Greatly Improving Electrochemical N₂ Reduction over TiO₂ Nanoparticles by Iron Doping

Abstract

Titanium-based catalysts are needed to achieve electrocatalytic N 2 reduction to NH 3 with alarge NH 3 yield and ahigh Faradaic efficiency (FE). One of the cheapest and most abundant metals on earth, iron, is an effective dopant for greatly improving the nitrogen reduction reaction (NRR) performance of TiO 2 nanoparticles in ambient N 2 -to-NH 3 conversion. In 0.5 m LiClO 4 ,F e-doped TiO 2 catalyst attains ah igh FE of 25.6 %a nd al arge NH 3 yield of 25.47 mgh À1 mg cat À1 at À0.40 Vv ersus ar eversible hydrogen electrode.This performance compares favorably to those of all previously reported titanium-and iron-based NRR electrocatalysts in aqueous media. The catalytic mechanism is further probed with theoretical calculations.Asanessential activated nitrogen source,NH 3 is extensively used to manufacture dyes,p olymers,f ertilizers,a nd explosives,and it also serves as carbon-neutral energy carrier with high energy density. [1][2][3] To date,the dominant industrial route for NH 3 synthesis is the Haber-Bosch process using N 2 and H 2 as the feeding gases,b ut this process operates at high temperature and high pressure,a nd consumes al arge amount of energy while emitting CO 2 . [4] Electrochemical N 2 reduction offers an attractive alternative in an environmentally benign and sustainable manner,b ut the strong N N bond is fairly inert and thus difficult to break in ac hemical reaction, underlying the need for electrocatalysts with high activity in the nitrogen reduction reaction (NRR). [5][6][7][8] Noble metals perform the NRR efficiently;however, their scarcity and high cost limits their application in large-scale N 2 reduction. [8][9][10][11] NRR research has thus shifted to development of noble-metal-free alternatives. [12][13][14][15][16][17][18][19][20][21][22][23][24] TiO 2 is highly adaptable as asemiconductor catalyst because of its long-term thermodynamic stability,n atural abundance,a nd nontoxicity. [25] Recent studies have demonstrated that TiO 2 with oxygen defects has good electrocatalytic activity for the NRR, [26,27] and heteroatoms (B, [28] C, [29] V, [30] and Zr, [31] )a re effective dopants to enhance the NRR performances of TiO 2 catalysts. However,T i-based catalysts that simultaneously achieve al arge NH 3 yield with ah igh Faradaic efficiency (FE) are not available thus far.As one of the cheapest and most abundant metals on the earth, [32] Fe also exists in biological nitrogenases for natural N 2 fixation. [33] Fe compounds have been widely utilized as catalysts for artificial N 2 fixation in the Haber-Bosch [4] and electrochemical [34][35][36][37][38][39] processes.I ti st hus natural for us to explore use of Fe as ad opant for TiO 2 ,w hich has not been reported before.Herein, we report on our recent experimental results that Fe-doped TiO 2 is superior in performances for electrocatalytic N 2 reduction under ambient conditions.I n 0.5 m LiClO 4 ,t his catalyst achieves ah igh FE of 25.6 %a nd al arge NH 3 yield of 25.47 mgh À1 mg cat À1 at À0.40 Vv ersus ar eversible hydrogen electrode (...

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