Studies On Photocatalytic And Electrocatalytic Hydrogen Production By Splitting Water

Energy and environment are two major themes of social development.At present,the main supply of energy still comes from fossil fuels such as coal,natural gas and oil,facing the problems of exhaustion and environmental pollution.Therefore,research and development of cheap,efficient,clean and sustainable access to energy is a great and difficult task for contemporary energy workers.As a new clean energy,hydrogen energy is the most ideal substitute for coal,oil and other traditional energy.Water is abundant on earth,almost inexhaustible and clean,so hydrogen production from water decomposition is a sustainable,clean and environmentally friendly energy production method.Among the many methods for hydrogen production from water splitting,photocatalysis and electrocatalysis are widely considered as simple and easy to operate.Due to the recombination of photo-generated electrons and holes as well as the back reaction of the products hydrogen and oxygen,the efficiency of photo splitting water to produce hydrogen in a pure water system is very low.Therefore,it is necessary to add an electron donor in the reaction system to irreversibly consume holes,thereby obtaining higher hydrogen production efficiency.Biomass as an electron donor,can be obtained cheaply and sustainably.Thus,it is a promising method to use biomass as an electron donor for photocatalytic hydrogen production.In the process of electrocatalytic water splitting to produce hydrogen,the oxygen evolution process consumes large energy,which is the main factor restricting the development of industrial water splitting technology.Precious metals such as Pt,Ru,Ir and their oxides?RuO₂ and IrO₂?have high catalytic properties for hydrogen evolution and oxygen evolution,but they are expensive and scarce.Non-precious metals such as copper,iron,cobalt,lead,molybdenum,and zinc can form oxides of different valence states.If these metal oxides are used as electrodes to decompose water to produce hydrogen at a very low potential.This is of great significance for saving energy consumption in the process of hydrogen production by electrolyzed water and realizing large-scale industrialization of hydrogen production.In this dissertation,the semiconductor photocatalytic splitting water to produce hydrogen by using biomass as an electron donors has been investigated firstly.Secondly,some explorations have been made on the electrocatalytic splitting water to produce hydrogen.The dissertation mainly includes the following five parts:???The photocatalytic hydrogen production behavior of?-D-and?-D-glucose as electron donors on Pt/TiO₂ was studied.?-D-glucose has a higher photocatalytic hydrogen evolution activity than?-D-glucose,The effect of the initial pH of the isomer solution on the photocatalytic hydrogen evolution was studied.The results show that weak alkaline conditions are favorable for photocatalytic hydrogen evolution.?-D-and?-D-glucose hydrogen-producing activities have the largest difference under neutral conditions,and the difference in hydrogen-releasing activity under alkaline or acidic conditions is small.The reaction mechanism was discussed.???MoS₂is a new and low-cost hydrogen evolution catalyst.In the presence of polyvinylpyrrolidone?PVP?,a thin layer of MoS2 was synthesized by hydrothermal method using sodium molybdate and thioacetamide as raw materials.The MoS2 was exfoliated in N,N-dimethylformamide?DMF?solvent and loaded on P25 TiO₂ to prepare a thin layer MoS2/TiO2 photocatalyst.The photocatalyst was characterized by XRD,SEM,UV-Vis diffuse reflectance spectroscopy,and ZETA potential.MoS2is well dispersed on TiO₂.MoS₂ loading increases the light absorption of TiO₂ and reduces its charge zero pH.The effects of MoS2 preparation and exfoliation conditions on the photocatalytic hydrogen production activity of MoS2/TiO2 with biomass as an electron donor were studied.The results show that the hydrogen evolution activity of MoS2 after DMF exfoliating is much higher than that unexfoliating;the optimal loading of MoS₂ is 1.60 wt%,and the hydrogen evolution rate of MoS₂/TiO₂prepared by ultrasonic exfoliating MoS₂ is 4.18 times higher than that of unexfoliating.DMF as a solvent,the ultrasonic dispersion effect is significantly better than water;the surfactant PVP also promotes the exfoliation of MoS₂ to improve the hydrogen production activity.The optimal pH of the reaction system was consistent with the charge zero pH of MoS2/TiO2;the effect of glycerol concentration on the hydrogen evolution activity was consistent with the Langmuir-Hinshelwood kinetic model.The reaction mechanism is discussed in detail.???Fe³⁺-modified CdS was prepared by impregnation-roasting method and characterized by XRD and UV-visible diffuse reflection.XRD showed that the raw materials CdS and Pt/CdS were cubic crystals,which were transformed into hexagonal phases after roasting.The photocatalytic decomposition of water to produce hydrogen under glycerol as an electron donor was investigated.The optimum amount of Fe3+impregnation is 0.50 wt%.With this modified amount,the photocatalytic hydrogen evolution activity of the modified CdS is 1.61 times as high as the unmodified one.The pH experiment showed that the photocatalytic hydrogen evolution efficiency was high when pH<3.5,whereas under neutral and weak alkaline conditions the hydrogen evolution activity was very low.???CdS with mixed crystal forms of cubic phase and hexagonal phase was prepared by sodium phosphate-assisted hydrothermal method.The catalyst was characterized by XRD.The photocatalytic hydrogen production over CdS photocatalyst with MoSx as a cocatalyst and lactic acid as an electron donor was studied.For the preparation of the photocatalyst,the optimal concentration of sodium phosphate and the optimal loading of MoS_x are 0.05 mol.L^-11 and 0.20 wt%,respectively.Pt control experiments show that MoS_x as co-catalyst produces much more hydrogen than Pt.The synergistic effect of MoSx and lactic acid protected CdS from photocorrosion.???Cheap and efficient electrocatalytic decomposition of water to produce hydrogen?HER?and oxygen?OER?catalyst have attracted much attention.Here,a self-supporting copper oxide electrode directly coated on a copper wire is reported as a working electrode for decomposing water.The electrode generates OER in an alkaline solution at a potential range of 0.00 to+0.21 V and HER in a potential range of+0.85 to+1.25 V.In the interactive OER and HER process,the electrode activity can be regenerated and is quite stable.Using this electrode can save energy consumption for industrial electrolyzed water.This led us to re-evaluate copper oxides and other metal oxides,and provided a new design idea for low-energy-consumption hydrogen production by electrolysis.