Reduction of N2 to NH3 catalyzed by a Keggin-type polyoxometalate-supported dual-atom catalyst

Abstract

Because of the harsh reaction conditions and relatively low ammonia (NH₃) yield of the Haber–Bosch process in the synthetic NH₃ industry, it is highly desirable to develop an alternative route for efficient dinitrogen (N₂) fixation under milder conditions. Dual-atom catalysts (DACs), in which metal dimers are anchored on an appropriate substrate, not only possess the advantage of single-atom catalysts, but also boast higher metal atom loading and more flexible active sites. In the present paper, a tantalum (Ta) atom was anchored onto a mono-Ta-substituted Keggin-type polyoxometalate (POM) support to form a homo-nuclear DAC for the nitrogen reduction reaction (NRR). According to our density functional theory (DFT) calculations, we found that the metal–support interaction of the DAC studied here was mainly determined by the bonding interaction, rather than the electrostatic force. Moreover, the DAC studied here possesses matching energy levels of frontier molecular orbitals (FMOs) for the activation of an inert N₂ molecule. The electronic structural and geometric analyses show that the suitable Ta–Ta distance and unique molecular orbital topology are responsible for the effective electron transfer from d_xy orbitals of two Ta centers to the phase matching π*_2px unoccupied orbital of the N₂ molecule. Free energy calculations show that the elemental steps for the reduction of N₂ to NH₃ on the DAC studied here are all thermodynamic allowed. The unique tilted arrangement of the adsorbed N₂ molecule significantly decreases the reaction free energy for the hydrogenation of the adsorbed N₂ molecule to the N₂H intermediate, which is always found to be the rate-determining step of the NRR in most catalytic systems. We hope that our findings would provide new insights into the catalytic mechanism of DACs for the NRR at the atomic level.