Cobalt-Porphyrin Catalyzed Electrochemical Reduction of Carbon Dioxide in Water II: Mechanism from First Principles

We apply first principles computational techniques to analyze the two-electron, multi-step, electrochemical reduction of CO2 to CO in water using cobalt porphyrin as a catalyst. Density Functional Theory calculations with hybrid functionals and dielectric continuum solvation are used to determine the steps at which electrons are added. This information is corroborated with ab initio molecular dynamics simulations in an explicit aqueous environment which reveal the critical role of water in stabilizing a key intermediate formed by CO2 bound to cobalt. Using potential of mean force calculations, the intermediate is found to spontaneously accept a proton to form a carboxylate acid group at pH<9.0, and the subsequent cleavage of a C-OH bond to form CO is exothermic and associated with a small free energy barrier. These predictions suggest that the proposed reaction mechanism is viable if electron transfer to the catalyst is sufficiently fast. The variation in cobalt ion charge and spin states during bond breaking, DFT+U treatment of cobalt 3d orbitals, and the need for computing electrochemical potentials are emphasized.Comment: 33 pages, 7 figure

Ab initio molecular dynamics study of manganese porphine hydration and interaction with nitric oxide

The authors use ab initio molecular dynamics and the density functional theory+U (DFT+U) method to compute the hydration environment of the manganese ion in manganese (II) and manganese (III) porphines (MnP) dispersed in liquid water. These are intended as simple models for more complex water soluble porphyrins, which have important physiological and electrochemical applications. The manganese ion in Mn(II)P exhibits significant out-of-porphine plane displacement and binds strongly to a single H2O molecule in liquid water. The Mn in Mn(III)P is on average coplanar with the porphine plane and forms a stable complex with two H2O molecules. The residence times of these water molecules exceed 15 ps. The DFT+U method correctly predicts that water displaces NO from Mn(III)P-NO, but yields an ambiguous spin state for the MnP(II)-NO complex.Comment: 10 pages, 6 figure

Density functional theory and DFT+U study of transition metal porphines adsorbed on Au(111) surfaces and effects of applied electric fields

We apply Density Functional Theory (DFT) and the DFT+U technique to study the adsorption of transition metal porphine molecules on atomistically flat Au(111) surfaces. DFT calculations using the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional correctly predict the palladium porphine (PdP) low-spin ground state. PdP is found to adsorb preferentially on gold in a flat geometry, not in an edgewise geometry, in qualitative agreement with experiments on substituted porphyrins. It exhibits no covalent bonding to Au(111), and the binding energy is a small fraction of an eV. The DFT+U technique, parameterized to B3LYP predicted spin state ordering of the Mn d-electrons, is found to be crucial for reproducing the correct magnetic moment and geometry of the isolated manganese porphine (MnP) molecule. Adsorption of Mn(II)P on Au(111) substantially alters the Mn ion spin state. Its interaction with the gold substrate is stronger and more site-specific than PdP. The binding can be partially reversed by applying an electric potential, which leads to significant changes in the electronic and magnetic properties of adsorbed MnP, and ~ 0.1 Angstrom, changes in the Mn-nitrogen distances within the porphine macrocycle. We conjecture that this DFT+U approach may be a useful general method for modeling first row transition metal ion complexes in a condensed-matter setting.Comment: 14 pages, 6 figure

LDRD final report on adaptive-responsive nanostructures for sensing applications.

Functional organic nanostructures such as well-formed tubes or fibers that can easily be fabricated into electronic and photonic devices are needed in many applications. Especially desirable from a national security standpoint are nanostructures that have enhanced sensitivity for the detection of chemicals and biological (CB) agents and other environmental stimuli. We recently discovered the first class of highly responsive and adaptive porphyrin-based nanostructures that may satisfy these requirements. These novel porphyrin nanostructures, which are formed by ionic self-assembly of two oppositely charged porphyrins, may function as conductors, semiconductors, or photoconductors, and they have additional properties that make them suitable for device fabrication (e.g., as ultrasensitive colorimetric CB microsensors). Preliminary studies with porphyrin nanotubes have shown that these nanostructures have novel optical and electronic properties, including strong resonant light scattering, quenched fluorescence, and electrical conductivity. In addition, they are photochemically active and capable of light-harvesting and photosynthesis; they may also have nonlinear optical properties. Remarkably, the nanotubes and potentially other porphyrin nanostructure are mechanically responsive and adaptive (e.g., the rigidity of the micrometers-long nanotubes is altered by light, ultrasound, or chemicals) and they self-heal upon removal the environmental stimulus. Given the tremendous degree of structural variation possible in the porphyrin subunits, additional types of nanostructures and greater control over their morphology can be anticipated. Molecular modification also provides a means of controlling their electronic, photonic, and other functional properties. In this work, we have greatly broadened the range of ionic porphyrin nanostructures that can be made, and determined the optical and responsivity properties of the nanotubes and other porphyrin nanostructures. We have also explored means for controlling their morphology, size, and placement on surfaces. The research proposed will lay the groundwork for the use of these remarkable porphyrin nanostructures in micro- and nanoscale devices, by providing a more detailed understanding of their molecular structure and the factors that control their structural, photophysical, and chemical properties

Tetracycloalkenyl-meso-tetraphenylporphyrins as Models for the Effect of Non-planarity on the Light Absorption Properties of Photosynthetic Chromophores

As the size of the cycloalkenyl ring in synthetic C5-C7 tetracycloalkenyl-meso-tetraphenylporphyrins (TCnTPP) is increased the porphyrin core changes from planar to non-planar, showing that porphyrins with well-defined degrees of ruffling can be synthesized for use in systematic studies to determine the effects of non-planarity

LDRD final report on nanovehicle light-driven propulsion.

Having demonstrated the possibility of constructing nanoscale metallic vehicular bodies as described in last year's proposal, our goals have been to make uniform preparations of the metallized lipid assemblies and to determine the feasibility of powering these nanostructures with biological motors that are activated and driven by visible light. We desired that the propulsion system be constructed entirely by self-assembly and powered by a photocatalytic process partially already built into the nanovehicle. The nanovehicle we desire to build is composed of both natural biological components (ATPase, kinesin-microtubules) and biomimetic components (platinized liposomes, photosynthetic membrane) as functional units. The vehicle's body was originally envisioned to be composed of a surfactant liposomal bilayer coated with platinum nanoparticles, but instead of the expected nanoparticles we were able to grow dendritic 2-nm thick platinum sheets on the liposomes. Now, we have shown that it is possible to completely enclose the liposomes with sheeting to form porous platinum spheres, which show good structural stability as evidenced by their ability to survive the stresses of electron-microscopy sample preparation. Our goals were to control the synthesis of the platinized liposomes well enough to make uniform preparations of the coated individual liposomes and to develop the propulsion system for these nanovehicles a hydrogen-evolving artificial photosynthetic system in the liposomal bilayer that generates the pH gradient across the membrane that is necessary to drive the synthesis of ATP by ATP-synthase incorporated in the membrane. ATP produced would fuel the molecular motor (kinesin) attached to the vehicle, needing only light, storable ADP, phosphate, and an electron donor to be produced by ATP-synthase in the membrane. These research goals appear to be attainable, but growing the uniform preparations of the liposomes coated with dendritic platinum sheeting, a necessary accomplishment that would simplify the task of incorporating and verifying the photosynthetic function of the nanovehicle membrane, has proved to be difficult. The detailed understanding of the relative locations of surfactant and Pt in the liposomal bodies has also forced a change in the nanovehicle design strategies. Nevertheless, we have found no insurmountable obstacles to making these nanovehicles given a larger and longer term research effort. These nanovehicles could potentially respond to chemical gradients, light intensity, and field gradients, in the same manner that magnetic bacteria navigate. The cargo might include decision-making and guidance components, drugs and other biological and chemical agents, explosives, catalytic reactors, and structural materials

Improving electronic structure methods to predict nano-optoelectronics and nano-catalyst functions.

This report focuses on quantum chemistry and ab initio molecular dynamics (AIMD) calculations applied to elucidate the mechanism of the multi-step, 2-electron, electrochemical reduction of the green house gas molecule carbon dioxide (CO{sub 2}) to carbon monoxide (CO) in aqueous media. When combined with H{sub 2} gas to form synthesis ('syn') gas, CO becomes a key precursor to methane, methanol, and other useful hydrocarbon products. To elucidate the mechanism of this reaction, we apply computational electrochemistry which is a fledgling, important area of basic science critical to energy storage. This report highlights several approaches, including the calculation of redox potentials, the explicit depiction of liquid water environments using AIMD, and free energy methods. While costly, these pioneering calculations reveal the key role of hydration- and protonation-stabilization of reaction intermediates, and may inform the design of CO{sub 2}-capture materials as well as its electrochemical reduction. In the course of this work, we have also dealt with the challenges of identifying and applying electronic structure methods which are sufficiently accurate to deal with transition metal ion complex-based catalyst. Such electronic structure methods are also pertinent to the accurate modeling of actinide materials and therefore to nuclear energy research. Our multi-pronged effort towards achieving this titular goal of the LDRD is discussed

LDRD final report on light-powered nanovehicles.

We have investigated the possibility of constructing nanoscale metallic vehicles powered by biological motors or flagella that are activated and powered by visible light. The vehicle's body is to be composed of the surfactant bilayer of a liposome coated with metallic nanoparticles or nanosheets grown together into a porous single crystal. The diameter of the rigid metal vesicles is from about 50 nm to microns. Illumination with visible light activates a photosynthetic system in the bilayer that can generate a pH gradient across the liposomal membrane. The proton gradient can fuel a molecular motor that is incorporated into the membrane. Some molecular motors require ATP to fuel active transport. The protein ATP synthase, when embedded in the membrane, will use the pH gradient across the membrane to produce ATP from ADP and inorganic phosphate. The nanoscale vehicle is thus composed of both natural biological components (ATPase, flagellum; actin-myosin, kinesin-microtubules) and biomimetic components (metal vehicle casing, photosynthetic membrane) as functional units. Only light and storable ADP, phosphate, water, and weak electron donor are required fuel components. These nano-vehicles are being constructed by self-assembly and photocatalytic and autocatalytic reactions. The nano-vehicles can potentially respond to chemical gradients and other factors such as light intensity and field gradients, in a manner similar to the way that magnetic bacteria navigate. The delivery package might include decision-making and guidance components, drugs or other biological and chemical agents, explosives, catalytic reactors, and structural materials. We expected in one year to be able only to assess the problems and major issues at each stage of construction of the vehicle and the likely success of fabricating viable nanovehicles with our biomimetic photocatalytic approach. Surprisingly, we have been able to demonstrate that metallized photosynthetic liposomes can indeed be made. We have completed the synthesis of metallized liposomes with photosynthetic function included and studied these structures by electron microscopy. Both platinum and palladium nanosheeting have been used to coat the micelles. The stability of the vehicles to mechanical stress and the solution environment is enhanced by the single-crystalline platinum or palladium coating on the vesicle. With analogous platinized micelles, it is possible to dry the vehicles and re-suspend them with full functionality. However, with the liposomes drying on a TEM grid may cause the platinized liposomes to collapse, although probably stay viable in solution. It remains to be shown whether a proton motive force across the metallized bilayer membrane can be generated and whether we will also be able to incorporate various functional capabilities including ATP synthesis and functional molecular motors. Future tasks to complete the nanovehicles would be the incorporation of ATP synthase into metallized liposomes and the incorporation of a molecular motor into metallized liposomes

DOE/BES/NSET annual report on growth of metal and semiconductor nanostructures using localized photocatalysts.

Our overall goal is to understand and develop a novel light-driven approach to the controlled growth of unique metal and semiconductor nanostructures and nanomaterials. In this photochemical process, bio-inspired porphyrin-based photocatalysts reduce metal salts in aqueous solutions at ambient temperatures to provide metal nucleation and growth centers. Photocatalyst molecules are pre-positioned at the nanoscale to control the location and morphology of the metal nanostructures grown. Self-assembly, chemical confinement, and molecular templating are some of the methods used for nanoscale positioning of the photocatalyst molecules. When exposed to light, the photocatalyst molecule repeatedly reduces metal ions from solution, leading to deposition and the synthesis of the new nanostructures and nanostructured materials. Studies of the photocatalytic growth process and the resulting nanostructures address a number of fundamental biological, chemical, and environmental issues and draw on the combined nanoscience characterization and multi-scale simulation capabilities of the new DOE Center for Integrated Nanotechnologies, the University of New Mexico, and Sandia National Laboratories. Our main goals are to elucidate the processes involved in the photocatalytic growth of metal nanomaterials and provide the scientific basis for controlled synthesis. The nanomaterials resulting from these studies have applications in nanoelectronics, photonics, sensors, catalysis, and micromechanical systems. The proposed nanoscience concentrates on three thematic research areas: (1) the creation of nanoscale structures for realizing novel phenomena and quantum control, (2) understanding nanoscale processes in the environment, and (3) the development and use of multi-scale, multi-phenomena theory and simulation. Our goals for FY03 have been to understand the role of photocatalysis in the synthesis of dendritic platinum nanostructures grown from aqueous surfactant solutions under ambient conditions. The research is expected to lead to highly nanoengineered materials for catalysis mediated by platinum, palladium, and potentially other catalytically important metals. The nanostructures made also have potential applications in nanoelectronics, nanophotonics, and nanomagnetic systems. We also expect to develop a fundamental understanding of the uses and limitations of biomimetic photocatalysis as a means of producing metal and semiconductor nanostructures and nanomaterials. The work has already led to a relationship with InfraSUR LLC, a small business that is developing our photocatalytic metal reduction processes for environmental remediation. This work also contributes to science education at a predominantly Hispanic and Native American university