Nicolas Bion

Design of nanocatalysts for green hydrogen production from bioethanol.

Bioethanol is an interesting feedstock that may be used for hydrogen production by steam or autothermal reforming. However, the impurities (heavy alcohols, esters, acids, N compounds) contained in the raw feedstock require a costly purification, as they have a dramatic impact on catalyst activity and stability. Thus, a method that can utilize the raw feedstock without severe degradation of the catalyst would be desirable. In this Minireview, the composition of bioethanol from first and second generation biomass, the reactions involved in the catalytic ethanol steam reforming process and the design of catalysts adapted for hydrogen production from a real bioethanol feed are surveyed.

Ordered benzene–silica hybrids with molecular-scale periodicity in the walls and different mesopore sizes

Mesoporous benzene–silica hybrid materials have been obtained via surfactant-mediated synthesis using alkyltrimethylammonium surfactants with alkyl chain lengths from 14 to 18. By combining N2 adsorption–desorption experiments and powder X-ray diffraction (PXRD) analysis we showed that these samples exhibited mesopore sizes in the range 3.2 to 3.9 nm and that the molecular-scale periodicity in the walls of these materials is maintained. Transmission electron microscopy (TEM), solid state nuclear magnetic resonance (NMR) and thermogravimetric analyses (TGA) have also been used to characterise the structure and stability of the samples.

Understanding the high catalytic activity of propylsulfonic acid-functionalized periodic mesoporous benzenesilicas by high-resolution 1H solid-state NMR spectroscopy

Propylsulfonic acid-functionalized periodic mesoporous benzenesilica (Ph-PMO-SO3H, 1) has been shown to be exceptional solid catalysts in the acid-catalyzed condensation of indole on benzaldehyde. The reasons for this distinct behavior are so far not completely understood. Here, we present a study involving the combination of advanced high-resolution solid state magic-angle spinning (MAS) NMR characterization with the results of the application of hydrated and dehydrated 1 with different acid loadings in the acid-catalyzed condensation of indole on benzaldehyde attempting an explanation of the higher performance of these materials when compared with the conventional solid catalysts. 1H MAS NMR investigations show the displacement of the propylsulfonic –SO3H protons to higher 1H chemical shifts with increase of the sulfonic acid strength suggesting the formation of hydrogen bonds involving neighboring –SO3H groups. The acid strength of 1 is lowered by the presence of water. At low sulfonic acid loading the catalytic activity is surprisingly high and independent of the water presence. The 2D 1H–1H recoupling MAS NMR experiments indicate that the phenyl rings may protect the acidic sites against water solvation, thus affording a plausible explanation for the negligible effect of water on the catalytic activity of 1 with low acid loading. For a proton loading higher than 0.40 mmol g−1, we observed a linear relationship between the catalyst TOF and the chemical shift value of the –SO3H proton, thus showing that solid 1H NMR appears to be a convenient tool to predict the catalytic activity of 1 in water.

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.

A Study of 15N/14N Isotopic Exchange over Cobalt Molybdenum Nitrides

The 14N/15N isotopic exchange pathways over Co3Mo3N, a material of interest as an ammonia synthesis catalyst and for the development of nitrogen transfer reactions, have been investigated. Both the homomolecular and heterolytic exchange processes have been studied, and it has been shown that lattice nitrogen species are exchangeable. The exchange behavior was found to be a strong function of pretreatment with ca. 25% of lattice N atoms being exchanged after 40 min at 600 °C after N2 pretreatment at 700 °C compared to only 6% following similar Ar pretreatment. This observation, for which the potential contribution of adsorbed N species can be discounted, is significant in terms of the application of this material. In the case of the Co6Mo6N phase, regeneration to Co3Mo3N under 15N2 at 600 °C occurs concurrently with 14N15N formation. These observations demonstrate the reactivity of nitrogen in the Co–Mo–N system to be a strong function of pretreatment and worthy of further consideration.

Clear microstructure-performance relationships in Mn-containing perovskite and hexaaluminate compounds prepared by activated reactive synthesis.

Microstructural properties of mixed oxides play essential roles in their oxygen mobility and consequently in their catalytic performances. Two families of mixed oxides (perovskite and hexaaluminate) with different microstructural features, such as crystal size and specific surface area, were prepared using the activated reactive synthesis (ARS) method. It was shown that ARS is a flexible route to synthesize both mixed oxides with nano-scale crystal size and high specific surface area. Redox properties and oxygen mobility were found to be strongly affected by the material microstructure. Catalytic activities of hexaaluminate and perovskite materials for methane oxidation were discussed in the light of structural, redox and oxygen mobility properties.

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.

Bioethanol reforming for H2 production. A comparison with hydrocarbon reforming

Hydrogen is essentially produced by steam reforming (SR) of hydrocarbon fractions (natural gas, naphtha, …) on an industrial scale. Replacing fossil fuels by biofuels for H2 production has attracted much attention with an increased interest for bioethanol steam reforming. Kinetics and mechanisms of hydrocarbon-SR and alcohol-SR present some similarities but also some very important differences due to alcohol reactivity much more complex than that of hydrocarbons. The scope of this report is to compare the two processes in terms of reaction mechanisms. Attention will also be paid to the case of crude bioethanol.

Isotopic Oxygen Exchange over Pd/Al2O3 Catalyst: Study on C18O2 and 18O2 Exchange

Oxygen mobility on Pd/Al2O3 catalysts is generally considered to be rather slow compared with other noble metal catalysts. This study shows that oxygen can be exchanged from gas‐phase CO2 produced during, for example, methane oxidation, more rapidly than from O2 molecules. The oxygen exchange from C18O2 occurs mainly on Al2O3, whereas 18O2 exchange needs to be carried out in the presence of palladium. However, the presence of Pd on Al2O3 slightly enhances C18O2 exchange according to the number of exchanged atoms in the gas phase. In this case, the calculation of initial rate for C18O2 exchange is not adequate to evaluate oxygen mobility; information on the number of exchanged atoms and on the evolution of the concentrations of the different isotopomers is also needed. The enhancement of oxygen exchange is due to carbonate formation on the catalytic surface, as evidenced by in situ FTIR spectroscopic measurements. A five‐step mechanism is suggested to explain these findings.

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.