Oxidation of Ethanol to Acetic Acid by Supported PtCu Nanoparticles Stabilized by a Diamine Ligand.

Diamine-capped PtCu nanoparticles have been synthesized by the simultaneous reduction of the corresponding bis-imine metal complexes with hydrogen and supported onto a high-surface-area carbon. The obtained heterogeneous catalyst was tested in thermally conducted aerobic oxidation of ethanol to acetic acid in water as well as in the electrochemical oxidation of ethanol. Both types of catalyses mediated by the PtCu alloy confirmed a notable increase in catalytic activity compared to the pure Pt- and Cu-based counterparts due to a clear bimetallic effect.

Remarkable stability of a molecular ruthenium complex in PEM water electrolysis.

The dinuclear Ru diazadiene olefin complex, [Ru2(OTf)(µ-H)(Me2dad)(dbcot)2], is an active catalyst for hydrogen evolution in a Polymer Exchange Membrane (PEM) water electrolyser. When supported on high surface area carbon black and at 80 °C, [Ru2(OTf)(µ-H)(Me2dad)(dbcot)2]@C evolves hydrogen at the cathode of a PEM electrolysis cell (400 mA cm-2, 1.9 V). A remarkable turn over frequency (TOF) of 7800 molH2 molcatalyst -1 h-1 is maintained over 7 days of operation. A series of model reactions in homogeneous media and in electrochemical half cells, combined with DFT calculations, are used to rationalize the hydrogen evolution mechanism promoted by [Ru2(OTf)(µ-H)(Me2dad)(dbcot)2].

Interlayer Coordination of Pd-Pd Units in Exfoliated Black Phosphorus.

The chemical functionalization of 2D exfoliated black phosphorus (2D BP) continues to attract great interest, although a satisfactory structural characterization of the functionalized material has seldom been achieved. Herein, we provide the first complete structural characterization of 2D BP functionalized with rare discrete Pd2 units, obtained through a mild decomposition of the organometallic dimeric precursor [Pd(η3-C3H5)Cl]2. A multitechnique approach, including HAADF-STEM, solid-state NMR, XPS, and XAS, was used to study in detail the morphology of the palladated nanosheets (Pd2/BP) and to unravel the coordination of Pd2 units to phosphorus atoms of 2D BP. In particular, XAS, backed up by DFT modeling, revealed the existence of unprecedented interlayer Pd-Pd units, sandwiched between stacked BP layers. The preliminary application of Pd2/BP as a catalyst for the hydrogen evolution reaction (HER) in acidic medium highlighted an activity increase due to the presence of Pd2 units.

Reshaping the Cathodic Catalyst Layer for Anion Exchange Membrane Fuel Cells: From Heterogeneous Catalysis to Homogeneous Catalysis.

In anion exchange membrane fuel cells, catalytic reactions occur at a well-defined three-phase interface, wherein conventional heterogeneous catalyst layer structures exacerbate problems, such as low catalyst utilization and limited mass transfer. We developed a structural engineering strategy to immobilize a molecular catalyst tetrakis(4-methoxyphenyl)porphyrin cobalt(II) (TMPPCo) on the side chains of an ionomer (polyfluorene, PF) to obtain a composite material (PF-TMPPCo), thereby achieving a homogeneous catalysis environment inside ion-flow channels, with greatly improved mass transfer and turnover frequency as a result of 100 % utilization of the catalyst molecules. The unique structure of the homogeneous catalysis system comprising interconnected nanoreactors exhibits advantages of low overpotential and high fuel-cell power density. This strategy of reshaping of the catalyst layer structure may serve as a new platform for applications of many molecular catalysts in fuel cells.

Selective Electrocatalytic H2 O2 Generation by Cobalt@N-Doped Graphitic Carbon Core-Shell Nanohybrids.

Electrocatalytic oxygen reduction (ORR) is an emerging synthetic strategy to prepare H2 O2 in a sustainable fashion. N-doped graphitic carbon with embedded cobalt nanoparticles was selected as an advanced material able to selectively trigger the ORR to form H2 O2 with a faradaic efficiency of almost 100 % at very positive applied potentials. The production of H2 O2 proceeded with high rates as calculated by bulk electrolysis (49 mmol g-1 h-1 ) and excellent current densities (≈-0.8 mA cm-2 at 0.5 V vs. reversible hydrogen electrode). The totally selective behavior depended on the combination of concomitant material features, such as the textural properties, the nature of the metal, the distribution of N moieties, the acidic environment, and the applied potential.

Electrochemical CO2 reduction in water at carbon cloth electrodes functionalized with a fac-Mn(apbpy)(CO)3Br complex.

The organometallic complex (fac-Mn(apbpy)(CO)3Br) (apbpy = 4-(4-aminophenyl)-2,2'-bipyridine) grafted electrochemically onto carbon cloth serves as an electrocatalyst in the aqueous reduction of CO2 to syngas. A faradaic efficiency of around 60% for CO and 40% for H2 at -1.35 V is achieved together with a productivity rate higher than 870 NlCO h-1 gMn-1 at turnover numbers of up to 33 200 during 10 hours of operation.

Effect of Electrode Shape and Flow Conditions on the Electrochemical Detection with Band Microelectrodes.

In this work, we report the analysis of the electrochemical detection of electroactive species with band microelectrodes that operate under controlled convection. The study focuses on the determination of the collection efficiency of the analyte as a function of inlet flow velocity and microband geometry (inlaid, bumped and recessed), also providing a straightforward method for the theoretical determination of the lower detection limit. The analysis has been carried out by simulating the dimensionless mass transport with the finite element method, delivering the stationary limiting current density. Simulations have been performed on systems consisting of single and double band electrodes to investigate the trail effect on the electrochemical detection. We show that the obtained dimensionless results can be easily turned into dimensional data, providing a tool for the design of devices. The proposed method is general and can easily be extended to systems with different geometry.

Operando SXRD study of the structure and growth process of Cu2S ultra-thin films.

Electrochemical Atomic Layer Deposition (E-ALD) technique has demonstrated to be a suitable process for growing compound semiconductors, by alternating the under-potential deposition (UPD) of the metallic element with the UPD of the non-metallic element. The cycle can be repeated several times to build up films with sub-micrometric thickness. We show that it is possible to grow, by E-ALD, Cu2S ultra-thin films on Ag(111) with high structural quality. They show a well ordered layered crystal structure made on alternating pseudohexagonal layers in lower coordination. As reported in literature for minerals in the Cu-S compositional field, these are based on CuS3 triangular groups, with layers occupied by highly mobile Cu ions. This structural model is closely related to the one of the low chalcocite. The domain size of such films is more than 1000 Å in lateral size and extends with a high crystallinity in the vertical growth direction up to more than 10 nm. E-ALD process results in the growth of highly ordered and almost unstrained ultra-thin films. This growth can lead to the design of semiconductors with optimal transport proprieties by an appropriate doping of the intra metallic layer. The present study enables E-ALD as an efficient synthetic route for the growth of semiconducting heterostructures with tailored properties.

Heat treated carbon supported iron(ii)phthalocyanine oxygen reduction catalysts: elucidation of the structure-activity relationship using X-ray absorption spectroscopy.

This paper focuses on studying the influence of the heat treatment on the structure and activity of carbon supported Fe(ii)phthalocyanine (FePc/C) oxygen reduction reaction (ORR) catalysts under alkaline conditions. The FePc macrocycle was deposited onto ketjen black carbon and heated treated for 2 hours under inert atmosphere (Ar) at different temperatures (400, 500, 600, 700, 800, 900 and 1000 °C). The atomic structure of Fe in each sample has been determined by XAS and correlated to the activity and ORR mechanisms determined in electrochemical half cells and in a complete H2/O2 anion exchange membrane fuel cells (AEM-FC). The results show that the samples prepared at 600 and 700 °C have the highest electrochemical catalytic activity for the ORR, consistent with the findings that the FeN4 active sites are thermally stable up to 700 °C, confirmed by both XANES linear combination fittings and EXAFS fittings. Upon annealing at temperatures above 800 °C, the FeN4 structure partially decomposes to small iron nanoparticles. The transition from the FeN4 structure to metallic Fe results in a significant loss in ORR activity and an increase in the production of undesirable HO2- during catalysis.

A Pd/C-CeO2 Anode Catalyst for High-Performance Platinum-Free Anion Exchange Membrane Fuel Cells.

One of the biggest obstacles to the dissemination of fuel cells is their cost, a large part of which is due to platinum (Pt) electrocatalysts. Complete removal of Pt is a difficult if not impossible task for proton exchange membrane fuel cells (PEM-FCs). The anion exchange membrane fuel cell (AEM-FC) has long been proposed as a solution as non-Pt metals may be employed. Despite this, few examples of Pt-free AEM-FCs have been demonstrated with modest power output. The main obstacle preventing the realization of a high power density Pt-free AEM-FC is sluggish hydrogen oxidation (HOR) kinetics of the anode catalyst. Here we describe a Pt-free AEM-FC that employs a mixed carbon-CeO2 supported palladium (Pd) anode catalyst that exhibits enhanced kinetics for the HOR. AEM-FC tests run on dry H2 and pure air show peak power densities of more than 500 mW cm(-2) .

Synergy of Cobalt and Silver Microparticles Electrodeposited on Glassy Carbon for the Electrocatalysis of the Oxygen Reduction Reaction: An Electrochemical Investigation.

The combination of two different metals, each of them acting on different steps of the oxygen reduction reaction (ORR), yields synergic catalytic effects. In this respect, the electrocatalytic effect of silver is enhanced by the addition of cobalt, which is able to break the O-O bond of molecular oxygen, thus accelerating the first step of the reduction mechanism. At the same time, research is to further reduce the catalyst's cost, reducing the amount of Ag, which, even though being much less expensive than Pt, is still a noble metal. From this point of view, using a small amount of Ag together with an inexpensive material, such as graphite, represents a good compromise. The aim of this work was to verify if the synergic effects are still operating when very small amounts of cobalt (2-10 µg·cm(-2)) are added to the microparticles of silver electrodeposited on glassy carbon, described in a preceding paper from us. To better stress the different behaviour observed when cobalt and silver are contemporarily present in the deposit, the catalytic properties of cobalt alone were investigated. The analysis was completed by the Levich plots to evaluate the number of electrons involved and by Tafel plots to show the effects on the reaction mechanism.

Direct alcohol fuel cells: toward the power densities of hydrogen-fed proton exchange membrane fuel cells.

A 2 µm thick layer of TiO2 nanotube arrays was prepared on the surface of the Ti fibers of a nonwoven web electrode. After it was doped with Pd nanoparticles (1.5 mgPd cm(-2) ), this anode was employed in a direct alcohol fuel cell. Peak power densities of 210, 170, and 160 mW cm(-2) at 80 °C were produced if the cell was fed with 10 wt % aqueous solutions of ethanol, ethylene glycol, and glycerol, respectively, in 2 M aqueous KOH. The Pd loading of the anode was increased to 6 mg cm(-2) by combining four single electrodes to produce a maximum peak power density with ethanol at 80 °C of 335 mW cm(-2) . Such high power densities result from a combination of the open 3 D structure of the anode electrode and the high electrochemically active surface area of the Pd catalyst, which promote very fast kinetics for alcohol electro-oxidation. The peak power and current densities obtained with ethanol at 80 °C approach the output of H2 -fed proton exchange membrane fuel cells.

Energy and chemicals from the selective electrooxidation of renewable diols by organometallic fuel cells.

Organometallic fuel cells catalyze the selective electrooxidation of renewable diols, simultaneously providing high power densities and chemicals of industrial importance. It is shown that the unique organometallic complex [Rh(OTf)(trop2NH)(PPh3)] employed as molecular active site in an anode of an OMFC selectively oxidizes a number of renewable diols, such as ethylene glycol , 1,2-propanediol (1,2-P), 1,3-propanediol (1,3-P), and 1,4-butanediol (1,4-B) to their corresponding mono-carboxylates. The electrochemical performance of this molecular catalyst is discussed, with the aim to achieve cogeneration of electricity and valuable chemicals in a highly selective electrooxidation from diol precursors.

Electrooxidation of ethylene glycol and glycerol on Pd-(Ni-Zn)/C anodes in direct alcohol fuel cells.

The electrooxidation of ethylene glycol (EG) and glycerol (G) has been studied: in alkaline media, in passive as well as active direct ethylene glycol fuel cells (DEGFCs), and in direct glycerol fuel cells (DGFCs) containing Pd-(Ni-Zn)/C as an anode electrocatalyst, that is, Pd nanoparticles supported on a Ni-Zn phase. For comparison, an anode electrocatalyst containing Pd nanoparticles (Pd/C) has been also investigated. The oxidation of EG and G has primarily been investigated in half cells. The results obtained have highlighted the excellent electrocatalytic activity of Pd-(Ni-Zn)/C in terms of peak current density, which is as high as 3300 A g(Pd)(-1) for EG and 2150 A g(Pd)(-1) for G. Membrane-electrode assemblies (MEA) have been fabricated using Pd-(Ni-Zn)/C anodes, proprietary Fe-Co/C cathodes, and Tokuyama A-201 anion-exchange membranes. The MEA performance has been evaluated in either passive or active cells fed with aqueous solutions of 5 wt % EG and 5 wt % G. In view of the peak-power densities obtained in the temperature range from 20 to 80 °C, at Pd loadings as low as 1 mg cm(-2) at the anode, these results show that Pd-(Ni-Zn)/C can be classified amongst the best performing electrocatalysts ever reported for EG and G oxidation.

Electrochemical milling and faceting: size reduction and catalytic activation of palladium nanoparticles.

Improved performance through milling: A method for enhancing the catalytic activity of supported metal nanoparticles is reported. This method enhances the activity for the ethanol electro-oxidation of a supported palladium catalyst. The much higher catalytic performance is ascribed to the increased electrochemically active surface area as well as the generation of high-index facets at the milled nanoparticle surface.

Energy efficiency enhancement of ethanol electrooxidation on Pd-CeO(2)/C in passive and active polymer electrolyte-membrane fuel cells.

Pd nanoparticles have been generated by performing an electroless procedure on a mixed ceria (CeO(2))/carbon black (Vulcan XC-72) support. The resulting material, Pd-CeO(2)/C, has been characterized by means of transmission electron microscopy (TEM), inductively coupled plasma atomic emission spectroscopy (ICP-AES), and X-ray diffraction (XRD) techniques. Electrodes coated with Pd-CeO(2)/C have been scrutinized for the oxidation of ethanol in alkaline media in half cells as well as in passive and active direct ethanol fuel cells (DEFCs). Membrane electrode assemblies have been fabricated using Pd-CeO(2)/C anodes, proprietary Fe-Co cathodes, and Tokuyama anion-exchange membranes. The monoplanar passive and active DEFCs have been fed with aqueous solutions of 10 wt% ethanol and 2 M KOH, supplying power densities as high as 66 mW cm(-2) at 25 °C and 140 mW cm(-2) at 80 °C. A comparison with a standard anode electrocatalyst containing Pd nanoparticles (Pd/C) has shown that, at even metal loading and experimental conditions, the energy released by the cells with the Pd-CeO(2)/C electrocatalyst is twice as much as that supplied by the cells with the Pd/C electrocatalyst. A cyclic voltammetry study has shown that the co-support ceria contributes to the remarkable decrease of the onset oxidation potential of ethanol. It is proposed that ceria promotes the formation at low potentials of species adsorbed on Pd, Pd(I)-OH(ads), that are responsible for ethanol oxidation.

Cobalt monolayer islands on Ag(111) for ORR catalysis.

The design of a catalyst for one of the most important electrocatalytic reactions, the oxygen reduction reaction (ORR), was done following the most recent guidelines of theoretical studies on this topic. Aim of this work was to achieve a synergic effect of two different metals acting on different steps of the ORR. The catalytic activity of Ag, already known and characterized, was enhanced by the presence of a monolayer of cobalt subdivided into nanosized islands. To obtain such a controlled nanostructure, a novel method utilizing self-assembled monolayers (SAMs) as templates was employed. In a recent study, we were able to perform a confined electrodeposition of cobalt onto Ag(111) in a template formed by selectively desorbing a short-chain thiol (3-mercaptopropionic acid, MPA) from binary SAMs using 1-dodecanthiols (DDT). This method allows for an excellent control of the morphology of the deposit by varying the molar ratio of the two thiols. Because cobalt does not deposit on silver at an underpotential, the alternative approach of surface limited redox replacement (SLRR) was used. This method, recently developed by Adzic et al., consists of the use of a monolayer of a third metal, which can be deposited at an underpotential, as a template for the spontaneous deposition of a more noble metal. Herein, we choose zinc as template for the deposition of cobalt. Ag(111) crystals were covered by monolayer islands consisting of cobalt, with the surface atomic ratios ranging from 12 to 39% for cobalt. The catalytic activity of such samples towards ORR was evaluated and the best improvement in activity was found to be that of the sample with a cobalt percentage of approximately 30% with respect to the bare silver, which is in good agreement with theoretical hypotheses.

In situ FTIR spectroelectrochemical study on the mechanism of ethylene glycol electrocatalytic oxidation at a Pd electrode.

The adsorption and electrooxidation pathways of ethylene glycol (EG) on polycrystalline palladium surfaces have been investigated in both alkaline and acidic media by in situ FTIR spectroscopy in conjunction with cyclic voltammetry. Palladium exhibits a high electrocatalytic activity in alkaline solution with low onset oxidation potentials and high current densities, depending on the pH, as well as on the supporting electrolyte. Higher potentials are required for EG oxidation in acidic solutions, where the catalytic performance decreases with increasing the pH. The products and intermediates of EG oxidation on Pd are influenced by the pH. In alkaline media, both C(2) species (glycolate, glyoxal, glyoxylate and oxalate) and C(1) species (formate and carbonate) are formed in mutual concentrations depending on the pH. In contrast, CO(2) is selectively produced in acidic aqueous solution.

Self-sustainable production of hydrogen, chemicals, and energy from renewable alcohols by electrocatalysis.

The selective and simultaneous production of hydrogen and chemicals from renewable alcohols, such as ethanol, glycerol, and ethylene glycol, can be accomplished by means of electrolyzers in which the anode electrocatalyst is appropriately designed to promote the partial and selective oxidation of the alcohol. In the electrolyzers described herein, the production of 1 kg of hydrogen from aqueous ethanol occurs with one-third the amount of energy required by a traditional H(2)/O(2) electrolyzer, by virtue of the much lower oxidation potential of ethanol to acetate vs. water to oxygen in alkaline media (E(0)=0.10 V vs. 1.23 V). The self-sustainability of H(2) production is ensured by the simultaneous production of 25 kg of potassium acetate for every kg of H(2), if the promoting co-electrolyte is KOH.