Efficient Bifunctional Polyalcohol Oxidation and Oxygen Reduction Electrocatalysts Enabled by Ultrathin PtPdM (M = Ni,Fe, Co) Nanosheets

Despite intense research in past decades, the development of high‐performance bifunctional catalysts for direct ethylene glycol or glycerol oxidation reaction (EGOR or GOR) and oxygen reduction reaction (ORR) remains a grand challenge in realizing fuel‐cell technologies for portable electronic devices and fuel‐cell vehicle applications. Here, a general method is reported for controllable synthesis of a class of ultrathin multimetallic PtPdM (M = Ni, Fe, Co) nanosheets (NSs) with a thickness of only 1.4 nm by coreduction of metal precursors in the presence of CO and oleylamine. With the optimized composition and components, ultrathin Pt₃₂Pd₄₈Ni₂₀ NSs exhibit the highest electrocatalytic activity for EGOR, GOR, and ORR among all different ultrathin PtPdM NSs, ultrathin PtPd NSs, and the commercial catalysts. The mass activities of ultrathin Pt₃₂Pd₄₈Ni₂₀ NSs for EGOR, GOR, and ORR are 7.7, 5.4, and 7.7 times higher respectively than a commercial catalyst, and they are the most efficient nanocatalysts ever reported for EGOR/GOR. The ultrathin PtPdNi NSs are also very stable for EGOR/GOR/ORR. It is further demonstrated that these ultrathin multimetallic NSs can be readily generalized to other sensor‐related electrocatalysis system such as high‐sensitivity electrochemical detection of H₂O₂.     

Structurally Ordered Fe3Pt Nanoparticles on Robust Nitride Support as a High Performance Catalyst for the Oxygen Reduction Reaction

The commercialization of fuel cell technologies requires a significant reduction in the amount of expensive platinum catalyst in the cathode while still maintaining high catalytic activity and stability. Herein a cost‐effective, highly durable, and efficient catalyst consisting of ordered Fe₃Pt nanoparticles supported by mesoporous Ti_0.5Cr_0.5N (Fe₃Pt/Ti_0.5Cr_0.5N) is demonstrated. The Fe₃Pt/Ti_0.5Cr_0.5N catalyst exhibits a five‐fold increase in mass activity relative to a Pt/C catalyst at 0.9 V for the oxygen reduction reaction. More importantly, the catalyst shows a minimal loss of activity after 5000 potential cycles (9.7%). The enhanced activity of the ordered Fe₃Pt/Ti_0.5Cr_0.5N catalyst, in combination with its enhanced stability, makes it very promising for the development of new cathode catalysts for fuel cells.     

Sequential Synthesis and Active‐Site Coordination Principle of Precious Metal Single‐Atom Catalysts for Oxygen Reduction Reaction and PEM Fuel Cells

Carbon‐supported precious metal single‐atom catalysts (PM SACs) have shown promising application in proton exchange membrane fuel cells (PEMFCs). However, the coordination principle of the active site, consisting of one PM atom and several coordinating anions, is still unclear for PM SACs. Here, a sequential coordination method is developed to dope a large amount of PM atoms (Ir, Rh, Pt and Pd) into a zeolite imidazolate framework (ZIF), which are further pyrolyzed into nitrogen‐coordinated PM SACs. The PM loadings are as high as 1.2–4.5 wt%, achieving the highest PM loadings in ZIF‐derived SACs to date. In the acidic half‐cell, Ir₁‐N/C and Rh₁‐N/C exhibit much higher oxygen reduction reaction (ORR) activities than nanoparticle catalysts Ir/C and Rh/C. In the contrast, the activities of Pd₁‐N/C and Pt₁‐N/C are considerably lower than Pd/C and Pt/C. Density function theory (DFT) calculations reveal that the ORR activity of PM SAC depends on the match between the OH* adsorption on PM and the electronegativity of coordinating anions, and the stronger OH* adsorption is, the higher electronegativity is needed for the coordinating anions. PEMFC tests confirm the active‐site coordination principle and show the extremely high atomic efficiency of Ir₁‐N/C. The revealed principle provides guidance for designing future PM SACs for PEMFCs.     

Molecular-Dynamics Simulation of the Surface Diffusion of n-Alkanes on Pt(111)

Abstract

In the present study, the equilibrium adsorption and the dynamics of surface diffusion in a model of ethane and n-butane on a Pt(111) surface were simulated with molecular dynamics. At low temperatures, we found that both admolecules adsorb in a specific binding site. Through analysis of the trajectories, several features of the dynamics were resolved. At low temperature, we observed that diffusion occurs through a nearest-neighbor hopping mechanism involving both lateral rotation and axial translation. At high temperatures, the admolecule makes multiple-site hops and nonlocalized long flights. The temperature dependence of the diffusion coefficients was analyzed and was found to exhibit good Arrhenius behavior. The apparent diffusion coefficients follow trends seen in related experimental studies. In the case of ethane, a comparison between the diffusion barrier measured in the molecular-dynamics simulations and the theoretical barrier predicted by transition-state theory indicates that the simulated barrier is larger than the theoretical value. This finding is consistent with conclusions in recent studies of metal-atom diffusion on metal surfaces, where it was found that systems with low corrugation exhibit a non-unique relationship between the dynamical diffusion barrier and the potential-energy-surface topology.     

DNA interaction and antiproliferative behavior of the water soluble platinum supramolecular squares [(en)Pt(N-N)]4(NO3)8 (en=ethylenediamine, N-N=4,4'-bipyridine or 1,4-bis(4-pyridyl)tetrafluorobenzene)

The interaction with DNA of two water soluble platinum supramolecular squares [(en)Pt(N-N)]4(NO3)8 (en=ethylenediamine, N-N=1,4-bis(4-pyridyl)tetrafluorobenzene, compound 1, N-N=4,4'-bipyridine, compound 2) has been studied by circular dichroism, electrophoretic mobility and atomic force microscopy. the two complexes drastically modify the second and tertiary structures of DNA, but compound 2 does it strongly due probably to its smaller size by comparison with compound 1 and its more suitable structural features for intercalation between base pairs. The two supramolecular squares were assayed against the HL-60 tumor cell line for 24 and 72 h. The IC50 values for 24 h are smaller than that of cisplatin for this time, however for 72 h the IC50 have higher values being the corresponding to compound 2 comparable to that of cisplatin. Apoptotic assays were also carried out for the compounds 1 and 2 against the tumor cell line.     

The Role of Active Oxide Species for Electrochemical Water Oxidation on the Surface of 3d‐Metal Phosphides

Transition metal phosphides (TMPs) have recently been utilized as promising electrocatalysts for oxygen evolution reaction (OER) in alkaline media. The metal oxides or hydroxides formed on their surface during the OER process are hypothesized to play an important role. However, their exact role is yet to be elucidated. Here unambiguous justification regarding the active role of oxo(hydroxo) species on O‐Ni_(1?x)Fe_xP₂ nanosheet with pyrite structure is shown. These O‐Ni_(1?x)Fe_xP₂ (x = 0.25) nanosheets demonstrate greatly improved OER performance than their corresponding hydroxide and oxide counterparts do. From density function theory (DFT) calculations, it is found that the introduction of iron into the pyrite‐phased NiP₂ alters OER steps occurred on the surface. Notably, the partially oxidized surface of O‐Ni_(1?x)Fe_xP₂ nanosheets is vital to improve the local environment and accelerate the reaction steps. This study sheds light on the OER mechanism of the 3d TMP electrocatalyst and opens up a way to develop efficient and low‐cost electrocatalysts.     

Ni3+‐Induced Formation of Active NiOOH on the Spinel Ni–Co Oxide Surface for Efficient Oxygen Evolution Reaction

Efficient and earth abundant electrocatalysts for high‐performance oxygen evolution reaction (OER) are essential for the development of sustainable energy conversion technologies. Here, a new hierarchical Ni–Co oxide nanostructure, composed of small secondary nanosheets grown on primary nanosheet arrays, is synthesized via a topotactic transformation of Ni–Co layered double hydroxide. The Ni³⁺‐rich surface benefits the formation of NiOOH, which is the main redox site as revealed via in situ X‐ray absorption near edge structure and extended X‐ray absorption fine structure spectroscopy. The Ni–Co oxide hierarchical nanosheets (NCO–HNSs) deliver a stable current density of 10 mA cm^?2 at an overpotential of ≈0.34 V for OER with a Tafel slope of as low as 51 mV dec^?1 in alkaline media. The improvement in the OER activity can be ascribed to the synergy of large surface area offered by the 3D hierarchical nanostructure and the facile formation of NiOOH as the main active sites on the surface of NCO–HNSs to decrease the overpotential and facilitate the catalytic reaction.     

Preparation and Use of Samarium Diiodide (SmI2) in Organic Synthesis: The Mechanistic Role of HMPA and Ni(II) Salts in the Samarium Barbier Reaction

Although initially considered an esoteric reagent, SmI₂ has become a common tool for synthetic organic chemists. SmI₂ is generated through the addition of molecular iodine to samarium metal in THF.^1,2-3 It is a mild and selective single electron reductant and its versatility is a result of its ability to initiate a wide range of reductions including C-C bond-forming and cascade or sequential reactions. SmI₂ can reduce a variety of functional groups including sulfoxides and sulfones, phosphine oxides, epoxides, alkyl and aryl halides, carbonyls, and conjugated double bonds.^2-12 One of the fascinating features of SmI-₂-mediated reactions is the ability to manipulate the outcome of reactions through the selective use of cosolvents or additives. In most instances, additives are essential in controlling the rate of reduction and the chemo- or stereoselectivity of reactions.^13-14 Additives commonly utilized to fine tune the reactivity of SmI₂ can be classified into three major groups: (1) Lewis bases (HMPA, other electron-donor ligands, chelating ethers, etc.), (2) proton sources (alcohols, water etc.), and (3) inorganic additives (Ni(acac)₂, FeCl₃, etc).³Understanding the mechanism of SmI₂ reactions and the role of the additives enables utilization of the full potential of the reagent in organic synthesis. The Sm-Barbier reaction is chosen to illustrate the synthetic importance and mechanistic role of two common additives: HMPA and Ni(II) in this reaction. The Sm-Barbier reaction is similar to the traditional Grignard reaction with the only difference being that the alkyl halide, carbonyl, and Sm reductant are mixed simultaneously in one pot.^1,15 Examples of Sm-mediated Barbier reactions with a range of coupling partners have been reported,^1,3,7,10,12 and have been utilized in key steps of the synthesis of large natural products.^16,17 Previous studies on the effect of additives on SmI₂ reactions have shown that HMPA enhances the reduction potential of SmI₂ by coordinating to the samarium metal center, producing a more powerful,^13-14,18 sterically encumbered reductant^19-21 and in some cases playing an integral role in post electron-transfer steps facilitating subsequent bond-forming events.²² In the Sm-Barbier reaction, HMPA has been shown to additionally activate the alkyl halide by forming a complex in a pre-equilibrium step.²³Ni(II) salts are a catalytic additive used frequently in Sm-mediated transformations.^24-27 Though critical for success, the mechanistic role of Ni(II) was not known in these reactions. Recently it has been shown that SmI₂ reduces Ni(II) to Ni(0), and the reaction is then carried out through organometallic Ni(0) chemistry.²⁸These mechanistic studies highlight that although the same Barbier product is obtained, the use of different additives in the SmI₂ reaction drastically alters the mechanistic pathway of the reaction. The protocol for running these SmI₂-initiated reactions is described.     

Steric effects on the substitution of pentaamine-chromium(III) and -rhodium(III) complexes. Anation reaction rate constants as an indicative of the dissociative shift of the mechanism on crowding the [M(RNH2)5H2O] (M = Rh, R = H, Me, Et, Pr; M = Cr, R = H, Me, Pr) complexes

The kinetics of the anation reactions of [M(RNH₂)₅H₂O]³⁺ (M = Rh, R = H, Me, Et, Pr; M = Cr, R = H, Me, Pr) with several ligands (H₃PO₄/H₂PO₄⁻, H₃PO₃/H₂PO₃⁻, CF₃COO⁻, Br⁻, Cl⁻, SCN⁻) have been studied at different temperatures and acidities at I = 1.0 M (LiClO₄. Results obtained for the anation rate constants and thermal activation parameters are compared with the previously published data for R = H, in order to establish the effects of the amine substituents in the reaction mechanism proposed for the substitution reactions of these complexes. The results obtained are interpreted on the basis of a mechanism where the bond formation process is more important in the substitution on M = Cr complexes than in that of the M = Rh complexes, as already pointed out for the published ΔΛ^≠ values for the water exchange on these systems. A simple Langford-Gray classification becomes inadequate to describe these situations where the increase of the steric demand of the amine substituents shifta the I_a-I_d classification to the I_d side, although no dramatic changes in the reaction mechanism are found. It is concluded that a More O'Ferall ‘continuous’ type of approach to the mechanism classification of the substitution reactions is much more useful in this case.     

Nanoscale Probing of Voltage Activated Oxygen Reduction/Evolution Reactions in Nanopatterned (LaxSr1‐x)CoO3‐δ Cathodes

Bias‐dependent mechanisms of reversible and irreversible electrochemical processes on a (La_0.5Sr_0.5)₂CoO_4±δ modified (La_xSr_1‐x)CoO₃‐ surface are studied using dynamic electrochemical strain microscopy (D‐ESM). The reversible oxygen reduction/evolution process is activated at voltages as low as 3–4 V and the degree of transformation increases linearly with applied bias. The irreversible processes associated with static surface deformation become apparent above 10–12 V. Post‐mortem focused‐ion milling combined with atomic resolution scanning transmission electron microscopy and electron energy loss spectroscopy is used to establish the mechanisms of irreversible transformations and attribute it to amorphization of the top layer of material. These studies both establish the framework for probing irreversible electrochemical processes in solids and illustrate rich spectrum of electrochemical transformations underpinning electrocatalytic activity in cobaltites.     

Three-centre, two-electron bonds in complexes of Mn, Ni, Co and Cd with 3-(4-benzonitrile)pyrazolyl scorpionates

A series of complexes of the form [M(Bp^3(4Bz))(Tp^3(4Bz))] containing a mixture of novel dihydrobis(3-(4-benzonitrile)pyrazolyl)borato (Bp^3(4Bz)) and hydrotris(3-(4-benzonitrile)pyrazolyl)borato (Tp^3(4Bz)) ligands have been synthesised and structurally characterised. The ligands, both containing 3-(4-benzonitrile)pyrazole arms, form isostructural complexes with a variety of transition metals although different crystalline solvates have been obtained depending upon the crystallisation conditions, namely [M(Bp^3(4Bz))(Tp^3(4Bz))] (M = Ni, Co), [Ni((Bp^3(4Bz))(Tp^3(4Bz))]·2MeOH and [M(Bp^3(4Bz))(Tp^3(4Bz))]·MeOH (M = Mn, Cd). In all cases the metal atoms are five-coordinate with an additional agostic B-H interaction from the bis(pyrazolyl)borate ligand.