Defective 1Tˊ-MoX2 (X = S, Se, Te) monolayers for electrocatalytic ammonia synthesis: Steric and electronic effects on the catalytic activity

Abstract

Point defects, including both vacancy (Vd) and antisite defects (Ad) are common in transition metal dichalcogenides (TMDs). Both types of defects are produced due to the deviation from stoichiometric ratio of precursor materials and both point defects can serve as active sites in catalytic reactions. In this study, by means of first-principles calculations, the activities of single-point defective 1T′–MoX2 (X = S, Se, Te) monolayers for the electrocatalytic nitrogen reduction reaction (eNRR) and nitric oxide reduction reaction (eNORR) are investigated. Our results show that these catalysts have great potential toward eNRR and eNORR by exhibiting low limiting potentials (UL). For eNRR, the antisite defects (UL’s are −0.45, −0.31 and −0.27 V for 1T′–MoS2@SAd, 1T′–MoSe2@SeAd and 1T′–MoTe2@TeAd) have better catalytic activity than the vacancy defects (UL’s are −0.94, −0.53 and −0.36 V for 1T′–MoS2@SVd, 1T′–MoSe2@SeVd and 1T′–MoTe2@TeVd). All the calculated UL’s are much smaller than that of the Ru (0001) surface (UL = −0.98 V), the benchmark catalyst for eNRR. For eNORR, except for the 1T′–MoS2@SVd catalyst, antisite defects still perform better (UL’s are correspondingly −0.45, 0 and 0 V for 1T′–MoS2@SVd, 1T′–MoSe2@SeVd and 1T′–MoTe2@TeVd; for vacancy defects, they are 0, −0.13 and −0.36 V). In addition, for eNRR, the hydrogen evolution reaction (HER) is effectively inhibited over the 1T′–MoTe2@TeVd, 1T′–MoS2@SAd, 1T′–MoSe2@SeAd and 1T′–MoTe2@TeAd catalysts. Meanwhile, for eNORR, HER is inhibited over all six catalysts. During the eNORR process at at high NO concentration, except for the 1T′–MoSe2@SeVd and 1T′–MoTe2@TeVd catalysts, the other four show better selectivity toward NH3 than the byproducts of N2O/N2. The activity superiority of the antisite defects is mainly steric, i.e., the steric hindrance is larger at the bowl-shaped vacancy defect. In contrast, the activity difference among the catalysts with the same type of defects is largely electronic, which can be represented by work function. For example, the capability of N2/NO activation and electron transfer increases with the decreasing value of work function:1T′–MoS2@SAd (5.62 eV) < 1T′–MoSe2@SeAd (5.10 eV) < 1T′–MoTe2@TeAd (4.53 eV). Overall, these catalysts are promising for electrochemical ammonia synthesis from N2 and NO. This work may also be useful to the rational design of TMD-based catalysts.