Amanda J. Morris

Using a One-Electron Shuttle for the Multielectron Reduction of CO2 to Methanol: Kinetic, Mechanistic, and Structural Insights

Pyridinium and its substituted derivatives are effective and stable homogeneous electrocatalysts for the aqueous multiple-electron, multiple-proton reduction of carbon dioxide to products such as formic acid, formaldehyde, and methanol. Importantly, high faradaic yields for methanol have been observed in both electrochemical and photoelectrochemical systems at low reaction overpotentials. Herein, we report the detailed mechanism of pyridinium-catalyzed CO(2) reduction to methanol. At metal electrodes, formic acid and formaldehyde were observed to be intermediate products along the pathway to the 6e(-)-reduced product of methanol, with the pyridinium radical playing a role in the reduction of both intermediate products. It has previously been thought that metal-derived multielectron transfer was necessary to achieve highly reduced products such as methanol. Surprisingly, this simple organic molecule is found to be capable of reducing many different chemical species en route to methanol through six sequential electron transfers instead of metal-based multielectron transfer. We show evidence for the mechanism of the reduction proceeding through various coordinative interactions between the pyridinium radical and carbon dioxide, formaldehyde, and related species. This suggests an inner-sphere-type electron transfer from the pyridinium radical to the substrate for various mechanistic steps where the pyridinium radical covalently binds to intermediates and radical species. These mechanistic insights should aid the development of more efficient and selective catalysts for the reduction of carbon dioxide to the desired products.

Solvothermal Preparation of an Electrocatalytic Metalloporphyrin MOF Thin Film and its Redox Hopping Charge-Transfer Mechanism

A thin film of a metalloporphyrin metal-organic framework consisting of [5,10,15,20-(4-carboxyphenyl)porphyrin]Co(III) (CoTCPP) struts bound by linear trinuclear Co(II)-carboxylate clusters has been prepared solvothermally on conductive fluorine-doped tin oxide substrates. Characterization of this mesoporous thin film material, designated as CoPIZA/FTO, which is equipped with large cavities and access to metal active sites, reveals an electrochemically active material. Cyclic voltammetry displays a reversible peak with E(1/2) at -1.04 V vs ferrocyanide attributed to the (Co(III/II)TCPP)CoPIZA redox couple and a quasi-reversible peak at -1.45 V vs ferrocyanide, which corresponds to the reduction of (Co(II/I)TCPP)CoPIZA. Analysis of the spectroelectrochemical response for the (Co(II/I)TCPP)CoPIZA redox couple revealed non-Nernstian reduction with a nonideality factor of 2 and an E(1/2) of -1.39 V vs ferrocyanide. The film was shown to retain its structural integrity with applied potential, as was demonstrated spectroelectrochemically with maintenance of isosbestic points at 430, 458, and 544 nm corresponding to the (Co(III/II)TCPP)CoPIZA transition and at 390 and 449 nm corresponding to the (Co(II/I)TCPP)CoPIZA transition. The mechanism of charge transport through the film is proposed to be a redox hopping mechanism, which is supported by both cyclic voltammetry and spectroelectrochemistry. A fit of the time-dependent spectroelectrochemical data to a modified Cottrell equation gave an apparent diffusion coefficient of 7.55 (±0.05) × 10(-14) cm(2)/s for ambipolar electron and cation transport throughout the film. Upon reduction of the metalloporphyrin struts to (Co(I)TCPP)CoPIZA, the CoPIZA thin film demonstrated catalytic activity for the reduction of carbon tetrachloride.

Non-Nernstian Two-Electron Transfer Photocatalysis at Metalloporphyrin–TiO2 Interfaces

A long-standing question in the photochemical sciences concerns how to integrate single-electron transfers to catalytic multielectron transfer reactions that produce useful chemical fuels. Here we provide a strategy for the two-electron formation of C-C bonds with molecular catalysts anchored to semiconductor nanocrystallites. The blue portion of the solar spectrum provides band gap excitation of the semiconductor while longer wavelengths of light initiate homolytic cleavage of metal-carbon bonds that, after interfacial charge transfer, restore the catalyst. The semiconductor utilized was the anatase polymorph of TiO(2) present as a nanocrystalline, mesoporous thin film. The catalyst was cobalt meso-5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin chloride, Co(TCPP)Cl. For this catalyst and iron protoporphyrin IX chloride, Fe(PPIX)Cl, two distinct and sequential metal-based M(III/II) and M(II/I) reductions were observed under band gap illumination. Spectroelectrochemical characterization indicated that both reductions were non-Nernstian, behavior attributed to an environmentally dependent potential drop across the molecule-semiconductor interface. Reaction of Co(I)(TCPP)/TiO(2) with organobromides (RBr = 1-Br-hexane or benzyl bromide) resulted in the formation of Co(III)-R(TCPP)/TiO(2). Visible light excitation induced homolytic cleavage of the Co-C bond and the formation of C-C-bonded products. The reactions were catalytic when band gap excitation or an electrochemical bias provided TiO(2) electrons to the oxidized catalyst. Sustained photocurrents were quantified in photoelectrosynthetic solar cells under forward bias.

Controlling Morphological Parameters of Anodized Titania Nanotubes for Optimized Solar Energy Applications

Anodized TiO2 nanotubes have received much attention for their use in solar energy applications including water oxidation cells and hybrid solar cells [dye-sensitized solar cells (DSSCs) and bulk heterojuntion solar cells (BHJs)]. High surface area allows for increased dye-adsorption and photon absorption. Titania nanotubes grown by anodization of titanium in fluoride-containing electrolytes are aligned perpendicular to the substrate surface, reducing the electron diffusion path to the external circuit in solar cells. The nanotube morphology can be optimized for the various applications by adjusting the anodization parameters but the optimum crystallinity of the nanotube arrays remains to be realized. In addition to morphology and crystallinity, the method of device fabrication significantly affects photon and electron dynamics and its energy conversion efficiency. This paper provides the state-of-the-art knowledge to achieve experimental tailoring of morphological parameters including nanotube diameter, length, wall thickness, array surface smoothness, and annealing of nanotube arrays.

Cooperative electrochemical water oxidation by Zr nodes and Ni–porphyrin linkers of a PCN-224 MOF thin film

Here, we demonstrate a new strategy for cooperative catalysis and proton abstraction via the incorporation of independent species competent in the desired reactivity into a metal–organic framework (MOF) thin film. The highly porous MOF, designated as PCN-224-Ni, is constructed by Zr–oxo nodes and nickel(II) porphyrin linkers. Films of PCN-224-Ni were grown in situ on FTO and were found to electrochemically facilitate the water oxidation reaction at near neutral pH.

A New Class of Metal-Cyclam-Based Zirconium Metal–Organic Frameworks for CO2 Adsorption and Chemical Fixation

Metal-organic frameworks (MOFs) have shown great promise in catalysis, mainly due to their high content of active centers, large internal surface areas, tunable pore size, and versatile chemical functionalities. However, it is a challenge to rationally design and construct MOFs that can serve as highly stable and reusable heterogeneous catalysts. Here two new robust 3D porous metal-cyclam-based zirconium MOFs, denoted VPI-100 (Cu) and VPI-100 (Ni), have been prepared by a modulated synthetic strategy. The frameworks are assembled by eight-connected Zr6 clusters and metallocyclams as organic linkers. Importantly, the cyclam core has accessible axial coordination sites for guest interactions and maintains the electronic properties exhibited by the parent cyclam ring. The VPI-100 MOFs exhibit excellent chemical stability in various organic and aqueous solvents over a wide pH range and show high CO2 uptake capacity (up to ∼9.83 wt% adsorption at 273 K under 1 atm). Moreover, VPI-100 MOFs demonstrate some of the highest reported catalytic activity values (turnover frequency and conversion efficiency) among Zr-based MOFs for the chemical fixation of CO2 with epoxides, including sterically hindered epoxides. The MOFs, which bear dual catalytic sites (Zr and Cu/Ni), enable chemistry not possible with the cyclam ligand under the same conditions and can be used as recoverable stable heterogeneous catalysts without losing performance.

Study of Electrocatalytic Properties of Metal–Organic Framework PCN-223 for the Oxygen Reduction Reaction

A highly robust metal-organic framework (MOF) constructed from Zr6 oxo clusters and Fe(III) porphyrin linkers, PCN-223-Fe was investigated as a heterogeneous catalyst for oxygen reduction reaction (ORR). Films of the framework were grown on a conductive FTO substrate and showed a high catalytic current upon application of cathodic potentials and achieved high H2O/H2O2 selectivity. In addition, the effect of the proton source on the catalytic performance was also investigated.

Thermodynamic Study of CO2 Sorption by Polymorphic Microporous MOFs with Open Zn(II) Coordination Sites.

Two Zn-based metal organic frameworks have been prepared solvothermally, and their selectivity for CO2 adsorption was investigated. In both frameworks, the inorganic structural building unit is composed of Zn(II) bridged by the 2-carboxylate or 5-carboxylate pendants of 2,5-pyridine dicarboxylate (pydc) to form a 1D zigzag chain. The zigzag chains are linked by the bridging 2,5-carboxylates across the Zn ions to form 3D networks with formulas of Zn4(pydc)4(DMF)2·3DMF (1) and Zn2(pydc)2(DEF) (2). The framework (1) contains coordinated DMF as well as DMF solvates (DMF = N,N-dimethylformamide), while (2) contains coordinated DEF (DEF = N,N-diethylformamide). (1) displays a reversible type-I sorption isotherm for CO2 and N2 with BET surface areas of 196 and 319 m(2)/g, respectively. At low pressures, CO2 and N2 isotherms for (2) were not able to reach saturation, indicative of pore sizes too small for the gas molecules to penetrate. A solvent exchange to give (2)-MeOH allowed for increased CO2 and N2 adsorption onto the MOF surface with BET surface areas of 41 and 39 m(2)/g, respectively. The binding of CO2 into the framework of (1) was found to be exothermic with a zero coverage heat of adsorption, Qst(0), of −27.7 kJ/mol. The Qst(0) of (2) and (2)-MeOH were found to be −3 and −41 kJ/mol, respectively. The CO2/N2 selectivity for (1), calculated from the estimated KH at 296 K, was found to be 42. At pressures relevant to postcombustion capture, the selectivity was 14. The thermodynamic data are consistent with a mechanism of adsorption that involves CO2 binding to the unsaturated Zn(II) metal centers present in the crystal structures.

Visible light induced photocatalytic activity of Fe3+/Ti3+ co-doped TiO2 nanostructures

Black TiO2 nanostructures co-doped with Fe3+ and Ti3+ were synthesized by annealing Fe-deposited TiO2 nanotubes under vacuum. These vacuum-annealed samples exhibited improved visible light absorption and efficient photocatalytic activity under visible light illumination as compared to conventional TiO2 materials. XPS and EPR spectra confirmed the presence of Ti3+ in the bulk and Fe3+ dopant.

Rethinking Band Bending at the P3HT–TiO2 Interface

The advancement of solar cell technology necessitates a detailed understanding of material heterojunctions and their interfacial properties. In hybrid bulk heterojunction solar cells (HBHJs), light-absorbing conjugated polymers are often interfaced with films of nanostructured TiO2 as a cheaper alternative to conventional inorganic solar cells. The mechanism of photovoltaic action requires photoelectrons in the polymer to transfer into the TiO2, and therefore, polymers are designed with lowest unoccupied molecular orbital (LUMO) levels higher in energy than the conduction band of TiO2 for thermodynamically favorable electron transfer. Currently, the energy level values used to guide solar cell design are referenced from the separated materials, neglecting the fact that upon heterojunction formation material energetics are altered. With spectroelectrochemistry, we discovered that spontaneous charge transfer occurs upon heterojunction formation between poly(3-hexylthiophene) (P3HT) and nanocrystalline TiO2. It was determined that deep trap states (0.5 eV below the conduction band of TiO2) accept electrons from P3HT and form hole polarons in the polymer. This equilibrium charge separation alters energetics through the formation of interfacial dipoles and results in band bending that inhibits desired photoelectron injection into TiO2, limiting HBHJ solar cell performance. X-ray photoelectron spectroscopic studies quantified the resultant vacuum level offset to be 0.8 eV. Further spectroelectrochemical studies indicate that 0.1 eV of this offset occurs in TiO2, whereas the balance occurs in P3HT. New guidelines for improved photocurrent are proposed by tuning the energetics of the heterojunction to reverse the direction of the interfacial dipole, enhancing photoelectron injection.