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Descripcion teorica y experimental del equilibrio de adsorcion de soluciones ionicas en la superficie de los sistemas [gamma]-[Al.sub.2][O.sub.3] y Mo[O.sub.3]/[gamma]-[Al.sub.2][O.sub.3].

Experimental and theoretical description of the equilibrium adsorption of metal on solutions on the surface of [gamma]-[Al.sub.2][O.sub.3] and Mo[O.sub.3]/[gamma]-[Al.sub.2][O.sub.3] systems

1. Introduccion

The need for better hydrotreating catalysts has promoted the research into new catalytic systems. The new generation noble metal-promoted Mo[O.sub.3]/[Al.sub.2][O.sub.3] systems, noble metal/[Al.sub.2][O.sub.3] and different combinations of these can exhibit different performances not only in hydrotreatment reactions of oil fractions but also in other reactions, i.e. isomerization and hydrogenation processes, NOx abatement, partial oxidation and olefin metathesis, Kubota et al. (2010), Achchar et al. (2009), Baldovino-Medrano et al. (2009), Chianelli et al. (2009), Roukoss et al. (2009), Eliche-Quesada et al. (2007), Lamonier et al. (2007), Klimova et al. (2009), Kurhinen & Pakkanen (2000), Navarro et al. (2000), Schmal et al. (1999), Klimova et al. (1998), Regalbuto et al. (1999), Griboval et al. (1998).

Studies on the adsorption of metal ions on the surface of metal oxides are important in many fields, such as soil chemistry, geochemistry, colloid science and catalytic chemistry. In latter case, fundamental knowledge of the impregnation steps is crucial for the development of the new generation of catalysts, Kubicka & Kaluza (2010), Pashigreva et al. (2009), Semagina & Kiwi-Minsker (2010), Bourikas et al. (2006), Chianelli et al. (2009), Komiyama (1985), Vissenberg et al. (2000), van Veen et al. (1990), Lutra & Cheng (1987), Wang & Hall (1982), Hachiya et al. (1984), Gajardo et al. (1980), Giordano et al. (1975). Thus, in recent decades several adsorption models have been proposed to explain the equilibrium of adsorption of several metal cations and anions from liquid solutions. Most equations, that express the adsorption isotherms in an analytical way, have been worked out for gases, i.e. Langmuir, Freundlich and BET isotherms; however, adsorption from solutions is much more complex. This complexity is reflected in the shape of the adsorption curve, Pashigreva et al. (2009), Roukoss et al. (2009), Bourikas et al. (1998, 1996, 2006), Baumgarten & Kirchhausen-Dusing (1997), Pizzio et al. (1996), van Veen et al. (1987), Siri et al. (1985), Tewari et al. (1975). Study of the genesis of the equilibrium isotherms should lead to deeper insight into the nature of the processes responsible for the initial catalyst structure. All this information can be used to understand and to model catalytic phenomena.

Within this context, the present work was undertaken to determine the equilibrium parameters of Mo(VI) and Co (II) ion adsorption on alumina with different precursors and, simultaneously, to explore their effects in the adsorption process. As a further goal, the adsorption of Co(II) and Pt(IV) ions onto molybdenum oxide-loaded alumina was attempted.

2. Experimental Procedure

2.1 Catalyst Preparation

Three [gamma]-[Al.sub.2][O.sub.3] supports from Akzo (BET area of 189 [m.sup.2]/g; mean pore diameter determined by the BJH method of 7.64 nm, and pore volume of 0.51 [cm.sup.3]/g), Rhone-Poulenc (BET area of 196 [m.sup.2]/g; mean pore diameter of 8.84 nm, and pore volume of 0.50 [cm.sup.3]/g), and Procatalyse (BET area of 228 [m.sup.2]/g; mean pore diameter of 8.87 nm and pore volume of 0.50 [cm.sup.3]/g) were used.

Molybdena-alumina catalysts used like supports for adsorption tests were prepared by impregnation of [gamma]-[Al.sub.2][O.sub.3] supports with aqueous solutions of ammonium heptamolybdate (AHM, Merck), whose concentration was selected in order to obtain surface concentrations close to the theoretical monolayer of Mo[O.sub.3] on the [gamma]-alumina. The preparation conditions have been reported in a previous work, Machuca et al. (2001). Here, the samples are labeled as Mo[O.sub.3]/[gamma]-[Al.sub.2][O.sub.3] (AK), Mo[O.sub.3]/[gamma]-[Al.sub.2][O.sub.3] (RO) and Mo[O.sub.3]/[gamma]-[Al.sub.2][O.sub.3] (PR), where AK, RO and PR refer to the Akzo, Rhodia and Procatalyse supports, respectively.

2.2 Determination of Equilibrium Adsorption Isotherms

The equilibrium adsorption data were obtained from the liquid solid adsorption processes. The isotherms were obtained following a typical procedure, reported previously, Machuca et al. (2001). Aqueous molybdenum, cobalt and platinum solutions were prepared from ammonium heptamolybdate (AHM, Merck), sodium molybdate dihydrate (SoMo, Merck), cobalt (II), nitrate hexahydrate (CoNi, Sigma), cobalt (II) chloride hexahydrate (CoCl, Aldrich), cobalt (II) acetate tetrahydrate (CoAc, Aldrich) and hexachloroplatinic acid (Sigma); deionized water (Milli Q quality, 18.2[mu][OMEGA]) was used in all cases.

2.3 Catalyst Characterization

The metal content of the calcined samples was determined using a Perkin Elmer Optima 3300 DV inductively coupled plasma optical emission spectrometer (ICP-OES). The texture (area, mean pore diameter and pore volume) of the supports and calcined catalysts was determined from the nitrogen adsorption isotherms using a Micromeritics Digisorb 2600 automatic apparatus. DRS and UV-Vis spectra of the catalyst samples and the precursors (solid and aqueous solutions) were collected on a Shimadzu UV-2100 spectrophotometer, in the 190900 nm range, using BaS[O.sub.4], alumina or water as references, respectively.

3. Results and Discussion

It is generally accepted that two kinds of coordination structures of surface molybdate species (tetrahedral and octahedral) exist on the [gamma]-[Al.sub.2][O.sub.3] surface. Tetrahedral molybdenum oxide species are related to the isolated species, whereas octahedral molybdate species are polymerized structures. Despite this current consensus, the genesis and development of molybdate surface during preparation steps is still controversial, even though they have been studied in depth, Bejenaru et al. (2009), Baldovino-Medrano et al. (2009), De Lacaillerie & Gan (2007), Frizi et al. (2008), Bergwerff et al. (2008), Chianelli et al. (2009), Klimova et al. (2009), Lutra & Chebg (1987), Mestl & Srinivasan (1998), de Wilmar et al. (1998). From all the works carried out to date, several scenarios emerge. Firstly, Jeziorowski and Knozinger, Mestl & Srinivasan (1998), proposed that Mo[O.sub.4.sup.-2] ions are developed on the alumina surface irrespective of the impregnating solution pH, although these species become polymerized during the calcination step, thus leading to octahedral structures. The second model was advanced by Wang and Hall; Wang & Hall (1982), Mestl & Srinivasan (1998), who pointed out that the adsorbed molybdate species depend on the impregnation solution pH: at high pH, only tetrahedral molybdate ions are adsorbed onto the [gamma]-[Al.sub.2][O.sub.3] surface, whereas at low pH the adsorption of polymolybdate ions (octahedral symmetry) dominates. Then, Xiong et al. (2000) reported that the coordination structures of the surface species are essentially influenced by the net pH value at the surface region of the support. Rencently, Bergwerff et al. (2008), reported that there are interactions between Co Mo ions in solutions and that this can have consequences for the preparation of supported CoMo HDS catalysts. In this case, the CoMo-complexes can be formed on the catalyst surface, where the Co(II) is bound to the outside the molybdate-ions.

3.1 pH and adsorption behavior

3.1.1 Mo/[gamma]-[Al.sub.2][O.sub.3] systems

Spectroscopic studies (UV-Vis) were performed to determine the ionic species present in solution; in parallel, the DRS UV-Vis spectra of the impregnated solids were recorded. Although the use of aqueous molybdate solutions is debatable, Bergwerff et al. (2008), Duan et al. (2007), Weber (1995), Fournier et al. (1989), due to the complex chemistry of molybdate ions in solution, the approach undertaken here is reasonable.

Although the tetrahedral Mo[O.sub.4.sup.2-] ion can be protonated in solution and its coordination sphere is solvated by [H.sub.2]O, leading to an increase in the Mo coordination number, Fournier et al. (1989), this process will lead to an overlapping in the absorption UV-Vis region with the octahedral molybdenum species. Along the preparation of Mo[O.sub.3]/alumina samples, carried out by impregnation of alumina with aqueous solutions of [Na.sub.2]Mo[O.sub.4], Xiong et al. (2000) found that in the wet, dry and calcined states all the samples exhibited Mo species that were tetrahedrally coordinated by oxygen ions, even after calcination. In the present work, the UV-Vis spectra of Mo, Co and Pt solutions were used to identify the nature of molybdenum oxo-species in solution. On the other hand, the comparison of the DR spectra of the salt precursors and the impregnated samples, allows one to distinguish among the molybdenum species present on the support surface. Apart from this, other techniques have been used to unveil the symmetry and nature of the adsorbed species (i.e. optical band-gap energy, Eg and Gaussian analysis) from DRS/UV-Vis spectra, Weber (1995), Fournier et al. (1989).

Along the adsorption of metal ions on the alumina surface, the pH values of the impregnating solution were seen to change. Table 1 compiles both the initial and final pH ranges for all the systems under study. From these data, it is clear that the pH of the solution changes not only along the impregnation process but also with the initial concentration of the impregnating solution. Although the type of anionic species present in molybdate solutions depends on their pH value, Bergwerff et al. (2008), Regalbuto et al. (1999), Lutra & Cheng (1987), Mestl & Srinivasan 1998), under the nearly acid solutions employed in the present work most of the molybdenum species in solution were polymeric ions (octahedral symmetry) because the ionic equilibrium was shift to the left-hand side:

[Mo.sub.7][O.sub.24.sup.6-] + 4 [H.sub.2]O [??] 7 Mo[O.sub.4.sup.2-] + 8 H+ (1)

Therefore, the adsorption of molybdate ions [([Mo.sub.7][O.sub.24]).sup.6-] shifts the pH to values close to point of zero charge of the support at the end of impregnation. Figure 1 shows the pH change at adsorption equilibrium. In all cases, the final pH values are higher than the initial ones. This type of behavior is similar to that observed by Spanos et al. (2006, 1990) for a similar system. However, the comparison of the final pH values for the adsorption of molybdate ions on alumina revealed some differences, although they were considered no significant.


Contrariwise, most solution oxo-molybdenum species present during the impregnation with SoMo salt were monomeric (Mo[O.sub.4.sup.-2]) species in tetrahedral symmetry. In this case, the change in pH values became smaller than those originated upon AHM impregnation (see Table 1). This observation can be related to the extent of adsorption, which in turn was substantially lower than that observed for AHM solutions. This can be explained as being due to a weak interaction between the monomeric ions and the adsorption sites. These results are in agreement with the reported by several authors, Iannibello & Mitchell (1979), Weigold (1983), Butz et al. (1989), Plyuto et al. (1997) and Jiang et al. (2008).

Since the alumina sites involved in the impregnation with AHM and SoMo salts was virtually the same, the surface had greater affinity for the polymeric species. This behavior could be related with the species on surface of supports, this species could change with the adsorption conditions, especially the pH. These results are in agreement with the findings of Luthra & Cheng (1987), Hall & Wang (1982) and Al-Dalama et al. (2005).

The electronic spectra of AHM and SoMo solutions are shown in Figs. 2 and 3, respectively. The positions of the absorption bands are also compiled in Table 2. With increasing molybdenum concentrations, the main band shifted toward a higher wavelength. However, SoMo solutions exhibited a shoulder at 234 nm, whose position did not change with increasing Mo concentrations. This shoulder was also observed in AHM solutions, although it shifted slightly with the Mo concentration. For SoMo solutions, the shoulder was overshadowed by an intense band at around 250 nm, and disappeared at Mo concentrations over 250 ppm.



The energy of electronic transitions depends strongly on the ligand field symmetry surrounding the molybdenum center. For oxo-ligands, a more energetic transition is expected for a tetrahedral Mo(VI) than for octahedral Mo(VI), Fournier et al (1989). The comparison of the electronic spectra obtained for SoMo and AHM solutions revealed a greater difference in the shift of the main band of the former solutions when the Mo concentration was increased. The difference was found at 5 nm (0.138 eV) and 9.5 nm (0.259 eV), respectively, although a greater difference (28.5 nm - 0.715 eV) can be seen if the main band at 250 ppm is taken into consideration. Nevertheless, the differences between the tetrahedral and octahedral species in aqueous media were quite small. Thus, an overlapping of the bands in the electronic spectra of the tetrahedral and octahedral species is expected to occur. For both solutions the largest difference observed was 4 nm (0.106 eV).

Figure 4 shows the DR spectra for Mo/Al O samples prepared from AHM solutions in the oxidized (Mo[O.sub.3]) and impregnated (AHM ion) states. The spectrum of the Mo[O.sub.3]/[Al.sub.2][O.sub.3] exhibits a strong adsorption, with a maximum at 247.5 nm, similar to that of the dry impregnates exhibiting bands at 244.5 and 243 nm. The shape of the spectra was similar for all the samples. The absorption band has the typical shape of Mo(VI) species, Lutra & Cheng (1987), Xiong et al. (2000), Fournier et al. (1989). Since the Mo(VI) ion has a d0 electronic configuration, the only absorption band that is able to arise in the UV-Vis range of the electronic spectra comes from ligand-metal charge transfer. This band type is usually observed between 200 and 400 nm. However, since the alumina itself exhibits a broad band at 200-350 nm, the resulting spectra of the Mo[O.sub.3]/[Al.sub.2][O.sub.3] systems could be considered as the overlap of the adsorption bands at 225, 275 and 300 nm. The resulting broad band suggests that both molybdate species were developed in the wet state on the alumina surface. After drying and calcination, the band is still broad, suggesting the formation of some surface polymerized molybdate species. This inference is consistent with the observations of Fournier et al. (1989) and Ramirez et al. (1999).

The DR spectra for AHM and SoMo salt precursors only exhibit the characteristic wavelength bands for octahedral and tetrahedral species respectively, van Veen et al. (1987, 1990), Lutra & Cheng (1987), Hachiya et al. (1984), Gajardo et al. (1980), Fournier et al. (1989). There were larger differences in the intensity and wavelength positions for the octahedral and tetrahedral symmetries, due to different interactions between the Mo atoms and their surroundings, this it is related with the structure of Mo[O.sub.3] and its crystal faces obtained by molecular modeling, Gesarai et al (1997).


The analysis of the reflectance spectra using model compounds confirmed that both type species (octahedral and tetrahedral) were present in the impregnates. Although most octahedral species were in solution, a small proportion of monomeric Mo[O.sub.4.sup.-2] species always remained in equilibrium with the heptamer (Eq. 1), which may also lead to adsorption on the surface as minor species, Jiang et al. (2008) and Weigold (1983). Finally, we found that the hydration degree and the type of drying (RD, rotary dry; ND, natural dry) did not change the DR spectra, and exerted a negligible effect on the absorption bands of AHM/[Al.sub.2][O.sub.3] sample. Thus, as already reported by Fournier et al. (1989), the structure of the adsorbed species should not change.

3.1.2 Co and/or Pt on [gamma]-[Al.sub.2][O.sub.3] or Mo[O.sub.3]/[gamma]-[Al.sub.2][O.sub.3] systems

A similar behavior was observed for the adsorption of cobalt and platinum ions on the Mo[O.sub.3]/[gamma]-[Al.sub.2][O.sub.3] surface. Under the experimental conditions used, most ionic species in solution were Pt (IV) and Co(II), in agreement with the work of Regalbuto et al. (1999) for Pt, and of Tamura et al. (1997) and Bergwerff et al. (2008) for Co ions. Also, the UV-Vis spectra for cobalt solutions (not shown here) were essentially the same as those reported by Tomisic & Simeon (1999). However, the initial pH of the impregnating [H.sub.2]Pt[Cl.sub.6] solutions was lower than that reported by Ruckenstein & Karpe (1989) for Pt ions. The general equations for complete dissociation of the salts in aqueous medium are:

[H.sub.2]Pt[Cl.sub.6] [??] 2[H.sup.+] + Pt[Cl.sub.6.sup.2-] (2)

Co[(N[O.sub.3]).sub.2] [??] 2N[O.sub.3.sup.-] + [Co.sup.2+] (3)

For ensuring the adsorption equilibrium had been reached, each point of the adsorption isotherm (298 K) was extended for a much longer time (24 h). As pointed out in the previous section, the adsorption of ions on the alumina surface led to significant changes in the pH of the impregnating solution. The results offered in Table 1 indicate that the initial solution pH for most solutions shifted to values close to the point of zero charge of the support ([gamma]-[Al.sub.2][O.sub.3] or Mo[O.sub.3]/[gamma]-[Al.sub.2][O.sub.3]) at the end of impregnation. For a given support, the final pH values were in general higher than the initial pH for the impregnation of Pt on Mo[O.sub.3]/[gamma]-[Al.sub.2][O.sub.3] and Pt-Co on Mo[O.sub.3]/[gamma]-[Al.sub.2][O.sub.3], while pH was lower for the impregnation of Co on Mo[O.sub.3]/[gamma]-[Al.sub.2][O.sub.3]. This observation is consistent with the close proximity of the final pH solution and the point of zero charge of (p[H.sub.ZPC]) of the molybdena support at the adsorption equilibrium. It should be noted that the final pH values of the impregnating solutions approached the p[H.sub.ZPC] of the solid (Mo[O.sub.3]/[gamma]-[Al.sub.2][O.sub.3] and [gamma]-[Al.sub.2][O.sub.3] ). These results are in complete agreement with the p[H.sub.ZPC] values reported by Kohler et al. (1992) for Mo[O.sub.3]/[Al.sub.2][O.sub.3] system and with those of Tawei et al. (1997) for [gamma]-[Al.sub.2][O.sub.3]. A final remark is that for the concentration limit of Co(II) ions, the pH values are lower than expected.

3.2 Adsorption isotherms

The adsorption isotherms of molybdenum (cobalt and platinum) at a constant temperature of 298 K, expressed as the amount of metal ion retained per gram of solid ([C.sub.a]) as a function of the metal ion concentration of the impregnating solution at equilibrium ([C.sub.eq]), were determined. The experimental isotherms for the different supports are shown in (Figs. 5-11.) The isotherms for the metal ion/[gamma]-[Al.sub.2][O.sub.3] systems belong to the L-type of the Giles classification, Giles et al. (1974). Exceptions were the isotherms obtained using CoAc and CoCl salt precursors and a PR alumina (Fig. 10), and also Pt (Co) on Mo[O.sub.3]/alumina (Fig. 11).

For the adsorption of molybdate ions, the shape of the isotherms (Figs. 5 and 6) coincides with that reported in the literature for this system, Al-Dalama et al. (2005), Bourikas et al. (1998), Pizzio et al. (1996), van Veen et al. (1987), Mulcahy et al. (1987). However, depending on the range of [C.sub.eq] values investigated, the shape of these isotherms may vary. Thus, more complex shapes can be found, particularly if the isotherm covers a broad range of Ceq concentrations, Vissenberg et al. (2000), Bourikas et al. (1996, 1998, 2006). Ca values increased rapidly with [C.sub.eq] in the range of concentrations up to ca. 15 mg/ml and 5 mg/ml for the AHM and SoMo solutions, respectively, whereas the curves flattened off at higher concentrations. An important distinction among the three [gamma]-[Al.sub.2][O.sub.3] supports is that, whatever the concentration, higher amounts of Mo were retained by the [gamma]-[Al.sub.2][O.sub.3] (AK) sample. Moreover, the experimental isotherms determined using the impregnating SoMo salt precursor clearly shows up the differences in the adsorption capability of the three [gamma]-[Al.sub.2][O.sub.3] substrates. In this case, the sequence in adsorption capability was AK >> RO >> PR alumina. The adsorption capacity was highest when AHM salt was used as the impregnating solution, in which case the sequence was AK >> RO > PR.

The adsorption behavior is influenced by the surface properties of the alumina (adsorption sites, IEP and/or ZPC) and the solution characteristics (pH and nature of the ionic species), Lutra & Chang (1987), Mestl & Srinivasan (1998), Semagina & Kiwi-Minsker (2009). On the basis of these characteristic properties, an explanation of the differences found between the adsorption of AHM (octahedral species) and SoMo (tetrahedral) salt precursors can be offered. The AK-type alumina must have a higher affinity for octahedral than for tetrahedral species because the AK alumina showed a higher experimental S (mg ion/g support) value. By contrast, a poor affinity of the surface for tetrahedral molybdate species was reflected in the adsorption from SoMo solutions in which only tetrahedral Mo[O.sub.4.sup.2-] species, but in no case octahedral [Mo.sub.7][O.sub.24.sup.6-] units, are present. The RO and PR alumina samples showed a similar kind of behavior. Upon observing the adsorption isotherms in (Figs. 5 and 6), it appears that the RO and PR [gamma]-[Al.sub.2][O.sub.3] substrates would have similar proportions of adsorption sites for the octahedral species, although still lower than for the PR counterpart. It should be stressed that specific area and porosity would have only a minor influence in the adsorption of molybdate ions.

A similar type of behavior was observed for the adsorption of Co(II) ions on alumina, see (Figs. 7-10), Bergwerff et al. (2008), Wang & Hall (1982), Bourikas et al. (1998), Siri et al. (1985). In all cases, the PR-type alumina had a slightly higher adsorption capacity than the other aluminas. There is, however, an interesting point to be considered in (Fig. 10); although the initial concentration was the same for all the experiments, [C.sub.eq] values were very different at the end of the adsorption (a higher amount of Co(II) ions was adsorbed). Thus, PR-type alumina exhibited a higher adsorption capacity than the AK and RO counterparts. Since these gave rise to an S-type shaped isotherm within the concentration range used, differences in impurity levels and in surface chemical groups of each alumina could well be responsible for this behavior. The curve shape found for CoNi, CoCl and CoCl adsorption suggests that the adsorption sites in the AK y RO alumina are similar.








The adsorption isotherms of Co (Pt) and Pt (Co) on Mo[O.sub.3]/[gamma]-[Al.sub.2][O.sub.3] are shown in (Fig. 11). The isotherm for Pt (Co) belongs to the S-type; this shape is very different from the L type, which was found for the Pt adsorption on Mo[O.sub.3]/[Al.sub.2][O.sub.3], Machuca et al (2001). The presence of Co(II) ion in solution may alter both the kinetics and extent of Pt adsorption. Similarly, the Co adsorption on Mo[O.sub.3]/alumina changed when Pt ion was also present in solution. In this case, the shape of the isotherm was of the L-type, which differs from the S-type found for the adsorption of Co(II) ions on Mo[O.sub.3]/alumina. In a previous work, Machuca et al (2001), it was observed that adsorbed, irrespective of the Al O source used the adsorption sites on Mo[O.sub.3]/[gamma]-[Al.sub.2][O.sub.3], where the ionic species (Co(II) or Pt(IV)) were adsorbed, can be virtually the same in each case since the new surface is the Mo[O.sub.3] oxide deposited over the Al O substrate.

The observed behavior can be explained in terms of the two types of isotherms (S and L), which imply lateral interactions between the Pt(IV) and Co(II) species in solution. As a consequence, the equilibrium adsorption capacity of the Mo[O.sub.3]/[Al.sub.2][O.sub.3] surface may be modified by these interactions. In this sense, a reduction in the Co(II) ion adsorption capacity on Mo[O.sub.3]/alumina surface was observed when Pt(IV) ions were in solution and vice versa. Within the concentration range used, the equilibrium adsorption capacity ratio (with and without ions) was 8/1 and 13/4 for the Co and Pt, respectively. This different behavior can be explained in terms of the equilibrium distribution of adsorbate species between the surface of the support and the liquid phase, which depends on the concentration and nature of the ions, temperature, the chemical nature of the support and the interface (liquid-solid) and ion-ion interactions, Giles et al. (1974). Consequently, when more than one adsorbate is present in solution, the interaction between different ions for the adsorption sites must be taken into account.

This assumption is reinforced by the observation that Co(II) and Pt(IV) ions will interact electrostatically with a molybdate phase bound to the surface. As a consequence of the simultaneous participation of the three species, the adsorption process is more complex. Since an S-type shaped isotherm was found within the concentration range used, this reflects the additive effect of the incorporated ion. Noble metal (Pt,Ru,Pd)-Mo/alumina samples prepared by Merino et al. (2000) showed a decrease in the activity for the hydrodesulfurization reaction of dibenzothiophene when one monolayer of the Mo[O.sub.3]-like phase was developed on the alumina substrate and, subsequently, Pt(IV) ions were adsorbed on the Mo[O.sub.3]/[Al.sub.2][O.sub.3] system. However, a better activity of the catalysts can be obtained by keeping the Mo-loading below the monolayer capacity of the Al O surface, Pinzon et al. (2006), Giraldo et al. (2008), these results confirm that both the monolayer of Mo species on the surface and the ion coimpregnation influence the adsorption process during the impregnation steps of noble metal-Mo/alumina samples.

In addition, Spanos & Lycourghiotis (1994) have reported that mutual promotion exists in the adsorption of Mo(VI) and Co(II) ion species on Al O in the 4.1-6.1 pH range; this was attributed to the strong lateral interactions exerted between Mo(VI) and Co(II) ions in solution. Finally, most of the adsorption isotherms for Pt[Cl.sub.6.sup.2-] ions reported in literature correspond to the Pt/[Al.sub.2][O.sub.3] system, Lee & Aris (1985), Gavriilidis et al. (1993), for which the effects of several inorganic and organic acids as competing agents on the deposition of Pt on alumina have been extensively investigated. Thus, Gavriilidis et al. (1993) and Papageorgiou et al. (1996) have reported the use of the modified Langmuir model to quantitatively describe transient adsorption studies of the Pt-citric acid system, which in turn also accounted for the effects of the solution and steric hindrance.

3.3 Numerical Calculations

For the quantitative description of the above isotherms, numerical calculations were carried out. For this purpose, different equations relating the equilibrium concentration in solution and the amount of adsorbed species were examined, Bourikas et al. (1996, 2006), Giles et al. (1974), Ruthven (1984). In this sense, the main goal was to obtain an expression or relationship that could be easily used to model the impregnation steps and to predict the concentration profile of the active component inside the pellet. The classic Langmuir expression was taken as the starting point for these studies. The Langmuir equation establishes a relationship between Ca and [C.sub.eq] by:

[Ca.sub.eq] = [K.sub.e].S.[C.sub.eq]/1 + [K.sub.e].[C.sub.eq] (4)

where [K.sub.e] (ml/mg) is the equilibrium adsorption constant; S (mg ion/g support) is the corresponding surface concentration at equilibrium, and [C.sub.eq] (mg/ml) is the solution concentration.

Other expressions such as those proposed by Giles, Kitchener, Henry and Freundlich were also used to quantitatively describe the relationship between Ca and [C.sub.eq], Pizzio et al. (1996), Giles et al. (1974), Ruthven (1984), Bearing in mind the assumption of these authors for type S isotherms, the expressions for Giles and Kitchener are:

[Ca.sub.eq] = [K.sub.G].S.[C.sup.x.sub.eq]/1 + [K.sub.G].[C.sup.x.sub.eq] (5)

[Ca.sub.eq] = [K.sub.1].S.[C.sub.eq][10.sup.-k1[theta]]/1 + [K.sub.1].[C.sub.eq][10.sup.-k1[theta]] (6)

Where [K.sub.G] (ml/mg)x is the Giles equilibrium adsorption constant, S is the saturation concentration (mg ion/g support) at equilibrium; x is an exponential that describes the isotherm shape; [K.sub.1] and [k.sub.1] are constants, and [theta] = Ca/S is the fraction of adsorption sites occupied. The Giles and Kitchener expressions are related to the Lagmuir constant by Ke=[K.sub.G][C.sub.eq.sup.x-1] and Ke=[K.sub.1][10.sup.-k1[theta]] respectively.

Using a non-linear least-squares algorithm (Levenberg-Marquart), the equilibrium adsorption parameters for each system and equation were calculated from the fitting of the equilibrium data. This procedure was accomplished by minimizing the [chi square] parameter. The group of parameters showing the best fit to the experimental isotherm is summarized in the (Table 3). Although in most cases the difference between the equations is very small, the choice basically depends on the simplicity of the mathematical model chosen to describe the impregnation process.

From Figs. 5 and 6 and Table 3, it is clear that the [gamma]-[Al.sub.2][O.sub.3] (AK) sample shows a slightly higher adsorption capacity than its [gamma]-[Al.sub.2][O.sub.3] (RO and PR) counterparts for Mo ion adsorption, although this difference is clearer in the SoMo isotherm. All the [gamma]-[Al.sub.2][O.sub.3] samples studied exhibited a higher affinity for polymeric molybdate than for monomeric ions. This difference may be due to the presence of different impurities and the different amount and types of sites for adsorption (hydroxyl types) on those surfaces, since the textural parameters and p[H.sub.ZPC] values are very close, Giordano et al. (1975), Adachi et al. (1996). The [gamma]-[Al.sub.2][O.sub.3] (AK) sample has a higher S (mg Mo/g alumina) value than its [gamma]-[Al.sub.2][O.sub.3] (RO and PR) counterparts.

At low concentration values, up to 5 mg/mL, the isotherms show a Henry-type behavior. Thus, the Henry constant for the AK-type [Al.sub.2][O.sub.3] sample is higher than for the RO- and PR-type [gamma]-[Al.sub.2][O.sub.3] counterparts. This observation again suggests a higher availability of adsorption sites on the AK-type [gamma]-[Al.sub.2][O.sub.3] than on the RO- and PR-type [gamma]-[Al.sub.2][O.sub.3] surfaces.

The fitting parameters, the saturation coverage of Mo, and the equilibrium constant, calculated from the Giles, Langmuir and Freundlich model are shown in Table 3. The close values of x (1.45, 1.32 and 1.28 for AHM adsorption) indicate that the shape of the curves is quite similar for all three aluminas. There are differences between the SoMo isotherms on alumina, reflected in the parameter values. These results lie in the same range as many others reported in the literature. At this juncture it should be emphasized that the reported values for S and [K.sub.e] (Langmuir model) for the adsorption of molybdenum on alumina surfaces are scattered over broad ranges. Thus, Mo saturation coverages from 50 to 156 mg Mo/g alumina and adsorption equilibrium constants from 1.9 x 10-4 to 1.65 ml/mg Mo can be found in the literature, van Veen et al. (1987), Giles et al. (1974), Gavriilidis et al. (1993), Pizzio et al. (1996). Although the chemical properties associated with the presence of different levels of impurities may alter the proportion of adsorption sites, textural properties may also contribute to exacerbating the observed differences.

Table 3 and Figs. 7-10 show that the Co/[gamma]-[Al.sub.2][O.sub.3] systems display almost identical L-shaped isotherms, except for CoAc and CoCl adsorptions on PR alumina, which exhibit an S type. This type of behavior suggests that either the same type of adsorption sites for Co are available on the [gamma]-[Al.sub.2][O.sub.3] surface or that a similar deposition process is operative along the impregnation and drying steps, irrespective of the second ion in solution. For PR-type alumina and CoCl and CoAc systems, the Langmuir model obviously did not apply; however, the Giles equation revealed an appropriate fitting of the adsorption data. Within the [C.sub.eq] concentration range explored, a stronger adsorption of Co(II) ions on PR-type alumina than for the other two aluminas was found.

By contrast, the two Pt(Co)/Mo[O.sub.3]/[gamma]-[Al.sub.2][O.sub.3] and Co(Pt)/Mo[O.sub.3]/[gamma]-[Al.sub.2][O.sub.3] samples showed the opposite adsorption behavior. The data can be satisfactorily described by the Freundlich and Giles equations respectively (Fig. 11 and Table 3). Saturation values of 2.0 mg Co/g Mo[O.sub.3]-[Al.sub.2][O.sub.3] and 0.4 mg Pt/g Mo[O.sub.3]-[Al.sub.2][O.sub.3] for the AK-type substrate were derived from the fitting. These values are lower than the adsorption data reported for Pt/[gamma]-[Al.sub.2][O.sub.3] catalysts. This monometallic system showed values between 21 to 56 mg Pt/g [Al.sub.2][O.sub.3] (110-290 [micro]mol [H.sub.2]Pt[Cl.sub.6]/g [Al.sub.2][O.sub.3]) and 0.56 to 159 ml/mg Pt (110-31000 l/mol [H.sub.2]Pt[Cl.sub.6]) for the surface saturation coverage and the equilibrium adsorption constant, [K.sub.e], respectively, Lee & aris (1985), Papageorgiou et al. (1996). The large difference observed in the saturation values for Pt can be explained in terms of the notion that most of the adsorption sites are located on the alumina surface, and in turn disappear upon molybdenum incorporation and interaction with the ion in solution. Some discrepancies may also arise from the experimental conditions of the impregnation process used in the adsorption step.

4. Conclusions

The extent of Mo adsorption on the [gamma]-[Al.sub.2][O.sub.3] surface depended on the type of molybdate species (tetrahedral and octahedral) in solution. Moreover, the adsorption process suggested that polymeric and monomeric species are present in the wet, dry and calcined states, since surface groups and alumina impurities can modify the adsorption process. The [gamma]-[Al.sub.2][O.sub.3] (AK) sample showed a slightly higher adsorption capacity for Mo than its [gamma]-[Al.sub.2][O.sub.3] (RO and PR) counterparts. At low concentration values (until 5 mg/ml), the Mo isotherms were almost linear (Henry-type behavior), with the Henry constant for the Al O (AK) higher than for the other [gamma]-[Al.sub.2][O.sub.3]. This observation suggests a higher availability of adsorption sites on the [gamma]-[Al.sub.2][O.sub.3] (AK) than on the [gamma]-[Al.sub.2][O.sub.3] (RO and PR) surface.

For Co systems, it was observed that: (a) for Co adsorption, the same sites are present on the alumina surface when different precursors are used; (b) the shape of the adsorption isotherm of Pt on MoO3/[gamma]-[Al.sub.2][O.sub.3] surface changes from the S- to the L-type; (c) since L- and S-shaped isotherms were found for the Co/[gamma]-[Al.sub.2][O.sub.3] and Co-Mo[O.sub.3]/[gamma]-[Al.sub.2][O.sub.3] systems respectively, the adsorption of Co species was altered by the Mo[O.sub.3]/[gamma]-[Al.sub.2][O.sub.3] substrate.

5. Acknowledgements

Machuca-Martinez gratefully acknowledges COLCIENCIAS, UIS and UNIVALLE (Colombia) and ICP-CSIC (Spain) for a fellowship and financial support. The alumina supports used in the present work were kindly supplied by Akzo-Nobel (The Netherlands), Rhodia (France) and Procatalyse (France).

(Recibido: Enero 13 de 2010--Aceptado: Abril 3 de 2010)

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Fiderman Machuca-Martinez *, Jose L. G. Fierro ** ([seccion])

* Laboratorio de Investigacion en Catalisis y Procesos. Escuela de Ingenieria Quimica, Universidad del Valle, A.A. 25360. Cali, Colombia.

** Instituto de Catalisis y Petroleoquimica, CSIC, Cantoblanco, 28049 Madrid, Espana.

([seccion]) e-mail:
Table 1. pH values for metal ion adsorption on y-[Al.sub.2][O.sub.3] or

System (a)                   Alumina   Initial       Final
                                       Range pH      Range pH

AHM/[gamma]-                   AK      4.87 - 5.70   6.40 - 7.72
AHM/[gamma]-                   RO      4.87 - 5.70   6.11 - 8.10
AHM/[gamma]-                   PR      4.49 - 4.96   6.26 - 7.28
SoMo/[gamma]-                  AK      7.14 - 7.32   7.36 - 8.72
SoMo/[gamma]-                  RO      7.14 - 7.32   8.42 - 8.24
SoMo/[gamma]-                  PR      7.06 - 7.14   7.50 - 8.62
CoNi/[gamma]-                  AK      7.04 - 6.57   6.96 - 5.65
CoNi/[gamma]-                  RO      7.04 - 6.57   7.60 - 4.77
CoNi/[gamma]-                  PR      6.37 - 5.08   7.61 - 5.54
CoCl/[gamma]-                  AK      7.50 - 4.55   5.66 - 5.79
CoCl/[gamma]-                  RO      7.50 - 4.55   7.54 - 5.90
CoCl/[gamma]-                  PR      7.02 - 7.50   6.05 - 5.55
CoAc/[gamma]-                  AK      8.08 - 7.27   7.04 - 5.80
CoAc/[gamma]-                  RO      8.08 - 7.27   7.38 - 5.69
CoAc/[gamma]-                  PR      7.02 - 7.50   7.15 - 5.73
Co/Mo[O.sub.3]-[gamma]-        AK      6.71 - 6.57   3.30 - 3.68
Co/Mo[O.sub.3]-[gamma]-        RO      6.71 - 6.57   3.87 - 3.80
Pt/Mo[O.sub.3]-[gamma]-        AK      2.20 - 1.57   3.65 - 3.60
Pt/Mo[O.sub.3]-[gamma]-        RO      2.20 - 1.57   3.73 - 3.82
Pt-(Co)/Mo[O.sub.3]-           AK      2.67 - 4.04   3.39 - 3.86
Co-(Pt)/Mo[O.sub.3]            AK      2.67 - 4.04   3.39 - 4.86

System (a)                   Alumina   [ph.sub.zpc]  Initial content.
                                       Support       range
                                                     (mg /mL) (b)

AHM/[gamma]-                   AK      7.56          0.013 - 65.00
AHM/[gamma]-                   RO      7.84          0.013 - 65.00
AHM/[gamma]-                   PR      7.72          0.013 - 42.50
SoMo/[gamma]-                  AK      7.56          0.013 - 29.98
SoMo/[gamma]-                  RO      7.84          0.013 - 29.98
SoMo/[gamma]-                  PR      7.72          0.055 - 26.00
CoNi/[gamma]-                  AK      7.56          0.005 - 6.200
CoNi/[gamma]-                  RO      7.84          0.005 - 6.200
CoNi/[gamma]-                  PR      7.72          0.021 - 11.07
CoCl/[gamma]-                  AK      7.56          0.006 - 6.200
CoCl/[gamma]-                  RO      7.84          0.006 - 6.200
CoCl/[gamma]-                  PR      7.72          0.0016 - 5.80
CoAc/[gamma]-                  AK      7.56          0.008 - 6.300
CoAc/[gamma]-                  RO      7.84          0.008 - 6.300
CoAc/[gamma]-                  PR      7.72          0.0016 - 5.80
Co/Mo[O.sub.3]-[gamma]-        AK      3.62          0.750 - 13.00
Co/Mo[O.sub.3]-[gamma]-        RO      3.80          0.750 - 13.00
Pt/Mo[O.sub.3]-[gamma]-        AK      3.62          0.500 - 3.300
Pt/Mo[O.sub.3]-[gamma]-        RO      3.80          0.500 - 3.300
Pt-(Co)/Mo[O.sub.3]-           AK      3.62          0.030 - 0.120
Co-(Pt)/Mo[O.sub.3]            AK      3.62          0.270 - 8.500

(a) Solution with support; (b) Amount of metal ion in solution;
(c) Solution with two ions

Table 2. Principal bands in UV-Vis spectra and pH for AHM and SoMo

Solution   Dominant    Initial pH         Mo
System     molybdate                      Concentration
           species                        (ppm)

AHM        Polymeric   4.51               120
                       4.53               60
                       4.67               30
                       4.87               13

SoMo       Monomeric   7.17               250
                       7.31               130
                       7.10               65
                       7.24               33
                       7.14               13

Solution   Dominant    Mean Band Peak     Shoulder (nm)
System     molybdate   (nm)

AHM        Polymeric   231.0-234.5 (a)
                       214.0              235.5
                       209.5              234.5
                       209.0              234.0

SoMo       Monomeric   237.0 (b)
                       233.0 (b)
                       218.0              234.0
                       209.5              234.0
                       208.5              234.0

(a) Distortion spectra; (b) One band

Table 3. Equilibrium adsorption parameters derived from the fittings

System                                     Giles

                                        Kg        S

AHMM/[Al.sub.2][O.sub.3] (AK)       0.0658   119.00
AHM/[Al.sub.2][O.sub.3](RO)         0.0481   100.00
AHM/[Al.sub.2][O.sub.3](PR)         0.1692    73.27
SoMo/[Al.sub.2][O.sub.3] (AK)       0.2460    23.26
SoMo/[Al.sub.2][O.sub.3](RO)        0.2780     5.99
SoMo/[Al.sub.2][O.sub.3](PR)        0.6670     8.50
CoNi/[Al.sub.2][O.sub.3] (AK)       0.2140    28.62
CoNi/[Al.sub.2][O.sub.3](RO)        0.5960    12.05
CoNi/[Al.sub.2][O.sub.3](PR)        1.3760     9.88
CoCl/[Al.sub.2][O.sub.3] (AK)       0.7720    12.22
CoCl/[Al.sub.2][O.sub.3](RO)        1.1180     9.83
CoCl/[Al.sub.2][O.sub.3](PR)        92.300    39.63
CoAc/[Al.sub.2][O.sub.3] (AK)       5.8950     9.45
CoAc/[Al.sub.2][O.sub.3](RO)        1.9430    13.63
CoAcl/[Al.sub.2][O.sub.3](PR)       47.820    58.87
Co-(Pt)/Mo[O.sub.3]/                     -        -
[Al.sub.2][O.sub.3] (AK)
Pt-(Co)/Mo[O.sub.3]/                1535.8     0.51

System                                 Giles

                                   X   [chi square]

AHMM/[Al.sub.2][O.sub.3] (AK)   1.45          15.50
AHM/[Al.sub.2][O.sub.3](RO)     1.32           4.09
AHM/[Al.sub.2][O.sub.3](PR)     1.28           2.62
SoMo/[Al.sub.2][O.sub.3] (AK)   0.87           0.18
SoMo/[Al.sub.2][O.sub.3](RO)    0.55           0.02
SoMo/[Al.sub.2][O.sub.3](PR)    1.49           0.07
CoNi/[Al.sub.2][O.sub.3] (AK)   0.41           0.03
CoNi/[Al.sub.2][O.sub.3](RO)    0.38           0.01
CoNi/[Al.sub.2][O.sub.3](PR)    0.97           0.05
CoCl/[Al.sub.2][O.sub.3] (AK)   0.51           0.02
CoCl/[Al.sub.2][O.sub.3](RO)    0.55           0.17
CoCl/[Al.sub.2][O.sub.3](PR)    2.04           0.10
CoAc/[Al.sub.2][O.sub.3] (AK)   0.92           0.29
CoAc/[Al.sub.2][O.sub.3](RO)    0.56           0.21
CoAcl/[Al.sub.2][O.sub.3](PR)   1.61           0.18
Co-(Pt)/Mo[O.sub.3]/               -              -
[Al.sub.2][O.sub.3] (AK)
Pt-(Co)/Mo[O.sub.3]/            2.82           7E-4

System                                         Langmuir

                                        Ke        S   [chi square]

AHMM/[Al.sub.2][O.sub.3] (AK)        0.109      140          25.50
AHM/[Al.sub.2][O.sub.3](RO)          0.059      130           6.11
AHM/[Al.sub.2][O.sub.3](PR)          0.209       80           3.15
SoMo/[Al.sub.2][O.sub.3] (AK)        0.253       21           0.20
SoMo/[Al.sub.2][O.sub.3](RO)         0.393      3.9           0.05
SoMo/[Al.sub.2][O.sub.3](PR)         0.579      9.4           0.20
CoNi/[Al.sub.2][O.sub.3] (AK)        2.380      8.2           0.32
CoNi/[Al.sub.2][O.sub.3](RO)         5.838      5.9           0.25
CoNi/[Al.sub.2][O.sub.3](PR)         1.431      9.7           0.05
CoCl/[Al.sub.2][O.sub.3] (AK)        3.654      7.5           0.20
CoCl/[Al.sub.2][O.sub.3](RO)         3.359      7.3           0.42
CoCl/[Al.sub.2][O.sub.3](PR)             -        -              -
CoAc/[Al.sub.2][O.sub.3] (AK)        7.338      9.2           0.26
CoAc/[Al.sub.2][O.sub.3](RO)         8.636       10           0.49
CoAcl/[Al.sub.2][O.sub.3](PR)            -        -              -
Co-(Pt)/Mo[O.sub.3]/                 1.426      1.9           0.06
[Al.sub.2][O.sub.3] (AK)
Pt-(Co)/Mo[O.sub.3]/                    -        -              -

System                                           Freundlich

                                        A        B    [chi square]

AHMM/[Al.sub.2][O.sub.3] (AK)        22.75    0.455            110
AHM/[Al.sub.2][O.sub.3](RO)          10.58    0.622           19.2
AHM/[Al.sub.2][O.sub.3](PR)         18.550    0.408           25.6
SoMo/[Al.sub.2][O.sub.3] (AK)        5.249    0.411           1.07
SoMo/[Al.sub.2][O.sub.3](RO)         1.308    0.335           0.04
SoMo/[Al.sub.2][O.sub.3](PR)         3.070    0.366           1.63
CoNi/[Al.sub.2][O.sub.3] (AK)        4.899    0.353           0.04
CoNi/[Al.sub.2][O.sub.3](RO)         4.260    0.270           0.04
CoNi/[Al.sub.2][O.sub.3](PR)         4.392    0.383           1.03
CoCl/[Al.sub.2][O.sub.3] (AK)        4.913    0.319           0.12
CoCl/[Al.sub.2][O.sub.3](RO)         4.784    0.310           0.10
CoCl/[Al.sub.2][O.sub.3](PR)         252.0    1.231           1.43
CoAc/[Al.sub.2][O.sub.3] (AK)        6.680    0.294           1.25
CoAc/[Al.sub.2][O.sub.3](RO)         8.142    0.280           0.63
CoAcl/[Al.sub.2][O.sub.3](PR)        547.6    1.210           0.53
Co-(Pt)/Mo[O.sub.3]/                 1.023    0.291           0.01
[Al.sub.2][O.sub.3] (AK)
Pt-(Co)/Mo[O.sub.3]/                 8.144    1.403           1E-3
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Author:Machuca-Martinez, Fiderman; Fierro, Jose L.G.
Publication:Ingenieria y Competividad
Date:Jun 1, 2010
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