ 416CO2 For comparison, 3% V 2 O 5 /TiO2 was prepared by the conventional wet impregnation method. The NH 4 VO 3 was dissolved in oxalic acid solution, followed by stirring the mixture solution on a heating plate for a few minutes to obtain a dark blue solution of (NH4 )2 [VO(C 2 O4 )2 ]. Stoichiometric pretreatment nano-TiO 2 was added to the obtained solution, which was subsequently desiccated at 200 °C for 2 h and then calcinated at 500 °C for 3 h in air. All catalysts prepared were ground and sieved to 40
80 mesh for evaluation. 2.2. Catalytic Activity, Selectivity, and H2O and SO2 Tolerance Studies. The performances of various catalysts in NH3
SCR were investigated in a temperature programmed reaction (TPR) system equipped with a FTIR (Thermo NICOLET 6700). In each TPR run, 0.5 mL of fresh catalyst sample was packed in the center of a fixed-bed quartz reactor with an inner diameter of 9 mm. The catalyst sample was plugged with silica wool to prevent the catalyst from being blen away. The reactor was installed in a tubular electric resistance furnace whose temperature was measured by a chromel-alumel thermocouple dipped in the catalyst bed and regulated by a proportional integral derivative (PID) controller. The furnace temperature ranging from the room temperature to 450 °C was ramped at 5 °C/min. The synthetic feed gas mixture consisted of 600 ppm NO, 600 ppm NH3, 400 ppm SO2 (when used), 7% H2O (when used), and 5 vol % O2 (balanced with N2) was adjusted by mass flower controllers. Water vapor was generated by passing N2 through a heated gas-wash bottle containing deionized water. The effect of SO2 on the NOx removal efficiency was investigated by adding 400 ppm SO2 to the feed gas mixture. The typical total gas flow rate was 100 mL/min at the standard temperature and pressure (STP), and the corresponding gas hourly space velocity (GHSV) was 12 000 h
1. The effluent gas including NO, NO2, N2O, and unreacted NH3 was continuously quantified with an online high resolution FTIR spectrometer (Thermo NICOLET 12851

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6700, OMNIC Quantpad software) equipped with a gas-measuring cell. Because the reaction may occur at low temperatures, it must be confirmed that the decrease of NO was not caused by the low temperature adsorption. For this reason, the catalyst sample was purged with reactant gas at the room temperature at the beginning of each TPR run until no difference of NO concentration between the inlet and the outlet gas mixtures was detected. The NOx conversion (ηNOx) and the N2O selectivity (SN2O) were calculated as follows: ηNOx ð%Þ ¼

½NOx in
½NOx out  100% ½NOx in

ð1Þ

where NOx is the sum of NO and NO2 concentrations SN2 O ð%Þ ¼

2½N2 Oout 100% ½NOin þ ½NH3 in
½NOout
½NH3 out ð2Þ

During the NH3
SCR reaction process, no other nitrogencontaining component was detected in the effluent gas except NO, NO2, N2O, and NH3, so we define the N2 selectivity (SN2) as formula 3 SN2 ð%Þ ½NOin þ ½NH3 in
½NOout
½NH3 out
½NO2 out
2½N2 Oout ¼  100% ½NOin þ ½NH3 in
½NOout
½NH3 out

ð3Þ The NH3 conversion (ηNH3) and the selectivities for the formation of N2 (SN2), NOx (SNO2), and N2O (SN2O) during separate NH3 oxidation were defined by the following equations: ηNH3 ð%Þ ¼

½NH3 in
½NH3 out  100% ½NH3 in

ð4Þ

sNOx ð%Þ ¼

½NOout þ ½NO2 out  100% ½NH3 in
½NH3 out

ð5Þ

sN2 O ð%Þ ¼

2½N2 Oout  100% ½NH3 in
½NH3 out

ð6Þ

sN2 ð%Þ ¼ 100%
ðsNOx þ sN2 O Þ

ð7Þ

2.3. Characterization of Catalysts. N2 adsorption
desorption isotherms were obtained at
196 °C using a Quantachrome NOVA 1000a Automated Gas Sorption Instrument. Prior to N2 adsorption, all samples were degassed at 100, 150, 200, and 250 °C for 10 min and at 50 and 300 °C for 30 min under vacuum, respectively. The specific surface areas of the catalysts were determined from these isotherms by applying the Brunauer
Emmett
Teller (BET) method in the 0.05
0.3 partial pressure range. The total pore volume (at a relative N2 pressure (P/P0) of 0.987), average pore diameters, and pore size distributions were measured from the N2 desorption branches of the isotherms using the Barret
Joyner
Halenda (BJH) method. The XRD patterns of catalysts were checked using a computerized Rigaku D/max-2200/PC X-ray diffractometer equipped with Cu KR (λ = 0.15418 nm) radiation source. The samples were located on a sample holder with a depth of 1 mm. XRD patterns were recorded in the 2θ range of 20 to 90° at a scan rate of 2θ = 5°/min with a scan step size of 0.02°. The identification of

crystalline phase was made by matching the Powder Diffraction File-International Centre for Diffraction Data (PDF-ICDD). The surface morphology and microstructures of the catalysts prepared and the catalysts after SO2 tolerance reaction were observed by scanning electron microscopy (SEM) in a Hitachi S-2150 system at an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) was performed on a JEOL JEM-2010 analytical electron microscope operating at an accelerating voltage of 200 kV to obtain indications about the microscopic structure of the catalysts. Meanwhile, the dispersion of the elemental composition and semiquantitative determination of the ratio of Ti, V, Mn, Ce, and O in the catalysts were verified by the energy dispersive X-ray spectrometer (EDX) analysis in the Oxford INCA EDAX Detecting Unit. Before the TEM test, the catalyst powder was ultrasonically suspended in alcohol for 10 min, and the obtained suspension was deposited on copper-grid-supported amorphous carbon films. Then the impregnated mesh was dried in air for TEM analysis. Meanwhile, 10 sites of EDX data were collected to get an average value of the elemental composition. To identify the surface nature and concentration of the active species, the catalysts were analyzed using the X-ray photoelectron spectroscopy (XPS). XPS was collected on a RBD upgraded PHI 5000C ESCA scientific electron spectrometer (PerkinElmer) with Mg KR radiation (hν = 1253.6 eV) as the excitation source. The operation conditions were kept constant at a high voltage of 14.0 kV and 250.0 W. The base pressure of the analyzer was about 5  10
8 Pa. To subtract the surface charging effect, the binding energies of V (2p), Mn (2p), Ce (3d), Ti (2p), and O (1s) were calibrated using the C-(C
H) component of the contaminated carbon (C1s = 284.6 eV) as the standard. 2.4. NH3-TPD Studies. Temperature programmed desorption of NH3 (NH3-TPD) experiments were carried out in a fixed-bed quartz reactor. During the experiments, 0.3 mL of catalyst sample was first pretreated in pure N2 at 500 °C for 2 h to yield a clear surface and then cooled to the room temperature. The cooled sample was saturated with 0.6 vol % NH3 in N2 at a flow rate of 100 mL/min for 1 h and then purged by N2 for another 1 h until no NH3 could be detected by FTIR at the outlet. In the desorption process, the carried gas N2 was kept at a flow rate of 100 mL/min while the temperature increased from room temperature to 500 °C with a heating rate of 5 °C/min. The effluent composition was analyzed continuously with a FTIR spectrometer. 2.5. In Situ DRIFTS Studies. The in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) experiments were used to investigate the adsorption and the reaction of NH3, NO, and O2 on the catalyst surface. The in situ DRIFTS was performed on a Thermo NICOLET 6700 FTIR spectrometer equipped with an in situ diffuse reflection chamber with a high sensitive mercury
cadmium
telluride (MCT-A) detector. An approximately 70 mg sample was finely ground and pressed into a self-supported wafer. The wafer was placed in a sample holder assembly in a Harrick praying mantis DRIFTS high-temperature reaction chamber which was directly connected to a purging and adsorption gas control system. The temperature of the reaction chamber was controlled precisely by an Omega temperature controller. Prior to each experiment, the sample was treated at 450 °C in flowing N2 for 2 h and then cooled to the desired temperature. At each temperature, the background spectrum of the catalyst was recorded at a gas flow 12852

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

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

3. RESULTS AND DISCUSSION 3.1. Catalytic Activity and Selectivity Studies. 3.1.1. NH3
SCR activity of 3% V2O5/TiO2 and Ti0.9V0.1O2
δ Catalysts.

Roy reported that the Ti0.9Mn0.05Fe0.05O2
δ catalyst prepared with the SHS method was superior to the catalyst of 5% V2O5/ TiO2 prepared by the wet impregnation method.43 Figure 1 shows that the NOx conversion over Ti0.9V0.1O2
δ prepared by the SHS method and 3% V2O5/TiO2 prepared by the wet impregnation method. It can be seen that the NOx conversion over 3% V2O5/TiO2 is very low (no more than 30%) below 200 °C. However, the SHS catalyst of Ti0.9V0.1O2
δ shows superior catalytic activity within a wide temperature window of 170 to 390 °C, within which more than 90% NOx is reduced. Particularly, at the extremely low temperature of 75 °C, this catalyst removes more than 50% NOx. The decrease of NOx conversion above 380 °C can be ascribed to the unselective NH3 oxidation by O2, which usually takes place at high temperatures. The active reaction temperature window (75
450 °C, in which more than 50% NOx is reduced) of Ti0.9V0.1O2
δ is much wider than that of the conventional V2O5/TiO2 catalyst. This may be attributed to the fact that V and Ti in the catalyst prepared by the SHS method interact strongly and lead to high dispersion of active phases and excellent structural characteristics that benefit SCR reactions.50
53 3.1.2. Effect of Partial Substitution of V with Mn, Ce on the Activity and Selectivity of Ti0.9V0.1O2
δ. In order to further improve the activity of Ti0.9V0.1O2
δ at low temperatures, we partially substitute V with Mn and Ce. Figure 2a presents the effect of Mn and Ce on the NOx conversion and N2O formation. It can be seen that by substituting half of V with equivalent Mn (Ti0.9Mn0.05V0.05O2
δ), the NOx conversion in 50
80 °C is further improved. At 55 °C, the NOx conversion over Ti0.9Mn0.05V0.05O2
δ is greatly improved from 14.5% to 54.2%. It is worth noting that, when the V content is partially substituted with Mn and Ce, the catalysts show almost the same activity in the range of 100
450 °C. As shown in Figure 2b, the selectivities of N2 and N2O of Ti0.9Mn0.05V0.05O2
δ and Ti0.9Ce0.05V0.05O2
δ are obviously higher than that of Ti0.9V0.1O2
δ at high temperatures (250
450 °C). In Figure 2a, N2O appears in high concentration (approximate 70 ppm) above 400 °C over Ti0.9V0.1O2
δ catalyst. The formation of N2O over Ti0.9Mn0.05V0.05O2
δ increases with

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

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

elevating temperatures and reaches a maximum of 42 ppm at 310 °C, and then declines to 16 ppm at 450 °C. It is noticed that there is only a few ppm of N2O detected on Ti0.9Ce0.05V0.05O2
δ at any temperature. This indicates that the Ce substitution catalyst displays excellent N2 selectivity. In a word, we conclude that among the three catalysts in this study Ti0.9Ce0.05V0.05O2
δ catalyst has the best NOx conversion and N2 selectivity. 3.1.3. Effect of GHSV. GHSV is a very important parameter to be considered in the catalyst design. Therefore, the influence of GHSV on the NOx removal efficiency over Ti0.9Ce0.05V0.05O2
δ is investigated. The activities of Ti0.9Ce0.05V0.05O2
δ are tested over a wide range of GHSV from 12 000 to 60 000 h
1, and the results are shown in Figure 3. It can be seen that the NOx removal efficiency decreases dramatically with the increasing of GHSV, especially in the low temperature range below 250 °C. However, 12853

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Figure 4. Separate oxidation of NH3 over Ti0.9V0.1O2
δ, Ti0.9Mn0.05V0.05O2
δ, and Ti0.9Ce0.05V0.05O2
δ. (a) NH3 oxidation efficiency, (b) selectivity for N2, (c) selectivity for NOx, and (d) selectivity for N2O. Reaction conditions: [NH3] = 600 ppm, [O2] = 5 vol %, N2 as balance gas, total gas flow rate 100 mL/min, and GHSV = 12 000 h
1.

it should be noted that the NOx removal efficiency still could maintain above 85% in a range of 310
390 °C even though the GHSV reaches 60 000 h
1, although the active reaction temperature window is much narrower than that in the low GHSV condition. Furthermore, the influence of GHSV on the NOx removal efficiency in the high temperature range above 350 °C is not obvious. On the contrary, the NOx removal efficiency at high GHSV is relatively higher than at low GHSV at the temperature above 400 °C, which is explained by the inhibition effect on NH3 oxidation to NOx with increasing GHSV at high temperatures. In summary, the NH3
SCR reaction over Ti0.9Ce0.05V0.05O2
δ catalyst is very sensitive to the contact time. GHSV is an important factor worth considering to ensure the appropriate catalytic efficiency in designing the performed catalyst for industrial applications. 3.2. NH3 Oxidation. In the SCR reaction, the oxidation of NH3 gives rise to NO and N2O, which will decrease the selectivity of N2. Therefore, the selectivity of SCR is closely correlated with the oxidation degree and the oxidation products of NH3.55,56 As shown in eqs 8
11, there are competitive reactions in NH3
SCR, in which NH3 is consumed by NO and O2 in reaction 8 as well as is oxidized to N2, NOx, and N2O directly in reactions 9
11.25,36 4NO þ 4NH3 þ O2 f 4N2 þ 6H2 O

ð8Þ

4NH3 þ 3O2 f N2 þ 6H2 O

ð9Þ

4NH3 þ 5O2 f 4NO þ 6H2 O

ð10Þ

2NH3 þ 2O2 f N2 O þ 3H2 O

ð11Þ

To evaluate the performance of the catalysts on the NH3 oxidation, the NH3
O2 mixture was led over the catalysts at the room temperature. After a saturated adsorption of NH3, the

temperature is risen to 450 °C at a ramp of 5 °C/min. Figure 4a presents the oxidation rate of NH3 over Ti0.9V0.1O2
δ, Ti0.9Mn0.05V0.05O2
δ, and Ti0.9Ce0.05V0.05O2
δ as a function of the reactor temperature. The activity of the NH3 catalytic oxidation based on T50 (the temperature at which 50% NH3 being oxidized) follows this order: Ti0.9V0.1O2
δ < Ti0.9Ce0.05V0.05O2
δ < Ti0.9Mn0.05V0.05O2
δ. The products of the NH3 oxidation include N2, NOx, N2O, and H2O. Figure 4b
d presents the selectivities of N2, NOx, and N2O, respectively. The selectivity to NOx over three catalysts is almost the same in the whole range of reaction temperatures. It is shown in Figure 4b that most NH3 is oxidized into N2 over three catalysts, in which Ti0.9Ce0.05V0.05O2
δ shows the maximum selectivity for N2. The catalyst of Ti0.9Mn0.05V0.05O2
δ shows the minimum selectivity toward N2 and the maximum selectivity to N2O within 160
360 °C. When the temperature exceeds 270 °C, the N2O generated from Ti0.9Mn0.05V0.05O2
δ decreases markedly, whereas the N2O from Ti0.9V0.1O2
δ increases sharply. As a result, the N2 selectivity of Ti0.9Mn0.05V0.05O2
δ is superior to that of Ti0.9V0.1O2
δ over 360 °C. We propose that the selectivity in NH3 oxidation is correlated with the SCR reaction over the catalysts, and that the high N2 selectivity in NH3 oxidation is in favor of the total N2 selectivity in SCR reactions. 3.3. Catalyst H2O and SO2 Tolerance Durability Test. The diesel engine exhaust gas usually contains both H2O and SO2, which are likely to poison the catalysts and depress their activities. Therefore, the resistance of the catalysts to the deactivation by H2O and SO2 is very important for industrial applications in the DeNOx process. For this purpose, We investigated the H2O and SO2 tolerance of Ti0.9Ce0.05V0.05O2-δ that is the best catalyst in activity and N2 selectivity under the circumstance of containing 400 ppm SO2 alone, 7% H2O alone, and both 7% H2O and 400 ppm SO2, respectively. As illustrated in Figure 5, in the absence of SO2, a steady-state NOx conversion of 83.1% at 150 °C is obtained in the first 2 h. 12854

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Figure 5. Effect of SO2 on the NOx conversion over Ti0.9Ce0.05V0.05O2
δ at 150 °C. Reaction conditions: [NO] = [NH3] = 600 ppm, [O2] = 5 vol %, 400 ppm SO2, N2 as balance gas, total gas flow rate 100 mL/min, and GHSV = 12 000 h
1. Figure 7. SEM micrographs of various conditions of Ti0.9Ce0.05V0.05O2
δ surface (a) the fresh, (b) after SO2 tolerance test in the presence of 400 pmm SO2 at 150 °C for 26 h, (c) after calcined at 200 °C for 2 h, and (d) after calcined at 400 °C for 2 h. SEM operation conditions: accelerating voltage: 20 kV, magnification times: 1000, and marker length: 50 μm.

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

When 400 ppm SO2 is introduced into the feed gas mixture, the NOx conversion first increases slightly to 83.4% (shown in the insert graph in Figure 4) and then gradually decreases to 79% after 26 h. When SO2 is eliminated from the feed gas stream, the NOx conversion gradually recovers to 80.5% and then remains almost unchanged during the next 2 h. The result demonstrates that Ti0.9Ce0.05V0.05O2
-δ shows good resistance to SO2 during NOx reduction at low temperatures. The promoted effect of SO2 in the initial state may be attributed to the formation of SO42- on the catalyst surface, which can increase the catalyst surface acidity and promote the NH3 absorption and thus the reactions between adsorbed NH3 and NOx.57 The deactivation of the catalyst may result from the chemiadsorption competition between SO2 and the reactants (NH3 and NOx) on the activity sites of the catalyst surface.58
60 Besides, SO2 can easily combine with NH3 to form instable sulphite on the catalyst surface at low temperatures.40
42 The sulphite would react with chemisorbed oxygen to form sulfate that covers the active site and decreases the activity of the catalyst.40,41,61 The effect of H2O on the NOx conversion over the Ti0.9Ce0.05V0.05O2
δ catalyst was monitored as a function of time on stream at 150 °C. As indicated in Figure 6, before H2O is added, the SCR reaction is stabilized at a steady-state NOx conversion of 83.1% at 150 °C for 2 h. When only 7% H2O is introduced into the feed gas mixture, the NOx conversion decreases slowly to about 80.2% after 26 h. After the removal of H2O from the feed gas stream, the NOx removal efficiency recovers to the original level before poisoning, suggesting that the poisoning effect of H2O on the Ti0.9Ce0.05V0.05O2
δ catalyst

is reversible, and that the catalyst possesses strong resistance to H2O under our test conditions. The synergetic effect of H2O and SO2 on the SCR activity over the Ti0.9Ce0.05V0.05O2
δ catalyst at 150 °C is also illustrated in Figure 6. When 7% H2O and 400 ppm of SO2 are simultaneously added to the reaction gas mixture, the NOx removal efficiency shows a relatively significant decrease from 83.1% to 70.1% during the following 26 h and then returns quickly to 74.4% in the next 4 h after both H2O and SO2 are eliminated from the reaction gas. The results clearly indicate that the deactivation of the Ti0.9Ce0.05V0.05O2
δ catalyst is due to the synergistic poisoning effect of SO2 and H2O. In the presence of both H2O and SO2, SO2 can be oxidized to SO3 by O2 and eventually forms ammonium bisulfate (NH4HSO4) and ammonium sulfate ((NH4)2SO4) by reacting with NH3 and H2O.59 Meanwhile, the formation and deposition rate of ammonium bisulfate or ammonium sulfate salts is much greater than the one in the presence of H2O alone and the one in the presence of SO2 alone, respectively. On the one hand, the SOx compounds (SO2 þ SO3) are adsorbed on the active metal oxides, and they are difficult to be adsorbed at the low temperatures; on the other hand, the easy formation of the ammonium bisulfate and ammonium sulfate species results in blocking the catalyst pores and occupying the catalytic active sites, which then deactivates the catalyst. It is also noted that when the catalyst is heat treated in N2 at 400 °C for 2 h the activity could be rapidly restored to almost its original level in the absence of H2O and SO2. The recovery of the catalytic activity after the heating treatment at the high temperature is due to the fact that the SOx compounds may be desorbed and that the sulfate species formed on the surface of catalysts have been decomposed completely,57,59 which can be verified by the FTIR spectra results about the various conditions of the Ti0.9Ce0.05V0.05O2
δ catalyst shown in Figure 8. The results mentioned above indicate that the Ti0.9Ce0.05V0.05O2
δ catalyst is robustly tolerant to H2O and SO2. In order to investigate the species formed on the surface of Ti0.9Ce0.05V0.05O2
δ catalyst, the morphologies of the fresh and the poisoned catalysts are examined respectively by the scanning 12855

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

The Journal of Physical Chemistry C

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Table 1. Physical and Morphological Properties of Ti0.9V0.1O2
δ, Ti0.9Mn0.05V0.05O2
δ, and Ti0.9Ce0.05V0.05O2
δ BET surface

pore volume

average pore

area (m2/g)

(cm3/g)

diameter (nm)

Ti0.9V0.1O2
δ

146.4

0.2049

5.596

Ti0.9Mn0.05V0.05O2
δ

144.0

0.1438

3.993

Ti0.9Ce0.05V0.05O2
δ

146.2

0.1657

4.535

catalysts

Figure 8. Comparison of FTIR spectra of various conditions of Ti0.9Ce0.05V0.05O2
δ (a) the fresh, (b) after SO2 tolerance test in the presence of 400 ppm SO2 at 150 °C for 26 h, (c) after calcined at 200 °C for 2 h, and (d) after calcined at 400 h for 2 h. FTIR operation conditions: spectral resolution: 1 cm
1 and 64 scans per spectrum.

electron microscope (SEM). As presented in Figure 7a, the surface of the fresh catalyst is smooth and uniform. After we feed 400 ppm SO2 for 26 h at 150 °C, a significant agglomeration and bulk deposition with a size of 30
50 μm could be observed obviously in the SEM images as shown in Figure 7b. The deposition on the catalyst surface may be attributed to the formation of ammonium nitrate (NH4NO3) and ammonium sulfate ((NH4)2SO4). For comparison, the poisoned catalyst is calcined and its morphology is checked with SEM again. When the poisoning catalyst is calcined at 200 °C for 2 h, the deposition becomes thin (with the size of 10
20 μm) as shown in Figure 7c. As the decomposition temperature of NH4NO3 is about 170 °C, most of NH4NO3 has been decomposed at 200 °C, leaving only the residue of (NH4)2SO4, whose decomposition temperature is near 300 °C. When the poisoned catalyst is calcined further at 400 °C for 2 h, the deposition vanishes and the surface of the catalyst returns to sublimate, as illustrated in Figure 7d. The transmission IR analysis is performed to qualitatively analyze whether the ammonium nitrate and ammonium sulfate are actually formed and deposited onto the catalyst to clarify the trace of the deposition during calcination. Before the test, the samples are diluted by KBr and pressed into wafers and then placed in the Harrick Praying Mantis Sampling Kit. As shown in Figure 8, the spectrum of the fresh catalyst exhibits three characteristic peaks at 932 and 3677 cm
1 and a large band at about 3232 cm
1. The catalyst after SO2 exposure exhibits not only all of the characteristic IR bands of the fresh catalyst but also some new IR bands. The new bands at 2939, 3132, and 1443 cm
1 can be assigned to the absorbance peaks of ammonium nitrate,62
64 and the IR bands at 1280 and 1040 cm
1 are due to the absorbance peak of ammonium sulfate.41,59,61,65
67 The IR spectrum of the poisoned catalyst after 2 h calcination at 200 °C shows that the ammonium nitrate IR bands disappear, whereas the IR bands of ammonium sulfate survive and become weaker. The IR spectrum from the poisoned catalyst after 2 h calcination at 400 °C shows no difference to the spectrum of the fresh catalyst except that a weak band of 1040 cm
1 is residual. The IR results are consistent with those of SEM micrographs. It should be pointed out that, because of the high resolution of the FTIR spectrometer, traces of ammonium sulfate (at band of 1040 cm
1) can still be detected by FTIR but cannot be depicted by the SEM due to the lower magnification times.

It is known that the NH3
SCR system of the low speed marine diesel engine exhaust is in the low temperature range of 150
280 °C with relatively low GHSV from 6000 to 10 000 h
1, and usually contains several hundreds ppm of SO2 due to the combustion of the fuel with more than 4.5% sulfur. The results mentioned above indicate that the Ti0.9Ce0.05V0.05O2
δ catalyst prepared by the SHS method has the superior performance with more than 83% NOx removal efficiency and higher than 95% N2 selectivity over a broad temperature window of 150
400 °C. Besides, according to the H2O and SO2 tolerance and durability test results, the Ti0.9Ce0.05V0.05O2
δ catalyst is also robustly tolerant H2O and SO2. Therefore, the catalyst is especially suitable for the NOx removal in the SCR system of marine diesel engines. 3.4. Catalyst Characterization. 3.4.1. BET. Table 1 summarizes the specific surface area, the BJH desorption pore volume, and the average pore diameter of Ti0.9V0.1O2
δ, Ti0.9Mn0.05V0.05O2
δ, and Ti0.9Ce0.05V0.05O2
δ. As can be noticed from this table, the SHS catalysts prepared under appropriate conditions exhibit high specific surface areas. The BET surface areas of the three catalysts are almost identical and achieve more than 140 m2/g. The large specific surface areas may be attributed to the good dispersion of active components on the titania support.53,68 Among these catalysts, Ti0.9V0.1O2
δ has the largest pore volume and the largest average pore diameter. Large pore volume might be beneficial to the catalytic activity, but a large average pore diameter might be one of the reasons for the activity decline. It is also noticeable that the Ti0.9Mn0.05V0.05O2
δ and Ti0.9Ce0.05V0.05O2
δ catalysts possess more abundant micropores ( Ti0.9Ce0.05V0.05O2
δ. This is mainly due to the decrease of well-defined titania species and the increase of V and Mn (or Ce) on the catalyst surface.6,22,52 The O (1s) peaks could be fitted into two peaks, in which the one at lower binding energies (530.2
529.7 eV) is corresponding to the lattice oxygen O2- (denoted as OL), and the subpeak at higher binding energies (532.5
531.1 eV) is ascribed to the surface chemisorbed oxygen, such as O22- or O
(hereafter, denoted as OA).6,9,20,82 As shown in Figure 13e and Table 2, after V is partially substituted by Mn, the concentration of OA increases from 10.2% to 14.0%, and the OA/(OA þ OL) ratio has an obvious increase from 14.0% to 19.4%. When V is partially substituted by Ce, the concentration of OA increases further, reaching 17.0%, and the corresponding OA/(OA þ OL) ratio also increases greatly to 23.7%, which is much higher than that of Ti0.9V0.1O2
δ. The surface chemisorbed oxygen OA has been reported to be the most active oxygen and highly active in the oxidation reaction due to its higher mobility than the lattice oxygen OL.6,82 Moreover, the high relative concentration ratio of OA/(OA þ OL) on the catalyst surface could be correlated with the high SCR activity. The doping of the Ce and Mn in Ti0.9V0.1O2
δ could create a charge imbalance, the vacancies, and the unsaturated chemical bonds on the catalyst surface, and lead to the increase of chemisorbed oxygen on the surface.6,100 Surface chemisorbed oxygen has been reported to be the most active oxygen and plays an important role in oxidation reactions. The doped catalysts have more oxide defects or hydroxyl-like groups acting as Brønsted acid sites and benefiting the NH3 adsorption in the form of NH4þ.82
84 The formed NH4þ can react with adsorbed NO2 and nitrate species to produce active intermediate species, and then further react with gaseous NO to produce N2 and H2O. This means that the high OA/(OA þ OL) ratio of Ti0.9Ce0.05V0.05O2
δ might diminish the unselective oxidation of NH3 by O2 to N2O, NO, or NO2 and thus give rise to its superior N2 selectivity. The results of the activity and selectivity studies in section 3.1.2 are well consistent with the XPS findings. According to the results of BET, it is observed that the BET surface areas of Ti0.9V0.1O2
δ, Ti0.9Mn0.05V0.05O2
δ, and Ti0.9Ce0.05V0.05O2
δ catalysts are almost identical, indicating that the BET surface area may not be the main factor causing the different catalytic performances of the three catalysts. It is also

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

noticeable that the Ti0.9Mn0.05V0.05O2
δ and Ti0.9Ce0.05V0.05O2
δ catalysts possess more abundant micropores (