Oxidation of Pt(111) under Near-Ambient Conditions

The oxidation of Pt(111) at near-ambient ${\mathrm{O}}_2$ pressures has been followed in situ using x-ray photoelectron spectroscopy (XPS) and ex situ using x-ray absorption spectroscopy (XAS). Polarization-dependent XAS signatures at the O $K$ edge reveal significant temperature- and pressure-dependent changes of the Pt-O interaction. Oxide growth commences via a PtO-like surface oxide that coexists with chemisorbed oxygen, while an ultrathin $\alpha \mathrm{\text{−}}{\mathrm{PtO}}_2$ trilayer is identified as the precursor to bulk oxidation. These results have important implications for understanding the chemical state of Pt in catalysis.

Figure 1

Pt $4f$ (left) and O $1s$ (right) XPS spectra for $\mathrm{O}/\mathrm{Pt}(111)$, deconvoluted using asymmetric Voigt and Gaussian-broadened Doniach-Šunjić functions, respectively; a Shirley-type background (dashed line) was removed from all spectra. Excepting $\theta =2.1\mathrm{ML}$, which was prepared at 5 Torr, 720 K but whose spectrum was acquired at 0.5 Torr, 620 K, the same $P({\mathrm{O}}_2)$, $T$ conditions were employed for sample preparation and spectral acquisition.

Figure 2

Experimental O $K$-edge XAS spectra (solid lines) are shown in (a) and compared in (b) against simulations (dashed) of the adjacent model structures. Side views of the model structures are cross sections along the dashed lines; the dashed rhombus denotes a ($2\times 2$) supercell of the Pt(111) substrate. Results for the 1.0 ML surface oxide and isolated oxide chain are shown using dashed and dashed-dotted lines, respectively. Simulations were obtained by convoluting the GPAW output with a Gaussian of 0.45 eV full-width at half-maximum (fwhm), except for $\alpha \mathrm{\text{−}}{\mathrm{PtO}}_2/\mathrm{Pt}\left(111\right)$, for which 0.7 eV was used; these widths were determined from the fwhm of the associated O $1s$ XPS peaks. Experimental spectra reflect removal of background signal from the Pt substrate and were placed on a B.E. scale in (b) by subtracting the O $1s$ XPS B.E. All spectra are area normalized $\sim 30\mathrm{eV}$ above threshold.

Figure 3

In-plane O $K$-edge XAS spectra (left) and Pt $4f_{7/2}$ XPS spectra (right) obtained after ${\mathrm{H}}_2$ treatment ($2\times {10}^{-8}\mathrm{Torr}$ ${\mathrm{H}}_2$ at 300 K) of Pt(111) oxidized at (a) 0.5 Torr ${\mathrm{O}}_2$ and 570 K ($\theta =0.8\mathrm{ML}$) and (b) 5 Torr ${\mathrm{O}}_2$ and 720 K ($\theta =2.1\mathrm{ML}$). XAS spectra are area normalized $\sim 30\mathrm{eV}$ above threshold. (c) and (d) show the fraction of the total Pt $4f$ peak area associated with each oxygen phase as obtained by deconvoluting the Pt $4f$ XPS spectra in (a) and (b), respectively.