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Platinum and platinum-ceria catalysts for the electro-oxidation of ethanol.

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Vieira Ribeiro Paulo, Maria João (2018). Platinum and platinum-ceria catalysts for the electro-oxidation of ethanol. Thèse. Québec, Doctorat en sciences de l'énergie et des matériaux, Université du Québec, Institut national de la recherche scientifique, 174 p.

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Direct ethanol fuel cells (DEFCs) are a promising system and a potential sustainable power source for portable, mobile, and stationary applications. The successful application of the direct ethanol fuel cells depends on the availability of a catalyst capable of breaking the carboncarbon (C-C) bond leading to the complete oxidation of this fuel to carbon dioxide and water, releasing 12 electrons through an external circuit. Pure Pt is the state of the art electrocatalyst for the oxidation of ethanol but it suffers from some limitations such as poisoning by COintermediates, resulting in a slow ethanol oxidation reaction kinetics and incomplete oxidation with the formation of by-products such as acetaldehyde (two-electron oxidation) or acetic acid (four-electron oxidation). In this work we aim at increasing the activity of Pt for ethanol oxidation, but using an approach that differs from the traditional which consists of carbon-supported platinum catalysts. Instead, we show that the electrocatalytic activity of unsupported Pt nanoparticles (NPs) for the ethanol oxidation in acid medium can be significantly enhanced by the addition of very small amounts of CeO2 nanoparticles ( 5 wt%). In order to understand and to compare the effect of ceria on Pt nanoparticles with different particle sizes, catalysts with the same composition were synthesized by two different methods, the Pechini (sol-gel) and reduction (solution-based) methods. Variations explored in this work to control and compare the size of the Pt particles include: a 1, 5 and 10 times dilution of the polymeric matrix for the Pechini method, and the addition of a citrate stabilizer for the reduction method. In the first method, Pt NPs with sizes ranging from 4 to 40 nm were obtained, whereas in the reduction method much smaller nanoparticles, with sizes varying between 2 and 6 nm, were synthesized. Comparative studies on the Pt, Pt-CeO2-1wt% and Pt-CeO2-5wt% catalysts synthesized by both methods revealed the existence of two different effects. In the Pechini method (larger particles), a strong electronic interaction between the Pt and CeO2 nanoparticles was demonstrated by a systematic shift of the Pt 4f7/2 peak to lower binding energies (BE) with the addition of CeO2. As a result, the current density normalized to Pt’s electrochemical surface area for ethanol oxidation increased up to 10  by adding 1 wt% CeO2. This positive effect is however hindered by a high coverage of the Pt nanoparticles surface by CeO2 which results in the decrease of active sites. In this sense the addition of more CeO2 (PtCeO2-5wt%) was detrimental for the catalytic activity. Because there is no sharp distinction between the nanoparticle size and ceria effects, there is no evidence of a size effect on the catalysts produced by the Pechini method. In the reduction method, we observed sharp shifts of the binding energy to higher values of Pt 4f7/2 with a decreased of the size of Pt nanoparticles. Assuming the Wertheim’s equation that establishes an inverse proportionality between the binding energy and the particle size of spherical-like clusters, there is evidence for the existence of size effect (final state effect). As expected, this relation is even more pronounced on the citrate-stabilized nanoparticles that correspond to the smallest catalysts produced in this work. In this sense we can remark a progressive BE increase from 71.4 eV (Pt-cit) to 71.9 eV (PtCeO2-1wt%-cit) which is inversely related by a decrease in the Pt-cit particle size from 4 (Pt-cit) to 2 nm (PtCeO2-1wt%-cit). The addition of 5wt% CeO2 to Pt-cit corroborates the same trend but its BE varies less than the 1wt% CeO2 since the last two composites show almost the same particle size. Thus, this enhancement is more meaningful on samples containing Pt-CeO2 1 wt%-cit and in Pt-CeO2-5wt% with current densities for ethanol oxidation of 2.7 Am-2 and 4 Am-2 respectively. Moreover, PtCeO2-5wt%, presents the ideal size for ethanol oxidation (3 nm) to which is ascribed the best compromise between structural and electronic effects and/or oxophilicity effects of the Pt surface in order to favor the formation of CO2. The HNMR analysis of the liquid products obtained from a 5h ethanol oxidation reaction showed different selectivity: PtCeO2-1wt%-cit produced slightly more acetaldehyde than acetic acid, whereas PtCeO2-5wt% produced only a small amount of acetic acid. As PtCeO2-1wt%-cit revealed a higher amount of Ce (III) and liquid products, it is possible that the ethanol oxidation to acetaldehyde and acetic acid, that requires the presence of oxygen-containing species, occurs via Ce2O3/CeO2.

Type de document: Thèse Thèse
Directeur de mémoire/thèse: Tavares, Ana
Mots-clés libres: énergie; matériaux
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 22 oct. 2020 17:16
Dernière modification: 22 oct. 2020 17:16
URI: http://espace.inrs.ca/id/eprint/10424

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