Insights into the Effect of Metal Ratio on Cooperative Redox Enhancement Effects over Au- and Pd-Mediated Alcohol Oxidation

The aerobic oxidation of alcohols and aldehydes over supported heterogeneous catalysts can be considered as comprising two complementary and linked processes: dehydrogenation and oxygen reduction. Significant rate enhancements can be observed when these processes are catalyzed by independent active sites, coupled by electron transport between the two catalysts. This effect, termed cooperative redox enhancement (CORE), could significantly influence how researchers approach catalyst design, but a greater understanding of the factors which influence it is required. Herein, we demonstrate that the Au/Pd ratio used in physical mixtures of monometallic catalysts and phase-separated Au and Pd bimetallic catalysts dramatically influences the degree to which CORE effects can promote alcohol oxidation. Perhaps more interestingly, the roles of Au and Pd in this coupled system are determined to be interchangeable. Preliminarily, we hypothesize that this is attributed to the relative rates of the coupled reactions and demonstrate how physical properties can influence this. This deeper understanding of the factors which influence CORE is an important development in bimetallic catalysis.

Adsorption and removal of ethidium bromide from aqueous solution using optimized biogenic catalytically active antibacterial palladium nanoparticles

Due to being low cost and eco-friendly, biological nanomaterial synthesis and development have made broad spectral progress. This study aimed to optimize the phytomediated synthesis of catalytically active, antibacterial palladium nanoparticles (PdNPs) for adsorption-based removal of ethidium bromide (EtBr) from an aqueous solution. Optimization of synthesis demonstrated that a precursor to extract ratio of 4:1, pH 3, and incubation at 80 °C for 60 min were the optimum conditions that led to the synthesis of negatively charged, highly stable, polycrystalline, spherical, and monodispersed PdNPs of 5-10 nm. When tested as catalysts, PdNPs successfully catalyzed Suzuki-Miyaura cross-coupling between aryl halides and arylboronic acids resulting in the synthesis of 4-acetylbiphenyl. Furthermore, the antibacterial activity test demonstrated that biogenic PdNPs were most effective and potent against Staphylococcus aureus and Proteus vulgaris followed by Escherichia coli, Bacillus subtilis, and Bacillus cereus. In addition, PdNPs were found as an excellent adsorbent for adsorption of EtBr from water as the adsorption reaction obeyed pseudo-second-order kinetics with a linear regression coefficient (R² > 0.995). The adsorption reaction fitted well with the Freundlich and Temkin isotherm models, indicating multi-layer adsorption. Estimating thermodynamic parameters resulted in a positive value of ΔH⁰ and ΔG⁰, demonstrating adsorption was non-spontaneous and endothermic.

Emerging carbon shell-encapsulated metal nanocatalysts for fuel cells and water electrolysis

The development of low-cost, high-efficiency electrocatalysts is of primary importance for hydrogen energy technology. Noble metal-based catalysts have been extensively studied for decades; however, activity and durability issues still remain a challenge. In recent years, carbon shell-encapsulated metal (M@C) catalysts have drawn great attention as novel materials for water electrolysis and fuel cell applications. These electrochemical reactions are governed mainly by interfacial charge transfer between the core metal and the outer carbon shell, which alters the electronic structure of the catalyst surface. Furthermore, the rationally designed and fine-tuned carbon shell plays a very interesting role as a protective layer or molecular sieve layer to improve the performance and durability of energy conversion systems. Herein, we review recent advances in the use of M@C type nanocatalysts for extensive applications in fuel cells and water electrolysis with a focus on the structural design and electronic structure modulation of carbon shell-encapsulated metal/alloys. Finally, we highlight the current challenges and future perspectives of these catalytic materials and related technologies in this field.

Oxygen reduction reaction on Pd nanocatalysts prepared by plasma-assisted synthesis on different carbon nanomaterials

In this work He/H₂ plasma jet treatment was used to reduce Pd ions in the aqueous solution with simultaneous deposition of created Pd nanoparticles to support materials. Graphene oxide (GO) and nitrogen-doped graphene oxide (NrGO) were both co-reduced with the Pd ions to formulate catalyst materials. Pd catalyst was also deposited on the surface of carbon black. The prepared catalyst materials were physically characterized using transmission electron microscopy, scanning electron microscopy and x-ray photoelectron spectroscopy. The plasma jet method yielded good dispersion of small Pd particles with average sizes of particles being: Pd/rGO 2.9 ± 0.6 nm, Pd/NrGO 2.3 ± 0.5 nm and Pd/Vulcan 2.8 ± 0.6 nm. The electrochemical oxygen reduction reaction (ORR) kinetics was explored using the rotating disk electrode method. Pd catalyst deposited on nitrogen-doped graphene material showed slightly improved ORR activity as compared to that on the nondoped substrate, however Vulcan carbon-supported Pd catalyst exhibited a higher specific activity for oxygen electroreduction.

One-Pot Anchoring of Pd Nanoparticles on Nitrogen-Doped Carbon through Dopamine Self-Polymerization and Activity in the Electrocatalytic Methanol Oxidation Reaction

Exploration of advanced electrocatalysts to promote the sluggish methanol oxidation reaction (MOR) is of vital importance for developing high efficiency and low-cost direct methanol fuel cells. Highly dispersed palladium nanoparticles (Pd NPs) anchored on a nitrogen-doped carbon support were fabricated using a facile one-pot dopamine self-polymerization mediated redox strategy, in which dopamine not only acted as a moderate reductant to induce the formation of Pd NPs during self-polymerization but was also the precursor of the nitrogen-doped carbon support for Pd. The synthesized hybrid features the following characteristics: 1) High dispersity of Pd NPs, which exposed a high abundance of active surfaces and sites for heterogeneous electrocatalysis; 2) metal-support interactions, which may affect the surface chemistry and electron distribution of active Pd NPs; 3) the Pd NPs were partially imbedded or encapsulated into the support, thus reducing the possible agglomeration of Pd NPs during cyclic measurements. The electrocatalyst with such favorable features provided higher mass activity (2.2 times that of commercial Pd/C) and better durability (reduced loss of activity during simulated frequent startup-shutdown operations) for the MOR in alkaline media.

Active and Durable Hydrogen Evolution Reaction Catalyst Derived from Pd-Doped Metal-Organic Frameworks

The water electrolysis is of critical importance for sustainable hydrogen production. In this work, a highly efficient and stable PdCo alloy catalyst (PdCo@CN) was synthesized by direct annealing of Pd-doped metal-organic frameworks (MOFs) under N2 atmosphere. In 0.5 M H2SO4 solution, PdCo@CN displays remarkable electrocatalytic performance with overpotential of 80 mV, a Tafel slope of 31 mV dec(-1), and excellent stability of 10 000 cycles. Our studies reveal that noble metal doped MOFs are ideal precursors for preparing highly active alloy electrocatalysts with low content of noble metal.