CO-induced inversion of the layer sequence of a model CoCu catalyst

• Published in: Surface science 2016-06, Vol.648, p.74-83
• Main Authors: Collinge, Greg, Xiang, Yizhi, Barbosa, Roland, McEwen, Jean-Sabin, Kruse, Norbert
• Format: Article
• Language: English
• Subjects: Adsorption; Bimetallic CoCu; Catalysts; CO adsorption; CO-induced segregation; Copper; Density of states; DFT; Electronics; Fischer–Tropsch; Mathematical models; Surface chemistry; X-ray photoelectron spectroscopy; XPS
• ISSN: 0039-6028; 1879-2758
• DOI: 10.1016/j.susc.2015.10.014

Experimental X-ray photoelectron spectroscopy (XPS) and theoretical density functional theory (DFT) calculations reveal the electronic and structural properties of CoCu catalysts before and after CO adsorption. DFT calculations show that, prior to CO adsorption, CoCu has a high tendency to self-assemble into a Co@Cu core-shell structure, which is in accordance with previous atom probe tomography (APT) results for CoCu-based systems and the known mutually low miscibility of Co and Cu. We demonstrate that Co and Cu are electronically immiscible using a density of states (DOS) analysis wherein neither metal's electronic structure is greatly perturbed by the other in “mixed” CoCu. However, CO adsorption on Co is in fact weakened in CoCu compared to CO adsorption on pure Co despite being electronically unchanged in the alloy. Differential charge density analysis suggests that this is likely due to a lower electron density made available to Co by Cu. CO adsorption at coverages up to 1.00 ML are then investigated on a Cu/Co(0001) model slab to demonstrate CO-induced segregation effects in CoCu. Accordingly, a large driving force for a Co surface enrichment is found. At high coverages, CO can completely invert the layer sequence of Co and Cu. This result is echoed by XPS evidence, which shows that the surface Co/Cu ratio of CoCu is much larger in the presence of CO than in H2. [Display omitted] •DFT calculations on CoCu to show thermodynamic tendency to segregate.•Density of states and differential charge density demonstrate Co vs Cu interactions.•Experimental XPS shows surface Co/Cu ratio increases after CO activation.•CO-induced and coverage-dependent “Co pumping” investigated via DFT.•High CO coverage induces a complete inversion of CoCu layer sequence.