Effect of Zr Addition on the Low-Temperature SCR Activity and SO2

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Article

The Effect of Zr Addition on the Low-Temperature SCR Activity and SO Tolerance of Fe-Mn/Ti Catalysts 2

Boqiong Jiang, Boyang Deng, Zhanquan Zhang, Zuliang Wu, Xiujuan Tang, Shuiliang Yao, and Hao Lu J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 16 Jun 2014 Downloaded from http://pubs.acs.org on June 24, 2014

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The Effect of Zr Addition on the Low-Temperature SCR Activity and SO2 Tolerance of Fe-Mn/Ti Catalysts Boqiong Jiang,†,§ Boyang Deng,† Zhanquan Zhang,‡,§ Zuliang Wu,† Xiujuan Tang,*,† Shuiliang Yao,† Hao Lu† †

School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou 310012, China ‡

College of Mechanical and Electrical Engineering, China University of Petroleum, Qingdao 266580, China

§

Department of Civil and Environmental Engineering, University of Illinois, 205 N. Mathews Ave., Urbana, IL 61801, USA Corresponding author Tel.: +86 571 28008230. Fax.: +86 571 28008215 E-mail address: [email protected] (Xiujuan Tang)


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The Effect of Zr Addition on the Low-Temperature SCR Activity and SO2 Tolerance of Fe-Mn/Ti Catalysts ABSTRACT

In this paper, Zr is added to the Fe-Mn/Ti catalyst to increase NO conversion and improve SO2 tolerance. It is found that 0.03 is the optimal ratio for Zr/Ti+Zr. With this ratio, the NO conversion below 150 oC increases, and the SO2 poisoning is alleviated, while the further increase of Zr does not have a positive effect on NO conversion and SO2 tolerance. With Zr additive, more manganese oxides are reduced in the form of MnO2 and Mn2O3 at lower temperature in H2-TPR, and the total amount of H2 consumption rises, indicating better redox properties. It leads to the increase of NO complexes on the catalysts. Despite the small decrease of NH3 adsorption, the reaction via L-H way is promoted and NO conversion increases. Furthermore, more nitrates and NO2 are formed in the reaction with SO2 on Fe-Mn/Ti-Zr(0.03) compared to Fe-Mn/Ti, so the L-H reaction way is less inhibited by SO2 with Zr additive, and the SO2 tolerance of this catalyst is also improved.

Keywords: Zr addition; redox properties; NO2 formation; L-H mechanism


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INTRODUCTION Selective catalytic reduction (SCR) of NOx with NH3 reductant is a widely industrial technology for NOx removal from stationary sources. This technology with commercial V2O5WO3(MoO3)/TiO2 usually requires high reaction temperatures (300-400 oC), and the device should be placed upstream of particulate control devices and flue gas desulfurization scrubbers (FGD).1 The catalyst is deactivated by the high concentration of SO2 and alkaline metals from dus at this position. If the SCR device with commercial catalyst is placed downstream of FGD, it can only work at a very low GHSV. In addition, the risk of toxic vanadium pentoxide exposure to the environment, and the high conversion of SO2 to SO3 drive the upgrade of current technology.2 Until now, low-temperature SCR technology is gaining increased interest, since it can be operated downstream of particulate control devices and even FGD, which can effectively prevent the catalyst from deactivation by dust and SO2. Regarding the low temperature SCR catalysts, transition elements including Fe, Mn, Ce, Cu, and Ni have been investigated as the active component of the catalyst, and it is found that Mnbased catalysts show good activity at low temperatures.3,4 In spite of good activity of Mn-based catalysts, they are still easily poisoned by low concentration SO2 (50-300 mg/m3) in the gas stream after FGD.5,6 Efforts have been made to improve the catalytic activity and the SO2 tolerant performance of catalysts, such as introduction of Sn and Ca into the catalyst matrix.7,8 NO conversion without SO2 is improved. However, the introduction of Sn and Ca still cannot inhibit the deactivation by SO2 below 150 oC. Since this is the temperature of flue gas after particulate control and FGD devices, SO2 poisoning is still not solved for practical applications. In recent years, Zr has drawn more attention for its improvement on catalytic activity and tolerance.9 Tetragonal ZrO2 is believed to have stabilization effect on particle size and to inhibit


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the agglomeration, and smaller particle size exposes more active sites for NO reduction.10,11 It was also reported that compared with TiO2, ZrO2 not only improves the dispersion of active sites, but also offers better oxidation activity.12 In view of the properties of catalysts, ZrO2 shows both mildly acidic and basic properties, which can be transferred into strong acid sites in SCR reactions, and it was found that the lower concentration of basic sites by Zr additive could result in improved resistance to SO2 poisoning.13 When Zr was added to V2O5/WO3-TiO2 catalyst, it was revealed that Zr can inhibit the growth of TiO2 crystallite size, and therefore improves the thermal stability of the catalysts, rendering higher NO conversion.14 However, to the best of our knowledge, there is no report about the Zr additive to Fe-Mn/Ti catalysts, which has been confirmed to be effective for NO conversion at low temperature.15 In this manuscript, Zr was introduced to Fe-Mn/Ti catalyst, to investigate its effect on NO conversion and SO2 tolerance. It is found that the optimal amount of Zr is 0.03 for the ratio of Zr/Ti+Zr. With this ratio, the NO conversion and SO2 tolerance of the catalyst are both improved. The characterization results show that Zr additive improves redox properties and enhances the formation of medium strength NO complexes. With Zr addition, more NO complexes are formed under SO2, and the inhibition of SO2 on L-H reaction way is alleviated, revealing higher SO2 tolerance. MATERIALS AND METHODS Materials Butyl titanate and acetic acid were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Ethanol, manganese nitrate, ferric nitrate, and zirconium isopropoxide were provided by Shanghai Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals used were of analytical grade. Preparation of Fe-Mn/TiO2


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The catalyst was prepared by a sol-gel method reported elsewhere.15 In a typical synthesis, butyl titanate (0.1 mol), ethanol (0.8 mol), water (0.6 mol), acetic acid (0.3 mol), manganese nitrate (0.04 mol), ferric nitrate (0.01 mol) and zirconium isopropoxide (0.001-0.1 mol) were mixed under vigorous stirring, forming transparent red sol. After being stable at room temperature for several days (7-15 days), the sol was transformed to gel. Solid was obtained after drying at 378 K for 12 h. The resulting solid was grounded and sieved to 60-100 mesh, followed by the calcination at 773 K for 6 h in an air atmosphere in a tubular furnace. The catalyst is denoted as Fe-Mn/Ti-Zr(z), where z represented the molar ratio of Zr to Ti+Zr. Activity Test of the Catalyst The activity measurement of Fe-Mn/Ti-Zr catalyst was carried out in a fixed-bed, quartz flow reactor. The experiments were performed under atmospheric pressure at 80-180 oC. The typical reactant gas consisted of 1000 ppmv NO, 1000 ppmv NH3, 4 vol % O2, 100 ppmv SO2 (when used), and 8% H2O (when used), and balance N2. The total flow rate was 1000 mL/min. The experiments are performed under two conditions. First, 2 mL of catalyst was packed into the quartz tubes (internal diameter 1 cm). and the GHSV for the experiment was 30,000 h-1. Second, 0.4 mL of catalyst was packed into the quartz tubes (internal diameter 0.6 cm), and the GHSV for the experiment was 150,000 h-1.The tubing of the reactor system was heated (200 oC) to prevent the formation and deposition of ammonium nitrate. NO, NO2, SO2, N2O, and O2 concentrations were monitored by two flue gas analyzer (Quintox Kane International Limited, KM9106 and Madur Gas-21Plus). Catalyst Characterization N2 sorption was carried out at -196 oC on a surface analyzer (Adsorbed Quantchrome). The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) equation.


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X-ray diffraction patterns (XRD) were recorded on a XD-2 X-ray diffractometer using Cu Kα radiation with a scanning speed of 4o/min in the range of 2θ=20-80o. X-ray photoelectron spectrum (XPS) measurements were used to determine the atomic concentration and the state of the elements on the catalyst surface with a V.G. Scientific Escalab 250 with Al Kα X-rays. The surface concentrations of Fe, Mn, Ti, Zr, O, N, and S were calculated from the integral of peak areas of the XPS data divided by each sensitivity factor of the element. Temperature programmed reduction (TPR) and temperature programmed desorption (TPD) experiments were carried out on Quantachrome Chembet TPR/TPD (p/n 02138-1). Prior to the TPR/TPD test, the samples were pretreated at 500 oC for 30 min in He gas (120 mL/min) and then cooled down to 50 oC. TPR were carried out with a ramp of 10 oC/min in 10 vol% H2/He (120 min/min), from 50 to 800 oC. For NH3-TPD and NO-TPD experiments, the samples were exposed to 10 vol% NH3/He (120 mL/min) for 10 min or 10 vol% NO/He (120 mL/min) for 30 min at 50 oC, followed by He purge for another 30 min. Finally, the temperature programed desorption was run to 500 oC in He flow with a rate of 10 oC/min. Fourier transform infrared spectroscopy (FTIR) spectra were acquired using an in-situ diffuse reflectance infrared Fourier transform (DRIFT) cell (high temperature chamber with ZnSe window) equipped with the gas flow system. The DRIFT measurements were performed with Nicolet 6700 FTIR spectrometers at 4 cm-1 resolution with 64 co-added scans. The balance gas was He, and the gas flow rate was 30 mL/min. The catalyst was treated at 500 oC in He for 2 h, then cooled to 150 oC. The background spectrum recorded in He was subtracted from the sample spectrum. The final differential sample spectra were calculated by Kubelka-Munk function.


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To investigate the SO2 effect on the adsorbed species of NH3 and/or NO, the protocol was run as follows. Firstly, the catalyst was pretreated at 500 oC in He for 2 h. After cooling down to 150 o

C, 500 ppmv SO2+3 vol% O2 was introduced to the cell, and then the catalyst was purged by

pure He for 30 min. The background spectrum recorded in He was subtracted from the sample spectrum. And then NO + O2/NH3 + O2 were introduced to investigate their adsorption on SO2 pretreated catalysts. RESULTS AND DISCUSSION The effect of Zr on catalytic activity and SO2 tolerance Figure 1 shows the influence of the Zr additive on NO conversion and SO2 tolerance of the catalysts. NO conversion increases with temperature without Zr. After introducing Zr into the catalyst, NO conversion is promoted in the whole temperature (80-180 oC) when the Zr/Ti+Zr ratio is less than 0.05. For Fe-Mn/Ti-Zr(0.05), the low-temperature activity is improved, but the NO conversion at 180 oC decreases. With further increasing Zr content, NO conversion is lower than Fe-Mn/Ti-Zr (0.05). When the ratio of Zr/Ti+Zr is higher than 0.1, NO conversion in the whole temperature window is lower than Fe-Mn/Ti, showing a negative effect of Zr. Furthermore, the addition of Zr is beneficial to the N2 selectivity and H2O tolerance. N2O formed during the reaction decreases with the increasing of Zr, and the negative effect of H2O is alleviated by Zr, as shown in Figure S1 and Figure S2, respectively. Figure 1b shows the transient NO conversion profiles with investigated catalysts, and the SO2 tolerance of the catalysts with different Zr. Obviously, the addition of Zr improves the SO2 tolerant performance, alleviating the SO2 deactivation rates. When SO2 was introduced, NO conversion of Fe-Mn/Ti decreased following a stepwise approach including a rapid decrease by 20% within 30 min, and subsequently gradual decrease. Note that Zr content has an influence on


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transient NO reduction profiles, as well as the NO conversion. When the ratio of Zr/Ti+Zr reaches 0.03, decrease of NO conversion in the first step is alleviated, and the gradual decline dominates the deactivation of SO2. The reduction of the NO conversion during the 6 h reaction with SO2 is shown in Figure 1c. At 150 oC, it shows that the addition of Zr obviously slows the drop of the first step, but the drop of NO conversion in the second step decreases firstly and increases a little with Zr addition. The same trend is also observed at 180 oC. The poisoning effect of SO2 is also weakened by the addition of Zr at 180 oC. When the experiment was carried out at higher GHSV at 150 oC (Figure S3), the initial reaction rate is very close when the ratio of Zr/Ti+Zr is below 0.1. However, the transient NO conversion profiles are different when SO2 is added to the reaction system. In the first 10 min, NO conversion decreases rapidly on Fe-Mn/Ti. This decrease trend is alleviated by the addition of Zr. On Fe-Mn/Ti-Zr(0.05) and Fe-Mn/TiZr(0.1), NO conversion even increases a little in the first a few minutes when SO2 is added. These behaviors are totally different with that on Fe-Mn/Ti. The optimal ratio of Zr/Ti+Zr is 0.03, combining NO conversion and SO2 tolerant performance.


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Figure 1. The effect of Zr addition on NO conversion and SO2 tolerance. (a) activity at different temperatures without SO2: (1) Fe-Mn/Ti, (2) Fe-Mn/Ti-Zr(0.01), (3) FeMn/Ti-Zr(0.03), (4) Fe-Mn/Ti-Zr(0.05), (5) Fe-Mn/Ti-Zr(0.1), (6) Fe-Mn/Ti-Zr(0.3), (7) FeMn/Ti-Zr(0.5); (b) NO conversion in the presence of SO2 at 150 oC; (c) the reduction of NO conversion in the 6 h reaction with SO2. [NO]=[NH3]=1000 ppmv, O2=3%, SO2=100 ppmv (when used), GHSV=30,000 h-1. Physical and chemical properties of the catalysts The BET surface areas of the catalysts are summarized in Table 1. For the fresh catalysts, the surface area decreases with the addition of Zr, and when the catalysts are used in SCR reaction


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with SO2 for 6 h, the surface area is decreased by 15.4 m2/g for Fe-Mn/Ti. Despite the negative effect of Zr additive on the BET surface area, it slows down the decrease of the BET surface area in the reaction with SO2, and the reduction is 11.34 m2/g when the ratio of Zr/Ti+Zr is 0.03. However, further increase of the amount of Zr does not abate the decrease of surface area, which is consistent with the second decline rate of the NO conversion with SO2 (Figure 1b and 1c). Table 1 summarizes the surface atomic concentrations of the catalysts. The addition of Zr adjusts the distribution of surface atomic concentrations (Fe, Ti, Mn, and O). Notably, when the stoichiometric ratio of Zr/Ti+Zr is less than 0.1, this ratio of the fresh catalysts is much higher than the theoretical value. In addition, the introduction of Zr results in the decrease of relative atomic concentration of Ti and correspondingly the increase of relative atomic concentration of O and Mn when the ratio of Zr/Ti+Zr is lower than 0.05. The atomic concentration of Mn is around 10% for all the catalysts except Fe-Mn/Ti-Zr(0.1). Mn is believed to be the active component in the catalyst. The enhancement of surface active sites can rationalize the observed improved NO conversion (Figure 1a). Regarding Fe-Mn/Ti-Zr(0.1), the ratio of Zr/Ti+Zr is below the theoretical ratio, and low atomic concentrations of surface Mn and Fe are observed. The obvious contrast indicates the content of Zr affects the distribution of the metal ions, revealed by the surface atomic concentration and relevant NO conversion. Table 1 The BET surface area and surface atomic concentration of the catalysts. BET surface area 2

Surface atomic concentration (%) S/ O

Fe

Zr

N

Ti

Mn

S

Mn

Zr/Ti +Zr

--

--

(m /g) Fe-Mn/Ti (flesh)

112.6

66.21

2.56 --

0.92 18.54

11.76

--

Fe-Mn/Ti(used)

97.2

66.75

2.71 --

2.45 14.40

9.82

3.87 0.39 --


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Fe-Mn/Ti-Zr(0.01) (flesh)

106.9

69.03

1.93 0.30 0.73 17.30

10.71

--

Fe-Mn/Ti-Zr(0.01) (used)

93.1

67.11

1.62 0.20 1.43 14.45

12.54

2.85 0.23 0.014


93.8

67.86

2.00 0.87 0.71 16.75

11.81

--


82.5

67.91

2.66 0.73 1.48 14.23

11.06

1.94 0.17 0.049


73.3

64.31

1.44 1.34 0.64 21.86

10.41

--


55.4

63.01

1.60 1.42 1.43 17.86

12.45

2.24 0.18 0.073


51.3

66.68

1.24 2.20 0.67 21.18

8.04

--


33.7

64.22

1.44 2.04 1.60 19.60

8.62

2.48 0.29 0.094

--

--

--

--

0.017

0.049

0.058

0.094

In terms of the spent catalysts (upon SCR reaction with SO2 for 6 h), sulfur is detected on the surface. It is expected that with the addition of Zr, the ratio of S/Mn is lower than that of FeMn/Ti. According to a previous study,16 the deactivation of catalysts is partially due to the interaction of SO2 with the active component MnOx, inhibiting NO adsorption over active sites. The lower ratio of S/Mn achieved over Zr added catalysts indicates less Mn active sites are occupied by adsorbed SO2/SO3, offering enough active sites for NO reduction. Consistent with catalytic results (Figure 1b and 1c), Fe-Mn/Ti-Zr(0.03) has the lowest ratio of S/Mn. Increasing the Zr content higher than 0.03, the S/Mn ratio changes conversely. XRD studies The X-ray powder diffraction patterns are shown in Figure 2. The reference catalyst FeMn/Ti shows the characteristic peaks of TiO2 anatase phase (JCPDS, 21-1272). Peaks due to ferric oxides or manganese oxides are not detected, suggesting that these two compounds are


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present in the amorphous phase. In comparison, the added Zr has an influence on the catalyst’s crystalline structure, particularly the peaks of TiO2. With the amount of Zr addition increasing, the relative crystallinity of TiO2 decreases gradually, and finally results in an amorphous structure with the disappearance of associated peaks.17 It is observed that the TiO2 peaks of Zradded catalysts all shift to higher 2θ with larger lattice spacing. It is probably attributed to the formation of ZrxTi1-xO2 solid solution,18 which inhibits the formation of TiO2 crystal phase.19 In addition to the disappearance of the peaks associated with TiO2, some new peaks appear. When the ratio of Zr/Ti+Zr is 0.03, the peaks at 36.4o and 41.6o are observed, and also, the peak at 54.4o overlapped with the peak associated with TiO2. These peaks are ascribed to the crystal phase of Fe2Ti3O9 (JCPDS 47-1777). The intensity of these peaks decreases with the continued increase of Zr content, and then finally disappears when the ratio of Zr/Zr+Ti reaches 0.1. Therefore, the addition of Zr can influence the microscopic crystal structure. Conclusively, with the Zr content increasing, the microscopic crystal structure undergoes the phase transition: TiO2, ZrxTi1-xO2, ZrxTi1-xO2-Fe2Ti3O9, and amorphous TiO2. It is generally assumed that the amorphous phase can cause high concentration of active component on the surface of the catalysts, resulting in high activity.15 However, the amorphous phase of Fe-Mn/Ti-Zr(0.1) does not have promotional effects on surface concentrations of Mn and accessible surface area, accompanied with the lowest NO conversion. It indicated that high amount of Zr addition does not benefit the reaction.


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Figure 2. XRD patterns of the catalysts. (■, TiO2; ▼, Fe2Ti3O9 ) (a) Fe-Mn/Ti, (b) Fe-Mn/TiZr(0.01), (c) Fe-Mn/Ti-Zr(0.03), (d) Fe-Mn/Ti-Zr(0.05), and (e) Fe-Mn/Ti-Zr(0.1). Temperature programmed reduction (TPR) measurement H2-TPR was employed to investigate the effect of Zr addition on the reducibility of the catalysts (Figure 3). When the ratio of Zr/Ti+Zr is lower than 0.1, the reduction peaks of the catalyst are similar in shape, including three main peaks ranging from 300-470 oC, 470-550 oC, and 600-700 oC, respectively. It is worthy of noting that the former two peaks shift to lower temperatures with the increase of Zr additive. The first peak corresponds to a stepwise reduction of MnOx, including MnO2 to Mn2O3 (303 oC), Mn2O3 to Mn3O4 (392 oC), and Mn3O4 to MnO (463 oC), 3 with the help of peak-fitting processing. The ratio of the hydrogen consumption for the reduction of MnO2 to Mn2O3 and Mn2O3 to Mn3O4 is 1:1.20:1.29:1.03:0.91 and 1:1.03:1.59:1.37:1.16 for the five catalysts, respectively. This indicates that the reducible MnO2 and Mn2O3 in the catalysts first increases and then decreases with the Zr addition. When MnOx is used for SCR, MnO2 and Mn2O3 are the primary active phases for NO reduction, while MnO2 is more active than Mn2O3.20 So the change of hydrogen consumption can partly explain the higher NO conversion with Fe-Mn/Ti-Zr(0.03). Another interesting phenomenon is the peak at around


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530 oC, which is attributed to the reduction of Fe species.21 With the increase of the amount of Zr , the intensity of this peak increases rapidly, accompanied with the decrease of the peak due to the reduction of Mn3O4 to MnO. Therefore, the introduced Zr could adjust the structure and interaction of Fe and Mn, which further influences the redox properties of the catalyst. When the ratio of Zr/Zr+Ti reaches 0.1, the peaks due to reduction of MnOx and Fe oxides are overlapped, and only a new peak at 480 oC is observed. Combining the XRD results, high amounts of Zr addition not only reduces the crystallinity of the catalysts, but also influences the reduction of Fe species by making it easily reduced, along with the reduction of Mn3O4 to MnO at lower temperature. The reduction of MnOx to MnO completes at 500oC and the isolated Fe3+/Fe2+ reduction should occur up to 520 oC.22 Furthermore, neither the reduction of Mn2+ to metallic Mn0 nor the reduction of iron oxo species takes place below 800 oC.23,24 Therefore, the broad peak ranging from 610 to 680 oC can be ascribed to the reduction of Mn/Fe cations which have strong interaction with TiO2. With the addition of Zr, the intensity of this peak increases, which indicates that Zr promotes the formation of the products from the interaction. Actually, the strong interaction between Mn/Fe cations and TiO2 inhibits the sintering of MnOx/FeOx and TiO2 during the calcination process, and therefore improves the dispersion of active phases, leading to the high activity of NO conversion at low temperature.15 This can also rationalize the increase of the catalytic activity of the catalysts below 150 oC. For the whole TPR procedure from 50 to 800 oC, the ratio of the total hydrogen consumption is 1:1.10:1.59:1.43:0.91 for the five catalysts with the increase of Zr addition presenting a “volcano” profile. It is reasonable to conclude that in Fe-Mn/Ti-Zr(0.03), Zr makes more metal (e.g. Mn) exist at higher vacancy. The inclusion of Zr is believed to result in more surface lattice oxygen


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species, rendering higher surface reducibility,25 which indicates better redox properties. However, high content of Zr may decrease the number of labile oxygen.26 Therefore, low content of Zr can promote the multi-interaction of metal oxides phases, and then improve the redox properties of the catalysts.

Figure 3. H2-TPR profiles for the catalysts. Temperature programmed desorption (TPD) measurement The NH3-TPD profiles of the samples in the range of 100-500 oC are shown in Figure 4. NH3 desorption is observed over a wide temperature range, due to the variability of adsorbed NH3 species with different thermal stabilities. The peaks of desorption around 150 oC are assigned to weakly adsorbed NH3, which are mainly due to the physical/weakly adsorbed NH3.27 The peaks between 150 and 390 oC can be assigned to the chemical adsorbed NH3 at Lewis and/or Brønsted acidity sites.28 It is suggested that the NH4+ ions with 3H structure are more stable than the other NH3 adsorbed species.29 Therefore, the peak around 390 oC is speculated to be the NH4+ ions with 3H structure. The chemical NH3 species formed on the catalyst were mainly divided to coordinated NH3 and NH4+,15 so it is reasonable to attribute the peak at 200-350 oC to the desorption of coordinated NH3 on the catalysts. The total NH3 adsorption capacity on Fe-Mn/Ti


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is higher than is obtained on Zr-added catalysts, and the amount of NH3 adsorption goes down gradually with the increase of Zr. In terms of the NH3 species, the decrease of the physical adsorbed NH3 is the most obvious, and it may be due to the decline of the BET surface area after the addition of Zr. For Zr-added catalysts with the ratio of Zr/Zr+Ti lower than 0.05, physical adsorbed NH3 can be identified from the corresponding NH3-TPD profiles, and the amounts of coordinated NH3 and NH4+ do not decrease significantly. When the ratio of Zr/Zr+Ti reaches 0.05, only a broad peak is found, and the amount of chemical adsorbed NH3, especially NH4+, shows an apparent decrease. Therefore, Zr addition not only changes the adsorbed NH3 species, but also adjusts the relative amount of each adsorbed species. Low Zr content (

Effect of Zr Addition on the Low-Temperature SCR Activity and SO2

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