cobalt(II) Tetraphenylporphyrin Research Articles

Cobalt(II) tetraphenylporphyrin trapped in the pores of Cu2O to enhance the C2+ selectivity towards acidic CO2 electroreduction

Achieving the high selectivity of multicarbon (C2+) products during the acidic CO2 electroreduction remains a formidable challenge due to the serious hydrogen evolution reaction. Herein we report a Cu-based tandem catalyst by implanting molecular cobalt(II) tetraphenylporphyrin (CoTPP) into a porous Cu2O (Cu2O-CoTPP) for improving the C2+ selectivity in acidic media. Specially, CoTPP, as a CO-selective catalyst, is predominantly trapped in the nanopores of Cu2O with the assistance of negative-pressure ultrasonic. The CO2-CO and CO-C2+ tandem catalysis on Cu2O-CoTPP, cooperating with the pore confinement effect of porous Cu2O, boosts the local CO concentration and *CO coverage on Cu sites, evidenced by in-situ Raman spectra and finite element simulations. As a result, Cu2O-CoTPP attains a C2+ Faradaic efficiency of 91.2 % at an industrial-level current density of 800 mA/cm2 in an acidic environment (pH = 1.9), higher than the one (70.6 %) on pristine Cu2O. Meanwhile, such a tandem catalyst obtains a high single-pass carbon efficiency of 52.6 % and the good catalytic stability of at least 20 h. This study offers a spatially-confined Cu-based tandem catalyst for selective C2+ production, presenting the promising application in acidic environments.

Construction of Cobalt Porphyrin-Modified Cu2O Nanowire Array as a Tandem Electrocatalyst for Enhanced CO2 Reduction to C2 Products.

Here, the molecule-modified Cu-based array is first constructed as the self-supporting tandem catalyst for electrocatalytic CO2 reduction reaction (CO2RR) to C2 products. The modification of cuprous oxide nanowire array on copper mesh (Cu2O@CM) with cobalt(II) tetraphenylporphyrin (CoTPP) molecules is achieved via a simple liquid phase method. The systematical characterizations confirm that the formation of axial coordinated Co-O-Cu bond between Cu2O and CoTPP can significantly promote the dispersion of CoTPP molecules on Cu2O and the electrical properties of CoTPP-Cu2O@CM heterojunction array. Consequently, as compared to Cu2O@CM array, the optimized CoTPP-Cu2O@CM sample as electrocatalyst can realize the 2.08-fold C2 Faraday efficiency (73.2%vs 35.2%) and the 2.54-fold current density (‒52.9vs ‒20.8mAcm-2) at ‒1.1V versus RHE in an H-cell. The comprehensive performance is superior to most of the reported Cu-based materials in the H-cell. Further study reveals that the CoTPP adsorption on Cu2O can restrain the hydrogen evolution reaction, improve the coverage of *CO intermediate, and maintain the existence of Cu(I) at low potential.

Temperature dependent product distribution of electrochemical CO2 reduction on CoTPP/MWCNT composite

Electrochemical reduction of CO2 to valuable products on molecular catalysts draws attention due to their versatile structures allowing tuning of activity and selectivity. Here, we investigate temperature influence on CO2 conversion product selectivity over a Cobalt(II)-tetraphenyl porphyrin (CoTPP)/multiwalled carbon nanotube (MWCNT) composite in the range of 20–50 ℃. Faradaic efficiency of products changes with temperature and potential so that two-electron transfer product CO formation is enhanced at low potentials and temperatures while the competing hydrogen formation shows an opposite trend. Multi-electron transfer product methanol formation is more favorable at low temperatures and potentials whereas reverse applies for methane. Activity and selectivity are analyzed with DFT simulations identifying the key differences between the binding energies of CH2O and CHOH, the binding strength of CO, and the protonation of CHO intermediate. This novel experimental and theoretical understanding for CO2 reduction provides insight in the influence of the various conditions on the product distribution.

Probing the Roughness of Porphyrin Thin Films with X-ray Photoelectron Spectroscopy.

Thin‐film growth of molecular systems is of interest for many applications, such as for instance organic electronics. In this study, we demonstrate how X‐ray photoelectron spectroscopy (XPS) can be used to study the growth behavior of such molecular systems. In XPS, coverages are often calculated assuming a uniform thickness across a surface. This results in an error for rough films, and the magnitude of this error depends on the kinetic energy of the photoelectrons analyzed. We have used this kinetic‐energy dependency to estimate the roughnesses of thin porphyrin films grown on rutile TiO2(110). We used two different molecules: cobalt (II) monocarboxyphenyl‐10,15,20‐triphenylporphyrin (CoMCTPP), with carboxylic‐acid anchor groups, and cobalt (II) tetraphenylporphyrin (CoTPP), without anchor groups. We find CoMCTPP to grow as rough films at room temperature across the studied coverage range, whereas for CoTPP the first two layers remain smooth and even; depositing additional CoTPP results in rough films. Although, XPS is not a common technique for measuring roughness, it is fast and provides information of both roughness and thickness in one measurement.

Surface-Dependent Phenomena in Cobalt-Based Electrocatalysts for Improved Nitrogen Oxyanion Reduction

More than half of the world is fed and fueled through the Haber-Bosch process, and ammonia consumption and global population continue to grow in tandem. However, resulting nitrogen oxyanion (NOx) waste threatens aquatic ecosystems by feeding harmful algal blooms. Molecular transition metal electrocatalysts are poised to selectively reduce these NOx waste products: macrocyclic complexes are particularly robust to electrocatalytic N–O bond activations. Here, two cobalt-based macrocycles with redox-active ligand frameworks prove capable of reducing nitrate and nitrite, with an intriguing dependence on surface interactions. With an applied potential under monochromatic irradiation, a cobalt(III)-based diiminomethyl macrocycle, [Co(DIM)Br2]+, can more efficiently reduce NO2 – to ammonia by bypassing an auto-destructive ligand-based radical. Similarly, an intermediate with ligand-radical character is active toward NO3 – reduction in cobalt(II) tetraphenylporphyrin (CoTPP) in bulk systems, but can be stabilized in aqueous conditions for more affordable onset potentials once immobilized. Inspired by surface influence, homogeneous and heterogeneous systems are blended into molecular catalyst-support assemblies: physisorbing CoTPP on nanosupports imparts catalyst durability and nitrate electroreduction in coordinating buffers. These hybrid assemblies remarkably improve onset potentials in previously inaccessible conditions, ultimately overcoming many compromises between overpotential and turnover in purely homogeneous designs. Figure 1

Sensitization of La modified NaTaO3 with cobalt tetra phenyl porphyrin for photo catalytic reduction of CO2 by water with UV–visible light

Lanthanum modified sodium tantalate, Na(1−x)LaxTaO(3+x), in conjunction with cobalt (II) tetra phenyl porphyrin (CoTPP) as sensitizer, has been explored for photo catalytic reduction of carbon dioxide (PCRC) with water. HOMO and LUMO energy level characteristics/redox potentials for ground (S0) and excited states (S1 singlet) of CoTPP have been calculated by Density Functional Theory (DFT). HOMO and LUMO energy levels enable sensitization of the tantalate, a typical wide band gap semi-conductor, with visible light. Visible light absorption by CoTPP results in the direct transfer of photo generated electrons to the conduction band of the tantalate, in addition to the intrinsic UV light excitation. Besides, sensitization also retards charge carrier recombination rate, as indicated by the photo luminescence spectral data for the pristine and sensitized Na(1−x)LaxTaO(3+x). A co-operative effect of these factors contributes towards nearly 3 fold increase in apparent quantum yield value for PCRC with the 1% w/w CoTPP/tantalate composite vis-à-vis pristine tantalate. After 20h of irradiation, rate of methanol formation remains constant with pristine and sensitized tantalates, while the rate of formation of ethanol increases on sensitization, indicating multi electron reduction process. Chemical composition and structural characteristics of the composite are preserved even after 20h of irradiation.

Controlling spins in adsorbed molecules by a chemical switch.

The development of chemical systems with switchable molecular spins could lead to the architecture of materials with controllable magnetic or spintronic properties. Here, we present conclusive evidence that the spin of an organometallic molecule coupled to a ferromagnetic substrate can be switched between magnetic off and on states by a chemical stimulus. This is achieved by nitric oxide (NO) functioning as an axial ligand of cobalt(II)tetraphenylporphyrin (CoTPP) ferromagnetically coupled to nickel thin-film (Ni(001)). On NO addition, the coordination sphere of Co2+ is modified and a NO–CoTPP nitrosyl complex is formed, which corresponds to an off state of the Co spin. Thermal dissociation of NO from the nitrosyl complex restores the on state of the Co spin. The NO-induced reversible off–on switching of surface-adsorbed molecular spins observed here is attributed to a spin trans effect.

Edge and terrace structure of CoTPP on Au(1 1 1) investigated by ultra-high vacuum scanning tunnelling microscopy at room temperature

Edge adsorption and terrace molecular domain structures of Cobalt(II) tetraphenylporphyrin (CoTPP) on Au(1 1 1) were investigated using STM at room temperature. Two different terrace domain structures were observed. These two arrangements were found to be enantiomorphous arrangements of the molecular assemblies, where the molecular rows rotate ±16° with respect to the [1 2 1] direction of Au(1 1 1). In both arrangements, most of the CoTPP molecules were imaged as one bright dot with four legs, corresponding to a planar conformation of the macrocycle. A small proportion of the CoTPP molecules appear as two bright dots, corresponding to a saddle shape of the macrocycle. Our results show that most of the saddle-deformed CoTPP molecules are distributed in the vicinity of the bridging sites of the reconstructed gold surface. Besides terrace domains, we found that several edge adsorption structures of CoTPP are also stable enough to be imaged and analysed in detail. Furthermore, the relationship between edge structures and terrace domains was revealed.

Adsorption of cobalt (II) octaethylporphyrin and 2H-octaethylporphyrin on Ag(111): new insight into the surface coordinative bond

The adsorption of cobalt (II) octaethylporphyrin (CoOEP) and 2H-octaethylporphyrin (2HOEP) on Ag(111) was investigated with scanning tunneling microscopy (STM) and photoelectron spectroscopy (XPS/UPS), in order to achieve a detailed mechanistic understanding of the surface chemical bond of coordinated metal ions. Previous studies of related systems, especially cobalt (II) tetraphenylporphyrin (CoTPP) on Ag(111), have revealed adsorption-induced changes of the oxidation state of the Co ion and the appearance of a new valence state. These effects were attributed to a covalent interaction of the Co ion with the silver substrate. However, recent studies show that the porphyrin ligand of adsorbed CoTPP undergoes a pronounced saddle-shape distortion, which could alter the electronic structure and thus provide an alternative explanation for the new valence state previously attributed to the formation of a surface coordinative bond. With the octaethylporphyrins investigated here, which were found to adsorb in a flat, undistorted conformation on Ag(111), the effects of geometric distortion can be separated from those of the electronic interaction with the substrate. The CoOEP monolayer gives rise to an adsorption-induced shift of the Co 2p signal (−1.9 eV relative to the multilayer), a new valence state at 0.6 eV below the Fermi energy, and a work-function shift of −0.84 eV (2HOEP: −0.44 eV) relative to the clean surface. Comparison with data for the distorted CoTPP confirms the existence of a covalent ion–surface interaction that is insensitive to the conformation of the ligand.

Bimolecular Two-Dimensional Organization of Porphyrins on Au(111): Site Selective, Metal Ion-Dependent Adsorption of Tetraphenylporphyrin

Adlayers consisting of two components selected from the group of cobalt(II) tetraphenylporphyrin (CoTPP), copper(II) tetraphenylporphyrin (CuTPP), and zinc(II) tetraphenylporphyrin (ZnTPP) were prepared by immersing Au(111) substrate in a benzene solution containing those molecules. The bimolecular adlayers thus prepared were investigated in 0.1 M HClO4 by electrochemical scanning tunneling microscopy (EC-STM). The mixed adlayers consisting of CoTPP and CuTPP formed structurally ordered but compositionally disordered arrays on Au(111). The ratio of CoTPP to CuTPP molecules in the mixed adlayer was proportional to the ratio of CoTPP to CuTPP molecules in the solution phase. Accordingly, the composition of CoTPP and CuTPP in the adlayer on the Au(111) surface was independent of absolute concentrations of these species in the solution and immersion time. In contrast, the structural feature of the mixed adlayer consisting of CoTPP and ZnTPP was similar to that of the mixed adlayer of CoTPP and CuTPP when these adlayers were prepared in solutions containing those mixtures at a total concentration of 100 microM, whereas when the total concentration was lower, adsorption was site-selective depending on the coordinated metal ion. This finding indicates that the herringbone structure of reconstructed Au(111) served as a template for the bimolecular assembly of CoTPP and ZnTPP. The characteristic phase separation of CoTPP and ZnTPP molecules assisted by reconstructed Au(111) surface can be controlled by the subtle balance between kinetics and thermodynamics.

OFF−OFF−ON Switching of Fluorescence and Electron Transfer Depending on Stepwise Complex Formation of a Host Ligand with Guest Metal Ions

Stepwise complex formation is observed between 2,3,5,6-tetrakis(2-pyridyl)pyrazine (TPPZ) and a series of metal ions (M(n+) = Sc3+, Y3+, Ho3+, Eu3+, Lu3+, Nd3+, Zn2+, Mg2+, Ca2+, Ba2+, Sr2+, Li+), where TPPZ forms a 2:1 complex [(TPPZ)2-M(n+)] and a 1:1 complex [TPPZ-M(n+)] with Mn+ at low and high concentrations of metal ions, respectively. The fluorescence intensity of TPPZ begins to increase at high concentrations of metal ions, when the 2:1 (TPPZ)2-M(n+) complex is converted to the fluorescent 1:1 TPPZ-M(n+) complex. This is regarded as an "OFF-OFF-ON" fluorescence sensor for metal ions depending on the stepwise complex formation between TPPZ and metal ions. The fluorescence quantum yields of the TPPZ-M(n+) complex vary depending on the metal valence state, in which the fluorescence quantum yields of the divalent metal complexes (TPPZ-M2+) are much larger than those of the trivalent metal complexes (TPPZ-M3+). On the other hand, the binding constants of (TPPZ)2-M(n+) (K1) and TPPZ-M(n+) (K2) vary depending on the Lewis acidity of metal ions (i.e., both K1 and K2 values increase with increasing Lewis acidity of metal ions). Sc3+, which acts as the strongest Lewis acid, forms the (TPPZ)2-Sc3+ and TPPZ-Sc3+ complexes stoichiometrically with TPPZ. In such a case, "OFF-OFF-ON" switching of electron transfer from cobalt(II) tetraphenylporphyrin (CoTPP) to O2 is observed in the presence of Sc3+ and TPPZ depending on the ratio of Sc3+ to TPPZ. Electron transfer from CoTPP to O2 occurs at Sc3+ concentrations above the 1:2 ratio ([Sc3+]/[TPPZ]0 > 0.5), when the (TPPZ)2-Sc3+ complex is converted to the TPPZ-Sc3+ complex and TPPZ-(Sc3+)2, which act as promoters of electron transfer (ON) by the strong binding of O2*- with Sc3+. In sharp contrast, no electron transfer occurs without metal ion (OFF) or in the presence at Sc3+ concentrations below the 1:2 ratio (OFF), when the (TPPZ)2-Sc3+ complex has no binding site available for O2*-.

Conformation-dependent photoinduced hydrogen evolution with Co(II)tetraphenylporphyrin–poly( l-glutamate) complex

Hydrogen production was performed under visible-light and combined UV/visible-light irradiation by using a system consisting of cobalt(II)tetraphenylporphyrin (CoTPP), methylviologen (MV 2+), ethylenediaminetetraacetic acid disodium salt and Pt-loaded poly( l-glutamate) (Poly(Glu)) in aqueous decylammonium chloride (DeAC) solution. CoTPP was solubilized in hydrophobic clusters of DeAC induced by its cooperative binding to Poly(Glu). About 0.15 and 3.5 μmol of hydrogen gas were obtained after irradiation for 3 h using visible-light and combined UV/visible-light, respectively. The rate of hydrogen evolution depended on the change in the conformation of Poly(Glu) induced by the cooperative binding with DeAC. An electron was transferred from CoTPP to MV 2+ in the Poly(Glu)–DeAC complex system via the singlet state, finally resulting in the hydrogen evolution.

Interaction of Cobalt(II) Tetraarylporphyrins with a Ag(111) Surface Studied with Photoelectron Spectroscopy

The interaction of cobalt(II) tetraphenylporphyrin (CoTPP) and cobalt(II) tetrakis-(3,5-di-tert-butylphenyl)porphyrin (CoTTBPP) with a Ag(111) surface has been investigated with photoelectron spectroscopy (XPS/UPS). It is demonstrated that these adsorbed metal complexes are excellent model systems for studying the electronic interaction between a coordinated metal ion and a metal surface. The photoelectron spectra and work function data provide evidence that the electronic interaction between the cobalt ion and the silver surface results in a transfer of electron density from the surface to the ion. The presence of an additional electronic state located ∼1 eV above the singly occupied molecular orbital (SOMO) of the metalloporphyrins is consistent with a molecular orbital (MO) model of the Co−Ag interaction as is the fact that the energetic position of this state depends on the distance between the Co ion and the Ag surface. The adsorbate-induced work function changes for the saturated monolayers amount t...

Fullerene Complexes with Cobalt(II) Tetraphenylporphyrin: From Molecular to Ionic Crystals

AbstractFor Abstract see ChemInform Abstract in Full Text.

Scanning Tunneling Microscopy Study of the Structure and Orbital-Mediated Tunneling Spectra of Cobalt(II) Phthalocyanine and Cobalt(II) Tetraphenylporphyrin on Au(111): Mixed Composition Films

Binary thin films of cobalt(II) phthalocyanine (CoPc) and cobalt(II) tetraphenylporphyrin (CoTPP) were prepared at submonolayer coverage on Au(111)/mica substrates byvapor deposition. All sample preparation and analysis were done under an ultrahigh vacuum. Scanning tunneling microscopy (STM) constant-current images of CoPc/CoTPP mixtures showed two close-packed surface structures, with different compositional percentages and some disorder. CoPc was also observed exclusively in one-dimensional chains and as single, isolated molecules below 220 K. Occupied and unoccupied orbital energy levels were identified by STM and tunnel-diode-based orbital-mediated tunneling spectroscopy. Occupied energy levels were also confirmed by ultraviolet photoelectron spectroscopy. The transient oxidation of the Co d(z2) orbital is identified in STM dI/dV(V) curves just negative of the 0 V sample bias for both molecules. Nearly identical constant-current contours are observed over the central Co2+ ions of CoTPP and CoPc, indicating that the attenuation of the d(z)2 orbital-mediated tunneling current induced by the structure of TPP relative to Pc is at most a factor of about 10. The orbital-mediated tunneling spectra of CoTPP and CoPc are distinctly different and allow these structurally similar species to be differentially identified.

Rearrangement of unsaturated cyclic peroxides obtained by photooxygenation of 2,3-dimethylene-7-oxabenzonorbornene and 2,3-dimethylene-1,4-etheno-1,2,3,4-tetrahydronaphthalene

Cobalt(II) tetraphenylporphyrin (CoTPP)-catalyzed rearrangement of peroxide 11 obtained by the photooxygenation of 5 gave a mixture of epoxides 15 (or 16) and 17. However, CoTPP-catalyzed rearrangement of peroxide 20 arising by photooxygenation of 2,3-dimethylene-1,4-etheno-1,2,3,4-tetrahydronaphthalane 6 gave 1,4-etheno-1,2,3,4-tetrahydronaphtho[ b]-furan 21. The different behaviour of these endoperoxides 11 and 20 is discussed in terms of ring strain and the degree of the pyramidalization of the CC bond.

Catalytic Action of Iron(II) Phthalocyanine as a Tryptophan-2,3-Dioxygenase Model in Oxygenolysis of 3-Methylindole with Metallophthalocyanines

Abstract Although the enzyme action of protohaem-containing tryptophan-2,3-dioxygenase has recently been simulated by the oxygenation of 3-substituted indoles to ring-opened keto-amides with bis(salycylidene)ethylenediaminatocobalt(II)(CoSalen),1–3 cobalt(II) tetraphenylporphyrin (CoTPP),4,5 manganese(II) phthalocyanine (MnPc),6 and a copper(I) chloride-pyridine complex (CuCl[sbnd]Py),7–9 the catalytic efficiency of iron(II or III) complexes as models of tryptophan-2,3-dioxygenase has hitherto been the subject of limited investigation in the oxygenolysis of indoles related to tryptophan; there is only one report dealing with the oxygenation of indoles to the ring-opened products catalyzed by FeTPP(Py)2.10 The present paper describes the catalytic action of iron(II) phthalocyanine (FeIIPc) in the oxidative ring-opening of 3-methylindole with metallophthalocyanines(MIIPc: M=Fe, Co, Cu, and/or Mn) as shown in the following equation.