MnO₂ doped CeO₂ with tailored 3-D channels exhibits excellent performance for NH₃-SCR of NO

Abstract

Diffusion behaviour during NO conversion by selective catalytic reduction by NH₃ (NH₃-SCR) was simulated by a molecular dynamics method. MnCeO_x with tailored 3-D channels was obtained from a KIT-6 template. The remarkable performance of the NH₃-SCR results from a large surface area for chemisorption and appropriately sized channels for mass diffusion.

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Introduction

NO_x are unavoidable during the combustion of fossil fuels. Unfortunately, the world will continue to rely on fossil fuels as the primary energy source for a long time. It is well known that selective catalytic reduction (SCR) is one of the most effective methods for NO removal that has been proven practically.¹ Commercial catalysts such as V₂O₅/TiO₂ and V₂O₅–WO₃/TiO₂ have been frequently employed due to their high activity for NO_x removal.^2,3 However they also suffer from contamination by SO₂ and fly ash. A simple solution is placing a SCR system following the de-sulfurizer, with a temperature range of 80 °C–175 °C. Therefore improving low temperature activity in SCR systems is a continuous goal. Manganese based catalysts exhibit high activities at low temperatures which has been well proven.^4–32 NO conversion over Mn_xCo_3−xO₄ nano-cages with a surface area of 77 m² g⁻¹ was about 90% at 125 °C while it was about 40% over nano-particle Mn_xCo_3−xO₄ with areas of 33 m² g⁻¹ under the same conditions as reported by Lei Zhang et al.²⁴ The highest activity for SCR was present on Ni(0.4)–MnO_x (400 °C) which owned the largest surface area (90.5 m² g⁻¹) in the series of catalysts prepared by Yaping Wan et al.²⁵ It is evident that surface area plays an important role in SCR. MnO_x supported on porous materials (nano-tubes, activated carbon, molecular sieves and silicon materials etc.) to enhance the surface area have been well studied.^5,6,26 A Mn–Ni(0.4)/TiO₂ catalyst reported by B. Thirupathi and Panagiotis G. Smirniotis showed a remarkable performance by exhibiting 100% NO conversion at 200 °C with a GHSV of 50 000 h⁻¹.²⁷ Mn/FER, Mn/Mont-10, Mn/ETS-10 and Mn/TiO₂ were prepared by Asima Sultana et al.²⁸ on which NO conversions were about 90% at a temperature range from 160 °C to 400 °C. MnCe@CNTs-R and Mn(25%)Ce(28%)/CNTs-S prepared by Dengsong Zhang et al.^9,29 exhibited the best activities at about 220 °C with NO conversion levels up to 95%. Tae Sung Park et al.³⁰ investigated the activity of MnO₂/γ-Al₂O₃ with a surface area of 243 m² g⁻¹ for NH₃-SCR on which NO conversion leveled up to 100% across the temperature range of 150 °C to 250 °C.

Another factor of the same importance as surface area is pore size. The pore size of MnO₂ doped Fe₂O₃ hollow nano-fibers has been successfully enlarged to 6–8 nm by using an electrospinning method which exhibited nearly 100% NO conversion from 150 °C to 300 °C.³¹ By using Pluronic F127 as a pore agent, a series of hierarchical Mn/TiO₂ catalysts have been synthesized by Yanni Shi et al., among which HM(0.012)–Mn/TiO₂ with a surface area of 112 m² g⁻¹ and macro-meso pores exhibited an NO conversion of 100% at 100 °C.³² It is obvious that the pore size has a great influence on SCR reactions. However, to the best of our knowledge, the mechanism is still unclear.

Molecular dynamics simulations

During the SCR process NO is converted to N₂ as described by the following equation:

4NH₃ + 4NO + O₂ → 4N₂ + 6H₂O (1)

Compared with the reactants and other products, the produced water has difficulty diffusing, especially in the micro-pores as shown in Fig. 1(a), which results from strong hydrogen bonds combined with the superficial oxygen on the catalysts.

Fig. 1 (a) Water diffusion in a slit pore; (b) water diffusion coefficients for different sized pores.

Increasing the pore size appropriately can ameliorate diffusion. However a large pore size generally results in a low space efficiency which is not what we want. So water diffusion behaviour in slits with different sizes was simulated. Diffusion coefficients were calculated from mean square displacement (MSD) curves shown in Fig. S1 (ESI†) according to the Einstein equation. The details are described in the ESI and the results are listed in Table S1.† It was found that the diffusion coefficient increased with an increase of the slit size then at a slower rate when the slit size was larger than 4 nm (Fig. 1(b)). In slits >4 nm the hydrogen bonds mainly come from a single surface of MnO₂. Adsorptive interactions from the opposite side are insufficient. The probability of collision with the opposite side is considerably low. Diffusion in slits larger than 4 nm is similar to that on an open surface. As a result, a slit size of 4 nm is large enough for water diffusion during the SCR reaction.

Experimental

According to the simulation results, a Mn based catalyst with a high surface area and tailored three dimensional channels was prepared by using KIT-6 as a template and applied in denitrification for the first time. A N₂ isotherm at 77 K was collected for surface area and pore size calculations. Powder X-ray diffraction (PXRD), Transmission Electron Microscopy (TEM), Energy Dispersive Spectrometry (EDS) and X-ray Photoelectron Spectroscopy (XPS) were performed for structure and component analysis. The low temperature activities for SCR of NO were tested. Effects of temperature and gas hourly space velocities (GHSV) were present.

All reagents were of analytical grade and purchased from Alfa Aesar. Deionized water was used for the catalyst synthesis. KIT-6 was prepared as the template with a BET surface area of 888 m² g⁻¹ according ref. 33. A 77 K N₂ isotherm on KIT-6 is presented in Fig. S2 (ESI†) and the pore distribution calculated by the BJH method is shown in Fig. S3 (ESI†).

The catalysts (MnCeO_x) were synthesized by dissolving the corresponding amounts of Ce(NO₃)·6H₂O (0.002 mol) and Mn(AC)₂·4H₂O (0.006 mol) in 30 ml of deionized water, followed by adding 2 g of KIT-6 as the template. The resulting slurry was stirred at 80 °C until dry. The pale pink powders were finally calcined at 400 °C for 4 hours in ambient air. After cooling to room temperature the dark brown powder was soaked in 100 ml NaOH solution (4 mol L⁻¹) with constant stirring for 20 min at 70 °C for template removal. It was subsequently filtered and washed with deionized water until the pH value reached 7. The obtained solids were dried overnight at 120 °C under vacuum and crushed to 60–80 mesh for use in the experiments.

The equipment used to measure the catalytic activities is shown in Scheme S2 (ESI†). The mixture gas continuous flow stainless steel reactor (i.d. = 4 mm) had a volume of 0.47 ml and 300 mg catalyst (60–80 mesh) was compacted in the reactor for each test. The NO, NO_x and NH₃ concentration of the inlet and outlet gases were measured using an FTIR spectrometer Model QGS-08C purchased from Beijing BAIF-Maihak Analytical Instrument Co. Ltd. All the feed gases were purchased from Beijing AP BAIF Gas Industry Co. Ltd. (Beijing, China) (1000 ppm NO, 1000 ppm NH₃ and 21% O₂ all balanced by N₂). Each flow rate was controlled by mass flow controllers, Model SY9312 purchased from Beijing Shengye Sci. & Tech. Dev. Co. The mixed gas used in the experiment consisted of NO (0.04%), NH₃ (0.04%) and O₂ (4%) balanced by N₂.

Results and discussion

A N₂ isotherm at 77 was collected using a Surface Area Analyzer Micromeritics (ASAP 2020) which is shown in Fig. 2(a). A surface area of 340 m² g⁻¹ and a pore volume of 0.45 cm³ g⁻¹ were calculated using the BET method and a N₂ uptake at p/p⁰ (0.995) respectively. The pore size is centred around 4.8 nm as calculated using the BJH method.

Fig. 2 (a) N₂ isotherm of the catalyst at 77 K (b) PXRD pattern of the catalyst; (c) TEM image of the catalyst; (d) EDX mapping of the catalyst.

PXRD patterns were recorded using a Rigaku D/max 2250VB/PC diffractometer (Rigaku, Japan) with CuKα radiation (λ = 1.5406 Å). Diffraction peaks of MnO_x and CeO₂ can be observed in Fig. 2(b). The wide half peak width illustrates that the structure of the catalyst is almost amorphous. Three dimensional porous structures were proven by the TEM image (Fig. 2(c)). Mn and Ce were well dispersed in the catalyst which has been proven by the EDX mapping image (Fig. 2(d)).

XPS analysis was performed using an ESCALAB 250 multi-technique X-ray photoelectron spectrometer (UK) using a monochromatic AlKα X-ray source (hν = 1486.6 eV). All XPS spectra (Fig. S4, ESI†) were recorded using an aperture slot of 300 × 700 microns, and survey spectra were recorded with a pass energy of 160 eV, and high resolution spectra with a pass energy of 29.35 eV. 20 scans per region were taken with a step size of 0.25 eV. Two main peaks, shown in Fig. 3(a), can be observed from 642 eV to 660 eV due to Mn 2p3/2 and Mn 2p1/2. By performing a peak-fitting deconvolution, the Mn 2p3/2 spectrum was separated into two peaks, Mn³⁺ (640 eV) and Mn⁴⁺ (641.6 eV). It was also observed from the XRD results. As shown in Fig. 3(b), complicated satellite structures were observed in the Ce3d XPS spectrum. The Ce3d XPS peaks are labelled for identification. The results indicate that the cerium in the catalyst is present in both the Ce⁴⁺ and Ce³⁺ oxidation states.^34,35 The ratio of Ce/Mn on the surface was approximately 0.26. It is noteworthy that the O content on the surface is about 36.4% (ESI†) which is more than the theoretical O content in the lattice of the MnCeO_x. The increased content of oxygen may be attributed to chemisorbed oxygen on the surface which is beneficial to the oxidation process during the NH₃-SCR.^41,42

Fig. 3 Mn (a) and Ce (b) XPS spectra of the meso-porous catalyst.

Catalyst performance was tested at a temperature range as low as 50 °C to 175 °C. Presented in Fig. 4 the NO conversion yield was 60% at 75 °C. With raising of the temperature from 30 °C to 100 °C the NO conversion level increased from 17.4% to 100%. NO conversion was kept at a high value of 100% when the temperature was greater than 100 °C with a GHSV of 89 000 h⁻¹. Below 80 °C the produced water has difficulty diffusing and occupies the active sites on MnCeO_x thereby restricting the reaction progress. Above 100 °C the metallic surface can be refreshed in time because the adsorptive water molecules overcome the constraints and disperse from the pore. On the other hand, the molecules were supplied with enough energy for overcoming the active energy barrier with an increase in the temperature. From the view of the reaction kinetics and diffusion, a high temperature is beneficial for NO conversion.

Fig. 4 The effect of temperature on NO conversion with GHSV 8.9 × 10⁴ h⁻¹.

GHSV is an important criterion to estimate catalytic activity. Under a relatively low GHSV, the molecules contact the surface intensively resulting in complete conversion. With an increase of GHSV NO conversion decreases (Fig. 5(a)) which results from restriction of the diffusion rate and the chemical reaction rate. By increasing the temperature the molecules were supplied enough energy to overcome the active energy barrier, resulting in an enhancement of the collision frequency. Gas molecules can be adsorbed on the active sites and dispersed from the inner pores immediately at high temperatures even under a high GHSV.

Fig. 5 (a) The effect of GHSV on NO conversion; (b) comparison of GHSV with other catalysts.^{9,10,31,36–40}

Targeting a NO conversion at 95% the maximum value of GHSV has been presented in Fig. 5(b) and compared with reported results. It is evident that the GHSV in the present work is much higher than others under the same conditions due to the high surface area (340 m² g⁻¹) and probably the channel size (4.8 nm). The maximum value of GHSV was 480 000 h⁻¹ at 170 °C and 89 000 h⁻¹ at 100 °C.

Insight into the kinetics was sought. The NH₃-SCR on the MnO_x–CeO₂ was generally considered to be a first-order reaction with respect to NO.¹² The effective first-order rate constant is related to NO conversion (η_NO) by the following equation:

where k is the first-order rate constant (cm³ g⁻¹ min⁻¹) of the reaction, F is the gas flow rate (cm³ min⁻¹) and A_f is the catalyst surface area (cm² g⁻¹).

According to the Arrhenius formula there are the following relations

ln(−ln(1 − η_NO)) = ln(P/A_f) − E_a/RT (4)

where E_a is the apparent activation energy (kJ mol⁻¹), R is the gas constant (8.314 J mol⁻¹ K⁻¹), T is the temperature (K) and P is the pre-exponential factor. The apparent activation energy and pre-exponential factor can be calculated from the slope and intercept of the plots of ln(−ln(1 − η)) vs 1/T. NO conversions of MnCeO_x (300 mg) were determined with a GHSV of 656 500 h⁻¹ across a temperature range of 75 °C to 175 °C for E_a calculation. The results are listed in Table S2 (ESI†).

The plots of ln(−ln(1 − η_NO)) vs. 1/T are shown in Fig. 6. According to the difference of the slope the plots could be divided into a low temperature section (75 °C–125 °C) and a high temperature section (125 °C–175 °C). The apparent activation energy and pre-exponential factor of each section are yield summed in Table S3 (ESI†).

Fig. 6 Arrhenius plots of NO conversion.

The low E_a value and low pre-exponential factor demonstrate that the SCR reaction rate is controlled by diffusion at high temperature. Gas molecules must diffuse into the meso-pore first then adsorb on the surface of the catalyst before conversions happen. The produced water can not depart from the surface immediately and occupy the active sites leading to the reduction of the reaction rate. While a high temperature is beneficial for diffusion and enhancing collision frequency, both of them can improve the reaction rate and NO conversion. The pre-exponential factor increased sharply with an increase in temperature which supported the above conclusions.

Conclusions

In summary, water diffusion behaviour on MnO₂ was simulated by a molecular dynamics method. An optimal slit size of 4 nm was obtained by analysis. By using a template method MnCeO_x with tailored 3-D channels was successfully synthesized. A significant improvement of low-temperature NH₃-SCR activity was shown on MnCeO_x with a surface area of 340 m² g⁻¹ and a 3-D porous structure. NO conversion can reach above 95% at a temperature as low as 100 °C with a GHSV of 89 000 h⁻¹ or at a temperature of 170 °C with a high GHSV of 480 000 h⁻¹. Kinetic analysis demonstrated that the SCR reaction was controlled by diffusion above 125 °C. The catalyst designed for NO conversion in this study is quite promising in overcoming the current bottleneck of SCR technology.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21206108 and 21406004) and the Tian Jin Natural Science Foundation (10JCZDJC23900).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra01355k

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