Bulk Palladium Research Articles

Evolution of surface and bulk structure of supported palladium nanoparticles by in situ X-ray absorption and infrared spectroscopies: Effect of temperature, CO and CH4 gas

Interaction of CO and CH4 with palladium is a relevant process in numerous catalytic reactions, which may induce structural changes in the metal affecting its catalytic properties This work establishes a systematic study of the evolution of the atomic and electronic structure in alumina- and carbon-supported palladium nanoparticles in presence of CO and CH4 gases at temperatures from 50 to 350 °C by a combination of in situ X-ray absorption and infrared spectroscopies. Immediate adsorption of CO molecules with formation of surface carbon deposits due to the disproportionation reaction occurs at low temperatures, while above 250 °C, active formation of bulk palladium carbide is observed. In CH4 the later process starts only at ca. 350 °C. Reduction of palladium and contraction of the Pd-Pd interatomic distances were observed with increasing temperature irrespective of the gas atmosphere, including the inert one, particle size and the type of support. For carbon supported samples formation of palladium carbide occurred above 200 °C due to the decomposition of the support.

Roles of metal and acid sites in the reductive depolymerization of concentrated lignin over supported Pd catalysts

Lignin is a natural polymer contained in lignocellulose and is a potential feedstock for the production of phenolics or small aromatic molecules. However, lignin depolymerization is typically performed using low lignin concentrations and insufficiently active catalysts, which hinders industrial applications. To address this issue, we herein investigated the depolymerization of concentrated kraft lignin and oak-extracted organosolv lignin promoted by supported bifunctional metal (palladium)-acid catalysts and elucidated the roles of acid and metal sites. In addition to acid sites, which are known to be critical for lignin degradation, metal sites were found to be required to initiate lignin depolymerization. Palladium atoms promoted depolymerization even in the absence of acid sites, while acid site–promoted depolymerization required the presence of surface palladium atoms. Catalysts with bulk palladium particles were much more active than those with highly dispersed palladium particles, which suggested that efficient depolymerization required the bulky lignin polymer to be adsorbed on the palladium surface in a multidentate fashion. Thus, our work provides valuable insights into the mechanism of catalytic lignin depolymerization and paves the way to the industrial-scale application of this process.

Low-resistivity Pd nanopatterns created by a direct electron beam irradiation process free of post-treatment steps

The ability to create metallic patterned nanostructures with excellent control of size, shape and spatial orientation is of utmost importance in the construction of next-generation electronic and optical devices as well as in other applications such as (bio)sensors, reactive surfaces for catalysis, etc. Moreover, development of simple, rapid and low-cost fabrication processes of metallic patterned nanostructures is a challenging issue for the incorporation of such devices in real market applications. In this contribution, a direct-write method that results in highly conducting palladium-based nanopatterned structures without the need of applying subsequent curing processes is presented. Spin-coated films of palladium acetate were irradiated with an electron beam to produce palladium nanodeposits (PdNDs) with controlled size, shape and height. The use of different electron doses was investigated and its influence on the PdNDs features determined, namely: (1) thickness of the deposits, (2) atomic percentage of palladium content, (3) oxidation state of palladium in the deposit, (4) morphology of the sample and grain size of the Pd nanocrystals and (5) resistivity. It has been probed that the use of high electron doses, 30000 μC cm−2 results in the lowest resistivity reported to date for PdNDs, namely 145 μΩ cm, which is only one order of magnitude higher than bulk palladium. This result paves the way for development of simplified lithography processes of nanostructured deposits avoiding subsequent post-treatment steps.

The Electro-oxidation of Hydrazine with Palladium Nanoparticle Modified Electrodes: Dissecting Chemical and Physical Effects: Catalysis, Surface Roughness, or Porosity?

Palladium nanoparticles in the form of a layer on the surface of an electrode are shown to be electrocatalytic with respect to the four-electron oxidation of hydrazine to form dinitrogen. Quantitative voltammetry shows that the reduced overpotential in comparison with both carbon and bulk palladium electrodes partly arises from the increased surface area of the interface and partly from an increased catalytic activity of the nanoparticles relative to the bulk material. The relative catalytic activity per unit surface area of the nanoparticles as compared with the bulk material is shown to be ca. 35-45.

Lattice dynamics of palladium in the presence of electronic correlations

We compute the phonon dispersion, density of states, and the Gr\"uneisen parameters of bulk palladium in the combined density functional theory (DFT) and dynamical mean-field theory (DMFT). We find good agreement with experimental results for ground state properties (equilibrium lattice parameter and bulk modulus) and the experimentally measured phonon spectra. We demonstrate that at temperatures $T \lesssim 20~K$ the phonon frequency in the vicinity of the Kohn anomaly, $\omega_{T1}({\bf q}_{K})$, strongly decreases. This is in contrast to DFT where this frequency remains essentially constant in the whole temperature range. Apparently correlation effects reduce the restoring force of the ionic displacements at low temperatures, leading to a mode softening.

Absorption of Hydrocarbons on Palladium Catalysts: From Simple Models Towards Machine Learning Analysis of X-ray Absorption Spectroscopy Data

Element selectivity and possibilities for in situ and operando applications make X-ray absorption spectroscopy a powerful tool for structural characterization of catalysts. While determination of coordination numbers and interatomic distances from extended spectral region is rather straightforward, analysis of X-ray absorption near-edge structure (XANES) spectra remains a highly debated and topical problem. The latter region of spectra is shaped depending on the local 3D geometry and electronic structure. However, there is no straightforward procedure for the unambiguous extraction of these parameters. This work gives a critical vision on the amount of information that can be practically extracted from Pd K-edge XANES spectra measured under in situ and operando conditions, in which adsorption of reactive molecules at the surface of palladium with further formation of subsurface and bulk palladium carbides are expected. We investigate how particle size, concentration of carbon impurities, and their distribution in the bulk and at the surface of palladium particles affect Pd K-edge XANES features and to which extend they should be implemented in the theoretical model to adequately reproduce experimental data. Then, we show how the step-by-step increasing the complexity of the theoretical model improves the agreement with experiment. Finally, we suggest a set of formal descriptors relevant to possible structural diversity and construct a library of theoretical spectra for machine-learning-based analysis of the experimental data.

In situ formation of surface and bulk oxides in small palladium nanoparticles.

Evolution of surface and bulk palladium oxides in supported palladium nanoparticles was followed in situ using X-ray absorption spectroscopy. The surface oxide was found to be easily reducible in hydrogen at room temperature, while removal of bulk oxide required heating in hydrogen above 100 °C. We also found that the co-presence of hydrogen and oxygen favours a stronger oxidation of palladium particles compared to pure oxygen.

Evolution of surface and bulk carbon species derived from propylene and their influence on the interaction of hydrogen with palladium

The evolution of carbon species on the surface and in the bulk of palladium (Pd) foils during exposure to C3H6/H2/N2 gas mixtures is quantitatively analyzed in this work as a function of temperature (300–500 °C), C3H6 concentration (5–20% in 80% H2/balance N2), and C3H6 exposure time (0–16 h) by temperature-programmed oxidation (TPO) of the carbonaceous deposits. The influence of these carbonaceous deposits on the interaction of H2 with Pd is analyzed by measurements of H2 fluxes across the Pd foils during exposure to C3H6/H2/N2 gas mixtures in the 300–500 °C range with C3H6 concentrations in the 5–20% range (in 80% H2 with balance N2). TPO results indicate that there are at least four different carbon species that are deposited onto the Pd foil during C3H6 exposure, which we hypothesize are associated with (1) a surface-adsorbed CxHy species, (2) a sub-surface C species, (3) a bulk solid solution C species, and (4) a bulk carbidic C species. The bulk C species are not observed at low temperatures (300–400 °C) due to the large activation barrier for C diffusion in the bulk of Pd (~123 ± 7 kJ/mol). The carbonaceous deposits inhibit hydrogen permeation across the Pd foils to an extent that increases with increasing temperature and increasing C3H6 concentration. There is a particularly strong correlation between the TPO peak temperature associated with the surface-adsorbed CxHy species and the H2 flux across the Pd foil. As the TPO peak temperature associated with the surface-adsorbed carbonaceous species increases with increasing C3H6 exposure time, C3H6 concentration, and C3H6 exposure temperature, the H2 flux across the membrane decreases. This could indicate that surface-adsorbed CxHy species are primarily responsible for inhibiting H2 permeation, and their structure evolves to become a more potent poisoning species with increasing C3H6 exposure time, C3H6 concentration, and C3H6 exposure temperature. This is an important step towards a molecular-level understanding of how different carbon species control the distribution of H atoms on the surface and in the bulk of Pd, which is important for many Pd-based H2 technologies.

Inverse temperature hysteresis and self-sustained oscillations in CO oxidation over Pd at elevated pressures of reaction mixture: Experiment and mathematical modeling

Studying the kinetics of CO oxidation over Pd foil at elevated CO and O2 pressures, we observed an inverse hysteresis: CO conversion was much lower under sample cooling than under foil heating. It was established that under heating the oxidation of metallic palladium to bulk palladium oxide occurred through formation of the palladium surface oxide. Under cooling the bulk oxide reduced directly to metallic species, and in this case some self-sustained oscillations of reaction rate were observed. The catalytic scheme including main various oxygen states of the palladium surface and bulk with transitions between them under reaction conditions in heating/cooling catalytic cycle was proposed. On the base of this scheme the kinetic model of CO oxidation reaction was developed. Both most important mechanisms: Langmuir- Hinshelwood and Mars-van Krevelen, were involved for description of the observed kinetic nonlinear phenomena. The performed simulations demonstrated the capability of this model to describe well the inverse hysteresis of CO oxidation rate together with its oscillatory behavior at constant temperature within the hysteresis loop.

Recent Advances in Palladium Nanoparticles-Based Hydrogen Sensors for Leak Detection.

Along with the development of hydrogen as a sustainable energy carrier, it is imperative to develop very rapid and sensitive hydrogen leaks sensors due to the highly explosive and flammable character of this gas. For this purpose, palladium-based materials are being widely investigated by research teams because of the high affinity between this metal and hydrogen. Furthermore, nanostructured palladium may provide improved sensing performances compared to the use of bulk palladium. This arises from a higher effective surface available for interaction of palladium with the hydrogen gas molecules. Several works taking advantage of palladium nanostructures properties for hydrogen sensing applications have been published. This paper reviews the recent advances reported in the literature in this scope. The electrical and optical detection techniques, most common ones, are investigated and less common techniques such as gasochromic and surface wave acoustic sensors are also addressed. Here, the sensor performances are mostly evaluated by considering their response time and limit of detection.

Self‐sustained oscillations within the temperature hysteresis in CO oxidation over Pd: Mathematical model of a cascade of continuous stirred‐tank reactors

AbstractThe results of temperature‐programmed reaction experimental studies and mathematical modeling of self‐sustained oscillations within an inverse temperature hysteresis in CO oxidation over Pd catalyst are presented. The experimental data demonstrate the influence of the reaction medium on the catalyst activity under reaction conditions. Under oxygen excess in the feeding gas mixture and high temperature, the defects appeared on the initially flat surface of metallic palladium due to deep oxidation of palladium (three‐dimensional PdO nanoparticles were observed). The palladium oxide reduced under cooling of the catalytic system, and the catalyst surface became flat again. To take into account these variations of the palladium surface structure, we consider the piecewise‐constant dependence of the rate constant of some reaction step on the concentrations of oxygen species, namely, the dissolved oxygen or oxide in the palladium bulk. The proposed model of the process in the cascade of continuous stirred‐tank reactors that account for these dependences qualitatively describes the inverse temperature hysteresis as well as the oscillatory dynamics within the hysteresis loop which were obtained experimentally.

Optical, charge transport and magnetic properties of palladium retrieved from photometric measurements: approaching the quantum mechanics background

A parametric Drude–Lorentz (DL) model is used to describe the spectral variation of the dielectric functions of bulk palladium samples at low and room temperature. In addition to the contribution of conduction electrons, the contribution of holes is also explicitly accounted for in the model. A simulated annealing method is applied to obtain the optimized values of the parameters involved in the model: volume plasma frequency of conduction electrons, high frequency dielectric constant, collision frequency of holes and corresponding relaxation time, and two additional parameters from which the effective mass of holes and collision frequency of conduction electrons are evaluated. Oscillatior strengths, resonance frequencies, and widths entering in the Lorentz contribution to the dielectric function are also optimized. Renormalization of the oscillator strengths requires the introduction of a new parameter in the context of the DL model: the ratio between number density of conduction electrons and number density of metal atoms, whose optimized value fits very well with its evaluation from band structure calculations and from independent measurements. Inclusion of this parameter in the framework allows us to evaluate additional quantities related to the charge-carrier transport: average effective masses, Fermi energies and electronic densities of states at the corresponding Fermi energies, intrinsic electrical resistivity, intrinsic mean free paths, heat capacities, mobilities, as well as paramagnetic and diamagnetic susceptibilities, for both electrons and holes. The optimized resonance frequencies are compared with energy differences between plausible interband transitions, in accordance with reported band structure diagrams and with our own band structure obtained from density functional theory calculations.

Effect of surface and bulk palladium doping on the catalytic activity of La2Sn2O7 pyrochlore oxides for diesel soot oxidation

The soot combustion on Pd-doped La2Sn2O7 pyrochlore oxides was studied. Palladium promoter was doped into the pyrochlore system via two ways: bulk substitution (La2Sn1.9Pd0.1O7) and surface impregnation (Pd/La2Sn2O7). The catalysts were characterized by XRD, BET, SEM, HRTEM, PL, XPS and H2-TPR measurements. Pd modifications increased the concentration of surface oxygen vacancy, especially Pd/La2Sn2O7. The different morphologies of palladium promoters were revealed in the two palladium-doped catalysts. Palladium was present primarily in the form of a lattice Pd2+ in the bulk-doping sample, with trace of metal palladium deposited on the surface. As for the surface-doping sample, PdO was the predominant form, accompanied with some of highly dispersed metallic palladium. Due to the adhesion of massive surface palladium species on Pd/La2Sn2O7, a strong metal oxide interaction occurred in the interface between Pd species and La2Sn2O7. The activity evaluation results show that Pd/La2Sn2O7 possesses the highest performance, exhibiting the lowest activation energy (98.83 kJ/mol) for soot combustion. Surface Pd/PdO redox couples as well as Pd–O–Sn interface species were identified to be the active phases for the superior catalytic behavior of Pd/La2Sn2O7. A possible oxidation mechanism for the reactions has been speculated.

Tight-Binding Approximation-Enhanced Global Optimization

Solving and predicting atomic structures from first-principles methodologies is limited by the computational cost of exploring the search space, even when relatively inexpensive density functionals are used. Here, we present an efficient approach where the search is performed using density functional tight-binding, with an automatic adaptive parametrization scheme for the repulsive pair potentials. We successfully apply the method to the genetic algorithm optimization of bulk carbon, titanium dioxide, palladium oxide, and calcium hydroxide, and we assess the stability of the unknown crystal structure of palladium hydroxide.

Temperature- and concentration-dependent hydrogen diffusivity in palladium from statistically-averaged molecular dynamics simulations

Solid-state hydrogen storage materials undergo complex phase transformations in which kinetics are often limited by hydrogen diffusion that significantly changes during hydrogen uptake and release. Here we perform robust statistically-averaged molecular dynamics simulations to obtain a well-converged analytical expression for hydrogen diffusivity in bulk palladium that is valid throughout all stages of the reaction. Our studies confirm the experimentally observed dependence of the diffusivity on concentration and temperature and elucidate the underlying physics. Whereas at low hydrogen concentrations, a single dilute hopping barrier dominates, at high hydrogen concentrations, diffusion exhibits multiple hopping barriers corresponding to hydrogen-rich and hydrogen-poor local environments.

Palladium Carbide and Hydride Formation in the Bulk and at the Surface of Palladium Nanoparticles

The presence of a core/shell behavior in Pd nanoparticles (NPs) during the formation of the metal-hydride phase has recently been highlighted combining X-ray absorption and scattering experiments [J. Phys. Chem. C 2017, 121, 18202]. In this work, we focus on the formation of the carbide phase in the bulk region and on the surface of supported palladium NPs because it affects the catalytic activity and selectivity in hydrogenation reactions. We present in situ X-ray absorption spectroscopy study of carbide formation and decomposition in 2.6 nm palladium nanoparticles supported on carbon during exposure to acetylene, hydrogen, and their mixtures at 100 °C, taken as a representative temperature for hydrogenation reactions. Fourier analysis of extended X-ray absorption fine structure (EXAFS) spectra was used to determine the average Pd–Pd bond distance in the NPs, reflecting the formation of bulk palladium carbide, while theoretical calculation of X-ray absorption near-edge structure (XANES) using the finite ...

The impact of surface composition on Tafel kinetics leading to enhanced electrochemical insertion of hydrogen in palladium

Our previous work experimentally demonstrated the enhancement of electrochemical hydrogen insertion into palladium by modifying the chemical composition of the cathode surface with Pb, Pt and Bi, referred to as surface promoters. The experiment demonstrated that an optimal combination of the surface promoters led to an increase in hydrogen fugacity of more than three orders of magnitude, while maintaining the same current density. This manuscript discusses the application of Density Functional Theory (DFT) to elucidate the thermodynamics and kinetics of observed enhancement of electrochemical hydrogen insertion into palladium. We present theoretical simulations that: (1) establish the elevation of hydrogen’s chemical potential on Pb and Bi surfaces to enhance hydrogen insertion, (2) confirm the increase of a Tafel activation barrier that results in a decrease of the reaction rate at the given hydrogen overpotential, and (3) explain why the surface promoter’s coverage needs to be non-uniform, namely to allow hydrogen insertion into palladium bulk while simultaneously locking hydrogen below the surface (the corking effect). The discussed DFT-based method can be used for efficient scanning of different material configurations to design a highly effective hydrogen storage system.

Effects of elevated temperatures and contaminated hydrogen gas mixtures on novel ultrathin palladium composite membranes

Transmembrane hydrogen flux of a novel 1 μm palladium membrane, supported by a nickel microstructured support grid, is characterized when exposed to carbon dioxide, carbon monoxide and water vapor over a range of temperature from 235° to 320 °C. At all temperatures, the palladium membrane is found to be primarily hydrogen transport rate limited due to surface reactions, not atomic hydrogen solution-diffusion though the bulk palladium membrane. Hydrogen flux decreases rapidly as contaminate gas concentration increases, before reaching a temperature dependent steady state, due to nearly complete surface coverage of the palladium by adsorbed contaminate. Carbon monoxide and carbon dioxide have the largest impact, while water vapor has a lesser impact. The likely source of deactivation is the blocking of surface active sites by adsorbed contaminate molecules. Independent of contaminate gas effects, this study reveals a permanent hydrogen flux decrease due to diffusion of nickel from the microstructured support grid into the palladium membrane at temperatures above 360 °C.

Ultrathin Layers of PdPX (X=S, Se): Two Dimensional Semiconductors for Photocatalytic Water Splitting

On grounds of first principles calculations we propose two-dimensional semiconductors, ultrathin mono-, bi- and trilayers of PdPX (X=S, Se), as photocatalysts with band gaps suitable for harvesting the visible part of the solar spectrum and band edge energies matching the electrochemical potentials required for water splitting reaction. Layered bulk palladium phosphochalcogenides, among them the title compounds, have been known since 1971. The cleavage energy of ≈0.6 J m-2 suggests that single and few layer PdPX can be produced by exfoliation from the bulk phase. PdPX monolayer possesses a moderate band gap of 2.12 (1.97) eV for PdPS (PdPSe) and shows high, albeit anisotropic, carrier motilities. Bandgap (and edge) engineering is possible by applying strain (in the range of 1.5∼2.3 eV), and by varying the number of layers, that is, to 1.95(1.80) eV for the bi- and 1.75 (1.62) eV for the trilayer, to 1.70 (1.43) eV in the bulk.

Vibrational states of atomic hydrogen in bulk and nanocrystalline palladium studied by neutron spectroscopy

The vibrational states of hydrogen atoms in bulk and nanocrystalline palladium were examined in a wide energy region $0\ensuremath{\le}\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\le}300$ meV using neutron spectroscopy. In bulk ${\mathrm{PdH}}_{0.73}$, the vibrational excitations of H atoms were roughly reproduced by the quantum harmonic oscillator (QHO) model. In ${\mathrm{PdH}}_{0.42}$ nanocrystals with a diameter of 8 nm, however, additional vibrational excitations were found at energies above 80 meV. The energies and intensities of the additional states were not explained by the QHO but reasonably described as vibrations in a highly anharmonic trumpetlike potential. The additional excitations are attributed to the vibrations of H atoms at tetrahedral sites in the subsurface region stabilized by surface effects. Thus, this is an experimental work which clearly detects hydrogen vibration inside metal nanoparticles.