Kinetic studies of the electrolytic reduction of carbon dioxide on the mercury electrode☆
Abstract
The reduction of CO2 to HCOOH has been studied for the Hg electrode in neutral and acidic aqueous solutions in the potential range between −0.8 and −1.9 V(sce). In the neutral pH range all the current is consumed in the production of formic acid, while in acid solutions both HCOOH and H2 are produced. Steady-state polarization curves, cathodic galvanostatic charging curves. current-efficiency measurements, reaction orders with respect to CO2 partial pressure, and double-layer variation have been used to determine possible reaction pathways. In neutral solutions the mechanism may be described by a direct reduction of CO2 in which two consecutive charge-transfer steps occur. In acidic solutions the above process occurs in parallel with the reduction of H3O+ ions. The H atoms formed react in a branching mechanism either with H3 O+ ions and electrons to yield H2, or with CO2 to yield eventually HCOOH.
Résumé
Etude de la réduction de CO2 en HCOOH an moyen de l'électrode de Hg en solutions aqueuses neutres et acides, dans un domaine de potentiel compris entre −0,8 et 1,0 V (ecs). Dans le domaine de pH neutre, tout le courant est consommé pour la production d'acide formique, tandis qu'en solution acide il y a formation conjointement de HCOOH et H2. Les courbes de polarisation à vitesse stationnaire, les courbes galvanostatiques d'evolution cathodique, les mesures de rendement de courant, les ordres des réactions par rapport à la pression partielle de CO2 et la variation de la couche double, ont été utilisés en vue de déceler les mécanismes réactionnels possibles. Dans des solutions neutres, le mécanisme peut être interprété par une réduction directe de CO2, pour laquelle apparaissent deux étapes de transfers de charges successives. En solution acide, le processus ci-dessus apparaît parallèlement à la réduction des ions H3 O+. Les atomes d'H formés réagissent suivant un mécanisme en chaîne, soit pour donner H2 avec les ions H3O+ et les électrons, soit pour former éventuellement HCOOH avec CO2.
Zusammenfassung
Die Reduktion von CO2 zu HCOOH an einer Hg-Elektrode in neutraler und saurer Lösung wurde im Potentialbereich zwischen −0,8 und −1,9 V (ges. Kalomelelektrode) untersucht. Im neutralen Gebiet bildet sich nur Ameisensäure, in saurer Lösung jedoch entsteht auch noch Wasserstoff. Gleichgewichts-Polarisationskurven, galvanostatische kathodische Ladungskurven, Messungen der Stromausbeute, Reaktionsordnungen im Bezug auf den Partialdruck von CO2 und Doppelschichtvariationen wurden zur Bestimmung möglicher Reaktionswege verwendet. In neutraler Lösung kann der Mechanismus als direkte Reduktion von CO2 mit zwei aufeinanderfolgenden Ladungsübertragungen beschrieben werden. In saurer Lösung läuft parallel zum obigen Mechanismus die Reduktion von H3O+-Ionen ab. Die entstehenden Wasserstoffatome reagieren einerseits mit H3O +-Ionen und Elektronen und geben H2, oder mit CO2, wobei dann HCOOH entsteht.
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Manuscript received 10 June 1968.
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Present address: John Harrison Laboratory, University of Pennsylvania, Philadelphia, Pa., U.S.A.