J.M. Campos-Martin

Retracted article: Towards near zero-sulfur liquid fuels: a perspective review

We, the named authors, hereby wholly retract this Catalysis Science & Technology article, due to significant similarity with previously published work. Signed: B. Pawelec, R. M. Navarro, J. M. Campos-Martin and J. L. G. Fierro, October 2012. Retraction endorsed by Jamie Humphrey, Editor, Catalysis Science & Technology.

A density functional theory study of the dissociation of H2 on gold clusters: Importance of fluxionality and ensemble effects

Density functional theory was employed to calculate the adsorption/dissociation of H2 on gold surfaces, Au(111) and Au(100), and on gold particles from 0.7 (Au14) to 1.2 nm (Au29). Flat surfaces of the bulk metal were not active towards H2, but a different effect was observed in gold nanoclusters, where the hydrogen was adsorbed through a dissociative pathway. Several parameters such as the coordination of the Au atoms, ensemble effects and fluxionality of the particle were analyzed to explain the observed activity. The effect of the employed functional was also studied. The flexibility of the structure, i.e., its adaptability towards the adsorbate, plays a key role in the bonding and dissociation of H2. The interaction with hydrogen leads to drastic changes in the structure of the Au nanoparticles. Furthermore, it appears that not only low coordinated Au atoms are needed because H2 adsorption/dissociation was only observed when a cooperation between several (4) active Au atoms was allowed.

Soybean oil epoxidation with hydrogen peroxide using an amorphous Ti/SiO2 catalyst

We report a study of the epoxidation of soybean oil and soybean methyl esters with hydrogen peroxide in dilute solution (6 wt%) using an amorphous heterogeneous Ti/SiO2 catalyst in the presence of tert-butyl alcohol. The influence of some relevant process variables such as temperature and the hydrogen peroxide-to-double bond molar ratio on performance is examined. The highest yields of epoxidized olefins were obtained upon using a H2O2 ∶ substrate molar ratio of 1.1 ∶ 1. Higher ratios than this were not effective for speeding up the reaction. Under the experimental conditions employed in this work, no degradation of the oxirane ring was observed.

Support Effect in Supported Ni Catalysts on Their Performance for Methane Partial Oxidation

Supported nickel catalysts were prepared by impregnation of La2O3, MgO and ZrO2 substrates and tested in the partial oxidation of methane to synthesis gas at atmospheric pressure. Nickel interacted strongly with La2O3 forming a deficient LaNiO3-δ perovskite structure upon calcination. Upon reduction at 973 K, the Ni/La2O3 catalysts that resulted were highly active and selective for syngas production. By contrast, a separate and readily reducible NiO phase was formed on the ZrO2 support. Because the interaction of metallic nickel particles on ZrO2 is weak, the catalysts underwent deactivation by sintering of metal particles during on-stream operation as confirmed by photoelectron spectroscopy. The relatively high activity of the Ni/MgO systems was associated with the formation of a highly stable cubic Ni-Mg-O solid solution, in which nickel remains highly dispersed during the methane partial oxidation reaction.

Direct evidence of the SMSI decoration effect: the case of Co/TiO2 catalyst

First direct images of cobalt nanoparticles covered by a few atomic layers thick TiO(x) moieties after reduction treatment of a Co/TiO(2) system with the simultaneous formation of Co-O-Ti bonds confirm the development of the SMSI decoration effect.

Silica–alumina-supported transition metal sulphide catalysts for deep hydrodesulphurization

Abstract Deep hydrodesulphurization (HDS) of dibenzothiophene (DBT) and gas-oil has been carried out on amorphous-silica–alumina (ASA)-supported transition metal sulphides (TMS) under conditions which approach industrial practice. The activity and selectivity of the binary Ni-, Ru- and Pd-promoted Mo catalysts were compared with the monometallic ones (Ru, Ir, Pd, Ni, Mo on ASA). For both HDS of DBT and gas-oil, the observed activity trends were similar; thus, all catalysts were more active with model feed than with gas-oil, and less active than commercial CoMo/Al 2 O 3 . The binary catalysts showed larger activity than monometallic ones, with Ni–Mo catalyst being more effective than Ru–Mo or Pd–Mo. For Ni–Mo sample, the X-ray photoelectron and temperature-programmed reduction techniques confirmed that incorporation of Mo minimises metal–support interaction, although the formation of nickel hydrosilicate was not prevented. The consecutive impregnation of calcined Mo/ASA catalyst with precursor solution followed by calcination enhances molybdenum surface exposure in binary samples. As a consequence, the temperature of reduction of MoO 3 to molybdenum suboxides is decreased.

Removal of refractory organosulfur compounds via oxidation with hydrogen peroxide on amorphous Ti/SiO2 catalysts

Efficient removal of benzothiophene (BT), dibenzothiophene (DBT) and 4,6-dimethyl dibenzothiophene (DMDBT) has been successfully achieved via oxidation with hydrogen peroxide in liquid phase using an amorphous silica-loaded titanium oxide catalyst. Both BT and DBT are easily oxidized to the corresponding sulfones, however in the case of DMDBT the steric hindrance of the alkyl groups makes the approach of the S-atom to the catalyst active centre (an isolated Ti(IV) species) difficult and therefore its reactivity is inhibited. The concentration of the organosulfur compound, the H2O2 concentration and the nature of the solvent play a key role in the rate of S-removal.

Grafting Strategy to Develop Single Site Titanium on an Amorphous Silica Surface

Titanium/silica systems were prepared by grafting a titanium alkoxide (titanium isopropoxide and titanium (triethanolaminate) isopropoxide) precursor onto amorphous silica. The grafting process, which consisted of the hydrolysis of the Ti precursor by the hydroxyl groups on the silica surface, yielded samples containing Ti-loadings of 1-1.6 wt %. The as synthesized and calcined TiO(2)-SiO(2) samples were characterized by UV-vis, FTIR, XPS, and XANES spectroscopic techniques. These systems were tested in the liquid-phase epoxidation of oct-1-ene with hydrogen peroxide reaction. Spectroscopic data indicated that titanium anchoring takes place by reaction between the alkoxide precursor and surface OH groups of the silica substrate. The nature of surface titanium species generated by chemical grafting depends largely on the titanium precursor employed. Thus, the titanium isopropoxide precursor yields tetrahedrally coordinated polymeric titanium species, which give rise to a low-efficiency catalyst. However, if an atrane precursor (titanium (triethanolaminate) isopropoxide) is employed, isolated titanium species are obtained. The fact that these species remain isolated even after calcination is due to the protective effect of the triethanolaminate ligand that avoids titanium polymerization. These differences in the titanium environment have a pivotal role in the performance of these systems in the epoxidation of alkenes with hydrogen peroxide.

Sulfonic acid-functionalized silica through quantitative oxidation of thiol groups.

A simple procedure for the preparation of amorphous silica containing thiol groups which quantitatively affords sulfonic acid groups has been developed, resulting in site densities and activity for the esterification of acetic acid with methanol greater than a commercial Nafion silica composite.

Evaluation of silica-alumina-supported nickel catalysts in dibenzothiophene hydrodesulphurisation

Abstract The influence of both the nickel–support interaction and the Pd promotion on the activity and thiotolerance during the hydrodesulphurisation (HDS) of dibenzothiophene (DBT) was studied in a variety of amorphous silica-alumina-supported nickel catalysts (Ni/ASA). The catalysts were characterized by means of X-ray diffraction (XRD), N 2 adsorption–desorption measurements, temperature-programmed reduction (TPR), temperature-programmed desorption (TPD) of NH 3 , CO chemisorption and X-ray photoelectron spectroscopy (XPS) techniques. In the low-nickel-content catalysts, a difficulty in reducing the Ni 2+ –support phase was observed, whereas in the high nickel-content catalysts a major reducible NiO phase was developed. Catalysts containing low amounts of nickel exhibited higher activity and selectivity toward ring-opening (RO) products than those containing higher nickel loadings. The larger ring-opening activity and the greater resistance to deactivation of the low nickel-loading catalysts appear to be related to the morphology of the nickel phases at the catalyst surface, consisting of very small Ni clusters separated by non-reduced Ni 2+ species. The high activity of the catalysts in the ring-opening reaction can be explained assuming that non-reduced Ni 2+ species might to force DBT adsorption on Ni 0 phase via CC bonds perpendicular to the surface. In addition, the greater resistance of the catalysts to deactivation was related to H 2 S adsorption on Ni 2+ species, thus maintaining Ni 0 locations available for performing the ring-opening reaction.