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La-dopant Location in La-doped #-AlO Nanoparticles Synthesized Using a Novel One-pot Process Stacey J. Smith, Baiyu Huang, Calvin H. Bartholomew, Branton J Campbell, Juliana Boerio-Goates, and Brian F. Woodfield J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07256 • Publication Date (Web): 07 Oct 2015 Downloaded from http://pubs.acs.org on October 8, 2015

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The Journal of Physical Chemistry

La-Dopant Location In La-Doped γ-Al2O3 Nanoparticles Synthesized Using A Novel One-Pot Process Stacey J. Smith1*, Baiyu Huang,1 Calvin H. Bartholomew2, Branton J. Campbell3, Juliana BoerioGoates1, Brian F. Woodfield1 1

Brigham Young University, Chemistry & Biochemistry 2

Brigham Young University, Chemical Engineering

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Brigham Young University, Physics & Astronomy

*

[email protected] ABSTRACT. We have recently developed a ‘solvent-deficient’ method of synthesizing high surface

area γ-Al2O3 nanoparticles that show promise for catalyst support applications. Here, we investigate doping the alumina with La3+ to stabilize the gamma phase to higher temperatures. The One-pot method for synthesizing La-doped γ-Al2O3 nanoparticles developed here has several advantages over conventional methods including requiring only 3 hours instead of 3 days and wasting no lanthanum. TEM, X-ray PDF, and BET analyses as a function of calcination temperature show that the La stabilizes the gamma phase by 100°C. In order to begin understanding the mechanism of stabilization, the location of the La3+ atoms in our doped γ-Al2O3 nanoparticles is investigated via X-ray PDF and EXAFS analyses which indicate that the La dopant adsorbs as single, isolated atoms on the γ-Al2O3 surface. As calcination temperature increases, the immediate oxygen coordination shell of the La becomes increasingly like La2O3 though an extended La2O3 lattice is not formed due to the sparse (3 wt%) concentration of La atoms. During the γ-to-α transition, the La environment changes drastically to

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become more like LaAlO3 than La2O3, suggesting that the La is enveloped by the alumina lattice during the alpha phase transition though an extended LaAlO3 lattice is not formed. Keywords: γ-Al2O3 nanoparticles, La-doping, solvent-deficient synthesis, EXAFS, X-ray PDF

1. Introduction Al2O3 in the gamma phase is widely used as a catalyst support material due to its high surface area, porous nature, and catalytic activity.1 We have recently developed a ‘solvent-deficient’ method2-3 of synthesizing high surface area γ-Al2O3 nanoparticles that show promise in these applications.4 However, the metastable gamma phase transforms to α-Al2O3 (corundum) at lower temperatures (1000-1050°C, see Figure 1a) in nanoparticles than in larger γ-Al2O3 particles (>1100°C),4-5 which transformation is accompanied by two ill effects; first, a large decrease in surface area (Figure 1b) occurs which is correlated with a drop in the catalytic activity, and second, there is a substantial shrinkage in volume which creates voids in the catalytic bed and leads to attrition of the catalyst.6 The α phase transformation is thus problematic for catalytic applications that involve high temperatures (>1000°C) such as automotive exhaust gas processing and the combustion of hydrocarbons.7 To prevent these effects, γ-Al2O3 is commonly doped with a few weight percent of one of several elements including zirconium, titanium, thorium, boron, silicon, alkaline earth, and rare earth elements.6-16 These ‘structural promoters’ delay the onset of the transition to α-Al2O3 by roughly 100°C,17 thereby stabilizing the gamma phase in the range of the catalytic applications. Of these promoters, La3+ is the most widely used in industry.7 Numerous studies have shown that La3+ dopant percentages of 3-5% (by weight) are sufficient to achieve the stabilizing effect,7, 18-22

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though several

studies have indicated that loadings as low as ~0.5 wt% may be sufficient.24-25 Increasing the La percentage beyond 5% appears to diminish the surface area19 and pore structure of the Al2O3 support22


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and may have negative effects on the activity of some catalysts.9, 20 Thus, a La dopant level of about 3 wt% is generally considered to be optimal.19-22 La-doping is typically accomplished in one of two ways. If the alumina is synthesized using the solgel method, the La is added during the preparation of the alumina. Otherwise, the impregnation method is used to modify pre-formed alumina.7 In the sol-gel method, La(NO3)3⋅6H2O is added to the alumina sol at a carefully controlled acidic pH prior to the typical drying and calcination steps.8, 14, 26 Of the several variations of the impregnation method,27 incipient wetness and soaking are perhaps the most widely used.7,

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With impregnation by soaking, the commercially produced Al2O3 is soaked in a

La(NO3)3⋅6H2O solution over a period of days to allow the alumina to adsorb/absorb the lanthanum nitrate. With incipient wetness, a volume of La solution roughly equal to that of the Al2O3 pore volume is added to the dry alumina slowly until the pores can absorb no more and the alumina begins to appear wet. With either method, the impregnated alumina is dried and then calcined to remove the nitrate ions. Both the impregnation and sol-gel processes typically require 2-3 days to complete. While it is generally agreed that the La3+ dopant stabilizes the γ-Al2O3 phase by roughly 100°C no matter the method of doping,17 there is no such consensus on the mechanism of stabilization. Part of the disagreement centers on the location and environment of the La3+, which has been much debated.7-8, 25, 30-32

For alumina doped via impregnation by soaking, the majority of studies have concluded that the

La3+ is deposited on or near the surface of the alumina.7, 28-29 Several have suggested that the La dopant forms a La2O3 or LaAlO3 layer on the Al2O3 surface,7, 17 but most recent studies assert that the La3+ dopant remains dispersed as isolated atoms on the Al2O3 surface,25,

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particularly at the low dopant

levels (< 5%) typically employed.34-35 In contrast, when the La3+ dopant is added during the alumina synthesis via the sol-gel method, the dopant has been reported to incorporate into the alumina structure,32, 36 though isolated, surface-bound dopant sites have also been reported. In this study, we used our solvent-deficient method to perform a one-pot approach to La-doping reminiscent of that used in the sol-gel process but which takes a fraction of the time to complete. This paper describes and evaluates the resulting method in which the La3+ is added during the solventACS Paragon Plus Environment

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deficient Al2O3 synthetic reaction rather than after the Al2O3 formation. To investigate the suitability of our La-doped Al2O3 for catalytic and adsorption applications, the structure and morphology of our nanomaterials are characterized as a function of calcination temperature using TEM, X-ray PDF, and BET analyses. In order to begin understanding the mechanism of stabilization provided by the La3+, the location of the La3+ atoms in our doped γ-Al2O3 nanoparticles is investigated via X-ray PDF and EXAFS analyses. The EXAFS data also provide information on the variations in La3+ dopant location and environment as a function of temperature.

2. Experimental Methods 2.1 Synthesis A one-pot procedure for La-doping was easily accomplished using the solvent-deficient method by simply adding La(NO3)3⋅6H2O (3% by weight) to the typical 1:3 mole ratio of Al(NO3)3•9H2O and NH4HCO3 reagents used in the solvent-deficient synthesis of Al2O3. The method then proceeded as described previously37 for pure Al2O3; the reagents were ground together using a mortar and pestle, causing the mixture to liquefy and bubble. Grinding continued until the bubbling/popping subsided, and the resulting slurry/paste containing the precipitated precursor material was calcined for 2 hours at the desired temperature (typically 700-800°C) to produce the γ-Al2O3 nanoparticles. To study how the properties and La environment of these La-doped Al2O3 nanomaterials change as a function of calcination temperature, we produced a suite of La-doped Al2O3 samples. One large batch of the La-doped precursor material was prepared by using a mortar and pestle to grind 267.66g of Al(NO3)3⋅9H2O and 2.993 g of La(NO3)3⋅6H2O (both of reagent grade purity, VWR) with 170.85g NH4HCO3 (reagent grade purity, VWR) for 15-20 minutes until bubbling ceased and a precipitate (the precursor) had formed. The slurry containing the precursor was then dried for 12 hours in air at 100°C using a Thermo Scientific Lindberg Blue M oven. The dried precursor was split into 23 portions, roughly 8.1 g each. Using the Thermo Scientific oven, one sample was calcined at each 50°C increment


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between 100°C and 1200°C by heating it in air at a rate of 5°C/min to its set temperature and then holding this temperature for 2 hours before allowing the sample to cool to room temperature. 2.2 Characterization Methods The presence of the La dopant in the calcined samples was verified by XEDS (X-ray energydispersive spectrometry) analysis using a FEI Philips Technai F20 Analytical STEM operating at 200 kV. For the analysis, a small amount of La-doped Al2O3 calcined at 700°C was suspended in ethanol, and a drop of the dilute suspension was placed on a formvar/carbon film supported by a 200 mesh Cu grid (Ted-Pella Inc.). The solvent was allowed to evaporate, and images were recorded in standard high resolution TEM mode before switching to STEM (scanning transmission electron microscopy) mode to perform the XEDS analysis. A HAADF (high-angle annular dark-field) detector was used in the XEDS analysis. Preliminary powder X-ray diffraction (XRD) data were collected using a PANalytical X’Pert Pro diffractometer with a Cu source and a Ge-111 monochromator providing Cu-Kα1 (λ = 1.5406 Å) radiation. The structural evolution of the La-doped Al2O3 was studied via X-ray Pair Distribution Function (PDF) analyses to determine the phase transition temperatures and thereby reveal the extent to which the La-doped γ-Al2O3 was stabilized relative to the pure, un-doped Al2O3. X-ray data for the PDF analyses were collected at the 11 ID-B beamline38 at the Advanced Photon Source (APS) at Argonne National Laboratory using synchrotron radiation of energy 60 keV (λ = 0.2128 Å). For each sample, ~10 mg of powder were loaded into a 0.0395 inch inner-diameter polyimide capillary, and 2-D images of the diffraction data were collected out to a maximum value of Q = 29.5 Å-1 in reciprocal space under ambient conditions using a Perkin Elmer area detector. The Fit2D software package39 was used to integrate the 2D ring patterns into 1D powder diffraction patterns. The PDFgetX2 software package40 was used to extract G(r), the experimental PDF,41 using a maximum momentum transfer of Q = 24.5 Å-1 in the Fourier transform. PDF refinements were performed using the PDFgui program.42 Brunauer-Emmett-Teller (BET) specific surface areas and pore sizes for each of the La-Al2O3 samples were determined from N2 adsorption at 77 K using a Micromeritics TriStar II instrument. For these ACS Paragon Plus Environment

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measurements, between 200 and 300 mg of each sample were degassed at 200°C for 12-24 hours to remove superfluous moisture. The samples were allowed to cool to room temperature prior to data collection. Specific surface area (SA) was calculated by the Brunauer-Emmett-Teller (BET) method, using a P/P0 range between 0.05 and 0.2.43 Pore volume (PV) was calculated from the adsorption isotherm at the relative pressure of 0.99, and the mean pore size distribution (MPD) were determined using a newly developed method for the Kelvin equation and structural corrections for area and volume, while fitting the data to a log normal distribution function.44 La LIII-edge EXAFS (extended X-ray absorption fine structure) spectra were collected for the Ladoped Al2O3 samples calcined every 100°C from 300-1200°C. For the sake of comparison, data were also collected for samples of La2O3 (Alpha Aesar, reagent grade), LaAlO3 (Alpha Aesar, reagent grade), and La-doped Al2O3 in which the La dopant was added to the surface of a γ-Al2O3 support using the impregnation method. For this last comparable, a γ-Al2O3 support from Alfa Aesar was impregnated with La using an aqueous chelated metal complex solution of ethylenediaminetetraacetic acid (EDTA, Mallinckrodt Chemicals, 99.4%) and La nitrate (La(NO3)3·6H2O,Fisher Scientific, >98% pure) and then calcined at 700°C, as described in a previous publication.45 Data for all samples were collected at room temperature using the 10 BM beamline38 at the Advanced Photon Source (APS) at Argonne National Laboratory using the La LIII-edge (5.4827 keV). The low concentration of La3+ (~3% by weight) proved to be too low for transmission measurements to provide acceptable statistics, so a 16-element Ge detector was used to collect the fluorescence signal. The samples were pressed into pellets prior to data collection to increase the fluorescence signal. Three scans of each sample were collected using an energy range of 5.2827 keV to 5.88579 keV with 3 different step sizes (0.005 keV for the pre-edge between 5.2827- 5.4527 keV, 0.0025 for the edge between 5.4527-5.5127 keV, and 0.009 keV for the post edge between 5.5127-5.88579 keV). The Athena program was used to average the three scans of each sample, fit pre- and post-edge backgrounds, and obtain the normalized EXAFS signal as a function of the wavevector k. Fourier transforms into direct space were performed using the range 2.0 ≤ k ≤ 8.2 with no window and with a k-weight of 2. ACS Paragon Plus Environment

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Linear combination fits using the spectra of the La2O3, LaAlO3, and La-doped Al2O3 standards were generated in the Athena program using the EXAFS (chi) signal in k-space in the range 2.03 ≤ k ≤ 6.665 with a k-weight of 2. Weights in the linear combinations were constrained to sum to 1.

3. Results and Discussion 3.1 Sample Characterization and Method Validation XEDS analysis (Figure 2a) confirmed the presence of the La dopant on the alumina produced using the one-pot, solvent-deficient method. The TEM image in Figure 2b reveals the same agglomerated morphology present in the pure Al2O3 samples produced by the solvent-deficient method,4 and the XRD pattern of the La-doped Al2O3 shown in Figure 2d verifies the γ-Al2O3 structure of the 3-5 nm Al2O3 crystallites highlighted in Figure 2c. Likewise, the BET surface area and pore size estimates given in Table 1 for our pure and La-doped alumina are essentially identical. Thus, the addition of the 3% La3+ dopant during the Al2O3 synthetic reaction successfully dopes the alumina with La while neither impeding the formation of the γ-Al2O3 lattice nor altering the morphology of the Al2O3 produced by the solvent-deficient method; the La-doped γ-Al2O3 is just as favorable for catalytic and adsorption applications as the pure γ-Al2O3. Having determined that our synthesis method results in the desired La-doped Al2O3 nanomaterial, we performed X-ray PDF and BET analyses as a function of calcination temperature to study the structural evolution of the La-doped Al2O3 and determine how effectively the La dopant stabilizes the γ-Al2O3 phase and surface area relative to pure γ-Al2O3. The PDF data of the La-doped Al2O3 between 850 and 1200°C (shown in Figure 3) were modeled using the same combination of gamma46 and/or alpha47 Al2O3 structures used for our pure γ-Al2O3 nanoparticles (for the fit parameters see the supporting information).4 The gamma phase fractions resulting from the fits of both the La-doped and pure γ-Al2O3 are plotted together in Figure 4a. As the figure shows, the γα transition for the La-doped Al2O3 occurs between ~1100-1200°C whereas the transition occurs between 1000-1100°C for pure Al2O3. The La dopant thus appears to stabilize the γ-Al2O3 by roughly 100°C. ACS Paragon Plus Environment

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BET analyses of the pure and La-doped Al2O3 (Figure 4b) reveals that the decrease in surface area accompanying the γα transition is likewise delayed by ~100°C for the La-doped Al2O3 sample. These are the same margins of stabilization observed for La-doped γ-Al2O3 produced by other methods of doping,16, 34 thus the one-pot method developed here appears to be a viable way to synthesize La-doped γ-Al2O3 suitable for catalytic applications. 3.2 La dopant Location As with the sol-gel method, the La3+ dopant in our new method is added during the Al2O3 synthetic reaction instead of after the Al2O3 is formed. But considering the variation in the reported La dopant locations/environments and the unique solvent-deficient synthetic environment of our method, we deemed it necessary to determine the location of the La3+ atoms in our La-doped Al2O3. Based on previous reports of the La dopant location, at least four possible scenarios (illustrated in Figure 5) could be envisioned for the location of the La dopant in our La-doped Al2O3: the La could be (1) intercalating into the alumina lattice by substituting for some of the octahedrally-coordinated Al atoms,48-49 (2) forming La2O3 or LaAlO3 nanoparticles completely separate from the γ-Al2O3 nanoparticles,34

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(3)

forming a protective shell of La2O3 or LaAlO3 around the γ-Al2O3 nanoparticles,52-53 or (4) residing as singly-adsorbed atoms on the γ-Al2O3 surface. To test these scenarios, we attempted to model them against the experimental PDF data for the 950°C sample.i To model the first scenario, La atoms were substituted into the partially occupied 8c and 8d octahedral Al sites in the γ-Al2O3 structure given in the supplemental material. Like the La2O3 phase fraction, the La atom occupancies in the 8c and 8d sites refined to negative and therefore unphysical values. In addition to this negative result, the larger atomic radius of La should require longer La-O

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The 950°C sample was used in the modeling for two primary reasons. First, 950°C is the highest calcination temperature than can be employed without inducing a transition to the alpha phase of the Al2O3. Thus, of the γ-Al2O3 samples, the 950°C sample would likely contain the most well-crystallized (and therefore detectable) La2O3 or LaAlO3 particles, if they were indeed present. Second, the 950°C sample had the highest degree of crystallization of any of the γ-Al2O3 samples, as evidenced by it having the sharpest, most distinct peaks at higher 2θ values than the samples calcined at lower temperatures. This allowed for less ambiguity in the fits. ACS Paragon Plus Environment

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bond distances than the lattice positions in the γ-Al2O3 structure typically allow, as judged by the 2.372.82 Å La-O bond distances in the LaAlO3 and La2O3 structures compared to the 1.8 Å Al-O bond distances in Al2O3. A lattice substitution should thus require significant distortions in the γ-Al2O3 lattice near the La dopant atoms. If such distortions are indeed responsible for deterring all of the γ-Al2O3 particles from transitioning to the alpha phase at the usual temperature, then they should be widespread enough to be detectable. However, such distortions are not reflected in the PDF data, which is modeled well by the pure γ-Al2O3 phase (Figure 6a). Thus, it seems unlikely that the La atoms substitute for Al atoms inside the alumina lattice. To model the second and third scenarios, separate La2O3 and LaAlO3 phases51-53 were combined with the γ-Al2O3 phase in multiphase refinements. The La2O3 phase fraction refined to a negative and therefore unphysical value. The LaAlO3 phase fraction remained positive, but was very close to zero (0.2%) and did not visually improve the fit over the pure γ-Al2O3 phase. Additionally, rough calculations indicate that 3% dopant levels do not provide enough La to cover the available surface area (measured by BET analysis) with even a single layer, one unit cell thick, of either La2O3 or LaAlO3. Indeed, previous studies have found that La atoms do not even cluster until the dopant level is increased past 3 wt%,35 with a separate LaAlO3 phase not observed until dopant levels of at least 5%.34 Thus, it seems unlikely that separate particles or surface layers of La2O3 or LaAlO3 are forming. For the remaining scenario in which the La atoms exist as singly adsorbed atoms on the γ-Al2O3 surface, the La-O bond distances could theoretically be represented in the PDF. Based on the LaAlO3 and La2O3 structures, the La-O bond distance could range from 2.37-2.82 Å. Unfortunately, most of this range overlaps with the large Al2O3 peak at 2.8 Å, and even though the La-doped data display a slightly larger pre-edge shoulder at about 2.6 Å than the pure Al2O3 PDF (as shown in the inset of Figure 6b), the difference is not large enough to be conclusive; so, the PDFs of the La-doped Al2O3 and the pure Al2O3 are essentially identical within experimental uncertainty. Thus, despite the increased scattering strength of La over Al, the 3% La dopant concentration may be too small to be detectable by the PDF method. ACS Paragon Plus Environment

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As the PDF results could not be definitive, we turned to EXAFS analysis to determine the La dopant location. The first noteworthy observation from the EXAFS data seen in Figure 7 is the exceedingly intense and sharp La LIII-edge absorption peak at 5900 eV. This shape is often observed for oxide materials, hence the assumption that La is bound to oxygen atoms appears to be valid. Another noteworthy observation is that the fine structure in the absorption signal for all of our samples tapers to zero by 5650 eV. The EXAFS signal of the impregnated La-Al2O3 sample likewise tapers by 5650 eV, but the fine structure in the La2O3 and LaAlO3 samples persists through the full range of data collection, which was nearly to 5900 eV. Such rapid decay of the EXAFS signal indicates the La coordination is not as well defined in those samples, and it was early evidence of the similarity between our samples and the surface La-Al2O3 sample. The EXAFS data (Fourier transformed to direct-space) for all the La-doped Al2O3 samples are shown in Figure 8 where they are compared with the spectra of the La2O3, LaAlO3, and impregnated La-doped Al2O3 samples. These data not only allow a closer examination of the scenarios given in Figure 5, they also provide insight into the La environment as a function of temperature. Hence, the data are discussed below in order of increasing temperature. The discussion is primarily qualitative because fits of the coordination shells proved too ambiguous to provide any useful information. As the first row of spectra in Figure 8 illustrates, the La environment of the 300°C La-doped Al2O3 sample matches neither La2O3 nor LaAlO3 very closely but matches the environment of the impregnated La-Al2O3 sample almost exactly. Because the La dopant in the impregnated sample resides on the alumina surface,45 we can infer that our 3-hour, one-pot doping method places the La on the Al2O3 surface in the same environment as the 3-day impregnation method. Thus, even though the La dopant is present while the alumina particles form, the La atoms do not substitute inside the Al2O3 lattice, and the first scenario in Figure 5 can be eliminated with more confidence. At calcination temperatures between 400-1100°C (in the second row of spectra in Figure 8), the La environment still closely resembles the impregnated La-Al2O3 sample beyond R = 3 Å, but the first coordination shell (1.5 < R < 3) has gained a small shoulder around R = 2.5 Å, increasing its ACS Paragon Plus Environment

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resemblance to La2O3. In fact, as Figure 9 illustrates, the first coordination shell increasingly becomes more like La2O3 as the calcination temperature increases. These minor perturbations in its immediate coordination suggest that the La binding is strong but not stiff; as temperature increases, the La is able to distort the local Al2O3 structure to have an oxygen lattice more similar to La2O3 immediately surrounding the La. It does not appear that an extended La2O3 phase or layer forms, however, because the coordination beyond the first shell (R > 3 Å) does not resemble La2O3. Thus, at calcination temperatures between 300-1100°C, none of the doped samples (including the impregnation method sample) form an extended structure of La2O3 or LaAlO3. Therefore, not only can we eliminate the second scenario in Figure 5 involving separate particles of these phases, we can also discount the third scenario in which surface layers of these phases form. We hence deduce with reasonable confidence that the La dopant in our La-doped Al2O3 samples is adsorbed as single, isolated atoms on the Al2O3 surface in accordance with scenario #4 in Figure 5. A drastic change in the La environment occurs at a calcination temperature of 1200°C, as shown in the third row of spectra in Figure 8. Here, the La environment no longer closely resembles any of the La2O3, LaAlO3, or impregnated La-doped Al2O3 samples. This is the temperature at which the γ-Al2O3 has transformed to α-Al2O3. Thus, it seems that during the γ-to-α transition, the La environment changes to become unlike any stable lanthanum oxide compound. To gain some insight into the nature of this new environment, we fit the data at 1200°C with linear combinations of the data from the La2O3, LaAlO3, and impregnated La-doped Al2O3 samples. Though the data could not be replicated exactly with these fits, a fit with peaks of the correct shape (slightly shifted in bond distance) was produced using a linear combination of the LaAlO3 and impregnated LaAl2O3 samples (Figure 10f). Given the increasing similarity between La2O3 and the first coordination shell of the samples from 400-1100°C (Figure 9), the inclusion of LaAlO3 and the lack of La2O3 in the fit producing the correct shape at 1200°C was surprising, though it agrees with some previous reports in which the LaAlO3 phase appeared at high temperatures.11


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Due to this surprising result, we also fit the data below 1200°C to linear combinations of the La2O3, LaAlO3, and impregnated La-Al2O3 samples in order to better quantify the similarities of these phases to our La-Al2O3 samples. The results of some of these fits are given in Table 2 and are shown in Figure 10. The data from samples below 1200°C were generally fit best by the La-Al2O3 impregnated model alone, but a fit nearly as good could be achieved by including an increasing amount of La2O3 between 4001100°C, as may be expected from Figure 9. At temperatures above 800°C, a very small amount of LaAlO3 was present in the best linear combination fits, and this amount increased with temperature. Close examination of the data suggests that LaAlO3 is included at these temperatures because it best models the peak at k = 5 Å–1 seen in the 1200°C data, which is only just beginning to appear above 800°C. Thus, as the alpha phase transition approaches and occurs, the La nearest-neighbor environment changes to become more like LaAlO3 rather than La2O3. This suggests that the La is being enveloped inside the alumina lattice as the particles sinter.

Conclusion Using our solvent-deficient synthetic method, we have developed a one-pot method for producing Ladoped γ-Al2O3 nanoparticles that show promise for catalytic applications. Our one-pot approach has several advantages over other doping methods. First, it is much faster than impregnation methods, requiring only 3 hours instead of 3 days to produce the La-doped γ-Al2O3 nanomaterials. Second, the synthetic procedure is much easier than either the sol-gel or impregnation methods which require careful monitoring of pH, concentration levels, solution temperatures, and soaking times. Third, it provides a more accurate means of controlling the amount of La added than impregnation methods; all the La added to the reaction mixture is utilized, so less La is used because none is discarded in the excess solution required by soaking impregnation methods. This also means that less aqueous waste is generated, so the method is more environmentally benign than soaking impregnation methods. The onepot, solvent deficient method thus provides a new and favorable alternative method for producing Ladoped γ-Al2O3 nanomaterials. ACS Paragon Plus Environment

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In La-doped γ-Al2O3 samples produced by both the impregnation and one-pot methods, the La dopant appears to adsorb as single, isolated atoms on the Al2O3 surface. The immediate oxygen coordination shell of the La in our nanomaterials becomes increasingly like La2O3 as calcination temperature increases from 300-1100°C, though an extended La2O3 lattice is not formed presumably due to the sparse (3%) concentration of La atoms. During the γ-to-α transition, the La environment undergoes a drastic change and becomes more like LaAlO3 than La2O3, suggesting that the La is enveloped by the alumina lattice during the alpha phase transition even though an extended LaAlO3 lattice is not formed. Hopefully this understanding of the location of the La will lead to a better understanding of its role in the stabilization of the γ-Al2O3.

Supporting Information. Fit parameters for the gamma phase in the X-ray PDF refinements. The structure was based on the tetragonally distorted Al2O3 structure reported by Paglia et al. (reference 46) in space group I41/amd in which O atoms occupy the 16h site and Al atoms partially occupy the tetragonal 4a site (occupancy 0.88) and the octahedral 8c and 8d sites (occupancies 0.31 and 0.58, respectively) (Table S1); Fit parameters for the alpha phase in the X-ray PDF refinements (Table S2). This information is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DEAC02-06CH11357. Other funding for this work was provided by the U.S. Department of Energy under grant DE-FG02-05ER15666. The laboratory diffractometer was purchased using funds from the National Science Foundation under grant CHE-0959862.


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Stacey J. Smith, Baiyu Huang, Calvin H. Bartholomew, Branton J. Campbell, Juliana Boerio-Goates, and Brian F. Woodfield La3+ adsorbs as isolated atoms on the alumina surface of La-doped γ-Al2O3 nanomaterials synthesized using a novel one-pot process.


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Table 1. BET surface area and BJH pore size estimates for both pure Al2O3 and La-doped Al2O3 (calcined at 700°C) produced by the solvent-deficient method.

700°C γ-Al2O3 700°C La-doped γ-Al2O3

surface area 231 m2/g 242 m2/g

pore diameter 3.7 nm 3.5 nm

Table 2. Results of fits using La-Al2O3 alone or a linear combination of La-Al2O3, La2O3, and La-Al2O3.

Phases 300 400 600 800 1100 1200

La-Al2O3 La-Al2O3 + La2O3 La-Al2O3 + La2O3 La-Al2O3 + La2O3 + LaAlO3 La-Al2O3 + La2O3 + LaAlO3 La-Al2O3 + LaAlO3

Linear Combinations La-Al2O3 La2O3 LaAlO3 weight weight weight 0.88 0.00 0.00 0.61 0.39 0.00 0.59 0.41 0.00 0.51 0.48 0.01 0.45 0.50 0.05 0.54 0.00 0.46

La-Al2O3 alone R-factor

R-factor

0.027 0.056 0.071 0.121 0.123 0.385

0.027 0.063 0.070 0.089 0.116 0.463


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Figure Captions

Figure 1. Phase fractions (a) and BET surface area (b) as a function of temperature for Al2O3 nanoparticles formed via the solvent-deficient method, as determined previously.4 Figure 2. (a) XEDS results confirming the presence of La3+ in the La-doped Al2O3 produced by the onepot, solvent-deficient synthetic method. (Pt and Cl peaks are present due to the Pt catalyst precursor also deposited on this particular sample.) (b) TEM image of our La-doped Al2O3 (calcined at 700°C), illustrating the agglomerated nature of the 3-5 nm Al2O3 crystallites whose individual size is evident in the dark field TEM image in (c) of a separate Al2O3 sample. (d) XRD pattern of the La-doped Al2O3 (black) compared with a γ-Al2O3 pattern (gray) from the ICDD database (reference number 00-0100425), confirming the γ-Al2O3 structure of the La-doped material. The intensity scale has been normalized to 100 counts for both the data and the standard to enable a clearer overlay of the two patterns. Figure 3. X-ray PDF data (gray) and corresponding fits (red) of La-doped Al2O3 synthesized via the one-pot, solvent-deficient method and calcined at the given temperatures. Figure 4. γ-Al2O3 phase fractions (a) and BET surface areas (b) as a function calcination temperature during the γα transition for pure γ-Al2O3 (red) and La-doped γ-Al2O3 (blue). Figure 5. Illustrations of four possible La dopant locations: (1) substitutionally doped inside the Al2O3 lattice at the partially occupied 8c and 8d octahedral Al sites highlighted in light blue, (2) separate particles of La2O3 or LaAlO3, (3) layers of La2O3 or LaAlO3 on the Al2O3 surface, or (4) singly adsorbed La atoms on the Al2O3 surface. In each of the depictons, red coloring represents the Al2O3 particle, and bright blue coloring indicates the La species in that scenario. In the unit cell included in the first scenario, the red atoms are oxygen and the dark blue atoms are tetrahedrally-coordinated Al. Figure 6. (a) PDF data for the La-doped Al2O3 sample calcined at 950°C fit using the pure γ-Al2O3 structrue and (b) comparison of the pure and La-doped Al2O3 at 950°C. The inset of (b) highlights the region where La-O bond distances should theoretically be. ACS Paragon Plus Environment

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Figure 7. Normalized absorption data at the La LIII-edge for the La-doped Al2O3 samples. Figure 8. EXAFS data for the La-doped Al2O3 compared with the EXAFS data for La2O3, LaAlO3, and impregnation-method La-Al2O3. The first, second, and third rows show, respectively, the 300°C sample (red), the 400-1100°C samples (multiple colors), and the 1200°C sample (red). The first, second, and third columns compare these samples to La2O3, LaAlO3, and the impregnated La-doped Al2O3 samples in blue, respectively. Figure 9. Comparison of the La2O3 and impregnated La-Al2O3 samples with our La-doped Al2O3 calcined at increasing temperatures between 400-1100°C,

highlighting the increasing similarity

between the first coordination shells (1.5 < R < 3) of La2O3 and our La-doped Al2O3 but also revealing that the shells beyond this still resemble the impregnated La-Al2O3 sample. Figure 10. The best linear combination fits for the 300 (a), 400 (b), 600 (c), 800 (d), 1100 (e), and 1200 (f) samples whose specifications are given in Table 2. Note that a slight shift to lower k values would enable a combination of LaAlO3 and La-Al2O3 to replicate the 1200°C data passably well, as opposed to lower temperatures at which a combination of La2O3 and impregnated La-Al2O3 passably replicate the data. Although these fits are not a representation of reality, they provide some insight into the nature of the La environment at different temperatures.


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(a)

(b)

Figure 1.


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(b) (a)

(c)

(d)

Figure 2.


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Figure 3.


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(a)

(b)

Figure 4.


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Figure 5.


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(a)

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(b) Figure 6.


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Figure 7.


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Figure 8.


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Figure 9.


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(a)

(b)

(c)

(d)

(e)

(f)

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Figure 10.


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La-Dopant Location in La-Doped Î³-Al2O3 ... - ACS Publications

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