Hydrogen Spillover to Oxygen Vacancy of TiO_2–<i>x</i>H_<i>y</i>/Fe: Breaking the Scaling Relationship of Ammonia Synthesis

Abstract

Optimizing kinetic barriers of ammonia synthesis to reduce the energy intensity has recently attracted significant research interest. The motivation for the research is to discover means by which activation barriers of N₂ dissociation and NH_<i>z</i> (<i>z</i> = 1-2, surface intermediates) destabilization can be reduced simultaneously, that is, breaking the "scaling relationship". However, by far only a single success has been reported in 2016 based on the discovery of a strong-weak N-bonding pair: transition metals (nitrides)-LiH. Described herein is a second example that is counterintuitively founded upon a strong-strong N-bonding pair unveiled in a bifunctional nanoscale catalyst TiO_2-<i>x</i>H_<i>y</i>/Fe (where 0.02 ≤ <i>x</i> ≤ 0.03 and 0 < <i>y</i> < 0.03), in which hydrogen spillover (H) from Fe to cascade oxygen vacancies (O_V-O_V) results in the trapped form of O_V-H on the TiO_2-<i>x</i>H_<i>y</i> component. The Fe component thus enables facile activation of N₂, while the O_V-H in TiO_2-<i>x</i>H_<i>y</i> hydrogenates the N or NH_<i>z</i> to NH₃ easily.