Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
pubs.acs.org/IECR
NOx Removal over V2O5/WO3−TiO2 Prepared by a Grinding Method: Influence of the Precursor on Vanadium Dispersion Lina Gan,†,‡ Jianjun Chen,‡ Yue Peng,*,‡ Jian Yu,*,† Tuyetsuong Tran,† Kezhi Li,‡ Dong Wang,‡ Guangwen Xu,† and Junhua Li‡ †
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China S Supporting Information *
ABSTRACT: V2O5/WO3−TiO2 (VWT) catalysts for selective catalytic reduction (SCR) of NOx removal were prepared by a mechanical grinding method using different vanadium precursors. The SCR performances were evaluated in simulated flue gas and explained through characterizations by X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, H2 temperature-programmed reduction, and in situ diffuse reflectance Fourier transform spectroscopy. VO(acac)2−VWT catalyst prepared using a vanadyl acetylacetonate (VO(acac)2) precursor exhibited the highest catalytic activity and excellent resistance to SO2 and H2O poisoning in 200−400 °C compared to the catalysts obtained using other precursors. The VO(acac)2 precursor could enrich V on the surface remarkably and promote the formation and dispersion of polymeric vanadia species. Furthermore, a relatively high percentage of low-valent vanadium atoms were found on the VO(acac)2−VWT surface, facilitating electron transfer between V4+ and V5+. Surfaceadsorbed NH3 species on VO(acac)2−VWT were much more reactive. The initial geometry of vanadium precursors determined the tendency of V to accumulate and further influenced the dispersion and final form of V species.
1. INTRODUCTION The control of the emission of nitrogen oxides is a huge challenge to protect air quality and human health. The gases participate in photochemical reactions inducing smog events, which have attracted more attention recently.1,2 In addition, nitrogen oxides contribute to global warming effect and acid rain.1,2 The selective catalytic reduction (SCR) of NOx with NH3 is a widely applied technology for the elimination of NOx from stationary sources.1,3−5 Commercial V2O5−WO3/TiO2 shows good catalytic activity because of its highly stable activity for NOx removal and low sensitivity to SO2 poisoning.6,7 Vanadium oxide is the active species in the catalyst recipes. The status of the vanadia species dispersed on the surface has been studied extensively.8−11 The monomeric vanadyl species is likely present on the catalyst surface at low vanadium loadings, while polymeric vanadium oxides and crystalline V2O5 can be formed on the surface at relatively high vanadium loadings. In addition, the activity of polymeric vanadia species is much higher than that of the monomeric vanadyl species in NH3-SCR de-NOx reaction, especially at low temperatures.10,11 The dispersion of vanadia is the key factor determining the catalyst performance. Typically, methods were used to prepare supported vanadium oxides, such as impregnation, precipitation, chemical vapor deposition, and liquid-phase grafting.12,13 For these preparation techniques, research efforts have been mostly focused on improving the active species dispersion and © XXXX American Chemical Society
strengthening the interaction between the active species and the support (chemical interaction of vanadium oxides with the TiO2 surface to generate a V−O−Ti bond). The possible precursors of vanadia sources used for the impregnation include ammonium metavanadate,14 vanadyl oxalate,15 and vanadium or vanadyl chlorides, while precursors for adsorption are vanadyl acetylacetonate16 and vanadyl tert-butylate.17 The physicochemical properties of vanadia species are directly related to the oxidation state of the vanadium precursor solution.14 Nonetheless, there have been few studies on the influence of vanadium precursors on the state and dispersion of the active VOx species and the performance of the V2O5/ WO3−TiO2 catalyst. Previous studies have systematically examined the preparation methods of V2O5/TiO2,12,18 V2O5/Al2O3,19,20 and V2O5− TiO2/SO42−21 by grinding V2O5 or VOSO4·xH2O and nano TiO2. V2O5/Al2O3 showed similar vanadyl complexes as the impregnated vanadia−alumina. In our previous study, we developed a V2O5/WO3−TiO2 catalyst using VO(acac)2 precursor by a solvothermal method.22 The catalyst showed better activity in the 220−300 °C temperature range and a Received: Revised: Accepted: Published: A
October 8, 2017 November 12, 2017 December 7, 2017 December 7, 2017 DOI: 10.1021/acs.iecr.7b04060 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 1. SCR of NO at different temperatures over different catalysts: (a) NO conversion and (b) N2O concentration in exit gas. (Reaction conditions: NO 500 ppm, NH3/NO = 0.8, O2 5 vol %, N2 as balance gas, GHSV 60 000 h−1.)
In addition, the reaction rate constant is another key parameter to value the catalytic activity; kinetic parameters were given as
higher tolerance for SO2 and H2O than the traditional catalyst. However, the preparation method of this catalyst is harmful to the environment because of the use of toluene as the solvent. Therefore, to eliminate the influence of the organic solvent and to develop an environmentally friendly synthesis method, we used a grinding method for the V2O5/WO3−TiO2 catalyst preparation. Four different vanadium precursors, namely, ammonium metavanadate (NH 4 VO 3 ), vanadyl sulfate (VOSO 4·xH2O), vanadyl oxalate (VOC2O4·xH2O), and vanadyl acetylacetonate (VO(acac)2), were selected. The catalysts were then carefully evaluated to elucidate the relationship between the surface morphology of the vanadium oxides and the performance of the catalyst.
k=−
in out C NO − C NO in C NO
(2)
where k, V, and W are reaction rate constant (mL/g·s), gas flow rate, and catalyst mass, respectively, and x is the NO conversion ratio. 2.3. Catalyst Characterization. The crystal structure of samples was characterized on a powder X-ray diffraction (XRD) instrument (Rigaku, D/max 2550/PC) by Cu Kα radiation (λ = 0.15418 nm). The Brunner−Emmet−Teller (BET) area and the properties of the pore were test at 77 K (Micromeritics, ASPS 2020). The surface atomic concentration on a catalyst was obtained with an Escalab 250Xi X-ray photoelectron spectroscopy (XPS) instrument using 100 W Al Kα radiation (hν = 1486.6 eV). The data were fitted by subtracting a nonlinear background. For each sample, a 100 mg sample was placed in a quartz U-tube reactor to conduct hydrogen temperature-programmed reduction (H2-TPR). The temperature was accelerated from 80 to 1000 °C with the rate of 10 °C/min after preheating for moisture removal, and the consumed H2 was continuously online monitored versus temperature using a process MS (Ametek, Proline Mass Spectrometer). The phase of vanadium oxides was examined using a Raman microscope (Raman, labRAM HR800) with the excitation wavelength of 514.5 nm and power of 25 mW. Elemental analysis was performed using X-ray fluorescence spectroscopy (XRF, PANalytical Axios). NH3 temperature-programmed desorption (NH3-TPD) experiments were conducted with the same reactor as that used in the activity tests. Each catalyst (100 mg) was pretreated at 350 °C for 60 min in N2 flow at 200 mL/min, saturated with 500 ppm NH3 until adsorption saturation at 100 °C, followed by N2 purging to remove the excess NH3 and physical absorption species. The TPD experiment was performed in the 100−700 °C range with a rate of 10 °C/min. The desorbed NH3 was continually tested by a Fourier transform infrared (FTIR) spectrometer (MKS, MultiGas 2030HS). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra were recorded using a Nicolet NEXUS 6700 spectrometer. The in situ DRIFTS test was performed under 500 ppm NH3, 500 ppm NO, and 5% O2.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. V2O5/WO3−TiO2 catalysts containing 2 wt % V2O5 were prepared by mixing and grinding WO3−TiO2 (WO3 5 wt %, Nippon Shokubai Co.) with ammonium metavanadate (NH4VO3), vanadyl sulfate (VOSO4· xH2O), vanadyl oxalate (VOC2O4·xH2O), and vanadyl acetylacetonate (VO(acac)2) in an agate mortar for 15 min. The obtained samples were first dried at 110 °C for 4 h and then calcinated at 500 °C for 4 h. Herein, the catalysts are denoted as X−VWT; for example, NH4VO3−VWT represents the V2O5/WO3−TiO2 catalyst prepared using NH4VO3 as the vanadium precursor. 2.2. Catalytic Activity. All the catalytic performances were tested in a fixed-bed quart reactor. During a typical experiment, 1.0 g of catalyst sample (50−100 meshes) was put in the reactor. A 1000 mL/min simulation flue gas balanced with N2 gas was injected to the system. The flow gas consisted of NO = 500 ppm, NH3 = 400 ppm, O2 = 5 vol %, SO2 = 350 ppm (selected), and H2O = 10 vol % (steam when applied). Each flow rate was monitored by mass flow meters. The gas hourly space velocity (GHSV) of this experiment was 60 000 h−1, with NH3/NO (molar ratio) = 0.8. The concentrations of the inlet and outlet gases were collected by an exhaust gas analyzer (ABB, AO2020). The removal efficiency of NO was evaluated as NO conversion =
V ln(1 − x) W
× 100% (1)
where CinNO and Cout NO are the NO concentration of the upstream inlet gas and that at the outlet of the reactor, respectively. B
DOI: 10.1021/acs.iecr.7b04060 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research Table 1. Surface Areas, Reaction Rates, and Surface Atomic Ratios from XPS Spectra and XRF Results XRF results (wt %)b
a
2
a
XPS results (%)c
catalyst
BET (m /g)
k (mL/g·s)
V2O5
WO3
SO42−
NH4VO3−VWT VOSO4−VWT VOC2O4−VWT VO(acac)2−VWT
74 73 74 74
1.76 1.03 9.37 18.48
1.98 1.94 2.00 2.02
5.45 5.56 5.68 5.61
2.30 2.29 1.32 1.20
V/Ti
V4+/V
0.78 0.97 4.82 5.15
− − 21.86 23.26
NO conversion at 200 °C. bMass percent from XRF. cSurface atomic ratio from XPS.
3. RESULTS AND DISCUSSION 3.1. Catalytic Performance for SCR of NO. The SCR catalytic activities of the NH4VO3−VWT, VOSO4−VWT, VOC2O4−VWT, and VO(acac)2−VWT catalysts in the 150− 450 °C temperature range are shown in Figure 1a. The NO conversion efficiency accelerated in the temperature range of 150−350 °C but declined above 400 °C. The VO(acac)2− VWT catalyst exhibited the best catalytic activity in the 150− 300 °C range. NO conversion achieves approximately 80% at 250−400 °C to reach the present NH3/NO ratio. The VOC2O4−VWT showed higher activity and a wider active temperature range than the NH4VO3−VWTi and VOSO4− VWTi catalysts. As shown in Figure 1b, minor N2O was detected below 300 °C, but the concentration of N2O increased up to 450 °C. Nonetheless, the amounts of formed N2O over VOC2O4−VWT and VO(acac)2−VWT were relatively lower than those of the other catalysts. Nearly no N2O was observed up to 450 °C on the VOC2O4−VWT catalyst. The results show that VO(acac)2−VWT and VOC2O4−VWT exhibited better catalytic activity with high N2 selectivity. The pseudo-first-order rate constants based on mass constants at 200 °C are listed in Table 1. The reaction rate constant of VO(acac)2−VWT is nearly 10 times higher than that of NH4VO3−VWT (1.76 to 18.48 mL/g·s). The results confirm that the catalysts prepared by different vanadium precursors showed remarkable differences in catalytic activity and that VO(acac)2−VWT displays excellent activity. The stability of the catalytic activity was tested at 220 °C for VO(acac)2−VWT in simulated flue gas containing 10 vol % steam and/or 350 ppm SO2 (Figure 2). When 10 vol % steam was introduced, the NO conversion initially decreased by approximately 5% and was then stable for 5 h. When 350 ppm SO2 was introduced alone, the NO conversion increased
slightly for 4 h. When we introduced both 10 vol % steam and 350 ppm SO2, the NO conversion was stable for 16 h; however, the NO conversion decreased approximately 6% relative to that without steam and SO2. When the steam and SO2 were shut down, NO conversion was restored to its original level. Therefore, the steam and SO2 suppression on de-NOx activity was due to the competitive adsorption between NH3 and H2O. The VO(acac)2−VWT catalyst exhibited stable resistance to poisoning by steam and SO2. 3.2. Structural and Textural Characteristics. The XRD patterns (Figure 3) showed that the main diffraction patterns of
Figure 3. (a and b) XRD patterns of the NH4VO3−VWT, VOSO4− VWT, VOC2O4−VWT, and VO(acac)2−VWT catalysts.
the catalysts can be attributed to the anatase phase TiO2 (JCPDS 21-1272). Both WOx and VOx were not seen on the VOC2O4−VWT and VO(acac)2−VWT catalysts, suggesting that the WO3 and V2O5 species were highly dispersed on the support as monolayer or polymer species on the surface,23 whereas some weak peaks of NH4VO3−VWT and VOSO4− VWT in Figure 3b are attributed to crystalline V2O5 (JCPDS 41-1426). The BET area and the properties of pores are summarized in Tables 1 and S1, respectively. The BET area was 75 m2/g for the WO3−TiO2 carrier, and no considerable changes in the surface area were found for the four catalysts (74 m2/g). The calcination process may also cause the growth of the WO3− TiO2 grain size (Table S1). These results suggest that the vanadium precursors do not change the structure and texture of the catalyst prepared by the grinding method. 3.3. Redox Properties (XPS, Raman, and H2-TPR). To study the chemical state of V and the atomic ratio on the surface layer, XPS spectra were recorded. The results of the bulk (XRF) and surface (XPS) analyses for all the catalyst samples are summarized in Table 1. The V2O5 contents were approximately 2 wt % for all samples, and the relative bulk atomic ratio of V/Ti was approximately 0.019. The SO42− contents of NH4VO3−VWT and VOSO4−VWT are higher than those of the other two catalysts. Compared to NH4VO3−
Figure 2. Activity stability resisting poisoning of SO2 and water steam at 220 °C for the VO(acac)2−VWT catalyst. (Reaction conditions: NO 500 ppm, NH3/NO = 0.8, O2 5 vol %, SO2 350 ppm, H2O 10 vol %, N2 as balance gas, GHSV 60 000 h−1.) C
DOI: 10.1021/acs.iecr.7b04060 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research VWT and VOSO4−VWT, both VOC2O4−VWT and VO(acac)2−VWT had remarkably more vanadia species on their surfaces. Both NH4VO3−VWT and VOSO4−VWT had less surface vanadia species than expected from the XRF analysis. Figure 4 shows the XPS spectrum for V 2p of catalysts consisting of V 2p1/2 and 2p3/2 peaks. To detect the vanadium
Figure 4. XPS spectra of V 2p for the NH4VO3−VWT, VOSO4− VWT, VOC2O4−VWT, and VO(acac)2−VWT catalysts.
oxidation states, the V 2p3/2 signal was further analyzed. The NH4VO3−VWT and VOSO4−VWT curves could not be fitted because of the low peak intensity. The binding energies at 515.7 and 516.6 eV can be assigned to V4+ 2p3/2 and V5+ 2p3/2, respectively.24 Therefore, VO(acac)2−VWT presented 23% V4+/V, indicating that the use of the VO(acac)2 precursor facilitated the formation of reduced VOx (V4+) over the WO3− TiO2 support. Figure 5 shows the results of the Raman spectroscopy investigation of the structures of the surface species dispersed on the TiO2 surface in all catalysts. As shown in Figure 5a, three major peaks at 395, 513, and 636 cm−1 were observed and can be assigned to the anatase TiO2 structure.25 As shown in Figure 5b, the broad band at 950−990 cm−1 belongs to the overlap of WO symmetrical stretching and VO symmetrical stretching.26 Because the signal intensity attributed to VO is four times greater than that assigned to WO on the TiO2 surface, the band attributed to WOx was overshadowed by the strong V species band.25,27 Hence, the shift of this band is assigned to VOx structural transformation. Therefore, all investigated catalysts reveal broad Raman bands at ∼978 cm−1 and form two-dimensional VOx. In addition to the band at 978 cm−1, the band at 793 cm−1 is attributed to the secondorder feature of anatase TiO2.28 Figure 5c shows the optical photographs of the NH4VO3−VWT and VO(acac)2−VWT catalysts. The optical photograph of VO(acac)2−VWT is uniform yellow. However, for NH4VO3−VWT, brown spots are observed in the optical photograph. The Raman spectra of the brown spot are shown in Figure 5a, and the peaks at 994, 701, 528, 408, 305, 286, and 144 cm−1 can be assigned to crystalline V2O5.28−30 All the results are consistent with the XRD profile. H2-TPR was conducted to study the reduction behavior of metal oxides on the surfaces of all catalysts. As shown in Figure 6, two bands roughly at 400−550 and 840 °C were found for all catalysts. The peak at low temperature belongs to the reduction of V5+ to V3+, W6+ to W4+, and SO42− to SO2, while the band at
Figure 5. (a and b) Raman spectra and (c) optical photographs of the (A) NH4VO3−VWT, (B) VOSO4−VWT, (C) VOC2O4−VWT, and (D) VO(acac)2−VWT catalysts. Scale bar = 200 μm.
Figure 6. H2-TPR profiles of the NH4VO3−VWT, VOSO4−VWT, VOC2O4−VWT, and VO(acac)2−VWT catalysts at 200−1000 °C.
840 °C belongs to the reduction of W4+ to W0.31−33 Because of the content of the sulfate species (Table 1), the H 2 consumptions of NH4VO3−VWT and VOSO4−VWT are greater than those of VO(acac)2−VWT and VOC2O4−VWT. For the tested catalysts, the low-temperature peak shifted considerably from 545 to 412 °C and followed the order VO(acac)2−VWT > VOC2O4−VWT > VOSO4−VWT > NH4VO3−VWT. Vanadium precursors exert a considerable D
DOI: 10.1021/acs.iecr.7b04060 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
1360 cm−1 were attributed to the coordination of NH3 to titanium and tungsten hydroxyl groups, VO and WO species, and the residual sulfate (SO) species.39 Similar to the NH4VO3−VWT catalyst, these intermediate species of adsorbed NH3 were also detected over the other three catalysts. The number of Brønsted acid sites (1425 cm−1) is remarkably increased over VO(acac)2−VWT. The amounts of Lewis acid sites (1240 cm−1) of the four catalysts show only slight differences. NH3 desorption at above 135 °C of the four catalysts are demonstrated by NH3-TPD profiles (Figure S2). The two larger NH3 desorption peaks at around 238 and 370 °C belong to weak and strong acid sites on NH4VO3−VWT catalyst, respectively. Compared to NH4VO3−VWT, the main NH3 desorption peak (280 °C) of VOSO4−VWT shifted to higher temperature, and the main desorption peaks was slightly shifted to lower temperature (°C) for VOC2O4−VWT and VO(acac)2−VWT (232 and 225 °C, respectively). The amount of total acid sites on VO(acac)2−VWT (2.68 μmol/m2) is higher than on the other catalysts, i.e., NH3 should first adsorb on acid sites and then participate in SCR reaction. Thus, the greater the NH3 adsorption, the better the SCR program.40 Figure 7b shows the integrated peak areas for Lewis and Brønsted acid sites from 100 to 300 °C (Figure S3). Both Lewis and Brønsted acidities decline with increasing temperature, and the acidities of VO(acac)2−VWT are always higher than those of the other catalysts, which is supported by the NH3-TPD results. 3.5. Reactivity of Surface-Adsorbed Species. To study the reactivity of adsorbed NH3 and NOx species, transient in situ DRIFTS measurements were carried out on the NH4VO3− VWT, VOSO4−VWT, VOC2O4−VWT, and VO(acac)2−VWT catalysts. Because the promotional effects on the catalytic activity of the different vanadium precursors are observed mainly at low temperatures (
