ARTICLE pubs.acs.org/IECR
Aluminum Anodization in Oxalic Acid: Controlling the Texture of Al2O3/Al Monoliths for Catalytic Aplications Oihane Sanz,*,† F. Javier Echave,† Jose Antonio Odriozola,‡ and Mario Montes† †
Department of Applied Chemistry, University of the Basque Country (UPV/EHU), Manuel de Lardizabal 3, ES-20018 San Sebastian, Spain ‡ Inorganic Chemistry Department and Material Sciences Institute, Universidad de Sevilla-, CSIC, Seville, Spain ABSTRACT: The anodization and postanodization processes of aluminum in order to prepare monoliths for catalytic applications have been studied in this work using oxalic acid as electrolyte. The effect of anodization variables (anodization time, current density, temperature, and electrolyte concentration) and postanodization processes on the surface morphology and textural properties of AAO (anodic aluminum oxide) films is analyzed. The anodization variables affect the two main processes taking part in the Al2O3 layer formation: alumina generation and its dissolution that are controlled by temperature, electrolyte concentration and time. The proper combination of both processes, as a result of the anodization variables choice, produces adherent alumina layers with tailored porosity and surface morphology that show excellent properties to be used as catalyst structured support. Larger pore sizes and the complete absence of sulfur that may poison reduced metal-supported active phases are main differences with the classical, most often used, sulfuric acid anodization process.
1. INTRODUCTION Structured catalysts and reactors became one of the most relevant and economically significant applications of catalytic reactor engineering and industrial catalysis so far, mostly due to the commercial success of well-known environmental catalytic processes.1 For practical applications, the catalyst should be supported on a structured support to treat large gas flows with low-pressure drop. Structured supports are stiff 3D-structures with channels or macropores that exhibit high lateral surface where a thin layer of catalyst is applied. Monoliths of parallel channels,1 foams,2 membrane catalyst,3 catalytic fibers and cloths4,5 are within the several structured supports reported. More recently, a considerable interest in microstructured reactor has shown up. These devices are often built of multiple parallel channels with diameters ranging between 10 and several hundred micrometers where chemical transformations occur.6-8 The use of micrometer-scale reaction spaces allows the precise control of diffusion, heat exchange, retention time, and flow patterns in the chemical reaction.9 Both ceramic and metallic structured supports have been used. However, the main drawback of metallic structured supports is the difficulty in obtaining adherent catalytic layers.1 Anodic aluminum oxide films (AAO) grown on aluminum foils provide excellent adherence between the catalytic oxide support and the metallic structure.7,10-12 AAO has been used to disperse noble metals2,13 and manganese oxide,14 or using the singular morphology of the cracked alumina surface to fix catalytic coatings on aluminum.15-17 Nevertheless, on using H2SO4 as electrolyte some sulfur remains in the alumina even after calcination.10 Such sulfur may act as a poison;18,19 hence, the use of sulfur-free electrolytes is of importance for catalytic purposes. In this work, we study the anodization of aluminum foils in oxalic acid, a sulfur-free electrolyte, with the focus on the Al2O3 r 2011 American Chemical Society
anodic layer properties. We study the influence of electrolyte concentration, temperature, current density, and process time with the objective of producing sulfur-free AAO layers but with textural properties similar to those obtained when using H2SO4 as electrolyte. The effect of the postanodization process, dissolution process after anodization, was studied as well, in order to obtain monoliths with a wide range of textural properties.
2. EXPERIMENTAL SECTION The aluminum foils (120 μm thick) used for the anodization were obtained from INASA (Industria Navarra del Aluminio S.A.), and their composition is given in Table 1. Prior to use, the aluminum foils were cleaned with detergent and water and rinsed with water. Acetone was then used for removing organic impurities, and finally the foils were dried. Monoliths were prepared by rolling up around spindle alternate flat and corrugated foils previously treated (anodization and postanodization). The final monolith is a cylinder of 3 cm height, 1.6 cm diameter, and a cell density of 55 cell/cm2 (Figure 1). Anodization. Anodization was carried out in an anodization polypropylene tank using oxalic acid as electrolyte. A cooling PTFE coil connected to an external chiller and an electrical heater connected to a PID temperature controller provide the temperature control; this system resulted in temperature oscillations below 0.1 °C. An Agilent HP 6692A power supply that operates between 0 and 60 V and 0-110 A, allowing current or voltage control, was used. All along this work, the current control option was used. Vigorous air bubbling ensures agitation inside the bath. After anodization, foils were removed from the electrolytic bath, Received: October 19, 2010 Accepted: December 23, 2010 Revised: November 30, 2010 Published: January 14, 2011 2117
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thoroughly washed with water to eliminate the acid, dried at 60 °C for 1 h, and calcined at 500 °C for 2 h. In order to study the influence of the variables affecting the anodization process and therefore the final properties of the generated alumina on the aluminum foils using oxalic acid, the following ranges for the process variables were chosen: • Anodization time (t): 10-100 min • Current density (I): 1-3 A 3 dm-2 • Electrolyte concentration (C): 0.4-1.6 M • Electrolyte temperature (T): 30-50 °C Postanodization. To increase the pore size maintaining constant the pore density, a postanodization process was carried out. The anodized aluminum foil (40 °C, 40 min, 2 A/dm2, and 1.6 M) was kept in the electrolytic bath at 40 °C from 10 to 80 min without electric current. After this treatment, foils were removed from Table 1. Weight Percentage of the Composition of the Aluminum Foil metal Fe %
Si
Mg
Mn
Cu
Cr
Pb
Ti
Zn
Al
0.34 0.1 0.002 0.005 0.002 0.0009 0.001 0.011 0.005 balance
Figure 1. Monoliths.
the bath, thoroughly washed with water to eliminate the acid, dried at 60 °C for 1 h, and further calcined at 500 °C for 2 h. Characterization. The amount of alumina generated during anodization was determined by a gravimetric method. The anodized foil was submitted for 10 min to a solution containing 35 mL of phosphoric acid (PROBUS 85%) and 20 g of chromic acid (PANREAC) in 1 L of distilled water kept at 80-100 °C. This treatment resulted in the selective dissolution of the AAO film. Thus, the weight difference between the anodized foils before and after the alumina dissolution determines the amount of alumina formed. Conventional SEM images (HITACHI S-2700) were obtained from gold-sputtered samples (2 min, BioRad SC 500 Sputter Coater). High-resolution SEM micrographs were recorded in a FE-SEM (HITACHI S-4800) at low acceleration potentials avoiding, this way, the gold coating procedure. The textural properties of the AAO films were measured by N2 adsorption-desorption isotherms at liquid nitrogen temperature with a homemade cell that allows the analysis of the whole monolith (Micromeritics ASAP 2020). Specific surface area, pore size, and pore size distribution (PSD) were calculated using the BET equation20 and the BJH method.21 Before analysis, the monoliths were degassed for 4 h at 120 °C in vacuum. Surface pore density and diameter were also calculated through the analysis of FE-SEM micrographs and the results compared with those obtained by N2 adsorption-desorption measurements. Top view FE-SEM images of the AAO films taken at different conditions were used to measure pore diameters utilizing the Leica Application Suit V3. PSDs were obtained by measuring over 100 pores from each micrograph (see Figure 2). The textural properties of the generated AAO films were calculated on the basis of the amount of alumina generated per square meter of aluminum foil (MAlumina, g Al2O3/m2 Al). This value is calculated from the AAO film dissolution in phosphoricchromic acid solution. The specific surface area and pore volume of the anodic alumina film (SAlumina, m2/g Al2O3 and Vp-Alumina, cm3/g Al2O3) are then the ratio of the measured surface area of the monolith to the amount of alumina and of the pore volume to the amount of alumina, respectively. The reported pore diameter (DpMax, nm) was associated to the diameter at which the pore size distribution obtained from the BJH calculation reaches a maximum. The thickness of the alumina layer (L, μm) was
Figure 2. Pore density and pore diameter measurement by SEM: FESEM micrograph of anodized aluminum at 40 °C, 1.6 M H2C2O4, 2 A/dm2, and 40 min (A), pore size distribution obtained by SEM (B) and N2 adsorption data and SEM data comparison (C). 2118
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estimated ((20%) averaging the thicknesses measured in several SEM micrographs. The surface pore density, number of pores per square meter of aluminum foil, was calculated from the pore volume, the thickness of the alumina layer, and the mean pore diameter, assuming constant cross-section cylindrical pores. Assuming a 2D dense packing of pores, the cell size (DCell, nm) is calculated from the surface pore density. As stated above, the
Figure 3. Potential variation during aluminum anodization at constant current density.
surface pore density and the pore diameter were also calculated by analyzing several FE-SEM images.
3. RESULTS During the anodization process, at constant current density, a variation of the voltage was observed as shown in Figure 3. Initially the potential increases reaching a maximum and then slowly decreases until a constant value was achieved. This trend observed in all the experiments reflects the different oxide formation steps occurring in the anodization process. When the current starts to pass through the sample, a barrier layer of nonporous oxide begins to form (I). As this layer grows, the potential increases due to the increment of resistance to the oxide layer. Due to this potential increment, small fractures appear on the top of the oxide layer (II). At one moment, the fractures increase until they become pores by oxide dissolution in the alumina layer, decreasing the resistance, which produces a voltage decrease when working at constant current density (III). Finally a constant voltage is observed due to the constant formation-dissolution of the oxide layer at the bottom of the pores (IV).10 Table 2 shows the textural properties of the anodic alumina layer formed on aluminum foils as a function of the anodization conditions. The excellent agreement between pore size and surface pore density calculated by direct observation (FE-SEM images) and by the BJH method from N2 adsorption-desorption isotherms is remarkable.
Table 2. Properties of the AAO Films As a Function of Anodization Conditions anodization conditions temper-
N2 adsorption
oxalic
current
MAlumina
SAlumina
Vp-Alumina
pore
acid
density
(gAl2O3/
(m2/
(cm3/
density
DpMax
DCell
gAl2O3)
gAl2O3)
(Np/m2Al)
(nm)
(nm) (μm) (Np/m2Al)
4
0.042
3.8 1014
31
86
17
3.8 1014
30
smooth
14
42
78
25
4.6 1014
36
smooth
ature
time
(°C)
(min) (mol/L) (A/dm2)
30
40
FE-SEM
0.4
2
m2Al) 115.8
pore L
density
DpMax
surface
(nm)
morphology
40
40
0.4
2
149.2
6
0.047
4.7 10
50
40
0.4
2
70.0
23
0.153
15.0 1014
54
43
16
-
30 40
40 40
0.8 0.8
2 2
109.4 141.0
5 8
0.047 0.068
5.2 1014 6.6 1014
27 36
74 65
16 25
5.2 1014 6.7 1014
50
40
0.8
2
61.4
29
0.187
15.6 1014
55
43
15
-
-
cracked
30
40
1.2
2
108.6
5
0.049
6.2 1014
27
68
17
6.2 1014
26
smooth
40
40
1.2
2
104.0
11
0.090
7.6 1014
36
61
20
7.5 1014
36
smooth
50
40
1.2
2
50.8
34
0.217
17.5 1014
46
40
13
-
30
40
1.6
2
110.6
7
0.056
7.3 1014
25
62
17
40
40
1.6
2
92.2
16
0.096
9.7 1014
37
54
50 40
40 40
1.6 1.6
2 1
47.2 44.6
36 17
0.219 0.104
15.2 1014 16.4 1014
43 28
43 42
40
40
1.6
2
92.2
16
0.096
9.7 1014
37
40
40
1.6
3
147.0
8
0.078
5.1 1014
43
50
40
1.6
1
16.8
52
0.324
33.8 1014
50
40
1.6
2
47.2
36
0.219
50
40
1.6
3
76.2
24
40
10
1.6
2
33.2
40 40
20 30
1.6 1.6
2 2
40
40
1.6
2
-
small cracks
25 37
smooth smooth
-
cracked
6.4 1014
26
smooth
18
9.3 1014
36
smooth
12 9
12.0 1014
28
highly cracked smooth
54
18
9.0 1014
35
smooth
74
27
4.8 1014
40
smooth
27
29
5
-
-
small cracks
15.2 1014
37
43
12
-
-
cracked
0.169
13.9 1014
43
45
18
-
-
highly cracked
6
0.038
8.9 1014
16
56
5
9.1 1014
11
smooth
56.4 74.2
8 11
0.055 0.078
9.0 1014 8.9 1014
23 28
56 56
10 14
8.9 1014 8.9 1014
26 27
smooth smooth
92.2
16
0.096
9.7 1014
37
54
18
9.3 1014
36
smooth
14
37
54
23
9.6 1014
39
smooth
40
50
1.6
2
117.8
16
0.101
9.7 10
40
60
1.6
2
132.2
17
0.103
9.4 1014
37
55
26
9.5 1014
40
smooth
40
80
1.6
2
147.6
17
0.105
9.3 1014
38
55
29
9.4 1014
42
smooth
40
100
1.6
2
170.0
19
0.121
11.0 1014
48
51
35
-
-
cracked
2119
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Figure 4. Variation of pore diameter distribution with the electrolyte temperature at 0.4 M H2C2O4, 40 min, and 2 A/dm2 (N2 adsorption data).
3.1. Anodization. First we studied the influence of the anodization time keeping constant the rest of variables (T = 40 °C, C = 1.6 M, and I = 2 A/dm2). By increasing the anodization time the amount of generated alumina per square meter increases. Similarly, an increase in surface area, pore volume, and pore size is observed. However, the surface pore density and consequently the cell size hardly changes with time. For studying the influence of electrolyte concentration and temperature on the AAO properties, the anodization time was fixed in 40 min and the current density in 2 A/dm2. The amount of alumina generated follows a trend with temperature that depends on the electrolyte concentration. Thus, for the highest electrolyte concentration tested (1.2 and 1.6 M) the amount of generated alumina continuously decreases on increasing the electrolyte temperature, however for the lowest concentration tested (0.4 and 0.8 M) the amount of generated alumina shows a maximum when the electrolyte temperature is fixed at 40 °C. On the other hand, the specific surface area, pore volume, and surface pore density continuously increase with temperature whatever the electrolyte concentration tested. The increase in porosity causes the increase in surface area as pointed out by the observed linear correlation between these two magnitudes. Surface pore density and pore volume increase with temperature, being the change particularly important when increasing temperature from 40 to 50 °C. Pore diameters and cell sizes show opposite behavior. Therefore, pore diameter increases while cell size decreases on increasing temperature. It is worth noting that at 50 °C, pore diameter is bigger than cell size. Figure 4 presents the PSD (40 min, 0.4 M, 2 A/dm2) as a function of temperature clearly showing that on increasing temperature, the pore distribution moves toward high pore diameters and that the area under the PSD, the pore volume, increases. The increase in the electrolyte concentration results in a decrease in the amount of generated alumina as well as in surface area and pore volume, while the thickness of the alumina layer decreased, even considering the low sensitivity of this magnitude. In this case, the increase in the specific surface area is related to an increase in the pore density since pore size slightly decreases on increasing the electrolyte concentration. However, in this case, the increase in surface pore density is less pronounced than in the case of the bath temperature. The PSD curves obtained at 40 °C, as a function of the oxalic acid concentration, are shown in
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Figure 5. Variation of the pore diameter distribution with the oxalic acid concentration at 40 °C, 40 min, and 2 A/dm2 (N2 adsorption data).
Figure 6. Variation of the pore diameter distribution with the current density at 40 °C, 40 min, 1.6 M H2C2O4, and 2 A/dm2 (N2 adsorption data).
Figure 5. The pore size distribution curves move toward the left, smaller pore sizes, on increasing the electrolyte bath concentration, while the area underneath the curve hardly changes indicating similar pore volume per square meter of foil. Therefore, the decrease in the amount of generated alumina on increasing the electrolyte bath concentration is compensated by the increase in pore volume per gram of alumina. Finally, by modifying the current density between 1 and 3 A/ dm2, while keeping constant the electrolyte bath temperature at 40 °C, the anodization time and the electrolyte concentration (40 min and 1.6 M), a linear increase in the amount of alumina generated is observed on increasing the current density and, in consequence, a parallel increase in the oxide layer thickness. On the contrary, the specific surface area of the obtained alumina decreases on increasing the current density. The pore diameter, Figure 6, and the cell size increase on increasing the current density, while the surface pore density decreases steeply accounting for the reduction in surface area. This must be associated with the reduction in porosity. The same behavior is observed when the electrolyte bath temperature is set at 50 °C, but the decrease 2120
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Figure 7. SEM micrographs of the alumina layer obtained (40 min, 2 A/dm2) at different temperature and electrolyte concentrations.
in surface area and pore volume is more pronounced. However, pore size remains almost unchanged on increasing the current density, and the increase in cell size is less pronounced on increasing the current density setting the electrolyte bath at 50 °C than by setting it at 40 °C. The microstructures resulting from the different anodization treatments were studied by SEM. Figure 7 presents top view images of the AAO film surface as a function of the anodization temperature and electrolyte concentration. For all the studied conditions two images are shown, a conventional SEM image (x6000) and a high-resolution FESEM image (x130000), accounting for a general view of the alumina surface morphology and a detailed view of the structure of the pore mouth. In general the top view of the AAO films is flat and smooth showing a regular array of cylindrical pores. However, for electrolyte concentrations above 0.8 M and a bath temperature of 50 °C the smooth surface disappears resulting in a superposition of grooves that mask the cylindrical pore structure. In these cases, the whole surface is cracked, covered by irregular grooves with crests in between. A FESEM image of the AAO film cross-section (Figure 8) shows the fibrous structure of crests and valleys. Bundles of alumina fibers form the valleys that pack together in the outermost part of the AAO films resulting in interconnected pores, Figure 9. The current density also affects the surface roughness, Table 2, but only for the highest temperature tested. At 40 °C the cracking phenomenon is not observed, but at 50 °C cracks appear and grow in size as the current density increased. Table 2 shows the influence of anodization time on surface morphology. It can be seen that at these conditions (40 °C, 1.6 M, 2 A/dm2), the surface of the produced alumina coating remained smooth for anodization time up to 80 min and only longer anodization times, 100 min, allow deep cracks appearing. 3.2. Postanodization. In order to widen the alumina pore size while keeping constant the surface pore density a postano-
Figure 8. FESEM micrographs of the lateral view of the AAO layer obtained at 50 °C, 40 min, 2 A/dm2, and 1.6 M oxalic acid.
dization process was carried out. After the anodization process (40 °C, 40 min, 2 A/dm2, and 1.6 M) the samples were kept in the electrolytic bath for 10 to 80 min at the anodization temperature. Table 3 shows the textural properties of the postanodized foils. On increasing the anodization time the amount of alumina and its thickness decreases. However, during the first 20 min of treatment the anodized film remains almost unchanged. For postanodization times above 20 min the alumina dissolution is important as evidenced by the decrease in amount and thickness of the anodic layer. Nevertheless, the alumina surface area increased with the postanodization time as a result of the porosity increment of the remaining alumina layer. The pore diameter increased with the immersion time from 37 to 67 nm. It 2121
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should be also pointed out that for postanodization periods above 40 min the cell size is smaller than the pore diameter (see Table 3). FE-SEM images of the surface of postanodized samples confirm the increase in pore diameter for treatments up to 40 min and the further interconnection of these pores for postanodization times between 40 and 80 min resulting in a rough surface (Figure 10).
4. DISCUSSION The amount of generated alumina as well as their surface area increases with the anodization time (see Table 2). The increase
Figure 9. FESEM micrographs of the alumina layer obtained at 50 °C, 40 min, 2 A/dm2, and 1.6 M oxalic acid.
in the alumina layer thickness (from 5 to 35 μm) as the anodization time increases result in a local temperature increase caused by both the Ohmic resistance increment22 and resistance to the entrance of fresh electrolyte.10,23 This temperature increase favors the dissolution of alumina and increases the pore diameter (from 18 to 48 nm) and hence the specific surface area of the alumina. The pore density and therefore the cell size remained constant until 100 min, when the pore density increased slightly, becoming similar the pore and the cell size. The increase in the electrolyte temperature gives rise to the increase in the number of pores also increasing their diameter, which results in a porosity increase (Table 2). The temperature increase leads to an enhancement of the alumina dissolution rate, which results in a decrease of the Ohmic resistance and hence of the potential of the process. As previously reported, the potential of the process is directly proportional to the pore size and inversely proportional to the pore density.24-26 Therefore,
Figure 10. SEM micrographs of the alumina layer obtained (40 °C, 1.6 M oxalic acid, 2 A/dm2) at different immersion times.
Table 3. Properties of the AAO at Different Submerged Times (10-80 min) N2 adsorption post anodization conditions time submerged (min) 0
MAlumina
SAlumina
Vp-Alumina
(gAl2O3/m2Al) (m2/gAl2O3) (cm3/gAl2O3) 92.2
16
FE-SEM
pore number DpMax DCell (Np/m2Al)
L
(nm) (nm) (μm)
pore number DpMax (Np/m2Al)
(nm)
0.096
9.6 1014
37
54
18
9.3 1014
14
surface morphlogy
36
smooth
10
92.8
16
0.096
9.6 10
38
54
18
9.3 1014
38
smooth
20
90.2
17
0.108
9.5 1014
44
55
18
9.1 1014
42
smooth
40 60
66.2 37.6
24 43
0.163 0.140
9.2 1014 8.5 1014
45 60
55 55
15 8
9.2 1014 -
45 -
smooth cracked
80
16.2
78
0.232
10.0 1014
67
53
4
-
-
highly cracked
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Industrial & Engineering Chemistry Research increasing the electrolyte temperature the potential of the process falls increasing the number of the pores formed but the pore size becomes smaller. Nevertheless the ability of the electrolyte to dissolve the alumina increases with temperature leading to larger pores. This trend becomes evident on increasing the electrolyte temperature from 40 to 50 °C. At 50 °C the pore size is 10 nm larger and the pore density more than twice that found at 40 °C (Table 2). The cell size is inversely proportional to the pore density. Increasing the pore density, the lattice parameter of the hexagonal structure is smaller. Moreover, at 50 °C, the pore diameter is larger than the cell size. This result suggests that a new phenomenon appeared at high temperature that modifies the trend observed at lower temperature. The electrolyte, oxalic acid, concentration affects the alumina dissolution rate in such a way that as the higher the oxalic acid concentration the higher the alumina dissolution resulting in higher surface areas and porosity (Table 2).10,22,27,28 However, Burgos et al.10 reported that the increase in porosity when anodizing aluminum in sulfuric acid is the result of the increase in surface pore density even though the pore diameter is larger. The potential of the process falls as a consequence of the increased dissolution rate of the alumina film,10,29 and therefore the pore density increases while the pore size decreases. The alumina dissolution capacity and the falling in the potential of the process affect the cell size; the first tends to increase the pore size while the second tends to decrease the pore size. Therefore, at 30 and 40 °C the cell size decreases on increasing electrolyte concentration since the alumina dissolution capacity is not enough to compensate the pore size reduction due to the reduction in the process potential. However, at 50 °C the cell size does not vary with the electrolyte concentration and, surprisingly, is smaller than the pore size. The amount of generated alumina is higher at higher current densities as previously reported,10,30-33 although the surface pore density decreases.10,29,34 Patermarakis et al.35 observed that an increase in the current density leads to an increase in pore diameter since the current amount through each pore increases, this causes an increase of the produced heat, which enlarges the pore diameter by dissolving alumina walls.10,22 The potential of the process increases as the current density increases and therefore the pore density decreases29,33-35 resulting in a lower pore density and a bigger cell size that follows the same trend that pore size, although the cell size is bigger than pore size at 40 °C and similar to pore size at 50 °C. By maintaining the anodized aluminum on the anodizing electrolyte bath, the postanodization process takes place. The modification of the textural properties produced by this process depends on the rate of alumina dissolution in the oxalic acid solution (Table 3). The AAO film reduces its thickness from 18 to 4 μm between 20 and 80 min postanodization treatment, which stands for a dissolution rate of 0.23 μm/min. This smaller film thickness is accompanied by an increase in pore size diameter from 37 to 67 nm in 80 min, at a rate of 0.375 nm/min. These data show slower dissolution rates than those reported by Graham et al.36 for the rate of dissolution of the AAO film when treated in 2% phosphoric acid at 45 °C. These authors measured 9 nm/min as the mean rate of dissolution that is an order of magnitude higher than that measure in this work. As the surface pore density and consequently the cell size remained constant, the pore size enlargement led again to the paradox of an AAO film with pores larger than the corresponding cell for postanodization treatments above 80 min.
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Figure 11. Images of the evolution of the grow of the diameter of pore.
The observed discrepancies between pore and cell size must be related with the anodization and postanodization processes that influence the surface morphology of the AAO films. When these processes are pushed to extreme conditions, the alumina dissolution process becomes important and the AAO surface turns into a cracked surface. Enhanced alumina dissolution on increasing electrolyte concentration and/or temperature was previously observed by us when anodizing aluminum foils or foams in sulfuric acid;16,17 this dissolution produced a modification of the surface morphology of the alumina film similar to that observed in this study (Figure 7). The coherent growing of alumina pillars resulting in a periodical 2D structure of pores is interrupted giving rise to deep cracks and grooves all over the alumina surface for anodization process taking place at 50 °C and oxalic acid concentration above 0.8 M. Moreover, crack size increases on increasing electrolyte concentration. The increase in current density from 1 to 3 A/dm2 for the anodization process carried out at low electrolyte temperatures, 40 °C, and for short periods of time, 40 min, results in an increase in the amount of formed alumina as well as in the textural properties without the presence of cracks (Table 2). However, on increasing electrolyte temperature the textural properties and the amount of formed alumina continue to grow with the current density but surface cracking occurs for all the current densities tested being higher for higher current densities, Table 2.16,17 For mild anodization conditions (40 °C, 1.6 M, and 2 A/dm2) cracks are undetectable for anodization times below 80 min, Table 2; however, significant cracking appears in these conditions for the highest anodization time studied, 100 min. Postanodization processes may also change the surface morphology (Figure 10). Thus, while the AAO surface layer remains smooth for short postanodization time processes, up to 60 min, the surface appears heavily cracked for the highest postanodization time tested, above 80 min. Therefore, surface cracking is a phenomenon that appears at severe anodization conditions or after a severe postanodization treatment. The analysis of the textural data (Tables 2 and 3) and the SEM observation of the morphology of the AAO surface (Figures 7 and 10 and Table 2) let us conclude the cracking process appears when pore size approaches the size of the hexagonal cell that contains it, as a result of an enhanced alumina dissolution process. This dissolution process that is always present competes favorably with the alumina growing process on increasing time, electrolyte concentration and local temperature, this later depending on bath temperature and current density. Pores are formed during aluminum anodization, on the barrier layer that coats the metal surface, Figure 11.A. They grow 2123
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Industrial & Engineering Chemistry Research
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In previous works, monoliths anodized in oxalic and sulfuric acid were used to prepare MnOx-Al2O3/Al systems for volatile organic compounds combustion.15 The low activity of aluminum monoliths obtained in sulfuric acid was due to the presence of residual SO42- ions from the anodization process that poisons the catalyst, forming MnSO4. This problem was solved when the anodization process was carried out with oxalic acid. A structured catalyst prepared using H2C2O4 that shows a cracked surface morphology (50 °C, 40 min, 2 A/dm2, and 1.6 M) showed excellent adherence of manganese oxide (3-5%). The mesopores of anodic alumina provided channels and nanosized walledges for supplying Al3þ and highly active reaction sites.12 Zincaluminum hydrotalcite-like thin films were prepared by direct precipitation on the surface of Al2O3/Al monoliths obtained combining anodization and postanodization processes.
Figure 12. Image of lateral view of cracked morphology.
Table 4. Geometric Properties of the Anodized Monoliths Obtained cylinders (length diameter)
3 1.6 cm
geometric volume
6 cm3
geometrical surface
0.024 m2
cell number empty fraction
55 cells cm-2 76%
wall thickness
0.12 mm
Table 5. Anodization Conditions and Main Properties of the Prepared Monoliths anodization conditions 40-10- 50-40- 40-100- 40-40-1.6-2 (°C - min [M] - A/dm-2)
1.6-2
0.4-2
1.6-2
(P-A 80 min)a
amount of alumina (g)
0.398
0.84
2.04
0.194
surface area
2.55
19.2
38.9
15.2
pore volume
0.015
0.153
0.247
0.045
(cm3/monolith) pore diameter (nm)
16
54
48
67
alumina thickness
5
16
35
4
NO
NO
YES
YES
(m2/monolith)
(μm) cracked a
Postanodized at the standard conditions used in this work.
5. CONCLUSIONS Anodization of aluminum in oxalic acid has been performed as a function of electrolyte temperature and concentration for different anodization times and taking into account the effect of the postanodization treatments. The effect of these anodization variables on the two main processes taking place in the Al2O3 layer formation during anodization (alumina generation and dissolution) is reported, highlighting the role played by local temperature and electrolyte concentration. The proper combination of both anodizing and postanodizing processes, resulting of the choice of the different process variables, led to an adherent alumina layer with excellent properties as catalyst support, including high thermal stability, sulfur-free alumina layer preventing poisoning of the catalyst. When anodization and postanodization conditions are extreme (high temperature, high electrolyte concentration and/or long anodization and postanodization times), an important cracking of the surface appears with width and depth cracks that could be used to fix the catalytic coatings. This study allows selecting anodization conditions for preparing alumina/aluminum monoliths with a wide range of textural morphologic parameters. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. (Figure 11.B) until their diameter is higher than cell size when pore mouths overlap (Figure 11.C). This overlap interconnects the pores producing alumina fiber-like structures at the intersection point of three cells (Figure 11.C), these structures further coalesce forming fiber bunches (Figures 8 and 12). In order to show the versatility of this technique to prepare structured catalytic supports with tailored textural properties a series of microstructured monolithic reactors were prepared using different anodization conditions. The monoliths were prepared by rolling up together flat (16 3 cm) and corrugated (22 3 cm) anodized aluminum sheets.10 The geometrical characteristics and textural properties of the monoliths prepared this way are shown in Tables 4 and 5, respectively. The amount of alumina, surface area, and pore volume are reported for the complete 6 cm3 monolithic reactor. As can be seen, monoliths can be prepared having a wide range of textural properties: amount of alumina from 0.194 to 2.04 g; surface area from 2.55 to 38.9 m2; pore volume from 0.015 to 0.247; pore diameter from 16 to 67 nm; layer thickness (that means pore length) from 5 to 35 μm.
’ ACKNOWLEDGMENT Financial support by the MICINN (ENE 2009-14522-C05, MAT2006-12386-C05, and FPU fellowship to F.J.E.), the UPV/ EHU (GIU07/63 and the “Ayuda de especializacion de investigadores doctores en la UPV/EHU” to O.S.), and Gobierno Vasco (CTP08-02) are gratefully appreciated. ’ REFERENCES (1) Avila, P.; Montes, M.; Miro, E. E. Monolithic reactors for environmental applications: A review on preparation technologies. Chem. Eng. J. 2005, 109, 11. (2) Sanz, O.; Echave, F. J.; Sanchez, M.; Monzon, A.; Montes, M. Aluminium foams as structured supports for volatile organic compounds (VOCs) oxidation. Appl. Catal., A 2008, 340, 125. (3) Gryaznov, V. M.; Orekhova, N. V. In Structured Catalyst and Reactors; Cybuslki, A., Moulijn, J. A., Eds.; Marcel Dekker Inc.: New York, 1998. 2124
dx.doi.org/10.1021/ie102122x |Ind. Eng. Chem. Res. 2011, 50, 2117–2125
Industrial & Engineering Chemistry Research (4) Twig, M. V.; Webster, D. E. In Structured Catalyst and Reactors; Cybuslki, A., Moulijn, J. A., Eds.; Marcel Dekker Inc.: New York, 1998. (5) Matatov-Meytal, Y.; Sheintuch, M. Catalytic fibers and cloths. Appl. Catal., A 2002, 231, 1. (6) Kiwi-Minsker, L.; Renken, A. Microstructured reactors for catalytic reactions. Catal. Today 2005, 110, 2. (7) Karim, A. M.; Federici, J. A.; Vlachos, D. G. Portable power production from methanol in an integrated thermoeletric/microreactor system. J. Power Sources 2008, 179, 113. (8) Almeida, L. C.; Echave, F. J.; Sanz, O.; Centeno, M. A.; Arzamendi, G.; Gandía, L. M.; Sousa-Aguiar, E. F.; Odriozola, J. A.; Montes, M. Fischer-Tropsch synthesis in microchannels. Chem. Eng. J. 2010Accepted. (9) Gavriilidis, A.; Angeli, P.; Cao, E. Technology and Applications of Microengineered Reactors. Chem. Eng. Res. Des. 2002, 80, 3. (10) Burgos, N.; Paulis, M.; Montes, M. Preparation of Al2O3/Al monoliths by anodization of aluminium as structured catalytic supports. J. Mater. Chem. 2003, 13 (6), 1458. (11) Almeida, L. C.; Echave, F. J.; Sanz, O.; Centeno, M. A.; Odriozola, J. A.; Montes, M. Washcoating of metallic monoliths and microchannel reactors. Stud. Surf. Sci. Catal. 2010, 175, 25. (12) Echave, F. J.; Sanz, O.; Almeida, L. C.; Odriozola, J. A.; Montes, M. Highly porous hydrotalcite-like film growth on anodized aluminium monoliths. Stud. Surf. Sci. Catal. 2010, 175, 639. (13) Burgos, N.; Paulis, M.; Antxustegi, M. M.; Montes, M. Deep oxidation of VOC mixtures with platinum supported on Al2O3/Al monoliths. Appl. Catal., B 2002, 251. (14) Barrio, I.; Legorburu, I.; Montes, M.; Domínguez, M. I.; Centeno, M. A.; Odriozola, J. A. New redox deposition-precipitation method for preparation of supported manganese oxide catalyst. Catal. Lett. 2005, 101, 151. (15) Aguero, F. N.; Barbero, B P.; Sanz, O.; Echave, F. J.; Montes, M.; Cadus, L. E. Influence of the Support on MnOx Metallic Monoliths for the combustion of Volatile Organic Compounds. Ind. Eng. Chem. Res. 2010, 49, 1663. (16) Sanz, O.; Almeida, L. C.; Zamaro, J. M.; Ulla, M. A.; Miro, E. E.; Montes, M. Washcoating of Pt-ZSM5 onto aluminium foams. Appl. Catal., B 2008, 78, 166. (17) Sanz, O.; Martinez T, L. M.; Echave, F. J.; Dominguez, M. I.; Centeno, M. A.; Odriozola, J. A.; Montes, M. Aluminium anodization for Au-CeO2/Al2O3-Al monoliths preparation. Chem. Eng. J. 2009, 151, 324. (18) Aguinaga, A.; Montes, M.; Malet, P.; Capit, M. J.; Carrizosa, I.; Odriozola, J. A. Extended X-ray absorption fine structure study of sulphur poisoned Ni/SiO2 catalyst. Appl. Catal., A 1994, 110, 197. (19) Bartholomew, C. H.; Farrauto, R. J. Fundamentals of Industrial Catalytic Processes, 2nd ed.; J. Wiley & Sons, Inc.: Hoboken, NJ, USA, 2006; Chapter 5. (20) Brunauer, S.; Emmett, P. H.; Teller, E. Adsorption of Gases in Multimolecular Layers. J. Am. Chem. Soc. 1938, 60, 309. (21) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. Thedetermination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms. J. Am. Chem. Soc. 1951, 73, 373. (22) Fedotiev, N. P; Grilijes, S. P. Electropulido y Anodizacion de Metales; Gustavo Gili, S. A., Eds.; Barcelona, 1972. (23) Rach, E.; Heitbaum, J. XPS analysis of the oxide layer formed on Pt by anodization in sulfuric acid. Electrochim. Acta 1986, 31, 477. (24) Wood, G. C.; O’Sullivan, J. P. The anodizing of aluminium in sulphate solutions. Electrochim. Acta 1970, 15, 1865. (25) Setoh, S.; Miyata, A. Anodic film of aluminium. I and II. Sci. Pap. Inst. Phys. Chem. Res. (Jpn.) 1932, 17, 189. (26) Ebihara, K.; Takahashi, H.; Nagayama, M. Structure and density of anodic oxide films formed on aluminum in sulfuric acid solutions. J. Met. Finish. Jpn. 1982, 33, 156. (27) Lizarbe Ruiz, L. Teoría y Practica de la Lucha Contra la Corrosion; Gonzalez Fernandez, , J. A., Grafimad, S. A., Eds.; Madrid, 1984.
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(28) Nourbakhsh, N.; Tsotsis, T. T.; Webster, I. A. Model planar alumina catalyst preparation and aging under hydrotreating conditions. Appl. Catal. 1989, 50, 65. (29) Patermarakis, G.; Moussoutzanis, K. Electrochemical kinetic study on the growth of porous anodic oxide films on aluminium. Electrochim. Acta 1995, 40, 699. (30) Patermarakis, G.; Papandreadis, N. Study on the kinetics of growth of porous anodic Al2O3 films on Al metal. Electrochim. Acta 1993, 38, 2351. (31) Patermarakis, G.; Lenas, P.; Karavassilis, Ch.; Papayiannis, G. Kinetics of growth of porous anodic Al2O3 films on A1 metal. Electrochim. Acta 1991, 36, 709. (32) Patermarakis, G.; Kerassovitou, P. Study on the mechanism of oxide hydration and oxide pore closure during hydrothermal treatment of porous anodic Al2O3 films. Electrochim. Acta 1992, 37, 125. (33) Patermarakis, G.; Karayannis, H. S. The mechanism of growth of porous anodic Al2O3 films on aluminium at high film thicknesses. Electrochim. Acta 1995, 40, 2647. (34) Patermarakis, G.; Nicolopoulos, N. Catalysis over Porous Anodic Alumina Film Catalysts with Different Pore Surface Concentrations. J. Catal. 1999, 187, 311. (35) Patermarakis, G.; Tzouvelekis, D. Development of a strict kinetic model for the growth of porous anodic Al2O3 films on aluminium. Electrochim. Acta 1994, 39, 2419. (36) Graham, A.; Bowen, C. R.; Robbins, J.; Taylor, J. Formation of a porous alumina electrode as a low-cost CMOS neuronal interface. Sens. Actuators, B 2009, 38-1, 296.
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