Oxidation of Pt(110)

Using scanning tunneling microscopy and temperature programed desorption we investigate the Pt(110) surface under strongly oxidizing conditions involving either high-pressure ${\mathrm{O}}_2$ or atomic oxygen exposure. At low temperatures, only disordered Pt oxide structures are observed. After annealing ordered surface oxide islands are observed to coexist with a highly stable reconstructed $(12\times 2)\mathrm{\text{−}}\mathrm{O}$ chemisorption structure. From density functional theory calculations a model for the surface oxide phase is revealed. The phase is found to be metastable, and its presence is explained in terms of stabilizing defects in the chemisorption layer and reduced Pt mobility.

Figure 1

STM images of the Pt(110) surface after exposure to (a) 5 min atomic O at 298 K ($300\times 300{Å}^2$). Inset: 0.01 mbar ${\mathrm{O}}_2$ for 10 min at 355 K ($150\times 150{Å}^2$). (b) 60 min atomic O at 500 K ($300\times 300{Å}^2$). (c) 60 min atomic O at 500 K. High-resolution image ($60\times 60{Å}^2$) of surface oxide island. The middle part of the island reveals a surface oxide while the protruding stripes are second layer Pt rows with chemisorbed O. (d) 75 min atomic O at 600 K ($500\times 500{Å}^2$). Insets: high-resolution image ($46\times 77{Å}^2$) of the chemisorption phase and associated ball model of the $(12\times 2)\mathrm{\text{−}}22$ O chemisorption structure. (e) A line scan along the white line in (c) and a ball model based on the DFT calculations presented below. All STM measurements were made in UHV at 298 K with $I=1.5–2\mathrm{n}\mathrm{A}$, $V=1–1.75\mathrm{V}$.

Figure 2

(a) TPD spectra of ${\mathrm{O}}_2$ obtained after 5 to 20 min atomic O exposures at 300 K, corresponding to 4.4, 5.0, 5.7, 7.3, and 8.7 ML, as well as after 500 L ${\mathrm{O}}_2$ exposure (1.2 ML, dashed line). An additional weak peak (${\beta }_1$) appears at $\sim 400\mathrm{K}$ at the highest exposures (not shown). (b) TPD spectra obtained after atomic O exposure at 500 and 600 K, respectively. Heating rate: $2.5\mathrm{K}/\mathrm{s}$.

Figure 3

Ball-and-stick models of (a) the chemisorption, (b)–(e) surface oxide, and (f),(g) thicker oxide structures modeled with DFT. Red, yellow, and blue spheres: oxygen, oxidic, and metallic platinum atoms.

Figure 4

(a) The formation energy per O atom, $\Delta E_{\mathrm{O}}$, as a function of O coverage. Extra Pt atoms are taken from bulk reservoirs. (b) Formation free energy per (strained) surface $(1\times 2)$ area with respect to $\Delta {\mu }_{\mathrm{O}}$. The reference point (dotted vertical line) is ${\mu }_{\mathrm{O}}=1/2E_{{\mathrm{O}}_2}$, while the dot-dashed vertical line corresponds to the heat of formation of $\alpha \mathrm{\text{−}}{\mathrm{P}\mathrm{t}\mathrm{O}}_2$. Temperature scales at two typical pressures of ${\mathrm{O}}_2$ have been indicated.