Oscillatory instabilities during the electrocatalytic oxidation of methanol on platinum

Abstract

Descreve-se neste artigo a observação experimental de dinâmica oscilatória durante a oxidação eletrocatalítica de metanol sobre platina. Além das, previamente relatadas, oscilações de potencial, oscilações de corrente obtidas sob controle potenciostático também são apresentadas. A região de existência de oscilações de corrente é mapeada no plano de bifurcação voltagem aplicada x resistência. Conjuntamente com investigações eletroquímicas, espectroscopia FTIR in situ também foi aplicada nestes estudos. Apesar de não ter sido possível acompanhar eventuais variações de intermediários reacionais durante as oscilações, tais experimentos revelaram que a cobertura média de monóxido de carbono permanece consideravelmente alta durante as oscilações. Os resultados são discutidos e comparados com as oscilações observadas na eletrooxidação de ácido fórmico, um sistema cujo comportamento é mais entendido e amplamente fundamentado por dados espectroscópicos obtidos in situ.It is described in this paper the experimental observation of oscillatory dynamics during the electrocatalytic oxidation of methanol on platinum. Besides the previously reported potential oscillations, current oscillations obtained under potentiostatic control are also presented. The existence region of current oscillations is mapped in an applied voltage x resistance bifurcation diagram. Conjointly with electrochemical investigations, in situ FTIR spectroscopy was also employed in the present studies. Although we were not able to follow eventual intermediate coverage changes during the oscillations, those experiments revealled that the mean coverage of adsorbed carbon monoxide remains appreciably high along the oscillations. Results are discussed and compared with the oscillations observed in the electrooxidation of formic acid, a system whose behavior is more understood and widely supported by in situ spectroscopic data. Keywords: current and potential oscillations, methanol, electrocatalysis, in situ FTIR IntroductionMethanol has been pointed as one of the most promising organic molecules to be used in large scale energy conversion systems, as in the so-called direct methanol fuel cells (DMFC). [1][2][3] Limitations associated to the high overpotential and the existence of parallel reaction pathways makes its understanding a rather challenging task. Studies have been carried out mainly on platinum surfaces, both polycrystalline and single crystals, by means of different electrochemical approaches sometimes coupled to other in situ and on line techniques. [4][5][6][7][8][9][10][11][12][13][14][15][16] In most of these reports, the focus stands on the electrocatalytic aspects of the methanol oxidation and encompasses questions such as the relationship between reaction rate and applied potential, the impact of the interfacial structure on reaction rate and selectivity, and the nature and geometry of adsorbates, to list a few. By far less studied however are the issues related to the complex kinetic aspects associated to the electrooxidation of methanol.In o...

Mechanism and model of the oscillatory electro-oxidation of methanol

“…[3][4][5][6][7][8][9][10] An important intermediate is HCO which has been shown to occupy three adjacent sites. It can be further dehydrogenated to CO, which subsequently reacts with adsorbed oxygen species ͑OH or O͒ to form CO 2 .…”

“…A promising candidate as energy source is methanol for so-called direct methanol fuel cells 2 on which numerous mechanistic and kinetic studies have been carried out. [3][4][5][6][7][8][9][10] The oxidation of MeOH on noble metal electrodes ͑Pt or Pt/Ru͒ is known to exhibit kinetic instabilities, e.g., under galvanostatic conditions the process exhibits kinetic oscillations as well as a hard transition to high potential. 4 Such dynamic instabilities can be exploited for model development as will be shown in the present study where we identify the core mechanism explaining the experimentally observed bifurcation sequence.…”

The dual pathway in action: decoupling parallel routes for CO2 production during the oscillatory electro-oxidation of methanol

“…25,26 As in the case of many small organic molecules, 27 the electro-oxidation of methanol is known to display potential/ current oscillations under some circumstances. [28][29][30][31][32][33][34][35][36][37] The main aspect to be pointed out in Fig. 3 is certainly the presence of additional peaks for the CO 2 production for each potential oscillation.…”

Complex Oscillatory Response of a PEM Fuel Cell Fed with H[sub 2]/CO and Oxygen

“…[1][2][3] Particularly in the case of low temperature fuel cell relevant reactions, such as the catalytic electrooxidation of hydrogen and small organic molecules, oscillatory dynamics seems to be a rule rather than an exception. [4][5][6][7][8][9][10][11] Current and potential oscillations during the electro-oxidation of hydrogen have been very often observed since the first half of the last century. [12][13][14][15][16][17][18][19] The current understanding of this reaction is that oscillations arise due to the overlap between two potential-dependent adsorption isotherms.…”