Application of isotopic exchange technique to innovative catalytic systems study : O2 activation and mobility on YSZ in dual catalytic bed and reactivity of lattice nitrogen in nitrides materials for heterogeneous catalysis

Ce travail porte sur l’étude de systèmes catalytiques innovants par la technique d’échange isotopique (EI) permettant d’apprécier des propriétés fondamentales (activation des molécules en surface, mobilité et réactivité des atomes de réseau) pour comprendre les mécanismes de réaction mis en jeu en catalyse hétérogène et développer des systèmes plus performants. Aussi, l’identification d’espèces adsorbées intermédiaires est possible en couplant la spectrométrie de masse (analyse de la phase gaz) à l’observation de la surface catalytique par spectroscopie DRIFT.L’EI 16O/18O montre des effets de dispersion ou de synergie de LaMnO3 (LM) supportée sur YSZ ou TiO2 expliquant les performances de cette structure pérovskite pour l’oxydation catalytique de C7H8 via un mécanisme suprafacial. L’activité en EI C16O2/C18O2 démontre la mobilité exceptionnelle des atomes O de réseau de YSZ dès 150 °C via la formation d’espèces (hydrogéno)carbonates en surface. En catalyse d’oxydation, à T < 800 °C, cette mobilité est pourtant limitée par l’activation d’O2 à la surface de YSZ. La solution proposée ici est la génération préalable d’une espèce oxygène réactive sur un lit de matériau réductible type LM. Le double-lit LM-YSZ montre d’excellentes performances pour abaisser la température d’oxydation de CH4 à 425 °C via un mécanisme Mars van Krevelen (MvK) où les atomes O de YSZ participent à la réaction par l’intermédiaire d’espèces formiates.L’EI 14N/15N est également utilisé dans ce travail pour analyser la réactivité des atomes N de réseau dans les nitrures métalliques. En particulier, Co3Mo3N et Ni2Mo3N montrent des propriétés remarquables, dépendant de la méthode de préparation ou du prétraitement appliqué. Leur comportement suggère la participation des atomes N dans la réaction de synthèse de NH3 sur le principe d’un mécanisme MvK.This work concerns the study of new catalytic systems by isotopic exchange (IE) technique allowing to appreciate basic properties (molecules surface activation, mobility and reactivity of lattice atoms) to better understand catalytic mechanisms and to develop efficient catalysts. The identification of intermediate adsorbed species is possible by coupling mass spectrometry (gas-phase analysis) with the catalytic surface analysis by DRIFT spectroscopy.IE 16O/18O shows dispersal and synergetic effect of supported LaMnO3 perovskite (LM) on YSZ or TiO2 which explain catalytic performances of this perovskite structure for toluene oxidation via a suprafacial mechanism. IE C16O2/C18O2 activity demonstrates the remarkable lattice O atoms mobility of YSZ from 150 °C via adsorbed (hydrogeno)carbonates. To the contrary, in oxidation catalysis, under 800 °C, this mobility is very limited by O2 activation on YSZ surface. The solution proposed in this work is the previous generation of reactive oxygen species on a first catalytic bed of reducible material as LM. LM+YSZ dual-bed shows very efficient activity to reduce methane oxidation temperature at 425 °C via a Mars-van Krevelen (MvK) mechanism in which lattice O atoms of YSZ take part in the reaction by intermediate formate species.IE 14N/15N is thereafter used to analyse lattice N atoms reactivity of metal nitrides materials. In particular, Co3Mo3N and Ni2Mo3N show interesting properties depending on preparation or pre-treatement routes. This behaviour supposes that ammonia synthesis reaction could be procced via MvK type mechanism with the participation of lattice N of this nitrides

Investigation of hysteresis phenomena during CO oxidation over Pt-based catalyst: a SSITKA-IR study

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Investigation of CO+NO reaction mechanism on Ag/Al2O3 by time-resolved IR analysis of reactant isotopic exchange

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Investigating the origin of hysteresis in CO oxidation using Steady State Isotopic Transient Kinetic Analysis

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Insight into the true role of hydrogen-carbonate species in CO oxidation over Pd/Al2O3 catalyst using SSITKA-transmission IR technique

International audienceThe powerful combination of steady state isotopic transient kinetic analysis (SSITKA) and operando infrared spectroscopy (IR) was applied to study the CO oxidation reaction on Pd-supported Al2O3 catalyst at low temperature. The aim was to reveal the true role of hydrogen‑carbonate species in the reaction mechanism. The SSITKA-IR experiments, conducted in presence and absence of CO2 in the gas feed, have confirmed that hydrogen‑carbonate species behave as spectator species in the reaction. Its formation was due to re-adsorption of the CO2 product itself on the alumina surface as confirmed by the direct 12CO2/13CO2 exchange observed on the bare support

Identifying the true role of hydrogen-carbonate species in CO oxidation over Pd/Al2O3 using SSITKA-IR technique

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Investigation of hysteresis phenomenon on Pt/Al2O3 during CO oxidation with Steady-State Isotopic Transient Kinetic Analysis coupled to operando Infrared spectroscopy

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Investigation of hysteresis phenomena on PGM during CO oxidation: SSITKA-IR study

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Revealing Origin of Hydrogen-Carbonate Species in CO Oxidation Over Pt/Al2O3: A SSITKA-IR Study

International audienceThe catalytic oxidation of CO by O2 to form CO2 over Pt based catalysis is one of the most studied catalytic reaction leading to controversial mechanism descriptions. Steady State Isotopic Transient Kinetic Analysis (SSITKA) coupled with InfraRed (IR) spectroscopy is a powerful technique to study heterogeneous reaction mechanisms combining both the observation of adsorbed species on the catalyst surface and kinetic measurements. In this paper, SSITKA-IR technique was applied to study the CO oxidation reaction in presence and absence of CO2 in order to distinguish between active and spectator intermediates formed during the reaction. Linear carbonyl and bridged carbonyl type species adsorbed on metallic Pt0 were clearly identified as active intermediates at low temperature (131 °C). In contrast, the hydrogen-carbonate species formed during CO oxidation reaction were proven to be inactive species in CO2 formation but rather due to the re-adsorption of CO2 product itself on alumina surface

Evaluating Different Strategies to Minimize cold-start Emissions from Gasoline Engines in steady-state and Transient Regimes

International audienceExhaust car emissions increase significantly at particular gasoline engine driving cycle such as cold-start when the three-way catalyst has not reached its light-off temperature. More efficient technologies are needed to reduce these extra emissions. This study focuses on comparing two strategies to lower cold-start pollutants on a commercial monolithic catalyst: (i) a high content of PGMs (Pd and Rh) loading with a variable concentration distribution along the catalyst, called zone-coating, was investigated in order to take advantages of an in situ pre-heating due to exothermic oxidation processes. And (ii) the use of external device for heating the monolith with the aim to shorten the laps of time to reach the required temperature for their conversion. Both approaches were compared below 300 °C in terms of NO, CO and hydrocarbons conversions as well as N2O formation with respect to homogeneously wash-coated catalyst. For evaluation, complex exhaust gas compositions were considered at different steady-state air-to-fuel λ ratios and high frequency transient lean/rich regime to mimic real operation in gasoline engine exhaust. Results show that a pre-heating of the catalyst at 300 °C is necessary to avoid completely N2O formation from NO reduction with CO. Remarkably higher NO and CH4 conversions were observed during transient regime rather than steady-state lean, rich or stoichiometric conditions at 200 and 300 °C