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Catalysts 2017, 7(4), 110; https://doi.org/10.3390/catal7040110

Article
Study of the V2O5-WO3/TiO2 Catalyst Synthesized from Waste Catalyst on Selective Catalytic Reduction of NOx by NH3
by Chunping Qi 1,2,3, Weijun Bao 1, Liguo Wang 1, Huiquan Li 1,2,3,* and Wenfen Wu 1,2
1
Key Laboratory of Green Process and Engineering, National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100090, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
3
Sino-Danish Center for Education and Research, University of Chinese Academy of Sciences, Beijing 100049, China
*
Correspondence: [email protected]; Tel.: +86-10-8254-4825; Fax: +86-10-8254-4830
This paper is an extended version of an abstract published in the 10th NCECM Conference of China, Hangzhou, China, 4–7 November 2016.
Academic Editors: Shaobin Wang, Xiaoguang Duan and Keith Hohn
Received: 1 December 2016 / Accepted: 6 April 2017 / Published: 8 April 2017

Abstract

:
V2O5-WO3/TiO2 catalysts were synthesized from waste selective catalytic reduction (SCR) catalyst through oxalic acid leaching and impregnating with various V2O5 mass loadings. The denitration (deNOx) activity and physiochemical properties of the catalysts were investigated. All the catalysts were characterized by N2 adsorption/desorption, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and H2-temperature programmed reduction. The evaluation result revealed that the deNOx activity of newly synthesized catalyst with 1.0% V2O5 was almost recovered to the level of fresh catalyst, with NO conversion being recovered to 91% at 300 °C, and it also showed a good resistance to SO2 and H2O. The characterization results showed that the decrease of impurities, partial recovery of the V4+/V5+ ratio, and increased reducibility were mainly responsible for the recovery of catalytic activity.
Keywords:
V2O5-WO3/TiO2 catalyst; leaching; oxalic acid; deNOx activity

1. Introduction

Selective catalytic reduction (SCR) of nitrogen oxides with ammonia is one of the most effective methods for eliminating the hazardous NOx emission from stationary sources [1]. The honeycomb monolith V2O5-WO3/TiO2 catalyst is widely used for commercial processes because of its high activity and SO2 resistance [2]. For the V2O5-WO3/TiO2 catalyst, anatase TiO2 acts as a carrier, V2O5 is the active component, and WO3 performs as the promoter to stabilize the catalyst and prevents the catalyst from sintering. In coal-fired power plants, the SCR catalyst suffers from gradual deactivation during the operation period because of poisoning, sintering, fouling, surface masking, attrition/crushing, and loss of vanadium or change in ratio value of the vanadium in different valence states [3,4,5,6,7,8].
Deactivated catalysts that retain a relatively intact monolith structure and low activity are always liable for regeneration by special regeneration equipment. A typical regeneration process generally involves mechanical blowing (dust removal), depth cleaning (toxic component elimination and inner channel dredging), active component impregnation, drying, and calcination [9,10,11]. By contrast, deactivated catalysts that involve physical breakage or attrition of the monolith and low activity are supposed to be disposed of as waste. Reportedly, 70%–80% of deactivated SCR catalysts could be regenerated, but the regeneration time is limited, and all deactivated SCR catalysts will eventually end up as waste [12].
In China, waste SCR catalyst was classified as hazardous waste in 2014, and effective highly cautious disposal is a matter of great urgency [13]. To date, researchers are mainly focused on a variety of recycling and reuse technologies including mineral filter applications, incorporation into a wet-bottom boiler slag, cement kiln co-processing, iron/steel-making applications [14,15], and its use in fresh catalyst manufacture and materials recovery [12,16,17]. Therein, the reuse of waste SCR catalysts to synthesize fresh SCR catalysts is supposedly an economical and high resource utilization method [15]. However, the direct utilization of waste SCR catalysts in the synthesis of fresh SCR catalysts invariably results in low catalytic activity. This reason is due to the powder of titanium and tungsten more easily agglomerating under the effect of binder during mixing, leading to an uneven distribution [18]. In addition, the deposited sulfates and metals, such as Fe, K, and Na, are toxic to catalysts, and reduce their activity significantly. Moreover, the valence alternation of activated vanadium results in the decrease of the concentration of V4+ species, so the catalyst activity cannot recover to the level of the fresh catalyst only by only removing the deposited contaminants.
Therefore, prior to reuse, cleaning of waste SCR catalysts should be required. At present, various aqueous solutions including acids, alkalis, and salts have been proposed for washing waste SCR catalyst. Diluted H2SO4 is proven to be effective in recovery of the activity of waste catalyst by removing alkali metals and alkaline earth metals, and recovering the porosity [19,20]. To our knowledge, the alkali solution might be more effective in dissolving sulfates [21]. Yu et al. [21] investigated the alkali washing of deactivated commercial SCR catalyst, which could effectively remove the deposited Al2(SO4)3 and recover porosity. However, the element Na deposited on the catalyst during alkali washing may exert an adverse impact on catalyst activity. With this consideration, alkali washing is always followed by acid washing. Washing by dilute NaOH and HNO3 solutions in sequence is found to be more effective with respect to activity recovery [22]. Furthermore, washing with ammonium salt solutions, such as NH4Cl and (NH4)2SO4 solution, are also found to be an effective method to regain deNOx activity [20,23].
Among these aqueous solutions, oxalic acid (OA), a strong organic acid, is effective in removing alkali metals, Fe, and other contaminants. It has been widely used in the regeneration of deactivated catalysts. Christou et al. [24] found oxalic acid was effective in removing P- and S-containing compounds from aged commercial three-way catalysts, and the investigation of oxalic acid in removing Pb, Zn, Ca, Mn, Fe, Cu, and Ni metal contaminants was also performed [25,26]. On the other hand, OA is reducing and has been used as a complexing agent to selectively extract vanadium from the spent catalyst [27], so vanadium in different valence states can be eluted forcedly by aqueous OA solution. In this work, the valence of vanadium changed in the waste catalyst, and resulted in the decrease of V4+/V5+ ratio. In addition, Fe was also significantly deposited on the catalyst, resulting in the increase of the SO2 oxidation rate [28]. OA solution was reported to have a high extract efficiency of V and Fe due to its strong acidity and good complexing effects [29]. More significantly, the obtained leaching residue is mainly anatase TiO2, which can be reused as a carrier for the synthesis of new SCR catalyst.
In this work, the V2O5-WO3/TiO2 catalysts were synthesized from waste SCR catalyst after oxalic acid leaching and impregnating with various V2O5 mass loadings. The deNOx activity and physiochemical properties were investigated. The surface structural, textural, and redox properties of the fresh waste and resynthesized V2O5-WO3/TiO2 catalysts were characterized by N2 adsorption/desorption, Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and H2-temperature programmed reduction (H2-TPR).

2. Results and Discussion

2.1. Characteristics of the Fresh and Waste SCR Catalyst

The main compositions of the fresh (donated as Fresh) and waste (donated as Waste) SCR catalysts are listed in Table 1. Compared with the fresh catalyst, waste catalyst showed 20% lower V content. Moreover, the concentrations of Al, Si, Ca, Fe, K, and Na increased to varying degrees. Notably, the K concentration was 10 times higher than that in fresh catalyst. In addition, Fe and Al elements also evidently accumulated in the waste catalyst, with the concentrations being six and two times higher than those of the fresh catalyst, respectively. Evidently, the waste catalyst could not be directly reused by blending with fresh catalyst, because the deposited alkali metals and alkaline earth metals exert an adverse effect on catalytic activity. Therefore, the waste catalyst was first washed with OA solution, and the compositions of OA leaching residue are also presented in Table 1. The contents of alkali metals (K, Na) and Fe in OA leaching residue markedly declined and were similar to that in the fresh catalyst. In addition, the contents of Al and Ca also dropped by different degrees, and approximately 44% of Al and 17% of Ca were removed. A large percentage of V was dissolved at the same time.
Figure 1 displays the X-ray diffraction (XRD) patterns of the fresh, waste, and OA leaching residue. Similar to the fresh and waste catalysts, the anatase type TiO2 carrier was not destroyed after leaching, and the obtained residue was mainly composed of anatase TiO2.
Based on the results of the analysis of compositions and XRD, the OA leaching residue was expected to synthesize the new V2O5-WO3/TiO2 catalyst after impregnation of the active components. So the V2O5-WO3/TiO2 catalysts loaded with 0.5%, 1.0%, and 1.5% V2O5 were resynthesized from the OA leaching residue.

2.2. Catalytic Activity Test

The deNOx activity profiles of the fresh, waste, and resynthesized (donated as Re-x%, where x was the overall V2O5 content in the catalyst after oxalic acid leaching and vanadium impregnation) V2O5-WO3/TiO2 catalysts are presented in Figure 2. The waste catalyst showed a similar deNOx activity to the fresh catalyst at temperatures exceeding 350 °C, but exhibited a lower activity in the temperature range of 150–350 °C. In particular, the NO conversion of the waste catalyst was 80% at 300°C, which was less than that of the fresh catalyst (96%). Furthermore, a direct impregnation of additional 0.2% V2O5 on a waste catalyst was tested, but no improvement in the NO reduction efficiency was observed (Figure S1). Therefore, V2O5-WO3/TiO2 catalysts were resynthesized from waste catalyst after oxalic acid leaching and impregnating with various V2O5 mass loadings.
Among all the catalysts, the resynthesized catalyst loaded with 0.5% V2O5 exhibited the poorest activity, even lower than that of the waste catalyst in the temperature range of 150–350 °C. For the resynthesized catalyst containing 1.0% V2O5, deNOx activity was significantly increased and almost recovered to the level of fresh catalyst. The NO conversion was recovered up to 91% at 300 °C. In comparison, the 1.5% V2O5-WO3/TiO2 catalyst showed even better deNOx activity at low temperature, with a high NO conversion value of almost 100%, but the conversion decreased markedly at temperature exceeding 400 °C, which was probably caused by the oxidation of NH3 (2NH3 + 2O2 → N2O + 3H2O) [8], because the increased N2O formation with increasing V2O5 content was also detected simultaneously. The results revealed that the deNOx efficiency (both activity and selectivity) of the resynthesized catalysts with different V2O5 contents varied considerably, and with V2O5 content of 1.0%, the resynthesized catalyst regained a similar activity to the fresh catalyst.
In the real circumstance of NH3-SCR, SO2 and H2O both unavoidably existed in the flue gas, so it is very important for industrial application to investigate the effect of SO2 and H2O on the SCR performance of catalysts. In this work, the effects of SO2 and H2O on deNOx activities over fresh, waste and resynthesized 1.0% V2O5-WO3/TiO2 catalysts were tested, and the results are illustrated in Figure 3. Figure 3a presents the combined effects of SO2 and H2O over different catalysts at various temperatures (350, 300 and 250 °C). It can be seen that the NO conversion of fresh, waste, and Re-1.0% catalysts was stabilized during the first 7.5 h run in the absence of SO2 and H2O. When 200 ppm SO2 and 10 vol % H2O were added into the reaction gas at 350 °C, a slight decline of NO conversion for in the first 0.5 h, and then it nearly stabilized. With the temperature decreasing from 350 to 250 °C, a more obvious decrease of NO conversion for all catalysts was observed in the coexistence of SO2 and H2O. Particularly, sustained declines of NO conversions for fresh, waste, and Re-1.0% catalysts were detected over 8 h at 250 °C, from 43.9% to 33.7%, from 17.6% to 13.0%, and from 43.2% to 42.0%, respectively. This was mainly caused by the easy formation of ammonium bisulfate with increasing NH3 slip due to the lower NO conversion, and the difficult decomposition of ammonium bisulfate with decreasing temperature [30].
In addition, the individual and combined effects of SO2 and H2O over different catalysts were also investigated at a typical temperature (300 °C), as shown in Figure 3b. The individual introduction of 10 vol % H2O or 200 ppm SO2 into the reaction gas stream was found to have little effect on the deNOx activities over all the catalysts. However, when both 200 ppm SO2 and 10 vol % H2O were simultaneously added to the reaction gas stream, a significant decrease of NO conversion was found over the fresh and waste catalysts, which was 8% and 12%, respectively during the following 8 h. By contrast, the NO conversion of Re-1.0% catalyst decreased from 91% to 84% at 300 °C, after the introduction of 200 ppm SO2 and 10 vol % H2O. After cutting off SO2 and H2O, the NO conversion of fresh and Re-1.0% catalyst was almost restored to its original level, and then remained unchanged during the test period, while the NO conversion over waste catalyst was gradually restored to 72%, which was less than the initial value. The above results suggested that, over the resynthesized 1.0% V2O5-WO3/TiO2 catalyst, the coexistence of water vapor and sulfur dioxide had a negative effect, but still exhibited a good resistance to H2O and SO2 during N reduction by NH3.

2.3. Textural Properties

The actual V2O5 content, specific surface area and VOx coverage of the abovementioned catalysts are presented in Table 2. The specific surface area of the waste catalyst was similar to that of the fresh catalyst. After OA leaching, the specific surface area of the residue was recovered to 76.9 m2/g owing to the removal of impurities and dissolution of vanadium. It is worth noting that the specific surface area of the resynthesized catalyst decreased to 50.1 m2/g after vanadium impregnation. On one hand, OA leaching would break down the portion of the original microstructure of the catalyst [31]. On the other hand, more vanadium was impregnated into the pores of the TiO2 carrier, covering the carrier surface, resulting in a decrease of specific surface area.
The calculation of vanadium (VOx) coverage was based on specific surface area. The deNOx activity of V2O5-WO3/TiO2 catalyst is highly dependent on the VOx coverage on titania, and the monolayer dispersion capacity of vanadia on titania is reported to be 1.14 mmol V/100 m2 TiO2 [32]. Baiker et al. [33] varied the vanadia loading from 0.08 to 0.75 mmol V/100 m2 TiO2 and found that deNOx activity increased when the vanadia loading increased from 0.18 to 0.3 mmol V/100 m2 TiO2, but decreased with further increases in vanadia loading. From Table 2, the VOx coverage of the waste catalyst decreased to 0.104 mmol V/100 m2 TiO2, which was considerably lower than that of fresh catalyst (0.139 mmol V/100 m2 TiO2). For the resynthesized catalysts, the VOx coverage was improved after impregnation of vanadium, and increased significantly with increasing V2O5 loadings. In particular, the Re-1.0% catalyst sample had a VOx coverage of 0.193 mmol V/100 m2 TiO2, which was higher than that of the fresh one, and did not exceed the monolayer dispersion capacity of V2O5 on anatase. In addition, the VOx surface coverage (the fraction of the theoretical monolayer) was also calculated according to the study by Marberger et al. [34]. As listed in Table 2, the waste catalyst had a lower VOx coverage (15.60%) than the fresh catalyst (20.85%). The resynthesized catalysts showed an increasing surface coverage with increasing V2O5 loadings. The surface coverage of the Re-0.5% catalyst sample was 18.30%, which seemed to not have enough active sites for the SCR reaction, and the Re-1.0% catalyst sample had a VOx coverage of 28.95% and supplied more active sites for the SCR reaction. However, the VOx coverage of the Re-1.5% catalyst sample increased to 55.95%, and it was reported that the catalyst would have low selectivity when the VOx coverage was above 50% [34], so the NO conversion of the Re-1.5% catalyst (shown in Figure 2) decreased at temperatures exceeding 400 °C. Therefore, the VOx coverage of the resynthesized catalysts was improved and the Re-1.0% catalyst sample had an optimized VOx coverage of 28.95%.

2.4. FT-IR Spectra

The FT-IR spectra shown in Figure 4 were investigated to study the supported species arising from V2O5 and WO3 components in the catalysts. The FT-IR spectra were taken with KBr pressed disks exposed to the air and the spectrum of the TiO2 support has been subtracted. Compared with the fresh catalyst, the waste catalyst showed a broad and strong band at 1122 cm−1, which was assigned to SO42− [10,35], and might result from the deposition of sulfates during operation. After OA leaching, the peaks of sulfates on the spectrum were clearly weakened, indicating that the deposited sulfates can be removed, favoring the improvement of catalytic activity of the resynthesized catalysts. Furthermore, the band was shifted from 1122 to 1108 cm−1 for the resynthesized catalysts, indicating that the deposited free surface sulfates was changed to bulk sulfates after the resynthesis of catalysts [36]. In addition, the weak band at 1063 cm−1, which showed increased intensity with increasing V2O5 content, was associated with the υ (S–O) stretching of the SO42− species interacting with the TiO2 surface [34]. The broad bands at 988–993 cm1 were associated with W=O and V=O stretching modes of wolframyl and vanadyl species that are superimposed on each other, and the bands increased slightly in intensity with increasing V2O5 content for the resynthesized catalysts [37,38,39]. The bands in the range of 873–895 cm1 and the weak broad bands at 942 cm1 were assigned to the V–O stretching mode of the metavanadate species [37,38] and the intensity of both bands decreased slightly with increasing V2O5 content for the resynthesized catalysts. This was because with increasing V2O5 content, the strongly interacting and stabilized isolated vanadium ions produced by the reaction with surface OH groups of TiO2, was gradually transformed into vanadium oxide clusters, more weakly interacting with the TiO2 surface [37].

2.5. XPS Analysis

The V4+/V5+ ratio on the SCR catalyst is known to be significantly correlated with deNOx activity, and a higher V4+/V5+ ratio contributed to better deNOx activity [40,41]. To understand the initial valence states and surface atomic concentration of vanadium, we characterized the catalysts using XPS. The results of V 2p XPS spectra are illustrated in Figure 5.
According to the V 2p photoelectron peak, V5+ is known to appear at 516.4–517.0 eV, V4+ at 515.7–516.2 eV [42], and V3+ at 513.1–514.7 eV [40], respectively. Clearly, in addition to V4+ and V5+, V3+ species were observed on the waste catalyst. After leaching, vanadium in the resynthesized catalyst was recovered to exist as V4+ and V5+. The concentration ratios of tri-, tetra-, and pentavalent vanadium ions are summarized in Table 3. The concentration of V4+ species on the waste catalyst decreased to 51.5% compared with the fresh catalyst (62.5%), and the ratio of V4+/V5+ decreased from 1.67 to 1.13. The resynthesis of the catalyst evidently influenced the atomic concentrations, and the V/Ti ratio and the concentration of V4+ species on the synthesized catalyst was gradually restored. When the V2O5 loading of the resynthesized catalyst increased from 0.5% to 1.5%, the V4+/V5+ ratio increased from 0.88 to 2.24. In particular, the concentration of V4+ species on the Re-1.0% catalysts sample was restored to 60.8%, and the V4+/V5+ ratio was 1.55. Therefore, it can be envisaged that after OA leaching and vanadium impregnating, the ratio of V4+ to V5+ in the resynthesized catalysts was improved, thus, contributing to the recovery of deNOx activity.

2.6. H2-TPR

It has been established that the oxidative dehydrogenation of the adsorbed ammonia species by vanadia species is a key step in the SCR of NO by NH3 [43,44,45,46]. According to this view, the reduction ability of catalysts was investigated, and the results of H2-TPR profiles are shown in Figure 6.
From Figure 6, all catalysts presented two main reduction peaks. The profile of the fresh catalyst showed two reduction peaks located at around 420 and 780 °C, respectively. The first peak was ascribed to the superimposed reduction of V5+ → V3+, W6+ → W4+,and sulfates to SO2, and the second peak was attributed to the reduction of W4+ → W0 [47]. Compared with the fresh catalyst, the reduction peak of V species on the waste catalyst shifted apparently to higher temperature (470 °C), implying the decrease in reduction ability. The main cause was the deposition of alkali metals and alkaline earth metals, which hindered the reducibility of the SCR catalysts and shifted the reduction peak of V species to a higher temperature [48]. Based on this analysis, the decrease of the reducibility should be another reason for the decrease of the activity of the waste catalyst, especially the low-temperature activity.
After OA leaching, the maximum reduction temperature of vanadium decreased to 458 °C, revealing that most of the impurities were removed and that the reducibility of the resynthesized catalysts were partially promoted.

3. Experimental

3.1. Raw Materials and Resynthesis Protocols

The fresh (named Fresh) and waste (named Waste) SCR catalysts used in this work were commercial honeycomb monolith catalysts obtained from Jiangsu Longyuan Catalyst Co., Ltd., Wuxi, China. The catalyst had been used for about 35,000 h with a collapsed honeycomb structure and a dark yellowish gray color.
The waste catalyst was ground and sieved to particles (<100 mesh) and leached by 1.5 mol/L OA (H2C2O4·2H2O, Beijing Chemical Works, Beijing, China, 99.5%). Then, the leaching residue was co-impregnated with an aqueous solution of ammonium metavanadate (NH4VO3, Xilong Chemical Co., Ltd., Shantou, China, 99.9%) dissolved in OA. The slurry was standing at 80 °C overnight, dried at 105°C for 6 h, and subsequently calcined at 600 °C in air for 3 h. The catalysts obtained with a nominal V2O5 mass loading amount of 0.5%, 1.0%, and 1.5% were denoted by Re-0.5%, Re-1.0%, and Re-1.5%, respectively.

3.2. Catalytic Activity Test

The catalytic deNOx activity over all prepared catalysts was evaluated in a fixed quartz reactor (i.d. = 20 mm) operating at atmospheric pressure. The prepared catalysts (50–100 mesh) were loaded into the reactor between quartz wool. A type-K thermocouple was inserted into the catalyst bed to monitor the reaction temperature. The reaction was performed at several temperature levels in the range of 150–500 °C with an interval of 50 °C. The simulated flue gas consisted of 1000 ppm NO, 1000 ppm NH3, 200 ppm SO2 (when used), 10 vol % H2O (when used), and 5 vol % O2, and the balance gas used was N2. This reaction stream was fed into the reactor through mass flow controllers with a total flow rate of 1600 mL/min at a GHSV of 27,430 h−1. At each temperature measurement point, all the catalysts were kept on stream for 1 h. Then NO conversion was calculated according to the following equation:
η NO = C NO , in C NO ,   out C NO , in × 100 %
where η NO is the NO conversion, C NO , in and C NO ,   out are the NO concentrations in the inlet and outlet gas, as measured by a flue gas analyzer (Lancom 4, Ametek, Bowen, PA, USA), respectively.

3.3. Catalyst Characterization

The chemical compositions of catalysts were determined by inductively coupled plasma atomic emission spectrometer (ICP-AES, Thermo Scientific, Waltham, MA, USA) according to the standard of YS/T 514.9-2009. Each sample was repeated for three times. The XRD patterns of catalysts were obtained by X-ray Diffraction (Empyrean, PANalytical, Almelo, The Netherlands,) by using Cu-Kα radiation (λ = 1.54056 Å, 40 mA and 40 kV) at the scanning range of 5–90 °C. The specific surface area and pore properties of catalysts were determined by N2 physisorption at −196 °C on an absorption unit (Autosorb-1, Quantachrome, Boynton Beach, FL, USA). Prior to analysis, approximately 0.1–0.2 g samples were evacuated under N2 atmosphere for 3 h at 300 °C.
FT-IR spectra were recorded in a TENSOR 27 FT-IR apparatus (Bruker, Karlsruhe, Germany). The valence states of elements and surface atomic concentrations in the catalysts were examined by XPS, which was conducted under an ultrahigh vacuum using an ESCALAB 250Xi spectrometer (Thermo Fisher Science, Waltham, MA, USA). Al-Kα X-rays were used as excitation light source, with a power of 150 W (15 kV, 10 mA), and the binding energy was corrected with the C 1s peak (284.8 eV).
H2-TPR was conducted on a chemisorption analyzer (AutoChem II 2920, Micromeritics, Norcross, GA, USA). Prior to analysis, the sample (about 100 mg) was purged in He (50 mL/min) at 300 °C for 60 min, and then cooled to 50 °C. Then the sample was heated from 50 to 900 °C at a rate of 10 °C/min in a mixture of 10% H2/Ar. The consumption of H2 was then detected by a thermal conductivity detector (TCD).

4. Conclusions

In this work, the V2O5-WO3/TiO2 catalysts were synthesized from the waste SCR catalyst after oxalic acid leaching and impregnating with various V2O5 mass loadings. The result showed that the deNOx activity of the newly synthesized catalyst with 1.0% V2O5 virtually recovered to the level of fresh catalyst, with NO conversion being recovered to 91% at 300 °C, and it also showed a good SO2/H2O resistance. The VOx coverage for the newly synthesized 1.0% V2O5/WO3/TiO2 catalyst was optimized to 29% after washing with oxalic acid and impregnation with vanadium, generating more active sites for the SCR reaction. The results of XPS and H2-TPR analysis demonstrated that the newly synthesized catalyst possessed a higher ratio of V4+ to V5+ and a lower reduction temperature of the dispersed V species, compared to the waste catalyst. These results appeared to be beneficial to the recovery of low-temperature deNOx activity of the resynthesized catalyst.

Supplementary Materials

The following are available online at www.mdpi.com/2073-4344/7/4/110/s1, Figure S1: NO conversion of the fresh, waste catalyst, and waste catalyst loaded with 0.2% V2O5.

Acknowledgments

The work was financially supported by the National Natural Science Foundation of China (No. 51574214).

Author Contributions

The experimental work was conceived and designed by Weijun Bao and Huiquan Li; Chunping Qi performed the experiments, analyzed the data and drafted the paper. The manuscript was amended through the comments of Liguo Wang, Weijun Bao and Wenfen Wu. All authors have given approval for the final version of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. X-ray diffraction (XRD) patterns of the (a) Fresh, (b) Waste, and (c) OA leaching residue.
Figure 1. X-ray diffraction (XRD) patterns of the (a) Fresh, (b) Waste, and (c) OA leaching residue.
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Figure 2. NO conversion of the fresh, waste and resynthesized V2O5-WO3/TiO2 catalysts prepared from OA leaching residue with different V2O5 loadings (Feed: NO = 1000 ppm, NH3/NO = 1.0, O2 = 5 vol %. Conditions: total flow rate = 1600 mL/min, gas hourly space velocity (GHSV) = 27,430 h−1).
Figure 2. NO conversion of the fresh, waste and resynthesized V2O5-WO3/TiO2 catalysts prepared from OA leaching residue with different V2O5 loadings (Feed: NO = 1000 ppm, NH3/NO = 1.0, O2 = 5 vol %. Conditions: total flow rate = 1600 mL/min, gas hourly space velocity (GHSV) = 27,430 h−1).
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Figure 3. Effects of SO2 and H2O on NO conversion over fresh, waste and resynthesized 1.0% V2O5-WO3/TiO2 catalysts (a) Combined effects of SO2 and H2O at 350, 300 and 250 °C; (b) Individual and combined effects of SO2 and H2O at 300 °C (Feed: NO = 1000 ppm, NH3/NO = 1.0, O2 = 5 vol %, H2O = 10 vol %, SO2 = 200 ppm. Conditions: total flow rate = 1600 mL/min, GHSV = 27,430 h−1).
Figure 3. Effects of SO2 and H2O on NO conversion over fresh, waste and resynthesized 1.0% V2O5-WO3/TiO2 catalysts (a) Combined effects of SO2 and H2O at 350, 300 and 250 °C; (b) Individual and combined effects of SO2 and H2O at 300 °C (Feed: NO = 1000 ppm, NH3/NO = 1.0, O2 = 5 vol %, H2O = 10 vol %, SO2 = 200 ppm. Conditions: total flow rate = 1600 mL/min, GHSV = 27,430 h−1).
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Figure 4. FT-IR spectra for the (a) Fresh (b) Waste (c) Re-0.5% (d) Re-1.0% and (e) Re-1.5% samples after subtraction of TiO2 support (KBr pressed disk exposed to the laboratory atmosphere).
Figure 4. FT-IR spectra for the (a) Fresh (b) Waste (c) Re-0.5% (d) Re-1.0% and (e) Re-1.5% samples after subtraction of TiO2 support (KBr pressed disk exposed to the laboratory atmosphere).
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Figure 5. V 2p XPS spectra of the (a) Fresh (b) Waste (c) Re-0.5% (d) Re-1.0% (e) Re-1.5% samples.
Figure 5. V 2p XPS spectra of the (a) Fresh (b) Waste (c) Re-0.5% (d) Re-1.0% (e) Re-1.5% samples.
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Figure 6. H2-TPR results of the (a) Fresh (b) Waste (c) Re-0.5% (d) Re-1.0% (e) Re-1.5% samples.
Figure 6. H2-TPR results of the (a) Fresh (b) Waste (c) Re-0.5% (d) Re-1.0% (e) Re-1.5% samples.
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Table 1. Main compositions of the fresh, waste V2O5-WO3/TiO2 catalysts and oxalic acid (OA) leaching residue as analyzed by an inductively coupled plasma atomic emission spectrometer (ICP-AES).
Table 1. Main compositions of the fresh, waste V2O5-WO3/TiO2 catalysts and oxalic acid (OA) leaching residue as analyzed by an inductively coupled plasma atomic emission spectrometer (ICP-AES).
SampleComposition (wt %)
TiVWAlSiCaFeKNa
Fresh55.040.393.480.440.870.840.040.020.07
Waste51.880.314.570.920.920.900.240.240.13
OA leaching residue52.420.123.940.521.020.750.070.040.03
Table 2. V2O5 content, specific surface area, and VOx coverage of the fresh and waste catalysts, OA leaching residue, and resynthesized V2O5-WO3/TiO2 catalysts.
Table 2. V2O5 content, specific surface area, and VOx coverage of the fresh and waste catalysts, OA leaching residue, and resynthesized V2O5-WO3/TiO2 catalysts.
CatalystV2O5 Content a (wt %)SBET (m2/g)VOx Coverage
mmol V/100 m2 TiO2 b% c
Fresh0.7165.60.13921
Waste0.5366.30.10416
OA leaching residue0.1276.90.0173
Re-0.5%0.5751.50.12218
Re-1.0%0.9850.10.19329
Re-1.5%1.4543.60.37356
a Measured by ICP; b wt   % / M V 2 O 5 × 2 × 1000 S BET / 100 , where wt % is the V2O5 content, and M V 2 O 5 is the molecular weight of V2O5 [33]; c Calculated according to the study by Marberger et al. [34].
Table 3. Valence states of V on the fresh, waste, and resynthesized V2O5-WO3/TiO2 catalysts obtained from X-ray photoelectron spectroscopy (XPS) spectra.
Table 3. Valence states of V on the fresh, waste, and resynthesized V2O5-WO3/TiO2 catalysts obtained from X-ray photoelectron spectroscopy (XPS) spectra.
CatalystBinding Energy (eV)Surface Atomic Concentration (%)Surface Atomic Ratio
V3+V4+V5+V3+V4+V5+V/TiV4+/V5+
Fresh-515.7516.6-62.537.50.0461.67
Waste514.6515.8516.63.051.545.50.0361.13
Re-0.5%-515.7516.6-46.953.10.0370.88
Re-1.0%-515.9516.9-60.839.20.0471.55
Re-1.5%-516.1517.1-69.130.90.0762.24

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