Nitrogen-Doped Graphene-Activated Iron-Oxide-Based Nanocatalysts for Selective Transfer Hydrogenation of Nitroarenes

Nanoscaled iron oxides on carbon were modified with nitrogen-doped graphene (NGr) and found to be excellent catalysts for the chemoselective transfer hydrogenation of nitroarenes to anilines. Under standard reaction conditions, a variety of functionalized and structurally diverse anilines, which serve as key building blocks and central intermediates for fine and bulk chemicals, were synthesized in good to excellent yields

Selective Oxidation of Alcohols to Esters Using Heterogeneous Co₃O₄–N@C Catalysts under Mild Conditions

Novel cobalt-based heterogeneous catalysts have been developed for the direct oxidative esterification of alcohols using molecular oxygen as benign oxidant. Pyrolysis of nitrogen-ligated cobalt­(II) acetate supported on commercial carbon transforms typical homogeneous complexes to highly active and selective heterogeneous Co₃O₄–N@C materials. By applying these catalysts in the presence of oxygen, the cross and self-esterification of alcohols to esters proceeds in good to excellent yields

Effective Quenching and Excited-State Relaxation of a Cu(I) Photosensitizer Addressed by Time-Resolved Spectroscopy and TDDFT Calculations

<p>Homogenous photocatalytic systems based on copper photosensitizers are promising candidates for noble metal free approaches in solar hydrogen generation. To improve their performance a detailed understanding of the individual steps is needed. Here, we study the interaction of a heteroleptic copper (I) photosensitizer with an iron catalyst by time-resolved spectroscopy and ab-initio calculations. The catalyst leads to rather efficient quenching of the ³MLCT state of the copper complex, with a bimolecular rate being about three times smaller than the collision rate. Using control experiments with methyl viologen an appearing absorption band is assigned to the oxidized copper complex demonstrating that electron transfer from the sensitizer to the iron catalyst occurs and the system reacts along an oxidative pathway. However, only about 30% of the quenching events result in an electron transfer while the other 70% experience deactivation indicating that the photocatalytic performance could be improved by optimizing the intermolecular interaction.</p><p><br></p

Inner- versus Outer-Sphere Ru-Catalyzed Formic Acid Dehydrogenation: A Computational Study

A detailed hybrid density functional theory study was carried out to clarify the mechanism of Ru-catalyzed dehydrogenation of formic acid in the presence of the octahedral complexes [Ru­(κ⁴-NP₃)­Cl₂] (<b>1</b>) and [Ru­(κ³-triphos)­(MeCN)₃]­(PF₆)₂ (<b>2·PF</b>_<b>6</b>) [NP₃ = N­(CH₂CH₂­PPh₂)₃, triphos = MeC­(CH₂­PPh₂)₃]. It was shown that Ru-hydrido vs Ru-formato species are pivotal to bringing about the efficient release of H₂ and CO₂ following either a metal-centered (inner-sphere) or a ligand-centered (outer-sphere) pathway, respectively

Inner- versus Outer-Sphere Ru-Catalyzed Formic Acid Dehydrogenation: A Computational Study

A detailed hybrid density functional theory study was carried out to clarify the mechanism of Ru-catalyzed dehydrogenation of formic acid in the presence of the octahedral complexes [Ru­(κ⁴-NP₃)­Cl₂] (<b>1</b>) and [Ru­(κ³-triphos)­(MeCN)₃]­(PF₆)₂ (<b>2·PF</b>_<b>6</b>) [NP₃ = N­(CH₂CH₂­PPh₂)₃, triphos = MeC­(CH₂­PPh₂)₃]. It was shown that Ru-hydrido vs Ru-formato species are pivotal to bringing about the efficient release of H₂ and CO₂ following either a metal-centered (inner-sphere) or a ligand-centered (outer-sphere) pathway, respectively

Inner- versus Outer-Sphere Ru-Catalyzed Formic Acid Dehydrogenation: A Computational Study

A detailed hybrid density functional theory study was carried out to clarify the mechanism of Ru-catalyzed dehydrogenation of formic acid in the presence of the octahedral complexes [Ru­(κ⁴-NP₃)­Cl₂] (<b>1</b>) and [Ru­(κ³-triphos)­(MeCN)₃]­(PF₆)₂ (<b>2·PF</b>_<b>6</b>) [NP₃ = N­(CH₂CH₂­PPh₂)₃, triphos = MeC­(CH₂­PPh₂)₃]. It was shown that Ru-hydrido vs Ru-formato species are pivotal to bringing about the efficient release of H₂ and CO₂ following either a metal-centered (inner-sphere) or a ligand-centered (outer-sphere) pathway, respectively

New Insights into the Photocatalytic Properties of RuO₂/TiO₂ Mesoporous Heterostructures for Hydrogen Production and Organic Pollutant Photodecomposition

Photocatalytic activities of mesoporous RuO₂/TiO₂ heterojunction nanocomposites for organic dye decomposition and H₂ production by methanol photoreforming have been studied as a function of the RuO₂ loading in the 1–10 wt % range. An optimum RuO₂ loading was evidenced for both kinds of reaction, the corresponding nanocomposites showing much higher activities than pure TiO₂ and commercial reference P25. Thus, 1 wt % RuO₂/TiO₂ photocatalyst led to the highest rates for the degradation of cationic (methylene blue) and anionic (methyl orange) dyes under UV light illumination. To get a better understanding of the mechanisms involved, a comprehensive investigation on the photogenerated charge carriers, detected by electron spin resonance (ESR) spectroscopy in the form of O^–, Ti³⁺, and O₂^– trapping centers, was performed. Along with the key role of superoxide paramagnetic species in the photodecomposition of organic dyes, ESR measurements revealed a higher amount of trapped holes in the case of the 1 wt % RuO₂/TiO₂ photocatalyst that allowed rationalizing the trends observed. On the other hand, a maximum average hydrogen production rate of 618 μmol h^–1 was reached with 5 wt % RuO₂/TiO₂ photocatalyst to be compared with 29 μmol h^–1 found without RuO₂. Favorable band bending at the RuO₂/TiO₂ interface and the key role of photogenerated holes have been proposed to explain the highest activity of the RuO₂/TiO₂ photocatalysts for hydrogen production. These findings open new avenues for further design of RuO₂/TiO₂ nanostructures with a fine-tuning of the RuO₂ nanoparticle distribution in order to reach optimized vectorial charge distribution and enhanced photocatalytic hydrogen production rates

Solar Hydrogen Production by Plasmonic Au–TiO₂ Catalysts: Impact of Synthesis Protocol and TiO₂ Phase on Charge Transfer Efficiency and H₂ Evolution Rates

The activity of plasmonic Au–TiO₂ catalysts for solar hydrogen production from H₂O/MeOH mixtures was found to depend strongly on the support phase (anatase, rutile, brookite, or composites thereof) as well as on specific structural properties caused by the method of Au deposition (sol-immobilization, photodeposition, or deposition–precipitation). Structural and electronic rationale have been identified for this behavior. Using a combination of spectroscopic in situ techniques (EPR, XANES, and UV–vis spectroscopy), the formation of plasmonic Au particles from precursor species was monitored, and the charge-carrier separation and stabilization under photocatalytic conditions was explored in relation to H₂ evolution rates. By in situ EPR spectroscopy, it was directly shown that abundant surface vacancies and surface OH groups enhance the stabilization of separated electrons and holes, whereas the enrichment of Ti³⁺ in the support lattice hampers an efficient electron transport. Under the given experimental conditions, these properties were most efficiently generated by depositing gold particles on anatase/rutile composites using the deposition–precipitation technique

Electron- and Energy-Transfer Processes in a Photocatalytic System Based on an Ir(III)-Photosensitizer and an Iron Catalyst

The reaction pathways of bis-(2-phenylpyridinato-)­(2,2′-bipyridine)­iridium­(III)­hexafluorophosphate [Ir­(ppy)₂(bpy)]­PF₆ within a photocatalytic water reduction system for hydrogen generation based on an iron-catalyst were investigated by employing time-resolved photoluminescence spectroscopy and time-dependent density functional theory. Electron transfer (ET) from the sacrificial reagent to the photoexcited Ir complex has a surprisingly low probability of 0.4% per collision. Hence, this step limits the efficiency of the overall system. The calculations show that ET takes place only for specific encounter geometries. At the same time, the presence of the iron-catalyst represents an energy loss channel due to a triplet–triplet energy transfer of Dexter type. This loss channel is kept small by the employed concentration ratios, thus favoring the reductive ET necessary for the water reduction. The elucidated reaction mechanisms underline the further need to improve the sun light’s energy pathway to the catalyst to increase the efficiency of the photocatalytic system

Structure–Activity Relationships in Bulk Polymeric and Sol–Gel-Derived Carbon Nitrides during Photocatalytic Hydrogen Production

Photocatalytic hydrogen evolution rates and structural properties as well as charge separation, electron transfer, and stabilization have been analyzed in advanced sol–gel-derived carbon nitrides (SG-CN) pyrolyzed at different temperatures (350–600 °C) and in bulk polymeric carbon nitride reference samples (CN) by XRD, XPS, FTIR, UV–vis, Raman, and photoluminescence as well as by in situ EPR spectroscopy. SG-CN samples show about 20 times higher H₂ production rates than bulk CN. This is due to their porous structure, partial disorder, and high surface area which favor short travel distances and fast trapping of separated electrons on the surface where they are available for reaction with protons. In contrast, most of the excited electrons in bulk polymeric CN return quickly to the valence band upon undesired emission of light, which is responsible for their low catalytic activity