Fabien Can

Remarkable Enhancement of O2 Activation on Yttrium-Stabilized Zirconia Surface in a Dual Catalyst Bed

Yttrium-stabilized zirconia (YSZ) has been extensively studied as an electrolyte material for solid oxide fuel cells (SOFC) but its performance in heterogeneous catalysis is also the object of a growing number of publications. In both applications, oxygen activation on the YSZ surface remains the step that hinders utilization at moderate temperature. It was demonstrated by oxygen isotope exchange that a dual catalyst bed system consisting of two successive LaMnO3 and YSZ beds without intimate contact drastically enhances oxygen activation on the YSZ surface at 698 K. It can be concluded that LaMnO3 activates the triplet ground-state of molecular oxygen into a low-lying singlet state, thereby facilitating the activation of the O2 molecule on the YSZ oxygen vacancy sites. This phenomenon is shown to improve the catalytic activity of the LaMnO3-Pd/YSZ system for the partial oxidation of methane.

Waste-free scale up synthesis of nanocrystalline hexaaluminate: properties in oxygen transfer and oxidation reactions

Synthesis of nanocrystalline hexaaluminate is reported using an original activated reactive synthesis process. Starting from a classical ceramic solid, exhibiting low surface area and a micrometric crystal size, a two-step grinding process allows reduction of the crystal size down to a few nanometers and development of high surface areas. The synthetic process was then used to produce transition metal- and noble metal-doped structures. The effects of (i) morphological and structural properties and (ii) substitution on oxygen transfer properties and catalytic properties in CO and CH4 oxidation reactions were studied. Crystal size was shown to be a key parameter in controlling the bulk oxygen transfer. Study of the catalytic properties in low and high temperature oxidation reactions also shows the crucial effect of the morphological parameters. Highest activities were achieved over nanocrystalline high surface compositions. Finally, even if less active than classical palladium supported solids, these new structures exhibited extremely high thermal stability.

Ionic Liquid‐Mediated α‐Fe2O3 Shape‐Controlled Nanocrystal‐Supported Noble Metals: Highly Active Materials for CO Oxidation

Shape‐controlled iron oxide nanocrystals were prepared by using an ionic liquid‐mediated hydrothermal process. Different morphologies can be synthesised, such as cubes and porous nanotubes. Owing to the different morphologies developed, accessible surface area varies from a few m2 g−1 to more than 50 m2 g−1. These differences result in different oxygen mobilities, and the porous nanorods demonstrate the highest bulk oxygen mobility. Thus, all these shaped materials demonstrate higher activity for the oxidation reaction compared to the commercial reference. In addition, the favourable physical properties, that is high surface area, enable the easy dispersion of noble metal nanoparticles (platinum, palladium and gold); some of these high‐surface area noble metal‐containing materials demonstrate remarkable catalytic activities. Porous nanorod‐supported gold nanoparticles enable the conversion of CO below 100 °C, which is far better than on commercial α‐Fe2O3‐supported gold for which dispersion of gold remains difficult owing to the low surface area of the commercial support.

Investigation of Methane Oxidation Reactions Over a Dual‐Bed Catalyst System using 18O Labelled DRIFTS coupling

Low loading Pd-supported (0.2 wt % Pd) Y-stabilized zirconia (YSZ) and LaMnO3 (LM) perovskite were associated to study the partial oxidation of methane using labelled 18 O2 in the gas phase. Synthesis gas production was demonstrated to occur through an indirect reaction in which oxygen is first consumed in the total methane combustion. A Mars-van Krevelen mechanism was observed over Pd/YSZ at 425 °C to yield C16 O2 and C16 O. A significant enhancement of the Pd/YSZ catalyst activity was achieved by the association of LM-Pd/YSZ in a dual catalyst bed, resulting in a significant increase of the oxidation rate. Vibration bands of adsorbed formate species, assumed to be intermediates to the gas production, were observed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) coupling experiments. It was proposed that LM enables the generation of highly active singlet O2 , which is activated on the YSZ oxygen vacancies to assist a rapid recovery of surface PdO and increase formate decomposition into CO and H2 in Pd-supported catalyst.

Study of Lanthanum Manganate and Yttrium-Stabilized Zirconia-Supported Palladium Dual-Bed Catalyst System for the Total Oxidation of Methane: A Study by 18O2/16O2 Isotopic Exchange

A significant enhancement of the yttrium‐stabilized zirconia‐supported palladium (0.2 wt % Pd/YSZ) catalytic activity for the total oxidation of methane was achieved in a dual‐bed system by the association of LaMnO3+Pd/YSZ materials. The combination with the best performance was 40 wt % LaMnO3+60 wt % Pd/YSZ. Pd/YSZ and LaMnO3 perovskite were characterized by using the 18O2 exchange technique. A Mars–van Krevelen mechanism was demonstrated over Pd/YSZ at 425 °C. Additionally, it was evidenced that LaMnO3 activated the O2 molecule on the YSZ oxygen vacancy sites to result in a significant increase of the oxidation rate characterized by a correlating decrease of the light‐off temperature of 65 °C.

NSR–SCR Combined Systems: Production and Use of Ammonia

This chapter gives a critical overview of the recent advances in NOx abatement in excess of oxygen based on the combination of the NOx storage-reduction (NSR) and Selective Catalytic Reduction (SCR) processes. Ammonia may be produced during the regeneration step of NSR catalyst, by the direct reaction (NOx + H2) or/and the isocyanate route. Recent literature highlights that the ammonia production rate is higher than the ammonia reaction rate with the remaining NOx in order to form N2. In order to optimize the use of the in situ produced ammonia, a catalyst dedicated to the NOx–SCR by NH3 can be added. Zeolites are the main studied materials for this application. Catalytic reduction of NOx by NH3 relates a complex mechanism, in which the nuclearity of the active sites is still an open question. Over zeolites, the NO to NO2 oxidation step is reported as the rate-determining step of the SCR reaction, even if the first step of the reaction is ammonia adsorption on zeolite Bronsted acid sites. Thus, the addition of a NH3–SCR material to the NSR catalyst is a possible way to increase the global NOx abatement and maximize the N2 selectivity, together with the prevention of the ammonia slip.

Transition metal oxides for combustion and depollution processes

Abstract Transition metal oxides (M=Mn, Co, Fe, Ni,…) are potential catalysts for application in combustion and depollution processes. Owing to huge improvements in their preparation, perovskites, spinels, hexaaaluminates, and some other oxide structures can replace noble metals in a number of processes. In this chapter, the most recent advances in the use of oxides for total oxidation (CO, methane, COV, wet air oxidation) and for the treatment of nitrogen compounds (NOx, NH3, urea) will be reviewed. In every case, the most probable mechanism (Langmuir–Hinshelwood, Eley–Rideal, Mars–van Krevelen…) and the nature of active sites (Mn+/Mn+1 ion pairs, acid–base sites…) as well as the role of reactive oxygen species will be examined in the light of recent results and up-to-date concepts. Finally the outstanding progresses in the oxide synthesis allow to apply these concepts to the development of extremely active and more stable catalysts.

Palladium, Iridium, and Rhodium Supported Catalysts: Predictive H2 Chemisorption by Statistical Cuboctahedron Clusters Model

Chemisorption of hydrogen on metallic particles is often used to estimate the metal dispersion (D), the metal particle size (d), and the metallic specific surface area (SM), currently assuming a stoichiometry of one hydrogen atom H adsorbed per surface metal atom M. This assumption leads to a large error when estimating D, d, and SM, and a rigorous method is needed to tackle this problem. A model describing the statistics of the metal surface atom and site distribution on perfect cuboctahedron clusters, already developed for Pt, is applied to Pd, Ir, and Rh, using the density functional theory (DFT) calculation of the literature to determine the most favorable adsorption sites for each metal. The model predicts the H/M values for each metal, in the range 0–1.08 for Pd, 0–2.77 for Ir, and 0–2.31 for Rh, depending on the particle size, clearly showing that the hypothesis of H/M = 1 is not always confirmed. A set of equations is then given for precisely calculating D, d, and SM for each metal directly from the H chemisorption results determined experimentally, without any assumption about the H/M stoichiometry. This methodology provides a powerful tool for accurate determination of metal dispersion, metal particle size, and metallic specific surface area from chemisorption experiments.

Enhancement of Oxygen Activation and Mobility in CaTi x Fe 1− x O 3− δ Oxides

The study of oxygen activation and mobility in CaTi0.9Fe0.1O3−δ was investigated by using the isotopic exchange technique. The thermomechanical and chemical stability of CaTi0.9Fe0.1O3−δ makes it a promising mixed ionic and electronic conducting oxide for catalytic membrane reactors. However, its oxygen flux performance is still lower than that of reference membranes. Two strategies were studied to improve oxygen transport in this perovskite oxide, that is, higher Fe doping and utilization of a dual‐bed system composed of LaMnO3/CaTi0.9Fe0.1O3−δ.

Study of the remarkable reactivity of HNCO/urea with NO2 in the NOx SCR by urea process over an oxide-based catalyst

An exceptional reactivity between NO2 and probably HNCO was demonstrated over an oxide-based catalyst during NO2-SCR experiments with urea as the injected reductant. This was observed when only NO2 was used as NOx, but not with gaseous NH3 as the reductant. Roughly, one third of the injected reductant reacted with NO2 and two thirds appeared to be oxidized by O2, the main product being N2. 28 reactions may be involved in the observed results. A cooperative effect of NO2 with O2 was demonstrated and the observed global stoichiometry did not depend on the NO2 concentration for (NO2/eq. NH3)inlet ratio ≤1.