Single-Atom Catalysts: A New Frontier in Heterogeneous Catalysis

Abstract

Supported metal nanostructures are the most widely used type of heterogeneous catalyst in industrial processes. The size of metal particles is a key factor in determining the performance of such catalysts. In particular, because low-coordinated metal atoms often function as the catalytically active sites, the specific activity per metal atom usually increases with decreasing size of the metal particles. However, the surface free energy of metals increases significantly with decreasing particle size, promoting aggregation of small clusters. Using an appropriate support material that strongly interacts with the metal species prevents this aggregation, creating stable, finely dispersed metal clusters with a high catalytic activity, an approach industry has used for a long time. Nevertheless, practical supported metal catalysts are inhomogeneous and usually consist of a mixture of sizes from nanoparticles to subnanometer clusters. Such heterogeneity not only reduces the metal atom efficiency but also frequently leads to undesired side reactions. It also makes it extremely difficult, if not impossible, to uniquely identify and control the active sites of interest.The ultimate small-size limit for metal particles is the single-atom catalyst (SAC), which contains isolated metal atoms singly dispersed on supports. SACs maximize the efficiency of metal atom use, which is particularly important for supported noble metal catalysts. Moreover, with well-defined and uniform single-atom dispersion, SACs offer great potential for achieving high activity and selectivity.In this Account, we highlight recent advances in preparation, characterization, and catalytic performance of SACs, with a focus on single atoms anchored to metal oxides, metal surfaces, and graphene. We discuss experimental and theoretical studies for a variety of reactions, including oxidation, water gas shift, and hydrogenation. We describe advances in understanding the spatial arrangements and electronic properties of single atoms, as well as their interactions with the support. Single metal atoms on support surfaces provide a unique opportunity to tune active sites and optimize the activity, selectivity, and stability of heterogeneous catalysts, offering the potential for applications in a variety of industrial chemical reactions.

Homogeneity of Surface Sites in Supported Single-Site Metal Catalysts: Assessment with Band Widths of Metal Carbonyl Infrared Spectra

“…Details of the experiments with solid samples are reported elsewhere. [16][17][18][19]21,22,24,34,35 The spectrum of Ir(CO) 2 (acac) in hexanes (EM Science, 98.5%) was measured with a saturated solution in a sealed liquid IR cell (International Crystal Laboratories) with a 0.5-mm path length.…”

Magnetically Separable and Sustainable Nanostructured Catalysts for Heterogeneous Reduction of Nitroaromatics

“…In particular, the remaining challenges, e.g., improving the structural stability, efficiency and durability of the engineered catalysts, can be rectified [64]. The syntheses, characterizations and practical applications of effective core-shell and yolk-shell nanostructured catalysts have been introduced as a unique strategy to resolve the aforementioned problems.…”

Single Atoms of Pt-Group Metals Stabilized by N-Doped Carbon Nanofibers for Efficient Hydrogen Production from Formic Acid

“…Catalysis for energy applications is presented by electrocatalytic reactions 14,22 and by catalytic hydrogen-producing reactions such as reforming of naphtha, 23 formic acid decomposition, 24 and water-gas shift reaction. 12,25 For the production of hydrogen by formic acid decomposition, catalysis with single-atoms and sub-nanometer clusters was reported for gold catalysts. 24,26,27 Evidently, the properties and 4 catalytic applications of single-atom catalysts should be strongly dependent on the catalyst's support.…”