Understanding the Reducibility of CeO2 Surfaces by Proton-Electron Transfer from CpCr(CO)3H.

CeO2 is a popular material in heterogeneous catalysis, molecular sensors, and electronics and owes many of its special properties to the redox activity of Ce, present as both Ce3+ and Ce4+. However, the reduction of CeO2 with H2 (thought to occur through proton-electron transfer (PET) giving Ce3+ and new OH bonds) is poorly understood due to the high reduction temperatures necessary and the ill-defined nature of the hydrogen atom sources typically used. We have previously shown that transition-metal hydrides with weak M-H bonds react with reducible metal oxides at room temperature by PET. Here, we show that CpCr(CO)3H (1) transfers protons and electrons to CeO2 due to its weak Cr-H bond. We can titrate CeO2 with 1 and measure not only the number of surface Ce3+ sites formed (in agreement with X-ray absorption spectroscopy) but also the lower limit of the hydrogen atom adsorption free energy (HAFE). The results match the extent of reduction achieved from H2 treatment and hydrogen spillover on CeO2 in a wide range of applications.

Ruthenium Complexes of Polyfluorocarbon Substituted Terpyridine and Mesoionic Carbene Ligands: An Interplay in CO2 Reduction.

In recent years terpyridines (tpy) and mesoionic carbenes (MIC) have been widely used in metal complexes. With the right combination with a metal center, both of these ligands are individually known to generate excellent catalysts for CO2 reduction. In this study, we combine the potentials of PFC (PFC=polyfluorocarbon) substituted tpy and MIC ligands within the same platform to obtain a new class of complexes, which we investigated with respect to their structural, electrochemical and UV/Vis/NIR spectroelectrochemical properties. We further show that the resulting metal complexes are potent electrocatalysts for CO2 reduction in which CO is exclusively formed with a faradaic efficiency of 92 %. A preliminary mechanistic study, including the isolation and characterization of a key intermediate is also reported.

Remarkable Enhancement of Catalytic Activity of Cu-Complexes in the Electrochemical Hydrogen Evolution Reaction by Using Triply Fused Porphyrin.

A bimetallic triply fused copper(II) porphyrin complex (1) was prepared, comprising two monomeric porphyrin units linked through ß-ß, meso-meso, ß'-ß' triple covalent linkages and exhibiting remarkable catalytic activity for the electrochemical hydrogen evolution reaction in comparison to the analogous monomeric copper(II) porphyrin complex (2). Electrochemical investigations in the presence of a proton source (trifluoroacetic acid) confirmed that the catalytic activity of the fused metalloporphyrin occurred at a significantly lower overpotential (≈320 mV) compared to the non-fused monomer. Controlled potential electrolysis combined with kinetic analysis of catalysts 1 and 2 confirmed production of hydrogen, with 96 and 71 % faradaic efficiencies and turnover numbers of 102 and 18, respectively, with an observed rate constant of around 107  s-1 for the dicopper complex. The results thus firmly establish triply fused porphyrin ligands as outstanding candidates for generating highly stable and efficient molecular electrocatalysts in combination with earth-abundant 3d transition metals.

Electrochemical CO2 reduction toward multicarbon alcohols - The microscopic world of catalysts & process conditions.

Tackling climate change is one of the undoubtedly most important challenges at the present time. This review deals mainly with the chemical aspects of the current status for converting the greenhouse gas CO2 via electrochemical CO2 reduction reaction (CO2RR) to multicarbon alcohols as valuable products. Feasible reaction routes are presented, as well as catalyst synthesis methods such as electrodeposition, precipitation, or sputtering. In addition, a comprehensive overview of the currently achievable selectivities for multicarbon alcohols in CO2RR is given. It is also outlined to what extent, for example, modifications of the catalyst surfaces or the use of bifunctional compounds the product distribution is shifted. In addition, the influence of varying electrolyte, temperature, and pressure is described and discussed.

Electrochemical CO2 reduction - The macroscopic world of electrode design, reactor concepts & economic aspects.

For the efficient electrochemical conversion of CO2 into valuable chemical feedstocks, a well-coordinated interaction of all electrolyzer compartments is required. In addition to the catalyst, whose role is described in detail in the part "Electrochemical CO2 Reduction toward Multicarbon Alcohols - The Microscopic World of Catalysts & Process Conditions" of this divided review, the general cell setups, design and manufacture of the electrodes, membranes used, and process parameters must be optimally matched. The authors' goal is to provide a comprehensive review of the current literature on how these aspects affect the overall performance of CO2 electrolysis. To be economically competitive as an overall process, the framework conditions, i.e., CO2 supply and reaction product treatment must also be considered. If the key indicators for current density, selectivity, cell voltage, and lifetime of a CO2 electrolyzer mentioned in the techno-economic consideration of this review are met, electrochemical CO2 reduction can make a valuable contribution to the creation of closed carbon cycles and to a sustainable energy economy.

Assembling Metal Organic Layer Composites for High-Performance Electrocatalytic CO2 Reduction to Formate.

2D metal-organic-framework (MOF) based composites have emerged as promising candidates for electrocatalysis due to their high structural flexibility and fully exposed active sites. Herein, a freestanding metal-organic layer (MOL) with a 2D kgd (kagome dual) lattice was constructed with abundant surface oxygenate groups serving as anchoring sites to immobilize diverse guests. Taking Bi as an example, tetragonal Bi2 O3 nanowires can be uniformly grown on MOLs after solvothermal treatment, the structural evolution of which was followed by ex situ electron microscopy. The as-prepared Bi2 O3 /MOL exhibits excellent CO2 electroreduction activity towards formate reaching a specific current of 2.3 A mgBi -1 and Faradaic efficiencies of over 85 % with a wide potential range from -0.87 to -1.17 V, far surpassing Bi2 O3 /UiO (a 3D Zr6 -oxo based MOF) and Bi2 O3 /AB (Acetylene Black). Such a post-synthetic modification strategy can be flexibly extended to develop versatile MOL composites, highlighting the superiority of optimizing MOL-based composites for electrocatalysis.

Integration of aprotic CO2 reduction to oxalate at a Pb catalyst into a GDE flow cell configuration.

Electrochemical CO2 reduction to oxalic acid in aprotic solvents could be a potential pathway to produce carbon-neutral oxalic acid. One of the challenges in aprotic CO2 reduction are the limited achievable current densities under standard conditions, despite the increased CO2 solubility compared to aqueous applications. The application of aprotic solvents can reduce CO2 rather selectively to oxalate, and faradaic efficiencies (FEs) of up to 80% were achieved in this study with a Pb catalyst in acetonitrile, the FE being mainly dictated by the local CO2 concentration at the electrode. This process was integrated into a flow cell employing a two-layered carbon-free lead (Pb) gas diffusion electrode (GDE) and a sacrificial zinc (Zn) anode. With the application of this GDE the applicable current densities could be improved up to a current density of j = 80 mA cm-2 at a FE(oxalate) = 53%, which is within the range of the highest j reported in the literature. In addition, we provide an explanation for the deactivation mechanism of metal catalysts observed in the aprotic CO2 reduction literature. The deactivation is not related to a mass transport limitation but to cathodic corrosion observed at highly negative potential when employing quaternary ammonium supporting electrolyte cations, promoting catalyst leaching.

Solvents and Supporting Electrolytes in the Electrocatalytic Reduction of CO2.

Different electrolytes applied in the aqueous electrocatalytic CO2 reduction reaction (CO2RR) considerably influence the catalyst performance. Their concentration, species, buffer capacity, and pH value influence the local reaction conditions and impact the product distribution of the electrocatalyst. Relevant properties of prospective solvents include their basicity, CO2 solubility, conductivity, and toxicity, which affect the CO2RR and the applicability of the solvents. The complexity of an electrochemical system impedes the direct correlation between a single parameter and cell performance indicators such as the Faradaic efficiency; thus the effects of different electrolytes are often not fully comprehended. For an industrial application, a deeper understanding of the effects described in this review can help with the prediction of performance, as well as the development of scalable electrolyzers. In this review, the application of supporting electrolytes and different solvents in the CO2RR reported in the literature are summarized and discussed.

In Situ Generated Gold Nanoparticles on Active Carbon as Reusable Highly Efficient Catalysts for a C sp 3 -C sp 3 Stille Coupling.

Gold nanoparticle catalysts are important in many industrial production processes. Nevertheless, for traditional C sp 2 -C sp 2 cross-coupling reactions they have been rarely used and Pd catalysts usually give a superior performance. Herein we report that in situ formed gold metal nanoparticles are highly active catalysts for the cross coupling of allylstannanes and activated alkylbromides to form C sp 3 -C sp 3 bonds. Turnover numbers up to 29 000 could be achieved in the presence of active carbon as solid support, which allowed for convenient catalyst recovery and reuse. The present study is a rare case where a gold metal catalyst is superior to Pd catalysts in a cross-coupling reaction of an organic halide and an organometallic reagent.

Synthesis of Acrylonitrile from Renewable Lactic Acid.

Acrylonitrile (ACN) is widely used as monomer in the synthesis of polymers and carbon fibers. Nowadays, its production is based on fossil resources. Herein, an alternative process based on renewable resources is presented. Lactic acid (LA), which can be obtained by fermentation of biomass, was converted to ACN in two steps with an overall selectivity of 57 %. In the first step, a direct amidation of LA in the presence of water was conducted at 230 °C. Zeolites can catalyze the formation of lactamide, and a selectivity of 92 % was reached at 33 % conversion with NH4 -ZSM-5. In the second step, the dehydration of lactamide to ACN was performed with acetic anhydride, and an ACN selectivity of 62 % was achieved at full conversion.

Reactive Extraction of Lactic Acid by Using Tri-n-octylamine: Structure of the Ionic Phase.

Lactic acid is a promising biogenic platform chemical which can be produced by fermentation of cellulose and hemicellulose. However, separating lactic acid from the fermentation broth is extremely costly and technically complex. We therefore investigated whether liquid/liquid extraction of lactic acid with tri-n-octylamine is a cost-effective alternative to the existing downstream processing method. In order to find an answer to this question, the structure of the middle phase of the occurring three-phase region, which is enriched with up to 20 wt. % lactic acid, was explored. The results of our IR, small-angle X-ray scattering and NMR measurements show that this phase is ionic and has a bicontinuous structure. Due to the analogy with bicontinuous microemulsions, it should be possible to further enrich the lactic acid, which could lead to a rethink regarding the design of extraction processes.

Coordination frameworks assembled from Cu(II) ions and H2-1,3-bdpb ligands: X-ray and magneto structural investigations, and catalytic activity in the aerobic oxidation of tetralin.

The syntheses and crystal structures of H2-1,3-bdpb·MeOH, [Cu(II)2(1,3-bdpb)(OCH3)2] (CFA-5) and [Cu(I)Cl(H2-1,3-bdpb)] (H2-1,3-bdpb = 1,3-bis(3,5-dimethyl-1H-pyrazol-4-yl)benzene) are described. The copper(II) containing metal-organic framework (termed Coordination Framework Augsburg University-5, CFA-5) crystallizes in the trigonal crystal system, within the space group R3̄ (no. 148) and the unit cell parameters are as follows: a = 26.839(3), c = 15.8317(16) Å, V = 9876.2(19) Å(3). CFA-5 features a two-fold interpenetrated 3-D microporous framework structure of cross-linked wheel-shaped {Cu(II)(pz)(OMe)}12 fundamental building units, each containing twelve copper(II) ions, µ2-bridging MeO(-) groups and pyrazolate (pz(-)) ligands. Replacing copper(II) acetate by copper(II) chloride in the synthesis leads to compound [Cu(I)Cl(H2-1,3-bdpb)], which crystallizes in the orthorhombic crystal system, within the space group Pnma (no. 62) and the unit cell parameters are as follows: a = 6.1784(8), b = 6.1784(8), c = 6.1784(8) Å, V = 1583.8(4) Å(3). In contrast to the former compound, CuCl(H2-1,3-bdpb) is a non-porous compound consisting of Cu(I)-Cl zigzag chains expanding in the direction [100] and H2-1,3-bdpb ligands. CFA-5 is characterized by elemental and thermogravimetric analyses, variable temperature powder X-ray diffraction and IR-spectroscopy; and its porosity and magnetic properties are described in detail. CFA-5 shows a promising catalytic activity in the heterogeneously catalyzed aerobic oxidation of tetralin, which is compared with other catalytically active metal-organic frameworks.

A rechargeable hydrogen battery based on Ru catalysis.

Apart from energy generation, the storage and liberation of energy are among the major problems in establishing a sustainable energy supply chain. Herein we report the development of a rechargeable H2 battery which is based on the principle of the Ru-catalyzed hydrogenation of CO2 to formic acid (charging process) and the Ru-catalyzed decomposition of formic acid to CO2 and H2 (discharging process). Both processes are driven by the same catalyst at elevated temperature either under pressure (charging process) or pressure-free conditions (discharging process). Up to five charging-discharging cycles were performed without decrease of storage capacity. The resulting CO2/H2 mixture is free of CO and can be employed directly in fuel-cell technology.

CFA-2 and CFA-3 (Coordination Framework Augsburg University-2 and -3); novel MOFs assembled from trinuclear Cu(I)/Ag(I) secondary building units and 3,3',5,5'-tetraphenyl-bipyrazolate ligands.

The syntheses of H2-phbpz, [Cu2(phbpz)]·2DEF·MeOH (CFA-2) and [Ag2(phbpz)] (CFA-3) (H2-phbpz = 3,3',5,5'-tetraphenyl-1H,1'H-4,4'-bipyrazole) compounds and their crystal structures are described. The Cu(I) containing metal-organic framework CFA-2 crystallizes in the tetragonal crystal system, within space group I4(1)/a (no. 88) and the following unit cell parameters: a = 30.835(14), c = 29.306(7) Å, V = 27 865(19) Å(3). CFA-2 features a flexible 3-D three-connected two-fold interpenetrated porous structure constructed of triangular Cu(I) subunits. Upon exposure to different kinds of liquids (MeOH, EtOH, DMF, DEF) CFA-2 shows pronounced breathing effects. CFA-3 crystallizes in the monoclinic crystal system, within space group P2(1)/c (no. 14) and the following unit cell parameters: a = 16.3399(3), b = 32.7506(4), c = 16.2624(3) Å, ß = 107.382(2)°, V = 8305.3(2) Å(3). In contrast to the former compound, CFA-3 features a layered 2-D three-connected structure constructed from triangular Ag(i) subunits. Both compounds are characterized by elemental and thermogravimetric analyses, single crystal structure analysis and X-ray powder diffraction, FTIR- and fluorescence spectroscopy. Preliminary results on oxygen activation in CFA-2 are presented and potential improvements in terms of framework robustness and catalytic efficiency are discussed.

The role of Pd2+/Pd0 in hydrogenation by [Pd(2-pymo)2]n: an X-ray absorption and IR spectroscopic study.

The metal-organic framework (MOF) [Pd(2-pymo)(2)](n) (2-pymo=2-pyrimidinolate) was used as catalyst in the hydrogenation of 1-octene. During catalytic hydrogenation, the changes at the metal nodes and linkers of the MOF were investigated by in situ X-ray absorption spectroscopy (XAS) and IR spectroscopy. With the help of extended X-ray absorption fine structure and X-ray absorption near edge structure data, Quick-XAS, and IR spectroscopy, detailed insights into the catalytic relevance of Pd(2+)/Pd(0) in the hydrogenation of 1-octene could be achieved. Shortly after exposure of the catalyst to H(2) and simultaneously with the hydrogenation of 1-octene, the aromatic rings of the linker molecules are hydrogenated rapidly. Up to this point, the MOF structure remained intact. After completion of linker hydrogenation, the linkers were also protonated. When half of the linker molecules were protonated, the onset of reduction of the Pd(2+) centers to Pd(0) was observed and the hydrogenation activity decreased, followed by fast reduction of the palladium centers and collapse of the MOF structure. Major fractions of Pd(0) are only observed when the hydrogenation of 1-octene is almost finished. Consequently, the Pd(2+) nodes of the MOF [Pd(2-pymo)(2)](n) are identified as active centers in the hydrogenation of 1-octene.

Application of a chiral metal-organic framework in enantioselective separation.

A modular approach for the synthesis of highly ordered porous and chiral auxiliary (Evans auxiliary) decorated metal-organic frameworks is developed. Our synthesis strategy, which uses known porous structures as model materials for incorporation of chirality via linker modification, can provide access to a wide range of porous materials suitable for enantioselective separation and catalysis. Chiral analogues of UMCM-1 have been synthesized and investigated for the enantioseparation of chiral compounds in the liquid phase and first promising results are reported.