Applied Catalysis A, General (v.213, #2)


Soft X-ray absorption spectroscopy proved to be a very powerful tool in different disciples and particularly in heterogeneous catalysis. The fine structure of the transition metals, which constitutes the metallic part of catalysts, is present at the L- and M-edges, while a shoulder is generally measured at the K-edge (3d and 4d transition metals). The observation of such fine details associated with the emergence of a significant theoretical formalism, allows a more precise description of the structural and electronic characteristics of this family of materials as compared to the one obtained through more classical Exafs based on hard X-ray photons. Moreover, X-ray absorption spectra can be collected at the K-edge of light elements such as carbon, nitrogen or oxygen. Thus, adsorption process of molecules of major interest in heterogeneous catalysis can be studied completely from the two actors of the chemical bond leading to a precise knowledge of intermediate species.At first, a brief presentation of the theoretical formalism as well as a description of several experimental devices implemented at LURE and dedicated to soft X-ray absorption are done. Then, different publications based on the analysis of soft X-ray spectra collected either at the K-edge of light elements or at the L- and M-edges of transition metals are presented. Finally, we will take some selected examples to demonstrate the nature of information obtained through this technique and discuss the advantages and the limits of such approach.
Keywords: Soft X-ray absorption spectroscopy; Heterogeneous catalysis;

The properties and performance of an O2–H2 system used for the direct oxidation of olefins have been studied on the example of direct oxidation of propylene. Ti-catalysts modified by impregnation with palladium tetraminenitrate and platinum tetraminechloride and by followed reduction under various conditions were used as bifunctional catalysts. New investigations showed that the pre-impregnation of the catalyst with salt and the using of alcohols or ketones as solvent for the impregnation of the catalyst with precious metal improved the selectivity of propylene oxide (PO). Also the influence of other precious metals such as Ir or Au as promoters on titanium silicalite-1 (TS-1) has been studied. In addition to propylene, other olefins like styrene or terpenes were also tested in direct oxidation with an O2–H2 system by using Pd/Pt/TS-1 or Pd/Pt/Ti-MCM-41, respectively. The use of these olefins as substrate lead to low yields of the desired product and showed the limits of the O2–H2 oxidation system, so far.
Keywords: Direct oxidation; Propylene oxide; Precious metal containing TS-1 and Ti-MCM-41;

Some paramagnetic superoxide ions detectable by electron paramagnetic resonance (EPR) can be generated on Au/ZnO catalyst by oxygen adsorption at room temperature as well as at 553 K. In both the cases, the O2 ions are present on the catalyst surface. The disappearance of the O2 signal by the introduction of carbon monoxide over the catalyst surface implies that the O2 ions are either the active oxygen species or the precursors of the active oxygen species. The CO3 species produced are also detected by EPR.
Keywords: Supported gold catalyst; Active oxygen species; Low-temperature CO oxidation; In situ EPR technique;

Monolithic catalysts — non-uniform active phase distribution by impregnation by Theo Vergunst; Freek Kapteijn; Jacob A. Moulijn (179-187).
During preparation of monolithic catalysts via impregnation of monolithic structures, an inhomogeneous active phase distribution can evolve on the scale of the whole monolithic structure. This is demonstrated by the Ni/Al2O3/cordierite system, investigated as a model system. Macroscopic redistribution of the active phase precursor occurred during drying of the monolithic structure after impregnation due to capillary suction, resulting in an accumulation of the active phase in an outer shell of the monolithic structure. Other solvent removal techniques like freeze-drying or microwave drying, or preparation techniques like deposition–precipitation can prevent this accumulation by immobilization of the precursor phase, resulting in a uniform active phase distribution throughout the monolithic catalyst.
Keywords: Impregnation; Monolithic catalyst; Metal distribution; Drying; Microwave drying; Freeze-drying; Deposition–precipitation;

Studies on magnesia supported mono- and bimetallic Pd-Fe catalysts prepared by microwave irradiation method by N Lingaiah; P.S Sai Prasad; P.Kanta Rao; Lesley E Smart; Frank J Berry (189-196).
Magnesia supported mono- and bimetallic Pd-Fe catalysts were prepared by conventional and microwave irradiation method. The prepared catalysts were characterised by N2 adsorption, temperature programmed reduction, selective hydrogen chemisorption and X-ray diffraction (XRD). The conventionally prepared catalysts after high temperature reduction (HTR) showed the formation of Pd-Fe alloy. In the case of microwave irradiated catalysts the formation of Pd-Fe alloy is limited during HTR reduction. These catalysts activity was tested in hydrodechlorination (HDC) of chlorobenzene. The catalysts prepared by microwave irradiation method are more active compared to conventionally prepared ones. The microwave irradiation leads to different morphology of the bimetallic catalysts. The HDC activity results are discussed in terms of structural differences between the catalysts prepared by conventional and microwave irradiation.
Keywords: Hydrodechlorination; Chlorobenzene; Pd-Fe bimetallic catalysts; Magnesia; Microwave irradiation;

The oxidative methylation of toluene with methane was studied over a series of BaO/NaX zeolite catalysts. The catalytic properties were estimated with respect to the conversion of toluene, selectivity and yield of C8 hydrocarbons (ethylbenzene+styrene). The basicity of the catalysts was investigated by temperature-programmed desorption (TPD) of CO2. It was shown that the increase in the barium content leads to an increase in the amount of basic sites and a corresponding increase in activity. The state of the zeolite crystal structure and that of barium before and after use of the catalysts was registered by X-ray diffraction (XRD) analysis. A preserved crystal structure, existence of differently dispersed BaO and formation of BaCO3 were detected in the catalysts.
Keywords: Basic zeolites; Barium; Methane; Toluene methylation (oxidative); TPD of CO2; XRD;

Anhydrous formaldehyde by sodium catalysis by Steffen Ruf; Alexander May; Gerhard Emig (203-215).
Sodium containing catalysts are convenient catalysts for the pure dehydrogenation of methanol to anhydrous formaldehyde. Elemental sodium catalyses the reaction in a homogeneous vapour phase reaction at mild reaction conditions with high yields. The variation of several process parameters revealed the potential of the new way supplying the catalyst. Additional experiments gave insights into the reaction mechanism showing sodium to act as chain carrier in the free radical reaction.
Keywords: Formaldehyde; Dehydrogenation; Thermodynamics; Sodium; Sodium methanolate;

Promoting effect of ruthenium on NiMo/Al2O3 hydrotreating catalysts by M Cattenot; C Geantet; C Glasson; M Breysse (217-224).
Ruthenium (Ru) has been tested as a dopant for NiMo on alumina catalyst. A promotion effect is observed for the hydrodesulfurization (HDS) of dibenzothiophene (DBT), and the hydrogenation (HYD) of tetralin. Maximum activity is obtained at a loading in the range of 0.25–0.5 wt.% Ru added to a NiMo catalyst. Temperature programmed reduction (TPR) and TEM were used to characterize the catalysts. The possibility to create a mixed decorated site (NiRu)MoS was investigated by varying the Ni/Ru atomic ratio. Substitution of Ni atoms by Ru may explain the promoting effect.
Keywords: Hydrotreatment; Sulfide catalysts; Ruthenium sulfide; Hydrogenation; TPR; TEM;

The influence of the support on the activity and selectivity of Pd in CO hydrogenation by Wen-Jie Shen; Mitsutaka Okumura; Yasuyuki Matsumura; Masatake Haruta (225-232).
Catalytic hydrogenation of carbon monoxide was studied over Pd supported on Al2O3, SiO2, TiO2, and ZrO2. Both the activity and the selectivity of the catalysts were strongly affected by the nature of the support. As a major product, Pd/Al2O3 produces dimethyl ether, Pd/SiO2 favors methanol formation, Pd/TiO2 produces CH4, and Pd/ZrO2 yields methanol. Higher CO conversions over Pd/ZrO2 and Pd/TiO2 were ascribed to the presence of cationic palladium species formed through the metal–support interaction. Significant dimethyl ether formation over Pd/Al2O3 was attributed to the acidity of the support metal oxide. The catalytic natures of Pd catalysts were discussed based on the results of catalyst characterization by XPS, TEM, and temperature-programmed reduction and desorption.
Keywords: CO hydrogenation; Methanol synthesis; Palladium catalyst; Support effect; Metal–support interaction;

The kinetics of methanol decomposition: a part of autothermal partial oxidation to produce hydrogen for fuel cells by Peter Mizsey; Esmond Newson; Than-binh Truong; Peter Hottinger (233-237).
Methanol is considered as a renewable energy source for fuel processor–fuel cell systems. The determination of the kinetics of methanol autothermal partial oxidation can be simplified if the problem is divided into subproblems. As a first step, the kinetics of methanol decomposition (Me) are studied. To describe this subproblem, a reaction system consisting of four reactions is assumed. The reactions are: Me, dimethyl ether (DME) formation, steam reforming, and water gas shift (WGS) which are studied on a commercial copper containing catalyst (5 wt.% copper on alumina). The dehydration of methanol to DME is a second order reaction and essentially provides an excess of water over the whole reactor allowing first order kinetic assumptions for steam reforming and WGS. The WGS reaction proves to be slow, compared to the other reactions, and equilibrium limited. The activation energies (kJ/mol) are estimated: Me 76 (±4%), DME formation 117 (±2%), steam reforming 81 (±7%), WGS 50 (±25%). Turnover frequencies for Me are estimated based on copper surface areas determined by the nitrous oxide pulse reduction method. At 220°C, values of 0.05 s−1 are estimated compared to a literature value of 0.019 s−1 at 200°C.
Keywords: Methanol decomposition; Kinetics; Fixed bed reactor; Fuel cell;

1-Hexene isomerization and n-hexane cracking over HMCM-22 by Anup K. Talukdar; Krishna G. Bhattacharyya; Toshihide Baba; Yoshio Ono (239-245).
Isomerization of 1-hexene and cracking of n-hexane over HMCM-22 were carried out in a continuous flow glass reactor. The results were compared with those obtained over HZSM-5 under similar conditions of conversion. The catalyst HMCM-22 caused much more double bond shift and less skeletal isomerization in 1-hexene isomerization than HZSM-5. Also, in n-hexane cracking, HMCM-22 gave a higher yield of olefins and a lower yield of aromatics than HZSM-5. The results can be explained on the basis of the lower acidic strength and the inferior hydrogen transfer capacity of HMCM-22 compared to those of HZSM-5.
Keywords: HMCM-22; 1-Hexene isomerization; n-Hexane cracking;

Hydroisomerization–cracking of n-octane on Pt/WO4 2−–ZrO2 and Pt/SO4 2−–ZrO2 by Javier Mario Grau; Juan Carlos Yori; José Miguel Parera (247-257).
The hydroconversion (isomerization–cracking) of n-octane was studied at 300°C, 1.5 MPa, WHSV=4 and H2/nC8=6 using oxoanion promoted zirconia with different Pt concentrations (0.1, 0.5 and 1%) as catalysts. Tungstate was added to zirconia by impregnation with ammonium meta-tungstate or tungstic acid solutions and calcined at 800°C. Sulfate was added by impregnation with 2N H2SO4 and calcined at 620°C. Isooctanes are intermediate products that are cracked to C3–C5 alkanes with predominance of isoalkanes. Sulfate zirconia is the most active and selective catalyst to cracking products. The addition of 0.1% of Pt produces an increase in activity and stability, respect of the support without metal. However, the increase in Pt content (0.5 and 1.0%) produces a decrease in nC8 conversion and in cracking products. The metallic properties of Pt are decreased by its strong interaction with the support, mainly with sulfate zirconia; at the same time, Pt decreases the acid cracking activity of the support.
Keywords: Tungstate–zirconia; Sulfate–zirconia; Pt load influence; n-Octane hydroisomerization–cracking; Light isomers;

Oxidative transformation of methane on 1.5 mol% Sr2+/La2O3-supported nickel catalysts by Sergio L. González-Cortés; José Orozco; Bernardo Fontal (259-271).
This work examines oxidative transformation of methane on 1.5 mol% Sr2+/La2O3 catalyst with nickel loadings up to 55 mol% nickel, prepared by wetness impregnation. The solids were characterized by X-ray diffraction, infrared spectroscopy, temperature-programmed reduction, BET surface area measurements, and thermogravimetric analysis. The NiO/Sr2+-La2O3 catalysts show a strong nickel oxide interaction with La2O3, involving anionic vacancies or structural defects that induce LaSrNiO x solid solutions with island-like structure formation. Two different reaction mechanisms are suggested depending on nickel loading. The 1.5 mol% Sr2+/La2O3 catalysts (0% Ni) favor oxidative coupling of methane (OCM) to C2 hydrocarbons, whereas the Ni-modified ones favor syngas formation through partial oxidation of methane (POM). Anionic vacancies and coordinatively-unsaturated oxygen atoms (OCUS) leading to hydroxycarbonate phase formation are proposed. Nickel carbide and hydroxycarbonate phases influence on syngas formation is analyzed.
Keywords: Methane; Lanthanum; Nickel loading; Strontium; Hydroxycarbonate; Syngas;

Montmorillonite as a versatile solid acid catalyst for tert.-butylation of resorcinol by Sankarasubbier Narayanan; K.V.V.S.B.S.R. Murthy (273-278).
The liquid phase alkylation of resorcinol with tert.-butyl alcohol was carried out over HZSM-5, HY, HM, MCM-41, alumina, montmorillonite-K10 and modified montmorillonite-K10 (mont.-K10). The major products of this reaction on these catalysts were 4-tert.-butyl resorcinol and 4,6-di-tert.-butyl resorcinol. Unmodified montmorillonite-K10 shows a very low conversion compared to modified montmorillonite. The conversion in terms of catalytic activity increases in the order modified mont.-K10⪢HZSM-5>HY>HM>MCM-41>Al2O3>mont.-K10. The activity and selectivity differences between the catalysts are explained based on their physical properties and acidity differences. The study brings out the significantly high butylation activity of modified montmorillonite-K10 compared to zeolite and oxide catalysts.
Keywords: Butylation of resorcinol; 4-tert.-Butyl resorcinol; 4,6-di-tert.-Butyl resorcinol; Solid acid catalysts; Acidity correlation;

An XPS study of dispersion and chemical state of MoO3 on Al2O3-TiO2 binary oxide support by Benjaram M Reddy; Biswajit Chowdhury; Ettireddy P Reddy; Asunción Fernández (279-288).
X-ray photoelectron spectroscopy (XPS) technique was employed to characterize Al2O3-TiO2 support and MoO3/Al2O3-TiO2 catalyst calcined at different temperatures from 773 to 1073 K. The Al2O3-TiO2 (1:1.3 mole ratio) binary oxide support was obtained by a coprecipitation procedure with in situ generated ammonium hydroxide. A nominal 12 wt.% MoO3 was impregnated over the calcined (773 K) support by a wet impregnation method. The initial characterization by X-ray powder diffraction, Fourier transform-infrared (FT-IR), and O2 chemisorption techniques revealed that the impregnated MoO3 is in a highly-dispersed state on the surface of the support. XPS electron binding energy (E b) values indicate that the MoO3/Al2O3-TiO2 catalyst contains the mixed-oxide elements in the highest oxidation states, Ti(IV), Al(III), and Mo(VI), respectively. However, the core level E b of Al 2p slightly increased with increase of calcination temperature, and this effect was more prominent in the case of molybdena-doped samples. A better resolved Mo 3d doublet was observed at all calcination temperatures. This was explained as due the coverage of alumina surface by titania, thereby lowering the interaction between molybdena and alumina. The XPS atomic ratios indicate that the Ti/Al ratio is sensitive to the calcination temperature. The Mo/Al ratio was found to be more than that of Mo/Ti ratio and decreased with increasing calcination temperature. A clear difference between the Al2O3 and the TiO2 surfaces, in terms of surface free energy, isoelectric point, and surface hydroxyl distribution was considered to be responsible for different distributions of molybdena over these supports.
Keywords: Molybdena; Al2O3-TiO2; MoO3/Al2O3-TiO2; Mixed oxide; XPS; XRD; Dispersion; Thermal stability;

CALENDAR (289-291).

AUTHOR INDEX (293-294).

SUBJECT INDEX (295-299).