High-performance FeaTibOx catalyst loaded on ceramic filter for NOx reduction

The FeaTibOx catalyst was loaded into the pores of porous ceramic using impregnation method for removing nitrogen oxides with selective catalytic reduction (SCR) technology. The catalytic performance over FeaTibOx catalyst was studied in detail. The Fe6Ti1Ox catalyst displayed more than 95% NOx conversion within the range of 280 °C–400 °C, and still maintained 97% NOx conversion when 300 ppm SO2 and 10 vol.% water vapor were introduced at the testing temperature of 300 °C. The influence factors (including filtration velocity, NH3/NO mole ratio and oxygen concentration) for NOx conversion over Fe6Ti1Ox catalyst were also studied. The suitable NH3/NO mole ratio and oxygen concentration was 0.9 and 6%, respectively. The XRD, N2-BET, H2-TPR, NH3-TPD and XPS were employed to investigate physicochemical property of the FeaTibOx catalyst. The results illustrated that Fe6Ti1Ox catalyst has hematite Fe2O3 and TiO2 structure, better redox properties and stronger surface acid.


Introduction
Industrial flue gas emission will result in seriously air pollution matters, such as photochemical smog, greenhouse effects, haze and so on [1,2]. Particulates and nitrogen oxides usually co-exist in the flue gas. Porous ceramic filter usually applied for particulates removal in hot filtration [3,4]. However, porous ceramic filter has no way in getting rid of nitrogen oxides. At present, NH 3 -SCR technology was an efficient and economical way to diminish NO x emissions [5,6]. De-NO x catalyst is very important for SCR technology. In order to removing particulates and NO x simultaneously, many researchers focus on combining de-NO x catalysts with porous ceramic filter.
Heidenreich et al [7] developed catalytic ceramic filter by integrating TiO 2 -V 2 O 5 -WO 3 catalyst and ceramic filter. The effect of filtration velocity and operating temperature on catalytic performance were studied. Results showed that catalytic ceramic filter can achieve 98% NO conversion under the condition of 300°C and 2 cm s −1 filtration velocity. Choi et al [8] used rotational coating method to deposit V 2 O 5 -WO 3 /TiO 2 catalyst on SiC porous ceramic. The experiment of V 2 O 5 content and ball milling on NO conversion was carried out. The research result revealed that ball milling can reduce catalyst particle size, so that catalyst could go into pores of SiC ceramic filter easily. The best V 2 O 5 content was 3% (M3V3), because M3V3 exhibited remarkable catalytic activity in reaction temperature range of 280°C-320°C. Chen et al [9] prepared mullite ceramic filter with loaded MnO x -CeO 2 catalyst. The effect of Mn/Ce ratio and deposited MnO x -CeO 2 content upon NO conversion was studied. The suitable Mn/Ce ratio and catalyst loading was 6/4 and 4%, respectively. Catalytic filter can achieve over 90% NO conversion within reactive temperature of 120°C-250°C.
Iron-based de-NO x catalyst not only have good catalytic performance at medium-high temperature range, but also inexpensive and environmentally friendly [10,11]. Titanium dioxide is extensively employed as a de-NO x catalyst supports in the SCR technology. As a result, most of researcher work based on Fe 2 O 3 or TiO 2 , like Fe-Ti [12], Fe-Ce-Ti [13], Fe-Nb-Ce [14], Fe-W-Zr [15], Fe-Ni-Ti [16], and Fe-W [17] composite oxides. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
In this work, the Fe a Ti b O x catalyst was loaded on ceramic filter by simple impregnation method. Porous ceramic filter has been developed in our previous work [18]. The de-NO x performance over Fe a Ti b O x catalyst was studied in detail. And various characterizations associated with the physicochemical properties, reduction behavior and ammonia adsorption ability for catalyst were performed.

Experimental
2.1. Preparation of porous ceramic filter with Fe a Ti b O x catalyst Firstly, we prepared cylindrical porous ceramic filter. The ceramic short fibres, carboxymethyl cellulose, deionized water and glass powder were mixed together with stirring. After aging and pugging, the mixture was pressed into cylindrical green body (diameter=22 mm, height=10 mm) followed by sintering at 1150°C for 1 h. The detailed preparation procedure of porous ceramic filter was described in our previous work [18]. Secondly, catalyst precursor solution was prepared by mixing TiOSO 4 ·xH 2 SO 4 ·xH 2 O (93%., Aladdin Reagent Co., Ltd), Fe(NO 3 ) 3 ·9H 2 O (AR., Sinopharm Chemical Reagent Co., Ltd) in deionized water. Thirdly, the prepared ceramic filter was impregnated in the catalyst precursor solution for 10 min. The ceramic filter with loaded Fe a Ti b O x catalyst was dried at 95°C for 10 h and then sintered at 400°C for 5 h. The preparation procedure was schematically shown in figure 1.

Measurement of catalytic activity
The catalytic performance of ceramic filter with loaded Fe a Ti b O x catalyst was measured in the fixed-bed reactor (schematic of activity test system was shown in figure 2). The fixed-bed reactor was consisted of two quartz tubes (diameter=22 mm and 38 mm). The feed stream was made up of 600 ppm NO x , 600 ppm NH 3 , 6 vol.% O 2 , 10 vol.% H 2 O (when used), 300 ppm SO 2 (when used), and balance N 2 . The mixed gas flow rate is 314 ml min −1 in total. The filtration velocity was 1 m min −1 (equal to GHSV of ∼50,000 h −1 ). Cylindrical porous ceramic filter (diameter=22 mm, height=10 mm, mass=3.6 g) with catalyst loading of 6% was put in the testing tube of diameter=22 mm. The concentration of nitrogen oxides at the inlet and outlet of the reactor was detected by the gas analyzer (ECOM-D, RBR, Germany). Data was recorded after the reaction was stabilized. The NO x conversion was determined by following formula:

Characterizations
The crystal structure of catalyst was identified via x-ray diffraction (XRD) analysis (D Max/RB) with Cu Kα radiation. The scan speed was 10°min −1 and the 2θ scans covered 10∼80°. The morphology of the ceramic filter with catalyst was observed by scanning electron microscope (SEM, JSM-6510). The BET surface area and pore size distribution were tested by N 2 adsorption/desorption isotherms via surface-area analyzer (Micromeritics, 2020 M V3.00H). The TPD profiles of NH 3 (NH 3 -TPD) was obtained on an automated chemisorption analyzer (Quantachrome Instruments, CHEMBET-3000). Before the measurement, the catalyst was treated at 400°C for 1 h with He gas, then cooled to 50°C. The measure temperature was raised to 500°C at 10°C min −1 . The temperature programmed reduction of H 2 (H 2 -TPR) test was executed on a Semiautomatic Micromeritics TPD/TPR 2900 instrument. Catalyst samples were preheated at 400°C for 1 h in the pure Ar gas. Afterwords, the temperature was lowered to 50°C and introducing 5% H 2 /Ar. Finally, the test temperature was risen to 700°C at 10°C min −1 . The x-ray photoelectron spectroscopy (XPS) was performed by the PHI 5600 spectrometer with a Mg-Kα radiation source. The degree of vacuum in the XPS equipment was 10 −7 Pa. The catalyst was dried at 80°C for 24 h to eliminate moisture.   [12].
The NO x conversion of catalytic ceramic filter with Fe 6 Ti 1 O x catalyst was comparable to that of Fe x TiO y catalyst ( [12]).
In actual application of catalytic ceramic filter, different filtration velocity will change reaction time for SCR process. The influence of different filtration velocity on catalytic performance was showed in figure 3(b). When the filtration velocity was 0.5 m min −1 (equal to GHSV of ∼25,000 h −1 ), the catalytic performance was best within temperature of 240°C-400°C. Fe 6 Ti 1 O x catalyst achieved more than 95% NO x conversion in the temperature range of 260°C-400°C. With the increasing of filtration velocity, NO x conversion was decreased apparently at lower temperature range of 240°C-300°C. At high temperature range (320°C-400°C), this phenomenon is not obvious. The experiment results suggested that NO x conversion in the temperature range of 240°C-300°C was sensitive for higher filtration velocity.
It is generally acknowledged that ammonia acts a pivotal part in the catalytic reaction process. Figure 3(c) presented the influence of NH 3 /NO mole ratio on NO x conversion at the testing temperature of 300°C. When NH 3 /NO mole ratio was 0.6, the NO x conversion was 62%. As the increase of NH 3 /NO mole ratio, the NO x conversion improved immediately. When NH 3 /NO mole ratio achieved 0.9, the NO x conversion reached 99%. According to 4NH 3 +4NO+O 2 →4N 2 +6H 2 O, oxygen concentrations will affect the NO x conversion of ceramic filter with Fe 6 Ti 1 O x catalyst. Figure 3(d) illustrated the influence of oxygen concentrations on the catalytic performance at the testing temperature of 300°C. The Fe 6 Ti 1 O x catalyst owned 75% NO x conversion with 0.5% O 2 concentration. As the elevating of O 2 concentration, the NO x conversion exhibited monotonic increase and then becoming steady. When oxygen concentrations achieved 6.0%, the NO x conversion reached maximum value (98%).

Influence of SO 2 and water vapor on the catalytic performance
As the industrial flue gas always have water vapor and SO 2 , water vapor and SO 2 in flue gas will influence the catalytic performance usually [19,20]. The figure 4 displayed the NO x conversion over Fe 6 Ti 1 O x catalyst in the presence of SO 2 and water vapor at the testing temperature of 260°C, 280°C and 300°C. Before the introduction of water vapor and SO 2 , the SCR process has been maintained for 0.5 h at testing temperature. When SO 2 and water vapor was injected at the testing temperature of 300°C, the NO x conversion maintained stable (97%). After removing SO 2 and water vapor, the catalytic activity regained to the first value (about 98%). At measuring temperature of 280°C and 260°C, the NO x conversion went down evidently after breathe into SO 2 and water vapor with same condition. The NO x removal efficiency maintained 85% and 65% at the testing temperature of 280°C and 260°C, respectively. When water vapor and SO 2 were removed, catalytic activity recovered to initial value. The decline of catalytic activity may be related with the formation of (NH 4 ) 2 SO 4 or NH 4 HSO 4 , these metal sulfates deposited on catalyst surface readily and prevented the SCR reaction [21][22][23]. In a word, the Fe 6 Ti 1 O x catalyst has outstanding catalytic performance at the testing temperature of 300°C even though SO 2 and water vapor were existed.   Figure 5(a) Figure 5(b) showed the cross section of ceramic filter with Fe 6 Ti 1 O x catalyst. It can be seen that the ceramic fibers interconnected each other to construct the inner pores of ceramic filter and the Fe 6 Ti 1 O x catalyst was coated on the ceramic fibers or the inner pore surface of ceramic filter.

Physicochemical properties
The N 2 adsorption-desorption isotherms over the Fe 2 Ti 1 O x , Fe 6 Ti 1 O x and Fe 10 Ti 1 O x catalysts were demonstrated in figure 6(a). With the increasing of Fe content, the Fe 2 Ti 1 O x , Fe 6 Ti 1 O x and Fe 10 Ti 1 O x catalysts showed type-IV, type-III and type-II pattern isotherms, respectively. In the region of P/P 0 >0.4, the Fe 2 Ti 1 O x catalyst displayed H2 type hysteresis loop, which implying the existence of an ink-bottle-shaped pore. Both Fe 6 Ti 1 O x and Fe 10 Ti 1 O x catalysts showed H3 type hysteresis loop, it means the formation of slit-shape pore structure. The pore size distribution of three catalyst samples was showed in figure 6(b). Table 2    volume and pore diameter compared with Fe 2 Ti 1 O x catalyst. For Fe 6 Ti 1 O x catalyst, slit-shape pore structure, larger pore volume and pore diameter was beneficial to the diffusion of SCR reactive gas.

Redox behavior
The x-ray photoelectron spectra were conducted to investigate the atomic concentrations and chemical states for Fe a Ti b O x catalysts. Figure 7 presented the XPS results of Fe 2p, Fe 2p 3/2 , O 1s and Ti 2p. The content of each element and iron with different valence was determined and presented in table 3. In figure 7(a), the peaks of Fe 2p 3/2 and Fe 2p 1/2 were detected for all catalyst samples. The appeared peaks of Fe 2p 3/2 and Fe 2p 1/2 indicated that Fe 3+ was the major valence state for three catalysts [24,25]. From figure 7(b), the Fe 2p 3/2 spectrum was fitted with two peaks, the peak turned up at higher binding energies can be ascribed to Fe 3+ , the lower one could be owned to Fe 2+ [26,27]. The content of Fe 3+ /(Fe 2+ +Fe 3+ ) on the surface of Fe a Ti b O x catalyst was listed in table 3. For the most part, Fe 3+ possessed stronger oxidative ability than Fe 2+ , which was contributed to the oxidation process of NO to NO 2 . And it could promote the 'fast SCR' process: [28][29][30]. The higher content of Fe 3+ /(Fe 2+ +Fe 3+ ) provide a guarantee for better redox properties in the selective catalytic reduction reaction.
The O 1s spectra could be deconvoluted into two peaks. In figure 7(c), the peak emerged at higher binding energies was surface adsorbed oxygen species (marked as O α ), in the shape of O 2− or O − with regard to defectoxide or hydroxyl-like group; another peak arise at lower binding energies was allocated to the lattice oxygen O 2− (marked as O β ) [31,32]. In figure 7(d), intensity of Ti 2p 3/2 and Ti 2p 1/2 peak decreased from Fe 2 Ti 1 O x to Fe 10 Ti 1 O x catalyst. Fe 10 Ti 1 O x catalyst only showed Ti 2p 3/2 peak. Combining with XRD pattern, the missing of Ti 2p 1/2 peak for Fe 10 Ti 1 O x catalyst may ascribe to higher Fe amount.
With regard to the NH 3 -SCR process, the reduction ability was one of the most significant factors for the catalyst. The H 2 -TPR profiles of Fe a Ti b O x catalyst was illustrated in figure 8(a) figure 8(b). Well known to us all, the position of the NH 3 desorption peak is correlated with the strength of acid sites [35]. It is evident that the temperature of the NH 3 desorption peak for Fe 6 Ti 1 O x catalyst was higher than that of other two catalyst. It means that Fe 6 Ti 1 O x catalyst has stronger surface acid than the other two catalyst; the NH 3 desorbed at higher temperature may be associated with the chemisorbed ammonia [36]. Wherefore, the higher SCR activity for Fe 6 Ti 1 O x catalyst may be assigned to its stronger surface acid.

Conclusions
In this study, high-performance Fe a Ti b O x catalyst was prepared for selective catalytic reduction of NO x with NH 3 . The Fe 6 Ti 1 O x catalyst enjoyed outstanding catalytic activity, more than 95% NO x removal efficiency was obtained at measuring temperature from 280 to 400°C. When the filtration velocity was 0.5 m min −1 , Fe 6 Ti 1 O x catalyst obtained exceed 95% NO x conversion in the temperature range of 260°C-400°C. The suitable O 2 concentration and NH 3 /NO mole ratio should maintain about 0.9 and 6%, respectively. Furthermore, the Fe 6 Ti 1 O x catalyst has attractive catalytic activity when SO 2 and water vapor were existed at the testing temperature of 300°C. The NO x conversion maintained 97% when 300 ppm SO 2 and 10 vol.% water vapor were introduced. A series of characterization result showed that Fe 6 Ti 1 O x catalyst presented a higher catalytic activity depending on hematite Fe 2 O 3 and TiO 2 structure, excellent redox properties and strong surface acid.