The scientific community now agrees that the rise in atmospheric CO(2), the most abundant green house gas, comes from anthropogenic sources such as the burning of fossil fuels. This atmospheric rise in CO(2) results in global climate change. Therefore methods for photochemically transforming CO(2) into a source of fuel could offer an attractive way to decrease atmospheric concentrations. One way to accomplish this conversion is through the light-driven reduction of carbon dioxide to methane (CH(4(g))) or methanol (CH(3)OH((l))) with electrons and protons derived from water. Existing infrastructure already supports the delivery of natural gas and liquid fuels, which makes these possible CO(2) reduction products particularly appealing. This Account focuses on molecular approaches to photochemical CO(2) reduction in homogeneous solution. The reduction of CO(2) by one electron to form CO(2)(*-) is highly unfavorable, having a formal reduction potential of -2.14 V vs SCE. Rapid reduction requires an overpotential of up to 0.6 V, due at least in part to the kinetic restrictions imposed by the structural difference between linear CO(2) and bent CO(2)(*-). An alternative and more favorable pathway is to reduce CO(2) though proton-assisted multiple-electron transfer. The development of catalysts, redox mediators, or both that efficiently drive these reactions remains an important and active area of research. We divide these reactions into two class types. In Type I photocatalysis, a molecular light absorber and a transition metal catalyst work in concert. We also consider a special case of Type 1 photocatalysis, where a saturated hydrocarbon links the catalyst and the light absorber in a supramolecular compound. In Type II photocatalysis, the light absorber and the catalyst are the same molecule. In these reactions, transition-metal coordination compounds often serve as catalysts because they can absorb a significant portion of the solar spectrum and can promote activation of small molecules. This Account discusses four classes of transition-metal catalysts: (A) metal tetraaza-macrocyclic compounds; (B) supramolecular complexes; (C) metalloporphyrins and related metallomacrocycles; (D) Re(CO)(3)(bpy)X-based compounds where bpy = 2,2'-bipyridine. Carbon monoxide and formate are the primary CO(2) reduction products, and we also propose bicarbonate/carbonate production. For comprehensiveness, we briefly discuss hydrogen formation, a common side reaction that occurs concurrently with CO(2) reduction, though the details of that process are beyond the scope of this Account. It is our hope that drawing attention both to current mechanistic hypotheses and to the areas that are poorly understood will stimulate research that could one day provide an efficient solution to this global problem.

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.

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.

The persistence of silver nanoparticles in aquatic environments and their subsequent impact on organisms depends on key transformation processes, which include aggregation, dissolution, and surface modifications by metal-complexing ligands. Here, we studied how cysteine, an amino acid representative of thiol ligands that bind monovalent silver, can alter the surface chemistry, aggregation, and dissolution of zero-valent silver nanoparticles. We compared nanoparticles synthesized with two coatings, citrate and polyvinylpirrolidone (PVP), and prepared nanoparticle suspensions (approximately 8 μM total Ag) containing an excess of cysteine (400 μM). Within 48 h, up to 47% of the silver had dissolved, as indicated by filtration of the samples with a 0.025-μm filter. Initial dissolution rates were calculated from the increase of dissolved silver concentration when particles were exposed to cysteine and normalized to the available surface area of nanoparticles in solution. In general, the rates of dissolution were almost 3 times faster for citrate-coated nanoparticles relative to PVP-coated nanoparticles. Rates tended to be slower in solutions with higher ionic strength in which the nanoparticles were aggregating. Xray absorption spectroscopy analysis of the particles suggested that cysteine adsorbed to silver nanoparticles surfaces through the formation of Ag(+I)sulfhydryl bonds. Overall, the results of this study highlight the importance of modifications by sulfhydrylcontaining ligands that can drastically influence the long-term reactivity of silver nanoparticles in the aquatic environment and their bioavailability to exposed organisms. Our findings demonstrate the need to consider multiple interlinked transformation processes when assessing the bioavailability, environmental risks, and safety of nanoparticles, particularly in the presence of metalbinding ligands.

The reactivity of reduced pyridinium with CO(2) was investigated as a function of catalyst concentration, temperature, and pressure at platinum electrodes. Concentration experiments show that the catalytic current measured by cyclic voltammetry increases linearly with pyridinium and CO(2) concentrations; this indicates that the rate-determining step is first order in both. The formation of a carbamate intermediate is supported by the data presented. Increased electron density at the pyridyl nitrogen upon reduction, as calculated by DFT, favors a Lewis acid/base interaction between the nitrogen and the CO(2). The rate of the known side reaction, pyridinium coupling to form hydrogen, does not vary over the temperature range investigated and had a rate constant of 2.5 M(-1)  s(-1). CO(2) reduction followed Arrhenius behavior and the activation energy determined by electrochemical simulation was (69±10) kJ mol(-1).