Kinetic hindrance during the initial oxidation of Pd(100) at ambient pressures

Formation of Epitaxial PdO(100) During the Oxidation of Pd(100)

The catalytic oxidation of CO and CH4 can be strongly influenced by the structures of oxide phases that form on metallic catalysts during reaction. Here, we show that an epitaxial PdO(100) structure forms at temperatures above 600 K during the oxidation of Pd(100) by gaseous O atoms as well as exposure to O2-rich mixtures at millibar partial pressures. The oxidation of Pd(100) by gaseous O atoms preferentially generates an epitaxial, multilayer PdO(101) structure at 500 K, but initiating Pd(100) oxidation above 600 K causes an epitaxial PdO(100) structure to grow concurrently with PdO(101) and produces a thicker and rougher oxide. We present evidence that this change in the oxidation behavior is caused by a temperature-induced change in the stability of small PdO domains that initiate oxidation. Our discovery of the epitaxial PdO(100) structure may be significant for developing relationships among oxide structure, catalytic activity, and reaction conditions for applications of oxidation catalysis.

Oxidation and de-alloying of PtMn particle models: a computational investigation

We present a computational study of the energetics and mechanisms of oxidation of Pt-Mn systems. We use slab models and simulate the oxidation process over the most stable (111) facet at a given Pt₂Mn composition to make the problem computationally affordable, and combine Density-Functional Theory (DFT) with neural network potentials and metadynamics simulations to accelerate the mechanistic search. We find, first, that Mn has a strong tendency to alloy with Pt. This tendency is optimally realized when Pt and Mn are mixed in the bulk, but, at a composition in which the Mn content is high enough such as for Pt₂Mn, Mn atoms will also be found in the surface outmost layer. These surface Mn atoms can dissociate O₂ and generate MnO_x species, transforming the surface-alloyed Mn atoms into MnO_x surface oxide structures supported on a metallic framework in which one or more vacancy sites are simultaneously created. The thus-formed vacancies promote the successive steps of the oxidation process: the vacancy sites can be filled by surface oxygen atoms, which can then interact with Mn atoms in deeper layers, or subsurface Mn atoms can intercalate into interstitial sites. Both these steps facilitate the extraction of further bulk Mn atoms into MnO_x oxide surface structures, and thus the progress of the oxidation process, with typical rate-determining energy barriers in the range 0.9-1.0 eV.

Ab Initio Simulations of Water/Metal Interfaces

Structures and processes at water/metal interfaces play an important technological role in electrochemical energy conversion and storage, photoconversion, sensors, and corrosion, just to name a few. However, they are also of fundamental significance as a model system for the study of solid-liquid interfaces, which requires combining concepts from the chemistry and physics of crystalline materials and liquids. Particularly interesting is the fact that the water-water and water-metal interactions are of similar strength so that the structures at water/metal interfaces result from a competition between these comparable interactions. Because water is a polar molecule and water and metal surfaces are both polarizable, explicit consideration of the electronic degrees of freedom at water/metal interfaces is mandatory. In principle, ab initio molecular dynamics simulations are thus the method of choice to model water/metal interfaces, but they are computationally still rather demanding. Here, ab initio simulations of water/metal interfaces will be reviewed, starting from static systems such as the adsorption of single water molecules, water clusters, and icelike layers, followed by the properties of liquid water layers at metal surfaces. Technical issues such as the appropriate first-principles description of the water-water and water-metal interactions will be discussed, and electrochemical aspects will be addressed. Finally, more approximate but numerically less demanding approaches to treat water at metal surfaces from first-principles will be briefly discussed.

Surface Properties of the Hydrogen-Titanium System

Titanium is an excellent getter material, catalyzes gas-solid reactions such as hydrogen absorption in lightweight metal hydrides and complex metal hydrides and has recently been shown as a potential ammonia synthesis catalyst. However, knowledge of the surface properties of this metal is limited when it absorbs large quantities of hydrogen at operation conditions. Both the conceptual description of such a surface as well as the experimental determination of surface hydrogen concentration on hydride-forming metals is challenging due to the dynamic bulk properties and the incompatibility of traditional surface science methods with the hydrogen pressure needed to form the metal hydride, respectively. In this paper, the surface pressure-composition isotherms of the titanium-hydrogen system are measured by operando reflecting electron energy loss spectroscopy (REELS). The titanium thin films were deposited on and hydrogenated through a palladium membrane, which provides an atomic hydrogen source under ultrahigh vacuum conditions. The measurements are supported by density functional theory calculations providing a complete picture of the hydrogen-deficient surface of TiH₂ being the basis of its high catalytic activity.

Hydrogen Solubility and Atomic Structure of Graphene Supported Pd Nanoclusters

We investigated the atomic structure of graphene supported Pd nanoclusters and their interaction with hydrogen up to atmospheric pressures at room temperature by surface X-ray diffraction and scanning tunneling microscopy. We find that Ir seeded Pd nanocluster superlattices with 1.2 nm cluster diameters can be grown on the graphene/Ir(111) moiré template with high structural perfection. The superlattice clusters are anchored through the rehybridized graphene to the Ir support, which superimposes a 2.0% inplane compression onto the clusters. During hydrogen exposure at 10 mbar pressure and room temperature, a significant part of the clusters gets unpinned from the superlattice. The clusters in registry undergo an out-of-plane expansion only, whereas the detached clusters expand in in- and out-of-plane directions. The formation of a hydrogen rich PdH_x α' phase was not observed. After exposure to 1 bar, the majority of the clusters are unpinned from superlattice sites, due to their surface interaction with hydrogen and possible spill over to the graphene support. Only minor sintering was observed, which is more pronounced for the unpinned clusters. The results give evidence that ultrasmall Pd clusters on graphene are a stable hydrogen storage system with reduced hydrogen storage hysteresis and maintain a large surface area for hydrogen chemisorption.

Epitaxy and Shape Heterogeneity of a Nanoparticle Ensemble during Redox Cycles

The role of metal-support epitaxy on shape and size heterogeneity of nanoparticles and their response to gas atmospheres is not very well explored. Here we show that an ensemble of Pd nanoparticles, grown on MgO(001) by deposition under ultrahigh vacuum, mostly consists of two distinctly epitaxially oriented particles, each having a different structural response to redox cycles. X-ray reciprocal space patterns were acquired in situ under oxidizing and reducing environments. Each type of nanoparticle has a truncated octahedral shape, whereby the majority grows with a cube-on-cube epitaxy on the substrate. Less frequently occurring and larger particles have their principal crystal axes rotated ±3.7° with respect to the substrate's. Upon oxidation, the top (001) facets of both types of particles shrink. The relative change of the rotated particles' top facets is much more pronounced. This finding indicates that a larger mass transfer is involved for the rotated particles and that a larger portion of high-index facets forms. On the main facets of the cube-on-cube particles, the oxidation process results in a considerable strain, as concluded from the evolution to largely asymmetric facet scattering signals. The shape and strain responses are reversible upon reduction, either by annealing to 973 K in vacuum or by reducing with hydrogen. The presented results are important for unraveling different elements of heterogeneity and their effect on the performance of real polycrystalline catalysts. It is shown that a correlation can exist between the particle-support epitaxy and redox-cycling-induced shape changes.

Microkinetic Modeling of the Oxidation of Methane Over PdO Catalysts—Towards a Better Understanding of the Water Inhibition Effect

Water, which is an intrinsic part of the exhaust gas of combustion engines, strongly inhibits the methane oxidation reaction over palladium oxide-based catalysts under lean conditions and leads to severe catalyst deactivation. In this combined experimental and modeling work, we approach this challenge with kinetic measurements in flow reactors and a microkinetic model, respectively. We propose a mechanism that takes the instantaneous impact of water on the noble metal particles into account. The dual site microkinetic model is based on the mean-field approximation and consists of 39 reversible surface reactions among 23 surface species, 15 related to Pd-sites, and eight associated with the oxide. A variable number of available catalytically active sites is used to describe light-off activity tests as well as spatially resolved concentration profiles. The total oxidation of methane is studied at atmospheric pressure, with space velocities of 160,000 h−1 in the temperature range of 500–800 K for mixtures of methane in the presence of excess oxygen and up to 15% water, which are typical conditions occurring in the exhaust of lean-operated natural gas engines. The new approach presented is also of interest for modeling catalytic reactors showing a dynamic behavior of the catalytically active particles in general.

Hot-Atom Mechanism in Syngas Methanation on Precovered Pd(100) Surfaces

The excess energy of subsurface hydrogen species may facilitate overcoming reaction barriers and remarkably alters the reaction pathways. We present an in-depth study on the different reactivity of surface and subsurface hydrogen species in syngas methanation on the O/C-covered Pd(100) by using density functional theory calculations and microkinetic simulations. It is shown that the apparent energy barriers to form H₂O and CH₄ are reduced by 0.87 and 0.61 eV for the case in which the hot subsurface hydrogen species are involved in the whole hydrogenation process. The activity of O-covered Pd(100) is better than that of the C-covered surface, and the reactivity of subsurface hydrogen species is much higher than that of surface hydrogen species under ambient conditions. Increasing CO partial pressure strongly enhances the reactivity of subsurface hydrogen species in syngas methanation on the O-covered Pd(100). These important results are helpful for understanding the hot-atom mechanism through subsurface heterogeneous catalysis.

Evidence of the O–Pd–O and Pd–O4 structure units as oxide seeds and their origin on Pd(211): revealing the mechanism of surface oxide formation

The formation of surface oxides on metal surfaces is not only important in materials science, but also of significance in heterogeneous catalysis due to the fact that during most oxidation reactions the metal catalysts are inevitably oxidized, which may cause dramatic consequences in the reactions.

Modelling complete methane oxidation over palladium oxide in a porous catalyst using first-principles surface kinetics

Modeled turnover frequencies for varying temperature and total pressure combined with reaction rate controlling regions.

In Situ Optical Reflectance Difference Observations of CO Oxidation over Pd(100)

Using a home-built reflectometer, we have investigated the changes in the optical reflectivity of a Pd(100) model catalyst during CO oxidation under high-pressure, high-temperature conditions. We observe changes in optical contrast when exposing the surface to CO oxidation conditions at 200 mbar from room temperature up to 400 °C. These changes in reflectivity are a result both of the formation of a surface oxide layer and of a change in surface roughness because of gas exposure. However, the reflectivity is more sensitive to the presence of a thin, flat oxide layer than to surface roughness. CO oxidation plays an important role in the decrease of the reflectivity. Since adding a reducing agent to the gas mixture renders it unlikely that the oxide thickness increases, we conclude that the observed decrease in reflectivity is dominated by increased surface roughness because of the catalytic reaction. We contribute this observed surface roughening to a Mars-van Krevelen-type reaction mechanism.

From dull to shiny: A novel setup for reflectance difference analysis under catalytic conditions

We have developed an experimental setup for optically monitoring a catalytically active surface under reaction conditions. A flow reactor with optical access allows us to image the behavior of an active catalyst surface down to the millimeter length scale. We use reflectance difference measurements with 625 nm light to investigate CO oxidation on Pd(100) at 300 mbar and 320 °C. We conclude that the changes in visible contrast result from the formation of an oxide layer after surface oxidation.

Surface science under reaction conditions: CO oxidation on Pt and Pd model catalysts

Application of surface-science techniques, such as XPS, SXRD, STM, and IR spectroscopy under catalytic reactions conditions yield new structural and chemical information. Recent experiments focusing on CO oxidation over Pt and Pd model catalysts were reviewed.

Role of RuO2(100) in surface oxidation and CO oxidation catalysis on Ru(0001)

Oxidation of Ru(0001) induces the simultaneous formation of RuO₂(100) and RuO₂(110) and a structure-sensitive oxygen spillover during CO oxidation.

Release of Oxygen from Palladium Oxide Cluster Ions by Heat

Palladium oxide cluster ions, PdnOm(+), were prepared in the gas phase using laser ablation of a palladium rod in the presence of oxygen. The cluster ions were heated to 1000 K downstream from the cluster source (post heating), and the abundance of PdnOm(+) (n = 2-7) was examined using mass spectrometry. Temperature-programmed desorption experiments revealed that an oxygen molecule is released from oxygen-rich PdnOm(+), forming oxygen-deficient PdnOm-2(+). It was found that Pd6O4(+) was thermally stable up to 1000 K. The activation energy for oxygen molecule desorption has been obtained and compared with previous results by Lang et al.

Structure of palladium nanoparticles under oxidative conditions

Using density functional theory (DFT) and thermodynamic considerations we study the shape and stability of Pd nanoparticles in oxygen-lean and oxygen-rich atmospheres. We find that at very high oxygen coverage cubes exposing (100) faces will form, which are stabilized due to the formation of a O/(√5 × √5)R27° overlayer. The shape of oxygen-covered Pd and Pt nanoparticles is compared in this study.

Oxidation of palladium on Au(111) and ZnO(0001) supports

The oxidation behavior of supported Pd-deposits on Au(111) and ZnO(0001) single crystals has been studied by x-ray photoemission spectroscopy (XPS). Oxidation has been carried out ex situ in a high-pressure cell and subsequent vacuum-transfer and characterization by ultra-high vacuum XPS, as well as in situ characterization by synchrotron based near ambient pressure XPS. On Au(111) alloying of Pd with the substrate competes with oxidation and only for sufficiently thick Pd films oxidation is obtained. For Pd deposits on ZnO the oxidation condition depends on the amount of deposited Pd. Thicker Pd-deposits behave similar to bulk Pd-samples, while for thinner films the oxidation temperatures may be lowered. Interestingly, for very small amounts of Pd, in situ XPS shows full oxidation at room temperature and at less than 0.6 mbar O2 pressure. This indicates lowering of the kinetic barriers for oxidation of very small supported Pd-clusters. The formed oxide is, however, not stable in ultra high vacuum and a slow reduction is observed. The instability of this oxide in UHV indicates that the formed Pd-oxide at the interface to ZnO may have different chemical properties compared to bulk PdO or surface oxides on Pd.

Negatively charged ions on Mg(0001) surfaces: appearance and origin of attractive adsorbate-adsorbate interactions

Adsorption of electronegative elements on a metal surface usually leads to an increase in the work function and decrease in the binding energy as the adsorbate coverage rises. Using density-functional theory calculations, we show that Cl adsorbed on a Mg(0001) surface complies with these expectations, but adsorption of {N,O,F} causes a decrease in the work function and an increase in the binding energy. Analyzing the electronic structure, we show that the presence of a highly polarizable electron spill-out in front of Mg(0001) causes this unusual adsorption behavior and is responsible for the appearance of a hitherto unknown net-attractive lateral electrostatic interaction between same charged adsorbates.

Highly efficient Pd-based core–shell nanowire catalysts for O2 dissociation

With the consideration of the stability and cost, we found that the Fe, Co, Ni, Cu, Ru, Ir atoms have lower price than the Pd and favor at the core even with O adatom at the surface. The formed M@Pd core–shell nanowires are active for O₂ dissociation with activation barriers no larger than 0.25 eV.

In situ oxidation study of Pd–Rh nanoparticles on MgAl2O4(001)

Oxidation induced dealloying of PdRh nanoparticles: Rh wins the oxidation race.

In situ x-ray photoelectron spectroscopy of model catalysts: at the edge of the gap

We present high-pressure x-ray photoelectron spectroscopy (HP-XPS) and first-principles kinetic Monte Carlo study addressing the nature of the active surface in CO oxidation over Pd(100). Simultaneously measuring the chemical composition at the surface and in the near-surface gas phase, we reveal both O-covered pristine Pd(100) and a surface oxide as stable, highly active phases in the near-ambient regime accessible to HP-XPS. Surprisingly, no adsorbed CO can be detected during high CO(2) production rates, which can be explained by a combination of a remarkably short residence time of the CO molecule on the surface and mass-transfer limitations in the present setup.

The Rh(100)-(3 × 1)-2O structure

The O adsorption on Rh(100) has been studied using high resolution core level spectroscopy, low energy electron diffraction and scanning tunnelling microscopy. In addition to the well known (2 × 2), (2 × 2)-pg and c(8 × 2) structures at coverages of 0.25, 0.5 and 1.75 ML respectively, an intermediate (3 × 1) structure with a coverage of 2/3 ML is identified.