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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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|
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\usepackage[version=3]{mhchem} % this is a great package for formatting chemical reactions |
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\hline |
| 325 |
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& Calculated & Experimental \\ |
| 326 |
|
\hline |
| 327 |
< |
\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} |
| 334 |
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\end{table} |
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+ |
|
| 336 |
|
|
| 337 |
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\subsection{Forcefield validation} |
| 338 |
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The CO-metal cross interactions were compared directly to DFT results |
| 339 |
+ |
found in the supporting information of Tao {\it et al.} |
| 340 |
+ |
\cite{Tao:2010} These calculations are estimates of the stabilization |
| 341 |
+ |
energy provided to double-layer reconstructions of the perfect 557 |
| 342 |
+ |
surface by an overlayer of CO molecules in a $c (2 \times 4)$ pattern. |
| 343 |
+ |
To make the comparison, metal slabs that were five atoms thick and |
| 344 |
+ |
which displayed a 557 facet were constructed. Double-layer |
| 345 |
+ |
(reconstructed) systems were created using six atomic layers where |
| 346 |
+ |
enough of a layer was removed from both exposed 557 facets to create |
| 347 |
+ |
the double step. In all cases, the metal slabs contained 480 atoms |
| 348 |
+ |
and were minimized using steepest descent under the EAM force |
| 349 |
+ |
field. Both the bare metal slabs and slabs with 50\% carbon monoxide |
| 350 |
+ |
coverage (arranged in the $c (2 \times 4)$ pattern) were used. The |
| 351 |
+ |
systems are periodic along and perpendicular to the step-edge axes |
| 352 |
+ |
with a large vacuum above the displayed 557 facet. |
| 353 |
+ |
|
| 354 |
+ |
Energies using our force field for the various systems are displayed |
| 355 |
+ |
in Table ~\ref{tab:steps}. The relative energies are calculated as |
| 356 |
+ |
$E_{relative} = E_{system} - E_{M-557-S} - N_{CO} E_{CO-M}$, |
| 357 |
+ |
where $E_{CO-M}$ is -1.84 eV for CO-Pt and -0.39 eV for CO-Au. For |
| 358 |
+ |
platinum, the bare double layer is slightly less stable then the |
| 359 |
+ |
original single (557) step. However, addition of carbon monoxide |
| 360 |
+ |
stabilizes the reconstructed double layer relative to the perfect 557. |
| 361 |
+ |
This result is in qualitative agreement with DFT calculations in Tao |
| 362 |
+ |
{\it et al.}\cite{Tao:2010}, who also showed that the addition of CO |
| 363 |
+ |
leads to a reversal in stability. |
| 364 |
+ |
|
| 365 |
+ |
The DFT calculations suggest an increased stability of 0.1 kcal/mol |
| 366 |
+ |
per Pt atom, while our force field gives an approximately 0.4 kcal/mol |
| 367 |
+ |
increase in stability per Pt atom. |
| 368 |
+ |
|
| 369 |
+ |
The gold systems show much smaller energy differences between the |
| 370 |
+ |
single and double layers. The weaker binding of CO to Au is evidenced |
| 371 |
+ |
by the much smaller change in relative energy between the structures |
| 372 |
+ |
when carbon monoxide is present. Additionally, as CO-Au binding is |
| 373 |
+ |
much weaker, it would be unlikely that CO would approach the 50\% |
| 374 |
+ |
coverage levels operating temperatures. |
| 375 |
+ |
|
| 376 |
+ |
%Table of single step double step calculations |
| 377 |
+ |
\begin{table}[H] |
| 378 |
+ |
\caption{Minimized single point energies of (S)ingle and (D)ouble |
| 379 |
+ |
steps. The addition of CO in a 50\% $c(2 \times 4)$ coverage acts as a |
| 380 |
+ |
stabilizing presence and suggests a driving force for the observed |
| 381 |
+ |
reconstruction on the highest coverage Pt system. All energies are |
| 382 |
+ |
in kcal/mol.} |
| 383 |
+ |
\centering |
| 384 |
+ |
\begin{tabular}{| c | c | c | c | c | c |} |
| 385 |
+ |
\hline |
| 386 |
+ |
\textbf{Step} & \textbf{N}\textsubscript{M} & \textbf{N\textsubscript{CO}} & \textbf{Relative Energy} & \textbf{$\Delta$E/M} & \textbf{$\Delta$E/CO} \\ |
| 387 |
+ |
\hline |
| 388 |
+ |
Pt(557)-S & 480 & 0 & 0 & 0 & - \\ |
| 389 |
+ |
Pt(557)-D & 480 & 0 & 114.783 & 0.239 & -\\ |
| 390 |
+ |
Pt(557)-S & 480 & 40 & -124.546 & -0.259 & -3.114\\ |
| 391 |
+ |
Pt(557)-D & 480 & 44 & -34.953 & -0.073 & -0.794\\ |
| 392 |
+ |
\hline |
| 393 |
+ |
\hline |
| 394 |
+ |
Au(557)-S & 480 & 0 & 0 & 0 & - \\ |
| 395 |
+ |
Au(557)-D & 480 & 0 & 79.572 & 0.166 & - \\ |
| 396 |
+ |
Au(557)-S & 480 & 40 & -157.199 & -0.327 & -3.930\\ |
| 397 |
+ |
Au(557)-D & 480 & 44 & -123.297 & -0.257 & -2.802 \\ |
| 398 |
+ |
\hline |
| 399 |
+ |
\end{tabular} |
| 400 |
+ |
\label{tab:steps} |
| 401 |
+ |
\end{table} |
| 402 |
+ |
|
| 403 |
+ |
|
| 404 |
|
\subsection{Pt(557) and Au(557) metal interfaces} |
| 405 |
|
Our Pt system is an orthorhombic periodic box of dimensions |
| 406 |
|
54.482~x~50.046~x~120.88~\AA~while our Au system has |
