Enhanced CO tolerance of Pt clusters supported on graphene with lattice vacancies

The adsorption of CO on ${\mathrm{Pt}}_4$ clusters supported on graphene with lattice vacancies is studied theoretically using the first-principles calculation. Our results show that the electronic structure of the graphene-supported ${\mathrm{Pt}}_4$ clusters is significantly modified by the interaction with carbon dangling bonds. As a result the adsorption energy of CO at a Pt site decreases almost linearly with the lowering of the Pt $d$-band center, in analogy with the linear law previously reported for CO adsorption on various Pt surfaces. An exceptional behavior is found for ${\mathrm{Pt}}_4$ supported on graphene with a tetravacancy, where CO adsorption is noticeably weaker than predicted by the shift in the $d$-band center. Detailed electronic structure analyses reveal that the deviation from the linear scaling can be attributed to lack of Pt $d$ states near the Fermi level that hybridize with CO molecular orbitals. The weakening of CO adsorption on the ${\mathrm{Pt}}_4$ clusters is considered as a manifestation of the support effect of graphene, and leads to the enhancement of CO poisoning tolerance that is crucial for developing high-performance Pt cluster catalysts.

Figure 1

Schematic views of graphene sheets without (a) and with carbon vacancies (b)–(e), and the corresponding graphene-supported ${\mathrm{Pt}}_4$ clusters and CO adsorption structures. In the first column, crosses ($\times$) stand for the positions of the carbon vacancies. In the second column, inequivalent Pt atoms in each ${\mathrm{Pt}}_4/{\mathrm{V}}_n$ are indicated by different labels.

Figure 2

Formation energies of graphene sheets with a lattice vacancy ${\mathrm{V}}_n$ ($\circ$) and graphene-supported ${\mathrm{Pt}}_4$ clusters ${\mathrm{Pt}}_4/{\mathrm{V}}_n$ ($\square$). Filled circles ($•$) and squares ($\blacksquare$) correspond to the energy differences defined by Eqs. (3) and (8), respectively.

Figure 3

LDOS projected onto the Pt $d$ states plotted as a function of energy measured from the Fermi level. The peaks are smeared by a Gaussian of width 0.1 eV. The results for ${\mathrm{Pt}}_4/{\mathrm{V}}_n$ with $n=0,1,...,4$ are shown in panels (a)–(e), respectively. In each panel, the $d$-band centers are indicated by vertical lines. The LDOSs projected onto Pt $d_{z^2}$ and ($d_{yz}+d_{zx}$) states are also shown in the results for ${{\mathrm{Pt}}_{\mathrm{A}}/\mathrm{V}}_n$, where the $z$ axis is perpendicular to the face of ${\mathrm{Pt}}_4$ opposite to ${\mathrm{Pt}}_{\mathrm{A}}$.

Figure 4

CO adsorption energy plotted as a function of the Pt–C bond length (a), the C–O bond length (b), the $d$-band center (c), the total overlap population (d), and the number of Pt $d$ states near ${\varepsilon }_{\mathrm{F}}$ (e). In each panel, pluses ($+$), crosses ($\times$), circles ($\circ$), triangles ($▵$), and squares ($\square$) correspond to the results for ${\mathrm{Pt}}_4/{\mathrm{V}}_n$ with $n=0,1,...,4$, respectively. Solid, dashed, and dotted lines are drawn to guide the eye. In panel (e), the number of Pt $d$ states near ${\varepsilon }_{\mathrm{F}}$ is estimated by integrating the LDOS in Fig. 3 from $-0.25$ eV to 0.25 eV with respect to ${\varepsilon }_{\mathrm{F}}$.

Figure 5

COOP plotted as a function of energy measured from the Fermi level. The peaks are smeared by a Gaussian of width 0.1 eV. The results for ${\mathrm{Pt}}_4/{\mathrm{V}}_n$ with $n=0,1,...,4$ are shown in panels (a)–(e), respectively. In each panel, the COOP curves for CO $4\sigma$ (blue), $1\pi$ (red), $5\sigma$ (orange), and $2\pi$ (green) are shown.