Low-temperature selective catalytic reduction of NO with NH₃ over Ni–Mn–O_x catalysts

Abstract

A series of manganese–nickel oxide catalysts with different Ni/Mn ratios were prepared using a hard template method with KIT-6 as the template. These mesoporous nanomaterials were characterized using Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy and X-ray photoelectron spectroscopy (XPS). The low temperature selective catalytic reduction (SCR) of NO with NH₃ in the presence of excess O₂ was investigated. At a gas hourly space velocity (GHSV) of 38 000 h⁻¹, the Ni(1 : 3)–MnO_x (Ni/Mn = 1 : 3) catalyst had the highest activity, giving an NO conversion of 81.3% at 60 °C and 100% from 100 to 220 °C. The Ni(1 : 3)–MnO_x catalyst had high N₂ selectivity, excellent stability, good H₂O resistance and functioned well at various GHSVs. The BET, XRD and XPS results showed that the Ni(1 : 3)–MnO_x catalysts had large surface areas (187 m² g⁻¹) and that a more amorphous phase was formed as the nickel content was increased. The Mn⁴⁺ and chemisorbed oxygen are the main active species for the SCR reaction. These properties are desirable for low-temperature SCR catalysts.

---

1. Introduction

Nitrogen oxide (NO_x), which is emitted from the combustion of fossil fuels, is a major atmospheric pollutant which contributes to acid rain, photochemical smog, and haze.^1,2 At present, many methods including selective noncatalytic reduction (SNCR),^3,4 selective catalytic reduction (SCR),⁵ and plasma storage-reduction,⁶ have been employed to deal with these environmental problems. Among these methods, the selective catalytic reduction of NO with NH₃ (NH₃-SCR) is one of the most promising methods due to the production of only environmentally benign products N₂ and H₂O.^7,8

The commercial catalyst V₂O₅–WO₃(MoO₃)/TiO₂ has been widely used for the reductive elimination of NO_x.^5,7,9 However, this catalyst is susceptible to deactivation and vanadium is volatile and toxic. Moreover, this catalyst is only active within a narrow temperature window (300–400 °C) and it has a poor activity at low-temperatures.^10,11 To meet the requirements of high SCR temperature range, SCR reactors are always located upstream of the desulfurizer and electrostatic precipitator, which can cause some problems, such as the possible oxidation of SO₂ to SO₃ and the deactivation of the catalysts due to the deposition of dust.¹² Thus, it is important to develop highly efficient low temperature SCR catalysts for power plants which can be placed downstream of the desulfurizer and electrostatic precipitator.¹³

The NH₃-SCR activity of many transition metal mixed oxides, such as NiCo₂O₄,¹ Fe₂O₃–CeO₂,¹⁴ Fe₂O₃–TiO₂,¹⁵ and Fe₂O₃–CeO₂/TiO₂,¹⁶ have been investigated. However, the low temperature SCR of NO by these catalysts is not satisfactory. In contrast, many manganese-based catalysts including MnO_x–TiO₂,¹⁷ Fe–MnO_x,¹⁸ Cr–MnO_x,¹⁹ Cu–MnO_x,²⁰ MnO_x–CeO₂ (ref. 21) and Ce–Mn/TiO₂ (ref. 22 and 23) are highly active for the low temperature SCR of NO with NH₃. Moreover, Qiu et al. reported that NO conversion over 3D-MnCo₂O₄ with a surface area of 92.9 m² g⁻¹ was about 98% at 100 °C and was about 83% over 2D-MnCo₂O₄ with an area of 69.7 m² g⁻¹ under the same conditions.²⁴ Wei et al. reported that NO conversion of MnCeO_x with a surface area of 340 m² g⁻¹ and 3-D porous structure 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⁻¹.²⁵ Liu et al. reported that nearly 100% NO_x conversion was obtained over the Mn–Ce catalysts prepared by the surfactant-template method in the temperature range of 100–200 °C.²⁶ Zuo et al. found that a Mn(0.5)–ZrO_x-450 (Mn/(Mn + Zr) = 0.5) catalyst gave 100% NO_x conversion at 100 °C at a space velocity of 30 000 h⁻¹.¹³ Wan et al. prepared a series of catalysts and reported that the Ni(0.4)–MnO_x (400 °C) catalyst with the largest surface area (90.5 m² g⁻¹) had the highest SCR activity.¹² They also found that the catalytic activity of MnO_x was enhanced by the addition of nickel which was due to a synergetic catalytic effect. Thirupathi et al. prepared a series of Mn–Ni/TiO₂ catalysts by adopting incipient wetness technique and found that 100% of NO conversion with 100% N₂ selectivity was obtained over the Mn–Ni(0.4)/TiO₂ catalyst at 200 °C.²⁷

The excellent SCR activities of manganese-based transition metal catalysts may be attributed to the following reasons. Transition metal elements have multiple valences states which facilitate the transfer of electrons and the storage of oxygen in catalytic reactions, and the dope of metal elements can also reduce sintering in preparation processes.²⁸ Hence, the thermal stability and longevity of mesoporous transition metal oxides are improved and relatively high SCR activities were obtained.

Mn–Ni mixed oxides have been reported to be suitable catalysts for de-NO_x.¹² However, there were few reports about the low temperature NH₃-SCR of Mn–Ni catalysts prepared by hard template method previously. Therefore, in this work, a series of Ni–Mn–O_x catalysts with different molar ratios of Ni/Mn were synthesized via a hard template method. The catalysts were characterized using Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The effects of the Ni/Mn molar ratios and different gas hourly space velocities (GHSVs) on the low temperature NH₃-SCR activities of the catalysts were investigated. In addition the stability and H₂O resistance of the catalysts were studied.

2. Experimental section

2.1 Catalysts preparation

Mesoporous Ni–Mn–O_x was prepared via a hard template method using 3D porous KIT-6 as the hard template and ethanol as the dispersing agent. The template KIT-6 was synthesized according to previously reported studies.²⁹ Briefly, given amounts of Ni(NO₃)₂·6H₂O and Mn(AC)₂·4H₂O with the desired molar ratios (Ni/Mn = 0, 1 : 6, 1 : 3, 1 : 1) were dissolved in ethanol (20 mL). The solution was then added to the flask containing KIT-6 which had been evacuated at 120 °C. The amount of template was calculated using the pole volume of KIT-6 and the molar mass and density of the precursors. After removing the vacuum, the suspension was dried at 70 °C and then the product was calcined in a tube furnace at 400 °C for 4 h with a heating rate of 2 °C min⁻¹. Finally, the silica host was removed by soaking in 100 mL of 2 M NaOH at 80 °C with stirring for 1.5 h. The resulting sample was centrifuged and washed several times with deionized water and absolute ethanol and then dried at 80 °C. The composite catalysts were denoted as Ni(n)–MnO_x (where “n” represents the molar ratio of Ni/Mn, n = 0, 1 : 6, 1 : 3, or 1 : 1).

2.2 Characterization

The catalysts were characterized using N₂ adsorption–desorption isotherms at 77 K. The specific surface areas were calculated by the BET method, and the pore size distributions were derived from the adsorption branches of the isotherms, using the Barrett–Joyner–Halenda (BJH) model.

XRD patterns were obtained using a Rigaku D/MAX 2500v/PC instrument with Cu Kα radiation (λ = 1.5406 Å). The scanning range was 10° to 90° at a scanning speed of 10° min⁻¹.

TEM images were collected using a FEI Tecnai G2 F20 instrument operated at 200 KV. SEM images were acquired with a Hitachi S-4800 field emission scanning electron microscope.

XPS analyses were carried out on an ESCALAB 250 multi-technique X-ray photoelectron spectrometer (UK) using a monochromatic AlKα X-ray source (1486.6 eV) radiation as the excitation source. The binding energies were corrected by referencing the C 1s peak at 284.6 eV.

2.3 SCR activity measurements

The equipment used to measure the SCR activities is shown in Fig. 1. The low-temperature NH₃-SCR activity measurements were carried out in a stainless steel reactor (i.d. = 4 mm) in the temperature range of 60–220 °C. Prior to being placed into the reactor, 100 mg of catalyst was sieved with 60–80 mesh. The reactant gas was made up of 1000 ppm NO, 1000 ppm NH₃, 4% O₂ and 10% H₂O (when employed) with a balance of N₂. The total flow rate was varied between 120 and 850 mL min⁻¹, which correspond to GHSVs of about 38 000 to 270 000 h⁻¹ respectively. The concentrations of reactants and products were measured by an FTIR spectrometer (Model QGS/08C, Beijing BAIF-Maihak Analytical Instrument Co. Ltd). All catalysts were kept on stream until a steady state was obtained (30–40 min). The NO conversion and N₂ selectivity were calculated from the following equation:

where “in” and “out” represent the inlet and outlet concentrations at steady state, respectively.

Fig. 1 SCR apparatus. (a) two-way value; (b) mass flow controller; (c) three-way valve; (d) reacting furnace; (e) desiccant; (f) temperature controller; (g) NO analyzer.

3. Results and discussion

3.1 Characterizations of catalysts

3.1.1 BET analysis. The N₂ adsorption–desorption isotherms and the pore size distribution of the Ni(n)–MnO_x catalysts are shown in Fig. 2. The surface area and pore data are given in Table 1. As displayed in Fig. 2(A), the isotherms of Ni(n)–MnO_x are type IV adsorption curves with H3-type hysteresis loops, indicating the existence of mesoporous structures.³⁰ Fig. 2(B) shows obvious mesoporous peak for the Ni(n)–MnO_x catalysts and this peak is the most distinct for Ni(1 : 3)–MnO_x. Table 1 show that the surface area of the catalysts increased as the content of nickel increased, so the surface area of the pure MnO_x was much lower than those of the mixed oxides. This indicates that adsorptive centers were created by the addition of nickel. From Ni(1 : 6)–MnO_x to Ni(1 : 1)–MnO_x, the pore volumes also increased slightly with increasing Ni content, however, the pore volume of pure MnO_x was between the pore volume of Ni(1 : 3)–MnO_x and that of Ni(1 : 1)–MnO_x.

Fig. 2 (A) N₂ adsorption–desorption isotherms and (B) pore size distributions of catalysts with different Ni/Mn molar ratios.

Table 1 Surface area and pore data for the catalysts with different Ni/Mn molar ratios

Catalyst	BET surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Average pore diameter (nm)
MnOx	120	0.237	3
Ni(1 : 6)–MnOx	129	0.218	2.5
Ni(1 : 3)–MnOx	187	0.220	3
Ni(1 : 1)–MnOx	229	0.270	2

---

3.1.2 XRD analysis. The XRD patterns of the pure MnO_x and the mixed oxide catalysts with various ratios of Ni/Mn are presented in Fig. 3. The XRD pattern of pure MnO_x features intense sharp peaks which correspond to Mn₃O₄ (PDF card 75-1560). These peaks are indicative of high crystallinity. When Ni was added, the MnO_x diffraction peaks gradually weakened with increasing Ni content and almost disappeared in the Ni(1 : 1)–MnO_x catalyst. A diffraction peak for NiO (PDF card 47-1049) can be seen in the Ni(1 : 3)–MnO_x catalyst (Fig. 3(C)) and the peak is even stronger for Ni(1 : 1)–MnO_x (Fig. 3(D)). In addition, as the amount of doped nickel was increased, the peaks for bi-metal oxide Ni₆MnO₈ (PDF card 42-0479), can be observed. However, the diffraction peaks of both NiO and Ni₆MnO₈ are rather weak, indicating poor crystallinity of the catalysts and the presence of amorphous phase.³¹ This is especially true for Ni(1 : 3)–MnO_x.

Fig. 3 XRD patterns of Ni(n)–MnO_x catalysts: (A) MnO_x, (B) Ni(1 : 6)–MnO_x, (C) Ni(1 : 3)–MnO_x and (D) Ni(1 : 1)–MnO_x.

3.1.3 TEM and SEM analysis. The Ni(1 : 3)–MnO_x catalyst was further examined via TEM (Fig. 4). The TEM image shows that plenty of pore structures can be observed in Ni(1 : 3)–MnO_x catalyst (Fig. 4(A)). The TEM result combined with the BET results (Fig. 2) proves that the catalyst is porous. The HRTEM image (Fig. 2(B)) clearly shows lattice fringes, revealing the crystal lattice structure of the Ni(1 : 3)–MnO_x frameworks. There are two distinct inter-planar distances of 0.24 nm and 0.25 nm which correspond to (222) and (311) planes, respectively. The EDX spectrum of Ni(1 : 3)–MnO_x in Fig. 4(C) confirms that the main elements in the catalyst are Mn, Ni and O.

Fig. 4 (A) TEM image, (B) HRTEM image and (C) EDX spectrum of the Ni(1 : 3)–MnO_x catalyst.

The SEM images of the pure MnO_x and Ni(1 : 3)–MnO_x catalyst (Fig. 5) show how the morphology of the catalysts changed when nickel was added. The MnO_x catalyst contains aggregated spherical particles whereas the Ni(1 : 3)–MnO_x has distinct rod-like structures.

Fig. 5 SEM images of (A) MnO_x and (B) Ni(1 : 3)–MnO_x.

3.1.4 XPS analysis. In order to determine the composition of the surface and the oxidation states of the manganese and nickel ions on the surface, XPS was performed on the Ni(1 : 3)–MnO_x catalyst. Fig. 6 shows the XPS spectra of Mn 2p, Ni 2p and O 1s. The different valence states for each element and their percentages are presented in Table 2. As showed in Fig. 6(A), there are two main peaks at 641.7 and 653.9 eV which are due to Mn 2p_3/2 and Mn 2p_1/2 respectively. By performing a peak-fitting deconvolution, the Mn 2p_3/2 spectrum was separated into three peaks, Mn²⁺ (640.5 eV), Mn³⁺ (641.7 eV) and Mn⁴⁺ (643.2 eV).³² This is in good agreement with the XRD results (Fig. 3). The Mn⁴⁺ species is the predominant valence state of the surface manganese (Table 2). Previous studies have demonstrated that manganese species with higher oxidation states are more active for redox reactions.³³ Therefore, a high Mn⁴⁺ concentration should be beneficial for enhancing low-temperature SCR activity.

Fig. 6 XPS spectra of (A) Mn 2p, (B) Ni 2p and (C) O 1s.

Table 2 Distribution of different valence states for each element in Ni(1 : 3)–MnO_x

Catalyst	Percent of valence state (%)
	Mn^2+/Mn	Mn^3+/Mn	Mn^4+/Mn	Oα/(Oα + Oβ)	Ni^2+/Ni	Ni^3+/Ni
Ni(1 : 3)–MnOx	18.0	39.5	42.5	61.1	67.2	32.8

---

By the same peak-fitting deconvolution technique, the Ni 2p_3/2 spectra (Fig. 6(B)) were separated into two peaks at 852.3 eV and 855 eV which can be assigned to Ni²⁺ and Ni³⁺, respectively.^34,36 The weak satellite peak at around 861.7 eV is due to the multiple splitting of the Ni-containing oxides energy levels.³⁴ This type of satellite peak has also been observed in NiO, LiNiO₂, Li(Ni_1/2Mn_1/2)O₂, Li(Ni_1/3Co_1/3Mn_1/3)O₂, and Li(Mn_1.5Ni_0.5)O₄.^34–37

The O 1s peaks in Fig. 6(C) could be fitted into two peaks. The lower-binding-energy peak at 529.4 eV can be ascribed to the surface lattice oxygen (O_β) and the higher-energy peak at 531.4 eV corresponds to chemisorbed oxygen from oxide defects or hydroxyl groups (O_α).³⁸ Due to their higher mobilities, surface chemisorbed oxygens (O_α) have been reported to be the most active oxygens in oxidation reactions.^38,39 Therefore, the relatively higher ratio of O_α/(O_α + O_β) on the surface of the Ni(1 : 3)–MnO_x catalyst should be favorable for high NH₃-SCR activity.

3.2 Catalytic performance

3.2.1 SCR activity of Ni(n)–MnO_x with different Ni/Mn molar ratios. The NH₃-SCR activities of Ni(n)–MnO_x with different Ni/Mn molar ratios at different temperatures are shown in Fig. 7(A). In the low temperature region (60–200 °C), the NO conversion of all the Ni(n)–MnO_x catalysts increased with temperature and Ni(1 : 3)–MnO_x showed the highest activity with 81.3% NO conversion at 60 °C and 98% conversion at 100 °C. This high activity may be attributed to the high surface area and abundant mesoporous channels in the catalyst both of which provide many active sites. The NO conversion for Ni(1 : 6)–MnO_x and Ni(1 : 1)–MnO_x were both above 80% at 80 °C and 98% at 100 °C. At higher temperatures (120–200 °C), the NO conversion was 100% for all the mixed oxide catalysts at a space velocity of 38 000 h⁻¹.

Fig. 7 (A) SCR activities of Ni(n)–MnO_x with different molar ratios of Ni/Mn. (B) N₂ selectivity of Ni(1 : 3)–MnO_x and MnO_x catalysts. Reaction conditions: [NO] = [NH₃] = 1000 ppm, [O₂] = 4 vol%, balance = N₂, GHSV = 38 000 h⁻¹.

At 60 °C, the SCR activity of the pure MnO_x was about 60% and clearly at temperatures less than 80 °C, the SCR activity was enhanced by the presence of nickel. However, between 97 °C and 120 °C, the Ni/Mn ratio (n = 1 : 6 and 1 : 1) was not beneficial for low-temperature SCR processes. Finally it should be noted that the NO conversion of both the pure MnO_x and Ni(1 : 6)–MnO_x decreased when the temperature was 220 °C, whereas the conversion of Ni(1 : 3)–MnO_x and Ni(1 : 1)–MnO_x remained at about 100%.

These results indicate that the optimum Ni/Mn ratio for facilitating high NO conversion of Ni(n)–MnO_x catalysts at low-temperature SCR processes is 1 : 3. In the previous results, the surface area increased with increasing Ni/Mn ratios (Table 1), however, the SCR activities did not present the same results. These also indicate that the surface area is not the only factor that affects SCR activity.

More importantly, nearly 100% N₂ selectivity was obtained over Ni(1 : 3)–MnO_x catalyst in the whole temperature range showed in Fig. 7(B). However, the N₂ selectivity of pure MnO_x catalyst decreased when the temperature was over 140 °C. The result indicated that the addition of nickel leads to the increased N₂ selectivity.

3.2.2 SCR activity with different gas hourly space velocities on Ni(1 : 3)–MnO_x. GHSV is an important criterion which affects catalytic activity. Therefore, the NH₃-SCR activity of the Ni(1 : 3)–MnO_x catalyst at different GHSVs was investigated, and the results are shown in Fig. 8. As the GHSV increased, the NO conversion decreased at all temperatures (Fig. 8(A)). This may be attributed to the restriction of the diffusion rates and the chemical reaction rates at higher velocities. At relatively low GHSVs (38 000–102 000 h⁻¹), the NO conversions were higher than 90% (Fig. 8(B)). At these flow rates, the molecules had more time to come in contact with the surface and be completely converted.

Fig. 8 (A) The effect of GHSV and (B) the effect of temperature on NO conversion with the Ni(1 : 3)–MnO_x catalyst. Reaction conditions: [NO] = [NH₃] = 1000 ppm, [O₂] = 4 vol%, balance = N₂.

At 140 and 180 °C, NO conversions of almost 95% were achieved even at a very high GHSV of 270 000 h⁻¹. This suggests that at high temperatures, the molecules had enough energy to overcome the energy barrier resulting in more frequent collisions.²² Overall, these results show that the Ni(1 : 3)–MnO_x catalyst is effective for NO removal over a wide range of GHSVs.

3.2.3 Stability of the catalyst. The change of catalytic activity with time on stream is a measure of the stability of a catalyst. The NO conversion by the Ni(1 : 3)–MnO_x catalyst at 150 °C over time is shown in Fig. 9. The conversion remained above 98% during 16 h of continuous operation. As shown in the SEM images (Fig. 10), the morphology of the Ni(1 : 3)–MnO_x catalyst did not change after 16 h of use. These results indicate that the Ni(1 : 3)–MnO_x catalyst possesses excellent stability.

Fig. 9 The stability test of Ni(1 : 3)–MnO_x catalyst at 150 °C. Reaction conditions: [NO] = [NH₃] = 1000 ppm, [O₂] = 4 vol%, balance = N₂, GHSV = 38 000 h⁻¹.

Fig. 10 SEM images of Ni(1 : 3)–MnO_x catalyst: (A) before and (B) after 16 h of use.

In a low temperature NH₃-SCR process, the device is placed behind the flue gas desulfurization device. So a certain amount of H₂O still exists after the desulfurization process. Hence, low temperature NH₃-SCR catalysts must not only have high activities at low temperatures but also need to have strong moisture resistance. The influences of H₂O on the NH₃-SCR activities of Ni(1 : 3)–MnO_x and MnO_x catalysts were measured in order to observe the effect of Ni on the resistance against H₂O (Fig. 11). The temperature of the flue gas downstream of the desulfurizer is about 150–160 °C,⁴⁰ so the water resistances of the Ni(1 : 3)–MnO_x and MnO_x catalysts were tested at 150 °C with a GHSV of 38 000 h⁻¹. The catalysts were allowed to stabilize for 1 h and then 10% H₂O was added. With the addition of water, the NO conversion of the Ni(1 : 3)–MnO_x catalyst decreased to about 91% and remained at this value until the water was turned off. When the supply of water was turned off, the activity rapidly increased back to 100%. The same process was carried out in pure MnO_x, the NO conversion decreased to about 85% with the addition of water, while the activity did not restored to 100% (about 93%) when the supply of water was cutted off. These results show that the Ni(1 : 3)–MnO_x catalyst has good H₂O resistance and the resistance was enhanced with the addition of nickel.

Fig. 11 The H₂O resistance of Ni(1 : 3)–MnO_x and MnO_x catalysts at 150 °C. Reaction conditions: [NO] = [NH₃] = 1000 ppm, [O₂] = 4 vol%, 10 vol% H₂O, balance = N₂, GHSV = 38 000 h⁻¹.

4. Conclusions

A series of mesoporous Ni(n)–MnO_x catalysts with high activity for the low temperature SCR of NO with NH₃ in the presence of O₂ has been synthesized by a hard template method using KIT-6 as the template. The Ni(1 : 3)–MnO_x catalyst displayed the best catalytic activity, giving 81.3% NO conversion at 60 °C and nearly 100% conversion from 100 to 220 °C with a space velocity of 38 000 h⁻¹. The Ni(1 : 3)–MnO_x catalyst also had high N₂ selectivity, high stability, functioned well at various GHSVs and good H₂O resistance. The excellent catalytic performance of the catalyst can be attributed to its large specific area which provides abundant active sites. In addition the presence of amorphous phase and high concentrations of surface Mn⁴⁺ and chemisorbed oxygen contributed to the superior catalytic activity. According to these excellent properties, the Ni(1 : 3)–MnO_x catalyst in this study might be a promising candidate for the NH₃-SCR of NO with NH₃.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21206108; 21406004) and the Tianjin Municipal Science Technology Commission (14JCYBJC21200).

Notes and references

1. X. Wang, W. Wen, J. Mi, X. Li and R. Wang, Appl. Catal., B, 2015, 176–177, 454–463.
2. C. Franch-Martí, C. Alonso-Escobar, J. L. Jorda, I. Peral, J. Hernández-Fenollosa, A. Corma, A. E. Palomares, F. Rey and G. Guilera, J. Catal., 2012, 295, 22–30.
3. M. T. Javed, N. Irfan and B. M. Gibbs, J. Environ. Manage., 2007, 83, 251–289.
4. S. W. Bae, S. A. Roh and S. D. Kim, Chemosphere, 2006, 65, 170–175.
5. P. Forzatti, I. Nova and E. Tronconi, Angew. Chem., Int. Ed., 2009, 48, 8366–8368.
6. D. H. Lee, K. T. Kim, H. S. Kang, Y. H. Song and J. E. Park, Environ. Sci. Technol., 2013, 47, 10964–10970.
7. P. G. W. A. Kompio, A. Brückner, F. Hipler, G. Auer, E. Löffler and W. Grünert, J. Catal., 2012, 286, 237–247.
8. Y. Peng, J. Li, X. Huang, X. Li, W. Su, X. Sun, D. Wang and J. Hao, Environ. Sci. Technol., 2014, 48, 4515–4520.
9. Z. Chen, Q. Yang, H. Li, X. Li, L. Wang and S. Chi Tsang, J. Catal., 2010, 276, 56–65.
10. P. G. Smirniotis, D. A. Pena and B. S. Uphade, Angew. Chem., Int. Ed., 2001, 32(40), 2479–2482.
11. R. Gao, D. Zhang, X. Liu, L. Shi, P. Maitarad, H. Li, J. Zhang and W. Cao, Catal. Sci. Technol., 2013, 3, 191–199.
12. Y. Wan, W. Zhao, Y. Tang, L. Li, H. Wang, Y. Cui, J. Gu, Y. Li and J. Shi, Appl. Catal., B, 2014, 148–149, 114–122.
13. J. Zuo, Z. Chen, F. Wang, Y. Yu, L. Wang and X. Li, Ind. Eng. Chem. Res., 2014, 53, 2647–2655.
14. J. Han, J. Meeprasert, P. Maitarad, S. Nammuangruk, L. Shi and D. Zhang, J. Phys. Chem. C, 2016, 120, 1523–1533.
15. R. Q. Long and R. T. Yang, J. Catal., 2002, 207, 158–165.
16. Y. Shu, H. Sun, X. Quan and S. Chen, J. Phys. Chem. C, 2012, 116, 25319–25327.
17. Z. Wu, B. Jiang and Y. Liu, Appl. Catal., B, 2008, 79, 347–355.
18. S. Yang, C. Wang, J. Li, N. Yan, L. Ma and H. Chang, Appl. Catal., B, 2011, 110, 71–80.
19. M. Qiu, S. Zhan, D. Zhu, H. Yu and Q. Shi, Catal. Today, 2015, 258, 103–111.
20. M. Kang, E. D. Park, J. M. Kim and J. E. Yie, Catal. Today, 2006, 111, 236–241.
21. G. Qi, R. T. Yang and R. Chang, Appl. Catal., B, 2004, 51, 93–106.
22. Z. Liu, J. Zhu, J. Li, L. Ma and S. I. Woo, ACS Appl. Mater. Interfaces, 2014, 6, 14500–14508.
23. T. Boningari, P. R. Ettireddy, A. Somogyvari, Y. Liu, A. Vorontsov, C. A. McDonald and P. G. Smirniotis, J. Catal., 2015, 325, 145–155.
24. M. Qiu, S. Zhan, H. Yu, D. Zhu and S. Wang, Nanoscale, 2015, 7, 2568–2577.
25. Y. Wei, Y. Sun, W. Su and J. Liu, RSC Adv., 2015, 5, 26231–26235.
26. Z. Liu, Y. Yi, S. Zhang, T. Zhu, J. Zhu and J. Wang, Catal. Today, 2013, 216, 76–81.
27. B. Thirupathi and P. G. Smirniotis, J. Catal., 2012, 288, 74–83.
28. Y. Ren, Z. Ma and P. G. Bruce, Chem. Soc. Rev., 2012, 41, 4909–4927.
29. F. Kleitz, S. H. Choi and R. Ryoo, Chem. Commun., 2003, 2136–2137.
30. S. Sun, Z. Wen, J. Jin, Y. Cui and Y. Lu, Microporous Mesoporous Mater., 2013, 169, 242–247.
31. X. Tang, J. Hao, W. Xu and J. Li, Catal. Commun., 2007, 8, 329–334.
32. S. Cai, H. Hu, H. Li, L. Shi and D. Zhang, Nanoscale, 2016, 8, 3588–3598.
33. D. Zhang, L. Zhang, L. Shi, C. Fang, H. Li, R. Gao, L. Huang and J. Zhang, Nanoscale, 2013, 5, 1127–1136.
34. K. M. Shaju, G. V. Subba Rao and B. Chowdari, Electrochim. Acta, 2003, 48, 1505–1514.
35. A. N. Mansour, Surf. Sci. Spectra, 1994, 3, 279–286.
36. K. M. Shaju, G. V. Subba Rao and B. V. R. Chowdari, Electrochim. Acta, 2002, 48, 145–151.
37. K. Amine, H. Tukamoto, H. Yasuda and Y. Fujita, J. Electrochem. Soc., 1996, 143, 1607–1613.
38. L. Zhang, D. Zhang, J. Zhang, S. Cai, C. Fang, L. Huang, H. Li, R. Gao and L. Shi, Nanoscale, 2013, 5, 9821–9829.
39. M. Kang, E. D. Park, J. M. Kim and J. E. Yie, Appl. Catal., A, 2007, 327(2), 261–269.
40. G. Qi and R. T. Yang, Appl. Catal., B, 2003, 44(3), 217–225.

---