Sabatier Phenomenon in Hydrogenation Reactions Induced by Single-Atom Density

Abstract

The Sabatier principle is a fundamental concept in heterogeneous catalysis that provides guidance for designing optimal catalysts with the highest activities. For the first time, we here report a new Sabatier phenomenon in hydrogenation reactions induced by single-atom density at the atomic scale. We produce a series of Ir single-atom catalysts (SACs) with a predominantly Ir1-P4 coordination structure with densities ranging from 0.1 to 1.7 atoms/nm2 through a P-coordination strategy. When used as the catalysts for hydrogenation, a volcano-type relationship between Ir single-atom density and hydrogenation activity emerges, with a summit at a moderate density of 0.7 atoms/nm2. Mechanistic studies show that the balance between adsorption and desorption strength of the activated H* on Ir single atoms is found to be a key factor for the Sabatier phenomenon. The transferred Bader charge on these Ir SACs is proposed as a descriptor to interpret the structure–activity relationship. In addition, the maximum activity and selectivity can be simultaneously achieved in chemoselective hydrogenation reactions with the optimized catalyst due to the uniform geometric and electronic structures of single sites in SACs. The present study reveals the Sabatier principle as an insightful guidance for the rational design of more efficient and practicable SACs for hydrogenation reactions.

An Overview of Metal Density Effects in Single-Atom Catalysts for Thermal Catalysis

“…In order to reveal the connection between geometric structures and the catalytic performance of SACs with various densities, an ideal catalyst to avoid the influence of other factors (e.g., coordination atoms, coordination numbers, etc.) is essential. ,,, One facile strategy is to load different contents of metal on the same support matrix so that the resultant SACs can be studied to determine the density-dependent catalytic performance (Figure a). It is noticeable that the intersite distance decreases as the surface density of single atoms increases. ,, Moreover, because dense single atoms are embedded into support, the configurations of SACs will suffer extensive reconstruction, resulting in nonplanar and asymmetric atom arrangements .…”

“…is essential. ,,, One facile strategy is to load different contents of metal on the same support matrix so that the resultant SACs can be studied to determine the density-dependent catalytic performance (Figure a). It is noticeable that the intersite distance decreases as the surface density of single atoms increases. ,, Moreover, because dense single atoms are embedded into support, the configurations of SACs will suffer extensive reconstruction, resulting in nonplanar and asymmetric atom arrangements . These distinguishing features of high-density SACs may induce the synergistic effect to boost the catalytic performance compared to isolated SACs, especially for most multi-intermediate reactions. ,,, Overall, the developed high-density SACs can create the geometric structures of abundant adjacent single atoms that form an intersite interaction.…”

“…The key factor is to create abundant anchoring sites and a strong metal–support interaction. As shown in Figure , we also summarized the representative breakthrough results reported in the literature about densely populated SACs for different thermal catalytic reactions, such as selective hydrogenation, CO 2 hydrogenation, coupling reactions, alkene epoxidation, and Fenton-like reactions. ,,− ,,,,,,,− In the following section, we give detailed examples and analyze how single-atom density affects the electronic structure and catalytic performance in different kinds of reactions.…”

“…Cao et al found the Sabatier phenomenon induced by single-atom density in hydrogenation reactions. They produced a series of Ir SACs with predominantly an Ir 1 –P 4 coordination structure, while densities ranging from 0.1 to 1.7 atoms·nm –2 .…”

“…The concept of single-atom catalysts (SACs) was established in 2011, where all active sites are atomically dispersed on supports to realize a catalyst system with a theoretical 100% atomic utilization . Different from nanoparticle-based catalysts, the isolated single atoms of SACs are usually anchored with the heteroatoms of the supports. − The downsizing of metals to single atoms often results in unique electronic structures with multiple discrete and separated states compared to the continuous bands characteristic of their bulk counterparts. − However, on one hand, in contrast to the zero-valent character associated with bulk metals, single metal atoms frequently exhibit positive charge states because of the electron transfer from the central metal sites to the substrate, depriving them of the metallic property required for the efficient adsorption and activation of specific catalytic reactions. ,, On the other hand, the inherent adsorption–energy scaling relationship of SACs limits the reaction kinetics and thermodynamics, especially for multi-intermediate reactions. ,− …”