Controlled Photochemical Synthesis Of Platinum Nanomaterials And Its Catalytic Applications

Platinum?Pt?metal nanomaterials have high-efficiency catalytic properties due to their unique physical and chemical properties,and the controlled synthesis of Pt catalysts has always been a hot issue in scientific research.In this paper,using visible light as the light source and K₂[PtCl₄]as Pt source,we studied the hydrolysis of the K₂[PtCl₄],the physical and chemical adsorption of the substrate on the carrier surface,and the mechanism of kinetics-controlled synthesis of Pt nanocatalyst in different solution systems.The morphology and composition of the synthesized catalyst were studied by Fourier transform infrared spectroscopy?FT-IR?,Transmission electron microscope?TEM?,X-ray photoelectron spectroscopy?XPS?,X-ray diffraction?XRD?,Energy dispersive X-ray spectroscopy?EDS?.At last,the activity of the catalyst was evaluated by a typical 4-nitrophenol?4-NP?reduction and the methanol electrocatalytic oxidation experiment.The main works include the following parts:?1?In the aqueous solution of K₂[PtCl₄],the changes of pH and UV absorption spectra of the solution under different light conditions were measured.The results show that visible light can promote the hydrolysis of[Pt Cl₄]^2-and affect its species distribution.As the intensity of visible light becomes stronger and the wavelength becomes shorter,Cl^-in[Pt Cl₄]^2-is more easily replaced by H₂O and OH^-in the solution,and then[PtCl_n?H₂O?_4-n]^2-n?n?3?,[PtCl_n?H₂O?_m?OH?_4-n-m]^m-2?n+m?3?,[PtCl_n?OH?_4-n]^2-and other Pt?II?complexes are formed.The dehydrogenation and dechlorination step in the hydrolysis of[PtCl₄]^2-is the key to the formation of Platinum Nanoparticles?Pt NPs?with different morphology.Therefore,visible light is an effective way to achieve kinetic control of the rate of hydrolysis of K₂[PtCl₄].?2?Without added surfactants and reducing agents,Pt NPs were synthesized in the aqueous solution of K₂[PtCl4]precursor under visible light by using multi-walled carbon nanotubes?MWCNTs?as carriers.The results have shown that the loading of hydrolysate on the surface of MWCNTs take place first during the formation of the catalyst which are PtOx?x?2??Pt?OH?_x?x?4?NPs with the average size of 2.04 nm.After in-situ reduction by methanol,the average particle diameter of Pt NPs with 2.09 nm which are basically unchanged in the morphology and dispersion.This shows that under the control of visible light,the formation of Pt NPs can be completed through two processes:loading and reduction.?3?In the cyclodextrin?CD?solution system,using?,?,?-CD with different chemical structure as stabilizers,Pt NPs were synthesized under visible light.In this system,the special cavity structure of CD can prevent the aggregation of Pt NPs,and its mild reducibility can easily control the growth and nucleation rate of Pt NPs.Finally,a highly dispersed Pt-?-CD catalyst with the average size of 1.6 nm were synthesized,and its activation energy?E_a?was12.44 kJ/mol in the catalytic reduction of 4-NP.?4?In the polyethylene glycol?PEG?solution system,when pH is10 and V_PEG:V_H2Ois 1:9,the average size of Pt NPs with 1.29 nm were loaded on the surface of MWCNTs under visible light irradiation.The Pt?II?complex produced by hydrolysis of K₂[PtCl₄]which combines with PEG in the solution to form a large number of nucleation sites,and then the high dispersion and small size of Pt NPs were synthesized by the in situ reduction.As a result,the reaction rate constant?k?of the Pt/MWCNTs catalyst is 0.6244 min^-11 in the 4-NP reduction experiment.?5?In the ethylene glycol?EG?solution system,using EG as a reducing agent and solvent,the average size of Pt nanoclusters with 2.5 nm were synthesized at pH=7 under visible light irradiation.The characterization results show that the Pt NPs loaded on the surface of MWCNTs have good crystallinity,and the catalyst still maintains a high catalytic activity and good stability after being used 5 times in the reduction 4-NP reaction.