Reactivity indexes for the electrocatalytic oxidation of hydrogen peroxide promoted by several ligand-substituted and unsubstituted Co phthalocyanines adsorbed on graphite

Low-Voltage Hydrogen Production via Hydrogen Peroxide Oxidation Facilitated by Oxo Ligand Axially Coordinated to Cobalt in Phthalocyanine Moiety

A cobalt phthalocyanine having an electron-poor CoN4 (+δ) in its phthalocyanine moiety was presented as an electrocatalyst for hydrogen peroxide oxidation reaction (HPOR). We suggested that hydrogen peroxide as an electrolysis medium for hydrogen production and therefore as a hydrogen carrier, demonstrating that the electrocatalyst guaranteed high hydrogen production rate by hydrogen peroxide splitting. The electron deficiency of cobalt allows CoN4 to have the highly HPOR-active monovalent oxidation state and facilitates HPOR at small overpotentials range around the onset potential. The strong interaction between the electron-deficient cobalt and oxygen of peroxide adsorbates in Co─OOH- encourages an axially coordinated cobalt oxo complex (O═CoN4 ) to form, the O═CoN4 facilitating the HPOR efficiently at high overpotentials. Low-voltage oxygen evolution reaction guaranteeing low-voltage hydrogen production is successfully demonstrated in the presence of the metal-oxo complex having electron-deficient CoN4 . Hydrogen production by 391 mA cm-2 at 1 V and 870 mA cm-2 at 1.5 V is obtained. Also, the techno-economic benefit of hydrogen peroxide as a hydrogen carrier is evaluated by comparing hydrogen peroxide with other hydrogen carriers such as ammonia and liquid organic hydrogen carriers.

A theoretical study on the ORR electrocatalytic activity of axial ligand modified cobalt polyphthalocyanine

In this work, the catalytic activity in the oxygen reduction reaction (ORR) of cobalt polyphthalocyanine whose central Co atom is coordinated at the axial position by ligands (L = -F, -OH, -OCH3, -N3, -Cl, -Br, -I, -SCN, and -CN) (CoPPc-L) was investigated using theoretical calculations in alkaline medium. Among all CoPPc-L, CoPPc-N3 exhibited the lowest ORR overpotential of 0.23 V vs. a standard hydrogen electrode, which is significantly lower than those of CoPPc (0.48 V) and Pt(111) (0.43 V). There is a good linear relationship between ΔG*OOH and the electronegativity of ligating atoms in axial ligands of CoPPc-L. The greater the electronegativity, the stronger the adsorption of the catalyst to the intermediate. Additionally, the adsorption strength of CoPPc to the intermediate is modified by the axial ligands, which adjust the distribution of anti-bonding electronic states of dz2, dxz, and dyz orbitals near the Fermi level, Ef. A larger Mayer bond order of the Co-L bond resulted in a smaller bond order of the Co-O bond. CoPPc-N3 exhibited a moderate Co-O bond order of 0.737, corresponding to moderate adsorption energy to the OOH intermediate. This study demonstrates that the interaction strength between CoPPc and ORR intermediates can be adjusted by selecting appropriate axial ligands, which can modulate the ORR catalytic activity.

Evidence of carbon-supported porphyrins pyrolyzed for the oxygen reduction reaction keeping integrity

Fe(III) 5,10,15,20-(tetraphenyl)porphyrin chloride (FeTPP) and Co(III) 5,10,15,20-(tetraphenyl)porphyrin chloride (CoTPP) were adsorbed on carbon Vulcan and studied as electrocatalysts for the oxygen reduction reaction (ORR) before and after pyrolysis. The pyrolysis process was also simulated through ab initio molecular dynamic simulations and the minimum energy path for the O₂ dissociation after the interaction with the metal center of the FeTPP and CoTPP were calculated. After the pyrolysis the FeTPP showed the best performances reducing O₂ completely to H₂O with increased limiting current and lower overpotential. Tafel slops for the various catalysts did not change after the pyrolytic process suggesting that the mechanism for the ORR is not affected by the heat treatment. TEM images, X-ray diffraction, XPS spectroscopy, ⁵⁷Fe Mössbauer, and DFT simulations, suggest that there is no breakdown of the macrocyclic complex at elevated temperatures, and that the macro cyclic geometry is preserved. Small variations in the Metal-O₂ (M-O₂) binding energies and the M–N bond length were observed which is attributed to the dispersive interaction between the macrocycles and the irregular surface of the Vulcan substrate induced by the heat treatment and causing better interaction with the O₂ molecule. The theoretical strategy herein applied well simulate and explain the nature of the M–N–C active sites and the performances towards the ORR.

Unprecedented Isomerism-Activity Relation in Molecular Electrocatalysis

The role of electrocatalysts in energy storage/conversion, biomedical and environmental sectors, green chemistry, and much more has generated enormous interest in comprehending their structure-activity relations. While targeting the surface-to-volume ratio, exposing reactive crystal planes and interfacial modifications are time-tested considerations for activating metallic catalysts; it is primarily by substitution in molecular electrocatalysts. This account draws the distinction between a substituent's chemical identity and isomerism, when regioisomerism of the -NO₂ substituent is conferred at the "α" and "β" positions on the macrocycle of cobalt phthalocyanines. Spectroscopic analysis and theoretical calculations establish that the β isomer accumulates catalytically active intermediates via a cumulative influence of inductive and resonance effects. However, the field effect in the α isomer restricts this activation due to a vanishing resonance effect. The demonstration of the distinct role of isomerism in substituted molecular electrocatalysts for reactions ranging from energy conversion to biosensing highlights that isomerism of the substituents makes an independent contribution to electrocatalysis over its chemical identity.

Evans–Showell-type polyoxometalate constructing novel 3D inorganic architectures with alkaline earth metal linkers: syntheses, structures and catalytic properties

Two 3D frameworks and two 2D networks with an excellent catalytic effect of cyanosilylation were successfully obtained, originating from Evans–Showell-type polyoxoanions [Co₂Mo₁₀H₄O₃₈]⁶⁻ and alkaline earth metal cations (Sr²⁺, Ba²⁺).

Comparison of the catalytic activity for O2reduction of Fe and Co MN4 adsorbed on graphite electrodes and on carbon nanotubes

Metal phthalocyanines adsorbed on CNTs deliver much higher electrocatalytic currents for the ORR because of the high concentration of catalyst.