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The hydrogenation of pyridine adsorbed on zeolite is dependent on the distance between acid and metal sites. Hydrogen species produced in the metal diffuse into pyridine and promote hydrogenation, suggesting a bifunctional mechanism is occurring.

This study proposes the steam reforming of a synthetic biogas stream containing 200 ppm of H2S, carried out in a non-commercial supported Pd-Au/Al2O3 membrane reactor (7-8 mu m selective layer thickness) at 823 K and 150 kPa over a non-commercial Rh(1%)/MgAl2O4/Al2O3 catalyst. This system is able to recover almost 80% of the total hydrogen produced during the reaction and shows good resistance to the H2S contamination, as confirmed by stable methane conversions for more than 400 h under operation. For comparison, the same reaction was carried out in a commercial self-supported Pd-Ag membrane (150 mu m wall thickness), yielding a hydrogen recovery equal to 40% at 623 K and 200 kPa, and presenting stable methane conversions for less than 200 h under operation due to the effect of the H2S contamination. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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fff; International audience; MnOx/SAPO-34 bifunctional catalysts are efficient for the conversion of syngas to light olefins. However, the reaction mechanism is still debated in particular the nature of the intermediate formed on MnOx (ketene vs methanol). In this study, it was evidenced from catalytic data and in situ DRIFT measurements that methanol is a key reaction intermediate produced on MnOx that synergistically reacts with SAPO-34 to produce light olefins.

Context and objectivesLight olefins (C2–C4) are key building-block chemicals mostly produced either by steam cracking or fluid catalytic cracking of oil resources, two highly energy-consuming processes. Among alternative processes, the syngas via methanol to olefins (SMTO) and the OX-ZEO single step processes are very promising since very high selectivity to light olefins can be reached. They combine one hydrogenating catalyst to synthesize either methanol or ketene and one acidic zeotype to convert the intermediate into light olefins. In both cases, the intermediate has to diffuse from the first catalyst to the other and their intimacy can significantly influence the catalytic performances.In this work, Cu/ZnO/Al2O3 or MnOx was used as hydrogenating catalyst and SAPO-34 as acidic zeotype The role of intimacy has been investigated by modulating the density of contact between the two functions in different bed arrangements and the distance through preparing MnO@SiO2 core shell structures or diluting the two catalysts in SiO2.Material and methodsCu/ZnO/Al2O3 was commercial catalyst while MnOx and MnOx@SiO2 were prepared from MnCO3 powder or monodispersed suspension by different routes. SAPO-34 powders with different acidities were prepared by hydrothermal method. The catalysts were characterized before and after reaction by various techniques, including ICP/XRF, BET, H2 and CO-TPR, NH3-TPD, XRD, SEM, STEM-HAADF, NMR as well as by in situ DRIFT and Raman spectroscopies. Catalytic performances were compared using a fixed-bed reactor with a feed H2/CO/N2:62/23/15 at 390-450 °C and 25 bar.Main resultsFor both catalytic systems, methanol was shown to be a key intermediate from catalytic data and from the in situ DRIFT observation and evolutions of formates (Fig. 1a) which are formed before methoxy species. The two catalytic systems were active and selective (>50%) to C2-C4 hydrocarbons with a low Olefin/Paraffin (O/P) ratio (

International audience; Copper-supported mesoporous carbon nanocatalysts (Cu/FDU-15) were synthesized using an easy and convenient one-pot soft-template method for low-overvoltage CO2 electroreduction. TEM imaging revealed the presence of large Cu nanoparticles (diameter 140nm) with Cu2O nanoparticles (16nm) as an additional phase. From the electron tomography observations, we found that the copper particles were placed inside and on the exterior surface of the porous FDU-15 support, providing an accessible surface for electrocatalytic reactions. CO2 electrolyses showed that the mesostructured Cu/FDU-15-350 cathode materials were active towards CO2 conversion to formic acid with 22% Faradaic efficiency at a remarkably low overpotential of 290mV, hydrogen being the only side-product. The catalyst's activity correlates to the calculated metallic surface area, as determined from a geometrical model, confirming that the mesoporous channels act as a diffusion path for the CO2 molecule, and that the whole Cu surface is accessible to CO2, even if particles are entrapped in the carbon matrix.

Mesoporous carbon structures containing copper particles were synthesized according to different synthesis methods in order to tune the morphostructural properties of these electrocatalysts for the CO2 reduction reaction. Microwave polyol synthesis, hard templating, one pot soft templating and evaporation induced self assembly gave access to different mesoporous structures. Copper particles were identified inside the mesoporous structures, while the porous channels remained unaffected leading to confined nano reactors. Bulk electrolyses showed that only two CO2 reduction products are obtained: HCOOH as the main product with a low amount of CO, H2 completing the faradaic balance. Differences in steady state current densities during electrolyses were correlated to the Cu metallic specific surface area. The latter was determined according to a geometrical model based on truncated octahedron clusters developed for copper clusters. This correlation evidences that confined Cu particles remains still electroactive and paves a way for the efficient development of 3D electrodes.

International audience; Hydrogenolysis of methylcyclopentane was studied in the presence of supported platinum catalysts, with particle sizes between 0.9 and 2.4 nm. The distribution of ring-opening products (n-hexane, 2-methylpentane, 3-methylpentane), which are virtually the only products at 573 K, mainly depended on the platinum particle size, with no effect of the methylcyclopentane conversion. This distribution was well described by the selective or non-selective mechanisms previously proposed in the literature except for the smallest particles (d

Copper-supported mesoporous carbon nanocatalysts (Cu/FDU-15) were synthesized using an easy and convenient one-pot soft-template method for low-overvoltage CO

International audience; Chemisorption of probe molecules such as hydrogen and carbon monoxide on the surface of Pt particles is the most common chemical technique used to estimate the crucial parameters of metal catalysts, namely the dispersion (D), the particle size (d), and the metallic specific surface area (SPt). However, it remains a controversy concerning the stoichiometry of adsorbate per surface metal atom, leading to an inaccurate estimation of D, d, and SPt. A model describing the statistics of the surface atoms and sites on perfect cuboctahedron clusters was developed to assess values of D, d, and SPt, assuming the most favorable adsorption sites based on density functional theory (DFT) calculation of the literature. This model successfully predicted the experimental values of D, d, and SPt determined from H or CO chemisorption data, and it allowed providing a set of simple equations for the accurate determination of these parameters from chemisorption experiments on Pt.

Highly selective catalysts for methanol synthesis from CO 2 and H 2 are required for increasing the process efficiency that actually produces large amounts of CO. The understanding of the Cu–ZnO synergy allowed the possibility to design core–shell catalysts of the type Cu@ZnO x with enhanced selectivity for methanol. This type of core–shell structure was synthesized with variable compositions and the catalytic efficiency as well as the physicochemical properties were scrutinized in order to build structure–reactivity relationships. Zn migration is responsible for the formation of the Cu x Zn (1− x ) O y active phase. The experimental results were compared with theoretical calculations based on a geometrical model taking into account the amount of Zn migrated. A direct correlation was found between theory and experiment. Finally, a linear correlation of catalyst activity versus Zn migrated is shown for three different catalyst designs, thus confirming that a high Zn migration is needed for obtaining an efficient methanol synthesis catalyst.

International audience; Chemisorption of hydrogen on metallic particles is often used to estimate the metal dispersion (D), the metal particle size (d), and the metallic specific surface area (SM), currently assuming a stoichiometry of one hydrogen atom H adsorbed per surface metal atom M. This assumption leads to a large error when estimating D, d, and SM, and a rigorous method is needed to tackle this problem. A model describing the statistics of the metal surface atom and site distribution on perfect cuboctahedron clusters, already developed for Pt, is applied to Pd, Ir, and Rh, using the density functional theory (DFT) calculation of the literature to determine the most favorable adsorption sites for each metal. The model predicts the H/M values for each metal, in the range 0–1.08 for Pd, 0–2.77 for Ir, and 0–2.31 for Rh, depending on the particle size, clearly showing that the hypothesis of H/M = 1 is not always confirmed. A set of equations is then given for precisely calculating D, d, and SM for each metal directly from the H chemisorption results determined experimentally, without any assumption about the H/M stoichiometry. This methodology provides a powerful tool for accurate determination of metal dispersion, metal particle size, and metallic specific surface area from chemisorption experiments.

International audience; The use of raw bioethanol is of major importance for a cost effective industrial application. Raw bioethanol contains higher alcohols as the main impurities and also aldehydes, amines, acids and esters. The effect of these impurities on the catalytic performances for ethanol steam reforming (ESR) has been studied, using a reference catalyst, Rh/MgAl2O4. It was shown that the aldehyde, the amine and methanol have no negative effect on the catalytic performances, contrary to the ester, acid and higher alcohols. The deactivation is mainly explained by coke formation favored by the presence of these impurities in the feed. In order to improve the stability of the catalyst and its performances in the presence of these deactivating impurities, the catalyst formulation, i.e. the composition of the support and of the metallic phase, was modified. The addition of rare earth elements instead of magnesium to the alumina support leads to a decrease of the strong and medium acid sites and to an increase of the basicity. On these modified supports, the dehydration reaction, leading to olefins, which are coke precursors, is disfavored, the ethanol conversion and the hydrogen yield are increased. The best catalytic performances were obtained with Rh/Y-Al2O3. Then, the metallic phase was also modified by adding a second metal (Ni, Pt or Pd). The Rh-Ni/Y-Al2O3 catalyst leads to the highest hydrogen yield. This catalyst, tested in the presence of raw bioethanol during 24 h was very stable compared to the reference catalyst Rh/MgAl2O4, which was strongly deactivated after 2 h of time-on –stream.

International audience; The effect of the addition of a wide range of rare earth elements (Sc, Y, La, Er and Gd) on the catalytic behavior of Rh/Al2O3 base catalyst in the ethanol steam reforming (ESR) reaction was investigated. Physical−chemical properties of Rh−REOx−Al2O3 materials have been evaluated by H2 chemisorption, XRD, BET, TPR, IR spectroscopy and TPO. It was established that the catalytic behavior of the Rh/Al2O3 base catalyst is widely improved by the promotion with rare earth (RE) elements. It has been shown that the RE elements do not have an important impact on the oxidation state and the dispersion of the metallic centers but greatly alter the acid−base properties of the support. It was demonstrated that the catalytic performances of Rh−REOx−Al2O3 catalysts are mainly due to the increase of the support basicity and to the rearrangement of acid sites. The presence of a new type of Lewis acid sites (LAS) of weak strength on Rh−REOx−Al2O3 materials leads to decrease the production of coke precursor (C2H4). Finally, the most basic support leads to the highest hydrogen yield and to the less stable coke.

CuO – CeO 2 catalyst precursors were synthesized by homogeneous thermal decomposition of urea. Catalysts containing different Cu/Ce ratios were obtained after calcination of the precursors at 450 ∘ C and characterized by ICP-OES, N 2 adsorption, XRD and TPR. Catalytic tests were performed in the temperature range 100 – 300 ∘ C to evaluate the activity of these samples for the COPROX reaction. A strong synergy between Cu and Ce is evidenced by the difference in the catalytic activity of the individual oxides (CuO and CeO 2 ), a mechanical mixture of both oxides and a Cu–Ce mixed catalyst. The synergy between Cu and Ce is confirmed by the improvement of the reducibility of both cations in the mixed samples. Nitric acid and ammonium carbonate washed catalysts, especially high-copper-content samples, are more active than the untreated catalysts suggesting that the washing procedure could provoke a destruction of the polycrystalline structure that improves the contact between the reactants and the catalytic sites.

A one-step CO2 hydrogenation reaction into hydrocarbons (HC) using a bifunctional system constituted by a methanol synthesis catalyst [Cu–ZnO–Al2O3 (CZA)] and a zeolite (HZSM-5) has been studied. The influence of the catalyst bed configuration on activity, selectivity, and HC yield has been evaluated. The results obtained at TR = 623 K, PR = 3.0 MPa and WHSV = 6000 h−1 show that CO2 hydrogenation and hydrocarbon selectivity were strongly influenced by the proximity between oxide and zeolite, whatever the disposition of the two catalytic active sites. Indeed, the highest conversion and the best yield of hydrocarbons (mainly C2) were obtained with the M1 bifunctional catalysts in which the oxide–zeolite proximity is the lowest. This is ascribed to the hydrogen spillover phenomenon, which does not promote the carbon chain growth.

Electroreduction is an attractive way for converting CO2 to energetically relevant molecules due to the mild conditions of temperature and pressure used in these processes. CO2 reduction to formic acid is also of great interest since the latter is a very promising hydrogen carrier1. Therefore, there is a need in the development of cheap and efficient electrocatalysts able to perform the CO2 reduction reaction at low overpotential for the further development of energetically efficient CO2 conversion systems2. One of the most attractive metal catalyst for this reaction is copper due to its unique ability to reduce CO2 into a wide class of compounds. The way the catalyst is synthesized can dramatically tune the selectivities. Thus, a careful control of the catalyst surface properties as well as the supporting material are critical to reach high current densities and high faradaic yields. In this study, we discuss on the role of the carbon support morphology on the global performances of this electrochemical reaction. Copper nanoparticles are supported on several carbon morphologies, from amorphous (Vulcan X72R) to highly organized mesoporous structures (CMK3, FDU15). The nucleation and growth of the nanoparticles are also investigated by using the microwave assisted polyol synthesis and a one pot soft templating method. Electron tomography allowed evidencing that copper nanoparticles can be embedded inside the mesoporous structure while keeping the pore structure unaffected (Fig. 1a), thus ensuring efficient diffusion path for CO2 access to the catalytic sites. Cu nanoparticles supported onto mesoporous carbon are found to be ideal candidates to increase the active electrode surface area and to reach current densities in the range of 10 mA cm-2 (Fig. 1b) in a complete electrolyser configuration. The activity of the different electrocatalysts is following the trend of the exposed metallic surface area, as determined by a novel methodology3 based on the surface statistics on a truncated octahedron cluster structure. Nevertheless, the competitive water reduction reaction leading to hydrogen levels off the faradaic yield for formic acid to ca. 24%. This intrinsic limitation is related to the onset potentials of the two competitive reactions, as evidenced by a coupled electrochemical and mass spectroscopic study. The strategy of alloying Cu with another metal to improve formic acid faradaic yield is also discussed. References 1. Lu, X.; Leung, D. Y. C.; Wang, H.; Leung, M. K. H.; Xuan, J., Electrochemical Reduction of Carbon Dioxide to Formic Acid. ChemElectroChem 2014, 1, 836-849. 2. Tatin, A.; Comminges, C.; Kokoh, B.; Costentin, C.; Robert, M.; Savéant, J.-M., Efficient electrolyzer for CO2 splitting in neutral water using earth-abundant materials. Proceedings of the National Academy of Sciences 2016, 113, 5526-5529. 3. Le Valant, A.; Comminges, C.; Can, F.; Thomas, K.; Houalla, M.; Epron, F., Platinum Supported Catalysts: Predictive CO and H2 Chemisorption by a Statistical Cuboctahedron Cluster Model. The Journal of Physical Chemistry C 2016.in press (doi 10.1021/acs.jpcc.6b09241). Figure 1

International audience; A series of Cu-ZnO/Al2O3 catalysts prepared by coprecipitation were used for methanol synthesis by CO2 hydrogenation in a fixed bed reactor system. The effect of the catalysts composition and the reaction temperature on the catalytic activity was investigated. The main results show that the highest CO2 conversion and the best yield of methanol are obtained with the catalyst containing 51 wt% Cu and 22 wt% Zn. This result is assigned to the highest metallic copper surface area and to the interaction between copper and zinc oxide. However, the reaction temperature increase is disadvantageous for the methanol synthesis reaction.

The object of the present study was to study the role of alcohols, which are the major impurities found in a crude bioethanol feed, on the catalytic performances of a 1%Rh/MgAl2O4/Al2O3 catalyst during ethanol steam reforming. The alcohols studied were methanol, propan-1-ol, butan-1-ol, pentan-1-ol, isopropanol, 2-methyl propan-1-ol, 3-methyl butan-1-ol. Whereas the presence of 1% of methanol in the ethanol and water feed only slightly increased the hydrogen yield, the addition of the same amount of higher alcohols strongly decreased the stability of the catalyst during ethanol steam reforming, with a direct impact on the ethanol conversion and the hydrogen yield. It was shown that the deactivation is increased when the amount of carbon atoms in the molecule is increased. This effect is more pronounced in the presence of branched alcohols compared to the linear ones. It was demonstrated by studying the steam reforming of these higher alcohols that they are dehydrated to the corresponding olefin. The strong deactivation of the catalyst observed in the presence of higher alcohols was explained in terms of coke deposition.