The Role Of CoO_x-Mo₂N Non-noble Metal Composite Cocatalyst In Photocatalytic Overall Water Splitting

The energy crisis and environmental pollution are the great challenges for humanity in the 21st century.The establishment of clean energy system is imminent.In the field of new energy,hydrogen energy has generally been considered as the most ideal green energy without pollution in the new century.Utilizing the abundant natural solar energy to produce hydrogen for photocatalytic water splitting is one of the best way to fundamentally address energy and environmental pollution problems.The current major problems in photocatalytic hydrogen production are the low hydrogen yield and the high cost of photocatalysts.Cocatalysts as an important factor to improve photocatalytic hydrogen production have been widely studied.The cocatalyst in the photocatalytic water splitting generally is composed of sole cocatalyst for hydrogen or oxygen production.Also,the cocatalysts are composed of two kinds of cocatalysts separated in space for hydrogen and oxygen production respectively,and most of them are noble metals,which are expensive and contradictory to low-cost industrial demand.Therefore,it is of great theoretical and practical value to develop an efficient non-noble metal cocatalyst adjacent in space and capable of simultaneously producing hydrogen and oxygen.In this research,a kind of composite cocatalyst was synthesized,which consisted of molybdenum nitride and cobalt oxide.The main catalyst was germanium nitride,and a composite photocatalyst system was constructed to perform photocatalytic water splitting.The specific research content is as follows.First,the Mo2N/Ge3N4 photocatalyst.Mo2N/Ge3N4 composite photocatalyst was prepared by high temperature nitriding method with ammonium molybdate as precursor.TEM characterization shows that Mo2N and Ge3N4 were tightly bound together.Photocatalytic water splitting experiments show that when the loading amount of Mo2N was 0.25%,the activity of photocatalytic water splitting was the highest,and the amounts of hydrogen and oxygen production reached 17 and 7.5?mol/h respectively,which is approximately 3.4 times of pure Ge3N4.Transient photocurrent experiments show that the photocurrent of Mo2N/Ge3N4(4.3?A/cm2)is about 1.4 times of Ge3N4.Cyclic voltammogram curves show that the oxidation current density of Mo2N/Ge3N4(0.15 mA/cm2)is larger than Ge3N4.Linear voltammetry scan curve shows that the overpotential of hydrogen production of Mo2N/Ge3N4 is smaller than that of Ge3N4,which indicates that the addition of cocatalyst Mo2N reduces the overpotential of hydrogen production.Mo2N enhances the efficiency of photocatalytic hydrogen production as hydrogen evolution cocatalyst.Second,the CoOx/Ge3N4 photocatalyst.CoOx/Ge3N4 composite photocatalyst system was constructed by using hydrothermal method with cobalt nitrate as precursor.TEM characterization shows that CoOx and Ge3N4 were tightly bound together.Photocatalytic water splitting experiments show that when the loading amount of CoOx was 0.5%,the activity of photocatalytic water splitting was the highest,and the amounts of hydrogen and oxygen production are 48.0 and 19.5?mol/h respectively,which is about 9.6 times of Ge3N4 alone.Transient photocurrent experiments show that the photocurrent of CoOx/Ge3N4(3.6 ?A/cm2)is about 2.4 times of Ge3N4.Cyclic voltammograms show that the oxidation current density of CoOx/Ge3N4 is drastically increased compared to Ge3N4,while the reduction current density has a small change,indicating that CoOx mainly plays a role of oxidation cocatalyst.The linear voltammetry scan curve shows that overpotentials of CoOx/Ge3N4 for hydrogen and oxygen production are smaller than Ge3N4,indicating CoOx cocatalyst is a dual functional cocatalyst.CoOx as a dual functional cocatalyst improves the efficiency of photocatalytic hydrogen production and oxygen production in some degree.Third,CoOx-Mo2N/Ge3N4 composite photocatalyst.The CoOx-Mo2N/Ge3N4 composite photocatalyst system was constructed by high temperature nitriding and hydrothermal method.TEM results show that Mo2N and CoOx are successfully combined together with Ge3N4,and Mo2N and CoOx are combined together,indicating that Mo2N and CoOx constitute a CoOx-Mo2N composite cocatalyst.Photocatalytic water splitting experiments show that the loading of cocatalyst can significantly inprove the activity of hydrogen production and oxygen production.When the loading amount of Mo2N is 0.25%and the loading amount of CoOx is 0.5%,the activity of photocatalytic water splitting is the highest.The amounts of hydrogen and oxygen production are 118.0 and 32.0?mol/h respectively,which is 23.6 times of Ge3N4 alone,6.9 times of Mo2N/Ge3N4,2.4 times of CoOx/Ge3N4,and 1.4 times of RuO2/Ge3N4.The photocurrent experiment shows that the photocurrent of CoOx-Mo2N/Ge3N4 is the highest,reaching 9.0?/cm2.The cyclic voltammograms show that the oxidation current density of CoOx-Mo2N/Ge3N4 changed the most,reaching 1.89 mA/cm2,and the reduction current density has small changes,indicating that the oxidation reaction in this reaction playing the key role.Linear voltammetry scan curves show that CoOx-Mo2N/Ge3N4 has the lowest overpotential for hydrogen and oxygen production.The overpotential for hydrogen production is-1.16 V.The overpotential of oxygen production is 0.78 V.The composite cocatalyst can simultaneously reduce the overpotentials of hydrogen and oxygen production.CoOx mainly play a role of transferring holes to produce oxygen.Mo2N mainly play a role of transferring electrons to produce hydrogen.Mo2N and CoOx constitute a composite cocatalyst,which is beneficial to water splitting in space.Fourth,a simple catalytic synthesis method for germanium semiconductor material.GeO2 and MoO3 are mixed and the nitrided,when the MoO3 content is 5%and the temperature is or over 800?,the germanium is produced within half an hour.The size of germanium is between 5?m and 10?m,and it belongs to the micro-nano level material.By comparison of germanium dioxide in a hydrogen atmosphere(500 ?,2 h),the temperature of preparing germanium in the ammonia atmosphere is higher,but the time is shorter.The formation of germanium has undergone a two-step transformation.Firstly,in the ammonia atmosphere,GeO2 and MoO3 are transformed into Ge3N4 and Mo2N at high temperatures,respectively.Next,under the catalytic action of Mo2N,Ge3N4 undergoes a decomposition reaction to form germanium.In the ammonia atmosphere,M0O3 is transformed into Mo2N at 800 ?,indicating that Mo2N is a catalyst for the synthesis of germanium.The experiments of MoO3,Mo2N and Ge3N4 in N2 atmosphere respectively show that MoO3 cannot decompose Ge3N4,while Mo2N promotes the decomposition of Ge3N4 into elemental germanium.Once again,Mo2N is proved to be the catalyst for the synthesis of germanium.