Solar-driven photocatalytic hydrogen production from water splitting holds significant potential in addressing the energy crisis and environmental pollution. However, rapid recombination of photogenerated carriers and insufficient surface catalytic reaction kinetics hinder the photocatalytic process. Constructing the catalytic systems with multi-channel charge-separation can effectively alleviate those issues. In this work, ternary PdO@TiO 2 -Cu 2 O composites have been synthesized through precursor calcination and low-temperature reduction, where PdO and Cu 2 O nanoparticles couple with the TiO 2 nanorods. Comparing to single TiO 2 , binary PdO@TiO 2 and TiO 2 -Cu 2 O, the PdO@TiO 2 -Cu 2 O hybrid exhibits enhanced photocatalytic hydrogen production performance (5831μmol g -1 h −1 ), with a hydrogen production rate more than three times that of single TiO 2 . Based on the band structures of TiO 2 and Cu 2 O, along with the photochemical/electrochemical, in-situ EPR, Photoluminescence (PL) tests, and work function calculations, the formation of an S-scheme heterojunction at the TiO 2 -Cu 2 O interface under light illumination facilitates the recombination of electrons in the conduction band (CB) of TiO 2 with holes in the valence band (VB) of Cu 2 O. In this regard, the established built-in electric field significantly accelerates the transfer of photogenerated charges while preserving their respective strong oxidation and reduction capabilities. Meanwhile, the introduction of PdO enriches the holes and further enhances the rate of charge separation. This S-scheme heterojunction with directional charge transfer and oxidation cocatalyst collectively form multi-channels of charge separation, optimizing charge migration and enabling efficient hydrogen production, thus improving the photocatalytic capability of pristine TiO 2 . This study provides a rational approach to construct efficient S-scheme heterojunction/oxidation cocatalyst multi-component catalytic systems for water splitting into hydrogen.
The reduction of nitrobenzene to aniline is very important for both pollution control and chemical synthesis. Nevertheless, difficulties still remain in developing a catalytic system having high efficiency and selectivity for the production of aniline. Herein, it was found that PdO nanoparticles highly dispersed on TiO 2 support (PdO/TiO 2 ) functioned as a highly efficient catalyst for the reduction of nitrobenzene in the presence of NaBH 4 . Under favorable conditions, 95% of the added nitrobenzene (1 mmol/L) was reduced within 1 min with an ultra-low apparent activation energy of 10.8 kJ/mol by using 0.5%PdO/TiO 2 as catalysts and 2 mmol/L of NaBH 4 as reductants, and the selectivity to aniline even reached up to 98%. The active hydrogen species were perceived as dominant species during the hydrogenation of nitrobenzene by the results of isotope labeling experiments and ESR spectroscopic. A mechanism was proposed as follows: PdO activates the nitro groups and leads to in-situ generation of Pd, and the generated Pd acts as the reduction sites to produce active hydrogen species. In this catalytic system, nitrobenzene prefers to be adsorbed on the PdO nanoparticles of the PdO/TiO 2 composite. Subsequently, the addition of NaBH 4 results in-situ generation of a Pd/PdO/TiO 2 composite from the PdO/TiO 2 composite, and the Pd nanoclusters would activate NaBH 4 to generate active hydrogen species to attack the adsorbed nitro groups. This work will open up a new approach for the catalytic transfer hydrogenation of nitrobenzene to aniline in green chemistry.
Recently, as an important product of the CO2 hydrogenation reaction and a crucial hydrogen storage material, the transformation of formic acid (FA)/formate to H2 or active hydrogen species has attracted much attention. However, obtaining a robust and efficient solid catalyst constitutes a major challenge. In this work, nitrobenzene was used as the active hydrogen species capture agent, and various Pd-based catalysts were prepared and used to refine the influence factors of FA decomposition over TiO2-supported palladium catalysts. The catalysts’ deactivation and regeneration process, control reactions, and catalyst characterizations indicate that the electrical state of the Pd species and interface microsurroundings played decisive roles in the activity and reusability of PdO/TiO2. Based on the results obtained, a tentative mechanism with the active sites (Pdδ+–O–Ti) located at the PdO/TiO2 interface was proposed, which would be useful for the chemistry of the Pd catalyst and formate decomposition.
Hong Wang,Zhongming Wang,Zizhong Zhang,Yaming Fan,Xianzhi Fu,Wenxin Dai
A prepared p-type PdO/n-type TiO 2 junction was prepared as the catalyst for photocatalytic hydrogen evolution from formic acid (FA). It was found that PdO/TiO 2 exhibited 1298-fold higher activity than TiO 2 under ultraviolet and visible light irradiation at room temperature. Multiple characterization techniques were utilized to demonstrate that doping PdO into TiO 2 can regulate the Fermi level of the sample and the adsorption behavior of FA, resulting in different electron transfer behavior between FA and PdO/TiO 2 compared to that between FA and TiO 2 . Oxygen vacancies acted as electron donors to deliver photogenerated electrons from TiO 2 to the adsorbed bidentate FA, which increased the electron density on the O atom of FA and led to the formation of activated monodentate FA. This activated FA was more easily oxidized by the holes in PdO generated by visible light, leading to the production of H 2 and CO 2 . The mode of adsorption associated with the Pd 2+ site is the key factor in driving this reaction and leading to the reversible electron transfer phenomenon. This study provides a new promising route for the design of photocatalysts for self-decomposition reactions.
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