Research On Photocatalytic Nitrogen Fixation Activity And Influencing Factors Of Mo Oxides And Oxygenates

NH₃ is an indispensable chemical product in social production.At present,the industrial ammonia synthesis process is still dominated by Haber-Bosch method,with high energy consumption and serious pollution.It is urgent to develop a new nitrogen fixation process with low energy consumption and pollution.Photocatalytic nitrogen fixation has become an important research direction for researchers because of its advantages of energy saving and environmental protection.Improving the adsorption capacity of N₂ through the construction of active sites is an important research topic to improve the photocatalytic nitrogen fixation performance of semiconductor materials.Mo has become the research object of this paper due to its high selectivity,strong adsorption and activation capabilities of N₂.The theoretical calculation of density functional theory of MoO₂ and MoO₃ was carried out by CASTEP.The calculation results confirmed the feasibility of MoO₂ as a photocatalytic nitrogen fixation material,and pointed out that its hydrogenation process follows the distal/alternating path.MoO₂ and MoO₃ were prepared by in-situ carbon reduction method,meanwhile,the phase analysis,structure and morphology characterization,optical performance testing were carried out.The results showed that MoO₂ has a rough surface structure and a cubic appearance with particle size between 1.5-2 ?m.Moreover,MoO₂ has a low carrier recombination rate and a strong light response.Under the condition of not adding any sacrificial agent,the ammonia-producing capacity of MoO₂ was detected by indigo colorimetry,and it was found that the ammonia-producing rate of MoO₂ was88.33 ?mol·g^-1·h^-1,which could be compared with precious metals.The controlled experiment showed that MoO₂ has photocatalytic nitrogen fixation activity only in the presence of both light and nitrogen.The reason why MoO₂ has better nitrogen fixation activity can be attributed to the adsorption and activation of N₂ by low-valent Mo⁴⁺ in the form of coordination bond,which improves the selectivity of N₂.At the same time,the valence electrons of Mo⁴⁺ can be fed back to N₂,which reduces the activation difficulty of N₂,thus achieving the highly efficient photocatalytic nitrogen fixation ability of the material under environmental conditions.Based on the density functional theory calculation of Na₂Mo₂O₇,it is predicted that the nitrogen fixation performance of Na₂Mo₂O₇ is inferior to MoO₂,and the reason for the difference is the different valence state of molybdenum ions.Pure phase Na₂Mo₂O₇ catalytic material was prepared by in-situ carbon reduction.The Na₂Mo₂O₇ were characterized and tested by XRD,SEM,TEM,XPS,UV-vis and PL.The results showed that the particle size of Na₂Mo₂O₇ was about 2 ?m,besides,the carrier separation ability and optical properties of Na₂Mo₂O₇ were not as good as MoO₂.The results of the photocatalytic nitrogen fixation test table showed that the ammonia production rate of Na₂Mo₂O₇ is 63.24 ?mol·g^-1·h^-1.The calculation and experimental results of MoO₂,Na₂Mo₂O₇ and MoO₃ have confirmed that the valence state of molybdenum ions is the main factor affecting the photocatalytic nitrogen fixation performance of the three materials.The nitrogen fixation activity and selectivity of low valence Mo in MoO₂ are higher than that in Na₂Mo₂O₇ and MoO₃.The results showed that the construction of low valence metal active centers is of great significance for the realization of high efficiency of photocatalytic nitrogen fixation under environmental conditions.