Nature provides an amazing variety of creative and design inspirations for the development of human civilization. Photocatalytic reduction of CO2 into hydrocarbon fuels, inspired by the photosynthesis of green plants, is one of the most attractive solutions to both energy shortage problems and global warming. This reaction is based on the charge-carrier photo-generation of semiconductor photocatalysts, involving the reductive half-reaction of CO2 fixation with a matched oxidative half-reaction, such as water, to achieve a carbon neutral cycle. In recent years, a great deal of research has focused on the development of semiconductor photocatalysts [1-6], including TiO2 [7-9], ZnO [10], CdS [11], ZnGa2O4 [12], Zn2GeO4 [13, 14], and metal-organic frameworks [15]. A series of catalysts with novel nanostructures have also been synthesized, such as ultrathin nanoribbons [16-19] and ultrathin nanosheets [20，21] to achieve high conversion efficiency.
To make breakthrough in the photo-conversion efficiency of CO2, many features of photocatalysts, such as their composition and structure, have been carefully designed to exactly mimic the natural process of photosynthesis. CO2 activation is one of the most challenging in chemistry because CO2 is highly stable and inert. Chlorophyll plays an important role in photosynthesis and consists of porphyry in and Mg ions. Design of innovative MgO-modified photocatalysts may enhance the photo-conversion efficiency of CO2 [23]. MgO on TiO2 photocatalysts can activate adsorbed CO2 molecules [22], which promotes the production of HCO3− [23], a possible intermediate for the production of hydrocarbon fuels or CO when the dissociative H atoms are available [24，25].
Biologically, both the multi-scale pores and the interconnectivity between pores of leaf scaffolds facilitate efficient mass flow, which makes green leaves an ideal system for photosynthesis. Ye and co-workers [26] demonstrated an efficient mass flow network for improved gas diffusion and light harvesting relying on the morphological replacement of leaf 3D hierarchical architecture into perovskite titanates.We recently reported that 3D interconnected macro/mesoporous TiO2 sponges can significantly enhance the reaction efficiency because of strong gas diffusion of the reactants and the products and more reaction sites arising from high specific surface areas [27].
As the connection between leaf and root, the stem plays an important role in efficient hydraulic supply, delivery of nutrients and sugars to the whole plant, and biomechanical support. For some plants, stems also possess photosynthetic, nutrient storage and reproductive functions, with one such typical example being water convolvulus. The stem of water convolvulus represents a hierarchical, hollow structure on macro/micro scales, an optimized structure facilitating gas diffusion. Experimentally, we found that this morphology could be well preserved after being dried by lyophilization and subsequently treated with a HCl solution 5% to remove metallic ions. In the present work, we employed the stem of water convolvulus as a biotemplate for the replication of its optimized structure to synthesize bionic-macro/microporous MgO-modified TiO2 for efficient photo-conversion of CO2 into CH4.
The typical synthesis of bionic-macro/microporous MgO-modified TiO2 photocatalysts is as follows: First, fresh water convolvulus stems were shredded into 5 cm long pieces and washed with distilled water, then immersed in 5% diluted HCl for 5 h to remove Mg, K, Ca and other ions, with the color turning from green to yellow-brown. After rinsing several times with distilled water, the as-treated samples were fully dried by lyophilization. Secondly, the dried water convolvulus stems were successively dipped into a closed vessel containing 10 vol% solution of tetra-n-butyl titanium (TBT) in 2-propanol and 2-propanol/water (v:v = 1:1) for 1 h during which hydrolysis and condensation reactions occurred, and a Mg(NO3)2 solution (containing MgO at 0,0.05,0.1,0.2,0.3,0.4, or 0.5 wt%). After removal from the last solution, the as-treated samples were fully dried by lyophilization. Finally, the resulting samples were calcined in air at 280 ℃ for 2 h and 500 ℃ for 3 h with a ramping rate of 10 ℃/min to remove the templates and allow crystallization of the MgO-modified TiO2.
The crystallographic phase of the as-prepared products was determined by powder X-ray diffraction (XRD, Rigaku Ultima Ⅲ, Japan) using Cu-Kα radiation (λ =0.154178 nm) with a scan rate of 10 min−1 at 40 kV and 40 mA. The samples were analyzed with X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher Scientific, USA). The XPS spectrum was calibrated with respect to the binding energy of the adventitious C1s peak at 284.8 eV. The morphology of the samples was observed by field emission scanning electron microscopy (FE-SEM, FEI NOVA NanoSEM 230, USA) and transmission electron microscopy (TEM, JEOL 3010, Japan). The specific surface areas of the samples were measured by nitrogen sorption at −196 ℃ on a surface area and porosity analyzer (Micromeritics TriStar, USA) and calculated by the BET method. The CO2 surface absorption of the samples was evaluated with the above-mentioned adsorption apparatus under ambient pressure and 0 ℃.
In the photocatalytic reduction of CO2,0.1 g of samples was uniformly dispersed on a glass reactor with an area of 4.2 cm2. A 300 W xenon arc lamp was used as the light source for the photocatalytic reaction. The reaction system volume was approximately 230 mL. The reaction setup was vacuum-treated several times, and then high purity CO2 gas was flowed into the reaction setup to reach ambient pressure. Deionized water (0.4 mL) was injected into the reaction system as a reducing agent. The as-prepared photocatalysts were allowed to equilibrate in the CO2/H2O atmosphere for several hours to ensure that the adsorption of gas molecules was complete. During irradiation, approximately 1 mL of gas was taken from the reaction cell at given time intervals for subsequent CH4 concentration analysis using a gas chromatograph (GC-2014, Shimadzu Co., Japan).
The in situ Fourier transform infrared spectroscopy (FT-IR) studies were performed using a Praying Mantis DRIFTS accessory and a reaction chamber (Harrick Scientific). All IR spectra were recorded using an FT-IR spectrometer (IRPrestige-21, Shimadzu Co., Japan). Prior to testing, the samples were purged with N2 for 3 h at 150 ℃ to clean the catalyst surface, then cooled to room temperature, and the background spectrum in the presence of the sample was collected. The in situ FT-IR analysis was carried out in two sequential steps subsequent to a single drop of deionized water (about 0.05 mL) being injected into the system as a water vapor supply. First, the adsorption of reactants on the catalyst surface was studied by introducing CO2 to the IR cell for 30 min in the dark (step 1). Next, a stream of N2 was used to purge the catalyst surface for 30 min (step 2), after which the spectrum was collected.
XRD measurements were conducted to compare the composition and crystal structure of MgO-modified TiO2 samples templated from water convolvulus stems, as shown in Fig. 1. Almost all of the samples exhibited identical TiO2 crystal phases assigned to anatase. No MgO diffraction peaks were detected, which was likely due to the small amount of MgO. It is noteworthy that the diffraction peaks of TiO2 tended to sharpen as the MgO content increased. This trend suggests much stronger aggregation and increased crystallinity of the TiO2 nanoparticles as a result of the addition of Mg [28].
Fig. 2 shows the XPS spectra of the pure TiO2 and the 0.5% MgO-TiO2. Both of the whole XPS surveys (Fig. 2(a)) demonstrated that a small amount of S exists in the oxides, indicating that the S contained in the original templates was self-doped into the resulting samples. Elements such as Mg, K, Ca, Na and Fe were not detected in the TiO2 samples, proving that the dilute HCl effectively removed them, and that the Mg results entirely from the decomposition of adventitious Mg(NO3)2. The comparison of the high-resolution scanning of O 1s between pure TiO2 and 0.5% MgO-TiO2 (Fig. 2(b)) indicates that the Mg exists in the form of MgO in Mg-modified TiO2 [29-31].
Macroscopically, the 2D optical images of the water convolvulus stem exhibit a hollow structure on centimeter scales (Fig. 3(a)). Microscopically, the stem is composed of hierarchical cylindrical tubes with lengths of several to tens of micrometers (Fig. 3(b-d)), and each tube is composed of stack-by-stack layers of interconnected hollow sub-micro tubes (Fig. 3(e)), which results in the development of more efficient hydraulic supply and delivery of nutrients. During the sol-gel process, the abundant −OH functional groups on the cellulose and hemicellulose of the cell walls of vascular bundles reacted with the precursor to deposit it on the template. After drying and calcination, the precursor condensed and crystallized to form the MgO-TiO2 network. Fig. 4 shows the FE-SEM images of 0.2% MgO-TiO2.
Relative to the applied bio-templates, the cross section hierarchical porous network was well replicated (Fig. 4(a)). Microstructures in the longitudinal direction revealed that the walls of the MgO-TiO2 vascular bundles were also porous, with a pore length of several micrometers (Fig. 4(b) and (c)). The high-resolution TEM image further reveals the well-defined lattice fringe of the incorporated MgO nanoparticle. A mesopore with a diameter of approximately 3 nm was also observed (Fig. 4(d)), which coincides with the BET nitrogen adsorption-desorption isotherm below.
The effect of MgO on the BET surface area and porous structure of the MgO-TiO2 was investigated using nitrogen adsorption-desorption measurements. The nitrogen adsorption-desorption isotherms of all samples are type IV, which is associated with a typical mesoporous structure [32]. Typically, the pore diameter of 0.2% MgO-TiO2 is estimated to be approximately 3 nm (Fig. 5). The related BET specific surface areas and pore volumes of pure TiO2 and various ratios ofMgO-TiO2 are listed in Table 1, showing that the surface areas and pore volumes clearly decreased with increases in the MgO loading. This is likely because of the mesopores between TiO2 nanoparticles being occupied by MgO nanoparticles and much stronger aggregation of TiO2 nanoparticles because of the addition of Mg [23], as supported by the above XRD analysis. The amount of CO2 absorbed on the MgO-TiO2 under ambient pressure at 0 ℃ initially increased and then decreased with increasing MgO content. This tendency is closely related to the CO2 physisorption and chemisorption on the MgO-TiO2. Increasing MgO may enhance the number of CO2 activity sites, resulting in improvement of the chemisorption capacity of CO2. However, the decrease of the BET surface will reduce the CO2 physisorption when increasing the MgO quantity. The 0.2% MgO-TiO2 sample exhibited the maximum CO2 absorption capacity.
Photocatalytic conversion of CO2 to renewable hydrocarbons using solar energy is one of the best solutions to both energy shortage problems and global warming. Generally, in the presence of water vapor, CO2 can be photoreduced into CH4 using a wide band gap semiconductor as a photocatalyst through CO2 reduction of CO2 + 8e− + 8H+ → CH4 +2H2O (Eoredox = −0.24 V, vs. NHE) and water oxidation of 2H2O → O2 + 4H+ +4e−CB (Eoredox = 0.82 V, vs. NHE). To explore the effect of MgO on the activation of CO2 and the photocatalytic activities of the porous MgO-TiO2 samples derived from the water convolvulus stems, photoreduction of CO2 was investigated. Gas chromatographic analysis demonstrates that CH4 was exclusively obtained as the reduction product, without detectable CO or C2H6 as secondary products. A blank experiment with identical conditions but the absence of CO2 exhibited no appearance of CH4, verifying that the carbon source was derived entirely from the input CO2. Fig. 6 demonstrates that the gross yield of CH4 increases with photoreduction evolution time. The average rate of CH4 production initially increased and then decreased with increasing MgO content, which is similar but not identical to the trend of CO2 adsorption capacity. It should be noted that all of the MgO-TiO2 showed higher CH4 production relative to pure TiO2, demonstrating that loaded MgO can indeed activate CO2 to improve the photocatalytic process. In situ FT-IR analysis confirms the activation of CO2 on the MgO (Fig. 7). Compared with TiO2 and 0.05% MgO-TiO2,0.2% MgO-TiO2 shows obvious bicarbonate (HCO3−) peaks at 1211,1424,1544, and 1645 cm-1, formed as a result of reactive adsorption of CO2 on the MgO surface. Nevertheless, an excess amount of MgO decreases the catalytic activity as the insulating MgO, when overlapping the surface of TiO2, would possibly block the migration of photo-generated charge carriers to the catalyst surface. Therefore, an overall consideration of various factors involving CO2 adsorption capacity, migration of charge carriers to the photocatalyst surface, and the number of activity sites determines the photoconversion efficiency of the CO2 with MgO-modified TiO2. The 0.2% MgO-TiO2 sample exhibits the best photocatalytic activity.
Water convolvulus stems were employed as a biotemplate for the replication of their optimized 3D hierarchical architecture to synthesize MgO-TiO2 for photoconversion of CO2 into CH4. A certain amount of MgO plays an important role in improving CO2 adsorption and activation. Various factors involving CO2 adsorption capacity, migration of charge carriers to the photocatalyst surface, and the number of activity sites, which depend on the amount of added MgO, affect catalyst performance and determine the photocatalytic conversion efficiency.
2016,
Vol. 37 Issue (6): 863-868 
PDF (21886 KB)
2016,
Vol. 37 Issue (6): 863-868 PDF (21886 KB)