Pd–Ti-MCM-48 cubic mesoporous materials for solar simulated hydrogen evolution

https://doi.org/10.1016/j.ijhydene.2014.11.089Get rights and content

Highlights

  • TiO2 and Pd nanoparticles were dispersed in periodic and aperiodic SiO2.

  • MCM-48 support shows higher hydrogen production under solar simulated conditions.

  • Pd0 containing mesoporous materials exhibit superior activity compared to Pd2+.

Abstract

A facile synthetic method (in as little as four hours) for simultaneously loading high amounts of titania (Si/Ti = 3) and Pd0 co-catalyst (0.1 wt.% per gram of total catalyst) in cubic mesoporous MCM-48 material was developed at room temperature. The solar simulated photocatalytic hydrogen evolution from photocatalysts containing Pd0 and TiO2 nanoclusters in periodic cubic MCM-48 and aperiodic mesoporous silica was compared. The results indicate that the periodicity of the mesoporous silica support, the oxidation state of Pd, the location and dispersion of Pd0 have a significant impact on the photocatalytic activity. Periodic cubic MCM-48 mesoporous silica containing Pd0 in close contact with titania exhibit superior hydrogen evolution rates compared to Pd0-TiO2 containing aperiodic mesoporous silica. The highly ordered and open three-dimensional mesoporous cubic MCM-48 support has high surface area and facilitate good dispersion and close contact of titania and Pd0. At very low loadings of 0.1 wt.% of Pd, hydrogen yield was found to be 560 μ mol h−1, which is among the highest reported in the literature for Pd0 containing TiO2 based materials under solar simulated conditions. The results suggest that the pore architecture of the support is also an important parameter that governs the photocatalytic activity. In addition, the Pd0-mesoporous materials in general possess higher activity than Pd2+ containing mesoporous materials. The photocatalysts were extensively characterized by a variety of techniques such as powder X-ray diffraction (XRD), nitrogen sorption analysis, transmission and scanning electron microscopic studies, photoluminescence, diffuse reflectance spectroscopy (DRS), CO Chemisorption, and X-ray photoelectron spectroscopy (XPS).

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.

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