Construting Transistion Metal Based Catalysts For Highly Efficient Hydrogen Evolution Reaction

With the intensification of the energy crisis,environmental pollution and global warming,it is an urgent need for people to explore,and develop environmentally friendly and sustainable energy.Hydrogen is a kind of continuously renewable clean energy.Therefore,enormous researchers have drawn much attention to preparing the hydrogen according to the strategy of electrocatalytic hydrogen evolution and solar hydrogen production.The level of the overpotential of the electrode material determines the amount of energy consumed during hydrogen production in the process of hydrogen production by electrocatalytic water splitting.The higher energy will be consumed when the electrode owns high overpotential,which may lead to the great energy consumption.So,the overpotential of the electrode material is one of the critical indicators to evaluate the performance of the catalyst.Meanwhile,the overpotential of the cocatalyst also affects the hydrogen production performance of the catalyst in the photocatalytic hydrogen production.The catalyst with an excellent performance of electrocatalytic hydrogen evolution catalyst is likely to become a cocatalyst for photocatalytic hydrogen production.The noble metals,such as Pt,Pd,Ru exhibit excellent performance of electrocatalytic hydrogen evolution due to the overpotential of the catalysts are close to zero.Therefore,these analogical material used as cocatalysts can greatly reduce the hydrogen evolution overpotential of the semiconductor towards photocatalytic performance.However,the precious metals have been limited in their practical application due to its scarce reserves and high cost.Therefore,it is a great importance to explore non-noble metal-based catalysts?cocatalyst?for electro?photo?catalytic water splitting to hydrogen production.In recent years,transition metal-based catalysts such as molybdenum disulfide and cuprous phosphide have been attracted attention due to their excellent properties of electrocatalytic hydrogen evolution.In general,the electrocatalytic hydrogen evolution catalyst with excellent performance is likely to become a cocatalyst for photocatalytic hydrogen production to replace the noble metal Pt.In this thesis,our aim is to reduce the catalyst cost and improve the activity of the catalyst,the hydrogen catalytic properties of molybdenum disulfide/sulfur and nitrogen co-doped carbon nanocomposites?MoS₂/SNC-900-12h?and the cuprous phosphide/graphitic carbon nitride?Cu₃P/g-C₃N₄?are disscussed.The main research contents and results are summarized as follows:?1?Highly dispersed few-layer MoS₂ nanosheets/S,N co-doped carbon for electrocatalytic H₂ productionMost of the MoS₂-based catalysts are multi-layered sheet structure,which leads to insufficient exposure of catalytic active sites and severely affects their catalytic performance.Here,we developed a method to directly produced solid-state S,N co-doped carbon?SNC?solid with graphite structure andmultiple surface groups through one step hydrothermal route.When Na₂MoO₄ was added into the above reaction,polymolybdate was in situ embedded into the carbon materials,which help polymolybdate uniformly disperse into the SNC.After thermal treatment,polymolybdate was transferred into MoS₂ at 800 ^oC in the inert gas environment due to S^2-can be released from SNC in the thermal treatment.The SNC effectively prevent the MoS₂ from aggregating into large particles.We successfully prepared highly dispersed MoS₂ in the SNC matrix.TEM tests show that the number of MoS₂ layers is only one or few layers,which will expose more catalytically active sites.The D-band(1341 cm^-1)and G-band(1584 cm^-1)of the Raman spectra show that the material has a good graphitized structure,which gives the composite a good conductivity.The C-S and C-N bonds are observed in the XPS C1s spectrum.There are C-S-C,C=S and C-SO_x-C bonds in the S2p spectrum.Pyridine nitrogen and graphite nitrogen present in the N1s spectrum.Combined with elemental analysis,the carbon material is confirmed to be the sulfur-nitrogen co-doped carbon.Mo spectrum showed Mo 3d_5/2?229.4 eV?and Mo 3d_3/2?232.6 eV?,demonstrating that molybdenum disulfide was successfully embedded in the carbon materials.The electrochemical characterizations indicate that the MoS₂/SNC-900-12h exhibit low onset potential?115 mV?and a low overpotential?237 mV?in 0.5 M H₂SO₄ solution at 0^-0.3 V?vs.RHE?.Furthermore,MoS₂/SNC-900-12h also illustrate excellent stability with²%decay at 10 mAcm^-2 of current densityafter 5000 cycles test.These results indicate that the MoS₂/SNC-900-12h catalyst not only has excellent electrochemical hydrogen evolution performance but also has good stability performance.?2?Visible-light catalytic decomposition of hydrogen by Cu₃P/g-C₃N₄ composite catalystThe precursor complex was synthesized by a simple chemical adsorption and deposition method.Then,the precursor was phosphatized under the PH₃ environment?decomposition of sodium phosphite?to obtain the Cu₃P/g-C₃N₄ composite catalyst.Cu₃P exhibits a low hydrogen overpotential in the electrocatalytic hydrogen evolution,it can be used as a co-catalyst for photocatalytic hydrogen evolution to replace the precious metal Pt.The hydrogen production performance of the sample was optimized according to change the content of Cu₃P.Furthermore,combining with the photophysical properties of the catalysts,photoelectrochemical tests and selective deposition of platinum,the reaction mechanism of photocatalytic water splitting hydrogen production was illuminated that Cu₃P acts as a co-catalyst during the reaction.The results show that Cu₃P can improve the photocatalytic water splitting performance of g-C₃N₄.Under the irradiation of visible light??>420 nm?and triethanolamine?TEOA?as electron donating sacrificial agent,Cu₃P/g-C₃N₄ composite photocatalyst with the Cu₃P loading of 1%has the best hydrogen production performance.And the hydrogen production efficiency reached 823?mol h^-1 g^-1,which was 76.2 times that of pure g-C₃N₄ and 1.25 times of 1%Pt/g-C₃N₄,indicating that Cu₃P is an efficient non-precious metal co-catalyst for photocatalytic hydrogen evolution.