Kinetic Insights into H<sub>2</sub> Activation on Anatase TiO<sub>2</sub>(101)-Supported Single-Atom Catalysts
Qiang Li, George Yan, Dionisios G. Vlachos
Abstract
Hydrogen (H 2 ) activation is fundamental in catalysis. Single-atom catalysts (SACs) can be highly selective in many reactions invoking H 2 activation due to their tunable geometric and electronic properties. In this work, we employ density functional theory (DFT) and microkinetic modeling (MKM) to study H 2 activation (adsorption, dissociation, and diffusion) on the dehydroxylated (101) facet of anatase TiO 2 (corresponding to a water-free reaction environment) over 14 single-atom transition metals from 3d to 5d (Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Cd, Os, Ir, Pt, and Au) and Sn. The stability of intermediates from the dissociative adsorption of H 2 is first evaluated, and linear scaling relationships are explored for H···H dissociation and diffusion. We find that linear scalings are generally inadequate for H 2 activation. MKM simulations show that H 2 activation over the SA/TiO 2 sites occurs under kinetic control at moderate temperatures (below 400 K). Thermodynamically preferred H–H splitting states are achieved via kinetically favored splitting followed by subsequent diffusion steps. Overall, adsorption is faster for SA sites with weaker SA–H interactions as more empty surface sites are exposed. H–H dissociation takes place by following the path with the lowest barrier but may lead to metastable products, where the most stable surface intermediates are reached via H diffusion, potentially leading to site poisoning. Up to 400 K, the system generally cannot reach steady state within 3 h, leading to diverse hydride (M–H) or OH sites that depend on the SA, the temperature, and exposure time. Temperature-programmed desorption (TPD) simulations reveal that the observed H 2 desorption peaks strongly correlate with the exposure temperature and the SA’s chemical nature, further demonstrating the importance of kinetics in H 2 activation by SA sites.