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\usepackage{natbib} |
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\usepackage{multirow} |
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\usepackage{wrapfig} |
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\usepackage{fixltx2e} |
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%\mciteErrorOnUnknownfalse |
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\usepackage[version=3]{mhchem} % this is a great package for formatting chemical reactions |
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\hline |
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& Calculated & Experimental \\ |
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\hline |
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< |
\multirow{2}{*}{\textbf{Pt-CO}} & \multirow{2}{*}{-1.9} & -1.4 \bibpunct{}{}{,}{n}{}{,} |
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\multirow{2}{*}{\textbf{Pt-CO}} & \multirow{2}{*}{-1.84} & -1.4 \bibpunct{}{}{,}{n}{}{,} |
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(Ref. \protect\cite{Kelemen:1979}) \\ |
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& & -1.9 \bibpunct{}{}{,}{n}{}{,} (Ref. \protect\cite{Yeo}) \\ \hline |
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\textbf{Au-CO} & -0.39 & -0.40 \bibpunct{}{}{,}{n}{}{,} (Ref. \protect\cite{TPDGold}) \\ |
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\end{tabular} |
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\label{tab:co_energies} |
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\end{table} |
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|
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|
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\subsection{Forcefield validation} |
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The CO-metal cross interactions were compared directly to DFT results |
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found in the supporting information of Tao {\it et al.} |
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\cite{Tao:2010} These calculations are estimates of the stabilization |
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energy provided to double-layer reconstructions of the perfect 557 |
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surface by an overlayer of CO molecules in a $c (2 \times 4)$ pattern. |
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To make the comparison, metal slabs that were five atoms thick and |
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which displayed a 557 facet were constructed. Double-layer |
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(reconstructed) systems were created using six atomic layers where |
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enough of a layer was removed from both exposed 557 facets to create |
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the double step. In all cases, the metal slabs contained 480 atoms |
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and were minimized using steepest descent under the EAM force |
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field. Both the bare metal slabs and slabs with 50\% carbon monoxide |
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coverage (arranged in the $c (2 \times 4)$ pattern) were used. The |
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systems are periodic along and perpendicular to the step-edge axes |
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with a large vacuum above the displayed 557 facet. |
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|
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Energies using our force field for the various systems are displayed |
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in Table ~\ref{tab:steps}. The relative energies are calculated as |
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$E_{relative} = E_{system} - E_{M-557-S} - N_{CO} E_{CO-M}$, |
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where $E_{CO-M}$ is -1.84 eV for CO-Pt and -0.39 eV for CO-Au. For |
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platinum, the bare double layer is slightly less stable then the |
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original single (557) step. However, addition of carbon monoxide |
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stabilizes the reconstructed double layer relative to the perfect 557. |
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This result is in qualitative agreement with DFT calculations in Tao |
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{\it et al.}\cite{Tao:2010}, who also showed that the addition of CO |
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leads to a reversal in stability. |
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|
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The DFT calculations suggest an increased stability of 0.1 kcal/mol |
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per Pt atom, while our force field gives an approximately 0.4 kcal/mol |
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increase in stability per Pt atom. |
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|
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The gold systems show much smaller energy differences between the |
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single and double layers. The weaker binding of CO to Au is evidenced |
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by the much smaller change in relative energy between the structures |
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when carbon monoxide is present. Additionally, as CO-Au binding is |
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much weaker, it would be unlikely that CO would approach the 50\% |
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coverage levels operating temperatures. |
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|
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%Table of single step double step calculations |
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\begin{table}[H] |
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\caption{Minimized single point energies of (S)ingle and (D)ouble |
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steps. The addition of CO in a 50\% $c(2 \times 4)$ coverage acts as a |
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stabilizing presence and suggests a driving force for the observed |
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reconstruction on the highest coverage Pt system. All energies are |
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in kcal/mol.} |
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\centering |
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\begin{tabular}{| c | c | c | c | c | c |} |
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\hline |
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\textbf{Step} & \textbf{N}\textsubscript{M} & \textbf{N\textsubscript{CO}} & \textbf{Relative Energy} & \textbf{$\Delta$E/M} & \textbf{$\Delta$E/CO} \\ |
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\hline |
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Pt(557)-S & 480 & 0 & 0 & 0 & - \\ |
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Pt(557)-D & 480 & 0 & 114.783 & 0.239 & -\\ |
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Pt(557)-S & 480 & 40 & -124.546 & -0.259 & -3.114\\ |
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Pt(557)-D & 480 & 44 & -34.953 & -0.073 & -0.794\\ |
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\hline |
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\hline |
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Au(557)-S & 480 & 0 & 0 & 0 & - \\ |
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Au(557)-D & 480 & 0 & 79.572 & 0.166 & - \\ |
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Au(557)-S & 480 & 40 & -157.199 & -0.327 & -3.930\\ |
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Au(557)-D & 480 & 44 & -123.297 & -0.257 & -2.802 \\ |
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\hline |
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\end{tabular} |
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\label{tab:steps} |
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\end{table} |
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|
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|
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\subsection{Pt(557) and Au(557) metal interfaces} |
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Our Pt system is an orthorhombic periodic box of dimensions |
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54.482~x~50.046~x~120.88~\AA~while our Au system has |