Precise Synthesis Of Nickel Phosphide And Its Application As A Cocatalyst For Photocatalytic Decomposition Of Formic Acid

Using semiconductor as photocatalyst to convert solar energy into chemical energy in a sustainable manner is one of the most promising solutions to the energy crisis and environmental issues.Formic acid is deemed as a potential carrier for hydrogen production by virtue of its merits such as high hydrogen content,non-toxic and stable.Many researches have been carried out on the formic acid dehydrogenation in the field of thermal catalysis.Photocatalytic formic acid dehydrogenation has multiple advantages of low cost,non-pollution as well as mild reaction conditions.Compared with the noble metal catalysts widely used in thermal catalysis,the non-noble metal substitution strategy was adopted in the process of photocatalytic formic acid dehydrogenation to produce hydrogen,which not only guaranteed the activity,selectivity and stability of the catalyst,but also reduced the cost of the catalyst.The controllable preparation of the ternary photocatalyst composed of nickel phosphide（cocatalyst）/cadmium sulfide（photocatalyst semiconductor）/graphene oxide（carrier）and its application in the photocatalytic formic acid decomposition to hydrogen production system were studied.The precise synthesis of nickel phosphide with controllable ratio of phosphorus to nickel was realized.The results show that nickel phosphide with high P/Ni ratio can be controlled by increasing the concentration of phosphorus species,synthesis temperature and synthesis time.In this paper,the hydrogen production activity of nickel phosphide composite photocatalyst with different nickel phosphide crystal phase,nickel phosphide loading and formic acid concentration was studied,and the role of nickel phosphide in photocatalytic decomposition of formic acid for hydrogen production system was analyzed.Nickel phosphide phase with high P/Ni ratio shows better co-catalytic performance(Ni₅P₄>Ni₂P>Ni₁₂P₅);The prepared nickel phosphide/cadmium sulfide/graphene oxide composite photocatalyst exhibited excellent formic acid photocatalytic hydrogen production performance;the photocatalytic activity of the photocatalyst with 1wt.%Ni₅P₄/Cd S/GO exhibited the highest hydrogen production rate of 18.62μmol/mg/h,which is 6.18 times higher than that of pure Cd S.The co-catalytic effect of Ni₂P in the field of photocatalytic formic acid dehydrogenation on different photocatalysts（Ti O₂,Cd S and C₃N₄）was also studied.The optimal H₂ evolution rate reaches 41.69μmol/mg/h,22.45μmol/mg/h and 47.67μmol/mg/h under the optimal formic acid cocentration over 1 wt.%Ni₂P/Ti O₂,1wt.%Ni₂P/Cd S and 10 wt.%Ni₂P/C₃N₄ photocatalyst,respectively,which are 3.8,10and 210 times greater than those of pure Ti O₂,Cd S and C₃N₄.The result indicates that nickel phosphide has universality in photocatalytic decomposition of formic acid for hydrogen production.Finally,the mechanism of photocatalytic formic acid dehydrogenation was proposed.The introduction of Ni₂P cocatalyst significantly accelerates photogenerated electrons transfer from semiconductor to Ni₂P and the separation of photogenerated electrons-hole pair.At the same time,Ni₂P promote the formation of active species·OH and increase the hydrogen production rate of formic acid decomposition.These research results show that nickel phosphide has high hydrogen production activity.It has a good application prospects for constructing an efficient photocatalytic formic acid dehydrogenation system.