Hydroxylation of an ultrathin Co3O4(111) film on Ir(100) studied by in situ ambient pressure XPS and DFT

Abstract

In the present work, we have studied the interaction of water with spinel cobalt oxide (Co₃O₄), an effect which has been considered a major cause of its catalytic deactivation. Employing a Co₃O₄(111) model thin film grown on Ir(100) in (ultra)high vacuum, and ambient pressure X-ray photoelectron spectroscopy (APXPS), hydroxylation in 0.5 mbar H₂O vapor at room temperature was monitored in real time. The surface hydroxyl (OH) coverage was determined via two different models based (i) on the termination of a pristine and OH-covered Co₃O₄(111) surface as derived from density functional theory (DFT) calculations, and (ii) on a homogeneous cobalt oxyhydroxide (CoO(OH)) overlayer. Langmuir pseudo-second-order kinetics were applied to characterize the OH evolution with time, suggesting two regimes of chemisorption at the mosaic-like Co₃O₄(111) film: (i) plateaus, which were quickly saturated by OH, followed by (ii) slow hydroxylation in the “cracks” of the thin film. H₂O dissociation and OH formation, blocking exposed Co²⁺ ions and additionally consuming surface lattice oxygen, respectively, may thus account for catalyst deactivation by H₂O traces in reactive feeds.