Aggregated Pt–Pd nanoparticles on Nafion membrane for impulsive decomposition of hydrogen peroxide

Abstract

Pt–Pd aggregated nanoparticles were immobilized on a Nafion-117 membrane to decompose hydrogen peroxide. The Pt or Pd particles alone decompose hydrogen peroxide at an insignificant rate. But a bimetallic Pt–Pd catalyst, having a Pt to Pd composition between 0.33 and 0.75, causes steady decomposition at an appreciable rate at room temperature. The Pd and Pt sites have been proposed to initiate and to complete, respectively, the impulsive hydrogen peroxide decomposition reaction.

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Introduction

Hydrogen peroxide is an essential ingredient in widespread industrial processes such as food processing, textile bleaching, manufacturing of cosmetics and pharmaceuticals, etc.^1,2 The oxidizing capacity of hydrogen peroxide is so strong that it is considered as a highly reactive oxygen donating species. Its most recent additional uses includes using it as a fuel/propellant instead of carcinogenic hydrazines.³ Hydrogen peroxide is a well-established source of the hydroxyl radical (˙OH) in the so-called oxidative destruction of organic wastes.^4–6 Thus, in many instances hydrogen peroxide is often regarded as a harmful agent for living being. Conversely, regardless of destructive effects, hydrogen peroxide is a source of artificial oxygen too. For example, hydrogen peroxide may be used as a source of oxygen for medically sick peoples and also for aquatic living creatures in an aquarium because of its steady and continuous decomposition. The decomposition of aqueous hydrogen peroxide (H₂O₂) over various heterogeneous catalysts, such Ag, Cu, Fe, Mn, Ni, Pt and their oxides on silica, alumina, and zeolites, has been the subject of earlier investigations.^7–17 However, these materials often cannot be reused as they are consumed and regeneration is often cost-effective. Conversely, noble metals seem to show pitiable surface catalytic performance for numerous decomposition reactions. In this connection, several reports have been published based on metal (Pt, Pd, Au) catalysed decomposition of hydrogen peroxide.^18–24 Composite catalysts consisting of two different metal elements have appealed to great interests in the field of chemical processes because of their various applications. Predominantly, bimetallic nanocatalysts have been subject to rigorous studies because of their exclusive properties which are superior to their monometallic analogues. For example, reactivity, selectivity and durability of a catalyst could be regulated by selecting an appropriate combination of metals and their compositions. The Pt–Pd bimetallic catalyst has been applied for various processes, including oxidation, reduction, and decomposition reactions, because of its utmost efficiency.^25–37 Likewise other catalytic reactions, a bimetallic catalyst has been proven effective in attaining hydrogen peroxide decomposition.³¹

The aim of the present article is to resolve the functions of the active sites of a bimetallic Pt–Pd catalyst that causes auto-decomposition of hydrogen peroxide molecules at room temperature and neutral pH.

Experimental

All the chemicals used were of analytical grade, and were used without further purification. In this article, Nafion membrane was chosen as a support for bimetallic (Pt–Pd) catalysts, because of splendid stability of the particle lean films. In order to prepare a metal-membrane assembly, platinum was first chemically deposited from H₂PtCl₆ on a Nafion-117 membrane (Du Pont) supplied by Wako Inc., Japan. A Nafion membrane of 2 cm × 3 cm was first sand blasted and dried at 110 °C and then was immersed into 200 mL of a 7.5 mM solution of H₂PtCl₆. For Pd deposition, PdCl₂ was used instead of H₂PtCl₆. In order to ensure the dissolution of PdCl₂ particles, 2 mL of concentrated ammonia was added to the mixture. Sodium borohydride is reported to be an essential agent for the reduction of many heavy metals under extreme basic conditions because 1 mol of NaBH₄ can supply 8 moles of electrons. Consequently, this chemical was supposed to reduce Pt⁴⁺ using the following reaction pathway.

BH₄⁻ + 8OH⁻ → B(OH)₄⁻ + 4H₂O + 8e⁻ (1)

2Pt⁴⁺ + 8e⁻ → 2Pt⁰ (2)

In this context, a mixed solution of 2 M NaBH₄ and 4 M NaOH was added to the membrane-containing system at a rate of 2 mL h⁻¹. Meanwhile, the reaction mixture was heated from 35 °C at a rate of 5 °C h⁻¹. Metallic plating on both sides of the Nafion membrane was completed within 12 hours. The Pt–Pd films were fabricated on a Nafion membrane from a mixture of a 7.5 mM solution of H₂PtCl₆ + 4 mM PdCl₂ following the same procedure as discussed above. The total geometric area of the catalytic surface was 12 cm². The resistivity of the surfaces as prepared was less than 10 Ω cm⁻¹.

Morphology of Pt–Pd/Nafion materials was evaluated using a FESEM instrument (FESEM; JSM-7600F, Japan). The XRD data was recorded using a multiflex 2 kW diffractometer. Energy dispersive X-ray analysis (XEDS) was investigated using a FESEM-coupled XEDS from JEOL, Japan. The X-ray photoelectron spectroscopy (XPS) measurements were executed on a Thermo Scientific K-Alpha KA1066 spectrometer. A monochromatic AlKα X-ray radiation source was used as the excitation source, where the beam-spot size was kept within 300 μm. The spectra were recorded in the fixed analyser transmission mode, where pass energy was kept at 200 eV. Scanning of the spectra was performed at pressures less than 10⁻⁸ Torr.

In this research, decomposition efficiency of H₂O₂ was studied with respect to time in the presence of different halides and pH at room temperature. Briefly, the decomposition of H₂O₂ by a Pd–Pt/Nafion assembly was performed using a reactor with a volume of 250 mL and made of Pyrex glass. Light was excluded by wrapping the reactor with aluminium foil. A graduated glass vessel was used to measure the volume of O₂ at standard temperature and pressure. The decomposition extent of H₂O₂ was also determined quantitatively by an iodometric titration method. Both the iodometric titration and O₂ evolution measurements provided almost similar results.

Results and discussion

Fig. 1A shows relative oxygen evolution vs. time profiles of hydrogen peroxide (HP) decomposition using pure Pt, Pd, and bimetallic Pt–Pd catalysts supported on a Nafion membrane. Previously, it was reported that while HP molecules are permitted to decompose on a noble metal surface, at first a catalytic site adsorbs one oxygen atom of a HP molecule. In the following step, the adsorbed oxygen atom reacts with an unadsorbed HP molecule from the bulk to yield one molecule of oxygen and a water molecule.¹⁸ This type of reaction trail is known as Rideal–Eley (R–E) mechanism. According to the R–E model, decomposition of HP molecules on a catalytic surface (M) hence can simply be expressed as follows.

M + H₂O₂ → M–O + H₂O (3)

M–O + H₂O₂ → M + O₂ + H₂O (4)

Fig. 1 (A) Decomposition of 0.1 M H₂O₂ using Pt, Pd, and Pt–Pd/Nafion catalysts. (B) Effects of pH on decomposition rate attained by the Pt–Pd/Nafion catalyst. Volume of the reactor vessel: 250 mL; geometric area of the catalytic surface: 12 cm².

It is notable that since the rate of reaction is dependent on both adsorbed and free substrate species, therefore, the rate law of decomposition of hydrogen peroxide on a catalytic surface may be presented by eqn (5).

where k is the decomposition rate constant, and K is adsorption related constant. According to this equation, at a certain higher concentration (where 1 ≪ 2K[H₂O₂]), the decomposition reaction follows first order kinetics. Although a Pt or Pd catalyst decomposes HP molecules very slowly, their synergy in the bimetallic state is conducive to speed up the reactions (3) and/or (4), thus enhancing the reaction rate (0.015 mmol O₂ min⁻¹).

Here, it is worthwhile to mention that a solution parameter, such as pH, is very important and that it greatly influences the reactivity and selectivity of a catalytic process. Fig. 1B shows how medium pH effects the heterogeneous HP decomposition rate which was estimated in the presence of a Pt–Pd catalyst. In an acidic medium, a HP molecule is protonated to form the stable H₃O₂⁺ species.³⁸ At the same time, the potential of zero charge (V_pzc) of any metallic surface is increased if the proton concentration is increased. Therefore, the effective columbic repulsive force between the H₃O₂⁺ species and a catalytic surface determines the possibilities of a heterogeneous catalytic decomposition reaction. This is a fact reflected by the insignificant decomposition rate at pH ∼ 2 as shown in Fig. 1B. Conversely, in an alkaline medium, HP molecules undergo disproportion reactions ((6) and (7)) producing oxygen as end product.³⁹

H₂O₂ + OH⁻ → H₂O + HOO⁻ (6)

H₂O₂ + HOO⁻ → OH⁻ + H₂O + O₂ (7)

These reactions are exothermic, which was also experimentally confirmed by monitoring the solution temperature in the reactor. Hence, the evolved heat instigated the thermal decomposition of HP molecules (reaction (8)) as well.

2H₂O₂ → O₂ + 2H₂O (8)

As a result, in a basic medium in addition to the catalytic reactions ((3) and (4)), disproportion ((6) and (7)), and decomposition (8), reactions also took place showing a maximum HP decomposition rate (0.025 mmol O₂ min⁻¹) at the medium pH of 11; an evaluated rate in agreement with eqn (5).

However, the main objective of this article is to manifest how the catalytic sites execute HP decomposition reactions. In order to do this, we performed HP decomposition experiments in the presence of 2 mM of different halide ions with the supposition that any of the ions would be competitively adsorbed on the catalytic surface. It was observed that F⁻, Cl⁻ and Br⁻ did not affect the catalytic process. However, in the presence of iodide ions (I⁻), the catalytic reactions were found to be inhibited. It is seen from Fig. 2 that the HP decomposition process was even stopped after 100 min in presence of iodide; hence the oxygen evolution vs. time curve reached to a limiting state. The surface activity could not, however, be regenerated even by washing with ample amounts of water. This observation suggests that catalytic activity of Pt–Pd catalyst was irreversibly deactivated in presence of iodide ions. In order to resolve the reasons for catalytic deactivation in presence of iodide ions, physical characterizations of the catalyst were required. In the first instance, X-ray diffraction (XRD) patters of the catalytic films were evaluated to determine whether the catalytic particles exist as a mixture of elemental Pt and Pd or of a Pt–Pd alloy. The XRD patterns of Pt, Pd, Pt–Pd particles on a Nafion membrane substrate are exhibited in Fig. 3. Fig. 3b is an expanded view of the (111) peaks of these three catalysts. It is clearly seen that the position of the (111) peak of Pt–Pd (40.08) exists in between those of Pt (39.90) and Pd (40.23). This observation suggests that in the bimetallic matrix, Pt and Pd atoms do not exist randomly; rather they exist as an ordered array. The shifting of the diffraction peak is considered as an indication of the formation of Pt–Pd alloy, where Pd and Pt particles share the same face-centered cubic crystal structure.^40,41 Next, surface topography and the composition of fresh and used catalysts were compared before and after use. The particle sizes of the fresh catalyst were found to reside in a range of 107 to 161 nm with globular shapes and exist as an aggregated network as shown in panel (A) (FESEM) of Fig. 4. The composition of the catalyst was determined by EDS analysis as shown in panel (B). The Pt and Pd particles were homogeneously distributed over the Nafion surface, which was confirmed by analysing data collected from several locations. However, with fresh catalyst, the Pt to Pd ratio was determined to be 1 : 1.44. Panels (C) and (D) of Fig. 4 show the FESEM image and EDS spectra, respectively, of the catalyst after the iodide mediated experiment. It is seen from Fig. 4C that the shapes and sizes of the particles remained almost unaltered but from Fig. 4D, it is confirmed that the composition of the catalytic matrix was rehabilitated. The used surface contained a Pt : Pd ratio of 1 : 1.36. Subsequently, XPS analysis also provided almost similar information (Pt : Pd = 1 : 1.26 for fresh and 1 : 12 for the used catalyst, respectively). These data were evaluated by integrating the counts per second under the area of respective peaks of XPS spectra of Pt and Pd entities as shown in Fig. 5. This compositional change indicates that iodide ions probably were adsorbed on the catalytic surface and subsequently destabilized the catalytic matrix. However, it can be seen from Fig. 5A that after the iodide coordinated experiment, a spin–orbit doublet, i.e. 3d_5/2 at 619.40 eV and 3d_3/2 at 631.11 eV, respectively, appeared; this is a clear indication of iodide adsorption on the catalytic surface. From the deconvoluted data it was confirmed that on the catalytic surface, a smaller fraction of Pt(II) and Pd(II) species are coexisting along with those of metallic Pt⁰ and Pd⁰ particles (see S1 of ESI† for the curve fitting XPS spectra of Pt(4f) and Pd(3d) doublets).

Fig. 2 Hydrogen peroxide decomposition in absence and presence of iodide ions. Other conditions were the same as mentioned in Fig. 1.

Fig. 3 (A) XRD patterns of Pt, Pd and Pt–Pd films immobilized on Nafion membrane. (B) An expanded view of (111) peak.

Fig. 4 FESEM image ((A): fresh surface, (C): after iodide mediated experiment) of Pt–Pd catalyst and corresponding EDS spectra ((B): fresh surface, (D): after iodide mediated experiment).

Fig. 5 XPS spectra of adsorbed iodide (A), platinum (B) and palladium (C) recorded from the surface of fresh and used Pd–Pt/Nafion catalysts. The used samples were collected after HP decomposition in presence of 2 mM KI.

At this point, a question arises; on which site of the bimetallic catalyst were the iodide ions adsorbed? This may be resolved by accounting the probable charge redistribution that took place between the Pt and Pd sites in the alloyed form. When two different metal atoms coexist, charge redistribution among the dissimilar atoms often results causing an intra-cluster columbic interaction between them.^42–44 In the present case, we evaluated possible charge redistribution that occurred between Pt and Pd sites using XPS analysis. Note that the relative binding energies (BE) of the spin–orbit doublets of the Pt and Pd metals under various physical states are tabulated in Table 1 (see S2 in the ESI† for the spectra). In reference to a monometallic state, Pt metal showed doublets (Pt4f_7/2 and Pt4f_5/2) at the binding energies of 71.25 eV and 74.50 eV, respectively. However, in presence of Pd (i.e., in the bimetallic state), the doublet positions of Pt were shifted negatively to 71.10 and 74.20 eV, respectively. Meanwhile, an opposite effect is noticed in the case of Pd spectra. The Pd atoms in a monometallic state exhibited doublets at the BE of 336.74 (3d_5/2) and 342.31 eV (3d_3/2), which were shifted positively to 336.79 and 342.37 eV, respectively. These observations confirm that electronic redistribution between the Pt and Pd sites occurred under the alloyed state. Since electronegativity of Pt and Pd are 2.28 and 2.20, then under bimetallic form, Pt received charges from Pd rendering themselves partially negatively (Pt^δ−) and positively (Pd^δ+) charged, respectively. This assumption is consistent with the XPS results as indicated by shifting of the BE of Pt(4f) and Pd(3d) doublets. As a result, partially positive (Pd^δ+) sites probably induced the iodide ions (I⁻) to be adsorbed on them. XPS analysis of the used surface, after HP decomposition in presence of 2 mM KI, also supports this supposition. As can be seen from Fig. 5B, although the BE of Pt doublets (4f, Fig. 5B) remained almost unaltered after the iodide experiment, but the doublets of Pd(3d) were highly perturbed with respect to binding energy (Fig. 5C; 3d_5/2 at 336.79 to 336.41 eV, and 3d_3/2 at 342.37 to 341.50 eV) after the iodide coordinated experiment. The negative shifting of the doublet positions strongly advises that an electronic interaction occurred between the iodide ions and Pd sites leading to an adsorption of iodide ions on the catalytic surface.

Table 1 XPS details of (a) Pt(4f) and (b) Pd(3d) under various physical states

(a) Pt(4f)
Physical state of Pt	Pt(4f7/2)/eV	Pt(4f5/2)/eV
Pt/Nafion (mono metallic, fresh)	71.25	74.50
Pt in Pt–Pd/Nafion (fresh)	71.10	74.20
Pt in Pt–Pd/Nafion (used, 2 mM KI)	71.15	74.25

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(b) Pd(3d)
Physical state of Pd	Pd(3d5/2)/eV	Pd(3d3/2)/eV
Pd/Nafion (mono metallic, fresh)	336.74	342.31
Pd in Pt–Pd/Nafion (fresh)	336.79	342.37
Pd in Pt–Pd/Nafion (used, 2 mM KI)	336.41	341.50

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Consequently, referring to the partial charge redistribution and iodide adsorption phenomenon, it can be inferred that dissociation of the HP molecules was not preferentially (or directly) instigated by the Pt sites; rather the reaction was initiated by the Pd sites of the Pt–Pd bimetallic catalyst. The charge redistribution phenomenon, under the experimental conditions, satisfactorily facilitated Pd sites to form the Pd–O species by dissociating the HP molecules. In the later step, the Pd–O species reacted with free H₂O₂ molecules to complete the decomposition process. Hence, the general reaction mechanism eqn (3) and (4) can be expressed more explicitly by eqn (9) and (10) and the scheme shown in Fig. 6.

(Pt–)Pd + H₂O₂ → (Pt–)Pd–O + H₂O (9)

(Pt–)Pd–O + H₂O₂ → (Pt–)Pd + O₂ + H₂O (10)

Fig. 6 Proposed catalytic scheme of hydrogen peroxide decomposition over Pt–Pd alloyed surface.

However, a perfect catalyst enhances a selective process by lowering the free energy of activation. In the present context, this is possible when Pt and Pd sites are moderately present in the catalytic matrix. The synergy of the bimetallic Pt–Pd catalysts clearly has been tailored by the density function theory (DFT).^27,28 The DFT studies indicated that the d-band center position (w.r.t. Fermi level) of a catalyst can play a pivotal role in determining catalytic properties since this parameter is directly related to adsorption energies of the reactants on a catalyst along with their activation barriers. The location of the d-band center (ε_d) has been found to depend on both the strain and electronic coupling of dissimilar particles in the catalyst. Nørskov and co-workers reported that a compressive strain tends to down-shift the energy of the d-band center, triggering adsorbates to bind less strongly to the catalyst, whereas a tensile strain has the opposite effect.^27,28 In the case of a bimetallic catalyst, consisting of Pt and Pd sites, trivial compressive strain originating from their weak lattice contract causes a downshift of the d-band center, which reduces the binding strength for the adsorbed intermediates. This circumstance may reduce or enhance catalytic activity depending on whether adsorption or desorption of the intermediates limit the reaction rate. In the case of HP decomposition on a pure Pd metal, it was reported that desorption of an intermediate (e.g. O, reaction (10)) is the rate limiting step.¹⁸ However, to attain an ideal HP decomposition reaction, enhancement of both reactions (i.e. reactions (9) and (10)) is required. This is possible when Pt particles are moderately coexisted with Pd particles such that the ε_d value remains moderate. From EDS analysis, it was revealed that an efficient catalyst should contain a Pt to Pd composition between 0.33 and 0.75, and that a catalyst having a composition of 0.50 can exhibit maximum efficiency (as seen in Fig. 7). If the composition exceeds this range, then at least one of the processes ((9) and (10)) is interrupted. A catalyst having a Pt composition lying between 0.33 and 0.75 probably generated such a number of Pd^δ+ sites that adsorption of ‘O’ was enhanced followed by the splitting of the O–H bond (reaction (9)). Meanwhile, at the same time, the required number of Pt^δ− sites were also formed and that expedited reaction (10). Thus, the combination of these two actions favoured the overall kinetics of the impulsive HP decomposition reaction.

Fig. 7 Activity of variable catalytic composition towards hydrogen peroxide decomposition. The composition was determined by EDS analysis. Other conditions are the same as mentioned in Fig. 1.

Conclusions

Alloyed Pt–Pd/Nafion is an ideal catalyst for hydrogen peroxide decomposition. At higher pHs catalytic, disproportion and thermal processes caused rapid hydrogen peroxide decomposition. In an effective bimetallic catalyst both Pt and Pd sites should be present moderately. Iodide ions inhibited the catalytic process by adsorbing onto the Pd sites. The adsorption and desorption steps of auto-decomposition of hydrogen peroxide were facilitated by the Pd and Pt sites, respectively.

Acknowledgements

The authors acknowledge Prof. Masato Machida, Department of Nano Science and Technology, Kumamoto University, Japan for supplying required materials to prepare Pt films on a Nafion membrane. University grants commission of Bangladesh and Research Center (of Shahahjalal University of Science and Technology) are acknowledged for financial supports. The World Academy of Sciences (TWAS, 2014) is acknowledged for encouraging us by providing a research grant.

References

1. G. Centi and M. Misino, Catal. Today, 1998, 41, 287–296.
2. R. Hage and A. Lienke, Angew. Chem., Int. Ed., 2005, 45, 206–222.
3. M. A. Ak, A. Ulas, B. Sümer, B. Yazıcı, C. Yıldırım, L. O. Gönc and F. E. Orhan, Fuel, 2011, 90, 395–398.
4. W. P. Kwan and B. M. Voelker, Environ. Sci. Technol., 2003, 37, 1150–1158.
5. Y. Seol and I. Javandel, Chemosphere, 2008, 72, 537–542.
6. M. A. Hasnat, M. M. Uddin, A. J. F. Samed, S. S. Alam and S. Hossain, J. Hazard. Mater., 2007, 147, 471–477.
7. D. L. Sedlak and A. W. Andren, Water Res., 1994, 28, 1207–1215.
8. B. R. Petigara, N. V. Blough and A. C. Mignerey, Environ. Sci. Technol., 2002, 36, 639–645.
9. Y. H. Dao and J. D. Laat, Water Res., 2011, 45, 3309–3317.
10. D. B. Broughton and R. L. Wentworth, J. Am. Chem. Soc., 1947, 69, 741–744.
11. S. S. Lin and M. D. Gurol, Environ. Sci. Technol., 1998, 32, 1417–1423.
12. A. Hiroki and J. A. Laverne, J. Phys. Chem. B, 2005, 109, 3364–3370.
13. C. M. Miller and R. L. Valentine, J. Hazard. Mater., 1995, 41, 105–116.
14. M. A. Hasan, M. I. Zaki, L. Pasupulety and K. Kumari, Appl. Catal., A, 1999, 181, 171–179.
15. H. Huang, M. C. Lu and J. N. Chen, Water Res., 2001, 35, 2291–2299.
16. Y. Seol and I. Javandel, Chemosphere, 2008, 72, 537–542.
17. I. A. Salem, J. Mol. Catal., 1994, 87, 25–32.
18. Y. Voloshin, J. Manganaro and A. Lawal, Ind. Eng. Chem. Res., 2008, 47, 8119–8125.
19. V. R. Choudhary, C. Samanta and P. Jana, Appl. Catal., A, 2007, 332, 70–78.
20. V. R. Choudhary, A. G. Gaikwad and S. D. Sansare, Angew. Chem., Int. Ed., 2001, 40, 1776–1779.
21. D. A. MacInnes, J. Am. Chem. Soc., 1914, 36, 878–881.
22. D. W. McKee, J. Catal., 1969, 14, 355–364.
23. S. A. G. Evans, J. M. Elliott, L. M. Andrews, P. N. Bartlett, P. J. Doyle and G. Denuault, Anal. Chem., 2002, 74, 1322–1326.
24. H. Ye and R. M. Crooks, J. Am. Chem. Soc., 2007, 129, 3627–3633.
25. H. L. Jiang, N. Yang, J. Zhu and C. Song, Catal. Today, 2013, 216, 71–75.
26. S. M. Alia, K. O. Jensen, B. S. Pivovar and Y. Yan, ACS Catal., 2012, 2, 858–863.
27. B. Hammer and J. K. Nørskov, Adv. Catal., 2000, 45, 71–129.
28. J. Greeley, J. K. Nørskov and M. Mavrikakis, Annu. Rev. Phys. Chem., 2002, 53, 319–348.
29. J. L. Zhang, M. B. Vukmirovic, Y. Xu, M. Mavrikakis and R. R. Adzic, Angew. Chem., Int. Ed., 2005, 44, 2132–2135.
30. J. Zhang, J. Mo, M. B. Vukmirovic, R. Klie, K. Sasaki and R. R. Adzic, J. Phys. Chem. B, 2004, 108, 10955–10964.
31. M. A. Hasnat, M. Maria Rahman, S. M. Borhanuddin, A. Siddiqua, N. M. Bahadur and M. R. Karim, Catal. Commun., 2010, 12, 286–291.
32. S. M. Kim, L. Liu, S. H. Cho, H. Y. Jang and S. Park, J. Mater. Chem. A, 2013, 1, 15252–15257.
33. Y. Nie, L. Li and Z. Wei, Chem. Soc. Rev., 2015, 44, 2168–2201.
34. H. Zhang, M. Jinb and Y. Xia, Chem. Soc. Rev., 2012, 41, 8035–8049.
35. M. B. Pomfret, J. J. Pietron and J. C. Owrutsky, Langmuir, 2010, 26, 6809–6817.
36. B. Pawelec, V. La Parola, R. M. Navarro, S. Murcia–Mascaros and J. L. G. Fierro, Carbon, 2006, 44, 84–98.
37. M. A. Hasnat, M. S. Alam, M. H. Mahbub-ulKarim, M. A. Rashed and M. Machida, Appl. Catal., B, 2011, 107, 294–301.
38. P. Valtazanos, E. D. Simandiras and C. A. Nicolaides, Chem. Phys. Lett., 1989, 156, 240–244.
39. R. Sundra, Can. Chem. News, 1998, 50, 15–17.
40. X. Yang, Q. Yang, J. Xu and C.-S. Lee, J. Mater. Chem., 2012, 22, 8057–8062.
41. P. Zhang, R. Li, Y. Huang and Q. Chen, ACS Appl. Mater. Interfaces, 2014, 6, 2671–2678.
42. M. A. Ha, J. Dadras and A. Alexandrova, ACS Catal., 2014, 4, 3570–3580.
43. W. Tang and G. Henkelman, J. Chem. Phys., 2009, 130, 194504–194506.
44. G. Zhang, C. Huang, R. Qin, Z. Shao, D. An, W. Zhangaand and Y. Wang, J. Mater. Chem. A, 2015, 3, 5204–5211.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra05620a

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