| 141 |
|
\caption{Adsorption energies for a single CO at the atop site on M(111) at the atop site using the potentials |
| 142 |
|
described in this work. All values are in eV.} |
| 143 |
|
\centering |
| 144 |
< |
\begin{tabular}{| cc | cc |} |
| 144 |
> |
\begin{tabular}{| cc | ccc |} |
| 145 |
|
\hline |
| 146 |
< |
& Site & Calculated & Experimental \\ |
| 146 |
> |
& Site & Calculated & Theory & Experimental \\ |
| 147 |
|
\hline |
| 148 |
< |
\textbf{Pt-CO} & atop & -1.47 & \\ |
| 149 |
< |
& bridge & -1.13 & \\ |
| 150 |
< |
& hollow & -1.02 & \\ |
| 151 |
< |
\textbf{Pd-CO} & atop & -1.54 & \\ |
| 152 |
< |
& bridge & -1.65 & \\ |
| 153 |
< |
& hollow & -1.60 & \\ |
| 148 |
> |
\textbf{Pt-CO} & atop & -1.47 & & \\ |
| 149 |
> |
& bridge & -1.13 & & \\ |
| 150 |
> |
& hollow & -1.02 & & \\ |
| 151 |
> |
\textbf{Pd-CO} & atop & -1.54 & & \\ |
| 152 |
> |
& bridge & -1.65 & & \\ |
| 153 |
> |
& hollow & -1.60 & & \\ |
| 154 |
|
\hline |
| 155 |
|
\end{tabular} |
| 156 |
|
\label{tab:CO_energies} |
| 426 |
|
compositions of the various domains we observed. To perform this analysis, the |
| 427 |
|
exposed surfaces were first simplified by projecting the 3-dimensional surface |
| 428 |
|
onto a 2-dimensional grid, with two grids per system to capture the surfaces on |
| 429 |
< |
both sides of the system. The grids, Ising-like in nature with only two values |
| 430 |
< |
at each site, Pt or Pd, were then deconvoluted into separate domains based on |
| 431 |
< |
nearest-neighbor connectivity (up, down, left, right, corners were not |
| 432 |
< |
included). The resulting data was aggregated and normalized and is presented in |
| 433 |
< |
Figures \ref{fig:domainAreasPt} and \ref{fig:domainAreasPd}. |
| 429 |
> |
both sides of the system. The grids could only have one of two values at each |
| 430 |
> |
site, Pt or Pd. The resulting Ising-like grids were then deconvoluted into |
| 431 |
> |
separate domains based on nearest-neighbor connectivity (up, down, left, right; |
| 432 |
> |
corners were not included). The resulting data was aggregated and normalized |
| 433 |
> |
and is presented in Figures \ref{fig:domainAreasPt} and |
| 434 |
> |
\ref{fig:domainAreasPd}. Representative examples of the grids can be seen in |
| 435 |
> |
the supporting information. |
| 436 |
|
|
| 437 |
< |
The quantification of the surface composition that these figures display is helpful, but is more easily seen when the curves are integrated, which is shown in Table \ref{tab:integratedArea}. Especially interesting is constrating these figures with Figure \ref{fig:domainAreasNoCO}, which shows a small recovery of the Pt domain size upon removal of CO (is this true?) for the 25\% and 50\% systems. While our previous analyses focused on individual adatom moment, through diffusion and nearest neighbor calculations, an examination of the domain sizes |
| 437 |
> |
The quantification of the surface composition that these figures display is |
| 438 |
> |
helpful, but is more easily seen when the curves are integrated, which is shown |
| 439 |
> |
in Table \ref{tab:integratedArea}. Especially interesting is contrasting these |
| 440 |
> |
figures with Figure \ref{fig:domainAreasNoCO}, which shows a small recovery of |
| 441 |
> |
the Pt domain size upon removal of CO (is this true?) for the 25\% and 50\% |
| 442 |
> |
systems. This analysis allows us to focus on collective motion of the surface |
| 443 |
> |
atoms as measured by the domain sizes, rather than individual adatom movement. |
| 444 |
> |
At the beginning of the simulations, the surface layer of Pt makes up one |
| 445 |
> |
domain of size $\sim$2625~\AA\textsuperscript{2}. This domain begins to shrink |
| 446 |
> |
relatively quickly which involves the appearance of the underlying Pd and a |
| 447 |
> |
growth in the Pd domains. The presence of CO in the system appears to allow |
| 448 |
> |
further clustering (i.e. shrinking) of the Pt domain which leads directly to a |
| 449 |
> |
larger amount of exposed Pd of various domain sizes. For clarity purposes, the |
| 450 |
> |
small growth in 1-2 atom Pt domains is not shown in Figure |
| 451 |
> |
\ref{fig:domainAreasPt}, but can be seen in the supporting information. |
| 452 |
|
|
| 453 |
|
|
| 454 |
|
\subsection{Equilibrium state} |
| 460 |
|
vertical displacement benefits both layers of Pt. The now underlying Pt has |
| 461 |
|
approximately 9 nearest neighbors of Pt and 3 of Pd and is essentially in bulk. |
| 462 |
|
The upper layer of Pt also benefits because it is now experiencing 9 nearest |
| 463 |
< |
neighbor interactions, all with other Pt. This is of course the ideal case, but |
| 464 |
< |
as seen in Figure \ref{fig:systems}.B, the 557 crystal facet is still present, |
| 465 |
< |
just with Pt plateaus moved slightly forward and backward. However, without the |
| 466 |
< |
presence of CO, very little vertical displacement is observed, which is what is |
| 467 |
< |
hypothesized to facilite the multiple layer features observed in the higher |
| 468 |
< |
coverage systems. The systems were run for approximately 80 nanoseconds and |
| 469 |
< |
then stopped, primarily because, large scale changes had appeared to stop. |
| 470 |
< |
Additionally, results from various analyses were converging (see |
| 463 |
> |
neighbor interactions, all with other Pt. The ideal case would involve the |
| 464 |
> |
majority of Pt maximizing their Pt-Pt interactions which could lead to massive |
| 465 |
> |
disruption without any need for CO, but as seen in Figure \ref{fig:systems}.B, |
| 466 |
> |
the 557 crystal facet is still present, just with Pt plateaus moved slightly |
| 467 |
> |
forward and backward. Without the presence of CO, very little vertical |
| 468 |
> |
displacement is observed, which is what is hypothesized to facilite the |
| 469 |
> |
multiple layer features observed in the higher coverage systems. The systems |
| 470 |
> |
were run for approximately 80 nanoseconds and then stopped, primarily because, |
| 471 |
> |
large scale changes had appeared to stop. Additionally, results from various |
| 472 |
> |
analyses were converging (see |
| 473 |
|
Figures~\ref{fig:domainAreasPd},~\ref{fig:domainAreasPt}, and |
| 474 |
|
\ref{fig:nearestNeighbors}), suggesting that we were close to a equilibrium |
| 475 |
|
state, at least for the time scales we were able to explore. Increased runtime |
| 476 |
|
while possible, was not judged to be feasible at this time. |
| 477 |
|
|
| 460 |
– |
|
| 478 |
|
\subsection{Role of CO: Presence and Absence} |
| 479 |
+ |
As shown in the previous sections, the presence of CO plays a large role in the |
| 480 |
+ |
restructuring of the Pt@Pd shell systems. The small amount of restructuring due |
| 481 |
+ |
to favorable Pt-Pt interactions is greatly enhanced when CO is added to the |
| 482 |
+ |
system. As concluded in our previous paper\cite{Michalka:2012}, CO helps enable |
| 483 |
+ |
vertical displacement of adatoms between layers, which is also seen here by |
| 484 |
+ |
examining the degree of clustering that occurred for various CO coverages. One |
| 485 |
+ |
final test we performed, already mentioned in Figure \ref{fig:domainAreasNoCO}, |
| 486 |
+ |
is the removal of CO from the 25\% and 50\% systems. Figure |
| 487 |
+ |
\ref{fig:domainAreasNoCO} shows a slight increase in the Pt domain size, which |
| 488 |
+ |
would require the multi-layer Pt cluster to lose some of its stability and |
| 489 |
+ |
spread out. This is very similar to our previous work, where the removal of CO, |
| 490 |
+ |
led to the double-layer beginning to split back into individual steps. These |
| 491 |
+ |
systems were run for an additional 50 ns and despite the initial destablizing |
| 492 |
+ |
of the Pt clusters, appear to be stuck in a local thermodynamic minimum. It is |
| 493 |
+ |
possible that a slower removal of CO would remove the stability while still |
| 494 |
+ |
keeping enabling the vertical displacement that CO assists with and allow these |
| 495 |
+ |
systems to approach the equilibrium 0\% coverage system, but this would likely |
| 496 |
+ |
require a much longer run time outside the scope of this study. |
| 497 |
|
|
| 498 |
|
|
| 499 |
|
\section{Conclusion} |
| 505 |
|
|
| 506 |
|
This work suggests that bimetallic and subsurface alloys could be tailored to |
| 507 |
|
create and or expose active catalytic sites as a result of an adsorbates |
| 508 |
< |
presence or absence. |
| 508 |
> |
presence or absence. |
| 509 |
|
|
| 510 |
|
|
| 511 |
|
\begin{acknowledgement} |
