Surface segregation of Au–Pd alloys in UHV and reactive environments: Quantification by a catalytic atom probe

New frontiers in atom probe tomography: a review of research enabled by cryo and/or vacuum transfer systems

There has been a recent surge in the use of cryo and/or vacuum specimen preparation and transfer systems to broaden the scope of research enabled by the microscopy technique of atom probe tomography. This is driven by the fact that, as for many microscopes, the application of atom probes to air- and temperature-sensitive materials or wet biological specimens has previously been limited by transfer through air at room temperature. Here we provide an overview of areas of research that benefit from these new transfer and analysis protocols, as well as a review of current advances in transfer devices, environmental cells, and glove boxes for controlled specimen manipulation. This includes the study of catalysis and corrosion, biological samples, liquid-solid interfaces, natural aging, and the distribution of hydrogen in materials.

Altering CO binding on gold cluster cations by Pd-doping

The introduction of dopant atoms into metal nanoparticles is an effective way to control the interaction with adsorbate molecules and is important in many catalytic processes. In this work, experimental and theoretical evidence of the influence of Pd doping on the bonding between small cationic AuN+ clusters and CO is presented. The CO adsorption is studied by combining low-pressure collision cell reactivity and infrared multiple photon dissociation spectroscopy experiments with density functional theory calculations. Measured dissociation rates of cluster-CO complexes (N ≤ 21) allow the estimation of cluster-CO binding energies, showing that Pd doping increases the CO adsorption energy to an extent that is size-dependent. These trends are reproduced by theoretical calculations up to N = 13. In agreement with theory, measurements of the C-O vibrational frequency suggest that for the doped PdAuN-1+ (N = 3-5, 11) clusters, CO adsorbs on an Au atom, while for N = 6-10 and N = 12-14, CO interacts directly with the Pd dopant. A pronounced red-shifting of the C-O vibrational frequency is observed when CO interacts directly with the Pd dopant, indicating a significant back-donation of electron charge from Pd to CO. In contrast, the blue-shifted frequencies, observed when CO interacts with an Au atom, indicate that σ-donation dominates the Au-CO interaction. Studying such systems at the sub-nanometre scale enables a fundamental comprehension of the interactions between adsorbates, dopants and the host (Au) species at the atomic level.

Atom Probe Tomography for Catalysis Applications: A Review

Atom probe tomography is a well-established analytical instrument for imaging the 3D structure and composition of materials with high mass resolution, sub-nanometer spatial resolution and ppm elemental sensitivity. Thanks to recent hardware developments in Atom Probe Tomography (APT), combined with progress on site-specific focused ion beam (FIB)-based sample preparation methods and improved data treatment software, complex materials can now be routinely investigated. From model samples to complex, usable porous structures, there is currently a growing interest in the analysis of catalytic materials. APT is able to probe the end state of atomic-scale processes, providing information needed to improve the synthesis of catalysts and to unravel structure/composition/reactivity relationships. This review focuses on the study of catalytic materials with increasing complexity (tip-sample, unsupported and supported nanoparticles, powders, self-supported catalysts and zeolites), as well as sample preparation methods developed to obtain suitable specimens for APT experiments.

Adaptive kinetic Monte Carlo simulations of surface segregation in PdAu nanoparticles

Surface segregation in bimetallic nanoparticles (NPs) is critically important for their catalytic activity because the activity is largely determined by the surface composition. Little, however, is known about the atomic scale mechanisms and kinetics of surface segregation. One reason is that it is hard to resolve atomic rearrangements experimentally. It is also difficult to model surface segregation at the atomic scale because the atomic rearrangements can take place on time scales of seconds or minutes - much longer than can be modeled with molecular dynamics. Here we use the adaptive kinetic Monte Carlo (AKMC) method to model the segregation dynamics in PdAu NPs over experimentally relevant time scales, and reveal the origin of kinetic stability of the core@shell and random alloy NPs at the atomic level. Our focus on PdAu NPs is motivated by experimental work showing that both core@shell and random alloy PdAu NPs with diameters of less than 2 nm are stable, indicating that one of these structures must be metastable and kinetically trapped. Our simulations show that both the Au@Pd and the PdAu random alloy NPs are metastable and kinetically trapped below 400 K over time scales of hours. These AKMC simulations provide insight into the energy landscape of the two NP structures, and the diffusion mechanisms that lead to segregation. In the core-shell NP, surface segregation occurs primarily on the (100) facet through both a vacancy-mediated and a concerted mechanism. The system becomes kinetically trapped when all corner sites in the core of the NP are occupied by Pd atoms. Higher energy barriers are required for further segregation, so that the metastable NP has a partially alloyed shell. In contrast, surface segregation in the random alloy PdAu NP is suppressed because the random alloy NP has reduced strain as compared to the Au@Pd NP, and the segregation mechanisms in the alloy require more elastic energy for exchange of Pd and Au and between the surface and subsurface.

A Gas-Phase Reaction Cell for Modern Atom Probe Systems

In this work, we demonstrate a new system for the examination of gas interactions with surfaces via atom probe tomography. This system provides capability of examining the surface and subsurface interactions of gases with a wide range of specimens, as well as a selection of input gas types. This system has been primarily developed to aid the investigation of hydrogen interactions with metallurgical samples, to better understand the phenomenon of hydrogen embrittlement. In its current form, it is able to operate at pressures from 10-6 to 1000 mbar (abs), can use a variety of gasses, and is equipped with heating and cryogenic quenching capabilities. We use this system to examine the interaction of hydrogen with Pd, as well as the interaction of water vapor and oxygen in Mg samples.

Phase transition in crown-jewel structured Au-Ir nanoalloys with different shapes: a molecular dynamics study

We have studied the melting process for crown-jewel structured Ir₅₅, Ir₅₄Au, Ir₄₃Au₁₂, Ir₂₅Au₃₀, Ir₁₃Au₄₂, and Au₅₅ nanoclusters in the icosahedral, Ir₅₅, Ir₅₄Au, Ir₄₃Au₁₂, Ir₁₉Au₃₆, Ir₁₃Au₄₂, and Au₅₅ nanoclusters in the cuboctahedral, and Ir₅₄, Ir₅₃Au, Ir₄₇Au₇, Ir₁₇Au₃₇, Ir₇Au₄₇, and Au₅₄ nanoclusters in the decahedral morphologies. We have investigated the different thermodynamic, structural, and dynamical properties for the different nanoclusters in the different structures. Our thermodynamic results indicated that as the concentration of Au atoms in the nanoclusters increases, the absolute value of internal energy, and so the melting points, of the nanoclusters decrease. It is also shown that the Au atoms decrease the melting temperature of the pure cuboctahedral cluster more than that of the other structures. We have also found that the Au atoms were located in favorable positions at the surface sites of nanoalloys. Also, the doping of the Ir nanocluster by Au atoms makes the cluster more stable. It is also found that nanoclusters with different morphologies have almost the same stability. Our structural results indicated that after the melting process, the Au atoms generally tend to lie in the outer shell of the cluster, but the Ir atoms generally tend to lie in the core of the cluster (see the Ir₁₃Au₄₂ and Ir₇Au₄₇ nanoclusters, for example). We have also found the interesting result that the Ir₇Au₄₇ nanocluster shows a solid-solid transition from a decahedral structure to an icosahedral structure before melting. The Ir₄₃Au₁₂ nanocluster also shows a transformation from a cuboctahedral structure to an icosahedral-like structure before melting. Our dynamical results showed that doping of the Ir₅₅ cluster with an Au atom sharply increases the self-diffusion coefficient in the initial state in the solid phase, especially in icosahedral and cuboctahedral structures. It is also shown that the Ir₁₃Au₄₂ cluster in icosahedral and cuboctahedral and the Ir₇Au₄₇ and Ir₁₇Au₃₇ clusters in decahedral morphologies have smaller values of self-diffusion coefficients than other clusters after the melting point and that this could be due to the formation of core-shell structures.

Chiral adsorption studied by field emission techniques: the case of alanine on platinum

Chirality at surfaces has become an active research area targeting possible applications of enantioselective separation or detection. A curved single crystal imaged with nanometric resolution is used to prepare a number of enantiomorphous metallic facets and to assess chiral adsorption of alanine.

Imaging and chemically probing catalytic processes using field emission techniques: a study of NO hydrogenation on Pd and Pd–Au catalysts

Nitric oxide hydrogenation is investigated on palladium and gold–palladium alloy crystallites, i.e. the extremity of sharp tip samples aimed at modelling a single catalytic grain.

Investigation of thermal, structural and dynamical properties of (Aux–Cuy–Niy)N=32,108,256 ternary nanosystems: effect of Au addition to Cu–Ni bimetallic nanoclusters via MD simulation

In this work, we have investigated the heating and cooling processes for ternary metallic nanoclusters with different Au mole fractions using molecular dynamics simulation.

Density functional theory study of CO-induced segregation in gold-based alloys

This paper reports a systematic study of the effect of CO gas on the chemical composition at the surface of gold-based alloys. Using DFT periodic calculations in presence of adsorbed CO the segregation behavior of group 9-10-11 transition metals (Ag, Cu, Pt, Pd, Ni, Ir, Rh, Co) substituted in semi-infinite gold surfaces is investigated. Although, CO is found to be more strongly adsorbed on (100) than on the (111) surface, the segregation of M impurities is found to be more pronounced on the (111) surface. The results reveal two competitive effects: the effect of M on CO and the effect of CO on M. Thus, on one hand, if M exists on the (100) gold facet, CO would be strongly adsorbed on it. But if M is initially located in the bulk, it would segregate to the (111) facet instead of the (100) in order to bind to CO.

Synergy between Pd and Au in a Pd–Au(100) bimetallic surface for the water gas shift reaction: a DFT study

Density functional theory calculations were performed to model a reaction relevant bimetallic surface and study the water gas shift reaction.

Hydrogenation of NO and NO2over palladium and platinum nanocrystallites: case studies using field emission techniques

Hysteresis effects and kinetic instabilities have been characterised at the nanoscale for the hydrogenation of NO_xspecies far from thermodynamic equilibrium.