Fabrication of La-doped NaTaO3 via H2O2 Assisted Sol-Gel Route and Their Photocatalytic Activity for Hydrogen Production

• Main Authors: Husni Husin, 胡星妮
• Other Authors: Bing Joe Hwang
• Format: Others
• Language: en_US
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/89134179375109059893

博士 === 國立臺灣科技大學 === 化學工程系 === 99 === Hydrogen is an ideal source of clean energy as well as being a raw material in many chemical industries. Recently, hydrogen has been mainly obtained from non-renewable resources (e.g., fossil fuels) or from high-energy consumption processes that are neither environmentally friendly nor economical. Therefore, the development of new methods to produce hydrogen from sustainable materials, such as biomass and water, will become a hot topic of research in the coming decades. The photocatalytic production of hydrogen from water is an attractive and potentially rewarding approach because water is abundant and freely available. The reaction processes can occur in ambient conditions using only sunlight and a metal oxide semiconductor photocatalyst. Among various metal oxides, NaTaO3 was reported to be one of the most efficient photocatalysts for water decomposition. The general goal of this research has been to develop the potential of a La-doped NaTaO3 photocatalyst for use in hydrogen production. To achieve this, crystalline NaTaO3 nanoparticles (NPs) doped with different concentrations of La3+ were synthesized via a H2O2-assisted sol-gel route using a hydrogen peroxide-water based solvent system (HW-derived). In this reaction, TaCl5 was dissolved in aqueous H2O2 solution to form a stable transparent Ta-peroxo complex solution. The formation of tantalum-peroxo complexes and their chelation by citric acid enables a better control of crystal growth. The substitution of La3+ ions in the NaTaO3 lattice is verified by crystallographic simulation (CaRIne Crystallography version 3.1). These results indicate that La3+ ions occupy the Na+ ions sites, which agrees very well with the experimental data. The optimal content of La3+ ions effectively increases crystallinity without agglomeration, contributing to efficient charge separation while preventing recombination between photogenerated electrons and holes. The highest photocatalytic H2 production of 2.9 mmol g-1cat.h-1 was obtained for a 2.0 mol% La-doped NaTaO3 sample, 1.8 times-higher than the non-doped NaTaO3. The photocatalytic activity of water splitting on the photocatalyst HW-derived were compared with those prepared by conventional sol-gel samples made using ethanol as a solvent (ET-derived). The H2 evolution of the HW-derived sample is about 1.65 times higher than ET-derived sample. Compared to the conventional sol-gel method, the H2O2-assisted sol-gel route produced La-doped NaTaO3 with good crystallinity. These materials exhibited higher photocatalytic activity for the HW-derived samples in water splitting than the ET-derived produced material. The activity of the sample was able to be increased 10-fold by depositing nickel nanoparticles (NPs) as a cocatalyst on the surface of the La0.02Na0.98TaO3. The possible mechanisms of H2 evolution from pure water and from aqueous methanol solutions using nickel in three states (i.e. Ni metal, NiO oxide, and Ni/NiO core/shell)-La0.02Na00.98TaO3, are discussed systematically. It is clearly shown that the activity of hydrogen generation from pure water is in the sequence: Ni/NiO > NiO >Ni, whereas the activity sequence with respect to aqueous methanol is: Ni > Ni/NiO > NiO. In this work, a novel bimetallic Pd/NiO core/shell nanoparticles (NPs) was also deposited on La0.02Na0.98TaO3 photocatalyst using an impregnation method with heat treatment at low temperature. The Pd/NiO core/shell NPs were synthesized by controlling the coating of NiO on Pd NPs. A possible synthesis mechanism for Pd/NiO NPs on La0.02Na0.98TaO3 is proposed. The Pd NPs show higher hydrogen production from aqueous methanol solutions than do the Pd/NiO core/shells. In contrast, Pd NPs loaded on La0.02Na0.98TaO3 show negligible activity from pure water, due to rapid water formation. The effect of the NiO shell thickness on photocatalytic activity is discussed. The shell thickness increases with the amount of nickel. Pd/NiO core/shells (1 nm thick) with 0.1 wt% palladium and 0.2 wt% nickel, displayed the highest hydrogen evolution i.e. 3.42 mmol g-1h-1 and 26.2 mmol g-1h-1 from pure water and aqueous methanol solutions, respectively. The hydrogen evolution from aqueous methanol solutions was greatly enhanced by adding electron donors as sacrificial reagents. The recombination is interrupted by the effective capture of the holes by methanol acting as a sacrificial reagent, thereby leading to higher hydrogen evolution. However, the competition between the recombination and the charge-transfer reaction occurs in pure water leading to a possible back reaction between H2 and O2 on the photocatalyst’s surface. Hydrogen generation from pure water is in sequence: Pd/NiO > Pd, whereas the activity sequence with respect to aqueous methanol is: Pd > Pd/NiO. Metallic Ni and Pd present the most active sites favoring the formation of hydrogen from aqueous methanol. The NiO coated Ni and Pd NPs suppresses the O2 photo-reduction and/or promotes the H2O photo-reduction. The core-shell Ni/NiO and Pd/NiO NPs are of great significance in water splitting hydrogen production, thus Ni/NiO and Pd/NiO core-shell nanoparticles loaded on La0.02Na0.98TaO3 are very promising candidates for photocatalytic hydrogen production either from either pure water or aqueous methanol solutions. The NaTaO3 nanoparticles produced by this facile, environmentally friendly ‘green process’ have better crystallinity, smaller size and higher photocatalytic activity.