Converting CO2 to usable fuels was the topic of a symposium—CO2 Conversion: Thermo-, Photo- and Electro-Catalytic—on Sunday at the 246th National Meeting & Exposition of the American Chemical Society in Indianapolis, Indiana. (ACS is the world’s largest scientific society.)
Converting CO2 into a renewable energy sources would involve capturing the gas from the smokestacks of coal-fired electric power generating stations, for instance, and processing it with catalysts or other technology into fuels and raw materials for plastics and other products.
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 (∆H0act) 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 ∆H0act = 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.