DFT-D3 Study of Molecular N2 and H2 Activation on Co3Mo3N Surfaces

Constantinos D Zeinalipour-Yazdi,Justin S J Hargreaves,C Richard A Catlow

doi:10.1021/acs.jpcc.6b04748

Constantinos D Zeinalipour-Yazdi, Justin S J Hargreaves + Show 1 more

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https://doi.org/10.1021/acs.jpcc.6b04748

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Abstract

Cobalt molybdenum nitride (Co3Mo3N) is one of the most active catalysts for ammonia synthesis, although the atomistic details of the reaction mechanism are currently unknown. We present a dispersion-corrected (D3) DFT study of the adsorption and activation of molecular nitrogen and hydrogen on Co3Mo3N-(111) surfaces to identify possible activation sites for ammonia synthesis. H2 was found to adsorb both molecularly on the Mo3N framework and dissociatively on Co8 clusters or Mo3 clusters that were exposed due to N-vacancies. We find that there are two possible activation sites for N2 where both N2 and H2 can coadsorb. The first is a Mo3 triangular cluster that resides at 3f nitrogen vacancies, and the second is a surface cavity where N2 is activated by a Co8 cluster, the second being a more efficient activation site. N2 was found to adsorb in three adsorption configurations: side-on, end-on, and an unusual tilt end-on (155°) configuration, and the existence of these three adsorption configurations is expla...

Highlights

Co3Mo3N when synthesized using the procedure patented by Topsøe[1] is known to be active for ammonia synthesis at 400 °C and elevated pressures using a 3:1, H2:N2 mixture.[2,3] Co3Mo3N when doped with Cs is one of the most active catalysts for ammonia synthesis,[2,4] with turnover-frequencies (TOF) that are remarkably high compared to graphitesuuspedpoirntdedusRtruialalyndfoFrel−arKg2eOs−caAlel2Oam3,mthoenitawosyncathtaelsyisst.s2−c8urLreinnetalyr quantitative structure−property relationships (QSPR) between the dissociation energy of N2 and its adsorption energy on Co3Mo3N were derived via periodic planewave density functional theory (DFT) calculations[5,9] and the barrier for N2-dissociation was found to be the rate-determining step (RDS)
Co3Mo3N when doped with Cs is one of the most active catalysts for ammonia synthesis,[2,4] with turnover-frequencies (TOF) that are remarkably high compared to graphitesuuspedpoirntdedusRtruialalyndfoFrel−arKg2eOs−caAlel2Oam3,mthoenitawosyncathtaelsyisst.s2−c8urLreinnetalyr quantitative structure−property relationships (QSPR) between the dissociation energy of N2 and its adsorption energy on Co3Mo3N were derived via periodic planewave DFT calculations[5,9] and the barrier for N2-dissociation was found to be the rate-determining step (RDS)
It has been previously suggested that ammonia synthesis may proceed via Mars−van Krevelen (MvK) chemistry and that lattice nitrogen may act in nitrogen transfer,[10] which was confirmed by isotopicexchange studies that showed that the lattice-N in Co3Mo3N is exchangeable at elevated temperatures.[11]

Summary

INTRODUCTION

Co3Mo3N when synthesized using the procedure patented by Topsøe[1] is known to be active for ammonia synthesis at 400 °C and elevated pressures using a 3:1, H2:N2 mixture.[2,3] Co3Mo3N when doped with Cs is one of the most active catalysts for ammonia synthesis,[2,4] with turnover-frequencies (TOF) that are remarkably high compared to graphitesuuspedpoirntdedusRtruialalyndfoFrel−arKg2eOs−caAlel2Oam3,mthoenitawosyncathtaelsyisst.s2−c8urLreinnetalyr quantitative structure−property relationships (QSPR) between the dissociation energy of N2 and its adsorption energy on Co3Mo3N were derived via periodic planewave DFT calculations[5,9] and the barrier for N2-dissociation was found to be the rate-determining step (RDS). We have recently shown via DFT calculations that the mechanism can proceed via MvK type surface chemistry at low temperatures as there is a large number of nitrogen vacancies (∼1013 cm−2), which can activate N2 by weakening of the triple bond.[13] N-vacancies were found to participate in the mechanism for the electrochemical reduction of ammonia on VN and ZrN14 and in the two-step solar-energy driven ammonia synthesis on metal nitrides.[15,16] Here, we extend the earlier study by investigating the adsorption at every possible site compared to the adsorption of molecular hydrogen. We explain the various bonding configurations of N2 via molecular orbital (MO) calculations and the sphere-in-contact model

COMPUTATIONAL METHODS

RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES