Prepartion And Catalytic Hydrogen Evolution Performance Research Of Cobalt-containing Polyoxometallates Composites

As a new energy source,hydrogen can release much higher energy,and about 95%of the world’s annual hydrogen is obtained by methane steam reforming.Therefore,the development of new,low-cost and global-friendly hydrogen production processes is an attractive research area.Keggin-type polyoxometallates（POM）,with their rich structural types and excellent oxygen reduction properties are widely used in photocatalytic and electrocatalytic hydrogen evolution reactions due to their unique multi-electron transfer behavior.However,in homogeneous catalytic systems,POM suffer from easy aggregation,activity decay and difficult separation after the reaction,so the preparation of POM-based non-homogeneous materials has attracted more and more attention.In this paper,we prepared two kinds of non-homogeneous POM-based catalytic materials by metal substitution and loading with substrate materials,and investigated their catalytic performance in electrocatalytic and photocatalytic hydrogen evolution reactions,respectively.（1）Four Co-containing POM（denoted as Co-POM）,K₅[Co ^ⅢW₁₂O₄₀],K₆[Co ^ⅡW₁₂O₄₀],K₇[Co ^ⅡPW₁₁O₄₀],and K₈[Co ^ⅡSi W₁₁O₄₀],were firstly prepared by elemental doping.Then,a series of hybridized materials were prepared through electrostatic interaction between the prepared Co-POM anion and quaternary ammonium cation.The hybridized materials were characterized by FT-IR,XRD,SEM,and XPS,the results showed that these hybrid materials retained the Co-POM anionic structure well.In the electrocatalytic hydrogen evolution reaction of water in 0.5 M H₂SO₄solution,TMPA[Co ^ⅢW₁₂]exhibited excellent electrocatalytic hydrogen evolution performance because of the higher electrochemical surface active area and smaller electron transfer resistance.The overpotential from LSV curve of TMPA[Co^ⅢW₁₂]was 141 m V at a current density of 10 m A·cm^-2.By comparing the Tafel slopes of different hybrid materials,TMPA[Co ^ⅢW₁₂]has the smallest Tafel slope(89 m V·dec^-1),indicating that its catalytic hydrogen evolution process is in accordance with the Volmer-Heyrovsky mechanism and the hydrogen production step is the rate determining step.In addition,TMPA[Co ^ⅢW₁₂]remained stable on the electrode surface and showed efficient electrocatalytic hydrogen evolution performance after several cycles of voltammetry tests.（2）Carbon nitride in bulk phase（g-C₃N₄）was obtained by calcination by thermal polycondensation using melamine as precursor.Then the above[Co ^ⅡPW₁₁O₄₀]^-was loaded on the surface of g-C₃N₄and the Co ^ⅡPW₁₁/g-C₃N₄composites were finally obtained by hydrothermal method under different p H conditions.The morphology and structure of the composites were characterized by FT-IR,XRD,SEM and XPS.It was demonstrated that Co^ⅡPW₁₁was successfully loaded onto the surface of g-C₃N₄by intermolecular hydrogen bonding and retained its own original structural features without destroying the structure of g-C₃N₄.In the photocatalytic hydrogen evolution experiments,the Co ^ⅡPW₁₁/g-C₃N₄composite which prepared at p H=3 showed the best catalytic activity,and the H₂evolution amount reached 5475μmol·g^-1after 3 h,which was about 10.2 times higher than that of the bulk phase g-C₃N₄before loading.This is attributed to the improved separation and transfer efficiency of the photogenerated carriers from the substrate material due to the excellent electron acceptor of Co ^ⅡPW₁₁,and the more negative conduction band position after loading is also more favorable for the hydrogen evolution reaction.Finally,the chemical stability of the composite was demonstrated by four cycles of experiments.