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The mechanism by which pyridinium (pyrH(+)) is reduced at a Pt electrode is a matter of recent controversy. The quasireversible cyclic voltammetric wave observed at -0.58 V vs SCE at a Pt electrode was originally proposed to correspond to reduction of pyrH(+) to pyridinyl radical (pyrH(•)). This mechanistic explanation for the observed electrochemistry seems unlikely in light of recent quantum mechanical calculations that predict a very negative reduction potential (-1.37 V vs SCE) for the formation of pyrH(•). Several other mechanisms have been proposed to account for the discrepancy in calculated and observed reduction potentials, including surface adsorption of pyrH(•), reduction of pyrH(+) by two electrons rather than one, and reduction of the pyrH(+) proton to a surface hydride rather than a π-based radical product. This final mechanism, which can be described as inner-sphere reduction of pyrH(+) to form a surface hydride, is consistent with experimental observations.

The environmental and societal consequences of the increasing levels of carbon dioxide in our atmosphere are among the most significant challenges society currently faces. Carbon dioxide utilization, in which carbon dioxide is either used directly or converted into more valuable products, is likely to be one component of a broad strategy to reduce carbon dioxide emissions, a challenge that will require both technological and policy changes. Catalysis is crucial to the successful conversion of carbon dioxide into value-added products. Here, we provide a review on chemical and biological systems for carbon dioxide conversion directed toward the readers of ACS Catalysis, which focuses on providing a general perspective on the field, rather than technical details. We discuss both challenges related to the conversion of carbon dioxide into specific products such as carbon monoxide, formic acid, methanol, methane, ethylene, fuels, carboxylic acids, and polymers as well as general challenges for the field. We also compare and contrast different methods for carbon dioxide conversion, for example homogeneous versus heterogeneous catalysis or photosynthetic versus nonphotosynthetic biological conversion, and highlight areas where one approach may have advantages over another. In a concluding section, we identify problems related to carbon dioxide conversion that will need to be addressed for technology to be both viable and reduce carbon dioxide emissions.

In this study, FTIR spectroscopy is used to investigate surface reactions of carbon dioxide at the adsorbed water−oxide interface. In particular, FTIR spectra following CO2 adsorption in the presence and absence of coadsorbed water on hydroxylated nanoparticulate Fe2O3 and γ-Al2O3 at 296 K are reported. In the absence of coadsorbed water, CO2 reacts with surface O−H groups to form adsorbed bicarbonate on the surface. In the presence of coadsorbed water, this reaction is blocked as water hydrogen bonds to the reactive M−OH sites. Instead, CO2 reacts with adsorbed water to yield adsorbed carbonate and protonated surface hydroxyl groups, M−OH2 +, through a proposed carbonic acid intermediate. The carbonate spectra recorded between 10 and 90% RH are nearly identical to that of carbonate adsorbed on these surfaces in the presence of the liquid water. FTIR isotope studies show that there is extensive exchange between oxygen in adsorbed water and oxygen atoms in both adsorbed carbonate and gas-phase carbon dioxide. On the basis of these experimental results along with quantum chemical calculations, a mechanism is proposed for surface reactions of carbon dioxide at the adsorbed water−oxide interface.

The chemistry of the electrocatalytic reduction of CO 2 using nitrogen containing heteroaromatics is further explored by the direct comparison of imidazole and pyridine catalyzed CO 2 reduction at illuminated iron pyrite electrodes. The mechanism of imidazole based catalysis of CO 2 reduction is investigated by analyzing the catalytic activity of a series of imidazole derivatives using cyclic voltammetry. While similar product distributions are obtained for both imidazole and pyridine, the imidazole catalyzed reduction of CO 2 likely proceeds via a very different mechanism involving the C2 carbon of the imidazole ring.

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Recent electrochemical studies have reported aqueous CO 2 reduction to formic acid, formaldehyde and methanol at potentials of ca. -600 mV versus SCE, when using a Pt working electrode in acidic pyridine solutions. In those experiments, pyridinium is thought to function as a one-electron shuttle for the underlying multielectron reduction of CO 2 . DFT studies proposed that the critical step of the underlying reaction mechanism is the oneelectron reduction of pyridinium at the Pt surface through proton coupled electron transfer. Such reaction forms a H adsorbate that is subsequently transferred to CO 2 as a hydride, through a proton coupled hydride transfer mechanism where pyridinium functions as a Brønsted acid. Here, we find that imidazolium exhibits an electrochemical behavior analogous to pyridinium, as characterized by the experimental and theoretical analysis of the initial reduction on Pt. A cathodic wave, with a cyclic voltammetric half wave potential of ca. -680 mV versus SCE, is consistent with the theoretical prediction based on the recently proposed reaction mechanism suggesting that positively charged Brønsted acids could serve as electrocatalytic oneelectron shuttle species for multielectron CO 2 reduction.

A recently proposed mechanism for electrochemical CO 2 reduction on Pt (111) catalyzed by aqueous acidic pyridine solutions suggests that the observed redox potential of ca. −600 mV vs. SCE is due to the one-electron reduction of pyridinium through proton coupled electron transfer (PCET) to form H atoms adsorbed on the Pt surface (H ads ). The initial pyridinium reduction was probed isotopically via deuterium substitution. A combined experimental and theoretical analysis found equilibrium isotope effects (EIE) due to deuterium substitution at the acidic pyridinium site. A shift in the cathodic cyclic voltammetric half wave potential of −25 mV was observed, consistent with the theoretical prediction of −40 mV based on the recently proposed reaction mechanism where pyridinium is essential to establish a high concentration of Brønsted acid in contact with the substrate CO 2 and with the Pt surface. A prefeature in the cyclic voltammogram was examined under isotopic substitution and indicated an H ads intermediate in pyridinium reduction. Theoretical prediction and observation of an EIE supported the assignment of the cathodic wave to the proposed reduction of pyridinium through PCET forming H ads and eventually H 2 on the Pt surface.