Synthesis and Characterization of Alumina-Zirconia Powders Obtained by Sol-Gel Method : Effect of Solvent and Water Addition Rate

The influence of solvent and the rate of addition of water on the characteristics of alumina-zirconia powders obtained by sol-gel method were investigated. The Al2O3-ZrO2 powders (1:1 molar ratio) were prepared using aluminum tri-secbutoxide and zirconium n-propoxide as precursors. Ethanol (EtOH), isopropanol (iPrOH) and isobutanol (iBuOH) were used as solvents. The Al2O3-ZrO2 powders were characterized by nitrogen physisorption (SBET), Fourier transformed infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Prepared oxides calcined at 700 ̊C showed high specific surface area (200 240 m/g). Obtained results suggest that the homogeneity of the mixed oxides is favored by using a water addition rate of 0.06 and 0.10 mL/min with ethanol as solvent.


Introduction
The application of Al 2 O 3 -ZrO 2 mixed oxides on CH 4 steam reforming reactions has been investigated in recent years [1][2][3].Alumina is the most widely known catalyst support for naphtha reforming reactions in the industry; however, catalysts supported on alumina tend to deactivate rapidly by coke formation, which in turn yields less active sites on the surface of the catalyst, consequently decreasing its lifespan [4].Zirconium oxide is thermally and chemically more stable than aluminum oxide.It is a unique oxide because it possesses acid-base properties [5].Furthermore, by mixing it with other oxides (Cerium oxide, Lanthanum Oxide, etc.), it presents reducing and oxidizing capabilities [6].
The metal used in the active phase of the catalyst may take different reaction routes when present on other oxides as support, therefore offering the possibility of reducing the undesirable sub-product formation such as coke or acetaldehyde.Furthermore, the complete conversion of hydrocarbons is essential for the reforming process to be cost effective.The catalyst has an important role in attaining such status.Once the rate of the reaction is confirmed by working more efficiently, the system yields more desired products.Thus, the selection and composition of the catalytic support have a vital consideration in the hydrocarbons' steam reforming reactions [7,8].
The properties of mixed oxides depend on several synthesis parameters: The type of precursor, solvent, water content, pH, precursor concentration, calcination temperature, mechanical or magnetic agitation, speed of agitation, preparation method and type of catalyst [9][10][11][12].Several methods for the synthesis of oxides are reported in the literature.Some of them are: Co-precipitation, sol-gel and calcination [13,14].The reported advantages in using the sol-gel method are: 1) Large specific surface area; 2) Uniform pore size distribution; 3) Superior homogeneity and purity; 4) High degree of thermal and chemical stability for the supported metals; 5) Better mechanical resistance and 6) Better microstructural control for metallic cluster-formation [15,16].
Several researchers have used the sol-gel method to synthetize Al 2 O 3 -ZrO 2 powders [17][18][19][20][21]. Chen et al. [17,18] have prepared Al 2 O 3 -ZrO 2 powders with various ZrO 2 contents using the sol-gel route and the co-precipitation method as well .They reported that the powders obtained by the sol-gel route yielded larger surface areas, diameters and smaller pore-sizes than the powders obtained by the coprecipitation method.The surface area of the powders Synthesis and Characterization of Alumina-Zirconia Powders Obtained by Sol-Gel Method: Effect of Solvent and Water Addition Rate obtained by the sol-gel route was about twice as large as the powders made by the co-precipitation method, observing that the catalyst effect decreases by increasing the calcination temperatures.The Al 2 O 3 -ZrO 2 mixed oxides with a zirconia content of about 10 wt%, obtained by the sol-gel technique calcined at 700˚C has a specific surface area of 290 m 2 /g.The specific surface area decreased by increasing the zirconia content.Moreover, the average pore size for the alumina-zirconia powders is affected positively by the relative amount of zirconia incorporated into the mixed oxides synthesis.Moran-Pineda et al. [19] have reported the synthesis and characterization of Al 2 O 3 -ZrO 2 mixed oxides with 5, 10, and 20 wt% of zirconia by the sol-gel method.Their experimental results showed that the oxides, 299, 283 and 354 m 2 /g had relative specific surface areas of 5, 10 and 20 wt% respectively at 500˚C.According to their study, the specific surface area increased from 283 to 354 m 2 /g for the mixed oxides at 10 -20 wt% of zirconia, opposite from the results obtained by Chen et al., who reported that when zirconia goes from 10% to 20%, the surface area decreased from 330 to 290 m 2 /g at 500˚C.
Klimova et al. [20] investigated a series of Al 2 O 3 -ZrO 2 mixed oxides with variable amounts of ZrO 2 calcined at 300˚C, 400˚C, 600˚C and 800˚C.They observed that the specific surface area of mixed oxides at 600˚C, with a molar ratio was about 389 m 2 /g, whereas the pure alumina sample yielded a surface area of 349 m 2 /g, which suggested that the incorporation of small amounts of zirconia into alumina presumably would increase the specific surface area, but the area actually decreased when more zirconium was added to the alumina.Likewise, smaller average diameter pores in the mixed oxides were obtained when the molar relation increased in comparison with the pure alumina oxide.Dominguez et al. [21] prepared a series of Al 2 O 3 -ZrO 2 solid solutions at different molar ratios.The results obtained by these authors showed that when calcined at 550˚C, the mixed oxides had a specific surface area of between 200 and 250 m 2 /g.A maximum specific surface area was obtained with a molar ratio X = 0.2.Their results agreed with those obtained by Klimova et al. [20].
In the synthesis and characterization of Al 2 O 3 -ZrO 2 mixed oxides obtained by the sol-gel method, the supports were prepared using several solvents and different chemical precursors by modifying the process reported by Yoldas [22] for the synthesis of alumina by the sol-gel route.
In the present research, the objective is to investigate the effect of three different solvents on the textural and chemical properties of Al 2 O 3 -ZrO 2 , synthesized by the solgel method.The effect of four different water-addition rates was also investigated.The lowest value of the water-addition rates resulted in a homogeneous pore-size distribution.The research will allow the development of tai-lor-made supports to be used in particular hydrocarbon reactions.

Procedure
In the present study, Al 2 O 3 -ZrO 2 supports (1:1 molar ratio) were obtained by the sol-gel method, using a molar ratio of 80/30/0.3/1(alcohol/water/acid/alkoxide).Aluminum tri-sec-butoxide (ATB, 97%, Aldrich) and anhydrous ethanol (94.2%J. T. Baker) were mixed in a reactor vessel with continuous stirring for 20 minutes under an inert atmosphere of nitrogen.In another reactor vessel zirconium (IV) propoxide (ZrP, 70 wt%, Aldrich) and the appropriate amount of ethanol were mixed under an inert atmosphere of nitrogen and then finally stirred for minutes.Afterwards, both solutions were blended, by stirring the mixture vigorously for two hours until a clear solution at room temperature was obtained.An aqueous solution of HNO 3 (65%, J. T. Baker) was added at four addition rates (0.06, 0.1, 0.15 and 0.26 mL/min) to the reactor vessel containing the solution of ethanol and alkoxides under continuous stirring.The gel was aged for hours and dried in a vacuum oven.The starting heat treatment step consisted in drying the powder to 120˚C for 1 hour at a heating rate of 1˚C/min starting from room temperature.The resulting powder was finally calcined up to 700˚C for 2 hours at a heating rate of 0.5˚C/min.Figure 1 shows the schematic diagram for the experimental set-up for the preparation of Al 2 O 3 -ZrO 2 supports by the sol-gel method.The same procedure was used for the mixed oxides obtained with isopropanol and isobutanol.

Characterization
The mixed-oxide supports were characterized using a variety of techniques, which are mentioned below.Synthesis and Characterization of Alumina-Zirconia Powders Obtained by Sol-Gel Method: Effect of Solvent and Water Addition Rate 652

N 2 Adsorption-Desorption
The specific surface areas of the calcined samples were calculated from N 2 adsorption-desorption acquired data at liquid N 2 temperature on a Micromeritics ASAP 2010 instrument, and the pore size distributions were determined by the Barrett-Joyner-Halenda (B-J-H) method applied to the desorption profile-branch of the nitrogen isotherm.

Infrared Spectroscopy
FT-IR spectroscopy was performed with Perkin Elmer Spectrum 100 equipment on pellets of the mixed oxides at the 400 -4000 cm -1 wave number range.For this analysis the pellets were elaborated by the application of mechanical pressure on the powdered material formed by adding KBr to the samples to be analyzed.

X-Ray Diffraction
The crystalline degree of the materials was determined by a Kristalloflex Siemens D5000 instrument using monochromatic Cu-Kα radiation, scanning 2θ from 10˚ to 90˚, using standard procedure.

Thermogravimetric Analysis
The thermogravimetric (TGA) and differential thermal (DTA) analyses of Al 2 O 3 -ZrO 2 mixed oxides were carried out with a heating rate of 10˚C/min up to 1000˚C in air at 100 mL/min, using a SDT Q600 simultaneous TGA-DSC instrument.

Scanning Electron Microscopy
In this case, the microstructure of the Al 2 O 3 -ZrO 2 mixed oxides was observed in a Leica S-420 scanning electron microscope (SEM) with an acceleration voltage of 20 KeV.The samples were first covered with Au-Pd in a diode sputtering coater  1, it is possible to observe that the surface area decreased with the increase of the water addition rate.

Textural Properties of Supports
Figure 2 shows the pore size distribution of Al 2 O 3 -ZrO 2 supports prepared with ethanol, isopropanol and isobutanol.The mean pore size distribution observed at the inset plot of Figure 2 corresponds to that of a mesoporous material.
The average pore diameter was observed to decrease from 69 Å to 48 Å using ethanol as solvent.In the case of the isopropanol used as solvent, the average pore diameter decreased from 88 Å to 54 Å.This effect is observed when isobutanol is used as solvent as well.
The pore size distribution of the mixed oxides observed in Figure 2(c) shows a bimodal size distribution, with the first maxima around 27 Å to 35 Å, but when isobutanol alcohol is used as solvent, a second maxima is observed around 60 Å to 100 Å for all water addition rates.The pore size distribution analysis of Figures 2(a) and (b) suggests that for a water addition rate of 0.06 mL/min a bimodal pore size distribution is obtained when the mixed oxides are prepared using ethanol and isopropanol as solvents.The remaining water-addition rates for the latter solvents present only a monomodal pore size distribution.
The nitrogen adsorption-desorption isotherms of Al 2 O 3 -ZrO 2 supports were prepared with ethanol, isopropanol and isobutanol, which are shown in Figure 3.
The supports show E-type hysteresis loops, which are interpreted as type IV isotherms, indicating the probable existence of tubular pores with narrow openings or tubular pores with irregular narrow openings, otherwise known as ink-well type pores.

FT-IR Analysis
Figure 4 shows spectra obtained by means of FT-IR analyses of Al 2 O 3 -ZrO 2 supports calcined at 700˚C prepared with ethanol as solvent at two water-addition rates.Similar results were obtained using isopropyl and isobutyl alcohols as solvents.
The FT-IR spectra in the 4000 -2900 cm -1 region show a broad band for the two water addition rates, a characteristic assigned to the presence of structural OH -groups.One band is observed at 3442 cm -1 for the water-addition rate of 0.06 mL/min is shown in Figure 4( from a low water-addition rate is due to lattice structural rearrangement.The bending vibrations of Zr-OH bonds appear at 1630 -1631 cm -1 for both hydrolysis rates [24].
The bands appearing at 1395 and 1407 cm -1 are typical for the γ-alumina support.The Al-O vibration is observed at 1371 and 1363 cm -1 for a water-addition rate of 0.06 and 0.26 mL/min respectively.Another important adsorption band can be observed at 523 cm -1 , which corresponds to the vibration of the Zr-O-Al bond in the alumina-zirconia powders [25].

X-Ray Diffraction
The XRD spectra of the Al  spectra for Al 2 O 3 -ZrO 2 is not detectable; however a moderate plateau of tetragonal ZrO 2 is shown at 2 -30˚, indicating the presence of an incipient crystalline structure in the synthesized mixed oxides [26,27].This incipient crystalline structure is responsible for the low level definition of the bands at 523 cm -1 and 525 cm -1 in the FT-IR studies [27].

Thermogravimetric Analysis
The TGA-DTA curves of the fresh Al tion of organic compounds.The exothermic small peak in the temperature range 374˚C -481˚C is observed in the DTA curve.This signal is associated with the tetragonal phase ZrO 2 .A zone in the temperature range 481˚C -637˚C corresponds to an endothermic change due to the conversion of dehydroxylated boehmite to -Al 2 O 3 [28].Finally, in the temperature range 850˚C -1091˚C there is an exothermic change, which is caused by the transformation of the ZrO 2 tetragonal phase to a monoclinic phase.
For the water addition rate of 0.26 mL/min, Figure 6(b): A first zone of the DTA curve in the temperature range 21˚C -109˚C is due to the evaporation of adsorbed water from the Al 2 O 3 -ZrO 2 powders.A second zone in the temperature range 109˚C -234˚C is exothermic and can be attributed to a total dehydration of mixed oxides and to the incipient combustion of the organic groups.In the third decomposition zone which is exothermic, in the temperature range 234˚C -304˚C, the peak corresponds to the elimination of chemically bonded water.The fourth decomposition zone in the temperature range 304˚C -451˚C corresponds to the total combustion of organic compounds [19,26,27].A fifth zone in the temperature range 451˚C -678˚C is associated to an endothermic change due to the conversion of dehydroxylated boehmite to -Al 2 O 3 [24].
Finally, in the temperature range 850˚C -1091˚C, there is an exothermic change, which is caused by the transformation of the tetragonal into the monoclinic zirconia phase.

Scanning Electron Microscopy
Figure 7 shows the micrographs obtained from the scanning electron microscopy studies for the Al 2 O 3 -ZrO 2 powders where an appreciable formation of agglomerates can be seen.The results show that the particle size increased by increasing the water addition rate.The agglomerates are composed of mostly homogeneous-sized particles as can be observed in Figure 7(a) for the 0.06 mL/min water-addition rate.On the other hand, Figure 7(b), shows a dispersion of non-spherical particles, where serrated edges are observed for the 0.26 mL/min water-addition rate.The lowest water-addition rate allowed for a higher extent of polymerization due to a longer reaction time which yielded more homogenous-sized particles.At the largest water-addition rate, the polymerization extent was less complete due to both the greater amount of reactant and less reaction time, which yielded particles that were less rounded and had a lesser degree of aggregation.

Discussions
The main difference observed when using three different solvents was related to the textural properties of the materials.The use of ethanol as solvent achieved a narrow pore-size distribution centered at the lowest pore-diameter and with the highest surface area suitable for reactions where small molecules can participate.On the other hand, the use of isobutanol as solvent generated the highest average-pore diameter, yielding consequently the lowest surface area.The FT-IR, TGA-DTA and XRD analyses were obtained for all solvents, and the main features are described only for ethanol in this contribution, since the remaining solvents show equivalent behavior.
As the water-addition rate was increased the BET surface area decreased, this effect is confirmed by SEM micrographs where particle-agglomeration is observed, Figure 7(b).The latter concept is further confirmed by the FT-IR analysis when a water-addition rate of 0.26 mL/min is used, where a less intense peak at 3447 cm -1 associated to surface OH -groups indicates a larger agglomeration degree.The presence of an FT-IR signal at 2900 cm -1 associated to C-H vibrations indicates an incomplete polymerization assumed to be due to a short reaction time.The TGA-DTA analysis shows a larger zone at 300˚C -450˚C with respect to the lower water-addition rate in Figure 6, which indicates the presence of more chemically-bound hydrocarbons from non-reacted alkoxides, as verified by the FT-IR analysis.
The BET surface area determined at a water-addition rate of 0.06 mL/min is larger when using ethanol as solvent.The micrograph by SEM reveals in this case the formation of uniform-sized particles with facile agglomeration and no appreciable distortions.The agglomeration of small spherical particles generates an E-type hysteresis loop, confirmed by the N 2 adsorption determination.It is proposed that such configuration was achieved by a combination of variables; time is important during particle formation while the textural properties are affected by the solvent.Lower water-addition rates facilitate the growth of completely-polymerized particles.TGA-DTA analysis indicates the formation and particle agglomeration with minor incorporation of non-reacted components, which can be observed by a lesser weight loss in the range 300˚C -450˚C than that observed at the larger water-addition rate.The FT-IR signal at 3442 cm -1 associated to OH -groups is more intense.The latter has been ascribed to a larger concentration of OH -groups, tied to a higher degree of polymerization that turns out in a lack of C-H vibrations at 2900 cm -1 as observed in Figure 4(a).

Conclusions
According to the results and discussions, the following conclusions are: 1) The use of ethanol as solvent produced Al 2 O 3 -ZrO 2 mixed oxides with higher specific surface internal areas than employing isopropyl or isobutyl alcohols; 2) The average pore diameter is largest in this study when isobutyl alcohol is used as solvent for the synthesized Al 2 O 3 -ZrO 2 supports; 3) High pore volumes were favored by using the isopropyl alcohol as solvent to the obtained Al 2 O 3 -ZrO 2 mixed oxides; 4) The water-addition rate of 0.26 mL/min leads to significant structural changes as indicated by the analysis of the FT-IR spectra, which can be attributed to an incomplete polymerization in the synthesis of the supports; 5) X-ray diffraction patterns for all samples do not identify well-defined crystalline structures corresponding to ZrO 2 or Al 2 O 3 .This fact suggests that the samples are mostly amorphous solids; 6) For water addition at a rate of 0.10 mL/min the specific surface area of Al 2 O 3 -ZrO 2 powders is almost equal to that obtained at a water-addition rate of 0.06 mL/min, hence, this rate can be used to prepare catalytic supports.

Figure 1 .
Figure 1.Schematic diagram of experimental set-up for the preparation of Al 2 O 3 -ZrO 2 supports by the sol-gel method.

Figure 2 .Figure 3 .
Figure 2. Pore size distribution of Al 2 O 3 -ZrO 2 supports prepared with (a) Ethanol; (b) Isopropanol; and (c) Isobutanol, obtained using different water addition rates.rate of 0.26 mL/min is shown in Figure 4(b).A small extra band is observed at 2900 cm -1 for the water-addition rate of 0.26 mL/min, which is assigned to C-H vibration due to an incomplete polymerization, whereas this band is not observed at the lower water-addition rate[19,23].The band associated with the stretching vibrations for the Zr-O-Al bond appears at 2374 cm -1 at the water addition rate of 0.26 mL/min.The band decrease in the Al 2 O 3 -ZrO 2 mixed oxides obtained

Figure 7 .
Figure 7. Micrograph by SEM (20,000x magnification) of the Al 2 O 3 -ZrO 2 support (calcined at 700˚C) obtained with ethanol as solvent at a water addition rate of a) 0.06 mL/min; b) 0.26 mL/min.