TiO₂-supported Ni₄ quadruple-atom catalyst: A promising few-atom catalyst with high atomic utilization

Using density functional theory calculations, we investigate the growth habit and structural stability of Ni₄ tetramer on TiO₂ (Ni₄/TiO₂), which acts as a representative of oxide-supported few-atom catalysts (FACs) ideally with high atomic utilization. We further analyze the structural characteristics and valence state distribution of metals of two structurally different Ni₄/TiO₂ for comparative study in catalysis, typically as hydrogen-related applications. The planar rhombic and tetrahedral Ni₄/TiO₂ feature the coordination environment of central metal atoms and the interfacial bonding from support interactions, respectively. Both structure-dependent binding characteristics and metal valence state distributions determine the active sites, catalytic activity, and reaction pathways and mechanisms in hydrogen production of the two catalysts. The planar rhombic structure exhibits high atomic utilization and outstanding catalytic activity, far exceeding those of the tetrahedral structure in this reaction. According to the atomic utilization and structure-dependent catalytic performance, we define and conceptualize the rising FACs, independent of cluster catalysts. These findings have implications for the design of suitable FACs and the creation of favorable conditions for multi-step reactions.