Improved low-temperature activity of La–Sr–Co–O nano-composite for CO oxidation by phase cooperation

Abstract

La_0.7–Sr_0.3–Co–O nano-composite shows stable and improved low-temperature activity for CO oxidation, with temperatures for ignition and full CO oxidation of 70 and 174 °C, respectively. Benefitting from the perovskite component, the catalyst shows stable activity during the reaction without appreciable loss after 36 h. Together with the characteristic results from XRD, XPS and TEM, it is inferred that the high activity of La_0.7–Sr_0.3–Co–O is attributed to its nano-sized structure, optimized Sr substitution, and especially a phase cooperation effect between the perovskite and metal oxides.

---

Perovskite oxides with ABO₃ or A₂BO₄ structure are attractive materials in catalysis and have been widely investigated in recent years because of their unique structures, in which the cation at either the A- or B-site can be substituted by a foreign one without destroying the matrix structure, thus the oxidation state of the B-site cation and the content of oxygen vacancies can be controlled as desired, offering a feasible way to correlate the physicochemical properties with the catalytic performances of a material.^1–4 Also, their low-cost, high thermal and hydrothermal stability, straightforward synthesis as well as considerable catalytic performance make them potential materials to substitute noble metal catalysts for industrial use.^5–8

One application of perovskite oxides is for catalytic oxidation removal of CO, which is an unwanted gas produced in automobile exhaust and solid oxide fuel cell (SOFC).^9–12 In contrast to metal oxide or noble metal catalysts, the perovskite oxides need to be performed at a higher temperature. For instance, the temperature of full CO oxidation over LaCoO₃ is near to 200 °C,^13–15 which is far higher than that over Co₃O₄^16,17 and gold catalysts.^18,19 This limits largely their use in practice. The improvement of low-temperature activity of perovskite oxides for CO oxidation thus is essential before its possible industrialization.

Four strategies have been suggested to improve the catalytic activity of perovskite oxide in literature: the first is to use a suitable preparation method, to generate desired surface properties and compositions;^20,21 the second is to optimize the metals at either A- or B-site, to enhance the physicochemical properties;^22,23 the third is to fabricate the material in nano-sized or porous structure, to increase the surface area and to expose more active site on the surface;^24–26 and the last is to synthesize a multi-phase material, by which synergistic effect between the perovskite and other phases that facilitates the reaction can be induced.^27–29

In this work we report a highly active La_0.7–Sr_0.3–Co–O nano-composite for CO oxidation. This sample has all the virtues mentioned above, that is, optimized A-site cation, nano-sized structure and mixed phases that can induce synergistic effect for the catalytic reactions. As expected it shows improved low-temperature activity for CO oxidation relative to the perovskite (e.g., La_0.7Sr_0.3CoO₃) and the simple oxide (e.g., Co₃O₄), with the temperature for ignition and full CO conversion of 70 and 174 °C, respectively. Moreover, the catalyst is stable with no appreciable loss in the activity after reaction for 36 h, showing potential application for oxidation removal of CO in practice.

XRD results of the La–B–O (B = Fe, Co, Ni, Cu) samples show that La–Fe–O has a relatively good perovskite structure, while La–Co–O, La–Ni–O and La–Cu–O exhibit a lot of impurity peaks (see Fig. S1 of the ESI†). This indicates that Fe is a more suitable metal than Co, Ni and Cu in the formation of oxides with perovskite structure. A possible reason could be that Fe exist easily in +3 oxidation state while Co, Ni and Cu, exist mainly in +2 oxidation state, thus facilitating the formation of LaFeO₃ perovskite structure from the viewpoint of electroneutrality, since La has +3 and O has −2 oxidation state. However, screening tests on CO oxidation over the samples show that La–Co–O is the best catalyst for the reaction (see below, Fig. 3A). We thus choose cobalt as objective metal for further investigations.

Fig. 1A presents the XRD patterns of the series of cobalt based samples, showing that the samples are mixed oxides containing perovskite, e.g., LaCoO₃, and simple metal oxides, e.g., Co₃O₄, and the Sr addition suppresses the formation of perovskite structure, as can be seen from the intensity ratio of peak at 37° (for Co₃O₄) to that at 33° (for LaCoO₃), which increases from 0.83 to 1.26 and to 1.44 with the Sr content from 0 to 0.1 and to 0.3, respectively. The perovskite structure is fully collapsed at 70% Sr percentage (i.e., La_0.3–Sr_0.7–Co–O). The existence of mixed oxides could be due to the low temperature applied in the calcination step, thus pure perovskite structure can be formed by supplying sufficient energy to the system. For example, La_0.7Sr_0.3CoO₃ with pure perovskite structure was formed by increasing the calcination temperature to 800 °C with otherwise identical conditions (see Fig. 1A). It is noted that the peak at about 2θ = 33° was split, suggesting the formation of two possible perovskite phases: LaCoO₃ and SrCoO₃, which is similar to results reported by other authors.³⁰

Fig. 1 (A) Wide-angle XRD patterns for the series of investigated catalysts; (B) typical TEM image for La_0.7–Sr_0.3–Co–O.

Fig. 1B presents a typical TEM image for sample La_0.7–Sr_0.3–Co–O, showing a nano-sized structure, with the average particle size of ca. 60 nm. The nano-sized structure is also observed for Co₃O₄ and LaCoO₃ (see Fig. S2 of the ESI†), indicating that the current sol–gel method using ethylene glycol and formic acid as complexing agent is effective for preparing nano-sized oxides. N₂ physisorption measurements show that the BET (five points) surface areas of these nano-sized La_1−x–Sr_x–Co–O samples are considerable, in the range of 11–15 m² g⁻¹.

For CO oxidation conducted over oxide catalysts the surface oxygen and metal species are two major factors influencing the activity as they are components of the active site. For this XPS measurement, which is a surface-sensitive technique and provides information on the surface elemental compositions and surface chemistry of material, are conducted to supply information for the surface oxygen and cobalt species of the samples. Fig. 2 presents the XPS spectra surveyed from Co 2p and O 1s of the samples. For Co 2p the peak area decreases in order of Co₃O₄ > La_0.7–Sr_0.3–Co–O > La_0.7Sr_0.3CoO₃, indicating that La_0.7Sr_0.3CoO₃ has the least number of surface cobalt species, due to the enrichment of La and Sr on the surface as justified by the surface (La + Sr)/Co ratio listed in Table 1. In contrast La_0.7–Sr_0.3–Co–O shows high percentage of surface Co species, with (La + Sr)/Co ratio of 0.65. The higher percentage of exposed surface Co species suggests that there is more active site on the La_0.7–Sr_0.3–Co–O than that on the La_0.7Sr_0.3CoO₃. In order to analyze the surface composition of Co species, the peaks were deconvoluted using XPS peak analysis software (XPS Peak) and the calculated results are listed in Table 1. From literature and our previous work it is known that the peak fitted at 779.5 and 782.8 eV is assigned to surface Co³⁺ and Co²⁺, respectively, and the peak fitted at 789.4 eV is to a satellite peak of Co²⁺ species.^15,31 The Co³⁺/Co²⁺ ratio for La_0.7–Sr_0.3–Co–O is 1.38, which is lower than that for Co₃O₄ and La_0.7Sr_0.3CoO₃ (1.65 and 1.63, respectively). The low Co³⁺/Co²⁺ ratio implies that oxygen vacancy is generated and Co₃O₄ phase is formed in the sample. From the principle of electroneutrality and the composition of Co₃O₄, it is known that the generation of oxygen vacancy and the formation of Co₃O₄ would lead to the presence of Co²⁺ species. The closer to unity of Co³⁺/Co²⁺ ratio might also suggests a easier transformation between Co³⁺ and Co²⁺ (Co³⁺[e⁻] ↔ Co²⁺). This is important as catalysis is a redox reaction which requires the redox cycling of the active site. The binding energy of Co³⁺ species for Co₃O₄ shifts to a higher position relative to that of the other two, possibly due to a different coordinate environment.

Fig. 2 XPS spectra surveyed from the (A) Co 2p and (B) O 1s of Co₃O₄, La_0.7Sr_0.3CoO₃ and La_0.7–Sr_0.3–Co–O.

Table 1 Surface compositions of each atom for the samples, determined from XPS spectra

Sample	(La + Sr)/Co	Co^3+/Co^2+	Oads./Olatt.
Co3O4	—	1.65	1.25
La0.7–Sr0.3–Co–O	0.65	1.38	1.31
La0.7Sr0.3CoO3	1.78	1.63	2.61

---

Similarly, the O 1s spectra was deconvoluted and the peaks fitted at 528–529 eV and 530–532 eV are assigned to lattice oxygen and oxygen adsorbed on the oxygen vacancy, respectively, Fig. 2B.³² By comparison, it is seen that the peak fitted for La_0.7–Sr_0.3–Co–O is shifted to a higher position relative to that for La_0.7Sr_0.3CoO₃, indicating a more reactive oxygen species (the oxygen is more liberal). From the surface O_ads./O_latt. ratio it is seen that the value increases in sequence of Co₃O₄ < La_0.7–Sr_0.3–Co–O < La_0.7Sr_0.3CoO₃. This indicates that the generation of oxygen vacancy, on which adsorbed oxygen (O_ads.) is produced, is more feasible in perovskite than that in simple oxide. By comparing with the shift in oxygen binding energy and the O_ads./O_latt. ratio of the samples, it is inferred that the La_0.7–Sr_0.3–Co–O sample is composed of perovskite and simple metal oxide, that is, a multi-phase structure. The peak fitted at above 533 eV is suggested to be attributed to the adsorbed oxygen and/or water.^15,33 The XPS spectra for La 3d, Sr 3d, as well as the Co 2p and O 1s of samples with different Sr contents are presented in Fig. S3 of the ESI†.

O₂-TPD and H₂-TPR measurements are conducted to supply information for the physicochemical properties of oxygen and cobalt atoms. Results are presented in Fig. S5A and S5B of the ESI,† respectively. O₂-TPD profiles show that the temperature for oxygen desorption is the lowest (588 °C) for La_0.7–Sr_0.3–Co–O. Considering that this sample has low Co³⁺/Co²⁺ and O_ads./O_latt. ratio, and its composition is in between La_0.7Sr_0.3CoO₃ and Co₃O₄, the low oxygen desorption temperature must be due to a synergistic effect between the phases that facilitates the oxygen mobility. H₂-TPR profiles indicate that the reduction temperature for La_0.7–Sr_0.3–Co–O begins at 240 °C, which is the same as that of Co₃O₄ and is lower than that of La_0.7Sr_0.3CoO₃. This is acceptable as Co₃O₄ is one component of La_0.7–Sr_0.3–Co–O and simple metal oxide normally is more reducible than perovskite oxide. As expected the reduction peak area and the stability of La_0.7–Sr_0.3–Co–O are in between that of La_0.7Sr_0.3CoO₃ and Co₃O₄, the latter shows the largest reduction peak area and is completely reduced at temperature below 530 °C.

Catalytic tests on the series of La–B–O (B = Fe, Co, Ni, Cu, Zn) oxides show that La–Co–O is the best and La–Zn–O is the worst catalyst for CO oxidation, with an activity order of La–Co–O > La–Cu–O > La–Ni–O > La–Fe–O > La–Zn–O, Fig. 3A, indicating that the type of transition metal influences greatly on the CO oxidation activity. The high activity of La–Co–O could be that, on the one hand, La–Co–O with perovskite structure possesses more oxygen vacancy and has better redox ability relative to La–Ni–O and La–Cu–O, which are in mixed metal oxide status, as verified by H₂-TPR results (see Fig. S5), and on the other hand, the surface oxygen on La–Co–O is more liberal and reactive than that on La–Fe–O, because of the weaker affinity of La–Co–O to surface oxygen.³⁴

Fig. 3 CO oxidation over catalysts (A) La–B–O (B = Fe, Co, Ni, Cu, Zn) oxides; (B) La_1−x–Sr_x–Co–O (x = 0, 0.1, 0.3, 0.7); (C) Co₃O₄, La_0.7Sr_0.3CoO₃, La_0.7–Sr_0.3–Co–O and a mechanically mixed sample Co₃O₄–La_0.7Sr_0.3CoO₃; (D) the long-time stability of La_0.7–Sr_0.3–Co–O composite at reaction temperature 140 °C.

On preparing active perovskite catalyst for CO oxidation, Dai et al. report that LaCoO₃ fabricated in porous structure shows better activity for CO oxidation than that in the bulk structure due to the improved surface area and physicochemical properties.¹⁴ In this respect we compared the catalytic performances of LaCoO₃ prepared with and without PMMA template (which is used to create pores inside the structure) for CO oxidation, finding no appreciable change in the activity is observed throughout the reaction (see Fig. S2C, S4 and S6 of the ESI†). This indicates that the herein catalyst has comparable catalytic performances to porous sample, due to its nano-sized structure (see Fig. 1B). The nano-sized particles exhibit improved activity for the reaction similar to that of porous sample.

Further optimizations by substitution of Sr²⁺ for La³⁺ show that sample with Sr²⁺ molar percentage of 30%, i.e., La_0.7–Sr_0.3–Co–O, is more favorable for the reaction, Fig. 3B. The reason could be due to the segregation of simple metal oxide, e.g., Co₃O₄, in the sample induced by the Sr addition. The cooperation between Co₃O₄ and the perovskite oxide results in a synergistic effect which enhanced the activity.^29,35 By comparing with the phase structure and activity of La_1−x–Sr_x–Co–O, it is inferred that the presence of both Co₃O₄ and LaCoO₃ is essential to the reaction, with a preferred ratio at x = 0.3. At the beginning (x ≤ 0.3) the increase of Sr content leads to the segregation of Co₃O₄, which contributes to the reaction by cooperation with LaCoO₃, but with the further increase of Sr content (x = 0.7) the perovskite structure was destroyed and thus decreased the activity.

To confirm that synergistic effect between the phases occurs, catalytic activity of Co₃O₄, La_0.7Sr_0.3CoO₃, La_0.7–Sr_0.3–Co–O and a mechanically mixed sample containing 60% Co₃O₄ and 40% LaCoO₃ (Co₃O₄–LaCoO₃, prepared based on the phase composition of La_0.7–Sr_0.3–Co–O) for CO oxidation are tested and compared, Fig. 3C. The activity increases in the sequence of La_0.7Sr_0.3CoO₃ < Co₃O₄–LaSrCoO₃ < Co₃O₄ < La_0.7–Sr_0.3–Co–O, indicating that the high activity of La_0.7–Sr_0.3–Co–O should not be due to a simple overlapping in the activity of LaCoO₃ and Co₃O₄, as the physically mixed sample shows low activity for CO oxidation. In other words, a synergistic effect between the phases of La_0.7–Sr_0.3–Co–O nano-composite that contributes to the reaction is induced. It is noted that La_0.7–Sr_0.3–Co–O shows 96%, while La_0.7Sr_0.3CoO₃ shows only 10% CO conversion at temperature below 150 °C, demonstrating clearly the importance of the presence of Co₃O₄ in the sample.

For CO oxidation catalyzed by perovskite oxides it is generally accepted that the sample with more oxygen vacancy and/or higher metal oxidation state is more favorable for the reaction.^36,37 Herein, La_0.7–Sr_0.3–Co–O with lower surface Co³⁺/Co²⁺ and O_ads./O_latt. ratio (see Table 1) however exhibits higher activity than La_0.7Sr_0.3CoO₃. The reason is that the structure of the former is not totally perovskite, thus the activity is not solely contributed from the perovskite, but from a synergistic effect induced from the multi-phases. The synergistic effect can be verified by the dynamical results obtained from the O₂-TPD and H₂-TPR measurements (see Fig. S5 of the ESI†). For example, the oxygen desorption or mobility is more facilitated over La_0.7–Sr_0.3–Co–O than that over La_0.7–Sr_0.3–Co–O, although the former has lower O_ads./O_latt. ratio.

The long-time stability of La_0.7–Sr_0.3–Co–O composite for CO oxidation reaction was conducted to test if the material benefits from the thermal stability of perovskite oxide. To ensure that the results are reliable, the temperature is controlled at 140 °C with activity of ca. 80%. Result in Fig. 3D shows that the activity is stable and no appreciable change in the activity is observed after running for 36 h, indicating that the material has good stability in the reaction. In contrast, a slight decrease in the activity of Co₃O₄ is observed after 36 h (see Fig. S7 of the ESI†), indicating that the presence of perovskite oxides is essential in stabilizing the catalytic behavior. Characterizations by XRD, XPS and BET confirm that the sample has stable structure (see Fig. S8 of the ESI†). This directs a way of designing low-temperature active perovskite-based catalyst for CO oxidation.

In order to illuminate the superiority of the La_0.7–Sr_0.3–Co–O nano-composite, we compare the activity of CO oxidation over a large number of LaCoO₃ catalysts reported in literature, as listed in Table 2 (for reaction conditions see Table S1 of the ESI†). La_0.7–Sr_0.3–Co–O shows comparable activity to LaCoO₃ prepared with special structures (porous, spherical and 3DOM, see entries 3, 5 and 6) and the activity is higher than that of the others. Considering the easy synthesis procedure and energy savings of La_0.7–Sr_0.3–Co–O (the preparation of spherical and 3DOM LaCoO₃ needs special steps and is energy costing, which can be inferred by comparing the preparation procedures), we believe that this sample would be more interesting for future use.

Table 2 Comparison in the activity (T_ign., T₅₀ and T₁₀₀) of La_0.7–Sr_0.3–Co–O with that of LaCoO₃ reported in literature

Entry	Catalyst	Tign.^a (°C)	T50^b (°C)	T100^c (°C)	Refs.
a Ignition temperature for CO oxidation.b Temperature for 50% CO conversion.c Temperature for 100% CO conversion.
1	La0.7–Sr0.3–Co–O	75	130	174	This work
2	La–Co–O	100	150	200	This work
3	LaCoO3 (porous)	60	145	175	15
4	LaCoO3 (nano-sized)	100	161	200	38
5	LaCoO3 (spherical)	70	131	170	38
6	LaCoO3 (3DOM)	50	162	190	14
7	LaCoO3	120	182	215	14
8	LaCoO3 (porous)	110	170	207	13
9	LaCoO3	140	187	237	13
10	La0.9K0.1CoO3	160	245	335	39
11	LaCoO3	130	205	246	40
12	LaCoO3	160	238	270	41
13	LaCoO3	115	172	210	42
14	LaCoO3	260	280	400	36

---

In summary, we report a low-temperature active La_0.7–Sr_0.3–Co–O nano-composite for CO oxidation. The sample is prepared by sol–gel method using ethylene glycol and formic acid as complexing agents, and possesses a multi-phase and nano-sized structure. Because of the nano-sized structure and especially the phase cooperation effect, the La_0.7–Sr_0.3–Co–O shows improved low-temperature activity for CO oxidation, with the temperature for ignition and 100% CO conversion of 70 and 174 °C, respectively. Long-term stability test shows that the sample is stable in the reaction, without appreciable loss in the activity for at least 36 h. The improved catalytic performance and good stability demonstrate that the current strategy of designing nano-sized samples with mixed phases is an effective way to prepare low-temperature active oxides catalyst for CO oxidation in future.

Acknowledgements

Financial support from the National Science Foundation of China (21203254), the Scientific Research Foundation for Returned Scholars, Ministry of Education of China (BZY11055) and the National Demonstration Center of Experimental Teaching on ethnical pharmacy, South-Central University for Nationalities, is gratefully acknowledged.

Notes and references

1. P. Ciambelli, S. Cimino, S. De Rossi, M. Faticanti, L. Lisi, G. Minelli, I. Pettiti, P. Porta, G. Russo and M. Turco, Appl. Catal., B, 2000, 24, 243–253.
2. M. Misono, Stud. Surf. Sci. Catal., 2013, 176, 67–95.
3. M. A. Pena and J. L. G. Fierro, Chem. Rev., 2001, 101, 1981–2017.
4. N. Yamazoe and Y. Teraoka, Catal. Today, 1990, 8, 175–199.
5. Y. Nishihata, J. Mizuki, T. Akao, H. Tanaka, M. Uenishi, M. Kimura, T. Okamoto and N. Hamada, Nature, 2002, 418, 164–167.
6. J. J. Zhu and A. Thomas, Appl. Catal., B, 2009, 92, 225–233.
7. C. H. Kim, G. Qi, K. Dahlberg and W. Li, Science, 2010, 327, 1624–1627.
8. J. Zhu, H. Li, L. Zhong, P. Xiao, X. Xu, X. Yang, Z. Zhao and J. Li, ACS Catal., 2014, 4, 2917–2940.
9. H. X. Dai, H. He, W. Li, Z. Z. Gao and C. T. Au, Catal. Lett., 2001, 73, 149–156.
10. U. G. Singh, J. Li, J. W. Bennett, A. M. Rappe, R. Seshadri and S. L. Scott, J. Catal., 2007, 249, 349–358.
11. H. Taguchi, S. Yamasaki, A. Itadani, M. Yosinaga and K. Hirota, Catal. Commun., 2008, 9, 1913–1915.
12. W. Yang, R. Zhang, B. Chen, N. Bion, D. Duprez and S. Royer, J. Catal., 2012, 295, 45–58.
13. Y. Wang, X. Cui, Y. Li, Z. Shu, H. Chen and J. Shi, Microporous Mesoporous Mater., 2013, 176, 8–15.
14. X. Li, H. Dai, J. Deng, Y. Liu, S. Xie, Z. Zhao, Y. Wang, G. Guo and H. Arandiyan, Chem. Eng. J., 2013, 228, 965–975.
15. P. Xiao, J. Zhu, H. Li, W. Jiang, T. Wang, Y. Zhu, Y. Zhao and J. Li, ChemCatChem, 2014, 6, 1774–1781.
16. X. W. Xie, Y. Li, Z. Q. Liu, M. Haruta and W. J. Shen, Nature, 2009, 458, 746–749.
17. Y. Lv, Y. Li and W. Shen, Catal. Commun., 2013, 42, 116–120.
18. K. Qian, L. Luo, H. Bao, Q. Hua, Z. Jiang and W. Huang, Catal. Sci. Technol., 2013, 3, 679–687.
19. M. Haruta, T. Kobayashi, H. Sano and N. Yamada, Chem. Lett., 1987, 405–408.
20. G. Pecchi, P. Reyes, R. Zamora, C. Campos, L. E. Caduus and B. P. Barbero, Catal. Today, 2008, 133, 420–427.
21. C. Zhang, Y. Guo, Y. Guo, G. Lu, A. Boreave, L. Retailleau, A. Baylet and A. Giroir-Fendler, Appl. Catal., B, 2014, 148–149, 490–498.
22. N. A. Merino, B. P. Barbero, P. Ruiz and L. E. Cadús, J. Catal., 2006, 240, 245–257.
23. C. Zhang, W. Hua, C. Wang, Y. Guo, Y. Guo, G. Lu, A. Baylet and A. Giroir-Fendler, Appl. Catal., B, 2013, 134–135, 310–315.
24. C. A. Chagas, F. S. Toniolo, R. N. S. H. Magalhães and M. Schmal, Int. J. Hydrogen Energy, 2012, 37, 5022–5031.
25. J. Xu, J. Liu, Z. Zhao, J. Zheng, G. Zhang, A. Duan and G. Jiang, Catal. Today, 2010, 153, 136–142.
26. Y. G. Wang, J. W. Ren, Y. Q. Wang, F. Y. Zhang, X. H. Liu, Y. Guo and G. Z. Lu, J. Phys. Chem. C, 2008, 112, 15293–15298.
27. V. C. Belessi, A. K. Ladavos and P. J. Pomonis, Appl. Catal., B, 2001, 31, 183–194.
28. Y. J. Zhu, D. Wang, F. L. Yuan, G. Zhang and H. G. Fu, Appl. Catal., B, 2008, 82, 255–263.
29. J. Kirchnerova, M. Alifanti and B. Delmon, Appl. Catal., A, 2002, 231, 65–80.
30. V. C. Belessi, C. N. Costa, T. V. Bakas, T. Anastasiadou, P. J. Pomonis and A. M. Efstathiou, Catal. Today, 2000, 59, 347–363.
31. J. Deng, L. Zhang, H. Dai, H. He and C. T. Au, Ind. Eng. Chem. Res., 2008, 47, 8175–8183.
32. N. Yamazoe, Y. Teraoka and T. Seiyama, Chem. Lett., 1981, 1767–1770.
33. A. Machocki, T. Ioannides, B. Stasinska, W. Gac, G. Avgouropoulos, D. Delimaris, W. Grzegorczyk and S. Pasieczna, J. Catal., 2004, 227, 282–296.
34. Y. Yokoi and H. Uchida, Catal. Today, 1998, 42, 167–174.
35. L. Simonot, F. o. Garin and G. Maire, Appl. Catal., B, 1997, 11, 167–179.
36. B. Seyfi, M. Baghalha and H. Kazemian, Chem. Eng. J., 2009, 148, 306–311.
37. K. Rida, A. Benabbas, F. Bouremmad, M. A. Peña and A. Martínez-Arias, Catal. Commun., 2006, 7, 963–968.
38. Y. Liu, H. Dai, J. Deng, L. Zhang, Z. Zhao, X. Li, Y. Wang, S. Xie, H. Yang and G. Guo, Inorg. Chem., 2013, 52, 8665–8676.
39. Q. Thi Hoang Yen, T. Thi Minh Nguyet, T. Que Chi, N. Quoc Trung, N. Thi Toan and L. Dang Khuong, Adv. Nat. Sci.: Nanosci. Nanotechnol., 2011, 2, 045007.
40. S. Sun, L. Yang, G. Pang and S. Feng, Appl. Catal., A, 2011, 401, 199–203.
41. S. A. Nedil'ko, I. V. Fesich, A. G. Dzyaz'ko, O. Z. Didenko, G. R. Kosmambetova and P. E. Strizhak, Theor. Exp. Chem., 2011, 47, 183–187.
42. P. Doggali, S. Kusaba, Y. Teraoka, P. Chankapure, S. Rayalu and N. Labhsetwar, Catal. Commun., 2010, 11, 665–669.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental section, additional characterizations and activity tests. See DOI: 10.1039/c4ra10902c

---