Franck Morfin

Stereoselective stilbene epoxidation over supported gold-based catalysts

The gold reference catalyst Au/TiO(2) exhibits high activity in the stereoselective epoxidation of trans-stilbene in methylcyclohexane in the presence of 5 mol% TBHP, by taking part in a chain reaction involving the activation of molecular oxygen by a radical produced from methylcyclohexane.

Structures and associated catalytic properties of well-defined nanoparticles produced by laser vaporisation of alloy rods

Bimetallic clusters, all containing gold, have been produced by laser vaporisation of bulk alloys followed by deposition of the formed clusters onto Al2O3 and TiO2 powders or flat silica supports. This technique allows a narrow size distribution of highly dispersed gold-based nanoparticles on powders and nanocrystalline structured thin films on 2D supports to be obtained. The catalytic performances of the as-obtained AuFe, AuNi, AuTi powdery catalysts have been studied in the PROX reaction and compared with those obtained in the oxidation of CO in the temperature range 25-300 degrees C. By comparing the activities of the different catalysts, it is concluded that the nature of the gold partner directly affects the activity of gold. The following tendency is observed: AuFe and AuNi have rather similar activities, significantly lower than that of AuTi. In this paper, we also present a first attempt to study reactivity of original self-supported systems. We show that significant CO oxidation reactivity can be obtained over unsupported nanoporous AuTi and PdAu thin films. By completely excluding the support effect, unsupported catalysts could provide a way of understanding the relevant catalytic mechanisms more easily.

Ultrastable iridium–ceria nanopowders synthesized in one step by solution combustion for catalytic hydrogen production

Mesoporous ceria loaded with 0.06–0.93 wt% iridium was synthesized in one step by the ambient air combustion of an aqueous solution of ceric ammonium nitrate, ammonium hexachloroiridate, and glycine fuel. The structural properties of the powders, and the influence of such parameters as Ir loading and thermochemical post-treatments, were investigated combining aberration-corrected HRTEM, SEM, in situ XRD, XPS, DRIFTS, and Raman spectroscopy. The materials, which appeared spongy at the micrometer scale, exhibited ca. 30 nm-sized ceria crystallites with a layered structure at the nanoscale. After reducing treatments, Ir nanoparticles anchored at the surface of ceria grains were identified, and their size (ca. 2 nm) did not evolve upon further heating at up to 900 °C. A detailed picture of the Ir–CeO2 interface could be established, with the presence of Irx+–O2−–Ce3+ entities along with oxygen vacancies. The powders loaded with only 0.1 wt% Ir were successfully employed as catalysts for the production of hydrogen from methane and water in low-steam conditions at 750 °C. Due to their higher Ir dispersion and stronger Ir–CeO2 interaction, the combustion-synthesized materials outperformed their conventionally prepared counterparts in terms of activity and stability, making them promising as active catalytic layers for solid-oxide fuel cells integrating the gradual internal reforming concept.

Effect of the titania morphology on the preparation of Au/TiO2(/SiO2) catalysts

Abstract The deposition-precipitation method is applied to pure anatase titanias with high surface areas and silica mesostructures doped with Ti/TiO 2 . The effect of the type and density of surface functionalities both on the deposition of gold and on the catalytic properties of the resulting materials in the oxidation of CO in the presence of H 2 are discussed.

A versatile elevated-pressure reactor combined with an ultrahigh vacuum surface setup for efficient testing of model and powder catalysts under clean gas-phase conditions

A small-volume reaction cell for catalytic or photocatalytic testing of solid materials at pressures up to 1000 Torr has been coupled to a surface-science setup used for standard sample preparation and characterization under ultrahigh vacuum (UHV). The reactor and sample holder designs allow easy sample transfer from/to the UHV chamber, and investigation of both planar and small amounts of powder catalysts under the same conditions. The sample is heated with an infrared laser beam and its temperature is measured with a compact pyrometer. Combined in a regulation loop, this system ensures fast and accurate temperature control as well as clean heating. The reaction products are automatically sampled and analyzed by mass spectrometry and/or gas chromatography (GC). Unlike previous systems, our GC apparatus does not use a recirculation loop and allows working in clean conditions at pressures as low as 1 Torr while detecting partial pressures smaller than 10(-4) Torr. The efficiency and versatility of the reactor are demonstrated in the study of two catalytic systems: butadiene hydrogenation on Pd(100) and CO oxidation over an AuRh/TiO2 powder catalyst.

Investigation of the local structure of nanosized rhodium hydride

The local structure and the thermal stability of small and well-dispersed RhHx nanoparticles (average size of 1.4 nm) were studied by in situ X-ray Absorption Spectroscopy. The RhHx nanoparticles are stable at room temperature and undergo a structural transition from hydride (fcc) to metal phase (fcc) with a shrinking of the lattice volume due to the desorption of hydrogen. This phase transition occurs in the temperature range of 150-180 °C, in good agreement with the results from thermo-desorption spectroscopy. Above 180 °C, the desorbed nanoparticles undertake important coalescence. In situ transmission electron microscopy performed up to 300 °C proves that this process cannot be only thermal, thus it may be ascribed to a X-ray beam effect.

Absorbed hydrogen enhances the catalytic hydrogenation activity of Rh-based nanocatalysts

Carbon-supported Rh-based nanoparticles with an average size of 1.0 nm have been tested for the hydrogenation of butadiene in two different forms: either metal Rh or hydride RhHx nanoparticles. Laboratory tests demonstrated that the Rh hydride nanocatalyst is more active than its metal counterpart, irrespective of the gas feed composition and temperature. Moreover, this difference is significantly more important at the initial stage of the reaction compared to that under quasi-stationary conditions. However, the reaction mechanisms appear similar for the metal and hydride nanocatalysts, as suggested by the similar selectivities to butenes, apparent reaction energies, and reaction orders. The local structures of both Rh and RhHx nanocatalysts were studied by operando XAS under stationary conditions. EXAFS analyses confirm that the RhHx and Rh catalysts preserve their structure during the reaction as either the hydride or metal phase, respectively. We suggest that the Mars–van Krevelen mechanism might occur at the initial stage but becomes progressively less predominant. Under quasi-stationary conditions, only electronic effects are invoked to explain the activity difference between the hydride and the metal phases. We hypothesize that the stabilization of Rh–H bonds at the surface in the presence of subsurface hydrogen, consistent with previous theoretical findings, explains the higher activity of the hydride catalyst.

Uncovering the three-dimensional structure of combustion-synthesized noble metal-ceria nanocatalysts

With its unique redox properties, ceria is an oxide with a range of applications, including automotive catalytic converters, which consist of platinum‐group metal nanoparticles on ceria‐containing supports. In this work, the 3 D architecture of a ceria‐based material synthesized by the widely employed glycine‐nitrate solution combustion method is revealed for the first time. Together with N2 adsorption volumetry, scanning transmission electron microscopy (STEM) and scanning electron microscopy (SEM), STEM tomography provides a comprehensive picture of the multimodal porous network of a pre‐reduced Pt‐CeO2 catalyst, from the nanometer to the micrometer scale. This material consists of ceria nanocrystallites forming 3 D aggregates and puzzle‐like 2 D walls separating large roundish mesopores and macropores. The small voids between imperfectly assembled crystallites give rise to some microporosity. In addition, it is demonstrated that a significant proportion of platinum nanoparticles (3–4 nm) are not located at the ceria surface following the one‐step synthesis process, about half of them are buried within ceria. This result is valid for another metal (Rh) and another fuel (oxalyl dihydrazide), and has important implications for heterogeneous catalysis.

A First Orienting Investigation of the Interaction of Cable Fire Products with Passive Autocatalytic Recombiners

Abstract Passive autocatalytic recombiners (PARs) have been installed inside light water reactor containments in many countries to remove hydrogen and, thus, to mitigate the combustion risk during a severe accident (SA). Due to the challenging SA boundary conditions, PARs are exposed to several deactivation risks during operation, which may cause a reduction of the hydrogen removal capacity. Such a deactivation may occur through different mechanisms and could in principle affect the start-up behavior up to the full loss of catalytic activity. To assess the interaction of PARs with the products of cable fires, a set of PAR catalyst samples has been introduced to the atmosphere of cable fire tests performed at Institut de Radioprotection et de Sûreté Nucléaire (IRSN), France. The subsequent surface analyses performed at Forschungszentrum Jülich (Germany) reveal a significant amount of carbon, chlorine (a constituent of polyvinyl chloride), zinc, and antimony (a flame retardant) on all catalyst samples compared to reference samples. The subsequent performance tests confirm that all catalyst sheets suffer a significant start-up delay of between 17 and 45 min compared to the reference samples. However, after burning off the soot deposition, the catalyst samples reach full conversion capacity and show immediate start-up behavior in a subsequent test. The present results clearly demonstrate the adverse effect of cable fire products on the efficiency of hydrogen conversion in a PAR. To further understand and quantify the impact of cable fire products and to assess their relevance for SA scenarios, further experimental as well as theoretical investigations are required. In particular, the combined influence of cable fire products and humidity, which has intentionally been omitted in the present study, should be investigated in the future due to the possible corrosive impact on the catalyst as well as the influence of humidity on the nature of the soot deposition.