Methodical aspects in the surface analysis of supported molybdena catalysts (original) (raw)
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An XPS study of La2O3 and In2O3 influence on the physicochemical properties of MoO3/TiO2 catalysts
Applied Catalysis A: General, 2001
X-ray photoelectron spectroscopy (XPS) technique was employed to characterize Al 2 O 3 -TiO 2 support and MoO 3 /Al 2 O 3 -TiO 2 catalyst calcined at different temperatures from 773 to 1073 K. The Al 2 O 3 -TiO 2 (1:1.3 mole ratio) binary oxide support was obtained by a coprecipitation procedure with in situ generated ammonium hydroxide. A nominal 12 wt.% MoO 3 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 O 2 chemisorption techniques revealed that the impregnated MoO 3 is in a highly-dispersed state on the surface of the support. XPS electron binding energy (E b ) values indicate that the MoO 3 /Al 2 O 3 -TiO 2 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 Al 2 O 3 and the TiO 2 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. .in (B.M. Reddy).
An XPS study of dispersion and chemical state of MoO3 on Al2O3-TiO2 binary oxide support
Applied Catalysis A: General, 2001
X-ray photoelectron spectroscopy (XPS) technique was employed to characterize Al 2 O 3 -TiO 2 support and MoO 3 /Al 2 O 3 -TiO 2 catalyst calcined at different temperatures from 773 to 1073 K. The Al 2 O 3 -TiO 2 (1:1.3 mole ratio) binary oxide support was obtained by a coprecipitation procedure with in situ generated ammonium hydroxide. A nominal 12 wt.% MoO 3 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 O 2 chemisorption techniques revealed that the impregnated MoO 3 is in a highly-dispersed state on the surface of the support. XPS electron binding energy (E b ) values indicate that the MoO 3 /Al 2 O 3 -TiO 2 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 Al 2 O 3 and the TiO 2 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. .in (B.M. Reddy).
Angewandte Chemie International Edition, 2013
The properties of heterogeneous catalysts are directly correlated to the molecular structure of the active sites which often consists of nanometer-scale particles of transition metals (in a metallic, oxide or sulfide form) dispersed on an oxide support. The complexity of physico-chemical phenomena occurring during the catalyst synthesis/activation stages often leads to the formation of unknown supported phases featuring new ill-defined sites. [2] Their identification requires an indepth characterization at the molecular scale of the catalyst with the use of various spectroscopic tools. Despite the valuable information so-obtained, these techniques sometimes are not able to provide the overall structure of the active species responsible for the catalytic activity. The use of theoretical tools to establish direct correlation between spectroscopic fingerprints and structural/electronic properties of catalysts is a powerful and quite new approach for unraveling the structure of catalysts.
Applied Catalysis A: General, 2001
A series of titania supported molybdenum catalysts were prepared by incipient wetness impregnation method and characterized by BET surface area, XRD, TPR, FTIR, ESCA, and low temperature oxygen chemisorption. Thiophene, cyclohexene and tetrahydrofuran were taken as model compounds for evaluating catalytic activities for hydrodesulfurization (HDS), hydrogenation (HYD), and hydrodeoxygenation (HDO) reactions, respectively. XRD results indicate that molybdenum oxide species are dispersed as a monolayer on the support up to 8 wt.% Mo and the formation of crystalline MoO 3 is observed above this loading. FTIR and TPR results showed that molybdenum oxide species were present predominantly in tetrahedral form at lower loading and polymeric octahedral forms are dominant at higher loading. Both oxygen chemisorption and rates of reaction were found to increase with increasing Mo loading up to 8 wt.% and then decrease with further increase in loading. HDS and HYD activities are more or less same but HDO activity is two times higher than HDS and HYD activities. The results are also interpreted with the help of other parameters, like dispersion, equivalent molybdenum surface area, surface coverage, crystalline size, quasi-turnover frequencies and intrinsic activities. ESCA results suggest that electron transfer is taking place from support to metal. (J. Ancheyta-Juárez). than four to five times of the present level are required . Several approaches have been applied to achieve the above mentioned goal and variation of support is an important alternative in this direction. Support plays an important role in determining nature and number of active sites, and consequently, in the activity of the catalysts. With a view to find out better materials for supporting active components such as Mo and W, a wide variety of materials have been examined for their suitability as support especially with reference to hydrotreating and related reactions . Support such as ZrO 2 [4], SiO 2 [5], TiO 2 [6], carbon 0926-860X/01/$ -see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 -8 6 0 X ( 0 0 ) 0 0 5 6 7 -6
Catalysis Letters, 1992
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Determination of the Metal Dispersion of Supported Catalysts Using XPS
Topics in Catalysis, 2019
In this work, recent applications of XPS for the dispersion measurements of oxidic supports and metal catalysts were discussed. The most frequently used models, those of Kerkhof-Moulijn and Davis, were selected to give an overview of several applications in the study of catalysts, based on noble (Pd, Au, Ag, and Rh) and non-noble metals (Co, Ni). In the case of the dispersion of different promoters on the surface of oxidic supports, the Kerkhof-Moulijn model was often applied. Good correlations were achieved for high surface area Al 2 O 3 supports modified by La, B and W. This model was also applied to analyze the dispersion of Co, Ni, Pd and Rh on oxidic supports, making it possible to propose the limit of monolayer formation for homogeneously distributed metal particles. The Davis model was developed for the estimation of metal particle sizes. This method considered the morphology of the metal particles and assumed a diamond-shaped support. The XPS results combined with the information obtained through other techniques showed the potential application of this model in the determination of metal particle sizes for Rh, Au, Pd and Ni based catalysts. Keywords Davis model • Kerkhof-Moulijn model • Particle sizes • XPS quantification List of symbols α c/λ dimensionless crystallite size parameter β(d,λ) Attenuation factor t/λ dimensionless parameter δi Density λ i Attenuation length or inelastic mean free path σ i Photoelectron cross section θ Emission angle of the electron from the surface normal A 0 Analyzed area c Dimension of crystallites with cubic shape d Diameter or particle size dI i Photoelectric differential peak intensity D(Ei) Detector efficiency Ei Kinetic energy f Fraction of support covered with the promoter F(Ei) Instrumental factor h Height Ii Photoelectric peak intensity Jx X-ray flux l Traveling length of the electrons Li Angular asymmetry factor n i Atomic density P xy Fraction of electron travelling through another layer S i Sensitivity factor S 0 Support surface area t Support sheet thickness T(E i) Analyzer transmission x Promoter fraction (weight) in the final catalyst * Laura M.
A COMPARATIVE STUDY OF XPS MODELS FOR PARTICLE SIZE DETERMINATION OF MOLYBDENUM SUPPORTED CATALYSTS
The Kerkhof-Moulijn model and its version simplified by Leon are applied for size determination of the oxidic species formed after Kand Ni deposition on the Mo03l'y-Ah03 system with one and the same Mo0 3 loading. The single-component Mo sample (18 wt.% Mo0 3) was prepared by wet impregnation of the support followed by drying and calcination. Nickel (3 wt.% NiO) was added as second component and potassium (5 wt.% K20) was introduced either as a second or a third component, following the same preparation procedure. According to the both models, the size of the molybdenum oxidic species increased in the same order: 18Mo<K18Mo<Ni18Mo<KNi18Mo, varying between 2-4 nm. It may be concluded that the simplified Leon version of Kerkhof-Moulijn model may be succssefully applied to express study of the changes in the Mox species size after promoter addition, using XPS data.
The Journal of Physical Chemistry, 1986
Surface structural speciation of Si02-supported Mo(V1) as a function of Mo impregnation technique, Mo loading, and hydration-alcination cycles is investigated by electron spectroscopy for chemical analysis (ESCA), ion scattering spectrometry (ISS), and laser Raman spectroscopy. More Mo is surface segregated by use of aqueous impregnation techniques than by use of allyl fixation. In both cases, the surface species, which are dependent upon Mo loading, include crystalline MOO,, highly dispersed surface molybdate, and silicomolybdic acid. The symmetry within the silicomolybdic acid is destroyed by 500 'C O2 calcination and is subsequently restored by rehydration. Consequences of the presence of such surface species with respect to the photocatalytic conversion of propane are discussed.
Surface-Analytical Studies of Supported Vanadium Oxide Monolayer Catalysts
Journal of Physical Chemistry B, 2004
Supported vanadium oxide catalysts, consisting of surface vanadia species on Al 2 O 3 , ZrO 2 , CeO 2 , and Nb 2 O 5 oxide supports, were investigated by X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) to elucidate the effect of calcination treatments as well as exposures to (nonmonochromatized) X-rays and He ions on the surface properties. It was found that calcination in air at 730 K of samples that had been previously calcined in air at 773 K and exposed to ambient atmosphere results in significant increases of the V intensity relative to the support signal both in XPS and ISS. This indicates that the surface vanadia species aggregate under the influence of moisture, but spread during calcination. The surface V(V) species were reduced to V(IV) upon extended exposure to X-rays of a nonmonochromatized source, which was accompanied by clustering as detected by ISS. Following a new methodology that avoids these effects by studying freshly calcined samples transferred without exposure to ambient atmosphere, without previous illumination by X-rays, and takes account of the abrasive effect of He ions by extrapolating the results of sputter series, it was found that in supported V 2 O 5 /ZrO 2 , V 2 O 5 /CeO 2 , and V 2 O 5 /Nb 2 O 5 catalysts possessing a vanadia monolayer coverage or above, the supports are densely covered by two-dimensional surface vanadia species, and the underlying oxide support cations of Zr, Ce, or Nb are not exposed. For a supported V 2 O 5 /Al 2 O 3 catalyst containing a monolayer surface coverage of vanadia, however, a slight exposure of the oxide support cation (Al 3+ ) was noted, which may originate from the much higher surface area of this support (Al 2 O 3 . Nb 2 O 5 , ZrO 2 , and CeO 2 ) resulting in a higher curvature of the surfaces covered by the supported vanadia species. The current XPS and ISS surface studies confirm that supported vanadium oxide catalysts consist of close-packed monolayers of surface vanadia species.