DOI: 10.1002/cssc.201000379 Electrocatalytic Carbon Dioxide Activation: The Rate- Determining Step of Pyridinium-Catalyzed CO 2 Reduction Amanda J. Morris, Robert T. McGibbon, and Andrew B. Bocarsly* [a] Introduction Carbon dioxide, a potent greenhouse gas, is thought to be a main contributor to global climate change. [1] The concentration of atmospheric CO 2 is increasing; this is due in part to anthro- pogenic sources, including the burning of fossil fuels. [2] There- fore, the mitigation of industrial carbon dioxide emissions has become a priority, and much research is being dedicated to its efficient capture and sequestration. Present discussion appears to favor storage in underground geologic structures as an option for carbon mitigation. However, another approach that may be advantageous is the chemical conversion of carbon di- oxide to a value-added product. One potential approach that has garnered recent industrial interest is the formation of a chemical fuel, for example, an alcohol, starting with a CO 2 feedstock. [3–4] Bocarsly et al. reported the low overpotential conversion of CO 2 to methanol at hydrogenated palladium electrodes using pyridinium as a homogeneous catalyst in the 1990s. [5] That work was recently expanded to include photoelectrochemical CO 2 reduction at illuminated p-GaP electrodes. [6] The Faradaic efficiency for conversion of CO 2 to methanol neared 100 % when the p-GaP electrode was held at an underpotential of 220 mV (based on the standard reduction potential for CO 2 conversion to methanol by six electrons and protons, E o = 0.52 V vs. SCE at pH 5.3). To date, the pyridinium-catalyzed reduction of CO 2 is the most efficient reported method for photoelectrochemical methanol formation. Recent mechanistic investigations have suggested key inter- mediates involving the formation of pyridyl N C bonds in the overall reaction scheme. [7] These species are responsible for the reduction of CO 2 through a series of one-electron transfer steps. This is in direct contrast to previously reported CO 2 re- duction catalysts that are believed to operate by multielectron charge-transfer (MET) pathways. Multiple-electron-transfer cat- alysis can offer an immense advantage over single-electron- transfer pathways. For example, the reduction of CO 2 by one electron to form the CO 2 C  requires 2.14 V versus SCE. If in- stead CO 2 accepts six reducing equivalents and protons to form methanol this can occur at the more moderate potential of 0.62 V versus SCE (pH 7). Pyridinium-catalyzed reduction of CO 2 is the first case, to the best of our knowledge, in which six sequential one-electron proton-coupled transfers provide the low-energy route for catalysis. This is significant because many could have historically overlooked one-electron-transfer catalysts for traditional multiple-electron-transfer catalysis, that is, proton reduction, water oxidation, and carbon dioxide re- duction. Herein, we report the dependence of the aforementioned reactivity on catalyst concentration, CO 2 pressure, and temper- ature. The measurement of the reaction rate as a function of these physical properties reveals further mechanistic insight. Of particular importance is the determination of the overall rate-determining step. Although the ultimate aim of this re- search is the efficient light-driven reduction of CO 2 at a semi- conductor electrode, the complex electrochemical behavior of the reduction at such an interface precludes direct mechanistic studies. To overcome this complication, CO 2 reactivity was in- vestigated at platinum electrodes. The applicability of the mechanistic information gathered in this study to reactivity at illuminated p-GaP electrodes is discussed. In addition to funda- mental implications, this study examines the electroreduction of CO 2 under a set of conditions that may be of importance for the development of an industrial CO 2 conversion process based on this technology. The reactivity of reduced pyridinium with CO 2 was investigated as a function of catalyst concentration, temperature, and pres- sure at platinum electrodes. Concentration experiments show that the catalytic current measured by cyclic voltammetry in- creases 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 investi- gated and had a rate constant of 2.5 m 1 s 1 . CO 2 reduction fol- lowed Arrhenius behavior and the activation energy deter- mined by electrochemical simulation was (69  10) kJ mol 1 . [a] Dr. A. J. Morris, R. T. McGibbon, Prof. A. B. Bocarsly Department of Chemistry Princeton University Princeton, NJ 08536 (USA) Fax:(+1) 609-258-2902 E-mail : bocarsly@princeton.edu ChemSusChem 2011, 4, 191 – 196  2011 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim 191