Structural Rearrangement of Au–Pd Nanoparticles under Reaction

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Structural Rearrangement of Au−Pd Nanoparticles under Reaction Conditions: An ab Initio Molecular Dynamics Study Cong-Qiao Xu,†,‡ Mal-Soon Lee,*,‡ Yang-Gang Wang,*,‡,# David C. Cantu,‡ Jun Li,*,†,§ Vassiliki-Alexandra Glezakou,‡ and Roger Rousseau‡ †

Department of Chemistry, Tsinghua University, Beijing 100084, China Institute for Interfacial Catalysis and §William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States

‡

S Supporting Information *

ABSTRACT: The structure, composition, and atomic distribution of nanoalloys under operating conditions are of significant importance for their catalytic activity. In the present work, we use ab initio molecular dynamics simulations to understand the structural behavior of Au−Pd nanoalloys supported on rutile TiO2 under different conditions. We find that the Au−Pd structure is strongly dependent on the redox properties of the support, originating from strong metal−support interactions. Under reducing conditions, Pd atoms are inclined to move toward the metal/oxide interface, as indicated by a significant increase of Pd−Ti bonds. This could be attributed to the charge localization at the interface that leads to Coulomb attractions to positively charged Pd atoms. In contrast, under oxidizing conditions, Pd atoms would rather stay inside or on the exterior of the nanoparticle. Moreover, Pd atoms on the alloy surface can be stabilized by hydrogen adsorption, forming Pd−H bonds, which are stronger than Au−H bonds. Our work offers critical insights into the structure and redox properties of Au−Pd nanoalloy catalysts under working conditions. KEYWORDS: Au−Pd nanoalloy, TiO2, ab initio molecular dynamics, redox property, charge transfer

F

However, little is known on how the elements are structured in bimetallic NPs under catalytic operating conditions. Thus, determining the structural properties of a prototypical bimetallic Au−Pd NP under operating conditions is the key to understanding and controlling their catalytic function. Previous studies have indicated that a truncated octahedron (TO) with local fcc packing was the global minimum structure for an ordered Au38 NP,44−49 while a structure having two Au adatoms on the Au4@Au32 core−shell was later found to be lower in energy, similar to a structure determined for Au40.50 To study the structure of bimetallic Au−Pd NPs, a 38-atom cluster, Au32Pd6, is chosen in this work as a model NP to represent an Au-rich alloy NP of roughly 1 nm in diameter, yet be tractable enough for large-scale ab initio molecular dynamics (AIMD) simulations. The NP structure depends on its electronic structure and can be affected by various factors.51

inite-size nanoparticles have been widely studied in the past decades because of their distinct properties from bulk materials.1−3 Au nanoparticles possess remarkable electronic and optical properties and are particularly active in various catalytic reactions such as CO oxidation.4−8 Bimetallic alloy clusters have exhibited enhanced activity and selectivity compared with monometallic ones due to their well-known catalytic, optical, and electronic properties.9−15 Among the binary nanoalloys, gold−palladium (Au−Pd) nanoparticles (NPs) have attracted considerable attention because of their superior performance in various catalytic reactions, such as synthesis of H2O2 from H2 and O2;16−22 oxidation of CO, C− H bonds, methane, and alcohols;10,23−30 vinyl acetate synthesis;31,32 dehydration of formic acid;33 and N2O decomposition;34 as well as other functions.35−41 The structure of binary NPs, or clusters, can be quite different from singlecomponent ones. Their atomic properties and functionalities are different on the surface, in the core, or at the interface with a surface.42 Their catalytic activity will depend on the surface structure, composition, and distribution of the atomic species.43 © 2017 American Chemical Society

Received: November 3, 2016 Accepted: January 25, 2017 Published: January 25, 2017 1649

DOI: 10.1021/acsnano.6b07409 ACS Nano 2017, 11, 1649−1658

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Figure 1. Optimized structures following AIMD simulations for Au32Pd6-4H, Au32Pd6/TiO2−x, Au32Pd6-4H/TiO2−x, Au32Pd6/TiO2, Au32Pd6/ 2Oad-TiO2−x, and Au32Pd6-4H/2Oad-TiO2−x. Color code: Pd (blue), Au (yellow), Ti (gray), lattice O (red), Oad (green), and H (white).

emerge onto the surface at elevated temperatures. Adsorbed H atoms on the alloy can also alter the structures because of bonding competition between Pd−H and Au−H. Also, Pd and Au atoms present different distributions at the interface depending on the redox state of the support.

Previous works have shown that bimetallic NP restructuring may occur in the presence of NO, CO, or H2, as well as at different temperatures.52−55 Hydrogen chemisorption on Au− Pd clusters affects their structure and stability.56−58 However, to our knowledge, these factors have not been studied systematically. Support properties also play an important role in NP59−61 and single-atom62 properties. For example, TiO2 supports are ubiquitously employed in catalysis and have electronic properties that have been thoroughly studied.63−71 As a reducible metal oxide, the existence of oxygen vacancy defects affects the interactions between the support and clusters.62,72−77 Bonding patterns, catalytic activities, and mechanisms can be quite different when supports are reduced, hydroxylated, or oxidized.61,78 Herein, TiO2 under reduced and oxidized conditions was chosen to investigate the support effects on the structure dynamics and charge states of supported Au32Pd6 NPs. In this work, the effects of support material redox state, hydrogen and oxygen adsorption, and temperature on the structure of Au−Pd NPs are investigated with AIMD simulations. To address this, the following model systems are considered: (1) the gas phase cluster, Au32Pd6; (2) a gas-phase cluster in the presence of H2, Au32Pd6-4H; (3) a reduced TiO2(110)-supported cluster, Au32Pd6/TiO2−x, with a single surface oxygen vacancy (VO) leading to a 5% coverage similar to the experimentally observed 5−8% oxygen vacancies; (4) a Au32Pd6-4H cluster on the reduced TiO2 surface, Au32Pd6-4H/ TiO2−x; (5) Au−Pd NPs on a stoichiometric TiO2 surface, Au32Pd6/TiO2; (6) Au−Pd NPs on an oxidized TiO2 surface with one VO and two extra O atoms (Oad), Au32Pd6/2OadTiO2−x; and (7) a Au32Pd6-4H cluster on an oxidized TiO2 surface with one VO and two extra O atoms, Au32Pd6-4H/2OadTiO2−x. To model support oxidation, one O2 molecule was placed on the TiO2−x surface, which easily dissociated into two O atoms, 2Oad-TiO2−x. The oxygen atoms became strongly bound to Ti atoms with negligible Oad−Oad interactions. Simulations show that Au−Pd nanoalloy structures depend strongly on the reaction conditions. At low temperatures Pd atoms prefer to remain inside the core of the cluster, but they

RESULTS AND DISCUSSION Au32Pd6 Structures in the Gas Phase at 0 K. The TO structure of the ordered Au38 can be viewed as a Au6 octahedral core surrounded by a Au32 shell, Au6@Au32, with eight equivalent hexagonal fcc(111)-like faces, as shown in Figure S1A. There are three possible Pd doping sites: (a) Pdcore, with the Pd atom in the octahedral core; (b) Pdcent, with the Pd located at the centroid of the surface fcc(111) facet; (c) Pdhex, where Pd lies at the vertex of the hexagonal faces. Figures S1B− F show the relative energies with respect to the lowest energy structure of several isomers of the Au32Pd6 cluster. It reveals that the Pd6@Au32 core−shell structure with Oh symmetry is the most stable, in accord with previous work.49,79−86 In addition, we see that the energy of the cluster increases as Pd atoms are closer to the surface. It is energetically favored for Pd atoms to reside in core sites, potentially to reduce the elastic strain on the cluster due to the smaller size of Pd atoms compared to Au. Among the structures with Pd atoms closer to the surface (Figures S1C−G), Pd atoms are preferentially located at the centroid site of the (111) facet (positions b, b′) rather than the hexagonal site at the (100) facet (positions c, c′, c″), in agreement with previous results of Yuan et al.87 The positional preference of Pd atoms, Pdcore > Pdcent > Pdhex, is consistent with the fact that the Pd−Au bond energy is larger than that of Pd−Pd and Au−Au.84,88 Indeed, on the fcc(111) facet a larger number of Pd−Au bonds are formed with Pdcent, which gives rise to a larger binding energy. The stability trend of centb > centb′ and hexc > hexc′ > hexc″ is consistent with the stronger Pd−Au interactions. Temperature and Support Effects on the Au32Pd6 Structure. AIMD simulations at 700 K were performed to account for finite-temperature fluctuations, as well as to obtain optimized structures under different conditions. The optimized structures are shown in Figure 1, where differences in overall 1650

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Figure 2. Radial distribution functions (RDFs) of (a) Pd−Ti, Au−Ti, and (b) Pd−O, Au−O relative to the distance between relevant species. (c) Density profiles as a function of the distance between Pd and Au atoms to the uppermost Ti layer of TiO2, which is marked as Pd-Sf and Au-Sf.

with average coordination numbers of ∼2 for Pd−Pd, ∼5 for Au−Au, and ∼8 for Pd−Au. O2 activation preferably occurs on Pd sites over Au sites on Au32Pd6/TiO2−x because O2 binds on Pd atoms. Adsorption of O2 is repulsive on surface Au (Ead = 0.27 eV) but attractive on Pd (Ead = −0.12 eV). After adsorption, Pd−O distances are ∼2.10 Å and the O−O bond length increases to 1.27 Å, suggesting O2 activation toward a superoxide. This supports the conclusion that Pd atoms on cluster surfaces can perform dioxygen activation, as likely observed in H2O2 formation.30,93,94 The nanoparticle and support interactions can be described in terms of the interatomic Pd−Ti, Au−Ti, Pd−O, and Au−O bonding interactions. These are expressed in the form of radial distribution functions (RDFs) and quantified by the number of bonds for the different support models, i.e., stoichiometric TiO2 or under reduced/oxidized conditions shown in Figure 2a,b and Table 1. The support character has the most significant effect

structures and Au−Pd atom distributions can be clearly distinguished. Au−Pd nanoparticles display liquid-like properties at 700 K. Their shapes are easily altered when supported on TiO2(110), as shown by their moments of inertia (MOIs) along the x-, y-, and z-axes that describe their sphericity (Figure S2). Gas-phase clusters, with and without H, have similar MOIs along the three axes, indicating that the cluster is approximately spherical. However, all clusters on a support show elongated shapes along the xy-plane, evidenced by a large MOI oriented along the zaxis. The flattened shape of the Au−Pd cluster is due to the interactions between the cluster and support. To quantify the liquid-like character of Au32Pd6 clusters, the root-mean-square (rms) bond length fluctuations (δrms) were calculated using the following equation: δrms

2 = N (N − 1)

∑ i7 Å in the xy-plane) to avoid cluster−cluster interactions during the simulation. In addition, a 30 Å vacuum space was used between slabs to ensure minimal interference between slabs in the z direction. The 38-atom truncated octahedron Au32Pd6 cluster was chosen as a model of the Au−Pd nanoalloy because of its typical structure and ratio between the two atoms. AIMD simulations in the canonical ensemble (NVT) using a Nose−Hoover thermostat120,121 were performed at 700 K for 40−50 ps with a 1 fs time step. All simulations started with the lowest energy structure of Au32Pd6 to investigate the structural changes of gas-phase

largest entropy change is noted in Au32Pd6-4H/2Oad-TiO2−x due to strong bonding interactions between H and Au/Pd.

CONCLUSIONS The structure and electronic properties of TiO2-supported Au− Pd nanoalloys are dynamic and change with H adsorption, redox conditions of the support, and temperature. At 0 K in the gas phase the Pd6@Au32 core−shell structure is found to be the most stable, with positively charged Pd core atoms. At elevated temperatures entropy drives Pd atoms to migrate to the outer surface of the cluster, more so when adsorbed H atoms are present or the cluster is supported on titania. For supported clusters, we observe charge transfer from the reduced support to the Au−Pd cluster or from the cluster to the oxidized support. As a result, there are more Pd−Ti and Au−Ti contacts between the negatively charged cluster and the reduced support, whereas there are more Au−O bonds between the positively charged cluster and the oxidized surface. Electron density differences and free energy calculations show that interactions between the Au−Pd cluster and the stoichiometric surface are the weakest due to the relatively small charge transfer between the cluster and the support. On nonstoichiometric surfaces, the Au−Pd 1654

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ACS Nano and supported clusters. Simulations ran for a duration of ∼50 ps. At the end of each trajectory, the last configuration was quenched to a temperature of 0 K to obtain final structures under different conditions. For AIMD analysis, only equilibrated data of the last 25−30 ps were considered. Charge analysis was done using the Bader atoms-in-molecule approach.122,123 In addition, we calculated the adsorption free energy as follows:

ACKNOWLEDGMENTS The authors M.S.L., Y.G.W., D.C.C., V.A.G., and R.R. were supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, & Biosciences. All the calculations were performed at the Pacific Northwest National Laboratory (PNNL), which is operated by Battelle for the DOE. The authors C.Q.X. and J.L. were financially sponsored by NSFC (21521091) and NKBRSF (2013CB834603) of China. Computational resources were provided by the W. R. Wiley Environmental Molecular Science Laboratory (EMSL), a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research, and PNNL Institutional Computing (PIC) program, both located at Pacific Northwest National Laboratory.

ΔGads = ΔHads − T ΔSads where Hads and Sads are the adsorption enthalpy and entropy, respectively, and T is the temperature. ΔHads and ΔSads were obtained by subtracting the values of the support and the nanoalloy from that of the total system, where the deconvoluted adsorption entropy and enthalpy were calculated using ΔSads = ΔStrans + ΔSrot + ΔSvib

ΔHads = ΔET = 0K + ΔUtrans + ΔUrot + ΔUvib − RT

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with ΔET=0K being the energy of the optimized structure at 0 K. Vibrational entropy (Svib) was calculated by employing a quasiharmonic approximation:

Svib = 3kB

∫0

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∞

D(ω)[x coth(x) − log(2 sinh(x))] dω

where kB is the Boltzmann constant and x = ℏω/2kBT. D(ω) is the VDOS, obtained from the Fourier transform of the velocity autocorrelation function D(ω) =

∫0

∞

e−iωt ⟨v(τ ) v(τ + t )⟩ dt

where ⟨v(τ) v(τ + t)⟩ denotes the autocorrelation of the velocity during a time period t, and ω is the frequency. Translational (Strans) and rotational entropies (Srot) are calculated only for gas-phase clusters. Detailed methods to calculate the translational and rotational entropies and enthalpies are explained in our recent publication.124

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b07409. Detailed information on gas-phase Au−Pd isomers, atomic distributions, pair distribution functions, RDFs, moment of inertia, root-mean-square bond length fluctuations, charge transfer based on plate capacity model, work functions, electron density differences, the projected electron density of states, and vibrational density of states (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Cong-Qiao Xu: 0000-0003-4593-3288 Mal-Soon Lee: 0000-0001-6851-177X Yang-Gang Wang: 0000-0002-0582-0855 Present Address #

Fritz-Haber-Institut der Max-Planck-Gesellschaft, 14195 Berlin-Dahlem, Germany. Notes

The authors declare no competing financial interest. 1655

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