Nora S. Fígoli

XPS, FTIR and TPR characterization of Ru/Al2O3 catalysts

Abstract The properties of Ru/Al 2 O 3 catalysts prepared using RuCl 3 as precursor and subjected to different treatments after impregnation, were studied using different techniques: XPS, FTIR, hydrogen chemisorption and thermal programmed reduction. Differences in the Ru species (Ru 0 , ruthenium oxides and ruthenium oxychloride), metal dispersion and chlorine content in the catalysts as well as in their catalytic activity and selectivity during benzene selective hydrogenation to cyclohexene, were found according to the preparation procedure used.

Influence of the tungsten oxide precursor on WOxZrO2 and Pt/WOxZrO2 properties

The influence of the tungsten precursor and of the preparation conditions on the properties of platinum free or platinum containing WOx promoted zirconia has been studied. Catalysts were characterized by hydrogen chemisorption at room temperature, TPR, XRD and Laser Raman spectroscopy and during the n-hexane reaction at 200°C, 6 kg cm−2, and a hydrogen/n-hexane molar ratio 7. For all catalysts, the same WOx species were observed after calcination: crystalline WO3 and an amorphous WOx species were detected by Laser Raman. The best catalytic activity and stability were observed on catalysts in which platinum was added after calcination, independently of the WOx precursor used in the preparation.

The influence of coke deposition on the functions of a Pt/Al2O3-Cl bifunctional catalyst

Abstract An acidic (Al2O3-Cl), a metallic (Pt/Sio2) and a bifunctional catalyst (Pt/Al2O3-Cl) were used in naphtha reforming. Temperature programmed oxidation of the coked catalysts shows that the coke on the metal is oxidised at lower temperatures than that on the acid function. According to C1/C3 (metal to acid), coke is mainly produced on the metal at the start of the run. At increasingly severe coking conditions, coke is produced on both functions and at higher severities or times it is produced mainly on the acidic function. Coke on the metal is in equilibrium with gaseous hydrogen. On increasing the hydrogen pressure, the coke is eliminated more easily from the metal than from the acid.

The role of Re and S in the PtReSAl2O3 catalyst

Abstract The modifications in catalytic activity, selectivity and stability when a Pt Al 2 O 3 catalyst is promoted by Re and/or S is studied following several C 6 hydrocarbons reforming. A bifunctional reaction scheme, showing modifications from those previously proposed, is introduced in order to interpret the results. Even the catalyst properties of PtRe are not the sum of the ones of Pt and Re, PtRe has properties brought by Pt and by Re. The addition of Re and S modifies the hydrogenolysis-dehydrogenation capacity of Pt. Re decreases the dehydrogenating capacity of Pt, reducing coke formation (desired effect) and increasing the formation of lower molecular weight paraffins by hydrogenolysis (undesired effect). The undesired contribution of Re is partially suppressed by the addition of S, that decreases the hydrogenolytic capacity of PtRe and increases the production of benzene and coke. This opposite action of Re and S on Pt can be related to the opposite electronic transfers, from Re to Pt and from Pt to S. Both geometrical and electronic effects influence the changes in selectivity.

ZnOCr2O3 + ZSM-5 catalyst with very low Zn/Cr ratio for the transformation of synthesis gas to hydrocarbons

Abstract Three Zn Cr mixed oxide plus ZSM-5 compound catalysts with different compositions covering a wide range of Zn/Cr atomic ratios (0.064 to 1.913), aimed at the direct conversion of synthesis gas into hydrocarbons, were tested under different experimental conditions: temperature ranged from 356 to 410°C, pressure from 3.60 to 4.49 MPa and space velocity from 0.2 to 3.0 mmol reactants/(g cat min). Reaction products included carbon dioxide, water and hydrocarbons with methanol conversion (through which hydrocarbons are formed) being complete. The catalyst with the least content of zinc gave the highest yields of liquid hydrocarbons (up to 74% of total hydrocarbons). Different crystalline phases (ZnO, ZnCr 2 O 4 and Cr 2 O 3 ) were found in the methanol synthesis component as a function of Zn/Cr ratio. The low Zn/Cr catalyst was characterized by X-ray diffraction, differential thermal analysis, nitrogen adsorption, temperature-programmed reduction, Infrared and X-ray photoelectron spectroscopy. The only phases observed in this catalyst were ZnCr 2 O 4 and Cr 2 O 3 . Calcination temperature had an influence in both physical and chemical catalyst properties. After calcination, Cr VI and Cr III species could be seen on the catalyst surface, but only Cr III species were observed after reaction or reduction. The evidences gathered suggest that Cr 2 O 3 is mainly responsible for methanol synthesis, while ZnCr 2 O 4 contributes to increase the specific surface area of the catalyst and influences gas product distributions. Compound catalysts with the mixed oxide (Zn Cr) as the methanol synthesis component showed to be more active than those with the individual (Cr or Zn) oxides.

Operational conditions and coke formation on Pt-Al2O3 reforming catalyst

Abstract The influences of pressure, H2 to naphtha ratio, temperature, space velocity and time on coke formation during naphtha reforming over Pt-Al2O3 has been studied. A standard test was used which consisted of operating the catalyst at normal conditions during a first and a third period of 7 h each, and changing to more severe conditions during an intermediate period of 20 h. Correlations between the amount of coke at the end of the test, the changes in octane number and the operational conditions during the severe period were derived. The decrease in octane number depended on1y on the amount of coke and did not depend on how the severity was achieved. The temperature programmed oxidation (obtained by differential thermal analysis) of the used catalysts showed two zones, one from 396 to 642 K and the other from 642 to 828 K. The second zone corresponded to the burning of a more polymerized deposit, the amount of which increased with the severity of the operation much more than the first zone. The thermogram patterns were quite similar and independent of the operational conditions used to deposit the coke.

Influence of Pt concentration on tungsten oxide-promoted zirconia during n-hexane isomerization

Abstract The effect of platinum concentration on tungsten oxide-promoted zirconia over the catalytic activity for n -hexane isomerization was studied. Catalysts were prepared by impregnation of tungsten oxide-promoted zirconia reaching up to 1.50% platinum, followed by calcination at 500°C. The n -hexane reaction was studied at 200°C, 5.9 bar, WHSV 4 and H 2 : n -hexane (molar) ratio 7. It was found that catalytic activity and stability increase for platinum concentrations above 0.05% because of higher hydrogen availability at the surface, measured as a function of the methylcyclopentane/C 6 isomers ratio. Further increments in platinum concentration do not produce important modifications in catalytic activity or hydrogen availability. Mechanical mixtures of Pt/SiO 2 and tungsten oxide-promoted zirconia present a lower conversion than platinum over tungsten oxide-promoted zirconia due to the larger distance between the acid and metallic functions.

PtSO42−ZrO2: Characterization and influence of pretreatments on n-hexane isomerization

Abstract The n-hexane isomerization on Pt SO 4 2− ZnO 2 was studied at 473 K and 6 bar. The catalyst was prepared by adding platinum to zirconium hydroxide by the incipient wetness technique followed by dipping in 0.5 M sulfuric acid solution. Before reaction, the material was calcined and reduced under different conditions. While reduction is unnecessary, the calcination step is very important because it changes the catalytic activity level. The large increase in activity occurs by calcining at 773 K when the material begins to show crystalline structure in X-ray diffraction patterns. The catalytic activity reaches the highest values for calcination between 803 and 878 K, the selectivity to isomers being almost constant. The crystalline structure is not sufficient to generate an active material, and the catalytic activity is not proportional to the sulfur content and/or the total acidity. A minimum sulfur loading and sulfur species generated by calcination are necessary.

Hydroisomerization of n-hexane on Pt/SO4 2™™ZrO2: effect of total and hydrogen partial pressure

The effect of both total pressure and hydrogen partial pressure during the n-hexane isomerization on Pt/SO42™-ZrO2 at 200°C and 4 h™1 weight hourly space velocity was studied. Total pressure was varied between 1.0 and 10.0 kg cm-2; different hydrogen partial pressures were obtained by dilution with nitrogen keeping the total pressure at 6 kg cm-2. The n-hexane isomerization activity increases when increasing either total pressure or hydrogen partial pressure. Methylpentanes are the C6 isomers formed in major proportion and the 2-methylpentane/3-methylpentane ratio practically reaches the equilibrium value in most of cases. Isomerization is always accompanied by hydrocracking. Hydrogenolysis products were not observed, the main cracking products being propane, isobutane and isopentane; then, the cracking product distribution cannot be interpreted by considering an intramolecular rearrangement of carbocation intermediates. A slight deactivation takes place with time-on- stream. The lifetime of surface intermediates is longer in the absence of hydrogen or at a low hydrogen partial pressure; it allows oligomerization and cracking to prevail at short times and polymerization and coke formation at long times.

On the deactivation of supported palladium hydrogenation catalysts by thiophene poisoning

The deactivation of supported palladium catalysts for ethylbenzene hydrogenation by thiophene was studied. The presence of Pdn+ species on the catalyst surface, in samples prepared from acid solutions of PdCl2, and reduced at temperatures below 450 °C, was evidenced by XPS. A correlation between the concentration of electron-deficient Pd species and the sulfur resistance was found. Thus, the higher the value of the Pdn+/Pd0 ratio, the higher the sulfur resistance. Catalysts prepared from Pd(NO3)2 or from PdCl2 reduced at 450 °C, which essentially contain Pd0, are more quickly deactivated. Furthermore, our results also suggest that while the influence of the metal dispersion on the stability of the palladium catalysts toward sulfur is not significant, the nature of the support, on the other hand, plays an important role.