Assembly of Bimetallic PdAg Nanosheets and Their Enhanced Electrocatalytic Activity toward Ethanol Oxidation

Alkali-Free Oxidation of 1,2-Propanediol to Lactic Acid over Intermetallic Cu1Pt Compounds

Polylactic acid (PLA) plastics are the most popular biodegradable and biocompatible materials; however, their monomer, lactic acid (LA), is currently produced via an inefficient fermentation process. Herein, activated carbon-supported Pt-Cu intermetallic compounds were synthesized and utilized in the selective oxidation of 1,2-propanediol (1,2-PDO) toward LA without the participation of homogeneous alkali or a basic support. It was found that rhombohedral intermetallic Cu1Pt compounds exhibited excellent performance and the calculated turnover frequency of each surface Pt atom reached 10 049 h-1. The yield of free LA reached 64.6% with a complete conversion of 1,2-PDO at 100 °C within 2 h. Diverse characterization and DFT calculations revealed that Pt-Cu intermetallic compounds were active for the adsorption and/or activation of molecular oxygen and 1,2-PDO. Raman analysis of adsorbed 1,2-PDO further disclosed that 1,2-PDO contacted with rhombohedral intermetallic Cu1Pt compounds strongly, and then the oxidation of the terminal hydroxyl group was performed preferentially, which promoted the formation of LA.

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.

Tubular palladium-based catalysts enhancing direct ethanol electrooxidation

Direct ethanol fuel cell (DEFC) has the advantages of high power density, high energy conversion efficiency and environmental friendliness, but its commercialization is restricted by factors such as insufficient activity and low anti-poisoning ability of anode catalyst for incomplete oxidation of ethanol. It is of great significance to design and prepare anode catalyst with high activity and high anti-poisoning ability that can be recycled. In this work, tubular palladium-based (Pd-based) catalysts with abundant lattice defect sites were prepared by simple and reproducible electro-displacement reactions using Cu nanowires as sacrificial template. Pd is the main catalytic element which provides adsorption sites for ethanol oxidation. Ag and Cu introduced facilitates the formation of hydroxyl groups to oxidize toxicity intermediates, and changes the d-band center position of Pd, so as to adjust the adsorption and desorption of ethanol and its intermediates on the Pd surface. At the same time, Au introduced with high potential maintains the stability of the catalyst structure. The tubular structure exposes more active sites, improves the atomic utilization rate and enhances the ability of the catalyst resisting dissolution and aggregation. The series of PdAuAgCu tubular catalysts with outer layer dendrites were prepared by electro-displacement reactions using the mixture (ethylene glycol : ultra-pure water = 3 : 1) as the reaction solvent and fivefold twinned Cu nanowires as sacrificial template. The performance evaluation of ethanol electrocatalytic oxidation showed that the Pd₁₇Au₄₀Ag₁₁Cu₃₂ tubular catalysts were prepared at 120 °C and 10 mM CTAB had excellent overall performance, with a peak mass activity of 6335 mA mg_Pd^-1, which was 9.6 times of Pd/C (JM). The residual current density after the stability test of 3000 s was 249 mA mg_Pd^-1, which was 3.3 times of Pd/C (JM).

Two-Dimensional Metal Nanostructures: From Theoretical Understanding to Experiment

This paper reviews recent studies on the preparation of two-dimensional (2D) metal nanostructures, particularly nanosheets. As metal often exists in the high-symmetry crystal phase, such as face centered cubic structures, reducing the symmetry is often needed for the formation of low-dimensional nanostructures. Recent advances in characterization and theory allow for a deeper understanding of the formation of 2D nanostructures. This Review firstly describes the relevant theoretical framework to help the experimentalists understand chemical driving forces for the synthesis of 2D metal nanostructures, followed by examples on the shape control of different metals. Recent applications of 2D metal nanostructures, including catalysis, bioimaging, plasmonics, and sensing, are discussed. We end the Review with a summary and outlook of the challenges and opportunities in the design, synthesis, and application of 2D metal nanostructures.

Ru doping boosts electrocatalytic water splitting

Heteroatom doping plays a crucial role in improving the electrocatalytic performance of catalysts towards water splitting. Owing to the existence of Ru-O moieties, Ru is thus emerging as an ideal dopant for promoting the electrocatalytic performance for water splitting by modifying the electronic structure, introducing extra active sites, improving electronic conductivity, and inducing a strong synergistic effect. Benefitting from these advantages, Ru-doped nanomaterials have been widely investigated and employed as advanced electrocatalysts for water splitting, and many excellent Ru-doped electrocatalysts have been successfully developed. In an effort to obtain a better understanding of the influence of Ru doping on the electrocatalytic water splitting performance of nanocatalysts, we herein summarize the recent progress of Ru-doped electrocatalysts by focusing on the synthesis strategies and advantageous merits. Applications of these new materials in water electrolysis technology are also discussed with emphasis on future directions in this active field of research.

Unveiling the Synergistic Effects of Monodisperse Sea Urchin-like PdPb Alloy Nanodendrites as Stable Electrocatalysts for Ethylene Glycol and Glycerol Oxidation Reactions

In recent times, the fabrication of noble metal-based catalysts with controllable morphologies has become a research hotspot. Electrocatalytic devices with excellent catalytic performance and enhanced durability for the ethylene glycol oxidation reaction (EGOR) and the glycerol oxidation reaction (GOR) are significant for commercial direct fuel cells. Herein, a series of PdPb sea urchin-like nanodendrite (ND) structures with controllable molar ratios were synthesized as EGOR and GOR electrocatalysts of high efficiency. The optimized structurally regular Pd₃Pb NDs exhibit the best electrocatalytic activity and outstanding stability compared to other samples and commercial Pt/C. In addition, the integrated Pb on Pd₃Pb NDs can mitigate the bond energy the intermediates generate and further boost the electrooxidation of the intermediates by supplying enough active sites without considering its intrinsic structure, which is beneficial to the enhanced EGOR and GOR activity and stability. With the assistance of electrochemical measurement, the mechanism of the enhanced alloy was further investigated. This paper presents a promising strategy to fabricate catalysts with stable structures, which will elucidate a very promising approach for developing Pd-based catalysts for further applications in fuel cells.

PdAg/C Electrocatalysts Synthesized by Thermal Decomposition of Polymeric Precursors Improve Catalytic Activity for Ethanol Oxidation Reaction

An efficient ethanol oxidation reaction (EOR) is required to enhance energy production in alcohol-based fuel cells. The use of bimetallic catalysts promises decreasing reliance on platinum group metal (PGM) electrocatalysts by minimizing the use of these expensive materials in the overall electrocatalyst composition. In this article, an alternative method of bimetallic electrocatalyst synthesis based on the use of polymeric precursors is explored. PdAg/C electrocatalysts were synthesized by thermal decomposition of polymeric precursors and used as the anode electrocatalyst for EOR. Different compositions, including pristine Pd/C and Ag/C, as well as bimetallic Pd80Ag20/C, and Pd60Ag40/C electrocatalysts, were evaluated. Synthesized catalysts were characterized, and electrochemical activity evaluated. X-ray diffraction showed a notable change at diffraction peak values for Pd80Ag20/C and Pd60Ag40/C electrocatalysts, suggesting alloying (solid solution) and smaller crystallite sizes for Pd60Ag40/C. In a thermogravimetric analysis, the electrocatalyst Pd60Ag40/C presented changes in the profile of the curves compared to the other electrocatalysts. In the cyclic voltammetry results for EOR in alkaline medium, Pd60Ag40/C presented a more negative onset potential, a higher current density at the oxidation peak, and a larger electrically active area. Chronoamperometry tests indicated a lower poisoning rate for Pd60Ag40/C, a fact also observed in the CO-stripping voltammetry analysis due to its low onset potential. As the best performing electrocatalyst, Pd60Ag40/C has a lower mass of Pd (a noble and expensive metal) in its composition. It can be inferred that this bimetallic composition can contribute to decreasing the amount of Pd required while increasing the fuel cell performance and expected life. PdAg-type electrocatalysts can provide an economically feasible alternative to pure PGM-electrocatalysts for use as the anode in EOR in fuel cells.

Electronic structure optimization boosts Pd nanocrystals for ethanol electrooxidation realized by Te doping

Te doping greatly modifies the electronic structure of Pd and promotes the electrocatalytic performance towards EOR.

Alloyed Palladium-Lead Nanosheet Assemblies for Electrocatalytic Ethanol Oxidation

Synthesizing alloyed bimetallic electrocatalysts with a three-dimensional (3D) structure assembly have arouse great interests in electrocatalysis. We synthesized a class of alloyed Pd₃Pb/Pd nanosheet assemblies (NSAs) composed of a two-dimensional (2D) sheet structure with adjustable compositions via an oil bath approach at a low temperature. Both the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images reveal the successful formation of the nanosheet structure, where the morphology of Pd₃Pb/Pd NSAs can be regulated by adjusting the atomic mole ratio of Pb and Pb metal precursors. The power X-ray diffraction (XRD) pattern shows that Pd₃Pb/Pd NSA catalysts are homogeneously alloyed. Electrochemical analysis and the density functional theory (DFT) method demonstrate that the electrocatalytic activity of the alloyed Pd₃Pb/Pd NSAs can be improved by the doping of the Pb element. As a result of the addition of element Pb and change of the electron structure, the electrocatalytic activity toward ethanol oxidation of alloyed Pd₃Pb/Pd-15 NSA can reach up to 2886 mA mg^-1, which is approximately 2.8 times that of the pure Pd NSA counterpart (1020 mA mg^-1). The Pd₃Pb/Pd NSAs are favorable in a high catalytic temperature, high KOH concentration, and high ethanol concentration.

Enhanced Electrocatalytic Activity of Alloyed Palladium-Lead Nanoparticles toward Electrooxidation of Ethanol

Although many researchers have made great efforts to pursue promising high-efficiency electrocatalysts, a formidable challenge remains for designing excellent palladium-based electrocatalysts for commercializing direct liquid fuel cells. This study reports the synthesis of bimetallic PdPb nanoparticles (NPs) via a mixed solution containing cetyl trimethyl ammonium bromide as the capping agent. Alloyed PdPb NPs are formed, where the size of the NPs increases as Pb atoms are introduced gradually. However, Pd₃Pb NPs are obtained with the same molar ratio of Pd and Pb in the raw systems. Among all of the as-made NPs, Pd₉Pb₁ NPs exhibit superior catalytic activity (2620 mA mg^-1) toward ethanol electrooxidation, 4.3 times higher than commercial Pd/C catalysts (613 mA mg^-1). The overall rate of the EOR for PdPb NPs is determined, demonstrating that the electrocatalytic activity of the PdPb NPs increases at high catalytic temperatures, in high pH environments, and/or at high ethanol concentrations.

Silver nanomaterials: synthesis and (electro/photo) catalytic applications

In view of their unique characteristics and properties, silver nanomaterials (Ag NMs) have been used not only in the field of nanomedicine but also for diverse advanced catalytic technologies. In this comprehensive review, light is shed on general synthetic approaches encompassing chemical reduction, sonochemical, microwave, and thermal treatment among the preparative methods for the syntheses of Ag-based NMs and their catalytic applications. Additionally, some of the latest innovative approaches such as continuous flow integrated with MW and other benign approaches have been emphasized that ultimately pave the way for sustainability. Moreover, the potential applications of emerging Ag NMs, including sub nanomaterials and single atoms, in the field of liquid-phase catalysis, photocatalysis, and electrocatalysis as well as a positive role of Ag NMs in catalytic reactions are meticulously summarized. The scientific interest in the synthesis and applications of Ag NMs lies in the integrated benefits of their catalytic activity, selectivity, stability, and recovery. Therefore, the rise and journey of Ag NM-based catalysts will inspire a new generation of chemists to tailor and design robust catalysts that can effectively tackle major environmental challenges and help to replace noble metals in advanced catalytic applications. This overview concludes by providing future perspectives on the research into Ag NMs in the arena of electrocatalysis and photocatalysis.

Porous PdAg alloy nanostructures with a concave surface for efficient electrocatalytic methanol oxidation

Tuning the composition and surface structure of the metal nanocrystals offered viable avenues for enhancing catalytic performances. Herein, we report a facile one-pot strategy for the formation of PdAg porous alloy nanostructures (PANs) with a concave surface. Due to their highly open nanostructures and tunable d-band center features, PdAg PANs exhibit superior electrocatalytic activity and long-term durability than Pd nanoparticles (NPs) and Pd/C for methanol oxidation reaction (MOR) in alkaline media. Our results provide a feasible and efficient approach for the controlled synthesis of high-performance Pd-based nanomaterials for alkaline MOR.

One-pot synthesis of rugged PdRu nanosheets as the efficient catalysts for polyalcohol electrooxidation

Recently, intensive attention has been attracted to the two-dimensional metal nanosheets, owing to their excellent electrocatalytic performance for direct alcohol fuel cells (DAFCs). Herein, PdRu nanosheets have been synthesized successfully by a facile one-pot method. The rugged nanosheet structure provided plentiful surface active sites to enhance the electrocatalytic activity. Moreover, benefiting from the synergistic effect and improved electronic structure, PdRu NSs exhibited splendid electrocatalytic performance in ethylene glycol oxidation reaction (EGOR) and glycerol oxidation reaction (GOR). Specifically, the mass activity of PdRu NSs was 1.72 and 3.69 times over those of Pd NSs and Pd/C catalysts in EGOR. Moreover, PdRu NSs displayed the largest mass activity in GOR, 1.48 and 2.47 times as large as Pd NSs and Pd/C catalysts. The results of stability tests demonstrated that the durability of PdRu NSs was the highest among the obtained catalysts. This work plays a directive role on the in-depth engineering on Pd-based catalysts with nanosheet architectures.

Surfactant-Assisted Synthesis of Palladium Nanosheets and Nanochains for the Electrooxidation of Ethanol

The synthesis of metal nanometer electrocatalysts with a two-dimensional (2D) structure or rich active sites has become a research hotspot in electrocatalysis. In this work, surfactant hexadecyltrimethylammonium bromide (CTAB) was used to assist the synthesis and assembly of Pd ultrathin nanosheet with the help of Mo(CO)₆ in the start system. Pd nanochain composed of nanoparticles is obtained under the same condition, replacing CTAB with carrageenan only. Electrochemical measurements showed that the catalytic peak current density for the electrooxidation of ethanol can reach 2145 mA mg^-1 for the Pd nanosheet assembly (NSA) and 1696 mA mg^-1 for Pd nanochains. Pd nanosheet assembly also has a lower electron-transfer barrier, better catalytic stability, and antipoisoning performance than that of Pd nanochains. The mechanism of Pd nanosheets and nanochains catalysts the enhanced electrocatalytic activity toward ethanol oxidation has been discussed based on the experimental data.

Two-dimensional multimetallic alloy nanocrystals: recent progress and challenges

In this highlight article, the recent progress on the preparation and application of multimetallic alloy nanocrystals with 2D nanostructures is systematically reviewed, as well as perspectives on future challenges and opportunities.

Synthesis of Bimetallic PdAg Nanoparticles and Their Electrocatalytic Activity toward Ethanol

Palladium-based bimetallic nanoparticles (NPs) have been studied as important electrocatalysts for energy conversion due to their high electrocatalytic performance and the less usage of the noble metal. Herein, well-dispersed PdAg NPs with uniform size were prepared via oil bath accompanied with the hydrothermal method. The variation of the Ag content in PdAg NPs changed the lattice constant of the face-centered cubic alloy nanostructures continuously. The Pd/Ag molar ratio in the PdAg alloy NPs affected their size and catalytic activity toward ethanol electrooxidation. Experimental data showed that PdAg NPs with less Ag content exhibited better electrocatalytic activity and durability than pure Pd NPs owing to both the small size and the synergistic effect. PdAg-acac-4 with the Pd/Ag molar ratio of 4 : 1 in the start system possessed the highest catalytic current density of 2246 mA/mg for the electrooxidation of ethanol. The differences in the morphology and electrocatalytic activity of the as-made PdAg NPs have been discussed and analyzed.