Calibrating the Oxidative Capacity of Microdroplets.

Recently, redox reactions have been reported to occur spontaneously in microdroplets. The origins of such reactivity are still debated, and any systematic correlation of the yield with the reactivity of the reactant is yet to be established. We report the simple, outer-sphere, one-electron oxidation of a series of ferrocene derivatives spanning oxidation potentials from -0.1 V to +0.8 V vs. Ag/AgCl generated via nebulization and measured by mass spectrometry of the ferrocenium ions. The reaction environments and dynamics in the droplets are complex, and it is still unclear whether such reactivity correlates with bulk thermodynamic values. Our key finding is that the ion yields decrease monotonically with the oxidation potential of the ferrocenes, which is a thermodynamic quantity. The ion yields emphatically do not obey the Nernstian ratio, revealing the redox processes in the droplets do not follow the assumptions of bulk steady-state electrochemistry. Furthermore, oxidative competition in the mixture of several ferrocenes suggest a finite oxidative capacity or oxidant concentration. These results demonstrate that even though ion generation could be an out-of-equilibrium and kinetically limited process, the oxidative yield in microdroplets does correlate with thermodynamics, suggesting a possible free energy relationship between the kinetics and thermodynamics of the process.

Inferring the Energetics of CO2-Aniline Adduct Formation from Vibrational Spectroscopy.

Control of atmospheric CO2 is an important contemporary scientific and engineering challenge. Toward this goal, the reaction of CO2 with amines to form carbamate bonds is an established method for CO2 capture. However, controllable reversal of this reaction remains difficult and requires tuning the energetics of the carbamate bond. Through IR spectroscopy, we show that a characteristic frequency observed upon carbamate formation varies as a function of the substituent's Hammett parameter for a family of para-substituted anilines. We present computational evidence that the vibrational frequency of the adducted CO2 serves as a predictor of the energy of formation of the carbamate. Electron donating groups typically enhance the driving force of carbamate formation by transferring more charge to the adducted CO2 and thus increasing the occupancy of the antibonding orbital in the carbon-oxygen bonds. Increased occupancy of the antibonding orbital within adducted CO2 indicates a weaker bond, leading to a red-shift in the characteristic carbamate frequency. Our work serves the large field of CO2 capture research where spectroscopic observables, such as IR frequencies, are more easily obtainable and can stand in as a descriptor of driving forces.

Photoinduced Carbon Dioxide Release via a Metastable Photoacid in a Nonaqueous Environment.

Capturing carbon dioxide (CO2) from the atmosphere is a scientific and technological challenge. CO2 can be captured by forming carbamate bonds with amines, most notably monoethanolamine (MEA). Regenerating MEA by releasing captured CO2 requires that the carbamate solution be heated. Recently, photoacids were used to induce a pH change to release CO2 from aqueous carbonate solutions. We report a merocyanine photoacid that releases CO2 from nonaqueous carbamate solutions of MEA, which has a CO2 loading capacity that is higher than that of water. On the basis of the absorption spectra of the photoacid in the presence of acids and CO2, we show that the photoacid cycle and the CO2 capture of MEA are two separate equilibria coupled to each other via protons. We demonstrate that irradiating the sample with 405 nm light induces the release of CO2, which we detect using an in-line mass spectrometer. This work highlights an alternative path for optimizing a photoinduced CO2 capture and release system.