Ch. Kordulis

Molybdenum-oxo species deposited on alumina by adsorption: I. Mechanism of the Adsorption

Abstract The mechanism of adsorption of molybdates on the surface of γ-alumina was investigated by use of adsorption equilibrium experiments, potentiometric titrations, microelectrophoresis and UV/VIS spectroscopy. From the experimental results and the theoretical analysis of the isotherms it was concluded that Mo VI deposition was done by adsorption of the Mo x O y z − ions on energetically equivalent and distinct sites of the inner Helmholtz plane (IHP) of the double layer on the surface of the γ-alumina particles suspended in the aqueous medium. The creation of these sites was mainly attributed to the presence of the protonated surface hydroxyls of γ-alumina and not to the neutral hydroxyls. Analysis of the adsorption isotherms suggested that considerable lateral interactions are exerted between the adsorbed Mo x O y z − ions, resulting in the formation of Mo x O y z − … Mo x O y z − … oligomers in the IHP. It is proposed that these polymeric ions are deposited on the surface in the drying step.

On the relationship between the preparation method and the physicochemical and catalytic properties of the CoMo/γ-Al2O3 hydrodesulfurization catalysts

A series of CoMo/gamma-Al(2)O(3) catalysts have been prepared using various methodologies. One of them (EDF) was prepared by depositing the Mo species on the support via the equilibrium deposition filtration (EDF) technique and then the Co species by dry impregnation. Another catalyst (co-EDF) was prepared by depositing the Co and Mo species simultaneously via EDF. A third catalyst (co-WET) was prepared by depositing Mo and Co species simultaneously using the wet impregnation method. The fourth catalyst (WET) was prepared by depositing the Mo species through wet impregnation and then the Co species by dry impregnation. Finally, the fifth catalyst (s-DRY) was prepared by mounting the Mo species through successive dry impregnations and then the Co species by dry impregnation. In all cases the Mo and Co content was identical, giving a Co/(Co+Mo) ratio equal to 0.13. These catalysts were characterized using various physicochemical techniques (BET, NO chemisorption, DRS, LRS, TPR, and XPS), and their catalytic activity for the hydrodesulfurization of thiophene was determined. The trend observed for the HDS activity (namely, EDF>co-EDF>co-WET>s-DRY>WET) is attributed to similar trends observed for both the fraction of well-dispersed octahedral cobalt in the oxidic precursors and the concentration of the edge sulfur vacancies formed on the active phase of the sulfided samples. The EDF and co-EDF catalysts exhibited relatively low hydrogenating activity. The maximum HDS activity, achieved over the EDF catalyst, suggested the most suitable preparative strategy for the preparation of very active and less hydrogen-demanding CoMo/gamma-Al(2)O(3) HDS catalysts.

Selective oxidation of toluene over V2O5/TiO2 catalysts. Effect of vanadium loading and of molybdenum addition on the catalytic properties

Abstract The influence of vanadium loading and molybdenum presence on the catalytic performance of vanadia-titania (anatase) catalysts for the selective oxidation of toluene was investigated. Two series of V2O5/TiO2 and V2O5-MoO3/TiO2 catalysts were prepared. In the first series the loading of vanadium varied from 0 to 8 mole %, whereas in the second the atomic ratio V/(V + Mo) varied from 0 to 1 while the total loading of active elements (V + Mo) was kept constant and equal to 8 mole %. The samples were characterized by XPS, TPR, XRD and BET measurements. It was found that the activity for the oxidation of toluene and the selectivity for side chain partial oxidation products (benzaldehyde and benzoic acid) exhibited by V2O5/TiO2 catalysts increased with the vanadium loading up to monolayer coverage. This increase was attributed to the parallel increase of the surface concentration of easily reducible isolated vanadium species interacting with the anatase surface. When the vanadium content increased above monolayer coverage both activity and selectivity decreased. V2O5-MoO3/TiO2 catalysts were found to be less active and selective than the corresponding V2O5/TiO2 ones. Molybdenum species supported on anatase were less active for this reaction than vanadium and, in addition, the presence of molybdenum inhibited the interaction between vanadium and anatase leading to a poor vanadium dispersion.

Molybdenum-oxo species deposited on titania by adsorption: Mechanism of the adsorption and characterization of the calcined samples

The mechanism of the adsorption of molybdates on the titania surface has been investigated using adsorption equilibrium experiments, potentiometric titrations, and microelectrophoretic mobility measurements. Comparison of adsorption data with the surface charge of titania, regulated by changing the pH of the impregnating solution, demonstrated that the groups responsible for the creation of the adsorption sites are mainly the protonated surface hydroxyls of titania in addition to the neutral sites. Moreover, the results obtained by the combination of potentiometric titrations and microelectrophoretic mobility measurements, and the variation of pH before and after adsorption with the Mo(vI) concentration suggested that the MoO22− ions are adsorbed on the Inner Helmholtz Plane (IHP) of the electrical double layer, which is developed between the surface of the titania particles and the impregnating solution. Finally, from the analysis of the isotherms obtained it was concluded that the adsorbed MoO2− ions are located on energetically equivalent sites of the IHP and that relatively weak lateral interactions are exerted. On the basis of the abovementioned menhanism, an explanation of the dependence of the sorptive capacity of titania on the pH of the impregnating solution is provided. The combined use of NO and CO2 adsorption, as well as XPS, RAMAN spectroscopy, and temperature programmed reduction measurements, showed that at pH 7.3, corresponding to about 2 wt% MoO3, the titania surfaceis completely covered and the active surface reaches its maximum value. At pH values lower than 7.3, a second molybdena layer starts to form until the Mo03 content reaches the 5.4 wt% MoO3, a point at which Mo03 crystallites are formed.

Molybdenum-oxo species deposited on alumina by adsorption: II. Regulation of the Surface MoVI Concentration by Control of the protonated Surface Hydroxyls

Abstract The adsorption of Mo x O y z − species from aqueous solutions on the surface of pure and Na + or Li + -doped γ-aluminas was studied over a pH range between 3.0 and 8.5 and temperatures ranging from 10 to 55°C. The variations in pH with the concentrations of the impregnating solutions observed and analysis of the isotherms demonstrated that the following findings reported for adsorption on pure γ-alumina at T = 25°C and pH 5 are also valid for adsorption on pure and sodium-doped aluminas performed at various pH values and temperatures: (i) The contribution of precipitation to the deposition is negligible. (ii) The adsorption constant for the MoO 4 2− ions is larger than those for the isopolyanions. (iii) The adsorbed Mo x O y z − ions are located on energetically equivalent but distinct sites of the inner Helmholtz plane, created mainly by the protonated surface hydroxyls. (iv) Lateral interactions are exerted between the adsorbed Mo x O y z − ions resulting in the formation of Mo x O y z − … Mo x O z − oligomers. Moreover, it was demonstrated that the regulation of the concentration of the protonated surface hydroxyls makes it possible to deposit by adsorption very large amounts of Mo VI on the γ-alumina surface. Although the sodium doping was found to be the most attractive from the view point of maximization of the extent of adsorption, it may be suggested that it promotes the formation of the catalytically inert sodium molybdate. Therefore the second best method of regulation, namely the change in the impregnating temperature, may prove to be the most convenient in practice.

NiW/γ-Al2O3 catalysts prepared by modified equilibrium deposition filtration (MEDF) and non-dry impregnation (NDI) characterization and catalytic activity evaluation for the production of low sulfur gasoline in a HDS pilot plant

Abstract The aim of this study was to investigate the desulfurization of fluid catalytic cracking (FCC) gasoline using NiW supported on γ-Al 2 O 3 catalysts prepared by conventional non-dry impregnation (NDI) method and a new one, the equilibrium deposition filtration, (EDF). The latter has been modified (MEDF) to produce the required catalyst quantity for its evaluation in a hydrodesulfurization (HDS) pilot plant. The modification of the typical EDF method showed that it is possible to apply the principles of this technique for practical catalyst preparations. In the first part of this work, the catalyst preparation methodology and the catalyst characterization results are presented. In the second part, the catalyst evaluation is described, in relation to a commercial catalyst, concerning their HDS, hydrogenation (HYG) and isomerization (ISO) activities. The main conclusion of this study was that the MEDF method is more promising than the conventional NDI method for preparing NiW hydrotreatment catalysts. The catalyst prepared by MEDF exhibited higher HDS activity than that of the NDI one, comparable to that of a commercial CoMo catalyst and the highest HYG and ISO activities. The enhanced catalytic activity of the NiW–MEDF catalyst was attributed to the high dispersity of the supported phases achieved by the corresponding preparation method.

Influence of impregnation parameters on the axial Mo/γ-alumina profiles studied using a novel simple technique

Abstract The development of a novel, simple technique allowed us to investigate the influence of various impregnation parameters on the axial Mo profiles achieved on γ-alumina extrudates. The effects of pH, temperature, volume, and concentration of the molybdate solutions, as well as the impregnation time, the rate of drying, the modification of γ-alumina with F − ions, and the nature and concentration of various competitors, have been systematically studied. It was found that decreases in pH as well as increases in the concentration of the molybdate solutions, of the impregnation temperature, and of the rate of drying cause a progressive transformation of the Mo profile from uniform to eggshell type. Doping of γ-alumina with F − ions and the use of NH 4 F, H 3 PO 4 , and citric acid as competitors transformed the Mo profiles from eggshell to uniform. The change in volume of the impregnating solution had no effect on the Mo profiles. The effect of the impregnation time was found to be complicated. Increases in the impregnation time, until a critical value sufficient for the complete imbibition of the extrudates, allow for the transfer of great amounts of molybdate to the interior of the extrudates, leading to more uniform profiles. Further increases in the impregnation time caused a considerable increase in the sharpness of the Mo profile. Most of the above observations were explained on the basis of derived equations, adopting a very simple macrodistribution model. Finally, it was demonstrated that small chromatographic columns filled with powder supports may be used to study active ion profiles on catalytic supports.

Development of molybdena catalysts supported on. gamma. -alumina extrudates with four different Mo profiles: Preparation, characterization, and catalytic properties

The main goal of this work is to prepare, characterize, and determine the catalytic properties of molybdena supported γ-alumina extrudates with four Mo profiles. Concerning preparation, a procedure was established allowing us to prepare (both axially and radially) egg-shell, egg-white, egg-yolk, and uniform profiles with the same, relatively high, Mo loading. The preparation of the egg-shell and uniform profiles was achieved by impregnating γ-alumina extrudates with acidic and alkaline ammonium heptamolybdate solution, respectively. Moreover, it was found that an egg-white (egg-yolk) profile may be achieved from an egg-shell (uniform) profile by successive pore volume or simple nondry impregnations of the Mo-supported extrudates with NH4OH aqueous solutions. A qualitative mechanism, based on the relative rates of desorption and diffusion of the MoO42− ions occuring in the NH4OH impregnation, was developed to interpret the influence of desorption time, concentration of NH4OH solution, drying, and mode of impregnation on the characteristics of the egg-white and egg-yolk profiles obtained. The physicochemical characterization of the Mo/γ-alumina extrudates for which radial profiles have been achieved, done by using a number of techniques, showed a progressive increase of the “Mo supported-support” interactions and Mo dispersion following the order egg-yolk < uniform < egg-white < egg-shell. A similar trend (uniform < egg-yolk < egg-white < egg shell) was obtained for the number of active sites as estimated by the amount of the adsorbed NO. The trends mentioned above were related with the values of the preparative parameters used for obtaining each profile. Kinetic experiments were done, using the hds of thiophene as a probe reaction, over the characterized samples being in the form of extrudates and powders produced by crushing the extrudates. It was found that hds activity was mainly determined by the number and the quality of active centers and not by the type of Mo profile, though these suggested the presence of no significant diffusional effects. The relative yield of butane, produced by the hydrogenation of the unsaturated hydrocarbons formed during hds, increased with the distance of the maximum of the Mo profile from the periphery of the extrudate to its center. Finally, it is important to note that the most active radial profile proved to be the egg-white for hds and the egg-yolk for hydrogenation.

Preparation and characterization of various titanias (anatase) used as supports for vanadia-supported catalysts

Abstract Samples of anatase, prepared using different methodologies, and an industrial sample were characterized using X-ray photoelectron spectroscopy, X-ray diffraction analysis, electron microscopy, diffuse reflectance spectroscopy, temperature-programmed desorption of ammonia, nitrogen adsorption, microelectrophoresis and potentiometric titrations. It was found that anatase prepared by hydrolysis of the titanium isopropoxide exhibited the largest specific surface area, the highest Lewis acidity and the highest concentration of the protonated plus neutral surface hydroxyl groups. These hydroxyl groups are considered to be the deposition sites for the VO 3 − ions in the preparation of the V 2 O 5 /anatase catalysts by equilibrium deposition-filtration. The above findings as well as the absence of foreign ions on the surface of the anatase prepared by the hydrolysis of titanium isopropoxide renders this material the most suitable for preparing the above catalysts used in NO reduction.

Mechanistic aspects of the deposition of the Cr(VI) species on the surface of TiO2 and SiO2

Abstract The deposition of the negatively charged Cr(VI) species on TiO2 and SiO2 surfaces suspended in aqueous electrolyte media has been studied in the pH range 3.0–8.0. The investigation included deposition isotherm and microelectro-phoretic mobility measurements. From the experimental results and a theoretical analysis of the deposition isotherms, it was concluded that the CrxOyz− ions taken up were located in energetically equivalent sites at the inner Helmholtz plane (IHP) of the electric double layer that had developed between the support particles and the impregnating solution. The deposition sites were mainly created by protonated surface hydroxyl groups in the case of SiO2 and by protonated and undissociated surface hydroxyl groups in the case of TiO2. The lateral interactions operative among the deposited Cr(VI) species were, in general, strong, being stronger in SiO2 supports in comparison with those of TiO2. Moreover, it was found that upon a pH decrease from 6.0 to 3.0, the extent of deposition of the Cr(VI) species on silica increased from 0.01 to 0.13 μmol Cr(VI) m−2. The extent of deposition of the Cr(VI) species on titania increased also from 0.23 to 1.68 μmol Cr(VI) m−2 with a pH decrease from 8.0 to 4.8. Further decrease in the pH in the case of titania resulted in a considerable decrease in the extent of deposition.