Spongelike nanoporous Pd and Pd/Au structures: facile synthesis and enhanced electrocatalytic activity

Interfacial Engineering of Porous Pd/M (M = Au, Cu, Mn) Sponge-like Nanocrystals with a Clean Surface for Enhanced Alkaline Electrochemical Oxidation of Ethanol

The interfacial engineering of Pd-based alloys (i.e., PdM with distinct morphologies, compositions, and strain defects) is an efficient way for enhanced catalytic activity; however, it remains a grand challenge to fabricate such alloys in aqueous solutions without heating, organic solvents, and multiple reaction steps. Herein, we present a simple, aqueous-phase, one-step, and ultrafast approach for the interfacial engineering of surfactant-free porous PdM (M = Cu, Au, and Mn) nanocrystals with well-controlled spongy-like morphology and compositions. The electronic interaction in PdM nanocrystals and their effect on the alkaline electrochemical ethanol oxidation reaction (EOR) are investigated using XRD, XPS, and electrochemical tests. Notably, integrating M metals into Pd atoms results in upshifting the d-band center of Pd and subsequently modulating the EOR activity and stability substantially. The EOR mass activity (10.78 A/mgPd (6.93 A/mgPdCu)) of PdCu was 1.83, 3.09, 4.51, and 53.90 times higher than those of AuPd (5.90 A/mgPd (3.27 A/mgAuPd)), PdMn (3.48 A/mgPd (3.19 A/mgPdMn)), Pd (2.39 A/mgPd), and Pd/C (0.20 A/mgPd), respectively, besides substantial durability after 1000 cycles. This is due to the porous two-dimensional morphology, a low synergetic effect, higher interfacial interaction, and greater active surface area of PdCu, besides a high Cu content with more oxophilicity that facilitates activation/dissociation of H2O to generate -OH species needed for quick EOR electrocatalysis. The electrochemical impedance spectroscopy (EIS) reveals better electrolyte/electrode interfacial interaction and lower charge transfer resistance on PdCu. The EOR activity of PdCu porous sponge-like nanocrystals was superior to all previously reported Pd-based alloys for electrochemical EOR. This study indicates that binary Pd-based catalysts with less synergetic effect are preferred for boosting the EOR activity, which could help in manipulating the surface properties of Pd-based alloys to optimize EOR performance.

Nanoporous Gold: From Structure Evolution to Functional Properties in Catalysis and Electrochemistry

Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis.

Room-temperature coalescence of Pd nanoparticles with sacrificial templates and sintering agents, and their catalytic activities in the Suzuki coupling reaction

Porous metal structures are very useful for heterogeneous catalysts in organic syntheses. This study reports a novel method to fabricate porous Pd structures by room-temperature (RT) coalescence of Pd nanoparticles (Pd NPs). First, oleylamine-capped Pd NPs were synthesized, and then Pd NP pastes were fabricated by mixing with tri-n-octylphosphine oxide as a sacrificial template. Finally, the Pd NP paste was dipped into methanol containing a sintering agent. When KOH was used as the sintering agent, porous Pd structures could be successfully obtained at RT. The catalytic activities of porous Pd structures were investigated in the Suzuki coupling reaction and they increased with the increase of the KOH concentration in the sintering process. These results indicate that pre-activation of porous Pd structures by KOH increased the catalytic activities.

Synthesis of Supported Bimetal Catalysts using Galvanic Deposition Method

Supported bimetallic catalysts have been studied because of their enhanced catalytic properties due to metal-metal interactions compared with monometallic catalysts. We focused on galvanic deposition (GD) as a bimetallization method, which achieves well-defined metal-metal interfaces by exchanging heterogeneous metals with different ionisation tendencies. We have developed Ni@Ag/SiO₂ catalysts for CO oxidation, Co@Ru/Al₂ O₃ catalysts for automotive three-way reactions and Pd-Co/Al₂ O₃ catalysts for methane combustion by using the GD method. In all cases, the catalysts prepared by the GD method showed higher catalytic activity than the corresponding monometallic and bimetallic catalysts prepared by the conventional co-impregnation method. The GD method provides contact between noble and base metals to improve the electronic state, surface structure and reducibility of noble metals.

Metal-Free Carbon-Based Materials: Promising Electrocatalysts for Oxygen Reduction Reaction in Microbial Fuel Cells

Microbial fuel cells (MFCs) are a promising green approach for wastewater treatment with the simultaneous advantage of energy production. Among the various limiting factors, the cathodic limitation, with respect to performance and cost, is one of the main obstacles to the practical applications of MFCs. Despite the high performance of platinum and other metal-based cathodes, their practical use is limited by their high cost, low stability, and environmental toxicity. Oxygen is the most favorable electron acceptor in the case of MFCs, which reduces to water through a complicated oxygen reduction reaction (ORR). Carbon-based ORR catalysts possessing high surface area and good electrical conductivity improve the ORR kinetics by lowering the cathodic overpotential. Recently, a range of carbon-based materials have attracted attention for their exceptional ORR catalytic activity and high stability. Doping the carbon texture with a heteroatom improved their ORR activity remarkably through the favorable adsorption of oxygen and weaker molecular bonding. This review provides better insight into ORR catalysis for MFCs and the properties, performance, and applicability of various metal-free carbon-based electrocatalysts in MFCs to find the most appropriate cathodic catalyst for the practical applications. The approaches for improvement, key challenges, and future opportunities in this field are also explored.

Highly Efficient Silver-Cobalt Composite Nanotube Electrocatalysts for Favorable Oxygen Reduction Reaction

This paper reports the synthesis and characterization of silver-cobalt (AgCo) bimetallic composite nanotubes. Cobalt oxide (Co₃O₄) nanotubes were fabricated by electrospinning and subsequent calcination in air and then reduced to cobalt (Co) metal nanotubes via further calcination under a H₂/Ar atmosphere. As-prepared Co nanotubes were then employed as templates for the following galvanic replacement reaction (GRR) with silver (Ag) precursor (AgNO₃), which produced AgCo composite nanotubes. Various AgCo nanotubes were readily synthesized with applying different reaction times for the reduction of Co₃O₄ nanotubes and GRR. One hour reduction was sufficiently long to convert Co₃O₄ to Co metal, and 3 h GRR was enough to deposit Ag layer on Co nanotubes. The tube morphology and copresence of Ag and Co in AgCo composite nanotubes were confirmed with SEM, HRTEM, XPS, and XRD analyses. Electroactivity of as-prepared AgCo composite nanotubes was characterized for ORR with rotating disk electrode (RDE) voltammetry. Among differently synthesized AgCo composite nanotubes, the one synthesized via 1 h reduction and 3 h GRR showed the best ORR activity (the most positive onset potential, greatest limiting current density, and highest number of electrons transferred). Furthermore, the ORR performance of the optimized AgCo composite nanotubes was superior compared to pure Co nanotubes, pure Ag nanowires, and bare platinum (Pt). High ethanol tolerance of AgCo composite nanotubes was also compared with the commercial Pt/C and then verified its excellent resistance to ethanol contamination.

Pt skin on Pd-Co-Zn/C ternary nanoparticles with enhanced Pt efficiency toward ORR

Exploring highly active, stable and relatively low-cost nanomaterials for the oxygen reduction reaction (ORR) is of vital importance for the commercialization of proton exchange membrane fuel cells (PEMFCs). Herein, a highly active, durable, carbon supported, and monolayer Pt coated Pd-Co-Zn nanoparticle is synthesized via a simple impregnation-reduction method, followed by spontaneous displacement of Pt. By tuning the atomic ratios, we obtain the composition-activity volcano curve for the Pd-Co-Zn nanoparticles and determined that Pd : Co : Zn = 8 : 1 : 1 is the optimal composition. Compared with pure Pd/C, the Pd8CoZn/C nanoparticles show a substantial enhancement in both the catalytic activity and the durability toward the ORR. Moreover, the durability and activity are further enhanced by forming a Pt skin on Pd8CoZn/C nanocatalysts. Interestingly, after 10 000 potential cycles in N2-saturated 0.1 M HClO4 solution, Pd8CoZn@Pt/C shows improved mass activity (2.62 A mg(-1)Pt) and specific activity (4.76 A m(-2)total), which are about 1.4 and 4.4 times higher than the initial values, and 37.4 and 5.5 times higher than those of Pt/C catalysts, respectively. After accelerated stability testing in O2-saturated 0.1 M HClO4 solution for 30 000 potential cycles, the half-wave potential negatively shifts about 6 mV. The results show that the Pt skin plays an important role in enhancing the activity as well as preventing degradation.

Catalytic and peroxidase-like activity of carbon based-AuPd bimetallic nanocomposite produced using carbon dots as the reductant

In this report, carbon-based AuPd bimetallic nanocomposite (AuPd/C NC) was synthesized using carbon dots (C-dots) as the reducing agent and stabilizer by a simple green sequential reduction strategy, without adding other agents. The as synthesized AuPd/C NC showed good catalytic activity and peroxidase-like property. The structure and morphology of these nanoparticles were clearly characterized by UV-Vis spectroscopy, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The AuPd/C NC catalyst exhibits noticeably higher catalytic activity than Pd and Au nanoparticles in catalysis reduction of 4-nitrophenol (4-NP). Moreover, based on the high peroxidase-like property of AuPd/C NC, a new colorimetric detection method for hydrogen peroxide (H2O2) has been designed using 3,3',5,5'-tetramethyl-benzidine (TMB) as the substrate, which provides a simple and sensitive means to detect H2O2 in wide linear range of 5 μM-500 μM and 500 μM-4 mM with low detection limit of 1.6 μM (S/N = 3). Therefore, the facile synthesis strategy for bimetallic nanoparticles by the mild reductant of carbon dot will provide some new thoughts for preparing of carbon-based metal nanomaterials and expand their application in catalysis and analytical chemistry areas.

Electrochemically synthesized sulfur-doped graphene as a superior metal-free cathodic catalyst for oxygen reduction reaction in microbial fuel cells

Electrochemically synthesized S-GN was proved to be an efficient alternative cathode catalyst to Pt/C in microbial fuel cell.

Recent advances in noble metal based composite nanocatalysts: colloidal synthesis, properties, and catalytic applications

This Review article provides a report on progress in the synthesis, properties and catalytic applications of noble metal based composite nanomaterials. We begin with a brief discussion on the categories of various composite materials. We then present some important colloidal synthetic approaches to the composite nanostructures; here, major attention has been paid to bimetallic nanoparticles. We also introduce some important physiochemical properties that are beneficial from composite nanomaterials. Finally, we highlight the catalytic applications of such composite nanoparticles and conclude with remarks on prospective future directions.

A reusable microRNA sensor based on the electrocatalytic property of heteroduplex-templated copper nanoclusters

Repeat detection of miRNA is accomplished by reconstruction of DNA–RNA heteroduplexes-templated copper nanoclusters on an electrode surface.

Simple synthesis of bimetallic alloyed Pd–Au nanochain networks supported on reduced graphene oxide for enhanced oxygen reduction reaction

A simple and rapid wet-chemical co-reduction method was developed for synthesis of alloyed Pd–Au nanochain networks supported on RGO, with the assistance of caffeine as a capping agent and a structure directing agent.