Gilles Berhault

HRTEM and molecular modeling of the MoS2–Co9S8 interface: understanding the promotion effect in bulk HDS catalysts

As environmental regulations increase, more selective transition metal sulfide (TMS) catalytic materials for hydrotreating applications are needed. Highly active TMS catalysts become more and more desirable triggering new interest for unsupported Co-promoted MoS2-based systems that have high volumetric activity as reported here. Contrary to the common observation for alumina-supported MoS2-based catalysts, we found in our previous studies with dibenzothiophene (DBT) hydrodesulfurization (HDS) that the catalytic activity is directly proportional to the increase of surface area of the sulfide phases (Co9S8 and MoS2) present in Co-promoted MoS2 unsupported catalysts. This suggests that activity is directly connected with an increase of the contact surface area between the two sulfide phases. Understanding of the nature of the possible interaction between MoS2 and Co9S8 in unsupported catalytic systems is therefore critical in order to get a more generalized overview of the causes for synergy. This has been achieved herein through the detailed characterization by XRD, XPS, and HRTEM of the highly active Co9S8/MoS2 catalyst resulting in a proposed model for a Co9S8/MoS2 interface. This model was then subjected to a DFT analysis to determine a reasonable description of the surface contact region between the two bulk phases. Modelling of the interface shows the creation of open latent vacancy sites on Mo atoms interacting with Co and formation of direct Co–Mo bonds. Strong electron donation from Co to Mo also occurs through the intermediate sulfur atom bonded to both metals while an enhanced metallic character is also found. These changes in coordination and electronic properties are expected to favor a synergetic effect between Co and Mo at the proposed localized interface region between the two bulk MoS2 and Co9S8 phases.

Factors Governing the Catalytic Reactivity of Metallic Nanoparticles

Some recent advances in catalysis by metals are reviewed illustrating the versatility of supported nanoparticles and their high potential to be major contributors to solve energetic or environmental issues of the ongoing century. Since the early structure sensitivity concept, progresses have brought new insights and guidelines on structure ‐ reactivity relationships such as strain effects, the ring‐capture model, the particle surface density effect, or the influence of crystallographic planes. These phenomena are proposed to play a key role in the catalytic cycle. Systematic studies are however still required to estimate their real relevance and their respective contributions according to the type of chemical reactions and of operating conditions.

Study of Pd(II) adsorption over titanate nanotubes of different diameters

Hydrogenotitanates (HNTs) nanotubes with different diameters were prepared by hydrothermal treatment of TiO(2) (P25) followed by washing with HCl aqueous solution. The prepared samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermal analysis and N(2) adsorption/desorption. In order to determine the palladium uptake ability of different HNT samples, the interaction between HNTs and Pd(II) was subsequently studied in aqueous solution at pH 9 (NH(+)(4)/NH(3) buffer). Transmission electron microscopy showed that the diameter of the nanotubes depends on the preparation conditions. Chemical analysis of residual sodium and thermal studies showed that the chemical formula of the two prepared HNT was H(x)Na(2-x)Ti(2)O(5)H(2)O with x=1.61 or 1.65. The HNTs are mesoporous materials with a multi-walled nanotubular structure and high specific surface area. In order to determine the capacity of palladium retention of different HNTs samples, the interaction between HNTs and Pd(II) was subsequently studied in aqueous solution at pH 9 (NH(+)(4)/NH(3) buffer). The adsorption kinetics of Pd(II) on the HNTs was very fast. The isotherms of Pd(II) on the HNTs showed that the adsorption occurred (1) initially through cationic exchange and (2) when the concentration of Pd(II) is high by precipitation of different Pd salts. The adsorption capacity of Pd(II) is strongly altered by the morphology of the HNTs samples.

From core–shell MoSx/ZnS to open fullerene-like MoS2 nanoparticles

A new two-step solution route has been developed for the preparation of hollow inorganic fullerene-like (IF) MoS2 particles with a quantitative yield. First, core–shell MoSx/ZnS (x ∼ 3–4) particles were synthesized by depositing amorphous MoSx on the ZnS seeds in a solution containing ethylene glycol and elemental sulfur. Core–shell particles contain a 7–9 nm size ZnS core surrounded with an amorphous MoSx sulfide shell. Then the core–shell particles were thermally treated at 400–750 °C. As a result, 20–50 nm size IF-like MoS2 particles were crystallized, whereas ZnS left the interior of the particles and formed bulky crystals aside. At the moment of ZnS crystallization a burst-out of the MoS2 walls occurs and the resulting IF particles contain openings, which make the internal voids accessible for gas adsorption. By means of varying the Mo/Zn ratio in the solvothermal reaction mixture, the number of slabs in the onion-like particles can be controlled. Due to the intermediate presence of the ZnS core, relatively large voids are created in the resulting IF-MoS2. The solids as obtained possess high specific surface area, enhanced thiophene HDS activity and exceptional thermal stability. Particle size and wall thickness can be controlled by means of variation of the Mo/Zn atomic ratio in the solution.

One-pot deposition of palladium on hybrid TiO2 nanoparticles and catalytic applications in hydrogenation.

One-pot deposition of Pd onto TiO(2) has been achieved through directly contacting palladium(II) salt with nanosized functionalized TiO(2) support initially obtained by sol-gel process using titanium isopropoxide and citric acid. Citrate groups act as functional moieties able to directly reduce the Pd salt avoiding any further reducing treatment. Various palladium salts (Na(2)PdCl(4) and Pd(NH(3))(4)Cl(2)·H(2)O) and titanium to citrate (Ti/CA) ratios (20, 50, and 100) were used in order to study the effect of the nature of the precursor and of the citrate content on the final Pd particle size and catalytic properties of the as-obtained Pd/TiO(2) systems. Characterization was performed using N(2) adsorption-desorption isotherms, ICP-AES, FTIR, XRD, XPS, and TEM. The as-obtained hybrid Pd/TiO(2) catalysts were tested in the selective hydrogenation (HYD) of an α,β-unsaturated aldehyde, i.e. cinnamaldehyde. Citrate-free Pd/TiO(2)-based catalysts present lower selectivity into saturated alcohol. However, citrate-functionalized Pd/TiO(2) catalyst seems to control the selectivity, the particle size and dispersion of Pd NPs leading to high intrinsic activity.

In-situ HRTEM study of the reactive carbide phase of Co/MoS2 catalyst

Hydrotreatment catalytic operations are commonly performed industrially by layered molybdenum sulfide promoted by cobalt or nickel in order to remove heteroelements (S, N, O) from fossil fuels and biofuels. Indeed, these heteroelements are responsible of the emission of pollutants when these fuels are used in vehicles. In this respect, previous studies made by our research group have shown that the active phase under steady state conditions is partially carbided while strong bending effects of MoS2 slabs were also observed. However, up to now, the morphology of the resulting Co/MoSxCy carbided catalyst has not been fully characterized. In the present study, for the first time, a chemical reaction between the carbon content of a TEM Cu/C grid and a freshly sulfide Co/MoS2 catalyst was in situ observed at 300 °C and 450 °C by HRTEM experimental techniques at ~10 nm of resolution. Results indicate that bending of MoS2 layers occurred due to carbon addition on MoS2 edge sites, as observed in stabilized catalysts after HDS reaction. Using a silicon grid, only cracks of MoS2 slabs were observed without bending effect confirming the role of structural-carbon in this change of morphology.

p-Hydroxybenzoic acid degradation by Fe/Pd-HNT catalysts with in situ generated hydrogen peroxide

Abstract Pd- and Fe/Pd-HNT were elaborated by successive ionic exchange and wet impregnation at room temperature of hydrogenotitanate nanotubes (HNT) nanomaterials. Hydrogen peroxide was generated in situ via reaction of oxygen and formic acid (FA) over Fe/Pd-HNT catalysts. A high p-hydroxybenzoic acid (p-HBz) mineralization was achieved (52%) using Fe/Pd-HNT/FA/O 2 /UV system compared to the simulated reaction using Fe/Pd-HNT/H 2 O 2 /UV system (34%). The rate of hydrogen peroxide formation over Pd-HNT catalyst, during in situ generation, probably plays a key role in controlling the kinetics of the p-HBz degradation.

Monitoring in situ the colloidal synthesis of AuRh/TiO2 selective-hydrogenation nanocatalysts

AuRh nanoparticles (NPs) of various compositions and sizes in the 2–4 nm range were synthesized using a colloidal approach and were characterized at each preparation step by dynamic light scattering (DLS), ultraviolet-visible (UV-vis) spectroscopy, and liquid-phase transmission electron microscopy (liquid TEM). The AuRh colloids appear relatively instable, leading to their gradual coalescence. After fast immobilization of the metallic nanoparticles on rutile TiO2 nanorods, the materials were investigated by high-resolution transmission electron microscopy (HRTEM) and low-temperature CO adsorption monitored by Fourier transform infrared (FTIR) spectroscopy. The inherent lack of miscibility between Au and Rh leads to partial segregation inside the NPs, which is further exalted after a reducing thermal treatment applied for PVA removal. The catalytic properties in the liquid-phase selective hydrogenation of cinnamaldehyde to hydrocinnamaldehyde are strongly influenced by these nanostructural modifications. While in as-prepared samples the intermixing between Au and Rh phases promotes the catalytic performances for Rh-rich AuRh catalysts through Au-induced stabilization of Rh in its metallic form, segregation into Janus particles after reduction decreases the catalytic activity.

Metal Sulfides: Novel Synthesis Methods and Recent Developments

The present review about metal sulfides will describe the use of this class of catalysts for hydrotreating processes including the treatment of biomass feedstock but also their novel applications mainly for the hydrogen production. After a short presentation of the historical background related to the development of these important catalytic materials and the features related to their activity–structure relationship, focus will be paid to novel synthesis methods, novel applications to recent hydrotreating problems, and photoelectrocatalysis.

One-Pot deposition of palladium on hybrid TiO2 nanoparticles: Application for the hydrogenation of cinnamaldehyde

Abstract One-pot deposition of Pd onto TiO 2 has been achieved by direct impregnation in excess of Na 2 PdCl 4 contacted with citrate-functionalized TiO 2 support initially obtained by sol-gel process techniques using Ti(O i Pr) 4 and citric acid. Citrate groups act as functional moieties able to directly reduce the Pd salt avoiding any further reducing treatment. Various titanium to citrate (Ti/CA) ratios (20, 50, and 100) were used to prepare the hybrid TiO 2 support in order to study the effect of citrate on the final catalytic properties of Pd/TiO 2 catalysts. Characterizations were performed using N 2 adsorption-desorption, ICP-AES, FTIR, XRD, XPS, and TEM. The as-obtained Pd/TiO 2 catalysts were tested in the selective hydrogenation (HYD) of cinnamaldehyde. Using citrate-functionalized TiO 2 support increases both the activity and the selectivity to saturated alcohol.