Kinetic Insights into H2 Activation on Anatase TiO2(101)-Supported Single-Atom Catalysts

Hydrogen (H2) activation is fundamental in catalysis. Single-atom catalysts (SACs) can be highly selective in many reactions invoking H2 activation due to their tunable geometric and electronic properties. In this work, we employ density functional theory (DFT) and microkinetic modeling (MKM) to study H2 activation (adsorption, dissociation, and diffusion) on the dehydroxylated (101) facet of anatase TiO2 (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 H2 is first evaluated, and linear scaling relationships are explored for H···H dissociation and diffusion. We find that linear scalings are generally inadequate for H2 activation. MKM simulations show that H2 activation over the SA/TiO2 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 H2 desorption peaks strongly correlate with the exposure temperature and the SA’s chemical nature, further demonstrating the importance of kinetics in H2 activation by SA sites.