Synthesis,Characterization And Catalytic Performance Of Pb Nanoparticles/Basic Lead Carbonate Ultra-thin Nanoplate Composite Structures

There are numerous active sites on the surface of nanoparticles,which makes the nanoparticles have good heterogeneous catalytic activity.Many kinds of materials have been applied as heterogeneous catalysts,including noble metals?gold,silver,palladium,platinum,etc.??transition metal oxides and sulfides,carbon nanomaterials,such as graphene,Carbon nanotubes,carbon quantum dots,etc,polyoxometalates?POM?and non-noble metals?copper,cobalt,nickel,zinc,etc.?.The mechanism of the heterogeneous catalytic reaction is mainly due to the adsorption of the reactants on the catalyst surface and the electron transfer through the catalyst.The heterogeneous catalytic properties of noble metals and carbon nanomaterials should be mainly attributed to their high ability to transfer electrons.As semiconductor materials have electron conduction bands,they also have catalytic effects.Most of the semiconductor materials are formed by transition metals.Lead is a base metal,a main element,cheap and easy to get.Lead compounds,such as PbO₂,have important applications in electrocatalysis.In this paper,the ultra-thin nanoplates of lead-base compounds,basic lead carbonate,were synthesized.The in situ synthesis of lead nanoparticles on the nanoplates was investigated.The catalytic and photocatalytic performance of the composites was investigated.1.Preparation and Characterization of Basic Lead Carbonate Ultra-thin NanosheetsThe basic lead carbonate ultra-thin nanoplates were obtained facily by adding base?organic base,such as N,N-dimethylformamide?DMF?or sodium hydroxide?to the aqueous solution of lead acetate at ambient conditions.The structure and morphology of the obtained nanoplates were characterized by X-ray diffraction?XRD?,transmission electron microscopy?TEM?,selected area electron diffraction?SAED?and atomic force microscopy?AFM?.The TEM and AFM results indicate that the obtained basic lead carbonates have an ultra-thin hexagonal structure with dimensions ranging from several hundred nanometers to several micrometers,while its thickness is only about 10 nanometers.The results of XRD and SAED reveal that the ultra thin nanoplates formed a trigonal crystalline structure of basic lead carbonate.The composition of basic lead carbonate is Pb?OH?₂·2PbCO₃?B-2A?.The layered structure is formed by Pb?OH?₂ and PbCO₃ sublayers through stacking up in the sequence of BAABAA.Based on the measured thickness of the nanoplates and the crystal structure parameters of the basic lead carbonate,it can be concluded that the nanoplates are formed by the 3,4 or 5 BAA units that stacked along the c-axis direction of the unit cells.The composition and properties of the nanoplates were characterized by X-ray photoelectron spectroscopy?XPS?,Fourier transform infrared spectroscopy?FTIR?and UV-vis spectroscopy.The results showed that the nanoplates are composed of two lead components.The characteristic vibrational peaks of carbonates appeared in the FTIR spectra.These results further confirmed the formation of basic lead carbonate.The UV-vis spectra show that the obtained nanoplates have strong absorption in the UV region.The vibration bands of methyl appeared in the FTIR spectra of the nanoplates prepared using DMF,indicating that DMF molecules were adsorbed on the surface of nanoplates.The effects of experimental conditions,including temperature,reaction time and the composition of the reaction systems on the morphology and structure of the nanoplates were investigated.The results showed that the size of nanoplates could be tuned by altering the experimental conditions.2.Chemical Catalysis Performance of Basic Lead Carbonate NanoplatesThe heterogeneous catalytic performance of the synthesized basic lead carbonate nanoplates was investigated by the reduction of nitroaromatic compounds and reduction or degradation of organic dyes in aqueous solutions.The nitroaromatics include nitrobenzene?NB?,p-nitroaniline?4-NA?and p-nitrophenol?4-NP?,and the organic dyes are methylene blue?MB?and methyl orange?MO?.Catalytic hydrogenation or degradation reactions were carried out in aqueous systems with borohydride as reductant.The nanoplates that had catalyzed the reduction of 4-NA were investigated using TEM,SAED,HRTEM and XPS.The results suggest that lead nanoclusters were formed in situ during the catalytic reaction process.These nanoclusters are distributed on nanoplates homogeneously to form a zero-dimensional/two-dimensional?0D/2D?heterostructure.The crystal structure of the nanoplates maintained during the reaction.For the nanoplates synthesized by organic bases,the catalytic reaction speed is very fast for the first cycle;the speed slows down and remains stable since the second cycle.This should be related to the formation and growth of lead nanoclusters.When used for the first time,a large number of nanoclusters with very small particle size were formed,and the catalytic activity was high.After that,the nanometer clusters grew into larger nanoparticles,and the activity decreased.The catalytic reaction rate remained stable,indicating that the nanoparticles did not grow further.Thus,this composite catalyst has a self-stabilizing property.This demonstrates that the catalyst is lead nanoparticles which were formed in situ.We found that this composite structure exhibited different catalytic behavior when catalyzing the reduction of different compounds.The catalytic reaction rates were found to follow the order,4-NA>NB>4-NP when catalyzing the reduction of nitroaromatics.This should be related to the different adsorption and desorption properties of the reactants and the products on the surface of the catalyst.We also compared the catalytic performance of ultra-thin nanoplates prepared by different methods.It was found that the catalytic properties of nanoplates prepared with organic base DMF were significantly better than those prepared by adjusting the pH of the lead acetate aqueous solution with sodium hydroxide.This should be attributed to the strong adsorption of OH-on the surface of the nanoplates.3.The photocatalytic performance of basic lead carbonate nanoplatesThe synthesized basic lead carbonate nanoplates have very strong UV absorption and ultra-thin structure with high crystallinity,which are favorable for the generation and separation of photogenerated charge carriers.It is expected that these nanoplates can be acted as a photocatalyst.We investigated the degradation of MB in aqueous solutions under the irradiation of fluorescent lamp using the nanoplates as photocatalyst.The results indicated that the nanoplates have a certain photocatalytic effect.The composition and structure of the nanoplates after the photocatalytic reactions were investigated.The TEM and SAED results showed that the structure of the nanoplates maintained and nanoparticles homogeneously dispersed on the nanoplates.The XPS result indicates that the composition of these nanoparticles is lead.This suggests that under the irradiation of light,lead nanoparticles were generated in situ due to the reduction of Pb?II?by the photogenerated electrons,resulting in the formation of the 0D/2D heterostructure.The formation of lead nanoparticles can transfer and separate charge effectively.Therefore,the photocatalysis of this system is worth looking forward to.