Journal of Materials Chemistry A
Communication
The rate-limiting step is the nal *NH2 to NH3 reaction. In
addition, we also calculated the alternative NRR mechanism,
9 I. Dybkjaer in Ammonia, Catalysis and Manufacture, ed. A.
Nielsen, Springer, Heidelberg, 1995, pp. 199–308.
which was found to be energetically unfavorable (Fig. S13 and 10 M. Appl, Ammonia–Ullmann’s Encyclopedia of Industrial
S14†). Instead, we have found another energetically allowed Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, 2002.
mixed NRR mechanism (Fig. 4). In this mechanism, the rst 11 M. K. Chan, J. Kim and D. C. Rees, Science, 1993, 260, 792–
reaction steps i.e. from N2 to *NH–*NH2 are the same as those 794.
in the alternative mechanism. The difference is that in the 12 B. K. Burgess and D. J. Lowe, Chem. Rev., 1996, 96, 2983–
mixed one, a proton and electron pair is added to the *NH2 3011.
moiety of the *NH–*NH2 species generating a NH3 molecule, 13 K. A. Brown, D. F. Harris, M. B. Wilker, A. Rasmussen,
not the *NH moiety. As a result, the high-energy NH2–NH2
N. Khadka, H. Hamby, S. Keable, G. Dukovic, J. W. Peters,
intermediate is bypassed. This is also consistent with the
L. C. Seefeldt and P. W. King, Science, 2016, 352, 448–450.
experimental observation of not detecting the N2H4 species (see 14 C. J. M. van der Ham, M. T. M. Koper and
above). It should be noted that both distal and mixed mecha- D. G. H. Hetterscheid, Chem. Soc. Rev., 2014, 43, 5183–5191.
nisms have the same rate-limiting step. Nonetheless, the distal 15 V. Kyriakou, I. Garagounis, E. Vasileiou, A. Vourros and
mechanism is still preferred because of its much more signi-
cant exothermicity from *N–*NH to *NH (Fig. 4).
M. Stoukides, Catal. Today, 2017, 286, 2–13.
16 C. Guo, J. Ran, A. Vasileff and S. Qiao, Energy Environ. Sci.,
2018, 11, 45–56.
In summary, Ti3C2Tx MXene nanosheets have been experi-
mentally and theoretically veried as non-noble-metal 2D 17 M. Shi, D. Bao, B. R. Wulan, Y. Li, Y. Zhang, J. Yan and
catalysts to enable high-performance electrohydrogenation of Q. Jiang, Adv. Mater., 2017, 29, 1606550.
N2 to NH3 under ambient conditions. When tested in acids, this 18 D. Bao, Q. Zhang, F. Meng, H. Zhong, M. Shi, Y. Zhang,
catalyst is excellent in activity with a NH3 yield of 20.4 mg hꢀ1
J. Yan, Q. Jiang and X. Zhang, Adv. Mater., 2017, 29, 1604799.
ꢀ1
mgcat.
and a FE of 9.3%, with high electrochemical and 19 H. Huang, L. Xia, X. Shi, A. M. Asiri and X. Sun, Chem.
structural stability. DFT calculations reveal that the distal NRR
Commun., 2018, 54, 11427–11430.
mechanism is more favorable and the related *NH2 / NH3 20 J. Wang, L. Yu, L. Hu, G. Chen, H. Xin and X. Feng, Nat.
reaction is the rate-limiting step. This study not only provides us Commun., 2018, 9, 1795.
with an attractive earth-abundant catalyst material for electro- 21 Z. Wang, F. Gong, L. Zhang, R. Wang, L. Ji, Q. Liu, Y. Luo,
chemical NH3 synthesis, but it would open up exciting new
avenues for the rational design and development of MXene-
based nanocatalysts for articial N2 xation.
H. Guo, Y. Li, P. Gao, X. Shi, B. Li, B. Tang and X. Sun,
Adv. Sci., 2018, 5, 1801182.
22 X. Zhang, Q. Liu, X. Shi, A. M. Asiri, Y. Luo, X. Sun and T. Li,
J. Mater. Chem. A, 2018, 6, 17303–17306.
23 Y. Zhang, W.-B. Qiu, Y. Ma, Y. Luo, Z. Tian, G. Cui, F. Xie,
L. Chen, T. Li and X. Sun, ACS Catal., 2018, 8, 8540–8544.
24 Q. Liu, X. Zhang, B. Zhang, Y. Luo, G. Cui, F. Xie and X. Sun,
Nanoscale, 2018, 10, 14386–14389.
Conflicts of interest
There are no conicts to declare.
25 L. Zhang, X. Ji, X. Ren, Y. Ma, X. Shi, Z. Tian, A. M. Asiri,
L. Chen, B. Tang and X. Sun, Adv. Mater., 2018, 30, 1800191.
Acknowledgements
This work was supported by the National Natural Science 26 L. Zhang, X. Ji, X. Ren, Y. Luo, X. Shi, A. M. Asiri, B. Zheng
Foundation of China (No. 21575137).
and X. Sun, ACS Sustainable Chem. Eng., 2018, 6, 9550–9554.
27 X. Wu, L. Xia, Y. Wang, W. Lu, Q. Liu, X. Shi and X. Sun,
Small, 2018, 14, 180311.
28 R. Zhang, Y. Zhang, X. Ren, G. Cui, A. M. Asiri, B. Zheng and
X. Sun, ACS Sustainable Chem. Eng., 2018, 6, 9545–9549.
29 M. Naguib, V. N. Mochalin, M. W. Barsoum and Y. Gogotsi,
Adv. Mater., 2014, 26, 992–1005.
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