Atomic-Scale Surface Structure and CO Tolerance of NiCu Single-Atom Alloys

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.