Effects of K adsorption on the CO-induced restructuring of Co(11-20)

Abstract

The location of potassium (K) on Cobalt (Co) and its effect on adsorption and adsorption-induced surface restructuring is important for understanding the deactivation of Co Fischer-Tropsch catalysts and the nature of the active surface. Co(11-20) restructures by anisotropic migration of Co atoms upon CO exposure. Deposition of sub-monolayer amounts of K on Co(11-20) and the effect on the CO-induced restructuring were therefore investigated using scanning tunneling microscopy (STM), high resolution photoemission spectroscopy (HR-PES), and density functional theory calculations (DFT). The combined STM and DFT results suggest that the preferred adsorption site for K at low coverage is in the vicinity of step edges. DFT also found that diffusion of K along the [0001] direction, in between the zigzag rows of the topmost Co layer is facile. The restructuring under CO exposure with K pre-adsorbed proceeded on the terraces rather than from the step edges, in a slower and more disordered manner. HR-PES showed that the amount of CO adsorbed at saturation significantly decreased with predeposited K. The obstructed migration of Co atoms across the surface may be important in understanding why very low amounts of K on supported Co catalysts significantly reduces the activity towards hydrogenation of CO.