Alcohol Oxidation Reactions Research Articles

Subnanoscale Dual-Site Pd-Pt Layers Make PdPtCu Nanocrystals CO-Tolerant Bipolar Effective Electrocatalysts for Alcohol Fuel Cell Devices.

Finding a high-performance low-Pt bipolar electrocatalyst in actual direct alcohol fuel cells (DAFCs) remains challenging and desirable. Here, we developed a crystalline PdPtCu@amorphous subnanometer Pd-Pt "dual site" layer core-shell structure for the oxygen reduction reaction (ORR) and alcohol (methanol, ethylene glycol, glycerol, and their mixtures) oxidation reaction (AOR) in an alkaline electrolyte (denoted D-PdPtCu). The prepared D-PdPtCu/C achieved a direct 4-electron ORR pathway, a full oxidation pathway for AOR, and high CO tolerance. The ORR mass activity (MA) of D-PdPtCu/C delivered a 52.8- or 59.3-fold increase over commercial Pt/C or Pd/C, respectively, and no activity loss after 20000 cycles. The D-PdPtCu/C also exhibited much higher AOR MA and stability than Pt/C or Pd/C. Density functional theory revealed the intrinsic nature of a subnanometer Pd-Pt "dual site" surface for ORR and AOR activity enhancement. The D-PdPtCu/C as an effective bipolar electrocatalyst yielded higher peak power densities than commercial Pt/C in actual DAFCs.

High‐Entropy Nanomaterials for Advanced Electrocatalysis

High‐entropy alloys refer to near‐equimolar alloys of five or more elements and are receiving attention due to their unique physical and chemical properties. In electrocatalysis, they serve as active sites in multiple elements, favoring the optimized adsorption/desorption property toward the target reaction. High‐entropy nanomaterials (HENMs) are attractive candidates as electrocatalysts by taking advantage of a high surface‐to‐volume ratio and tailored composition. This review begins with the concept of high‐entropy materials and various strategies for designing electrocatalysts. Then, the recent advances in HENMs as electrocatalysts for various applications (water‐splitting reaction, carbon dioxide reduction reaction, alcohol oxidation reaction, etc.) are introduced with their catalytic performances. Finally, based on the current status of HENMs for electrocatalysis, the challenging aspects and the future insight of HENMs for advanced electrocatalysis are discussed and proposed.

Enhanced electrocatalytic activity and stability of Pd-based bimetallic icosahedral nanoparticles towards alcohol oxidation reactions

Enhanced electrocatalytic activity and stability of Pd-based bimetallic icosahedral nanoparticles towards alcohol oxidation reactions

Mechanistic Elucidation of an Alcohol Oxidation Reaction Promoted by a Nickel Azophenolate Complex

Borrowing hydrogen (BH) reactions by base metals have attracted significant focus with a perspective in green and sustainable chemistry. In a BH reaction, dehydrogenation of the alcohol motifs is a fundamental step, where several well-established catalysts exhibit metal–ligand cooperativity. We showcase here that the redox noninnocence of the azophenolate backbone in an earth-abundant nickel catalyst drives the alcohol dehydrogenation by a radical pathway. Detailed mechanistic probation including saturation kinetics supports the binding of the substrate alcohol to the nickel center. Thermodynamic data derived from such analysis also proves the azophenolate hemilability to pave the further steps. An Eyring analysis quantifies the kinetic barrier associated with a crucial hydrogen atom transfer step that is driven by the azo radical. Furthermore, both Hammett correlation and linearity in Evans–Polanyi plot implicate that C–H abstraction by radical mechanism as the rate-determining step. The overall mechanism offered by the azophenolate catalyst closely resembles the galactose oxidase-based copper phenoxide chemistry.

Cobalt nickel boride as electrocatalyst for the oxidation of alcohols in alkaline media

A mixed Co and Ni boride precursor was synthesized via chemical reduction and subsequently annealed at 400 or 500 °C with or without prior addition of the monomer benzoxazine (BO). The resulting mixed CoNiB materials were investigated as electrocatalysts for three alcohol oxidation reactions (AOR) in alkaline electrolyte: the oxidation of glycerol (GOR), ethylene glycol (EGOR) and ethanol (EOR). Comparison of the rotating disk electrode (RDE) cyclic voltammograms for the different catalysts revealed that CoNiB annealed at 500 °C without the addition of BO exhibited the lowest overpotentials in AORs at 10 mA cm−2, promoting GOR at 224 ± 6 mV lower potential compared to OER. When pyrolysis was conducted at 400 °C, the BO-containing catalyst showed a significant increase in the electrocatalytic activity for the AORs compared to the CoNiB catalyst only. The product selectivity on the different catalysts was investigated in a batch-type reactor with flow recirculation revealing formate as the main oxidation product during GOR and EGOR with faradaic efficiencies (FE) in a range of 60%–80%, while acetate was obtained during EOR (FE ∼ 85%–90%). The electrode potential, electrolyte composition and the type of ionomer were explored with respect to their influence on the GOR selectivity. The use of different ionomers resulted in significant differences in the activity trends between RDE and the batch-type reactor with flow recirculation measurements, indicating a strong influence of the two different substrates used, namely glassy carbon and carbon paper on the catalyst formation and thus on the recorded electrochemical activity.

Directional editing of self-supported nanoarray electrode for adaptive paired-electrolysis

Anodic oxidation assisted hydrogen production under mild conditions powered by renewable electricity represents a sustainable approach to energy conversion systems. Here, we fabricated a versatile and universal self-supported nanoarray platform that can be intelligently edited to achieve adaptive electrocatalysis for alcohol oxidation reactions and hydrogen evolution reaction (HER). The obtained self-supported nanoarray electrocatalysts exhibit excellent catalytic activity due to the integration of multiple merits of rich nanointerface-reconstruction and self-supported hierarchical structures. Particularly, the membrane-free pair-electrolysis system coupling HER and ethylene glycol oxidation reaction (EGOR) required an applied voltage of only 1.25 V to drive the current density of 10 mA cm−2, which is about 510 mV lower than that of the overall water splitting, showing the capability to simultaneously produce H2 and formate with high Faradic efficiency and stability. This work demonstrates a catalytic self-supported nanoarray platform for energy-efficient production of high-purity H2 and value-added chemicals.

Approaching Molecular Definition on Oxide-Supported Single-Atom Catalysts.

ConspectusSingle-atom catalysts (SACs) offer unique advantages such as high (noble) metal utilization through maximum possible dispersion, large metal-support contact areas, and oxidation states usually unattainable in classic nanoparticle catalysis. In addition, SACs can serve as models for determining active sites, a simultaneously desired as well as elusive target in the field of heterogeneous catalysis. Due to the complexity of heterogeneous catalysts bearing a variety of different sites on metal particles and the respective support as well as at their interface, studies of intrinsic activities and selectivities remain largely inconclusive. While SACs could close this gap, many supported SACs remain intrinsically ill-defined due to complexities arising from the variety of different adsorption sites for atomically dispersed metals, hampering the establishment of meaningful structure-activity correlations. In addition to overcoming this limitation, well-defined SACs could even be utilized to shed light on fundamental phenomena in catalysis that remain ambiguous when studies are obscured by the complexity of heterogeneous catalysts.In this Account, we describe approaches to break down the complexity of supported single-atom catalysts through the careful choice of oxide supports with specific binding motives as well as the adsorption of well-defined ligands such as ionic liquids on single metal sites. An example of molecularly defined oxide supports is polyoxometalates (POMs), which are metal oxo clusters with precisely known composition and structure. POMs exhibit a limited number of sites to anchor atomically dispersed metals such as Pt, Pd, and Rh. Polyoxometalate-supported single-atom catalysts (POM-SACs) thus represent ideal systems for the in situ spectroscopic study of single atom sites during reactions as, in principle, all sites are identical and thus equally active in catalytic reactions. We have utilized this benefit in studies of the mechanism of CO and alcohol oxidation reactions as well as the hydro(deoxy)genation of various biomass-derived compounds. More so, the redox properties of polyoxometalates can be finely tuned by changing the composition of the support while keeping the geometry of the single-atom active site largely constant. We further developed soluble analogues of heterogeneous POM-SACs, opening the door to advanced liquid-phase nuclear magnetic resonance (NMR) and UV-vis techniques but, in particular, to electrospray ionization mass spectrometry (ESI-MS) which proves powerful in determining catalytic intermediates as well as their gas-phase reactivity. Employing this technique, we were able to resolve some of the long-standing questions about hydrogen spillover, demonstrating the broad utility of studies on defined model catalysts.

Electrochemical Analysis Using Organic Nitroxyl Radicals

Organic nitroxyl radicals represented by 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) are known to be compounds that catalyze alcohol oxidation reactions. These catalytic reactions can be applied to a wide range of compounds with hydroxy and amino groups. It is also possible to selectively oxidize primary alcohols by designing the skeleton around the nitroxyl radical moiety for use in organic synthesis. Reactions can also be carried out by electrochemical methods, and the electrical current measured during the reaction can be used to quantify the substrates. Therefore, the combination of reactions catalyzed by nitroxyl radicals and electrochemical techniques is expected to be applied as a new analytical method. However, since the reaction does not proceed rapidly in neutral aqueous solutions, it has mostly been applied in basic aqueous solutions or organic solvents, and there have been no reports on sensor applications under physiological conditions. Herein, we have developed a novel catalyst, nortropine N-oxyl (NNO), which is highly active even in neutral aqueous solutions, and have found that it can be used for the analysis of biological components and drugs under physiological conditions. The combination of this method with enzymatic reactions made it possible to specifically detect certain compounds. In this review, we describe a novel analytical method that combines these nitroxyl radicals with electrochemical methods.

Conjugated dual size effect of core-shell particles synergizes bimetallic catalysis

Core-shell bimetallic nanocatalysts have attracted long-standing attention in heterogeneous catalysis. Tailoring both the core size and shell thickness to the dedicated geometrical and electronic properties for high catalytic reactivity is important but challenging. Here, taking Au@Pd core-shell catalysts as an example, we disclose by theory that a large size of Au core with a two monolayer of Pd shell is vital to eliminate undesired lattice contractions and ligand destabilizations for optimum benzyl alcohol adsorption. A set of Au@Pd/SiO2 catalysts with various core sizes and shell thicknesses are precisely fabricated. In the benzyl alcohol oxidation reaction, we find that the activity increases monotonically with the core size but varies nonmontonically with the shell thickness, where a record-high activity is achieved on a Au@Pd catalyst with a large core size of 6.8 nm and a shell thickness of ~2–3 monolayers. These findings highlight the conjugated dual particle size effect in bimetallic catalysis.

Electrocatalyst Fabrication Using Metal Nanoparticles Prepared in Ionic Liquids.

Metal nanoparticle-based electrocatalysts are widely used in electronic devices, which serve for electrochemical reactions like oxygen reduction reaction, alcohol oxidation and CO2 reduction reaction. These catalyst-dependent reactions are the key of the emerging clean energy systems. Catalyst design and synthesis therefore have received keen attention in past decades. We are motivated to study synthesis approaches of metal nanoparticle-based electrocatalysts using ionic liquids (ILs), which are promising solvents for the nanoparticle preparation because of their unique physicochemical properties. In this personal account, we review our previous and present works on nanoparticle preparation in IL and utilization of the obtained nanoparticles as electrocatalysts.

Ternary PtZrNi nanorods for efficient multifunctional electrocatalysis towards oxygen reduction and alcohol oxidation

Pt-based alloys with precise structure and composition design have been considered to be effective and robust novel electrocatalysts for fuel cells. Whereas, the sluggish kinetics of oxygen reduction reaction (ORR) and low intrinsic activity of Pt limited their real application on a large scale. Herein, a novel ternary PtZrNi nanorods (PtZrNi NRs) was synthesized via a facile wet-chemical method to achieve high electrocatalytic performance for both ORR and alcohol oxidation reaction owing to the synergism of chosen three elements and prominent one-dimensional morphology. Specifically, the PtZrNi NRs show enhanced mass and specific activities of 0.755 A mgPt-1 and of 0.97 mA/cm2 at 0.9 VRHE towards ORR in acidic media, which are 4.7 and 4.4 times of those of commercial Pt/C, respectively. Additionally, in alkaline media, the PtZrNi NRs also exhibit superior ORR mass and specific activities of 3.216 A mgPt-1and 4.13 mA/cm2, enhanced by 34.6 and 31.3 times compared with those of commercial Pt/C, respectively. The PtZrNi NRs retain the nanorod shape well without agglomeration after an accelerated durability test (20000 cycles). This work may offer a new perspective for engineering high-performance Pt-based electrocatalysts for commercial fuel cells.

Nepenthes-inspired hierarchical ZnIn2S4/Ru nanotubes with chemical bond-mediated interface enable efficient photocatalytic benzyl alcohol oxidation

In nature, Nepenthes has a unique tubular structure for harvesting and preserving goals. Herein, inspired by Nepenthes, we report the selective photocatalytic oxidation of benzyl alcohol by using the hierarchical ZnIn2S4/Ru nanotubes. The tubular chamber with abundant active sites not only facilitates enriching the target molecules but also shortening the bulk-to-surface distance, and the Zn-S-Ru interfacial bonds creates direct charge transfer channels for the activating O2 to generate •O2– species boosting the benzyl alcohol selective conversion to benzaldehyde. The photocatalytic benzyl alcohol oxidation reaction has a high conversion rate of 68.2 % as well as a benzaldehyde selectivity of 99 % under visible light irradiation. The effects of Zn-S-Ru interfacial bonds on modulating the carrier transfer dynamics for the selective catalytic oxidation reaction are investigated by electrochemical characterization and density functional theory simulation. This work offers an inspiration on regulating the interfacial charge transfer channel for the photo-driven selective oxidation reaction.

Native amine dehydrogenases can catalyze the direct reduction of carbonyl compounds to alcohols in the absence of ammonia

Native amine dehydrogenases (nat-AmDHs) catalyze the (S)-stereoselective reductive amination of various ketones and aldehydes in the presence of high concentrations of ammonia. Based on the structure of CfusAmDH from Cystobacter fuscus complexed with Nicotinamide adenine dinucleotide phosphate (NADP+) and cyclohexylamine, we previously hypothesized a mechanism involving the attack at the electrophilic carbon of the carbonyl by ammonia followed by delivery of the hydride from the reduced nicotinamide cofactor on the re-face of the prochiral ketone. The direct reduction of carbonyl substrates into the corresponding alcohols requires a similar active site architecture and was previously reported as a minor side reaction of some native amine dehydrogenases and variants. Here we describe the ketoreductase (KRED) activity of a set of native amine dehydrogenases and variants, which proved to be significant in the absence of ammonia in the reaction medium but negligible in its presence. Conducting this study on a large set of substrates revealed the heterogeneity of this secondary ketoreductase activity, which was dependent upon the enzyme/substrate pairs considered. In silico docking experiments permitted the identification of some relationships between ketoreductase activity and the structural features of the enzymes. Kinetic studies of MsmeAmDH highlighted the superior performance of this native amine dehydrogenases as a ketoreductase but also its very low activity towards the reverse reaction of alcohol oxidation.

Photoelectrocatalytic oxidation of ethylene glycol on trimetallic PdAgCu nanospheres enhanced by surface plasmon resonance

The notable surface plasmon resonance (SPR) effect of some metals has been applied to improve the efficiency of alcohol oxidation reactions, whereas the comprehensive investigation of Cu-assisted photoelectrocatalysis remains challenging. We herein successfully prepared trimetallic PdAgCu nanospheres (NSs) with abundant surface bulges for the advanced ethylene glycol oxidation reaction (EGOR) and compared them with bimetallic PdAg NSs to investigate the performance enhancement mechanism. Impressively, the as-optimized PdAgCu NSs exhibited superb mass activity and electrochemical stability. Moreover, under visible light illumination, the mass activity of PdAgCu NSs increased to 1.62 times compared to that in the dark, and in contrast, the mass activity of PdAg NSs only increased to 1.48 times that in the dark. A mechanistic study indicated that the incorporation of Cu not only strengthens the whole SPR effect of PdAgCu NSs but also further modifies the electronic structure of Pd. This work highlighted that the incorporation of Cu into PdAg NSs further enhanced the photoelectrocatalytic performance and increased noble metal atom utilization, which may provide guidance to fabricate novel and promising nanocatalysts in the field of photoelectrocatalysis.

Tungstic Acid: A Simple and Effective Solid Catalyst in Terpene Alcohol Oxidation Reactions with Hydrogen Peroxide

In this work, we report for the first time, the tungstic acid-catalyzed oxidation of terpene alcohols with hydrogen peroxide. This simple, solid, and commercially available catalyst efficiently promoted the conversion of borneol, geraniol and nerol to camphor and epoxide products, respectively. Effects of main reaction parameters, such as catalyst load, the molar ratio of oxidant to the substrate, time, and reaction temperature were investigated. Conversions and selectivity greater than 90% were achieved using 1.0 mol % of H2WO4 after 2 h of reaction at 90 °C. The activation energy was equal to 66 kJmol−1. We propose a reaction mechanism based on the experimental results. This solid catalyst was easily recovered and reused without loss of activity. As far as we know, it is the first time that tungstic acid was used as the catalyst in the oxidation reactions of terpene alcohols.

Active species identification in Zn and Mn-based materials for application in alcohol oxidation reactions

Surface and bulk properties and catalytic performance during benzaldehyde synthesis of inexpensive Zn modified-MnCO3 catalysts (xZnMn), prepared by impregnation method and covering a wide range of Zn loadings (1.0 wt% Zn < x < 22.6 wt% Zn) were deeply investigated for the first time. XRD, TPN2 (temperature-programmed decompositions in N2) and XPS characterization showed that different crystalline phases containing Mn and/or Zn species are present in xZnMn. In these phases Mn oxidation state changes from 2+ toward higher values (3+ and 4+ ) as Zn content increases entailing the transformation of MnCO3 to ZnMn2O4 spinel and MnOx oxides.During benzyl alcohol oxidation at mild reaction conditions using oxygen, the highest benzaldehyde yield (∼70 %) was obtained on 15.4ZnMn sample after 6 h of reaction. The superior catalytic performance of the 15.4ZnMn catalyst was attributed to a synergistic interaction of both metals.

Controlled alcohol oxidation reactions by supported non-noble metal nanoparticles on chitin-derived N-doped carbons

Non-noble metal nanoparticles supported on biomass-derived N-doped carbons as efficient and selective catalytic systems in alcohols oxidation reactions.

Coexistence of Au single atoms and Au nanoparticles on NiAl-LDH for selective electrooxidation of benzyl alcohol to benzaldehyde.

Introducing different active sites into heterogeneous catalysts provides new prospects to address the challenges in single-atom catalysis. Herein, the Au single atoms together and the Au nanoparticles were loaded onto NiAl-LDH by a facile impregnation-reduction method for the first time, resulting in the formation of Au1+n-NiAl-LDH, in which abundant Au single atoms are located around the Au nanoparticles with ∼5 nm size. When applied in the electrocatalytic benzyl alcohol oxidation reaction (BAOR), the as-prepared Au1+n-NiAl-LDH exhibits a remarkable selectivity of 91% and 177.63 μmol for benzaldehyde in 5 hours, while in contrast examples using solely Au single atom loaded NiAl-LDH (Au1-NiAl-LDH) and solely Au nanoparticle loaded NiAl-LDH (Aun-NiAl-LDH) can only realize 87.36 μmol production (75% selectivity) and 48.90 μmol production (28% selectivity) of benzaldehyde, respectively. Such a dramatic difference can be attributed to the synergistic effects of Au single atoms and Au nanoparticles. DFT calculation results reveal that for Au1+n-NiAl-LDH, Au single atoms promote the dehydrogenation capacity of LDH laminates, while Au nanoparticles offer adsorption sites for the electrophilic attachment of benzyl alcohol.

A high-index facet gold 12 tip nanostar for an improved electrocatalytic alcohol oxidation reaction with superior CO tolerance.

Direct alcohol fuel cells have a long and promising future, which will require the development of highly active electrocatalysts for alcohol electrooxidation reactions. To this end, high-index facet nanomaterial-based electrocatalysts provide significant promise for the successful oxidation of alcohols. However, the fabrication and exploration of high-index facet nanomaterials are seldom reported, especially in electrocatalytic applications. Herein, we successfully synthesized a high index facet {711} Au 12 tip nanostructure for the first time using a single-chain cationic TDPB surfactant. Electrooxidation results demonstrate that a {711} high-index facet Au 12 tip exhibited much higher electrocatalytic activity (∼10-fold higher) than the {111} low-index facet Au nanoparticles (Au NPs) without being poisoned by CO under identical conditions. Besides, Au 12 tip nanostructures offer appreciable stability and durability. The high electrocatalytic activity with excellent CO tolerance is due to the spontaneous adsorption of the negatively charged -OH on the high-index facet Au 12 tip nanostars, as evidenced by the isothermal titration calorimetry (ITC) analysis. Our findings suggest that high-index facet Au nanomaterials are ideal candidate electrode materials for the electrooxidation reaction of ethanol in fuel cells.

Oxidation Catalysis of Au Nano-Particles Immobilized on Titanium(IV)- and Alkylthiol-Functionalized SBA-15 Type Mesoporous Silicate Supports

Novel Au nano-particle catalysts immobilized on both titanium(IV)- and alkylthiol-functionalized SBA-15 type ordered mesoporous silicate supports were developed. The bi-functionalized SBA-15 type support could be synthesized by a one-pot method. To the synthesized supports, Au was immobilized by the reaction of the alkylthiol groups on the supports with AuCl4−, following reduction with NaBH4. The immobilized amount and the formed structures and the electronic property of the Au species depended on the loading of alkylthiol. The moderate size (2–3 nm) nano particulate Au sites formed on Ti(0.5)-SBASH(0.5) were negatively charged. The aerobic alcohol oxidation activity of the catalysts depended on the loading of alkylthiol and the structure of the Au nano-particles. The non-thiol-functionalized catalyst (Au/Ti(0.5)-SBASH(0)) composed of the large (5–30 nm) and the higher thiol-loaded catalyst (Au/Ti(0.5)-SBASH(8)) composed of the small cationic Au species were almost inactive. The most active catalyst was Au/Ti(0.5)-SBASH(0.5) composed of the electron-rich Au nano-particles formed by the electron donation from the highly dispersed thiol groups. Styrene oxidation activity in the presence of 1-phenylethanol with O2 depended on the loadings of titanium(IV) on the Au/Ti(x)-SBASH(0.5). The titanium(IV) sites trapped the H2O2 generated through the alcohol oxidation reaction, and also contributed to the alkene oxidation by activating the trapped H2O2.