Pd loading threshold for an efficient noble metal use in Pd/CeO2 methane oxidation catalysts

The influence of the noble metal-support interaction on the performance, activation behavior and stability of Pd/CeO$_2$ catalysts for total methane oxidation was systematically investigated. A series of samples with 1–3 wt% Pd loading supported on CeO2 with surface areas varying between 30 and 120 m2/g were tested as-prepared and after different reductive treatments. Distinct Pd structural states were identified, which influence the catalytic performance depending on the pre-treatment and the theoretical monolayer Pd coverage of CeO$_2$. Although reductive treatments improved the catalytic activity, the stability strongly depended on the Pd coverage on ceria and the type of reducing agent. A threshold of Pd concentration on the ceria surface was identified that ensures optimal noble metal efficiency, activity and long-term durability. Below this threshold, rapid catalyst deactivation occurs due to redispersion on the strongly interacting support whereas above this threshold Pd is prone to sintering irrespective of the atmosphere

Influence of air exposure on structural isomers of silver nanoparticles

Up to date, the influence of ambient air exposure on the energetics and stability of silver clusters has rarely been investigated and compared to clusters in vacuum. Silver clusters up to 3000 atoms in size, on an amorphous carbon film, have been exposed to ambient air and investigated by atomic-resolution imaging in the aberration-corrected Scanning Transmission Electron Microscope. Ordered structures comprise more than half the population, the rest are amorphous. Here, we show that the most common ordered isomer structures is the icosahedron. These results contrast with the published behaviour of silver clusters protected from atmospheric exposure, where the predominant ordered isomer is face-centred cubic. We propose that the formation of surface oxide or sulphide species resulting from air exposure can account for this deviation in stable isomer. This interpretation is consistent with density functional theory calculations based on silver nanoclusters, in the size range 147-201 atoms, on which methanethiol molecules are adsorbed. An understanding of the effects of ambient exposure on the atomic structure and therefore functional properties of nanoparticles is highly relevant to their real-world performance and applications

Devising More Efficient Pt Catalysts for Fuel Cell Applications from a First-Principle Approach

National audienc

Composition-Stability Relations for AuCu and PtNi Nanoalloys

National audienc

Theoretical insight of adsorption thermodynamics of multifunctional molecules on metal surfaces

International audienc

Modélisation, classification et hiérarchisation de nanoparticules métalliques par une approche atomistique de type DFT

International audienc

An Introduction to DFT Models of Metallic Nanoparticles: Applications from Synthesis to Reactivity

National audienc

Morphological Sensitivity of Silver Nanoparticles to Environment

Silver nanoparticles have been modeled by density functional theory calculations and ab initio molecular dynamics simulations to explore their stability in air at room temperature. The role of contaminants is considered by adsorbing monoshells of methanethiol molecules on silver nanoparticles of various morphologies (icosahedral, strongly irregular and regular truncated octahedral structures) in the range 147-201 atoms (∅ = 1.66-1.80 nm), under atmospheric nitrogen pressure. While ab initio molecular dynamics simulations suggest that the icosahedral and all the face-centered cubic clusters are stable at 300 K in vacuum, the ino-decahedral geometry progressively transforms into a complex structure composed of an irregularly icosahedral outershell and a decahedral core. Nanoparticle surface energies, computed at 0 K and 300 K, show a preference for FCC clusters in vacuum, as previously reported experimentally. In presence of air at 300 K, the icosahedral cluster presents the largest exothermicity in terms of adsorption surface energy of contaminant monoshells. This energetic gain is understood on the basis of the larger surface silver atomic density for the icosahedral structure, which better accommodates dense contaminant monoshells than FCC clusters. The methanethiol adlayers are composed of a complex phase of chemisorbed molecules bound to silver and in interaction with physisorbed contaminants through hydrogen bonds. This theoretical study agrees with measurements of silver nanoparticles exposed to air after synthesis in vacuum and also investigated in solution, and demonstrates that the air environment tunes the relative stability of morphologies in competition. This work paves the way on the understanding of nanoparticle ageing in environmental conditions at the atomic scale

Growth of a Pt film on non-reduced ceria: A density functional theory study

International audienceThe growth of platinum on non-reduced CeO2 (111) surface is studied by means of calculations based on the density functional theory. Particles of increasing size are formed on the oxide surface by incorporating the platinum atoms one by one until multilayer films are obtained. The main conclusion is that platinum atoms tend to maximize the number of metallic bonds and to approach the situation of the bulk, hence preferring films to particles, particles to isolated atoms, and a three-dimensional growth to a two-dimensional one. The supported particles and the films exhibit a contraction of the Pt–Pt distances, with respect to those of the Pt bulk, in order to match the ceria lattice. The density of states projected on the filmsurfaceplatinum atoms shows important differences in shape and energy (lower d-band center) compared to the Pt(111) reference surface, which could be the major reason for the observed changes in catalytic reactivity when deposited particles are compared with single crystal surfaces