Jean-Michel Tatibouët

Surface structure and reactivity of CrO3/SiO2 catalysts

The molecular structure of the two-dimensional chromium oxide overlayer on the silica support at different chromium oxide contents has been investigated by in situ Raman spectroscopy. Under dehydrated conditions, the surface chromium oxide species on silica consists of a highly distorted, tetrahedral monochromate species regardless of chromium oxide content. The catalysts above 2% CrO3/SiO2 also contain crystalline α-Cr2O3 particles in addition to the surface monochromate species. The methanol oxidation studies reveal that the catalytic activity per Cr atom, the turnover number (TON), decreases as the chromium oxide content increases. The somewhat higher TON of the initial rate as compared to the steady state rate for the methanol oxidation reaction reflects the higher activity of the fully oxidized chromium oxide species relative to the partially reduced chromium oxide species for the methanol oxidation reaction. The major reaction product is HCHO, while HCOOCH3, CO, and C02 are next in abundance. The selectivity for HCHO increases as the chromium oxide content increases and the precalcination temperature is increased. The opposite trend is observed for the selectivity of HCOOCH3. These selectivity changes are due to dehydroxylization of the silica surface by precalcining at elevated temperatures and increasing the surface chromium oxide species. The current results suggest that HCHO and HCOOCH3 are produced on two different catalytic sites: HCHO is formed on the Cr site, whereas HCOOCH3 is produced via hemiacetal intermediates, which are formed by interaction between HCHO adsorbed on the Cr site and CH3O adsorbed on the silica site. The CO and CO2 combustion products are produced on both the Cr and silica site.

Influence of chlorine ions in Pt/A12O3 catalysts for methane total oxidation

Residual chlorine ions on a Pt/Al2O3 catalyst surface prepared from chlorine-containing precursors appear to inhibit the total oxidation of methane. At 450°C, as chlorine is eliminated with time on stream, the reaction rate increases despite the sintering of the platinum particles. The steady state reaction rate which is reached after 60 h is identical to that obtained with a catalyst prepared from a precursor containing no chlorine. Whether chlorine is present or not in the initial state of the catalyst does not appear to have an influence on the evolution of the platinum particle size.

Properties of iron-based mesoporous silica for the CWPO of phenol: a comparison between impregnation and co-condensation routes.

Iron-based mesoporous silica materials were prepared according to different impregnation and co-condensation procedures. Several complementary techniques, including XRD, TEM/EDX and nitrogen sorption isotherms were used to evaluate the final structural and textural properties of the calcined Fe/SBA-15 materials. While Fe(2)O(3) isolated particles of which the size is close to the silica pore diameter ( approximately 7-8 nm) were obtained using classical wet impregnation procedure, smaller iron oxide particles ( approximately 2-4 nm) homogeneously dispersed within the hexagonal pore structure of the SBA15 host support were generated by self-combustion of an impregnated iron-glycinic complex. By contrast, the various co-condensation routes used in this work were less efficient to generate iron oxide nanoparticles inside the silica mesopores. Catalytic performances of the materials were evaluated in the case of total phenol oxidation by H(2)O(2) in aqueous solution at ambient conditions. Large differences in terms of catalytic activity and iron species stability were observed. While the impregnated solids proved to be the most active catalysts (highest Fe(2)O(3) nanoparticles dispersion), iron leaching was observed in aqueous solution, accounting for a homogeneous catalytic contribution. In contrast, the co-condensed samples exhibiting larger iron oxide clusters stabilized over the silica surface proved more efficient as active sites in Fenton catalysis.

Methane catalytic combustion on La-based perovskite type catalysts in high temperature isothermal conditions

The rate constants in isothermal conditions at 900C have been measured for a series of La1 x Srx FeO3 unsupported oxide (0x0.5) with the perovskite structure. The best catalytic performances were obtained, in terms of catalytic activity per weight of catalyst for xD0.2, and in terms of catalytic activity per unit surface area for 0.3 x0.5. A deactivation, mainly due to the sintering of the catalysts, was observed with the time on stream at 900C. This deactivation can be lowered by supporting the perovskite phase on BaAl12O19, but unfortunately, an important CO formation occurred, increasing with the time on stream. From pulses experiments, two oxidation sites behaving differently have been revealed: a very reactive one leading to a fast and selective CO2 formation and a less reactive one, selective for the COCH2 formation which could arise from the partial reduction of the very reactive site.

Combination of ball-milling and non-thermal atmospheric plasma as physical treatments for the saccharification of microcrystalline cellulose

Here we report that non-thermal atmospheric plasma (NTAP) can be used as a physical treatment for the extensive depolymerization of amorphous cellulose to low molecular weight cello-oligomers (DP = 36). Such NTAP treatment does not require addition of any solvent and catalyst, thus facilitating the recovery of cello-oligomers. Additionally, the NTAP treatment was found to be highly selective since degradation/oxidation of glucose units occurs to a rather small extent (purity of cello-oligomers >90%). These low molecular weight cello-oligomers recovered after NTAP treatment were then more prone to hydrolysis than native cellulose and were easily hydrolyzed to glucose over a cation exchange resin with an unprecedented yield of 58%.

Transient isotopic study of methanol oxidation on unsupported V2O5: Mechanism of methylal formation

The mechanism of methylal formation by methanol oxidation on unsupported V2O5 catalyst has been investigated in transient isotopic experiments, performing an exchange between CH3OH and CD3OD. Adsorbed methoxy groups and hemimethylal (CH3OCH2OH) have been identified as intermediate species, the hemimethylal being formed by reaction between an activated methanol species (CH2OH) and the surface methoxy groups. The methylal is then obtained by a dehydration reaction between hemimethylal and a methanol molecule or a surface methoxy group. We can also propose that the methyl formate formation (HCOOCH3) could occur by hemimethylal oxidative dehydrogenation, the hemimethylal (CH3OCH2OH) appearing then as the key intermediate in the methanol oxidation to methylal and methyl formate.

Methanol oxidation on MoO3/TiO2 catalysts

The reaction of methanol oxidation on MoO3/TiO2 catalysts shows different behaviour depending on the surface coverage with molybdena and on the specific surface area of the TiO2 support. The nature of the TiO2 support (anatase or rutile) seems to play only a minor role. Two different zones can be distinguished which correspond to a molybdena content higher and lower than a theoretical monolayer (mnl). The catalytic behaviour of samples with a low molybdena content (<1 mnl) depends largely on the specific surface area and on the surface coverage of the TiO2 support. Catalysts with a high molybdenum content (1 mnl) exhibit similar catalytic properties. The formation of methyl formate is observed only on high specific surface area catalysts and it involves methoxy groups formed by the reaction of methanol with the OH groups on the surface of the TiO2 support.

Total oxidation pathways in oxidative coupling of methane over lanthanum oxide catalysts

Abstract Lanthanum oxides calcined at low or high temperature are shown to present a surface composition corresponding to the phase La2O2(CO3) in the conditions of the oxidative coupling of methane. The C2 selectivity and productivity are improved on samples calcined at low temperature. At variance with the C2 formation, the total oxidation into CO2 would proceed mainly from surface reactions. The concentration and reactivity of the oxidation sites, determined from isotopic transient experiments, appear to depend on the procedure of catalyst preparation.

Effect of strontium doping on catalytic behaviour of lanthanum oxide on oxidative coupling of methane

The doping of lanthanum oxide with strontium maintains the good selectivity of the oxidative methane coupling for the catalysts prepared or calcined at high temperature (> 800 °C) by preserving the platelet shape of oxide particles.

Understanding of the oxygen activation on ceria- and ceria/alumina-supported gold catalysts: a study combining 18O/16O isotopic exchange and EPR spectroscopy

Gold supported on ceria or ceria–alumina mixed oxides are very active catalysts for total oxidation of a variety of molecules. The key step of the oxygen activation on such catalysts is still a matter of debate. Gold–ceria (Au/CeO2) and gold–ceria–alumina (Au/CeO2/Al2O3) catalysts were prepared by deposition–precipitation of gold precursor with urea as in former works where their efficiency to catalyze the oxidation of propene and propan-2-ol was demonstrated. To understand the phenomenon of oxygen activation over this class of catalysts, efficient techniques generally used to characterize the interaction between oxygen and cerium-based oxides were applied; the oxygen storage capacity (OSC) measurement, the 18O2/16O2 isotopic exchange study (OIE), as well as characterizations by in situ Raman and electron paramagnetic resonance (EPR) spectroscopies. Each of the techniques allowed showing the impact of the gold nanoparticles on the activation of dioxygen, on the kinetic governing the gas-phase/solid oxygen atom exchange, and on the nature and the location of the adsorbed oxygen species. Gold nanoparticles were shown to increase drastically the OSC values and the rate of oxygen exchange. OIE study demonstrated the absence of pure equilibration reaction (16O2(g) + 18O2(g) ↔ 2 16O18O(g)), indicating that gold did not promote the dissociation of dioxygen. Peroxo adspecies were observed by Raman spectroscopy only in the presence of gold. On the contrary, EPR spectroscopy indicated that the concentration of superoxo adspecies was lower for oxide-supported gold samples than for bare oxides. The combination of techniques allowed reinforcing the hypothesis that the gold nanoparticules promote the activation of dioxygen by generating extremely mobile diatomic-oxygenated species at the gold/ceria interfacial perimeter. This specific gold–ceria interaction, which leads to the increase in oxygen mobility, is probably also responsible for the higher catalytic performance of Au/CeO2 and Au/CeO2/Al2O3 in oxidation reaction compared to bare supports.