Enhanced Electrochemical Hydrogenation of Benzaldehyde to Benzyl Alcohol on Pd@Ni-MOF by Modifying the Adsorption Configuration.

Electrocatalytic hydrogenation (ECH) approaches under ambient temperature and pressure offer significant potential advantages over thermal hydrogenation processes but require highly active and efficient hydrogenation electrocatalysts. The performance of such hydrogenation electrocatalysts strongly depends not only on the active phase but also on the architecture and surface chemistry of the support material. Herein, Pd nanoparticles supported on a nickel metal-organic framework (MOF), Ni-MOF-74, are prepared, and their activity toward the ECH of benzaldehyde (BZH) in a 3 M acetate (pH 5.2) aqueous electrolyte is explored. An outstanding ECH rate up to 283 µmol cm-2 h-1 with a Faradaic efficiency (FE) of 76% is reached. Besides, higher FEs of up to 96% are achieved using a step-function voltage. Materials Studio and density functional theory calculations show these outstanding performances to be associated with the Ni-MOF support that promotes H-bond formation, facilitates water desorption, and induces favorable tilted BZH adsorption on the surface of the Pd nanoparticles. In this configuration, BZH is bonded to the Pd surface by the carbonyl group rather than through the aromatic ring, thus reducing the energy barriers of the elemental reaction steps and increasing the overall reaction efficiency.

Dielectric function of vanadium oxide thin films by thermal annealing.

The dielectric function of ${{\rm{VO}}_x}$ and ${{\rm{V}}_2}{{\rm{O}}_5}$ thin films is determined with the use of a spectroscopic Mueller matrix ellipsometer from 1.5 to 5.0 eV. The complex dielectric function of the films is calculated using the measured Mueller matrices filtered with the Cloude decomposition. ${{\rm{VO}}_x}$ shows high absorption in the UV region, a Tauc-Lorentz gap around 2.4 eV, and non-vanishing absorption in the visible. ${{\rm{V}}_2}{{\rm{O}}_5}$ shows a high absorption band centered at 2.87 eV, an indirect optical band gap at 1.95 eV, and a direct optical band gap at 2.33 eV. The ellipsometric characterization is supported by Raman, x-ray photoelectron, and photoluminescence spectroscopy.

Photodehydrogenation of Ethanol over Cu2O/TiO2 Heterostructures.

The photodehydrogenation of ethanol is a sustainable and potentially cost-effective strategy to produce hydrogen and acetaldehyde from renewable resources. The optimization of this process requires the use of highly active, stable and selective photocatalytic materials based on abundant elements and the proper adjustment of the reaction conditions, including temperature. In this work, Cu2O-TiO2 type-II heterojunctions with different Cu2O amounts are obtained by a one-pot hydrothermal method. The structural and chemical properties of the produced materials and their activity toward ethanol photodehydrogenation under UV and visible light illumination are evaluated. The Cu2O-TiO2 photocatalysts exhibit a high selectivity toward acetaldehyde production and up to tenfold higher hydrogen evolution rates compared to bare TiO2. We further discern here the influence of temperature and visible light absorption on the photocatalytic performance. Our results point toward the combination of energy sources in thermo-photocatalytic reactors as an efficient strategy for solar energy conversion.

Influence of Colloidal Au on the Growth of ZnO Nanostructures.

Vapor-liquid-solid processes allow growing high-quality nanowires from a catalyst. An alternative to the conventional use of catalyst thin films, colloidal nanoparticles offer advantages not only in terms of cost, but also in terms of controlling the location, size, density, and morphology of the grown nanowires. In this work, we report on the influence of different parameters of a colloidal Au nanoparticle suspension on the catalyst-assisted growth of ZnO nanostructures by a vapor-transport method. Modifying colloid parameters such as solvent and concentration, and growth parameters such as temperature, pressure, and Ar gas flow, ZnO nanowires, nanosheets, nanotubes and branched-nanowires can be grown over silica on silicon and alumina substrates. High-resolution transmission electron microscopy reveals the high-crystal quality of the ZnO nanostructures obtained. The photoluminescence results show a predominant emission in the ultraviolet range corresponding to the exciton peak, and a very broad emission band in the visible range related to different defect recombination processes. The growth parameters and mechanisms that control the shape of the ZnO nanostructures are here analyzed and discussed. The ZnO-branched nanowires were grown spontaneously through catalyst migration. Furthermore, the substrate is shown to play a significant role in determining the diameters of the ZnO nanowires by affecting the surface mobility of the metal nanoparticles.

Correction to "Oxidative Cleavage of Cellobiose by Lytic Polysaccharide Monooxygenase (LPMO)-Inspired Copper Complexes".

[This corrects the article DOI: 10.1021/acsomega.9b00785.].

Photoluminescence of ZnO Nanowires: A Review.

One-dimensional ZnO nanostructures (nanowires/nanorods) are attractive materials for applications such as gas sensors, biosensors, solar cells, and photocatalysts. This is due to the relatively easy production process of these kinds of nanostructures with excellent charge carrier transport properties and high crystalline quality. In this work, we review the photoluminescence (PL) properties of single and collective ZnO nanowires and nanorods. As different growth techniques were obtained for the presented samples, a brief review of two popular growth methods, vapor-liquid-solid (VLS) and hydrothermal, is shown. Then, a discussion of the emission process and characteristics of the near-band edge excitonic emission (NBE) and deep-level emission (DLE) bands is presented. Their respective contribution to the total emission of the nanostructure is discussed using the spatial information distribution obtained by scanning transmission electron microscopy-cathodoluminescence (STEM-CL) measurements. Also, the influence of surface effects on the photoluminescence of ZnO nanowires, as well as the temperature dependence, is briefly discussed for both ultraviolet and visible emissions. Finally, we present a discussion of the size reduction effects of the two main photoluminescent bands of ZnO. For a wide emission (near ultra-violet and visible), which has sometimes been attributed to different origins, we present a summary of the different native point defects or trap centers in ZnO as a cause for the different deep-level emission bands.

Conformational Effects of [Ni2 (µ-ArS)2 ] Cores on Their Electrocatalytic Activity.

Two nickel complexes supported by tridentate NS2 ligands, [Ni2 (κ-N,S,S,S'-NPh {CH2 (MeC6 H2 R')S}2 )2 ] (1; R'=3,5-(CF3 )2 C6 H3 ) and [Ni2 (κ-N,S,S,S'-NiBu {CH2 C6 H4 S}2 )2 ] (2), were prepared as bioinspired models of the active site of [NiFe] hydrogenases. The solid-state structure of 1 reveals that the [Ni2 (µ-ArS)2 ] core is bent, with the planes of the nickel centers at a hinge angle of 81.3(5)°, whereas 2 shows a coplanar arrangement between both nickel(II) ions in the dimeric structure. Complex 1 electrocatalyzes proton reduction from CF3 COOH at -1.93 (overpotential of 1.04 V, with icat /ip ≈21.8) and -1.47 V (overpotential of 580 mV, with icat /ip ≈5.9) versus the ferrocene/ferrocenium redox couple. The electrochemical behavior of 1 relative to that of 2 may be related to the bent [Ni2 (µ-ArS)2 ] core, which allows proximity of the two Ni⋅⋅⋅Ni centers at 2.730(8) Å; thus possibly favoring H+ reduction. In contrast, the planar [Ni2 (µ-ArS)2 ] core of 2 results in a Ni⋅⋅⋅Ni distance of 3.364(4) Šand is unstable in the presence of acid.

Oxidative Cleavage of Cellobiose by Lytic Polysaccharide Monooxygenase (LPMO)-Inspired Copper Complexes.

The potentially tridentate ligand bis[(1-methyl-2-benzimidazolyl)ethyl]amine (2BB) was employed to prepare copper complexes [(2BB)CuI]OTf and [(2BB)CuII(H2O)2](OTf)2 as bioinspired models of lytic polysaccharide copper-dependent monooxygenase (LPMO) enzymes. Solid-state characterization of [(2BB)CuI]OTf revealed a Cu(I) center with a T-shaped coordination environment and metric parameters in the range of those observed in reduced LPMOs. Solution characterization of [(2BB)CuII(H2O)2](OTf)2 indicates that [(2BB)CuII(H2O)2]2+ is the main species from pH 4 to 7.5; above pH 7.5, the hydroxo-bridged species [{(2BB)CuII(H2O) x }2(µ-OH)2]2+ is also present, on the basis of cyclic voltammetry and mass spectrometry. These observations imply that deprotonation of the central amine of Cu(II)-coordinated 2BB is precluded, and by extension, amine deprotonation in the histidine brace of LPMOs appears unlikely at neutral pH. The complexes [(2BB)CuI]OTf and [(2BB)CuII(H2O)2](OTf)2 act as precursors for the oxidative degradation of cellobiose as a cellulose model substrate. Spectroscopic and reactivity studies indicate that a dicopper(II) side-on peroxide complex generated from [(2BB)CuI]OTf/O2 or [(2BB)CuII(H2O)2](OTf)2/H2O2/NEt3 oxidizes cellobiose both in acetonitrile and aqueous phosphate buffer solutions, as evidenced from product analysis by high-performance liquid chromatography-mass spectrometry. The mixture of [(2BB)CuII(H2O)2](OTf)2/H2O2/NEt3 results in more extensive cellobiose degradation. Likewise, the use of both [(2BB)CuI]OTf and [(2BB)CuII(H2O)2](OTf)2 with KO2 afforded cellobiose oxidation products. In all cases, a common Cu(II) complex formulated as [(2BB)CuII(OH)(H2O)]+ was detected by mass spectrometry as the final form of the complex.

Easily reduced bis-pincer (NS2)2molybdenum(iv) to (NHS2)2Mo(ii) by alcohols vs. redox-inert (NS2)(NHS2)iron(iii) complexes.

Iron and molybdenum complexes supported by a pincer-type dianionic [NS2]2- donor were prepared to compare their structural, spectroscopic, and electrochemical properties. The versatility of the [NS2]2Mo(iv) complex (2) to access different oxidation states was evidenced in the activation of methanol and isopropanol, oxidising them to formaldehyde or acetone with concomitant reduction and protonation to afford [NHS2]2Mo(ii), complex (3). This redox behaviour contrasts with the null reactivity observed for the analogous ferric complex [NS2][NHS2]Fe(iii) (1). Complex 2 presents a quasi-reversible process at E1/2 = -0.80 V relative to the ferrocenium/ferrocene couple (Fc+/Fc), which is attributed to the Mo(iv)/Mo(v) redox couple. Two irreversible cathodic processes were observed at Ecp = -1.59 and -2.20 V, which are attributed to the Mo(iv)/Mo(iii) and Mo(iii)/Mo(ii) redox couples. Cyclic voltammetry and solid-state structures obtained by X-ray crystallography support a 2H+ and 2e- process, whereby the Mo(iv) centre in 2 is reduced sequentially to Mo(iii), and finally to Mo(ii) in 3. These redox events were observed at Ecp = -1.22 and -2.15 V (vs. Fc+/Fc) in the anodic cyclic voltammograms of 2 in THF in the presence of acid. A new reduction peak was detected under these conditions at Ecp = -2.30 V, consistent with electrocatalytic proton reduction. This was corroborated for 2 as a catalyst precursor in the presence of increasing amounts of p-toluenesulfonic acid, with the addition of 2 to 14 equivs resulting in an increase of the current measured.

Redox flexibility of iron complexes supported by sulfur-based tris(o-methylenethiophenolato)amine relative to its tripodal oxygen-based congener.

Tripodal ligands designed to generate a local C3 symmetry have resulted in novel types of metal complexes that feature unusual bonding and electronic properties. However, most complexes reported to date are characterised by strong field ligands that enforce low or intermediate-spin states for the metal centres. Moreover, anionic sulfur-based tripodal ligands are particularly scarce due to their challenging synthesis. In this context, we herein report the synthesis, spectral characterization, structural, and electronic properties of an iron complex supported by the tripodal, trianionic ligand [N(CH2ArS)3](3-) as the trigonal-bipyramidal complexes [Fe{N(CH2ArS)3}(X)] ((X), X = DMSO, THF). The solid-state structures reveal local C3v symmetry around the Fe(3+) ions, while electron spin resonance measurements established a high-spin state (S = 5/2). Electrochemical studies demonstrate the redox flexibility of the FeS3 fragment by direct comparison with the oxygen-based analogue N(CH2ArOH)3, which displays an irreversible reduction; in contrast, (THF) has a reversible Fe(3+)/Fe(2+) redox process at -0.83 V (relative to the ferrocenium/ferrocene redox couple). The high spin and redox properties of (THF) are attributable to the weak ligand field provided by the NS3 fragment, as confirmed by the electronic structure calculated by density functional theory, which reveals substantial electronic delocalisation and covalency of the Fe-S bonds in (X).

Electrocatalytic proton reduction by dimeric nickel complex of a sterically demanding pincer-type NS2 aminobis(thiophenolate) ligand.

Basic methanolysis of a sterically hindered aminobis(S-arylthiocarbamate) affords a novel aminobis(thiophenolate) pincer-type ligand NS22­; the in situ generated dianion reacts cleanly with Ni2+ and Zn2+ resulting in dimeric complexes with bridging thiophenolate ligands, as determined spectroscopically and by X-ray crystallography. The C2-symmetric [Ni(NS2)]2 dimer (1) has a square planar coordination geometry around the Ni2+ ions, while the [Zn(NS2)]2 analogue (2) is characterized by a distorted tetrahedral geometry around each independent Zn2+ ion. Addition of the neutral monodentate donor L = 2,6-xylylisocyanide to [Ni(NS2)]2 affords the monomeric complex [LNi(NS2)] (3), which is characterized in the solid state by a square planar geometry with the isocyanide donor trans to the tertiary amine of NS2. The pincer NS2 ligand provides redox plasticity to 1, manifested in the accessibility of the putative Ni+Ni+ and Ni3+Ni3+ dimeric complexes, based on comparative cyclic voltammetry studies with 2 and 3. The redox properties of 1 endow it with hydrogenase-type activity, as evidenced in the electrocatalytic reduction of protons in a mixed aqueous/organic phase, as well as the oxidation of hydrides from NaBH(OAc)3. Both 1 and 3 are resilient under protic and oxidative conditions, as evidenced in reactivity tests monitored by UV­vis spectroscopy.

Structural evolution of small gold clusters doped by one and two boron atoms.

The potential energy surfaces (PES) of a series of gold-boron clusters with formula Aun B (n = 1-8) and Aum B2 (m = 1-7) have been explored using a modified stochastic search algorithm. Despite the complexity of the PES of these clusters, there are well-defined growth patterns. The bonding of these clusters is analyzed using the adaptive natural density partitioning and the natural bonding orbital analyses. Reactivity is studied in terms of the molecular electrostatic potential.

Evolution of the vibrational spectra of doped hydrogen clusters with pressure.

The evolution of the vibrational spectra of the isoelectronic hydrogen clusters H26, H24He, and H24Li(+) is determined with pressure. We establish the vibrational modes with collective character common to the clusters, identify their individual vibrational fingerprints and discuss frequency shifts in the giga-Pascal pressure region. The results are of interest for the identification of doping elements such as inert He and ionic Li(+) in hydrogen under confinement or, conversely, establish the pressure of doped hydrogen when the vibrational spectrum is known. At high pressure, the spectra of the nanoclusters resemble the spectrum of a solid, and the nanoclusters may be considered crystals of nanometer scale. The computations are performed at the gradient-corrected level of density functional theory. The investigation is the first of its kind.

Copper versus thioether-centered oxidation: mechanistic insights into the non-innocent redox behavior of tripodal benzimidazolylaminothioether ligands.

A series of Cu(+) complexes with ligands that feature varying numbers of benzimidazole/thioether donors and methylene or ethylene linkers between the central nitrogen atom and the thioether sulfur atoms have been spectroscopically and electrochemically characterized. Cyclic voltammetry measurements indicated that the highest Cu(2+)/Cu(+) redox potentials correspond to sulfur-rich coordination environments, with values decreasing as the thioether donors are replaced by nitrogen-donating benzimidazoles. Both Cu(2+) and Cu(+) complexes were studied by DFT. Their electronic properties were determined by analyzing their frontier orbitals, relative energies, and the contributions to the orbitals involved in redox processes, which revealed that the HOMOs of the more sulfur-rich copper complexes, particularly those with methylene linkers (-N-CH2-S-), show significant aromatic thioether character. Thus, the theoretically predicted initial oxidation at the sulfur atom of the methylene-bridged ligands agrees with the experimentally determined oxidation waves in the voltammograms of the NS3- and N2S2-type ligands as being ligand-based, as opposed to the copper-based processes of the ethylene-bridged Cu(+) complexes. The electrochemical and theoretical results are consistent with our previously reported mechanistic proposal for Cu(2+)-promoted oxidative C-S bond cleavage, which in this work resulted in the isolation and complete characterization (including by X-ray crystallography) of the decomposition products of two ligands employed, further supporting the novel reactivity pathway invoked. The combined results raise the possibility that the reactions of copper-thioether complexes in chemical and biochemical systems occur with redox participation of the sulfur atom.