Photochemically Controlled Preparation And Catalytic Performance Of Ultrasmall Platinum Nanoparticles

Controlled preparation has always been a hot topic in the research field of nanocatalytic materials.Platinum-based nanomaterials have become one of the most concerned catalyst materials due to their excellent catalytic activity.The photochemical method is characterized by mild reaction conditions,simple operation,green environment protection and low toxicity.It can effectively control the kinetic process of metal precursor photoreduction by adjusting the light intensity,wavelength range and irradiation time,which in turn affects the size,shape and surface chemistry of nanoparticles to achieve the purpose of controlled preparation.In this work,ultrasmall sized supported Pt-based nanocatalysts were prepared in one step in the frozen state and liquid phase without adding stabilizers,using visible and UV light as the inducing light sources and K₂[Pt Cl₄]as the Pt source,respectively.And their catalytic performance was investigated.The main contents are as follows:(1)Under the conditions of no addition of reducing and stabilizing agents,the uniformly dispersed and ordered ultrasmall Pt nanocatalysts(Pt/SWCNTs)were prepared by irradiating a mixture of frozen single-walled carbon nanotubes(SWCNTs)and potassium chloroplatinate(K₂[Pt Cl₄])with visible light and UV light,respectively.HR-TEM images shows that the samples have nano-rectangular array morphology with an average particle size of about 1.0 nm.The results of XPS spectra proves that platinum metal mainly exists in the form of Pt⁰.The catalytic performance test results show that the prepared Pt/SWCNTs can effectively catalyze the reduction of 4-NP to4-AP with the highest apparent rate constant of 0.36 min^-1.After seven cycles,the conversion rate remained about 87%.Compared with literatures,the samples obtained in this experiment have better catalytic activity and stability than the literature.It was shown that photoinduced,support-substrate interactions and the natural domain limiting effect of the ice lattice could effectively control the aggregation of Pt⁰,resulting in well dispersed ultrasmall Pt nanoparticles.(2)Ultrasmall Pt nanoparticles with narrow size distribution(Pt/SWCNTs-M)were prepared by irradiating the mixed solution of SWCNTs and K₂[Pt Cl₄]with visible light and UV light,without the addition of stabilizer and using methanol as the reducing agent.The XRD diffraction pattern shows that the high content of Pt(111)crystalline surface plays a key role in improving the catalytic activity of Pt nanoparticles.The average particle size of Pt/SWCNTs-M2 prepared by visible light is about 2.0 nm,as shows by the TEM.XPS results confirms that the prepared Pt nanoparticles mainly existed in the form of Pt⁰.The catalytic performance test results shows that the platinum nanocatalyst(Pt/SWCNTs-M2)obtained under the condition of 650 nm visible light has the highest activity for catalyzing the reduction of 4-NP,and its apparent rate constant of k=0.24 min^-1.The conversion remains about 80%after six cycles,which has a good reusability.The result shows that the energy of 650nm visible light excitation is more matched with the reduction rate of methanol,which hinders the agglomeration of Pt⁰to a certain extent,so the obtained Pt nanoparticles have higher catalytic activity and smaller particle size.(3)Pt nanoparticles(Pt/SWCNTs-E)with ultrasmall size were prepared by irradiating the mixed solution of SWCNTs and K₂[Pt Cl₄]using visible and UV light,without the addition of stabilizers and using the ethylene glycol as the reducing agent.The XRD diffraction pattern shows a high content of Pt(111)crystalline planes in the prepared samples,which was beneficial to improve the catalytic activity of Pt nanoparticles.TEM results shows that the average particle size of Pt/SWCNTs-E3prepared under UV light irradiation is about 2.2 nm.XPS results shows that the prepared Pt nanoparticles are mainly existed in the form of Pt⁰.The catalytic performance test results shows that the ultrafine Pt nanocatalyst obtained under 395nm UV light conditions had the highest activity for catalytic 4-NP reduction with an apparent rate constant of k=0.34 min^-1,which has good catalytic activity.After seven cycles,the conversion remained about 83%,which has good reusability and stability.The research shows that due to the strong reducibility of ethylene glycol,the excitation energy of 395 nm ultraviolet light is more matched with its reduction rate,which can effectively inhibit the aggregation of Pt⁰,so the obtained ultrasmall platinum nanoparticles have better catalytic performance.