Structural Rearrangement of Au-Pd Nanoparticles under Reaction Conditions: An ab Initio Molecular Dynamics Study (original) (raw)
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Journal of Catalysis, 2005
Model catalysts of Pd nanoparticles and films on TiO 2 (110) were fabricated by metal vapour deposition (MVD). Molecular beam measurements show that the particles are active for CO adsorption, with a global sticking probability of 0.25, but that they are deactivated by annealing above 600 K, an effect indicative of SMSI. The Pd nanoparticles are single crystals oriented with their (111) plane parallel to the surface plane of the titania. Analysis of the surface by atomic resolution STM shows that new structures have formed at the surface of the Pd nanoparticles and films after annealing above 800 K. There are only two structures, a zigzag arrangement and a much more complex "pinwheel" structure. The former has a unit cell containing 7 atoms, and the latter is a bigger unit cell containing 25 atoms. These new structures are due to an overlayer of titania that has appeared on the surface of the Pd nanoparticles after annealing, and it is proposed that the surface layer that causes the SMSI effect is a mixed alloy of Pd and Ti, with only two discrete ratios of atoms: Pd/Ti of 1:1 (pinwheel) and 1:2 (zigzag). We propose that it is these structures that cause the SMSI effect.
Direct interactions between metal nanoparticles and support: STM studies of Pd on TiO 2 (1 1 0
We have fabricated ultra-nanoparticulate model catalysts of Pd on TiO 2 (1 1 0) using metal vapour deposition (MVD) to form particles in the size range 1–50 nm, which can be imaged at very high spatial resolution (and in some cases at atomic resolution) using scanning tunnelling microscopy (STM). Using these methods we are able to identify the atomic level mechanism responsible for certain phenomena in catalysis, for which molecular level models have previously been proposed from macroscopic measurements. In this paper we address two such phenomena, namely spillover and the so-called strong metal–support interaction (SMSI) effect. Oxygen spillover from Pd particles to the titania support occurs due to the fast adsorption of oxygen on Pd compared with titania, and is driven by reaction with Ti 3+ ions in the vicinity of the particles. The SMSI state is identified at atomic resolution as being due to the appearance of Ti at the surface of the Pd particles. These Ti layers are partially oxidised and form very well defined structures of two main types—a rectangular lattice and hexagonal unit cells of large dimension. These layers passivate the surface for the adsorption of CO.
Non-Equilibrium Properties of Au-Pd Nanoparticles
Solid State Phenomena, 2011
We address the question of the evolution of a nanostructured system in a metastable state to equilibrium. To this purpose, we use the case study of the transition of an Au core Pd shell nanoalloy cluster containing up to about 600 atoms toward the equilibrium Au segregated configuration. We start from a molecular dynamics approach with an embedded atom potential. The way the transition develops at low temperatures is found to be very sensitive to the cluster morphology and the way energy is exchanged with the environment. The transition of icosahedral inverse core-shell Au-Pd clusters is predicted to nucleate locally at the surface contrary to clusters with other morphologies, and starting at lower temperatures compared to them.
The Bifurcation Point of the Oxygen Reduction Reaction at Au-Pd Nanoalloys
Faraday Discuss., 2016
The oxygen reduction reaction is of major importance in energy conversion and storage. Controlling electrocatalytic activity and its selectivity remains a challenge of modern electrochemistry. Here, first principles calculations and analysis of experimental data unravel the mechanism of this reaction on Au–Pd nanoalloys in acid media. A mechanistic model is proposed from comparison of the electrocatalysis of oxygen and hydrogen peroxide reduction on different Au–Pd ensembles. A H2O production channel on contiguous Pd sites proceeding through intermediates different from H2O2 and OOHσ adsorbate is identified as the bifurcation point for the two reaction pathway alternatives to yield either H2O or H2O2. H2O2 is a leaving group, albeit reduction of H2O2 to H2O can occur by electrocatalytic HO–OH dissociation that is affected by the presence of adsorbed OOHσ. Similarities and differences between electrochemical and direct synthesis from H2 + O2 reaction on Au–Pd nanoalloys are discussed.
Potential-Dependent Structural Memory Effects in Au–Pd Nanoalloys
The Journal of Physical Chemistry Letters, 2012
Alloying of metals offers great opportunities for directing reactivity of catalytic reactions. For nanoalloys, this is critically dependent on near-surface composition, which is determined by the segregation energies of alloy components. Here Au−Pd surface composition and distribution of Pd within a Au 0.7 Pd 0.3 nanoalloy were investigated by monitoring the electrocatalytic behavior for the oxygen reduction reaction used as a sensitive surface ensemble probe. A time-dependent selectivity toward the formation of H 2 O 2 as the main oxygen reduction product has been observed, demonstrating that the applied potential history determines surface composition. DFT modeling suggests that these changes can result both from Pd surface diffusion and from exchange of Pd between the shell and the core. Importantly, it is shown that these reorganizations are controlled by surface adsorbate population, which results in a potential-dependent Au−Pd surface composition and in remarkable structural memory effects.
On the Structure of Au/Pd Bimetallic Nanoparticles
Journal of Physical Chemistry C, 2006
We performed a study on bimetallic Au/Pd nanoparticles using aberration corrected electron microscopy along with molecular dynamics simulations to investigate the features of specific atomic sites at the surface, which can be related to the high catalytic activity properties of the particles. The calculations mimic the growth of nanoparticles through a cooling process from a molten solid to a crystalline structure at room temperature. We found that the final structure for the alloy particles is neither a cuboctahedral nor an icosahedral, but a complex structure that has a very rough surface and unique isolated Pd sites surrounded by Au atoms. We also found that there is predominance of three specific Pd sites at the surface, which can be directly related to the catalytic activity of the nanoparticles.
First Principle Studies of Au-Pd Nanoalloys
hpc-europa.eu
± Global optimization of Pd-Au bimetallic clusters over a wide size range (N 2 À 50) have been performed using a genetic algorithm, coupled with the Gupta many-body empirical potential (EP) to model interatomic interactions. Two sets of EP parameters have been used in this work : (1) an average of pure Pd and Au bulk properties (2) experimental Pd-Au fitted parameters. Stability criteria, such as binding energy (E b ) and second difference in energy (D 2 E) has been used in order to determine the lowest energy structures, i.e. global minima (GM). Density Functional Theory (DFT) local-relaxations have been performed on all the``putative'' GM structures on 1:1 compositions (Pd-Au) N up to N 50 for both set of parameters. Specific 38-atom size Pd-Au clusters was selected on the basis of previous work on bimetallic systems. DFT relaxations were performed on the lowest``excess energy'' Á Gupta QV compositions (Pd 19 Au 19 ± Pd 13 Au 25 ) allowing us to have a clearer description of the energy landscape for both sets of parameters at this cluster size.
Entropic Control of HD Exchange Rates over Dilute Pd-in-Au Alloy Nanoparticle Catalysts
ACS Catalysis, 2021
Dilute Pd-in-Au alloy catalysts are promising materials for selective hydrogenation catalysis. Extensive surface science studies have contributed mechanistic insight on the energetic aspect of hydrogen dissociation, migration and recombination on dilute alloy systems. Yet, translating these fundamental concepts to the kinetics and free energy of hydrogen dissociation on nanoparticle catalysts operating at ambient pressures and temperatures remains challenging. Here, the effect of the Pd concentration and Pd ensemble size on the catalytic activity, apparent activation energy and rate limiting process is addressed by combining experiment and theory. Experiments in a flow reactor show that a compositional change from 4 to 8 atm% Pd of the Pd-in-Au alloy catalyst leads to strong increase in activity, even exceeding the activity per Pd atom of monometallic Pd under the same conditions, albeit with an increase in apparent activation energy. First-principles calculations show that the rate and apparent activation enthalpy for HD exchange increase when increasing the Pd ensemble size from single Pd atoms to Pd trimers in a Au surface, suggesting that the ensemble size distribution shifts from mainly single Pd atoms on the 4 atm% Pd alloy to larger Pd ensembles of at least three atoms for the 8 atm% Pd/Au catalyst. The DFT studies also indicated that the rate-controlling process is different: H 2 (D 2) dissociation determines the rate for single atoms whereas recombination of adsorbed H and D determines the rate on Pd trimers, similar to bulk Pd. 2 Both experiment and theory suggest that the increased reaction rate with increasing Pd content and ensemble size stems from an entropic driving force. Finally, our results support hydrogen migration between Pd sites via Au and indicate that the dilute alloy design prevents the formation of subsurface hydrogen, which is crucial in achieving high selectivity in hydrogenation catalysis.
The Journal of Physical Chemistry C, 2010
Using first principles calculations, we examine how the ensemble effect on the performance of bimetallic catalysts is affected by the change of surface electronic structure associated with their geometric parameters. We look at H 2 O 2 formation from H 2 and O 2 based on three different Pd monomer systems including AuPd adlayers with a Pd monomer each on Pd(111) [AuPd M /Pd(111)] and Au(111) [AuPd M /Au(111)] and a 55atom cluster with Au 41 Pd shell and Pd 13 core [Au 41 Pd@Pd 13 ]. Our calculations show that H 2 O 2 selectivity tends to be significantly deteriorated in the Au 41 Pd@Pd 13 and AuPd M /Au(111) cases, as compared to the AuPd M /Pd(111) case. This is largely due to enhancement of the activity of corresponding surface Pd and its Au neighbors, while isolated Pd surface sites surrounded by less active Au are responsible for the H 2 O 2 formation by suppressing O-O cleavage. This study highlights that ensemble contributions in multimetallic nanocatalysts can be a strong function of their geometric conditions, particularly local strain and effective atomic coordination number at the surface, that are directly related to surface electronic states.
Investigation of finite-size effects in chemical bonding of AuPd nanoalloys
The Journal of Chemical Physics, 2015
In this paper, the size-dependent changes in energetic, vibrational, and electronic properties of C–O gas molecule interacting with surface Pd atom of a variety of AuPd nanoalloy structures are investigated by means of first principles calculations. The variation in C–O adsorption energies, C–O vibration frequencies (νC−O), and Pd d-bond centers (εd) on a series of non-supported Aun−1–Pd1 nanoparticles (with n varying from 13 to 147) and on two semi-finite surfaces are inspected with cluster size. We demonstrate for the first time that, with small AuPd bimetallic three-dimensional clusters as TOh38, one can reach cluster size convergence even for such a sensitive observable as the adsorption energy on a metal surface. Indeed, the results show that the adsorbate-induced perturbation is extremely local and it only concerns the isolated Pd interacting with the reactive gas molecule. Except for 13 atom clusters, in which molecular behaviour is predominant, no finite-size effects are obs...