Bulk Pd Research Articles

Unraveling the formation of Tb/Pd films by coupled reduction as targets for heavy ion-beam irradiation

This study provides insights into the formation of Tb/Pd films produced by electroplating in combination with the so-called coupled reduction technique. The use of 149Tb as an α-emitting radionuclide enabled for the first time the study of the distribution of Tb across the surface and along the thickness of the Pd bulk by means of radiography and α-spectroscopy combined with Monte Carlo simulations of the energy loss of emitted α-particles, respectively. Additionally, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy were employed for surface analysis, whereas X-ray diffraction provided conclusive evidence of the formed crystalline phases. The obtained results indicate the formation of either the intermetallic phase TbPd3 or a Tb/Pd solid solution, depending on the heating time and temperature as well as the local Tb/Pd atomic ratio. These insights pave the way to the production of stable targets for heavy ion-beam irradiations as well as other applications.

Influence of Catalyst Surface Structure on the Electrochemical Hydrogenation of Cis,Cis-Muconic Acid

Adipic acid (AA) production from petroleum-based cyclohexane is one of the largest chemical processes in terms of annual volumetric turnover accounting for 3 million tons as of 2022. The current commercial approach to synthesize AA is through the oxidation of petroleum-derived cyclohexane using nitric acid. The release of N2O in stoichiometric amounts makes it one of the most significant greenhouse gas (GhG) emitting processes. The abundance of biomass in the Midwestern United States has driven significant research efforts to synthesize bio-derived and sustainable commodity products and lower the chemical industry’s carbon footprint.1 cis,cis-Muconic acid (ccMA), a biobased C6 diunsaturated diacid platform molecule can be transformed to commodity chemicals like caprolactam, adipic acid, and terephthalic acid, with AA being of particular interest for the synthesis of nylon 6,6.2 For the first time in literature, we demonstrate appreciable production of AA (conversion: 91%, selectivity: >95%) through electrochemical hydrogenation (ECH) of ccMA on PGM catalysts. Carbon-supported palladium (Pd/C) and platinum (Pt/C) selectively produced AA while bulk Pd and Pt yielded the monounsaturated trans-3-hexenedioic acid (t3HDA) intermediate and H2 under all tested conditions. Such an influence of surface structure on the electrochemistry of organics is not unprecedented. Koper et. al3 studied Pt single crystals for the electrochemical hydrogenation of acetone and showed how activity, selectivity, and extent of catalytic poisoning are highly sensitive to the Pt surface structure. Similarly, we used density functional calculations to elucidate the structure-sensitive nature of ccMA ECH and its intermediates by identifying thermodynamically preferred reaction pathways as a function of surface geometry (terrace vs steps). The energetics coupled with experimental observations suggest that the reduction of ccMA to t3HDA occurs through an outer sphere proton-coupled electron transfer mechanism, while the reduction of HDA to AA preferentially occurs on Pd and Pt (111), the most abundant facet in Pd/C and Pt/C. Our calculations also explain the exceptional performance of Pd/C compared to Pt/C by the weaker binding of Pd terrace sites compared to Pt. Overall, combining renewable electricity, in-situ hydrogen generation from water, and a biologically-produced intermediate enabled us to open a sustainable route for bio-adipic acid production. This allows us to further design and optimize the catalyst for the selective production of biobased AA.

Top‐down Surfactant‐Free Synthesis of Supported Palladium‐Nanostructured Catalysts

Nanostructured palladium (Pd) is a universal catalyst that is widely used in applications ranging from catalytic converters of combustion engine cars to hydrogenation catalysts in industrial processes. Standard protocols for synthesizing such nanoparticles (NPs) typically use bottom‐up approaches. They utilize special and often expensive physical techniques or wet‐chemical methods requiring organic surfactants. These surfactants should often be removed before catalytic applications. In this article, the synthesis of Pd NPs immobilized on carbon support by electrochemical erosion without using any surfactants or toxic materials is reported. The Pd NPs synthesis essentially relies on a Pd bulk pretreatment, which causes material embrittlement and allows the erosion process to evolve more efficiently, producing homogeneously distributed NPs on the support. Moreover, the synthesized catalyst is tested for hydrogen evolution reaction. The activity evaluations identify optimal synthesis parameters related to the erosion procedure. The electrocatalytic properties of the Pd NPs produced with sizes down to 6.4 ± 2.9 nm are compared with a commercially available Pd/C catalyst. The synthesized catalyst outperforms the commercial catalyst within all properties, like specific surface area, geometric activity, mass activity, specific activity, and durability.

An Electrochemical Cell for Operando Grazing-Incidence X-Ray Absorption Spectroscopic Studies of Low-Loaded Electrodes

X-ray absorption spectroscopy (XAS) is a powerful technique that can provide element specific information on the local electronic and structural properties of newly developed electrocatalysts, especially when performed under operating conditions (i.e., operando). However, the large amounts of catalyst typically needed to achieve sufficiently high spectral quality and temporal resolution can result in working electrodes of several micrometers in thickness. This can in turn lead to an inhomogeneous potential distribution across the electrode, delamination and/or incomplete utilization of the catalyst layer, severe bubble formation and accumulation due to poor mass transport properties.[1, 2] In addition, the activity and selectivity of a catalyst are often measured in a different cell geometry (e.g., using rotating disk electrodes (RDEs)), leading to uncertainties in comparison with the results inferred from spectroscopic data.[3]To tackle these problems we have developed a new spectroelectrochemical XAS flow cell that enables spectral acquisition in fluorescence mode using an X-ray beam incidence angle of ≤ 0.1° with regards to the working electrode’s substrate plane, i.e., in a so called grazing incidence (GI) configuration (see Figure 1). In this acquisition configuration we successfully tracked the formation of palladium hydride with a time resolution of 10 seconds per spectrum while using a Pd-loading as low as 50 µgPd∙cm-2. Moreover, the careful design of the working electrode flow field allows the study of faradaic processes (e.g., O2-reduction) under mass-transport controlled conditions entailing currents comparable to those attained in RDE measurements (≤ 10 mA∙cm−2). The combination of these features with an ion-conductive membrane to separate the working- and counter-electrode compartments additionally enables the detection of gaseous products (e.g., from CO2-electroreduction) by degassing them out of the electrolyte and analyzing them in time-resolved fashion by means of mass spectrometry.[4] References Diklić, N., et al., Potential Pitfalls in the Operando XAS Study of Oxygen Evolution Electrocatalysts. ACS Energy Letters, 2022. 7(5): p. 1735-1740. Diercks, J.S., et al., Spectroscopy vs. Electrochemistry: Catalyst Layer Thickness Effects on Operando/In Situ Measurements. Angew Chem Int Ed Engl, 2023: p. e202216633. Diercks, J.S., et al., Interplay between Surface-Adsorbed CO and Bulk Pd Hydride under CO2-Electroreduction Conditions. ACS Catalysis, 2022: p. 10727-10741. Khanipour, P., et al., Electrochemical Real-Time Mass Spectrometry (EC-RTMS): Monitoring Electrochemical Reaction Products in Real Time. Angew Chem Int Ed Engl, 2019. 58(22): p. 7273-7277 Figure 1

Facile Synthesis of Pd Nanosheets and Implications for Superior Catalytic Activity.

As a member of the 2D materials family, 2D metal nanosheets (metallenes) have received increasing attention due to their intriguing properties distinct from those of graphene and other inorganic 2D nanosheets. However, the synthesis of metallenes is still challenging, owing to the lack of an efficient synthetic approach. Here we present a facile one-pot approach to the controlled synthesis of Pd nanosheets. A key feature of this process is a stepwise reaction using 2,4,6-trichlorophenyl formate (TCPF); TCPF emits carbon monoxide gas, which acts as both a reductant and a surface capping agent, promoting the anisotropic 2D growth of the Pd nanosheets. Photoemission spectroscopy revealed some peculiar features of the surface charge and valence band states due to suppressed electron transfer at the 2D surface. This surface state caused improved catalytic activity for the hydrogen evolution reaction compared to that of bulk Pd.

PdNi alloys with surface segregation of Pd@hydrophilic N, O doped carbons toward hydrogen production with high current densities

A series of catalysts, which consist of PdNi alloy cores and N, O co-doped carbon (NOC) shells, are developed by pyrolysis of metal–organic framework (PdNi) for hydrogen production with high current densities. The bulk or surface Pd compositions (Pd/(Pd + Ni) molar ratios) of PdNi alloys can be well controlled by the fed ones. Meanwhile, some structural features that are favorable to HER activity, especially to achieving high HER current densities under low overpotentials, are further adjusted by the Pd compositions, including d-band center (εd) of PdNi alloy affected by surface segregation of Pd, hydrophilicity of NOC shell, and charge transfer and mass diffusion efficiency of an entire catalyst. When the bulk Pd/(Pd + Ni) molar ratio is ∼ 0.21, Pd0.21Ni0.79@NOC has a moderate εd, the highest hydrophilicity, the highest charge transfer and mass diffusion efficiency, and finally the highest electric double-layer capacitance (or electrochemical active surface area). Accordingly, it affords the optimal apparent or specific activity for hydrogen production with high current densities under low overpotentials in 0.5 M H2SO4, including a low apparent η1000 of ∼ 240 mV or specific η2500(BET) of ∼ 240.9 mV, and a high current density of ∼ 1590 mA cm−2 at only ∼ 242 mV.

Low temperature hydrogen superpermeation in vanadium composite metal foil pumps

Palladium-based foil membranes are an effective option for hydrogen isotope recovery from the plasma exhaust of future fusion plants, but cost and availability are concerns. Vanadium (V) is a relatively low cost, neutron tolerant material with high hydrogen permeability. It has been well-studied as a superpermeable membrane at high temperature (>500 °C), but V displays negligible superpermeation at low temperature (75–200 °C) due to catalytic limitations. Composite membranes were fabricated by depositing thin layers (∼100 nm) of either Pd or BCC PdCu on sputter-cleaned vanadium foils (100 μm). Symmetric membranes elevated superpermeation to levels approaching bulk Pd or PdCu foils, with ∼5X higher flux in the latter reflecting the superior properties of PdCu. Asymmetric membranes revealed that the Pd-based catalyst layer was critical for both efficient absorption of superthermal hydrogen upstream as well as catalyzing re-combinative desorption downstream. At T ≥ 150 °C composite membrane superpermeation was equivalent to the Pd-based foils, but the flux was attenuated by a factor of 2-3X as the temperature was reduced. This deviation from pure foil performance coincided with the formation of vanadium hydride (β-V2H), which also impacted the transient response. Nevertheless, no embrittlement was observed under the conditions examined and elevating the temperature >150 °C removed the hydride and restored full performance. The achievement of palladium-level performance with a >99% reduction in Pd inventory makes these V composite metal foils pumps an attractive option for low temperature hydrogen isotope recovery in future fusion plants.

Size-Dependent Energy and Adhesion of Pd Nanoparticles on Graphene on Ni(111) by Pd Vapor Adsorption Calorimetry

Carbon-supported late transition-metal nanoparticles are promising catalysts and electrocatalysts for wide-ranging applications. However, experimental investigations of the bonding energetics of metal nanoparticles on carbon supports are very limited. Here, we report heats of adsorption of Pd vapor deposited onto single-layer graphene(0001) supported on Ni(111) at 100 and 300 K as Pd grows particles of well-defined size in the range from three atom clusters to 6 nm diameter. Sizes were determined from He+ low-energy ion scattering (LEIS). In this size range, the differential heat of Pd adsorption increases from 228 kJ/mol to within 10 kJ/mol of the heat of sublimation of bulk Pd (377 kJ/mol). The chemical potential of metal atoms in these nanoparticles as a function of average particle size was determined from these results. The adhesion energy at the Pd/graphene(0001)/Ni(111) interface was extracted from these data and found to be 3.5 J/m2 for the largest Pd particles. For the three metal elements that have now been studied (Pd, Ni, and Ag), we found an increase in metal/graphene(0001)/Ni(111) adhesion energy with metal carbophilicity, which we defined here as the heat of C atom adsorption on that metal’s (111) surface estimated from published density functional theory calculations.

The normalization of the active surface sites of bimetallic Pd-Pt catalysts, their inhomogeneity, and their roles in methane activation.

Multi-metallic catalysts containing Pt species are widely used. As there is no methodology to evaluate the quantity of active surface sites of Pt or other metal species, researchers have only published the total conversion or selectivity of all active surface sites. This study focuses on Pt-Pd bimetallic catalysts and uses both methane reaction kinetics and infrared (IR) spectroscopy to characterize the surface Pd and Pt sites. The surface Pt sites, which were determined from the fitted rate coefficients, were evaluated in the reaction region where the catalyst structure was insensitive to catalytic performance. Another methodology involves IR spectroscopy to normalize the active surface sites. As three typical absorption bands of Pt species were observed during CO chemisorption, spectral deconvolution was conducted to obtain the integrated intensity of the Pd and Pt species, and the quantity of surface Pd and Pt sites was calculated. The two methods have good consistency, and the IR spectra are considered to be more suitable for calculating the quantity of active surface sites. In addition, the IR spectra revealed a correlation between oxidative Pd surface sites and methane reactivity. The ionic Pd sites provide abundant oxygen intermediates in the catalytic reaction and improve the catalytic performance. As for the surface Pd species and bulk Pd species, the XPS results indicate a similar variation in the Pdδ+/(Pdδ+ + Pd0) ratio vs. Pd/Pt ratio.

Harvesting PdH Employing Pd Nano Icosahedrons via High Pressure.

Palladium hydrides (PdHx ) have important applications in hydrogen storage, catalysis, and superconductivity. Because of the unique electron subshell structure of Pd, quenching PdHx materials with more than 0.706 hydrogen stoichiometry remains challenging. Here, the 1:1 stoichiometric PdH ( is successfully synthesized using Pd nano icosahedrons as a starting material via high-pressure cold-forging at 0.2 GPa. The synthetic initial pressure is reduced by at least one order of magnitude relative to the bulk Pd precursors. Furthermore, PdH is quenched at ambient conditions after being laser heated ≈2000 K under ≈30 GPa. Corresponding ab initio calculations demonstrate that the high potential barrier of the facets (111) restricts hydrogen atoms' diffusion, preventing hydrogen atoms from combining to generate H2 . This study paves the way for the high-pressure synthesis of metal hydrides with promising potential applications.

Single Pd atom embedded Janus HfSeTe as promising sensor for dissolved gas detection in transformer oil: A density functional theory study

Exploring high-performance sensing materials aimed at dissolved gases in transformer oil is of crucial importance to guarantee the safe and stable operation of power transformer. This study investigates the adsorption and sensing of five kinds of dissolved gas molecules (H2, CO, CH4, C2H2, and C2H4) on single Pd atom doped Janus HfSeTe monolayer (Pd-HfSeTe). The adsorption structure, adsorption energy, electron transfers, density of states (DOS), band structure, and work function are calculated. The results demonstrate that single Pd atom has much larger binding energy on HfSeTe surface compared to bulk Pd, and shows high thermal stability at 500 K. In addition, the doping of Pd significantly enhances the adsorption strength of CO, C2H2, and C2H4. The electronic properties illustrate that the band gap experiences apparent increase for CO adsorption on Pd site for both Se and Te surface, and there shows obvious overlapping of Pd 4d and C 2p orbitals in atomic DOS for CO, C2H2, and C2H4 adsorption. Furthermore, taking electronic properties, work function, and occupation function into consideration, Pd-HfSeTe can detect C2H4 with higher selectivity as both resistance-type and work-function type gas sensor. This theoretical study allows novel opinions of Janus transitional-metal dichalcogenides (TMDCs) doped with single transition-metal atom as sensing materials for gas detection in numerous application area.

High Selective Direct Synthesis of H2O2 over Pd1@γ‐Al2O3 Single‐Atom Catalyst

AbstractPd‐based catalysts are essential for direct synthesis of H2O2 from H2 and O2, which still need further improvement of the activity and selectivity by engineering the status of Pd active species. In this work, isolated Pd atoms anchored on Al vacancy of the γ‐Al2O3(100) surface (Pd1@γ‐Al2O3(100)) are proposed to act as a heterogeneous catalyst for direct synthesis of H2O2 from first‐principles theoretical study and micro‐kinetic analysis. The thermodynamic stability and whole catalytic mechanism for conversion of O2 to H2O2 on Pd1@γ‐Al2O3(100) was studied. It is found that the high selectivity toward H2O2 of Pd1@γ‐Al2O3(100) is attributed to the high oxidation state of isolated Pd active sites with high stability against atom‐aggregation, derived from the strong electronic metal‐support interaction. The formation of 3+ cation state of isolated Pd atoms activates adsorbed oxygen molecule for hydrogenation and simnutaously restrains the formation of intermediate toward by‐product, compared with traditional bulk Pd catalyst in the metallic state. This work provides theoretical insights into the feasibility of atom‐level dispersion Pd to catalyze direct synthesis of H2O2 and guidance for its future development.

Influence of the Molecular Structure on the Electrocatalytic Hydrogenation of Carbonyl Groups and H2 Evolution on Pd

We investigated the electrocatalytic hydrogenation (ECH) of model aldehydes and ketones over carbon-supported Pd in the aqueous phase. We propose reaction mechanisms based on kinetic measurements and on spectroscopic and electrochemical characterization of the working catalyst. The reaction rates of ECH and of the H2 evolution reaction (HER) vary with the applied electric potential following trends that strongly depend on the organic substrate. The intrinsic rates of hydrogenation and H2 evolution are influenced, in opposing ways, by the sorption of the reacting organic substrate. Strong interactions, that is, higher standard free energies of adsorption of the organic compound, induce high hydrogenation rates. The fast hydrogenation kinetics produces a hydrogen-depleted environment that kinetically hinders the HER and the bulk phase transition of Pd to a H-rich bulk Pd hydride, which is triggered by the applied potential in the absence of reacting organic compounds. As a consequence of strong organic–metal interactions, hydrogenation dominates at low overpotential. However, the coverages of organic substrates on the metal surface decrease, and the rates of H2 evolution surpass those of hydrogenation with increasingly negative electric potential. We determined the range of electric potential favoring hydrogenation on Pd and quantitatively deconvoluted the effects of the sorption of the organic compound, and of the rates of proton-coupled electron transfers, on the kinetics of both ECH and HER. The results indicate that electrocatalysis offers hydrogenation pathways for polar molecules which are different and, in some cases, faster than those dominating in the absence of an external electric potential.

Role of quantum well in Pd(111) thin film magnetism

The face-centered cubic palladium (Pd) is paramagnetic but exhibits ferromagnetism in special environments, such as nanostructures and thin films. In this study, first-principles density functional calculations are performed to investigate the magnetism of the Pd(111) films. Results show that a ferromagnetic state is more stable than a paramagnetic state with a period of ∼2.5 monolayers. A Fermi surface nesting wave vector of 1.15 Å−1 in bulk Pd is observed along the [111] direction, which matches well with the periodicity of the thin film ferromagnetism. Quantum well states cross the Fermi level periodically in tune on Fermi surface nesting, which is the main cause of the ferromagnetism, satisfying the Stoner criterion. Strain effect is discussed, and tensile strain may strengthen the ferromagnetism. In addition, the surface energy of the Pd(111) film is affected by the quantum well states, showing the oscillatory film stability with thickness.

Embedding functions for Pt and Pd: recalculation and verification on properties of bulk phases, Pt, Pd, and Pt–Pd nanoparticles

Embedding functions for Pt and Pd: recalculation and verification on properties of bulk phases, Pt, Pd, and Pt–Pd nanoparticles

Interplay between Surface-Adsorbed CO and Bulk Pd Hydride under CO2-Electroreduction Conditions

Palladium is an increasingly investigated electrocatalyst for the electrochemical reduction of carbon dioxide due to its unique ability to yield carbon monoxide or formate with large selectivities at high vs low overpotentials (i.e., ∼−0.5 to ∼−1.0 vs ∼−0.1 to ∼−0.4 V vs the reversible hydrogen electrode), respectively. While this behavior has been described multiple times on different Pd-electrocatalysts, previous studies disagree with regard to palladium’s ability to form a hydride phase (PdHx) under CO2 reduction reaction (CO2RR) conditions, as well as on the influence of this PdHx on its CO2RR-selectivity. These inconsistencies are partially related to the known poisoning of the Pd-surface during the CO2RR with adsorbed CO, whose precise influence on the formation of PdHx and corresponding effects on the CO2RR-mechanism and product selectivity remain poorly understood. With this motivation, herein we used an unsupported Pd-aerogel to investigate CO adsorption and PdHx formation at CO2RR potentials in electrolytes with different compositions (i.e., in the presence or absence of CO2 and bicarbonate) and as a function of the applied potential and duration of the potential hold. The results of these electrochemical measurements unveiled the strong influence of surface-adsorbed CO on the formation rate of Pd-hydride, for which the final stoichiometry was nevertheless found to be independent of the presence of CO2 or bicarbonate. This finding was supported by in situ X-ray absorption spectroscopy (XAS) measurements that confirmed the complete formation of β-phase PdHx (with x ≈ 0.6) at all applied potentials under CO2RR conditions, thus shedding light on the contradictory results in this regard reported in previous operando XAS studies.

Self-sustained oscillations in oxidation of methane over palladium: Experimental study and mathematical modeling.

An experimental study of the catalytic oxidation of methane over Pd foil in a flow reactor revealed that regular temporal oscillations in the reaction rate can arise at atmospheric pressure under methane-rich conditions. CO, CO2, H2, and H2O were detected as products. The oscillations of partial pressures of products and reactants in the gas phase were accompanied by oscillations of the catalyst temperature. According to an operando x-ray diffraction and mass-spectrometry study, the oscillations originate due to spontaneous oxidation and reduction of palladium; the high active catalyst surface is represented by metallic palladium, and the transition to the low-active state is accompanied by the formation of the PdO phase. In addition, it was detected that carbon dissolves in near-surface layers of palladium to form the PdCx phase. To describe the oscillations in the oxidation of methane, a 17-step reaction mechanism and a corresponding kinetic model were developed. The mechanism considers direct dissociative adsorption of methane and oxygen, pyrolytic activation of methane, oxidation and reduction of palladium, and direct formation and desorption of CO, CO2, H2, and H2O. Numerical solutions from the mathematical model of the continuously stirred-tank reactor qualitatively reproduce experimentally observed oscillatory dynamics. We have also developed a model, which considers the reversible diffusion of adsorbed oxygen and carbon atoms into the Pd bulk that allows us to explain the long induction period preceding the appearance of the oscillations. Mathematical modeling shows that the concentrations of dissolved oxygen and carbon atoms also oscillate under reaction conditions.

(Invited) Pd Monolayer Catalyst As Transformative Concept for Electrolytic Hydrogen Isotope Separation

The effect of strain in Pd monolayer catalyst is explored for electrolytic hydrogen isotope separation. Positive/negative strain increases/decreases strength of adsorbed hydrogen bond in overpotential region where recombination of hydrogen atoms is the rate determining step in hydrogen evolution reaction. As a result, the increased/decreased hydrogen isotope separation efficiency of Pd monolayers is expected as compared to bulk Pd. The positive/negative strain rises/lowers diffusion barrier for adsorbed hydrogen atoms. This effect favors/retards recombination of isotopes with smaller mass. As the surface is stretched/compressed, the neighboring adsorption sites separate/approach each other which affects heavier hydrogen isotopes more/less and result in their lower/higher probability for recombination. All these fundamental consideration indicate that Pd monolayers with different level of strain should have much different separation factors than corresponding bulk electrodes. To study described effects, Pd monolayers we synthesized electrochemically on Au(111) and Ru(0001) electrodes each yielding qualitatively different strain levels (Au-positive, Ru-negative). The adsorption strength of hydrogen isotopes is studied by infra-red spectroscopy. These results are input in classical models for isotope separation[1] T calculations. The calculated ratio between the rates of hydrogen and deuterium recombination and separation factors for Pd monolayers and corresponding bulk electrodes are compared to experimentally measured ones. The following discussion focuses on understanding and quantification of strain effects on separation efficiency of Pd monolayers. [1] B. E. Conway, Proceedings of The Royal Society of London, A. Mathematical and Physical Sciences, 247, 400 (1958).

Elucidation of Structure-Activity Relations in Proton Electroreduction at Pd Surfaces: Theoretical and Experimental Study.

The structure-activity relationship is a cornerstone topic in catalysis, which lays the foundation for the design and functionalization of catalytic materials. Of particular interest is the catalysis of the hydrogen evolution reaction (HER) by palladium (Pd), which is envisioned to play a major role in realizing a hydrogen-based economy. Interestingly, experimentalists observed excess heat generation in such systems, which became known as the debated "cold fusion" phenomenon. Despite the considerable attention on this report, more fundamental knowledge, such as the impact of the formation of bulk Pd hydrides on the nature of active sites and the HER activity, remains largely unexplored. In this work, classical electrochemical experiments performed on model Pd(hkl) surfaces, "noise" electrochemical scanning tunneling microscopy (n-EC-STM), and density functional theory are combined to elucidate the nature of active sites for the HER. Results reveal an activity trend following Pd(111) > Pd(110) > Pd(100) and that the formation of subsurface hydride layers causes morphological changes and strain, which affect the HER activity and the nature of active sites. These findings provide significant insights into the role of subsurface hydride formation on the structure-activity relations toward the design of efficient Pd-based nanocatalysts for the HER.

Hierarchical palladium catalyst for highly active and stable water oxidation in acidic media.

Acidic water electrolysis is of great importance for boosting the development of renewable energy. However, it severely suffers from the trade-off between high activity and long lifespan for oxygen evolution catalysts on the anode side. This is because the sluggish kinetics of oxygen evolution reaction necessitates the application of a high overpotential to achieve considerable current, which inevitably drives the catalysts far away from their thermodynamicequilibrium states. Here we demonstrate a new oxygen evolution model catalyst-hierarchical palladium (Pd) whose performance even surpasses the benchmark Ir- and Ru-based materials. The Pd catalyst displays an ultralow overpotential (196mV), excellent durability and mitigated degradation (66μVh-1) at 10mAcm-2 in 1M HClO4. Tensile strain on Pd (111) facets weakens the binding of oxygen species on electrochemical etching-derived hierarchical Pd and thereby leads to two orders of magnitudes of enhancement of mass activity in comparison to the parent Pd bulk materials. Furthermore, the Pd catalyst displays the bifunctional catalytic properties for both oxygen and hydrogen evolutions and can deliver a current density of 2Acm-2 at a low cell voltage of 1.771V when fabricated into polymer electrolyte membrane electrolyser.