D. Duprez

Catalytic oxidation of organic compounds in aqueous media

Abstract Ru, Pt and Rh catalysts supported on titania, ceria or active carbon and a Mn/Ce composite oxide catalysts were prepared and their catalytic behavior in Wet Air Oxidation ( 20 bar O 2 ) of phenol and acetic acid were investigated. Phenol was found to be an easily oxidizable compound as 170°C, while acetic acid was a very refractory molecule, even at 200°C. Ru(5 wt.-%)/C is a very efficient catalyst for the WAO of acetic acid without any leaching of noble metal.

Preparation of zirconia-ceria materials by soft chemistry

Abstract New zirconia–ceria materials were prepared by soft chemistry with thermal treatments at temperatures below 1000°C. Samples were characterized by XRD, DTA–TGA, BET area measurements and oxygen storage capacity (OSC) measurements. For the preparation, cerium nitrate and different zirconium precursors were used: zirconyl nitrate or chloride for coprecipitation and zirconium propoxide for the modified sol–gel method. Both the structure and the texture of those solids depend on the synthesis and the zirconium precursor. For solids prepared by the modified sol–gel method, with a surface area of 60xa0m 2 xa0g −1 , a new cubic phase (Zr 0.25 Ce 0.75 O 2 ) is obtained, while an orthorhombic zirconia phase was identified for solids prepared either by coprecipitation or sol–gel methods. The sol–gel method was particularly efficient for preparation of Ce–Zr–O mixed oxides with high cerium contents. OSC values were significantly higher than in the case of coprecipitated oxides.

Oxygen Storage and Mobility on Model Three-Way Catalysts

Oxygen storage capacity (OSC) of TW catalysts has a strong impact on their performances under oscillatory conditions. This paper reviews the different methods developed for OSC measurements, the effects of catalyst composition on OSC as well as the impact of OSC on TW catalysis, concentrating on the results obtained in our laboratory. Use of OSC measurements for catalysts control (on-board diagnostic) is also examined. The role of superoxide species as intermediates in the oxygen storage process will be emphasized. Finally, we shall review the recent developments on CeZr-based oxides.

Study of surface mobility by isotopic exchange: recent developments and perspectives

Oxygen and hydrogen surface mobility can be involved in many catalytic processes. However, quantitative measurements of surface diffusion are relatively scarce. We present the recent developments of the isotopic methods which are based on the determination of the rates of exchange between 18O2 or of D2 with 16O and H species of oxides, via metal particles (mainly Rh) deposited on these oxides. O and H surface mobility on SiO2, Al2O3, ZrO2, MgO and CeO2 is reviewed. Great differences between these various oxides can be observed. H and O species are very mobile on ceria while they are not on silica. The impact of surface mobility in steam reforming, three-way catalysis (oxygen storage capacity) and in selective oxidations is also discussed.

Oxygen Storage Capacity of three-way catalysts : a global test for catalyst deactivation.

ABSTRACT Two commercial catalysts (PtRh and PdRh) were treated in N 2 xa0+xa0H 2 O (or Airxa0+xa0H 2 O) at 900, 1000 or 1100xa0°C (Lab. ageing) or aged on an engine bench for 25, 50 or 200xa0h. OSC (transient CO oxidation) and catalytic activity for CO, HC and NO abatment (light-off temperatures and conversion at 450xa0°C) were measured over these aged catalysts. Correlations between OSC values and catalytic activity are discussed for a possible On-Board Diagnostic application.

Surface mobility of oxygen species on mixed-oxides supported metals

Cerium-zirconium mixed oxides-supported metals (Rh, Pt, Pd) were prepared by impregnation of Ce x Zr (1-x) O 2 (x=0, 0.63 and 1) supports with the salt of the corresponding metal. These materials were studied on the basis of their oxygen Storage Capacity (OSC) and activity in the isotopic exchange of oxygen. For all of them oxygen as been found especially reactive and mobile above 400°C. Introduction of a metal markedly enhances the OSC of the supports and their activity in the exchange of oxygen (Rh>Pt>Pd). The OSC at 400°C of Ce 0.63 Zr 0.37 O 2 is multiplied by a factor of almost 4 in the presence of 0.3%Rh. Metal particles are asimilated to portholes for the subsequent migration of oxygen on the support. In the same way, the activity at 350°C of 0.3%Rh/CeO 2 in the isotopic exchange of 18 O 2 is 3 orders of magnitude larger than in the absence of rhodium.

Surface migration of oxygen and of hydrogen in supported metal catalysts

Abstract Coefficients of oxygen and hydrogen surface diffusion on various supports (Al2O3, SiO2, Z1O2, CeO2) were determined by isotopic exchanges (18O/16O and 2H/1H). Rhodium particles were used as a source of diffusing species. A good correlation was obtained between the oxygen mobility and the basicity of the oxide (CO2 chemisorption). However, hydrogen mobility depends in a complex manner on the acidity (determined by probe reactions of isomerisation).

Study of the oxygen diffusion on three-way catalysts: a kinetic model

A computer model was developed to take into account all the phenomena that can occur in 18O/16O isotopic exchange over Pt/CeZrOx materials: adsorption/desorption on the metal, surface and bulk O diffusion. Discriminating each step of the exchange process is no longer necessary: kinetic parameters and O diffusivity can be calculated in a single experiment.

Spillover of oxygen species in the steam reforming of propane on ceria-containing catalysts

Several kinds of experiments were carried out to show that oxygen species (OH, O) can be transferred from a Rh/Al 2 O 3 (RhA) catalyst to pure oxides and vice-versa. Ceria physically mixed with RhA increases the catalytic activity of the Rh catalyst for the steam reforming of propane. Conversely, RhA can promote the exchange of 18 O 2 with the 16 O of pure oxides, normally inactive for the 18 O/ 16 O exchange in the absence of RhA. Moreover, large amounts of steam reforming products (H 2 , CO) were obtained by injecting pulses of pure propane over a PtRh/CeO 2 /Al 2 O 3 catalyst. OH groups stored on the support migrate to the metal particles where the reaction with CH x fragments (issued from the C 3 H 8 activation) can occur.

A Study of the NOx Selective Catalytic Reduction with Ethanol and Its By-products

The NOx selective catalytic reduction (SCR) with ethanol has been investigated over alumina supported silver catalyst with a special attention to the main involved reactions depending on the temperature test. With this aim, the possible reducers from ethanol transformations were also evaluated (C2H5OH, CH3CHO, C2H4, CO). In addition, the contributions of the gas phase reactions and the alumina support were also pointed out. Based on the C-products and N-compounds distributions, it is assumed that at low temperature (Txa0<xa0300xa0°C), ethanol reacts firstly with NOxa0+xa0O2 to produce acetaldehyde and N2. For higher temperatures, two reaction pathways have been proposed, supported by the CH3CHO-SCR results: a direct reaction between NO2 and CH3CHO, or via –NCO species.