Mesoporous palladium—the surface electrochemistry of palladium in aqueous sodium hydroxide and the cathodic reduction of nitrite

Sublayer Stable Fe Dopant in Porous Pd Metallene Boosts Oxygen Reduction Reaction

Engineering the morphology and electronic properties simultaneously of emerging metallene materials is an effective strategy for enhancing their performance as oxygen reduction reaction (ORR) electrocatalysts. Herein, a highly efficient and stable ORR electrocatalyst, Fe-doped ultrathin porous Pd metallene (Fe-Pd UPM) composed of a few layers of 2D atomic metallene layers, was synthesized using a simple one pot wet-chemical method and characterized. Fe-Pd UPM was measured to have enhanced ORR activity compared to undoped Pd metallene. Fe-Pd UPM exhibits a mass activity of 0.736 A mg_Pd^-1 with a loss of mass activity of only 5.1% after 10 000 cycles at 0.9 V versus the reversible hydrogen electrode (vs RHE) in 0.1 M KOH solution. Density functional theory (DFT) calculations reveal that the stable Fe dopant in the inner atomic layers of Fe-Pd UPM delivers a much smaller overpotential during O* hydrogenation into OH*. The morphology, porous structure, and Fe doping were verified to have enhanced ORR activity. We believe that the rational design of metallene materials with porous structures and interlayer doping is promising for the development of efficient and stable electrocatalysts.

In situ electrochemical activation as a generic strategy for promoting the electrocatalytic hydrogen evolution reaction and alcohol electro-oxidation in alkaline medium

In situ electrochemical activation as a new pre-treatment method is extremely effective for enhanced electrocatalytic performances for different applications. With the help of this method, in situ surface modification of electrocatalyst is achieved without using pre-made seeds or complex synthesis procedure. Herein, with the purpose of finding an in situ and simple electrochemical activation protocol, the green synthesis of Au/Pd nanoparticles (AuPd) by means of polyoxometalate (POM) is reported. Structural analysis of the AuPd nanohybrid unveil the Au-core/Pd-shell structure which surrounded by POM. We propose a novel cathodic electrochemical activation in phosphate buffer solution which can greatly boost the electrocatalytic activity of the as-prepared AuPd and Pd electrocatalyst not only for hydrogen evolution reaction (HER) as a model of electro-reduction, but also for methanol and ethanol electro-oxidation reaction (MOR & EOR). For the HER in 1 M NaOH solution, after the electrochemical activation, the needed potential to drive a geometrical current density of 10 mA cm-2 significantly decreases from - 400 mV vs. the reversible hydrogen electrode (RHE) to -290 mV vs. RHE. For the EOR and MOR, electrochemically activated AuPd realized 3.4- and 2.9- fold increase in mass current density (mA mgPd -1) with respect to the pristine AuPd electrocatalyst, respectively.

Tunable Periodically Ordered Mesoporosity in Palladium Membranes Enables Exceptional Enhancement of Intrinsic Electrocatalytic Activity for Formic Acid Oxidation

Developing superior electrocatalysts for formic acid oxidation (FAO) is the most crucial step in commercializing direct formic acid fuel cells. Herein, we electrodeposited palladium membranes with periodically ordered mesoporosity obtained by asymmetrically replicating the bicontinuous cubic phase structure of a lyotropic liquid-crystal template. The Pd membrane with the largest periodicity and highest degree of order delivered up to 90.5 m²  g^-1 of electrochemical active surface area and 3.34 A mg^-1 electrocatalysis capability towards FAO, 3.8 and 7.8 times the values of the commercial Pd/C catalyst, respectively. By controlling the temperature and potential of the electrodeposition procedure, the periodicity area and order degree of the mesoporosity are highly tunable. These Pd membranes gave prototype formic acid fueled cells with 4.3 and 2.4 times the maximum current and power density of the commercial Pd/C catalyst.

Adsorption Behavior and Electron Structure Engineering of Pd-Based Catalysts for Acetylene Hydrochlorination

Adsorption and activation for substrates and the stability of Pd species in Pd-based catalysts are imperative for their wider adoption in industrial and practical applications. However, the influence factor of these aspects has remained unclear. This indicates a need to understand the various perceptions of the structure–function relationship that exists between microstructure and catalytic performance. Herein, we revisit the catalytic performance of supported-ionic-liquid-phase stabilized Pd-based catalysts with nitrogen-containing ligands as a promoter for acetylene hydrochlorination, and try to figure out their regulation. We found that the absolute value of the differential energy, |Eads(C2H2)-Eads(HCl)|, is negative correlated with the stability of palladium catalysts. These findings imply that the optimization of the electron structure provides a new strategy for designing highly active yet durable Pd-based catalysts.

Design strategies for the development of a Pd-based acetylene hydrochlorination catalyst: improvement of catalyst stability by nitrogen-containing ligands

Acetylene hydrochlorination is an attractive chemical reaction for the manufacture of polyvinyl chloride (PVC), and the development efforts are focused on the search for non-mercury catalyst systems. Supported Pd-based catalysts have relatively high activity in the catalytic hydrochlorination of acetylene but are still deactivated rather quickly. Herein, we demonstrated that the atomically dispersed (NH₄)₂PdCl₄ complex, distributed on activated carbon, enabled the highly active and stable production of the vinyl chloride monomer (VCM) through acetylene hydrochlorination under low temperature conditions. We found that the presence of nitrogen-containing ligands in the structure of the active center could remarkably improve the stability of the Pd-based catalysts when compared with the case of the conventional PdCl₂ catalyst. Further analyses via X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR) show that the variations in the Pd dispersion, chemical state and reduction property are caused by the nitrogen-containing ligands. Temperature-programmed desorption (TPD) characterizations illustrated that the N-containing ligands over the (NH₄)₂PdCl₄/AC catalyst might enhance the adsorption of HCl. These findings suggest that in addition to strategies that target the doping modification of support materials, optimization of the structure of the active center complexes provides a new path for the design of highly active and stable Pd-based catalysts.

The activation of substrates over Pd active sites and the corresponding dispersion could be enhanced by the introduction of N-containing ligands.

The unexpected activity of Pd nanoparticles prepared using a non-ionic surfactant template

Pd deposits on vitreous carbon substrates were prepared by electrodeposition from liquid crystal phases (both micellar and hexagonal phases) consisting of self-assembled non-ionic surfactant molecules. The morphology of the deposits varied significantly with the concentration of the surfactant but all are made up of aggregated nanoparticles circa 9 nm in diameter. The deposits from the micellar phase of the surfactant offer the largest electroactive area and specific activity for the hydrogen evolution, oxygen evolution and reduction reactions and formic acid and ethanol oxidations. Unexpectedly the deposits lead to an increase in catalytic activity far in excess of that expected from an enhancement in electroactive area.

Nanopatterning palladium surface layers through electrochemical deposition and dissolution of zinc in ionic liquid

Cracklike nanopatterned palladium surface layers have been produced by a green chemistry method based on in situ electrochemical deposition-dissolution of zinc (Zn-ECDD) in an ionic liquid bath. During the cathodic process, reactive Zn was electrochemically deposited onto a polycrystalline Pd substrate. During the subsequent anodic process, Zn was removed from the substrate through electrochemical dissolution. Scanning electron microscope (SEM) measurements showed that repetitive Zn-ECDD mediated by potential cycles results in the nanopatterning of Pd surface layers, characterized by uniform crack appearance with well-distributed concave spacings separated by nanowidth cracks. Energy-dispersive X-ray microscopy (EDX) studies revealed that the nanopatterned surface layers chemically contain a small amount of Zn. A mechanism based on the development of stress induced by the Zn-ECDD on Pd surfaces was proposed to be responsible for the nanopatterning of Pd surface layers. Electrochemical oxidation of formic acid and reduction of nitrite were studied as model reactions to demonstrate potential applications of the nanopatterned Pd electrode to electrocatalysis and electrochemical determination of environmental contaminants. Highly improved electrochemical responses were obtained on the nanopatterned Pd for the two reactions, compared to the untreated Pd.

Nanoporous palladium with sub-10 nm dendrites by electrodeposition for ethanol and ethylene glycol oxidation

High surface area Pd foams with roughness factors of more than 1000 and a specific surface area of 60 m(2) g(-1) are obtained by electrodeposition. The foams are composed of dendrites with branches on the 10 nm scale. The resulting electrodes show high activity towards the oxidation of C(2) alcohols.

Low-coordination sites in oxygen-reduction electrocatalysis: their roles and methods for removal

Low-coordination sites, including edges, kinks, and defects, play an important role in oxygen-reduction electrocatalysis. Their role was studied experimentally and theoretically for various Pt surfaces. However, the roughness effect on similar-sized nanoparticles that could elucidate the role of low-coordination sites has attracted much less attention, with no studies on Pd nanoparticles. Here, using Br- adsorption/desorption, we introduce an effective approach to reduce surface roughness, yielding Pd nanoparticles with smoother surfaces and an increased number of (111)-oriented facets. The resulting nanoparticles have a slightly contracted structure and narrow size distribution. Pt monolayer catalysts that contain such nanoparticles as the cores showed a 1.5-fold enhancement in specific and Pt mass activities for the oxygen reduction reaction compared with untreated ones. Furthermore, a dramatic increase in durability was observed with bromide-treated Pd(3)Co cores. These results demonstrate a simple approach to preparing nanoparticles with smooth surfaces and confirm the adverse effect of low-coordination sites on the kinetics of the oxygen-reduction reaction.