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.

Elucidation of reaction mechanisms and the geometric and electronic structure of the active sites themselves is a challenging, yet essential task in the design of new heterogeneous catalysts. Such investigations are best implemented via a multi-pronged approach that comprises ambient pressure catalysis, surface science, and theory. Herein, we employ this strategy to understand the workings of NiAu single-atom alloy (SAA) catalysts for the selective non-oxidative dehydrogenation of ethanol to acetaldehyde and hydrogen. The atomic dispersion of Ni is paramount for selective ethanol to acetaldehyde conversion, and we show that even the presence of small Ni ensembles in the Au surface results in the formation of undesirable byproducts via C-C scission. Spectroscopic, kinetic, and theoretical investigations of the reaction mechanism reveal that both C-H and O-H bond cleavage steps are kinetically relevant and single Ni atoms are confirmed as the active sites. X-ray absorption spectroscopy studies allow us to follow the charge of the Ni atoms in the Au host before, under, and after a reaction cycle. Specifically, in the pristine state the Ni atoms carry a partial positive charge which increases upon coordination to the electronegative oxygen in ethanol and decreases upon desorption. This type of oxidation state cycling during reaction is similar to the behavior of single-site homogenous catalysts. Given the unique electronic structure of many single-site catalysts, such a combined approach in which the atomic-scale catalyst structure and charge state of the single atom dopant can be monitored as a function of its reactive environment is a key step towards developing structure function relationships that inform the design of new catalysts.

Silver-based heterogeneous catalysts, modified with a range of elements, have found industrial application in several reactions in which selectivity is a challenge. Alloying small amounts of Pt into Ag has the potential to greatly enhance the somewhat low reactivity of Ag while maintaining high selectivity and resilience to poisoning. This single-atom alloy approach has had many successes for other alloy combinations but has yet to be investigated for PtAg. Using scanning tunneling microscopy (STM) and STM-based spectroscopy, we characterized the atomic-scale surface structure of a range of submonolayer amounts of Pt deposited on and in Ag(111) as a function of temperature. Near room temperature, intermixing of PtAg results in multiple metastable structures on the surface. Increasing the alloying temperature results in a higher concentration of isolated Pt atoms in the regions near Ag step edges as well as direct exchange of Pt atoms into Ag terraces. Furthermore, STM-based work function measurements allow us to identify Pt rich areas of the samples. We use CO temperature programmed desorption to confirm our STM assignments and quantify CO binding strengths that are compared with theory. Importantly, we find that CO, a common catalyst poison, binds more weakly to Pt atoms in the Ag surface than extended Pt ensembles. Taken together, this atomic-scale characterization of model PtAg surface alloys provides a starting point to investigate how the size and structure of Pt ensembles affect reaction pathways on the alloy and can inform the design of alloy catalysts with improved catalytic properties and resilience to poisoning.

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.