Strain and coordination effects in the adsorption properties of early transition metals: A density-functional theory study

Strain and coordination effects in the adsorption on early transition metals were studied using density-functional theory. We show that, in contrast to late transition metals, several early transition-metal surfaces with a less than half-filled local $d$ band exhibit lower adsorption energies upon lattice expansion and on low-coordinated sites, in agreement with predictions based on the $d$-band model. This demonstrates that the $d$-band model can be extended to early transition metals. Implications of these results for hydrogen storage materials are discussed.

Figure 1

(Color online) Illustration of the effect of tensile strain or reduced coordination on the width and position of the $d$ band of an early transition metal: (a) less than half-filled $d$ band; (b) reduced width of the $d$ band due to a smaller overlap or lower coordination; and (c) downshift of the $d$ band because of charge conservation.

Figure 2

(Color online) Calculated shift of the $d$-band center as a function of the lattice strain for several transition-metal surfaces.

Figure 3

(Color online) Calculated change in the adsorption energies for H at the most favorable sites (a) and for CO at the ontop sites (b) as a function of the lattice strain for several early transition-metal surfaces.

Figure 4

(Color online) CO adsorption energies at the ontop sites of the strained early transition-metal surfaces as a function of the local $d$-band center. The inset shows the corresponding data for the CO adsorption on Pt surfaces using the same scale.

Figure 5

(Color online) Calculated change in the adsorption energies CO at the ontop sites of a step-edge atom and an adatom with respect to the adsorption on the close-packed terraces.