Investigation of the Pt/YSZ interface at low oxygen partial pressure by solid electrochemical mass spectroscopy under high vacuum conditions

The Pt/YSZ interface was investigated at low oxygen partial pressure under high vacuum (HV) conditions at 400 A degrees C. Two different electrochemical techniques were coupled to mass spectrometric gas analysis using a new solid electrochemical mass spectrometric monitoring device. Under cathodic polarization, the lack of oxygen in the gas phase induces the reduction of the YSZ solid electrolyte which acts as oxygen source for the formation of O2- ions migrating to the anode. Under anodic polarization, both platinum oxidation and oxygen evolution reaction are identified. PtO (x) is formed at both the Pt/YSZ and the Pt/gas interface according to two different mechanisms. At the Pt/YSZ interface, PtO (x) formation is an electrochemical process following a parabolic growth law, while the presence of PtO (x) at the Pt/gas interface is related to the diffusion of PtO (x) formed at the triple phase boundary towards the Pt/gas interface. It is proposed that the side oxygen evolution reaction stabilizes thermodynamically the PtO (x) diffusion toward the gas exposed interface during the anodic polarization.

Solid electrochemical mass spectrometry (SEMS) for investigation of supported metal catalysts under high vacuum

Christos Comninellis, Cyril Falgairette, Wolfgang Harbich, Chun Xia

A new experimental set-up, coupling electrochemistry and mass spectroscopic techniques, for the investigation of a solid electrochemical cell under high vacuum conditions (HV) is presented. Two configurations are realized allowing the investigation of both the electrochemical and electrocatalytical behavior of a thin Pt layer on yttria stabilized zirconia (YSZ). We can readily select the atmosphere down to 10(-6) Pa partial pressure and determine the response of the system in less than 1 s. Under HV conditions, YSZ appears electrochemically active and we have identified, in the cathodic potential domain, the reduction/oxidation process of zirconia and in the anodic domain, the platinum oxidation/reduction and the oxygen evolution reactions. In a catalytic active gas mixture, despite the Faradaic enhancement of the CO oxidation observed over Pt/YSZ during an anodic polarization, an intriguing sustainable enhanced Pt/YSZ catalyst activity is achieved after current interruption.

Springer Netherlands2010

Stored Electrogenerated Promoters Inducing Sustainable Enhanced Pt Catalyst Activity

Cyril Falgairette

This work concerns the investigation of the electrochemical promotion of Pt/YSZ catalyst giving more emphasis to the sustainable enhanced catalytic activity after current interruption. The permanent electrochemical promotion (P-EPOC) of C2H4 combustion over Pt/YSZ is investigated at 375°C under atmospheric pressure. Under anodic polarization, a non-faradaic enhancement of the reaction rate is observed (ρ = 4.2 and Λ = 370). However, after current interruption, the sustainable enhanced catalytic activity (P-EPOC) increases with the holding polarization time (γ =2,2 after 10 hours) giving evidence that a storage mechanism of oxygen promoters is involved in P-EPOC. In fact, a model involving two different types of promoters is proposed. Oδ1- promoters, highly mobile and reactive at the Pt/gas, are proposed to be responsible of EPOC while Oδ2- promoters, slow and very stable at the Pt/gas, are proposed to account for P-EPOC. Further electrochemical investigations of the Pt/YSZ interface realized at both atmospheric pressure and under high vacuum (HV) conditions gave strong evidence that the electrogeneration of the Oδ2- promoters is related to the formation of PtO taking place during an anodic polarization. In fact, the investigations of the O2(g),Pt/YSZ systems at atmospheric pressure, have revealed that, under anodic polarization, two electrochemical reactions take place: PtO formation at the Pt/YSZ interface and O2 evolution at the triple phase boundary (tpb). The current efficiencies of each process (ηPtO and ηO2) are determined allowing estimating the effective rate of PtO formation at the Pt/YSZ interface. In addition, CV-MS and DSCP-MS measurements, performed under HV conditions, have confirmed this process of oxygen storage at the Pt/YSZ interface and reveal a cooperative mechanism between O2 evolution reaction and PtO formation which allows the slow diffusion of oxygen strongly bonded (Oδ2- promoters) toward the Pt/gas interface.

EPFL2010

Promotion électrochimique des catalyseurs à base de rhodium et d'iridium

Justyna Eaves

In this thesis, the electrochemical promotion of lr and Rh based catalysts was studied for the complete combustion of ethylene and propylene. The study was performed by using two types of electrocatalytical cells: a classical, pellet type cell and a tubular cell. This work has two major goals. The first one is the optimization of the employed electrocatalytical systems from electrochemical point of view. The second one concerns the study of the mechanism of the electrochemical promotion. The major part of this work concerns the electrochemical characterization of two cell types: a pellet type cell and a tubular cell. On the basis of the obtained results the modélisation of the potential and current distribution was performed, providing key parameters for the development of more efficient cell configurations. On one hand, these configurations ensure an uniform current distribution on the working electrode and enable correct catalyst potential measurement. On the other hand, it minimizes the current bypass in the bipolar tubular cell. The aim of the investigation in a two-compartment reactor was to evaluate and to compare the Au and Ir electrodes used in this work. Under 400°C, gold was found to be much worse material than lrÛ2 both as reference and counter electrode. Its potential was affected by oxygen and propylene adsorption and no equilibrium of electrochemical reaction was obtained at such low temperature. On the base of this knowledge, several materials better adapted to electrochemical promotion conditions have been proposed. Using the two-compartment reactor potential measurements under steady state conditions allowed to verify the mechanism of the propylene combustion on Rh and that of ethylene combustion on lr based catalysts proposed in the literature. Ameliorations of the existent kinetic models were proposed. The kinetic measurements were then analyzed with the additional help of ex situ XPS analysis. This study allowed to link the oxidation state of the catalyst surface with the extent of electrochemical promotion of the catalysts. It also allowed the ex situ observation of the appearance of new oxygen (oxide) species on the gas exposed surface of Ir catalyst. This new type of oxygen was different from the oxygen of Ir02 oxide, and it can represent the promoting species, according to backspiïïover theory. The same oxygen species was observed on well oxidized Ir02 catalyst surface, even before the polarization of this catalyst. Its presence was associated to the catalyst-support effect. It explains the high activity of Ir02 catalyst by comparison with the catalytic activity of lr metal. XPS analysis of Rh based catalysts indicated the possibly formation of an inferior Rh oxide (close to RhO) due to polarization. This may be a possible explanation of the observed permanent electrochemical promotion. Voltammetric charge measurements in very narrow potential window were performed while changing the catalyst thickness. The charge was demonstrated to be proportional to the catalyst thickness, suggesting the presence of the charged species on the gas exposed catalyst surface. The analogy between this situation and that of the emerged electrodes in liquid state electrochemistry was emphasized. The voltammograms obtained in this study were interpreted in the quite new, original way, which allowed to calculate the capacity of the different catalyst interfaces, the gas/catalyst interface, and the triple phase boundary, in good agreement with reference values from impedance spectroscopy measurements. Finally, the reaction rate transients during galvanostatic polarization of lr based catalyst have been moralized. The model fits well the experimental results.