Palladium Single Crystals Research Articles

Visualization of Electrooxidation on Palladium Single Crystal Surfaces via In Situ Raman Spectroscopy.

The electrooxidation of catalyst surfaces is across various electrocatalytic reactions, directly impacting their activity, stability and selectivity. Precisely characterizing the electrooxidation on well-defined surfaces is essential to understanding electrocatalytic reactions comprehensively. Herein, we employed in situ Raman spectroscopy to monitor the electrooxidation process of palladium single crystal. Our findings reveal that the Pd surface's initial electrooxidation process involves forming *OH intermediate and ClO4 - ions facilitate the deprotonation process, leading to the formation of PdOx. Subsequently, under deep electrooxidation potential range, the oxygen atoms within PdOx contribute to creating surface-bound peroxide species, ultimately resulting in oxygen generation. The adsorption strength of *OH and the coverage of ClO4 - can be adjusted by the controllable electronic effect, resulting in different oxidation rates. This study offers valuable insights into elucidating the electrooxidation mechanisms underlying a range of electrocatalytic reactions, thereby contributing to the rational design of catalysts.

Visualization of Electrooxidation on Palladium Single Crystal Surfaces via In Situ Raman Spectroscopy

AbstractThe electrooxidation of catalyst surfaces is across various electrocatalytic reactions, directly impacting their activity, stability and selectivity. Precisely characterizing the electrooxidation on well‐defined surfaces is essential to understanding electrocatalytic reactions comprehensively. Herein, we employed in situ Raman spectroscopy to monitor the electrooxidation process of palladium single crystal. Our findings reveal that the Pd surface's initial electrooxidation process involves forming *OH intermediate and ClO4− ions facilitate the deprotonation process, leading to the formation of PdOx. Subsequently, under deep electrooxidation potential range, the oxygen atoms within PdOx contribute to creating surface‐bound peroxide species, ultimately resulting in oxygen generation. The adsorption strength of *OH and the coverage of ClO4− can be adjusted by the controllable electronic effect, resulting in different oxidation rates. This study offers valuable insights into elucidating the electrooxidation mechanisms underlying a range of electrocatalytic reactions, thereby contributing to the rational design of catalysts.

Twinned and Single-Crystal Palladium Nanocrystals for the Electrooxidation of HCOOH

Twinned and Single-Crystal Palladium Nanocrystals for the Electrooxidation of HCOOH

A modeling analysis of molecular orientation at interfaces by polarization-dependent sum frequency generation vibrational spectroscopy

Sum frequency generation (SFG) vibrational spectroscopy has been proven an excellent tool to measure the molecular structures, symmetries and orientations at surfaces/interfaces because of its strong polarization dependence. However, a precise quantitative analysis of SFG spectral intensity and molecular orientation at interfaces must be carefully performed. In this work, we summarized the parameters and factors that are often ignored and illustrated them by evaluating studies of CO adsorption on the (111) facet of platinum (Pt) and palladium (Pd) single crystals at the gas (ultra-high vacuum, UHV)/solid interfaces and methanol (water) adsorption at the air/liquid (solid/liquid) interfaces in the presence of sodium iodide (chloride) salts. To intuitively estimate the influence of incidence angles and refractive indices on the SFG intensity, solely a defined factor of | F yyz | was discussed, which can be individually separated from the macroscopic second-order non-linear susceptibility term and represents the SSP intensity. Moreover, effects of refractive indices and the molecular hyperpolarizability ratio ( R ) were discussed in the orientational analysis of interfacial CO and methanol molecules. When I PPP / I SSP was identical, molecules with a larger R had smaller tilting angles ( θ ) on Pt (assuming θ < 51°), and CO molecules on Pd would tilt much closer to the surface than they did on Pt. A total internal reflection (TIR) geometry enhanced the SFG intensity, but it also amplified the influence of refractive index on SFG intensity at the solid (silica)/liquid interface. The refractive index and R-value had similar influence on the methanol orientation in the presence of sodium iodide salts at air/liquid and solid/liquid interfaces. This work should provide a guideline for analyzing the orientation of molecules with different R , which are adsorbed on catalysts or located at liquid interfaces involving changes of refractive indices. Even slight changes of refractive index (medium 2) and hyperpolarizability ratio ( R = βaac/βccc), which are usually rather ignored, have significant effects on the SFG spectral intensity and molecular orientation of gas (UHV)/solid, air/liquid and solid/liquid interfaces.

One-Pot Synthesis of Single-Crystal Palladium Nanoparticles with Controllable Sizes for Applications in Catalysis and Biomedicine

Palladium (Pd) nanoparticles are of particular interest to various fundamental studies and emerging areas of technology. The properties of Pd nanoparticles have a strong dependence on particle size...

Periodic domain boundary ordering in a dense molecular adlayer: Sub-saturation carbon monoxide on Pd(111)

We describe a previously unreported ordered phase of carbon monoxide adsorbed on the (111) facet of single crystal palladium at near-saturation coverage. The adlayer superstructure is identified from low energy electron diffraction to be c(16×2) with respect to the underlying Pd(111) surface net. The ideal coverage is determined to be 0.6875ML, approximately 92% of the 0.75‐ML saturation coverage. Density functional theory calculations support a model for the molecular packing characterized by strips of locally-saturated (2×2) regions, with the CO bound near high-symmetry surface sites, separated by antiphase domain boundaries. The structure exists in a narrow coverage range and is prepared by heating the saturated adlayer to desorb a small fraction of the CO. Comparison of the c(16×2) domain-boundary structure with structural motifs at lower coverages suggests that between 0.6 and 0.6875ML the adlayer order may be more strongly influenced by interadsorbate repulsion than by adsorption-site-specific interactions. The system is an example of the structural complexity that results from the compromise between adsorbate–substrate and adsorbate–adsorbate interactions.

Shape anisotropy and instability of holes formed during dewetting of single-crystal palladium and nickel films

This study investigates the shape anisotropy and instability of holes formed during dewetting of single-crystal palladium and nickel films. The length ratios of edges constrained to expose {111} facets to other edges of the holes are found to be greater in palladium than in nickel films. The pinch-off is found to occur earlier in palladium than in nickel films. These morphological differences are explained in terms of oxygen adsorption and its effect on the surface energy anisotropy of the film materials.

Fabrication of ordered arrays of micro- and nanoscale features with control over their shape and size via templated solid-state dewetting.

Templated solid-state dewetting of single-crystal films has been shown to be used to produce regular patterns of various shapes. However, the materials for which this patterning method is applicable, and the size range of the patterns produced are still limited. Here, it is shown that ordered arrays of micro- and nanoscale features can be produced with control over their shape and size via solid-state dewetting of patches patterned from single-crystal palladium and nickel films of different thicknesses and orientations. The shape and size characteristics of the patterns are found to be widely controllable with varying the shape, width, thickness, and orientation of the initial patches. The morphological evolution of the patches is also dependent on the film material, with different dewetting behaviors observed in palladium and nickel films. The mechanisms underlying the pattern formation are explained in terms of the influence on Rayleigh-like instability of the patch geometry and the surface energy anisotropy of the film material. This mechanistic understanding of pattern formation can be used to design patches for the precise fabrication of micro- and nanoscale structures with the desired shapes and feature sizes.

Low-Temperature CO Oxidation Catalyzed by Free Palladium Clusters: Similarities and Differences to Pd Surfaces and Supported Particles

The catalytic low-temperature oxidation of CO to CO2 with molecular oxygen is of particular industrial and ecological interest. Gas-phase reaction kinetics measurements in conjunction with first-principles calculations provide comprehensive insight into the mechanisms and energetics of the low-temperature CO combustion reaction catalyzed by small free palladium clusters Pdx+ (x = 2–7). Similar to the cases of extended palladium single crystals and supported nanoparticles, the catalytic activity of the free palladium clusters was found to be largely determined by the fast adsorption and dissociation of molecular oxygen and the binding strength of carbon monoxide. In particular, Pd4+, Pd5+, and Pd6+ were found to catalyze the oxidation of CO at room temperature, with Pd6+ being most active. Detailed mechanistic investigations of the CO oxidation reaction catalyzed by Pd6+ reveal a Langmuir–Hinshelwood reaction mechanism, similar to that found earlier for CO oxidation on palladium single crystals, with compa...

Synthesis of cubic and spherical Pd nanoparticles on graphene and their electrocatalytic performance in the oxidation of formic acid.

Single-crystal palladium nanoparticles (NPs) with controllable morphology were synthesized on the surface of reduced graphene oxide (RGO) by a novel procedure, namely reducing palladium acetylacetonate [Pd(acac)2] with the N-methylpyrrolidone (NMP) solvent in the presence of poly(vinylpyrrolidone) (PVP). The resulting Pd nanocrystals (8 nm in diameter) were uniformly distributed on the RGO. A possible formation mechanism is discussed. The electrocatalytic performance of Pd nanocrystal/RGO catalysts during formic acid oxidation was investigated, which revealed that the cubic Pd/RGO catalyst performed significantly better than the spherical Pd/RGO catalyst. The shape of Pd nanocrystals on the surface of graphene nanosheets can be easily controlled via tuning the synthesis parameters, resulting in tunable catalytic properties. Moreover, this method can be easily extended to fabricate other noble metal nanostructures.

Liquid organic hydrogen carriers: surface science studies of carbazole derivatives.

We review recent results towards a molecular understanding of the adsorption and dehydrogenation of carbazole-derived liquid organic hydrogen carriers on platinum and palladium single crystals and on Al2 O3 -supported Pt and Pd nanoparticles. By combining synchrotron-based high-resolution X-ray photoelectron spectroscopy, infrared reflection-absorption spectroscopy, advanced molecular beam methods and temperature-programmed desorption spectroscopy, detailed insights into the reaction mechanism are obtained. On Pt(111), dehydrogenation of perhydro-N-ethylcarbazole, H12 -NEC, starts with activation of the hydrogen atoms at the pyrrole unit, yielding H8 -NEC as the first stable reaction intermediate at ∼340 K, followed by further dehydrogenation to NEC at ∼380 K. Above 390 K, dealkylation starts, yielding carbazole as an undesired byproduct. On small supported Pt particles, the dealkylation sets in at lower temperatures, due to the higher reactivity of low-coordinated sites, while on larger particles with (111) facets a reactivity as on the flat surface is observed. Carbazole derivatives with ethyl, propyl and butyl chains show an overall very similar reactivity, both on Pt(111) and on Pt nanoparticles. When comparing the dealkylation behavior of H12 -NEC on Pt(111) and Pt nanoparticles to that on Pd(111) and Pd nanoparticles, we find a higher reactivity for the Pd systems.

Organized Surfaces of Highly Faceted Single-Crystal Palladium Structures Seeded by Sacrificial Templates

The templated synthesis of periodic arrays of highly faceted single-crystal structures of palladium is described. The devised route confines an array of sacrificial gold templates between a palladium foil and a single-crystal oxide substrate. At elevated temperatures, the gold templates are annihilated and replaced with palladium structures situated at the precise locations formerly occupied by gold. The assembly process relies on the concurrent sublimation of palladium and gold, which results in the complete transfer of the templated gold from the substrate to the foil but not before the templates act as heterogeneous nucleation sites for palladium adatoms arriving to the substrate surface. The so-formed palladium structures take on the equilibrium Wulff shape expected for elements with a face-centered cubic crystal structure, where the shape and crystallographic alignment of the arrayed structures is promoted through a heteroepitaxial relationship with the underlying substrate. The assembled structures provide an excellent platform for advancing applications reliant on the unique hydrogen storage, catalytic, and plasmonic properties of palladium.

High-resolution angle-resolved photoemission study of electronic structure and electron self-energy in palladium

In this study, we investigate the electronic structure and electron self-energy of palladium single crystals using polarization-dependent high-resolution angle-resolved photoemission spectroscopy. The observed Fermi surfaces and energy-band dispersions agree with those given by the band-structure calculation. A detailed comparison between the observed and theoretical band dispersions of the ${\ensuremath{\Sigma}}_{1}$ band forming the electronlike Fermi surface indicates an electron-electron coupling parameter of ${\ensuremath{\lambda}}_{ee}\ensuremath{\sim}0.02$. Near the Fermi level, a kink structure in the energy-band dispersion exists at $\ensuremath{\sim}\ensuremath{-}20$ meV, in agreement with the Debye energy. The electron-phonon coupling parameter is estimated to be ${\ensuremath{\lambda}}_{ep}=0.39\ifmmode\pm\else\textpm\fi{}0.05$ at 8 K for the ${\ensuremath{\Sigma}}_{1}$ band, which is consistent with the theoretical values of ${\ensuremath{\lambda}}_{ep}=0.35--0.41$. Furthermore, analyses of the self-energy indicate a possible contribution from the electron-paramagnon interaction in the energy range of $\ensuremath{-}50\ensuremath{\sim}\ensuremath{-}150$ meV. The evaluated electron-paramagnon coupling parameter is ${\ensuremath{\lambda}}_{em}\ensuremath{\sim}0.06$ for the ${\ensuremath{\Sigma}}_{1}$ band. We found that the magnitudes of ${\ensuremath{\lambda}}_{ep}$ and ${\ensuremath{\lambda}}_{em}$ depend on the Fermi surface points. The total effective mass enhancement factor is estimated to be $1+{\ensuremath{\lambda}}_{ep}+{\ensuremath{\lambda}}_{ee}+{\ensuremath{\lambda}}_{em}\ensuremath{\sim}1.5$ for the ${\ensuremath{\Sigma}}_{1}$ band, which is close to the values ${m}^{*}/{m}_{b}\ensuremath{\sim}1.5--1.7$ given by the de Haas--van Alphen and electron specific-heat measurements.

Self-assembled palladium nanocrystals on helical carbon nanofibers as enhanced electrocatalysts for electro-oxidation of small molecules

We present a novel approach to prepare helical carbon nanofibers homogeneously functionalized with single crystal palladium nanoparticles via a phase-transfer method. The materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA) and electrochemical measurements. We find that homogeneous and small single-crystal Pd nanoparticles can be easily functionalized with phenyl mercaptan, transferred into the toluene phase from the dimethyl sulfoxide (DMSO) phase and then non-covalently self-assembled onto the surface of helical carbon nanofibers with a very good dispersion and homogeneous diameters of 4.5 ± 0.6 nm. The palladium–helical carbon nanofiber composite exhibits significantly higher electrochemical active area and electrocatalytic activity towards the electrooxidation of formic acid, ethanol and methanol than the commercial electrocatalyst Pd/Vulcan XC-72. Our results show that the prepared material can be potentially used as an advanced nano-electrocatalyst in a direct alkaline fuel cell system.

ChemInform Abstract: Heterogeneously Catalyzed Cyclization Reactions of Ethyne Over Single Crystal Palladium and Palladium Catalysts

ChemInform Abstract: Heterogeneously Catalyzed Cyclization Reactions of Ethyne Over Single Crystal Palladium and Palladium Catalysts

Elastic moduli of single crystal palladium hydride near the tri-critical point.

Palladium has long been studied as a model metal hydride system because of the high absorption rates and levels of hydrogen. Even moderately high pressures and temperatures can result in almost one to one hydrogen to palladium atom ratios. Depending on the ambient temperature and hydrogen pressure, the structural phase of hydrogen atoms in the palladium lattice can have three different forms: alpha, beta, and a mixed phase (alpha + beta). There exists a triple point in the pressure versus temperature phase diagram near 290° C and 400 psi. A recent model for absorption/desorption hysteresis in metal hydride systems predicts a sharp drop in the shear modulus near this triple point. Results from recent elastic constants and acoustic attenuation measurements as a function of both temperature and pressure near the triple point will be shown [R. B. Schwarz and A. G. Khachaturyan, Acta Mater. 54, 313 (2006)]. The experimental procedures for resonant ultrasound spectroscopy measurements at high temperatures and pressures will be also discussed.

Nitrate reduction on Pt single crystals with Pd multilayer

Nitrate reduction on palladium multilayers deposited on platinum single crystal electrodes was studied by cyclic voltammetry and FTIR spectroscopy in acid and alkaline media. The results are compared with those obtained with bulk palladium single crystals. The reaction is sensitive to the electrode surface structure, the reactivity depending on the solution pH. In acid solution nitrate was reduced at potentials below the potential of zero total charge (pztc), when the electrode is negatively charged. Competition between nitrate, hydrogen and anion adsorption and NO formation and accumulation at the surface are proposed as the main reasons for the slow reaction rate. On the bulk palladium single crystal electrodes, NO formation leads to a fast blockage of the surface resulting in a very low activity for nitrate reduction. In alkaline solution, nitrate is reduced at more positive potentials with significantly higher current being measured on the Pd multilayer on Pt(1 0 0) electrode.

Elastic moduli and internal friction of nanocrystalline Pd and PdSi as a function of temperature

Resonant ultrasound spectroscopy was used to study the elastic constants and internal friction of two nanocrystalline palladium samples over the temperature range 3–300 K. The first material, nc-Pd, had a grain size of 80–100 nm and a density 93% of that of single-crystal bulk palladium. The second material, nc-PdSi containing 0.5 at.% Si, had a grain size of 15–22 nm and a density 97% of the single-crystal value. The bulk and shear moduli were significantly reduced in the nc-Pd material from that expected based on single-crystal data, the effect being greater for the bulk modulus. The moduli of nc-PdSi were reduced 4–5% from that based on crystalline Pd. As compared to previous reports of the elastic moduli of nanocrystalline palladium (grain size 5–15 nm) the present values for the larger-grained nc-Pd are comparable, but the present values for the smaller-grained nc-PdSi are considerably higher. An internal friction peak and a modulus defect were found in the nc-Pd material, but not in the nc-PdSi material. These effects are attributed to a relaxation process at the grain boundaries. The temperature dependence of the moduli is similar to that of crystalline palladium and is strongly influenced by electronic effects.

Catalytic combustion of methane on palladium single crystals

Catalytic methane combustion was studied over the palladium single crystals Pd(1 1 1), Pd(1 0 0) and Pd(1 1 0). Under lean reaction conditions at 600 K (O 2:CH 4 = 10:1), stoichiometric palladium oxide was formed with an increase in surface area by a factor of approximately two. The oxide phase formed a “cauliflower-like” surface structure composed of approximately 4 nm sized semispherical oxide agglomerates. This oxide structure was independent of the original metal single crystal orientation. The turnover rates over the oxide structure starting with metal single crystals were 0.7 s −1 on Pd(1 1 1), 0.9 s −1 on Pd(1 0 0) and 0.9 s −1 on Pd(1 1 0) at 600 K, 160 Torr O 2, 16 Torr CH 4, 1 Torr H 2O and N 2 balance to 800 Torr, suggesting that the methane combustion reaction is independent of the initial structure of the catalyst. Methane combustion on palladium single crystals experienced an activation period in which the initial turnover rates based on the initial Pd surface area were about 1/8–1/4 of the steady-state rates determined based on the oxide surface area. This activation period was caused by the slow oxidation of palladium single crystals and concomitant surface area increase during reaction. The increase in surface area happened mostly in the first 10 min of reaction. Carbon dissolution into the crystal was not found during methane combustion under reaction conditions in excess oxygen.

The kinetics of ethylene hydrogenation catalyzed by metallic palladium

The activity of Pd(111) for ethylene hydrogenation is measured using a high-pressure reactor incorporated into an ultrahigh vacuum chamber for temperatures between 300 and 475 K, ethylene pressures between 50 and 300 Torr and hydrogen pressures from 45 to 600 Torr. The reaction rate is found to be rapid with turnover frequencies up to ∼400 reactions/site/s (where rates are referenced to the atom site density on the (111) face of palladium). The measured activation energy is ∼35 kJ/mol. A hydrogen reaction order of ∼1.02 was found at a reaction temperature of 300 K and an ethylene pressure of 100 Torr, where the hydrogen reaction order was found to depend on temperature. A negative reaction order of ∼−0.22 was found in ethylene pressure at a reaction temperature of 320 K and a hydrogen pressure of 100 Torr. The reaction rates are in good agreement with values obtained on silica-supported palladium and with other work on palladium single crystals.