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\usepackage[version=3]{mhchem} % this is a great package for formatting chemical reactions |
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\usepackage{url} |
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\title{CO-induced island formation on Pt@Pd(557) subsurface alloys: A |
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\title{\ce{CO}-induced island formation on \ce{Pt}/\ce{Pd}(557) subsurface alloys: A |
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molecular dynamics study} |
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\section{Introduction} |
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Bimetallic alloys, subsurface alloys, and core-shell nanostructures are |
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currently under intense investigation\cite{Kim:2013jt} because of their large accesible |
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currently under intense investigation\cite{Kim:2013jt, Gao:2009oj, Gao:2009wo} because of their large accesible |
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design space for various catalytic processes. The presence of two (or more) |
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components in these structures allows for a high degree of tuning of the |
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specific characteristics, whether that be catalytic activity\cite{a}, thermal |
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specific characteristics, whether that be catalytic activity\cite{Kim:2013jt}, thermal |
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stability\cite{a}, or resistance to deactivation\cite{a}. As seen in many |
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experimental\cite{Ertl:1989} and theoretical studies\cite{a}, the potential |
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energy landscape of the surface is often modified by the presence of adsorbates |
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leading to large-scale reconstructions of the surface. This reconstruction |
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could be a simple refaceting or a more complicated process that leads to the |
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formation of significant nano-features. Both situations will provide additional |
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or different active sites, chaning the activity and selectivity of the |
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or different active sites, changing the activity and selectivity of the |
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catalyst. |
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|
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Tuning catalyst work... |
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|
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As Tao et al.\cite{Tao:2010} showed and we modeled\cite{Michalka:2013}, the |
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steps of the Pt(557) system when exposed to a CO atmosphere undergo doubling. |
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To the extent of our knowledge there has been no similar work done with CO on |
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Pd(557) and this work is an attempt to explore that system as well as what |
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happens to a bimetallic system containing both Pt and Pd. |
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steps of the Pt(557) system when exposed to a \ce{CO} atmosphere undergo doubling. |
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To the extent of our knowledge there has been no similar work done with \ce{CO} on |
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\ce{Pd}(557) and this work is an attempt to explore that system as well as what |
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happens to a bimetallic system containing both \ce{Pt} and \ce{Pd}. |
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|
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This work is an investigation into the effect of CO adsorption on surface |
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restructuring of a Pd(557) and Pt@Pd(557) shell surface using molecular |
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This work is an investigation into the effect of \ce{CO} adsorption on surface |
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restructuring of a \ce{Pd}(557) and \ce{Pt}/\ce{Pd} (557) subsurface alloy using molecular |
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simulation. Since the mechanism and dynamics of the restructuring are of |
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particular interest, classical force fields which balance computational |
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efficiency against chemical accuracy were employed. A more complete |
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neighbor interactions during parameterization.\cite{a} |
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|
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In this work, we have employed the embedded atom method (EAM) to |
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describe the Pt and Pd electron densities, embedding functionals, and |
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describe the \ce{Pt} and \ce{Pd} electron densities, embedding functionals, and |
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pair potentials,\cite{EAM} utilizing the Johnson mixing rules for the |
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Pt-Pd cross-interactions.\cite{johnson89} |
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\ce{Pt\bond{-}Pd} cross-interactions.\cite{johnson89} |
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|
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The carbon monoxide (\ce{CO}) self-interactions were modeled using a |
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rigid three-site model developed by Straub and Karplus for studying |
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photodissociation of CO from myoglobin.\cite{Straub} This model |
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photodissociation of \ce{CO} from myoglobin.\cite{Straub} This model |
| 147 |
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accurately captures the large linear quadrupole (and weak dipole) of |
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the CO molecule. |
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the \ce{CO} molecule. |
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|
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The Pt-CO interactions have been modified from previous fits to |
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The \ce{Pt\bond{-}CO} interactions have been modified from previous fits to |
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account for recently-published DFT |
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data.\cite{Michalka:2013,Deshlahra:2012} This modification yields a |
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slightly weaker Pt-CO binding energy. |
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slightly weaker \ce{Pt\bond{-}CO} binding energy. |
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|
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The Pd-CO interaction potential was parameterized as part of this |
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work, and uses similar functional forms to the Pt-CO |
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The \ce{Pd\bond{-}CO} interaction potential was parameterized as part of this |
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work, and uses similar functional forms to the \ce{Pt\bond{-}CO} |
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model.\cite{Michalka:2013} Our starting point is a model introduced by |
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Korzeniewski \textit{et al.}\cite{Pons:1986} The parameters were |
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modified to reflect binding energies and binding site preferences on |
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the M(111) surfaces. One key difference from the potential in |
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Ref. \citenum{Michalka:2013} is that the M-O bond is modeled using a |
| 160 |
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the \ce{M} (111) surfaces. One key difference from the potential in |
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Ref. \citenum{Michalka:2013} is that the \ce{M\bond{-}O} bond is modeled using a |
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purely repulsive Morse potential, $D e^{-2\gamma(r-r_e)}$. The |
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functional forms and the broad repulsive M-O contribution are flexible |
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enough to reproduce the atop preference for Pt-CO as well as the |
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bridge/hollow - preference for Pd-CO. Parameters for the potentials |
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functional forms and the broad repulsive \ce{M\bond{-}O} contribution are flexible |
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enough to reproduce the atop preference for \ce{Pt\bond{-}CO} as well as the |
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bridge/hollow - preference for \ce{Pd\bond{-}CO}. Parameters for the potentials |
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are given in Table~\ref{tab:CO_parameters} and the calculated binding |
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energies at various binding sites are shown in |
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Table~\ref{tab:CO_energies}. |
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\begin{table} |
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\caption{Parameters for the metal-CO cross-interactions. Metal-Carbon |
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\caption{Parameters for the metal-\ce{CO} cross-interactions. Metal-Carbon |
| 171 |
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interactions are modeled with Lennard-Jones potentials, while the |
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metal-Oxygen interactions are fit using repulsive Morse potentials. |
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Distances are given in \AA~and energies in |
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\hline |
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& $\sigma$ & $\epsilon$ & & $r_e$ & $D$ & $\gamma$ (\AA$^{-1}$) \\ |
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\hline |
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\textbf{Pt-C} & 1.41 & 45 & \textbf{Pt-O} & 4.4 & 0.05 & 1.8 \\ |
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\textbf{Pd-C} & 1.6 & 40 & \textbf{Pd-O} & 4.95 & 0.05 & 1.45\\ |
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\textbf{\ce{Pt\bond{-}C}} & 1.41 & 45 & \textbf{\ce{Pt\bond{-}O}} & 4.4 & 0.05 & 1.8 \\ |
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\textbf{\ce{Pd\bond{-}C}} & 1.6 & 40 & \textbf{\ce{Pd\bond{-}O}} & 4.95 & 0.05 & 1.45\\ |
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\hline |
| 183 |
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\end{tabular} |
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\end{table} |
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|
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%Table of energies |
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\begin{table} |
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\caption{Adsorption energies for a CO molecule at the three special sites |
| 189 |
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on M(111) using the potentials described in table |
| 188 |
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\caption{Adsorption energies for a \ce{CO} molecule at the three special sites |
| 189 |
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on \ce{M} (111) using the potentials described in table |
| 190 |
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\ref{tab:CO_parameters}. These values are compared with DFT |
| 191 |
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calculations of XXX along with relevant experimental desorption |
| 192 |
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data. Reference \citenum{Deshlahra:2012} values are reported at $\frac{1}{4}$ ML. All values are in eV.} |
| 191 |
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calculations of XXX along with experimental desorption |
| 192 |
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data when available. Reference \citenum{Deshlahra:2012} values are reported at $\frac{1}{4}$ ML. All values are in eV.} |
| 193 |
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\centering |
| 194 |
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\begin{tabular}{| cc | ccc |} |
| 195 |
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\hline |
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& Site & This Model & DFT & Experimental \\ |
| 197 |
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\hline |
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\textbf{Pt-CO} & atop & -1.47 & -1.48\cite{Deshlahra:2012} & -1.39\cite{Kelemen:1979}, -1.43\cite{Ertl:1977}, -1.90\cite{Yeo:1997} \\ |
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\textbf{\ce{Pt\bond{-}CO}} & atop & -1.47 & -1.48\cite{Deshlahra:2012} & -1.39\cite{Kelemen:1979}, -1.43\cite{Ertl:1977}, -1.90\cite{Yeo:1997} \\ |
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& bridge & -1.13 & -1.47\cite{Deshlahra:2012} & \\ |
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& hollow & -1.02 & -1.45\cite{Deshlahra:2012} & \\ |
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\textbf{Pd-CO} & atop & -1.54 & -1.44\cite{Honkala:2001sf} & \\ |
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\textbf{\ce{Pd\bond{-}CO}} & atop & -1.54 & -1.44\cite{Honkala:2001sf} & \\ |
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& bridge & -1.65 & -1.83\cite{Honkala:2001sf} & \\ |
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& hollow & -1.60 & -1.99\cite{Honkala:2001sf} & -1.47\cite{Ertl:1970}, -1.54\cite{Guo:1989} \\ |
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\hline |
| 206 |
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\label{tab:CO_energies} |
| 207 |
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\end{table} |
| 208 |
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|
| 209 |
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This Pd-CO model does not have a strong preference for either the |
| 209 |
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This \ce{Pd\bond{-}CO} model does not have a strong preference for either the |
| 210 |
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bridge or hollow binding sites, so it may overestimate the bridge-site |
| 211 |
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binding at low coverages, but at higher coverages, the situation is |
| 212 |
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somewhat less clear.\cite{Wong:1991ta} Studies using low-energy |
| 213 |
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elecron diffraction (LEED) and C-O stretching frequencies of \ce{CO} |
| 213 |
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elecron diffraction (LEED) and \ce{C\bond{-}O} stretching frequencies of \ce{CO} |
| 214 |
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bound to \ce{Pd}(111) suggest that the 3-fold hollow sites are |
| 215 |
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preferred at low |
| 216 |
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coverages,\cite{Bradshaw:1978uf,Conrad:1978fx,Ohtani:1987zh} where it |
| 221 |
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reported to lie between 1.3 and 1.54 eV. |
| 222 |
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|
| 223 |
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At higher \ce{CO} coverages (e.g. $> 0.5$ ML), the preferred binding |
| 224 |
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of CO on Pd(111) appears to be a $c(4\times2)$ ordered structure with |
| 225 |
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the CO bound to the bridge sites.\cite{Bradshaw:1978uf} |
| 224 |
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of \ce{CO} on \ce{Pd}(111) appears to be a $c(4\times2)$ ordered structure with |
| 225 |
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the \ce{CO} bound to the bridge sites.\cite{Bradshaw:1978uf} |
| 226 |
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|
| 227 |
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Theoretical work by Honkala \textit{et al.}\cite{Honkala:2001sf} using |
| 228 |
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DFT with the generalized gradient approximation (GGA) to describe |
| 229 |
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electron exchange correlation and pseudopotentials for the Pd atoms |
| 229 |
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electron exchange correlation and pseudopotentials for the \ce{Pd} atoms |
| 230 |
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also reported the fcc site as the most favorable binding position with |
| 231 |
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a binding energy of 2.00 eV compared to the bridge site binding |
| 232 |
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energy of 1.83 eV at $1/3$ monolayer. |
| 242 |
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energy difference is 0.06 eV (-1.85 hollow, -1.79 bridge). |
| 243 |
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|
| 244 |
|
Although the weak preference for hollow vs. bridge sites is not |
| 245 |
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captured by the Pd-CO fit, it does represent a significant change from |
| 246 |
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the atop preference of the Pt-CO model. The dynamics of the metal |
| 247 |
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bound to the CO is significantly altered as a result of this |
| 245 |
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captured by the \ce{Pd\bond{-}CO} fit, it does represent a significant change from |
| 246 |
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the atop preference of the \ce{Pt\bond{-}CO} model. The dynamics of the metal |
| 247 |
> |
bound to the \ce{CO} is significantly altered as a result of this |
| 248 |
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difference. |
| 249 |
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|
| 250 |
|
\subsection{557 interfaces and subsurface alloys} |
| 251 |
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The Pd(557) model is an orthorhombic periodic box with dimensions of |
| 251 |
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The \ce{Pd}(557) model is an orthorhombic periodic box with dimensions of |
| 252 |
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$55.09 \times 49.48 \times 120$~\AA~ while the subsurface alloys |
| 253 |
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(Pt(557) surface layers, with Pd bulk) have dimensions of $54.875 |
| 254 |
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\times 49.235 \times 120$~\AA. The Pd system consists of 9 layers of |
| 255 |
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Pd while our subsurface alloys consist of 7 layers of Pd sandwiched |
| 256 |
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between 2 layers of Pt. Both the pure Pd slab and the subsurface |
| 257 |
< |
alloy systems are $\sim$22~\AA~ thick. The lattice constants for Pd |
| 258 |
< |
and Pt, 3.89 and 3.92~\AA, respectively, provide minimal strain energy |
| 254 |
> |
\times 49.235 \times 120$~\AA. The \ce{Pd} system consists of 9 layers of |
| 255 |
> |
\ce{Pd} while our subsurface alloys consist of 7 layers of \ce{Pd} sandwiched |
| 256 |
> |
between 2 layers of \ce{Pt}. Both the pure \ce{Pd} slab and the subsurface |
| 257 |
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alloy systems are $\sim$22~\AA~ thick. The lattice constants for \ce{Pd} |
| 258 |
> |
and \ce{Pt}, 3.89 and 3.92~\AA, respectively, provide minimal strain energy |
| 259 |
|
in the alloy, and the relaxed geometries of the two interfaces are |
| 260 |
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therefore quite similar. |
| 261 |
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|
| 266 |
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|
| 267 |
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Simulations of the metal without any adsorbate present were performed |
| 268 |
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at temperatures ranging from 300 to 900~K to establish the stability |
| 269 |
< |
of the 557 surface without a CO overlayer. The bare systems were run |
| 269 |
> |
of the 557 surface without a \ce{CO} overlayer. The bare systems were run |
| 270 |
|
in the canonical (NVT) ensemble at 850~K for 200 ps and the |
| 271 |
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microcanonical (NVE) ensemble for 1 ns, and displayed no changes in |
| 272 |
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the 557 structure during this period. |
| 273 |
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|
| 274 |
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Ten systems were constructed, corresponding to five CO-coverage levels |
| 275 |
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for each metallic system. The number of CO molecules (0, 48, 240, |
| 274 |
> |
Ten systems were constructed, corresponding to five \ce{CO}-coverage levels |
| 275 |
> |
for each metallic system. The number of \ce{CO} molecules (0, 48, 240, |
| 276 |
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320, and 480) yield surface coverages of 0, 0.05, 0.25, 0.33, and 0.5 |
| 277 |
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monolayers (ML) assuming that every CO adsorbs on the surface. |
| 277 |
> |
monolayers (ML) assuming that every \ce{CO} adsorbs on the surface. |
| 278 |
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|
| 279 |
|
Simulation boxes of the same sizes as the metallic systems were |
| 280 |
< |
constructed with appropriate densities of CO and equilibrated to |
| 281 |
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850~K. The gas-phase CO and surface simulation boxes were then |
| 282 |
< |
combined, using a 5~\AA~ cutoff between metallic atoms and CO to |
| 283 |
< |
prevent overlap. The remaining CO population was further reduced to |
| 280 |
> |
constructed with appropriate densities of \ce{CO} and equilibrated to |
| 281 |
> |
850~K. The gas-phase \ce{CO} and surface simulation boxes were then |
| 282 |
> |
combined, using a 5~\AA~ cutoff between metallic atoms and \ce{CO} to |
| 283 |
> |
prevent overlap. The remaining \ce{CO} population was further reduced to |
| 284 |
|
match the required number for the correct surface coverage. |
| 285 |
|
Velocities were resampled from a Boltzmann distribution, and any net |
| 286 |
|
linear momentum was subtracted from the entire system. The combined |
| 287 |
|
systems were run for 1 ns in the NVT ensemble, before being run in the |
| 288 |
|
NVE ensemble for data collection. |
| 289 |
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|
| 290 |
< |
All of the Pd systems were run in the microcanonical ensemble for a |
| 291 |
< |
minimum of 40 ns to collect statistics. The Pt/Pd subsurface alloy |
| 290 |
> |
All of the \ce{Pd} systems were run in the microcanonical ensemble for a |
| 291 |
> |
minimum of 40 ns to collect statistics. The \ce{Pt/Pd} subsurface alloy |
| 292 |
|
systems, which were observed to undergo significant restructuring, |
| 293 |
|
were each run for a total simulation time of 110 ns. All simulations |
| 294 |
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were carried out with the open source molecular dynamics package, |
| 295 |
|
OpenMD.\cite{openmd,OOPSE} |
| 296 |
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|
| 297 |
|
\section{Results} |
| 298 |
< |
In our earlier work on Pt(557) we observed CO-induced restructuring |
| 299 |
< |
into relatively clean double-layer structures. For the pure Pd(557) |
| 298 |
> |
In our earlier work on \ce{Pt}(557) we observed \ce{CO}-induced restructuring |
| 299 |
> |
into relatively clean double-layer structures. For the pure \ce{Pd}(557) |
| 300 |
|
studied here, the 557 facet retains the plateaus and steps with only |
| 301 |
|
minimal adatom movement, and with almost no surface reconstruction. |
| 302 |
< |
Higher CO coverages appear to have minimal effect on the pure Pd(557) |
| 302 |
> |
Higher \ce{CO} coverages appear to have minimal effect on the pure \ce{Pd}(557) |
| 303 |
|
systems. |
| 304 |
|
|
| 305 |
< |
However, the Pt/Pd subsurface alloy exhibits a CO-induced speedup of |
| 305 |
> |
However, the \ce{Pt/Pd} subsurface alloy exhibits a \ce{CO}-induced speedup of |
| 306 |
|
the diffusion of surface metal atoms, as well as a large-scale |
| 307 |
< |
restructuring of the well-ordered surface into Pt-rich islands, and |
| 307 |
> |
restructuring of the well-ordered surface into \ce{Pt}-rich islands, and |
| 308 |
|
will therefore be the focus of most of our analysis. |
| 309 |
|
|
| 310 |
|
\begin{figure} |
| 311 |
|
\includegraphics[width=\linewidth]{../figures/SystemFigures/systems_ochre2.png} |
| 312 |
|
\caption{Snapshots of the some of the simulated systems. Panel A is the |
| 313 |
< |
pure Pd(557) $\sim$40 ns after being dosed with $\frac{1}{3}$ |
| 314 |
< |
monolayer of CO. Panels B-D are the subsurface alloy 80 ns after |
| 313 |
> |
pure \ce{Pd}(557) $\sim$40 ns after being dosed with $\frac{1}{3}$ |
| 314 |
> |
monolayer of {CO}. Panels B-D are the subsurface alloy 80 ns after |
| 315 |
|
being dosed with 0, $\frac{1}{3}$, and |
| 316 |
< |
$\frac{1}{2}$ ML of CO, respectively. Pt atoms are shown in gray, |
| 317 |
< |
Pd in orange, while the CO molecules are shown in black / red. } |
| 316 |
> |
$\frac{1}{2}$ ML of \ce{CO}, respectively. \ce{Pt} atoms are shown in gray, |
| 317 |
> |
\ce{Pd} in orange, while the \ce{CO} molecules are shown in black / red. } |
| 318 |
|
\label{fig:systems} |
| 319 |
|
\end{figure} |
| 320 |
|
|
| 321 |
|
Figure \ref{fig:systems} shows representative configurations of the |
| 322 |
< |
various systems after significant exposure to the CO. We see that the |
| 322 |
> |
various systems after significant exposure to the \ce{CO}. We see that the |
| 323 |
|
Pd system highlighted in panel A has undergone no surface |
| 324 |
|
restructuring. The other three panels highlight the effect of varying |
| 325 |
< |
CO concentrations on the subsurface alloys, which do exhibit |
| 325 |
> |
\ce{CO} concentrations on the subsurface alloys, which do exhibit |
| 326 |
|
structural reorganization. |
| 327 |
|
|
| 328 |
|
\subsection{Diffusion of Surface Metal Atoms in the Subsurface Alloy} |
| 337 |
|
|
| 338 |
|
However, there is significant movement of surface Pt in the subsurface |
| 339 |
|
alloys, and the mobility of the surface Pt layer increases with |
| 340 |
< |
increasing CO coverage. To estimate the surface diffusion, we define a |
| 340 |
> |
increasing \ce{CO} coverage. To estimate the surface diffusion, we define a |
| 341 |
|
``mobile'' atom as one which moves at least 2~\AA~ in any 10 ps window |
| 342 |
|
during the simulation. Once an atom has been labeled as mobile, we |
| 343 |
|
analyze the entire simulation to find the planar ($xy$) diffusion |
| 344 |
|
constant for the mobile atoms of a particular type. The calculated |
| 345 |
|
diffusion constants of mobile Pt atoms from the subsurface alloys are |
| 346 |
|
shown in Table \ref{tab:diffusion}. The absolute number of mobile Pt |
| 347 |
< |
atoms ($\sim 600$) was similar between all systems, independent of CO |
| 348 |
< |
coverage. There is a correlation between increasing CO coverage and Pt |
| 347 |
> |
atoms ($\sim 600$) was similar between all systems, independent of \ce{CO} |
| 348 |
> |
coverage. There is a correlation between increasing \ce{CO} coverage and Pt |
| 349 |
|
diffusion rates of $\sim 1.6$ \AA\textsuperscript{2}/ns/ML. |
| 350 |
|
|
| 351 |
|
\begin{table} \centering \begin{tabular}{| c | c |} \hline |
| 352 |
< |
CO Coverage & Diffusion Constant\footnotemark[1] (\AA\textsuperscript{2}/ns) \\ |
| 352 |
> |
\ce{CO} Coverage & Diffusion Constant\footnotemark[1] (\AA\textsuperscript{2}/ns) \\ |
| 353 |
|
\hline |
| 354 |
|
0 & 2.779(2) \\ |
| 355 |
|
0.05 & 3.992(6) \\ |
| 357 |
|
0.33 & 4.180(7) \\ |
| 358 |
|
0.50 & 3.935(5) \\ |
| 359 |
|
\hline \end{tabular} |
| 360 |
< |
\caption{Diffusion constants of mobile Pt |
| 360 |
> |
\caption{Diffusion constants of mobile \ce{Pt} |
| 361 |
|
atoms for the subsurface alloys.\label{tab:diffusion}} |
| 362 |
|
|
| 363 |
|
\footnotemark[1]{Uncertainties in the last digit |
| 366 |
|
|
| 367 |
|
\subsection{Island Formation and Clustering in the Subsurface Alloy} |
| 368 |
|
|
| 369 |
< |
In a similar manner to the Pt(557) surfaces, the structural |
| 369 |
> |
In a similar manner to the \ce{Pt} (557) surfaces, the structural |
| 370 |
|
reconstructions that occur for the subsurface alloy are influenced by |
| 371 |
< |
the presence of the CO adsorbate. In Figure \ref{fig:domainAreasPd}, |
| 372 |
< |
the area of exposed Pd increases both over time, and as a function of |
| 373 |
< |
CO coverage. The appearance of the subsurface Pd requires a |
| 374 |
< |
simultaneous reduction in the surface area of the outer Pt skin. Two |
| 375 |
< |
scenarios could explain the reduction of exposed Pt: either the Pt |
| 376 |
< |
atoms are being buried under the Pd bulk, or islands of Pt are forming |
| 377 |
< |
on top of the Pd surface. |
| 371 |
> |
the presence of the \ce{CO} adsorbate. In Figure \ref{fig:domainAreasPd}, |
| 372 |
> |
the area of exposed \ce{Pd} increases both over time, and as a function of |
| 373 |
> |
\ce{CO} coverage. The appearance of the subsurface \ce{Pd} requires a |
| 374 |
> |
simultaneous reduction in the surface area of the outer \ce{Pt} skin. Two |
| 375 |
> |
scenarios could explain the reduction of exposed \ce{Pt}: either the \ce{Pt} |
| 376 |
> |
atoms are being buried under the \ce{Pd} bulk, or islands of \ce{Pt} are forming |
| 377 |
> |
on top of the \ce{Pd} surface. |
| 378 |
|
|
| 379 |
< |
Both mechanisms would explain the decreased Pt surface area (see Fig. |
| 379 |
> |
Both mechanisms would explain the decreased \ce{Pt} surface area (see Fig. |
| 380 |
|
\ref{fig:domainAreasPt}). To discern which of these mechanisms is |
| 381 |
|
taking place, the identity of nearest metal atom neighbors can be |
| 382 |
< |
tabulated. Single-layer Pt skins have atoms with 6 Pt nearest |
| 383 |
< |
neighbors. Islands of Pt require the presence of Pt atoms with 7-9 Pt |
| 382 |
> |
tabulated. Single-layer \ce{Pt} skins have atoms with 6 \ce{Pt} nearest |
| 383 |
> |
neighbors. Islands of \ce{Pt} require the presence of \ce{Pt} atoms with 7-9 \ce{Pt} |
| 384 |
|
nearest neighbors. In figure \ref{fig:nearestNeighbors}, we see an |
| 385 |
< |
increase in Pt population with 9 Pt nearest neighbors along with the |
| 386 |
< |
simultaneous decrease in Pt atoms with only 6 Pt nearest neighbors. |
| 387 |
< |
This is evidence for the formation of multi-layer Pt features since |
| 388 |
< |
single layers of Pt are restricted to having 6 Pt nearest neighbors. |
| 385 |
> |
increase in \ce{Pt} population with 9 \ce{Pt} nearest neighbors along with the |
| 386 |
> |
simultaneous decrease in \ce{Pt} atoms with only 6 \ce{Pt} nearest neighbors. |
| 387 |
> |
This is evidence for the formation of multi-layer \ce{Pt} features since |
| 388 |
> |
single layers of \ce{Pt} are restricted to having 6 \ce{Pt} nearest neighbors. |
| 389 |
|
We note that nearest-neighbor population analysis provides information |
| 390 |
|
similar to the information one might obtain from an XAFS experiment, |
| 391 |
|
which could make this phenomenon experimentally observable. |
| 393 |
|
\begin{figure} |
| 394 |
|
\includegraphics[width=\linewidth]{../figures/domainAreas/domainSize_Pd_110ns_deCluttered_color.pdf} |
| 395 |
|
%\includegraphics[width=\linewidth]{../figures/domainAreas/final_domain_Pd.pdf} |
| 396 |
< |
\caption{Distributions of Pd domain size as a function of time and CO coverage. |
| 396 |
> |
\caption{Distributions of \ce{Pd} domain size as a function of time and \ce{CO} coverage. |
| 397 |
|
Data is averaged over $\sim$20~ns segments to help show progression, |
| 398 |
|
additionally, the data is shown as a percentage of the total surface area of |
| 399 |
< |
the Pt@Pd system with the integration of the curves equaling the percentage |
| 400 |
< |
surface area of Pd, shown in Table \ref{tab:integratedArea}. The presence of CO |
| 401 |
< |
leads to more exposure of the underlying Pd, which is quantified here by an |
| 402 |
< |
increasing number and increasing size of Pd domains. The bare Pt@Pd surface, |
| 399 |
> |
the \ce{Pt/Pd} system with the integration of the curves equaling the percentage |
| 400 |
> |
surface area of \ce{Pd}, shown in Table \ref{tab:integratedArea}. The presence of \ce{CO} |
| 401 |
> |
leads to more exposure of the underlying \ce{Pd}, which is quantified here by an |
| 402 |
> |
increasing number and increasing size of \ce{Pd} domains. The bare \ce{Pt/Pd} surface, |
| 403 |
|
as seen in Figure \ref{fig:systems}.B, undergoes some restructuring, however, the |
| 404 |
|
extent is much less when compared to the 25\% and 50\% monolayer (ML) systems.} |
| 405 |
|
\label{fig:domainAreasPd} |
| 408 |
|
\begin{figure} |
| 409 |
|
\includegraphics[width=\linewidth]{../figures/domainAreas/domainSize_Pt_110ns_deCluttered_color.pdf} |
| 410 |
|
%\includegraphics[width=\linewidth]{../figures/domainAreas/final_domain_Pt.pdf} |
| 411 |
< |
\caption{Distributions of Pt domain size as a function of time and CO coverage. |
| 412 |
< |
Here the presence of CO facilitates the clustering of Pt into smaller domains |
| 413 |
< |
by forming multilayer features which leads to a reduction of Pt surface coverage and concomitant increased exposure of the Pd.} |
| 411 |
> |
\caption{Distributions of \ce{Pt} domain size as a function of time and \ce{CO} coverage. |
| 412 |
> |
Here the presence of \ce{CO} facilitates the clustering of \ce{Pt} into smaller domains |
| 413 |
> |
by forming multilayer features which leads to a reduction of \ce{Pt} surface coverage and concomitant increased exposure of the \ce{Pd}.} |
| 414 |
|
\label{fig:domainAreasPt} |
| 415 |
|
\end{figure} |
| 416 |
|
|
| 417 |
|
|
| 418 |
|
\begin{figure} |
| 419 |
|
\includegraphics[width=\linewidth]{../figures/nearestNeighbor/NearestNeighbor_110ns_color.pdf} |
| 420 |
< |
\caption{Population of Pt atoms with either 6 (solid) or 9 (hollow) |
| 421 |
< |
Pt nearest neighbors averaged over similar blocks of time as in |
| 420 |
> |
\caption{Population of \ce{Pt} atoms with either 6 (solid) or 9 (hollow) |
| 421 |
> |
\ce{Pt} nearest neighbors averaged over similar blocks of time as in |
| 422 |
|
Figure \ref{fig:domainAreasPd} and \ref{fig:domainAreasPt}. At |
| 423 |
< |
time 0, the majority ($\frac{2}{3}$) of Pt is located in the (111) |
| 424 |
< |
plateaus where the number of Pt nearest neighbors is 6. A sizeable |
| 423 |
> |
time 0, the majority ($\frac{2}{3}$) of \ce{Pt} is located in the (111) |
| 424 |
> |
plateaus where the number of \ce{Pt} nearest neighbors is 6. A sizeable |
| 425 |
|
minority ($\frac{1}{3}$) is located at the step edge, or beneath a |
| 426 |
< |
step edge with a nearest neighbor number of 5. The decrease in Pt |
| 427 |
< |
with 6 nearest neighbors, while Pt with 9 nearest neighbors rises |
| 428 |
< |
implies that Pt atoms are being incorporated into multilayer |
| 426 |
> |
step edge with a nearest neighbor number of 5. The decrease in \ce{Pt} |
| 427 |
> |
with 6 nearest neighbors, while \ce{Pt} with 9 nearest neighbors rises |
| 428 |
> |
implies that \ce{Pt} atoms are being incorporated into multilayer |
| 429 |
|
features. } \label{fig:nearestNeighbors} |
| 430 |
|
\end{figure} |
| 431 |
|
|
| 432 |
|
The small amount of restructuring observed in the zero coverage system suggests |
| 433 |
< |
that there are two driving forces for restructuring, with the CO playing one |
| 433 |
> |
that there are two driving forces for restructuring, with the \ce{CO} playing one |
| 434 |
|
role. |
| 435 |
|
|
| 436 |
|
|
| 437 |
|
|
| 438 |
|
\begin{table} |
| 439 |
< |
\caption{Percent Pd surface coverage as a function of time. The following values were obtained by integrating the data in Figure \ref{fig:domainAreasPd}.} |
| 439 |
> |
\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}.} |
| 440 |
|
\begin{tabular}{| c || c | c | c | c | c | c |} |
| 441 |
|
\hline |
| 442 |
|
& 0-18 ns & 19-37 ns & 38-56 ns & 57-75 ns & 76-94 ns & 95-113 ns \\ |
| 455 |
|
\section{Discussion} |
| 456 |
|
|
| 457 |
|
Explaining figure 1: The minor restructuring in B is due to the energy benefit gained when |
| 458 |
< |
Pt maximizes Pt-Pt bonds. (C) and (D) have undergone greater |
| 459 |
< |
remodeling because the presence of CO helps speed up adatom mobility |
| 460 |
< |
and enables the vertical displacement of Pt adatoms leading to more |
| 458 |
> |
\ce{Pt} maximizes \ce{Pt\bond{-}Pt} bonds. (C) and (D) have undergone greater |
| 459 |
> |
remodeling because the presence of \ce{CO} helps speed up adatom mobility |
| 460 |
> |
and enables the vertical displacement of \ce{Pt} adatoms leading to more |
| 461 |
|
clustering. |
| 462 |
|
|
| 463 |
< |
The stronger Pd-CO binding energy when compared to Pt-CO is hypothesized to |
| 463 |
> |
The stronger \ce{Pd\bond{-}CO} binding energy when compared to \ce{Pt\bond{-}CO} is hypothesized to |
| 464 |
|
play a role in disrupting the surface and in the case of the shell system in |
| 465 |
< |
revealing the underlying Pd by causing clustering and island formation of the |
| 466 |
< |
Pt shell. |
| 465 |
> |
revealing the underlying \ce{Pd} by causing clustering and island formation of the |
| 466 |
> |
\ce{Pt} shell. |
| 467 |
|
|
| 468 |
|
\subsection{Diffusion} |
| 469 |
< |
As noted above, their is limited movement of Pd in any of the systems we |
| 470 |
< |
examined. In a few instances, inversion is observed where a Pd and a Pt atom |
| 471 |
< |
are swapped in the shell systems. But on the whole the Pd is overwhelmingly |
| 469 |
> |
As noted above, their is limited movement of \ce{Pd} in any of the systems we |
| 470 |
> |
examined. In a few instances, inversion is observed where a \ce{Pd} and a \ce{Pt} atom |
| 471 |
> |
are swapped in the shell systems. But on the whole the \ce{Pd} is overwhelmingly |
| 472 |
|
stationary. Time scales and kinetic barriers are possible explanations for the |
| 473 |
|
lack of movement, but for the shell systems what seems to be the most likely is |
| 474 |
< |
that the Pt is acting as a protective layer. Even with significant |
| 475 |
< |
restructuring of the Pt overlayer, the underlying Pd is unlikely to be located |
| 474 |
> |
that the \ce{Pt} is acting as a protective layer. Even with significant |
| 475 |
> |
restructuring of the \ce{Pt} overlayer, the underlying \ce{Pd} is unlikely to be located |
| 476 |
|
in a position where an energetically easier break from a step-edge will be |
| 477 |
|
possible. However, this explanation does not explain the stability of the pure |
| 478 |
< |
Pd systems and is an area for further exploration. |
| 478 |
> |
\ce{Pd} systems and is an area for further exploration. |
| 479 |
|
|
| 480 |
< |
An analysis of Pt's perpendicular (across the plateaus) and parallel (along the |
| 480 |
> |
An analysis of \ce{Pt}'s perpendicular (across the plateaus) and parallel (along the |
| 481 |
|
steps) diffusion constants on the various shell systems is shown in the |
| 482 |
|
supporting information. Unlike in our previous work\cite{Michalka:2013}, where |
| 483 |
|
the step-edges were maintained throughout the restructuring of the surface from |
| 489 |
|
previous work which is easily explained by the lower temperature these systems |
| 490 |
|
were run at (850~K compared to 1000~K). While the 5\% data is abnormally high, |
| 491 |
|
the other coverages show a strong correlation of increasing diffusion with |
| 492 |
< |
increasing CO coverage. This correlation likely stems from the same mechanism |
| 493 |
< |
we reported previously, where the presence of CO, coupled with its large |
| 492 |
> |
increasing \ce{CO} coverage. This correlation likely stems from the same mechanism |
| 493 |
> |
we reported previously, where the presence of \ce{CO}, coupled with its large |
| 494 |
|
quadrupolar moment assists in the initial break-up of the step-edges allowing |
| 495 |
< |
for consistent adatom formation. Once the Pt adatoms are formed, the barrier |
| 495 |
> |
for consistent adatom formation. Once the \ce{Pt} adatoms are formed, the barrier |
| 496 |
|
for diffusion is negligible ($<$4 kcal/mol using the EAM forcefield) and the |
| 497 |
|
adatom will continue to diffuse until it is reincorporated, with most diffusion |
| 498 |
< |
occuring along the front of the step edges. Thus, the more CO present on the |
| 498 |
> |
occuring along the front of the step edges. Thus, the more \ce{CO} present on the |
| 499 |
|
surface, the more likely adatoms will form and explore the surface before |
| 500 |
|
reaching a more stable state. |
| 501 |
|
|
| 502 |
|
\subsection{Relative Metallic Binding Energies} |
| 503 |
< |
The presence and amount of CO is one of the driving forces for the observed |
| 503 |
> |
The presence and amount of \ce{CO} is one of the driving forces for the observed |
| 504 |
|
reconstruction, however, this doesn't explain the minor restructuring observed |
| 505 |
< |
for the shell system that had no CO present. Rather, there appears to be two |
| 505 |
> |
for the shell system that had no \ce{CO} present. Rather, there appears to be two |
| 506 |
|
factors that are both responsible for aspects of the restructuring. This other |
| 507 |
< |
driving force is that Pt-Pt interactions are stronger and thus more favored |
| 508 |
< |
when compared to Pt-Pd interactions, as established by the EAM forcefield. |
| 509 |
< |
Removing a Pt surface atom from a (111) plateau on a pure Pt (557) surface, |
| 510 |
< |
shows that the Pt was contributing (-$\infty$ kcal/mol) to the energy of the |
| 511 |
< |
system, while a Pt taken from a similar spot in our shell system was only |
| 512 |
< |
contributing (-$\infty$ kcal/mol). In the first instance, the Pt had 9 |
| 513 |
< |
nearest neighbors, all Pt, while in the second the three atoms underneath the |
| 514 |
< |
surface are now Pd, which contribute a smaller electron density, leading to a |
| 515 |
< |
weaker binding between the Pt and Pd. As Figure \ref{fig:nearestNeighbors} |
| 516 |
< |
shows, over the 110 ns of the simulation, the number of Pt with increasing |
| 517 |
< |
number of Pt-Pt nearest neighbors grows. Thus, the restructuring of the surface |
| 518 |
< |
for the 0\%~coverage system can be explained by the stronger Pt-Pt binding |
| 519 |
< |
interaction, while the presence of CO is what appears to allow or speed up the |
| 507 |
> |
driving force is that \ce{Pt\bond{-}Pt} interactions are stronger and thus more favored |
| 508 |
> |
when compared to \ce{Pt\bond{-}Pd} interactions, as established by the EAM forcefield. |
| 509 |
> |
Removing a \ce{Pt} surface atom from a (111) plateau on a pure \ce{Pt} (557) surface, |
| 510 |
> |
shows that the \ce{Pt} was contributing (-$\infty$ kcal/mol) to the energy of the |
| 511 |
> |
system, while a \ce{Pt} taken from a similar spot in our shell system was only |
| 512 |
> |
contributing (-$\infty$ kcal/mol). In the first instance, the \ce{Pt} had 9 |
| 513 |
> |
nearest neighbors, all \ce{Pt}, while in the second the three atoms underneath the |
| 514 |
> |
surface are now \ce{Pd}, which contribute a smaller electron density, leading to a |
| 515 |
> |
weaker binding between the \ce{Pt} and \ce{Pd}. As Figure \ref{fig:nearestNeighbors} |
| 516 |
> |
shows, over the 110 ns of the simulation, the number of \ce{Pt} with increasing |
| 517 |
> |
number of \ce{Pt\bond{-}Pt} nearest neighbors grows. Thus, the restructuring of the surface |
| 518 |
> |
for the 0\%~coverage system can be explained by the stronger \ce{Pt\bond{-}Pt} binding |
| 519 |
> |
interaction, while the presence of \ce{CO} is what appears to allow or speed up the |
| 520 |
|
mechanism of step traversal, leading to larger scale reconstructions and for |
| 521 |
|
the shell systems, clustering and island formation. |
| 522 |
|
|
| 527 |
|
exposed surfaces were first simplified by projecting the 3-dimensional surface |
| 528 |
|
onto a 2-dimensional grid (with two grids per system to capture the surfaces on |
| 529 |
|
both sides of the system). The grids could only have one of two values at each |
| 530 |
< |
site, Pt or Pd. The resulting Ising-like grids were then deconvoluted into |
| 530 |
> |
site, \ce{Pt} or \ce{Pd}. The resulting Ising-like grids were then deconvoluted into |
| 531 |
|
separate domains based on nearest-neighbor connectivity (up, down, left, right; |
| 532 |
|
corners were not included). The resulting data was aggregated and normalized |
| 533 |
|
and is presented in Figures \ref{fig:domainAreasPd} and |
| 536 |
|
|
| 537 |
|
This analysis allows us to focus on collective motion of the surface atoms as |
| 538 |
|
measured by the domain sizes, rather than individual adatom movement. At the |
| 539 |
< |
beginning of the simulations, the surface layer of Pt makes up one domain of |
| 539 |
> |
beginning of the simulations, the surface layer of \ce{Pt} makes up one domain of |
| 540 |
|
size $\sim$2625~\AA\textsuperscript{2}. This domain begins to shrink relatively |
| 541 |
< |
quickly and is matched by a growth in the number and size of Pd domains. The |
| 542 |
< |
presence of CO in the system allows further clustering |
| 543 |
< |
of the Pt domains, which requires a larger amount of exposed |
| 544 |
< |
Pd of various domain sizes. For clarity purposes, there is a small peak in the |
| 545 |
< |
Pt graphs around 0-100~\AA~that is not shown in Figure \ref{fig:domainAreasPt} |
| 541 |
> |
quickly and is matched by a growth in the number and size of \ce{Pd} domains. The |
| 542 |
> |
presence of \ce{CO} in the system allows further clustering |
| 543 |
> |
of the \ce{Pt} domains, which requires a larger amount of exposed |
| 544 |
> |
\ce{Pd} of various domain sizes. For clarity purposes, there is a small peak in the |
| 545 |
> |
\ce{Pt} graphs around 0-100~\AA~that is not shown in Figure \ref{fig:domainAreasPt} |
| 546 |
|
but can be seen in the supporting information. These data poins arise from 1 to |
| 547 |
< |
2 atom clusters of Pt embedded in the Pd. |
| 547 |
> |
2 atom clusters of \ce{Pt} embedded in the \ce{Pd}. |
| 548 |
|
|
| 549 |
|
The quantification of the surface composition that these figures display is |
| 550 |
|
helpful, but is more easily seen when the curves are integrated, which is shown |
| 553 |
|
|
| 554 |
|
\subsection{Equilibrium state} |
| 555 |
|
As shown in Figure \ref{fig:systems}.B, the 0\% coverage system has undergone a |
| 556 |
< |
small but significant amount of restructuring, despite no CO being present. |
| 557 |
< |
This is due to the stronger Pt-Pt compared to Pt-Pd binding energy. Movement |
| 558 |
< |
of Pt from one layer onto the top of another layer without vertical |
| 559 |
< |
displacement benefits both layers of Pt, and the small energy barrier |
| 556 |
> |
small but significant amount of restructuring, despite no \ce{CO} being present. |
| 557 |
> |
This is due to the stronger \ce{Pt\bond{-}Pt} compared to \ce{Pt\bond{-}Pd} binding energy. Movement |
| 558 |
> |
of \ce{Pt} from one layer onto the top of another layer without vertical |
| 559 |
> |
displacement benefits both layers of \ce{Pt}, and the small energy barrier |
| 560 |
|
preventing it is overcome by the increased thermal motion at elevated |
| 561 |
< |
temperatures. The now underlying Pt has approximately 9 nearest neighbors of Pt |
| 562 |
< |
and 3 of Pd and is essentially in bulk. The upper layer of Pt also benefits |
| 561 |
> |
temperatures. The now underlying \ce{Pt} has approximately 9 nearest neighbors of \ce{Pt} |
| 562 |
> |
and 3 of \ce{Pd} and is essentially in bulk. The upper layer of \ce{Pt} also benefits |
| 563 |
|
because it is now experiencing 9 nearest neighbor interactions, all with other |
| 564 |
< |
Pt. The ideal case would involve the majority of Pt maximizing their Pt-Pt |
| 565 |
< |
interactions which could lead to massive disruption without any need for CO, |
| 564 |
> |
\ce{Pt}. The ideal case would involve the majority of \ce{Pt} maximizing their \ce{Pt\bond{-}Pt} |
| 565 |
> |
interactions which could lead to massive disruption without any need for \ce{CO}, |
| 566 |
|
but as seen in Figure \ref{fig:systems}.B, the (557) crystal facet is still |
| 567 |
< |
present, just with Pt plateaus moved slightly forward and backward. Without the |
| 568 |
< |
presence of CO, very little vertical displacement is observed, which is what is |
| 567 |
> |
present, just with \ce{Pt} plateaus moved slightly forward and backward. Without the |
| 568 |
> |
presence of \ce{CO}, very little vertical displacement is observed, which is what is |
| 569 |
|
hypothesized to facilite the multiple layer features observed in the higher |
| 570 |
|
coverage systems. The systems were run for approximately 110 nanoseconds and |
| 571 |
|
then stopped, primarily because, large scale changes had drastically slowed. |
| 575 |
|
state, at least for the time scales we were able to explore. Increased run time |
| 576 |
|
while possible, was not judged to be useful at this time. |
| 577 |
|
|
| 578 |
< |
\subsection{Role of CO: Presence and Absence} |
| 579 |
< |
As shown in the previous sections, the presence of CO plays a large role in the |
| 580 |
< |
restructuring of the Pt@Pd shell systems. The small amount of restructuring due |
| 581 |
< |
to favorable Pt-Pt interactions is greatly enhanced when CO is added to the |
| 582 |
< |
system. As concluded in our previous paper\cite{Michalka:2013}, CO helps enable |
| 578 |
> |
\subsection{Role of \ce{CO}: Presence and Absence} |
| 579 |
> |
As shown in the previous sections, the presence of \ce{CO} plays a large role in the |
| 580 |
> |
restructuring of the \ce{Pt/Pd} shell systems. The small amount of restructuring due |
| 581 |
> |
to favorable \ce{Pt\bond{-}Pt} interactions is greatly enhanced when \ce{CO} is added to the |
| 582 |
> |
system. As concluded in our previous paper\cite{Michalka:2013}, \ce{CO} helps enable |
| 583 |
|
vertical displacement of adatoms between layers, which is also seen here by |
| 584 |
< |
examining the degree of clustering that occurred for various CO coverages. |
| 584 |
> |
examining the degree of clustering that occurred for various \ce{CO} coverages. |
| 585 |
|
|
| 586 |
|
%One |
| 587 |
|
%final test we performed, already mentioned in Figure \ref{fig:domainAreasNoCO}, |
| 599 |
|
|
| 600 |
|
|
| 601 |
|
\section{Conclusion} |
| 602 |
< |
The favorable Pt-Pt interactions, coupled with the stronger Pd-CO binding |
| 603 |
< |
energy help to explain the clustering seen on the Pt@Pd (557) systems. The lack |
| 604 |
< |
of any surface disruption on the Pd (557) surfaces at all coverages, suggests |
| 605 |
< |
that the presence of CO is not enough of a perturbation to overcome the |
| 602 |
> |
The favorable \ce{Pt\bond{-}Pt} interactions, coupled with the stronger \ce{Pd\bond{-}CO} binding |
| 603 |
> |
energy help to explain the clustering seen on the \ce{Pt/Pd} (557) systems. The lack |
| 604 |
> |
of any surface disruption on the \ce{Pd} (557) surfaces at all coverages, suggests |
| 605 |
> |
that the presence of \ce{CO} is not enough of a perturbation to overcome the |
| 606 |
|
thermodynamic barriers hindering reconstruction. |
| 607 |
|
|
| 608 |
|
This work suggests that bimetallic and subsurface alloys could be tailored to |
