Nitrogen oxides (NOx) are a major component of air pollution. Selective catalytic reduction (SCR) of NOx with NH3 is a well-known promising technique for removing NOx [1, 2]. The industrial catalysts for NH3-SCR usually comprise V2O5/TiO2 in combination with either WO3 or MoO3, however, there are also some disadvantages, such as the narrow operating temperature window (300–400 °C) [3, 4] and the environmental toxicity of vanadium species at high temperatures [5]. Therefore, novel high performance catalysts with transition metal alternatives to vanadium have been extensively investigated [6,7,8,9,10,11].
Mn/TiO2 anatase catalyst reported by Ettireddy et al. [4] shows high activity for low temperature SCR of NO with NH3 and shows stable NO conversion and N2 selectivity, even after 10 days of time-on-stream in the presence of 11 vol% water vapor at 175 °C. However, the low temperature SCR performance of Mn-based catalysts is reduced by poisoning from sulfur residuals in fuels and engine oils. A series of catalysts composed of manganese oxide and iron-manganese oxide supported on TiO2 were studied for low-temperature SCR in the presence of excess oxygen. It was found that the addition of iron oxide not only increased the NO conversion and N2 selectivity but also increased catalyst resistance to H2O and SO2 [12].
Roy et al. [8] have prepared serial Ti0.9M0.1O2-δ (M = Cr, Mn, Fe, Co, and Cu) catalysts using a self-propagating high-temperature synthesis (SHS) method. It was found that the optimum Ti0.9Mn0.05Fe0.05O2-δ catalyst showed good activity in the 150–500 °C temperature range and more than 80% N2 selectivity even at 450 °C. SCR catalysts prepared by the SHS method exhibited reasonably high specific surface areas, appropriate pore volumes, average pore diameters, and typical nanoparticle sizes [10], which significantly enhanced the catalyst performance.
The Mn-Fe based catalysts exhibit high activities at low temperature, which is possibly caused by the strong interactions of Fe and Mn. However, the SCR mechanism for this catalyst is still uncertain. For the Fe-Ce-Mn/ZSM-5 catalyst [13], two possible reaction pathways were proposed. One was that NO2 could react with NH4+ on Brönsted acid sites and the formed NO2[NH4+]2 would react with NO, producing N2 and H2O. Another pathway involves NH3 adsorption and subsequent reaction with NO or HNO2. Possible intermediates NH4NO2 and NH2NO were unstable and would decompose into N2 and H2O. Additionally, the SCR reaction over (Fe2.5Mn0.5)1-δO4 mainly follows an Eley-Rideal (E-R) mechanism [14]. Adsorbed ammonia species are activated to form amide species (–NH2) by Mn4+ and Fe3+ on the surface. Then, gaseous NO is reduced by –NH2 on the surface to form N2 and H2O. For the case of the Ti0.9Mn0.05Fe0.05O2-δ prepared by Roy et al. [8], according to a Langmuir-Hinshelwood (L-H) mechanism, the adsorbed NH3 or NH4+ can react with adsorbed NO to form an NH2NO adduct, which can then dissociate into N2 and H2O. Overall, the process of adsorption and activation of reactant NH3 or NO is important because it is related to the intermediate species and determines the reaction pathways.
The purpose of this study is to obtain information on the reaction mechanism of manganese and iron oxide catalysts at low temperatures through the respective adsorption of NH3 and NO, as well as the reaction between NH3 and NO. SHS is an effective, low-cost method for production of various industrially useful materials [15]. In this paper, an SCR catalyst, Ti0.9Mn0.05Fe0.05O2-δ was also prepared by the SHS method [16]. The catalyst sample was exposed to simulated exhaust gas mixtures (NO/O2/NH3/N2) to evaluate the SCR performance and was characterized by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to investigate the possible mechanism of NH3-SCR.
Catalyst Ti0.9Mn0.05Fe0.05O2-δ was prepared by the SHS method. First, the required amount of absolute alcohol was added dropwise into the precursor tetrabutyl titanate (Ti(OCH3(CH2)3)4), and deionized water was likewise added to the product of metatitanic acid (TiO(OH)2). Next, in order to get titanyl nitrate (TiO(NO3)2), the right amount of HNO3 solution was added. At the same time, a solution of metal nitrate (Mn(NO3)2·4H2O and Fe(NO3)3·9H2O) in deionized water was prepared in stoichiometric amounts. Then, the nitrate solution was combined with the titanyl nitrate. Glycine (CH2NH2COOH) was sequentially added to fuel the reaction. After stirring at 50 °C for 1 h, the aqueous solution was placed in a muffle furnace at 350 °C in air. During the first few minutes, the solution boiled with froth and combusted with smoke. The resulting product was finally calcined at the ignition temperature for 1 h. The synthesized catalyst was allowed to cool and was sieved to 40–80 mesh for further analysis.
Catalytic activity tests were carried out using the temperature-programmed reaction (TPR) method. The powder sample (0.5 mL) was used in a fixed-bed quartz reactor (i.d. 9 mm), and the reaction temperature was raised from room temperature to 450 °C at a ramp rate of 5 °C/min. A simulated exhaust gas mixture (0.06% NO, 0.06% NH3, 5% O2, and N2 for balance) was fed to the catalyst sample through a set of mass flow controllers. The total flow rate was set to 100 mL/min, corresponding to a gas hourly space velocity (GHSV) of 12000 h–1. The outlet gas composition containing NO, NO2, N2O, and NH3 was measured on-line with an FTIR gas analyzer (Thermo Nicolet 6700). Before the experiment, concentrations of the gases were quantified based on the classic least squares methodology using the Omnic Quantpad software. A calibration run of each gas consisting of four spectral absorption standards was performed and used to generate each standard curve within Quantpad.
X-ray diffraction (XRD) was used to reveal the catalyst crystal phases. XRD continuous measurements were performed on a Rigaku D/max-2200/PC system. Diffraction patterns were obtained from 20° to 90° (2θ) with a scanning rate of 5°/min, and a step size of 0.02° at ambient temperature.
The microscopic morphology of catalyst was interrogated by transmission electron microscopy (TEM) on a JEOL JEM-2010 analytical electron microscope operating at an accelerating voltage of 200 kV. Before TEM characterization, the catalyst powder was ultrasonically suspended in alcohol for 10 min, after which the obtained suspension was deposited on copper-grid-supported amorphous carbon films. The impregnated mesh was allowed to dry in air for TEM analysis.
In situ DRIFTS spectra were measured by an FTIR gas analyzer (Thermo Nicolet 6700) equipped with a diffuse reflectance optics accessory (HVC-DRP-4). The inlet gas concentrations were identical to those used in the catalytic activity test (0.06% NO, 0.06% NH3, 5% O2, and N2 for balance). A 60 mg of powder sample was used, and the total flow rate was 50 mL/min. To clean the catalyst surface, the sample was purged in N2 at 450 °C for 2 h prior to acquisition of DRIFTS spectra. Thereafter, the background spectra were recorded from 50 to 450 °C, with 50 °C increments. In this experiment, the IR range was from 4000 to 650 cm–1, and the resolution was set to 4 cm–1.
TPR was performed to evaluate the SCR performance of the Ti0.9Mn0.05Fe0.05O2-δ catalyst, as shown in Fig. 1. The calculation methods to determine NOx conversion, N2O selectivity, and N2 selectivity were described in our earlier paper [10]. More than 80% of NO is reduced by NH3 on the Ti0.9Mn0.05Fe0.05O2-δ catalyst in a wide temperature window of 100 to 350 °C (Fig. 1(a)). Ti0.9Mn0.05Fe0.05O2-δ reaches the maximum NO conversion of 100% at an especially low temperature of 120 °C, and this NO conversion rate remains at its peak until 350 °C. NO conversion subsequently decreases sharply above 350 °C. The main reason for this is unselective NH3 oxidation by O2. The N2 yield initially reaches over 95% at ambient temperature to 100 °C, but then decreases to a lower level of around 70% at 250 °C (Fig. 1(b)). With further rise in temperature, the N2 yield again increases. From 350 °C, N2 selectivity decreases monotonically due to oxidation of NH3. This can be seen from the NO2 concentration curve in Fig. 1(b). At the same time, under 250 °C, N2O selectivity shows the reverse trend as N2 selectivity. A fast drop in selectivity happens between 250 and 300 °C. N2O selectivity then decreases slightly with increasing temperature. Overall, in a broad temperature window of 100–350 °C, Ti0.9Mn0.05Fe0.05O2-δ has high SCR activity, which is comparable to that reported by Roy et al. [8].
The power XRD pattern of the Ti0.9Mn0.05Fe0.05O2-δ catalyst sample is shown in Fig. 2, with peaks indicated as identified in the ICDD (anatase TiO2 phase: PDF #21-1272). The XRD peaks of the catalyst closely conform to anatase phase TiO2, with no evidence for the rutile phase. No diffraction peaks for FeOx, MnOx, or any other mixed oxides are seen, which indicates that the Mn and Fe are either in a highly dispersed state and assumes an amorphous form on TiO2, or that the formed crystallites cannot be detected by XRD. This is due to strong Fe-Mn interactions, which leads to enhanced dispersion and lower crystallization for both [12].
TEM was used to examine the morphology and size of the Ti0.9Mn0.05Fe0.05O2-δ. As presented in Fig. 3, there are many nanoparticles dispersed on the catalyst, and no obvious particle agglomeration is apparent. The TEM image shows that the typical nanoparticles are well dispersed with sizes on the order of 10 nm. At higher magnification, a crystal plane spacing of 0.346 nm is detected, which is slightly smaller than the spacing of the anatase TiO2 (101) plane. Any morphology of the manganese and iron oxides are not observed, which is consistent with the XRD result.
NH3 adsorption measurements were performed in a feed of 0.06% NH3 in N2 at 50 °C for 30 min. Catalyst Ti0.9Mn0.05Fe0.05O2-δ was then purged with N2 for 30 min. Surface adsorption species with IR bands at 1188 and 1598 cm–1 are formed (Fig. 4), which are assigned to coordinated NH3 adsorbed on Lewis acid sites [10]. In the NH vibration region, IR bands at 3160, 3253, and 3334 cm–1 are seen and are also assigned to coordinated NH3 bound to Lewis acid sites. The band at 1306 cm–1 is probably due to NH3 adsorbed on different Lewis sites [17]. At the same time, IR bands observed at 1458 and 1680 cm–1 are from asymmetric and symmetric deformations of NH4+ on Brönsted acid sites. It seems that there are less Brönsted acid sites than Lewis acid sites on the Ti0.9Mn0.05Fe0.05O2-δ catalyst. It can also be seen that the intensity of the bands at 1188 and 1598 cm–1 are far stronger than those at 1458 and 1680 cm–1. Additionally, the two stronger bands reach their peak intensities after NH3 is saturated on the catalyst at 50 °C.
All surface-adsorbed NH3 species are quite stable during subsequent purging with N2 at 50 °C. The spectra of these surface species at different temperatures are also presented in Fig. 4. With increasing temperature, the intensity of the bands at 1188 and 1598 cm–1 drops off gradually. At 450 °C, these bands still obviously exist. The intensities of the bands in the 3400–3100 cm–1 region also decrease slightly. Meanwhile, the intensities of the bands at 1458 and 1680 cm–1 decline drastically, and the band at 1458 cm–1 can no longer be observed above 200 °C. These results demonstrate that NH3 desorbs at increased temperatures. Moreover, Brönsted acid sites have lower thermal stability than Lewis acid sites.
In this experiment, the Ti0.9Mn0.05Fe0.05O2-δ catalyst is treated with NH3 + O2 in the same way. Both NH4+ ions and coordinated NH3 are formed (Fig. 5). The intensities of the coordinated NH3 bands at 1188 and 1598 cm–1 decrease with increasing temperature as NH3 desorbs. However, when compared to the spectra of NH3 adsorbed on Lewis acid sites in Fig. 4, the variations between spectra at different temperatures are larger. The intensities of the bands at 1188 and 1598 cm–1 initially decrease gradually beginning at 150 °C. When reaching a temperature of 450 °C, the coordinated NH3 almost vanishes, without any obvious intensity. According to the literature [18], this may be caused by oxidation of NH3 to NH2 at a relatively low temperature range and oxidation of NH3 to N2 at a relatively high temperature. Additionally, a new band begins to appear at 1245 cm–1 at 400 °C, which was attributed to a bridging nitrate. This nitrate could be due to side reactions taking place, yielding a certain amount of NO.
NO adsorption measurements were performed in a feed of 0.06% NO in N2 for 30 min. The catalyst was then purged with N2 for 30 min. Surface nitrate species are observed at 1245, 1276, 1470, 1576, and 1612 cm–1 (Fig. 6). The IR band at 1245 cm–1 can be assigned to a bridging nitrate, and that at 1276 cm–1 and 1470 cm–1 are assigned to monodentate nitrate, while bands at 1576 cm–1 are attributed to bidentate nitrate. According to results of Long et al. [17], the band at 1612 cm–1 is not easy to assign. They assigned it to an NO2 adspecies (nitro or adsorbed NO2 molecule) on the catalyst, because this species had a different thermal stability from that of other nitrate species. It can be also seen that the species appeared at 1612 cm–1 exhibits a similar thermal stability as the bridging nitrate. Thus, we assign it to the bridging nitrate.
IR bands in these regions decrease with increasing temperature (Fig. 6), indicating desorption of NO. IR bands at 1276 and 1470 cm–1 show the most rapid disappearance, occurring at 200 °C. The intensities of the bands at 1245 and 1612 cm–1 decrease slightly slower, such that they can still be observed at 350 °C. The bidentate nitrate at 1576 cm–1 does not disappear until 450 °C. The thermal stability of the nitrate species follows the order: bidentate nitrate > bridging nitrate > monodentate nitrate.
To investigate the influence of gaseous O2, we carried out the adsorption of NO and NO + O2 over Ti0.9Mn0.05Fe0.05O2-δ at 150 °C. First, the catalyst was treated with NO and NO + O2 in N2 for 30 min, and then was purged with N2 for an extended 90 min. The recorded IR spectra are shown in Fig. 7. It is obvious that NO + O2 adsorption is much stronger than NO adsorption because gaseous O2 could enhance the oxidation of NO to NO2, leading to the formation of surface nitrate species. In the NO only treatment case, oxidation of NO is simply caused by lattice oxygen. After purging with N2 for 90 min, only a slight decrease in the intensities of the surface nitrate species bands is seen in the two cases, which implies that surface NO adsorption species are caused by fairly stable chemisorption, regardless of O2. This is consistent with results reported by Centi et al. [19], who proposed that chemisorption of NO on the catalyst copper sites was significant and that chemisorption is enhanced by the presence of oxygen.
To investigate the reaction mechanism of SCR by NH3 over the Ti0.9Mn0.05Fe0.05O2-δ catalyst, the sample was first exposed to NH3 at 150 °C until it was saturated. The catalyst was then purged with N2 and subsequently exposed to NO. IR spectra were recorded as a function of time (Fig. 8). Because of exposure to NH3, IR bands assigned to coordinated NH3 or NH4+ appear. However, after 70 min of NO exposure, no apparent decrease in the adsorbed NH3 is seen, and no surface NO adsorption species are seen to accumulate on the catalyst. At the same time, we could not find evidence for the formation of any new adsorption species. This can be explained from several aspects [17, 20, 21]. First, the absence of gaseous O2 inhibits NO oxidation, suppressing the formation of nitrate species. Additionally, our observations are possibly caused by competition between NO and NH3 for the same adsorption sites on the catalyst surface. Moreover, gaseous NO has a low reaction probability with adsorbed NH3, owing to lack of gaseous O2 in the reaction conditions. The activation of adsorbed NH3 by gaseous O2 to form NH2 is a main step in the catalytic mechanism.
For the case of reaction between NO + O2 and adsorbed NH3, the catalyst sample was first exposed to NH3 at 150 °C until it was saturated, then purged with N2. After that, NO+O2 was introduced into the cell. The results are shown in Fig. 9. IR bands at 1188, 3160, 1598, and 3253 cm–1, assigned to coordinated NH3 on Lewis acid sites as noted above, decease gradually with time until they disappear. The vibrational signature of NH4+ on Brönsted acid sites at 1680 cm–1 vanishes quickly after exposure to NO for about 25 min, indicating that NO rapidly reacts with adjacent adsorbed NH4+ 25 min later. Thus, it is suggested that both Brönsted and Lewis acid sites are involved in NO reduction.
Furthermore, it can be seen that the rate of SCR reaction becomes faster after 25 min. For the competitive adsorption of NO and NH3 on the catalyst surface, no nitrate species formation is seen in the first 25 min. Coordinated NH3 is first activated by active oxygen from the dehydration between manganese oxides, yielding the amide species from H abstraction [21]. At this point, the intermediate species NH2 further reacts with gaseous NO to form N2 and H2O following an E-R mechanism. Then as the adsorbed NH3 species are consumed, sufficient adsorption sites are available for NO. The SCR process then begins to follow a L-H mechanism, in which adsorbed NH4+ and coordinated NH3 react with nitrate species. Comparing with the results presented in Section 3.3.4.1., it is clear that oxygen plays an important role in the reaction between NO and NH3 [21], as a result of the oxidation of NH3 and NO. At a low temperature (150 °C), it is difficult to form NH2 species because of the high activation energy [20]. Conversely, NO oxidation occurs a little faster by the high oxidative ability of Mn. As more nitrate species adsorb, the reaction rate is accelerated. This is why the catalysts with Mn have outstanding low-temperature performance. As a result, at 150 °C both L-H and E-R mechanisms are involved in the SCR reaction, while the L-H mechanism is dominant for the Ti0.9Mn0.05Fe0.05O2-δ catalyst.
To investigate the reaction between NH3 and adsorbed gas mixture NO + O2, the Ti0.9Mn0.05Fe0.05O2-δ catalyst sample was first treated with gas mixtures NO + O2 at 150 °C until it was saturated, followed by N2 purging. When the treated sample is exposed to NH3, the intensities of the bands at 1245, 1276, 1576, and 1612 cm–1, assigned to surface nitrate species, decrease (Fig. 10). The band intensity for the bridging nitrate at 1245 cm–1 decreases quickly, disappearing within 15 min, while the band intensity of the monodentate nitrate at 1276 cm–1 decreases slightly slower, disappearing after about 35 min of NH3 exposure. The bridging nitrate band at 1612 cm–1 can still be observed after 40 min. Exposure to NH3 results in little change to the band intensity of the bidentate nitrate at 1576 cm–1. These intensity changes for the nitrate species indicate that monodentate and bridging nitrates have high reactivities with NH3, while the bidentate nitrate seems to be little reactive towards NH3. This is consistent with the results of reaction between NO + O2 and adsorbed NH3.
Meanwhile, IR bands at 1188, 1230, 1295, 1458, 1680, 3160, 3253, and 3334 cm–1 emerge, belonging to a group of surface NH3 adsorption complexes on the Ti0.9Mn0.05Fe0.05O2-δ catalyst. The intensity of all these bands increases quickly within about 10 min, because surface acid sites have a strong adsorption capacity of NH3 due to the effect of the active Fe metal [20]. The intensity of the band at 1188 cm–1 remains strong after 10 min exposure to NH3, suggesting continuous accumulation of coordinated NH3 on the catalyst. However, almost no changes are seen in the ionic NH4+ bands at 1458 and 1680 cm–1 after 10 min. New bands at 1230 and 1295 cm–1 are detected, and they increase with time in NH3 flow. So these bands could not be attributed to surface nitrate species. In contrast to the results of adsorbed NH3 shown in Fig. 4, no adsorbed species are seen at 1230 and 1295 cm–1 when the catalyst is first treated with NH3 and then purged by N2. Therefore, we assigned these bands to weak adsorbed NH3 species.
On the basis of the above analysis, the L-H mechanism is dominant for the Ti0.9Mn0.05Fe0.05O2-δ catalyst at 150 °C, and both adsorbed NH4+ and coordinated NH3 participate in the L-H reaction path. During the reaction of NH3 and adsorbed NO + O2, surface NH4+ on Brönsted acid sites remain stable, maintaining their intensity, probably because the rate of NH3 adsorption on Brönsted acid sites is equal to the rate of NH4+ reaction with NO. The amount of coordinated NH3 on Lewis acid sites continues to increase due to the relatively slow rate of reaction between NO and coordinated NH3. This suggests that Brönsted acid sites are more active than Lewis acid sites on the Ti0.9Mn0.05Fe0.05O2-δ catalyst.
According to the results of in situ DRIFTS, the reaction mechanism can be deduced as following the reaction scheme depicted in equations (1)–(4):
The formation of monodentate nitrate Mn+–O–NO2 (M = Fe and Mn) is involved in reaction with adsorbed NH4+ or NH3 on neighboring acid sites, to produce an active intermediate Mn+–O–NO2[NH4+]2 or Mn+–O–NO2[NH3]2, which finally reacts with gaseous NO to form N2 and H2O [17, 20].
The Ti0.9Mn0.05Fe0.05O2-δ catalyst shows high SCR performance over a broad temperature window of 100–350 °C. The active components of Mn and Fe exist in a highly dispersed state and amorphous form on TiO2. In situ DRIFTS measurements suggest that Brönsted and Lewis acid sites both take part in the SCR reaction, and Brönsted acid sites may play a more important role in determining the activity of Ti0.9Mn0.05Fe0.05O2-δ. The monodentate nitrate species is key active intermediate, in comparison to the less important bridging and bidentate nitrate species. At 150 °C, both L-H mechanism and E-R mechanisms are involved in the SCR reaction, while the L-H mechanism dominates catalytic activity of Ti0.9Mn0.05Fe0.05O2-δ. The presence of O2 significantly affects NO oxidation and coordinated NH3 activation. Hence, at a low temperature, NO oxidation is a main step, which depends on the oxidative ability of the active metal.
2014, Vol. 35 Issue (3): 294-301
PDF (692KB)
2014, Vol. 35 Issue (3): 294-301
PDF (692KB)