François Devred

Hydrogenation of NO and NO2 over palladium and platinum nanocrystallites: case studies using field emission techniques

In this work, we investigate the catalytic hydrogenation of NO over palladium and platinum and of NO2 over platinum surfaces. Samples are studied using field emission techniques including field emission/ion microscopies (FEM/FIM). The aim of this study is to obtain detailed information on the non-linear dynamics during NOx hydrogenation over nanocrystallites at the atomic scale. The interaction between Pd and pure NO has been studied between 450 K and 575 K and shows the dissociative adsorption of NO. After the subsequent addition of hydrogen in the chamber, a surface reaction with the oxygen-adlayer can be observed. This phenomenon is reversible upon variation of the H2 pressure, exhibits a strong hysteresis behaviour but does not show any unstable regime when control parameters are kept constant. On platinum, NO is dissociated and the resulting O(ads) layer can also react with H2. Although occurring on both Pd and Pt metals, the reaction mechanism seems to be different. On palladium, NO dissociation takes place on the whole visible surface area leading to a “surface oxide” that can be reacted off by raising the H2 pressure whereas on Pt, the catalytic reaction is self-sustained and restricted to 〈001〉 zone lines comprising {011} and {012} facets and where self-triggered surface explosions are observed. Two kinetic phase diagrams were established for the NO–H2 reaction over palladium and platinum samples under similar experimental conditions. Their shapes reflect a different chemical reactivity of metal surfaces towards oxygen species resulting from the dissociation of NO. NO2 hydrogenation is followed over Pt samples and shows self-sustained kinetic instabilities that are expressed as peaks of brightness that are synchronized over the whole active area (corresponding to the 〈001〉 zone lines as in the NO case) within 40 ms, the time resolution of the video-recorder used for this work.

Probing the structural changes and redox behavior of mixed molybdate catalysts under ammoxidation conditions: an operando Raman spectroscopy study

Mixed model molybdate catalysts that contain CoMoO4, Bi2Mo3O12, and Fe2Mo3O12 were investigated in the ammoxidation reaction of propene to acrylonitrile to study the redox behavior of the iron molybdate component. The flexible changes of the oxidation state of the Fe component during ammoxidation were followed by examining the intensity changes of the characteristic bands of Fe2Mo3O12 and FeMoO4 in the Raman spectra and could be correlated with the catalyst performance examined by simultaneous MS analysis. Depending on the catalyst composition and the oxygen content in the feed, Fe2Mo3O12 is reduced to a different extent, and consequently, FeMoO4 is formed, which is accompanied by a restructuring of the catalyst and the formation of nanostructured MoOx species. In accordance with previous investigations, operando Raman studies revealed that CoMoO4 and Bi2Mo3O12 influence the redox behavior of Fe2Mo3O12 in different ways.

Hydrotalcites promoted by NaAlO2 as strongly basic catalysts with record activity in glycerol carbonate synthesis

A new type of highly basic catalysts is obtained by promoting Mg–Al layered double hydroxides with sodium aluminate. The Mg–Al mixed oxides obtained by the calcination of pristine hydrotalcites are poorly active in the synthesis of glycerol carbonate from glycerol and dimethyl carbonate (DCM). Pure sodium aluminate on the other hand is highly active in this reaction, but it is also highly corrosive, making its handling problematic. Remarkably, promoting hydrotalcites with low amounts of sodium aluminate is sufficient to reach high yields. At 90 °C, with 3 wt % catalyst and with a DMC/glycerol ratio of 2:1, a glycerol conversion of 92 % was achieved after 30 min over the 10 wt % NaAlO2/hydrotalcite catalyst with almost 100 % selectivity towards glycerol carbonate. The texture and the crystallinity of the catalysts were strongly affected by the addition of NaAlO2. The high activity was clearly correlated with the boost in basicity brought about by sodium aluminate promotion. Whereas pristine hydrotalcites possess only weak basic sites, the basicity of the catalysts increased drastically upon promotion with NaAlO2, both in amount and strength. Diffuse reflectance infrared spectroscopy coupled with CO2 adsorption measurements revealed the presence of surface carbonates arising from strongly basic sites. Importantly, our study demonstrates that these basic catalysts are truly heterogeneous, stable, and reusable.

Elementary Steps in Heterogeneous Catalysis

In this chapter, a comprehensive introduction is provided along with a few illustrative examples to demonstrate adsorption, diffusion, reaction, and desorption as the elementary steps in heterogeneous catalysis. This is followed by examining in more detail three examples, CO adsorption/oxidation, CH4 activation, and NOx adsorption/reduction/oxidation from both a theoretical and an experimental point of view.

Highly Efficient Low-Temperature N-Doped TiO2 Catalysts for Visible Light Photocatalytic Applications

In this paper, TiO2 prepared with an aqueous sol-gel synthesis by peptization process is doped with nitrogen precursor to extend its activity towards the visible region. Three N-precursors are used: urea, ethylenediamine and triethylamine. Different molar N/Ti ratios are tested and the synthesis is adapted for each dopant. For urea- and trimethylamine-doped samples, anatase-brookite TiO2 nanoparticles of 6–8 nm are formed, with a specific surface area between 200 and 275 m2·g−1. In ethylenediamine-doped samples, the formation of rutile phase is observed, and TiO2 nanoparticles of 6–8 nm with a specific surface area between 185 and 240 m2·g−1 are obtained. X-ray photoelectron spectroscopy (XPS) and diffuse reflectance measurements show the incorporation of nitrogen in TiO2 materials through Ti–O–N bonds allowing light absorption in the visible region. Photocatalytic tests on the remediation of water polluted with p-nitrophenol show a marked improvement for all doped catalysts under visible light. The optimum doping, taking into account cost, activity and ease of synthesis, is up-scaled to a volume of 5 L and compared to commercial Degussa P25 material. This up-scaled sample shows similar properties compared to the lab-scale sample, i.e., a photoactivity 4 times higher than commercial P25.