CO Adsorption on a Single-Atom Catalyst Stably Embedded in Graphene

Abstract

Confined single metal atoms in graphene-based materials have proven to be excellent catalysts for several reactions and promising gas sensing systems. However, whether the chemical activity arises from the specific type of metal atom or is a direct consequence of the confinement itself remains unclear. In this work, through a combined density functional theory and experimental surface science study, we address this question by investigating Co and Ni single atoms embedded in graphene (Gr) on a Ni(111) support. These two single atom catalysts (SACs) exhibit opposite behavior toward carbon monoxide (CO) gas molecules: at RT, CO binds stably to Co, whereas it does not to Ni. We rationalize this difference by the energy position of trapped metal dxz and dyz states involved in π backdonation to CO: while for Co, these states lie at the Fermi level, for Ni are located deep below it. This conclusion is corroborated by a proof-of-concept experiment, where a Gr/Ni(111) sample containing both stable Ni and Co single atoms was exposed to a CO partial pressure of 5 ‧ 10-7 mbar. Scanning tunnelling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD) measurements confirm the selective adsorption of CO on Co at RT.