YANG Zhengyu, HUANG Bangfu*, ZHANG Guifang, DAI Meng,WEN Zhenjing, LI Wanjun
(1. Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China; 2. Clean Metallurgy Key Laboratory of Complex Iron Resources, University of Yunnan Province, Kunming 650093, China)
Abstract: To study the modification mechanism of activated carbon (AC) by Fe and the low-temperature NH3-selective catalytic reduction (SCR) denitration mechanism of Fe/AC catalysts, Fe/AC catalysts were prepared using coconut shell AC activated by nitric acid as the support and iron oxide as the active component.The crystal structure, surface morphology, pore structure, functional groups and valence states of the active components of Fe/AC catalysts were characterised by X-ray diffraction, scanning electron microscopy, nitrogen adsorption and desorption, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy,respectively. The effect of Fe loading and calcination temperature on the low-temperature denitration of NH3-SCR over Fe/AC catalysts was studied using NH3 as the reducing gas at low temperature (150 ℃). The results show that the iron oxide on the Fe/AC catalyst is spherical and uniformly dispersed on the surface of AC,thereby improving the crystallisation performance and increasing the number of active sites and specific surface area on AC in contact with the reaction gas. Hence, a rapid NH3-SCR reaction was realised. When the roasting temperature remains constant, the iron oxide crystals formed by increasing the amount of loading can enter the AC pore structure and accumulate to form more micropores. When the roasting temperature is raised from 400 to 500 ℃, the iron oxide is mainly transformed from α-Fe2O3 to γ-Fe2O3, which improves the iron oxide dispersion and increases its denitration active site, allowing gas adsorption. When the Fe loading amount is 10%,and the roasting temperature is 500 ℃, the NO removal rate of the Fe/AC catalyst can reach 95%. According to the study, the low-temperature NH3-SCR mechanism of Fe/AC catalyst is proposed, in which the redox reaction between Fe2+ and Fe3+ will facilitate the formation of reactive oxygen vacancies, which increases the amount of oxygen adsorption on the surface, especially the increase in surface acid sites, and promotes and adsorbs more reaction gases (NH3, O2, NO). The transformation from the standard SCR reaction to the fast SCR reaction is accelerated.
Key words: nitric acid activation method; coconut shell activated carbon; Fe/AC catalyst; NH3-SCR; low-temperature denitrification mechanism
The iron and steel industry is an important establishment of a nation’s economy. Sintering is one of the most polluting processes in the iron and steel production processes, with high energy consumption,high flue gas emissions and serious pollution. It contains NOxpollutants that cause serious harm to human life, health and the ecological environment[1-3].Currently, the main technologies for the denitrification of sintering flue gas include physical adsorption,forced oxidation, catalytic oxidation, selective catalytic reduction (SCR) and other methods, with SCR is commonly used in industry because of its high denitrification rate[4,5], and the core of the its catalyst. In industries, V2O5-WO3/TiO2catalyst is widely used. It has been commonly used in thermal power plants and coal-fired boilers, but its operating temperature window is too high, generally between 300 and 400 ℃. There have been numerous domestic and foreign reports on the research of different types of low-temperature NOxremoval catalysts[6-8]. Currently, one of the main carriers of low-temperature NOxremoval catalysts is AC. Among them, coconut shell AC has a large specific surface area, rich pore structure, good adsorption performance, high mechanical strength and stable chemical properties, allowing for treating multiple pollutants in the sintering flue gas to be coordinated[9,10].However, when coconut shell AC is used as an adsorbent, its denitration performance is limited, and it cannot achieve ultra-low emission directly. Therefore,developing high-efficiency SCR catalysts with lowtemperature activity is critical for treating sintering flue gas.
Several studies have shown that coconut shell AC should be activated by various acids[11,12]to destroy its pore structure, change the mesopores into a micropore,and change the pore volume and specific surface to enhance its adsorption performance. The metaldoping modification improves the catalyst structure,component activity and denitration performance of coconut shell AC, achieving an ultra-low emission.Currently, catalysts containing transition elements[13-15](V, Fe, Mn, Cu,etc) and rare earth elements[16-18](Ce,Ho,etc) as active metal components exhibit good denitration activity at temperatures below 200 ℃.Among them, Fe2O3is a good catalyst active substance in the NO reaction and has the best dispersion on the AC surface, and the active site of NO molecular activation is related to Fe2O3species[19-21]. Xieet al[22]found that NH3molecules chemically absorb with FexOy/AC surface-active Fe atoms to form coordination NH3, which is conducive to NH3adsorption and thus accelerates the NH3-SCR denitration reaction. Lianget al[23]used nano-sizedγ-Fe2O3as a catalyst to conduct experiments at low-temperature experiments, and the results showed that nano-sizedγ-Fe2O3catalyst has good NOxremoval activity, with a NOxremoval efficiency of 97% under aerobic conditions. These experimental studies demonstrate that Fe/AC catalyst exhibits excellent NOxremoval efficiency and is suitable for development as a removal NOxremoval catalyst. Currently, there are few reports on the effect of different iron crystal oxides on the catalyst activity.Among them,γ-Fe2O3is a commonly used magnetic material that has a wider active temperature window,better SCR activity, higher nitrogen selectivity and wider active temperature window thanα-Fe2O3[24,25].However, the mechanism of AC modification by Fe2O3and the influence mechanism ofα-Fe2O3andγ-Fe2O3on denitration in the low-temperature denitration process have not been fully revealed.
To clarify the modification mechanism of AC by Fe after nitric acid activation and the low-temperature NH3-SCR denitration mechanism of Fe/AC catalyst, the Fe/AC catalysts were prepared with coconut shell AC after activation of nitric acid using a constant volume ultrasonic impregnation method. The Fe/AC catalysts were characterised using SEM, BET, FITR, XRD and XPS. The modification mechanism of Fe/AC catalysts with different roasting temperatures and Fe loading were investigated, including changes in pore structure,specific surface area, active substance and acidic oxygen-containing functional groups. The influence mechanism ofα-Fe2O3andγ-Fe2O3on denitration over Fe/AC catalysts in the process of low-temperature denitration process was investigated.
Iron nitrate (Fe(NO3)3·9H2O), analytically pure,was produced by Tianjin Komio Chemical Reagent Co.,Ltd. Coconut shell AC (particle size of 20-40 mesh)was from Henan Gongyi Lantian Water Purification Technology Co., Ltd. The water used in the experiment was deionised.
The coconut shell AC was sieved to 20-40 mesh and deionised by water 6-7 times to remove surface ash. Then, the water bath was heated at 60 ℃ for 2 h at a constant temperature. After filtration, it was placed in a blast-drying oven at 110 ℃ for 12 h (marked as AC). The coconut shell AC (marked as HNO3/AC)was activated by nitric acid at 80 ℃ for 2 h. Then, four portions of HNO3/AC, each with a mass of 20.00 g,were soaked in 5% and 10% Fe(NO3)3solution at room temperature. The HNO3/AC was immersed in water bath at 60 ℃ for 2 h before drying in a clean air-blastdrying oven at 110 ℃ for 12 h. After drying, the Fe/AC catalysts were prepared by roasting at different temperatures in an N2atmosphere at 400 and 500 ℃for 4 h until the Fe(NO3)3was completely decomposed.The Fe/AC catalysts were recorded as Fe/AC-1 (5%,400 ℃), Fe/AC-2 (10%, 400 ℃), Fe/AC-3 (5%, 500℃) and Fe/AC-4 (10%, 500 ℃).
The load metal oxide was estimated using a rotating target XRD (TTR18 kW Cu target) to analyse the crystal phase pattern of Fe/AC, and the surface microstructure change of Fe/AC was observed using SEM (Tescan VEGAS SBH), nitrogen adsorption and a desorption tester, and the N2adsorption isotherm was used (BET, QDS-evo). At 77 K, the specific surface area, pore volume and average pore diameter of Fe/AC were measured, and the micropore volume was calculated using the Dubinin–Radushkevich method.Each sample was degassed under vacuum at 200 ℃ for 4 h before the N2adsorption. The surface functional group changes of the Fe/AC catalysts were measured using FTIR (Nicolet iS 10) in the range of 4 000-400 cm−1, and the surface atomic state of the catalysts was analysed using XPS (PHI5000 Versaprobe-II) at 50 W using AlKα X-ray (hv= 1 486.6 eV) radiation. A C1sbinding energy (BE) value of 284.8 eV was used to calibrate the BE.
The denitration activity test of the Fe/AC catalysts was conducted in a fixed-bed reactor. Fig.1 shows the catalyst denitration reaction device, which consists of a gas metering and mixing system, a reaction system and a tail gas detection system. At the same temperature,the denitration activity of Fe/AC with different loads was tested. The thermocouple in the tubular reactor was used to measure the reaction temperature; based on the measured temperature, the furnace temperature was adjusted, reaction temperature was determined and the simulated flue gas was passed into the reactor. The simulated flue gas (NO) flow rate was 40 mL·min−1,the NH3flow rate was 40 mL·min−1, and the O2flow rate was 120 mL·min−1(10%). The balance gas was N2,the total gas flow rate was 1 200 mL, the flow rate was 40 mL·min−1,and the GHSV was 30 000 h−1. The final exhaust gas was detected using a Testo-340 flue gas analyser (Germany Testo Instruments) for 60 min.
Fig.1 Catalyst denitration reaction device1. N2; 2. NO; 3. NH3; 4. O2; 5. Vertical fixed bed; 6. Catalysts;7. Tail gas treatment unit; 8. Gas buffer bottle; 9. Detection gun;10. Flue gas analyser; 11. Computer for analysis
The conversion rate of NOxwas calculated using the following formula:
whereCin(NO)is the NO concentration at the reactor inlet, ppm;Cout(NO+NO2)is the NO + NO2concentration at the reactor outlet, ppm; andηis the denitration rate, %.
Fig.2 shows the XRD patterns of AC after hydrothermal treatment and HNO3/AC after nitric acid activation. The figure shows that after carbon or nitric activation, AC was mainly composed of graphite crystallites. Therefore, graphite peaks are usually found in the XRD pattern. The two types of AC are 23.813°and 43.467° (100 crystal planes) in 2θ. Each position has a wide dispersion characteristic peak, indicating that the coconut shell AC is mainly composed of amorphous carbon. The peak at 23.813° is the diffraction peak of loosely stacked graphene layers, and the peak at 43.467° is graphite crystals. The diffraction peak, the sharp peak at 26.468° in the figure, is the diffraction peak of the graphite (002) crystal plane[26],indicating that the diffraction peak of the graphene layer and graphite crystal becomes sharp and enhanced after nitric acid activation treatment. As shown in Fig.3, according to the standard card ofα-Fe2O3(JCPDS Card No.33-664), the peaks at 2θof 24.123°, 35.569°,49.378°, 53.988°, 57.409° and 62.381° correspond to the diffraction peaks ofα-Fe2O3. From the standard card ofγ-Fe2O3(JCPDS Card No.39-1346) in the figure, the diffraction peak ofγ-Fe2O3is at 30.109°,33.088°, 35.735°, 43.216°, 43.412°, 57.732° and 62.991°. The XRD patterns of Fe/AC-1, 2 show that the peaks at 2θof 24.123°, 35.569°, 49.378°, 57.409°and 62.381° correspond to the diffraction peak ofα-Fe2O3, and the diffraction peak ofγ-Fe2O3at 30.255°,as shown in Fig.3. The results show that there is a significant amount ofα-Fe2O3in the iron oxides formed by roasting iron nitrate at 400 ℃, as well as a part ofγ-Fe2O3. The XRD pattern of Fe/AC-3, 4 shows that the diffraction peak of γ-Fe2O3corresponds to the peak at 2θ of 30.109°, 35.735°, 43.216°, 43.412°, 57.732° and 62.381°, and a small peak ofγ-Fe2O3at 33.236°, and the diffraction peak at 35.735° has sharp symmetry,indicating that the iron oxides produced by roasting iron nitrate at 500 ℃ are mainlyγ-Fe2O3crystals, and only part ofα-Fe2O3exists. It was concluded that with the increase of roasting temperature from 400 to 500℃,γ-Fe2O3crystal cells gradually grow up, the degree of crystallisation increases, and the lattice diffraction peak increases, while the characteristic peak ofα-Fe2O3weakens, and theα-type crystal cell structure changes toγ-type[27]. The XRD diagram of Fe/AC-4 shows that the peaks at various angles are not sharp, indicating that the dispersion ofγ-Fe2O3on AC load is more uniform,and the crystallisation performance is better.
Fig.2 XRD pattern of AC and HNO3/AC
Fig.3 XRD pattern of Fe/AC catalysts
SEM characterisation was conducted to further study the changes in the surface morphology of the material before and after AC loading Fe, and the results are shown in Fig.4. Due to different loading and calcination temperatures, the iron oxides present different granular spheres on the surface and pores of coconut shell AC under the same immersion time condition. A scanning electron micrograph with a 10 000 times magnification is shown in Fig.4.According to Fig.4(a) and (b) of the AC of coconut shell before Fe loading, the AC of coconut shell without treatment has developed a pore structure,smooth surface and clear pore structure. After activation by nitric acid, the AC of the coconut shell showed an obvious porous structure in the same region,with mainly microporous and smooth pore walls,indicating that the coconut shell AC had improved physical and chemical properties, as well as enhanced adsorption performance. The surface morphology of the catalyst changed after impregnation with Fe(NO3)3.The pore structure of the coconut shell AC carrier was still retained after loading Fe. Few particles were dispersed in the pores of AC, as shown in Figs.4(c) and 4(d). As the loading capacity increased, iron oxide was uniformly dispersed on the surface of the AC channel(Fig.4(e) and 4(f)). The catalyst particles were spherical with uniform size, thereby drastically increasing the number of active sites and specific surface area in contact with the reaction gas. Most iron oxides (the main body isα-Fe2O3) generated by roasting iron nitrate at high temperatures are attached to the surface of the coconut shell AC with a certain thickness plate, as shown in Fig.4(e). The pores of coconut shell AC have irregular granular crystals and few blocks embedded in the pores, indicating poor dispersion of the iron oxides.The pore structure of AC is visible, iron oxide (the main body isγ-Fe2O3) is spread on the surface and pore structure of AC, and some small pieces are embedded in the pores of the AC, and the connectivity between the pores is good, relatively independent without adhesion, and no large plate is found, as shown in (f).Therefore, comparing Fe/AC-3 with Fe/AC-4 showed that when the roasting temperature was 400 ℃, the main iron oxide wasα-Fe2O3, and the pore of AC was blocked, affecting the denitrification performance of the modified AC. When the roasting temperature is 500℃, the iron oxide crystals can enter the pore structure of AC and accumulate to form more micropores. Iron nitrate mainly formedγ-Fe2O3. The increase in roasting temperature improves the dispersion of iron oxides and makes the loadγ-Fe2O3distribution more uniform.Simultaneously, it also accords with the XRD analysis results.
Fig.4 SEM micrographs of different catalysts: (a) AC; (b) HNO3/AC; (c) Fe/AC-1; (d) Fe/AC-2; (e) Fe/AC-3; (f) Fe/AC-4
The specific surface area of a catalyst affects its SCR denitration activity to some extent, and a larger specific surface area can provide more active sites for the reaction. Hence, the increased contact area between the reaction gas and the catalyst surface accelerates the denitration reaction. Therefore, the BET was used to characterise and analyse the Fe/AC catalysts.The specific surface area, average pore volume and pore size of Fe/AC catalysts are shown in Table 1.The nitrogen isothermal adsorption-desorption curve and the pore size distribution of Fe/AC catalysts are shown in Fig.5 and Fig.6. According to the IUPAC classification, it is type IV isotherms with H1-type hysteresis loops[28,29], indicating that the structures have highly ordered micropore channels, which is similar to the SEM results. In summary, the specific surface area of AC decreased after nitric acid activation treatment.After nitric acid activation, the average pore size of AC decreases while the average pore volume increases.This is because nitric acid activation removes the ash on the original pore while generating new pores. The carbon skeleton of AC will shrink during the Fe/AC roasting process at high temperatures in a nitrogen atmosphere, reducing the size of the original pores.In the process of impregnation of AC, Fe3+can enter the pore of AC. In the process of roasting Fe(NO3)3at high temperatures, the crystal position will become the heat aggregation point, resulting in the ablation of part of AC. The interior pore structure of AC can be developed. In the Fe/AC prepared by roasting with equal volume impregnation of Fe(NO3)3, Fe/AC-4 is the best, and the micropore volume, micropore specific surface area, total pore volume and total specific surface area are the highest among the four types of Fe/AC, indicating that as loading capacity increases, some small pores are blocked, which increases the average pore size. However, this will not affect the increase in the specific surface area or denitration reaction. As the number of active components increases, new channels will be generated based on the original AC channels.Iron oxide is mainly transformed fromα-Fe2O3,blocking the pores of AC toγ-Fe2O3with a more uniform dispersion, which is also verified by the SEM and XRD analysis results.
Table 1 Specific surface area and pore structure of Fe/AC
Fig.5 Pore diameter distribution
Fig.6 N2 adsorption-desorption
The surface chemical properties of coconut shell AC are mainly determined by the type and quantity of surface functional groups, and most of the functional groups on the surface of AC are oxygen-containing functional groups, such as carboxyl, phenolic hydroxyl,carbonyl, ester and ether groups[30]. After HNO3activation treatment, AC is somewhat ablated, releasing CO and CO2[31]. The basic pore structure shows partial defects, and O and H are adsorbed on the pores to form various oxygen-containing functional groups[32]. The FTIR spectra after nitric acid activation and metal oxide loading are shown in Fig.7 and Fig.8, respectively.According to a previous study[33-35], the infrared band 3 600-3 200 cm−1is attributed to the stretching vibration of –OH; the infrared band 1 740-1 480 cm−1is attributed to the stretching vibration of C=O or C=C.The 1 300-1 000 cm−1band is attributed to C–O–C or C–O stretching vibrations. Fig.7 shows that after activation with nitric acid, the AC spectrum shows clear peaks at 3 438 cm−1(–OH stretching vibration),1 573 cm−1(–C=O stretching vibration) and 1 135 cm−1(–CO stretching vibration). After nitric acid activation,the state was enhanced, and two new absorption peaks appeared at 1 631 cm−1(–C=O stretching vibration) and 1 450 cm−1(–CH). More acidic oxygen-containing and nitrogen-containing functional groups, such as carboxyl groups, lactone groups, hydroxyl groups and nitro groups, which are easy to react with NO, are formed on the AC surface after nitric acid activation[36,37]. The basic types of Fe/AC functional groups are the same under different loading and calcination temperatures,as shown in Fig.8. However, as the calcination temperature increased, the -OH stretching vibration absorption peak in the -COOH of 3 419 cm−1and chemically adsorbed water was enhanced, and the asymmetric vibration absorption peak of 1 638 cm−1lactone group becomes more obvious. The results showed that as the roasting temperature increased,the adsorption sites of oxygen-containing functional groups on the AC surface, such as carboxyl and lactone groups, increased continuously, and the increase in oxygen-containing functional groups would provide more active sites for the denitration reaction, as well as, more highly adsorbed oxygen (Oβ) for the first stage of the denitration reaction[38], thus promoting the denitration reaction of NH3-SCR.
Fig.7 FTIR spectrogram of AC and HNO3/AC
Fig.8 FTIR spectrogram of Fe/AC
This experiment used XPS characterisation on the Fe/AC catalysts activated by nitric acid before denitration to study the valence state and proportion of active elements on the surface of the catalyst. The figures show the energy spectra of O1s and Fe2p orbitals. The energy spectra of the Fe2p orbitals of different samples are described in Fig.10. The Fe2p of each catalyst showed two obvious strong peaks, corresponding to Fe2p3/2 (BE about 711 eV)and Fe2p1/2 (BE about 725 eV), respectively. The binding energies of Fe2p3/2 were 710.7 and 712.5 eV, which correspond to Fe2+and Fe3+, respectively,indicating that Fe coexisted with Fe2+and Fe3+in the four catalysts. The O1s spectra of different samples are shown in Fig.9. Because of the unique surface structure of AC, the surface oxygen species can be classified as lattice oxygen (Oα), surface-adsorbed oxygen (Oβ) and chemical oxygen (Oγ)[39]. Fitting the peaks of O1s yielded three peaks with binding energies of approximately 530.1, 532.8 and 531.3 eV, corresponding to Oα, Oγand Oβ, respectively. The increase in the Fe/AC-4 catalyst (Oβ) ratio,i e, ((Oβ) /(Oα) + (Oβ) + (Oγ)), was pronounced. However, as shown in the FTIR diagram, nitric acid activation produced more hydroxyl and carbonyl groups in addition to other oxygen-containing functional groups.However, as the roasting temperature increased, a redox reaction occurred between Fe2+and Fe3+, forming more active oxygen vacancies, thereby increasing Oβ,i e, the amount of oxygen adsorbed on the surface. In summary, as the roasting temperature increased, the content ofγ-Fe2O3significantly increased, resulting in a significant increase in the Fe3+content (Fe3+/Fe2++ Fe3+), allowing the catalyst surface to have more surface adsorption oxygen (Oβ), which has a higher flow rate. Simultaneously, the results of XRD were verified.
Fig.9 XPS survey spectrum of O1s
Fig.10 XPS survey spectrum of Fe2p
Fig.11 NOx conversion for different Fe/AC catalysts
In the experiment, the denitration activity of the prepared catalyst was measured to investigate the mechanism of the effect of Fe loading and roasting temperature on the denitration mechanism of NH3-SCR. In the presence of GHSV of 30 000 h−1and 10%O2, low-temperature NH3-SCR denitrification activity of Fe/AC catalysts loaded with AC at different loadings and calcination temperatures at 150 ℃ was studied.Fig.7 showed that the denitration effect of HNO3/AC after nitric acid activation is higher than that of AC after hydrothermal treatment. This may be due to the formation of oxygen-containing functional groups due to the activation of nitric acid, which promotes the adsorption and oxidation of NO[40], allowing metal oxides to disperse in AC. When the roasting temperature is 400 ℃, the NO conversion rate is relatively low, which may be because the metal oxides on the AC surface are mainlyα-Fe2O3.As the proportion ofα-Fe2O3increases, the pores of the AC will be blocked, affecting the denitration activity of the Fe/AC catalyst. When the roasting temperature is 500 ℃, the denitrification efficiency is relatively stable, reaching>90%. It is possible that the AC surface metal oxides,mainlyγ-Fe2O3, will significantly increase, resulting in Fe/AC having good thermal stability and an increase in surface adsorption oxygen, especially with the increase of surface acid sites and the adsorption of more reactive gases[41,42]. In conclusion, when the roasting temperature is 500 ℃, the denitration efficiency of the Fe/AC-4 catalyst is the highest, which may be attributed to an increase of surface-adsorbed oxygen, which may help to improve the catalytic activity ofγ-Fe2O3in NH3-SCR reaction[43].
As a carbon-based carrier of porous catalysts,AC has numerous tiny channels, and its structure with abundant graphite microcrystals can provide a surface for gas–solid reactions. AC was used as the basis for Fe/AC catalysts. During the impregnation process, the AC carrier is immersed in Fe(NO3)3mixed solutions with different mass fractions required by an ultrasonic water bath at a temperature of 60 ℃ to make the metal ions move vigorously into the interior of the AC carrier void and attach to the AC surface during the drying process. In the process of N2protective roasting,Fe(NO3)3decomposes into different crystalline iron oxides at 400 and 500 ℃. As shown in the XRD figure,as the increase of roasting temperature increased, the number ofγ-Fe2O3crystal cells increased, as did the degree of crystallisation, and the lattice diffraction peak increased, while the characteristic peak ofα-Fe2O3weakened, andα-Fe2O3toγ-Fe2O3transformation. As shown in the XPS diagram, the redox reaction between Fe2+and Fe3+formed more active oxygen vacancies,which increased the amount of oxygen adsorption(Oβ) on the surface. As shown in the FTIR diagram,the number of adsorption sites, such as carboxyl and lactone groups containing oxygen-functional groups on the AC surface, increased continuously as the roasting temperature increased. The increase in oxygenfunctional groups will provide more active sites for denitration and provide more highly adsorbed oxygen(Oβ) for the first stage of denitrification. The Fe/AC catalysts’ preparation schematic diagram is shown in Fig.12.
Fig.12 Schematic diagram of the preparation of Fe/AC catalyst
Based on these characterisation observations,the NH3-SCR low- temperature denitration reaction on the Fe/AC catalyst surface is a representative heterogeneous reaction. The NH3-SCR denitrification mechanism is proposed based on the characterisation results, as shown in Fig.13, and can be divided into three steps:
Fig.13 Denitration mechanism diagram of Fe/AC catalysts
a) The reaction gases of NH3, NO and O2are continuously diffused from the catalyst’s outer surface to the inner surface of Fe/AC. Numerous reactant gases can be stored, and many reaction units can be generated on the graphite microcrystalline structure inside the catalyst because of the large specific surface area and rich pore structure of AC[44]. The reaction gas is adsorbed on the surface of the catalyst in an adsorption state on the active sites of the catalyst,and the active sites then activate the reaction gas to form molecules. For example, many acidic oxygencontaining functional groups and C-H groups detected by FTIR will preferentially adsorb N; additionally, XPS can also detect that the adsorbed oxygen (Oβ) content on the surface of Fe/AC catalyst is relatively high.Since surface-absorbed oxygen (Oβ) has high mobility,it can promote the oxidation of NO to NO2and provide a highly active adsorption site for NH3[45](Eqs.(1)-(4)).
b) A reaction occurs between NO and NH3.During the denitration process, an oxidation-reduction reaction between Fe2+and Fe3+may occur (Eqs.(5)and (6)). As demonstrated by XPS characterisation,the surface-adsorbed oxygen (Oβ) is more active than lattice oxygen (Oα) because of the formation of oxygen vacancies between Fe2+and Fe3+in the redox reaction, which significantly promotes the continuous replenishment of (Oβ). The transformation from the standard SCR reaction to the fast SCR reaction (Eqs.(7) and (8)) is accelerated. This phenomenon may be due to the increased mobility of the active substance or oxygen, which is conducive to the denitration reaction.
c) The gas (N2and H2O) generated from the surface of the catalyst is desorbed and diffused to the inner surface through the AC pores, and then the gas diffuses from the inner surface of the AC to the outer surface of the Fe/AC catalyst (Eqs.(9) and (10)).
Based on the above findings, the valence of Fe ions on the surface of the Fe/AC catalyst increases,as does the amount of adsorbed oxygen (Oβ) on the surface increases (Oβ). The Fe/AC catalyst can absorb more reaction gases as the surface acid functional groups increase. The accumulation of all factors results in Fe/AC achieving significant NH3-SCR denitration efficiency.
In this study, an equal volume impregnation method was used to prepare a series of Fe/AC catalysts with different loads and calcination temperatures. The effects of different loads and roasting temperatures on the efficiency of NH3-SCR denitration were studied,and the following conclusions were drawn:
a) when the roasting temperature remains constant, iron oxide crystals formed by increasing the loading amount can enter the AC pore structure; these crystals are stacked to form more micropores, thereby improving the adsorption performance of the reaction gas.
b) When the loading amount of Fe is kept constant and the roasting temperature is increased from 400 to 500 ℃, the iron oxide is mainly converted fromα-Fe2O3toγ-Fe2O3, thereby enhancing iron oxide dispersion and providing more active sites for denitration; these changes are conducive to gas adsorption, and they cause an enhanced denitration effect.
c) Fe2O3, the active component of Fe/AC catalysts,is highly dispersed as spherical particles on the AC surface and has good thermal stability. Increasing the number of acidic oxygen-containing functional groups on the surface of Fe/AC catalysts, such as carboxyl group and lactone groups, will provide more active adsorption sites for the denitration reaction.
d) Based on the redox reaction between Fe2+and Fe3+, active oxygen vacancies are formed, which increases the amount of adsorbed oxygen (Oβ) on the surface, especially the increase of surface acid sites,which promotes and adsorbs more reaction gases (NH3,O2, NO), resulting in the conversion of the standard SCR reaction to fast SCR reaction.
Conflict of interest
All authors declare that there are no competing interests.
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