Oxygen reduction reaction at binary and ternary nanocatalysts based on Pt, Pd and Au

Recent advances in Pt-based electrocatalysts for PEMFCs

In order to reduce the cost and improve the performance of proton exchange membrane fuel cells (PEMFCs), it is imperative to further enhance the activity and durability of Pt based electrocatalysts for the oxygen reduction reaction (ORR). This article analyzes the latest advances in Pt-based ORR electrocatalysts, including the Pt alloys, Pt–M core–shell structures, particle size effects, support effects, doping in Pt/PtM and post treatment. In addition, the performance of some of the developed novel electrocatalysts in membrane electrode assemblies (MEA) is also included for comparison, as they are rarely available and the superior activity and durability exhibited in RDE frequently doesn't translate into MEA.

In this paper, the latest progress in the design of Pt-based ORR electrocatalysts is reviewed, including the understanding of research progress in the synthesis of high activity and high stability catalysts.

Towards High Efficacy of Pd-Au/C Catalyst for Tetrachloromethane Hydrodechlorination

We present an efficient strategy for synthesising the PdAu catalysts with a homogeneous PdAu alloy phase for environmentally important hydrodechlorination of tetrachloromethane in the gas phase. The synthesis of carbon-supported catalysts involved two major steps: (i) incorporation of palladium and gold nanoparticles into carbon support and (ii) activation of the catalysts. The critical part of this work was to find the optimal conditions for both steps. Thus, the incorporation of the nanoparticles was carried out in two ways, by impregnation and direct redox reaction method using acetone solutions of metal precursor salts. The activation was performed either by a conventional thermal reduction in hydrogen or flash irradiation in a microwave oven. The homogeneity and structure of the PdAu alloy were found to depend on the catalyst activation method critically. In all cases, we observed better homogeneity for catalysts that were subject to microwave irradiation. Moreover, the flash microwave irradiation of prepared catalysts provided catalysts of better stability and selectivity towards the desired products (hydrocarbons) in the hydrodechlorination of tetrachloromethane as compared to the catalyst obtained by conventional thermal activation in hydrogen.

Electroreforming of Glucose and Xylose in Alkaline Medium at Carbon Supported Alloyed Pd3Au7 Nanocatalysts: Effect of Aldose Concentration and Electrolysis Cell Voltage

The effects of cell voltage and of concentration of sugars (glucose and xylose) on the performances of their electro-reforming have been evaluated at a Pd3Au7/C anode in 0.10 mol L−1 NaOH solution. The catalyst synthesized by a wet chemistry route is first comprehensively characterized by physicochemical and electrochemical techniques. The supported catalyst consists in alloyed Pd3Au7 nanoparticles of circa 6 nm mean diameter deposited on a Vulcan XC72 carbon support, with a metal loading close to 40 wt%. Six-hour chronoamperometry measurements are performed at 293 K in a 25 cm2 electrolysis cell for the electro-conversion of 0.10 mol L−1 and 0.50 mol L−1 glucose and xylose at cell voltages of +0.4 V, +0.6 V and +0.8 V. Reaction products are analyzed every hour by high performance liquid chromatography. The main products are gluconate and xylonate for glucose and xylose electro-reforming, respectively, but the faradaic yield, the selectivity and the formation rate of gluconate/xylonate decrease with the increase of aldose concentration, whereas lower faradaic yields and higher formation rates of gluconate/xylonate are observed at +0.8 V than at +0.4 V (higher chemical yields).

Pd-Shaped Nanoparticles Modified by Gold ad-Atoms: Effects on Surface Structure and Activity Toward Glucose Electrooxidation

Palladium nanoparticles (Pd-NPs) with controlled distributions of sizes and shapes (nanospheres-Pd-NS-, nanocubes -Pd-NC-, and nanooctahedrons -Pd-NO-) are synthesized by wet chemistry methods and characterized by TEM/HRTEM. The surfaces of Pd-NPs are modified by spontaneous adsorption of gold and characterized by cyclic voltammetry in acidic medium. It is shown that the modification of Pd-NPs by dipping in HAuCl4 solutions of different concentrations allows controlling the surface coverage by gold. It is also shown that the modification of Pd-NPs surfaces involves first the formation of PdAu surface alloys. For higher coverages, both PdAu surface alloys and pure Au structures are formed. The activity toward the glucose electrooxidation reaction is determined by linear scan voltammetry (LSV). Higher activity is observed on pure Pd-NC presenting extended (100) surfaces than on Pd-NO with mainly (111) surface orientation and on Pd-NS without preferential surface orientation, both these latter Pd-NPs displaying almost the same activity. The modification of the surface by spontaneous adsorption of gold greatly improves the activity of all Pd-NPs. However, Au-modified Pd-NC materials remain the most active catalysts. PdAu surface alloys seem to be involved in the improvement of the catalytic activity at low potentials, although the role of pure gold structures on Pd-NPs toward the enhancement of the catalytic activity cannot be excluded for high gold coverage. The study allows a better understanding of the material structure/electrocatalytic behavior relationship.

Tuning Catalytic Properties of Supported Bimetallic Pd/Ir Systems in the Hydrogenation of Cinnamaldehyde by Using the “Water-in-Oil” Microemulsion Method

Supported Pd/Ir bimetallic catalysts were synthesized by the “water-in-oil” microemulsion method at different precursor concentrations and characterized by XRD, XPS, SEM, TEM, and cyclic voltammetry. Depending on the preparation conditions, formation of bimetallic catalysts with different metal segregation and surface composition can be easily obtained, thus tuning the bimetallic structure of catalysts as well as their relative catalytic properties. Bimetallic Pd/Ir systems were efficiently tested in the hydrogenation of cinnamaldehyde showing a better performance than analogous monometallic catalysts.

Nanoscale mechanism of the stabilization of nanoporous gold by alloyed platinum

Nanoporous gold (NPG) is usually made by electrochemical dealloying of Ag from binary AgAu alloys. The resulting nanoscale ligaments are not very stable, and tend to coarsen with time by surface self-diffusion, especially in electrolyte, which may lead to inferior electrocatalytic properties. Addition of a small amount of Pt to the precursor alloy is known to refine and stabilize the nanoporous product (NPG-Pt). However, the mechanisms by which Pt serves to refine the microstructure remain poorly understood. The present study aims to expand our knowledge of the role of Pt by examining NPG-Pt at atomic resolution with Atom Probe Tomography (APT), as well as by aberration-corrected Transmission Electron Microscopy. Atomic level observation of Pt enrichment on ligament surfaces sheds light on the underlying mechanisms that give rise to Pt's refining effect. Owing to improved Ag retention with higher Pt content, NPG-Pt₁ (made by dealloying Ag₇₇Au₂₂Pt₁) was shown to have the highest surface area-to-volume ratio, compared to NPG-Pt₃ (made by dealloying Ag₇₇Au₂₀Pt₃). Quantitative estimates reveal up to 5-fold enrichment of Pt at nanoligament surfaces, compared to the precursor content, in NPG-Pt. The interface between the dealloyed layer and the substrate was captured by APT, for the first time. The findings of this investigation add insight into the functionality of NPG-Pt and its prospective catalytic performance.

A facile and surfactant-free route for nanomanufacturing of tailored ternary nanoalloys as superior oxygen reduction reaction electrocatalysts

Laser ablation synthesis in solution-galvanic replacement reaction (LASiS-GRR) enables tuning of elemental ratios and bonding properties for Pt based ternary nanoalloys as ORR electrocatalysts.

Decoupling strain and ligand effects in ternary nanoparticles for improved ORR electrocatalysis

Ternary Pt–Au–M (M = 3d transition metal) nanoparticles show reduced OH adsorption energies and improved activity for the oxygen reduction reaction (ORR) compared to pure Pt nanoparticles, as obtained by density functional theory.