Ch. Papadopoulou

A comparative study of Pt/γ-Al2O3, Au/α-Fe2O3 and CuO-CeO2 catalysts for the selective oxidation of carbon monoxide in excess hydrogen

Three different catalysts, namely Pt/γ-Al 2 O 3 , Au/α-Fe 2 O 3 and CuO-CeO 2 were prepared (by wet impregnation, coprecipitation and a sol-gel method, respectively) and their catalytic performance for the selective oxidation of carbon monoxide in the presence of excess hydrogen was evaluated and compared. The effects of the presence of CO 2 and H 2 O in the reactant feed on the activity and selectivity of these catalysts, as well as their stability under realistic reaction conditions were also investigated. Regardless of whether CO 2 or both CO 2 and H 2 O are present in the reactant feed, the Au/α-Fe 2 O 3 catalyst is superior to the other two for the selective CO oxidation at relatively low reaction temperatures (<80-120°C, depending on contact time and feed composition employed), while at higher reaction temperatures, best results are obtained with the CuO-CeO 2 catalyst, which proved to be more active and remarkably more selective than the Pt/γ-Al 2 O 3 catalyst. The Au/α-Fe 2 O 3 catalyst was the most sensitive, while the Pt/γ-Al 2 O 3 the most resistant towards deactivation caused by the presence of CO 2 and H 2 O in the feed. Finally, while the Au/α-Fe 2 O 3 catalyst lost a considerable portion of its activity during the first 80 h under reaction conditions, the CuO-CeO 2 and Pt/γ-Al 2 O 3 catalysts exhibited a stable catalytic performance, at least during the time period tested in this work (7-8 days).

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.

Fluorinated hydrotreatment catalysts: characterization and hydrodesulphurization activity of fluorine-nickel-molybdenum/γ-alumina catalysts

The oxidic and sulphided state of a series of FNiMo/γ-Al2O3 catalysts containing constant MoO3 and NiO loadings (11.5% and 7.3%, respectively) and various amounts of F−, 0.0–2.0%, introduced after the deposition of the Mo(VI) and Ni(II) were characterized using X-ray photoelectron spectroscopy (XPS), X-ray powder analysis, diffuse reflectance spectroscopy, nitric oxide adsorption, B.E.T. and pore-volume distribution measurements. The results were correlated with the catalytic hydrodesulphurization activity of the specimens, performed in the temperature range 255–325°C using a differential fixed-bed reactor operating under atmospheric pressure. Concerning the oxidic precursor state, the formation of the following species takes place: (i) a supported molybdate phase in which a fraction of the supported Ni(II) has been incorporated in octahedral sites; (ii) α-NiMoO4 and (iii) NiAl2O4. The relative concentrations of these phases are independent of the fluoride content. Low fluoride concentration (≤ 0.8%) has little effect on the texture of the solids. Further increase in the concentration of the modifier results in a marked decrease in the specific surface area which can be partly attributed to the breaking of the inner walls of the relatively thin pores and the creation of larger ones in the range 15–50A. The XPS spectra of the oxide precursors showed that fluorination does not change the structure of the supported phase. No AlF3 was detected. The surface coverage of the active phase was estimated in the sulphided state by the total amount of nitric oxide chemisorbed (per m2 of the catalyst) at 25°C. This decreases slightly up to 0.3 wt% F− and then increases linearly with the fluoride content. The XPS spectra of the sulphided specimens showed that fluorination inhibits the extent of sulphidation and/or reduction of Mo(VI) and the sulphidation of Ni(II) as well. The intensity of both effects increased with the fluoride content. The intrinsic hydrodesulphurization activity was found to increase with the fluoride content. The variation in the activity with the surface coverage of the active phase in the sulphided state showed that the promoting action of F− is mainly due to the fact that these ions increase the surface coverage of the supported Mo and Ni and, therefore, the number of active sites per unit surface area of catalyst. The increase in the surface coverage was attributed to both the increase in the dispersion and the decrease in the specific surface area.

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.

Fluorinated hydrotreatment catalysts Effect of the deposition order of F- ions on F-CoMo/γ-Al2O3 catalysts

Abstract In order to elucidate the influence on the hydrodesulphurization (HDS) activity of the order of deposition of F− ions on CoMo/γ-Al2O3-F catalysts, we prepared, characterized (using BET, NO chemisorption, DRS and XPS) and determined the HDS activity (at atmospheric pressure and various temperatures using the HDS of thiophene as model reaction) of four samples. Specifically, we prepared one undoped CoMo/γ-Al2O3 catalyst (denoted as MC) and three fluorinated catalysts in which the F− ions were deposited before (FMC), after (MCF) and simultaneously with the MoVI and CoII phases ((MCF)). All the specimen which were prepared contained a fixed amount of Mo (11.5 wt.-% MoO3), Co (3.5 wt.-% C03O4) and F (0.8 wt.-% F). It was found that a change in the deposition order provoked a change in the specific surface area, in the dispersity of the supported Mo and Co species and in the intrinsic activity of each HDS active site. The last change was found to be related to the ratio Mo(octahedral)/Mo(tetrahedral). Moreover, it was inferred that fluorination increased the number of active sites per unit active surface. The above explained the changes in the density of the HDS active sites, in the number of these sites and finally on the HDS activity which follows the order MCF>FMC>MC>(MCF) in all temperatures studied. Finally, an attempt was made to convincingly develop pictures related to the preparation which explain the influence of the F doping on the specific surface area and dispersity of the Mo and Co supported species.

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.

Influence of fluorination on the catalytic activity of (Ni, Mo)/γ-Al2O3 catalysts in the hydrogenolysis of thiophene

Deposition of F− ions, after the Ni(II) and Mo(VI) ions, on γ-Al2O3 promotes the activity of (Ni, Mo)/γ-Al2O3 catalysts for the hydrodesulfurization of thiophene in the temperature range of 275–325°C. X-ray diffraction and X-ray photoelectron spectra show that the fluorine ions do not form AlF3 on the γ-Al2O3 surface.AbstractНанесение ионов F− на γ-Al2O3 после нанесения ионов Ni(II) и Mo(VI) повышает активность катал изаторов (Ni, Mo)/γ-Al2O3 в гидродесульфуировании тиофена в интервале температур 275–325°C.

Creation of New Selective Sites by Spill-Over Oxygen via α-Sb2O4 in the Oxidation of Ethanol

Two system were studied. The oxidative dehydrogenation of ethanol to acetaldehyde over Fe 2 (MoO 4 ) 3+ α-Sb 2 O 4 catalysts and the oxidation of ethanol to acetic acid over SnO 2+ MoO 3+ α-Sb 2 O 4 catalysts. Catalysts were formed by mechanical mixtures. In catalysts containing α-Sb 2 O 4 an important increase in the selectivity was observed. To explain the synergy it is proposed that α-Sb 2 oO 4 produces spill-over oxygen which creates (or maintains) selective sites on Fe 2 (MoO 4 ) 3 and on SnO 2 MoO 3 . Two mechanisms could explain the creation of selective sites by spill-over oxygen: i) maintaining the surface of Fe 2 (MoO 4 ) 3 in a high oxidation state, avoiding the segregation of reduced phases and ii) promoting the cristallization of SnO 2+ MoO 3 mixtures, avoiding reduction and degradation of MoO 3