Initial Carbonation of Ni(111) Surfaces.

Understanding the carbon formation on Ni surfaces is critical for the controlled Ni-based nanofabrication and heterogeneous catalysis. Due to the high solubility of carbon in nickel and the complicated migrations of carbon in the near-surface area, achieving a fundamental understanding of the initial carbonation of a Ni surface at an atomic level is experimentally challenging. Herein, the initial formation of surface carbon adsorbates on Ni(111) from the Boudouard reaction (2CO ↔ CO + C) is studied by scanning tunneling microscopy (STM) in combination with density functional theory (DFT) calculations. The initial carbon formation is site-selective: carbon adsorption at step edges is isolated and strongly bonded, acting as the precursor of carbide formation; the adsorption on terrace sites is weaker and mobile, acting as the initial graphene clusters on Ni(111). The difference in kinetics of C adsorption on the Ni(111) may play a role in determining the future growth of carbide or graphene. Upon further carbon adsorption, new evidence is presented to resolve the debate over the atomic structure of the well-recognized (√39 × √39) R16.1° carbide structure. Our results based on combined STM measurements and DFT calculations are further extended to other surfaces, such as Ni(110) and Ni(211), and a wide range of temperatures and pressures. This provides valuable insights into controlling the chemical processes related to carbon-nickel interactions.