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Design Strategies for Development of SCR Catalyst: Improvement of Alkali Poisoning Resistance and Novel Regeneration Method Yue Peng, Junhua Li,* Wenbo Shi, Jiayu Xu, and Jiming Hao State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: Based on the ideas of the additives modification and regeneration method update, two different strategies were designed to deal with the traditional SCR catalyst poisoned by alkali metals. First, ceria doping on the V2O5−WO3/TiO2 catalyst could promote the SCR performance even reducing the V loading, which resulted in the enhancement of the catalyst’s alkali poisoning resistance. Then, a novel method, electrophoresis treatment, was employed to regenerate the alkali poisoned V 2O 5−WO 3/TiO2 catalyst. This novel technique could dramatically enhance the SCR activities of the alkali poisoned catalysts by removing approximately 95% K or Na ions from the catalyst and showed less hazardous to the environment. Finally, the deactivation mechanisms by the alkali metals were extensively studied by employing both the experimental and DFT theoretical approaches. Alkali atom mainly influences the active site V species rather than W oxides. The decrease of catalyst surface acidity might directly reduce the catalytic activity, while the reducibility of catalysts could be another important factor. area, but a significant decrease of surface acidity.6 Kröcher et al found that K strongly reduced the adsorption equilibrium constant of NH3.7 Tang et al proposed that the reducibility of the catalysts could be a key factor for the poisoning of V2O5/ TiO2 doped with Na+ and Ca2+ ions.8 However, which is the predominant factor is still controversial. Calatayud et al used the density functional theory (DFT) calculations to investigate the effects of alkali atoms locating on the V2O5/TiO2 model and concluded that the dopants could significantly change the VO groups.9 Gao et al selected Pb as a probe atom to reveal the poisoning mechanism on V2O5/TiO2 and proposed the active sites V can be covered by Pb, resulting in decreased acid formation and reducibility.10 On the other hand, the dispose or regeneration of the used catalysts is still a significant problem. Khodayari et al have extensively studied the regeneration of traditional SCR catalyst by washing and sulphation methods, and found that washing with water could not effectively regenerate the vanadia-based catalysts, whereas water washing followed by sulphation could improve the activity.11,12 Recently, Shang et al. successfully regenerated the used traditional catalysts by water and dilute sulfuric acid washing, and proposed that the regenerated catalysts can yield superior activity and low SO2 oxidation

1. INTRODUCTION Nitrogen oxide emissions from stationary and automobile sources are a serious threat to the environment, since they can form acid rain or photochemical smog. The selective catalytic reduction (SCR) of NO with NH3 is the most effective technology for abating NO in gas flue. The traditional catalyst is the V2O5−WO3/TiO2.1,2 However, this catalyst has some disadvantages, such as the toxicity of V species, high activity for the oxidation of SO2, and a narrow temperature window of 300−450 °C. Furthermore, the deactivation by alkali metals is another shortcoming. Therefore, many researchers attempted to modify the recipe or to develop new catalysts to avoid the defects above. Chen et al3 doped ceria to the V2O5−WO3/ TiO2, and found that the SCR activity can be maintained, even though decreasing the V loading. They attributed this effect to the well-dispersion of active component and high reaction rate by ceria. Sun et al4 prepared new materials, Ti0.5Sn0.5O2 supported V2O5−WO3, and demonstrated that Sn can highly improve the dispersion of V and W species on the support and prevent the crystallization of WO3 on the surface of catalysts, leading the enhancement of the activity.5 Alkali metals in fly ash are a major concern for the SCR catalysts used at coal-fired plants, since they may plug the pores of the catalyst channel and react with the active sites, leading to reduce the activity. Although great effort has been expended in studying the effects of SO2 and H2O on catalysts, few studies have been related to the deactivation effects of alkali metals. Lisi et al proposed that K and Na did not cause the loss of surface © 2012 American Chemical Society

Received: Revised: Accepted: Published: 12623

July 15, 2012 October 25, 2012 October 25, 2012 October 25, 2012 dx.doi.org/10.1021/es302857a | Environ. Sci. Technol. 2012, 46, 12623−12629

Environmental Science & Technology

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rate.13 However, the utilization of sulfuric acid brings about another problem: the disposition of the used acid solution. In our previous study, we have already investigated K and Na doped on CeO2−WO3, and found it can provide more alkali poisoning resistance than the traditional catalyst, and water washing can regenerate this catalyst effectively.14 In this work, two different strategies were used to deal with the traditional V2O5−WO3/TiO2 alkali metals poisoned problem: ceria doping improvement and novel electrophoresis treatment. Finally, the poisoning mechanisms of alkali metals on the catalyst were extensively investigated by both experimental and theoretical methods.

atomic emission spectroscopy (ICP-AES) with an IRIS Intrepid II XSP apparatus from Thermo Fisher Scientific Inc. The removing efficiency (Re %) of alkali metals were calculated as follows: Re% =

ICP result × 0.1 L 0.1 g × 1% ×

(

2μ 2μ + 16

)

× 100% (1)

where μ is the molar mass of potassium or sodium. For example, 0.1 g, 1% K samples washed in 100 mL water, the ICP result is 0.20 mg/L. The Re % is equal to 24.1%. This means during this process, 24.1% of K atom on the poisoned catalyst can be removed. 2.5. Computational Details. Dunn et al believed that W could preferentially coordinate with the TiO2 support rather than the surface V species, and there is no evidence for electronic interaction between the W and V of the V2O5− WO3/TiO2.6,16 Choo et al proposed that the WO3 simply promotes the formation of polymeric V2O5 by studying the V2O5/sulfated TiO2 and P25.17 A recent study attributed the promotional effect of W to form a well-mixed surface oxide phase, leading to the formation of highly active V−O−V paired structures.18 The alkali metals poisoned on the CeO2−WO3 catalyst has been studied in our previous study, the dopant atoms could cause surface’s electronic redistribution and cover the active site.15 Moreover, the stability of main low-index TiO2 anatase surfaces are as follows: (101)>(100)>(001); from a chemical point of view, stability implies low catalytic activity, suggesting that the (001) surfaces could play an important role during the reaction. From the study of Vittadini et al, VOx species should occur especially on the (001) surface.19 To directly study the poisoning effect of alkali metals on the active site, we selected the V2O5 and WO3 clusters respectively bonded to the TiO2 anatase (001) surface slab models. All the calculations were based on DFT, and were performed using Material Studio 5.5 modeling DMol3.20 Doublenumerical-quality basis set with polarization functions (DNP) and GGA-PBE21,22 were used for all calculations. Core electrons are treated with DFT semicore pseudopotentials (DSPP).23 Spin polarization is also applied and the real space cutoff radius was maintained as 4.2 Å. The Brillouin zone was sampled using a Monkhorst Pack grid to ensure the convergence of the whole systems. The convergence criteria for the self-consistent field energy and displacement were set to 1 × 10−6 Ha and 5 × 10−3 Å, respectively. The slab model of TiO2 (001) with Ti24O48 supercell was selected widely accepted for DFT calculations,24,25 and the vacuum region was set to 15 Å. The adsorption energies (Ead) of NH3 molecule are calculated as follows

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The traditional and ceria doped catalysts were prepared by the impregnation method. The ammonium metavanadate, ammonium paratungstate and/or cerium nitrate were mixed in the oxalic acid solution in the desired proportion. The appropriate amount TiO2 anatase was impregnated by stirring for 2 h, dried overnight at 120 °C and then calcined at 500 °C for 5 h in static air. The catalyst denoted as V0.4-Ce5W5/Ti means that the amount of V, Ce, and W are 0.4, 5, and 5 wt %, respectively. 2.2. Poisoning and Regeneration. K, Na metals were induced to the fresh catalyst by impregnation from the potassium and sodium nitrate solutions with 1 wt % loading. After drying at 120 °C for 12 h, the samples were calcined at 500 °C for 5 h. The deactivated catalysts were regenerated by washing with deionized water and by electrophoresis method. Washing method: the poisoned catalysts (1 g) were washed in water (100 mL water at 30 °C) for 36 h, and then dried at 120 °C for 6 h in the oven. Electrophoresis method: two titanium electrode plates were inserted in the both sides of a container with 100 mL water, and the poisoned catalysts (1 g) were wrapped in the filter paper and suspended between the two plates. Direct current was passed into. The experimental facilities were shown in Figure S1 in the Supporting Information (SI). Then the catalysts were dried at 120 °C for 6 h and calcined at 500 °C for 5 h. 2.3. Catalytic Performance Tests. Activity measurements were performed in a fixed-bed quartz reactor (inner diameter 9 mm) using 200 mg catalyst of 40−60 mesh. The feed gas mixture contained 500 ppm NO, 500 ppm NH3, 3% O2, and the balance N2. The total flow rate of the feed gas was 200 mL/ min. The gas hourly space velocity (GHSV) was about 60 000 mL·g−1·h−1. The concentrations of inlet and outlet gases were continually monitored by an FTIR spectrometer (Gasmet DX4000). The activity data were collected when the reaction reached a steady state after 30 min at each temperature. 2.4. Catalyst Characterization. The BET surface area of the samples was carried out by a Micromeritics ASAP 2020 apparatus. Temperature programmed reduction of H2 (TPR) and temperature programmed desorption of NH3 (TPD) were performed on a chemisorption analyzer (Micromeritics, ChemiSorb 2720 TPx). For TPR, the sample was under a 10% H2/Ar gas flow at a rate of 10 °C/min up to 900 °C; for TPD, the sample first exposed on a 500 ppm NH3/He gas flow for 1 h, and isothermal desorption under He for 30 min, then desorbed under a He gas flow at a rate of 10 °C/min up to 600 °C. The gas flow was set as 50 mL/min. Each sample was pretreated at 300 °C in He for 1 h before testing. The alkali metal concentrations dissolved in the deionized water by the two methods were determined by inductively coupled plasma

Ead = Esurface + Eammonia − Eammonia/surface

(2)

Esurface is the energy of surface, Eammonia is the energy of an isolated NH3 molecule, and Eammonia/surface is the total energy of the same molecule adsorbed on surface. Note that a positive value for Ead suggests a stable adsorption.

3. RESULTS AND DISCUSSION 3.1. Promotion Effect of Ceria on Low V Catalysts. The SCR activities of the low V loading catalysts at temperature ranging from 150 to 450 °C under a GHSV of about 60 000 mL·g−1·h−1 are shown in Figure 1. When the W content increased up to 10 wt %, the catalytic activities can be 12624

dx.doi.org/10.1021/es302857a | Environ. Sci. Technol. 2012, 46, 12623−12629


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Three possible reasons could be responsible for this effect: (1) Though ceria does not provide remarkable surface acidity to the catalyst, its good oxygen storage capacity (OSC) and redox properties of Ce4+ to Ce3+ could enhance the reducibility of catalyst, which is another important factor of SCR catalyst alkali metal poisoning (discussed below); (2) The deactivation of the traditional catalysts is due to the occupying active site vanadia by alkali atoms. Based on our previously DFT study, K or Na atoms could facilely bonded to the CeO2 (110) surface, which could decrease the adsorption amount of alkali on the active V species. The ceria here can be believed as an alkali metals’ reservoir to reduce the amount of alkali metal on the surface; (3) By carefully studying the activity of K poisoned catalyst and the fresh one in Figure 2, with increased the reaction temperature, the alkali resistance effect of ceria become more apparent. Considering the ions radii of alkali metals and the bond length of Ce−O, the alkali metal could transfer to the subsurface or body phase of ceria rather titanium dioxide with increasing the temperature. Another phenomenon presented in Figure 2 should be paid attention: the deactivation of 1% K doped catalysts without ceria seems more severe than that of 1% Na doped one, which is in contrast to most published literatures.3 Whereas the active order is reversed with ceria doping especially at high temperature region, and our previous studies of deactivation effect of K and Na on ceria containing catalysts exhibit the similar results (SI Figure S2). This can be attributed this to the existence of ceria on the catalysts’ surface, and will be discussed in Section 3.3. 3.2. Comparisons of Different Regeneration Methods. Though doping additives ceria could promote catalysts alkali poisoning resistance, another way to lengthen the catalyst lifetime should be paid more attention: regeneration of the poisoned catalysts. The V1−W5/Ti poisoned by 1 wt % K and Na oxides were then treated by water washing and electrophoresis, the activity profiles are shown in Figure 3(a). Water washing could increase the catalytic activity of the poisoned samples to some extent. The results show that washed catalyst could provide about 50% of NO conversion at 400 °C. Moreover, the alkali metal ion concentrations were also studied by ICP-AES, and the Re % were calculated by the formula 1 and labeled in Figure 3(a). Only one-third of the alkali metal can be removed from the catalysts, which is responsible for the low activity of regenerated samples. The activity cannot be further enhanced by washing with water for twice or more, since seldom alkali metals could be removed from the catalyst by the second or third washing (SI Figure S3). The results indicate that the water washing method is not fit for the traditional catalyst.11 Then, electrophoresis method was used for the same poisoned catalysts, and the results show that the catalytic activity was almost recovered at 350 °C and above, especially for the 1% K poisoned catalyst, on which 95% NO can be reduced at 400 °C. For 1% Na poisoned catalyst, the regenerated sample provides 90% NO conversion at 400 °C. The Re % of the 1% K and Na poisoned catalysts are 92.6% and 98.6%, respectively. These reveal that nearly all the alkali metal ions can be dissolved in the water by electrophoresis treatment, and the activity can significantly recovered. During the whole treatment, furthermore, no hazardous solutions or gases were produced, which indicating that this method is effective and environmental benign to the catalysts regeneration. The influences of the voltage and treating time on the poisoned

Figure 1. Activity and N2 selectivity of the low V loading catalysts.

significantly enhanced below 300 °C, nearly 97% of NO can be removed. This indicates that rising up the W loading could be a possible way to promote the catalysts’ activity. However, considering the cost of tungsten oxides raw materials, it is not favorable in industry. Based on our pervious study,3 5 wt % of ceria was substituted for the W loading. The results show the activity of the V0.4-Ce5W5/Ti is even higher at 250 °C than the V0.4-W10/Ti, and could be maintained up to 450 °C. Furthermore the N2 selectivity of these catalysts was also studied, and the V0.4-W10/Ti exhibited poor selectivity. Figure 2 shows the deactivation of 1 wt % K and Na doped on the V0.4-Ce5W5/Ti and compared with the V0.4-W10/Ti. One

Figure 2. Activity of 1% alkali doped V0.4-Ce5W5/Ti and V0.4-W10/Ti catalysts.

wt % K and Na are of the same magnitude order as the amount of alkali metals doped into SCR catalysts after 600−700 h on stream.7 When the alkali metals are doped on the V0.4-Ce5W5/ Ti, the activities decrease significantly, exhibiting only 40% of NO conversion at 450 °C. To reveal the effect of ceria on alkali poisoning resistance, the same amount of alkali metals were doped on the V0.4-W10/Ti, and the activity profiles shows even lower (less than 20% at 400 °C for 1 wt % K doped) than the poisoned V0.4-Ce5W5/Ti during the whole temperature region.26 Compared with our earlier study about the alkali metals poisoning on different catalysts (SI Figure S2), 1 wt % K lead to the NO conversion decreased only to 80% and 68% at 450 °C for the Ce1W1 and Ce5W5/Ti, respectively.15 These results suggest that ceria could not only enhance the SCR activity and N2 selectivity, but also the alkali poisoning resistance. 12625



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Figure 3. SCR activity for (a) the 1% K and Na poisoned V1−W5/Ti and (b) the V0.4-Ce5W5/Ti catalysts regenerated by water washing or electrophoresis methods.

Figure 4. (a) H2-TPR and (b) NH3-TPD profiles of the V0.4-Ce5W5/ Ti and 1% alkali-doped V0.4-Ce5W5/Ti catalysts.

are almost the same. However, in view of the Mg, Ca poisoned catalysts (SI Figure S5), the locations of peaks are the same as K, Na poisoned, which is not in good accordance with activity. These results suggest that the dopant atoms could decrease the catalysts reducibility, but this is not the main reason responsible for the catalysts’ deactivation.9,26 Figure 4(b) shows the NH3-TPD profiles of the V0.4-Ce5W5/ Ti and poisoned catalysts. The fresh catalyst exhibits a shoulder peak centered at about 140 °C and a broad peak at 280 °C. They can be attributed to weakly and strongly bond NH3 related to two different acidities on intensity, respectively.3,26 For Na doped catalyst, the amount of weakly bonded NH3 is nearly unchanged comparing to the fresh catalyst; however, the strongly bonded NH3 is significantly decreased. For K doped catalyst, both the weak and strong acid sites are decreased. The results indicate that the amount of NH3 adsorption is the major difference between the fresh and poisoned catalysts, which is responsible for the reaction activity. Moreover, the surface acidity for Na poisoned catalyst at high temperature is weaker than that for K poisoned catalyst. Alkali metals atoms can directly bonded to the active site of vanadia for the traditional V2O5−WO3/TiO2 catalyst and lower the surface acidity and reducibility by prohibiting the NH3 adsorptions as Lewis or Brønsted acidities on the V−O−V or VO, which could be

catalyst was also investigated (SI Figure S4). The results suggest that 1 g catalyst treated for 36 h at 1.6 eV could display the highest SCR performance. Furthermore, the V0.4-Ce5W5/Ti sample was also regenerated by the electrophoresis method (Figure 3 (b)). The results show that this method is also available and effective to the ceria doped traditional SCR catalyst. 3.3. Redox Properties and Surface Acidities Experimental Section. The BET surface areas for the V0.4-Ce5W5/ Ti and K, Na poisoned catalysts are 59.7, 54.2, and 56.7 m2/g, respectively, indicating that the influences of dopants on the surface areas are negligible. A similar conclusion was proposed by Lisi et al on the deactivation of the traditional SCR catalysts and they also suggested that no detectable formation peaks of alkali metals occurs on the poisoned catalysts from the XRD patterns.7 Figure 4(a) displays the H2-TPR profiles of the V0.4-Ce5W5/ Ti and poisoned catalysts. Two main peaks can be observed in the curve of the fresh catalyst. It is found that the reduction peaks of the surface-capping oxygen (surface Ce4+ to Ce3+) and the bulk oxygen (bulk Ce4+ to Ce3+) of ceria are centered at 488 and 748 °C, respectively.27,28 When poisoned by alkali metals, the reduction peaks of cerium moved to higher temperature regions, the shifts for the K, Na doped catalysts 12626



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account for the active order of K and Na doped ceria containing catalysts at high temperature. TPR and TPD profiles about the V1−W5/Ti and corresponding poisoned catalysts were also studied and show in SI Figure S6 and S7, revealing the same conclusions as low V catalysts. The promotion effect of ceria to the alkali metal resistance is also related by comparing the NH3TPD profiles of V0.4-Ce5W5/Ti and V0.4-W5/Ti catalysts (SI Figure S8), and the results show that the amount of NH3 desorption is not significantly increased by doping with ceria, only the peak of V0.4-W5/Ti shifts slightly to the low temperature, when doped by ceria. 3.4. Redox Properties (Theoretical Section). Figure 5 (a−c) shows the optimized models of TiO2 (001) and V2O5,

Figure 5. Optimized structures of the (a) TiO2 (001), (b) V2O5/TiO2 (001), and (c) WO3/TiO2 (001) surface models. Titanium atoms are gray, oxygen atoms are red, vanadium atoms are green, and tungsten atom is dark cyan.

Figure 6. PDOS of (a) clean and K doped on V2O5/TiO2 (001) and (b) clean and K doped on WO3/TiO2 (001) surface models, only the uppermost level (clusters and one TiO2 layer) are calculated.

WO3 bonded to TiO2 (001) surface. Considering the proposal of Giakoumelou et al,28 monolayer V species were predominant when the V loading was less than 6 wt %. Thus we only consider the monolayer V model. The bond length of VO is 1.60 Å and the bond length range of V−O−V is from 1.76 to 1.82 Å. While for the WO3/TiO2 (001) model, the bond length of WO is 1.76 Å and the bond length of W−O is 1.94 Å. To elucidate the deactivation effect of alkali metals on the catalyst in a molecular level, K atom was selected as a probe to locate on the surfaces. The reducibility of catalyst can also be investigated by DFT calculation. Electronic structures of the optimized models are calculated in terms of the density of states (DOS), which are related to redox properties.29 Projected DOS (PDOS) on the outermost layers are shown in Figure 6(a) and (b) for the V2O5/TiO2 (001) and WO3/ TiO2 (001) models. As we known, the top of valence band (VB) and the bottom of conduction band (CB) is usually associated to the reactivity: a system with a small or no band gap is more reactive than a system with big band gap.29,30 The CB of the K free surface model is composed of V 3d orbitals and O 2p orbitals, with a band gap 0.74 eV. When K dope on the V2O5/TiO2 (001) model, the 3d orbitals in the CB shift to high energy region, which is responsible for the broadening of the band gap (0.81 eV). Even the small O 2p orbitals of VB above the Fermi energy (0 eV) disappear for the 2 V2O5/TiO2 (001) model, leading to a wider band gap (2.12 eV). However, for the K doped on WO3/TiO2 (001) model, the band gap is nearly unchanged. These results suggest that doping of K could

lower the reducibility of the active component V2O5 rather than WO3 on TiO2 (001) surface. 3.5. The Surface Acidities (Theoretical Section). To study the surface acidities when doped with K atom, the adsorption energy of NH3 (Ead) and the Mulliken charge populations are both calculated.31 First, an NH3 molecule is located at the proper position on the V2O5/TiO2 (001) model with Lewis and Brønsted acid sites, and the optimized structures are shown in Figure 7(a, b). The Ead calculate by formula 2 are 1.23 and 0.81 eV for Lewis and Brønsted acid site adsorptions, respectively. The results suggest that adsorptions on Lewis acid sites are more stable than those on Brønsted acid sites. The distance of N to the O of VO bond is about 2.24 Å, Yuan et al have obtained this results (2.21 Å) by optimizing

Figure 7. Optimized configurations of the NH3 adsorption on (a) Lewis acid site, Brønsted acid site and (c) K doped of V2O5/TiO2 (001) surface models. 12627



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the V6 cluster.32 However, they failed to acquire a stable configuration for the Lewis acid site adsorption, since they neglected the influence of support TiO2. Then the NH3 molecule is located at the proper site of the V2O5/TiO2 (001) model with K atom doped on (Figure 7(c)). This could suppress the formation of hydroxyl on the catalytic surface (Brønsted acid site). After optimization, NH3 molecule move far away from the V atom and the Ead is decreased from 1.23 to 0.84 eV, suggesting that doping K could significantly lower the NH3 adsorption on the models: occupy the Brønsted acid sites and weaken the Lewis acid adsorption. The Ead on the WO3/TiO2 (001) models were also calculated (SI Figure S9. Only Brønsted acid sites (0.94 eV) were stably exist on the surface (0.52 eV for Lewis acid site). When K atom doped on the surface, the Ead of NH3 is 0.86 eV. The Mulliken charges of O atoms bonded to V on V2O5/ TiO2 (001) model are calculated. The charge of the terminal O decreased from −0.38 e (VO) to −0.53 e (V−O−K) and the bridging O (V−O−V) also have a small decline (−0.60 e to −0.64 e), indicating that some electrons transfer from K atom to terminal O atom, which decrease the surface acidity. The results from DFT calculations present a well accordance with the experiments.

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ASSOCIATED CONTENT

S Supporting Information *

Some related figures are shown. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 10 62771093; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the national High-Tech Research and the Development (863) Program of China (Grant No. 2010AA065002, 2012AA062506) and the help of Jun Wang are gratefully acknowledged.

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REFERENCES

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