Surface X-ray Scattering Observation of High-Density CO Monolayers on Pt(111)

We report on surface x-ray scattering studies of high-density long-range ordered CO adsorbates on Pt(111) in equilibrium with gas phase at near-atmospheric CO partial pressures. We adsorbed CO with an electrochemical method and transferred the sample into a controlled gas atmosphere. At CO partial pressure $P_{\mathrm{CO}} \sim 1\,\mathrm{atm}$ we found long-range ordered $\left(2\! \times\!2\right)$-3CO, at $P_{\mathrm{CO}} < 0.5\,\mathrm{atm}$ we found $(\sqrt{19}\!\times\!\sqrt{19})\!R23.4^{\circ}$-13CO. These structures do not depend on the electrochemically controlled initial coverage, but are only a function of CO partial pressure and temperature. Based on our x-ray studies, we propose a phase diagram of CO monolayers on Pt(111) under near atmospheric pressure of CO. We will discuss the parallel behavior of CO monolayers in high-pressure atmospheres and those in electrochemical environment. However, there is disagreement between our results and those of recent studies of CO adsorbed from the gas phase, where $(\sqrt{19}\!\times\!\sqrt{19})$ has been reported for $P_{\mathrm{CO}}=1\,\mathrm{atm}$ and room temperature [Longwitz \emph{et al.}\ JPCB \textbf{108} 14497 (2004)]. Similar differences have been reported for studies in UHV when comparing gas adsorbed CO structures and electrochemically prepared structures studied \emph{in vacuo} [Zurawski \emph{et al.}\ JPC \textbf{94} 2076 (1990)]. We will address plausible causes for these discrepancies.