Infoscience

Thesis

Stored Electrogenerated Promoters Inducing Sustainable Enhanced Pt Catalyst Activity

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.

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