Structural Characterization of Ni-Substituted Hexaaluminate Catalysts

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Structural Characterization of Ni-Substituted Hexaaluminate Catalysts Using EXAFS, XANES, XPS, XRD, and TPR Todd H. Gardner* National Energy Technology Laboratory, U.S. Department of Energy, 3610 Collins Ferry Road, P.O. Box 880, Morgantown, West Virginia 26507-0880

James J. Spivey Louisiana State UniVersity, Cain Department of Chemical Engineering, Baton Rouge, Louisiana 70803

Edwin L. Kugler West Virginia UniVersity, Chemical Engineering Department, Morgantown, West Virginia 26506

Andrew Campos Louisiana State UniVersity, Cain Department of Chemical Engineering, Baton Rouge, Louisiana 70803

Jason C. Hissam† West Virginia UniVersity, Chemical Engineering Department, Morgantown, West Virginia 26506

Amitava D. Roy Louisiana State UniVersity, J. Bennett Johnson, Sr., Center for AdVanced Microstructures and DeVices, Baton Rouge, Louisiana 70806 ReceiVed: December 12, 2009; ReVised Manuscript ReceiVed: March 13, 2010

The structure of five Ni-substituted Ba0.75NiyAl12-yO19-δ hexaaluminate catalysts at various Ni loadings (y ) 0.2, 0.4, 0.6, 0.8 and 1.0) was investigated using EXAFS, XANES, XPS, XRD, and TPR. As Ni-substitution into the hexaaluminate lattice is increased, the unit cell dimension decreases along the c axis. This systematic change is consistent with Ni substitution for Al3+ in the hexaaluminate crystalline structure. XANES analysis suggests that Ni-O bonding is stronger for Ni substituted into the hexaaluminate lattice, relative to that of bulk NiO. The average coordination numbers obtained from EXAFS indicate that Ni is preferentially exchanging with tetrahedrally coordinated Al3+ in the structure which predominates in regions of the hexaaluminate unit cell near the mirror plane. It is at these sites that, preferential substitution of Ni2+ likely occurs to minimize strain in the crystalline lattice. 1. Introduction Substituted hexaaluminates are crystalline compounds with the general formula MI(MII)yAl12-yO19-δ where MI is a cation (e.g., MI ) Br, La, or Sr) located near the mirror plane between the spinel blocks of the hexaaluminate framework, and MII is a metal dopant (e.g., MII ) Ni) substituted for Al3+ in the lattice. Regardless of the cation or dopant, all hexaaluminate compounds consist of stacked spinel blocks of close packed oxide ions that are charge balanced by two large mono-, di-, or trivalent cations.1 These cations reside within a mirror plane at opposite ends of a spinel block. Depending on the ionic radius and/or valence of the mirror plane cation, hexaaluminate compounds possess either β-alumina or magnetoplumbite structure as shown in Figure 1.1,2

Figure 1. Hexaaluminate unit cell.

* Corresponding author. E-mail: [email protected]. Phone: (304) 285-4226. Fax: (304) 285-0903. † Present address: National Energy Technology Laboratory, U.S. Department of Energy, 3610 Collins Ferry Road, P.O. Box 880, Morgantown, WV 26507-0880.

The high thermal stability of hexaaluminate compounds in both oxidizing and reducing environments is due to this layered structure of alternately stacked spinel blocks separated by mirror planes.3 This structure suppresses interlayer diffusion of oxide

10.1021/jp9117634  2010 American Chemical Society Published on Web 04/09/2010

Characterization of Substituted Hexaaluminates ions and crystal growth along the c axis at high temperatures,4-6 as shown by the fact that these materials retain surface areas of more than 10-15 m2/g after calcination at temperatures of 1300 °C or more.5,6 Oxygen within the mirror plane resides as a monatomic layer that has been shown to be less tightly bound than oxygen present in the spinel block, providing a preferential path for the diffusion of oxide ions.5,7,8 By substituting catalytically active metals for Al3+ in the hexaaluminate structure, disperse active catalytic sites can be created with little effect on the thermal stability of the hexaaluminate structure.2,3,9 This substitution is allowed to satisfy the condition of charge neutrality which is possible because of the presence of cation vacancies in the mirror planes of the hexaaluminate structure.10,11 Under reducing conditions, the labile oxygen ions in the mirror plane will preferentially reduce, exposing the active metal sites.1 Because these sites are bound within the hexaaluminate framework, and yet are reducible, they provide atomically disperse, thermally stable, catalytically active sites that have been widely studied for reactions requiring high temperatures.1-3,5 Maintaining high dispersion of the active metal is particularly important in minimizing deactivation by sulfur poisoning and carbon deposition, which are widely believed to be promoted by large ensembles of metal atoms.1,12-14 A number of reactions requiring these properties have been studied using substituted hexaaluminate compounds including catalytic combustion,5,10 partial oxidation of methane,6,14,15 fuel reforming,1,16 and the decomposition of N2O.3,17 Many of these studies have examined the effect of the mirror cation (e.g., Sr, La, and Ba) and catalytic metal (e.g., Ni, Fe, Mn, etc.) on the catalytic properties of metal substituted hexaaluminate. BaNiyAl12-yO19-δ is one of a number of substituted hexaaluminate catalysts that has been studied for these reactions,1,3,5,7,8,14,18-20 but we are aware of only the work of Chu et al.14 in which a systematic study of the degree of Ni substitution into the lattice of a BaAl12O19-δ hexaaluminate compound has been carried out. That study used a glycol precipitation synthesis route, and focused on the effect of Ni substitution on the stability of the catalyst in the catalytic partial oxidation of methane, and characterization of the carbon deposition and postreaction catalyst. Here, we use an ammonium carbonate precipitation synthesis method,11 which can affect the final structure of the hexaaluminate compound compared to other methods.8 We examine the effect of Ni substitution on the structure of the as-prepared Ba0.75NiyAl12-yO19-δ (y ) 0.2, 0.4, 0.6, 0.8, and 1.0) catalysts, the relationship between surface and bulk hexaaluminate structure, and the reducibility of Ni substituted into the structure using EXAFS, XANES, and conventional methods such as XPS, XRD, and TPR. 2. Experimental Section 2.1. Catalyst Synthesis. Five Ni-substituted hexaaluminate catalysts with the general formula Ba0.75NiyAl12-yO19-δ (y ) 0.2, 0.4, 0.6, 0.8, and 1.0) were prepared by coprecipitation of nitrate precursors. The catalysts were prepared by first dissolving nitrate precursor salts in 300 mL of deionized water at 60 °C to form a 1 M solution. In a separate vessel, a sufficient amount of ammonium carbonate was added to 300 mL of deionized water to neutralize the metal nitrate solution. The ammonium carbonate solution was then heated and maintained at 60 °C. Once both solutions reached 60 °C, the nitrate solution was then added dropwise, over the course of 30 min, to the ammonia carbonate solution under vigorous mixing conditions until the pH of the solution reaches 7.5.

J. Phys. Chem. C, Vol. 114, No. 17, 2010 7889 The resultant gel was then aged at 60 °C under vigorous mixing for six hours. After aging, the gel was then separated by vacuum filtration and rinsed three times with deionized water to remove the excess nitrate. The filtered gel was then dried for 12 h at 110 °C, crushed. The crushed samples were then calcined using a stepwise heating procedure from 110 to 1400 °C with the temperature being held isothermally for 1 h with every 100 °C increase in temperature. 2.2. Catalyst Characterization. 2.2.1. Catalyst Surface Area and Bulk Composition. N2 BET surface areas of the catalysts were determined using a Quantachrome Surface Area Analyzer 2000. A Perkin-Elmer Optima 3000 inductively coupled plasma emission spectrometer was utilized to determine the bulk composition of the hexaaluminate catalysts. Analyte standards were obtained from Alfa Aesar. Samples were first digested by mixing 0.1 g of sample with 1.0 g of lithium tetraborate and heating in a platinum crucible at 950 °C for 20 min. The crucibles were then cooled to room temperature and submerged in a dilute HCl solution to dissolve the melt. The solution was taken to volume with water to give a clear sample solution. This procedure was then performed in duplicate with the average taken as the final value. Sample precision was determined to be within (2% for all samples. 2.2.2. Temperature Programmed Reduction. Temperature programmed reduction was carried out using a Micromeritics ASAP 2910 automated catalyst characterization system. The catalysts, as prepared in their oxide state, were heated from ambient to 1100 °C at a rate of 10 °C per min in a binary gas mixture containing 5.15 vol% H2, balance Ar. The H2 consumption was measured continuously as a function of increasing temperature using a thermal conductivity detector. 2.2.3. Catalyst Surface Composition by X-ray Photoelectron Spectroscopy (XPS). XPS spectra were recorded with a cylindrical mirror analyzer with a 15 kV X-ray source (PerkinElmer TNBX) as previously described by Gardner et al.1 The system consisted of a detection chamber which was operated within a pressure range of 10-9 to 10-8 Torr. The detector angle used was 54.7°. The instrument was calibrated by using the following photo emission lines: EB(Cu 2 p3/2) ) 932.4 eV, EB(Au 4 f7/2) ) 83.3 eV. The binding energies were referenced to the C 1s level at 284.6 eV for adventious carbon. Spectra were recorded at low X-ray fluxes both at 15 kV and 22 mA and 12 kV and 10 mA in order to avoid X-ray induced reduction. 2.2.4. Phase Analysis, Unit Cell Refinement, and Crystallite Size by X-ray Diffraction (XRD). Powder XRD was used to identify the oxide catalyst phases, the unit-cell dimensions, volume and the average crystallite size for the series of Nisubstituted hexaaluminate catalysts. Diffraction measurements were made using filtered monochromatic Cu KR synchrotron radiation with a wavelength of 0.0922 nm at the National Synchrotron Light Source on beamline X7B. Sample preparation consisted of mild grinding in an agate mortar and pestle prior to mounting the samples in the sapphire capillary tube. In these experiments, the catalyst was placed in a single-crystal sapphire capillary tube that was positioned in the X-ray beam. X-ray diffraction patterns were taken at 25 °C under He over a scanned region of 0 to 55° (2θ) using a Rayonix Mar345 image plate detector. The Debye-Scherrer powder rings were processed using FIT2D software (European Synchrotron Research Facility) to provide diffraction angle and intensity data. Lanthanum hexaboride was used as an external calibration standard. The XRD intensity and position data was analyzed using Jade Plus 7.5 (Materials Data Incorporated). Cell refinement was performed

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TABLE 1: Ni K-Edge and Ba LIII Edge XANES Scanning Parameters

TABLE 3: Binding Energies for Ba0.75NiyAl12-yO19-δ Hexaaluminate Catalysts

Ni K Edge (8.333 KeV)

binding energies (eV)

-150,-20,50,400,580 2,1,2,2 1,1,2,2

scan interval relative to edge energy step size (eV) integration time (s)

in Jade Plus 7.5 with Ba0.75Al11O17.25 [ICSD 04-010-2927] used as the baseline pattern. 2.2.5. Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorbance Near Edge Spectroscopy (XANES). The Ba0.75NiyAl12-yO19-δ catalyst samples were prepared for EXAFS and XANES by first grinding the samples in a mortar and pestle followed by mounting on Kapton tape. The Double Crystal Monochromator (DCM) beamline at LSU’s J. Bennett Johnston, Sr., Center for Advanced Microstructures and Devices (CAMD) was used which employs a Si-(110) crystal monochromator and a single-element Ge diode fluorescence detector which operates at 1.3 GeV and a current between 100 and 200 mA. The scanning parameters for the Ni K-edge study are given in Table 1. The samples were scanned ten times with a 5 µm thick Ni foil (8.333 KeV) placed after the sample for calibration purposes; the resolution of the beamline was 2 eV at the energies tested. The Ni standards used were γ-Ni (5 µm) and NiO (99% metals basis, Alfa Aesar), which was scanned in transmission mode. EXAFS data reduction was performed using Athena (v. 0.8.050) software.21 The first shell fitting was performed using Artemis (v. 0.8.007) software. The amplitude reduction factor (s20) was fit from the foil analysis and was found to have a value of 0.821. The final model structure used for the first shell fitting of the Ni-substituted hexaaluminate catalysts was Ba2.333Al2.333O34.333 [ICSD-81999], with Ni-substituted for tetrahedrally coordinated Al3+ sites. 3. Results and Discussion 3.1. Catalyst Characterization. 3.1.1. Catalyst Bulk Composition, Surface Area, Pore Volume, and Crystallite Size. The BET surface area, average pore volume, bulk Ni concentration and the average crystallite size are given in Table 2. The surface area of the hexaaluminate compounds are known to be influenced by their planar morphology that created by oxygen diffusion through the mirror planes during the heat treatment process.5 This morphology is responsible for their excellent high temperature stability.3,4,6 The BET surface areas for the catalysts show no general trend with Ni-substitution, and are within a narrow range from 7.8 to 12.2 m2/g, which is comparable to the values reported by others for Ba Ni-substituted hexaaluminate compounds [1, 5, 7]. This, coupled with the lack of a consistent trend in pore volume, and average size of the Nisubstituted hexaaluminate crystallites, as determined by XRD, shows that increasing the Ni-substitution for aluminum in the crystalline lattice did not produce a significant change in the hexaaluminate structure. As expected, increasing Ni-substitution is confirmed by the measured monotonic increase in bulk Ni content, increasing from 1.32 to 6.91 wt %.

Al

O

Ba

Ni

catalyst

2p

1s

3 d5/2

3 d3/2

2 p3/2

2 p1/2

Ba0.75Ni0.2Al11.8O19-δ Ba0.75Ni0.4Al11.6O19-δ Ba0.75Ni0.6Al11.4O19-δ Ba0.75Ni0.8Al11.2O19-δ Ba0.75Ni1.0Al11.0O19-δ

73.9 73.9 73.9 74.3 74.2

530.9 530.7 530.6 531.1 531.1

780.1 779.9 779.8 780.2 780.1

795.3 795.1 795.0 795.4 795.4

855.7 855.8 855.2 855.3 855.5

873.3 873.3 872.8 872.9 873.1

These results are consistent with prior studies on Ba-β-Al2O3 phase formation4,10 in which monophasic materials with high surface area containing highly interspersed Ba species are synthesized. These species hinder extraneous alumina transitions until the final phase is formed. Similarly, the coprecipitation technique used in this investigation atomically disperses the chemical components which favor high surface area formation during sintering. The negligible effect of low Ni-substitution (Ni/Al e 1/11) on the morphological properties is consistent with these prior investigations. 3.1.2. Surface Composition by XPS. Al 2p, Ba 3d, Ni 2p, and O 1s photoelectron peaks for the as prepared Ba0.75NiyAl12-yO19-δ (y ) 0.2, 0.4, 0.6, 0.8, and 1.0) catalyst series were compared to determine if chemical modification had occurred as a result of Ni substitution into the hexaaluminate lattice. The binding energies of the constituent elements are given in Table 3. The values of the binding energies for Al, Ba, Ni, and O do not change with the extent of Ni substitution in the lattice. The observed Al 2p binding energy at 74.0 ( 0.2 eV is consistent with the Al3+ values reported in the literature1,6,19,23,24 for Al2O3 and hexaaluminate compounds. The Ba 3d binding energies observed at 780.0 ( 0.2 for Ba 3d5/2 and 795.2 ( 0.2 for Ba 3d3/2 are consistent with prior literature observations for Ba2+ values reported for BaO25 and barium hexaaluminate compounds.1,19 The O 1s binding energy is observed at 530.9 ( 0.2 eV which is consistent with the presence of O2- in the lattice of the Ni-substituted barium hexaaluminate catalysts.1,6,19 The Ni 2p binding energy values are observed at 855.5 ( 0.3 for Ni 2p3/2 and 873.1 ( 0.3 eV for Ni 2p1/2 which indicates that the oxidation state of Ni on the hexaaluminate catalyst surface is Ni2+. The binding energies of the Ni 2p peaks, where Ni is substituted in the hexaaluminate lattice, are consistent with those from Chu et al.6 but slightly lower than those given by Xu et al.19 and slightly higher than those previously given by Gardner et al.1 This difference may be due to the different calibrations of the spectrometers or due to the use of different reference standards. The literature reports that both the Ni 2p3/2 peak location and energy separation between the main peak and the shakeup satellite peak for Ni bound as NiAl2O3 and NiO are different and can be used to differentiate between Ni that is substituted in the hexaaluminate structure, which is comprised of alternative stacks of spinel blocks,2,3,5 and NiO residing on the surface. Figure 3 shows the binding energy versus the relative intensity

TABLE 2: Textural Properties and Bulk Composition for Ba0.75NiyAl12-yO19-δ Hexaaluminate Catalysts catalyst

BET surface area (m2/g)

pore volume (cm3/g)

bulk Ni content in catalyst (wt %)

crystallite size (Å)


12.1 12.2 10.4 7.8 11.9

0.3166 0.1127 0.0567 0.2219 0.1044

1.32 2.70 4.52 6.05 6.91

266 264 273 273 262

Characterization of Substituted Hexaaluminates

Figure 2. Powder XRD of Ba0.75NiyAl12-yO19-δ hexaaluminate catalysts: y ) (a) 1.0, (b) 0.8, (c) 0.6, (d) 0.4, and (e) 0.2.

Figure 3. Ni(2p1/2) and Ni(2p3/2) spectra for Ba0.75NiyAl12-yO19-δ hexaaluminate catalysts: y ) (a) 1.0, (b) 0.8, (c) 0.6, (d) 0.4, and (e) 0.2.

for the Ni(2p1/2) and Ni(2p3/2) spectrum. The Ni 2p3/2 shakeup satellite peak for Ni2+ bound in the NiAl2O3 spinel phase is reported to have an energy separation, ∆E, of 6.3 ( 0.3 eV from the main core peak6,23,24,26 that differs from the energy separation for NiO, which is observed to be 7.1 ( 0.1 eV.23,24,26 The observed Ni 2p3/2 shakeup satellite peak from the Ni substituted hexaaluminate catalysts is located at 861.9 ( 0.2 eV which gives an energy separation of 6.4 eV from the main core peak located at 855.5 ( 0.3 eV. This energy separation is consistent with surface Ni being present in the NiAl2O3 spinel phase and substituted into spinel blocks which comprise the hexaaluminate structure.

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Figure 4. Surface composition and bulk compositions of Ba0.75NiyAl12-yO19-δ hexaaluminate catalysts: (b) bulk Al/Ni atomic ratio and (O) the surface Al/Ni atomic ratio.

The bulk and surface nickel Al/Ni ratios are given in Figure 4. Logarithmic line fits to the upper and lower data sets produced correlation coefficients of 0.96 and 0.95, respectively. The surface shows Al enrichment beyond that of the bulk phase which is consistent with the results of Gardner et al.1 for BaNi0.4Al11.6O19-δ and Xu et al.19 for BaNiAl11O19-δ. In addition, as Ni substitution with the lattice is increased, the surface Al/ Ni ratio also decreases, approaching that observed in the bulk hexaaluminate catalysts. This segregation behavior suggests that as Ni substitution into the hexaaluminate lattice is increased, the surface becomes more like that of the bulk. Gardner et al.1 and Xu et al.19 have suggested that the surface Ni concentration of hexaaluminate catalysts correlates with both the size and valence of the mirror cation, i.e., the mirror cation affects the chemical bonding of nickel in the hexaaluminate lattice. 3.1.3. Temperature Programmed Reduction. A key property of solid oxide catalysts is the reduction temperature of the metals that are substituted into the lattice. The TPR profiles for the Ba0.75NiyAl12-yO19-δ (y ) 0.2, 0.4, 0.6, 0.8, and 1.0) catalysts are given in Figure 5. Within this series, the two catalysts with the lowest level of Ni substitution (Ba0.75Ni0.2Al11.6O19-δ and Ba0.75Ni0.4Al11.6O19-δ) exhibited two H2 consumption peaks, a small peak located at 590 °C, and a high temperature peak that begins at about 850 °C and has a maximum at about 1050 °C. The small peaks at 590 °C are not bulk NiO, which reduces at 400 °C.6,18,27,28 Yin et al.8 observed a major reduction peak at 582 °C for BaAl11O19, containing no Ni, suggesting that at least a portion of the 590 °C peak in Figure 5 may be due to the reduction of the bulk hexaaluminate, not nickel oxide(s). However, Yokota et al.16 found that “pre-reduction” of BaNiAl11O19 (corresponding to y ) 1.0 in our series of catalysts) at 700 °C caused metallic Ni particles to form on the surface of the catalyst. Though no TPR data are provided, this result suggests that a small amount of Ni (from the small size of the TPR peak in Figure 5) may segregate from the hexaaluminate structure and account for the 590 °C peak observed here. If so, this nickel species is not bulk NiO since its reduction temperature is much higher than 400 °C. The major peak observed for the two lowest levels of Ni substitution (y ) 0.2 and 0.4; i.e., Ba0.75Ni0.2Al11.6O19-δ and Ba0.75Ni0.4Al11.6O19-δ; Figure 5a,b is at 1050 °C. Chu et al.6 identified two peaks for BaNi0.3Al11.7O19-δ and BaNi0.6Al11.4O19-δ (i.e., y ) 0.3 and 0.6) at 920 °C, attributed to the reduction of NiAl2O4 spinel, and 1100 °C, attributed to reduction of nickel

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Gardner et al. TABLE 4: Lattice Parameters for Ba0.75NiyAl12-yO19-δ Hexaaluminate Catalysts catalyst

a,b axis (Å)

c axis (Å)

unit cell volume (Å3)


5.588 ( 0.020 5.596 ( 0.021 5.603 ( 0.022 5.608 ( 0.010 5.610 ( 0.010

22.689 ( 0.004 22.683 ( 0.004 22.668 ( 0.005 22.643 ( 0.003 22.641 ( 0.004

613.50 ( 3.116 615.06 ( 3.194 616.27 ( 3.441 616.75 ( 1.597 617.13 ( 1.525

TABLE 5: EXAFS Parameters for Ba0.75NiyAl12-yO19-δ Hexaaluminate Catalysts

Figure 5. Temperature programmed reduction of Ba0.75NiyAl12-yO19-δ hexaaluminate catalysts: y ) (a) 1.0, (b) 0.8, (c) 0.6, (d) 0.4, and (e) 0.2.

which is more strongly bonded into the BaNiyAl12-yO19-δ phase. The single peaks observed in Figure 5d,e are closer to the 1100 °C peak seen by Chu et al.6 and can be attributed to the reduction of nickel within the hexaaluminate structure. There is a small, but discernible trend in the temperature at which the reduction begins for all five catalystssthe higher the Ni substitution, the lower the temperature at which the reduction begins (vertical lines in Figure 5a-e). This trend suggests an increase in the concentration of less stable Ni-O-Ni bonds relative to Al-O-Ni bonds, producing slightly more reducible Ni sites. The oxygen in the mirror plane has been reported to be less tightly bound than oxygen within the spinel block,5,7,8 which suggests that as nickel substitution increases, a somewhat greater portion of the nickel residing in this region of the unit cell may be in the more reducible Ni-O-Ni coordination, reducing at a lower temperature than nickel present in other regions of the spinel block. As expected, the areas under the TPR curves also increase with nickel substitution, corresponding to more reducible nickel. These results are consistent with Chu et al.6 for a series of Nisubstituted BaNiyAl12-yO19-δ catalysts. Literature results show that at 1100 °C, nickel in comparable BaNiyAl12-yO19-δ hexaaluminate compounds reduces either completely5 or nearly completely,14,19 as observed here. 3.2. Structure of Catalysts. 3.2.1. Phase Analysis by XRD. As previously mentioned, the hexaaluminate structure consists of alternate stacking along the c axis of Al3+ containing spinel blocks and mirror planes in which the large Ba2+ cations are located. The powder XRD patterns for the series of Ba0.75NiyAl12yO19-δ (y ) 0.2, 0.4, 0.6, 0.8 and 1.0) hexaaluminate catalysts are given in Figure 2. Analysis of the XRD pattern indicates that the compounds synthesized are hexaaluminate with β-alumina structure consistent with the Ba0.75Al11O17.25 compound.29 It is evident that changing the Ni concentration causes only small changes in some of the peak positions, but the peaks retain similar sharpness and no new structures are shown to appear. However, the intensity of the peaks does increase with increasing

catalyst

coordination number

Ni-O bond distance

Debye-Waller factor


4.2 ( 1.3 4.0 ( 0.8 3.9 ( 0.5 4.2 ( 1.1 3.8 ( 0.5

1.927 ( 0.024 1.979 ( 0.020 1.951 ( 0.014 1.988 ( 0.026 1.963 ( 0.014

0.0016 ( 0.0031 0.0018 ( 0.0028 0.0028 ( 0.0020 0.0040 ( 0.0041 0.0039 ( 0.0021

Ni substitution indicating that the crystallinity of the Ba-Al-O hexaaluminate compound is changing with increasing Ni substitution. The average crystallite size calculated from the Scherrer equation,30 given in Table 2, shows that all five samples have an average crystallite size in a narrow range between 262 and 277 Å. The unit cell parameters and volume for the Ni-substituted hexaaluminate catalysts are reported in Table 4. The presence of Ni in the hexaaluminate structure is confirmed by the absence of Ni-containing phases present in the XRD pattern and by comparing the values of the calculated cell parameters for the hexaaluminate catalysts at different Ni loadings. The average dimension along the a,b axis is statistically independent of Ni substitution, but there is a statistically significant contraction of the c axis in the unit cell with higher levels of Ni substitution into the lattice. It has been suggested that the contraction of the hexaaluminate unit cell along the c axis and expansion along the a and b axes occurs when a divalent species with a larger ionic radii is substituted for trivalent aluminum into the hexaaluminate lattice.9,31 To maintain electroneutrality, as Al3+ substitution occurs, the Ba2+ concentration in the mirror plane increases resulting in stronger local bonding between the spinel block and the mirror plane, which contracts the mirror plane, decreasing the c axis dimension.9,31 Table 4 also shows that there is no statistically significant change in the total unit cell volume with nickel substitution. The change in the unit cell c dimension clearly indicates that Ni, which has a larger ionic radius than aluminum, is being substituted for Al3+ in the hexaaluminate lattice. 3.2.2. XANES and EXAFS. Analysis of the EXAFS data shows that the average first shell Ni-O coordination number is 4 which suggests tetrahedral Ni2+ site-substitution into the hexaaluminate lattice, for all levels of Ni substitution, as expected.10,11 The EXAFS fitting results are based on Ba2.333Al2.333O34.333 [ICSD-81999] as the model structure, with Ni-substituted for tetrahedrally coordinated Al-sites, are listed in Table 5. This observation is consistent with the literature which suggests that preferentially substitutable tetrahedral Al3+ sites are located in a localized region of high strain near the mirror plane.32 The substitution of larger divalent cations, like Ni2+, which are both larger and less positively charged than Al3+ ions, reduces the strain. For example, Roth32 has suggested that the preferential substitution of Mg2+ cations into tetrahedral Al3+ sites localized near the mirror plane occurs because the strain is reduced when the Al3+ ion is replaced by the larger

Characterization of Substituted Hexaaluminates

Figure 6. EXAFS [k*(k)] of NiO, Ni foil standards and Ba0.75NiyAl12-yO19-δ hexaaluminate catalysts: y ) (a) 1.0, (b) 0.8, (c) 0.6, (d) 0.4, and (e) 0.2.

and less positive Mg2+. It is suggested here that since Ni2+ is larger than Al3+, Ni2+shows the same site preference. The EXAFS data in Table 5 show no clear correlation between the Ni loading and the first shell Ni-O bond distance. Only a 3% variation between the largest and smallest Ni-O bond distances is observed. In addition, the Ni-O bond distances reported (with a mean of 1.962 Å) are significantly different from the previously reported barium hexaaluminate Al-O bond distances (with a mean of 1.793 Å for tetrahedral sites).33 The tetrahedral bond distance of Ni-O is greater than that of Al-O and is consistent with the XRD data which shows an increase in the average unit cell volume with increasing Ni substitution into the lattice. The Debye-Waller factor for the Ni-O bond in all Ni-substituted hexaaluminate catalysts has similar values to those found by Corrias et al.34 for crystalline NiO/SiO2 which is indicative of a well-ordered Ni crystalline structure.34,35 Since three of the five Debye-Waller factors are not statistically different from zero, the apparent increase in the Debye-Waller factor as a function of Ni loading is inconclusive. The EXAFS spectra for the series of Ni-substituted hexaaluminate catalysts and γ-Ni and NiO reference compounds are given in Figure 6. The similarity between the EXAFS spectra for the different Ni-substituted hexaaluminate catalysts suggests the presence of a single Ni-containing phase which is confirmed by principal component analysis. This similarity is further confirmed by contrasting the spectra from the γ-Ni and NiO reference compounds which produced dissimilar EXAFS spectra. The Ni XANES spectrum for the Ba0.75Ni1.0Al11O19-δ catalyst is shown in Figure 7. From this figure, it is observed that the inflection point, located at an energy of 8341.8 eV, for the Ba0.75Ni1.0Al11O19-δ catalyst overlaps that of NiO which further indicates that Ni is present in the 2+ oxidation state. The preedge feature, located at 8.333 KeV, is the result of 1sf3d transitions in the XANES spectrum of Ba0.75Ni1.0Al11O19-δ and NiO and indicates the presence of an oxide-type phase. However, the Ba0.75Ni1.0Al11O19-δ spectrum has a more pronounced pre-edge feature, suggesting stronger Ni-O bonding, relative to NiO, which is consistent with Ni-substitution into the hexaaluminate lattice. The FEFF 8.436 calculations shown in Figure 7 use a modified Ba2.333Al2.333O34.333 [ICSD-81999] structure with Ni-substitution for tetrahedral Al3+ sites which is consistent with the EXAFS data. The FEFF calculations are in agreement with the Ba0.75Ni1.0Al11.0O19-δ XANES spectrum as well as the substitution for Al3+ tetrahedral sites from the EXAFS results.

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Figure 7. XANES analysis of the NiO standard, the Ba0.75Ni1.0Al1O19-δ hexaaluminate catalyst, as synthesized, and a FEFF calculation of Ni-substitution into the hexaaluminate reference compound Ba2.333Al2.333O34.333 [ICSD-81999].

4. Conclusions The crystalline structure of five hexaaluminate catalysts is studied by EXAFS, XANES, XPS, XRD, and TPR. XRD indicates that all five samples exhibit the β-alumina phase consistent with Ni-substitution into the crystalline lattice of the Ba0.75Al11O17.25 compound. As Ni substitution into the lattice increases, the unit cell c axis dimension decreases. This systematic change is direct evidence that Ni2+ is substituting for Al3+ in the hexaaluminate crystal structure. XPS analysis indicates that Ni is present in the spinel phase which is consistent with the hexaaluminate structure that is comprised of alternative stacks of spinel blocks. As Ni loading in the structure is increased, the surface Al/Ni ratio is shown to approach that observed in the bulk hexaaluminate structure. The influence that the mirror cation has on the surface Al/Ni ratio is consistent with Ni substitution in a region near the mirror plane and cation. TPR experiments suggest that the substitution of Ni into the hexaaluminate lattice stabilizes the Ni-O bond toward reduction. Consistent with the TPR data, XANES analysis of the Ba0.75Ni1.0Al11.0O19-δ indicates that Ni-O bonding is stronger for Ni substituted into the hexaaluminate lattice, relative to that of NiO. The average coordination numbers obtained from EXAFS analysis are consistent with Ni preferentially exchanging with tetrahedrally coordinated Al3+ in the structure. Tetrahedral Al3+ predominates in regions of the hexaaluminate unit cell structure near the mirror plane where the preferential substitution of Ni at these sites likely occurs to minimize strain in the crystalline lattice. The similarity of the EXAFS spectra for the different Ni substitution levels, within the hexaaluminate catalyst, indicates the presence of a single Ni-containing phase which is confirmed by principal component analysis. Acknowledgment. XRD measurements were conducted at beamline X7B of the National Synchrotron Light Source at Brookhaven National Laboratory, Upton, NY. EXAFS data was collected at the DCM beamline, J. Bennett Johnston, Sr., Center for Advanced Microstructures and Devices at Louisiana State University. Mr. James Poston contributed invaluable assistance to this investigation. Funding assistance was provided through the NETL-URI program. References and Notes (1) Gardner, T. H.; Shekhawat, D.; Berry, D. A.; Smith, M. W.; Salazar, M.; Kugler, E. L. Appl. Catal., A 2007, 323, 1.

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JP9117634

Structural Characterization of Ni-Substituted Hexaaluminate Catalysts

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