Structure, Chemical Order and Reactivity of Gold-Palladium Nanoparticles: from vacuum to reaction conditions – AuPd-Seg

Thanks to the skills of the different partners of this project we combine a large panel of theoretical and experimental methods useful for our study. From theoretical point of view, we developed a dual approach which is a quantum calculations based on the Density of Functional Theory (DFT)) vs Classical method based on Ising model and semi-empirical (SMA) approximation. This dual approach will allow as identifying the main parameters governing the surface segregation of the palladium in the Gold-Palladium alloys and to predict the structure and the chemical composition of nanoalloys under different environments.
From experimental point of view, beside investigating several elaboration methods able to select the best Au-Pd ratio (a ratio favoring the formation of Pd dimers and avoiding the formation of shell palladium), many characterization techniques as the Transmission Electron Microscopy (TEM) and the Diffuse Reflectance Infrared Fourier Transform (DRIFT) were used to check the evolution of the nanoparticles composition and structure under gas. These results were confronted to DFT energetic and harmonic calculations of different configurations of Au-Pd to study the structure and the reactivity of the alloy .

Extensive DFT calculations were performed to calculate the segregation energies and the metal-metal pair interactions, both in the infinite dilute limits of Au-Pd alloy. Based on these calculations we adjusted SMA N-body inter-atomic potential. SMA-Monte Carlo simulations performed in the semi-gran-canonical ensemble allowed us to draw the segregation isotherms as a function of the chemical potential of the alloy or of the bulk concentration at fixed temperature.
By the exploitation of these curves via the mean filed approximation we identified the segregation enthalpies and entropies in all concentration range of the alloy. This dual approach, already used for other alloys, allows us to analyze the physical origin of the superficial segregation by determining the alloy components monitored by cohesive, alloy and size effects. This represents a crucial step for developing the effective Ising model including the effect of structural relaxation and able to describe the segregation before and after the gas exposure. One of the major results of this successful methodology, applied under vacuum, seems to indicate that Pd is preferentially located on the sub-surface of Au-Pd alloy.
Concerning the study of reactivity, DFT method was used to validate the experimental interpretations of CO-Pd IR spectra. Thus, thanks to the combined experimental and theoretical methods we have identified the surface structure of Pd (site, coordination’s, binding, electronic structure etc…) of elaborated Au-Pd nanoparticles and explained their structural evolution with time CO exposure (see illustration).

The DFT determination of energetic alloy parameters in presence of adsorbed CO is now achieved. Currently, we are working on the Monte-Carlo simulation via the mean filed approach of the segregation of Pd in presence of CO on the (111) and (100) surface. The next step is to develop a kinetic Monte-Carlo formalism that account the presence of gas on the surface of nanoparticles.

Publications

1.«Evidence of Pd segregation and stabilization at edges of AuPd nano-clusters in the presence of CO: a combined DFT and DRIFTS study.« B. Zhu, G. Thrimurthu, L. Delannoy, C. Louis, C. Mottet, J. Creuze, B. Legrand and H. Guesmi; Journal of Catalysis, submitted.

2. «Theoretical insights on the effect of reactive gas on the chemical ordering of gold based nanoalloys.« H. Guesmi; Gold Bulletin, xxxx, December (2013).

Communications:

1.Nanoalloy Action Conference, Antalya (November 2012) Turkey. «Remarkable reactivity of specific Pd ensembles in Au-Pd nanoalloys: CO oxidation reaction« (oral)

2.Nanoalloy Action Conference, Antalya (November 2012) Turkey. «CO adsorption on AuPd nano-cluster: a combined DFT and DRIFT study« (oral)

3.Réunion plénière du GDR OrNano 2012 (Nanoparticules d’or : de la théorie à l’application), Poitier (Mars 2012) France. «Etude DFT du phénomène de ségrégation dans les alliages à base d’or en présence de gaz.« (Poster).

4. 13èmes Journées de la Matière Condensée, Montpellier (Aout 2012) France. «Palladium distribution in Au-Pd nanoalloy surfaces and its effect on CO oxidation« (oral).

5. Réunion plénière du GDR Nano-alliage, Orléans (Décembre 2012) France. «Pd ensembles in Au-Pd nanoalloys with remarkable reactivity for CO oxidation« (oral)

6.Réunion plénière du GDR Nano-alliage, Orléans (Décembre 2012) France. «CO adsorption on AuPd nano-cluster: a combined DFT and DRIFT study« (Poster)

The knowledge of the composition and surface structure of nanoalloy particles is crucial to explain their catalytic performance. In addition, the bonding of adsorbates may, in some cases, induce modifications in local atomic composition and surface structure, changing the activity and selectivity of the catalyst. These facts were observed for Au-Pd nanoparticles. Indeed, in the presence of reactive gas (for example CO and O2) the segregation of the Pd to the surface was reported.
Because the extent to which segregation occurs can have implications for the performance and the lifetime of the catalyst, it is important to understand whether the particular configuration is stable under the operating environment for a specific application. For this purpose the present project aims to study the structure, chemical order and the reactivity of Au-Pd surface nanoparticles under vacuum and in the presence of adsorbates.
Theoretical studies of catalytic properties are often investigated on model systems such as surfaces or model aggregates and rarely on “realistic” systems similar to those observed experimentally. In addition, one feature of the vast majority of these studies is that no account is taken for the possibility that the surface composition can be modified after the gas exposure. These are serious drawbacks that may prevent reliable description of the nanoparticle reactivity who mainly depend on the structure configuration of the surface. In order to address this challenging task, we intend to combine original theoretical approaches (DFT based Ising model and semi-empirical based simulations) to model Au-Pd nanoparticles (FCC based symmetry cluster and icosahedron) and to study segregation and transition state properties under vacuum and reaction conditions. In addition experimental work combined with DFT investigations (taking in to account adsorbate-induced surface change) will be devoted to the study of the reactivity of these nanoalloys in CO oxidation reaction. This project will be developed in collaboration with physicists and chemists experts which are very active in the domain of nanoparticles and nanoalloys.