Article pubs.acs.org/est
Deactivation Mechanism of Potassium on the V2O5/CeO2 Catalysts for SCR Reaction: Acidity, Reducibility and Adsorbed-NOx Yue Peng,† Junhua Li,*,†,‡ Xu Huang,† Xiang Li,† Wenkang Su,† Xiaoxu Sun,† Dezhi Wang,§ and Jiming Hao†,‡ †
State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, People’s Republic of China ‡ State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Tsinghua University, Beijing 100084, People’s Republic of China § Beijing Guodian Longyuan Environmental Engineering Co., LTD, Beijing 100084, People’s Republic of China S Supporting Information *
ABSTRACT: A series of V2O5/CeO2 catalysts with different potassium loadings were prepared to investigate alkali deactivations for selective catalytic reduction of NOx with NH3. An alkali poisoning mechanism could be attributed to surface acidity, reducibility, and NOx adsorption/desorption behaviors. The detailed factors are as follows: (1) decrease of surface acidity suppresses NH3 adsorption by strong bonding of alkali to vanadia (major factor); (2) low reducibility prohibits NH3 activation and NO oxidation by formation bonding of alkali to vanadia and ceria (important factor); (3) active NOx− species at low temperature diminish because of coverage of alkali on the surfaces (minor factor); and (4) stable, inactive nitrate species at high temperature increase by generating new basic sites (important factor). and nitrite species (NOx−) have not been given more attention.9,10 The oxidation process of NO to NO2 would result in significant promotions for SCR reaction under 300 °C by forming key intermediate species NH4NO3.11−13 However, considerable NO3− cannot take part in the reaction with NH3 for the vanadia-based or nonvanadia catalysts.14,15 In our previous studies, the structure activity relationships of V2O5/ CeO2 for the SCR reaction have been investigated by the combination of in situ IR and Raman spectroscopies.16 The results indicate that polymeric VOx and CeVO4 species served as major acid sites under 300 °C and parts of NOx− are reactive above 350 °C. However, the influence of NOx− on the reaction mechanism coupled with alkali poisoning has not been systematically investigated. The objective of the present work is to identify surface variations of acidity, reducibility, and NOx− adsorption/desorption behaviors by alkali and their impact on SCR performance.
1. INTRODUCTION Selective catalytic reduction (SCR) with NH3 is one of the most widely adopted technologies for NOx removal from stationary sources or diesel vehicles exhaust. Although commercial catalysts, V2O5−WO3(MoO3)/TiO2, display good deNOx efficiency within 300−400 °C, some defects still remain, including the toxicity of vanadia and high oxidation ability of SO2 to SO3.1 Furthermore, alkali metals might restrain the performance of vanadia-based catalysts and lower their lifetime. Alkali in fly ash is a major concern for coal-fired plants, while some fuels, lubrication oil additives, and urea solutions are another origin for diesel vehicles.2−4 Therefore, many scientists are working to elucidate the deactivation mechanism of alkali and to develop poisoning resistance catalysts. Some researchers suggested that the competitive adsorption of NH3 molecules with alkali led to the low SCR activity of the poisoned catalysts, rather than the loss of surface area or catalyst micropore occlusion. Others proposed that low reducibility might be another factor prohibiting the activation process of NH3 from further reacting with NOx.5−8 Our previous studies suggested that ceria could not only enhance catalysts alkali resistance, but also consequently promote regeneration from the views of acidity and reducibility. Nonetheless, interactions between alkali and adsorbed nitrate © 2014 American Chemical Society
Received: Revised: Accepted: Published: 4515
December 16, 2013 March 2, 2014 March 17, 2014 March 17, 2014 dx.doi.org/10.1021/es405602a | Environ. Sci. Technol. 2014, 48, 4515−4520
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2. EXPERIMENTAL SECTION 2.1. Catalysts Preparation and Poisoning. The 4 wt % V2O5 supported on CeO2 was selected as the nonpoisoned catalyst (fresh).16 Potassium oxides were added to as-prepared catalysts by impregnating KNO3 solutions with different concentrations. The K2O loadings on the catalysts were set to be 0.5, 1, and 2 wt %, where 0.5 wt % of K2O content (1.0 wt % K) is of the same order of magnitude as the alkali metal doped on SCR catalysts after 600−700 h on stream.4 The poisoned catalyst with 0.5 wt % of K2O content was denoted as K0.5. Other detailed prepartion methods can be found in the Supporting Information (SI). 2.2. Catalysts Performance. The activity measurements were carried out in a fixed-bed quartz reactor (inner diameter 5 mm) using 100 mg catalyst of a 40−60 mesh. The feed gas mixture contained 0.05% NO, 0.05% NH3, 3% O2, and was balanced with N2. The total flow rate of the feed gas was 200 mL·min−1, and the space velocity was about 120 000 mL·gcat−1· h−1. The concentrations of NO, NO2, N2O, and NH3 were continually monitored by an FTIR spectrometer (Gasmet FTIR DX-4000). 2.3. Catalysts Characterization. The BET surface area was carried out with a Micromeritics ASAP 2020 apparatus. Raman spectroscopy was measured using a Renishaw Raman microscope. Temperature programmed reduction (TPR) of H2 was performed on a ChemiSorb 2720 TPx chemisorption analyzer under 10% H2/Ar gas flow (50 mL·min−1) at a rate of 10 °C·min−1 up to 1000 °C. Temperature programmed desorption (TPD) of NO was performed in a fixed-bed quartz reactor. The sample was purged under 1000 ppm NO at room temperature after being pretreated in N2 at 350 °C for 1 h. When isothermal removal of the physically adsorbed NO was accomplished, the temperature was raised to 630 °C at a rate of 10 °C·min−1. In-situ IR spectra were recorded on a Nicolet 6700 FTIR equipped with a Harrick cell. The background spectrum was collected in N2 and was subtracted from each sample spectrum. The spectra were recorded by accumulating 32 scans. Here, to diminish the influence of absorbance from different samples, the absorbance intensity was set to 3.00 for every sample at 300 °C.
Figure 1. The SCR performance and N2O formation of V2O5/CeO2 (fresh) catalysts and alkali-doped catalysts. Reaction conditions: catalysts = 100 mg, [NO] = [NH3] = 500 ppm, [O2] = 3%, total flow rate = 200 mL/min, and GHSV = 120 000 mL·gcat−1·h−1.
influence of H2O is also studied (SI Figure S1), and the results indicate that H2O could decrease the catalytic activity, as soon as one shuts down the H2O gas flow, and then NOx conversion might be restored. The BET surface areas of the four samples were nearly the same, limited between 30 to 32 m2·g−1. The dehydrated Raman spectra are shown in Figure 2. All the profiles are normalized to
3. RESULTS AND DISCUSSION 3.1. SCR Activity and Physical Structure. Figure 1 shows the SCR performance of fresh and poisoned V2O5/CeO2 in the range of 200−400 °C under a space velocity of 120 000 mL· gcat−1·h−1. Previous kinetic studies have proposed that 4 wt % V2O5/CeO2 exhibited a higher reaction rate than V2O5−WO3/ TiO2 below 300 °C, and the energy barriers dramatically decreased when surface polymeric VOx and CeVO4 formed.16 So we only compared NOx conversion among the samples. The activity of poisoned catalysts significantly decreased, and the deactivation order depended directly on the potassium loading below 400 °C, while the sequence of poisoned catalysts was reversed at 400 °C, resulting from the suppression of NH3 oxidation at high temperature. As a side reaction involved in the SCR process, NH3 oxidation is directly related to the number of surface vanadia sites. By the coverage of potassium on the vanadia sites at high temperature, the NH3 oxidation was slightly restricted. Moreover, N2O formations during the SCR process were also obtained and displayed a promotional trend with increasing potassium loading, indicating the prohibition of deNOx efficiency and N2 selectivity after poisoning. The
Figure 2. Raman spectra (532 nm laser) of the fresh and poisoned catalysts (dehydration).
464 cm−1. Peaks at 464, 599, and 1168 cm−1 can be attributed to the F2g mode of CeO2 fluorite phase, defect-induced mode and second-order longitudinal optical mode, respectively.17 For poisoned ones, the fluorite phase did not vary. Peaks at 715 and 831 cm−1 can be due to the V−O−V vibration mode and CeVO4 species, and peaks at 982 and 1053 cm−1 can be due to the VO vibration mode.18,19 The disappearance of the peak at 831 cm−1 after poisoning is due to the coverage of potassium on CeVO4, leading to the decrease of surface acidity.16 While the Raman peak at 1053 cm−1 was intensified, which can be 4516
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°C for fresh and 370 °C for K2), exhibiting the same trend as the SCR performance. Moreover, the reduction peaks of ceria also moved to a somewhat higher temperature, suggesting a slight reducibility loss of ceria. Combined with Raman results, potassium bonds to both surface vanadium and cerium atoms, prohibiting the reduction of V5+ (major) or Ce4+ (minor). The loss of surface acidity and reducibility can be responsible for the low activity of poisoned catalysts. When alkali form oxides with surface oxygen bonding to active components (K− O−V or K−O−Ce), alkali may prevent adsorption and activation of gaseous NH3. The adsorption and activation process of NH3 are the most important steps in SCR reaction, which were primarily controlled by acid sites and redox sites, respectively.25 Once they both decrease, the deNOx efficiency might be restrained. 3.3. NO-TPD. In order to investigate the influences of alkali on NO adsorption/desorption behaviors, NO-TPD experiments are carried out and shown in Figure 5. The fresh sample
attributed to the aggregation of surface VOx species resulting from the strong bonding of potassium to cerium.20,21 3.2. Surface Acidity and Reducibility. Figure 3 shows the integral areas calculated from the IR absorbance peaks ranging
Figure 3. The desorption amounts of NH3 molecules preadsorbed on the catalysts calculated from in situ IR spectra.
from 1150 to 1300 cm−1, representing the catalyst surface acidity.22 The original IR spectra are shown in SI Figure S2. The catalyst acidities were significantly improved by vanadia but diminished by potassium at 100 °C. With elevated temperature, the acidity of the fresh catalyst reduced rapidly between 100 and 200 °C and relatively mildly diminished between 200 and 300 °C. For K1 and K2 catalysts, nearly all the NH3 molecules were desorbed from the surface at 200 °C, which led to lower SCR activity. Figure 4 shows the H2-TPR Figure 5. NO-TPD carried out on a fix-bed reactor in the temperature range of 30−630 °C.
yielded considerable NO2 at 190 and 450 °C, whereas for the poisoned ones, only NO desorption peaks can be detected. The results present a good accordance with NO oxidation (SI Figure S3) that oxidation ability suppressed by potassium. Two NO desorption peaks appeared at 150 and 470 °C for fresh, which could be assigned to NOx−. Considering the unfavorable adsorption of NOx on vanadia,26,27 the two peaks might bond to ceria. For poisoned catalysts, the low temperature (LT) peak shifted to 280 °C, and the high temperature (HT) peak shifted to 500 °C, respectively. The results suggest that doping with potassium might strengthen the bonding of NO with surfaces. The intensities of the LT peak decreased and the intensities of HT peak increased with elevating potassium loading. That is, a different potassium amount would result in the redistribution of stable and metastable NOx− species. This can account for newly generated basic sites provided by potassium. However, NOTPD cannot distinguish the configuration types of NOx−, especially the assignment of LT and HT peaks, we carried out the desorption again on IR spectra. 3.4. NOx Desorption on in Situ IR Spectra. Figure 6(a) shows IR spectra of NOx desorption in the range of 100−550 °C. At 100 °C, several peaks at 1038, 1241, 1286, 1461, 1530, 1576, 1605, and 1739 cm−1 can be observed. They can be attributed to surface nitrate species (1038 and 1605 cm−1 for
Figure 4. H2-TPR profiles of the fresh and poisoned catalysts.
profiles of fresh and poisoned catalysts. Three peaks can be observed from the fresh sample: a shoulder peak at 361 °C can be attributed to the reduction of vanadia, and peaks at 495 and 757 °C can be assigned to the surface and bulk reduction of ceria.23,24 For poisoned catalysts, the reduction peaks of vanadia weakened and gradually disappeared, and the initial H2 consumption point also moved to a higher temperature (250 4517
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Figure 6. NO-TPD carried out from in situ IR spectroscopy at 100, 300, 400, 500, and 550 °C, respectively.
bridged-NO3−, 1576 cm−1 for bidentate-NO3−, and 1530 cm−1 for monodentate-NO3−), nitrite species (1461 and 1241 cm−1 for monodentate-NO2−, 1286 cm−1 for bridged-NO2−), and adsorbed NO (1739 cm−1), respectively.22 With increasing the temperature to 300 °C, monodentate-NO2− and adsorbed NO disappeared, while a new peak at 1203 cm−1 occurred, owing to chelated-NO2−. On the basis of NO-TPD, the vanished NOx− species could be directly desorbed from the catalyst surface or changed to chelated-NO2−. Up to 400 °C, parts of bridgedNO3− and bidentate-NO3− weakened and nearly all NOx− species desorbed at 500 °C. For K0.5 sample (Figure 6(b)), except for the existence of peaks at 1743, 1604, 1571, 1526, 1457, 1024 cm−1 on the spectra, two new peaks at 1314 and 1272 cm−1 appeared at 100 °C. For the other two poisoned samples, similar peaks were also observable (1317 and 1272 cm−1 for K1 and K2). Considering the NO-TPD results, the two peaks could be tentatively attributed to nitrite species on new adsorbed sites. Elevating the temperature, the two peaks weakened and vanished above 500 °C, suggesting that they can be assigned to HT peaks in Figure 5. Increasing the potassium loading could enhance the quantities of adsorption configurations, which might be originated from newly formed basic sites
provided by potassium. Furthermore, the stability of nitrate species was also improved. For fresh catalyst, they desorbed totally at 500 °C, while for poisoned ones, they can be still obtained above 500 °C. Chelated-NO2− species at 1203 cm−1 for the fresh catalyst shifted a little higher to a wavenumber of 1207 cm−1 for K0.5 and cannot be detected for K1 and K2 samples, indicating that potassium oxides could suppress the formation of chelatedNO2− species. For the IR spectra over K1 (Figure 6(c)) and K2 (Figure 6(d)), bridged-NO3− and bidentate-NO3− species cannot be observed and monodentate-NO3− species can be obtained. The results indicate that bridged-NO3− and bidentate-NO3− species mainly bond to ceria and monodentate-NO3− species bond to both ceria and K2O. New peaks at 1489 cm−1 for K1 and 1469 cm−1 for K2 occurred, and parts of the two peaks became weak in quantities at 300 °C. Combined with NO-TPD results, these peaks could be attributed to LT peak discussed above. That is, NOx desorbing at low temperature might originate from nitrite species (1461 cm−1), which mainly bond to cerium atoms. Active cerium atoms exposed on the surface diminish with increasing the potassium loading. These decrease in amount but improve in bond strengthening of NOx adsorption 4518
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Figure 7. Sequential in situ IR spectra of the fresh sample recorded under various atmospheres: (a) the dehydrated sample was first pretreated with NH3, and then NO and O2 were introduced, and (b) the reversed order at 250 °C.
at low temperature. However, NOx desorbed at high temperature can be attributed to nitrate species (for all samples), adsorbed NO (for poisoned), and nitrite species (1272 and 1314 cm−1). The quantities of bridged-NO3− and bidentateNO3− is restrained, whereas the stability of both monodentateNO3− and newly generated nitrite species bonding to potassium is enhanced at high temperature. 3.5. Reaction Details of Fresh at 250 °C. Finally, the reaction details of fresh sample at 250 °C on in situ IR spectra were performed. In Figure 7(a), the sample first adsorbed NH3 saturated then purged by N2 to remove physic-sorbed NH3. After introducing NO and O2 for 5 min, parts of the NH3 peak (1163 cm−1) became weakened and nearly disappeared at 10 min, indicating that the surface NH3 are active.16 Then the order of the gas flow was reversed, and the NOx species can stably bond to the catalysts (Figure 7(b)). When NH3 and O2 were induced into the gas flow for 30 min, most of the NOx− species (HT peaks) cannot react with gaseous NH3. Therefore, the increased NOx− species by alkali doping display good stability under thermal treatment and are inactive during SCR reaction. After 2 min, the peak at 1210 cm−1 weakened and the peak at 1034 cm−1 also decreased but still existed in 30 min, while the peak at 1178 cm−1 appeared, which can be due to the vibration of NH3. The results indicate that only chelated-NO2− and parts of bridged-NO3− (LT peaks) can be involved in the reaction at 250 °C. On the basis of the study about NOx adsorption/desorption behaviors, active NOx− species bonding to ceria decrease and inactive NOx− species bonding to basic sites increase by alkali doping, which could be another factor for SCR deactivation. 3.6. Speculations on the Weights of Poisoning Factors. Finally, the importance of the four deactivation factors is tried to quantify. (1) Surface acid: the major factor suppressing the deNOx efficiency of the poisoned catalyst, because the adsorption of NH3 is the first step and seems quite fast on acid catalysts, such as vanadia-based ones. If the surface acidity is prohibited, then this step will become the rate-determining step to dramatically decrease the reaction rate.
(2) Reducibility: an important factor but less crucial compared to surface acid. Common titanium or cerium supports could more or less provide surface oxygen to activate adsorbed NH3 or NOx to more active ones. Unless the reducibility was significantly suppressed, this factor cannot determine the SCR activity. (3) Active NOx− species: these species show some merit to SCR reactions but are not important because the adsorption and activation of NOx are not the key steps, especially at medium and high temperature, Eley−Rideal mechanism plays more important role than Langmuir− Hinshelwood mechanism. (4) Inactive NOx− species: an important factor because the bond of NOx− to alkali metals might form KNO3-like species, which could stabilize surface oxygen to hinder the activation of NH3. Moreover, these species are thermally stable and might accumulate with increasing working time.
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ASSOCIATED CONTENT
S Supporting Information *
Catalysts preparation and poisoning; influence of H2O; IR spectra of NH3 adsorptions; NO oxidations; and some related figures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 10 62771093; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial supports from National Natural Science Foundation of China (21325731 and 21221004), and National High-Tech Research and the Development (863) Program of China (2013AA065401) and the International Postdoctoral Exchange Fellowship Program of China (20130032). 4519
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