Approche multi-échelle appliquée à la catalyse hétérogène en milieu oxydant sur Pt/Al2O3 hautement dispersé

L’objectif de ces travaux réside dans une meilleure compréhension de la catalyse d’oxydation sur un catalyseur Pt/γ-Al2O3 hautement dispersé. La thématique est porteuse dans les domaines de la catalyse de dépollution, de l’oxidation sélective des hydrocarbrures ainsi que du développement des piles à combustible. Des phénomènes d’adsorption et d’oxydation ont été étudiés suivant une approche multi-échelle. L’évaluation de phénomènes isolés a été réalisées expérimentalement par désorption et réaction en température programmée, couplée à des analyses de spectrométrie de masse, de FTIR operando ainsi que microGC. La modélisation moléculaire ab initio a été utilisée afin de comprendre le comportement microscopique de l’interaction adsorbat/catalyseur/support lors de ces réactions. Les données thermodynamiques extraites ont été intégrées à des modèles micro-cinétiques détaillés, et ont permis de comparer les expériences simulées et réelles. Trois systèmes ont été explorés en détail : (i) l’adsorption dissociative de O2 en oxygène atomique, (ii) l’adsorption de monoxyde de carbone et (iii) son oxidation en CO2. L’impact de la structuration du catalyseur a été discuté par rapport aux surfaces idéales telles que Pt(111). L’oxydation d’hydrocarbures légers tels que le méthanol et le propène ont fait l’objet d’une étude préliminaire. Un générateur de réactions de surface a été spécialement développés pour automatiser la génération exhaustive de réactions de surface concernant l’oxydation des hydrocarbures.This work tackles at a better understanding of oxidation catalysis on highly dispersed Pt/Al2O3 surface. This thematic is of great interest for depollution catalysis, for hydrocarbon selective oxidation or even for fuel cell developments. Adsorption and oxidation phenomena were studied with a multi-scale approach. Experimental evaluation of isolated phenomenon were performed using temperature programmed adsorption/reaction coupled with mass spectrometry, operando FTIR and microGC. Ab initio molecular modelling investigations were undertaken to handle the microscopic behaviour of adsorbate/catalyst/support interactions. Thermodynamic data were integrated into exhaustive micro-kinetic models that allow to compare experiments and modelled data. Three systems were fully explored: (i) the dissociative adsorption of O2 into atomic oxygen, (ii) adsorption of carbon monoxide and (iii) oxydation of CO towards CO2. The impact of the catalyst structuration has been discussed with ideal surfaces such as Pt(111). Light hydrocarbons oxidations such as methanol and propene were also investigated in a preliminary way. Surface reaction mechanism generator was specially developped to handle exhaustive hydrocarbon oxidation surface reactions

Impact de l’état d’oxydation du platine sur l’oxydation catalytique du CO : Apport de la méthodologie SSITKA-IR

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Thermokinetic and Spectroscopic Mapping of Carbon Monoxide Adsorption on Highly Dispersed Pt/γ-Al 2 O 3

International audienceThe understanding and quantification of the CO adsorption modes and strength on ultradispersed platinum catalysts supported on γ-Al2O3 is of prominent importance for analytic and catalytic purposes. We report a multiscale experimental (AEIR, CO-TPD) and theoretical approach to provide vibrational properties, adsorption enthalpies, and desorption behaviors. First principles calculations on Pt13(CO)m/γ-Al2O3 and Pt(111) surface models (using various exchange-correlation functionals) provide a complementary view to experimental approches. Adsorption enthalpies computed with the RPBE functional appear to be the most compatible with the AEIR results. The occupation of top sites by CO dominates the behavior of supported Pt clusters. CO coverage reaches higher values in comparison to Pt(111) for similar operating conditions, and considerable cluster reconstruction is observed at high coverage. First principles calculations also confirm the IR assignment related to the various adsorption modes on top and bridge sites and demonstrate a particle size effect, lowering the frequency of linear adsorption at top sites with respect to extended Pt(111) surfaces. Finally, first principles-based microkinetic modeling of CO-TPD experiments shows that the adsorption strengths predicted on the small-size cluster by DFT are compatible with the experimental values. We discuss possible reasons for the experimental desorption pattern to be much broader than the computed pattern

Atmosphere-dependent stability and mobility of catalytic Pt single atoms and clusters on gamma-Al2O3

MICROSCOPIE+ECI2D:ING+FMO:JRO:MAO:PAF:LPIInternational audienceAtomically dispersed metals promise the ultimate catalytic efficiency, but their stabilization onto suitable supports remains challenging owing to their aggregation tendency. Focusing on the industrially-relevant Pt/-Al2O3 catalyst, in situ X-ray absorption spectroscopy and environmental scanning transmission electron microscopy allow us to monitor the stabilization of Pt single atoms under O2 atmosphere, as well as their aggregation into mobile reduced subnanometric clusters under H2. Density functional theory calculations reveal that oxygen from the gas phase directly contributes to metal-support adhesion, maximal for single Pt atoms, whereas hydrogen only adsorbs on Pt, and thereby leads to Pt clustering. Finally, Pt cluster mobility is shown to be activated at low temperature and high H2 pressure. Our results highlight the crucial importance of the reactive atmosphere on the stability of single-atom versus cluster catalysts