324 |
|
\hline |
325 |
|
& Calculated & Experimental \\ |
326 |
|
\hline |
327 |
< |
\multirow{2}{*}{\textbf{Pt-CO}} & \multirow{2}{*}{-1.84} & -1.4 \bibpunct{}{}{,}{n}{}{,} |
327 |
> |
\multirow{2}{*}{\textbf{Pt-CO}} & \multirow{2}{*}{-1.81} & -1.4 \bibpunct{}{}{,}{n}{}{,} |
328 |
|
(Ref. \protect\cite{Kelemen:1979}) \\ |
329 |
|
& & -1.9 \bibpunct{}{}{,}{n}{}{,} (Ref. \protect\cite{Yeo}) \\ \hline |
330 |
|
\textbf{Au-CO} & -0.39 & -0.40 \bibpunct{}{}{,}{n}{}{,} (Ref. \protect\cite{TPDGold}) \\ |
335 |
|
|
336 |
|
|
337 |
|
\subsection{Forcefield validation} |
338 |
< |
The CO-metal cross interactions were compared directly to DFT results |
338 |
> |
The CO-Pt 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 |
340 |
> |
\cite{Tao:2010}, while the CO-Au results are interpreted on their own. |
341 |
> |
These calculations are estimates of the stabilization |
342 |
> |
energy provided to double-layer reconstructions of the perfect (557) |
343 |
> |
surface by an overlayer of CO molecules in a $c (2 \times 4)$ pattern. |
344 |
> |
To make the comparison, metal slabs of both Pt and Au that were five atoms thick and |
345 |
> |
which displayed a (557) facet were constructed. Double-layer |
346 |
|
(reconstructed) systems were created using six atomic layers where |
347 |
< |
enough of a layer was removed from both exposed 557 facets to create |
347 |
> |
enough of a layer was removed from both exposed (557) facets to create |
348 |
|
the double step. In all cases, the metal slabs contained 480 atoms |
349 |
|
and were minimized using steepest descent under the EAM force |
350 |
|
field. Both the bare metal slabs and slabs with 50\% carbon monoxide |
351 |
|
coverage (arranged in the $c (2 \times 4)$ pattern) were used. The |
352 |
|
systems are periodic along and perpendicular to the step-edge axes |
353 |
< |
with a large vacuum above the displayed 557 facet. |
353 |
> |
with a large vacuum above the displayed (557) facet. |
354 |
|
|
355 |
< |
Energies using our force field for the various systems are displayed |
356 |
< |
in Table ~\ref{tab:steps}. The relative energies are calculated as |
357 |
< |
$E_{relative} = E_{system} - E_{M-557-S} - N_{CO} E_{CO-M}$, |
358 |
< |
where $E_{CO-M}$ is -1.84 eV for CO-Pt and -0.39 eV for CO-Au. For |
359 |
< |
platinum, the bare double layer is slightly less stable then the |
360 |
< |
original single (557) step. However, addition of carbon monoxide |
361 |
< |
stabilizes the reconstructed double layer relative to the perfect 557. |
362 |
< |
This result is in qualitative agreement with DFT calculations in Tao |
363 |
< |
{\it et al.}\cite{Tao:2010}, who also showed that the addition of CO |
364 |
< |
leads to a reversal in stability. |
365 |
< |
|
365 |
< |
The DFT calculations suggest an increased stability of 0.08 kcal/mol |
366 |
< |
(0.7128 eV) per Pt atom for going from the single to double step |
367 |
< |
structure in the presence of carbon monoxide. |
355 |
> |
Energies calculated using our force field for the various systems are |
356 |
> |
displayed in Table ~\ref{tab:steps}. The relative energies are calculated |
357 |
> |
as $E_{relative} = E_{system} - E_{M-557-S} - N_{CO}*E_{M-CO}$, |
358 |
> |
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 |
> |
arrangement of CO on the single and double steps however, leads to a |
362 |
> |
slight displacement from the minimum. For a 1 ps run at 3 K, the single |
363 |
> |
step Pt-CO average bond length was 1.60~\AA, and for the double step, |
364 |
> |
the bond length was 1.58~\AA. This slight increase is likely due to small |
365 |
> |
electrostatic interactions among the CO and the non-ideality of the surface. |
366 |
|
|
367 |
< |
The gold systems show much smaller energy differences between the |
368 |
< |
single and double layers. The weaker binding of CO to Au is evidenced |
369 |
< |
by the much smaller change in relative energy between the structures |
370 |
< |
when carbon monoxide is present. Additionally, as CO-Au binding is |
371 |
< |
much weaker than CO-Pt, it would be unlikely that CO would approach |
372 |
< |
the 50\% coverage levels operating temperatures for the gold surfaces. |
367 |
> |
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 |
> |
when compared to single step system with CO on the order of 230 kcal/mole |
371 |
> |
for this system size. The absolute difference is minimal, but this result is in |
372 |
> |
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 |
+ |
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 |
|
\caption{Minimized single point energies of (S)ingle and (D)ouble |
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 & 114.783 & 0.239 & -\\ |
397 |
< |
Pt(557)-S & 480 & 40 & -124.546 & -0.259 & -3.114\\ |
398 |
< |
Pt(557)-D & 480 & 44 & -34.953 & -0.073 & -0.794\\ |
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 & 79.572 & 0.166 & - \\ |
403 |
< |
Au(557)-S & 480 & 40 & -157.199 & -0.327 & -3.930\\ |
404 |
< |
Au(557)-D & 480 & 44 & -123.297 & -0.257 & -2.802 \\ |
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} |