Ab Initio Molecular Dynamics Investigation of the Coadsorption of

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Ab Initio Molecular Dynamics Investigation of the Coadsorption of Acetaldehyde and Hydrogen on a Platinum Nanocluster Angelo Vargas,*,† Gianluca Santarossa,‡ and Alfons Baiker* Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, H€onggerberg, HCI, CH-8093 Zurich, Switzerland ABSTRACT: By means of ab initio molecular dynamics, we have investigated the molecular adsorption of acetaldehyde on Pt 13 nanoparticles in the presence of coadsorbed hydrogen on the surface of the metal particle. The acetaldehyde molecules exclusively interact with low-coordinated metal atoms of the nanoparticle while they remain inert toward direct interaction with adsorbed hydrogen, thus confirming the key role of the metal as intermediate binding site for hydrogenation. At room temperature within a time scale of picoseconds aldehyde
metal bonds are formed. Coadsorbed hydrogen decreases the reactivity of the aldehyde molecules toward the metal. Kinetically, the first adsorption modes to occur are of the η1 type, either via the oxygen or via the carbon atom. A tendency for double adsorption on the same metal site is observed. Upon addition of ammonia molecules to the simulation box, the interesting phenomenon of the conversion of η1 to η2 carbonyl bonding appears, mediated by the adsorption of the ammonia nitrogen to a platinum atom. This investigation highlights the richness of the interaction modes of a carbonyl group with a platinum nanoparticle, reached in the very brief time scale of a few picoseconds. In particular the adsorption modes of the aldehyde are modified by the presence of a second electron-donor molecule, such as another aldehyde molecule or an ammonia molecule, in the latter case even changing the adsorption mode of the carbonyl moiety from η1 to η2.

1. INTRODUCTION The adsorption and reactivity of carbonyl groups on platinum surfaces have been addressed by several experimental and theoretical studies in the past years,1
22 with the aim of achieving an atomistic understanding of the elementary steps involved in catalytic hydrogenation of carbonyl compounds and the reverse process of alcohol dehydrogenation. Considerable effort has been dedicated to the rationalization of the selective hydrogenation between the carbonyl and CdC double bond in unsaturated aldehydes, typically using model low-Miller index surfaces.1,5
9 If the molecule bearing the carbonyl group is a prochiral ketone, and if the platinum surface is modified by a chiral organic molecule, usually an alkaloid of the Cinchona family, the hydrogenation process can also by asymmetric, thus leading to the selective formation of one of the possible enantiomers of the resulting chiral alcohol.23
27 The full process of saturation of carbonyl groups by hydrogen is typically interpreted within the Horiuti-Polianyi mechanism for the hydrogenation of ethylene.28 Such mechanism implies an η2 adsorption step of the carbonyl moiety to the platinum surface, followed by the reduction of the carbonyl group by surface hydrogen atoms. This mechanism has been thoroughly calculated from first principles, on a model Pt(111) slab, in the case of formaldehyde.12 The adsorption modes of different ketones and aldehydes have been analyzed using static calculations, in the attempt to understand the hydrogenation step following the adsorption process.3,7
11 Both η1 and η2 surface configurations have been proposed to be the r 2011 American Chemical Society

reactive surface species for ketones.10,11 Even for simple ketones and aldehydes, first principles calculations of hydrogen transfer are typically preceded by an investigation of the adsorption modes of the molecules, which sets the premises to further reactivity studies. Such investigations make use of models of the surface usually constituted by a platinum slab or nanoparticle reproducing a low-Miller index surface, as for example Pt(111). Real catalysts are composed of nanoparticles that possess a more complex surface structure that can only partially be reproduced by symmetric surfaces.29,30 To increase the understanding of the reaction processes on a catalytic surface, an important step forward consists of the simulation of the chemical interactions directly on the surface of a nanoparticle.31,32 In the present investigation we use ab initio molecular dynamics (MD) to study the adsorption modes of acetaldehyde on a platinum nanoparticle in the presence of coadsorbed hydrogen. We addressed the problem of the reactivity of the platinum nanoparticle toward acetaldehyde by changing the amount of coadsorbed hydrogen and by addition of ammonia in the simulation supercell. The addition of ammonia is used to simulate the effect of the coadsorption of an amine on the adsorption process of the carbonyl compound. The interest for amines originates from the fact that currently used chiral surface modifiers for asymmetric Received: January 7, 2011 Revised: March 16, 2011 Published: May 09, 2011 10661

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hydrogenation of carbonyl groups are alkaloids of the Cinchona family,23
26 and it has been previously shown that amino groups have critical influence on the reaction rate of carbonyl group hydrogenation and therefore on the formation of the activated complex on a metal surface.33 In the real asymmetric catalytic systems tertiary amines such as quinuclidine or other complex molecules bearing tertiary amines are typically used, but in the present investigation, simple ammonia has been utilized in order to reduce the complexity of the system. It has also been previously shown by means of ab initio molecular dynamics that for entropic reasons during a simulation tertiary amines tend to interact with the platinum surface of a nanoparticle mainly through the alkyl chain, therefore simulations based on tertiary amines for observing the interaction between nitrogen and the metal surface would require too long a simulation time.31 On the other hand, simple ammonia makes nitrogen available in the system without interference of the alkyl side chains and has no entropic hindrance due to the alkyl groups.

2. METHODS 2.1. Computational Details. Gaussian and Plane Wave (GPW) formalism34,35 as implemented in the CP2K code36 has been used for the Born
Oppenheimer molecular dynamics (BOMD) simulations. We use the latest implementation of the GPW formalism in the Quickstep module of the CP2K program package, a suite of programs aimed at performing efficient electronic structure calculations and molecular dynamics at different levels of theory.37 The Kohn
Sham orbitals are expanded in terms of contracted Gaussian type orbitals (GTO)

ψi ðrÞ ¼

∑R CRi jR ðrÞ

ð1Þ

where ψi is the molecular orbital corresponding to the ith Kohn
Sham state, {jR} are the basis set functions, and {CRi} the expansion coefficients. The auxiliary PW basis set is, instead, used only to expand the electronic charge density for the calculation of the Hartree potential. In order to limit the number of PW basis functions, the interaction of the valence electrons with frozen atomic cores is described using norm-conserving, dual-space type pseudopotentials (PP).38 For Pt atoms we used a PP including all the electrons up to the 5s levels in the core, thus treating the 18 electrons, corresponding to the 5p and 5d, explicitly in the valence. The GTO basis set adopted are optimized for the specific pseudopotentials based on the Goedecker
Teter
Hutter (GTH) method. We used triple-ζ valence (TZV) basis set for the metal and TZV2P for the lighter elements. For the auxiliary PW expansion of the charge density, the energy cutoff has been set at 300 Ry. The exchange and correlation term was modeled using the Perdew
Burke
Ernzerhof functional.39 For the solution of the SCF equations, we used an optimizer based on orbital transformations, which scales linearly in the number of basis functions.40 It has been already demonstrated that this optimization algorithm, in combination with the GPW linear scaling calculation of the Kohn
Sham matrix, can be used for applications with several thousands of basis functions.41 A time step of 0.5 fs and a wave function convergence of 10
5 guarantee energy conservation during the dynamics. The sampling of the canonical ensemble at 300 K is obtained by coupling to a thermostat using the Canonical Sampling through Velocity Rescaling (CSVR) scheme42 with a time constant of 100 fs.

Geometry optimizations of frames isolated from the MD trajectories have been carried out using the Broyden
Fletcher
Goldfarb
Shanno minimization algorithm (BFGS),43
47 and the structures have been optimized until the atomic displacements were lower than 3  10
3 Bohr and the forces lower than 4.5  10
4 Ha/Bohr. For the optimized configurations, the charges have been calculated using the scheme suggested by L€owdin.48 2.2. Description of the Supercells. We started from a square supercell with a side of 22 Å with a Pt 13 nanoparticle in its optimal configuration32 placed at its center and surrounded the metal with 126 H2 molecules. The MD simulation of this system at room temperature allowed the hydrogen atoms to rapidly adsorb on the metal surface. By stopping the simulation when only 6 hydrogen atoms were adsorbed on the surface of the nanoparticle, we obtained an unsaturated nanoparticle model. By letting the system evolve until equilibrium had been reached, we obtained a saturation of the platinum nanoparticle. Under the chosen conditions, equilibrium was reached after 3 ps, when 28 hydrogen atoms adsorbed on the cluster. The simulation was run for additional 60 ps, during which no relevant changes took place. Therefore, the nanoparticle with 28 hydrogen atoms adsorbed at the surface is considered as fully saturated. From this initial simulation, three systems have been set up for further investigation. The first simulation was run by including in the simulation box 30 molecules of acetaldehyde and the hydrogen-saturated platinum nanoparticle, with 28 hydrogen atoms adsorbed on the metal suface. In the second simulation 30 acetaldehyde molecules were added in the simulation box to the unsaturated platinum nanoparticle, with only 6 H atoms adsorbed on its surface. A third simulation was run by using the surface unsaturated platinum nanoparticle, including in the simulation box 27 acetaldehyde molecules and 10 ammonia molecules.

3. RESULTS AND DISCUSSION 3.1. Acetaldehyde and Hydrogen Saturated Platinum Nanoparticle. The first MD simulation was performed using a

platinum nanoparticle, the surface of which was saturated by 28 hydrogen atoms. The saturated cluster was generated through the MD simulation of the Pt13 cluster in the presence of an excess of H2 molecules at high pressure and at 300 K as described in the previous section. The supercell representing the equilibrated system in its starting configuration is shown in Figure 1A. Since the hydrogen atoms are chemisorbed on the metal cluster, this condition is stable, and the adsorbed atoms cannot desorb in the form of molecular hydrogen even under vacuum at room temperature. Under these conditions the metal atoms are highly, although not completely, coordinated. Both highly coordinated (4
5 hydrogen atoms per metal atom) and low coordinated (0
2 hydrogen atoms per metal atom) platinum sites coexist in the cluster during the entire length of the simulation. The highly coordinated platinum atoms are completely saturated and inert toward the reactants, whereas the low-coordinated metal sites are not entirely saturated and are more reactive to the presence of substrate molecules. The simulation was run for 70 ps in order to observe the interactions between the aldehyde and the nanoparticle. During the simulation two adsorption events have been isolated: (i) the interaction, illustrated in Figure 1B, between the oxygen of one of the aldehyde molecules with a low-coordinated platinum atom (Ads. mode 1), and (ii) the interaction, illustrated 10662

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Figure 2. Time evolution of the distances between the interacting aldehydes and the metal nanoparticle saturated with 28 H atoms. (A) Distances between the Pt atom of the nanoparticle and the O (thick line) and C (thin line) atoms of the acetaldehyde molecule interacting through the carbonyl oxygen; (B) distances between the Pt atom of the nanoparticle and the H (thick line) and C (thin line) atoms of the acetaldehyde molecule interacting through the aldehyde hydrogen. The dotted vertical line indicates the point chosen for the snapshots of Figure 1 and the values of Table 1. Between 15 and 70 ps, the values were constant and are therefore not reported.

Figure 1. (A) Simulation cell with hydrogen saturated Pt13 nanoparticle and 30 acetaldehyde molecules; (B) η1 interaction between one acetaldehyde molecule and the metal through the carbonyl oxygen (Ads. mode 1); (C) η1 interaction between one acetaldehyde molecule and the metal through the aldehyde hydrogen (Ads. mode 2). Note that hydrogen atoms are represented in yellow for better visibility.

in Figure 1C, between the aldehyde hydrogen and another low coordinated platinum atom (Ads. mode 2). The adsorption modes were η1 in both cases. The evolution of the interactions in time are measured by the distance between the bonded atoms of the adsorbed acetaldehyde molecules and the interacting metal atom of the nanoparticle (Figures 2, 4, and 6). The graphs are cut after 15 ps to focus on the most relevant events occurring during the first stage of the simulation. After 15 ps, no new events could be observed. The results for Ads. modes 1 and 2 are shown in Figure 2. Figure 2A shows the evolution of the aldehyde interaction with the metal through the carbonyl oxygen of Figure 1B (Ads. mode 1). At the beginning of the simulation, the acetaldehyde molecule was at 3.7 Å from the metal nanoparticle. After 1 ps the molecule was bound to the Pt nanoparticle through its oxygen atom, at an average Pt
O distance of 2.2 Å. This is an indication of the formation of a chemisorption event. The constant value of the

Table 1. Changes in the L€ owdin Charges Due to the Formation of a Bonda Ads. mode

Pt

O
C

1


0.06

0.25

2


0.06

3 4


0.00 0.15

5


0.08

H
C

C

1.5
0.04

5.0
0.10
0.22

0.00

time (ps)

2.8 4.5 14.0

a

The charges before the interaction always refer to the starting frame of the dynamics, whereas the times of the frames for the interactions are reported in the table. The atoms involved in the interaction are the metal atom (Pt) and the aldehyde atoms directly involved in the interaction (H
C, O
C, or C), which depend on the adsorption mode.

C
Pt distance shown in Figure 2A (3.2 Å) indicates a stable η1 adsorption mode. The bonds were constant throughout the whole duration of the simulation (70 ps). Table 1 reports the changes in L€owdin charges of the atoms involved in the bond due to the interaction. They are calculated as the difference between the electronic fraction of charges before and after the bonding event. Negative values indicate electron enrichment of the atoms due to the interaction, whereas positive values indicate depletion of the atom charge due to the bond formation. The charges before the interaction always refer to the 10663

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The Journal of Physical Chemistry C starting frame of the dynamics, whereas the times of the frames for the interactions differ from each other and are reported in Table 1. In the case of Ads. mode 1, the oxygen and carbon atoms of the aldehyde donate electron density to the metal, confirming the presence of a chemisorption event. This fact is reflected in the negative value of the Pt charge (
0.06) and the positive value of the O and C charges (þ0.25). The charge depletion from the aldehyde does not transfer only to the Pt atom directly involved in the interaction; it is distributed to the neighboring metal atoms. Ads. mode 2, characterized by the interaction between the nanoparticle and the acetaldehyde molecule through its aldehyde hydrogen, is shown in Figure 2B. At the beginning of the simulation, the aldehyde was at 4.0 Å from the nanoparticle and did not experience any attraction by the metal. During the first picoseconds of dynamics, the aldehyde molecule could move away from the nanoparticle, reaching a distance of 6.2 Å, and could freely rotate around its center of mass. At ca. 2 ps of simulation it started moving toward the metal particle and suddenly bound to the metal nanoparticle through its hydrogen atom. After the interaction, the average distance between the aldehyde hydrogen and the Pt atom was ca. 1.9 Å, which is an indication of the formation of a chemical bond. The interaction was stable during the entire trajectory. As for Ads. mode 1, after the bonding event the C
Pt distance was constantly at 3.0 Å, indicating that the molecule was rigidly kept on the surface of the nanoparticle and was not allowed to evolve toward different adsorption modes. The charge transfer for Ads. mode 2 (Table 1) shows an electron enrichment of the whole set of atoms involved (
0.06 for Pt and
0.04 for H and C). This is an indication of a back-donation interaction from the metal to the aldehyde molecule. The electron density comes from different sites of the metal nanoparticle and the negative charge difference on the Pt atom can be explained by a partial electron donation from the aldehyde to the metal. The stability of these interactions during all of the trajectory showed that at room temperature, once bound, the molecule tends to stay coordinated. This simulation evidenced that a saturated platinum nanoparticle seems to be inert toward interaction with a carbonyl group. The keto group of the acetaldehyde keeps a high distance to adsorbed hydrogen and within the simulation time sampled cannot displace surface hydrogen. The chemisorption can only occur on the low-coordinated Pt atoms of the cluster, which are not completely saturated by the hydrogen. 3.2. Acetaldehyde and Platinum Nanoparticle with Six Adsorbed Hydrogen Atoms. A second simulation was run in the presence of a platinum nanoparticle bearing only 6 adsorbed hydrogen atoms (Figure 3). In this case, the platinum atoms are low coordinated and never completely saturated by the hydrogen atoms. Ca. 70 ps of simulation were performed, resulting in a richer sequence of adsorption events with respect to the case where the metal was saturated with hydrogen. In fact the two adsorption modes shown in Figure 1, panels B and C, were found, along with two other modes that are shown in Figure 3, panels A and B. Figure 3A shows an adsorption mode where the methyl group of acetaldehyde interacts with the surface and loses a hydrogen that passes on the metal (Ads. mode 3). A nearby second molecule of aldehyde then binds to the same metal center, giving rise to a double interaction mode. Figure 4A shows the time evolution of this chemisorption event. Initially, the interaction between the aldehyde and the metal occurred through the methyl hydrogen, which formed a bond at ca. 2.1 Å for the first 1.9 ps of MD. Suddenly, the Pt
H distance

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Figure 3. New interactions occurring between acetaldehyde and Pt13 nanoparticle when only 6 hydrogen atoms are adsorbed on the metal: (A) the methyl group of the aldehyde loses an hydrogen to the metal, binds to a Pt atom, and promotes the coordination on the same site of another acetaldehyde in η1 mode (Ads. mode 3); (B) the aldehyde loses the hydrogen to the metal and binds to it via the carbon (Ads. mode 4); (C) a single metal atom coordinates two molecules of acetaldehyde in η1 mode.

dropped to 1.5 Å, and the hydrogen atom detached from the aldehyde and moved to the metal nanoparticle. After the breakage, the hydrogen atom was undistinguishable from the other H atoms adsorbed on the nanoparticle. This hydrogen atom was adsorbed on the nanoparticle for the rest of the simulation, but it was free to hop from one Pt site to another, as indicated by a distance from the Pt atom larger than 3.0 Å (Figure 3A). At the same time of the H
C breaking, the methyl carbon atom of the aldehyde interacted with the metal nanoparticle, forming a stable bond (C
Pt distance of 2.2 Å) through the whole simulation. The charge differences shown in Table 1 for Ads. mode 3 indicate that, in analogy with Ads. mode 2, the bond is characterized by back-donation from the metal to the acetaldehyde. The C atom of the aldehyde, which is involved in the new bond, becomes negatively charged (
0.10). It seems that the electronic charge does not come only from the interacting Pt atom (charge difference 0.00 on the directly bound Pt atom), but comes from parts of the metal nanoparticle distant from the reaction event, confirming the presence of free electrons on the metal particle. The second new adsorption mode, Ads. mode 4, is shown in Figure 3B. In this case, the initial interaction occurs between the aldehydic hydrogen of the molecule and a platinum atom of the 10664

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Figure 5. New interactions occurring in the presence of ammonia: (A) NH3 molecules bind to the metal, and form intermolecular hydrogen bonds; (B) the adsorption of NH3 to a platinum atom activates the formation of an η2 binding mode of the carbonyl group of the aldehyde (Ads. mode 5). Figure 4. Time evolution of the distances between the interacting aldehydes and the unsaturated metal nanoparticle with 6 H atoms. (A) Distances between the Pt atom of the nanoparticle and the H (thick line) and C (thin line) atoms of the acetaldehyde molecule interacting through the methyl group; (B) distances between the Pt atom of the nanoparticle and the H (thick line) and C (thin line) atoms of the acetaldehyde molecule interacting through the aldehydic hydrogen. The dotted vertical line indicates the point chosen for the snapshots of Figure 3 and the values of Table 1. Between 15 and 70 ps, the values were constant and are therefore not reported.

cluster, in the same fashion as Ads. mode 2 seen in Figure 1C. Later, the molecule lost the aldehydic group hydrogen to the metal and could bind to the platinum surface directly with its carbon atom. The time evolution of this interaction is shown in Figure 4B. The interaction took place after 0.5
1.0 ps of simulation. For the following 3 ps the aldehyde interacted with the metal through its hydrogen atom. The H
Pt distance (2.0 Å) and C
Pt distance (3.0 Å) are in agreement with the values shown in Figure 2B. After 4 ps of simulation, the hydrogen detached from the aldehyde and adsorbed to the metal. This is evidenced in the graph of Figure 4B by the increased oscillations of the Pt
H bond. Compared to the interaction discussed in Figure 4A, the H atom did not hop to a far Pt atom. This might be due to local potential of the Pt atoms of the nanoparticle, keeping the H atom in its vicinity. As in the case of Ads. mode 3, again its behavior was undistinguishable compared to any other adsorbed H atom on the surface. During the reaction event, while the H atom moved to the surface, a new bond was formed between the C of the aldehyde and the Pt of the nanoparticle. For the rest of the simulation, the C
Pt distance was constantly at ca. 2.0 Å. For Ads. mode 4, the changes of the charge due to the interaction evidence a strong back-donation from the metal to the aldehyde (Table 1). The C atom acquires a large fraction of charge (
0.22), which is mainly donated by the interaction with the directly interacting Pt atom (0.15). No evidence of electron donation from the aldehyde molecule to the cluster could be identified. It is

worth mentioning that the charge transfers between Ads. modes 3 and 4 differ. While the former is characterized by a combination of electron donation and back-donation, the latter is dictated mainly by the back-donation from the interacting platinum atom to the aldehyde. These differences are an indication of different reaction processes occurring at the surface. Finally another adsorption mode occurred in the same time scale as the others (Ads. modes 5). This is shown in Figure 3C and is constituted by a double η1 interaction of the carbonyl oxygen atom of two acetaldehyde molecules with the same platinum center. The characteristics of this adsorption mode do not differ from the single adsorption events already described in Figures 1B and 2A; therefore, they will not be repeated. In the presence of free, unsaturated platinum surface sites, the aldehyde increased its interaction modes with the metal, in particular (i) losing hydrogen atoms to the platinum, therefore decomposing its structure, and (ii) giving rise to complex adsorption modes where two aldehyde molecules interact with the same platinum atom. The adsorption of the aldehyde was in all cases of the η1 type. In particular such double interaction modes show that given the availability of platinum sites, a first interaction activates the metal center toward a second one. Also during this simulation no interaction occurred between acetaldehyde and adsorbed hydrogen. 3.3. Acetaldehyde and Platinum Nanoparticle with Six Adsorbed Hydrogen in the Presence of Ammonia. A third simulation was run, as for the previous one in the presence of the Pt13 nanoparticle with only 6 adsorbed hydrogen atoms but reducing the number of acetaldehyde molecules to 27 and adding 10 ammonia (NH3) molecules in the simulation box. The simulation was run for 60 ps, during which analogous interactions as in the previous simulations occurred, along with two new ones (Figure 5). The ammonia molecules interacted with the unsaturated atoms of the platinum cluster, as shown in Figure 5A, and gave rise to binding interactions among themselves through 10665

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Figure 6. Time evolution of the distances between the interacting aldehyde and the unsaturated metal nanoparticle with 6 H atoms, in the presence of ammonia. The graph shows the distances between the Pt atom of the nanoparticle and the O (thick line) and C (thin line) atoms of the acetaldehyde molecule interacting through the aldehydic group. The dotted vertical line indicates the point chosen for the snapshot of Figure 5B and the values of Table 1. Between 15 and 70 ps, the values were constant and are therefore not reported.

hydrogen bonding. In particular while hydrogen bonding interactions between NH3 molecules which were not bound to the surface occurred and disappeared within the ps time scale due to the Brownian motion of the molecules at the simulation temperature of 300 K, the intermolecular interactions between ammonia molecules where one NH3 was bound to the metal were persistent (Figure 5A). Interestingly, the interaction of ammonia with the platinum nanoparticle activated the adsorption on the same site of an acetaldehyde molecule (Figure 5B), in a similar fashion as previously observed for two molecules of aldehyde. In the present case the conversion from η1 to η2 adsorption mode was observed (Ads. mode 5), albeit with a single platinum center. The losing of planarity (rehybridization) of the carbonyl double bond can be seen in Figure 5B. The time evolution of Ads. mode 5 is shown in Figure 6. The interaction between the oxygen atom and the metal nanoparticle was constant through the whole duration of the simulation. During the first part of the MD, the interaction was analogous to Ads. mode 1 (Figure 2A), corresponding to the η1 adsorption mode. The Pt
O distance is ca. 2.2 Å, and the Pt
C distance is 3.2 Å. After 4 ps of simulation, the adsorption mode switched from η1 to η2, as confirmed by the lowering of the Pt
C distance from ca. 3.0 to ca. 2.2 Å. The O
Pt interaction was only marginally affected by this change. Note that the changes in the electronic charge were minor in the case of Ads. mode 5 (Table 1). The C and O atoms do not show any electronic change compared to the unbound case, while the donation to the Pt atom in the η2 adsorption mode (
0.08) is comparable to the η1 adsorption of Ads. mode 1 (
0.06). The additional electron density stabilizing the η2 adsorption mode with respect to the η1 is due to the presence of the electron-rich ammonia molecule. Interacting with the particle, the amine is partially deprived of electron density (þ0.15, not shown in Table 1) in favor of the metal. The negative charge of the platinum atoms is partially transferred to the aldehyde molecule, thus counterbalancing the molecule-to-particle donation evidenced for the η1 adsorption mode and favoring the η2 adsorption.

4. CONCLUSIONS The adsorption modes of acetaldehyde on a Pt13 nanoparticle bearing adsorbed hydrogen have been investigated. In none of

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the simulations the aldehyde interacted with adsorbed hydrogen. The hydrogen atoms seem to have a shielding effect on the platinum nanoparticle preventing the interaction with the aldehyde. While only two weak bonds between the aldehyde and the metal occur when the Pt13 nanoparticle is saturated with hydrogen, in the presence of less adsorbed hydrogen more binding events take place, among which loss of hydrogen from the aldehyde to the platinum nanoparticle, and double interactions on the same metal site. Interactions of acetaldehyde with the metal are of the η1 type. When ammonia is added to the simulation cell, the binding of NH3 to the metal promotes also an η2 binding mode of the carbonyl to the metal, albeit with a single metal center. The changes in the electronic charges confirm the presence of chemisorption events on the surface, characterized by molecule-to-metal donation and metal-to-molecule backdonation. Each adsorption mode exhibits a different adsorption behavior, in terms of the donation/back-donation ratio. The present investigation shows that the complexity of the interactions between a simple aldehyde and a platinum nanoparticle increases in comparison to the case of symmetric surfaces. The reactive metal sites are invariably low coordination sites of the nanoparticle, to a large extent deviating from the bulk surface structure of symmetric surfaces. The presence of an electron donor molecule such as another aldehyde molecule or an amine seem to play a role in activating single metal sites and in defining the adsorption structure and therefore also its reactivity. Such single-site double interactions occur in a very short time scale, and are likely to be critical for the chemical events following the adsorption, such as hydrogenation processes.

’ AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]; [email protected]. Tel: þ41 44 6323153. Fax: þ41 44 6321163. Present Addresses †

BASF Construction Chemicals Europe Ltd., 8048 Zurich, Switzerland. ‡ Institute of Process Engineering, Department of Mechanical and Process Engineering, ETH Zurich, CH-8092 Zurich, Switzerland.

’ ACKNOWLEDGMENT Financial support from the Swiss National Science Foundation is kindly acknowledged. The Swiss National Supercomputing Centre (CSCS, Manno) and ETHZ are acknowledged for providing computing resources. ’ REFERENCES (1) Haubrich, J.; Loffreda, D.; Delbecq, F.; Sautet, P.; Jugnet, Y.; Krupski, A.; Becker, C.; Wandelt, K. J. Phys. Chem. C 2008, 112, 3701– 3718. (2) Laliberte, M. A.; Lavoie, S.; Hammer, B.; Mahieu, G.; McBreen, P. H. J. Am. Chem. Soc. 2008, 130, 5386. (3) Vargas, A.; Reimann, S.; Diezi, S.; Mallat, T.; Baiker, A. J. Mol. Catal. A-Chem. 2008, 282, 1–8. (4) Bako, I.; Palinkas, G. Surf. Sci. 2006, 600, 3809–3814. (5) Loffreda, D.; Delbecq, F.; Vigne, F.; Sautet, P. Angew. Chem., Int. Ed. 2005, 44, 5279–5282. (6) Loffreda, D.; Delbecq, F.; Vigne, F.; Sautet, P. J. Am. Chem. Soc. 2006, 128, 1316–1323. 10666

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dx.doi.org/10.1021/jp200179g |J. Phys. Chem. C 2011, 115, 10661–10667