The Roles Of Noble Metal/Mo-based Non-noble Metal Compounds Composite Co-catalysts In The Photocatalytic H₂ Production

With the continuous development of human society,how to solve the shortage of traditional fossil fuel and the environmental problems caused by the use of fossil fuel has become the research hotspots in the field of energy.Photocatalytic hydrogen evolution utilizing abundant solar energy is a feasible way to solve the energy crisis.In photocatalytic hydrogen evolution,the separation of photogenerated carriers is the rate-determining step that affects the photocatalytic efficiency.Loading a suitable co-catalyst can effectively achieve the separation of photogenerated carriers.At present,photocatalytic hydrogen production co-catalysts are mainly divided into two categories:noble metals and non-noble metals.However,the mechanisms of the two are different,and each has advantages and disadvantages.How to integrate the advantages of the two types of co-catalysts,design a suitable noble metal/non-noble metal co-catalyst system,and study the different roles of noble metal and non-noble metal co-catalysts are of great significance.The research finds that:in the composite system,the molybdenum-based co-catalyst plays the role of storing electrons,and the noble metal plays the role of supporting catalytic active center;the capacitance and the co-catalyst performance of the co-catalyst form a volcanic distribution relationship;With the same amount of noble metals,the photocatalytic activity of the co-catalyst with large capacitance is increased by multiples.The specific research content is as follows:(1)Studies of the different roles of the noble metal Pt and the non-noble metal Mo2N in the Pt/Mo2N composite co-catalyst.Three hydrogen evolution photocatalysts were prepared:Mo2N/CdS,Pt/CdS and(Pt/Mo2N)/CdS.It was found that(Pt/Mo2N)/CdS has the highest photocatalytic hydrogen evolution activity,which is 1.6 times than that of Mo2N/CdS,6.2 times of Pt/CdS,7.2 times of CdS alone.Transient photocurrent experiment shows that(Pt/Mo2N)/CdS has the highest photocurrent,representing the strongest photo-generated carrier separation ability;the linear scanning voltammetry curve and electrochemical impedance spectroscopy proves that Pt has the lowest hydrogen-producing overpotential,the introduction of Pt in Pt/Mo2N can effectively reduce the overpotential of hydrogen evolution and the interface charge transfer resistance;the cyclic voltammetry curve proves that Mo2N/CdS has the largest capacitance,which mainly plays the role of storing electrons,and noble metal co-catalyst mainly plays the role of supporting catalytic active center.It was clarified that photocatalytic hydrogen production efficiency of ?photocatalytic hydrogen production=?light absorption × ?charge separation × ?proton catalyzed,non-noble metal co-catalyst corresponds to ?charge separation,and noble metal co-catalyst corresponds to ?roton catalyzed.(2)Studies of the effect of capacitance on the performance of Mo2N co-catalyst.Three different shapes of Mo2N co-catalysts were prepared.The hexagonal rod-shaped Mo2N/CdS(h-Mo2N/CdS)has the highest photocatalytic hydrogen production activity which is 1.9 times than that of sheet Mo2N/CdS and 3.1 times that of sphere Mo2N/CdS,6.1 times of 1wt.%Pt/CdS,and 30.1 times of CdS alone.Photoelectrochemical experimental studies have shown that the h-Mo2N/CdS has the strongest photocurrent response intensity,which means that it has the strongest photogenerated carrier separation efficiency;linear scanning voltammetry found three Mo2N do not have obvious difference which shows the overpotential is not the main factor affecting the catalytic performance;the electrochemical cyclic voltammetry curve proves that h-Mo2N has the largest specific capacitance of 19.5 F/g,the influence of capacitance on the storage of electrons by non-noble metal co-catalyst has been confirmed.The larger the capacitance,the better the storage of photo-generated electrons.The capacitance is an index to measure the storage of electrons by non-noble metal co-catalysts.Capacitance is one of the essential factors that affect the performance of non-noble metal co-catalysts.(3)Studies of the relationship between the ability of store electrons and the catalytic performance in hydrogen production.Six kinds of Mo2C with increasing capacitance values were prepared by organic-inorganic hybrid method(capacitance values were:9.75,16.51,18.95,28.65,33.44 and 36.62 F/g).The study found that the relationship between the activity and the capacitance value is a volcanic distribution.The co-catalyst with a capacitance value of 18.95 F/g has the highest co-catalytic activity,and its catalytic performance is 1.5 times than that of the co-catalyst with the lowest capacitance value,2.1 times of the highest capacitance value,and 18.5 times of CdS alone.By assembling different amounts of noble metals with a certain amount of non-noble metals,it was found the relationship between the ability and the loading amount also shows a volcanic curve relationship;when the same amount of noble metal Pt was combined with the different Mo2C,the improvement of the photocatalytic activity is positively correlated with the capacitance value,the highest fold reaches 2.08 times.The same amount of noble metal has a limit to the promotion of co-catalytic performance.With the increase of the capacitance value,it eventually tends to be a platform.By comparing different photocatalysts with Pt loading amounts of 0.002%,0.02%,and 0.2%,the linear scanning voltammetry curve proves that Pt has a strong proton catalytic capacity,and the introduction of Pt can effectively reduce the overpotential of hydrogen evolution.Electrochemical impedance spectroscopy proves that the introduction of Pt can effectively reduce the interface charge transfer resistance.The transient photocurrent response curve shows that(0.02%Pt/Mo2C-a4)/CdS has the largest photocurrent,which shows that has the highest photogenerated charge separation efficiency,which further proves the reason for its high photocatalytic activity.