Search

Recent work has demonstrated that, for bimetallic Au–Pd systems, the rate of catalytic alcohol and formyl dehydrogenation (DH) is intrinsically linked to the rate of oxygen reduction (ORR) within the same system.

In oxidation reactions catalysed by supported metal nanoparticles with oxygen as the terminal oxidant, the rate of the oxygen reduction can be a limiting factor. This is exemplified by the oxidative dehydrogenation of alcohols, an important class of reactions with modern commercial applications1–3. Supported gold nanoparticles are highly active for the dehydrogenation of the alcohol to an aldehyde4 but are less effective for oxygen reduction5,6. In contrast, supported palladium nanoparticles are less active than gold for dehydrogenation but offer high efficacy for oxygen reduction5,6. This imbalance can be overcome by alloying gold with palladium which gives enhanced activity to both reactions7,8; however, the electrochemical potential of the alloy is a compromise between that of the two metals meaning that although the oxygen reduction is improved in the alloy, the dehydrogenation activity is poorer. Here we show that by separating the gold and palladium components in bimetallic carbon-supported catalysts we can almost double the reaction rate beyond that achieved with a corresponding alloy catalyst. We demonstrate this using physical mixtures of carbon-supported monometallic gold and palladium and a bimetallic catalyst comprising separated gold and palladium regions. Furthermore, we demonstrate electrochemically that this enhancement is attributable to the coupling of separate redox processes occurring at isolated gold and palladium sites. The discovery of this novel catalytic effect, a cooperative redox enhancement (CORE), offers a new approach to the design of multi-component heterogeneous catalysts.

Low temperature oxidation of alcohols over heterogeneous catalysts is exceptionally challenging, particularly under neutral conditions. Herein, we report on an efficient, base-free method to oxidise glycerol over a 0.5%Pd-0.5%Fe/SiO2 catalyst at ambient temperature in the presence of gaseous H2 and O2. The exceptional catalytic performance was attributed to the in situ formation of highly reactive surface-bound oxygenated species, which promote the dehydrogenation on the alcohol. The PdFe bimetallic catalyst was determined to be significantly more active than corresponding monometallic analogues, highlighting the important role both metals have in this oxidative transformation. Fe leaching was confirmed to occur over the course of the reaction but sequestering experiments, involving the addition of bare carbon to the reactions, confirmed that the reaction was predominantly heterogeneous in nature. Investigations with electron paramagnetic resonance spectroscopy suggested that the reactivity in the early stages was mediated by surface-bound reactive oxygen species; no homogeneous radical species were observed in solution. This theory was further evidenced by a direct H2O2 synthesis study, which confirmed that the presence of Fe in the bimetallic catalyst neither improved the synthesis of H2O2 nor promoted its decomposition over the PdFe/SiO2 catalyst.

TiO2 nanocrystals with controlled facets have been extensively investigated due to their excellent photocatalytic performance in sustainable and green energy field. However, the applications in thermal catalysis without applying UV irradiation are comparably less and the identification of their intrinsic roles, especially the different catalytic behaviors of each crystal facet, remains not fully recognized. In this study, bimetallic AuPd nanoparticles supported on anatase TiO2 nanosheets exposing {0 0 1} facets or TiO2 nanospindles exposing {1 0 1} as a catalyst were prepared by sol-immobilization method and used for solvent-free benzyl alcohol oxidation. The experimental results indicated that the exposed facet of the support has a significant effect on the catalytic performance. AuPd/TiO2-001 catalyst exhibited a higher benzyl alcohol conversion than that of the AuPd/TiO2-101. Meanwhile, all the prepared AuPd/TiO2 catalysts were characterized by XRD, ICP-AES, XPS, BET, TEM, and HRTEM. The results revealed that the higher number of oxygen vacancies in TiO2-sheets with the exposed {0 0 1} facets of higher surface energy could be responsible for the observed enhancement in the catalytic performance of benzyl alcohol oxidation. The present study displays that it is plausible to enhance the catalytic performance for the benzyl alcohol oxidation by tailoring the exposed facet of the TiO2 as a catalyst support.

In oxidation reactions catalysed by supported metal nanoparticles with oxygen as the terminal oxidant, the rate of the oxygen reduction can be a limiting factor. This is exemplified by the oxidative dehydrogenation of alcohols, an important class of reactions with modern commercial applications

TiO

International audience; Today there is a consensus regarding the potential of NaBH4 as a good candidate for hydrogen storage and release via hydrolysis reaction, especially for mobile, portable and niche applications. However as gone through in the present paper two mains issues, which are the most investigated throughout the open literature, still avoid NaBH4 to be competitive. The first one is water handling. The second one is the catalytic material used to accelerate the hydrolysis reaction. Both issues are object of great attentions as that can be noticed throughout the open literature. This review presents and discusses the various strategies which were considered until now by many studies to manage water and to improve catalysts performances (reactivity and durability). Published studies show real improvements and much more efforts might lead to significant overhangs. Nevertheless the results show that we are still far from envisaging short-term commercialization.

Cobalt chloride (CoCl2) is one of the most reactive catalysts in the hydrolysis of sodium borohydride (NaBH4). The present study reports hydrogen release results for NaBH4CoCl2 systems, both solids having been mixed together via a ‘solvent’ route. Actually, the ‘solvent’ route using tetrahydrofuran (THF), compared to the mechanical mixing (‘mortar’ route), improves the CoCl2 reactivity in terms of catalyst activation time, hydrogen generation rate (HGR) and total conversion. SEM and XRD observations suggest that the ‘THF’ route favors the ex-situ reduction of Co2+ to an active cobalt phase. Other metal (cobalt or ruthenium) salts are also reported.

Acid-catalyzed hydrolysis of sodium borohydride (NaBH4) has been studied (reactivity and kinetics) at high acid concentration (0.32 M). A mineral (hydrochloric acid, HCl) and an organic benign (acetic acid, CH3COOH) acid have been chosen. Our study has three distinct objectives, namely: (i) combining the simplicity of the storage of solid NaBH4 with the simplicity of the aqueous solution of acid; (ii) showing CH3COOH can be as reactive as HCl in specific well-chosen operating conditions; and (iii) emphasizing the relative greenness of the CH3COOH-based process. All of these objectives have been fulfilled and show that CH3COOH is a benign relatively green acid catalyst of choice for catalyzing hydrogen generation from NaBH4, the acid–water–NaBH4 system being quite simple.

The effect of cobalt-based catalysts, i.e. CoCl 2 (20 wt% Co)/Al 2 O 3 treated by different acids, on NaBH 4 hydrolysis was investigated. Five acids were used: oxalic acid, citric acid, acetic acid, sulfuric acid and hydrochloric acid. Two ways of acid treatment were considered: (i) ex-situ addition of acid to CoCl 2 (20 wt% Co)/Al 2 O 3 at room temperature and (ii) in-situ addition by mixing CoCl 2 , Al 2 O 3 and acid (one-step process). Both ways showed that adding an acid to the catalyst contributed to an important increase of the catalytic activity towards the NaBH 4 hydrolysis. The best performances were obtained with the catalysts treated with either HCl or CH 3 COOH as the global activity of CoCl 2 (20 wt% Co)/Al 2 O 3 was increased up to 50%.

Hydrogen generation through sodium borohydride (NaBH4) hydrolysis has attracted much attention. This reaction has to be catalyzed by metal-based materials. We studied the catalytic potential of cobalt (II) and (III) salts. Some of them have never been studied, and compared to e.g. cobalt nanoparticles or powder, and cobalt borides. CoCl2 showed the best performance. In our opinion, CoCl2 should not be dismissed from the large number of catalysts. One could conceive portable applications using CoCl2; this is briefly discussed. CoCl2 was compared to both commercial cobalt boride and in-situ formed (through our hydrolysis conditions) cobalt boride. Their hydrogen generation rates were 86.3, 1.0 and 1.6 L min−1 g−1(Co), respectively. The hydrogen generation rate of CoCl2 is one of the highest ones reported so far. It is assumed that cobalt boride surface evolves during the reaction and depends on the hydrolysis medium features. Further studies are required to fully explain the complex reaction mechanisms.

Oxidative reforming of biomass derived ethanol over an inexpensive Ni–Cu/SiO 2 catalyst has been carried out with respect to solid polymer fuel cell (SPFC) applications. Two types of runs were performed, either under diluted conditions (with helium as diluent) or under conditions corresponding to an on-board reformer. Selectivities of ethanol reforming have been analyzed as a function of operating parameters: reaction temperature, H 2 O/EtOH molar ratio and O 2 /EtOH molar ratio of the feed to the reformer. The hydrogen content and the CO 2 /CO x molar ratio in the outlet gases were used as parameters to optimize the operating conditions in the reforming reactor. The tests carried out at on-board reformer conditions evidenced that an H 2 O/EtOH molar ratio=1.6 and an O 2 /EtOH molar ratio=0.68 at 973 K allow a hydrogen rich mixture (33%) that can be considered of high interest for SPFC. Furthermore, the use of oxygen decreases the production of methane and coke which increases in turn the lifetime of the catalyst. The stability of this catalyst has been fully demonstrated by long time runs.

Catalysis is an important research topic in the field of hydrogen generation by hydrolysis of boron-based hydrides. A typical example is hydrolysis of ammonia borane (AB, NH3BH3), 1 mol of which is able to liberate up to 3 mol H2 at temperatures lower than 80 °C. However, the presence of a catalyst, generally metal-based, is necessary. The present work was thus conducted in this framework. Herein, we propose a bottom-up approach to prepare cobalt-based bimetallic supported catalysts. The general idea was: first, to screen cobalt-based bimetallic nanoparticles and select the best combination, which was found to be CoCu with a weight ratio 70:30 – its reactivity was discussed in terms of electronic and geometric effects; second, to prepare Ni foam-supported CoCu through a 2-stage process – CoCu/Ni showed a hydrogen generation rate of ca. 25 mL min−1, which almost 5 times better than that observed for the monometallic counterparts Co/Ni and Cu/Ni; third, to propose a new concept of CoCu supported catalysts using a plastic film (light, easy to handle and to prepare) – it showed to be stable and, despite a low hydrogen generation rate (because most of the nanoparticles were embedded in the film), totally converted AB. Our main results are reported and discussed herein.

Porous nanowires are of great interest for applications in solar cells, thermoelectrics, gas sensors, fuel cells and catalysis. Here we report on a general methodology for NWs with highly ordered connected macroporosity using a low-cost and scalable synthesis method based on the combination of hard templating with electrodeposition. The greatly improved catalytic performance of these porous NWs (ca. 600% increasing compared to dense NWs) in the hydrolysis of ammonia borane makes them exciting materials for the implementation of hydrolytic boron hydrides as hydrogen carriers for fuel cell applications.

Electrodeposition was used to deposit cobalt over polycarbonate membrane (PCM), which was used as stable supported catalyst in hydrolysis of sodium borohydride NaBH 4 . We selected PCM as support owing to its lightness, easy handling, stability, and porous structure with nanosized channels. Our primary objective was to obtain a catalytic film resistant to both physical degradation and delamination while H 2 bubbled on its surface. A thin film consisting of mushroom-like cobalt nanoarchitectures were prepared. By SEM, we observed that it is strongly embedded into the PCM thickness, with the anchoring occurring through the channels. This shaped catalyst was mechanically stable and did not show degradation during the reaction. The main results are reported and discussed in details herein.

Cobalt is an efficient metal catalyst in hydrolysis of sodium borohydride NaBH4 owing to its tunable reactivity and cost-effectiveness. However, it is sensitive to the reaction medium, deactivating over cycles. The present work is a contribution in highlighting the fundamental importance of the in situ forming borate-based compounds in the deactivation. Cobalt electrodeposited over nickel foam was used as supported catalyst. Its reactivity and its surface evolution (by SEM, EDS, XRD, XPS, TGA and 11B NMR) were followed up after reaction. It stands out that a surface coating consisting of borates forms already after the first hydrolysis. Actually, the borates are strongly surface-adsorbed, being resistant to thorough washing with deionized water, and are clearly at the origin of the catalyst deactivation. To regain the initial reactivity of cobalt, it is proposed to wash it with a slightly acid solution, which can be done after each hydrolysis. In doing so, the catalyst shows a good stability over cycles. Interestingly, it was observed by SEM that the coating peels off and, by NMR, that the coating is made of borates. Our main results and the main SEM images are reported herein.

International audience

Hydrazine borane N2H4BH3 (HB, 15.4 wt% H) is a promising chemical hydrogen storage material. Herein, we report high-extent dehydrogenation of HB (i.e.hydrolysis of the group BH3 and decomposition of the moiety N2H4) in the presence of nickel–platinum nanoparticles (NPs) at 50 °C. Using Ni0.89Pt0.11 NPs, 5.79 ± 0.05 equiv. H2 + N2 per HB can be released, corresponding to H2 selectivity as high as 93 ± 1%.

Hydrolysis of NaBH4 to release molecular hydrogen is today an intensely investigated reaction and most of the studies focus on the material used as catalyst. Among the various metals tested up to now, cobalt has soon showed to be the most attractive in terms of reactivity and cost. Nevertheless, in order to further decrease its cost by decreasing its amount as well as to increase its reactivity, cobalt has been dispersed over supports. The as-formed supported catalysts have showed to be more efficient. This is the topic of the present study. Herein it is showed that CoCl2 supported over an Al2O3 support with a specific surface area of 180 m2 g-1 is more reactive than CoCl2 supported over a high-surface-area activated carbon (780 m2 g-1), CoCl2 being in-situ reduced into the Co-based active phase. CoCl2-Al2O3 is besides as reactive as another CoCl2-Al2O3 catalyst, the latter support having a higher specific surface area (i.e. 250 m2 g-1). In fact, CoCl2-Al2O3 is more performing than neat CoCl2 whereas the latter has been often showed as being one of the best catalytic systems. To further gain in reactivity, a new, alternative strategy has been envisaged. The Al2O3 was mixed together with a controlled amount of another oxide, namely TiO2. The CoCl2- Al2O3-TiO2(20 wt%) was found to be more reactive than CoCl2-Al2O3. All of these reactivity data are reported and briefly discussed hereafter. Further studies are in progress to highlight the reasons of such improved reactivity.

The present paper reports preliminary results relating to a search for durable cobalt-based catalyst intended to catalyze the hydrolysis of sodium borohydride (NaBH 4 ). Fluorination of Co [Suda S, Sun YM, Liu BH, Zhou Y, Morimitsu S, Arai K, et al. Catalytic generation of hydrogen by applying fluorinated-metal hydrides as catalysts. Appl Phys A 2001; 72: 209–12.] has attracted our attention whereas the fluorination of Co boride has never been envisaged so far. Our first objective was to compare the reactivity of fluorinated Co with that of Co boride. We focused our attention on the formation of Co boride from fluorinated Co. Our second objective was to show the fluorination effect on the reactivity of Co. Our third objective was to find an efficient, durable Co catalyst. It was observed a limited stabilization of the Co surface by virtue of the fluorination, which made the formation of surface Co boride more difficult while the catalytic activity was unaltered. The fluorination did not affect the number of surface active sites. Nevertheless, it did not prevent the formation of Co boride. The fluorination of Co boride was inefficient. Hence, fluorination is a way to gain in stabilization of the catalytic surface but it is quite inefficient to hinder the boride formation. Accordingly, it did not permit to compare the reactivity of Co boride with that of Co.

Akdim, Ouardia Cai, Weijie Fierro, Vanessa Provendier, Helene van Veen, Andre Shen, Wenjie Mirodatos, Claude; The effect of metal and support on ethanol oxidative steam reforming (OSR) has been investigated over a series of stable and active catalytic systems with noble (Rh or Ir) and non noble metal (Ni-Cu) supported over neutral (SiO2), amphoteric (Al2O3) or redox (CeO2) supports. Ethanol decomposition and oxidative steam reforming was investigated by in situ diffuse reflectance infrared spectroscopy under temperature-programmed desorption and surface reaction conditions. Different mechanisms were established for these systems, from the initial steps of ethanol activation to the final equilibration of the decomposition products CO, CO2, H-2 and H2O. Various intermediates such as formates, acetates and/or carbonates were found to play different roles depending on the catalyst composition. The stability and activity of the investigated systems were finally assigned to specific features of these mechanisms.

Ouardia Akdim , Rita Chamoun , Umit B. Demirci *, Youssef Zaatar , Antonio Khoury , Philippe Miele c Universite Lyon 1, CNRS, UMR 5615, Laboratoire des Multimateriaux et Interfaces, 43 boulevard du 11 Novembre 1918, F-69622 Villeurbanne, France Universite Libanaise, Faculte des Sciences II, Laboratoire de Physique Appliquee, 90656 Jdeidet El Metn, Lebanon c Institut Europeen des Membranes, Universite Montpellier 2, CNRS, UMR 5635, Place Eugene Bataillon, F-34095 Montpellier Cedex 5, France