Design And Catalytic Properties Of Co-Based And Ni-Based Noble-Metal-Free Catalysts For Photocatalytic (Photoelectrocatalytic) Hydrogen Production

Due to the exhaustible sources of fossil fuels and environmental pollutions by using those fuels,it is highly desired to to develop renewable clean energies.Hydrogen(H2),known as the "the oil of future",has attracted intensive attention nowadays.Splitting water into H2 via solar energy is a promising approach for H2 production.In this thesis,we describe the design and catalytic properties of a few noble-metal-free catalysts for photocatalytic(photoelectrocatalytic)hydrogen production.The main contents of this thesis are as follows:1.One dimensional CdS/ZnS core-shell nanorods are developed as an efficient photocatalyst for hydrogen production.The results demonstrate that a thin layer(-2-5 nm)of ZnS on CdS can promote the photocatalytic hydrogen production rate,which is about 10 times higher than that of bare CdS.The apparent quantum efficiency is 16.8%at 420 nm.The enhanced performance could be attributed to a quasi-type ? structure and the passivation effect in the presence of ZnS.2.NiS decorated CdS/ZnS composite photocatalytst for photocatalytic H2 production reaction is studied.In the presence of Na2S/Na2SO3,the photocatalytic H2 production rate reaches an optimal value of 574 ?mol/h after the loading of NiS,which is more than 38 times higher than the catalytic activity of pure CdS NRs under the same conditions.The average apparent quantum yield is?43.2%during 5 h of irradiation under monochromatic 420 nm light.This study demonstrates the advantage of synergetic strategies to form not only semiconductor heterojunctions but also photocatalyst-cocatalyst interfaces to enhance the catalytic activity for photocatalytic H2 production.3.Ni-based salen complexes are used as cocatalysts on CdS/ZnS core-shell heterojunctions for photocatalytic hydrogen production.The complexes can efficiently transport photogenerated electrons and holes over a heterojunction photocatalyst to hamper charge recombination,leading to highly improved catalytic efficiency and durability of a heterojunction photocatalyst-molecular cocatalyst system.The optimal apparent quantum yield is approximately 58.3%after 7 h of irradiation under monochromatic 420 nm light.Further studies reveal that the Ni-based salen complex is a stable molecular cocatalyst in the present system during photocatalytic hydrogen production.4.Co modified CdS nanorods are used as an efficient photocatalyst in photocatalytic dehydrogenation of benzyl alcohol for simultaneous hydrogen and benzaldehyde production.The system can produce hydrogen and benzaldehyde with high selectivity and efficiency.The optimal selectivity for benzaldehyde production is-94.4%.Mechanism studies indicate that the carbon-centered radical is the key intermediate during the oxidation of benzyl alcohol and plays a very important role in the selectivity for benzaldehyde production.Under optimal conditions,the rate of hydrogen evolution in the present noble-metal-free system is 848 ?mol/h and an apparent quantum yield of 63.2%(?=420±5 nm)is achieved.5.Cobalt phosphide(CoP)as a highly active cocatalyst on hematite photoanode for high-performance photoelectrochemical water splitting is reported for the first time.Under optimal conditions,the as-prepared CoP modified Ti-PH photoanode exhibits excellent and stable photoelectrochemical water oxidation performance with a current density of 2.1 mA/cm2 at 1.23 VRHE.At an applied bias of 1.23 VRHE the hole injection efficiency of Ti-PH/CoP is?90%,which is close to 96%at a higher potential.In addition,the photocurrent onset potential cathodically shifts by?180 mV compared to Ti-doped hematite photoanode under AM 1.5G illumination(100 mW/cm2).Further studies show that the enhanced PEC performance is attributed to the improved ?inj and decreased Rct in the interface through loading with CoP cocatalyst.6.A composite cobalt hydroxide/nickel based oxygen evolution catalyst(Co(OH)2/Ni-OEC)is decorated on nanoporous BiVO4 for photoelectrochemcial water spliting reaction.Co(OH)2/Ni-OEC is prepared by successive electrodeposition and photoelectrodeposition methods.Spectroscopic characterization and electrochemical studies demonstrate that the composite Co(OH)2/Ni-OEC electrocatalyst not only accelerates water oxidation catalysis,but also efficiently reduces the charge recombination at the surface trapping states and enhances the catalytic stability,leading to a remarkable photocurrent density of 4.5 mA/cm2 at 1.23 VRHE.The highest applied bias photon-to-current efficiency of 1.69%at 0.64 V is achieved under AM 1.5G illumination(100 mW/cm2).