Enhanced catalytic activity of α-Fe₂O₃ nanorods enclosed with {110} and {001} planes for methane combustion and CO oxidation

Abstract

Hematite (α-Fe₂O₃) nanorods, nanotubes, and nanocubes were selectively synthesized via a hydrothermal process and subsequent calcination treatment. The nanorods showed higher catalytic activity than the nanotubes and the nanocubes for methane combustion and CO oxidation. It may be attributed to the highest occupancy of iron ions on the surface of the nanorods.

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1. Introduction

The performance of catalysts depends not only on their chemical composition, size, morphology, but also greatly on the arrangement manners of their surface atoms.^1–3 For example, the Pt (111)/Ru electrode was found to be the best fuel-cell anode for methane oxidation;⁴ ceria nanorods with predominantly exposed planes {001} and {110} showed enhancement of catalytic activity for CO oxidation than the irregular ceria nanoparticles with predominantly exposed planes {111};⁵Co₃O₄ nanorods predominantly exposing facets {110} revealed higher activity and stability for CO oxidation than Co₃O₄ polycrystals at temperatures as low as −77 °C;⁶ the high index {112} crystal planes of Co₃O₄ nanocrystals are more reactive than the basic {001} and {011} planes for methane combustion.⁷ However, most of the heterogeneous catalysts at present consist of assorted polycrystals with various geometries and exposed surfaces. Although they possess a few active sites, they still show lower catalytic activity because of lack in predominantly exposed facets. Therefore, it is an effectual way to design and synthesize micro/nano-crystals with predominantly specific exposed facets to enhance catalytic activity.

Hematite (α-Fe₂O₃), the most stable iron oxide with n-type semiconducting properties (E_g = 2.1 eV) under ambient conditions, is widely used as catalysts, pigments, gas sensors, and electrode materials,⁸ because of its low cost, high resistance to corrosion, and environmentally friendly properties. Over several years its nanostructures such as nanocubes,^9,10nanorods,^11–13 nanobelts,^14,15nanotubes,^16–18 nanoflowers,¹⁹ nanorings²⁰ and dendritic micro-pines²¹ have been prepared and showed a few specific applications. To lower the cost of catalytic combustion, Kwon et al. once used nano-Fe₂O₃ as catalysts for carbon monoxide and methane combustion, and discovered that nano-Fe₂O₃ is more active than micro-Fe₂O₃.²² Liu et al. once used α-Fe₂O₃ nanorods, and nanocubes as catalysts for CO oxidation, and discovered that the nanorods with more reactive crystal planes {110} showed a higher activity than the nanocubes, but they dealt with neither methane combustion nor catalytic performance of α-Fe₂O₃ nanotubes.²³ Herein we selectively synthesized α-Fe₂O₃ nanorods, nanotubes, and nanocubes via a hydrothermal method, and discovered that α-Fe₂O₃ nanorods enclosed with {110} and {001} planes exhibited an enhanced catalytic activity for both methane combustions and CO oxidation.

2. Experimental methods

Sample preparation

Preparation of α-Fe₂O₃ nanorods: 2.1881 g of Fe(NO₃)₃·9H₂O was dissolved in 30 mL of deionized water, and then a 10 wt% NaOH solution was added dropwise to adjust the pH value of the solution to 12 under vigorous stirring. Here a brown precipitate appeared. After continuously stirring for about 30 min, all of the suspension (about 45 mL) was transferred into a 65 mL Teflon-lined stainless steel autoclave and treated at 180 °C for 12 h. The products were separated by centrifugation, rinsed several times with deionized water and absolute ethanol, respectively. Finally the products were dried at 80 °C for 4 h in air, and calcined at 600 °C for 1 h.

Preparation of α-Fe₂O₃ nanotubes: 2.0 mL of FeCl₃ aqueous solution (0.5 mol L⁻¹) and 1.8 mL NH₄H₂PO₄ aqueous solution (0.02 mol L⁻¹) were mixed with vigorous stirring. Then deionized water was added in order to reach a final volume of 50 mL. After stirring for 10 min, the mixture was transferred into a Teflon-lined stainless-steel autoclave with a capacity of 65 mL for hydrothermal treatment at 220 °C for 48 h. The subsequent treatment is as above.

Preparation of α-Fe₂O₃ nanocubes: 1.62 g of FeCl₃·6H₂O was dissolved in 50 mL of a cetyltrimethylammonium bromide (CTAB) aqueous solution (C_CTAB = 0.04 mol L⁻¹). The whole mixture was stirred for 30 min, and then transferred into a Teflon-lined stainless steel autoclave with a capacity of 65 mL, sealed, and maintained at 120 °C for 32 h. The subsequent treatment is as above.

Characterization

All chemicals were of analytical grade and were used as received without further purification. The as-synthesized products were characterized by an X-ray diffractometer (XRD; Shimadzu XRD-6000) with graphite monochromatized Cu Kα1-radiation, a scanning electron microscope (SEM; S-4800) and high-resolution electron microscopy with a point resolution of 0.19 nm (HRTEM; JEOL model JEM-2100). Nitrogen adsorption–desorption isotherms were obtained at 77 K on a Micromeritics ASAP 2020 apparatus after degassing of the samples at 300 °C for 2 h, and the Brunauer–Emmett–Teller (BET) specific surface area were calculated using BET adsorption data acquired at the relative pressure range from 0.06 to 0.2, and the pore size distribution was calculated from the nitrogen desorption isotherm by the Barrett–Joyner–Halenda (BJH) method.

Catalytic activity measurements

The catalytic activity for methane combustion was measured in a quartz-tube plug flow reactor (inner diameter, 6 mm; length, 250 mm) using 40 mg of catalysts (for each test) in a gas mixture of 1.99 vol% CH₄ and 80.01 vol% air at a space velocity of 25 500 cm³ g⁻¹ h⁻¹. Similarly, the catalytic activities for CO oxidation were measured with a mixed gas of 2 vol% CO, and 98 vol% air. The products were analyzed by an on-line gas chromatography with a 5A molecular sieve chromatography column for separating N₂, O₂, CO, CO₂ and CH₄, and thermal conduction detectors (TCDs). After the reaction was stabilized for 30 min, data were recorded.

3. Results and discussions

The X-ray diffraction (XRD) patterns of the nanorods, nanotubes, and nanocubes can be indexed into hexagonal α-Fe₂O₃ (JCPDS card No.: 33-0664, hematite, Fig. S1, ESI†). Fig. 1(a) and (b) show the transmission electron microscopy (TEM) image of the α-Fe₂O₃ nanorods and a single nanorod, respectively. The typical nanorod is about 71 nm in width, 7 nm in thickness, and 314 nm in length. Some positions of the nanorods are hollow (inset of Fig. 1b). Fig. 1(c)–(e) exhibit high-resolution transmission electron microscopy (HRTEM) images of part a, b and c in Fig. 1(b), respectively. Fringe spaces of 0.25 nm in Fig. 1(c)–(e) correspond to d-values of (110), (1–20), and (−210) planes, respectively. Their fast Fourier transform (FFT) patterns are displayed in insets, respectively. Based on the above analysis, the structure of the nanorod is drawn in Fig. 1(f). The nanorods are enclosed by six equivalent crystal planes with {110} and two equivalent planes with {001}. Fig. 2(a) and (b) are the SEM and TEM images of α-Fe₂O₃ nanotubes, respectively. The typical nanotube has an outer diameter of 90–110 nm, inner diameter of 40–80 nm, and lengths of 250–400 nm. Fig. 2(c) reveals a selected area electron diffraction (SAED) pattern of the box in Fig. 2(b), which can be indexed as α-Fe₂O₃. Fig. 2(d) exhibits the corresponding HRTEM image. Spaces of 0.37 and 0.27 nm correspond to d-values of (012) and (0−14) planes. The FFT pattern and the structure are displayed in top-left and down-right insets, respectively. The nanotubes are enclosed by two preferentially exposed planes with {010} and two planes with {001}. Fig. 3(a)–(c) show that the nanocubes are rhombus cubes with an edge length of about 680 nm. Fig. 3(d) displays the HRTEM image of a single nanocube in Fig. 3(c), and a fringe space of 0.37 nm corresponds to the d-value of (012) plane. Its SAED pattern (top-left inset in Fig. 3(d)) can be indexed as α-Fe₂O₃. As shown in the down-right inset of Fig. 3(d), the nanocubes are enclosed by planes with {012}, {102}, and {−11−2}.²³

Fig. 1 (a) TEM image of a pile of nanorods, (b) TEM image of a single nanorod. (c), (d), and (e) are HRTEM images of part a, b, and c in (b), respectively. (f) Structure of α-Fe₂O₃ nanorods.

Fig. 2 (a) SEM image of nanotubes; (b) TEM image of nanotubes. (c) SAED pattern of the box in (b). (d) HRTEM image of the box in (b); left-above inset is the FFT pattern of the nanotube; right-down inset shows the structure of nanotube.

Fig. 3 (a) SEM image of nanocubes. (b) TEM image of nanocubes. (c) TEM image of a nanocube. (d) HRTEM image in (c); left-above inset showing a SAED pattern in (c); right-down inset showing the structure of nanocube.

N₂ adsorption/desorption isotherms and the pore size distributions of nanorods and nanotubes are shown in Fig. S2(a) and (b) (ESI†), respectively. According to the original IUPAC classification,²⁴ the isotherms are classified as type IV isotherms with H3 type hysteresis loops. According to the extended classification of adsorption isotherms,²⁵ the isotherms are classified as type IIb isotherms. These results indicate typical slit-shaped mesopore performance, and the mesopore should be voids between particles in the aggregates. BJH calculations for the pore-size distribution, derived from desorption data, reveal a narrow distribution. The specific surface area of the nanorods approaches that of nanotubes. The nanotubes possess a higher pore capacity than the nanorods. Because there are no mesopores on the nanocubes, the nanocubes have a knockdown specific surface area. The results are included in Table 1.

Table 1 The pore parameters, specific surface areas and conversion temperatures of α-Fe₂O₃ nanorods, nanotubes, and nanocubes catalysts for methane combustion and CO oxidation

Catalyst	Specific surface area/m^2 g^−1	BJH pore size/nm	Pore volume (10^−4 cm^3 g^−1 nm^−1)	Temperature (°C) for 50% CH4 conversion	Temperature (°C) for 100% CH4 conversion	Temperature (°C) for 50% CO conversion	Temperature (°C) for 100% CO conversion
Nanorods	10.1	86.3	6.0	443	650	289	370
Nanotubes	13.7	52.0	17	610	750	400	640
Nanocubes	0.384			557	700	378	580

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Fig. 4(a) shows the conversions of methane combustion over the nanorods, nanotubes, and nanocubes as a function of reaction temperature. The temperatures of the complete combustions are 650, 750, and 700 °C, respectively. Fig. 4(b) shows the conversions of CO oxidation over the nanorods, nanotubes, and nanocubes as a function of reaction temperature. The temperatures of the complete CO oxidation are 370, 640, and 580 °C, respectively. The order of the catalytic activity is the nanorods > the nanocubes > the nanotubes (Table 1). The activity of the nanorods is higher than nano-Fe₂O₃.²² However, the order of their BET specific surface area is the nanotubes (13.7 m² g⁻¹) > nanorods (10.1 m² g⁻¹) > nanocubes (0.384 m² g⁻¹). The results mean that the BET surface area is not the sole criterion to determine the activity of catalyst, but the specific crystal facets of α-Fe₂O₃ nanocrystals are indeed factors influencing the catalytic performances.

Fig. 4 Conversion of (a) methane combustion and (b) CO oxidation as a function of temperature over α-Fe₂O₃ nanorods, nanotubes, and nanocubes at GHSV = 25 500 cm³ g⁻¹ h⁻¹.

Generally, the catalytic oxidation of carbon monoxide by metal oxides can be broken down into two steps. Firstly, metal oxides lose one oxygen atom to carbon monoxide to form carbon dioxide and metal. Secondly, the metal is oxidized into metal oxides.²⁶ Similarly, catalytic combustion of methane also adheres to this reaction process.²⁷ Therefore the tendency of metal oxides to lose one oxygen atom to carbon monoxide or methane is an important effect factor, which is related to the surface densities of the iron atoms (SDIAs) of these nanocrystals.

The SDIAs of {110} and {012} planes are 10.1 and 7.3 atom per nm², respectively.²³ The SDIAs of {102}, {001}, {010} and {−11−2} planes are calculated as 2.44, 4.56, 4.34 and 4.76 atom per nm², respectively, as shown in Fig. 5. The surface areas of {110} and {001} planes of the nanorods occupy about 11% and 89%, respectively, so {001} planes are the dominantly exposed facets. The surface areas of {010} and {001} planes of the nanotubes occupy about 96% and 4%, respectively, so {010} planes are the dominantly exposed facets. The surface areas of {012}, {102} and {−11−2} planes of the nanocubes occupy about 1/3, respectively, so there is no predominantly exposed facet. Because iron ions are active centres, and by calculation the SDIAs of the nanorods, nanocubes, and nanotubes are about 5.2, 4.9, and 4.3 atom per nm², respectively, their catalytic activities decrease in order.

Fig. 5 Atomic arrangement of various crystal planes and α-Fe₂O₃ structure. Blue globes are Fe ions (small); red globes are oxygen ions (large). Occupancy of (001) iron atoms = 1/(0.5035 × 0.5035 sin 60°) = 4.56 (atom per nm²); occupancy of (010) iron atoms = 3/(1.372 × 0.5035 sin 90°) = 4.34 (atom per nm²); occupancy of (102) iron atoms = 2/(1.625 × 0.5035 sin 90°) = 2.44 (atom per nm²); occupancy of (−11−2) iron atoms = 1.3/(0.5419 × 0.5035 sin 90°) = 4.76 (atom per nm²).

4. Conclusions

The nanorods, nanotubes, and nanocubes of α-Fe₂O₃ have been successfully synthesized via a hydrothermal process and subsequent calcination treatment. The nanorods are enclosed with {110} and {001} planes, and {001} planes are the dominantly exposed facets. The nanocubes are enclosed with {012}, {102}, and {−11−2} planes, and there are no dominantly exposed facets. The nanotubes are enclosed with {010} and {001} planes, and {010} planes are the dominantly exposed facets. The difference of the SDIAs of the nanorods, nanocubes, and nanotubes leads to that of the catalytic activities. These results show the importance of morphology control in the preparation of transition metal oxides as highly efficient oxidation catalysts.

Acknowledgements

The work was financially supported by the National Science Foundations of China (No. 20671050) and National Basic Research Program of China (973 Program, No. 2007CB936302).

References

1. T. S. Ahmadi, Z. L. Wang, T. C. Green, A. Henglein and M. A. EI-Sayed, Science, 1996, 272, 1924.
2. R. Narayanan and M. A. EI-Sayed, J. Am. Chem. Soc., 2004, 126, 7194.
3. V. Polshettiwar and R. S. Varma, Green Chem., 2010, 12, 743.
4. W. Chrzanowski and A. Wieckowski, Langmuir, 1998, 14, 1967.
5. K. B. Zhou, X. Wang, X. M. Sun, Q. Peng and Y. D. Li, J. Catal., 2005, 229, 206.
6. X. Xie, Y. Li, Z. Q. Liu, M. Haruta and W. Shen, Nature, 2009, 458, 746.
7. L. H. Hu, Q. Peng and Y. D. Li, J. Am. Chem. Soc., 2008, 130, 16136.
8. C. Z. Wu, P. Yin, X. Zhu, C. Z. O. Yang and Y. Xie, J. Phys. Chem. B, 2006, 110, 17806.
9. Z. Pu, M. Cao, J. Yang, K. Huang and C. Hu, Nanotechnology, 2006, 17, 799.
10. S. B. Wang, Y. L. Min and S. H. Yu, J. Phys. Chem. C, 2007, 111, 3551.
11. K. Woo, H. J. Lee, J. P. Ahn and Y. S. Park, Adv. Mater., 2003, 15, 1761.
12. Y. M. Zhao, Y.-H. Li, R. Z. Ma, M. J. Roe, D. G. McCartney and Y. Q. Zhu, Small, 2006, 2, 422.
13. X. Hu and J. C. Yu, Adv. Funct. Mater., 2008, 18, 880.
14. X. Wen, S. Wang, Y. Ding, Z. Wang and S. Yang, J. Phys. Chem. B, 2005, 109, 215.
15. R. Wang, Y. Chen, Y. Fu, H. Zhang and C. Kisielowski, J. Phys. Chem. B, 2005, 109, 12245.
16. J. Chen, L. Xu, W. Li and X. Gou, Adv. Mater., 2005, 17, 582.
17. Z. Sun, H. Yuan, Z. Liu, B. Han and X. Zhang, Adv. Mater., 2005, 17, 2993.
18. C. J. Jia, L. D. Sun, Z. G. Yan, L. P. You, F. Luo, X. D. Han, Y. C. Pang, Z. Zhang and C. H. Yan, Angew. Chem., Int. Ed., 2005, 44, 4328.
19. L. S. Zhong, J. S. Hu, H. P. Liang, A. M. Cao, W. G. Song and L. J. Wan, Adv. Mater., 2006, 18, 2426.
20. X. Hu, J. C. Yu, J. Gong, Q. Li and G. Li, Adv. Mater., 2007, 19, 2324.
21. M. Cao, T. Liu, S. Gao, G. Sun, X. Wu, C. Hu and Z. L. Wang, Angew. Chem., Int. Ed., 2005, 44, 4197.
22. S. Kwon, M. Fan and T. D. Wheelock, Environ. Eng. Sci., 2007, 24, 1065.
23. X. Liu, J. Liu, Z. Chang, X. Sun and Y. Li, Catal. Commun., 2011, 12, 530.
24. K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure Appl. Chem., 1985, 57, 603.
25. F. Rouquerol, J. Rouquerol and K. Sing, Adsorption by Powders and Porous Solids, Academic Press, London, 1999.
26. P. Li, D. E. Miser, S. Rabiei, R. T. Yadav and M. R. Hajaligol, Appl. Catal., B, 2003, 43, 151.
27. J. Zhu, J. G. van Ommen, H. J. M. Bouwmeester and L. Lefferts, J. Catal., 2005, 233, 434.

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Footnote

† Electronic supplementary information (ESI) available: XRD Patterns, N₂ adsorption–desorption isotherms. See DOI: 10.1039/c1cy00080b

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