Predicting Hydrogen Saturation on Small Transition Metal Nanoparticles

Hydrogen adsorption is important for understanding coverage effects for various chemical reactions and for characterizing the size of nanoparticles. Recent literature suggests that the often-assumed saturation coverage of unity may not be reasonable, at least for small nanoparticles. This includes our prior study which showed that Ir and Pt nanoparticles, which primarily bind H adsorbates in atop binding modes, can saturate at H/Msurf ratios well above 1, depending on the metal and particle size. This work examines H adsorption on small nanoparticles of Ru, Rh, Pd, and Os metals that preferentially bind H in 3-fold sites. We therefore investigate new adlayer structures to understand how undercoordinated atoms influence these H configurations. Calculations on 201-atom nanoparticles suggest H* (at low coverages) binds most favorably to undercoordinated bridge sites for all six metals of interest. As suggested by the slab model calculations, 3-fold sites are also among the most stable sites for Ru, Rh, and Pd particles, while atop sites are more stable for Os, Ir and Pt. These trends in single H* binding behavior do not significantly change with particle size. Saturation adlayers for 4d metals (Ru, Rh, and Pd) are dominated by H* bound to fcc 3-fold sites and undercoordinated bridge sites. With saturation coverages of 1.38, 1.48, and 1.28 ML respectively, Rh has the highest saturation coverage of the 4d metals, reflecting a nonmonotonic periodic trend. For the 5d metals (Os, Ir, Pt), saturated adlayers are dominated by the same undercoordinated bridge sites, but also generally favor atop-bound H* instead of 3-fold except Pt. Their saturation coverages are 1.87. 1.93, and 1.57 ML, respectively. Saturation coverage increases when moving from a 4d to a 5d metal, and the group 9 metals (Rh and Ir) exhibit the highest saturation in their respective series. Our results show that H* binding energy is largely independent of particle size for a given site type. Instead, changing the particle size primarily alters the distribution of available binding sites (corner, edge, and terrace). Therefore, the adlayer model developed for the 201-atom particle can be used to reliably estimate saturation coverages, allowing for extrapolation to both smaller and larger nanoparticles.