Hydrogen insertion in Pd core/Pt shell cubo-octahedral nanoparticles

We report a theoretical density functional theory study of the absorption and adsorption of hydrogen in Pd core/Pt shell cubo-octahedral nanoparticles of 55 and 147 atoms. We have explored all the possible sites available for hydrogen loading. The relative stability of H in the different sites is calculated and discussed in terms of the local geometrical and chemical environments. As a general trend, the most stable absorption sites are pyramidal interstices at the interface offering a volume for H around $2\times 2.60$ Å${}^3$, with a moderate atomic rearrangement upon H insertion and in which the H atom maximizes the number of Pd neighbors. Our theoretical findings give support to the recent hydrogen pressure-composition isotherms and NMR measurements carried out in this type of bimetallic nanoparticle [Kobayashi et al., J. Am. Chem. Soc. 130, 1818 (2008)].

Figure 1

View of one-eighth of the cubo-octahedral cluster ${\mathrm{Pd}}_x{\mathrm{Pt}}_y$ ($x+y=147$). The different colors correspond to the different cubo-octahedral layers within the cluster. The 55-atom cluster is formed with only the first three layers (blue, red, and white). The H atom will sit at different levels in interstitial positions between two layers or at the surface. The level $l=0$ corresponds, for both sizes, to adsorption at the surface, whereas the most internal position of H corresponds to the level $l=-2$ for the 55-atom cluster and to the level $l=-3$ for the 147-atom cluster.

Figure 2

All the nonequivalent positions for the insertion of H (light blue hexagon) at the different interstitial levels within the cluster as well as at the surface. For the squared faces of the cluster, a shift along the (001) direction has to be considered, unless shown otherwise, since the plot only shows the projection. For the triangular faces, a shift along the (111) direction has to be considered.

Figure 3

Spread in energy for H sitting at the different levels defined in Figs. 1 and 2, taking as a reference the energy of the most stable site. The black lines correspond to the surface sites (level $l=0$), the red lines correspond to the interface level ($l=-2$ for Pd${}_{13}$Pt${}_{134}$ and $l=-1$ for Pd${}_{55}$Pt${}_{92}$), the green lines correspond to the internal sites inside the Pd core ($l=-3$ for Pd${}_{13}$Pt${}_{134}$ and $l=-2,-3$ for Pd${}_{55}$Pt${}_{92}$), and the blue line corresponds to the internal sites inside the Pt shell ($l=-1$ for Pd${}_{13}$Pt${}_{134}$). The height of each bar measures the number of stable sites found for H at the corresponding level.