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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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|
\end{tabular} |
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\label{tab:co_energies} |
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+ |
\end{table} |
| 335 |
+ |
|
| 336 |
+ |
|
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+ |
\subsection{Validation of forcefield selections} |
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By calculating minimum energies for commensurate systems of |
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+ |
single and double layer Pt and Au systems with 0 and 50\% coverages |
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+ |
(arranged in a c(2x4) pattern), our forcefield selections were able to be |
| 341 |
+ |
indirectly compared to results shown in the supporting information of Tao |
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+ |
{\it et al.} \cite{Tao:2010}. Five layer thick systems, displaying a 557 facet |
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+ |
were constructed, each composed of 480 metal atoms. Double layers systems |
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+ |
were constructed from six layer thick systems where an entire layer was |
| 345 |
+ |
removed from both displayed facets to create a double step. By design, the |
| 346 |
+ |
double step system also contains 480 atoms, five layers thick, so energy |
| 347 |
+ |
comparisons between the arrangements can be made directly. The positions |
| 348 |
+ |
of the atoms were allowed to relax, along with the box sizes, before a |
| 349 |
+ |
minimum energy was calculated. Carbon monoxide, equivalent to 50\% |
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+ |
coverage on one side of the metal system was added in a c(2x4) arrangement |
| 351 |
+ |
and again allowed to relax before a minimum energy was calculated. |
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+ |
|
| 353 |
+ |
Energies for the various systems are displayed in Table ~\ref{tab:steps}. Examining |
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+ |
the Pt systems first, it is apparent that the double layer system is slightly less stable |
| 355 |
+ |
then the original single step. However, upon addition of carbon monoxide, the |
| 356 |
+ |
stability is reversed and the double layer system becomes more stable. This result |
| 357 |
+ |
is in agreement with DFT calculations in Tao {\it et al.}\cite{Tao:2010}, who also show |
| 358 |
+ |
that the addition of CO leads to a reversal in the most stable system. While our |
| 359 |
+ |
results agree qualitatively, quantitatively, they are approximately an order of magnitude |
| 360 |
+ |
different. Looking at additional stability per atom in kcal/mol, the DFT calculations suggest |
| 361 |
+ |
an increased stability of 0.1 kcal/mol per Pt atom, whereas we are seeing closer to a 0.4 kcal/mol |
| 362 |
+ |
increase in stability per Pt atom. |
| 363 |
+ |
|
| 364 |
+ |
The gold systems show a much smaller energy difference between the single and double |
| 365 |
+ |
systems, likely arising from their lower energy per atom values. Additionally, the weaker |
| 366 |
+ |
binding of CO to Au is evidenced by the much smaller energy change between the two systems, |
| 367 |
+ |
when compared to the Pt results. This limited change helps explain our lack of any reconstruction |
| 368 |
+ |
on the Au systems. |
| 369 |
+ |
|
| 370 |
+ |
|
| 371 |
+ |
%Table of single step double step calculations |
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+ |
\begin{table}[H] |
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+ |
\caption{Minimized single point energies of unit cell crystals displaying (S)ingle or (D)double steps. Systems are periodic along and perpendicular to the step-edge axes with a large vacuum above the displayed 557 facet. The addition of CO in a 50\% c(2x4) coverage acts as a stabilizing presence and suggests a driving force for the observed reconstruction on the highest coverage Pt system. All energies are in kcal/mol.} |
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\centering |
| 375 |
+ |
\begin{tabular}{| c | 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{Unit-Cell Energy} & \textbf{Energy per M} & \textbf{Energy per CO} & \textbf{Difference per M} \\ |
| 378 |
+ |
\hline |
| 379 |
+ |
Pt(557)-S & 480 & 0 & -61142.624 & -127.381 & - & 0 \\ |
| 380 |
+ |
Pt(557)-D & 480 & 0 & -61027.841 & -127.141 & - & 0.240 \\ |
| 381 |
+ |
\hline |
| 382 |
+ |
Pt(557)-S & 480 & 40 & -62960.289 & -131.167 & -45.442 & 0 \\ |
| 383 |
+ |
Pt(557)-D & 480 & 44 & -63040.007 & -131.333 & -45.731 & -0.166\\ |
| 384 |
+ |
\hline |
| 385 |
+ |
\hline |
| 386 |
+ |
Au(557)-S & 480 & 0 & -41879.286 & -87.249 & - &0 \\ |
| 387 |
+ |
Au(557)-D & 480 & 0 & -41799.714 & -87.084 & - & 0.165 \\ |
| 388 |
+ |
\hline |
| 389 |
+ |
Au(557)-S & 480 & 40 & -42423.899 & -88.381 & -13.615 & 0 \\ |
| 390 |
+ |
Au(557)-D & 480 & 44 & -42428.738 & -88.393 & -14.296 & -0.012 \\ |
| 391 |
+ |
\hline |
| 392 |
+ |
\end{tabular} |
| 393 |
+ |
\label{tab:steps} |
| 394 |
|
\end{table} |
| 395 |
|
|
| 396 |
+ |
|
| 397 |
|
\subsection{Pt(557) and Au(557) metal interfaces} |
| 398 |
|
Our Pt system is an orthorhombic periodic box of dimensions |
| 399 |
|
54.482~x~50.046~x~120.88~\AA~while our Au system has |
