Turbostratic boron nitride coated on high-surface area metal oxide templates

Article abstract of DOI:10.1002/ejic.200700839

Boron nitride coatings on high-surface area MgAl₂O₄ and Al₂O₃ have been synthesized and characterized by transmission electron microscopy and by X-ray powder diffraction. The metal oxide templates were coated with boron nitride using a simple nitridation in a flow of ammonia starting from ammonium borate adsorbed on MgAl₂O ₄ or γ-Al₂O₃. This procedure resulted in the formation of a turbostratic boron nitride film with a thickness of a few individual BN layers. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700839

SHORT COMMUNICATION
DOI: 10.1002/ejic.200700839
Turbostratic Boron Nitride Coated on High-Surface Area Metal Oxide
Templates
Søren K. Klitgaard,[a] Kresten Egeblad,[a] Michael Brorson,[b] Konrad Herbst,[b] and
Claus H. Christensen*[a]
Keywords: Boron nitride / Turbostratic / Metal oxide templates / Catalyst support material / Nanostructure
Boron nitride coatings on high-surface area MgAl2O4 and
Al2O3 have been synthesized and characterized by trans-
mission electron microscopy and by X-ray powder diffrac-
tion. The metal oxide templates were coated with boron ni-
tride using a simple nitridation in a flow of ammonia starting
from ammonium borate adsorbed on MgAl2O4 or γ-Al2O3.
This procedure resulted in the formation of a turbostratic
boron nitride film with a thickness of a few individual BN
layers.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
that normally involves considerable technical difficult-
ies.[3,13] Very recently, the use of organic-inorganic block co-
polymers have been used to produce BN and BCN nano-
structured ceramics with very high-surface areas.[14]
Introduction
The importance of boron nitride (BN) as a ceramic mate-
rial, a lubricant, and for other industrial purposes is in-
creasing. Whereas boron nitride in many respects is similar
to carbon because of the isoelectronic relation, some impor-
tant differences between the properties of the two materials
remain. On one hand, boron nitride, like carbon, exists in
several allotropic forms, including a diamond-like cubic
structure and a graphite-like hexagonal layer structure.
Even boron nitride nanotubes[1] and an intriguing boron
nitride nanomesh[2] were recently reported. On the other
hand, white hexagonal BN (occasionally referred to as
“white graphite”) is a good insulator with a bandgap
around ca. 5.3 eV,[3] whereas black graphite is a semimetal-
lic conductor. Both materials are resistant towards most
reagents, but boron nitride is slightly less so due to its more
polarized bonds. However, boron nitride is superior to car-
bon when it comes to thermal stability, in air and also in
hydrogen. This has led to numerous studies on the use of
boron nitride as a non-oxide ceramic material.[4–6] Due to
the excellent stability of boron nitride at high temperatures
and under harsh chemical conditions it has found applica-
tion as a support material for heterogeneous catalysts.[7–12]
The growing interest in using boron nitride as a catalyst
support material, in particular for reactions where carbon
is not stable, has increased the need for simple ways of syn-
thesizing high-surface area boron nitride – an undertaking
Results and Discussion
Here we present a simple, versatile procedure for the for-
mation of high-surface area turbostratic boron nitride by
high-temperature nitridation of ammonium borate ad-
sorbed on high-surface area MgAl2O4 or γ-Al2O3 with am-
monia.
Previously, reports of BN thin films on different sub-
strates have appeared in the literature.[3,7,15,16–19] Paine et al.
showed that large Al2O3, TiO2 and MgO single crystals
could be covered by BN thin films using pyrolysis of a solid
polymer formed by cross-linking chloroborazenes with
hexamethyldisilazane.[4,16] Also, the preparation of BN
films on metallic surfaces obtained by thermal decomposi-
tion of borazine or related compounds have been re-
ported.[17,18]
In the present work, BN was synthesized by impregnat-
ing high-surface area MgAl2O4 and Al2O3 with aqueous
precursor solutions of ammonium borate, using readily
available boric acid as the source of boron, followed by high
temperature nitridation in a flow of ammonia.[15,20] One ad-
vantage of the present procedure is its simplicity and the
fact that it can be easily modified to control the extent of
BN coverage on the metal oxide. Al2O3 and MgAl2O4 was
chosen as the templates because they are considered com-
pletely inactive in redox processes and both known as inert
support materials from the literature.[21,22] Given the sta-
bility, a removal of the metal oxides has not been attempted
here even though new interesting structures could be envis-
[a] The Center for Sustainable and Green Chemistry, Department
of Chemistry, Building 206, Technical University of Denmark,
2800 Lyngby, Denmark
E-mail: chc@kemi.dtu.dk
[b] Haldor Topsøe A/S,
Nymøllevej 55, 2800 Lyngby, Denmark
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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C. H. Christensen et al.
SHORT COMMUNICATION
aged. Other templating approaches to produce high-surface 0.333 nm interlayer distance of bulk hexagonal BN.[3] The
area BN have been reported recently. For instance, a soft- composition of the BN film was qualitatively confirmed by
template approach involving surfactant assembly[23] and elemental analysis using energy-dispersive X-ray (EDX)
also a hard-template method involving the use of carbon as analysis. TEM analyses of the samples with higher BN con-
mesopore hard-template for making boron nitride and bo- tent, i.e. samples impregnated two or three times with pre-
ron carbon nitride, BCN, using B2O3 as precursor was re- cursor solution, showed BN coating of agglomerates of
ported.[19] These materials were prepared by reaction with metal oxide particles already coated with BN (see electronic
N2 which require very high reaction temperatures (1500– supporting information, Figure S1).
1750 °C) due to the thermodynamic stability of N2. By con-
ducting the nitridation with ammonia the reaction tempera-
ture can be lowered.
The boron content of all the samples was determined by
elemental analysis using Inductively Coupled Plasma (ICP)
analysis. Prior to ICP analysis, the boron nitride coated
samples were pretreated with aqua regia in a microwave
oven leading to decomposition of boron nitride and its dis-
solution as boric acid. As shown in Table 1, the measured
boron contents are in good agreement with those calcu-
lated. Table 1 also lists the surface areas measured at liquid-
nitrogen temperature with a Micromeritics ASAP 2020, and
calculated using the BET (Brunauer–Emmett–Teller)
method.[24] As seen in Table 1, the BET surface areas of all
the samples are moderate to high, though not as high as
Figure 1. TEM image of BN on MgAl2O4 impregnated once. A
thin film of BN encapsulates the MgAl2O4 particles. The film con-
the parent metal oxide templates. The surface area of the
boron nitride is very much dependent on the choice of
metal oxide. By choosing a high-surface area metal oxide it
will give a high-surface area boron nitride. Not surprisingly,
a further decrease in the surface area is observed with suc-
cessive BN coatings of the MgAl2O4 sample. This can be
interpreted as encapsulation of the individual metal oxide
crystals into larger agglomerates giving an overall smaller
surface area.
sists of 2–6 layers of BN.
Figure 2 shows a TEM image of BN-coated Al2O3 ob-
tained by impregnating once with the precursor solution.
Al2O3 is encapsulated by BN in a manner very similar to
that of MgAl2O4, as 2–6 BN layers are formed. Multiple
impregnations of Al2O3 are expected to show a similar be-
haviour as those of MgAl2O4.
Figure 1 shows a representative high-resolution trans-
mission electron microscopy (TEM) image of the prepared
BN-coated MgAl2O4, impregnated once with the precursor
solution. The images were obtained by means of a JEM
2000FX microscope operated at an accelerating voltage of
300 kV. For sample preparation, ground powders were dis-
persed in ethanol and deposited on a 300 mesh copper grid
coated with a holey carbon film. Figure 1 shows MgAl2O4
particles coated with a thin film of BN. The BN films,
which coat and apparently fully encapsulate the metal oxide
particles, have thicknesses between 0.5 and 3 nm. These
films consist of 2 to 6 individual BN sheets. Judged from
Figure 1, the thickness of a stack of 6 sheets is estimated to
be ca. 1.8 nm resulting in an interlayer distance between the
Figure 2. TEM image of BN on Al2O3. The Al2O3 particles are
sheets of ca. 0.3 nm. This distance compares well with the coated with a thin BN film.
Table 1. Boron contents and BET surface areas.
Support material
Number of impregnations
Boron content
measured [wt.-%]
Boron content
calculated [wt.-%]
BET
Surface area [m2/g]
MgAl2O4
MgAl2O4
MgAl2O4
MgAl2O4
0
1
2
3
0
0
72
39
27
14
4.67
7.77
10.70
4.49
8.13
11.16
Al2O3
Al2O3
0
1
0
4.30
0
4.49
228
175
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Eur. J. Inorg. Chem. 2007, 4873–4876

Turbostratic BN Coatings on Metal Oxide Templates
X-ray powder diffraction (XRPD) patterns of all samples determined. The interlayer spacings d(002), obtained from
were recorded after nitridation. Figure 3 shows the XRPD the exact position of the (002) peak maxima, are 0.344 nm
patterns of the MgAl2O4 sample coated (a) twice and (b) in Figure 3, (a) and 0.339 nm in Figure 3, (b). This means
three times with BN; the remaining XRPD patterns are that the BN film is, on average, 7 individual layers of BN
available as supplementary information (Figures S2–S3). In thick confirming the impression from the TEM image
the XRPD patterns of BN-coated MgAl2O4 and Al2O3 shown in Figure S1. The slightly higher value for the d spac-
samples that were impregnated once with precursor solu- ing in samples containing turbostratic BN compared to
tion, only the parent metal oxide templates are observed, well-ordered hexagonal BN is consistent with the layer dis-
whereas the BN films are X-ray amorphous. This is because order in these samples, and several subpolytypes of turbos-
the BN film is too thin to give rise to any diffraction show- tratic BN have been discussed in the literature.[27] On the
ing that all BN in the samples is present as film and that other hand, the two-dimensional (10) reflection can be used
no bulk BN is present. However, when increasing the BN to calculate the width of the crystallite layer. However, in
content by impregnating a second or third time, the BN the diffraction patterns of the BN-coated MgAl2O4 sam-
phase appears as broad diffraction peaks at 2θ ca. 26° and ples, the (10) reflection at 2θ ca. 43° is partially covered by
43° in the diffraction pattern of BN-coated MgAl2O4. These the (400) reflection of spinel, preventing an unambiguous
diffraction maxima can be assigned to a boron nitride phase determination of the line width of the (10) reflection.
which has previously been described in the literature as tur-
The presence of a turbostratic boron nitride phase im-
bostratic BN.[25] Turbostratic materials are characterized by plies the presence of boron oxynitride (BNxOy) and
having a structure of roughly parallel layers, these layers, amorphous boric oxide zones interconnecting the peripher-
however, showing random rotation and translation about ies of the hexagonal planar boron nitride networks.[27–29]
the layer normal.[26] In an X-ray diffraction pattern, turbos- On the other hand, no indication for the presence of crystal-
tratic BN can be indexed on the basis of the diffraction line boric oxide with diffraction peaks at 2θ = 13.58° and
maxima of well-defined hexagonal BN, although some re- 27.98° is seen in the diffraction patterns shown in Figure 3.
flections of hexagonal BN alter their shape and other reflec- A removal of the oxygen impurities, coinciding with the
tions are missing. The most characteristic changes caused growth and ordering of the BN layers to form crystalline
by the layer disorder in turbostratic relative to hexagonal BN, requires temperatures above 1350 °C.[29,30]
BN are seen in a broadening of the (002) reflection at 2θ =
The diffraction pattern of BN coated on alumina (Figure
26.4°, the appearance of a two-dimensional (10) reflection S3) shows a mixture of two alumina phases, γ-alumina and
at 2θ = 42.4° merged from the (100) and (101) reflections θ-alumina, the latter being obtained by thermal degrada-
and the disappearance of higher indexed reflections (102), tion of γ-alumina at temperatures above 800 °C. Remarka-
(004) and (103) above 2θ = 45°.[25] These features, although bly, no indication was found in the diffraction pattern for
partially covered by the diffraction maxima of the spinel the formation of the thermodynamically stable α-alumina
phase, are most clearly visible in the diffraction pattern during the nitridation process, despite the thermal treat-
shown in Figure 3, (b).
ment of the coated material at 1100 °C for 24 h.
Conclusions
In summary, composite materials of turbostratic boron
nitride coated in various concentrations on high-surface
area metal oxides, MgAl2O4 and Al2O3, were prepared by
nitridation of ammonium borate with ammonia. This pro-
vides a simple way for the synthesis of high-surface area
BN which could be an ideal support material for many
heterogeneous catalytic applications.
Experimental Section
Figure 3. XRPD patterns of the BN samples coated (a) twice and
(b) three times on MgAl2O4. The turbostratic BN phase appears at
2θ ca: 26° and 43°.
In a typical synthesis of BN, a precursor solution was prepared by
suspending H3BO3 (5.72 g, containing 1 g of boron) in H2O (7 mL)
whilst stirring, followed by addition of aqueous NH3 (3 mL, 24 wt.-
%) resulting in a clear solution. Aqueous NH3 was used to raise
the solubility of H3BO3. The BN precursor solution (0.5 mL) was
impregnated once onto high-surface area powders of MgAl2O4 or
γ-Al2O3 (1 g) to incipient wetness, giving boron loads of ca. 4.5
wt.-% in each of the final the samples after the formation of boron
nitride. Prior to nitridation, all samples were dried for at least
30 min at 100 °C. Nitridation was performed by passing a gentle
Both diffraction maxima of turbostratic boron nitride
can be used for the calculation of the particle size, although
different information is obtained from the three-dimen-
sional (002) reflection and the two-dimensional (10) reflec-
tion. The (002) reflection renders information about the
crystallite size along the c axis, i.e. the thickness Lc of the
crystallite. From the (002) reflections in Figure 3, (a) and
(b), Lc parameters of 2.5 and 2.2 nm, respectively, can be flow of ammonia over the samples at 1100 °C for 24 h. Attempts
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C. H. Christensen et al.
SHORT COMMUNICATION
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Received: August 10, 2007
Published Online: September 28, 2007
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