Hydrogen Spillover
Hydrogen spillover is the process in which H2 dissociates at an active site (often a Pt-group metal) and the resulting H atoms migrate onto a neighboring material that could not dissociate H2 efficiently on its own. Whether spillover occurs depends on both kinetics (the dissociation barrier at the active site and diffusion barriers away from it) and thermodynamics (whether H is stable on the receiving surface relative to recombining and desorbing as H2). It is relevant to catalysts where a cheap metal such as Cu does the main chemistry but needs a supply of atomic hydrogen. Related: Single-atom alloys, Hydrogen adsorption free energy.
Spillover from isolated Pd atoms onto Cu(111), but not Au(111) (2009)
Tierney and coworkers imaged islands of H atoms on pure Cu(111) regions of a Pd/Cu(111) surface after a 10 L H2 dose at 420 K, while bare Cu(111) and Pd/Au(111) took up no hydrogen. With PW91 DFT, H adsorption is favorable at the Pd site in Cu(111) (about 0.24 eV/H) and at the neighboring Cu site (about 0.14 eV/H), but on Au(111) the site next to Pd is endothermic (+0.07 eV/H), so H recombines and desorbs instead of spilling over. Calculated diffusion barriers away from Pd were 0.12 to 0.15 eV, and two spillover pathways on Cu were both judged feasible. The authors note that spillover from Pd to Cu was already known in methanol synthesis and argue the concept is general across alloy combinations (Tierney et al. 2009). Summary: tierney2009.