Identification of Radiolytically-Active Thermal Transition Phases in Boehmite

Article abstract of DOI:10.1016/j.tca.2020.178611

The stability of aluminum oxyhydroxide (AlOOH, boehmite) to radiolysis and dehydration to alumina (γ-Al₂O₃) under vacuum was investigated using TGA followed by detailed, structural analysis with Raman, powder X-Ray Diffraction (pXRD)

Full text of DOI:10.1016/j.tca.2020.178611

ThermochimicaActa689(2020)178611
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Identification of Radiolytically-Active Thermal Transition Phases in
Boehmite
Patricia L. Huestisa,b, Trent R. Grahamc, Sebastian T. Mergelsbergc, Jay A. LaVernea,b,
*
a Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556, USA
b Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA
c Pacific Northwest National Laboratory, Richland, WA 99354, USA
A R T I C L E I N F O
A B S T R A C T
Keywords:
Boehmite
The stability of aluminum oxyhydroxide (AlOOH, boehmite) to radiolysis and dehydration to alumina (γ-Al2O3)
under vacuum was investigated using TGA followed by detailed, structural analysis with Raman, powder X-Ray
Diffraction (pXRD), high energy X-Ray Diffraction (heXRD), 27Al Magic Angle Spinning Nuclear Magnetic
Resonance (27Al MAS NMR) spectroscopy, Scanning Electron Microscopy (SEM), and nitrogen adsorption. Slight
structural deviations from annealing temperatures prior to phase transformation are evident using these tech-
niques. Radiolytic characterizations by room temperature Electron Paramagnetic Resonance (EPR) spectroscopy
showed that while boehmite preheated to 300 °C under vacuum remained crystalline boehmite as determined by
bulk structural characterizations, this phase produced the largest amount of EPR-detectable defects. A more
dramatic mass loss of 16% was achieved by preheating boehmite to 400 °C under vacuum that resulted in a phase
similar to γ-Al2O3, but enriched with residual OH groups. Complete dehydration to γ-Al2O3 was accomplished by
preheating to 550 °C, which showed no indication of residual OH groups. These results demonstrate that the
residual OH groups contribute to the hydrogen gas evolved in the radiolysis of boehmite.
Dehydration
Radiolysis
1. Introduction
dome space of the Hanford tanks [13–16]. Hydrogen gas is particularly
problematic for both interim and long term storage materials [17,18].
Transition alumina phases are widely used for various industrial
applications. In particular γ-Al2O3, which is formed by calcining boeh-
mite (aluminum oxyhydroxide, γ-AlOOH), is important for catalysis
[1–7]. Boehmite itself has several other industrial applications such as
use in ceramics and fire-retardants [8,9]. Importantly, boehmite is also
found in significant quantities within the Hanford waste tanks located in
Washington state [10]. These tanks contain Cold War-era legacy waste
whose complex thermal history includes temperatures in excess of 150 °C
with isolated readings reaching up to 350 °C [11]. Ongoing waste pro-
cessing steps involve subjecting boehmite to further, more extensive
heating. For example, during vitrification, waste streams containing
boehmite as well as radioactive products will be heated to extreme
temperatures to mix with molten borosilicate glass [10,12]. This glass
will be used as the main containment for permanent storage of the
Hanford waste. An understanding of how thermally degraded boehmite
behaves in a radiation field is important for its successful processing both
for a current and future understanding of its behavior.
Previously studies of pristine boehmite, partially dehydrated boehmite,
and alumina found that radiolysis generates interlayer H atoms that lead
to the production of H2 in the presence of adsorbed H2O films with strong
dependence on the thermal treatment [19]. Given this dependence of the
H2 gas evolution on boehmite dehydration, additional characterization is
performed here to detail structural transitions. This work utilizes Raman,
powder X-Ray Diffraction (pXRD), high energy X-Ray Diffraction (heXRD),
27Al Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR)
spectroscopy, and Electron Paramagnetic Resonance (EPR) spectroscopy
to acquire an improved understanding of these structural differences
arising from the dehydration of boehmite. Ultimately, this work provides
insight into H2 gas evolution and concurrent structural transformations in
boehmite in extreme environments such as stored radioactive wastes.
2. Materials and methods
2.1. Materials
Radiolysis mediated hydrogen gas (H2) generation is of general interest
to better understand the origin of significant quantities of it present in the
Boehmite powders were obtained from Nabeltec (APYRAL AOH 60).
⁎ Corresponding author at: Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556, USA
E-mail address: laverne.1@nd.edu (J.A. LaVerne).
https://doi.org/10.1016/j.tca.2020.178611
Received 4 March 2020; Received in revised form 6 April 2020; Accepted 8 April 2020
Availableonline18April2020
0040-6031/©2020ElsevierB.V.Allrightsreserved.

P.L. Huestis, et al.
ThermochimicaActa689(2020)178611
The same batch (Lot 121214) was used throughout all of the experi-
ments to ensure reproducibility of results. Prior to analysis, a known
mass of AlOOH was heated overnight under rough vacuum to one of the
following temperatures: 100, 200, 300, 400, or 550 °C. All heated
samples were cooled under vacuum, reweighed, and then placed into a
desiccating cabinet containing a slurry of Mg(NO3)2 to fix the relative
humidity of air at ambient temperature at 53%. Samples remained in
the cabinet until the materials were no longer adsorbing water, which
was determined gravimetrically.
loaded into commercial 2.5 mm Bruker rotors, equipped with Vespel
drive and bottom caps, in an N2-filled glove box. Single pulse, direct
excitation, 27Al MAS NMR spectra were acquired with 8192 transients
and an acquisition time of 9.8 ms. A delay between transients of 0.375 s
was used for all samples, which was found to be sufficient for essen-
tially full relaxation of all samples. Acquisition utilized a single, non-
selective, π/20 pulse corresponding to a duration 0.450 μs. The pulse
width corresponding to a π/20 excitation pulse was determined with a
pulse width nutation experiment for a sample of 1 M Al3+(H2O)6 pre-
pared by dissolution of aluminum chloride hexahydrate
(AlCl3·6H2O, > 99%, Sigma-Aldrich) in H2O (18 MΩ cm). Chemical
shifts were also referenced to this 1 M Al3+(H2O)6 solution in H2O in
which the hexaaqua aluminum ion resonance was set to 0 ppm. Spectra
were primarily collected with a MAS spin rate of 32 kHz, although
additional acquisitions consisting of only 512 transients were collected
with MAS spin rates of 0, 8 and 20 kHz with otherwise identical in-
strument parameters. MAS NMR spectra were processed in Mestrenova
(version 14.01-23559, released 2019-06-07, Mestrelab Research S.L.).
The free induction decay was zero-filled to 52.5 ms and 100 Hz line
broadening was applied.
2.2. Nitrogen Adsorption Analysis
Specific surface area and porosity was determined using
a
Quantachrome Autosorb-1 with nitrogen as the adsorbate. Prior to
analysis, material was baked at 100 °C overnight on the degassing sta-
tion of the instrument to remove excess surface adsorbed species.
Surface area was determined using the Brunauer-Emmett-Teller (BET)
methodology and porosity distributions using Barrett-Joyner-Halenda
(BJH) analysis.
2.3. Scanning Electron Microscopy (SEM)
2.8. High-energy X-Ray Diffraction (heXRD)
SEM to visualize the surface damage was performed using an FEI-
Magellan 400 FESEM. Samples were deposited onto an aluminum SEM
stub using a conductive carbon tab. To mitigate charging, samples were
then coated with a thin layer of iridium. An accelerating voltage of 5 kV
was used to achieve magnifications of up to 1,200x.
HeXRD was completed to obtain high-resolution patterns that could
be refined. Sample powders were loaded into 2 mm ID polyimide ca-
pillaries for heXRD at the Advanced Photon Source (APS), beamline 11-
ID-B [20]. The sample-to-detector distance and the detector non-or-
thogonality was calibrated using diluted CeO2 standard (NIST diffrac-
tion standard set 674a). The scattered radiation (energy of ∼58.6 keV,
λ =0.2114 Å) was measured in transmission mode using an amor-
phous-Si detector system manufactured by Perkin Elmer TM
(2048 × 2048 pixels, 200 × 200 μm2 pixel size). The program GSAS-II
was used to sum 2D scans and perform integration to 1D profiles using a
polarization correction and for all Rietveld refinements [21]. Direct
subtraction of the Kapton® background was accomplished by the in-
dependent measurement of an empty polyimide capillary. The CeO2
standard was used to calculate instrument peak broadening for the re-
levant q range (0.55 to 10.3 Å-1).
2.4. Thermogravimetric Analysis and Differential Scanning Calorimetry
(TGA/DSC)
Thermal gravimetric analysis (TGA/DSC) was used to establish
transition temperatures for the specific boehmite sample and was per-
formed using a Mettler Toledo TGA/DSC-1 with a heating rate of 10 °C/
minute. The aluminum crucible used for the experiment was first run
without boehmite to obtain a background spectrum that was subtracted
out using the included STARe software.
2.5. Powder X-Ray Diffraction (pXRD)
2.9. Gamma Irradiations
PXRD patterns to identify crystalline structure were obtained on a
Philips X’pert Multi-Purpose diffractometer (PANAlytical Almelo, The
Netherlands), using a fixed Cu anode operating at 40 mA and 50 kV.
Acquisition was between 2θ angles of 5 and 79.99° with a step size of
0.020° and a time per step of 1.00 second, which corresponds with a
scan speed of 0.02 °/second and 3750 steps.
After samples were saturated with adsorbed water, samples were
then transferred to 3.4 mm ID x 5.0 mm OD fused silica (Suprasil) tubes
and were evacuated using the freeze-pump-thaw method. Evacuated
tubes were flame-sealed and irradiated using gamma rays from a con-
tained Shepard 60Co source located at the University of Notre Dame
Radiation Laboratory with a dose rate in January 2019 of 88.4 Gy/min
as determined by Fricke dosimetry.
2.6. Raman Spectroscopy
2.10. Electron Paramagnetic Resonance (EPR) Spectroscopy
Raman measurements were taken with
a Jasco Micro-Raman
Spectrometer MRS-5100 using a 532 nm laser that had a power output
of approximately 4-5 mW. The instrument was configured to have a
resolution of 1.8 cm-1 and the estimated sampling depth was about
1 μm.
EPR measurements to determine paramagnetic species were com-
pleted using a Bruker EMXplus spectrometer. The analysis area was
partially shielded with lead during irradiation to prevent irradiation of
the tube in the analysis area that could potentially overwhelm the
signal seen in the sample. Measurements were taken with a frequency of
about 9.84 GHz while the magnetic field was swept. Fitting was ac-
complished using EasySpin [22].
2.7. Al Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR)
Spectroscopy
27Al MAS NMR spectroscopy was utilized to study the changes in
aluminum coordination brought about by preheating boehmite. The
measurements were performed on a Bruker NMR spectrometer at a field
strength of 14.0954 T corresponding to a 156.375 MHz 27Al Larmor
frequency. Spectra were acquired with a MASDVT600W2 BL2.5 X/Y/H
probe. To limit adsorption of adventitious H2O, analyzed powders were
placed in a vacuum oven (ADP-300C, Yamato Scientific) operating at
3. Results and discussion
3.1. Bulk Structural Analysis of Thermal Transition Phases
Solid-state characterization of the dehydrated boehmite phases was
conducted to explain H2 gas evolution following radiolysis [19]. The
phases studied were initially identified by analysis of TGA/DSC, which
80
2 °C at a gauge pressure of −100 kPa for 12 hours and then were
2

P.L. Huestis, et al.
ThermochimicaActa689(2020)178611
Table 1
Summary of mass and specific surface area (SSA) changes due to heating boehmite.
Annealing Temperature (°C)
Water lost (% of initial mass)
Water regained (% of initial mass)
Estimated material OH groups remaining (%)
BET calculated SSA (m2/g)
100
200
300
400
550
2
1.7
1.7
0.5
4
100
100
89
6.5
2
6.5
4
6.9
16
17.5
11
17.9
23.0
1.2
0
is shown in the Supporting Information in Fig. S1. From the data, the
temperatures of 25, 100, 200, 300, 400, and 550 °C were selected to
monitor the dehydration of boehmite under vacuum.
Gravimetric analysis of dehydration is shown in Table 1. Previous
experiments utilizing radiolytic H2 production to probe the efficiency of
energy transfer within the material suggested a minor transition oc-
curring in the phase obtained by preheating boehmite to 300 °C under
vacuum [19]. The sample preheated to 300 °C lost approximately 3% of
the total initial mass upon heating and readsorbed only 15% of that lost
mass when rehydrated in the 53% humidity chamber. The small loss of
structural water, likely from the loss of hydroxyl groups, suggests that
the sample preheated to 300 °C corresponds to the first endotherm [23].
Given the irreversibility of the mass loss, this shallow endotherm must
not be due to just a loss of strongly bound water as has been suggested
previously [24]. Furthermore, Density Functional Theory (DFT) calcu-
lations indicate that dissociatively bound surface water can be more
stable to high temperatures than bulk structural OH groups [25]. The
shift in temperature between the endothermic peak and this phase is
likely due to the difference in pressure between the TGA/DSC mea-
surement and the preheating done to form the 300 °C sample. Notably,
these endothermic peaks of boehmite are shifted to lower temperatures
by heating under vacuum [3]. The lack of perfect mass recovery fol-
lowing rehydration indicates that heating must decompose the boeh-
mite lattice, and this process was characterized with a combination of
diffraction and spectroscopy.
Raman spectroscopy and XRD were used to determine phase purity
and ascertain structural changes brought about by heat. Raman and
heXRD patterns for the heated samples are shown in Figs. 1 and 2,
respectively, and the pXRD patterns can be found in Fig. S2. Raman,
heXRD, and pXRD are considered bulk techniques for these materials
due to the size of the primary particles. The initial Raman spectrum
shows peaks at 234, 343, 362, 452, 498, 676, and 735 cm-1, which is in
good agreement with previous measurements [26–30]. The Raman
spectra are see very little structural changes until 400 °C, at which point
the material loses all boehmite characteristic peaks. In fact, the Raman
spectra in samples heated over 400 °C no longer show any peaks which
could be due to the formation of γ-alumina [31].
Fig. 1. Top: Raman spectra for the preheated samples. Bottom: Spectra for the
Focusing on the most intense Raman peak shown in Fig. 1B, it is
clear that the sample preheated to 300 °C shows a significantly nar-
rower band than the samples preheated to lower temperatures. In fact,
three Raman bands display smaller FWHMs for the sample preheated to
300 °C. All of the narrower bands (362, 498 and 676 cm-1) belong to the
Ag symmetry group of the Al-O vibration [27] in which all aluminum
and oxygen atoms move parallel to the b axis [30]. The widths of the
bands are 15-20% narrower for the sample preheated to 300 °C than for
samples preheated to lower temperatures. In addition to the Ag bands
narrowing, the most intense band becomes more symmetric after pre-
heating to 300 °C. This loss of asymmetry was noted by Doss and Zallen
during sol aging of boehmite hydrogels, though the effect was shown to
be due to an increase in crystallite size [30]. However, an increase in
crystallite size is unlikely following heating the material dry to rela-
tively low temperatures, as was done for this experiment [4]. Instead,
the loss of asymmetry in the 362 cm-1 band as well as the decrease in
FWHM for all of the Ag bands is attributed to a decrease in the electron-
phonon coupling. Electron-phonon coupling is strongest for Ag modes
because those modes retain the symmetry of the lattice [32]. The bands
most intense Raman peak at different temperatures.
belonging to other symmetry modes are therefore not as affected by the
decrease in electron-phonon coupling and do not experience an ob-
servable change in their FWHM.
Further analysis of the Raman spectra for samples preheated to
400 °C and 550 °C is intractable as there are no peaks for which to
compare. However, there are subtle differences between the two phases
that can be seen in the Rietveld refinement of the heXRD data.
Fig. 2 contains heXRD patterns where the diffractograms obtained
from samples heated below 300 °C are in excellent agreement with
boehmite [33,34] and the diffractograms of samples heated to at least
400 °C are consistent with γ-alumina [35,36]. Rietveld refinement of
high-energy X-ray diffraction (heXRD) patterns further resolved struc-
tural differences between samples (Table 2). Using previously published
structures of boehmite [37] and cubic γ-Al2O3 [38], the heXRD patterns
were fit by refining the unit cell, thermal parameters, preferred sample
orientation, particle size (only for cubic γ-alumina), and partial
3

P.L. Huestis, et al.
ThermochimicaActa689(2020)178611
exhibit discrete parameters. However, aluminum (oxy)hydroxides ex-
hibit a slight distribution of site parameters that often appear as quasi-
Lorentzian lineshapes [40]. In severely amorphous materials, a dis-
tribution of site parameters (CQ, η, and chemical shift) is used to de-
scribe the asymmetric lineshapes in 27Al NMR spectra [41]. Analysis of
the spectra in this work primarily focused on the transition between
octahedral coordinated aluminum in boehmite to tetrahedral co-
ordinated aluminum in the thermally treated samples. The collected
spectra are shown in Fig. 3(A) at a spinning rate of 32 kHz and addi-
tional results of select samples are shown at slower spinning rates in
Fig. S3.
The chemical shift of 27Al NMR resonances is dependent on the
coordination environment about the aluminum nucleus [42]. The 27Al
MAS-NMR spectrum of the aluminum of the as-synthesized boehmite
indicates that the overwhelming occupancy of aluminum is octahed-
rally coordinated, as expected based on the crystal structure [41]. The
octahedral coordinate aluminum resonates at ca. 9 ppm. However,
thermally treated samples exhibit a greatly increased amount of tetra-
hedral coordinated aluminum. The relative site occupancy is reported
in Fig. 3(B). Specifically, annealing at 300 °C in a vacuum oven perturbs
the coordination of Al such that 0.3% tetrahedral aluminum exhibits
tetrahedral coordination resonating at ca. 69 ppm. The generation of
the tetrahedral coordinate aluminum may relate to the subtle changes
in powder heXRD.
Fig. 2. HeXRD patterns collected with λ =0.2114 Å for the preheated samples.
occupancy of Al sites (only for cubic γ-Al2O3). Domain sizes for boeh-
mite were not refined because large particle sizes and some preferred
alignment drive the Rietveld model to overestimated unit cell para-
meters and underestimated particle sizes. Results show that samples
preheated to 25 and 100 °C are very similar, with a slight increase in
unit cell volume refined for the sample preheated to 200 °C. The 300 °C
sample shows significant shortening of the a cell parameter and an
increase in the fit residual, Rwp. This change is primarily due to a de-
crease in intensity of the (031) peak and a marked increase in the peak
intensity for the (120) peak with few other differences, indicating a
distinct difference in domain shapes rather than additional sample co-
alignment (Fig. S4).
In agreement with both Raman spectra and heXRD diffractograms,
the phase transformation of boehmite to γ-Al2O3 occurs following
heating under vacuum at 400 °C which produces clear changes in the
27Al MAS-NMR spectra. The spectra indicated that approximately 34%
of the Al in γ-Al2O3 exhibits tetrahedral coordination, in agreement
with the expected coordination distribution in γ-Al2O3 [41], often
produced from boehmite calcination [43,44].
There is no clear observation of pentahedral coordinated aluminum
resonance in this study, which would exhibit a peak ca. 35 ppm [41].
Others have well-demonstrated using dynamic nuclear polarization
based NMR techniques that penta-coordinate species are restricted to
the surface of γ-Al2O3 [45]. Related to the restricted location of these
species to surface sites, extensive homogenization by grinding (which
was not performed here) was shown to increase the abundance of
pentahedral resonances in thermally treated gibbsite [46] and γ-Al2O3
[47]. Other observations of pentahedral coordinated aluminum oc-
curred following thermal treatment of boehmite produced from an or-
ganic aluminum precursor that formed nanoparticles with smaller
crystal domain sizes [2]. Given the dependence of these pentahedral
coordinated Al species on the size of the boehmite and γ-Al2O3, SEM
was performed to ascertain the particle size.
Measurements of samples preheated to 400 and 550 °C reveal 100%
conversion from boehmite to cubic γ-Al2O3 with only minor crystal-
lographic differences between the two. The sample at 550 °C is slightly
more crystalline with a larger domain size. Al site occupancies are
different between the two samples, as well, indicating that heating to
550 °C produced a more ordered structure. The partial occupancy of
oxygen sites was not refined and assumed to be 1. Larger residuals for
these samples (10.85% and 11.75%, see Fig. S5) are largely due to a co-
alignment of domains within particles, size/strain effects, and disorders
that are not captured by the refinement model [38]. Strain is likely due
to residual OH groups still present in the structure for the 400 °C sample
[1,39].
The first isotherm of boehmite dehydration occurring at ca. 385 °C
was further investigated with 27Al MAS-NMR. 27Al is a high abundance
(100%), spin 5/2 nucleus which often leads to non-Lorentzian line-
shapes in solids. The lineshapes are described with the quadrupolar
coupling parameter (CQ), isotropic chemical shift (δ), and the asym-
metry parameter (η). In crystalline materials, the aluminum sites
SEM images and corresponding nitrogen adsorption analyses taken
of the heated samples are shown in Fig. 4. For the as received sample,
the SEM micrographs revealed that the particles were roughly rhom-
bohedral in shape with a (010) face size between 500 nm and 1 μm and
a thickness of approximately 200 nm. Nitrogen adsorption isotherms
Table 2
Rietveld fit for all six samples, refining unit cell parameters only for boehmite powders and unit cell parameters plus atom fractions for cubic γ-alumina phases.
Annealing Temperature (°C)
25
100
200
300
400
550
Phase
boehmite
3.694
12.22
2.859
129.1
–*
boehmite
3.694
12.21
2.861
129
boehmite
3.696
12.22
2.862
129.3
–*
boehmite
3.687
12.23
2.861
129
cubic γ-alumina
cubic γ-alumina
a (Å)
7.907
7.906
b (Å)
7.907
7.906
c (Å)
7.907
7.906
Vol. (Å3)
494.4
494.2
Crystal Domain Size (nm)
Al partial occupancy
Rwp (%)
–*
–*
8
9
–
–
–
–
0.609, 0.502, 0.135, 0.045
0.747, 0.503, 0.137, 0.038
5.937
3.64
5.865
3.46
6.046
3.33
6.429
2.97
10.82
11.75
8.46
RF (%)
16.92
χ2
1.112
0.00146
1.333
0.00175
1.238
0.00163
1.352
0.00179
1.596
1.693
0.00224
Reduced χ2
0.00219
Note: (*) Due to the large primary particle size of boehmite samples which lead to some preferred alignment, the unit cell was refined but not the crystal domain size.
4

P.L. Huestis, et al.
ThermochimicaActa689(2020)178611
Fig. 3. (A) 27Al MAS-NMR at 14.1 T at a spinning rate of 32 kHz. (B) The tetrahedral (○) and octahedral (⬤) site occupancy as a function of annealing temperature.
revealed no major porosity, and the BET calculated specific surface area
was 6.55 m2/g. The overall particle size does not appear to change even
after heating to 550 °C. It should be noted that the refined domain size
from the heXRD data is significantly smaller than the observed particle
sizes shown by the SEM micrographs; however, these refined values are
well within size ranges previously reported for γ-Al2O3 [48].
as the O- radical. The narrowest component has a g-factor of 2.0006 and
a peak-to-peak linewidth of 0.18 m T and is likely a trapped electron in
Suprasil. The two remaining components have g-factors of 2.001 and
2.0115 and linewidths of 1.1 and 0.32 m T, respectively. The first
component is also present in the unheated sample. In contrast, the
second component did not appear in the unheated sample and is
therefore due to emergence of a new defect. A defect with a similar g-
factor but larger peak-to-peak linewidth has been seen in boehmite with
smaller particle sizes and was assigned to an O3- radical [52]. However,
this assignment necessities production of dissolved O2 within the ma-
terial upon irradiation to create the O3- radical, and no appreciable
amount of O2 has been measured from the radiolysis of this material
[19]. It is therefore unlikely that the defect with a g-factor of ∼2.012
belongs to the O3- radical.
For the sample preheated to 400 °C, a small amount of pitting can be
seen on the surface. Surface analysis using nitrogen adsorption shows a
slight deviation between the adsorption and desorption isotherms, in-
dicative of the formation of pores. The sample heated to 550 °C shows a
significant amount of surface pitting and roughening. This sample also
exhibits significant hysteresis in the adsorption and desorption iso-
therms, confirming the formation of pores visible in SEM micrographs.
Using the desorption data points to calculate the BJH pore size dis-
tribution as is shown in Fig. S6, it is clear that the sample preheated to
550 °C has significantly more pores than the sample preheated to
400 °C. There is also a slight increase in the average pore size calculated
from the BJH pore size distributions, though this number is likely in-
fluenced by the small number of points available to calculate the
number. The formation of pores upon heating is expected due to the
large size of the starting material [3,49,50].
Three major components make up the central EPR spectra for the
sample preheated to 400 °C (Fig. 6(C)). A broad component with a g-
factor of 2.016 and a peak-to-peak linewidth of 4.6 m T is assigned to
the O- radical and the narrow component with a g-factor of 2.0007 and
a linewidth of 0.24 m T is assigned to trapped electrons in Suprasil. The
last component has a g-factor of 2.0116 and a linewidth of 0.35 m T.
This component was also measured in the sample preheated to 300 °C.
In addition, there are several smaller defects within the sample that
were not fit with the EasySpin software. These defects show no reg-
ularity which suggests they are not the result of hyperfine interactions,
so they were considered to be minor components and left out of the
analysis.
3.2. Radiolytic Defects of Thermal Transition Phases
EPR spectra of the central peaks for the preheated materials irra-
diated to 1 kGy are shown in Fig. 5 and summarized in Table 3. As
shown in a previous publication[19], all samples preheated up to 400 °C
sample show a doublet separated by approximately 50 m T, which is
due to the presence of trapped hydrogen radicals within the bulk of the
material. The central spectrum of the unheated boehmite sample
(Fig. 6(A)) shows contributions from three main components: a broad
component and two narrower components. The broad component has a
g-factor of 2.0145 and a peak-to-peak linewidth (ΔHpp) of 4.6 m T.
These values identify the defect as an O- which has been seen in various
aluminum oxide compounds [51–54]. The two narrower components
overlap with similar g-factors. One component has a g-factor of 2.0006
and a linewidth of 0.25 m T, and the other has a g-factor of 2.0009 and
a linewidth of 1.2 m T. The narrower component was observed in ir-
radiated Suprasil quartz and is due to electrons trapped in an oxygen
vacancy [55].
The central EPR signal for the sample preheated to 550 °C
(Fig. 6(D)) shows only 2 components. The broad component has a g-
factor of 2.013 and a peak-to-peak linewidth of 4.2 m T which is at-
tributed to the O- defect. Another narrower component with a g-factor
of 2.0007 and a peak-to-peak linewidth of 0.17 m T is due to trapped
electrons in Suprasil.
The first unassigned defect is found in both the unheated material as
well as the material preheated to 300 °C. The defect has a g-factor si-
milar to the trapped electron defect in Suprasil, but the unassigned
defect is significantly broader. The g-factor is smaller than that of a free
electron which classically identifies it as an electron center [52,56]. The
broadness of the defect could be explained by unresolved hyperfine
interactions that should be present for electrons close to aluminum
atoms. Interestingly, the defect does not appear in the samples pre-
heated to 400 and 550 °C, so the electron defect must be due to the
unique geometry of the unheated or mildly heated boehmite structure.
The second unassigned defect is only found in the samples pre-
heated to 300 and 400 °C. The g-factor is greater than that of a free
The central spectrum for the boehmite sample preheated to 300 °C
(Fig. 6(B)) is composed of four components: one broad component and
three significantly narrower components. The broad component has a g-
factor of 2.016 and a peak-to-peak linewidth of 4.6 m T that identifies it
5

P.L. Huestis, et al.
ThermochimicaActa689(2020)178611
Fig. 4. SEM images and nitrogen adsorption/desorption isotherms for Top: boehmite with no preheating. Middle: boehmite preheated to 400 °C. Bottom: boehmite
preheated to 550 °C. The middle figures show an enlarged version of the black box in the left figures and are provided for visual clarity.
electron, suggesting that the defect belongs to a trapped hole. The most
likely location of the trapped hole, based on the chemical composition,
is around an oxygen atom. The appearance of this defect corresponds to
the appearance of tetrahedrally coordinated aluminum. This result
suggests that the defect is a hole trapped on an oxygen atom bound to a
tetrahedrally coordinated aluminum atom. By 550 °C, the broad O-
signal has shifted to a lower g-factor and is likely overlapping with the
new defect.
than the unheated sample, as evidenced by a slight increase in the oc-
tahedral resonance linewidth in 27Al MAS-NMR (see Table S1) as well
as EPR results. Since this particular phase also produces the largest
amount of trapped hydrogen radicals, it is likely that the hydrogen
atom trap probed by room temperature EPR in boehmite is the result of
a defect in the crystal structure. Additionally, the structure was found to
produce more hydrogen gas during irradiation than the samples pre-
heated to lower temperatures [19]. Since a decrease in a solid state
Raman peak indicates a longer phonon lifetime, the increase in radi-
olytic activity seen for the sample preheated to 300 °C could be ex-
plained by the decrease in electron-phonon coupling leading to a longer
exciton migration within the material[58].
The removal of a small amount of structural water, represented by
the boehmite sample preheated to 300 °C, corresponds to the formation
of a new defect. It is likely that the new defect site is the result of an
oxygen vacancy created by the removal of hydroxyls during dehydra-
tion. Previous results also show that the sample preheated to 300 °C
produced the largest amount of trapped hydrogen radicals upon irra-
diation [19]. In that paper, the nature of the hydrogen atom trap within
the boehmite structure was questioned. DFT calculations indicate that a
perfect boehmite structure only contains one trapping site for hydrogen
radicals [57]. The same calculations also indicate that the diffusion into
and out of those trapping sites should be facile, which was at odds with
the observation of hydrogen radicals months after irradiation. The
boehmite sample preheated to 300 °C has a more defective structure
The boehmite samples preheated to 400 °C and 550 °C show a dif-
ference in porosity and EPR defects as well as a significant difference in
radiation response. As shown previously [19], the structure resulting
from preheating to 400 °C produces nearly 20 times as much hydrogen
gas upon irradiation as the unheated or mildly heated samples, and over
6 times as much as the sample preheated to 550 °C. Additionally, the
sample preheated to 400 °C still shows trapped hydrogen radicals. As
hydrogen radicals require highly symmetric cages to be trapped [59],
the presence of the hydrogen EPR signal indicates that the sample
6

P.L. Huestis, et al.
ThermochimicaActa689(2020)178611
Fig. 5. EPR spectra for boehmite preheated to (A) 25 °C, (B) 300 °C, (C) 400 °C, and (D) 550 °C and irradiated to a dose of 1 kGy. The dotted black line is the
experimental spectrum and the solid magenta line is the fit obtained from EasySpin.
preheated to 400 °C still has some amount of structural water and
trapping cavities present. Because the material is large and pre-
ferentially forms channels upon dehydration [60], it is possible that the
sample preheated to 400 °C retains some of the aluminum-oxygen
backbone of the original boehmite structure. This backbone could be
the result of residual structural OH groups that remain in the lattice
until high annealing temperatures [1], which can act to “pin” small
domains to maintain a boehmite-like structure. This conclusion is
supported by the EPR results that show trapping cages as well as a
defect not found in alumina [52–54].
that, for the sample preheated to 400 °C, not all Al atoms are in their
final positions for γ-Al2O3 so the structure is not entirely crystalline. By
550 °C, the residual structure OH groups have been removed and thus
the radiolytic H2 yield becomes that of γ-Al2O3. The Al-O skeleton of γ-
Al2O3 is not as efficient at transporting energy to attached OH groups
[19], thus the yield is significantly lower.
4. Conclusions
Boehmite was preheated to several different temperatures driving
its dehydration pathway to γ-Al2O3 and the structural properties of the
resultant transition phases were explored. Material heated to the early
stages of dehydration (300 °C for this particular sample) resulted in a
phase that exhibited enhanced energy transport properties as evidenced
by Raman linewidths and confirmed by prior experiments monitoring
The Al-O skeleton of boehmite has been shown to be very efficient
at transferring energy to the interlayer OH groups [19,61], but H atoms
created during irradiation are too easily trapped by the material. Upon
heating, the hydroxyl layers are preferentially damaged and removed
[2], exposing the Al-O skeleton. Rietveld refinement calculations reveal
Table 3
Summary of key results obtained from the radiolysis of thermal transition phases of boehmite. †Values taken from [19].
Radiolytic Hydrogen Production†
Radiolytic EPR Defects
Annealing Temperature (°C)
25
G(H2) (molecules H2/100 eV)
0.021
H atom present in EPR?
Yes
g-factor
2.0145
2.0006
2.0009
2.016
ΔHpp (mT)
Tentative Assignment
O (octahedral)
4.6
0.25
1.2
Trapped e- (quartz)
Trapped e- (AlOOH)
O- (octahedral)
300
0.051
Yes
4.6
2.0006
2.001
0.18
1.1
Trapped e- (quartz)
Trapped e- (AlOOH)
O- (tetrahedral)
2.0115
2.016
0.32
4.6
400
550
0.38
Yes
No
O- (octahedral)
2.0007
2.0116
2.013
0.24
0.35
4.2
Trapped e- (quartz)
O- (tetrahedral)
0.058
O- (both)
2.0007
0.17
Trapped e- (quartz)
7

P.L. Huestis, et al.
ThermochimicaActa689(2020)178611
the radiation response [19]. Techniques including pXRD, heXRD, and
27Al MAS-NMR indicate a small loss of structural water leading to that
particular phase. Material heated to just below the complete dehydra-
tion endotherm (400 °C) resulted in a phase that is almost structurally
indistinguishable from fully transformed γ-Al2O3 as seen in Raman and
pXRD; small differences were evident in the porosity, the Rietveld re-
finement parameters, the coordination of 27Al, and the EPR-detectable
defects. This particular phase has an incredible affinity to adsorb water
as well as transfer energy deposited in the material phase to the ad-
sorbed water where it can produce H2 gas. The fully dehydrated phase
(created by heating boehmite to 550 °C) is highly porous and behaves
exactly as expected for γ-Al2O3 using the techniques employed by this
study. The radiolytic H2 generation has therefore been shown to be
highly dependent on the structure of the transition phase, and even
subtle differences in structure can have drastic effects on the amount of
H2 produced.
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This work was supported as a part of IDREAM, an Energy Frontier
Research Center funded by the U.S. Department of Energy, Office of
Science. The authors acknowledge the Center for Sustainable Energy at
Notre Dame (cSEND) Materials Characterization Facilities for the use of
the Mettler Toledo TGA/DSC-1 and Jasco Micro-Raman Spectrometer
NRS-5100 as well as the Notre Dame Integrated Imaging Facility
(NDIFF) for the use of the Magellan 400 FESEM. NMR spectroscopy
utilized facilities at the Environmental Molecular Science Laboratory
(EMSL, grid.436923.9), a DOE Office of Science User Facility sponsored
by the Office of Biological and Environmental Research at Pacific
Northwest National Laboratory (PNNL). PNNL is a multiprogram na-
tional laboratory operated for DOE by Battelle Memorial Institute op-
erating under Contract No. DE-AC05-76RL0-1830. The 14.1 T NMR
spectrometer was acquired with support from Basic Energy Sciences
(BES)Chemical Sciences, Geosciences, & Biosciences (CSGB) Division.
This project used resources of the Advanced Photon Source, a U.S.
Department of Energy (DOE) Office of Science User Facility operated
for the DOE Office of Science by Argonne National Laboratory under
Contract [DE-AC02-06CH11357], where data acquisition was per-
formed at beam line 11-ID-B. The authors acknowledge Dr. Josiane
Kaddissy for help in setting up the EPR. The authors also thank
Professor Ian Carmichael for making available the facilities of the Notre
Dame Radiation Laboratory, which is supported by the Division of
Chemical Sciences, Geosciences and Biosciences, Basic Energy Sciences,
Office of Science, United States Department of Energy through grant
number DE-FC02-04ER15533. This contribution is NDRL-5274 from the
Notre Dame Radiation Laboratory.
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CRediT authorship contribution statement
[19] J.A. LaVerne, P.L. Huestis, H Atom Production and Reaction in the Gamma
Radiolysis of Thermally Modified Boehmite, J. Phys. Chem. C. 123 (2019)
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Patricia L. Huestis: Conceptualization, Investigation, Resources,
Formal analysis, Writing
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original draft. Trent R. Graham:
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Declaration of Competing Interest
[22] S. Stoll, A. Schweiger, EasySpin, a comprehensive software package for spectral
simulation and analysis in EPR, J. Magn. Reson. 178 (2006) 42–55, https://doi.org/
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The authors declare that they have no conflict of interest.
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