Theoretical understanding of the stability of single-atom catalysts

Abstract

As a new frontier, the rapid development of single-atom catalysts (SACs) in heterogeneous catalysis has attracted extensive attention since the concept of single-atom catalysis was first coined in 2011 [1,2]. Supported metal single atoms usually possess unique chemical and physical properties and have a special local chemical environment that is distinctly different from conventional supported nanoparticles and metal catalysts. Studies in the past several years have shown four main advantages of SACs: high selectivity, possibly high stability, high atomic efficiency and tunable high activity, as shown in Fig. 1. For efficiency, SAC maximizes the utilization of expensive metals by exposing each single metal atom to reactants. The local configurations of the active centers of an ideal SAC can be highly identical, leading to excellent selectivity compared to supported nanoparticles and metal surfaces that often have multiple yet rather diverse active sites. Unique coordination of single metal atoms with neighboring atoms of support may lead to high activity for specific reactions. All the above merits are on the basis of the stability of catalysts that people are most concerned about. To avoid aggregation under catalytic reaction conditions, single metal atoms can be stabilized by anchoring at specific sites on the support, including embedding and surface-adsorbing. Considering the complex realistic conditions, the stability of SACs may also depend on the surface condition, support type, reactant species, and finite temperature and pressure, which are difficult to ascertain and vary from one system to another. It remains a grand challenge to guide the prediction and design of highly stable and reactive SACs today. Examples illustrating the advantages of single-atom catalysts (SACs) include activity, stability, selectivity and atomic efficiency. (a) High loadings of palladium atoms on TiO2 exhibit high catalytic activity in hydrogenation of C = C bonds [3]. (b) Pt1/CeO2 and Au1/FeOx are stable at 800°C and 400°C in oxidation conditions, respectively [4,5]. (c) Iron single atom embedded on a SiO2 surface exhibits high selectivity for conversion of methane to ethylene, aromatics [6]. (d) SACs are less expensive than supported nanoparticles and metal catalysts. Four types of single atoms on an oxide surface are shown in Fig. 2b. The single atom can be adsorbed on perfect or defective surfaces and can also be embedded (doped) into cation or oxygen vacancy. Most fabricated SACs from experiments are the embedded type, with strong covalent metal–support interaction (CMSI), while the sintering of single atoms and dispersion of nanoparticles should involve the diffusion of supported zerovalent single atoms. Therefore, both the supported and embedded single-atom active centers are possible under realistic conditions and worth thorough investigation. The intrinsic stability of SAC arises from the support-assisted lower chemical potential when compared to nanoparticles. When the free-energy change from nanoparticles to single atoms is negative, nanoparticles can be dispersed to single atoms spontaneously, which leads to thermodynamic stability of SACs (Fig. 2a, black curve). However, even if the free-energy change is positive, single atoms can also be stable when the aggregation barrier is high enough to prevent sintering, which is the kinetic stability of SACs (Fig. 2a, red curve). Following the principles of the atomistic theory of Ostwald ripening developed by W-X Li and coworkers, we have achieved a quantitative description and comparison of the two models by taking the reaction environment, particle size and morphology, support type and defects, and metal–reactant interaction into account [7,8]. We have proposed a complete theoretical model to predict the chemical potential of supported single metal atoms and supported nanoparticles, and taken the CO oxidation reaction as an example to guide the design of stable SACs. As a measure of the degree of change of free energy of a system by adding 1 mole of metal atoms to the system at a given temperature and pressure, the chemical potentials for SACs and metal particles were discussed in our recent work [8]. Both thermodynamic and kinetic criteria are considered to determine the stability of SACs. The thermodynamic part includes (i) the energetics of supported metal particles, which is based on the Gibbs−Thomson (G–T) relation with considering the adsorbed reactants and (ii) the chemical potential of monomers (both the metal adatoms and metal–reactant complexes) on supports. And the kinetic part includes (i) the diffusion barrier of monomers on perfect surfaces and defects and (ii) the barrier of moving one metal atom from a supported metal nanoparticle to a substrate surface with corresponding sintering rate equations. Here, we discuss the support effect on the stability of supported metal atoms first using selected examples [9]. We have found that the quantum primogenic e