Pd–Ti-MCM-48 cubic mesoporous materials for solar simulated hydrogen evolution
Graphical abstract
Introduction
Solar hydrogen generated through photocatalytic splitting of water offers a sustainable and clean way to solve the energy problem and also alleviate the environmental impact caused by using conventional fossil based energy sources. Since the pioneering work of Fujishima and Honda [1], extensive research has been devoted into photoelectrochemical and photocatalytic splitting of water [2], [3], [4]. Among the myriad number of photocatalytic systems developed, TiO2 has attracted attention due to its ease of preparation and robustness under diverse sets of conditions [5], [6]. Also, the favorable band structures of TiO2 in principle should enable the evolution of both hydrogen and oxygen from water. However, the photocatalytic activities of TiO2 based materials are limited by the large band-gap energy and fast charge-carrier recombination. The wide band-gap energy of TiO2, for instance ∼3.2 eV in the case of anatase, limits its application, since only Ultra-Violet radiation can be used to excite the electrons from the valence band to the conduction band. Also, the fast charge-carrier recombination in bulk TiO2 is detrimental to its photocatalytic efficiency. Extensive efforts have been made to overcome these drawbacks to increase the photocatalytic performance.
A commonly adopted strategy to extend the response of wide band-gap semiconductors and minimize electron-hole recombination is to deposit noble metals, such as Pt, Pd, Ag, and Au etc. [7], [8], [9], [10]. The noble metals in the form of metallic nanoparticles such as Pt, Au etc. serve as electron traps (due to their high electron affinity) and minimize electron-hole recombination at optimal loading levels. In addition, several metals such as Pt, Pd, etc. can also act as co-catalysts for hydrogen evolution. Therefore, the addition of noble metal nanoparticles is conducive in general in improving the efficiencies of TiO2 based photocatalysts.
In addition to loading noble metals, another approach to modulate the photocatalytic activity of TiO2 is to disperse it in high surface area supports such as silica and alumina. The advantages of dispersing TiO2 in other metal oxides can be summarized as follows: i) effective separation of the charge carriers is facilitated by spatial separation of the titania particles [11], [12], ii) the support provides a robust matrix for the dispersion of TiO2 and the high porosity of the support facilitate diffusion of the reactant molecules to the active titania sites, iii) the particle size of TiO2 can be constrained to small sizes so that volume recombination of charge carriers can be minimized [13], and iv) photocatalytic activity of the composite binary metal oxides may be enhanced in certain circumstances owing to the increased surface acidity in these materials [14], [15]. With these factors in mind, binary metal oxide composite TiO2–SiO2 photocatalysts have been prepared and investigated [16], [17], [18], [19], [20], [21], [22].
In recent years, periodic mesoporous materials have attracted immense interest for applications ranging from drug delivery to catalysis [23]. In particular, because of their large surface area and pore volume and tunable pore size, they have received great attention in the field of catalysis [24]. Among the various classes of mesoporous materials, the M41S series originally developed by Mobil researchers continues to be a popular choice of support for various reactions [25]. The cubic form, MCM-48 is an interesting material since it possesses three-dimensional network of pores compared to the popular MCM-41 that contains uni-dimensional network of pores. Titania containing MCM-48 materials have been synthesized and explored for oxidation reactions [26], [27], [28], [29]. However, MCM-48 material has not been explored extensively as a support for photocatalytic reactions and literature reports are limited [30], [31], [32]. In particular, their applications for photocatalytic water splitting are scarce, barring work from our group.
Our group has worked extensively with TiO2 containing periodic mesoporous silica materials for photocatalytic splitting of water. In our previous studies, we have systematically investigated and compared the photocatalytic activities of various sets of TiO2–SiO2 based photocatalysts including: i) TiO2 supported on aperiodic mesoporous SiO2 (commercially available, 220–400 mesh size) [33], ii) TiO2 supported on periodic mesoporous MCM-48 [34], and iii) TiO2 supported on MCM-41 [35] materials. Our studies have indicated the following: i) the large surface areas of mesoporous SiO2 facilitate the high dispersion of TiO2 and limit the growth of TiO2 in SiO2 matrices, ii) both aperiodic and periodic mesoporous SiO2 are robust host materials for dispersing TiO2, iii) the photocatalytic activity of spatially isolated and tetrahedrally coordinated Ti4+ is superior to that of octahedrally coordinated Ti4+, iv) in general, smaller sized TiO2 exhibit higher photocatalytic efficiency until an optimal value is attained, and most interestingly, v) the geometry of the mesoporous SiO2 host material has an overriding influence on the photocatalytic activities compared with other factors such as surface area of the TiO2–SiO2 composite materials, particle size of TiO2 clusters, and the coordination of Ti4+ ions. Therefore, it is important to probe and unveil the effect of mesoporous SiO2 support in photocatalytic processes. We have demonstrated that TiO2 supported on periodic cubic MCM-48 shows higher activity compared to TiO2 supported on hexagonal MCM-41 mesoporous materials, however, there is no literature comparing the activity of TiO2 supported on highly ordered periodic mesoporous materials and irregular or disordered (random orientation of pores) mesoporous materials for photocatalytic splitting of water. Towards understanding this factor, in this work, we have selected two supports, a highly ordered and periodic mesoporous cubic MCM-48 support and an aperiodic (non-ordered) mesoporous silica support. It will be interesting to compare if the periodicity of the pores have an influence in the generation of hydrogen from photocatalytic splitting of water under identical loadings of titania. Herein, we use relatively high loading of TiO2 (Si/Ti = 3). Also, the mesoporous materials were loaded with Pd (0.1 wt.% per gram of total catalyst) as a co-catalyst with a purpose of evaluating the photocatalytic splitting of water under solar simulated conditions.
In this work, Pd has been selected as a co-catalyst in our TiO2 containing MCM-48 mesoporous photocatalyst system since it has been widely applied in several photocatalytic reactions [36], [37], [38], [39], [40], [41], [42], [43], [44]. The photocatalytic activity of Pd containing TiO2 materials for photocatalytic splitting of water has been investigated using both powdered suspensions and thin films [43], [45], [46], [47], [48] with yields of hydrogen ranging from 3.8 μ mol h−1 to 1500 μ mol h−1 under UV light illumination. In contrast, only limited studies have been carried out using Pd0 containing titania semiconductors under solar simulated conditions and these have resulted in relatively poor yields of hydrogen in the range of 0.8 μ mol h−1 to 150 μ mol h−1 [49], [50], [51], [52].
The preparation of high quality cubic 3-D MCM-48 mesoporous materials is much more difficult compared to MCM-41 since the cubic phase is extremely sensitive to small variations in the experimental conditions [53]. In particular, the preparation of MCM-48 with high loadings of metal oxide is a challenge at room temperature. This work is the first to incorporate high loadings of TiO2 into both periodic and aperiodic mesoporous SiO2 and compare their activities towards hydrogen generation from photocatalytic splitting of water under solar simulated conditions. In this work, the preparation of cubic MCM-48 periodic and aperiodic mesoporous materials were achieved by simply varying the sequence of addition of the Ti precursor. This observation proves that the formation of cubic mesoporous MCM-48 is extremely sensitive to the experimental conditions. All the materials were elaborately characterized using a wealth of techniques ranging from powder X-ray diffraction (XRD), nitrogen physisorption, CO-pulse chemisorption, UV–vis diffuse reflectance spectra (DRS), transmission electron microscopy (TEM), photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS).
Section snippets
Materials
Sodium terachloropalladate (II) (Na2[PdCl4]) as the precursor of palladium source was purchased from Pressure Chemicals. Titanium (IV) isopropoxide (98+%), Tetraethyl orthosilicate (TEOS, 98%) and sodium borohydride (98%) were obtained from Acros. Cetyltrimethylammonium bromide (CTAB, 98%) and tetra-n-octylammoniumbromide (TOABr, 98+%) were purchased from Alfa Aesar. Ammonium hydroxide and ethanol were obtained from Fisher and Pharmo-AAPER respectively. Deionized water was used throughout this
XRD analysis
Fig. 1a shows low angle powder XRD patterns of all the studied materials. The presence of a good quality cubic MCM-48 phase is indicated from the strong d211 and weak d220 reflection peaks in the range of 2.5–3.5°. For the materials that were prepared via an in-situ route, it can be seen from the XRD patterns that the different timing of the addition of the Ti source can result in materials with different morphologies (cubic and disordered) in the final outcomes. For the two materials (Pd2+
Conclusions
Pd0 nanoparticles and TiO2 (at high loadings) incorporated into a high surface area cubic MCM-48 mesoporous materials presented highly efficient activities and long-term stabilities for photocatalytic hydrogen evolution under irradiation of solar simulated light. Periodic cubic phased 3-D MCM-48 mesoporous material proved to be a better support compared to an aperiodic mesoporous support. The high surface area, the open and inter-connected network of pores, the good dispersion and contact of Pd0
Acknowledgments
Thanks are due to NSF-CHE-0722632, NSF-EPS-0903804, DE-EE0000270, and SD NASA-EPSCOR NNX12AB17G. We are thankful to Mr. S. Mishra and Dr. Phil Ahrenkiel at South Dakota School of Mines and Technology for assistance with TEM studies.
References (57)
- et al.
Photodegradation of phenol in water using silica-supported titania catalysts
Appl Catal B Environ
(1997) - et al.
Synthesis of nanosized TiO2/SiO2 particles in the microemulsion and their photocatalytic activity on the decomposition of p-nitrophenol
Catal Today
(2003) - et al.
Photocatalytic degradation of aqueous pollutants using silica-modified TiO2
Water Res
(2003) - et al.
Sol–gel synthesis of titania–silica photocatalyst for cyanide photodegradation
J Photochem Photobiol A Chem
(2004) - et al.
Photoresponse and AC impedance characterization of TiO2–SiO2 mixed oxide for photocatalytic water decomposition
Catal Today
(2003) - et al.
Water decomposition on TiO2–SiO2 and RuS2/TiO2–SiO2 photocatalysts: the effect of electronic characteristics
Catal Commun
(2004) - et al.
Ordered mesoporous materials as catalysts
- et al.
Ti containing mesoporous silica submicrometer-sphere, with tunable particle size for styrene oxidation
Appl Surf Sci
(2013) - et al.
The epoxidation of allyl alcohol on Ti-complex/MCM-48 catalyst
Microporous Mesoporous Mater
(2008) - et al.
Synthesis of Ti-containing MCM-48 by using TiF4 as titanium source
Mater Lett
(2008)
Synthesis and structural properties of titanium containing microporous/mesoporous silicate composite (Ti, Al)-Beta/MCM-48
Microporous Mesoporous Mater
One-step synthesis of highly active Ti-containing Cr-modified MCM-48 mesoporous material and the photocatalytic performance for decomposition of H2S under visible light
Appl Surf Sci
TiO2–SiO2 mixed oxides: organic ligand templated controlled deposition of titania and their photocatalytic activities for hydrogen production
Int J Hydrogen Energy
Rapid and facile synthesis of Ti-MCM-48 mesoporous material and the photocatalytic performance for hydrogen evolution
Int J Hydrogen Energy
Catalysis at the metal-support interface: exemplified by the photocatalytic reforming of methanol on Pd/TiO2
J Catal
Characterization and activity of Pd-modified TiO2 catalysts for photocatalytic oxidation of NO in gas phase
J Hazard Mater
Photooxidation of xylenol orange in the presence of palladium-modified TiO2 catalysts
Catal Commun
Palladium enhanced resistance to deactivation of titanium dioxide during the photocatalytic oxidation of toluene vapors
Appl Catal B Environ
A composite visible-light photocatalyst for hydrogen production
J Power Sources
Relationship between Pd oxidation states on TiO2 and the photocatalytic oxidation behaviors of nitric oxide
Chemosphere
Pd-Gardenia-TiO2 as a photocatalyst for H2 evolution from pure water
Int J Hydrogen Energy
Comparative investigation on photocatalytic hydrogen evolution over Cu-, Pd-, and Au-loaded mesoporous TiO2 photocatalysts
Catal Commun
Decomposition of water in the separate evolution of hydrogen and oxygen using visible light-responsive TiO2 thin film photocatalysts: effect of the work function of the substrates on the yield of the reaction
Appl Catal A General
Modification of the photocatalydic properties of self doped TiO2 nanoparticles for hydrogen generation using sunlight type radiation
Int J Hydrogen Energy
Enhanced photocatalytic activity of indium and nitrogen co-doped TiO2–Pd nanocomposites for hydrogen generation
Appl Catal A General
Synthesis of substituted acetylenes, aryl–alkyl ethers, 2-alkene-4-ynoates and nitriles using heterogeneous mesoporous Pd-MCM-48 as reusable catalyst
Tetrahedron
Preparation of TiO2-SiO2 aperiodic mesoporous materials with controllable formation of tetrahedrally coordinated Ti4+ ions and their performance for photocatalytic hydrogen production
Int J Hydrogen Energy
Electrochemical photolysis of water at a semiconductor electrode
Nature
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