Hydrothermal Stability of FeâBEA as an NH3-SCR Catalyst - Industrial

Article pubs.acs.org/IECR

Hydrothermal Stability of Fe−BEA as an NH3‑SCR Catalyst Soran Shwan,*,† Radka Nedyalkova,† Jonas Jansson,‡ John Korsgren,‡ Louise Olsson,† and Magnus Skoglundh† †

Competence Centre for Catalysis, Chalmers University of Technology, SE-41296, Gothenburg, Sweden Volvo Group Trucks Technology, SE-40508, Gothenburg, Sweden

‡

ABSTRACT: The hydrothermal stability of Fe−BEA as a selective catalytic reduction (SCR) catalyst was experimentally studied. Cordierite supported Fe−BEA samples were hydrothermally treated at 600 and 700 °C for 3−100 h to capture the effect of aging time and temperature. Before and after aging the samples were characterized with BET, XPS, XRD, and NH3-TPD. The catalytic performance of the samples with respect to NO oxidation, NH3 oxidation, and NO reduction (NH3-SCR) was studied by flow reactor experiments to correlate changes of the catalytic performance with structural changes of the zeolite and the iron phases. The NH3-SCR experiments did not show any significant decrease in activity after a short time of aging (3 h at 700 °C) even though the ammonia storage capacity decreased by 40% and the oxidation state of iron slightly increased. A longer time of aging resulted in decreased activity for NO reduction during low temperatures (150−300 °C), while at higher temperatures (400−500 °C) the activity remained high. The results indicate that the NO reduction is more sensitive at low temperatures to changes in the oxidation state of iron caused by hydrothermal aging than at higher temperatures. Furthermore, a maximum in activity for NO oxidation and increased oxidation state of iron (Fe3+) indicate Fe2O3 particle growth. regeneration of the particulate filter, which usually is placed close to the SCR catalyst. The regeneration occurs typically between 600 and 700 °C.19 Hence, the hydrothermal stability of metal-based zeolites is crucial. Previous studies of Fe−ZSM-5 as an SCR catalyst have shown stable catalytic properties below 500 °C under hydrothermal conditions. However, at higher temperatures deactivation becomes a problem.1,10,20,21 Different mechanisms have been proposed for the deactivation caused by hydrothermal treatment. The two major mechanisms are (i) the loss of catalytic surface area due to pore collapse of crystallites of the active phase and (ii) the loss of support area due to support collapse and crystallite growth.22 At temperatures above 500 °C Brønsted acidity has been shown to decrease due to dealumination.1,3,23−26 Water also favors migration of iron in Fe-exchanged zeolites which results in formation of metal oxide clusters and particles with reduced activity.27 Brandenberger et al. proposed a mechanism for the hydrothermal deactivation of Fe−ZSM-5 which is dominated by three parallel processes: (i) rapid dealumination of Brønsted sites (i.e., Al−OH−Si), (ii) depletion of dimeric iron species, and (iii) migration of isolated iron ions followed by dealumination.23 The impact from loss of active iron sites and Brønsted acidity on the SCR activity has been thoroughly investigated, but no general conclusions have been able to be drawn. Recent studies show that high numbers of Brønsted acid sites are not required for high SCR activity.23,28 It has been concluded that NO oxidation is the rate-determining step3,29 in the SCR reaction at low temperatures where the reaction preferentially proceeds over the ionexchanged metal.1,27 Devadas et al. showed that Fe2O3 particles

1. INTRODUCTION Catalyst deactivation is a significant and inevitable problem in many important applications. The deactivation causes negative economic as well as environmental effects, and there is considerable motivation for understanding the mechanisms for catalyst decay to design stable catalysts and catalytic processes. Selective catalytic reduction with ammonia (NH3SCR) is a well-established and effective method to eliminate nitrogen oxides (NOX) under oxygen excess for stationary and, more recently, mobile applications.1 Vanadia-based catalysts were initially used for SCR applications. However, problems including decrease in activity and selectivity, and also the toxicity of vanadia, which may form volatile compounds at high temperatures, have promoted the development of alternatives to vanadia-based catalysts, especially for mobile applications.1 Metal-ion-exchanged zeolites, particularly based on copper2−7 or iron,1,3,8−11 have in this connection proven to be very active and promising alternatives to vanadia in SCR catalysts.1,12,13 Iron-based zeolites are generally more active for SCR at higher temperatures compared to zeolites based on copper, which have good low-temperature performance but lower hightemperature activity. Furthermore, Kamasamudram et al. have compared iron- and copper-based zeolites and shown higher SCR performance for zeolites based on iron under transient conditions.14 Recently, promising results have been shown for simultaneously exchanged Fe/Cu zeolites combining the advantages of both metals in the same catalyst.15 Several challenges arise when using these materials in exhaust after-treatment systems for lean burn or diesel vehicles. One problem is catalyst deactivation by poisoning caused by hydrocarbons,16−18 especially during cold start conditions. The resistance of metal-exchanged zeolites toward hydrocarbon poisoning can be enhanced by the addition of a protection layer of mordenite.16 Another problem is thermal deactivation due to the high-temperature conditions in connection with the © 2012 American Chemical Society

Received: Revised: Accepted: Published: 12762

June 11, 2012 September 11, 2012 September 12, 2012 September 12, 2012 dx.doi.org/10.1021/ie301516z | Ind. Eng. Chem. Res. 2012, 51, 12762−12772

Industrial & Engineering Chemistry Research

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and NH3 oxidation experiments) were performed using a continuous flow reactor system which is described in detail in ref 2. Briefly, the reactor consists of a horizontal quartz tube, 800 mm long with an inner diameter of 22 mm, equipped with a heating coil and insulation. The heating unit is regulated by a temperature regulator (Eurotherm). The sample is placed in the quartz tube with a thermocouple placed about 10 mm in front of the catalyst to control the inlet gas temperature. A second thermocouple is placed inside the center of the catalyst to measure the sample temperature. The gas mixing system contains separate mass flow controllers (Bronkhorst) for NO, NH3, O2, and Ar. Water is added downstream of the mixed gases in an evaporator which is heated to 150 °C. The amount of water added to the feed is regulated by a mass flow controller (Bronkhorst). Gas phase analysis is performed using a gas phase Fourier transform infrared (FTIR) spectrometer (MKS 2000 FTIR). 2.4. Catalyst Characterization. After aging the samples were characterized in the flow reactor where NH3-TPD experiments and activity measurements were performed. The samples were further characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and nitrogen physisorption (BET). Before each ammonia TPD experiment the samples were exposed to 8% O2 in Ar for 15 min at 500 °C. During the subsequent NH3-TPD experiment the sample was exposed to 400 ppm NH3 and 5% H2O for 40 min at 150 °C followed by a 30 min Ar flush with 5% water. This was followed by a temperature ramp with a heating rate of 10 °C/min from 150 to 500 °C in 5% water and Ar. The specific surface areas of the samples were measured using an ASAP 2010 sorptometer by means of N2 physisorption isotherms at −196 °C. Before N2 adsorption, the samples were thermally dried at 200 °C for 2 h under reduced pressure in order to remove adsorbed water. The specific surface area was determined according to the standard Brunauer−Emmett− Teller (BET) method.31 X-ray photoelectron spectroscopy studies were performed to determine the chemical state of iron in the samples. The XPS measurements were performed using a PerkinElmer PHI 5000C ESCA system described in detail in ref 32. The XPS spectra were obtained using monochromatic Al Kα radiation and a 45° takeoff angle. The monolith samples were cut into 1 × 2 mm and placed in the pretreatment chamber where the pressure was reduced to about 10−9 mbar before the samples were transferred to the ultrahigh vacuum chamber. The Fe 2p and O 1s binding energy levels were thoroughly studied. Correction for charging of the samples was performed by normalizing the spectra using the C 1s peak at 284.6 eV33 as reference. Deconvolution of the Fe 2p3/2 peak from each sample was performed by fitting a Gaussian−Lorentzian (GL) function with a Shirley background.34 The Lorentzian function models the lifetime broadening (natural broadening) due to the uncertainty principle relating lifetime and energy of the ejected electrons,35 while the Gaussian model describes the measurement process (e.g., response from instrument, X-ray line shape, and thermal broadening).35 The GL function was fitted to the experiments and compared to other studies. The peak positions were optimized according to the same procedure for all samples to achieve the smallest standard deviation, χ2, and were found to be 710.37 and 712.18 eV for Fe2+ and Fe3+, respectively. The Gaussian−Lorentzian ratio was fixed to 20, meaning 80% Gaussian and 20% Lorentzian. The aged sample 24 h−600 °C

are not active in the SCR reaction but are active in the oxidation of NO.30 It was observed that oxidation of NO to NO2 over Fe3+ is favored by Fe2O3 particles in Fe−ZSM-5 and is a prerequisite for the SCR reaction. Furthermore, as an SCR catalyst, Rahkamaa-Tolonen et al.3 showed that Fe−BEA is more thermally stable than Fe−ZSM-5 catalysts. The focus of this work is to investigate the hydrothermal stability of iron-exchanged zeolite BEA as an NH3-SCR catalyst with the aim to gain increased understanding of the fundamental deactivation mechanisms. Fe−BEA coated monoliths were hydrothermally treated at 600 and 700 °C for 3−100 h to capture the effect of aging time and temperature. The catalytic performance of the samples with respect to NO oxidation, NH3 oxidation, and NH3-SCR was studied by flow reactor experiments with the focus on correlating changes of the catalytic performance with structural changes of the zeolite and iron phases. The aged samples were characterized by temperature programmed desorption of ammonia, nitrogen physisorption, X-ray photoelectron spectroscopy, and X-ray diffraction.

2. EXPERIMENTAL METHODS 2.1. Sample Preparation. A 6 wt % Fe−BEA model SCR catalyst with a molar SiO2/Al2O3 ratio of 29 was used for the studies in this work. The catalyst consists of a washcoated ceramic monolith (400 cpsi) with 21 wt % active material (i.e., zeolite and iron) from which seven 20 mm long, 22 mm diameter samples were cut out. Any loosely bound washcoat in the samples was removed using pressurized nitrogen before further usage. 2.2. Hydrothermal Treatment. The fresh samples were hydrothermally treated in an aging reactor. The reactor rig consists of a gas preheater, an oven where the samples are placed, mass flow controllers, and a temperature and gas flow regulating system. The oven is vertically placed and consists of a preheater and an oven chamber where the samples are placed. The preheater has two thermocouples placed in the front and at the end of the preheater. The oven chamber has six thermocouples which are placed above each sample. The gas preheater contains a thermocouple for gas temperature regulation. The oven chamber has six additional small chambers were the samples are placed. The gas flow through the oven during the hydrothermal treatment was 4000 mL/min and consisted of 5% H2O, 6% O2, and N2 as inert gas. Three catalyst samples were hydrothermally treated at 600 °C for 24, 48, and 100 h, respectively. The remaining three samples were hydrothermally treated at 700 °C for 3, 24, and 48 h, respectively. One catalyst (denoted “fresh”) was kept untreated for comparison with the aged catalysts. Before analysis, all samples were degreened at 500 °C using 400 ppm NO, 400 ppm NH3, 8% O2, and 5% H2O for 2 h. The samples hydrothermally treated at 600 °C are referred to as 24 h−600 °C, 48 h−600 °C, and 100 h−600 °C, and the samples hydrothermally treated at 700 °C are referred to as 3 h−700 °C, 24 h−700 °C, and 48 h−700 °C. 2.3. Flow Reactor System. Before and after aging the following experiments were performed to characterize and evaluate the catalytic properties of the samples: temperature programmed desorption of ammonia (NH3-TPD), selective catalytic reduction of NOX with ammonia (NH3-SCR), NO oxidation, and NH3 oxidation. The NH3-TPD experiments and the experiments for evaluation of the catalytic properties (NH3-SCR, NO oxidation, 12763

dx.doi.org/10.1021/ie301516z | Ind. Eng. Chem. Res. 2012, 51, 12762−12772


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Table 1. Sample Weight, Specific Surface Area, Relative Amount of Fe3+, Ammonia Adsorption−Desorption Characteristics, and the Calculated Zeolite BEA Mean Crystal Size by the Debye−Scherrer Equation aging time (h)

aging temp (°C)

sample wta (g)

sp surf. areab (m2/g)

Fe 3+/(Fe2+ + Fe3+) [%]

adsorbed amt NH3c (mmol/g)

desorbed amt NH3c,d (mmol/g)

BEA mean cryst. size (nm)

fresh 24 48 100 3 24 48

− 600 600 600 700 700 700

3.91 3.88 3.79 3.94 4.07 3.88 3.93

114 113 107 106 109 107 103

51 53 62 63 58 61 73

8.26 5.39 4.77 4.36 5.26 4.15 4.10

8.36 5.59 5.09 4.67 5.47 4.42 4.17

52 48 47 45 49 45 44

a

Sample weight includes the mass of the monolith and the washcoat. bSpecific surface area for entire washcoated monolith. cNormalized based on sample weight (mmolNH3/gsample). dWeakly bound ammonia included in the calculated desorbed amount NH3.

had the highest standard deviation, also shown in Figure 3, with an χ2 value of 0.19 which is quite low compared to other studies.33 X-ray diffraction was used to identify the crystalline phases and to estimate the particle size in the washcoat of the different samples. The instrument used was a Siemens D500 X-ray diffractometer with Bragg−Brentano geometry and a Cu Kα source. The main BEA zeolite diffraction peak at 2θ = 22.7°36,37 was fitted with a Lorentzian function for all the samples, where the most severely aged sample (48 h−700 °C) had the highest standard deviation with a χ2 value of 1.15. The size of the crystalline phases was estimated using the Debye−Scherrer equation.38 2.5. Activity Measurements. Steady-state activity tests were performed for all samples in the presence of 5% water using a gas composition of 400 ppm NO and/or 400 ppm NH3 and 8% O2 in Ar in the flow reactor. In all experiments the total gas flow was kept constant at 3500 mL/min, which corresponds to a space velocity (GHSV) of 27 600 h−1. Before each experiment the sample was exposed to 8% O2 in Ar for 15 min. To investigate the activity for NO oxidation, the samples were exposed to 400 ppm NO, 8% O2, and 5% H2O, whereby the temperature was increased stepwise from 150 to 500 °C (150, 200, 250, 300, 400, and 500 °C). The duration was 40 min for the first step, 30 min for steps two and three, and 20 min for the last three temperature steps. The heating rate between each temperature step was 20 °C/min, and the temperature did not exceed the set point more by than 4 °C before stabilization. The activity for NH3 oxidation was investigated accordingly by exposing the samples to 400 ppm NH3, 8% O2, and 5% H2O, whereby the temperature was increased stepwise from 150 to 500 °C (150, 200, 250, 300, 400, and 500 °C). The duration was 40 min for the first step, 30 min for steps two and three, and 20 min for the last three temperature steps. The heating rate between each temperature step was 20 °C/min. The activity for selective catalytic reduction of NOX with ammonia as the reducing agent was studied for all samples. The gas feed consisted of 400 ppm NO, 400 ppm NH3, 8% O2, and 5% H2O. The samples were exposed to the gas mixture at 150 °C for 40 min, at 200, 250, and 300 °C for 30 min, and at 400 and 500 °C for 20 min.

Figure 1. NH3 uptake and desorption spectra for the studied catalysts. (a) Catalysts aged at 600 °C for 24, 48, and 100 h. (b) Catalysts aged at 700 °C for 3, 24, and 48 h. They are compared to a fresh sample and empty reactor experiment. The samples were exposed to 400 ppm NH3 and 5% H2O for 40 min at 150 °C, followed by 30 min of Ar and 5% H2O flowing over the catalyst, and finally a temperature ramp of 10 °C/min up to 500 °C was applied. The total flow rate is 3500 mL/ min.

empty reactor experiment. Figure 1a shows the ammonia response for the samples hydrothermally treated at 600 °C. During the storage phase the fresh sample shows complete ammonia uptake for about 5 min followed by a sloping NH3 response up to the saturation point, which corresponds to a high amount of ammonia adsorbed (8.26 mmol/g, catalyst) compared to the hydrothermally treated samples. For the samples aged at 600 °C the time for total uptake is almost constant, about 2 min. However, the slope of the ammonia response after the initial phase with complete uptake is lower for the sample aged for 24 h at 600 °C (5.39 mmol/g, catalsyt) compared to the samples aged at 48 and 100 h (4.77 and 4.36 mmol/g, catalsyt, respectively) which correspond to higher amounts of NH3 adsorbed. Figure 1b shows the ammonia responses for the corresponding experiment with the samples hydrothermally treated at 700 °C. There is a complete uptake of ammonia for about 3 min for the sample aged for 3 h (5.26 mmol/g, catalsyt). For the sample aged for 24 h there is a complete uptake for about 1 min, and for the sample aged at 48 h the total uptake is observed for about 2 min. However, the slope of the NH3 response is sharper for the sample aged at 48 h compared to the sample aged for 24 h, which corresponds to almost the same amount of adsorbed ammonia (4.15 and 4.10

3. RESULTS 3.1. Characterization. The results for the ammonia storage and desorption experiments are summarized in Table 1 and displayed in Figure 1 together with the NH3 response from an 12764



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mmol/g, catalsyt, respectively) in these samples. For the desorption part of the experiment, weakly bound ammonia desorbs during the entire initial period (30 min) at constant temperature for all samples. The amount of weakly bound ammonia is not significantly affected by the hydrothermal treatment for all samples. However, more pronounced differences between the samples can be seen during the subsequent temperature ramp corresponding to desorption of more strongly bound NH3. The fresh sample shows the NH3 desorption maximum at about 314 °C, the samples aged at 600 °C show the NH3 desorption maximum at about 303 °C, and the samples aged at 700 °C show the NH3 desorption maximum at about 295 °C. The results of the surface area measurements are summarized in Table 1. The specific surface area for the sample aged for 24 h at 600 °C decreases by about 1% compared to the fresh sample. For the samples aged for 48 and 100 h at 600 °C the decrease is about 7%. Furthermore, for all samples aged at 700 °C, the decrease in specific surface area is between 4 and 10% compared to the fresh sample. The results from the XPS measurements can be seen in Table 1 and in Figures 2 and 3. Figure 2 shows the XPS spectra of Fe for the fresh and all aged samples. Two peaks centered around 712 and 725 eV, corresponding to Fe 2p3/2 and Fe 2p1/2, respectively, are clearly seen in the spectra. The Fe 2p3/2 peak is more narrow and intense than the Fe 2p1/2 peak, and the peak area is also higher due to the spin−orbit (j−j) coupling, where the Fe 2p3/2 peak has degeneracy of four states while the Fe 2p1/2 peak only has two states.33 Furthermore, a satellite is associated with the Fe 2p3/2 peak. The satellite is located approximately 8 eV higher in binding energy than the main Fe 2p3/2 peak. The peak position of the Fe 2p3/2 peak for FeIIO and FeIII2O3 is reported to be around 710 and 712 eV, respectively.39−43 In Table 2 the positions of the Fe 2p3/2 peak for FeO, Fe 2 O3, and Fe 3 O 4 from several studies are summurized. Compared to the fresh sample, there is no notable shift in the peak positions for the aged samples. The position of the Fe 2p3/2 peak is approximately 711.6 eV for all samples. However, it can be noted that the small satellite of the Fe 2p3/2 peak becomes less pronounced with aging compared to the fresh sample, which indicates that the Fe 2p3/2 peak consists of Fe3O4.42,43 Fe3O4 can be stoichiometrically expressed as FeIIO·FeIII2O3. The obtained XPS spectra indicate a mixture of FeO and Fe2O3 in the aged samples at the higher binding energies. Figure 3 shows the resulting deconvolution of the Fe 2p3/2 peak for the fresh and aged samples. Experimental values are shown as black dots, while the fitted peaks, attributed to Fe2+ and Fe3+, and the sum of these peaks are shown by the solid black lines. The peak areas for Fe2+ and Fe3+ are marked with dark and light gray, respectively. In Table 1 the ratios between the peak areas for Fe3+ and the sum of Fe2+ and Fe3+ are summarized. For the fresh sample the relative amount of Fe3+ is 51%. The data in Table 1 show an increasing relative amount of Fe3+ with increased aging time and temperature. This trend can be seen by following the area of Fe2+ which becomes lower relative to the Fe3+ area with increasing aging time and temperature. For the most severely aged sample, 48 h−700 °C, the relative amount of Fe3+ has increased to 73%. Figure 4 and Table 1 show the results from the XRD analysis. Figure 4 shows the X-ray diffractograms for the samples aged at 600 and 700 °C together with the diffractogram for the fresh sample. No diffraction peaks from Fe2O3 at 2θ = 33.2 or 35.5°18 could be observed, neither for the fresh sample nor for the

Figure 2. XPS spectra of Fe 2p3/2 and Fe 2p1/2 regions for all samples. The positions of Fe2+ and Fe3+ in the deconvoluted peaks are marked with dotted lines at 710.37 and 712.18 eV, respectively. The associated satellite for Fe 2p3/2 is marked with a dotted line, around 719 eV.

samples aged at 600 °C, which indicates that the Fe2O3 particles, if present, are too small to be identified with XRD or that iron oxide is present as noncrystalline phases. However, a small peak at 2θ = 33.2° from crystalline Fe2O3 can be observed for the samples aged at 700 °C. This diffraction peak becomes more intense with increasing aging time, which indicates growth of Fe2O3 crystals. Diffraction peaks at 2θ = 7.74 and 22.7°, characteristic for zeolite BEA, can be observed for all samples. The intensities of these diffraction peaks change with increasing aging. For the fresh sample the intensity of the zeolite BEA peaks is lower and the peaks are broader than for the aged samples. The full width at half-maximum of the diffraction peak at 2θ = 22.7° was used to calculate the mean size of the zeolite crystals using the Debye−Scherrer equation. The mean size of the zeolite crystals is 52 nm for the fresh 12765



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Figure 3. Deconvolution of the XPS spectra of only the Fe 2p3/2 peak for all studied catalysts, including the fresh one. Dark gray and light gray represent Fe3+ and Fe2+, respectively The sums of the deconvoluted peaks are shown by solid black lines and compared to the experimental data in dots. (a) Samples aged at 600 °C, (b) samples aged at 700 °C, and (c) the fresh sample.

Table 2. Binding Energies for Fe 2p3/2 Peaks in Various Works in Comparison with the Fitted Peaks in Figure 3 species FeIIO

FeIII2O3

Fe3O4

binding energy [eV]

reference

709.5 709 710.2 710.5 711.6 711 712 712.3 712.7 714 710.6

McIntyre and Zetaruk57 Thomas et al.58 Pratt et al.59 Janas et al.60 McIntyre and Zetaruk57 Thomas et al.58 Thomas et al.58 Janas et al.60 Pratt et al.59 Thomas et al.58 Yamashita et al.33

sample. The crystal size decreases with increased aging time and temperature and is 44 nm for the most severely aged sample (48 h−700 °C). 3.2. Oxidation of NO and NH3. The results for the NO oxidation experiments where the samples were exposed to 400 ppm NO, 8% O2, and 5% H2O and the temperature was increased stepwise from 150 to 500 °C are shown in Figure 5. Figure 5a shows the outlet concentration of NO and NO2 from the experiment for the samples aged at 600 °C. At the lowest temperatures, 150 and 200 °C, only minor oxidation of NO can be observed. However, at 300 °C a notable increase of the oxidation rate can be observed. Interestingly, it can be noted that NO is more efficiently oxidized over the aged samples

Figure 4. X-ray diffraction spectra of Fe−BEA catalysts. The positions of H−BEA zeolite are marked at at 2θ = 7.74 and 22.7°. 12766



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Figure 5. NO and NO2 evolution during NO oxidation experiments: (a) aged samples at 600 °C compared to fresh sample; (b) samples aged at 700 °C compared with fresh sample. The samples were exposed to 400 ppm NO, 8% O2, and 5% H2O and increasing temperature stepwise from 150 to 500 °C (150, 200, 250, 300, 400, and 500 °C). The total flow rate is 3500 mL/min.

Figure 6. Evolution of NH3 concentration during NH3 oxidation experiments: (a) aged samples at 600 °C compared to fresh sample; (b) samples aged at 700 °C compared to fresh sample. The samples were exposed to 400 ppm NH3, 8% O2, and 5% H2O and increasing temperature stepwise from 150 to 500 °C (150, 200, 250, 300, 400, and 500 °C). The total flow rate is 3500 mL/min.

treatment and NO2 is most efficiently produced after the shortest treatment, 3 h. The results for the NH3 oxidation experiment where the samples were exposed to 400 ppm NH3, 8% O2, and 5% H2O and the temperature was increased stepwise from 150 to 500 °C are shown in Figure 6. The outlet concentrations of NH3 from the experiments with fresh and aged samples at 600 and 700 °C are compared in Figure 6. At 150 °C ammonia storage can be observed during the initial part of the experiment, and

compared to the fresh one. It can also be noted that the oxidation is most efficient after shorter hydrothermal treatment, meaning a higher NO oxidation over the sample aged for 24 h compared to the samples aged for 48 and 100 h at 600 °C. Figure 5b shows the outlet concentrations of NO and NO2 from the corresponding experiment for the samples aged at 700 °C. Almost no oxidation of NO can be observed at 150 and 200 °C, whereas at 300 °C NO starts to be oxidized. Also for these samples the NO oxidation is promoted by the hydrothermal 12767



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Figure 7. Evolution of NOX and NH3 concentrations during SCR reaction for samples aged at 600 °C: (a) NOX outlet concentration during SCR reaction compared to the fresh sample. (b) NH3 outlet concentration during SCR reaction compared to the fresh sample. The samples were exposed to 400 ppm NO, 400 ppm NH3, 8% O2, and 5% H2O and increasing temperature stepwise from 150 to 500 °C (150, 200, 250, 300, 400, and 500 °C). The total flow rate is 3500 mL/min. Temperature is represented by a dotted line.

Figure 8. Evolution of NOX and NH3 concentrations during SCR reaction for samples aged at 700 °C. (a) NOX outlet concentration during SCR reaction compared to the fresh sample. (b) NH3 outlet concentration during SCR reaction compared to the fresh sample. The samples were exposed to 400 ppm NO, 400 ppm NH3, 8% O2, and 5% H2O and increasing temperature stepwise from 150 to 500 °C (150, 200, 250, 300, 400, and 500 °C). The total flow rate is 3500 mL/min. Temperature is represented by a dotted line.

at 600 °C (24 h) and 700 °C (3 h) NH3 conversion is 30 and 40%, respectively. For the more severely aged samples at 600 and 700 °C the NH3 conversion is about 20 and 10%, respectively. 3.3. SCR. The results for the SCR experiments where the samples were exposed to 400 ppm NO, 400 ppm NH3, 8% O2, and 5% H2O and the temperature was increased stepwise from 150 to 500 °C are shown in Figures 7 and 8. Parts a and b of Figure 7 show the outlet concentrations of NOX and NH3, respectively, for the samples hydrothermally treated at 600 °C.

during the temperature increase after each temperature step, significant NH3 desorption peaks are seen (cf. the NH3-TPD experiments in Figure 1). However, between the highest temperatures the NH3 desorption is considerably lower than between the temperature steps at lower temperature. For temperatures below 300 °C no ammonia oxidation can be observed. For the fresh sample ammonia starts to react with oxygen at 300 °C. At 500 °C about 75% conversion is reached. At this temperature the hydrothermal treatment clearly affects the activity for ammonia oxidation. For the least aged samples 12768



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At 200 °C, there is a small difference between the fresh sample and the sample aged for 3 h. However, a more pronounced effect can be observed for the samples hydrothermally treated at 700 °C for 24 and 48 h. Generally, it can be noted that for both aging temperatures, 600 and 700 °C, the samples hydrothermally treated at 24 and 48 h show a high change in SCR activity compared to the fresh sample while the samples aged for 100 h at 600 °C do not show any further marked change in SCR activity.

At 150 °C the SCR reaction is very slow and almost no NOX reduction can be observed. However, at 200 °C significant conversion of NOX can be observed, where the SCR reaction is markedly affected by the hydrothermal treatment of the samples. At 400 and 500 °C the conversion of NOX is high for all samples. At 400 °C, it can be noted that the NOX conversion is slightly higher for the aged samples than for the fresh sample. However, at 500 °C no difference can be seen. Furthermore, when the temperature is increased from 250 to 300 °C, it can be seen in Figure 7a that the conversion of NOX temporarily increases before reaching steady state. The same behavior can be seen when the temperature is increased to the next temperature level, 400 °C. Figure 7b shows the outlet concentration of NH3. Also here ammonia is stored during the initial part of the experiment. At 150 °C the conversion of ammonia is very low, as is the conversion of NOX at the same temperature. There is a notable conversion of ammonia already at 200 °C and a clear difference between the fresh and aged samples. The conversion of ammonia is very high after 250 °C. An overconsumption of ammonia during NH3-SCR over Fe zeolites has previously been observed.44 For all the samples almost complete conversion of NH3 is observed at 400 and 500 °C, which is not the case for NO conversion. Significant ammonia desorption peaks can be seen between all temperature steps except for the highest temperatures, 400 and 500 °C. The aged samples show considerably lower ammonia desorption peaks compared to the fresh sample. It can be noted that the hydrothermal treatment of the catalysts at 600 °C clearly affects the SCR reaction for aging times up to 48 h. Another interesting observation is that the fresh sample never reaches steady state at the lower temperatures (200−250 °C), where the NOX conversion gradually becomes higher with time during the interval. The same observation can be seen also for the ammonia response from the fresh sample, where the conversion of ammonia increases with time but never reaches steady state during the temperature steps. Figure 8 shows the outlet concentrations of NOX and NH3 from the experiments for the samples aged at 700 °C. As for the samples aged at 600 °C the SCR reaction at 150 °C is very slow. However, above 150 °C there is a notable difference in NOX conversion between the fresh and aged samples. The SCR activity is most affected for the samples with the most extended hydrothermal treatment. At 400 and 500 °C the NO X conversion is very high, and at 500 °C complete NOX conversion is reached for all samples. As for the samples aged at 600 °C, the conversion is temporarily higher during the temperature increase until steady state is reached. Another interesting observation in Figure 8a is the NOX conversion at 300 °C, which is lower over the samples aged at 24 h compared to the more severely aged sample (48 h−700 °C). However, at other temperatures the sample aged for 48 h has lower conversion. Figure 8b shows the outlet concentration of NH3. Also for these samples ammonia storage is observed during the low-temperature step at 150 °C, where the conversion of ammonia is very low, as is the conversion of NOX. The conversion of ammonia becomes higher at 200 °C, and a clear difference in conversion is observed between the fresh and aged samples. At 400 and 500 °C complete NH3 conversion is observed for the fresh sample and for the aged samples. Strong ammonia desorption peaks can be seen between all temperature steps except between 400 and 500 °C. The same trend can be seen here as in Figure 7b; a higher amount of ammonia desorbs from the fresh sample compared to the aged samples.

4. DISCUSSION 4.1. Ammonia Storage. From the ammonia storage experiments it can clearly be seen that the ammonia storage capacity is considerably higher for the fresh sample than for the aged samples. This may be explained by loss of Brønsted acidity during the initial period of the hydrothermal treatment at 600 °C (and 700 °C). Brandenberger et al.23 showed that already after a short time of aging at 650 °C the Brønsted acidity of H− ZSM-5 almost completely disappeared due to dealumination of the zeolite. As the capacity to store ammonia mainly is related to the Brønsted acidity and likely also to the number of iron sites,1,23,28,45,46 the NH3 storage capacity of the zeolite decreases if the number of Brønsted acid sites decreases during the hydrothermal treatment. Furthermore, the XRD results indicate a reduction of the mean size of the zeolite crystals during the aging. This indicates dealumination and hence loss of Brønsted acidity and shows that the ammonia storage capacity is very sensitive to aging. Figure 1b shows a drastic decrease in ammonia storage capacity already after 3 h of hydrothermal treatment at 700 °C compared to the fresh sample. Furthermore, a shift in the ammonia desorption maximum toward lower temperatures can be observed for all aged samples, which indicates a decrease in the adsorption strength of ammonia. This is in line with previous studies showing that the adsorption energy of ammonia in Cu−BEA decreases after aging.47 Pinto et al.48 showed that the ammonia desorption peak for H−BEA can be described by desorption from 16 different adsorption sites with different adsorption strengths. However, it was further suggested that the main desorption peak can be divided into two main peaks with different binding energies of ammonia.48,49 The shift in the ammonia desorption maximum toward lower temperatures with aging in the present study indicates that the stronger Brønsted sites are more sensitive toward hydrothermal treatment compared to the weaker Brønsted sites. 4.2. NO Oxidation. The NO oxidation experiments show an increased activity for NO oxidation for the hydrothermally treated samples. For Fe−ZSM-5 the oxidation of NO has been concluded to proceed over Fe3+ sites in Fe2O3 particles.50 In the present study the XPS results show an increase of the relative amount of Fe3+ with increased hydrothermal treatment which could explain the increase in NO oxidation. Thus, the number of Fe3+ sites active for NO oxidation increases with aging. However, whereas the deconvoluted Fe 2p3/2 peaks show a continuous increase of the relative Fe3+ concentration in the catalyst with increased aging time, the activity for NO oxidation passes a maximum after a relative short aging period. The samples aged the shortest time at each temperature show the highest activity for NO oxidation. However, previous studies have shown higher activity for NO oxidation with increased concentration of Fe2O3 particles.30,50 This could be explained by a lower active surface area with particle growth. If the Fe2O3 particles grow with aging, the number of Fe2O3 surface sites will 12769



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NH3 continues to desorb during the temperature steps and frees active sites for NO adsorption, which increases the SCR reaction. However, the loss of Brønsted acidity does not seem to affect the overall NOX conversion. The least aged samples at 700 °C do not show any significant change in NOX conversion even though the storage capacity of ammonia has decreased, indicating that high storage capacity of ammonia in the zeolite is not crucial to obtaining high NOX conversion during NH3SCR. Devas et al.30 recently showed that NO oxidation over Fe2O3 particles in the zeolite is the rate-determining step for the NH3SCR reaction over Fe−ZSM-5. The ammonia SCR experiments in the present study show lower activity over the aged samples compared to the fresh sample for temperatures up to 400 °C. However, the activity for NO oxidation is higher for the aged samples and can be observed already at 150 °C for the samples aged at 700 °C. These results seem contradictory. At high temperature (400 °C) the NOX conversion over the aged samples is slightly higher compared to the fresh sample, whereas at 500 °C the highest NOX conversion is observed over the fresh sample. When comparing the results from the NO oxidation and NH3-SCR experiments, it seems that the oxidation of NO does not result in an increased NH3-SCR reaction rate at the lower temperatures. After the catalyst, there is no NO2 observed during the NH3-SCR experiments. When the molar ratio between NO and NO2 becomes closer to 1, the fast SCR reaction is promoted.29,54−56 At high temperatures the activity for NO oxidation is higher for the aged samples compared to the fresh sample and more NO2 is produced compared to lower temperatures. This results in higher NO conversion during the SCR reaction, which could explain why the aged and fresh samples have similar NOX conversions despite the differences in low temperature NH3-SCR activity. Brandenberger et al.27 showed that monomeric iron species (Fe3+) are the governing iron species for low-temperature SCR whereas dimeric iron species (Fe2+) are the dominating species for high-temperature SCR and for NH3 oxidation. The XPS results in the present study show a small relative decrease in Fe2+ concentration and a corresponding increase in Fe3+ concentration after a short time of aging, whereas the NOX reduction was almost unaffected. However, the activity for NH3 oxidation decreased strongly. A longer time of aging showed a progressive decrease in the relative concentration of Fe2+ and further decrease in activity for NH3 oxidation. The increase in the relative concentration of Fe3+ is likely due to formation of larger iron particles from monomeric and dimeric iron species, hence the observed decrease in NOX reduction at low temperatures. However, the activity for NH3-SCR at high temperatures did not significantly decrease as the NH3-SCR activity at low temperatures showed. When correlating the XPS experiments to NH3-SCR activity, the results indicate that the NOX conversion at high temperatures is not as sensitive to the loss of dimeric iron (Fe2+) sites as the loss of isolated iron (Fe3+) species is for the lower temperature range. However, a large fraction of the catalyst is not used at high temperatures due to high reaction rates. Therefore, the deactivation must be very severe until it is observed at higher temperatures (>400 °C), which could be the reason for the low degree of deactivation observed at high temperature.

decrease compared to smaller Fe2O3 particles. From the XPS results in Figure 2 it can be observed that the satellite to the Fe 2p3/2 peak becomes smaller with aging, which indicates that the particles on the surface consist of Fe3O4 (FeO·Fe2O3).42,43 This can be correlated to larger particles with lower active surfaces of Fe2O3, which could explain why the NO oxidation is higher after shorter aging. 4.3. NH3 Oxidation. The reactor experiments clearly show decreased activity in NH3 oxidation for the aged samples. Already after a short time of aging (3 h−700 °C) the activity strongly decreased compared to that for a fresh sample. However, for the SCR reaction the decrease in activity was considerably less pronounced after short-time aging when compared to the activity loss in NH3 oxidation. For Fe−ZSM-5 Brandenberger et al.23 found a strong linear correlation between the concentration of dimeric Fe2+ and the activity of NH3 oxidation. The decreased activity for ammonia oxidation for the aged samples in the present study is thus likely related to a decreased number of sites on dimeric Fe2+ species. The decreased ammonia oxidation indicates a strong sensitivity of dimeric iron species to hydrothermal treatment, which also is supported by the XPS results showing a progressive decrease of the Fe2+ concentration with increased aging. 4.4. NH3-SCR. For temperatures up to 300 °C the NOX conversion during NH3-SCR is strongly affected by the hydrothermal treatment. At higher temperatures the NOX conversion is almost complete and the difference between fresh and aged samples is low. There seems to be a correlation between the changes in the XPS spectra and the NH3-SCR activity. Brandenberger et al.23 showed that growth of Fe particles occurred after migration of ion-exchanged iron species and proposed that migration of such species is the main reason for the observed decrease in NOX conversion. The relative increase of the Fe3+ concentration as revealed by XPS study is most likely related to the lower activity for NOX reduction observed during the NH3-SCR experiments in the present study. Different iron species cannot be separated here; however, the increases of the relative amount of Fe3+ and activity for NO oxidation indicate an increase of iron in the form of Fe2O3 which is in line with the results in the study by Brandenberger et al.23 The NH3-SCR results in Figures 7 and 8 show an initial period with temporarily higher NOX conversion during the temperature ramps between the temperature steps (between 250 and 400 °C). A probable reason for this effect can be competition of active sites. When the temperature is increased, strong ammonia desorption is observed and at the same time higher conversion of NOX is seen. High NOX conversion is dependent on a balanced coverage of adsorbed NOX and NH3 species on the active sites.46,51−53 Prior to the temperature ramp, the ammonia coverage is higher which hinders the adsorption of NOX, while during the temperature ramp ammonia desorbs and high NOX conversion is temporarily obtained until the set temperature is reached and the ammonia concentration reaches the equilibrium coverage level for the actual temperature. The reason why this is only observed at higher temperatures is probably due to faster desorption of ammonia, which results in decreased ammonia inhibition. At low temperature the conversion of NOX continuously increases after the set temperature is reached for the fresh sample. This effect can be correlated to the amount of ammonia stored, and to the adsorption and desorption rates of ammonia. The desorption rate of ammonia is slower at low temperatures and

5. CONCLUDING REMARKS The aim of the present study is to gain an increased understanding of the fundamental deactivation mechanisms of 12770



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Technology and Miroslawa Milh for help with the hydrothermal treatment of the samples.

Fe−BEA as an NH3-SCR catalyst caused by hydrothermal aging by correlating changes of the catalytic performance with structural changes of the zeolite and iron phases. A short time of hydrothermal aging strongly reduced the ammonia storage capacity and shifted the desorption peak of NH3 toward lower temperatures, which indicates a decreased adsorption strength of ammonia. This can be compared with other studies where the main desorption peak from zeolitebased SCR catalysts is related to desorption from two main Brønsted sites with different adsorption strengths to NH3. The results in the present study indicate that the stronger sites are more sensitive to hydrothermal treatment compared to weaker sites. Furthermore, the oxidation state of iron in the zeolite increases continuously with aging. The effect of the increased Fe3+ concentration can be clearly seen during the NH3-SCR reaction, where the NOX reduction at low temperatures strongly decreased while the SCR activity remained high at high temperatures. Furthermore, this effect can also be seen during NH3 oxidation, where a strong decrease in activity can be observed already after a short time of aging. A temporarily increased SCR activity can be observed for all samples during the temperature ramps between the steps with constant temperature, indicating NH3 inhibition. Furthermore, the increased NO oxidation for the aged samples did not show any significant improvement of the NOX reduction during NH3-SCR. The deactivation of NH3-SCR activity seemed to occur in two steps: (i) a shorter time of aging strongly decreased the ammonia storage capacity and slightly increased the oxidation state of iron, which clearly decreased the NH3 oxidation and improved the NO oxidation but did not change the NOX conversion, and (ii) a longer time of aging continued to increase the oxidation state of iron which in turn resulted in a significant decrease of the NOX reduction during lowtemperature NH3-SCR. During high-temperature NH3-SCR the NOX reduction is relatively unchanged due to the aging, indicating that high-temperature SCR (400−500 °C) is less sensitive to hydrothermal treatment compared to the lower temperature range (150−300 °C), which decreases continuously with aging. However, at high temperatures a large fraction of the catalyst is not used due to high reaction rates and the deactivation must be very severe until it can be observed at higher temperatures (>400 °C), which could be the reason for the low degree of deactivation observed.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +46 (0)31 772 2943. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been performed within the FFI program (Project No. 32900-1), which is financially supported by the Swedish Energy Agency and partly within the Competence Centre for Catalysis, which is hosted by Chalmers University of Technology and financially supported by the Swedish Energy Agency and the member companies AB Volvo, Volvo Car Corp. AB, Scania CV AB, Haldor Topsøe A/S, and ECAPS AB. Financial support from the Knut and Alice Wallenberg Foundation, Dnr KAW 2005.0055, is gratefully acknowledged. The authors would also like to thank Volvo Group Trucks 12771



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