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.