Ab initio study of alloying and straining effects on CO interaction with Pt

We present the systematic picture of alloying and straining effects on the Pt-CO interaction via ab initio density functional calculations. The results show that alloying neighbors (Ru, Sn, and Mo) and/or compressive strain increase the valence electron populations of the Pt atoms in the first layers of the modified Pt surfaces. The larger valence electron population of the Pt atom tends to prevent CO from donating electrons in the adsorption reaction, resulting in the weak Pt-CO bond. We suggest that the degree of the donation from CO determines the Pt-CO bond strength.

Figure 1

Schematic representation of the metal surface model constructed by 15 metal atoms, where 10 and 5 atoms are set in the first and second layers, respectively. $M_i$ and $M_f$ denote the positions of the CO-adsorbed metal atoms at the initial and final states, respectively. $X$ denotes the CO position relative to the $M_i$ on the $M_i\text{−}{M}_f$ axis. $a$ denotes the lattice constant.

Figure 2

Minimum potential energy curves for the (a) $\mathrm{C}\mathrm{O}∕\mathrm{Pt}\left(111\right)$, (b) $\mathrm{C}\mathrm{O}∕\mathrm{Ru}\left(0001\right)$, (c) $\mathrm{C}\mathrm{O}∕\mathrm{Ru}$-modified Pt(111), (d) $\mathrm{C}\mathrm{O}∕\mathrm{Sn}$-modified Pt(111), and (e) $\mathrm{C}\mathrm{O}∕\mathrm{Mo}$-modified Pt(111) as a function of the $M_i\text{−}X$ distance on the $M_i\text{−}{M}_f$ axis (see Fig. 1), respectively. Each of the minimum potential energies is relative to the total energy of the corresponding top CO-adsorbed substrate.

Figure 3

CO adsorption energies on the localized-strained Pt(111) as a function of the coordinate of the neighboring Pt atom at the $M_f$ along the $M_i\text{−}{M}_f$ axis relative to the original position, where the $M_i$ side corresponds to minus (see Fig. 1 and the inset in this figure).

Figure 4

CO adsorption energies on the Pt(111) [(a) Pt(111), (b) Ru-modified Pt(111), (c) Sn-modified Pt(111), (d) Mo-modified Pt(111), (e) Pt(111) $(a=2.60\mathrm{\AA })$, and (f) Pt(111) $(a=3.00\mathrm{\AA })$] as functions of the Pt-C and C-O distances, respectively.

Figure 5

CO adsorption energies on the Pt(111) [(a) Pt(111), (b) Ru-modified Pt(111), (c) Sn-modified Pt(111), (d) Mo-modified Pt(111), (e) Pt(111) $(a=2.60\mathrm{\AA })$, and (f) Pt(111) $(a=3.00\mathrm{\AA })$] as functions of the Pt-C and C-O stretching frequencies, respectively.

Figure 6

Energies of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of the isolated Pt(111), Mo-modified Pt(111) and ${\mathrm{Mo}}_{\mathrm{ML}}∕\mathrm{Pt}\left(111\right)$. Energies of the HOMO and LUMO of CO are also shown by the broken lines.

Figure 7

CO adsorption energies on the Pt(111) [(a) Pt(111), (b) Ru-modified Pt(111), (c) Sn-modified Pt(111), (d) Mo-modified Pt(111), (e) Pt(111) $(a=2.60\mathrm{\AA })$, and (f) Pt(111) $(a=3.00\mathrm{\AA })$] as a function of the $5d$ orbital population of the Pt atom centered in the first layer of the isolated Pt(111).