Alkali Metal Poisoning of a CeO2–WO3 Catalyst ... - ACS Publications

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Alkali Metal Poisoning of a CeO2−WO3 Catalyst Used in the Selective Catalytic Reduction of NOx with NH3: an Experimental and Theoretical Study Yue Peng,†,‡ Junhua Li,*,† Liang Chen,†,§ Jinghuan Chen,† Jian Han,‡ He Zhang,‡ and Wei Han1*,‡ †

School of Environment, Tsinghua University, Beijing, 100084, China Physical College, Jilin University, Changchun City, Jilin Province, 130012, China § CPI YUANDA Environmental-Protection Engineering Co., Ltd., Chongqing, 401122, China ‡

S Supporting Information *

ABSTRACT: The alkali metal-induced deactivation of a novel CeO2−WO3 (CeW) catalyst used for selective catalytic reduction (SCR) was investigated. The CeW catalyst could resist greater amounts of alkali metals than V2O5−WO3/ TiO2. At the same molar concentration, the K-poisoned catalyst exhibited a greater loss in activity compared with the Na-poisoned catalyst below 200 °C. A combination of experimental and theoretical methods, including NH3-TPD, DRIFTS, H2-TPR, and density functional theory (DFT) calculations, were used to elucidate the mechanism of the alkali metal deactivation of the CeW catalyst in SCR reaction. Experiments results indicated that decreases in the reduction activity and the quantity of Brønsted acid sites rather than the acid strength were responsible for the catalyst deactivation. The DFT calculations revealed that Na and K could easily adsorb on the CeW (110) surface and that the surface oxygen could migrate to cover the active tungsten, and then inhibit the SCR of NOx with ammonia. Hot water washing is a convenient and effective method to regenerate alkali metal-poisoned CeW catalysts, and the catalytic activity could be recovered 90% of the fresh catalyst.

1. INTRODUCTION Nitrogen oxide (NOx ) emissions from stationary and automobile sources are a serious threat to the environment, because they can form acid rain or photochemical smog. Selective catalytic reduction (SCR) of NOx with NH3 has been proven to be the most effective technology for abating NOx in flue gas. The current commercial catalyst for this reaction is V2O5−WO3/TiO2 (V−W/Ti).1,2 However, this catalyst is not without disadvantages, such as the toxicity of vanadium species, high activity for the oxidation of SO2 to SO3, and the alkali metal poisoning effect. Therefore, many researchers have worked to develop new catalysts to avoid these detects. Chen et al.3 prepared CeO2/TiO2 and CeO2−WO3/TiO2 catalysts and compared their SCR performance at temperatures from 150 to 500 °C. They reported that doping with tungsten was beneficial in forming Ce3+ and greatly enhanced the catalytic activity of the CeO2/TiO2 catalyst. Recently, Shan et al.4 developed a novel cerium−tungsten catalyst that yielded higher NOx conversion and N2 selectivity along with better H2O and CO2 durability in a wide temperature range compared to V− W/Ti. The ceria-based catalytic system could be competitive for practical applications.3−7 Alkali metals in fly ash are a major concern for SCR catalysts used at municipal solid waste incineration plants and coal-fired plants, because they may plug the pores of the catalyst and react with the active sites. For diesel vehicles, some fuel and © 2012 American Chemical Society

lubrication oil additives as well as urea solutions are another origin of alkali metals.8 These substances can significantly reduce the SCR performance. Although great effort has been expended in studying the effects of SO2 and H2O on catalysts, few studies have been devoted to the poisoning effects of K and Na.8−11 It has been reported that the deposition of alkali metals can decrease both the quantity and the strength of the Brønsted acid sites regarded as responsible for the SCR activity of V−W/ Ti.9 Tang et al.12 proposed that the reducibility of the catalysts could be another key factor for the poisoning of V2O5/TiO2 catalysts doped with Na+ and Ca2+ ions. In the theoretical calculations, Calatayud et al.13,14 investigated the stability and reactivity of V2O5 (110) and (001) surfaces by density functional theory (DFT) method. Moreover, they studied the effect of alkali doping on V2O5/TiO2 catalyst model and concluded that the dopant atoms significantly affect the VO groups.15 Hejduk et al.16,17 systematically studied the electronic structure and H2O adsorption/desorption behavior of V2O5 by ab-inito method. Recently, Gao et al.18 used lead atoms as a probe to reveal that the poisoning effect on V2O5/TiO2 can be attributed to the covering of vanadium active sites by DFT Received: Revised: Accepted: Published: 2864

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cooled by liquid N2. Diffuse reflectance measurements were performed in situ in a high temperature cell with a ZnSe window. The catalyst was heated to 350 °C under N2 at a total flow rate of 100 mL/min for 60 min to remove adsorbed impurities. The background spectrum was collected in a flowing N2 atmosphere and was subtracted from the sample spectra. The DRIFTS spectra were recorded by accumulating 200 scans with a resolution of 4 cm−1. Temperature-programmed reduction (H2-TPR) experiments were performed on a chemisorption analyzer (Micromeritics, ChemiSorb 2720 TPx) under a 10% H2 gas flow (50 mL/min) at a rate of 10 °C/min up to 1000 °C. Each sample was pretreated at 350 °C in helium for 1 h before testing. The crystal structure was determined by using an X-ray diffractometer (XRD) (Rigaku, D/max-2200/PC), operating at 30 kV and 30 mA using CuKα radiation.

calculations, resulting in decreased acid formation and reducibility. Previous researchers have studied alkali metal poisoning in V2O5-based catalysts, but there are few studies of ceria-based SCR catalytic systems. Hence, this study mainly investigates the poisoning of CeO2−WO3 (CeW) catalysts by potassium and sodium. NH3-TPD, DRIFTS, and H2-TPR were performed to experimentally reveal the relationship between the deactivation, surface acidity and reducibility. Meanwhile, DFT calculations were also performed to elucidate the interactions of alkali metal atoms with the catalytic surface.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. CeW catalysts were synthesized by a standard coprecipitation method,4 where the molar ratio of cerium to tungsten was 1:1. Appropriate amounts of cerium nitrate and ammonium metatungstate were dissolved in an oxalic acid solution, and an excess of urea solution was added with as a precipitator. The precipitate cakes were filtrated and washed with distilled water, followed by drying at 120 °C for 12 h and then calcining at 500 °C for 5 h. Finally the catalyst was crushed and put through a 40−60 mesh sieve. Samples prepared in this way are denoted ‘fresh’. Commercial V−W/ Ti catalysts were also prepared by the impregnation method and the loading of V2O5 is about 1%. 2.2. Poisoning and Regeneration. Potassium and sodium were added to the fresh catalyst by impregnation from KNO3 and NaNO3 solutions with different concentrations. After drying at 120 °C for 12 h in an oven, the samples were calcined at 500 °C for 5 h. The fresh catalysts were poisoned with 0.30, 1.00 wt % K, and 0.18, 0.58 wt % Na as well as both 0.50 wt % K and 0.29 wt % Na. These values of potassium and sodium contents represent equivalent molar ratios, and are of the same order of magnitude as the amount of alkali metals doped into SCR catalysts after 600−700 h on stream.11 These samples are denoted “0.3 K”, “Na&K”, etc. The BET of fresh and poisoned catalysts are nearly the same and approximately 64 m2/g. Poisoned catalysts of 40−60 mesh were regenerated in hot water (for example 300 mg of sample in 30 mL water) for 30 min three times at 60 °C, then drying at 120 °C for 6 h. The samples are denoted “0.58 Na Re”, etc. 2.3. Catalytic Performance Tests. Activity measurements were performed in a fixed-bed quartz reactor (inner diameter 9 mm) using 300 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 300 mL/ min. The concentrations of gases (NO, NO2, N2O, and NH3) were continually monitored by an FTIR spectrometer (Gasmet FTIR DX-4000). The activity data were collected when the reaction reached a steady state after 30 min at each temperature. 2.4. Catalyst Characterization. The temperature-programmed desorption of NH3 (NH3-TPD) was performed in a fixed-bed quartz reactor. A typical experiment used a 300 mg sample and a gas flow rate of 300 mL/min. The experiment consisted of four stages: (1) degasification of the catalyst under N2 at 350 °C for 1 h, (2) adsorption of 500 ppm NH3 at room temperature for 60 min, (3) isothermal desorption under N2 at 30 °C, and (4) temperature-programmed desorption under N2 at 10 °C/min up to 620 °C. In-situ DRIFTS spectra were recorded on a Fourier transform infrared spectrometer (FTIR, Nicolet NEXUS 870) equipped with a SMART collector and an MCT detector

3. COMPUTATIONAL DETAILS All calculations are based on DFT, and were performed using Material Studio modeling DMol3 from Accelrys.19 Joshi et al.7 studied the interaction of NH3 with pure and tungsten doped ceria using 5 layers. Zhou et al.20 synthesized and considered the CeO2 nanorods and revealed that the (110) and (100) surfaces are more reactive for CO oxidation than the stable (111) surface. A double-numerical-quality basis set with polarization functions (DNP) and GGA-PBE21,22 were used for all calculations. The core electrons were treated with effective core potentials (ECP). Spin polarization is also applied to our calculations, and the real space cutoff radius was maintained as 4.2 Å. The Brillouin zone was sampled using a (1 × 2 × 1) Monkhorst Pack grid to ensure the convergence of the whole systems. The convergence criteria for the self-consistent field (SCF) energy and displacement were set to 2 × 10−5 Ha and 4 × 10−3 Å, respectively. The selection of the models and methods selection are discussed in greater detail in the Supporting Information (SI). 4. RESULTS AND DISCUSSION 4.1. SCR Performance. Figure 1 shows the activity comparisons of V−W/Ti and CeW catalysts and corresponding

Figure 1. Comparison of NH3−SCR activity of V−W/Ti and CeW catalysts with corresponding 1 wt % K-doped catalysts. Reaction conditions: catalyst = 300 mg, [NO] = [NH3] = 500 ppm, [O2] = 3%, total flow rate = 300 mL/min, GHSV = 60 000 h−1.

1 wt % K-doped catalysts at temperature ranging from 100 to 300 °C under a GHSV of 60 000 h−1. Without K doping, the activity of the CeW catalyst was slightly higher than the V−W/ 2865

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Ti catalyst below 280 °C, with a maximum of nearly 99% NOx conversion at 220 °C, maintained up to 300 °C. When doped with 1 wt % of K, the activity of V−W/Ti decreased significantly, yielding 30% or less NOx conversion during the test. However, for the poisoned CeW catalyst, though the activity was lower compared to the fresh one either, it can still obtain 70% NOx conversion above 200 °C. This result indicates that the novel CeW catalyst can provide more alkali resistance than the traditional V−W/Ti catalyst below 300 °C. Figure 2 shows the SCR activity of different amounts of K-, Na-doped CeW catalyst and their N2 selectivity between 100

Figure 3. NH3-TPD profiles of the fresh CeW, 1 K and 0.58 Na poisoned CeW catalysts.

catalyst exhibits two NH3 desorption peaks around 174 and 236 °C that can be attributed to weakly bonded and strongly bonded NH3, respectively.5 It is expected that for alkali metal containing samples, the NH3 desorption should decrease compared to the fresh catalyst, and the order for the amount of acid is in the sequence: fresh >0.58 Na > 1 K, in good accordance with the SCR activity of these catalysts. Consequently, the results of the NH3-TPD experiments indicated that the doping with the alkali metal could decrease the NH3 adsorption, and lower the SCR performance. 4.3. DRIFTS Study of NH3 Adsorption. To further explore the type, strength and quantity of surface acid sites, DRIFTS spectra of NH3 adsorption were employed to investigate the poisoning effect of alkali metals on the acid sites of the CeW catalyst. SI Figure S2 (a−c) shows the spectra of NH3 species adsorbed on the fresh and K or Na poisoned CeW catalysts from 50 to 350 °C. All the spectra obtained at room temperature show NH3 adsorption bands at the similar positions. The bands at ca. 1600 and 1160 cm−1 can be assigned to the asymmetric and symmetric bending vibrations of the N− H bonds in NH3 linked to Lewis acid sites.5,24 The band at 1430 cm−1 can be attributed to the asymmetric bending vibrations of NH4+ chemisorbed on the Brønsted acid sites, whereas the broad bands in the range of 1680−1850 cm−1 are due to the symmetric bending vibrations of NH4+.5,24 Figure 4

Figure 2. The NH3−SCR activity and N2 selectivity of CeW and alkali-doped CeW catalysts. Reaction conditions: catalyst = 300 mg, [NO]=[NH3] = 500 ppm, [O2] = 3%, total flow rate = 300 mL/min, GHSV = 60 000 h−1.

and 300 °C. When alkali metals were doped into a fresh sample, the extent of deactivation depended on the concentration of the poisoning metals. The Na&K catalyst was less active at low temperatures but yielded higher NOx conversion above 200 °C compared with the 1 K and 0.58 Na catalysts. At a given molar concentration, K gave rise to more deactivation than Na below 200 °C, due to its more potent neutralizing properties.11 The N2 selectivity presented in figure 2 of the poisoned catalysts did not significantly decrease. Over the whole temperature range, N2O formation was less than 10%. The steady state kinetics of the SCR reaction was also considered and the results are shown in SI Table S1. Lisi et al.11 proposed that small amounts of alkali metals could lead to a dramatic decrease in catalytic activity for a V2O5−WO3/TiO2 monolith, which yielded ca. 40% NOx conversion at 300 °C. Chen et al.9 prepared nanoscale V2O5−WO3/TiO2 catalysts, and studied their poisoning by alkali (earth) metals. They reported that 1% Na could decrease the NOx conversion to 35%, and that doping with K caused a considerable (and similar) amount of deactivation. Zhang et al.23 investigated the doping of KCl into V2O5/AC catalysts, and found that 0.1% KCl could decrease the NOx conversion from 80% to 40% at 250 °C. Taken together, these results clearly indicate that the CeW catalyst is more resistant to alkali metals than V2O5−WO3/ TiO2 and V2O5/AC, and could be useful in practical applications. 4.2. NH3-TPD. The following sections are mainly focused on the effects and mechanisms of alkali metal poisoning in the CeW catalyst from the standpoint of acidity and reducibility. Figure 3 shows the TPD profiles of fresh, 1 K and 0.58 Na catalysts obtained at temperatures of 30−620 °C. The fresh

Figure 4. The strength and quantity of Lewis and Brønsted acid sites calculated from DRIFTS spectra, centered at 1160 cm−1 (Lewis acid) and 1430 cm−1 (Brønsted acid), respectively. 2866

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shows the values of the integrated peaks area centered at 1160 and 1430 cm−1. These represent the quantities of Lewis acid and Brønsted acid sites, respectively. At room temperature, the amounts of Lewis acid sites observed for the three catalysts were somewhat similar. However for the Brønsted acid sites, the amount observed varies as fresh >0.58 Na > 1 K, in accordance with the order of SCR performance below 200 °C. As we known, NH4+ species adsorbed on the Brønsted acid sites are significant for the SCR reaction especially at low temperature. The result indicates that K-doped catalyst could decrease more NH4+ adsorption at room temperature, which could critically low the SCR performance. In the presence of alkali metals the ammonia desorption was almost complete at 250 °C, while the fresh catalyst still held adsorbed NH3 at 300 °C, and the desorption was complete at approximately 350 °C. The results clearly indicated that alkali metals can reduce the adsorption activity of Brønsted acid sites, and thus lower the SCR activity. With increasing temperature, the amount of both Lewis and Brønsted acids with adsorbed ammonia decreased, but the desorption rates of Brønsted acid sites were nearly the same as those for the poisoned catalysts. This reveals that the Brønsted acid strength sequence of the fresh catalyst is not changed upon alkali metal doping, in contrast to the V−W/Ti.9 4.4. Reducibility (H2-TPR). The redox activity of the cerium and tungsten species is considered the most important property in the catalytic cycle of SCR.2,25 Based on this theory, the reducibility of the cerium and tungsten species doped with different alkali metal was investigated and the H2-TPR profiles are shown in Figure 5. Two main peaks and a shoulder peak

Figure 6. XRD patterns of ceria and the CeW catalysts.

attributed to the cubic fluorite structure of CeO2 (JCPD 81− 0792). We assume that tungsten atoms are well dispersed on the ceria surface and could substitute for surface cerium atoms to form point detects. The following calculations were all based on the ceria (110) surface. Based on the results shown in Figure 4 suggesting that alkali metals only influence the Brønsted acid sites, the following DFT calculations consider the “fresh” and “poisoned” Brønsted acid sites for clarity. 4.6. Optimized Structures. The optimized structures of the Brønsted acid site and the alkali poisoned sites of the CeW models are shown in Figure 7. From the calculated results, the

Figure 7. Front view and top view of the optimized structures of the (a) Brønsted acid site surfaces, (b) Na-poisoned and (c) K-poisoned surfaces of CeW (110). Cerium atoms are gray, oxygen atoms are red, tungsten atoms are violet, hydrogen atoms are white, sodium atoms are yellow, and potassium atoms are green.

Figure 5. H2-TPR profiles of the fresh CeW, 1 K, and 0.58 Na poisoned CeW catalysts.

were observed in the curve of the fresh CeW 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 were centered at 509 and 812 °C, respectively,26 and the shoulder peak located at 720 °C can be attributed to the reduction of tungsten (W6+ to W0).27 When alkali metals were doped into CeW catalysts, the reduction peaks of cerium and tungsten species shifted to higher temperature. These results show that doping alkali metals into CeW catalysts could reduce the degree of reduction of the cerium species. 4.5. XRD Patterns. Characterization of the crystal lattice of the prepared materials was required to model a reasonable and compact catalytic surface. Based on the XRD patterns of ceria and the CeW catalysts shown in Figure 6, no peaks related to oxidized tungsten species were detected, and all peaks were

ceria (110) surface is significantly changed when Na and K are substituted for H atoms. After geometry optimization, the alkali metals atoms were strongly adsorbed to the surface (the bond length of Na−O and K−O were 2.48 and 2.75 Å), and a surface oxygen atom (moved oxygen, bonded with tungsten and cerium) moved to and covered the top site of tungsten, which is considered the active site of the CeW catalyst.7 Recently, Witko et al. presented a hole site deactivation model to explain the K-poisoned V2O5 surface using ab initio methods, and proposed that alkali metals could be stable on nonatomic hole sites formed on the V2O5 (010) surface, which would inhibits the active sites.28 Nicosia et al. used results from XPS and DFT to infer that the poisoning elements occupy the “hole sites” in the middle of four vanadium centers.10 Based on our 2867

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interacting with the moved oxygen (1.38 Å). The bond length of H−O in H2O was 1.11 Å, which was longer than the standard H−O bond in H2O (0.98 Å) and the other H−O bond is remained unchanged. The Mulliken charge of the tungsten increased to +1.08, which was nearly the same as in the fresh models. This model indicated that H2O can be dissociated to H+ and OH− on the poisoned surface, where the H+ can be tightly bonded to a surface oxygen atom.14,17 This could heal the covering effect of the moved oxygen and form a new Brønsted acid site. However, this is only elementary discussed, more efforts need to be made for future study. We also tested the SO2 resistance of the fresh and poisoned CeW catalysts (SI Figure S4) and the SCR activity of the corresponding regenerated samples (SI Figure S5) at low and high temperatures. These experiments gave no evidence of a synergistic inhibition effect from the SO2 and alkali metals on the SCR activity and demonstrated that the hot water regeneration method is also useful for catalysts poisoned by both SO2 and alkali metals. On the basis of the above results, it may be concluded that alkali metal poisoning resistance of CeW catalyst is better than commercial V−W/Ti and V2O5/AC. The amounts of Brønsted acid and reducibility rather than acid strength are responsible for the alkali metal poisoning mechanism. The theoretical calculations have shown that for poisoned surfaces, oxygen atom could cover the active site, prohibiting the formation of NH3 chemisorption on Brønsted acid sites.

calculations, the alkali metals could not only adsorb on the catalyst surface easily, but also induce an adjacent oxygen atom to cover tungsten, which could inhibit the adsorption of NH3 molecule. The Mulliken charge of the tungsten atom was +1.09 for Brønsted acid sites, and when Na and K were doped in, the charges of tungsten decreased slightly (+1.03 for Na and +1.01 for K doping). On the other hand for the surface cerium atoms, the charges were nearly unchanged (+1.57 to +1.59). The slight electron transfer from surface atoms will produce electron reorganization on the catalyst surface and result in changes in the catalytic surfaces. The local density of states (LDOS) for the surface oxygen and tungsten atoms of the optimized structures are shown in SI Figure S3. 4.7. Regeneration. Figure 8 shows the activities of the poisoned catalysts after regeneration by hot water compared

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ASSOCIATED CONTENT

* Supporting Information S

The model and functional selections, some related tables and figures. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 8. The NH3−SCR activity of the regenerated poisoned catalysts. Reaction conditions: catalyst=300 mg, [NO] = [NH3] = 500 ppm, [O2] = 3%, total flow rate = 300 mL/min, GHSV = 60 000 h−1.

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with the fresh and poisoned samples. The regenerated catalysts yielded higher activities than the corresponding poisoned ones, which indicate that some of the surface alkali metal oxides can be dissolved in hot water and removed from catalysts surfaces. Furthermore, the NOx conversion of regenerated samples below 160 °C was higher than that of the fresh CeW catalyst. This is probably due to the promoting effects of surface hydroxyls (M−OH).29,30 DFT calculations were also employed to investigate H2O adsorbed on the poisoned surface, though here we studied only the Na-poisoned surface. The optimized structure is shown in Figure 9. H2O can bond to the surface with a hydrogen atom

AUTHOR INFORMATION

Corresponding Author

*Phone: +86 10 62771093; fax: +86043185167869; e-mail: [email protected] (J. L.), [email protected] (W. H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the national High-Tech Research and Development (863) Program of China (Grant No. 2010AA064806) and a special fund of the State Key Joint Laboratory of Environment Simulation and Pollution Control (10Z01ESPCT) is gratefully acknowledged.

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REFERENCES

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