Microwave-Assisted Synthesis of Pt-Au Nanoparticles with Enhanced Electrocatalytic Activity for the Oxidation of Formic Acid

UNLEASH: Ultralow Nanocluster Loading of Pt via Electro-Acoustic Seasoning of Heterocatalysts

The shift toward sustainable energy has fueled the development of advanced electrocatalysts to enable green fuel production and chemical synthesis. To date, no material outperforms Pt-group catalysts for key electrocatalytic reactions, necessitating advanced catalysts that minimize use of these rare and expensive constituents (i.e., Pt) to reduce cost without sacrificing activity. Whilst a myriad of routes involving co-synthesis of Pt with other elements have been reported, the Pt is often buried within the bulk of the composite, rendering a large proportion of it inaccessible to the interfacial electrocatalytic reaction. Surface decoration of Pt on arbitrary substrates is therefore desirable to maximize catalytic activity; nevertheless, Pt electrodeposition suffers from clustering and ripening effects that result in large (⌀ 0.1 - 1 μ m $\diameter \ \!0.1-1\ \umu{\rm m}$ ) aggregates that hinder electrocatalytic activity. Herein, an unconventional synthesis method is reported that utilizes high-frequency (10 MHz) acoustic waves to electrochemically 'season' a gold working electrode with an ultralow loading of Pt nanoclusters. The UNLEASH platform is shown to facilitate high-density dispersion of nanometer-order clusters at the bimetallic interface to enable superior atomic utilization of Pt. This is exemplified by its utility for methanol oxidation reaction (MOR), wherein a mass activity of 5.28 Amg Pt - 1 ${\rm mg}_{\rm Pt}^{-1}$ is obtained, outperforming all other Au/Pt bimetallic electrocatalysts reported to date.

Boosted formic acid electro-oxidation on platinum nanoparticles and "mixed-valence" iron and nickel oxides

The modification of Pt nanoparticles (nano-Pt, assembled electrochemically onto a glassy carbon (GC) substrate) with hybrid multivalent nickel (nano-NiO_x) and iron (nano-FeO_x) oxide nanostructures was intended to steer the mechanism of the formic acid electro-oxidation (FAO) in the desirable dehydrogenation pathway. This binary modification with inexpensive oxides succeeded in mediating the reaction mechanism of FAO by boosting reaction kinetics "electron transfer" and amending the surface geometry of the catalyst against poisoning. The sequence of deposition was optimized where the a-FeO_x/NiO_x/Pt/GC catalyst (where "a" denotes a post-activation step for the catalyst at -0.5 V in 0.5 mol L^-1 NaOH) reserved the best hierarchy. Morphologically, while nano-Pt appeared to be spherical (ca. 100 nm in average diameter), nano-NiO_x appeared as flowered nanoaggregates (ca. 56 nm in average diameter) and nano-FeO_x (after activation) retained a plate-like nanostructure (ca. 38 nm in average diameter and 167 nm in average length). This a-FeO_x/NiO_x/Pt/GC catalyst demonstrated a remarkable catalytic efficiency (125 mA mg_Pt^-1) for FAO that was ca. 12.5 times that of the pristine Pt/GC catalyst with up to five times improvement in the catalytic tolerance against poisoning and up to -214 mV shift in the FAO's onset potential. Evidences for equipping the a-FeO_x/NiO_x/Pt/GC catalyst with the least charge transfer resistance and the highest stability among the whole investigated catalysts are provided and discussed.

Surface-Plasmon-Mediated Alloying for Monodisperse Au-Ag Alloy Nanoparticles in Liquid

Plasmonic noble-metal nanoparticles with broadly tunable optical properties and catalytically active surfaces offer a unique opportunity for photochemistry. Resonant optical excitation of surface-plasmon generates high-energy hot carriers, which can participate in photochemical reactions. Although the surface-plasmon-driven catalysis on molecules has been extensively studied, surface-plasmon-mediated synthesis of bimetallic nanomaterials is less reported. Herein, we perform a detailed investigation on the formation mechanism and colloidal stability of monodisperse Au-Ag alloy nanoparticles synthesized through irradiating the intermixture of Au nanochains and AgNO₃ solution with a nanosecond pulsed laser. It is revealed that the Ag atoms can be extracted from AgNO₃ solution by surface-plasmon-generated hot electrons and alloy with Au atoms. Particularly, the obtained Au-Ag alloy nanoparticles without any surfactants or ligands exhibit superior stability that is confirmed by experiments as well as DLVO-based theoretical simulation. Our work would provide novel insights into the synthesis of potentially useful bimetallic nanoparticles via surface-plasmon-medicated alloying.

Nanocomposite Concept for Electrochemical In Situ Preparation of Pt-Au Alloy Nanoparticles for Formic Acid Oxidation

Herein, we report a straightforward approach for the in situ preparation of Pt-Au alloy nanoparticles from Pt + xAu/C nanocomposites using monometallic colloidal nanoparticles as starting blocks. Four different compositions with fixed Pt content and varying Pt to Au mass ratios from 1:1 up to 1:7 were prepared as formic acid oxidation reaction (FAOR) catalysts. The study was carried out in a gas diffusion electrode (GDE) setup. It is shown that the presence of Au in the nanocomposites substantially improves the FAOR activity with respect to pure Pt/C, which serves as a reference. The nanocomposite with a mass ratio of 1:5 between Pt and Au displays the best performance during potentiodynamic tests, with the electro-oxidation rates, overpotential, and poisoning resistance being improved simultaneously. By comparison, too low or too high Au contributions in the nanocomposites lead to an unbalanced performance in the FAOR. The combination of operando small-angle X-ray scattering (SAXS), scanning transmission electron microscopy (STEM) elemental mapping, and wide-angle X-ray scattering (WAXS) reveals that for the nanocomposite with a 1:5 mass ratio, a conversion between Pt and Au from separate nanoparticles to alloy nanoparticles occurs during continuous potential cycling in formic acid. By comparison, the nanocomposites with lower Au contents, for example, 1:2, exhibit less in situ alloying, and the concomitant performance improvement is less pronounced. On applying identical location transmission electron microscopy (IL-TEM), it is revealed that the in situ alloying is due to Pt dissolution and re-deposition onto Au as well as Pt migration and coalescence with Au nanoparticles.

PT-BI Co-Deposit Shell on AU Nanoparticle Core: High Performance and Long Durability for Formic Acid Oxidation

This work presents the catalysts of Pt-Bi shells on Au nanoparticle cores and Pt overlayers on the Pt-Bi shells toward formic acid oxidation (FAO). Pt and Bi were co-deposited on Au nanoparticles (Au NP) via the irreversible adsorption method using a mixed precursor solution of Pt and Bi ions, and the amount of the co-deposits was controlled with the repetition of the deposition cycle. Rinsing of the co-adsorbed ionic layers of Pt and Bi with a H2SO4 solution selectively removed the Bi ions to leave Pt-rich and Bi-lean (<0.4 atomic %) co-deposits on Au NP (Pt-Bi/Au NP), conceptually similar to de-alloying. Additional Pt was deposited over Pt-Bi/Au NPs (Pt/Pt-Bi/Au NPs) to manipulate further the physicochemical properties of Pt-Bi/Au NPs. Transmission electron microscopy revealed the core–shell structures of Pt-Bi/Au NPs and Pt/Pt-Bi/Au NPs, whose shell thickness ranged from roughly four to six atomic layers. Moreover, the low crystallinity of the Pt-containing shells was confirmed with X-ray diffraction. Electrochemical studies showed that the surfaces of Pt-Bi/Au NPs were characterized by low hydrogen adsorption abilities, which increased after the deposition of additional Pt. Durability tests were carried out with 1000 voltammetric cycles between −0.26 and 0.4 V (versus Ag/AgCl) in a solution of 1.0 M HCOOH + 0.1 M H2SO4. The initial averaged FAO performance on Pt-Bi/Au NPs and Pt/Pt-Bi/Au NPs (0.11 ± 0.01 A/mg, normalized to the catalyst weight) was higher than that of a commercial Pt nanoparticle catalyst (Pt NP, 0.023 A/mg) by a factor of ~5, mainly due to enhancement of dehydrogenation and suppression of dehydration. The catalytic activity of Pt/Pt-Bi/Au NP (0.04 ± 0.01 A/mg) in the 1000th cycle was greater than that of Pt-Bi/Au NP (0.026 ± 0.003 A/mg) and that of Pt NP (0.006 A/mg). The reason for the higher durability was suggested to be the low mobility of surface Pt atoms on the investigated catalysts.

Single-Nanoparticle Coulometry Method with High Sensitivity and High Throughput to Study the Electrochemical Activity and Oscillation of Single Nanocatalysts

To explore nanocatalysts with high electro-catalytic performance and less loading of precious metals, efforts have been made to develop electrochemical methods with high spatial resolution at the single nanoparticle level. Herein, a highly sensitive single-nanoparticle coulometry method is successfully developed to study the electrochemical activity and oscillation of single PtTe nanocatalysts. Based on microbattery reactions involving the formic acid electro-oxidation and the deposition of Ag on the single PtTe nanocatalyst surface, this method enables the transition from the undetectable sub-fA electric signal of the formic acid electro-oxidation into strong localized surface plasmon resonance scattering signal of Ag detected by dark-field microscopy. The lowest limiting current for a single nanocatalyst is found to be as low as 25.8 aA. Different trends of activity versus the formic acid concentration and types of activity of the single nanocatalyst have been discovered. Unveiled frequency-amplitude graph shows that the two electrochemical oscillation modes of low frequency with high amplitude and vice versa coexist in a single PtTe nanocatalyst, indicating the abundantly smooth surfaces and defects of nanocatalysts. This conducted study will open up the new avenue for further behavioral and mechanistic investigation of more types of nanocatalysts in the electrochemistry community.

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.

Tweaking the Interplay among Galvanic Exchange, Oxidative Etching, and Seed-Mediated Deposition toward Architectural Control of Multimetallic Nanoelectrocatalysts

Nanoscale galvanic exchange confined by metallic nanoparticles is an intriguing structure-remodeling process that transforms geometrically simple solid nanoparticles into multimetallic hollow nanoparticles with increased structural complexity and compositional diversity. Using liquid polyols with intrinsic reducing capabilities as the reaction medium for nanoparticle-templated galvanic exchange represents an interesting paradigm shift, allowing us to interface galvanic exchange with oxidative etching and seed-mediated deposition without introducing any additional oxidizing or reducing agents. By kinetically maneuvering the interplay among galvanic Cu-Pt exchange, oxidative Cu etching, and seed-mediated Pt deposition, we have been able to selectively transform AuCu₃ alloy nanoparticles into two architecturally distinct multimetallic heteronanostructures, namely, Au-Pt alloy skin-covered spongy nanoparticles and Pt nanodendrite-covered hollow nanoparticles, both of which exhibit unique structural features highly desirable for high-performance electrocatalysis. Using the formic acid oxidation and hydrogen evolution reactions in acidic electrolytes as model electrocatalytic reactions, we show that the multimetallic nanoparticles derived from AuCu₃ alloy nanoparticles through polyol-mediated galvanic exchange reactions markedly outperform the commercial Pt/C benchmark catalysts in terms of both activity and durability. This work not only provides important mechanistic insights on how galvanic exchange dynamically interplays with other redox processes to rigorously dictate the versatile structural transformations of multimetallic nanoparticles but also sheds light on the detailed structure-property relationships underpinning the intriguing electrocatalytic behaviors of architecturally complex multimetallic heteronanostructures.

Microwave-Driven Synthesis of Iron-Oxide Nanoparticles for Molecular Imaging

Here, we present a comprehensive review on the use of microwave chemistry for the synthesis of iron-oxide nanoparticles focused on molecular imaging. We provide a brief introduction on molecular imaging, the applications of iron oxide in biomedicine, and traditional methods for the synthesis of these nanoparticles. The review then focuses on the different examples published where the use of microwaves is key for the production of nanoparticles. We study how the different parameters modulate nanoparticle properties, particularly for imaging applications. Finally, we explore principal applications in imaging of microwave-produced iron-oxide nanoparticles.

Fabrication of CuOx-Pd Nanocatalyst Supported on a Glassy Carbon Electrode for Enhanced Formic Acid Electro-Oxidation

Formic acid (FA) electro-oxidation (FAO) was investigated at a binary catalyst composed of palladium nanoparticles (PdNPs) and copper oxide nanowires (CuOxNWs) and assembled onto a glassy carbon (GC) electrode. The deposition sequence of PdNPs and CuOxNWs was properly adjusted in such a way that could improve the electrocatalytic activity and stability of the electrode toward FAO. Several techniques including cyclic voltammetry, chronoamperometry, field-emission scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction were all combined to report the catalyst’s activity and to evaluate its morphology, composition, and structure. The highest catalytic activity and stability were obtained at the CuOx/Pd/GC electrode (with PdNPs directly deposited onto the GC electrode followed by CuOxNWs with a surface coverage, Г, of ca. 49%). Such enhancement was inferred from the increase in the peak current of direct FAO (by ca. 1.5 fold) which associated a favorable negative shift in its onset potential (by ca. 30 mV). The enhanced electrocatalytic activity and stability (decreasing the loss of active material by ca. 1.5-fold) of the CuOx/Pd/GC electrode was believed originating both from facilitating the direct oxidation (decreasing the time needed to oxidize a complete monolayer of FA, increasing turnover frequency, by ca. 2.5-fold) and minimizing the poisoning impact (by ca. 71.5%) at the electrode surface during FAO.

Efficient Direct Formic Acid Fuel Cells (DFAFCs) Anode Derived from Seafood waste: Migration Mechanism

Commercial Pt/C anodes of direct formic acid fuel cells (DFAFCs) get rapidly poisoned by in-situ generated CO intermediates from formic acid non-faradaic dissociation. We succeeded in increasing the Pt nanoparticles (PtNPs) stability and activity for formic acid oxidation (DFAFCs anodic reaction) by embedding them inside a chitosan matrix obtained from seafood wastes. Atop the commercial Pt/C, formic acid (FA) is predominantly oxidized via the undesired poisoning dehydration pathway (14 times higher than the desired dehydrogenation route), wherein FA is non-faradaically dissociated to CO resulting in deactivation of the majority of the Pt active-surface sites. Surprisingly, PtNPs chemical insertion inside a chitosan matrix enhanced their efficiency for FA oxidation significantly, as demonstrated by their 27 times higher stability along with ~400 mV negative shift of the FA oxidation onset potential together with 270 times higher CO poisoning-tolerance compared to that of the commercial Pt/C. These substantial performance enhancements are believed to originate from the interaction of chitosan functionalities (e.g., NH₂ and OH) with both PtNPs and FA molecules improving FA adsorption and preventing the PtNPs aggregation, besides providing the required oxygen helping with the oxidative removal of the adsorbed poisoning CO-like species at low potentials. Additionally, chitosan induced the retrieval of the Pt surface-active sites by capturing the in-situ formed poisoning CO intermediates via a so-called “migration mechanism”.