Xavier Courtois

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.

Chapter 8 The role of cerium-based oxides used as oxygen storage materials in DeNOx catalysis

Abstract Materials with high ‘oxygen storage capacity’ (OSC) are now widely used in automotive converter catalysts. They are mainly composed of Ce-based oxides (CeZrOx, CeZrPrOx, etc.) having both multiple cationic valencies and oxygen vacancies. These properties allow the catalyst to store active O species (O2−, superoxide, etc.) in O2 excess and to release them when the O2 concentration in gas phase decreases or becomes nil. After having briefly examined the main properties of these OSC materials and the methods employed for their characterization, their impact in automotive catalysis will be reviewed, with a special insight in DeNOx catalysis: (1) in three-way catalysis (2) in automotive catalysis under lean conditions (lean-burn spark ignition engine and diesel).

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.

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.