SURFACE AND INTERFACE ANALYSIS, VOL. 19, 139-144 (1992)
Adsorption of Hydrogen Fluoride on Alumina
R. G. Haverkamp,’ J. B. Metson,’ M. M. Hyland’ and B. J. Welch’
Department of Chemistry, University of Auckland, Private Bag, Auckland, New Zealand
* Department of Chemical Engineering, University of Auckland, Private Bag, Auckland, New Zealand
In the smelting of aluminium, HF fumes are produced that are subsequently trapped by absorption onto alumina.
The factors that affect the adsorption capacity of alumina have been studied previously and are weU established,
but the mechanism by which H F adsorbs onto the alumina surface is not well understood.
In this study, x-ray photoelectron spectroscopy (XPS)was used to investigate the nature of the surface adsorp
tion of HF on alumina. XPS is particularly weU suited to the study of this type of gas adsorption process.
Laboratory-prepared samples were studied with particular interest in evidence for AI-F bonding, or in fluoride
species formed by reaction with -OH or -0. Also of interest was the role of sodium, since it is segregated to the
surface of the alumina during calcination.
AI-F bonding was observed on only one sample type. An AI-F interaction was identified when the alumina had
been predried and dry HF was adsorbed. When moisture was present no AIF, formation was observed. This
suggests that under conventional conditions (i.e. moisture present) the adsorption of HF involves a weak interaction, probably hydrogen bonding, with intermediate layers of water. After heating the samples containing weakly
bound HF to 500 “C, no Al-F interaction was observed. Much of the HF was desorbed at 700 O C .
An Na-F interaction was observed in all fluoride-adsorbed samples; however, this can only account for a small
proportion of the total fluoride adsorbed.
INTRODUCTION
Aluminium-smelting cells produce emissions of fluoride
consisting of hydrogen fluoride and particulate sodium
aluminium fluorides. The hydrogen fluoride is formed
by the reaction of either water or hydrogen with cryolite. Particulate fluorides, volatile at cell temperatures,
are emitted and then condense in the off gas stream.
These emissions a can be captured by dry scrubbing
systems in which the reactive gases and particulates are
adsorbed onto alumina. The efficiency of the scrubber
will depend to a large extent on the fluoride adsorption
capacity of the alumina.
Cochran et al.’ studied the adsorption of H F on
alumina. They found that the chemisorbed fluoride is
initially ‘amorphous’ but forms AIF, on heating.
Cochran’ later postulated that the chemisorption of a
monolayer of H F on Al’O, takes place by the formation of F(O), tetrahedrons with H + as a central cation.
He suggested that at high temperatures this is converted
into AlF, . He also commented on the role of sodium in
the adsorption and concluded that there was insufficient
total sodium present to account for more than a small
proportion of the fluoride to be adsorbed by reaction
with sodium.
More recent studies on the mechanism of H F adsorption have generally concluded that water is involved in
the adsorption process.
Lamb3 found that multilayers of HF are adsorbed
and that water vapour has a large influence on the
amount of HF that can be held. Initial adsorption of
HF on alumina takes place by reaction with surface
sodium and with surface hydroxyl groups, with the
hydroxyl groups being substituted by fluoride. Further
adsorption then takes place by another mechanism to
0142-2421/92/240139-06 $08.00
0 1992 by John Wilcy & Sons,Ltd.
form multilayers of HF. He suggested that this could be
explained by hydrogen bonding between the fluoride
molecules already adsorbed and the incoming HF molecules or, in the presence of water, with hydrogen
bonded HF-H’O chains.
In the ‘bimolecular’ model of Baverez and de
Marco4*’it is proposed that multiple layers of HF molecules are bound to each other and to the alumina
surface via adsorbed water molecules. This model is
based on the observed dependence of H F adsorption on
relative humidity. Coyne et
have suggested that
H F is directly bound to the surface hydroxyl groups
since surface hydroxyl content (as determined by moisture on ignition) has a greater effect on HF capacity
than humidity. Coyne et al.’ found that the H F adsorption capacity of alumina increased slightly with increasing residual sodium content.
One of the commercial processes for manufacturing
AlF, is by the reaction of H F with Al(OH), at high
temperatures (500-600 “C).’ In the HF adsorption
studies referred to above,’-’ no direct evidence of the
formation of AIF, has been found at lower temperatures (20-120 “C)comparable to those in dry scrubber units. It appears that formation of AIF, by the
reaction of H F with alumina has not been identified
except at elevated temperatures.
This paper reports new studies using x-ray photoelectron spectroscopy (XPS)to determine whether AlF, can
be detected in samples prepared by the adsorption of
H F on alumina under conditions of varying humidity.
Evidence of an Na-F interaction was also sought. Al-F
interactions were identified only when the alumina had
been predried. When moisture was present, as in the
earlier studies, no AIF, formation was observed. Na-F
bonding was observed in all fluoride-adsorbed samples;
however, this can only account for a small proportion
~
1
.
~
9
’
R. G. HAVERKAMP ET AL.
140
of the total fluoride adsorbed. The work reported here
deals only with adsorption of H F and H F / H 2 0 gas
mixtures but not with aluminium reduction pot off
gases, which also contain particulate sodium aluminium
fluorides. In an earlier paper9 we reported XPS studies
on fluorinated alumina where Na-F bonding was identified and HF was shown to desorb on heating rather
than form AIF, .
EXPERIMENTAL
allowed variable gas concentrations of hydrogen fluoride in nitrogen to be produced. Warming of the H F
cylinder was required to maintain sufficient pressure of
HF gas. The exhaust gas was bubbled into a solution of
TISAB with a fluoride electrode monitoring the presence of dissolved fluoride.
Two samples were prepared from a batch of
commercial-grade aluminium-smelting-feed alumina.
For the first of these, DF1, the alumina was not
pretreated. This alumina had a weight loss due to
adsorbed water of 1.0% on heating to 300°C for 1 h.
For the second sample, DF2, the alumina was predried
for 12 h at 300°C before HF adsorption.
HF adsorption
Desorption of fluoride. The fluorinated aluminas were
Wet HF adsorption. A system for adsorbing fluoride onto
alumina using aqueous hydrogen fluoride was set up
based on the procedure of Coyne et aL6 The set-up is
illustrated in Fig. 1. A metered flow of preheated nitrogen was mixed with 3.7% hydrofluoric acid solution
delivered by a calibrated syringe pump. This mixture
then passed through a Teflon coil in a heated block
before passing through the heated alumina bed. The
heated block was maintained at 120"C.The adsorption
behaviour was followed by monitoring the exhaust gas
fluoride content. The exhaust gas was bubbled into a
solution of TISAB (total ionic strength adjustment
buffer) with a fluoride electrode monitoring the presence
of dissolved fluoride. The alumina bed held up to 1 g of
alumina, which was sufficient for XPS and XRD studies.
Several samples were prepared by this method and
the one referred to in this paper is labelled WF1.
Dry HF adsorption. An apparatus for adsorbing hydro-
gen fluoride gas, without added water, onto larger (80 g)
samples of alumina was developed. As illustrated in Fig.
2, this consisted of a vertical bed of alumina fluidized by
a nitrogen flow. Into this nitrogen flow, a small flow of
hydrogen fluoride gas was bled. This arrangement
1
--
X-ray photoelectron spectroscopy
X-ray photoelectron spectra were collected using a
Kratos XSAM 800 spectrometer with an Mg Ka x-ray
source. The presence in the analytical chamber was
typically in the 10-9-10-'0 Torr range during analysis.
Wide scans (0-1100 eV) of the alumina samples were
collected under analyser conditions that gave good
intensity but low-energy resolution. Narrow scans over
the appropriate Na, F, 0, C and A1 peaks were subsequently collected under high-energy resolution conditions (20 eV pass energy). Binding energies (20.1 ev)
are quoted using the 2p peak of A1 and the 1s peak for
0. For Na and F, the Auger parameter (a')is reported.
This parameter is the difference between the kinetic
energy (KE) of the major Auger line and the binding
energy (BE) of the major photoelectron peak (the 1s
level for both Na and F). It is more sensitive to changes
in the Na and F chemical environments than the BE of
(d)
/
~
subjected to heating in an electric furnace in air for 500,
700 and lo00 "C for 1 h. The objective was to determine
the effect of heating on the surface fluoride levels and on
the fluoride bonding.
\
1
ressure
'gauge
(0-SOokPa)
flow
meter
(0.2-2 Ilmin)
1
alumina bed
teflon
tTbe
needle
valve
'teflon tee
syringe pump
(HF solution)
TISAB solution
Figure 1. 'Wet' hydrogen fluoride adsorption apparatus.
ADSORPTION OF HYDROGEN FLUORIDE ON ALUMINA
141
(7
ressure
'gauge
'
i
\/'
i
,
p
i
valves
alumina
fluid bed
3
i
TlSAB solution
Figure 2. 'Dry' hydrogen fluoride adsorption apparatus.
the 1s peaks." Binding energies are referenced to
adventitious hydrocarbon, which is present on all
samples and set at 285.0 eV. The possibility of HF
desorption under vacuum conditions was checked by
monitoring the relative intensities of the A1 2p and F 1s
peaks over a 90 min residence period in the analytical
chamber. There was no detectable decrease in the F/Al
intensities and therefore H F was not being desorbed
during this phase of the analysis. It is possible that
weakly bound H F may be desorbed during the initial
pumpdown, but this could not be monitored.
The XPS spectra of the laboratory-prepared samples
of HF-adsorbed alumina, before and after heating, were
collected. In addition, XPS spectra of commercialsmelter-grade alumina used as the starting material
were collected. This was done using a pressed pellet of
the sample before and after heating at 300°C under a
vacuum for 2 h and for 18 h in the sample-holding
chamber of the XPS instrument. Also, the spectra of
AIF, , synthetic cryolite (Na3AlF,), NaF and y-Al,03
were run as BE standards.
X-ray diffraction
X-ray diffraction spectra were run on the fresh alumina
and dry scrubber alumina samples and on the
laboratory-fluorinated samples using a Philips x-ray diffractometer, scanning from 10" to 70" 28 using a Cu Ka
x-ray source.
RESULTS AND DISCUSSION
The surface elemental concentrations obtained by XPS,
for samples and standards, are listed in Table 1.
Standard alumina
The 0 1s spectra of the unreacted alumina shows a
small asymmetry on the high-binding-energy side of the
peak, which is due to some surface -OH groups on the
alumina. This is still present in the spectrum of alumina
Table 1. Surface elemental composition
Mass concentration
Atomic concentration
F
Na
%Al
%O
%F
% Na
Al
0
Na,AIF, (syn)
NaF
42.3
31.4
11.1
0
54.9
6.1
5.2
0
1.1
62.1
45.7
47.2
1.7
0.5
38.1
52.8
1
1
1
0
2.19
0.33
0.79
0
0.04
2.81
5.85
1.08
0.05
0.02
4.03
1
WF1
WF1 500°C 1 h
WF1 700°C 1 h
52.7
38.5
38.3
36.9
46.3
52.4
10.1
13.2
5.8
0.3
2.0
3.6
1
1
1
1.18
2.03
2.31
0.27
0.49
0.22
0.1
0.06
0.11
DF1
DF1 500°C 1 h
DF1 700°C 1 h
38.9
39.4
40.6
46.9
41.4
51.6
13.4
17.4
5.7
0.8
1.7
2.1
1
1
1
2.03
1.77
2.14
0.49
0.63
0.20
0.02
0.05
0.06
DF2
DF2 500°C 1 h
DF2 700°C 1 h
DF2 1000°C 1 h
29.3
28.8
34.5
44.6
25.6
12.5
40.2
53.1
44.8
55.9
21.4
1.4
0.3
2.7
3.9
0.9
1
1
1
1
1.47
0.73
1.97
2.01
2.17
2.76
0.88
0.04
0.01
0.11
0.13
0.02
Sample no.
A1203
Al F,
R. G . HAVERKAMP ET AL.
142
after heating at 300°C for 2 h and even after 18 h of
heating. This confirms that this asymmetry is due to terminal hydroxyl groups rather than to adsorbed water,
which would be removed at this temperature. A thermogravimetric study suggested that the hydroxyl
groups are removed at a temperature of 500-600 "C.
a)
HF adsorption levels
For the vaporized-HF-solution-adsorbed sample, WF1,
the adsorption behaviour was similar to that reported
by other^.^*^.' Adsorption continues quantitatively until
saturation is achieved. Once this equilibrium H F
adsorption capacity of the alumina under the particular
humidity conditions is reached, no further H F is
adsorbed. A surface H F level of 10.1 wt.% F was
achieved under the above conditions. This corresponds
to a surface F/Al atomic ratio of 0.3 : 1 atomic ratio.
For the HF-adsorbed sample using the apparatus
illustrated in Fig. 3, with a dry atmosphere but with the
alumina not predried (DFl), a surface HF level of 13.4
wt.% F was obtained, corresponding to a surface F/Al
atomic ratio of 0.5: 1. For adsorption on alumina that
had been dried prior to adsorption (DF2), a very high
level of surface fluoride was achieved. The 44.8 wt.% F
corresponds to a surface F/Al atomic ratio of 2.2: 1.
Fluorination of the sample was stopped before equilibrium was achieved. The reaction of HF with this
dried alumina was markedly exothermic, with a temperature of 120 "Cbeing reached.
b)
-,-Ad
/hh
c)
/ \
/
4f
"
I
,
,
,
.,-i
-
4
A1 2p binding energies
For the samples and the standards, Table 2 lists the
binding energies of the A1 2p and 0 1s peaks, as well as
the Auger parameters for the Na and F lines taken from
the narrow scans.
For the fluoride-adsorbed aluminas WF1 and DF1,
the A1 2p binding energies (summarized in Table 2) do
not show significant shifts from the value of 74.3 eV in
unreacted alumina. The binding energies of compounds
AlF, and Na,AlF6 , containing A1-F bonds are higher
at 77.0 and 76.0 eV, respectively. This suggests that in
the fluoride-adsorbed samples WF1 and !3F2 there is
Figure 3. Al 2p spectra of: (a) standard alumina; (b) wet HF
adsorbed alumina (WF1); (c) HF adsorbed predried alumina
(DF2); (d) AIF,.
Table 2. Binding energies of aluminium, oxygen, sodium and fluorine
Al 2p
0 1s
AIF,
Na,AIF,
NaF
74.3
77.0
76.0
-
531.5
533.0
531.8
-
WFl
WF1 500°C 1 h
WF1 700°C 1 h
74.5
74.5
74.3
DF1
DF1 500°C 1 h
DF1 700°C 1 h
DF2
Sample
A1203
DF2 500°C 1 h
DF2 700°C 1 h
DF2 1000°C 1 h
Binding energy
F a'
(eV)
Peak width (eV)
Fls
Na 1s
Na a'
A l2p
01s
1340.5
1339.9
1339.7
1339.8
2062.4
2060.3
2061 .O
2.2
2.1
2.1
-
2.8
3.3
3.3
-
1.4
2.4
2.9
2.2
1.8
2.5
2.2
531.8
531.8
531.4
1340.2
1340.8
1340.8
2061.0
2062.1
2062.0
2.3
2.4
2.4
2.8
2.7
2.7
2.6
3.1
3.0
1.7
2.4
74.7
74.7
74.9
531.8
531.8
531.8
1340.5
1340.4
1340.8
2060.6
2061.3
2061.7
2.2
2.3
2.2
2.7
2.6
2.5
3.1
3.2
2.7
2.0
2.2
75.7
76.4
75.1
74.4
532.7
533.7
531.8
531.4
1340.2
1339.5
1340.3
1340.9
2060.2
2060.0
2060.7
2062.1
3.1
4.3
2.4
1.8
4.1
3.5
2.5
2.2
3.0
3.3
3.8
1.9
2.8
2.7
-
ADSORPTION O F HYDROGEN FLUORIDE ON ALUMINA
not a strong interaction between the H F and the surface
A1 atoms.
On the other hand, in sample DF2, which was dried
prior to HF adsorption, there is a shift to higher BE of
the A1 2p peak of 1.4 eV from the unreacted alumina
value, This is clear evidence of A1-F bond formation.
The A1 2p BE peak for the DF2 sample can be
separated by computer curve-fitting into two unresolved
doublets, with 2p3,, peaks at 74.9 eV and 76.3 eV (Fig.
4). The area under each of these peaks is 49% and 51%,
respectively. The intensity of the 76.3 eV peak is evidence that about half of the surface A1 atoms are
involved in a strong interaction with F.
The x-ray diffraction patterns of the wet fluorinated
alumina, WF1, and one dry fluorinated alumina, DF1,
showed no peaks other than those due to alumina. This
indicates that the absorbed fluoride in these samples
does not form distinct crystalline compounds in SUEcient quantity to be detected. It therefore supports the
evidence from XPS that no AlF, is formed. The x-ray
diffraction pattern of the predried alumina that was
fluorinated (DF2) did show the presence of major
quantities of 8-AlF, 3H20, with lesser amounts of
a-AlF, 3H,O and possibly AlF,(OH).
The predried alumina may be able to react with the
dry H F to form AlF, because the otherwise protective
layer of surface-adsorbed water has been removed.
When this layer of water is present, the fluoride is
hydrogen bonded to the water layer and is not able to
approach the aluminium or the oxygen atoms of the
alumina closely enough to enable reaction and the formation of AlF, .
-
0 Is binding energies
The 0 1s peak position and the widths also provide
information on changes taking place with fluorination
of alumina. For DF2 there is a shift in the 0 1s peak,
relative to unreacted alumina, of
1.2 to 532.7 eV
(Table 2). The peak width is 4.1 eV, which is considerably larger than the 2.8 eV of fresh, undried alumina.
+
80
78
76
74
Binding Energy
72
Figure 4. Al 2p of dry HF-adsorbed predried alumina (DF2) with
two peaks fitted.
143
From the width and shape of the 0 1s peak it is clear
that this envelope is composed of several peaks. The
peak at 531.8 eV is assigned to oxygen in Al,O, as well
as to adsorbed water, which also lies at -531.8 eV.
Another broad peak, which makes up 50% of the area,
is centred at 534.0 eV (Fig. 5). This second peak is consistent with the compound 8-AIF, . 3H,O identified by
x-ray diffraction in DF2, in which the oxygen contained
in the lattice water is moved to higher BE by the adjacent fluoride.
For the HF-adsorbed samples where water was
present, i.e. WF1 and DF1, the 0 1s peak does not shift
from the value for pure alumina, indicating that there is
no strong F-0 interaction (or F-OH).
For compounds AlF, and Na,AlF,, which nominally
do not contain any oxygen, surface oxygen attributed to
adsorbed water was detected. The 0 1s binding energies
for these compounds were, respectively, 533.0 and 531.8
eV (Table 2). The approximate position for adsorbed
water is 531.8 eV. For AlF, the large shift in the 0 1s
peak suggests that water is structural rather than just
surface adsorbed as with Na,AlF,.
The fluorine Auger parameters for the samples
studied are listed in Table 2. There is little change in
this parameter between the different standards, e.g. NaF
and AlF, , and between the samples.
Fluoride desorption
The desorption of fluoride from the fluorinated aluminas can be seen from the data in Table 1. Heating at
500°C for 1 h does not result in a reduction in the
surface fluoride levels. After heating at 700°C for 1 h a
significant removal of fluoride occurs, while at lo00 "C
essentially all the fluoride is removed. The desorption
behaviour is similar for all the samples.
For the WFl and DF1 samples there is no observable increase in the higher binding energy component of
the A1 2p peak with heating to 500"C, suggesting that
AlF, is not formed. With further heating this surfaceadsorbed HF is volatilized. For the DF2 sample, where
538
536
534
532
Binding Energy
530
528
Figure 5. 0 1s of dry HF-adsorbed predried alumina (DF2) with
two peaks fitted.
144
R. G . HAVERKAMP ET AL.
an Al-F interaction is already apparent, the average BE
of the A1 2p peak shifts to a higher value with heating,
indicating an increase in the proportion of aluminium
atoms involved in the A1-F interaction. The 0 1s peak
also shifts to higher BE as the dehydration of P-AlF, .
3H20 takes place to form AlF, . 2 H 2 0 and AlF, *
H 2 0 , with a consequently stronger influence of fluoride
on the oxygen orbitals.
Sodium fluoride interaction
Sodium is involved with the bonding of some of the
adsorbed fluoride. Coyne et aL7 found that the HF
adsorption capacity of alumina increased slightly with
increasing residual sodium content. In an earlier paperg
we reported XPS studies on fluorinated alumina where
Na-F bonding was identified in fluorinated aluminas.
The sodium Auger parameters for the samples studied
here are listed in Table 2. There is a progressive shift in
the sodium Auger parameter in the order: unreacted
alumina > WF1 > DF1 > DF2. This shift in the Auger
parameter is a result of the sodium present on the
surface of the alumina reacting with the adsorbed HF to
form Na-F bonds. Despite the strong surface segregation of sodium, even if all of the surface sodium is
involved in HF bonding this can account for only a
small proportion of the adsorbed fluoride.
CONCLUSIONS
We have investigated hydrogen fluoride adsorption processes occurring at the surface of alumina. The alumina
surface contains terminal hydroxyl groups (Al-OH),
surface adsorbed water and surface segregated sodium.
X-ray photoelectron spectroscopy was used to look for
an A1-F interaction in samples prepared by the adsorption of H F on alumina under a variety of humidity conditions. Evidence of an Na-F interaction was also
sought.
An Al-F interaction was observed in only one of the
sample types, i.e. the case of the alumina that had dry
HF adsorbed after being predried. When moisture was
already present on the alumina, or water was present in
the H F gas stream, as in the earlier
no AlF,
formation was observed. As the previous sample has
shown that when an Al-F interaction occurs we can
detect this in the XPS spectrum, it can be confidently
said that there is no significant Al-F interaction in these
samples adsorbed with moisture present. This suggests
that the adsorption of H F involves a weak interaction,
probably hydrogen bonding with layers of water, as
proposed by other workers.,-'
On heating, no evidence of the formation of AlF, was
seen for the WF1 and DF1 samples up to 500"C, and
by 700°C the HF was being desorbed. For the DF2
sample, both the A1 2p and the 0 1s BEs increased with
heating to 500 "C, indicating more Al-F interaction and
the dehydration of the P-AlF, 3H2O. The fluoride was
desorbed >700 "C for all these samples.
Na-F
bonding was observed in all fluorideadsorbed samples; however, this can only account for a
small proportion of the total fluoride adsorbed.
The work reported here deals only with the HF and
HF/H,O gas mixtures but not with aluminium
reduction pot off gases, which also contain particulate
sodium aluminium fluorides.
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