Single-atom alloys (SAAs) play an increasingly significant role in the field of single-site catalysis and are typically composed of catalytically active elements atomically dispersed in more inert and catalytically selective host metals. SAAs have been shown to catalyze a range of industrially important reactions in electro-, photo-, and thermal catalysis studies. Due to the unique geometry of SAAs, the location of the transition state and the binding site of reaction intermediates are often decoupled, which can enable both facile dissociation of reactants and weak binding of intermediates, two key factors for efficient and selective catalysis. Often, this results in deviations from transition metal scaling relationships that limit conventional catalysts. SAAs also offer reduced susceptibility to CO poisoning, cost savings from reduced precious metal usage, opportunities for bifunctional mechanisms via spillover, and higher resistance to deactivation by coking that plagues many industrial catalysts. In this review, we begin by introducing SAAs and describe how model systems and nanoparticle catalysts can be prepared and characterized. We then review all available SAA literature on a per reaction basis before concluding with a description of the general properties of this new class of heterogeneous catalysts and presenting opportunities for future research and development.

The complexity of heterogeneous catalysts means that a priori design of new catalytic materials is difficult, but the well-defined nature of single-atom–alloy catalysts has made it feasible to perform unambiguous theoretical modeling and precise surface science experiments. Herein we report the theory-led discovery of a rhodium-copper (RhCu) single-atom–alloy catalyst for propane dehydrogenation to propene. Although Rh is not generally considered for alkane dehydrogenation, first-principles calculations revealed that Rh atoms disperse in Cu and exhibit low carbon-hydrogen bond activation barriers. Surface science experiments confirmed these predictions, and together these results informed the design of a highly active, selective, and coke-resistant RhCu nanoparticle catalyst that enables low-temperature nonoxidative propane dehydrogenation.

The selective hydrogenation of alkynes to alkenes is an important industrial process. However, achieving high selectivity and reducing the usage of precious platinum group metals are still challenging for the conventional hydrogenation catalysts. With atomically dispersed active metal atoms on the surface of a host metal, single-atom alloys (SAAs) have shown excellent hydrogenation selectivity and activity, but their hydrogenation mechanism is not fully understood. This work reports on the selective hydrogenation of 1-hexyne to 1-hexene on PdAu SAA catalysts. Au is a highly selective hydrogenation catalyst, but it is not active at low temperatures. Through measurements of reaction kinetics and in operando spectroscopy studies, we follow the much more facile activation of PdAu SAA catalysts and demonstrate the different hydrogenation chemistry of single Pd atoms and Pd nanoparticles (NPs). We further investigate the role of Pd atoms and the mechanism behind the improved hydrogenation selectivity through surface science and density functional theory. These studies indicate that the difference in reactivity stems from the relative energy barrier heights for over-hydrogenating the terminal C atom. The complementary catalysis-surface science-theory investigation described here is a powerful and general approach for understanding and controlling NP performance. The selective hydrogenation on PdAu SAAs is demonstrated and understood fundamentally, which serves as a guide for future designs of this type of catalyst.

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We demonstrate that PdAu single-atom alloy model catalysts offer a heterogeneous route to selective Würtz-type C–C coupling.

Ni is one of the most extensively utilized metals in industrial catalysis. For example, Ni is the catalyst of choice for the steam reforming of hydrocarbons. However, pure Ni also detrimentally catalyzes the formation of graphitic carbon, which in turn leads to coking and deactivation of the catalyst. It has been shown that alloying small amounts of a less reactive metal like Au into Ni can alleviate this issue by breaking up the larger Ni ensembles that promote coke formation. We are taking the opposite of this approach by alloying very small amounts of Ni into Cu, a catalytically less active host metal, to create single Ni atom sites. In this way our single-atom alloy approach has the potential to greatly enhance catalytic selectivity and reduce poisoning, analogous to other single-atom alloys such as PtCu and PdCu. Herein we report the atomic-scale surface structure and local geometry of low coverages of Ni deposited on a Cu(111) single crystal as determined by scanning tunneling microscopy. At 433 K, low concentrations of Ni alloy in the Cu host as a single-atom alloy in Ni-rich brims along ascending step edges. To support our STM assignments of the single-atom dispersion of Ni, reflection absorption infrared spectroscopy of CO on NiCu was performed. To access the binding strength of CO to isolated Ni sites, we used temperature-programmed desorption studies, which revealed that CO binds more weakly to single Ni atoms in Cu compared with Ni(111), indicating that NiCu single-atom alloys are promising for catalytic applications in which CO poisoning is an issue. Together, these results provide a guide for the preparation of NiCu single-atom alloy model catalysts that are predicted by theory to be promising for a number of reactions.

The reducibility of metal oxides, when they serve as the catalyst support or are the active sites themselves, plays an important role in heterogeneous catalytic reactions. Here we present an integrated experimental and theoretical study that reveals how the addition of small amounts of atomically dispersed Pt at the metal/oxide interface dramatically enhances the reducibility of a Cu2O thin film by H2. X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) results reveal that, upon oxidation, a PtCu single-atom alloy (SAA) surface is covered by a thin Cu2O film and is, therefore, unable to dissociate H2. Despite this, in situ studies using ambient-pressure (AP) XPS reveal that the presence of a small amount of Pt under the oxide layer can, at the single-atom limit, promote the reduction of Cu2O by H2 at room temperature. We built two density functional theory based surface models to better understand these experimental findings: a Cu2O/Cu­(111)-like surface oxide layer, known as the “29” oxide, in which Pt is alloyed into the Cu(111) surface, as well as a PtCu SAA. Our calculations suggest that the increased activity is due to the presence of atomically dispersed Pt under the surface oxide layer, which weakens the Cu–O bonds in its immediate vicinity, thus making the interface between subsurface Pt and the surface oxide a nucleation site for the formation of metallic Cu. This initial step in the reduction process results in the presence of surface Pt atoms surrounded by metallic Cu patches, and the Pt atoms become active in H2 dissociation, which consequently accelerates the reduction of the oxide layer. This work demonstrates how isolated Pt atoms at the metal/oxide interface of a Cu-based catalyst accelerate the reduction of the oxide and, therefore, help maintain the active, reduced state of the catalyst under the reaction conditions.

The conversion of surface-bound alkyl groups to alkanes and alkenes are important steps in many heterogeneously catalyzed reactions. While Pt is ubiquitous in industry because of its high activity toward C-H activation, many Pt based catalysts tend to over-bind reactive intermediates, which leads to deactivation by carbon deposition and coke formation. On the other hand, Cu binds intermediates more weakly than Pt but activation barriers tend to be higher on Cu. We examine the reactivity of ethyl, the simplest alkyl group that can undergo hydrogenation and dehydrogenation via β-elimination, and show that isolated Pt atoms in Cu enable low temperature hydrogenation of ethyl, unseen on Cu, while avoiding the decomposition pathways on pure Pt that lead to coking. Furthermore, we confirm the predictions of our theoretical model and experimentally demonstrate that the selectivity of ethyl (de)hydrogenation can be controlled by changing the surface coverage of hydrogen.