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A series of dimeric metalloporphyrin molecules has been synthesized in which the two porphyrin rings are constrained to lie parallel to one another by two amide bridges of varying length that link the rings together. These cofacial metalloporphyrins have been applied to the surface of graphite electrodes and tested for catalytic activity toward the electroreduction of dioxygen to water in aqueous acidic electrolytes. All molecules tested exhibited some catalytic activity, but hydrogen peroxide rather than water was the chief reduction product. However, the dicobalt cofacial porphyrin linked by four-atom bridges produced a catalyzed reduction almost exclusively to water and at exceptionally positive potentials. Rotating ring-disk voltammetric measurements were employed to diagnose the electrode reaction pathway and a possible mechanism for the observed catalysis is suggested. The results seem to demonstrate the participation of two metal centers in controlling the course of a multiple-electron process.

This paper focuses on a new method for measuring the diffusion coefficients of gases in room-temperature ionic liquids (RTILs) using a semi-infinite volume approach. Henry's constants were also measured under the same conditions as the diffusion coefficients. The relationship of the Henry's constants and diffusion coefficients with temperature is discussed along with the relationship of the diffusion coefficients with molecular size and viscosity. The RTIL used was ethylmethylimidazolium bis(trifluoromethanesulfonyl)amide ([emim])-([Tf 2 N]), and the gases used were ethane, ethene, propane, propene, and carbon dioxide.

Low energy separations for <1% CO2 gases would benefit gas treatment and CO2 sequestration. Theoretically, electrochemical pumping can separate and concentrate CO2 from the atmosphere or other gases with <1% CO2 at significantly lower energy cost than current systems. Principles of electrochemical pumping for CO2 separations are discussed and results for both organic solvent and ionic liquid working fluid systems are presented. Due to the large quantities of gases requiring processing during the separation/concentration of <1% CO2 gases, this work looked at solvents with negligible vapor pressures, specifically propylene carbonate and the room-temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate. Other important parameters, as illustrated by the data and models presented, are low CO2 solubility in the solvent, high CO2 carrier solubility, CO2 binding constants, and the CO2 carrier’s electrochemistry. Reported is the electrochemical pumping of CO2 from 0.5% (in nitrogen) to 100%, a 200-fold increase in partial pressure, using the CO2 carrier 2,6-di-tert-butyl-1,4-benzoquinone in a propylene carbonate solution. The ratio of CO2 moles pumped per electron mole was 0.43. The models determined the optimal CO2 solubility in the solvent and the required redox swing in the CO2 binding constants of the carrier. © 2003 The Electrochemical Society. All rights reserved.