Analysis of Porphyrines as Catalysts for Electrochemical Reduction of O₂ and Oxidation of H₂O

Bioinspired structures are promising as improved catalysts for various redox reactions. One example is metal hangman-porphyrines (MHP), which recently have been suggested for oxygen reduction/evolution reaction (ORR/OER). The unique properties of the MHPs are attributed to both the hangman scaffold and the C₆F₅ side groups. Herein, the OER/ORR over various transition metal MHPs is investigated by density functional theory calculations within an electrochemical framework. A comparison of the reaction landscape for MHP, metal porphyrine (MP) and metaltetrafluorophenyloporphyrines (MTFPP), allow for a disentanglement of the different roles of the hangman motif and the side groups. In agreement with experimental studies, it is found that Fe and Co are the best MHP metal centers to catalyze these reactions. We find that the addition of the three-dimensional moiety in the form of hangman scaffold does not break the apparently universal energy relation between *OH and *OOH intermediates. However, the hangman motif is found to stabilize the oxygen intermediate, whereas addition of C₆F₅ groups reduces the binding energy of all reaction intermediates. Our results indicate that the combination of these two effects allow new design possibilities for macromolecular systems with enhanced catalytic OER/ORR activity

Simulated Photoemission Spectra of Hydroxylated MgO(100) at Elevated Temperatures

Density functional theory has been used to investigate photoemission O1s core-level shifts (CLS) of hydroxylated MgO(100). Rapid proton exchange at elevated temperatures (300 K) yields broad features in the simulated photoemission signal, in good agreement with experimental observations. The results provide further evidence that the stable structure of hydroxylated MgO(100) consists of a partly dissociated water monolayer. Analysis of the CLS for adsorbed hydroxyl groups at different coverage reveals a pronounced effect on hydrogen bonding to neighboring H₂O molecules. The inclusion of exact exchange by use of the hybrid PBE0 functional leads to quantitatively similar results as the gradient corrected PBE functional

The Influence of Inert Ions on the Reactivity of Manganese Oxides

Inert ion doping is a possible method to modify electrical conductivity and catalytic activity of transition-metal oxide electrocatalysts. Despite the importance of dopants, little is known about the underlying mechanisms for the change of the system properties. We have performed density functional theory calculations to investigate the influence of different valent ions on the O₂ evolution reaction activity of β-MnO₂ and Mn₂O₃. While Mn₂O₃ is unaffected by dopants, β-MnO₂ is strongly affected by ions placed in a subsurface position. Doping does not affect the ion charge at the active site, but instead it effects the bond character. This is concluded through an analysis of the density overlap regions indicator and the density of states showing that the partially covalent nature of the bonds in β-MnO₂ is responsible for the changes in the adsorbate binding energies. This mechanism is not active in the mostly ionic Mn₂O₃. These results highlight the need to explicitly consider the detailed bonding situation and to go beyond simple charge transfer considerations when describing doping of transition metal oxide catalysts

The Influence of Inert Ions on the Reactivity of Manganese Oxides

Inert ion doping is a possible method to modify electrical conductivity and catalytic activity of transition-metal oxide electrocatalysts. Despite the importance of dopants, little is known about the underlying mechanisms for the change of the system properties. We have performed density functional theory calculations to investigate the influence of different valent ions on the O₂ evolution reaction activity of β-MnO₂ and Mn₂O₃. While Mn₂O₃ is unaffected by dopants, β-MnO₂ is strongly affected by ions placed in a subsurface position. Doping does not affect the ion charge at the active site, but instead it effects the bond character. This is concluded through an analysis of the density overlap regions indicator and the density of states showing that the partially covalent nature of the bonds in β-MnO₂ is responsible for the changes in the adsorbate binding energies. This mechanism is not active in the mostly ionic Mn₂O₃. These results highlight the need to explicitly consider the detailed bonding situation and to go beyond simple charge transfer considerations when describing doping of transition metal oxide catalysts

Electrooxidation of Glycerol on Gold in Acidic Medium: A Combined Experimental and DFT Study

Glycerol is a byproduct of biodiesel production and an abundant feedstock for the synthesis of high-value chemicals. A promising approach for valorization of glycerol is electrooxidation on gold. In this work, we investigate electrooxidation of glycerol on gold in acidic media using cyclic voltammetry and density functional theory calculations. Experimentally, we observe activity for electrooxidation above a potential of 0.5 V versus the reversible hydrogen electrode (RHE). A Pourbaix diagram is calculated to evaluate the surface coverage under reaction conditions, indicating that the surface is free from adsorbates at the measured onset potential. Computationally, we find that the onset potentials for partial dehydrogenation of glycerol to dihydroxyacetone, 2,3-dihydroxy-2-propenal, and glyceraldehyde are 0.39, 0.39, and 0.60 V versus RHE, respectively, while complete dehydrogenation to carbon monoxide requires 0.50 V versus RHE. Our theoretical and experimental findings are in agreement and show the possibility of using gold as a catalyst for the production of hydrogen and other valuable chemicals from glycerol

Methanol Desorption from Cu-ZSM‑5 Studied by <i>In Situ</i> Infrared Spectroscopy and First-Principles Calculations

The dynamic interaction of methanol and its derivatives with Cu-exchanged ZSM-5 during methanol temperature-programmed desorption from 30 to 450 °C has been investigated using <i>in situ</i> diffuse reflectance infrared Fourier transform spectroscopy and first-principles calculations. The results emphasize that defects in the framework structure of the zeolite and Brønsted acid sites constitute ion-exchange sites for Cu ions. The Cu sites introduced in ZSM-5 actively interact with methanol adsorbed at moderate temperature, i.e., below 250 °C, and take roles in further oxidation of the adsorbed species to formate and CO. Moreover, spectra recorded at higher temperatures, i.e., above 300 °C, after adsorption of methanol show strong interaction between methoxy groups and the zeolite framework, suggesting that, under mild conditions, proton extraction for methanol production during direct partial oxidation of methane to methanol over Cu-ZSM-5 is necessary