Supporting Information for: Metal-Free Single Atom Catalyst for N2 Fixation Driven by. Visible Light

Transcription

1 Supporting Information for: Metal-Free Single Atom Catalyst for N2 Fixation Driven by Visible Light Chongyi Ling, a,b Xianghong Niu, c Qiang Li, a Aijun Du,*,b Jinlan Wang*,a a School of Physics, Southeast University, Nanjing , People s Republic of China. b School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, Gardens Point Campus, Brisbane, QLD 4001, Australia. c School of Science, Nanjing University of Posts and Telecommunications, Nanjing , People s Republic of China. *jlwang@seu.edu.cn (J.W.); *aijun.du@qut.edu.au (A.D). S1

2 Free Energy Calculations The calculations of Gibbs free energy change ( G) for each elemental step was based on the computational hydrogen electrode model proposed by Nørskov et al. (Energy Environ. Sci. 2010, 3, 1311), which can be computed by: G = E + EZPE T S + eu + GpH where E is the electronic energy difference between the free standing and adsorption states of reaction intermediates; EZPE and S are the changes in zero point energies and entropy respectively, which are obtained from the vibrational frequency calculations and presented in Table S2; T is the temperature, which is set to be K in this work; e and U are the number of electrons transferred and the electrode potential applied; GpH is the free energy correction of ph, which can be calculated by: GpH = kbt ph ln10. The EZPE and TS for each reaction intermediates can be calculated by the following equations, respectively: E ZPE = 1 2 hv i i TS = K B T ln (1 e hv i K BT ) i 1 hv i ( hv i ) i ek BT 1 where h, v and KB are Planck constant, vibrational frequencies and Boltzmann constant, respectively. Moreover, only the calculation of zero point energy and entropy of reaction intermediates are needed as the contribution of substrate can be offset. To verify this point, the EZPE and TS values of B/g-C3N4 substrate under different environments (pure, N2 and N-NH adsorbed) are further calculated. As shown in Table S1, the contribution of B/g-C3N4 substrate to the EZPE and TS are identical with each other and therefore, can indeed be offset. S2

3 The onset potential (UOnset) of the whole reduction process is determined by the potential-limiting step which has the most positive G ( Gmax) as computed by: UOnset = - Gmax/e. Although the ph values will affect the free energy for each electrochemical reaction, the calculated overpotential (η, computed by: η = Uequilibrium - UOnset) is actually invariable. As presented above, the effect of ph on G can be calculated by: GpH = kbt ph ln10 = 0.06pH. Therefore, for all the six electrochemical reactions under a given ph, the calculated G will be shifted up by 0.06pH with respect to that at ph = 0 ( G(pH=0)): G = G(pH=0) pH. As a result, the rate-determining step is independent of the ph values and only the limiting potential will be more negative: UOnset = UOnset(pH=0) 0.06pH. Meanwhile, the variation of ph will also lead to the change of equilibrium potential. According to the Nernst equation, the equilibrium potential under the given ph (Uequilibrium) can be calculated by: Uequilibrium = Uequilibrium(pH=0) 0.06pH. Therefore, the overpotential under a given ph (η) will be: η = Uequilibrium UOnset = (Uequilibrium(pH=0) 0.06pH) (UOnset(pH=0) 0.06pH) = Uequilibrium(pH=0) UOnset(pH=0) = η(ph=0) Therefore, the ph will not change the overpotential and thus only the ph = 0 is considered in our work. S3

4 Table S1. Calculated EZPE and TS values (in ev) of B/g-C3N4 substrate under different environments (pure, N2 and N-NH adsorbed). E ZPE Pure g-c3n N2 adsorbed g-c3n N-NH adsorbed g-c3n TS Table S2. Calculated vibrational frequencies, zero point energies and entropy of different adsorption species, where the * denotes the adsorption site. Therefore, *N *N and *N N represent the side-on and end-on adsorption configurations, respectively. Adsorption Species Vibrational Frequencies (cm -1 ) EZPE (ev) TS (ev) *N *N *N=*NH *NH =*NH *NH *NH *NH2 *NH *NH2 *NH *N *NH *NH *NH *N N *N=NH *NH=NH *NH NH *NH2 NH *N NH S4

5 Table S3. Calculated binding energies (Eb) of B atom on VN3C2, Hex and VN2C1 sites of B/g-C3N4 substrate as well as the difference (ΔEb) with respect to Eb on VN3C2 (in ev). E b ΔE b VN3C Hex VN2C Figure S1. Structures of the reaction intermediates for N2 reduction on B/g-C3N4 through the distal pathway. S5

6 Figure S2. Structures of the reaction intermediates for N2 reduction on B/g-C3N4 through the alternating pathway. Figure S3. Structures of the reaction intermediates for N2 reduction on B/g-C3N4 through the enzymatic pathway. S6

7 Figure S4. Free energy diagrams for N2 reduction on B/g-C3N4 through (a) a minimum free energy pathway (MFEP) at different applied potentials and (b) enzymatic mechanism at different applied potentials by using RPBE functional. Figre S5. Band structures of (a) g-c3n4 and (b) B/g-C3N4. Fermi levels are set to zero and represented by red dash lines. S7

8 Figure S6. Variations of temperature and energy of against the time for the ab initio molecular dynamics simulations of B/g-C3N4, insert are top and side views of the snapshot of atomic configuration. The simulation is run under 400 K for 10 ps with a time step of 2 fs. Figure S7. Variations of temperature and energy of against the time for the ab initio molecular dynamics simulations of B/g-C3N4, insert are top and side views of the snapshot of atomic configuration. The simulation is run under 600 K for 10 ps with a time step of 2 fs. S8

9 Figure S8. Variations of temperature and energy of against the time for the ab initio molecular dynamics simulations of B/g-C3N4, insert are top and side views of the snapshot of atomic configuration. The simulation is run under 800 K for 10 ps with a time step of 2 fs. Figure S9. Variations of temperature and energy of against the time for the ab initio molecular dynamics simulations of B/g-C3N4, insert are top and side views of the snapshot of atomic configuration. The simulation is run under 1000 K for 100 ps with a time step of 3 fs. S9