Effect of O2 on Selective Catalytic Reduction of NO by C3H6 over Fe Catalysts Supported on Porous Clay Heterostructures (Fe-PCH)

Praparation of the Catalysts

Synthesis of PCH carrier: The raw material for synthesizing PCH carrier is K10 montmorillonite (Sinopharm Chemical Reagent, China). The clay particle size is less than or equal to 2μm, and the cation exchange capacity (CEC) is 102 meq/100g. First, the montmorillonite raw material is sodium treated. Then sodium montmorillonite (2g) was added to the surfactant solution (hexadecyltrimethylammonium chloride, 0.1M, 100ml) and stirred at 60 °C for 24h. Then, the clay was separated from the solution and washed with deionized water to pH 7. Next, add 2g of modified clay to neutral amine (dodecylamine) and organosilicon source (tetraethylorthosilicate-TEOS), the molar ratio of each substance is: modified clay/dodecylamine/TEOS=1 /20/150. After the mixture was continuously stirred at room temperature for 12 h, the modified clay material was separated from the solution and dried. Finally, PCHs materials were obtained by calcining at 600°C for 6 h in a muffle furnace. Synthesis of Fe-PCH catalyst: The Fe-PCH catalyst was prepared by impregnation method. The process was as follows: 1.5g PCH carrier (about 5 cm3 in volume) was added to 10 mL Fe(NO3)3·9H2O solution with a concentration of 0.27 mol/L. The mixture was sonicated for 12 h and then placed in a 60°C water bath to evaporate to dryness. The samples were dried in a drying oven at 80°C for 12h, and finally placed in a muffle furnace, and calcined at 500°C for 3h to obtain Fe- PCH catalysts. The iron loading of the Fe-PCH catalyst was 8.4 wt%. .

Characterization of Basic Physicochemical Properties

The microscopic morphological characteristics of the catalysts were observed by transmission electron microscopy (TEM) (model JEM-2100F) with an accelerating voltage of 100 kV. The microporous characteristics of the catalyst were characterized by N2 adsorption-desorption. The instrument used the ASAP-2460 adsorption instrument of Micromeritics. The detection results were calculated by BET equation and BJH model to obtain the specific surface area, pore volume and average pore size. The valence state distribution of each element on the catalyst surface can be analyzed by X-ray photoelectron spectroscopy (XPS) using a Thermo Escalab 250Xi spectrometer with Al-Kα excitation light source.

SCR-C3H6 Activity

A fixed-bed microreactor was used to evaluate the SCR performance, and a Fourier transform infrared spectrometer (Thermo Nicolet IS10, thermoFisher, USA) was used to monitor the gas components online. The reaction gas composition is 0.1% C3H6, 0.1% NO, 0~10% O2, N2 balance, total flow 100ml/min. The diameter of the fixed bed reactor is 6 mm, the catalyst dosage is 400 mg, and the corresponding GHSV space velocity is 15000 h-1. Before the start of the experiment, the residual air in the catalyst pores was removed by pretreatment in N2 atmosphere at 300°C for 1 h.

The NO conversion and N2 selectivity were calculated as follows:

[ ] -[ ] (%) 100% [ ]

$$ NO conversion (\%) = \frac {[ NO ] _ {in} - [ NO ] _ {out}}{[ NO ] _ {in}} \times 1 $$ in out in [ ] [ ] [ ] [ ] [ ] [ ]

$$ N _ {2} \text {selectivity} (\%) = \frac {\left[ N O \right] _ {i n} - \left[ N O \right] _ {o u t} - \left[ N O _ {2} \right] _ {o u t} - 2 \left[ N _ {2} O \right] _ {o u t}}{\left[ N O \right] _ {i n} - \left[ N O \right]} \times 100 \% $$ NO NO NO N O N selectivity NO NO $$ \frac {i _ {1} - \left[ N O \right] _ {o u t} - \left[ N O _ {2} \right] _ {o u t} - 2 \left[ N _ {2} O \right] _ {o u t}}{\left[ N O \right] _ {i n} - \left[ N O \right] _ {o u t}} \times 1 $$ in out Where [NO], [NO2] and [N2O] are the volume fraction of the NO, NO2 and N2O respectively, the subscripts ‘in’ and ‘out’ stands for the values at inlet and outlet of the reactor respectively, ppm or %.

In situ DRIFTS studies

The intermediates generated during the reaction were investigated in situ by diffuse reflectance Fourier transform infrared spectrometer (DRIFTS) (Nicolet IS 50). Before the test, 50 mg of the catalyst sample was placed in the in-situ cell of the DRIFTS spectrometer and pretreated for 1 h at 500°C in an N2 atmosphere to remove impurities adsorbed on the catalyst surface. During the experiment, the components of the reacting gas introduced were 0.1% C3H6, 0.1% NO and different concentrations of O2, N2 to be balanced, the total gas flow was 20 mL/min, and the reaction space velocity was 18000 h-1.

C3H6-SCR Catalytic Reactivity

Figure 1 shows the test results of the C3H6-SCR catalytic activity on the Fe-PCH catalyst surface under different oxygen concentrations. As reported by Valverde, et al. [18], the critical oxygen concentration ([O2]crit) can be defined as the minimum oxygen concentration required for complete oxidation of propylene. It can be seen from Figure 1b that [O2]crit is about 0.8% under the conditions studied in this paper.

Click to enlarge

As shown in Figure 1a, in the absence of oxygen supply, the Fe-PCH catalyst exhibits the best catalytic activity, and the NO conversion rate can reach 100% at 400 °C. When the oxygen concentration is increased in the range lower than [O2]crit, the catalytic activity gradually decreases, but the maximum NO conversion rate can still reach 100%. When the oxygen concentration is higher than [O2]crit, the selective catalytic reduction of NO activity on the catalyst surface drops sharply, and the maximum NO conversion rate drops to 20%-30%. However, at this time, the conversion rate of C3H6 has reached more than 90% at 350°C. This indicates that on the surface of Fe-PCH catalyst, propylene is preferentially combusted with O2 to be consumed. When the oxygen concentration is lower than [O2]crit, the remaining propylene can further react with NO to generate N2. However, when the oxygen concentration is higher than [O2]crit, only a small amount of C3H6 involves in the reduction reaction with NO. In addition, it can be seen from Figure 1c that with the increase of oxygen concentration, the N2 selectivity gradually decreases, which is due to the oxidation of unreduced NO2 by O2 to generate a large amount of NO2. In the range of 200~550°C, no by-product N2O was detected.

Click to enlarge

TEM Characterization

Transmission electron microscopy (TEM) images can clearly observe the microscopic features of the catalyst surface. As shown in Figure 2, the Fe-PCH catalyst has a lamellar structure, and a three-dimensional pore structure is formed between the layers after being pillared by organic intercalation. The α-Fe2O3 nanorods supported on the catalyst surface are the main active species in the process of C3H6-SCR catalytic reaction, and the crystal structure of this species corresponds to JCPDS card 33-0664. Its high-resolution image HRTEM (Figure 2c) shows that the interplanar spacings of the α-Fe2O3 crystal surface are 0.185 nm and 0.272 nm, respectively, and the electron diffraction pattern is selected to analyze SADP (Figure 2d) on the (024) and (104) crystal planes, respectively. The exposed (104) crystal face of the α-Fe2O3 nanorod crystal surface has a higher +3 valence Fe atomic density [20], which is conducive to the chemical adsorption of NO molecules [21], thereby improving the SCR reactivity.

N2 Adsorption/Desorption

The pore characteristics of the catalysts can be detected by N2 adsorption/desorption. The N2 adsorption/ desorption curves of the Fe-PCH catalyst sample and the original montmorillonite are shown in Figure 3, and the corresponding specific surface area, pore volume and pore size results are shown in Table 1.

The original N2 adsorption capacity and specific surface area of montmorillonite were lower, and its N2 adsorption isotherm belonged to IV type, and the hysteresis loop corresponds to H4 type. This shows that the mesoporous structure in the original montmorillonite is mainly formed by the irregular accumulation of lamellae [22]. The Fe- PCH catalyst has a large amount of N2 adsorption, the adsorption isotherm belongs to the IV type, the hysteresis loop corresponds to the H3 type, and the corresponding specific surface area and pore volume value increase significantly. This is because the silica column is between the montmorillonite layers. Column support, so as to form an open cylindrical channel structure in the interlayer slot corridor area [1].

On the other hand, the average pore size of Fe-PCH material decreases, indicating that the pore structure of montmorillonite becomes regular and orderly after organic intercalation pillaring, which is conducive to the uniform dispersion of active components on the catalyst surface, thereby exposing more multiple active sites. In addition, in the range of P/P0=0.05-0.2, the N2 adsorption curve of Fe- PCH catalyst gradually increased, indicating that the material contains ultra-micropore and small mesoporous structure [16], which is helpful for the physical properties of reacting gas molecules adsorption process.

The XPS spectrum of O 1s in Figure 4b can be fitted to two sub-peaks. The sub-peak located at the binding energy of 529.98 eV represents the lattice oxygen O2- (denoted as Oα), and the sub-peak located at 531.98 eV corresponds to

XPS Characterization

XPS characterization can further analyze the element valence and chemical properties of the catalyst surface active components. The XPS spectrum of Fe element on the surface of Fe-PCH catalyst and its peak fitting analysis results are shown in Figure 4a. Among them, the fitting separation peaks of Fe 2p3/2 peak are located at 710.88eV, 713.08eV and 714.88eV, respectively, and the satellite peak of Fe 2p3/2 is located at 718.18eV, which is consistent with the peak separation results of α-Fe2O3 species in the literature Li WG, et al. [23].

O 1s b)

Oβ 531.98eV

Intensity (a.u.)

O 1s

Oα 529.98eV

538 536 534 532 530 528 526

Binding Energy (eV)

the adsorbed oxygen species (denoted as Oβ). The lattice oxygen species Oα is beneficial to the activation of C3H6 on the catalyst surface [24], thereby improving the SCR activity of the catalyst. However, the adsorbed oxygen species Oβ will promote the complete oxidation of C3H6 by O2 to CO2 [25].

In situ DRIFTS Spectra

The in-situ DRIFTS technique can be used to detect the adsorbed species and reaction intermediates on the catalyst surface during the reaction. Combined with the test results of C3H6-SCR performance of Fe-PCH catalysts under different oxygen concentrations, three conditions of no oxygen, lower than [O2]crit (0.5% O2) and higher than [O2]crit (2% O2) were selected to study the adsorption species of C3H6 and NO on Fe-PCH catalyst surface. The reaction temperature is 300°C. Figure 5 shows the DRIFTS spectra of C3H6 adsorbed species on the Fe-PCH catalyst surface under different oxygen concentrations.

Click to enlarge

As shown in Figure 5, the characteristic peaks at 3100- 2800 cm-1 correspond to the asymmetric stretching of C-H bonds [26]. The characteristic peak around 1650cm-1 corresponds to the stretching vibration of C=C bond [27]. The characteristic peak near 1550cm-1 corresponds to the C-H bond vibration [27]. In addition, the characteristic peak at 2360cm-1 broadband represents CO2 species [28]. At the beginning of the adsorption process (0.5min), the spectra were all in a horizontal state, and there were no obvious characteristic peaks except for a small amount of CO2 gas. With the progress of the adsorption process, the intensity of the characteristic peaks of the adsorbed species on the catalyst surface gradually increased. It can be found from the figure that with the increase of oxygen concentration, the characteristic peak intensity at the broadband of 3100- 2800 cm-1 gradually increases, which indicates that the addition of O2 is beneficial to the activation process of C3H6 on the surface of Fe-PCH catalyst. According to the interfacial mechanism of hydrocarbon-catalyzed oxidation (Mars-van- Krevelen), lattice oxygen O2- species on the surface of Fe-PCH catalyst can promote the activation process of propylene. The active lattice oxygen is consumed by hydrocarbons to form anion vacancies, and then the process of regenerating lattice oxygen by adsorption of gaseous O2 on the surface.

Figure 6 shows the DRIFTS spectra of C3H6+NO adsorbed species on the Fe-PCH catalyst surface under different oxygen concentrations. The catalyst was first adsorbed in the C3H6 atmosphere with different oxygen concentrations for 30 min, and then NO was introduced. It can be found from the figure that a distinct characteristic peak appears at 2235 cm-1, which corresponds to the important intermediate isocyanate species (R-NCO) in the C3H6-SCR reaction [24]. As the oxygen concentration increased, the characteristic peak intensity of the R-NCO species gradually weakened, indicating that the formation of this species was inhibited.

a)

0.05 a.u.

1639 Figure 6: In situ DRIFTS results of C3H6+NO adsorbed species under (a) without oxygen; (b) 0.5% O2; (c) 2% O2 over Fe-PCH catalyst at 300°C. Reactive condition: 0.1% C3H6, 0.1% NO, N2 balance and GHSV=18000h-1.

The effect of oxygen concentration on the C3H6-SCR activity on the Fe-PCH catalyst surface can be further analyzed in combination with the in-situ DRIFTS spectra of the C3H6 + NO + O2 adsorbed species, as shown in Figure 7.

The characteristic peaks at 1588cm-1 and 1439cm-

1 correspond to CxHyOz and acetate (CH3COO-) species, respectively [26, 27]. The characteristic peak at 1676cm-1 corresponds to the nitrogen-containing organic intermediate (R-NO2) [27]. In addition, the characteristic peak at 3658 cm-1 may be generated by the shear bending vibration of the O-H bond [28]. The redox mechanism of selective catalytic reduction of NO by hydrocarbons is that hydrocarbons are activated and oxidized on the catalyst surface to CxHyOz(ads) (or acetate) species, and then react with NOx(ads) (or nitrate) species to form nitrogen-containing species Organic intermediates, which are converted into species such as isocyanates on the catalyst surface, and finally obtain the products N2, CO2 and H2O. Oxygen inhibits the formation of isocyanate species on the surface of Fe-PCH catalysts, thereby reducing their catalytic activity for C3H6-SCR. Reaction Mechanism Oxygen is required to activate the oxidation of hydrocarbons in the reaction pathway of the redox mechanism of HC-SCR. It was found that the pillared clay catalyst with isolated Cu2+ as the active center has almost no HC-SCR catalytic activity under anaerobic conditions [18]. However, Fe-PCH catalysts showed better C3H6-SCR catalytic performance under anaerobic conditions, and Zhou [24], Belver [10] also found similar phenomena. Therefore, the process of adsorption and decomposition of NO should still exist on the surface of Fe-PCH catalyst.

The reaction mechanism is shown in Figure 8. In the adsorption decomposition path [29], NO is decomposed into adsorbed nitrogen atoms (Nad) and adsorbed oxygen atoms (Oad) on the surface of Fe-PCH catalyst, and the two Nad can combine to generate N2. Part of Oad can oxidize C3H6 to generate C_x_H_y_O_z_ or CO2 and H2O, and Oad can also generate new lattice oxygen through oxygen migration to participate in the redox pathway of SCR.

2. More PM (2017) Effect of active component addition and support modification on catalytic activity of Ag/ Al2O3 for the selective catalytic reduction of NOx by hydrocarbon – a review. J Environ Manage 188(1): 43- 48.

3. Yang RT, Li WB (1995) Ion-exchanged pillared clays: a new class of catalysts for selective catalytic reduction of NO by hydrocarbons and by ammonia. J Catal 155(2): 414-417.

Click to enlarge

1. Under anaerobic conditions, the catalytic performance is the best, and the NO conversion rate can reach 100% at 400 °C. When the oxygen concentration is lower than [O2]crit, the catalytic activity of Fe-PCH gradually decreases with the increase of oxygen concentration. When the oxygen concentration is higher than [O2]crit, the catalytic performance drops sharply.
2. The characterization results of TEM, BET and XPS show that the Fe-PCH catalyst has a relatively regular pore structure, the α-Fe2O3 nanorods supported on the surface are the main active components, and the exposed (