This investigation confirms that the existence of the hydrogen spillover effect (HSPE) in the case of metal catalysts supported on non-reducible monoxides or zeolites is based on a strong corpus of experimental studies, enlarging and deepening previous statements. The structure of hydrogen spillover consists of H/OH pairs conjugated with Mm+/Op− pairs (p = 1 or 2). It is formed by dehydroxylation followed by OH/OH exchange or by the hydrogenation of conjugated pairs. Such a structure imposes the following chemical processes: (i) hydrogenations take place over OH Brönsted acid sites (BAS); (ii) they are excluded over Mm+/Op− Lewis acid sites (LASs), which are deactivating or dehydrogenating; (iii) surface diffusion of hydrogen spillover proceeds through the migration of H/H pairs from LASs to LASs; (iv) the diffusion rates are determined by the oxide supports’ basicity; and (v) H/D exchange is proof of the existence of hydrogen spillover. The nature of hydrogen spillover (radical/ionic) depends on the polarity of the H/OH pairs, which in turn, is determined by the basicity of the support. Our concept of conjugated active sites is a good descriptor of the reaction paths at the molecular level. The view of LASs bringing about additional activity to BAS is not pertinent.

Ni supported over activated carbon (AC) based on olive stones were tested for methane decomposition to produce hydrogen.Physical (by H 2 O) and chemical (by H 3 PO 4 ) activations were compared.Kinetic parameters of methane decomposition were determined depending on Ni load, methane partial pressure and reaction temperature.The catalysts were characterized before and after reaction by N 2 adsorption, X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM).The catalysts showed good initial activities that increased with temperature and nickel load, reactivity decreased with time.The reaction orders were 0.63 and 0.74 and the activation energies were 122 and 139 kJ/mol for physically and chemically activated carbon, respectively.BET surface areas and pore volumes decreased dramatically after reaction due to the deposit of carbon on the support.Ni stayed under its metallic form on the physically AC whereas it was mainly present as Ni 12 P 5 over the chemically activated one.TEM characterization revealed the formation of well-organized carbon nano-onions surrounding Ni particles on the physically activated carbon.Nano-onions were not formed around Ni 12 P 5 particles in the chemically activated carbon.The physical activation allowed the synthesis of catalysts with a better stability for methane conversion than what chemical activation would allow.

This review is based on a reflexion on the nature of the active sites of hydrogenation reactions and applied to the unsaturated C??O carbon-oxygen double bond. Catalysts considered were metal oxides or oxide supported metals and benzaldehyde (BzCHO) was chosen as the model molecule. Attention is paid to the reaction mechanisms and kinetics involved. Our analysis shows that metal oxides reactivity is determined by the mechanism of pre-reduction: nonreducible or slightly reducible (ZrO2) oxides are dehydroxylated whereas reducible oxides are dehydrated. In both cases this results in the incorporation of the H atoms as H/OH pairs from which they may desorb as gaseous H2 or react with the organic molecule, producing ?/O2- pairs (? is an oxygen vacancy). The dissociation of H2 on the ?/O2- pairs renews the H/OH active sites. The reactivity of non-reducible oxides is determined by their usual acid-base properties whereas that of reducible oxides may be correlated to the valence shell electrons structure of the reduced metal cations. For oxides metal catalysts, due the polar character of BzCHO, the hydrogenation would take place either on the support by the hydrogen spillover effect (over H/OH sites) or at the metal-support interface implying oxygen vacancies. The catalytic activity is determined by the nature of the metal and by metal-support interactions. By-products are toluene and benzene for both oxides and oxide supported metal catalysts formed on the same active sites as the BzCH2OH primary product. For each type of catalyst the selectivity order follows the same rules as that proposed for reactivity. Reaction thermodynamics have also to be considered in relation with the dissociation energy of the ?(C??O), ?(C?OH) or ?(C?C) bonds involved.

ABSTRACT The hydrogen spillover effect (HSPE) on metal catalysts supported on non-reducible oxides is still controversial. Our investigation shows that the controversy comes from a misunderstanding about statements of pioneer works. Papers accepting or rejecting the HSPE and based on these statements were found both not pertinent. Factually, the oxide surface OH groups play an important role in the formation, extent, and reactivity of hydrogen spillover. We propose that hydrogen spillover would consist in H/OH pairs, produced by an interfacial dehydroxylation then diffusing over the support by a thermodynamically neutral H/OH exchange mechanism and not by an H atom hopping process, as generally believed. The hydrogen atoms of the H/OH pairs may be consumed chemically or desorb as H2, giving rise to □/O2- pairs (where □ denotes an oxygen vacancy) which, in turn, may further dissociate H2, renewing the H/OH active sites. H/OH and □/O2 constitute conjugated pairs in the hydrogen spillover effect.

Dolomite is a cheap and robust catalyst used for biomass gasification, but its deactivation under relevant conditions of pilot-scale gasifiers has still been poorly understood. For this reason, the catalytic activity of fresh and used dolomites produced from an industrial air-blow fluidized bed was investigated. Fresh and used dolomites were characterized by BET, SEM-EDX, XPS, ICP-MS, XRD, TPD and TPO. Benzene steam reforming was selected as a surrogate reaction of tar conversion in order to probe the reactivity of the two dolomites. The activity of used dolomite was 25% lower than that of fresh dolomite. This difference could be explained by: (1) the deposition of a Si-based layer from biomass ashes at the surface of used dolomite, and (2) the production of coke during gasification. The reaction mechanism of benzene steam reforming over fresh and used dolomites was discussed. For used dolomite, the Si and coke depositions reduced the availability of the active sites (CaO, MgO) thus lowering the conversion of benzene. These deposits could also inhibit the interactions between CaO and MgO and enhance the formation of a stable coke.

Supported Pd catalysts on silica were prepared by different synthesis methods using Pd(Ac)(2) and PdCl2 as salts precursors. The obtained materials were characterized by X-Ray Diffraction (XRD), H-2 chemisorption, and temperature programmed desorption of hydrogen (H-2-TPD). The catalytic performances of these catalysts have been evaluated in the hydrogenation of benzene. The obtained results show that metal dispersion and catalytic activity are strongly dependent on the salts precursor and the method of preparation of the catalyst. The catalysts prepared by hydrazine reduction exhibit higher activity in benzene hydrogenation than that by the polyol reduction method. Moreover, the catalyst prepared with palladium acetate showed higher catalytic activity than those prepared with palladium chloride. The superior catalytic performance of this catalyst in the hydrogenation of benzene was ascribed to a significantly better dispersion of Pd particles on the silica support.

The chemisorption and hydrogenating properties of Ni/SiO2 catalysts prepared by the hydrazine method then calcined at 400 °C with various times were investigated. Metal dispersion and activity in benzene hydrogenation increased with increasing calcination time whereas desorbed amounts of hydrogen significantly decreased. Dilution of a calcined sample by the support led to a sharp increase of both hydrogen storage by the support and catalytic activity. Metal dispersion and hydrogen storage capacity influenced the reaction mechanisms of hydrogenation of benzene which, therefore, is believed to occur on the metal phase or/and on the support by the hydrogen spillover mechanism. The metal active phase would be composed of an ensemble of metallic and oxidized nickel species.

In this paper, we investigate the electronic and magnetic properties of Cu-doped nickel clusters by means of density functional theory. The stabilities of these clusters have also been studied in terms of the binding energies, second-order difference of energies, fragmentation energies and HOMO-LUMO energy gaps. The obtained results reveal that the N4Cu, N5Cu and Ni7Cu clusters are found to be more stable that than all other clusters. Higher HOMO-LUMO gap was observed for Ni5Cu cluster (2.265 eV), indicating its higher chemical stability. A half-metallic behaviour has also been observed for the NinCu clusters, which suggests that these clusters can be employed as nanocatalysts for several catalytic processes, particularly for hydrogenation and dehydrogenation reactions. The magnetism calculations show that the magnetic moment is mostly located on the Ni atoms, and the contribution of the Cu atom to the total magnetic moment in the NinCu clusters is very small. Furthermore, partial density of states analysis indicates that the 3d orbitals in Ni atoms are mostly responsible for the magnetic behaviour of these clusters, and the s orbitals have a very little contribution to the total magnetic moment. [GRAPHICS] .

Mesopores are “highways” for mass transfer inside zeolite crystals and enhance the formation of mono-aromatic hydrocarbons from biomass pyrolysis.

The reactivity of alumina-supported nickel catalysts can be improved by storing hydrogen in catalysts. This illustrates that a smooth correlation exists between the amount of stored hydrogen in catalysts and the catalytic activity.

The surface properties of Ni particles can be altered by the addition of a second metal. This influences its catalytic properties. Hence, the surface properties of a series of alumina (Al2O3) supported NiCu bimetallic catalysts prepared via chemical reduction using KBH4 were investigated. They were characterized using H-2-chemisorption, H-2-temperature programmed desorption, H-2-temperature programmed reduction, X-ray diffraction and scanning electron microscopy analysis and tested for the hydrogenation of styrene in liquid phase. In the bimetallic catalysts, Ni and Cu existed as NiCu species or as separate phases. The Ni active sites favored the hydrogenation of styrene to only ethylbenzene. However, the extent of hydrogenation was mainly determined by the types of Ni active sites available on the catalyst which depended on the Cu/Ni ratio. The surface structure of the nickel phase is modified by incorporation of copper.

The equilibrium geometries, relative stabilities, electronic and magnetic properties of small RhnCa (n = 1–9) clusters have been investigated by DFT calculations. The obtained results show that the three‐dimensional geometries are adopted for the lowest‐energy RhnCa clusters, and the doped Ca atom prefers locating on the surface of the cluster. Based on the analysis of the second‐order difference of energies, fragmentation energies and the HOMO‐LUMO energy gaps, we identify that the Rh4Ca, Rh6Ca, and Rh8Ca clusters are relatively more stable than their neighboring clusters, and the doping of Ca enhances the chemical reactivity of the pure Rhn clusters, suggesting that the RhnCa clusters can be used as nanocatalysts in many catalytic reactions. The magnetic moment for these clusters is mostly localized on the Rh atoms, and the doping Ca atom has no effect on the total magnetic moment of RhnCa clusters. The partial density of states, VIP, VEA, and η of these clusters in their ground‐state structures were also calculated and discussed. © 2015 Wiley Periodicals, Inc.

Biosourced aromatics (BTX (benzene, toluene, xylene) and phenols) could be produced by lignin pyrolysis coupled with catalytic hydrodeoxygenation (HDO) of uncondensed pyrolysis vapors. Guaiacol is used as a model compound to study the catalytic HDO over Fe/SiO2 catalyst. Experiments were conducted in a fixed bed reactor operated at 673 K (1 atm) with a gas mixture (guaiacol, H-upsilon, H2O, CO, CO2) that mimics the real gas composition from lignin pyrolysis. Fe/SiO2 catalyst was shown to be selective for guaiacol HDO into benzene and phenols because it does not catalyze the aromatic ring hydrogenation. Major and minor products are modeled by a semidetailed kinetic mechanism. A deactivation law is also determined. The kinetic model is then included in an Aspen Plus model of lignin to BTX process. Aspen Plus model handles (1) pyrolysis of lignin, including char, oligomers, gases and aromatic yields, (2) catalytic conversion of aromatics by the kinetic model, (3) heat exchangers, and (4) BTX vapors recovery by scrubbing with 1-methyl-naphthalene. Mass and carbon balances, heat demand, and selectivity in desired products are given for the overall process. The effect of gas dilution from pyrolysis reactor on BTX losses, heat demand, and scrubbing solvent flow rate is highlighted. High carrier gas flow rates (as required for biomass pyrolysis in fluidized bed) lead to the entrainment of fines and oligomers, dilute the products, and impact considerably the process intensification.

Partial oxidation of methane by molecular oxygen and nitrous oxide was studied in the presence of catalytic amounts of the Keggin-type heteropolyoxometalates of general formula [PW11MO39](7−n)− with M=Co(II), Ni(II), and Fe(III). The catalysts were prepared by refilling the vacant site of the lacunary precursor K7PW11O39 by the metal additives and characterized by 31P NMR, UV–vis and IR spectra, XRD, TGA/DTA and cyclic voltammetry. The oxidation reaction was performed at atmospheric pressure at 873 or 923K. Reaction products observed were methanol, formaldehyde, carbon oxides and water. Most prominent results are the following: (i) selectivity to oxygenates as high as 48% (conversion 5%) was obtained; (ii) cobalt and iron doped polyoxometalates were the most active and selective catalysts; (iii) N2O was more reactive and selective than O2. The activity rise was correlated with the increase of the oxidant character of the cluster metal. Kinetic study and catalyst behaviour suggested that reaction paths were different for nitrous oxide and molecular oxygen. For N2O, methane would be oxidized by MO2 centres to methoxy species, precursors of both methanol and formaldehyde. For O2, methane activation rather involves hydrogen abstraction by the lattice oxygen on M=O centres to form metal-methyl species, the key-intermediates in the oxidation processes.

Lignin is a potential renewable material for the production of bio-sourced aromatic chemicals. We present the first hydrotreatment of lignin pyrolysis vapors, before any condensation, using inexpensive and sustainable iron-silica (Fe/SiO2 ) and iron-activated carbon (Fe/AC) catalysts. Lignin pyrolysis was conducted in a tubular reactor and vapors were injected in a fixed bed of catalysts (673 K, 1 bar) with stacks to investigate the profile of coke deposit. More than 170 GC-analyzable compounds were identified by GCxGC (heart cutting)/flame ionization detector mass spectrometry. Lignin oligomers were analyzed by very high resolution mass spectrometry, called the "petroleomic" method. They are trapped by the catalytic fixed bed and, in particular, by the AC. The catalysts showed a good selectivity for the hydrodeoxygenation of real lignin vapors to benzene, toluene, xylenes, phenol, cresols, and alkyl phenols. The spent catalysts were characterized by temperature-programmed oxidation, transmission electron microscopy (TEM), and N2 sorption. Micropores in the Fe/AC catalyst are completely plugged by coke deposits, whereas the mesoporous structure of Fe/SiO2 is unaffected. TEM images reveal two different types of coke deposit: 1) catalytic coke deposited in the vicinity of iron particles and 2) thermal coke (carbonaceous particles ≈1 μm in diameter) formed from the gas-phase growth of lignin oligomers.

A series of Pt-100 and bimetallic PtNi catalysts were prepared on various supports namely amorphous silica (Degussa), crystalline silica (Chempure) and cerium oxide (CeO2). The samples were prepared via precipitation method using NaBH4 as a reducing agent. H-2-TPR analysis revealed that total reduction of the metal salts to metal particles occurred during this stage. All catalysts were tested for the hydrogenation of benzene to cyclohexane. It was found that the catalysts exhibited a decrease in the catalytic reactivity in the order of amorphous silica, crystalline silica and CeO2. This is mainly due to the surface area and acidity of the support. Comparison of the PtNi catalysts with their respective monometallic catalyst showed that only the PtNi prepared on Chempure exhibited an enhanced reactivity. This is due to alloying of PtNi. For catalysts prepared on Degussa, the low H-2-chemisorption properties as well as lack in Pt peak shift in the XRD profiles leads to the believe that Ni and Pt may exist separately. H-2-TPD analysis supports these findings.

Ethanol (EtOH) is now produced from fermentation of carbohydrates. Guerbet reaction is a possible w ay to couple C-C bonds of aliphatic alcohols towards higher carbon skeleton molecules that are suitable for di sel, gasoline, aviation fuel and chemicals. When vaporized EtOH is contacted to solids at temperatures near 673.15 K, a lot of different products are obtained containing olefins, paraffins, longer alcohols, diolefins, ketones, aldehydes, ethers, etc. Some H 2 is produced and carbonaceous deposits are formed. In this work, the EtOH to butanol (But-OH) probe reaction was studied. Most of catalytic data available on C-C coupling of EtOH are on the gas phase at 1 bar, for that reason this condition was studied. However, in fermentation process, EtOH is produced in a solution of water (at ~6 wt.% EtOH) [1], this aqueous solution was also studied. It is expected that higher alcohols (like octanol) separate spontaneously from water by decantation, avoiding energy-intensive distillation (consider that fuel EtOH should be anhydrous).

The radiolysis route is applied to synthesize nickel catalysts deposited on titanium dioxide. The TPR profile of radiation-induced Ni/TiO2 catalyst indicates a more complete reduction of the irradiated catalysts compared to the conventionally H2-reduced one. When tested in the benzene hydrogenation, the radiolytic Ni/TiO2 exhibits catalytic properties with higher efficiency than the H2-reduced catalyst. This observation is assigned to the presence of intermetallic Ni–Ti compounds (Ni2.66Ti1.33 and Ni3Ti) evidenced by XRD. In contrast, the calcined and H2-reduced catalyst contains predominantly the oxidized Ni5TiO7 phase, where the nickel is in strong interaction with the support. The TEM observations show highly dispersed nickel.

A detailed study on the surface properties of oleic acid-stabilized PtNi nanoparticles supported on silica is reported. The oleic acid-stabilized PtNi nanoparticles were synthesized using NaBH4 as the reducing agent at various temperatures and oleic acid concentrations, prior to incorporation onto the silica support. X-ray diffraction studies of the unsupported oleic acid-stabilized PtNi particles revealed that the PtNi existed as alloys. Upon incorporation onto silica support, surface properties of the catalysts were investigated using H2-temperature reduction (H2-TPR), H2-temperature desorption (H2-TPD) and H2-chemisorption techniques. It was found that for the bimetallic catalysts, no oxides or very little oxidation occurred. Furthermore, these catalysts exhibited both Pt and Ni active sites on its surface though the availability of Ni active sites was dominant. A comparison of the surface properties of these materials with those prepared without oleic acid in our previous work [N. H. H. Abu Bakar et al., J. Catal. 265, 63 (2009)] and how they affect the hydrogenation of benzene is also discussed.

Fe/SiO2 is shown to be a selective catalyst for guaiacol hydrodeoxygenation (HDO). Guaiacol is used as a model compound to study the conversion of lignin pyrolysis vapours into aromatics (benzene, phenols). The effect of each individual gas present in a pyrolysis gas (H2, CO, CO2, H2O, CH4) on the selectivity of a 10wt% Fe/SiO2 catalyst is studied (673K, atmospheric pressure, 50mol% H2, 1/WHSV=0.6gcath/ggua). The speciation of the iron phase (metallic (α-Fe), carbide (Fe5C2), oxide (Fe3O4), and super-paramagnetic) in spent catalysts is revealed by XRD and Mössbauer spectroscopy as a function of gases composition. At least 3 types of carbonaceous deposit were evidenced by TPO analysis. TEM observations showed that iron particles size is not markedly affected by the reaction and that carbon deposit mainly occurs in the vicinity of iron particles. When all the gases except methane (Guaiacol+H2+CO+CO2+H2O) are simultaneously in the feed stream, the conditions are still sufficiently reducing to maintain the activity of the catalyst (66% of benzene and toluene carbon yield, 7.5gcath/ggua). The effects of support (silica or activated carbon-AC) and iron loading (5, 10, 15wt% Fe/SiO2) were also studied. 10wt% Fe/AC has a higher selectivity in phenol and cresols production than Fe/SiO2. Active sites and reaction mechanisms are discussed.