Modulation of oscillatory CO oxidation on platinum using electrochemical promotion

L. LIZARRAGA; M. GUTH; A. BILLARD; P. VERNOUX

Toronto

Conferencia; 17th Conference on Solid State Ionics; 2009

The International Society for Solid State Ionics

Introduction The oxidation of carbon monoxide on platinum is one of the most studied catalytic reactions. Oscillations in the reaction rate have been observed over a wide range of pressures from 10-6 mbar to 1 bar [1]. Those under high pressure have been described by the oxide model [2]. The latter assumes that part of the active surface is covered by chemisorbed oxygen which is transformed into an inactive state as oxide. The electrochemical promotion of catalysis (EPOC) is based on the electrochemical pumping of ions by an applied potential between a solid electrolyte (for example: Y2O3-stabilized ZrO2, YSZ) and the surface of a porous catalyst (Pt) [3]. In the present work, this technique was used in order to modify the catalytic surface by pumping O2- ions from or to it [4]. It was possible to start or finish the oscillatory oxidation of CO when electrical currents were applied. Experimental Electrochemical catalysts were prepared from a polycrystalline Pt film deposited by DC-magnetron sputtering at low pressure onto solid electrolyte pellets (17 mm diameter and 1.5 mm thickness) composed by 8% mol Y2O3- stabilized ZrO2 [4]. The Pt film obtained presented a thickness of 60 nm, and was used as working electrode. Gold films were deposited onto the opposite site of the Pt-coating in order to act as counter and reference electrode. The experiments were carried out in a quartz reactor. This was specially designed to facilitate the electrical connection between the electrodes and the potentiostat, and also to allow the reactive mixture (CO, O2 and He) to reach the catalytic surface. The catalytic activity was analyzed at different CO/O2 ratios in open circuit voltage conditions (OCV), even when O2- ions were pumped to the Pt surface (positive currents) and from it (negative currents). All measurements were performed at 250 ºC, with overall flow rate of 4.5 L h-1 and 1200 ppm of CO. Results and Discussion Figure 1 presents the CO conversion and the measured potential at different conditions. The CO conversion was increased when negative currents were applied (change from B to C). In contrast, when positive currents were applied the catalytic activity diminished (E - F). Fig. 1. CO conversion and potential obtained with the different O2 concentrations and applied polarizations. A: 1900 ppm O2, OCV; B: 4000 ppm O2, OCV; C: 4000 ppm O2, -17 mA, D: 4000 ppm O2, OCV; E: 1900 ppm O2, OCV; F: 1900 ppm O2, +15 mA,; G: 1900 ppm O2, OCV. Figure 1 also shows that negative currents lead to the oscillations beginning in CO conversion. Simultaneously, the potential presented an oscillatory behaviour (condition E). When positive currents were applied, the oscillatory response could be stopped. Conclusions The oscillatory CO oxidation rate was modulated by the electrochemical pumping of O2- ions from or to the Pt surface. This behaviour can be understood using the oxide model for the oscillatory reactions at high pressure.