Electro-oxidation of small organic molecules : kinetic instabilities and spatiotemporal pattern formation
저자
발행사항
Berlin : Freien Universitat Berlin, 2001
학위논문사항
Thesis(doctoral)-- Freien Universitat Berlin: Biologie chemie und pharmazie 2001
발행연도
2001
작성언어
영어
주제어
KDC
431.47 판사항(4)
형태사항
180, vip. : Illustrations ; 26cm .
일반주기명
Bibliographical references: p. 171-179
소장기관
The electrocatalysed oxidation of small organic molecules (mostly C_(1) compounds) has seen a resurgence of interest in the search for suitable anodic reactions in devices for direct electrochemical energy conversion. Common to all reactions is the tendency towards self-poisoning of the reactive surface, mostly by carbon monoxide, and the existence of parallel reaction channels. Of all the striking features of the electrocatalytic oxidation reactions of organic C_(1) fuels, their tendency to exhibit self-organised and spatiotemporal instabilities at higher overpotentials or current loads is perhaps the most remarkable. Thus bistability, oscillations as well as standing and travelling waves were obtained.
In Chapter 3, to assist in the basic understanding of electrocatalytic oxidation of C_(1) molecules, CO oxidation on the Ru (0001) surface was investigated. In order to compare the electrocatalytic activity of rough Ru (0001) with that of a flat surface, cyclic voltammetry and current-time transient experiments were performed. Unlike the case of flat Ru (0001), a broader and featureless cyclic voltammetry (CV) curve was obtained on the rough surface. Three subsequent CVs on the flat Ru electrode are needed for complete oxidation of CO_(ad), while on rough Ru, a single oxidation peak initiated at lower overpotential of + 0.17 V gives rise to complete stripping of all CO_(ad). Current-time transients at different electro-oxidation potentials indicated that a rough Ru (0001) surface could decrease the oxidation overpotential by ca. 550 mV compared with the flat surface. A comparable increase of activity of the rough surface was obtained in HCOOH oxidation. Summarising the experimental data, this reaction can be considered as a reference for the development of a more effective catalyst for CO tolerance in a hydrogen fuel cell.
Chapter 4 focused on nonlinear behaviour such as spontaneous oscillations of current or potential, complex and chaotic oscillations, associated bifurcation scenarios and spatial pattern formation, which occurs when the system is maintained far from thermodynamic equilibrium. The electrocatalytic oxidation of HCOOH on Bi/Pt ringspontaneously underwent transitions from homogeneous catalytic activity to spatiotemporally inhomogeneous distributions of the interfacial electrode potential.
The pattern formation resulted from hybrid effects of the nonlinear chemistry during HCOOH oxidation and the long-range coupling of the interfacial potential, the form of which is determined by the chosen geometry (ring type) of the working electrode. With negative coupling conditions, the anti-phase dynamics (standing waves or pulses) could prevent complete poisoning of the electrode, since passivation at some point on the electrode would enhance the catalytic activity at a remote location. This generally results in a prolonged catalytic reactivity of the electrode compared with stationary operation conditions. Propagating pulses on the Pt ring were externally perturbed via a trigger electrode at one location of the ring. While usual phase resetting was obtained for small perturbation amplitudes, stronger perturbations resulted in reversal of the direction of pulse motion when applied behind the pulse over a wide phase interval.
Whether in-phase and anti-phase oscillations of the double layer potential were selected strongly depended on the reference electrode distance parameter β, which was defined as the ratio of the distance between the working electrode and the reference electrode to the outer radius of the ring-shaped working electrode. In-phase homogeneous oscillations were obtained at β ≥ 0.7, due to the positive global coupling, while anti-phase inhomogeneous patterns in the form of travelling pulses were observed at β ≤ 0.25, due to negative nonlocal coupling. Within an intermediate reference position range from β ∼ 0.70 down to β ∼ 0.3, there exist complex patterns and their theoretical interpretations are still unclear.
The mechanistic origin of electrochemical oscillations of HCOOH on Bi/Pt was discussed. Although CO formation is largely suppressed, a hidden negative differential resistance (HNDR), most likely due to metallic Bi, was identified from the electrochemical impedance spectra near the onset of potential oscillations, while a manifest NDR, probably due to adsorbed oxygen species, appeared to be responsible for the oscillations at higher overpotentials on the anodic scan in the electrocatalytic oxidation of HCOOH.
Chapter 5 dealt with experimental observations of edge effects on a Pt ribbon electrode in the electrocatalytic oxidation of HCOOH, predicted theoretically. The spatiotemporal pattern formation under bistable and oscillatory conditions were investigated on pure Pt and on a Bi-modified Pt electrode, respectively. On a thin ribbon, pattern formation along the short axis can be neglected. Theoretical calculation by J. Christoph [27] predicted that there would be in-phase and anti-phase edge oscillations due to the positive and the negative long-distance coupling, respectively. This was clearly confirmed by experimental measurements; i.e., inhomogeneous and homogeneous spatiotemporal patterns were observed at low and high β (β was defined as the ratio of the distance between WE and RE electrode to the length of ribbon WE).
Under bistable conditions, theory predicts that the local function of effective spatial conductivity diverges at the edges of the electrode. In other words, much smaller spatial resistance is observed at the two edges of the ribbon electrode, and experimentally it resulted in both higher migration current and higher double layer potential at the two edges compared with the centre.
In Chapter 6, experimental observations of the temporal dynamics in the electrocatalytic oxidation of methanol (CH₃OH) on pure and Ru-modified Pt electrodes were reported. Hidden negative differential resistance (HNDR) and instabilities of the system were investigated by means of an electrochemical impedance spectrum analysis, potential oscillations under galvanostatic control and current oscillations with the application of an appropriate external resistance. The Hopf bifurcation shifted to lower potential with Ru modification. On the ring, no spatial instabilities were obtained.
Many patterns observed in electrocatalysis have been well modelled and successfully proven by experiments. This could increase our understanding of spatiotemporal dynamics in many other systems where migration coupling plays a crucial role.
Die elektrokatalytische Oxidation kleiner organischer Molekule (uberwiegend C_1-Verbindungen) hat in letzter Zeit verstarktes Interesse gefunden, insbesondere in Zusammenhang mit der Suche nach geeigneten anodischen Reaktionen in Brennstoffzellen. Die verschiedenen undersuchten Reaktionen weisen einige Gemeinsamkeiten auf: So neigen sie zur Vergiftung der Elektrode, vorwiegend durch Bildung adsorbierten Kohlenmonoxids, und verlaufen uber verzweigte Reaktionswege. Diese mechanistischen Eigenschaften bedingen auch die wohl hervorstechendste Eigenschaft der elektrokatalytischen Oxidationen von C_1-Verbindungen, namlich ihre Tendenz zu selbstorganisierten raumzeitlichen Instabilitaten bei erhohter uberspannung bzw. ausreichend hohem Strom, welche zu Bistabilitat, Oszillationen sowie stehenden und laufenden Wellen fuhrten.
Um zum grundlegenden Verstandnis der Elektro-Oxidation von C_1-Molekulen beizutragen, wurde zunachst die CO-Oxidation auf Ru (0001) untersucht (Kapitel 3). Die elektrokatalytische Aktivitat einer glatten und rauhen Ru (0001)-Oberflache wurde durch Zyklovoltammetrie und Messung von Strom-Transienten verglichen. Im Gegensatz zur glatten Oberflache wies der aufgerauhte Kristall ein breites und strukturloses Zyklovoltammogramm auf. Die glatte Flache benotigte 3 Zyklen zur vollstandigen Oxidation von praadsorbiertem CO, wahrend auf der rauhen Flache alles CO in einem einzigen breiten Oxidationspeak ab + 0.17 V entfernt wurde. Die Strom-Transienten bei konstantem Potential zeigten, daß durch Aufrauhung der Oberflache die uberspannung um ca. 550 mV gesenkt werden konnte. Eine vergleichbare Aktivitatssteigerung ergab sich fur die HCOOH-Oxidation. Die Ergebnisse konnten fur die Entwicklung von CO-toleranten Elektrodenmaterialien in Brennstoffzellen relevant sein.
Kapitel 4 konzentrierte sich auf nichtlineare Phanomene in der Ameisensaure-Oxidation auf einem Bi-modifizierten Pt-Ring, wie Strom- oder Potentialoszillationen, komplexe Oszillationen, das zugehorige Bifurkationsverhalten und raumliche Musterbildung.
Die Musterbildung resultiert aus dem Zusammenspiel von nichtlinearer Chemie und langreichweitiger Migrationskopplung, die durch die Geometrie (Ring) gegeben ist. Bedingungen negativer Kopplung fuhrten zu Antiphasen-Dynamik (stehende Wellen oder Pulse), welche eine vollstandige Vergiftung der Elektrode verhindert, da Passivierung an einer Stelle erhohte Aktivitat gegenuber zur Folge hat. Dies fuhrt generell zu langerer katalytischer Reaktivitat der Elektrode im Vergleich zu homogenem stationaren Betrieb.
Durch eine Trigger-Elektrode an einer Stelle des Rings wurde ein rotierender Puls gestort. Bei niedriger Storamplitude ergab sich eine Neueinstellung der Phase (analog zum "phase resetting" bei Grenzzyklen), starkere Storungen ermoglichten es, die Ausbreitungsrichtung des Pulses umzukehren.
Das Auftreten homogener bzw. antiphasiger Oszillationen hing stark vom Entfernungsparameter β der Referenzelektrode ab (definiert als Verhaltnis des Abstandes zwischen Arbeits- und Referenzelektrode und außerem Radius der Arbeitselektrode). Fur β ≥ 0.7 wurden aufgrund positiver globaler Kopplung ausschließlich homogene Oszillationen erhalten, sich ausbreitende Pulse dominierten bei β ≤ 0.25 als Folge negativer langreichweitiger Kopplung. Dazwischen (0.3 < β < 0.7) traten komplexe Muster auf, deren theoretische Beschreibung noch unklar ist.
Der den Oszillationen in der HCOOH-Oxidation auf Bi/Pt zugrundeliegende Mechanismus wurde diskutiert. Obwohl die CO-Bildung weitgehend unterdruckt war, wurde aus den Impedanzspektren ein versteckter negativer differentieller Widerstand (HNDR) identifiziert, der vermutlich auf metallisches Wismut zuruckzufuhren ist. Ein manifester negativer differentieller Widerstand, wahrscheinlich hervorgerufen durch Adsorption von O-haltigen Spezies, durfte fur die Oszillationen bei hoheren uberspannungen verantwortlich sein.
Kapitel 5 behandelte die experimentelle Untersuchung von theoretisch vorhergesagten Randeffekten anhand einer streifenformigen Pt-Elektrode bei der HCOOH-Oxidation. Die raumzeitliche Musterbildung wurde im bistabilen und oszillatorischen Bereich an Pt und Bi-modifiziertem Pt untersucht. Auf den schmalen Streifen konnte Musterbildung entlang der kurzen Achse vernachlassigt werden. Von J. Christoph [27] vorhergesagte synchronisierte bzw. antiphasige Randoszillationen bei positiver bzw. negativer langreichweitiger Kopplung wurden experimentell bei großem bzw. kleinem Abstand zwischen Arbeits- und Referenzelektrode bestatigt.
Im bistabilen Bereich sagte die Theorie Divergenz der effektiven Leitfahigkeit am Elektrodenrand vorher. Der stark herabgesetzte Widerstand zum Rand hin zeigte sich im Experiment in hoheren Stromen und erhohtem Doppelschichtpotential.
Kapitel 6 widmete sich zeitlichen Instabilitaten in der elektrokatalytischen Oxidation von Methanol (CH₃OH) auf reinem bzw. Ru-modifiziertem Pt. Ein versteckter negativer differentieller Widerstand (HNDR), nachgewiesen durch Impedanzmessungen, fuhrte zu galvanostatischen Potentialoszillationen sowie zu potentiostatischen Oszillationen bei Verwendung eines ausreichend großen externen Widerstands. Die Hopf-Bifurkation verschob sich durch Ru-Modifikation zu niedrigerem Potential. Auf der Ring-Elektrode ergaben sich keine raumlichen Instabilitaten.
Zahlreiche raumzeitliche Muster in elektrokatalytischen Systemen sind mittlerweile experimentell untersucht und theoretisch mit Hilfe von Reaktions-Migrations-Gleichungen verstanden. Dies sollte auch zu einem besseren Verstandnis der Dynamik anderer (z.B. biologischer) Systeme fuhren, in denen Migrationskopplung eine entscheidende Rol
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