| 50 |
|
|
| 51 |
|
|
| 52 |
|
\section{Introduction} |
| 53 |
+ |
|
| 54 |
+ |
Pt-based and Pd-based bimetallic materials are crucial catalysts in |
| 55 |
+ |
oxygen reduction reactions (ORR),\cite{Lim:2009fk} and oxygen |
| 56 |
+ |
evolution reactions (OER),\cite{Reier:2012uq} that are important in |
| 57 |
+ |
charging and discharging of Li-air batteries\cite{Lu:2011vn} and in |
| 58 |
+ |
fuel cell processes.\cite{Bliznakov:2012kx} Oxide-supported noble |
| 59 |
+ |
metal particles are also important in the water-gas shift |
| 60 |
+ |
reaction.\cite{Bunluesin:1998ys} |
| 61 |
+ |
|
| 62 |
+ |
|
| 63 |
|
Bimetallic alloys, subsurface alloys, and core-shell nanostructures are |
| 64 |
|
currently under intense investigation\cite{Kim:2013jt, Gao:2009oj, Gao:2009wo} |
| 65 |
|
because of their large accesible design space for various catalytic processes. |
| 147 |
|
neighbor interactions during parameterization.\cite{a} |
| 148 |
|
|
| 149 |
|
In this work, we have employed the embedded atom method (EAM) to |
| 150 |
< |
describe the \ce{Pt} and \ce{Pd} electron densities, embedding functionals, and |
| 151 |
< |
pair potentials,\cite{EAM} utilizing the Johnson mixing rules for the |
| 152 |
< |
\ce{Pt\bond{-}Pd} cross-interactions.\cite{johnson89} |
| 150 |
> |
describe the \ce{Pt} and \ce{Pd} electron densities, embedding |
| 151 |
> |
functionals, and pair potentials,\cite{EAM} utilizing the Johnson |
| 152 |
> |
mixing rules for the \ce{Pt\bond{-}Pd} |
| 153 |
> |
cross-interactions.\cite{johnson89} |
| 154 |
|
|
| 155 |
|
The carbon monoxide (\ce{CO}) self-interactions were modeled using a |
| 156 |
|
rigid three-site model developed by Straub and Karplus for studying |
| 163 |
|
data.\cite{Michalka:2013,Deshlahra:2012} This modification yields a |
| 164 |
|
slightly weaker \ce{Pt\bond{-}CO} binding energy. |
| 165 |
|
|
| 166 |
< |
The \ce{Pd\bond{-}CO} interaction potential was parameterized as part of this |
| 167 |
< |
work, and uses similar functional forms to the \ce{Pt\bond{-}CO} |
| 168 |
< |
model.\cite{Michalka:2013} Our starting point is a model introduced by |
| 169 |
< |
Korzeniewski \textit{et al.}\cite{Pons:1986} The parameters were modified to |
| 170 |
< |
reflect binding energies and binding site preferences on the \ce{M} (111) |
| 171 |
< |
surfaces. One key difference from the potential in Ref. |
| 172 |
< |
\citenum{Michalka:2013} is that the \ce{M\bond{-}O} bond is modeled using a |
| 173 |
< |
purely repulsive Morse potential, $D e^{-2\gamma(r-r_e)}$. The functional |
| 174 |
< |
forms and the broad repulsive \ce{M\bond{-}O} contribution are flexible enough |
| 175 |
< |
to reproduce the atop preference for \ce{Pt\bond{-}CO} as well as the |
| 176 |
< |
bridge/hollow - preference for \ce{Pd\bond{-}CO}. Parameters for the |
| 177 |
< |
potentials are given in Table~\ref{tab:CO_parameters} and the calculated |
| 178 |
< |
binding energies at various binding sites are shown in |
| 179 |
< |
Table~\ref{tab:CO_energies}. |
| 166 |
> |
The \ce{Pd\bond{-}CO} interaction potential was parameterized as part |
| 167 |
> |
of this work, and uses similar functional forms to the |
| 168 |
> |
\ce{Pt\bond{-}CO} model.\cite{Michalka:2013} Our starting point is a |
| 169 |
> |
model introduced by Korzeniewski \textit{et al.}\cite{Pons:1986} The |
| 170 |
> |
parameters were modified to reflect binding energies and binding site |
| 171 |
> |
preferences on the \ce{M} (111) surfaces. One key difference from the |
| 172 |
> |
potential in Ref. \citenum{Michalka:2013} is that the \ce{M\bond{-}O} |
| 173 |
> |
bond is modeled using a purely repulsive Morse potential, $D |
| 174 |
> |
e^{-2\gamma(r-r_e)}$. The functional forms and the broad repulsive |
| 175 |
> |
\ce{M\bond{-}O} contribution are flexible enough to reproduce the atop |
| 176 |
> |
preference for \ce{Pt\bond{-}CO} as well as the bridge/hollow - |
| 177 |
> |
preference for \ce{Pd\bond{-}CO}. Parameters for the potentials are |
| 178 |
> |
given in Table~\ref{tab:CO_parameters} and the calculated binding |
| 179 |
> |
energies at various binding sites are shown in |
| 180 |
> |
Table~\ref{tab:CO_energies}. |
| 181 |
|
|
| 182 |
|
\begin{table} |
| 183 |
|
\caption{Parameters for the metal-\ce{CO} cross-interactions. Metal-Carbon |
| 219 |
|
\label{tab:CO_energies} |
| 220 |
|
\end{table} |
| 221 |
|
|
| 222 |
< |
This \ce{Pd\bond{-}CO} model does not have a strong preference for either the |
| 223 |
< |
bridge or hollow binding sites, so it may overestimate the bridge-site binding |
| 224 |
< |
at low coverages, but at higher coverages, the situation is somewhat less |
| 225 |
< |
clear.\cite{Wong:1991ta} Studies using low-energy elecron diffraction (LEED) |
| 226 |
< |
and \ce{C\bond{-}O} stretching frequencies of \ce{CO} bound to \ce{Pd}(111) |
| 227 |
< |
suggest that the 3-fold hollow sites are preferred at low |
| 228 |
< |
coverages,\cite{Bradshaw:1978uf,Conrad:1978fx,Ohtani:1987zh} where it forms a |
| 229 |
< |
$(\sqrt{3} \times \sqrt{3}) R~30^{\circ}$ pattern. These observations are |
| 230 |
< |
supported by temperature desorption spectroscopy,\cite{Guo:1989} and infrared |
| 231 |
< |
absorption spectroscopy~\cite{Szanyi:1992} where binding energies have been |
| 222 |
> |
This \ce{Pd\bond{-}CO} model does not have a strong preference for |
| 223 |
> |
either the bridge or hollow binding sites, so it may overestimate the |
| 224 |
> |
bridge-site binding at low coverages, but at higher coverages, the |
| 225 |
> |
situation is somewhat less clear.\cite{Wong:1991ta} Studies using |
| 226 |
> |
low-energy elecron diffraction (LEED) and \ce{C\bond{-}O} stretching |
| 227 |
> |
frequencies of \ce{CO} bound to \ce{Pd}(111) suggest that the 3-fold |
| 228 |
> |
hollow sites are preferred at low |
| 229 |
> |
coverages,\cite{Bradshaw:1978uf,Conrad:1978fx,Ohtani:1987zh} where it |
| 230 |
> |
forms a $(\sqrt{3} \times \sqrt{3}) R~30^{\circ}$ pattern. These |
| 231 |
> |
observations are supported by temperature desorption |
| 232 |
> |
spectroscopy,\cite{Guo:1989} and infrared absorption |
| 233 |
> |
spectroscopy~\cite{Szanyi:1992} where binding energies have been |
| 234 |
|
reported to lie between 1.3 and 1.54 eV. |
| 235 |
|
|
| 236 |
|
At higher \ce{CO} coverages (e.g. $> 0.5$ ML), the preferred binding |
| 237 |
< |
of \ce{CO} on \ce{Pd}(111) appears to be a $c(4\times2)$ ordered structure with |
| 238 |
< |
the \ce{CO} bound to the bridge sites.\cite{Bradshaw:1978uf} |
| 237 |
> |
of \ce{CO} on \ce{Pd}(111) appears to be a $c(4\times2)$ ordered |
| 238 |
> |
structure with the \ce{CO} bound to the bridge |
| 239 |
> |
sites.\cite{Bradshaw:1978uf} |
| 240 |
|
|
| 241 |
|
Theoretical work by Honkala \textit{et al.}\cite{Honkala:2001sf} using |
| 242 |
|
DFT with the generalized gradient approximation (GGA) to describe |
| 243 |
< |
electron exchange correlation and pseudopotentials for the \ce{Pd} atoms |
| 244 |
< |
also reported the fcc site as the most favorable binding position with |
| 245 |
< |
a binding energy of 2.00 eV compared to the bridge site binding |
| 246 |
< |
energy of 1.83 eV at $1/3$ monolayer. |
| 243 |
> |
electron exchange correlation and pseudopotentials for the \ce{Pd} |
| 244 |
> |
atoms also reported the fcc site as the most favorable binding |
| 245 |
> |
position with a binding energy of 2.00 eV compared to the bridge site |
| 246 |
> |
binding energy of 1.83 eV at $1/3$ monolayer. |
| 247 |
|
|
| 248 |
|
High resolution x-ray photoelectron spectroscopy (XPS) results from |
| 249 |
|
Surnev \textit{et al.}\cite{Surnev:2000uk} confirm that the preferred |
| 255 |
|
binding energy difference shrinks, as at $1/2$ ML the hollow to bridge |
| 256 |
|
energy difference is 0.06 eV (-1.85 hollow, -1.79 bridge). |
| 257 |
|
|
| 258 |
< |
Although the weak preference for hollow vs. bridge sites is not captured by the |
| 259 |
< |
\ce{Pd\bond{-}CO} fit, it does represent a significant change from the atop |
| 260 |
< |
preference of the \ce{Pt\bond{-}CO} model. The dynamics of the metal bound to |
| 261 |
< |
the \ce{CO} is significantly altered as a result of this difference. |
| 258 |
> |
Although the weak preference for hollow vs. bridge sites is not |
| 259 |
> |
captured by the \ce{Pd\bond{-}CO} fit, the slight favoring of the |
| 260 |
> |
bridge adsorption site in this model does result in an accurate |
| 261 |
> |
reproduction of the $c(4\times2)$ adsorption structure at higher |
| 262 |
> |
coverages, which would be the most relevant regime for catalytic |
| 263 |
> |
behavior. |
| 264 |
|
|
| 248 |
– |
|
| 249 |
– |
Although the weak preference for hollow vs. bridge sites is not captured by the |
| 250 |
– |
\ce{Pd\bond{-}CO} fit, the slight favoring of the bridge adsorption site in |
| 251 |
– |
this model does result in an accurate reproduction of the $c(4\times2)$ |
| 252 |
– |
adsorption structure at higher coverages, which is where most of the |
| 253 |
– |
restructuring was observed. |
| 254 |
– |
|
| 255 |
– |
|
| 265 |
|
\subsection{557 interfaces and subsurface alloys} |
| 266 |
< |
The \ce{Pd}(557) model is an orthorhombic periodic box with dimensions of |
| 267 |
< |
$55.09 \times 49.48 \times 120$~\AA~ while the subsurface alloys |
| 266 |
> |
The \ce{Pd}(557) model is an orthorhombic periodic box with dimensions |
| 267 |
> |
of $55.09 \times 49.48 \times 120$~\AA~ while the subsurface alloys |
| 268 |
|
(Pt(557) surface layers, with Pd bulk) have dimensions of $54.875 |
| 269 |
< |
\times 49.235 \times 120$~\AA. The \ce{Pd} system consists of 9 layers of |
| 270 |
< |
\ce{Pd} while our subsurface alloys consist of 7 layers of \ce{Pd} sandwiched |
| 271 |
< |
between 2 layers of \ce{Pt}. Both the pure \ce{Pd} slab and the subsurface |
| 272 |
< |
alloy systems are $\sim$22~\AA~ thick. The lattice constants for \ce{Pd} |
| 273 |
< |
and \ce{Pt}, 3.89 and 3.92~\AA, respectively, provide minimal strain energy |
| 274 |
< |
in the alloy, and the relaxed geometries of the two interfaces are |
| 275 |
< |
therefore quite similar. |
| 269 |
> |
\times 49.235 \times 120$~\AA. The \ce{Pd} system consists of 9 |
| 270 |
> |
layers of \ce{Pd} while our subsurface alloys consist of 7 layers of |
| 271 |
> |
\ce{Pd} sandwiched between 2 layers of \ce{Pt}. Both the pure \ce{Pd} |
| 272 |
> |
slab and the subsurface alloy systems are $\sim$22~\AA~ thick. The |
| 273 |
> |
lattice constants for \ce{Pd} and \ce{Pt}, 3.89 and 3.92~\AA, |
| 274 |
> |
respectively, provide minimal strain energy in the alloy, and the |
| 275 |
> |
relaxed geometries of the two interfaces are therefore quite similar. |
| 276 |
|
|
| 277 |
|
The systems are cut from a FCC crystal along the 557 plane, and are |
| 278 |
|
rotated so that they are periodic in the $x$ and $y$ directions, |
| 310 |
|
OpenMD.\cite{openmd,OOPSE} |
| 311 |
|
|
| 312 |
|
\section{Results} |
| 313 |
< |
In our earlier work on \ce{Pt}(557) we observed \ce{CO}-induced restructuring |
| 314 |
< |
into relatively clean double-layer structures. For the pure \ce{Pd}(557) |
| 315 |
< |
studied here, the 557 facet retains the plateaus and steps with only |
| 316 |
< |
minimal adatom movement, and with almost no surface reconstruction. |
| 317 |
< |
Higher \ce{CO} coverages appear to have minimal effect on the pure \ce{Pd}(557) |
| 318 |
< |
systems. |
| 313 |
> |
In our earlier work on \ce{Pt}(557), we observed \ce{CO}-induced |
| 314 |
> |
restructuring into relatively clean double-layer structures. For the |
| 315 |
> |
pure \ce{Pd}(557) studied here, the 557 facet retains the plateaus and |
| 316 |
> |
steps with only minimal adatom movement, and with almost no surface |
| 317 |
> |
reconstruction. Higher \ce{CO} coverages appear to have minimal |
| 318 |
> |
effect on the pure \ce{Pd}(557) systems. |
| 319 |
|
|
| 320 |
< |
However, the \ce{Pt/Pd} subsurface alloy exhibits a \ce{CO}-induced speedup of |
| 321 |
< |
the diffusion of surface metal atoms, as well as a large-scale |
| 322 |
< |
restructuring of the well-ordered surface into \ce{Pt}-rich islands, and |
| 323 |
< |
will therefore be the focus of most of our analysis. |
| 320 |
> |
However, the \ce{Pt}-coated \ce{Pd} alloy exhibits a \ce{CO}-induced |
| 321 |
> |
speedup of the diffusion of surface metal atoms, as well as a |
| 322 |
> |
large-scale restructuring of the well-ordered surface into |
| 323 |
> |
\ce{Pt}-rich islands, and will therefore be the focus of most of our |
| 324 |
> |
analysis. |
| 325 |
|
|
| 326 |
|
\begin{figure} |
| 327 |
|
\includegraphics[width=\linewidth]{../figures/SystemFigures/systems_ochre2.png} |
| 335 |
|
\end{figure} |
| 336 |
|
|
| 337 |
|
Figure \ref{fig:systems} shows representative configurations of the |
| 338 |
< |
various systems after significant exposure to the \ce{CO}. We see that the |
| 339 |
< |
Pd system highlighted in panel A has undergone no surface |
| 338 |
> |
various systems after significant exposure to the \ce{CO}. We see that |
| 339 |
> |
the Pd system highlighted in panel A has undergone no surface |
| 340 |
|
restructuring. The other three panels highlight the effect of varying |
| 341 |
< |
\ce{CO} concentrations on the subsurface alloys, which do exhibit |
| 341 |
> |
\ce{CO} concentrations on the surface alloys, which do exhibit |
| 342 |
|
structural reorganization. |
| 343 |
|
|
| 344 |
< |
\subsection{Diffusion of Surface Metal Atoms in the Subsurface Alloy} |
| 344 |
> |
\subsection{Diffusion of Surface Metal Atoms in the Surface Alloy} |
| 345 |
|
|
| 346 |
|
Figure \ref{fig:systems} suggests that there is limited to no mobility |
| 347 |
|
on the pure Pd systems. Analysis of the surface atom mobility showed |
| 382 |
|
|
| 383 |
|
\subsection{Island Formation and Clustering in the Subsurface Alloy} |
| 384 |
|
|
| 385 |
< |
In a similar manner to the \ce{Pt} (557) surfaces, the structural |
| 385 |
> |
In a similar manner to the \ce{Pt}(557) surfaces, the structural |
| 386 |
|
reconstructions that occur for the subsurface alloy are influenced by |
| 387 |
< |
the presence of the \ce{CO} adsorbate. In Figure \ref{fig:domainAreasPd}, |
| 388 |
< |
the area of exposed \ce{Pd} increases both over time, and as a function of |
| 389 |
< |
\ce{CO} coverage. The appearance of the subsurface \ce{Pd} requires a |
| 390 |
< |
simultaneous reduction in the surface area of the outer \ce{Pt} skin. Two |
| 391 |
< |
scenarios could explain the reduction of exposed \ce{Pt}: either the \ce{Pt} |
| 392 |
< |
atoms are being buried under the \ce{Pd} bulk, or islands of \ce{Pt} are forming |
| 393 |
< |
on top of the \ce{Pd} surface. |
| 387 |
> |
the presence of the \ce{CO} adsorbate. In Figure |
| 388 |
> |
\ref{fig:domainAreasPd}, the area of exposed \ce{Pd} increases both |
| 389 |
> |
over time, and as a function of \ce{CO} coverage. The presence of |
| 390 |
> |
\ce{CO} leads to more exposure of the underlying \ce{Pd}, measured by |
| 391 |
> |
the increasing number and size of \ce{Pd} domains. Without \ce{CO} |
| 392 |
> |
exposure, the bare \ce{Pt/Pd} surface does undergo some restructuring, |
| 393 |
> |
although both the rate and extent is significantly smaller than in the |
| 394 |
> |
0.25 and 0.50 monolayer (ML) systems. |
| 395 |
|
|
| 396 |
< |
Both mechanisms would explain the decreased \ce{Pt} surface area (see Fig. |
| 397 |
< |
\ref{fig:domainAreasPt}). To discern which of these mechanisms is |
| 398 |
< |
taking place, the identity of nearest metal atom neighbors can be |
| 399 |
< |
tabulated. Single-layer \ce{Pt} skins have atoms with 6 \ce{Pt} nearest |
| 400 |
< |
neighbors. Islands of \ce{Pt} require the presence of \ce{Pt} atoms with 7-9 \ce{Pt} |
| 390 |
< |
nearest neighbors. In figure \ref{fig:nearestNeighbors}, we see an |
| 391 |
< |
increase in \ce{Pt} population with 9 \ce{Pt} nearest neighbors along with the |
| 392 |
< |
simultaneous decrease in \ce{Pt} atoms with only 6 \ce{Pt} nearest neighbors. |
| 393 |
< |
This is evidence for the formation of multi-layer \ce{Pt} features since |
| 394 |
< |
single layers of \ce{Pt} are restricted to having 6 \ce{Pt} nearest neighbors. |
| 395 |
< |
We note that nearest-neighbor population analysis provides information |
| 396 |
< |
similar to the information one might obtain from an XAFS experiment, |
| 397 |
< |
which could make this phenomenon experimentally observable. |
| 396 |
> |
The appearance of \ce{Pd} from the bulk layers on the surface requires |
| 397 |
> |
a simultaneous reduction in the surface area of the outer \ce{Pt} |
| 398 |
> |
skin. Two scenarios could explain the reduction of exposed \ce{Pt}: |
| 399 |
> |
either the \ce{Pt} atoms are being buried under the \ce{Pd} bulk, or |
| 400 |
> |
islands of \ce{Pt} are forming on top of the \ce{Pd} surface. |
| 401 |
|
|
| 402 |
+ |
Both mechanisms would explain the decreased \ce{Pt} surface area (see |
| 403 |
+ |
Fig. \ref{fig:domainAreasPt}). To discern which of these mechanisms |
| 404 |
+ |
is taking place, the identity of nearest metal atom neighbors can be |
| 405 |
+ |
tabulated as a function of time of exposure to \ce{CO}. Single-layer |
| 406 |
+ |
\ce{Pt} skins have atoms with 6 \ce{Pt} nearest neighbors. Islands of |
| 407 |
+ |
\ce{Pt} require the presence of \ce{Pt} atoms with 7-9 \ce{Pt} nearest |
| 408 |
+ |
neighbors. In figure \ref{fig:nearestNeighbors}, we see an increase in |
| 409 |
+ |
\ce{Pt} population with 9 \ce{Pt} nearest neighbors along with the |
| 410 |
+ |
simultaneous decrease in \ce{Pt} atoms with only 6 \ce{Pt} nearest |
| 411 |
+ |
neighbors. This is evidence for the formation of multi-layer \ce{Pt} |
| 412 |
+ |
features since single layers of \ce{Pt} are restricted to having 6 |
| 413 |
+ |
\ce{Pt} nearest neighbors. |
| 414 |
+ |
|
| 415 |
+ |
The presence of \ce{CO} therefore appears to facilitate the clustering |
| 416 |
+ |
of \ce{Pt} into smaller domains by forming multilayer features which |
| 417 |
+ |
leads to a reduction of \ce{Pt} surface coverage and concomitant |
| 418 |
+ |
increased exposure of the \ce{Pd}. We note that nearest-neighbor |
| 419 |
+ |
population analysis provides information similar to the information |
| 420 |
+ |
one might obtain from an XAFS experiment, which could make this |
| 421 |
+ |
phenomenon experimentally observable. |
| 422 |
+ |
|
| 423 |
|
\begin{figure} |
| 424 |
|
\includegraphics[width=\linewidth]{../figures/domainAreas/domainSize_Pd_110ns_deCluttered_color.pdf} |
| 425 |
|
%\includegraphics[width=\linewidth]{../figures/domainAreas/final_domain_Pd.pdf} |
| 426 |
< |
\caption{Distributions of \ce{Pd} domain size as a function of time and \ce{CO} coverage. |
| 427 |
< |
Data is averaged over $\sim$20~ns segments to help show progression, |
| 404 |
< |
additionally, the data is shown as a percentage of the total surface area of |
| 405 |
< |
the \ce{Pt/Pd} system with the integration of the curves equaling the percentage |
| 406 |
< |
surface area of \ce{Pd}, shown in Table \ref{tab:integratedArea}. The presence of \ce{CO} |
| 407 |
< |
leads to more exposure of the underlying \ce{Pd}, which is quantified here by an |
| 408 |
< |
increasing number and increasing size of \ce{Pd} domains. The bare \ce{Pt/Pd} surface, |
| 409 |
< |
as seen in Figure \ref{fig:systems}.B, undergoes some restructuring, however, the |
| 410 |
< |
extent is much less when compared to the 25\% and 50\% monolayer (ML) systems.} |
| 426 |
> |
\caption{Distributions of \ce{Pd} domain sizes at different \ce{CO} |
| 427 |
> |
coverages and at different times after exposure to \ce{CO}.} |
| 428 |
|
\label{fig:domainAreasPd} |
| 429 |
|
\end{figure} |
| 430 |
|
|
| 431 |
|
\begin{figure} |
| 432 |
|
\includegraphics[width=\linewidth]{../figures/domainAreas/domainSize_Pt_110ns_deCluttered_color.pdf} |
| 433 |
|
%\includegraphics[width=\linewidth]{../figures/domainAreas/final_domain_Pt.pdf} |
| 434 |
< |
\caption{Distributions of \ce{Pt} domain size as a function of time and \ce{CO} coverage. |
| 435 |
< |
Here the presence of \ce{CO} facilitates the clustering of \ce{Pt} into smaller domains |
| 419 |
< |
by forming multilayer features which leads to a reduction of \ce{Pt} surface coverage and concomitant increased exposure of the \ce{Pd}.} |
| 434 |
> |
\caption{Distributions of \ce{Pt} domain sizes at different \ce{CO} |
| 435 |
> |
coverages and at different times after exposure to \ce{CO}.} |
| 436 |
|
\label{fig:domainAreasPt} |
| 437 |
|
\end{figure} |
| 438 |
|
|
| 423 |
– |
|
| 439 |
|
\begin{figure} |
| 440 |
|
\includegraphics[width=\linewidth]{../figures/nearestNeighbor/NearestNeighbor_110ns_color.pdf} |
| 441 |
< |
\caption{Population of \ce{Pt} atoms with either 6 (solid) or 9 (hollow) |
| 442 |
< |
\ce{Pt} nearest neighbors averaged over similar blocks of time as in |
| 443 |
< |
Figure \ref{fig:domainAreasPd} and \ref{fig:domainAreasPt}. At |
| 444 |
< |
time 0, the majority ($\frac{2}{3}$) of \ce{Pt} is located in the (111) |
| 445 |
< |
plateaus where the number of \ce{Pt} nearest neighbors is 6. A sizeable |
| 446 |
< |
minority ($\frac{1}{3}$) is located at the step edge, or beneath a |
| 432 |
< |
step edge with a nearest neighbor number of 5. The decrease in \ce{Pt} |
| 433 |
< |
with 6 nearest neighbors, while \ce{Pt} with 9 nearest neighbors rises |
| 434 |
< |
implies that \ce{Pt} atoms are being incorporated into multilayer |
| 435 |
< |
features. } \label{fig:nearestNeighbors} |
| 441 |
> |
\caption{Population of \ce{Pt} atoms with either 6 (solid) or 9 |
| 442 |
> |
(hollow) \ce{Pt} nearest neighbors averaged over 18 ns blocks of |
| 443 |
> |
time. At $t=0$, the majority ($\frac{2}{3}$) of \ce{Pt} is |
| 444 |
> |
located in the (111) plateaus where the number of \ce{Pt} nearest |
| 445 |
> |
neighbors is 6. The remaining \ce{Pt} is located at step edges, |
| 446 |
> |
with a nearest neighbor \ce{Pt} count of 5.} \label{fig:nearestNeighbors} |
| 447 |
|
\end{figure} |
| 448 |
|
|
| 449 |
< |
The small amount of restructuring observed in the zero coverage system suggests |
| 450 |
< |
that there are two driving forces for restructuring, with the \ce{CO} playing one |
| 451 |
< |
role. |
| 449 |
> |
The small amount of restructuring observed in the bare metal system |
| 450 |
> |
suggests that the relative surface energies of the two metals provides |
| 451 |
> |
some of the driving force for the restructuring, while the \ce{CO} |
| 452 |
> |
significantly speeds up the effects (and may help to drive the process |
| 453 |
> |
at lower temperatures). |
| 454 |
|
|
| 442 |
– |
|
| 443 |
– |
|
| 455 |
|
\begin{table} |
| 456 |
< |
\caption{Percent \ce{Pd} surface coverage as a function of time. The following values were obtained by integrating the data in Figure \ref{fig:domainAreasPd}.} |
| 456 |
> |
\caption{\ce{Pd} surface coverage (in \% of total surface area) |
| 457 |
> |
averaged over 18 ns blocks of time.} |
| 458 |
|
\begin{tabular}{| c || c | c | c | c | c | c |} |
| 459 |
|
\hline |
| 460 |
< |
& 0-18 ns & 19-37 ns & 38-56 ns & 57-75 ns & 76-94 ns & 95-113 ns \\ |
| 460 |
> |
\ce{CO} coverage & 0-18 ns & 19-37 ns & 38-56 ns & 57-75 ns & 76-94 ns & 95-113 ns \\ |
| 461 |
|
\hline |
| 462 |
|
0.00 & 6.6 & 16.2 & 20.1 & 21.7 & 23.5 & 25.2 \\ |
| 463 |
|
0.05 & 8.0 & 15.8 & 20.2 & 25.1 & 27.6 & 30.9 \\ |
