Study On Synthesis And Electrocatalytic Nitrogen Fixation Properties Of 1T And 1T’ Phase MoS₂ Composites

As a very important chemical raw material,ammonia can not only be used in the manufacture of chemical fertilizers,explosives,medicines,etc.,but also considered as an ideal carrier for hydrogen energy because of its high hydrogen content.Currently,the industrial ammonia synthesis method is based on the Haber-Bosch reaction.This process requires a high temperature and high pressure condition,which not only has high energy consumption,but also releases a large amount of carbon dioxide.Electrocatalytic reduction of nitrogen to ammonia can be performed under ambient condition,and the reaction process is environmentally friendly and efficient.It is considered to be one of the most promising ammonia synthesis technologies.In order to enable it to be used in a wide range of industrial applications,it is necessary to design and develop stable,efficient and inexpensive electrocatalyst materials.As a typical transition metal sulfide,the 1T phase MoS2 not only has high electrical conductivity and can promote electron transport during nitrogen fixation,but also has abundant exposed active sites on its edges and basal planes,which can be used for nitrogen molecular adsorption and activation.1T’-phase MoS2 has higher stability while retaining the physical and chemical properties of 1T-phase MoS2.Therefore,2T and 1T’-phase MoS2 are potential nitrogen-fixing electrocatalysts.By combining 1T and 1T’-MoS2 with different substrate materials to suppress competitive hydrogen evolution reactions,the electrocatalytic nitrogen fixation performance can be improved,and higher NH3 yield rate and Faraday efficiency can be obtained.The main research contents and results of this article are as follows:(1)A series of 1T-MoS2/Ti3C2 MXene composites were prepared by using Ti3C2 MXene as the substrate and supporting 1T-MoS2 nanospots with different contents by hydrothermal reaction.For the prepared 1T-MoS2/-Ti3C2 MXene composite,when the loading of 1T-MoS2 is 10 wt.%,its electrocatalytic nitrogen fixation performance is the best.In 0.1 M HCl,its NH3 yield rate and FEs at-0.3 V vs.RHE are 30.33 μg h-1 mg-1cat.and 10.94%,respectively,which are much higher than 3.25 μg h-1 mg-1cat.and 2.65%of TiC2 MXene.The excellent electrocatalytic nitrogen-fixing performance of 1T-MoS2/Ti3C2 MXene composite is mainly attributed to the sandwich-like structure composed of 1T-MoS2 nanospots,Ti3C2 MXene,and TiO2 nanolayers,which not only promotes electron transport during nitrogen fixation,but also effectively inhibition of the hydrogen evolution reaction of Ti3C2 MXene,so as to obtain higher NH3 yield rate and FEs.(2)1T-MoS2 nanodots were uniformly supported on g-C3N4 nanosheets by ultrasonic treatment,and 1T-MoS2/g-C3N4 composites were successfully prepared,and their electrocatalytic nitrogen fixation properties at room temperature and pressure were investigated.For 1T-MoS2/g-C3N4 composites,when the loading of 1T-MoS2 nanodots is 3.0 wt.%,its electrocatalytic nitrogen fixation performance is the best.In 0.1 M HCl,the NH3 yield rate and FEs were 29.97 μg h-1 mg-1cat、and 20.48%at-0.3 V vs.RHE,respectively.It also shows outstanding chemical stability and durability.In addition,the 15N isotope labeling experiments show that the nitrogen source of detected NH3 is an externally supplied gas rather than the decomposition of g-C3N4.The excellent electrocatalytic nitrogen fixation performance of 1T-MoS2/g-C3N4 composites is mainly attributed to the synergy between the g-C3N4 nanosheets and 1T-MoS2 nanodots.The g-C3N4 nanosheet has a large number of active sites,which can effectively adsorb nitrogen molecules.1T-MoS2 nanodots provide relatively aboundent active sites due to their nature and small size,and their high electrical conductivity is conducive to their electron transport in NRR.(3)The 1T’-MoS2 nanosheets were successfully assembled on the surface of urchin-like TiO2 hollow nanospheres to construct 1T’-MoS2/TiO2 composite nitrogen fixation electrocatalysts,which were prepared by hydrolysis reaction,hydrothermal directional etching and hydrothermal reaction.Their electrocatalytic nitrogen fixation performances under ambient condition were explored.For the prepared 1T’-MoS2/TiO2 composites,when the content of 1T’-MoS2 is 7.0 wt.%,its electrocatalytic nitrogen fixation performance is optimal.The NH3 yield rate and FEs reached 29.62 μg h-1 mg-1 cat and 12.4%at-0.75V vs.RHE,respectively,which is far superior to urchin-like TiO2 hollow nanospheres.The excellent electrocatalytic nitrogen fixation performance of 1T’-MoS2/TiO2 composites mainly depend on the unique structural properties of 1T’-MoS2 nanosheets,which provide a rich active site for the adsorption and activation of nitrogen molecules.Meanwhile,urchin-like TiO2 hollow nanosphere substrates not only provide a wide space for the loading of 1T’-MoS2 nanosheets,but also increase the chance of contact with nitrogen molecules,due to their high specific surface area.