Oxygen Reduction Reaction at Penta-Coordinated Co Phthalocyanines (original) (raw)
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ACS Applied Materials & Interfaces, 2020
Conversion of CO 2 to reduced products is a promising route to alleviate irreversible climate change. Here we report the synthesis of a Co-based phthalocyanine with pyridine moieties (CoPc-Pyr), which is supported on a carbon electrode and shows Faradaic efficiency ∼90% for CO at 490 mV of overpotential (−0.6 V vs reversible hydrogen electrode (RHE)). In addition, its catalytic activity at −0.7 V versus RHE surpasses other Co-based molecular and metal−organic framework catalysts for CO 2 reduction at this bias. Density functional theory calculations show that pyridine moieties enhance CO 2 adsorption and electron affinity of the Co center by an inductive effect, thus lowering the overpotential necessary for CO 2 conversion. Our study shows that CoPc-Pyr reduces CO 2 at lower overpotential and with higher activity than noble metal electrodes, such as silver.
Fuel, 2012
Deposited and heat-treated phthalocyanines are promising electrocatalysts for replacing platinum in the oxygen reduction reaction (ORR), the most important process in energy conversion systems such as fuel cells; and yet its key mechanistic features are not well understood. To optimize their use, it is necessary to understand their behavior in the absence of an electric field. In the pursuit of this goal, we pyrolyzed metal-free, cobalt and copper phthalocyanines between 550 and 1000°C and studied their structural and chemical changes by elemental analysis, N 2 and CO 2 adsorption, X-ray diffraction (XRD), Raman spectroscopy, X-ray analysis fine structure (XAFS) and X-ray photoelectron spectroscopy (XPS). Their catalytic activity was assessed by non-isothermal O 2 gasification and NO reduction reactions. A comparison of these results with their other properties allowed us to reach the following conclusions: (i) the loss of reactivity of metal-free phthalocyanine with heat treatment is attributed to its structural annealing and heteroatom loss, with the porosity changes having no effect; (ii) for metal phthalocyanines at intermediate heat treatment temperatures, the optimum in reactivity correlates with the micropore surface area and the presence of metal particles, with no influence of nitrogen content; (iii) the coordination metal increases phthalocyanine thermal stability in an inert atmosphere, but in an oxidizing atmosphere it acts as a gasification catalyst even below decomposition temperatures. The implications of these findings for catalytic oxygen-transfer mechanisms are discussed.
CO2 electrochemical catalytic reduction with a highly active cobalt phthalocyanine
Nature Communications
Molecular catalysts that combine high product selectivity and high current density for CO 2 electrochemical reduction to CO or other chemical feedstocks are urgently needed. While earth-abundant metal-based molecular electrocatalysts with high selectivity for CO 2 to CO conversion are known, they are characterized by current densities that are significantly lower than those obtained with solid-state metal materials. Here, we report that a cobalt phthalocyanine bearing a trimethyl ammonium group appended to the phthalocyanine macrocycle is capable of reducing CO 2 to CO in water with high activity over a broad pH range from 4 to 14. In a flow cell configuration operating in basic conditions, CO production occurs with excellent selectivity (ca. 95%), and good stability with a maximum partial current density of 165 mA cm −2 (at −0.92 V vs. RHE), matching the most active noble metal-based nanocatalysts. These results represent state-of-the-art performance for electrolytic carbon dioxide reduction by a molecular catalyst.
The Journal of Physical Chemistry B, 2008
Density functional theory (DFT) has been applied to the analysis of the structural and electronic properties of the alkyl-cobalt(III) phthalocyanine complexes, [Co III Pc]-R (Pc ) phthalocyanine, R ) Me or Et), and their pyridine adducts. The BP86/6-31G(d) level of theory shows good reliability for the optimized axial bond lengths and bond dissociation energies (BDEs). The mechanism of the reductive cleavage was probed for the [Co III Pc]-Me complex which is known as a highly effective methyl group donor. In the present analysis, which follows a recent study on the reductive Co-C bond cleavage in methylcobalamin (J. Phys. Chem. B 2007, 111, 7638-7645), it is demonstrated that addition of an electron and formation of the π-anion radical [Co III (Pc • )]-Mesignificantly lowers the energetic barrier required for homolytic Co-C bond dissociation. Such BDE lowering in [Co III (Pc • )]-Mearises from the involvement of two electronic states: upon electron addition, a quasi-degenerate π* Pc state is initially formed, but when the cobalt-carbon bond is stretched, the unpaired electron moves to a σ* Co-C state and the final cleavage involves the three-electron (σ) 2 (σ*) 1 bond. As in corrin complexes, the π* Pc -σ* Co-C states crossing does not take place at the equilibrium geometry of [Co III (Pc • )]-Mebut only when the Co-C bond is stretched to ∼2.3 Å. The DFT computed Co-C BDE of 23.3 kcal/mol in the one-electron-reduced phthalocyanine species, [Co III (Pc • )]-Me -, is lowered by ∼37% compared to the neutral Py-[Co III Pc]-Me complex where BDE ) 36.8 kcal/mol. A similar comparison for the corrin-containing complexes shows that a DFT computed BDE of 20.4 kcal/mol for [Co III -(corrin • )]-Me leads to ∼45% bond strength reduction, in comparison to 37.0 kcal/mol for Im-[Co III (corrin)]-Me + . These results suggest some preference by the alkylcorrinoids for the reductive cleavage mechanism.
Journal of Electroanalytical Chemistry, 2000
When the logarithm of the rate constant of the electroreduction of O 2 is plotted versus the Co(III)/Co(II) redox potential of different substituted phthalocyanines (Co-Pcs), for graphite electrodes modified with these complexes, a straight line is obtained. Log k decreases as the driving force of the phthalocyanine increases. The same is observed when log k is plotted versus the sum of the Hammett parameters of the substituents on the periphery of the phthalocyanine ligand. This is explained in terms of the hardness of the system, i.e. the more the separation between the energy of the frontier orbitals of the donor (Co -Pc) and the acceptor (O 2 ), the less the reactivity. The energies of the frontier orbitals of cobalt tetraneopentoxyphthalocyanine, cobalt octamethoxyphthalocyanine, cobalt phthalocyanine, cobalt tetrasulfophthalocyanine and cobalt hexadecafluorophthalocyanine, and of the O 2 molecule were calculated using the PM3 semi-empirical theoretical method. The intermolecular hardness of the Co-Pc/O 2 system was estimated by taking one half the energy difference of the SOMO of the Co -Pc and the SOMO of the dioxygen molecule. A plot of log k versus the intermolecular hardness gives a straight line of negative slope, which shows that the rate constants decrease with increasing intermolecular hardness of the system.
Cobalt phthalocyanine (CoPc)-impregnated functionalized multi-walled carbon nanotubes (CNTs) were used as nonprecious electrocatalysts for oxygen reduction reaction (ORR). The electrocatalysts were thermally treated at temperatures ranging from 450 to 850 C, and the effect of pyrolysis temperature and their relationship to the electrocatalytic activity for ORR were investigated. Thermo gravimetric analysis, X-ray diffraction, and electron microscopy were used to study the thermal stability, crystal structure, and morphology of these catalysts. Cyclic voltammetry and rotating disk electrode results showed that CoPc/CNTs pyrolyzed at a temperature of 550 C had the highest electrocatalytic activity for ORR, and the catalytic activity decreased with further increase in pyrolysis temperature. X-ray photoelectron spectroscopy showed decrease in functional groups at a temperature higher than 550 C, correlating with the decreased catalytic activity. The result suggests that oxygen functional groups introduced by acid oxidation for anchoring the CoPc on CNT plays a major role in determining the electrocatalytic activity.
Journal of Organometallic Chemistry, 2013
In this work, the new phthalonitrile derivatives 4, 5 bearing 1,3-bis(naphthalen-1-yloxy)propan-2-ol and 1,3-bis(naphthalen-2-yloxy)propan-2-ol, cobalt phthalocyanine complexes have been synthesized. The new cobalt phthalocyanines have been prepared by cyclotetramerization of phthalonitrile derivatives 4, 5 in the presence of the corresponding CoCl 2 salt in DMAE by using a microwave oven. The new compounds have been characterized by IR, 1 H NMR, 13 C NMR and MS spectral data. The new complexes have been tested as a catalyst for the oxidation of cyclohexene with different oxidants, such as tert-butylhydroperoxide (TBHP), m-chloroperoxybenzoic acid (m-CPBA) and hydrogen peroxide (H 2 O 2), in DMF. m-CPBA is also found as the most successful oxidant in the other oxygen sources. In this sense, the substrate/catalyst ratio, the influence of temperature, oxidant/cat ratio and type of oxidant have been studied. It is observed three oxidation products. First one is 2-cyclohexene-1-ol as dominant product and the others are 2-cyclohexene-1-one and cyclohexene oxide as minor product.