Nitrogen oxides are major contributors to worsening environment problems such as acid rain, photochemical smog, and the greenhouse effect [4]. Selective catalytic reduction (SCR) of NO with NH3 is the most widely used technique for the abatement of NO emissions. Currently, commercial V2O5(WO3, MO3)/TiO2 catalysts are mainly used [5, 6]. However, the operating temperature window for V2O5/TiO2 catalysts is narrow. V2O5-based catalysts suffer from low N2 selectivity and sublimation of V2O5 at high temperatures, even after modification with Mo and W species [7]. Increasing attention is therefore being paid to the development of novel environmentally friendly deNOx catalysts with high efficiencies and broad active temperature windows.
Because of its redox properties and high oxygen storage capacity, CeO2 has attracted much interest as a catalyst for a broad range of applications, e.g., fuel cells [8], photocatalysis [9], and oxygen permeation membranes [24, 25]. For NH3-SCR, the oxygen storage capacity of CeO2 can be increased by the introduction of other transition and non-transition metal ions, and many CeO2-based catalysts have been developed, e.g., Ce-Zr-Ox [26], CeO2/TiO2 [32], Ce-W-Ox/TiO2 [33], CeO2/Al2O3 [34], CeO2/TiO2-SiO2 [35, 36], Ce-Zr-Ox/TiO2 [2], Mn-Ce/TiO2 [37], Cu-Ce/TiO2 [2], Ce-Mo/TiO2 [18, 41] and Mn-Fe-Ce/Al2O3 [42, 43]. These catalysts have broad temperature windows for effective denitration activity.
There have been various reports of the promotional effects of Er addition [44, 45]. However, the promotional effects of Er incorporation on NH3-SCR of NO over Ce-based metal oxide catalysts have not been investigated. Er2O3 can interact with CeO2, and the defect concentration increases with increasing Er incorporation; Er2O3 addition therefore improves the NH3-SCR activities of catalysts. In this work, the promotional effects of Er incorporation in CeO2(ZrO2)/TiO2 (CZT) as a catalyst for NH3-SCR were investigated. CZT catalysts and CZT catalysts with added Er (ECZTs) were prepared using a sol-gel method. The catalysts had good redox properties, high defect concentrations, and large oxygen storage capacities, and therefore had high SCR activities.
The catalysts were prepared by thermal decomposition of aged Ce-Zr/Ti and Er-Ce-Zr/Ti composite gels calcined at 500 ℃ for 2 h. A Ce-Zr/Ti composite sol, with Ti:Zr:Ce molar ratios of 4:1:1, was synthesized using Ti(OC4H9)4 (CP, LingFeng, Shanghai, China), ZrOCl2·8H2O (AR, Sinopharm, Beijing, China), and Ce(NO3)3·6H2O (AR, Ruibo, Zibo, China) as precursors. Ti(OC4H9)4 and ethanol were mixed under vigorous stirring at room temperature to form a composite solution A, and composite solution B was synthesized by mixing glacial acetic acid, deionized water, ethanol, and nitric acid. Solution B was added dropwise to solution A, and then the required amounts of ZrOCl2·8H2O and Ce(NO3)3·6H2O were dissolved in the AB mixed solution under vigorous stirring for 1 h to obtain the CZT composite sol. Er(NO3)3·5H2O (GR, Xiya, Chengdu, China) was added to the CZT mixed solution to obtain the ECZT composite sol. Aged gels were obtained by aging the sols, and drying in air at 80 ℃ for 24 h and 110 ℃ for 12 h. The solid was calcined at 500 ℃ for 2 h.
The catalytic activities of the prepared catalysts in NH3-SCR of NO were investigated using a fixed-bed quartz reactor (6 mm inner diameter), 0.7 mL of catalyst (particle size 0.3-0.45 mm), and a gas flow rate of 833 mL/min, corresponding to a gas hourly space velocity (GHSV) of 71 400 h-1. The reactant gas typically consisted of 600 ppm NO, 600 ppm NH3, 6 vol% O2, and balance N2. The NO concentrations at the inlet and outlet of the reactor were monitored online using a flue gas analyzer (MRU VarioPlus, Germany). The catalytic activity (XNO) in NH3-SCR of NO is expressed by equation (1). Analysis was performed at selected temperatures after 30 min, when the reactor temperature had stabilized.
The temperature range in which the catalytic activity is equal to or more than 90% of the maximum catalytic activity is defined as the catalytically active temperature window, denoted by Tr. The lowest Tr is denoted by Tr-L, and the highest by Tr-H [46].
The N2 concentration at the reactor outlet was monitored online by gas chromatography (GC2014, Japan), using a 5 Å molecular sieve column (length 2 m, sorbent particle size 60-80 mesh); a single-point corrected external standard method was used. We selected two representative samples (CZT and ECZT-0.10) and collected data at various temperatures. The detecting conditions were as follows: injection volume 1 mL, injector temperature 90 ℃, chromatographic column temperature 80 ℃, thermal conductivity detector operated at 120 ℃, carrier gas Ar, and H2 flow rate 30 mL/min. We obtained the peak area for a known standard, i.e., N2/Ar (600 ppm, and balance Ar), and defined this area as S0. We then determined the peak area for N2 detected at the outlet and defined this area as S. Based on the reaction
the consumed NOx is converted to N2, i.e., [NOx]N2consu = [N2] = 600 S/S0 ppm, and the total NOx conversion is [NOx]conv = [NOx]inlet - [NOx]outlet. To facilitate quantitative analysis and determine the N2 selectivity of the catalysts in NH3-SCR of NOx, the N2 selectivity is expressed as [1]:
Powder X-ray diffraction (XRD) patterns were obtained using a D/MAX-RB X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation. The 2θ scans covered the range 10°-80°, and the accelerating voltage and applied current were 40 kV and 40 mA, respectively. Electron paramagnetic resonance (EPR) spectroscopy was performed at -163 ℃, using a Bruker EMX-10/12 spectrometer, at the X-band. Photoluminescence (PL) spectra were obtained at room temperature using a Labram-HR800 spectrophotometer (Jobin Yvon Co., France) with a He-Cd laser (λ = 325 nm) as the light source. X-ray photoelectron spectroscopy (XPS) was performed using an Axis Ultra DLD instrument, with monochromatic Al Kα radiation as the excitation source. After complete removal of moisture from the catalysts by drying at 100 ℃ for 24 h, the catalysts were analyzed without surface sputtering or etching so that the degree of vacuum in the XPS equipment was maintained at 1 x 10-7 Pa.
The numbers of acid sites in, and acid strengths of, the catalysts were evaluated using temperature-programmed NH3 desorption (NH3-TPD; CHEMBET-3000, Quantachrome). The samples were preheated to 450 ℃ in a He stream for 1 h, and then cooled to 100 ℃ for NH3 adsorption; NH3 was desorbed using He at a flow rate of 30 mL/min from 100 to 800 ℃ at a heating rate of 10 ℃/min. The NH3 desorption was monitored online using a Thermo ONIX ProLab mass spectrometer. Temperature-programmed H2 reduction (H2-TPR) was performed using a semiautomatic Micromeritics TPD/TPR 2900 apparatus. The samples were treated at 400 ℃ for 1 h in an Ar flow and then cooled to 50 ℃ before the H2-TPR experiments. Reduction profiles were obtained using 5% H2/Ar at a flow rate of 20 mL/min. The temperature was increased from 50 to 900 ℃ at a rate of 10 ℃/min. The specific surface areas and average pore diameters of the samples, determined using the Brunauer-Emmett-Teller (BET) method, were determined from the N2 adsorption/desorption isotherms at -196 ℃, obtained using a surface area analyzer (2020M V3.00H, Micromeritics). All the samples were degassed at 350 ℃ under vacuum for 3 h prior to the adsorption experiments.
Fig. 1 shows the NO conversion over catalysts calcined for 2 h at 450, 500, 550, and 600 ℃, denoted by CZT-450, CZT-500, CZT-550, and CZT-600, respectively. The catalytic activity decreased slightly with increasing calcination temperature from 450 to 550 ℃. The low-temperature (190-350 ℃) catalytic activity of the catalyst calcined at 600 ℃ was clearly lower than those of the other catalysts, but the catalytic activities were similar at higher temperatures. These results suggest that anatase transformation to rutile occurred at about 600 ℃, therefore the catalytic activity decreased. Precursor decomposition may be incomplete at a calcination temperature of 450 ℃, resulting in carbonation of CZT. Based on the test results, CZT and ECZT calcined at 500 ℃ were used; CZT-500 is referred to as CZT for brevity.
The catalytic activities of CZT and ECZT were investigated; the NO conversions as a function of temperature are shown in Fig. 2(a). The results show that CZT and ECZT were effective catalysts for NH3-SCR of NO, and Er incorporation strongly affected the catalytic activity. Fig. 2(a) shows that the maximum catalytic activity of CZT was 94.28% at 320 ℃, and the catalytically active temperature window was 230-390 ℃. However, addition of a small amount of Er (Er/Ce = 0.05:1) to CZT clearly increased the activity. This suggests that Er incorporation plays an important role in promotion of the catalytic activity. ECZT-0.10 (Er/Ce = 0.10:1) showed the best catalytic activity (98.85%) and the catalytically active temperature window was 220-395 ℃. The catalytic activity of ECZT decreased with further increases in the Er/Ce molar ratio, possibly because of a decrease in the number of active Ce sites on the catalyst surface.
Fig. 2(a) shows that all the catalytic activity versus temperature curves was parabolic: at temperatures lower than Tr-L, the quantity of activated molecules and the effective collision frequency on the catalyst surface increased with increasing temperature, which increased the reaction rate [11]. In addition, NH3 was oxidized at temperatures higher than Tr-H, which directly decreased the amount of reducing agent, therefore the catalytic activity in NOx reduction decreased with increasing temperature. Fig. 2(b) shows the N2 selectivity of the CZT and ECZT-0.10 catalysts in NH3-SCR of NO at various temperatures. The N2 selectivity decreased slightly on Er incorporation and the ECZT-0.10 catalyst showed high N2 selectivity at all test temperatures.
Fig. 3 shows the NO conversions over CZT and ECZT-0.10 under the reaction conditions 5 vol% H2O and/or 200 ppm SO2 at 320 ℃. The total catalytic activities of CZT and ECZT-0.10 were unchanged when water vapor was introduced separately during the tests, possibly because competitive adsorption among water vapor and NH3 or NO on the catalyst surface was weak. When 5 vol% H2O and 200 ppm SO2 were added, the NO conversions over CZT and ECZT-0.10 dropped by 18% and 13%, respectively, in 5 h. In the next 13 h, the NO conversions over CZT and ECZT-0.10 were almost unchanged. When SO2 and H2O additions were stopped, the NO conversions over CZT and ECZT-0.10 quickly recovered to 82% and 91%, respectively. The anti-sulfur ability was therefore enhanced slightly by Er addition, and ECZT-0.10 showed excellent resistance to catalyst poisoning caused by vapor and sulfur.
Fig. 4 shows the XRD patterns of the CZT and ECZT catalysts. All the patterns showed the reflections for anatase TiO2 (PDF 71-1168) and Ce0.5Zr0.5O2 (PDF 38-1436). In terms of the effect of the ionic radius, the ionic radii of Zr4+ and Ce4+ are 0.086 and 0.101 nm, respectively, therefore they satisfy equation (4). CeO2 could therefore react with ZrO2 and form a continuous solid solution [47, 48].
Fig. 4 shows that the intensities of the anatase TiO2 peaks decreased and the Ce0.5Zr0.5O2 peaks widened as a result of Er incorporation, i.e., crystallization of a CeO2-ZrO2 solid solution and TiO2 was inhibited by Er incorporation. In addition, no diffraction peaks attributable to Er species were detected in the XRD patterns, implying that the Er species were well dispersed in the ECZT samples or were present as amorphous species. The catalytic activities of the CZT and ECZT samples suggest that the catalytic activity can be increased by appropriate inhibition of TiO2 and CeO2-ZrO2 solid solution crystallization.
The BET surface areas, pore volumes, and pore sizes are listed in Table 1. The surface area of the CZT catalyst was 105.52 m2/g. The data in Table 1 show that the surface area decreased with increasing Er incorporation, possibly because of the small surface area of Er2O3 and because the small pores in CZT collapsed to form larger pores on Er incorporation. This could be one reason why the catalytic activities of ECZT-0.20 and ECZT-0.60 were lower than that of CZT at low temperatures.
Four samples, CZT, ECZT-0.10, ECZT-0.20 and ECZT-0.60, were examined using NH3-TPD. Fig. 5 shows the NH3-TPD profiles. All the profiles showed two distinct regions: one desorption peak centered at a low temperature, representing weak acid sites, and another centered at a higher temperature, representing strong acid sites. Two desorption peaks were observed in the NH3-TPD profiles of the CZT and ECZT samples.
Fig. 5 shows that the areas of the NH3 desorption peaks decreased in the order CZT ≈ ECZT-0.10 > ECZT-0.20 > ECZT-0.60 at low temperatures, implying that the number of weak acid sites decreased in this order. The areas of the NH3 desorption peaks at high temperatures clearly decreased on Er incorporation, indicating that the total amount of acid sites in CZT was higher than those in the ECZT catalysts. For ECZT-0.10, the low-temperature NH3 desorption peak shifted from 223 to 186 ℃, and the high-temperature peak shifted from 630 to 610 ℃. These observations indicate that the strengths of the weak and strong acid sites both decreased when a small amount of Er was added to the CZT catalyst. The low-temperature NH3 desorption peak shifted from 223 to 236 ℃ for ECZT-0.60; this could be one reason why the catalytic activity of ECZT-0.60 was lower than that of CZT. The catalytic activity therefore showed a parabolic trend on Er addition.
Fig. 6 shows the H2-TPR results for CZT and ECZT-0.10. For CZT, the broad peak at around 461 ℃ is assigned to reduction of surface oxygen in Ce4+-O-Ce4+ [49, 50]. The weak peak at around 611 ℃ is attributed to reduction of TiO2 and surface oxygen in Ce3+-O-Ce4+ [51]. The peak at around 848 ℃ corresponds to reduction of bulk CeO2, which only occurs above 750 ℃ [52]. However, ECZT-0.10 gave three clear reduction peaks, at about 480, 700 and 824 ℃, suggesting that Er incorporation adversely affects the redox properties. The integral areas of H2-TPR profiles are usually used to compare the oxygen storage capacities of catalysts, which is a significant parameter in the SCR activity of CeO2-based catalysts [53]. The total H2 consumption by ECZT-0.10 was higher than that by CZT, which indicates that the number of surface oxygen defects increased, i.e., the oxygen storage capacity of the catalyst increased on Er incorporation. This may be one reason why the catalytic activity of ECZT-0.10 was better than that of CZT.
PL emission spectra have been widely used to investigate surface defects [54]. When the defect concentration increases, more electrons are trapped and recombine with holes through nonradioactive paths. The PL intensity decreases with increasing number of surface defects [55]. Fig. 7 shows that the PL spectra of CZT and ECZT-0.10 are similar. Peak 1, at 558 nm, arises from surface defects, and peak 2, at 633 nm, results from the polarizability of the lattice ions surrounding the defects [56]. The PL peak intensity decreased on Er incorporation, i.e., Er incorporation increased the number of surface defects. These results suggest that an increase in the number of surface defects improves the catalytic activity.
Fig. 8 shows the EPR spectra of CZT and ECZT-0.10. Both catalysts gave a strong and broad peak at g = 2.006, which is attributed to oxygen vacancies [1]. The EPR intensity of ECZT-0.10 at g = 2.006 was higher than that of CZT, i.e., the oxygen vacancy concentration in the CZT catalyst increased on Er incorporation, and this provides active oxygen species for the NH3-SCR reaction, and improves the catalytic activity. The signal at g = 1.977 is attributed to Ti3+, and the EPR intensity increased on Er incorporation [2]. This suggests that Er incorporation increased the Ti3+ content. The presence of a large Ti3+ amount is beneficial in the SCR reaction because Ti3+ can create a charge balance, and form oxygen vacancies and unsaturated chemical bonds on the catalyst surface, leading to an increased amount of chemisorbed oxygen [3].
High-resolution Ce 3d, Ti 2p, Zr 3d, and O 1s XPS profiles of CZT and ECZT-0.01 were obtained to identify the states of surface species on the catalysts; the spectra are shown in Fig. 9. The spectra of CZT and ECZT-0.10 showed peaks at binding energy of 903 and 885 eV in Fig. 9(a), attributed to Ce4+ and Ce3+ [4]. The O 1s peaks Fig. 9(d) were fitted to a peak from chemisorbed oxygen (Oα) and one from lattice oxygen (Oβ) [5]. The Ti 2p Fig. 9(b) spectra had peaks attributable to Ti3+ and Ti4+ [6]. The Ce 3d, O 1s, Ti 2p, and Zr 3d core levels shifted to higher binding energies on Er incorporation, indicating that interactions among Ce, Ti, O, and Zr increased slightly.
Table 2 shows the surface atomic ratios for CZT and ECZT-0.10. The atomic ratios of Oα, Ce3+, and Ti3+ on the CZT catalyst surface were 0.22, 0.24, and 0.06, respectively, and the atomic ratios of Oα, Ce3+, and Ti3+ on the ECZT-0.10 catalyst surface were 0.42, 0.23, and 0.12 respectively. The atomic ratio of Ce3+ did not change much. The atomic ratios of Oα and Ti3+ increased on Er incorporation; this is in agreement with the EPR results. It has been reported that surface chemisorbed oxygen is the most active form of oxygen and plays an important role in oxidation [7]. The catalytic activity of ECZT-0.10 in oxidation of NO should therefore be higher than that of CZT, because an increase in the amount of surface chemisorbed oxygen has a positive effect on the catalytic activity.
CZT is a solid acid catalyst, and there is a strong correlation between catalytic activity and acid properties. As stated above, the strengths of the weak and strong acid sites decreased when Er was added to the CZT catalyst, and this is why Er incorporation widened the catalytically active temperature window. In addition, Ti3+ can create a charge balance, and form oxygen vacancies and unsaturated chemical bonds on the catalyst surface, and surface chemisorbed oxygen plays an important role in NO oxidation. Er incorporation increased the concentrations of chemisorbed oxygen and Ti3+, resulting in an excellent catalytic performance of ECZT-0.10. The oxygen storage capacity of the catalyst is also increased by Er incorporation, and this may be one of the reasons why the catalytic performance of ECZT-0.10 was better than that of CZT.
Er incorporation greatly improved the catalytic performance of CZT in NH3-SCR of NO. The ECZT-0.10 catalyst, with an Er/Ce molar ratio of 0.10, showed the highest catalytic activity and excellent resistance to catalyst poisoning by vapor and sulfur. The reasons for these results are as follows: (1) Er incorporation increased the oxygen storage capacity and decreases the acid strength; (2) the surface defect (oxygen vacancies and Ti3+ concentrations increased on Er incorporation; (3) Er incorporation inhibited growth of TiO2 and Ce0.5Zr0.5O2 crystal particles. All these features contribute to the excellent catalytic performance of ECZT.
2016,
Vol. 37 Issue (9): 1521-1529 
PDF
2016,
Vol. 37 Issue (9): 1521-1529 PDF