Electroreduction of N 2 to ammonia at ambient conditions on mononitrides of Zr, Nb, Cr, and V A DFT guide for experiments

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1 Electronic Supplementary Information Electroreduction of N to ammonia at ambient conditions on mononitrides of Zr, Nb, r, and V DFT guide for experiments Younes bghoui a, nna L. Garden b, Jakob G. Howalt c, Tejs Vegge c and Egill Skúlason a* a Science Institute and Faculty of Physical Sciences, VR-III, University of Iceland, IS-7 Reykjavik, Iceland b Department of hemistry, University of Otago, P.O. Box 56, Dunedin 95, New Zealand c Department of Energy onversion and Storage, Technical University of Denmark, DK-8 Kgs. Lyngby, Denmark * orresponding author. egillsk@hi.is DFT calculations are used to find the adsorption energy of the most stable intermediates along the reaction path by considering all the possible adsorption sites of proton on the surface. To construct free energy diagrams, the free energy of each elementary step is estimated at ph = according to: G = E + E ZPE T S () where E is the reaction energy calculated using DFT. The zero-point energy correction ( E ZPE ) and entropy difference ( S) are calculated within a harmonic approximation (given in table S below). The effect of an applied bias, U, is included for all electrochemical reaction steps by shifting the free energy for reactions involving n electrons by neu: G(U) = E + E ZPE T S neu () Table S. Zero point energy and entropy contributions to the free energy of gas phase and adsorbed molecules at K for RS nitrides (in ev). RS () TS ZPE NH.7.89 H..7 N.6.5 *N.9.7 *H on-m.. *H on-n..5 *H.7.69 *H.7. *N..8 *NH.6. *NH.7.69 *NH.8.99

2 RS () TS ZPE NH.7.89 H..7 N.6.5 *N.. *H.7.9 *H..59 *H..89 *N.. *NH.7.9 *NH *NH..99 ZB () TS ZPE NH.7.89 H..7 N.6.5 *N.. *H.5.7 *H.8.6 *H.89.9 *N.7.9 *NH.55.7 *NH.75.7 *NH.5. ZB () TS ZPE NH.7.89 H..7 N.6.5 *N..6 *H on-m.. *H on-n.7.9 *H..6 *H.5.96 *N.7.8 The values for adsorbed species are obtained from DFT calculations of vibrational normal modes. The gas phase values are taken from standard molecular tables. ccordingly, the free energy diagrams are constructed and shown for the overall mechanisms of NH formation on different facets of ZrN, NbN, rn and VN in Figure S.

3 ssociative MVK RS() ZrN.76 ev B) ssociative MVK RS() NbN.65 ev - NbH NH B NbH N E NH,B NbH N E NH,B NbH N E NH - - B NbH N E NH,B NbH N E NH

4 ssociative MVK RS() rn NH,E N B NH RDS:.85 ev :.76 ev B NH N E NH B NH N E NH N E NH N B N N B ssociative MVK RS() VN.5 ev NH,E N D E NH VN -.5 -

5 7 6 5 ssociative MVK RS() ZrN,D NH B NH NH. ev D NH B, NH,D NH B NH,E N E NH,D NH B NH.56 ev E NH E NH D,D NH,B N,D NH B NH N - ssociative MVK RS() NbN. ev B NH NH,D NH B NH B NH,E N RDS.58 ev,d NH,B N D B NH N -

6 ssociative MVK RS() rn NH,D NH,E N,D NH B NH,D NH,B N,D NH N - RDS:.77 ev :.7 ev - - ssociative MVK RS() VN NH.85 ev RDS.75 ev,e N,D NH B NH -

7 bridge Dissociative ssociative -D NH bridge. ev,e N RDS. ev - ZB() rn -5,D NH ssociative MVK ZB() VN NH.97 ev B NH. ev,e N,D NH,B N -

8 6 5 ssociative MVK -D ZrH ZrH B NH ZB() ZrN T NH. ev ZrH,E N ZrH,B N ZrH N ssociative MVK ZB() NbN.59 ev D -

9 ssociative MVK ZB() rn rh NH B NH.5 ev rh B NH N E NH RDS.76 ev ssociative MVK ZB() VN.6 ev B,D NH NH NH B,D NH NH B,D NH,E N RDS.58 ev. ev D,D NH,B N - Figure S. The free energy diagrams constructed at zero potential when vacancy is filled with N (g) either associatively (red) or dissociatively (blue). The different species formed at each mechanism is shown with the relevant colour code. The labels written bold are the species formed/remained at that step. If the step with the largest free energy is an electrochemical step including proton-electron transfer, it will be tuneable by external bias and therefore called potential determining step (). If the step does not include proton-electron transfer, it will not be tuned by external bias and therefore called rate-determining step (RDS).

10 Table S. Energy per metal-nitrogen pair (in ev) of the various metal nitride slabs. The energies are relative to the most stable facet for each nitride. Slabs that were found to be unstable upon relaxation are denoted with a hyphen. Data taken from Ref.. Nitride RS() RS() ZB() ZB() ScN TiN VN rn MnN FeN on NiN...6. un YN ZrN NbN MoN.... RuN RhN PdN gn HfN TaN WN ReN OsN IrN PtN un To investigate the stability of the N-vacancy against poisoning by O, OH or a proton from the electrochemical media, the free energies are calculated at the onset potential proposed for the ammonia formation. The free energy of adsorption of *X (X= O or OH or H) is calculated at the onset as well as the free energy of adsorption of nitrogen (*N). The addition of the onset potential neu to G(*N) and to G(*X) does not cancel out when calculating dg(*n-*x). That is because we considered adsorption of H or O or OH being affected by the value of the potential but not the adsorption of N. These results are as expected as when the bias is tuned towards more negative values, the electropositive O species bind weaker on the surface compared to H O, as they would rather form bonds with the surface when the bias is more positive: H O + * *O + H + + e - onversely, the H adsorption free energy becomes more negative when the bias is lowered: H + +e - *H While analysing different mechanisms for reduction of nitrogen to ammonia, we found that the Mars-van Krevelen mechanism is more energetically favourable on the surface of nitrides compared to the common associative or dissociative mechanisms. For the associative mechanism, the formation of *N H was found to be thermodynamically unfavored on the surface of these nitrides. For the dissociative mechanism, very large kinetic barriers of more than.5 ev were found for splitting of di-nitrogen on the surface. The

11 results of these two mechanisms are shown below. For ammonia formation via associative mechanism on the clean surface of RS() and ZB(), there was no stable site found on the surface for adsorption of di-nitrogen. Via dissociative mechanism as well, no stable adsorption site was found on the surface of the RS() and ZB() for addition of the second *N. On the surface of the ZB(), large surface reconstructions were observed upon addition of the second *N and therefore those facets are not included here. RS VN rn ZrN NbN *NNH *NNH *N VN rn ZB *NNH ZrN NbN *NNH *N Figure S. The first three elementary steps in the free energy diagram towards NH formation via an associative mechanism on the clean surface of the nitrides. The N adsorption on the surface is endothermic in most cases and the first protonation step of the adsorbed N is endergonic making this mechanism unlikely to occur at ambient conditions. On the surface of the RS() and ZB() facets of these nitrides, no stable adsorption site was found for adsorption of di-nitrogen. On the ZB() of ZrN, *NNH formation led to large reconstruction of the surface and therefore, the *NNH step is not shown here.

12 .5. rn VN E a / ev NbN.5 ZrN. - - ΔE / ev Figure S. ctivation energy (Ea, in ev) and reaction energy ( E, in ev) of N dissociation on the clean surface of the RS() facet of the most promising metal nitrides. For all nitrides, there is a very large energy barrier for splitting of dinitrogen on the surface making the dissociative mechanism very unlikely to occur at ambient conditions. No stable adsorption site was found on the surface of the RS() and ZB() for addition of the second *N. On the surface of the ZB(), large surface reconstructions were observed upon addition of the second *N and therefore those facets are not included here. References. tkins, W. Physical hemistry, 6 th ed.; Oxford University Press: Oxford, 998; pp 85, , and 9.. bghoui, Y.; Garden,. L.; Hlynsson, V. F.; Björgvinsdóttir, S.; Ólafsdóttir, H.; Skúlason, E. Phys. hem. hem. Phys. 5, 7,