Talks presented in the symposium included:

• Homogeneous reduction of formic acid by pyridine: A key step in CO₂ reduction to CHOH (Lim et al.) One key obstacle in CO₂ reduction that is often overlooked is the reduction of formic acid (HCOOH), a common intermediate en route to CH₃OH production. Most CO₂ reduction catalysts successfully convert CO₂ to HCOOH or its deprotonated form formate (HCOO^-), but fail to achieve the subsequent reduction of HCOOH to ultimately produce CH₃OH.

  Lim et al. addressed this challenge by showing how competent catalysts, when synergistically combined with optimum reaction conditions, can accomplish HCOOH reduction. They used high-level CCSD(T) quantum chemical calculations to demonstrate a viable concerted proton-coupled electron transfer (PCET) mechanism for the homogeneous reduction of formic acid (HCOOH) by pyridinium radical (PyH⁰) and hydronium ion (H₃O⁺).

  The predicted barrier (∆H⁰_act) for concerted PCET to convert HCOOH to PyH⁺•CH(OH)₂⁰ is 10.5 kcal/mol. PyH⁺•CH(OH)₂⁰ can then dissociate and complex with pyridine to form Py•CH(OH)₂⁰, which undergoes condensation and ultimately forms Py•OCH⁰, a proposed intermediate in formaldehyde formation. The highest barrier step for HCOOH reduction is the condensation reaction which results in C-O bond cleavage with ∆H⁰_act = 14.9 kcal/mol. The electron affinity and pKa quantitatively show that concerted PCET is favored over sequential PCET in the PyH⁰ catalyzed reduction of HCOOH due to the high cost of localizing electron density on HCOOH and deprotonating PyH⁰.

• CO₂ chemistry: Catalytic transformation of carbon dioxide based on its activation (He et al.) The challenge with the use of CO₂ as a feedstock for commodity chemicals, fuels and materials is to develop catalysts that are capable of activating CO₂ under low pressure (preferably at 1 atm), and thus incorporating CO₂ into organic molecules catalytically. CO₂ could be activated through the formation of carbamate/alkyl carbonate with Lewis basic nitrogen species.

  Lim et al. discussed and updated advances on C-N bond formation with the production of oxazolidinones, quinazolines, carbamates, isocyanates and polyurethanes using CO₂ as C₁ feedstock, and CO₂ capture by amino-functionalized ionic liquids presumably leading to CO₂’s activation and thus subsequent conversion through C-N bond formation pathway.

• Molecular catalysts for the reduction of CO₂ to CO or formate (Appel et al.). The efficient reduction of CO₂ requires the development of new catalysts for the interconversion of this substrate and the corresponding fuels. Inspiration from nature can provide a starting point for the design of catalysts through the incorporation of bifunctional interactions.

  Lim et al. presented the latest results from their lab for the catalytic reduction of CO2 using molecular complexes for both hydrogenation and electrocatalytic reduction, as well as related transformations, with a focus on balancing reaction energetics. For the electrocatalytic reduction of CO₂, the emphasis was on analogs of previously reported palladium triphosphine complexes, and for the hydrogenation of CO₂, they presented their efforts in using complexes of first row transition metals.

• Catalytic activation of CO₂ over lanthanum zirconate (La₂Zr₂O₇) pyrochlores and its role in dry (CO₂) reforming of CH₄ (Pakhare et al.) Dry reforming of methane (DRM) is an endothermic reaction and the catalysts used for studying this reaction are unsubstituted lanthanum zirconate (LZ), Rh substituted LRhZ (2 wt%) and LRhZ (5 wt%) pyrochlores.

  In-situ Fourier transform infrared spectroscopy (FTIR) studies in CO₂ and CH₄ were conducted on these three catalysts. The FTIR spectra obtained were then analyzed for the formation of carbonates and formates during DRM reaction.

  Lanthanum oxide (La₂O₃) and zirconium oxide (ZrO₂) were used as standards for analyzing the carbonate formation over the pyrochlores. The researchers observed that the absorption spectra of the lanthanum oxide carbonates were very similar to carbonates formed on LZ. There were no observable carbonates formed on zirconia. This suggests that the active site for activation of CO2 is the lanthanum phase of LZ. Lanthanum oxide forms carbonate which then oxidize activated CH₄ during the DRM reaction.

• Electrocatalytic reduction of CO₂ to CO by monodisperse Au nanoparticles (Zhu et al.) Converting CO₂ to active carbon forms, such as CO, formic acid, methanol and other hydrocarbons is considered an essential approach to sustainable use of fuels/chemicals. Recent studies have centered on CO₂ capture, secure storage, and chemical conversions.

  Among various chemical reactions studied, electrochemical reduction is considered a potentially efficient way to convert CO₂ selectively into CO or other hydrocarbon fuels over metal electrodes. Among all metals tested for the electrochemical reduction of CO₂, gold is the most attractive one for its catalytic reduction of CO₂ to CO.

  The researchers presented the electrochemical reduction of CO₂ to CO with high Faradic efficiency by monodisperse Au nanoparticles. The reduction reaction was tested in aqueous solution of potassium bicarbonate. The working electrode was prepared by the deposition of carbon supported Au NPs over the carbon paper via polyvinylidene fluoride.

  The electrocatalysis was performed at different potentials (-0.3 V to -0.9 V vs. RHE) and the reduction was found to be Au NP size-dependent. With all Au NPs tested, only CO was detected as the gas product while a very low amount of formic acid was found in the liquid phase product analysis. The highest Faradic efficiency of 90% was calculated for the 8 nm Au NPs catalyst at -0.6 V. The studies showed that Au NPs when properly synthesized and activated are selective for electrochemical reduction of CO₂ to CO.