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J. Phys. Chem. C 2008, 112, 9486–9492
Role of Pentacoordinated Al3+ Ions in the High Temperature Phase Transformation of γ-Al2O3 Ja Hun Kwak, Jianzhi Hu, Adrienne Lukaski,† Do Heui Kim, Ja´nos Szanyi, and Charles H. F. Peden* Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 ReceiVed: March 26, 2008
In this work, the structural stability of γ-alumina (γ-Al2O3) was investigated by a combination of XRD and high-resolution solid-state 27Al MAS NMR at an ultrahigh magnetic field of 21.1 T. XRD measurements show that γ-Al2O3 undergoes a phase transition to θ-Al2O3 during calcination at 1000 °C for 10 h. The formation of the θ-Al2O3 phase is further confirmed by 27Al MAS NMR; additional 27Al peaks centered at 10.5 and ∼78 ppm were observed in samples calcined at this high temperature. Both the XRD and NMR results indicate that, after calcination at 1000 °C for 10 h, the ratio of the θ-Al2O3 phase to the total alumina in samples modified by either BaO or La2O3 is significantly reduced in comparison with γ-Al2O3. 27Al MAS NMR spectra revealed that the reduction in the extent of θ-Al2O3 formation was highly correlated with the reduction in the amount of pentacoordinated aluminum ions, measured after 500 °C calcination, in both BaOand La2O3-modified γ-Al2O3 samples. These results strongly suggest that the pentacoordinated aluminum ions, present exclusively on the surface of γ-Al2O3, play a critical role in the phase transformation of γ-Al2O3 to θ-Al2O3. The role of the modifiers, in our case BaO or La2O3, is to convert the pentacoordinated aluminum ions into octahedral ones, thereby improving the thermal stabilities of the samples. Oxide additives, however, seem to have little, if any beneficial effect on preventing reductions in specific surface areas that occurred during high-temperature (e1000 °C) calcination. Introduction γ-Alumina (γ-Al2O3) is an important catalytic material both as an active catalyst and as a support for a variety of catalytically active phases (metals and oxides). The widespread applications of such catalysts range from petroleum refining to automotive emission control.1 It is also well known that γ-Al2O3 is one of the metastable “transition” alumina structural polymorphs.2,3 The γ-Al2O3 phase transforms into the δ- and θ-Al2O3 polymorphs with increasing calcination temperature and finally forms R-Al2O3, the thermodynamically stable structure.4 On the basis of NMR and IR spectroscopic measurements, this work showed that at ∼950 °C pentahedral Al3+ cations disappeared and θ-Al2O3 formed. Thus, the authors suggested that at sufficiently high temperatures, the pentahedral aluminum ions became unstable because of their coordinative unsaturation, resulting in structural rearrangements that lead to the formation of θ-Al2O3. There have also been numerous studies dedicated to improving the thermal stability of γ-Al2O3 because its phase transformations are thought to directly affect both the surface area and the number of active sites, which are very important with respect to its practical application.4–7 For example, it has been shown that the addition of lanthanum oxide improves the thermal stability of γ-Al2O3 by inhibiting both sintering and phase transformations.6 Other oxides have also been reported to be effective for the stabilization of the γ-Al2O3 polymorph at high temperatures.8,9 Despite extensive studies on the role of modifiers to stabilize the γ-Al2O3 structure, the origin of the * Corresponding author. E-mail:
[email protected]. † Department of Chemical Engineering, University of Delaware.
instability and the role that the modifiers play in the stabilization process remain unclear. Solid-state 27Al-NMR is a powerful technique for investigating the structural transformations and surface structure of alumina.4,5,10–13 In particular, distinctive NMR peaks are obtained for the tetra-, penta-, and octahedral aluminum ions present in transition aluminas. 27Al is a quadrupolar nucleus with a spin quantum I ) 5/2 and thus is subject to quadrupolar line broadening. Because such line broadening is inversely proportional to the external magnetic field strength,14 the resolution in MAS 27Al spectra of γ-alumina obtained at low to medium magnetic fields is relatively poor, hindering clear separation of the peak corresponding to the pentacoordinated aluminum ions from those corresponding to the tetrahedral and octahedral aluminum. As such, the use of MAS NMR to study the relationship of the γ-alumina surface structure to its material and catalytic properties has been correspondingly limited. For example, Q’Dell et al.10 have recently reported the 27Al MAS NMR spectra of sol-gel prepared alumina recorded at a moderately high magnetic field of 14.1 T field as a function of annealing temperature, where a peak due to pentacoordinated alumina centered at about 35 ppm was clearly observed at annealing temperatures of 600 and 800 °C. Still, this feature was partially overlapped with the tails of the dominant tetrahedral and the octahedral peaks, and these authors did not attempt to correlate changes in this peak’s intensity with structural transformations in the sol-gel prepared alumina. The multiple-quantum magic-angle spinning (MQMAS) technique, developed in 1995,15,16 allows the use of standard MAS probes to obtain well-resolved two-dimensional NMR spectra that contain an isotropic dimension unperturbed by
10.1021/jp802631u CCC: $40.75 2008 American Chemical Society Published on Web 05/30/2008
Role of Pentacoordinated Al3+ Ions
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quadrupolar broadening. The application of this technique to clearly resolve and assign spectral features to Al in tetrahedral, pentahedral, and octahedral coordination has been nicely demonstrated.17–19 Despite its remarkably rapid acceptance as an important technique for structural studies of a broad range of solid materials, especially microporous solids, glasses, and novel metal oxides, the MQMAS experiment has aspects that still require further development. Notably, the sensitivity of the technique needs to be improved, particularly for nuclei with very large quadrupolar coupling constants. Equally important, to simplify the quantification of the technique, the excitation and detection of multiple-quantum coherence as a function of the magnitude of the quadrupolar coupling needs to be made more uniform. In contrast, MAS spectra obtained at ultrahigh fields and high spinning rates are intrinsically quantitative and very sensitive experiments. In these cases, the second-order line broadening is not eliminated but rather progressively narrowed at increasing magnetic fields. Recently, we have reported that pentacoordinated aluminum ions were located exclusively on the surface of γ-Al2O3 by using NMR spectroscopy at an ultrahigh magnetic field of 21.1 T.20 In these experiments, the pentacoordinated alumina peak is well separated and clearly resolved from the dominant tetrahedral and the octahedral peaks, thus providing a simple way to obtain quantitative 27Al MAS NMR data. In the same study, we also found evidence for the specific interaction between the surface pentacoordinated aluminum ions and BaO, brought about during the synthesis of a BaO/γ-Al2O3 catalyst material. In this contribution, we report on a relationship between pentacoordinated aluminum ions and the thermal stability of γ-Al2O3. In particular, the specific interaction of modifiers with these pentacoordinated Al3+ ions is shown to correlate with the observed enhancement of the thermal stability of γ-Al2O3. Experimental Section The γ-Al2O3 samples used in this work were purchased from Sasol and Condea. The 2 and 8 wt % BaO/γ-Al2O3 samples were prepared by the incipient wetness method,21 using an aqueous solution of Ba(NO3)2 (Aldrich) and a γ-Al2O3 support (200 m2/g, Condea). After impregnation, the samples were dried at 120 °C and then activated via calcination at 500 °C in 20% O2 in nitrogen for 2 h. The alumina samples were calcined in a muffle furnace at 500 and 800 °C each for 2 h, and 1000 °C for 10 h. All 27Al MAS NMR experiments were performed at room temperature on a Varian-Inova 900.52 MHz NMR spectrometer, operating at a magnetic field of 21.1 T. The corresponding 27Al Larmor frequency was 234.669 MHz. All of the spectra were acquired at a sample spinning rate of 18 kHz, using a homemade, 3.2-mm pencil-type MAS probe. A single pulse sequence with a pulse width of about 30° was used. Each spectrum was acquired using a total of 512 scans with a recycle delay time of 1 s and an acquisition time of 66 ms. All spectra were externally referenced (i.e., the 0 ppm position) to an 1 M Al(NO3)3 aqueous solution. The general strategy we used in plotting the 27Al MAS spectra was to scale them to the same total NMR peak area for ease of comparison. XRD analysis was carried out on a Philips PW3040/00 XP`ert powder X-ray diffractometer using Cu KR1 radiation (λ ) 1.5406 Å) in step mode between 2θ values of 15 and 75°, with a step size of 0.02°/s. Data analysis was accomplished using JADE (Materials Data, Inc., Livermore, CA) as well as the Powder Diffraction File database (2003 Release, International Center for Diffraction Data, Newtown Square, PA).
Figure 1. XRD patterns (a), and solid-state 27Al MAS NMR spectra (b) of γ-Al2O3 samples calcined at (i) 500 °C for 2 h, (ii) 800 °C for 2 h, and (iii) 1000 °C for 10 h (inserted are ×5).
The specific surface areas of the alumina samples were determined by the BET method using an automated adsorption instrument (Micromeritics, TriStar-3000). Prior to N2 adsorption, all alumina samples were flushed with UHP He at 150 °C for 3 h. Results and Discussion Figure 1a shows the XRD patterns collected after calcination of a γ-Al2O3 sample at different temperatures. No noticeable structural changes are observed after extended calcination at either 500 °C (i) or 800 °C (ii). However, several new peaks at 2θ values of 31.8, 32.9, 51.1, and 60.3°, assigned to θ-Al2O3,22 are observed in the XRD pattern of the alumina calcined at 1000 °C for 10 h. These results are consistent with previous reports discussing the evolution of alumina structures as a function of calcination temperature.22 Figure 1b shows the corresponding high-resolution solid-state 27Al MAS NMR spectra. Figure 1b(i), obtained from the 500 °C
9488 J. Phys. Chem. C, Vol. 112, No. 25, 2008 calcined γ-Al2O3 sample, is identical to our result published previously.20 The spectrum consists of three features at 13.8, 38, and 72.8 ppm, due to Al3+ cations in octa-, penta-, and tetrahedral coordination, respectively (the weak feature at ∼90 ppm is a spinning sideband). Similar to the results obtained from XRD, no significant changes are observed in the spectra of the 500 °C (i) and 800 °C (ii) calcined samples. However, in the spectrum of the γ-Al2O3 sample calcined at 1000 °C for 10 h (iii), two additional peaks, present as shoulders of the main peaks, can be observed. The shoulder centered at 77.2 ppm is strongly overlapped with the tetrahedral Al3+ peak of the original γ-Al2O3 (centered at 72.8 ppm). The other new peak is centered at about 11.8 ppm and is overlapped with the octahedral aluminum peak of the γ-Al2O3 starting material (centered at 13.8 ppm). In a previous report by O’Dell, et al.,10 27Al MAS NMR peaks with chemical shifts at ∼80 and ∼10.5 ppm were assigned to tetra- and octahedral aluminum in the θ-Al2O3 structure, respectively. Therefore, we assign the peaks at 77.2 and 11.8 ppm in Figure 1b(iii) to tetrahedral and octahedral aluminum ions in θ-Al2O3, produced via a phase change during calcination at 1000 °C for 10 h. In general, the linewidths of the γ-Al2O3 features obtained at a 21.1 T field, defined as the full width at the half-peak height positions, are about 8.4 ppm for the octahedral peak centered at about 13.8 ppm and about 12 ppm for the tetrahedral peak centered at about 72.8 ppm for the spectra given in Figure 1b(i). O’Dell, et al.10 have reported that the isotropic value of the corresponding tetrahedral θ-Al2O3 peak, obtained from a rigorous simulation of the NMR spectrum from a sol-gel prepared alumina sample annealed at 1200 °C using two magnetic fields of 8.45 and 14.1 T, was 80 ( 1 ppm. The fact that the center of the peak (77.2 ppm) for the tetrahedral θ-Al2O3 in Figure 1b(iii) in this study is so close to the reported isotropic chemical shift value reported by O’Dell et al. suggests that the line broadening due to the second-order quadrupolar interaction is nearly quenched at the 21.1T field used in the experiments reported here. As a result, nearly isotropic chemical shift spectra are obtained at this ultrahigh magnetic field. This conclusion is further supported by the essentially symmetric lineshapes for both the tetrahedral and the octahedral peaks given in Figure 2b. Therefore, performing 27Al experiments at ultrahigh magnetic field offers a significant advantage in that the appearance and disappearance of peaks can be directly related to material structural changes. In our previous study,20 pentacoordinated Al3+ ions, that gave rise to a small NMR feature near 38 ppm, were found to be located exclusively on the γ-Al2O3 surface. BaO deposited onto γ-Al2O3 was found to selectively occupy these pentacoordinated Al3+ sites. Furthermore, we have reported23,24 that the crystalline structure of the γ-Al2O3 support did not change even after calcination at 1000 °C for 10 h in the presence of 8 wt % BaO. Taken together, these results suggest the intriguing possibility that the presence or absence of pentacoordinated aluminum sites is related to the thermal stability of γ-Al2O3. This possibility will be explored in the following paragraphs. Figure 2a shows the XRD patterns of BaO-loaded [0 (i), 2 (ii), and 8 wt % (iii)] γ-Al2O3 samples calcined at 1000 °C for 10 h. As we have shown above, a significant fraction of the pure (BaO-free) γ-Al2O3 transformed to θ-Al2O3 during this calcination procedure [Figure 2a(i)]. In contrast, in the diffraction pattern from the 1000 °C-calcined 2 wt % BaO-loaded γ-Al2O3 sample [Figure 2a(ii)], the amount of θ-Al2O3 formed is significantly reduced, which is evident from the considerable reduction of diffraction peak intensities at 2θ values of 31.8, 32.9, 51.1, and 60.3°, characteristic of the θ-Al2O3 phase. No
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Figure 2. XRD patterns (a) after being calcined at 1000 °C for 10 h, and the corresponding solid-state 27Al MAS NMR spectra (b) after being calcined at 500 °C for 2 h: (i) γ-Al2O3, (ii) 2 wt % BaO/Al2O3, and (iii) 8 wt % BaO/Al2O3.
diffraction peaks related to θ-Al2O3 are observed in the XRD pattern of the high-temperature calcined 8 wt % BaO-loaded alumina sample [Figure 2a(iii)]. These XRD results clearly indicate that the phase transformation of γ-Al2O3 into θ-Al2O3 is gradually suppressed as BaO is loaded onto γ-Al2O3 with increasing amounts. Figure 2b shows the solid-state 27Al MAS NMR spectra of 0, 2, and 8 wt % BaO-loaded γ-Al2O3 samples after calcination at 500 °C for 2 h under a dry air atmosphere. The spectra are blown up to highlight changes to the pentacoordinate Al3+ NMR feature near 38 ppm. It has been established in our recent report20 that the ratio of the number of pentacoordinate Al3+ ions to the total number of Al3+ ions (estimated from the integrated intensities of the NMR peaks) decreased with increasing BaO loading. At a BaO loading of approximately 4 wt %, the number of BaO molecules was found to be identical with the number of pentacoordinate aluminum ions (Alp) formed during calcination of the γ-Al2O3 sample at 500 °C for 2 h.20 Consequently, in the 2 wt % BaO-loaded γ-Al2O3 sample, approximately half
Role of Pentacoordinated Al3+ Ions of the Alp sites are occupied by BaO; the other half-remains coordinatively unsaturated. Alternatively, in the 8 wt % BaOloaded catalyst all of the Alp sites should be occupied and some excess BaO should also be present. In support of our previous study, Figure 2b demonstrates the changes to the population of pentacoordinate Al3+ surface sites with BaO loading. As noted above, the results presented in Figure 2a and b suggest a correlation between the number of the pentacoordinated aluminum sites (Figure 2b) present after 500 °C calcination and the extent of the phase transformation from γ- to θ-Al2O3 during calcination at 1000 °C for 10 h (Figure 2a). All of the results we have discussed so far lead us to conclude that (a) pentacoordinated aluminum ions, present exclusively on the surface of γ-Al2O3, are responsible for the structural instability of this metastable polymorph at high temperature; that is, the transformation from γ- to θ-Al2O3 occurs only in the presence of Alp sites and (b) the structural stability of the γ-Al2O3 phase can be significantly enhanced by eliminating these pentacoordinated aluminum ions through coordinative saturation (converting the pentacoordinated Al3+ ions into octahedral ones by metal oxide doping). To further validate these conclusions, we also investigated the thermal stability of a commercial lanthanum oxide-doped γ-Al2O3 material. Lanthanum oxide is a commonly used dopant to improve the thermal stability of the γ-Al2O3 phase (e.g., in three-way catalysts for automotive applications). Figure 3a summarizes the XRD patterns of a commercially available La2O3-modified alumina (Sasol, 3% La2O3-modified γ-Al2O3 (La2O3/γ-Al2O3)) that was subjected to calcination at 500 °C for 2 h (i), 800 °C for 2 h (ii), and 1000 °C for 10 h (iii). Comparing these results to those presented in Figure 2a, it is evident that no diffraction peaks corresponding to the θ-Al2O3 phase are observed after either 500 or 800 °C calcination. Furthermore, the intensities of the diffraction peaks corresponding to the θ-Al2O3 phase (31.8, 32.9, 51.1, and 60.3°) in Figure 3a(iii) for the sample calcined at 1000 °C for 10 h are much lower than those in Figure 1a(iii) for a nondoped, pure γ-Al2O3. These results indicate, as expected, that the thermal stability of lanthanum oxide-doped γ-Al2O3 is significantly increased relative to the pure γ-Al2O3 material. To substantiate the role of pentacoordinate Al3+ ions in the γ- to θ-Al2O3 phase transformation, we turned again to 27Al MAS NMR. Figure 3b shows the solid-state 27Al MAS NMR spectra of lanthanum oxide-doped γ-Al2O3 calcined at 500 °C for 2 h (i), 800 °C for 2 h (ii), and 1000 °C for 10 h (iii) (the corresponding XRD patterns are shown in Figure 3a). The 27Al MAS NMR spectra given in Figure 3b(i and ii) are essentially identical; that is, no features corresponding to Al3+ ions in the θ-Al2O3 phase (i.e., peaks at 10.5 and ∼78 ppm) are observed. In comparison with the spectrum given in Figure 1b(iii) for the γ-Al2O3 sample calcined at 1000 °C for 10 h, the intensities of the peaks corresponding to the tetra- and octahedrally coordinated Al3+ ions in θ-Al2O3 (11.8 and 77.2 ppm, respectively) in the sample of lanthanum oxide-doped γ-Al2O3 calcined under the same conditions [Figure 3b(iii)] are significantly reduced (by more than 50%). The reduction in the amount of θ-Al2O3 phase formed during the calcination at 1000 °C demonstrates again that lanthana doping significantly improves the thermal stability of the γ-Al2O3 support, consistent with the results obtained from the XRD measurements (Figure 3a). Furthermore, note the relatively small (compared to pure γ-Al2O3) NMR peak due to pentacoordinate Al3+ in the spectra from the lanthanadoped sample. The smaller quantity of these species is consistent
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Figure 3. XRD patterns (a), and solid-state 27Al MAS NMR spectra (b) of La2O3/Al2O3 after calcination at (i) 500 °C for 2 h, (ii) 800 °C for 2 h, and (iii) 1000 °C for 10 h (inserted are ×5).
with the proposal that their presence enhances and/or precipitates alumina phase changes. So far we have focused on the role of pentacoordinated Al3+ ions in the γ- to θ-Al2O3 phase transition during calcination up to 1000 °C. The most intriguing result of our study is that Alp ions, present exclusively on the surface of γ-Al2O3, seem to be involved in the phase transformation that affects the bulk structure of the material. We have also demonstrated that oxide additives (e.g., BaO and La2O3) can effectively prevent the γto θ-Al2O3 phase transformation at T e 1000 °C by coordinatively saturating the Alp ions. Beside the conversion of γ-Al2O3 to the other polymorphs under the calcination conditions we have discussed so far in this study, there is another important process that can take place on this catalyst-support material, namely, significant reduction of specific surface area that can dramatically influence the overall catalytic activities of aluminasupported catalysts. In many instances, decreases in specific surface area of γ-Al2O3 during high-temperature calcination have
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TABLE 1: BET Surface Areas for Alumina Samples as a Function of Calcination Temperature surface area(m2/g) sample γ-Al2O3 8 wt % BaO/Al2O3 γ-Al2O3-150 La2O3/Al2O3
500 °C × 2 h 800 °C × 2 h 1000 °C × 10 h 177 158 126 118
154 143 128 121
102 106 92 86
been related to the above-discussed phase transformations. Therefore, as a logical next step, we set out to investigate a possible relationship between the presence of Alp ions and the decrease in specific surface area of γ-Al2O3 during hightemperature calcination (e1000 °C). In addition, we were interested to determine whether oxide additives (BaO and La2O3) can prevent the surface area decrease, analogously to the suppression of the γ- to θ-Al2O3 phase transformation. A related fundamental question is whether the γ- to θ-phase transformation, clearly observed under the conditions of this study for pure γ-Al2O3, was responsible for decreases in surface area for this material. To this end, we examined pure and oxide-doped (BaO and La2O3) γ-Al2O3 samples with different specific surface areas using XRD and 27Al MAS NMR techniques. First, we compared the thermal stabilities of a pure, 150 m2/g specific surface area γ-Al2O3 (γ-Al2O3-150) and a lanthanadoped γ-Al2O3 that has a very similar specific surface area [after calcination at 500 °C for 2 h, the specific surface areas of these two materials, as determined by the BET method, were 126 and 118 m2/g, respectively (Table 1)]. The XRD data collected from these samples after calcination at 1000 °C for 10 h are shown in Figure 4a for γ-Al2O3-150 (i) and La2O3/γ-Al2O3 (ii). In the XRD pattern of γ-Al2O3-150, the intensities of the diffraction peaks corresponding to the θ-Al2O3 phase (31.8, 32.9, 51.1, and 60.3°) are intense, indicating that extensive γ- to θ-Al2O3 phase transformation has occurred. In addition, new peaks at 2θ values of 25.7, 35.1, 43.4, and 57.6° with considerable intensities are observed, and can be assigned to the R-Al2O3 phase (JCPDS no. 010-0173). In contrast, in the XRD pattern of La2O3/γAl2O3, features due to R-Al2O3 are absent and the peak intensities corresponding to θ-alumina are much lower, indicating a significantly lower extent of phase transformation in comparison with the γ-Al2O3-150 sample. Figure 4b displays the solid-state 27Al MAS NMR spectra of γ-Al2O3-150 (i) and La2O3/γ-Al2O3 (ii) samples that were calcined under identical conditions (at 500 °C for 2 h). Comparison of these two spectra reveals that the number of pentacoordinated aluminum ions in the La2O3/γ-Al2O3 sample is significantly lower than that in γ-Al2O3-150.14 Using the integrated peak areas to estimate the amount of Alp ions reveals that the relative amount of pentacoordinated aluminum ions in La2O3/γ-Al2O3 is only about 50% of that in γ-Al2O3-150. This result re-enforces our conclusion that La2O3 preferentially binds to the γ-Al2O3 surface at the pentacoordinated Al3+ sites and, consequently, inhibits the phase transformation of γ-Al2O3 to θ-Al2O3 during calcination up to 1000 °C. The results of both the XRD and NMR studies have again shown the beneficial effect of lanthana addition to γ-Al2O3 to preserve the γ-phase during calcination up to 1000 °C. However, no improvement can be seen in the retention of high specific surface area during high-temperature (e1000 °C) calcination by lanthana doping. As we have already mentioned, the starting materials (γ-Al2O3-150 and La2O3/γ-Al2O3) exhibited similar specific surface areas after calcination at 500 °C for 2 h. The results reported in Table 1 reveal no change in the specific
Figure 4. XRD patterns (a) after being calcined at 1000 °C for 10 h, and solid-state 27Al MAS NMR spectra (b) after being calcined at 500 °C for 2 h of γ-Al2O3-150 (i), and La2O3/Al2O3 (ii) samples.
surface area as the calcination temperature was increased form 500 to 800 °C (128 m2/g for γ-Al2O3-150, and 121 m2/g for La2O3/γ-Al2O3). However, large drops in specific surface areas were observed for both samples as the calcination temperature was raised to 1000 °C. The most striking observation is that the surface area of the γ-Al2O3-150 sample decreased to exactly the same extent as that of the lanthana-stabilized γ-Al2O3. The beneficial effect of La2O3 addition seen for the stabilization of the γ-alumina phase is not present for the preservation of the high specific surface area at least not up to 1000 °C, the maximum calcination temperature investigated in this study. We do note, however, that lanthana doping has been shown to stabilize alumina surface areas at higher (1200 °C) calcination temperatures.25 We have already discussed the phase stabilization effect of BaO in a sample where 8 wt % BaO was added to a γ-Al2O3 that, in the absence of BaO, exhibited a specific surface area of 177 m2/g after calcination at 500 °C for 2 h. The specific surface area of the corresponding 8 wt % BaO-doped γ-Al2O3 was 158 m2/g. Increasing the calcination temperature to 800 °C resulted
Role of Pentacoordinated Al3+ Ions
J. Phys. Chem. C, Vol. 112, No. 25, 2008 9491 obtained from a γ-Al2O3 sample calcined at 500 °C for 2 h (θ-Al2O3-free γ-Al2O3 standard). Spectra ii, iii, and iv correspond to γ-Al2O3, La2O3/Al2O3, and 8 wt % BaO/Al2O3 calcined at 1000 °C for 10 h, respectively. (Spectra i and ii are taken from Figure 1b and are shown again here for comparison purposes.) In order to highlight the differences observed for these materials, we show the NMR peak representing tetrahedral Al3+ ions and located between 50 and 100 ppm in Figure 5b. The spectrum of γ-Al2O3 that was calcined at 1000 °C for 10 h (ii) clearly shows that a significant amount of γ-Al2O3 was converted into θ-Al2O3 as is evidenced by the clearly identifiable feature at 77.2 ppm that appears at the expense of the signal of Al3+ ions in γ-Al2O3 (72 ppm). In comparison, the intensity of the 77.2 ppm peak in the spectrum of the La2O3/γ-Al2O3 sample calcined at 1000 °C for 10 h (iii) is much smaller, though still clearly visible, indicating a significantly reduced extent of γto θ-phase transformation. The line shape of the peak representing tetrahedrally coordinated Al3+ ions in the 8 wt % BaOmodified alumina, calcined at 1000 °C for 10 h (iv), is almost identical to that of γ-Al2O3 heated at 500 °C for 2 h, indicating that the extent of the γ- to θ-phase transformation was very limited or nonexistent. The degree of this phase transformation can again be directly compared to the relative amounts of pentacoordinate Al3+ in these samples. Thus, the NMR results obtained from samples calcined at 1000 °C for 10 h are consistent with the XRD results, and confirm that the thermal stabilities of the alumina materials can be dramatically enhanced with surface modifications; that is, by coordinative saturation of the Alp ions. Conclusions
Figure 5. Solid-state 27Al MAS NMR spectra in the chemical shift ranges of -30 to 120 ppm (a), and 50 to 100 ppm (b) for (i) γ-Al2O3 calcined at 500 °C for 2 h, (ii) γ-Al2O3 calcined at 1000 °C for 10 h, (iii) La2O3/Al2O3 calcined at 1000 °C for 10 h, and (iv) 8 wt % BaO/ Al2O3 calcined at 1000 °C for 10 h.
in very similar decreases in the specific surface areas of both of these materials, regardless of the presence or absence of BaO. Further calcination at 1000 °C for 10 h significantly reduced the specific surface areas of both of these materials (from 154 m2/g to 102 m2/g for γ-Al2O3, and from 143 m2/g to 106 m2/g for BaO/γ-Al2O3). Most importantly, the specific surface areas of both materials after the 1000 °C calcination were practically identical, whereas there were significant differences in their phase stabilities. These results suggest that oxide additives (that are very effective in preventing the γ- to θ-Al2O3 phase transformation) are less able to influence changes in the specific surface area of γ-Al2O3 support materials at temperatures e1000 °C. Finally, in Figure 5, we summarize the solid-state 27Al MAS NMR results for γ-Al2O3, La2O3/γ-Al2O3, and 8 wt % BaO/γAl2O3 samples calcined at 1000 °C for 10 h. Spectrum i was
In this work, we addressed the role of pentacoordinated Al3+ ions in the thermal phase transformation of γ-Al2O3 to θ-Al2O3 by using a combination of XRD and high-resolution solid-state 27Al MAS NMR at an ultrahigh magnetic field of 21.1 T. The results of the XRD measurements show that γ-Al2O3 undergoes a phase transition to θ-Al2O3 during calcination at 1000 °C for 10 h. The formation of the θ-Al2O3 phase was confirmed by 27Al MAS NMR spectroscopy, which showed the appearances of new peaks centered at ∼11.8 and ∼77.2 ppm in the samples calcined at high temperatures. The results of both the XRD and NMR studies consistently showed that the addition of BaO or La2O3 to γ-Al2O3 significantly reduced the extent of the γ- to θ-phase transformation during calcination at 1000 °C for 10 h. 27Al MAS NMR spectra revealed that the significantly enhanced thermal stabilities of both BaO- and La2O3-modified alumina samples were directly related to the amount of pentacoordinated aluminum ions present in these samples after calcination at 500 °C for 2 h. These results strongly suggest that the presence of pentacoordinated aluminum is directly related to the thermal instability of γ-Al2O3 at high temperatures and, therefore, likely responsible for initiating the phase transformation of γ- to θ-Al2O3. Upon coordinative saturation of these pentacoordinated Al3+ sites (e.g., by the addition of BaO or La2O3), the thermal stability of γ-Al2O3 support was dramatically improved. Contrary to the beneficial effect of these oxide additives on the retention of the γ-Al2O3 phase at high temperatures, these additives were not found to prevent significant reduction in the specific surface areas in these materials, at least for calcinations up to 1000 °C. Therefore, we conclude that the reduction in specific surface area of these alumina supports is not directly correlated with the γ- to θ-phase transformation at temperature up to 1000 °C.
9492 J. Phys. Chem. C, Vol. 112, No. 25, 2008 Acknowledgment. This work was supported by U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences. The research was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE Office of Biological and Environmental Research, and located at the Pacific Northwest National Laboratory. PNNL is operated for DOE by Battelle Memorial Institute under Contract No. DE-AC0676RLO-1830. A.L. thanks the U.S. DOE for financial support under grant FG02-84ER13290. We acknowledge Sasol North America, Inc. for providing some of the alumina samples. References and Notes (1) Taylor, K. C. Catal. ReV. Sci. Eng. 1993, 35, 457. (2) Levin, I.; Brandon, D. J. Am. Ceram. Soc. 1998, 81, 1995. (3) Pinto, H. P.; Nieminen, R. M.; Elliott, S. D. Phys. ReV. B 2004, 70, 125402. (4) Pecharroman, C.; Sobrados, I.; Iglesias, J. E.; Gonzalez-Carreno, T.; Sanz, J. J. Phys. Chem. B 1999, 103, 6160. (5) Das, R. N.; Hattori, A.; Okada, K. Appl. Catal., A 2001, 207, 95. (6) Ozawa, M.; Nishio, Y. J. Alloys Compd. 2004, 374, 397. (7) Machida, M.; Eguchi, K.; Arai, H. J. Catal. 1987, 103, 385. (8) Balint, I.; You, Z.; Aika, K.-i. Phys. Chem. Chem. Phys. 2002, 2002, 2501. (9) Castro, R. H. R.; Ushakov, S. V.; Genermbre, L.; Gouvea, D.; Novrotsky, A. Chem. Mater. 2006, 18, 1867.
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