Showing papers on "Disproportionation published in 2021"

Photodriven Disproportionation of Nitrogen and Its Change to Reductive Nitrogen Photofixation

Abstract: Nitrogen fixation is an essential process for sustaining life. Tremendous efforts have been made on the photodriven fixation of nitrogen into ammonia. However, the disproportionation of dinitrogen to ammonia and nitrate under ambient conditions has remained a grand challenge. In this work, the photodriven disproportionation of nitrogen is realized in water under visible light and ambient conditions using Fe-doped TiO2 microspheres. The oxygen vacancies associated with the Fe dopants activate chemisorbed N2 molecules, which can then be fixed into NH3 with H2 O2 as the oxidation product. The generated H2 O2 thereafter oxidizes NH3 into nitrate. This disproportionation reaction can be turned to the reductive one by loading plasmonic Au nanoparticles in the doped TiO2 microspheres. The generated H2 O2 can be effectively decomposed by the Au nanoparticles, resulting in the transformation of the disproportionation reaction to the completely reductive nitrogen photofixation.

Thermal Cycling of a MOF-Based NO Disproportionation Catalyst

Abstract: The metal–organic framework CuI-MFU-4l reacts with NO, initially forming a copper(I)-nitrosyl at low pressure, and subsequently generates NO disproportionation products CuII–NO2 and N2O. The therma...

Interplay between CO Disproportionation and Oxidation: On the Origin of the CO Reaction Onset on Atomic Layer Deposition-Grown Pt/ZrO2 Model Catalysts.

Abstract: Pt/ZrO2 model catalysts were prepared by atomic layer deposition (ALD) and examined at mbar pressure by operando sum frequency generation (SFG) spectroscopy and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) combined with differentially pumped mass spectrometry (MS). ALD enables creating model systems ranging from Pt nanoparticles to bulk-like thin films. Polarization-dependent SFG of CO adsorption reveals both the adsorption configuration and the Pt particle morphology. By combining experimental data with ab initio density functional theory (DFT) calculations, we show that the CO reaction onset is determined by a delicate balance between CO disproportionation (Boudouard reaction) and oxidation. CO disproportionation occurs on low-coordinated Pt sites, but only at high CO coverages and when the remaining C atom is stabilized by a favorable coordination. Thus, under the current conditions, initial CO oxidation is found to be strongly influenced by the removal of carbon deposits formed through disproportionation mechanisms rather than being determined by the CO and oxygen inherent activity. Accordingly, at variance with the general expectation, rough Pt nanoparticles are seemingly less active than smoother Pt films. The applied approach enables bridging both the "materials and pressure gaps".

Mechanism of mediated alkali peroxide oxidation and triplet versus singlet oxygen formation

Abstract: Aprotic alkali metal–O2 batteries face two major obstacles to their chemistry occurring efficiently, the insulating nature of the formed alkali superoxides/peroxides and parasitic reactions that are caused by the highly reactive singlet oxygen (1O2). Redox mediators are recognized to be key for improving rechargeability. However, it is unclear how they affect 1O2 formation, which hinders strategies for their improvement. Here we clarify the mechanism of mediated peroxide and superoxide oxidation and thus explain how redox mediators either enhance or suppress 1O2 formation. We show that charging commences with peroxide oxidation to a superoxide intermediate and that redox potentials above ~3.5 V versus Li/Li+ drive 1O2 evolution from superoxide oxidation, while disproportionation always generates some 1O2. We find that 1O2 suppression requires oxidation to be faster than the generation of 1O2 from disproportionation. Oxidation rates decrease with growing driving force following Marcus inverted-region behaviour, establishing a region of maximum rate. Redox mediators are important for improving the rechargeability of metal–air batteries, however, how they affect singlet oxygen formation and hence parasitic chemistry is unclear, hindering strategies for their improvement. Now, the mechanism of mediated peroxide and superoxide oxidation is elucidated, explaining how redox mediators either enhance or suppress singlet oxygen formation.

Hydrocracking, hydrogenation and isomerization of model biomass tar in a packed bed reactor over bimetallic NiMo zeolite catalysts: Tailoring structure/acidity

Abstract: In this experimental study, various NiMo-promoted (as follows: H-beta, H-mordenite, H-USY, H-Y, and H-ZSM-5) catalysts were prepared, tested, and compared with pristine zeolites for the purpose of the gas phase hydrocracking, hydrogenation, and isomerization in a packed bed reactor at 370 °C under atmospheric relative pressure. 5 wt.% naphthalene/95 wt.% 1-methylnaphthalene were selected as biomass tar model molecule compounds, based on real chemical compositions. A series of material characterization techniques were applied to determine the physical structural, morphological, textural, redox, and acidic properties of synthesized catalysts. An outstanding catalytic activity and stability were found over the 2.5 wt.% Ni–2.5 wt.% Mo/ZSM-5 with high carbon deposition resistance, 2-methylnaphthalene selectivity (96.0 mol.%) in the liquid with the products with a yielded total conversion of 96.3 mol.% after an 18 h time on stream, where ethylene/propane were main (94.2 wt.%). The latter can be attributed to the presence of mesopore volume/surface area, existing boundary interface, the amount of medium/strong acid sites, and the synergetic interaction phenomena between metal atom species/supports. Attention should be paid to particle size dimensions, diameters and acidity, which facilitated poly-aromatic hydrocarbon removal. Considering particular obtained distributions, intermediates reaction pathway was proposed. Cracking, synthetic ring opening, alkylation, condensation and disproportionation were additionally involved. Results were consistent with the occurrence of two competing mechanisms, a monomolecular, as well as a bimolecular one.

Quasilinear 3d-metal(i) complexes [KM(N(Dipp)SiR3)2] (M = Cr-Co) - structural diversity, solution state behaviour and reactivity.

Abstract: The synthesis and characterization of neutral quasilinear 3d-metal(I) complexes of chromium to cobalt of the type [KM(N(Dipp)SiMe3)2] (Dipp = 2,6-di-iso-propylphenyl) are reported. In solid state these metal(I) complexes either occur as isolated molecules (Co) or are part of a potassium ion linked 1D-coordination polymer (Cr–Fe). In solution the potassium cation is either ligated within the ligand sphere of the metal silylamide or is separated from the complex depending on the solvent. For iron, we showcase that it is possible to use sodium or lithium metal for the reduction of the metal(II) precursor. However, in these cases the resulting iron(I) complexes can only be isolated upon cation separation using an appropriate crown-ether. Further, the neutral metal(I) complexes are used to introduce NBu4+ as an organic cation in the case of cobalt and iron. The impact of the intramolecular cation complexation was further demonstrated upon reaction with diphenyl acetylene which leads to bond formation processes and redox disproportionation instead of η2-alkyne complex formation.

Highly stable titanium–manganese single flow batteries for stationary energy storage

Abstract: Manganese-based flow batteries have attracted increasing interest due to their advantages of low cost and high energy density. However, the sediment (MnO2) from Mn3+ disproportionation reaction creates the risk of blocking pipelines, leading to poor stability. Herein, a titanium–manganese single flow battery (TMSFB) with high stability is designed and fabricated for the first time. In the design, a static cathode without the tank and pump is employed to avoid blockage of pipelines by MnO2 particles. Benefiting from the deceasing disproportionation reaction rate, MnO2 from the disproportionation reaction is fully utilized, realizing nearly two electron capacity. The novel TMSFB exhibits coulombic efficiency (CE) of over 99.0% at a current density of 40 mA cm−2. Most importantly, the TMSFB can run stably over 1000 cycles without capacity decay, demonstrating very good stability. With low cost, high efficiency and long cycle life, TMSFBs exhibit remarkable potential for large scale energy storage.

Evidence for ligand- and solvent-induced disproportionation of uranium(IV)

Abstract: Disproportionation, where a chemical element converts its oxidation state to two different ones, one higher and one lower, underpins the fundamental chemistry of metal ions. The overwhelming majority of uranium disproportionations involve uranium(III) and (V), with a singular example of uranium(IV) to uranium(V/III) disproportionation known, involving a nitride to imido/triflate transformation. Here, we report a conceptually opposite disproportionation of uranium(IV)-imido complexes to uranium(V)-nitride/uranium(III)-amide mixtures. This is facilitated by benzene, but not toluene, since benzene engages in a redox reaction with the uranium(III)-amide product to give uranium(IV)-amide and reduced arene. These disproportionations occur with potassium, rubidium, and cesium counter cations, but not lithium or sodium, reflecting the stability of the corresponding alkali metal-arene by-products. This reveals an exceptional level of ligand- and solvent-control over a key thermodynamic property of uranium, and is complementary to isolobal uranium(V)-oxo disproportionations, suggesting a potentially wider prevalence possibly with broad implications for the chemistry of uranium. Disproportion of uranium(IV) is rare, as it is usually the stable product of uranium(III) or (V) disproportionation. Here, the authors report uranium(IV) disproportionation to uranium(III) and (V) revealing ligand and solvent control over a key thermodynamic property of uranium

A Quasi-Intramolecular Solid-Phase Redox Reaction of Ammonia Ligands and Perchlorate Anion in Diamminesilver(I) Perchlorate

Abstract: The reaction of ammoniacal AgNO3 solution (or aq. solution of [Ag(NH3)2]NO3) with aq. NaClO4 resulted in [Ag(NH3)2]ClO4 (compound 1). Detailed spectroscopic (correlation analysis, IR, Raman, and UV) analyses were performed on [Ag(NH3)2]ClO4. The temperature and enthalpy of phase change for compound 1 were determined to be 225.7 K and 103.04 kJ/mol, respectively. We found the thermal decomposition of [Ag(NH3)2]ClO4 involves a solid-phase quasi-intramolecular redox reaction between the perchlorate anion and ammonia ligand, resulting in lower valence chlorine oxyacid (chlorite, chlorate) components. We did not detect thermal ammonia loss during the formation of AgClO4. However, a redox reaction between the ammonia and perchlorate ion resulted in intermediates containing chlorate/chlorite, which disproportionated (either in the solid phase or in aqueous solutions after the dissolution of these decomposition intermediates in water) into AgCl and silver perchlorate. We propose that the solid phase AgCl-AgClO4 mixture eutectically melts, and the resulting AgClO4 decomposes in this melt into AgCl and O2. Thus, the final product of decomposition is AgCl, N2, and H2O. The intermediate (chlorite, chlorate) phases were identified by IR, XPS, and titrimetric methods.

Switching between mono and doubly reduced odd alternant hydrocarbon: designing a redox catalyst

Abstract: Since the early Huckel molecular orbital (HMO) calculations in 1950, it has been well known that the odd alternant hydrocarbon (OAH), the phenalenyl (PLY) system, can exist in three redox states: closed shell cation (12π e−), mono-reduced open shell neutral radical (13π e−) and doubly reduced closed shell anion (14π e−). Switching from one redox state of PLY to another leads to a slight structural change owing to its low energy of disproportionation making the electron addition or removal process facile. To date, mono-reduced PLY based radicals have been extensively studied. However, the reactivity and application of doubly reduced PLY species have not been explored so far. In this work, we report the synthesis of the doubly reduced PLY species (14π e−) and its application towards the development of redox catalysis via switching with the mono-reduced form (13π e−) for aryl halide activation and functionalization under transition metal free conditions without any external stimuli such as heat, light or cathodic current supply.

HCOOH disproportionation to MeOH promoted by molybdenum PNP complexes.

Abstract: Molybdenum(0) complexes with aliphatic aminophosphine pincer ligands have been prepared which are competent for the disproportionation of formic acid, thus representing the first example so far reported of non-noble metal species to catalytically promote such transformation. In general, formic acid disproportionation allows for an alternative access to methyl formate and methanol from renewable resources. MeOH selectivity up to 30% with a TON of 57 could be achieved while operating at atmospheric pressure. Selectivity (37%) and catalyst performance (TON = 69) could be further enhanced when the reaction was performed under hydrogen pressure (60 bars). A plausible mechanism based on experimental evidence is proposed.

Reactive pathways toward parasitic release of singlet oxygen in metal-air batteries

Abstract: The superoxide disproportionation reaction is a key step in the chemistry of aprotic metal oxygen batteries that controls the peroxide formation upon discharge and opens the way for singlet oxygen release. Here we clarify the energy landscape of the disproportionation of superoxide in aprotic media catalyzed by group 1A cations. Our analysis is based on ab initio multireference computational methods and unveils the competition between the expected reactive path leading to peroxide and an unexpected reaction channel that involves the reduction of the alkaline ion. Both channels lead to the release of triplet and singlet O2. The existence of this reduction channel not only facilitates singlet oxygen release but leads to a reactive neutral solvated species that can onset parasitic chemistries due to their well-known reducing properties. Overall, we show that the application of moderate overpotentials makes both these channels accessible in aprotic batteries.

Effect of Leu277 on Disproportionation and Hydrolysis Activity in Bacillus stearothermophilus NO2 Cyclodextrin Glucosyltransferase

Abstract: The disproportionation activity of cyclodextrin glucosyltransferase (CGTase; EC 24119) can be used to convert small molecules into glycosides, thereby enhancing their solubility and stability However, CGTases also exhibit a competing hydrolysis activity The +2 subsite of the substrate binding cleft plays an important role in both the disproportionation and hydrolysis activities, but almost all known mutations at this site decrease disproportionation activity In this study, Leu277 of the CGTase from Bacillus stearothermophilus NO2, located near both the +2 subsite and the catalytic acid/base Glu253, was modified to assess the effect of side chain size at this position on disproportionation and hydrolysis activities The best mutant, L277M, exhibited a reduced Km for the acceptor substrate maltose (048 mM versus 0945 mM) and an increased kcat/Km (1,175 s-1 mM-1 versus 6861 s-1 mM-1), compared with those of the wild-type enzyme The disproportionation-to-hydrolysis ratio of L277M was 24-fold greater than that of the wild type Existing structural data were combined with a multiple-sequence alignment and Gly282 mutations to examine the mechanism behind the effects of the Leu277mutations The Gly282 mutations were included to aid a molecular dynamics (MD) analysis and the comparison of crystal structures They reveal that changes to a hydrophobic cluster near Glu253 and the hydrophobicity of the +2 subsite combine to produce the observed effects IMPORTANCE In this study, mutations that enhance the disproportionation to hydrolysis ratio of a CGTase have been discovered For example, the disproportionation-to-hydrolysis ratio of the L277M mutant of Bacillus stearothermophilus NO2 CGTase was 24-fold greater than that of the wild type The mechanism behind the effects of these mutations is explained This paper opens up other avenues for future research into the disproportionation and hydrolysis activities of CGTases Productive mutations are no longer limited to the acceptor subsite, since mutations that indirectly affect the acceptor subsite also enhance enzymatic activity

An Isopolymolybdate-Incorporated Metal-Organic Framework with Sulfite Oxidase-Mimicking Activity for Photocatalytic Oxidation of Sulfides Utilizing In Situ-Generated Singlet Oxygen.

Abstract: Developing new photocatalysts for sulfide oxidation utilizing in situ-generated 1O2 is very significant. Inspired by natural enzymatic processes, we synthesized a mimic sulfite oxidase (SO), {[Co(Mo4O13)(TPT)2]} (CoMo-TPT), by incorporating an isopolymolybdate anion [Mo4O13]2- into a 2,4,6-tri(4-pyridyl)-1,3,5-triazine (TPT)-based metal-organic framework under mild hydrothermal conditions. In this structure, {Mo4O13} units with intrinsic SO-like catalytic sites are beneficial for the selective oxidation of sulfite and thioether. The ultraviolet-visible spectra of CoMo-TPT exhibited strong absorption from 250 to 650 nm and potential application in the utilization of solar energy. Mott-Schottky measurements indicated that CoMo-TPT is an n-type semiconductor with a LUMO value of -0.70 V (vs NHE) and a HOMO value of 1.39 V. The transient photocurrent responses with strong current density cycles with visible light indicated CoMo-TPT has a high photochemical activity. The lower resistance indicated that CoMo-TPT has a higher efficiency of photoinduced electron and hole separation. CoMo-TPT displayed a high efficiency of 99% and a selectivity of 97.3% in photocatalytic oxidation of sulfides by utilizing in situ-generated 1O2 through a tandem process of formation of H2O2 from O2 followed by catalyzed disproportionation of H2O2.

Catalytic Reduction of Dioxygen to Water by a Bioinspired Non-Heme Iron Complex via a 2+2 Mechanism.

Abstract: We report a bioinspired non-heme Fe complex with a tripodal [N3O]- ligand framework (Fe(PMG)(Cl)2) that is electrocatalytically active toward dioxygen reduction with acetic acid as a proton source in acetonitrile solution. Under electrochemical and chemical conditions, Fe(PMG)(Cl)2 selectively produces water via a 2+2 mechanism, where H2O2 is generated as a discrete intermediate species before further reduction to two equivalents of H2O. Mechanistic studies support a catalytic cycle for dioxygen reduction where an off-cycle peroxo dimer species is the resting state of the catalyst. Spectroscopic analysis of the reduced complex FeII(PMG)Cl shows the stoichiometric formation of an Fe(III)-hydroxide species following exposure to H2O2; no catalytic activity for H2O2 disproportionation is observed, although the complex is electrochemically active for H2O2 reduction to H2O. Electrochemical studies, spectrochemical experiments, and DFT calculations suggest that the carboxylate moiety of the ligand is sensitive to hydrogen-bonding interactions with the acetic acid proton donor upon reduction from Fe(III)/(II), favoring chloride loss trans to the tris-alkyl amine moiety of the ligand framework. These results offer insight into how mononuclear non-heme Fe active sites in metalloproteins distribute added charge and poise proton donors during reactions with dioxygen.

Lithium Peroxide Growth in Li–O2 Batteries via Chemical Disproportionation and Electrochemical Mechanisms: A Potential-Dependent Ab Initio Study with Implicit Solvation

Abstract: The growth of lithium peroxide via Li–air batteries is investigated using an electrochemical cell-free energy model based on density functional theory. The model builds on previous work to incorpor...