Using a Surface-Sensitive Chemical Probe and a Bulk Structure

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Using a Surface-Sensitive Chemical Probe and a Bulk Structure Technique to Monitor the γ- to θ-Al2O3 Phase Transformation Ja Hun Kwak,* Charles H. F. Peden, and Janos Szanyi Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352, United States ABSTRACT: In this work, we investigated the phase transformation of γ-Al2O3 to θ-Al2O3 by ethanol TPD and XRD. Ethanol TPD showed remarkable sensitivity toward the surface structures of the aluminas studied. Maximum desorption rates for the primary product of ethanol adsorption, ethylene, were observed at 225, 245, and 320 °C over γ-, θ-, and R-Al2O3, respectively. Ethanol TPD over a γ-Al2O3 sample calcined at 800 °C clearly shows that the surface of the resulting material possesses θ-alumina characteristics, even though only the γ-alumina phase was detected by XRD. These results strongly suggest that the γ-to-θ phase transformation of alumina initiates at oxide particle surfaces. The results obtained are also consistent with our previous finding that the presence of penta-coordinated Al3þ sites, formed on the (100) facets of the alumina surface, is strongly correlated with the thermal stability of γ-alumina.

’ INTRODUCTION The γ phase of aluminum oxide (γ-Al2O3) is one of the most important materials in heterogeneous catalysis. It has been widely used both as an active catalyst and as a support for a variety of catalytically active phases (metals and oxides).1 The widespread applications of such catalysts range from petroleum refining to automotive emission control. It is well known that γ-Al2O3 is one of the metastable “transition” alumina structural polymorphs.2,3 Upon calcination, the γ-Al2O3 phase goes through a series of transformations, first, into the δ- and θ-Al2O3 polymorphs and finally to the R-Al2O3 phase, which is the thermodynamically stable structure.4 A key consequence of these phase transformations is the dramatic reduction in surface area that seriously affects catalytic properties. For this reason, considerable efforts have been dedicated to understanding both the mechanisms of the phase transformations and the development of methods to prevent (delay) these processes.47 It is well known that the surface decoration of γ-Al2O3 by lanthanum oxide or barium oxide improves its thermal stability and inhibits both sintering and phase transformations (the onset of both processes shift to higher temperature).7 Addition of other oxides has also been reported to be effective for the stabilization of the γ-Al2O3 polymorph at high temperatures.8,9 Recently, we have reported that penta-coordinated aluminum ions (Alp3þ), formed on (100) facets of the γ-Al2O3 surface by dehydration and/or dehydroxylation at elevated temperatures, play a critical role in the phase transformation of γ-Al2O3 to θ-Al2O3.10,11 Notably, we have also shown that, when the coordinative unsaturation of these Alp3þ sites is eliminated by the oxide additives (i.e., La2O3 or BaO), the thermal stability of the γ-Al2O3 phase improved significantly. These results seem to suggest that the γ-to-θ phase transition of Al2O3 is initiated by surface processes occurring on specific facets of the oxide surface. The catalytic properties (activity, selectivity, distribution of supported catalytic materials) of oxides (e.g., alumina) are intimately linked to their surface properties, because these chemical r 2011 American Chemical Society

processes take place primarily on the surfaces of catalysts.12 Therefore, the chemical and physical characterization of Al2O3 surfaces is crucially important for the correlation of catalytic properties with surface geometric and electronic structure.13,14 However, the characterization of high-surface-area transition alumina surfaces by well-established analytical techniques is not straightforward, due to the intrinsic properties of these phases, such as low crystallinity, small particle size, etc. The characterization of the surface of this oxide is especially difficult, because it can go through phase transformations even under catalytically relevant conditions (e.g., moderately high temperatures). X-ray diffraction is the most commonly used experimental structure probe for determining the phases of crystalline materials. This bulk technique, however, is ineffective for the detection of molecularly thin surface layers that might possess structural properties (geometric, electronic, coordination) significantly different from those of the bulk. However, chemical probes (i.e., surface reactions) provide important information for inferring the nature of changes in the near-surface structural properties of solid materials. The information gained from such studies can also be used to directly correlate catalytic properties with surface structure. Recently, we have reported that temperatureprogrammed desorption (TPD) of ethanol is a very sensitive method to follow changes in the γ-Al2O3 surface during thermal dehydration/dehydroxylation.15 In this paper, on the basis of the results of ethanol TPD and XRD, we provide direct evidence that the γ-to-θ phase transformation of Al2O3 starts on the surface of γ-Al2O3. In the first stage of this phase transformation, a coreshell structure is formed, with the shell (near-surface region) showing characteristics of θ-Al2O3, whereas the core is still γ-Al2O3. The results obtained can now clearly explain how surface modification by oxide Received: April 15, 2011 Revised: May 20, 2011 Published: May 23, 2011 12575

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additives leads to significantly improved thermal stability of the γ-Al2O3 phase.

’ EXPERIMENTAL SECTION The γ-Al2O3 samples used in this work were obtained from Condea (surface area = 200 m2 g1) and Sasol (surface area = 150 m2 g1), and R-Al2O3 was obtained from Alpha-Aesar. θ-Al2O3 was prepared from γ-Al2O3 (Condea SBA-200) by calcination in a muffle furnace at 1000 °C for 10 h. Specific surface areas of the θ-Al2O3 and R-Al2O3 samples, as determined by BET analysis, were 102 and 5 m2/g, respectively. Ethanol TPD experiments were performed using the same protocol we have described in a previous report.15 Prior to ethanol TPD experiments, 0.05 g of γ-alumina was calcined at a specific temperature (at 200 or 500 °C) for 2 h under a He flow (1.0 mL/s) (note that every TPD run was carried out on a freshly calcined alumina sample). After calcination, the sample was cooled to room temperature, and ethanol adsorption was carried out for 30 min using a 2.0% ethanol/He gas mixture (1.0 mL/s), followed by a He purge for 1 h in order to remove most of the weakly bound ethanol molecules. After stabilization of the flame ionization detector (FID) signal of a Hewlett-Packard 5890 gas chromatograph (GC), a TPD experiment was carried out in flowing He (1.0 mL/s) with a heating rate of 10 K/min, with the reactor outlet flowing directly to the FID (i.e., no GC column separation). Desorption amounts were determined from FID signal intensities, with the FID sensitivity factor calibrated using 100 μL pulses of 2.0% ethanol in He. In this experimental configuration, only the total amount of desorbed hydrocarbons is measured. For the identification of specific desorption products in the TPD spectra, full gas chromatography (GC) analysis was performed at the maximum temperatures of the desorption peaks. The only product detected during the high-temperature desorption (T > 200 °C) under the experimental conditions of this study was ethylene. Note that, in the following text when we discuss the high-temperature ethanol desorption features in the TPD experiments, we will be referring to ethylene evolution that is produced from adsorbed ethanol. XRD analysis was carried out on a Philips PW3040/00 X’Pert powder X-ray diffractometer using the Cu KR radiation (λ = 1.5406 Å) in step mode between 2θ values of 10 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). The specific surface areas of the alumina samples were determined by the BET method using an automated adsorption instrument (Micromeritics, TriStar3000). Prior to N2 adsorption, the alumina samples were flushed with UHP He at 150 °C for 3 h. ’ RESULT AND DISCUSSION Figure 1 shows the XRD patterns obtained for the γ-, θ-, and R-Al2O3 standard samples studied here. The γ- and R-Al2O3 samples were commercial powders, and their diffraction patterns are consistent with the prior literature for γ-Al2O316 and R-Al2O3 (JCPDS no. 010-0173). The XRD of the alumina sample that was prepared from γ-Al2O3 by calcination at 1000 °C for 10 h included diffraction peaks at 2θ values of 31.8, 32.9, 51.1, and 60.3°, consistent with the θ-Al2O3 phase, as reported previously.16 Thus, these measurements provided baseline XRD patterns for the three phase-pure alumina polymorphs discussed in this study.

Figure 1. XRD patterns of γ- (black), θ- (red), and R-Al2O3 (blue) standard samples.

Because of the nature of the method used (XRD), only direct information is obtained about the bulk structures of the aluminas, but nothing about the surface structure of the different samples. Therefore, we used ethanol as a probe molecule to gain information about the chemical properties of the alumina samples present in different crystalline forms, and calcined at different temperatures. From the ethanol temperature-programmed desorption (TPD) data, we next show that the near-surface structure of these various alumina samples can be inferred. We first performed two series of ethanol TPD experiments over γ-, θ-, and R-Al2O3 samples that were precalcined in situ at 200 and 500 °C, and the results obtained are displayed in Figure 2. Each TPD profile shows three main desorption peaks with maximum desorption rates at ∼70 and ∼90 °C and above 200 °C, similar to our previously reported results for ethanol adsorption on γ-Al2O3.15 The position of the two low-temperature desorption peaks are very similar, regardless of the alumina phase present or calcination temperature, and are assigned to weakly bound (molecularly adsorbed) ethanol. These features are incidental to the conclusions of this paper, so they will not be discussed further.15 The high-temperature desorption peak observed for each sample varied significantly with both the alumina phase studied and the calcination temperature. For example, ethanol TPD spectra collected after 500 °C calcination (solid curves in Figure 2) showed ethylene desorption maxima at 225, 245, and 320 °C for γ-, θ-, and R-Al2O3, respectively. On the other hand, after calcination at 200 °C, the high-temperature desorption features over these samples were observed at 250 (γ), 270 (θ), and 320 °C (R). These results clearly demonstrate that the surface chemistry of alumina is dependent upon both the crystalline phase of the bulk and the extent of surface hydroxylation. We first summarize the results obtained for the γ-Al2O3 surface that we discussed in detail in a recent publication.15 In this prior work, we showed that the differences in surface properties for γ-Al2O3 as a result of calcining at 500 °C versus 200 °C could be attributed to dehydration/dehydroxlation of solely (100) facets. In particular, it has been shown by us15 and others13 that calcination of γ-Al2O3 at 200 °C is insufficient to 12576

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Figure 2. Ethanol TPD profiles for γ- (black), θ- (red), and R-Al2O3 (blue) after activation at 200 °C (dotted lines) and 500 °C (solid lines).

remove any hydroxyl groups from this material’s surface, whereas 500 °C calcination results only in removal of hydroxyls from the (100) facets, and they are nearly completely removed by this higher-temperature thermal treatment. As a consequence, we assigned the lower (225 °C) temperature ethylene desorption (Figure 2) feature to arise from ethanol decomposition at undercoordinated 5-fold Alp3þ sites on these (100) facets.15 Similarly, we concluded that the higher (250 °C) temperature ethylene desorption peak for the 200 °C calcined surface arises from ethanol decomposition at Brønsted acidic hydroxyls bound to these 5-fold Alp3þ sites. Critical evidence for these conclusions was that the quantity of ethylene desorbing from both 200 and 500 °C calcined surfaces was essentially identical and also equal (within experimental error) to the maximum possible number of 5-fold Alp3þ sites. The peak temperature for the ethylene desorption maxima was also sensitive to calcination temperature for the θ-Al2O3 surface, shifting from 270 to 245 °C for the 200 °C- and 500 °Ccalcined surfaces, respectively (Figure 2). In addition, the quantities of desorbing ethylene from these two surfaces were approximately equal, but lower than ethylene desorption quantities from the γ-Al2O3 surface, as expected due to the lower surface area of θ-Al2O3. There are very few published studies of the surface structure and chemistry of θ-Al2O3, which makes it more difficult to rationalize the TPD results for this surface. However, on the basis of the very similar ethylene peak temperature shifts to those observed for γ-Al2O3, we propose that these results can also be understood as arising from ethanol decomposition at either hydroxylated or under-coordinated 5-fold Alp3þ sites for 200 °C- and 500 °C-calcined surfaces, respectively. Interestingly, the ethylene desorption temperature is insensitive to calcination temperature for the R-Al2O3 surface. In support of the above conclusions, however, dehydroxylation of this surface is not expected to occur to any extent during 500 °C calcination. Thus, to summarize the results in Figure 2, we propose that dehydroxylation of 5-fold Alp3þ sites on γ- and θ-Al2O3 surfaces leads to a shift to lower temperatures in the peak position for ethylene desorption.

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Most important for the discussion here to note again that the ethylene desorption temperature occurs at a specific temperature as a function of both the phase of alumina and the precalcination temperature. These results indicate that ethanol adsorption/ desorption may be used to identify the phase present on the surface of alumina particles for samples that are treated under identical conditions (here, calcined at either 200 or 500 °C), therefore, providing information about the formation of surface phases that can possibly differ from bulk phases determined by techniques such as XRD. So far, we have shown that the pure θ-Al2O3 phase can be prepared from γ-Al2O3 by extended calcinations at 1000 °C, and the phase transformation can be conveniently confirmed by both the bulk technique XRD (Figure 1) and the surface-sensitive chemical probe of ethanol TPD (Figure 2). The next question we sought an answer to was how the γ-to-θ phase transformation was initiated. More specifically, is this a process that starts in the bulk phase, or on surface defects? In fact, the results of our prior studies11 seem to suggest that the γ-to-θ phase transformation initiates on specific under-coordinated defects sites located on the (100) facets of γ-Al2O3. These defect sites (pentacoordinate Alp3þ ions, as revealed by 27Al solid-state MAS NMR10) are created by annealing γ-Al2O3 above 300 °C, resulting in the removal of hydroxyl groups specifically from the (100) facets.13,15 When these “defect” sites are eliminated by oxide promoters (La2O3 or BaO), the phase transformation is delayed, that is, shifted to much higher temperature with respect to that observed for undoped γ-Al2O3.11 To investigate the early stages of the γ-to-θ phase transformation, we prepared a sample by calcining the commercial 200 m2 g1 γ-Al2O3 powders at 800 °C, a temperature higher than that required for the complete dehydroxylation of the (100) facets, but lower than that needed for the complete conversion of γ-Al2O3 to the θ phase. We then applied both XRD and ethanol TPD to characterize the bulk and the surface properties, respectively, of the thus prepared material and compared them to those obtained from the pure γ and θ phases. XRD patterns collected from two commercial γ-Al2O3 samples (with different BET surface areas of approximately 150 and 200 m2 g1), the above-discussed θ-Al2O3 obtained after extended 1000 °C calcination of the 200 m2 g1 γ-Al2O3 powder, and the 800 °Ccalcined γ-Al2O3 are displayed in Figure 3. Comparison of these patterns reveals no apparent change in the γ-Al2O3 phase during the 800 °C calcinations; the XRD pattern recorded from this sample was identical to those obtained from the two commercial γ-Al2O3 samples. This technique, however, is not sensitive to the possible changes on the surfaces of the γ-Al2O3 crystallites, and it only proves that there were no new bulk phases formed during the 800 °C calcination. The ethanol TPD profile collected from the γ-Al2O3 sample calcined at 800 °C for 2 h is displayed in Figure 4, together with those from the two other γ-Al2O3 samples, and from the phasepure θ-Al2O3 discussed above. All samples were again calcined at 500 °C prior to ethanol adsorption to ensure dehydroxlation of (100) facets for the γ-Al2O3 samples. The two commercial γ-Al2O3 used in these experiments possessed different BET surface areas; the original γ-Al2O3 powders that were used in the calcinations at 800 and 1000 °C had a specific surface area of 200 m2 g1. However, the 2 h calcination at 800 °C led to a BET surface area of 150 m2 g1. Thus, studying both the commercial 200 and 150 m2 g1 γ-Al2O3 samples allowed us to quantitatively compare the amount of ethylene desorbed (high-temperature 12577

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Figure 3. XRD patterns of γ-Al2O3 (200 m2/g, black), γ-Al2O3(150 m2/g, green), θ-Al2O3 (blue), and initially 200 m2/g γ-Al2O3 after calcination at 800 °C for 2 h (red).

Figure 4. Ethanol TPD profiles for γ-Al2O3 (200 m2/g, black), θ-Al2O3 (blue), initially 200 m2/g γ-Al2O3 after calcination at 600 °C (brown) and 800 °C (red) for 2 h, and γ-Al2O3 (150 m2/g, green).

desorption feature in the TPD profiles) from the 800 °C-calcined sample to a γ-Al2O3 sample with a similar surface area. The identical desorption temperatures (225 °C) observed for the two γ-Al2O3 samples once more demonstrates the sensitivity of the chemical probe used (i.e., ethanol TPD) for the characterization of phases present on the surfaces of the alumina. The ratio of the integrated intensity under the high-temperature desorption features is consistent with the surface area ratio of the two γ-Al2O3 samples studied here. (An additional ethanol TPD profile collected from a γ-Al2O3 sample after calcination at an intermediate temperature of 600 °C for 2 h is also shown in Figure 4. The temperature of the maximum desorption rate and the

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integrated intensity of the feature are identical to those measured on the sample calcined at 500 °C. These results are consistent with our understanding of this system: the surface structure of the γ-Al2O3 samples calcined at 500 and 600 °C is practically identical, and the γ-to-θ phase transformation begins at higher temperature (∼800 °C)). The high-temperature desorption feature from the γ-Al2O3 sample calcined at 800 °C appeared at 250 °C, the temperature identical to that observed for ethanol desorption from the phasepure θ-Al2O3 sample (see Figure 4). Furthermore, the amount of ethylene desorbed from the 800 °C-annealed sample was identical, within the error of the measurements, to that measured from the 150 m2 g1 γ-Al2O3 sample. These results strongly suggest that the surfaces of the γ-Al2O3 crystallites have already gone through the γ-to-θ phase transformation during the 800 °C calcination process, although, in the XRD pattern, no traces of the θ-Al2O3 phase could be observed. Because the XRD pattern collected from the 800 °C-calcined sample is identical to that of γ-Al2O3, we can conclude that the γ-to-θ phase transformation of aluminum oxide begins on the surfaces of the crystallites, which now display chemical properties identical to those of the θ phase, even though the dominant (bulk) phase is still γ-Al2O3. This conclusion is consistent with our previous finding that oxide additives, bound to the surface of γ-Al2O3, retard/delay the γ-toθ phase transformation of alumina. Notably, our prior highresolution 27Al solid-state MAS NMR study11 clearly established that the oxide additives (La2O3 and BaO) were exclusively bound to under-coordinated Al3þ sites (penta-coordinated Al3þ) that formed on the (100) facets of the γ-Al2O3 crystallites by dehydroxylation. Thus, the model we can propose for the initial stage of the γ-to-θ phase transformation, based on our current and previously published results, is a coreshell structure; that is, the bulk of the alumina crystallites is phase-pure γ-Al2O3, whereas the surfaces of the particles have structures of the θ-Al2O3 phase. The important evidence for this model is again that, although the bulk of the 800 °C-calcined crystallites is still in the γ phase, the chemical reactivity perfectly matches that of the θ phase. In view of the seemingly clear evidence for a surface-initiated phase transformation for γ-Al2O3, it is useful to consider how this compares to phase transformations observed for other catalytically important oxide materials. A particularly important process is the anatase-to-rutile phase transformation for TiO2. In a combined UV/visible Raman, XRD, and high-resolution TEM study, Li and co-workers have reported that this TiO2 phase transformation begins at the interfaces between small anatase particles that are present in larger aggragates.17 As the phase transformation was propagated from the interfaces of the small TiO2 particles in the agglomerates, the inner portions of the agglomerates sintered and transformed into the rutile phase. However, anatase crystallites that were at the outer portions of the agglomerates, that is, they had few, if any, interfaces with other anatase crystallites in the agglomerate, remained in the anatase phase. This resulted in the formation of large particles with a coreshell structure. As in the present study, the surface chemistry of the “shell” could be used to verify this structure.17b However, for the case of TiO2, the core had the more stable rutile phase, whereas the shell remained in the anatase structure. Interestingly, and similar to known behavior for γ-Al2O3, the addition of an oxide promoter (La2O3) to the anatase TiO2 prevented/delayed the anatase-to-rutile phase transformation.17 The oxide additive was proposed to bind to the defect sites on the 12578

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The Journal of Physical Chemistry C anatase crystallites formed by an elevated temperature calcination, similar to the case we presented here for γ-Al2O3. However, in the anatase TiO2 case, it was claimed that the phase transformation could only take place when anatase particles with defects created on their surfaces interacted with another anatase crystallite (with or without defects), but not on the surfaces of individual anatase crystallites. Therefore, the addition of La2O3 to the anatase TiO2 played two roles in preventing the anatase-to-rutile phase transformation: they bound to surface defect sites and also prevented intimate contact between anatase particles where the phase transformation was thought to be initiated. The thus stabilized La2O3/TiO2 system retained its anatase structures up to ∼900 °C. The phase transition of zirconia (ZrO2), from the tetragonal to the monoclinic structure, has also been studied by UV/visible Raman and XRD techniques.18 Very similar to the phase transformation of γ-Al2O3 presented in this study, the tetragonal-to-monoclinic phase transition of ZrO2 was initiated at the surface of the tetragonal crystallites and proceeded toward the bulk of the crystallites at higher temperatures and/or longer calcinations times. The transitional coreshell structures observed were, thus, very similar to those we propose for the alumina particles: the shell represents the new phase being formed (monoclinic), whereas the core has the original phase structure (tetragonal).

’ CONCLUSIONS In this work, we investigated the phase transformation of γ-Al2O3 to θ-Al2O3 using a surface structure-sensitive chemical probe (ethanol TPD) and a bulk technique (XRD). Ethanol TPD proved to be a very sensitive probe for monitoring the changes in the structure of the alumina surface. The maximum rates of high-temperature ethylene desorption (the decomposition product of adsorbed ethanol) were observed at 225, 245, and 320 °C over γ-, θ-, and R-Al2O3, respectively. The ethanol TPD profile obtained from the γ-Al2O3 sample calcined at 800 °C showed characteristics of the phase-pure θ-Al2O3 sample, even though XRD only evidenced the presence of the γ-Al2O3 phase. These results strongly suggest that the γ-to-θ phase transformation of Al2O3 initiates at the surfaces of the alumina crystallites, underscoring our previous finding11 that penta-coordinated Al3þ sites, formed on the (100) facets of the γ-Al2O3 surface, strongly correlate with the thermal stability of γ-Al2O3. A coreshell structure, with a θ-Al2O3 shell and γ-Al2O3 core, is proposed to form during the γ-to-θ phase transformation process.

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the U.S. DOE by Battelle Memorial Institute under contract number DE-AC05-76RL01830.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We gratefully acknowledge the U.S. Department of Energy (DOE), Basic Energy Sciences, Division of Chemical Sciences, and the DOE’s Vehicle Technologies Program for the support of this work. The research described in this paper was performed in the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for 12579

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