Recent studies have suggested that rare earth (RE) elements in catalysts significantly influence the performance of the ammonia synthesis. The REs appear in various forms in the ammonia synthesis catalysts including supports (oxides, hydrides, and nitrides), promotors, and intermetallic. Besides the conventional RE oxide-supporting catalysts (mainly Ru/REO), some new RE-containing catalyst systems, such as electrode and nitride systems, could drive the ammonia synthesis via a benign Mars–van Krevelen mechanism or multi-active-site mode, affording high ammonia synthesis performance under mild conditions. These works demonstrate the great potential of RE-containing catalysts for more efficient ammonia synthesis. This review summarizes the contributions of different kinds of RE-based catalysts and highlights the function mechanism of incorporated REs. Finally, an overview of this area and the challenges for further investigation are provided.

Although tin monoxide (SnO) is an interesting compound due to its p-type conductivity, a widespread application of SnO has been limited by its narrow band gap of 0.7 eV. In this work, we theoretically investigate the structural and electronic properties of several SnO phases under high pressures through employing van der Waals (vdW) functionals. Our calculations reveal that a metastable SnO (β-SnO), which possesses space group P2₁/c and a wide band gap of 1.9 eV, is more stable than α-SnO at pressures higher than 80 GPa. Moreover, a stable (space group P2/c) and a metastable (space group Pnma) phases of SnO appear at pressures higher than 120 GPa. Energy and topological analyses show that P2/c-SnO has a high possibility to directly transform to β-SnO at around 120 GPa. Our work also reveals that β-SnO is a necessary intermediate state between high-pressure phase Pnma-SnO and low-pressure phase α-SnO for the phase transition path Pnma-SnO →β-SnO → α-SnO. Two phase transition analyses indicate that there is a high possibility to synthesize β-SnO under high-pressure conditions and have it remain stable under normal pressure. Finally, our study reveals that the conductive property of β-SnO can be engineered in a low-pressure range (0–9 GPa) through a semiconductor-to-metal transition, while maintaining transparency in the visible light range.