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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 |
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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}{*}{-1.81} & -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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\hline |
| 332 |
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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-Pt 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}, while the CO-Au results are interpreted on their own. |
| 341 |
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These calculations are estimates of the stabilization |
| 342 |
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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 of both Pt and Au 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 |
| 349 |
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and were minimized using steepest descent under the EAM force |
| 350 |
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field. Both the bare metal slabs and slabs with 50\% carbon monoxide |
| 351 |
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coverage (arranged in the $c (2 \times 4)$ pattern) were used. The |
| 352 |
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systems are periodic along and perpendicular to the step-edge axes |
| 353 |
+ |
with a large vacuum above the displayed (557) facet. |
| 354 |
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|
| 355 |
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Energies calculated using our force field for the various systems are |
| 356 |
+ |
displayed in Table ~\ref{tab:steps}. The relative energies are calculated |
| 357 |
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as $E_{relative} = E_{system} - E_{M-557-S} - N_{CO}*E_{M-CO}$, |
| 358 |
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where $E_{M-CO}$ is -1.8 eV for CO-Pt and -0.39 eV for CO-Au. Our |
| 359 |
+ |
calculated CO-Pt minimum is actually at -1.83 eV at a distance of 1.53~\AA, |
| 360 |
+ |
which was obtained from single-atom liftoffs from a Pt(111) surface. The |
| 361 |
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arrangement of CO on the single and double steps however, leads to a |
| 362 |
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slight displacement from the minimum. For a 1 ps run at 3 K, the single |
| 363 |
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step Pt-CO average bond length was 1.60~\AA, and for the double step, |
| 364 |
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the bond length was 1.58~\AA. This slight increase is likely due to small |
| 365 |
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electrostatic interactions among the CO and the non-ideality of the surface. |
| 366 |
+ |
|
| 367 |
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For platinum, the bare double layer is less stable then the original single |
| 368 |
+ |
(557) step by about 0.25 kcal/mole per Pt atom. However, addition of carbon |
| 369 |
+ |
monoxide to the double step system provides a greater amount of stabilization |
| 370 |
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when compared to single step system with CO on the order of 230 kcal/mole |
| 371 |
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for this system size. The absolute difference is minimal, but this result is in |
| 372 |
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qualitative agreement with DFT calculations in Tao {\it et al.}\cite{Tao:2010}, |
| 373 |
+ |
who also showed that the addition of CO leads to a reversal in stability. |
| 374 |
+ |
|
| 375 |
+ |
The gold systems show a smaller energy difference between the clean |
| 376 |
+ |
single and double layers when compared to platinum. Upon addition of |
| 377 |
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CO however, the single step surface becomes much more stable. These |
| 378 |
+ |
results, while helpful, need to be tempered by the weaker binding energy |
| 379 |
+ |
of CO to Au. From our simulations we see that at the elevated temperatures |
| 380 |
+ |
we are running at, it is difficult for the gold systems to maintain > than 25\% |
| 381 |
+ |
coverage, despite their being enough CO in the system. |
| 382 |
+ |
|
| 383 |
+ |
%Table of single step double step calculations |
| 384 |
+ |
\begin{table}[H] |
| 385 |
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\caption{Minimized single point energies of (S)ingle and (D)ouble |
| 386 |
+ |
steps. The addition of CO in a 50\% $c(2 \times 4)$ coverage acts as a |
| 387 |
+ |
stabilizing presence and suggests a driving force for the observed |
| 388 |
+ |
reconstruction on the highest coverage Pt system. All energies are |
| 389 |
+ |
in kcal/mol.} |
| 390 |
+ |
\centering |
| 391 |
+ |
\begin{tabular}{| c | c | c | c | c | c |} |
| 392 |
+ |
\hline |
| 393 |
+ |
\textbf{Step} & \textbf{N}\textsubscript{M} & \textbf{N\textsubscript{CO}} & \textbf{Relative Energy} & \textbf{$\Delta$E/M} & \textbf{$\Delta$E/CO} \\ |
| 394 |
+ |
\hline |
| 395 |
+ |
Pt(557)-S & 480 & 0 & 0 & 0 & - \\ |
| 396 |
+ |
Pt(557)-D & 480 & 0 & 119.788 & 0.2495 & -\\ |
| 397 |
+ |
Pt(557)-S & 480 & 40 & -109.734 & -0.2286 & -2.743\\ |
| 398 |
+ |
Pt(557)-D & 480 & 48 & -110.039 & -0.2292 & -2.292\\ |
| 399 |
+ |
\hline |
| 400 |
+ |
\hline |
| 401 |
+ |
Au(557)-S & 480 & 0 & 0 & 0 & - \\ |
| 402 |
+ |
Au(557)-D & 480 & 0 & 83.853 & 0.1747 & - \\ |
| 403 |
+ |
Au(557)-S & 480 & 40 & -253.604 & -0.5283 & -6.340\\ |
| 404 |
+ |
Au(557)-D & 480 & 48 & -156.150 & -0.3253 & -3.253 \\ |
| 405 |
+ |
\hline |
| 406 |
+ |
\end{tabular} |
| 407 |
+ |
\label{tab:steps} |
| 408 |
|
\end{table} |
| 409 |
|
|
| 410 |
+ |
|
| 411 |
|
\subsection{Pt(557) and Au(557) metal interfaces} |
| 412 |
|
Our Pt system is an orthorhombic periodic box of dimensions |
| 413 |
|
54.482~x~50.046~x~120.88~\AA~while our Au system has |
