Controlable Synthesis And Photocatalytic Property Of CdS-TiO₂ For Hydrogen Production

Nowadays, the world is facing serious energy and environmental problems, so it is very urgent to produce a clean and sustainable hydrogen energy using renewable energy source to solve such problems. Hydrogen energy has many advantages, such as: it could be produced from many hydrogen-containing resources and utilized in various forms; it has excellent combustion performance and water is the only combustion product; moreover, the water produced from the burning of hydrogen can be reused to generate hydrogen. Considering above merits, hydrogen energy has been regarded as the most promising alternative energy resource in 21st century by the majority of scholars around the world. Among various hydrogen production paths, the heterogeneous photocatalytic water splitting technique is increasingly under the spotlight, due to the fact that the system is simple and sustainable, and solar energy could be utilized directly. As UV and visible light response photocatalysts for hydrogen generation, TiO₂ and CdS are two of the most representative materials, the research of which is of continuing popularity. They have become the preferred base material to study the microscopic mechanism of photocatalytic hydrogen generation reaction. They are still the favorable objects for researchers around the world today. They provide reference for the design and preparation of other types of photocatalytic materials for photocatalytic hydrogen generation. With the new method of materials science and the development of new ideas, different crystal form, different morphology and structure of TiO₂ and CdS were prepared, however, the apparent characteristics on the catalytic activity is still worth studying. According to the above idea, some new types of TiO₂, CdS, and CdS and Ti-based complex catalysts for photocatalytic water splitting were successfully synthesized in this paper. The preparation, characterization, photocatalytic water splitting properties and the visible light sensitization of them were studied in detail by using XRD, DRS, XPS, SEM, TEM, BET, PL and elemental analysis techniques. It revealed the structure-activity relationship between the morphology of the material and the activity of the hydrogen production from water splitting. Moreover, the system of the hydrogen production from photocatalytic steam reforming of methane was also researched. The content of this study and important conclusions were summarized as follows:1. By hydrolysis, hydrothermal, ion exchange and re-roasting process, fibrous monoclinic state TiO₂ (B) has been successfully prepared, employing tetrabutyl titanate as titanium source, and 10 M NaOH as hydrolysis solution. In this system, Na_xH_2-xTi₃O₇·nH2O nanotube and monoclinic state Na2Ti6O13 nanofiber were also obtained by adjusting the hydrothermal temperature. The the sequence of the hydrogen evolution rates was P25 > Na_xH_2-xTi₃O₇.nH2O > TiO₂( anatase )?TiO₂(B) > Na2Ti6O13. TiO₂(B) was still monoclinic state as the calcination temperature increased from 200 oC to 400 oC, while monoclinic state TiO₂(B) thoroughly tranalated into anatase TiO₂ as the roasting temperature increased to 500 oC. The photocatalytic hydrogen evolution decreased, then increased as the calcination temperature of TiO₂(B) increased. The specific surface area decreased and the phase transition were the two key factors.2. Various morphologies of anatase TiO₂ photocatalysts, such as solid nanosph- ere(s-TiO₂), hollow nanosphere (h-TiO₂), nantube (a-TNT), and mesoporous from (m-TiO₂), had been obtained by different methods. The s-TiO₂ were composed of small monodispersed primary particles of 10-20 nanometers in size, and the mosopores among the small primary particles produced a hierarchical porous structure in crystalline TiO₂, and the specific surface area was 142.4 m²/g. The h-TiO₂ was the hollow spherical structure with a shell, and the diameter was between 3 5.5?m, and the specific surface area was 68.2m²/g. The a-TNT was obtained by the titanate nanotubes calcined through the formation of dehydration. The high calcination temperature would lead to peeling rupture of nanotubes, so the best calcination temperature was about 350 oC. Length of the a-TNT was been varied from 50 to 150 nm and diameter was obout 10 nm. The high specific surface area was 264m²/g. The m-TiO₂ was a“straight”shape mesoporous structure with pore size distribution concentrated in the 7-9 nm, and the surface area was 39.8m²/g. The mesoporous structure is unstable by the ultrasonic treatment. The the sequence of the hydrogen evolution rates was s-TiO₂ > h-TiO₂ > m-TiO₂ > P25 > a-TNT. The sub-nanocrystal- lites of s-TiO₂ play an important role in the photocatalytic reaction. The small particles were beneficial to the migration of photogenerated electrons and holes to the surface and the suppression of electron/hole recombination in the bulk.3. Novel CdS nanomaterials had been synthesized by a simple“one-pot”hydrother- mal biomolecule-assisted method using glutathione (GSH) as the sulfur source and structure-directing reagent. Various morphologies of CdS photocatalysts, such as solid nanospheres (s-CdS), hollow nanospheres (h-CdS) and nanorods (r-CdS), were been obtained by controlling only the hydrothermal temperatures. The formation mechanism of different morphology might be due to complexation, S-C bond rupture, spherical aggregation and Ostwald ripening processes. In the "Ostwald" crystallization stage, the fast supply of reaction precursor resulted in isotropic growth of nuclei leading to the formation of h-CdS, while diffusion effect caused by fractal growth of the crystal (anisotropic crystallization) leads to [002] preferred growth and the formation of r-CdS. The visible light photocatalysis activities of the different morphology of CdS were been investigated through the photocatalytic water splitting. The sequence of the hydrogen evolution rates was s-CdS > h-CdS > commercial CdS > r-CdS. The sub-nanocrystallites of solid spheres play an important role in the photocatalytic reaction. The solid nanosphere is beneficial to the migration of photogenerated electrons and holes to the surface and the suppression of electron/hole recombination in the bulk.4. The three composite materials of Ti-based nano material were prepared by different methods on the basis of Ti-based nano material. Under??420 nm, the the sequence of the hydrogen evolution rates was CdS/TNT-1 > CdS/TNT-2 > 20 wt% CdS + 80 wt% TNT-1 > CdS > CdS/TNT-3.The apparent quantum yield for hydrogen production reached about 43.4% for the CdS/TNT-1 under visible light around?= 420 nm. The surface structures of concave shape might contribute to the formation of a“nest”on the inner surface of tubes. This structure promoted a uniform distribution of the small CdS particles, which were strongly associated with the“nest”, and restrained the aggregating and growing of these particles. The CdS nanoparticles incorporated into the TNT-1 were smaller size, and were more homogeneous dispersion than those particles formed outside or on the surface of TNT-2 (or TNT-3). These CdS particles will contribute to a efficient electron–hole pairs separation and the electrons fast transport. Moreover, in such system, the distance that the photoinduced holes and electrons have to diffuse before reaching the interface decreased. Because of the decreased distance, the holes and electrons could be effectively captured by the electrolyte in the solution. The apparent quantum yield was quite high for CdS/TNT-1, which resulted from a“synergetic effect”of CdS particles and TNT-1. The potential gradient at the interface between CdSNPs and TNT-1 helped to facilitate the diffusion of photoelectrons generated from CdS particles toward TNT-1, and led to high photocatalytic activity of hydrogen production. 5. Photocatalytic SRM to produce hydrogen could proceed on Pt/TiO₂ upon UV irradiation around room temperature in a fixed bed device under continuous dynamic conditions,ca.323K. CH₄ + 2 H₂O (g) hpc 4H₂+CO₂?Focuses on the different parameters on the reaction, the ratios of CH4/H₂O were almost 4, showing that the reaction reached the hightest hydrogen production rate and the maximum the ratios of H₂/CO₂. The optimum total feed flow rate was obout 0.5 ml/min. High activity was obtained on Pt/TiO₂ photocatalysts enough loaded of metallic Pt and NiOx. The catalyst prepared by the photodeposition mwthod showed much higher activity than that prepared by the impregnation method. The optimal amount of catalyst was about 20 mg/cm² in the fixed-bed unit. This reaction was promoted by the electrons and holes generated by the band gap (200 nm-300 nm) photoexcitation of TiO₂ semiconductor. Moreover, the catalyst has good stability by recycling, which has good application prospects. By comparing the TiO₂ with different morphologies with commercially P25, the sequence of the hydrogen evolution rates was s-TiO₂ > a-TNT > P25 > h-TiO₂ > m-TiO₂. The specific surface area and morphology of the catalyst were the important two factors in this gas-solid photocatalytic reaction. The reaction intermediates, possibly described as [CH₂O]n,ad were formed on the surface of the catalyst, and reacted with H₂O to produce H₂ and CO₂. The formation of the surface intermediates depended on the activity of the photocatalyst, the CH4 concentration in the reaction gas, and the incident light wavelength. The moderate accumulation of the surface intermediates considerably enhanced the reaction rate. The reaction mechanism was also proposed.