Preparation and characterization of high surface area alumina-titania solid acids

Article abstract of DOI:10.1023/A:1018628608830

As part of a systematic study of solid acids, a series of high surface area single-phase alumina-titania mixed oxides were prepared. These materials were found to have higher surface areas, greater porosity and enhanced specific surface acidity than compa

Full text of DOI:10.1023/A:1018628608830

JOURNAL OF MATERIALS SCIENCE 32 (1997) 5583 — 5592
Preparation and characterization of high surface
area alumina–titania solid acids
G. S. WALKER, E. WILLIAMS, A. K. BHATTACHARYA
Centre for Catalytic Systems and Materials Engineering, Department of Engineering,
University of Warwick, Coventry CV4 7AL, UK
As part of a systematic study of solid acids, a series of high surface area single-phase
alumina—titania mixed oxides were prepared. These materials were found to have
higher surface areas, greater porosity and enhanced specific surface acidity than
comparable alumina-titania solid acids reported in the literature. The thermal stability
of the phases and their textural and physicochemical properties have been studied
over a range of Al/Ti compositions and for calcinations up to 1000°C. The samples
were characterized by differential thermal analysis—thermogravimetry, X-ray
diffraction, nitrogen physisorption and X-ray photoelectron spectroscopy. Surface
acidity was measured by temperature programmed desorption and in situ Fourier
transform-infrared spectroscopy, using ammonia as a probe molecule. The strength and
density of the acid sites were proportional to the Ti-content.
1
. Introduction
extent than pure titania and that the anatase to rutile
transformation occurred at higher temperatures for
the mixed oxides (900—980 °C) than for pure titania
(560 °C) [13]. These mixed oxides were prepared by
adding a dispersible boehmite sol (AlOOH) to
a titania sol, which when dried produced an intimate
physical mixture. Both studies showed that for these
mixed phase samples, stability to sintering increased
with alumina content.
The aim of the work reported here was to prepare
single-phase alumina—titania materials which retained
their properties to high temperatures. A solid solution
should have enhanced thermal stability through inhi-
biting phase separation and thus inhibit the sintering
of the single metal oxides. Having homogeneous sam-
ples will also maximize the Al—Ti interaction. Samples
were prepared by the alkoxide sol—gel route because
the method produces materials with significant ad-
vantages for technological applications in this area.
Cogels produced by this method are, in general, more
homogeneous mixtures than those obtained by co-
precipitation, and the porosity can be controlled by
the preparation conditions [14—16]. Unlike previously
reported literature, this paper reports the preparation
of monophasic alumina—titania solid acids, with high
and controllable surface area and porosity. These
properties are retained to high temperatures and are
resistant to sintering at elevated temperatures. Further-
more, the materials are prepared chlorine-free, which
is important for the purposes of the present study, as
mentioned above.
Aluminium trichloride and sulfuric acid are extensive-
ly used as homogeneous acid catalysts. However, both
acids are toxic and environmentally hazardous, mak-
ing them difficult to handle and to dispose of the
waste. A solution to these problems is to use a solid
acid which is itself less hazardous than the acids used
and can be easily separated from the waste effluent.
One such material is alumina—titania which is re-
ported to have both acid and base sites [1—11] at the
surface. There is also interest in the use of alumina—
titania mixed oxides as supports for transition metal
catalysts [4—6, 12].
Reports in the literature on these systems mostly
concentrate on the surface acidity of the materials and
how this is affected by the Al/Ti composition and the
precursor used [2—5]. A factor often not addressed is
the heterogeneity of the materials. Alumina—titania is
often prepared by coprecipitation or impregnation
which will yield materials with multiple phases. These
papers also report the properties found for one calci-
nation temperature (typically 500 °C). Often catalytic
reactions are more economically viable at elevated
temperatures and for such reactions, catalysts which
retain their surface area, porosity and catalytic prop-
erties at higher temperatures, are required. Two re-
ports did study the change in properties of alumina—
titania with temperature. The structural and acidic
properties of alumina—titania oxides prepared by co-
precipitation from chloride salts were measured at
various calcination temperatures (400—900 °C) [2].
However, chloride contamination is inevitable in
routes based on chloride salts and is a well-known
catalyst poison. The other study looked at titania-rich
mixed oxides and found that they sintered to a lesser
The systematic study concentrates on alumina-rich
oxides because of their greater thermal stability
[2, 13]. Of particular interest are the effects arising
from the substitution of titanium for aluminium, the
0
022—2461 ( 1997 Chapman & Hall
5583

structural properties and the surface acidity. By cor-
relating these effects, we seek to gain an understanding
of the principles affecting surface acidity. Differential
thermal analysis—thermogravimetry (DTA—TG) was
used to identify the formation of oxides from the al-
cogels. The structural and textural properties were
measured by nitrogen physisorption and X-ray diffrac-
tion (XRD). X-ray photoelectron spectroscopy was
used to characterize the surface chemistry, and surface
acidity was measured using ammonia as a probe mol-
ecule; the adsorbed ammonia was characterized by
temperature programmed desorption and in situ
Fourier transform-infrared spectroscopy (FT-IR).
2.4. Surface area and porosity
measurements
The surface area and porosity of the samples were
measured by nitrogen physisorption, using a Micro-
meritics ASAP 2000. The samples were pre-treated by
degassing at 200 °C until the pressure was (2 mtorr
(&12 hs; 1 torr"133.322 Pa). The BET method was
used to calculate surface area and the Barrett, Joyner
and Halenda (BJH) model for pore volume distribu-
tion (the adsorption results were used).
2.5. TPD
Alcogel samples (&0.2 g) of particle size 200—400 lm
were loaded into a stainless steel reactor tube using
quartz wool plugs to keep the powder in the middle of
the tube. The gas pressure in the reactor was atmo-
spheric and the flow rate for all gases was
2
. Experimental procedure
2
.1. Preparation
The alcogels were prepared by the hydrolysis of alumi-
nium isopropoxide (98%, Aldrich) and titanium (IV)
isopropoxide (97%, Aldrich). The same procedure was
used for the single and mixed metal preparations,
which aimed to make enough gel to produce 0.05
moles of oxide upon calcination. Aluminium alkoxide
followed by titanium alkoxide (in the desired propor-
tions) were transferred to a flask under dry nitrogen
and weighed. After adding 80 ml isopropanol to the
flask, the mixture was heated up under nitrogen to
dissolve the aluminium isopropoxide and then re-
fluxed overnight. The hydrolysis was carried out by
adding a water/isopropanol mix drop-wise using
a syringe pump to the refluxing alkoxide solution
whilst vigorously stirring the contents of the flask. The
addition took an hour and a 50% excess of water was
used. After the addition, the mixture was stirred and
refluxed for a further hour. The alcogel was collected
by removing as much of the solvent as possible using
a Buchi rotary evaporator with a bath temperature of
5
0 ml min\ꢀ. Helium (cp grade, BOC) was dried by
passing through a liquid-nitrogen trap; other gases
were used as-received. The temperature of the sample
was controlled by a Newtronics micro96 temperature
controller. Samples were calcined in situ under flowing
dry air (BOC) at 600 °C for 6 h using a ramp rate of
1
°C min\ꢀ. After calcination, the sample was purged
with helium for 1 h at 100 °C prior to adsorbing am-
monia (99.9%, Linde) for 30 min at the same temper-
ature. To remove physisorbed ammonia the sample
was purged with helium for 4 h at 100 °C. The TPD
was carried out under flowing helium using a ramp
rate of 10 °C min\ꢀ heating the sample up to 600 °C.
The desorbed ammonia was detected using a Hiden
mass spectrometer to sample the exhaust gases. The
mass spectrum signal used to detect ammonia was
m/z"16, the contribution to this signal from water
was removed by subtracting a 0.9% of the m/z"18
intensity. The mass spectrometer was calibrated after
each experiment using 1.0 ml pulses of ammonia from
a Valco valve.
3
0 °C. Some alumina samples were dried by slow
evaporation at room temperature in a fume cupboard.
The solid gel was dried in an oven overnight at 105 °C.
The DTA—TG results were used to select calcination
temperatures and the alcogels were calcined at these
temperatures for 6 h in static air, using a muffle
furnace. Samples were loaded in alumina boats and
heated at a rate of 1°C min\ꢀ to the required
temperature.
2.6. IR spectroscopy
A Galaxy 7020 FT—IR spectrometer with an MCT
detector was used to collect IR spectra.
A
Graseby—Specac high-temperature high-pressure cell
was used to obtain in situ transmission spectra of the
samples. The samples were loaded as wafer-thin self-
supporting discs. Titania was studied by diffuse reflec-
tance IR spectroscopy using a Graseby—Specac
DRIFTS selector and environmental chamber. KBr
was found to be a suitable diluent for the DRIFTS
experiments (i.e. did not react with ammonia), and was
ground with the titania in &4 : 1 ratio. The temper-
ature of the cells were controlled by a Eurotherm 847
controller and gas treatment/supply was the same as
for TPD. The oxides were dried by heating to 200 °C
for 1 h under dry helium and then cooled to 100 °C.
Ammonia was passed over the sample for 30 min at
100°C, and then purged with helium at the same
temperature for 3—4 h to remove the physisorbed am-
monia. Spectra were taken of the dried sample and
after the helium purge.
2
.2. Thermal analysis
Simultaneous differential thermal analysis (DTA) and
thermogravimetric analysis (TGA) of the samples was
carried out using a Polymer Laboratories STA1500
thermal analyser. The samples (&12 mg) were heated
under flowing air up to 1500 °C using a ramp rate of
1
0 °C min\ꢀ.
2
.3. Powder XRD
X-ray powder diffraction patterns were recorded on
a Philips PW1710 diffractometer using CuK radi-
ation with a nickel filter. Diffractions were measured
for 2h values from 10°—80° with a scanning speed of
a
0
.25° min\ꢀ.
5
584

2
.7. XPS
There are many phases of alumina and the phase
produced depends on the precursor structure and heat
treatment [19, 20]. The alkoxide preparations pro-
duce a boehmite phase [21], c-AlOOH, which upon
heating forms cPdPh-alumina and finally a-
alumina at &1100 °C [19, 20]. On heating the alumi-
nium alcogel, Fig. 1b, there was an endotherm with
loss of sample weight, followed by two exothermic and
an endothermic events, all with varying degrees of
sample weight loss. At 900 °C there was a change in
the DTA signal and finally an exotherm 1180 °C.
There was no loss of sample weight during these last
two events. The product after thermal analysis was
still a white powder but it had sintered greatly. Con-
stant weight was achieved by 650 °C, with a total
weight loss of 23.4%. From this it was determined that
the sample had 0.35 moles of water per mole of boeh-
mite prior to the experiment. This water was desorbed
at 80 °C and the anhydrous boehmite decomposed in
three stages forming alumina by 600 °C (assumed to be
c). The phase transformations at 900 °C are probably
the conversion of c-alumina to d- and h-alumina.
These phase changes are slow and often overlap
making them difficult to determine by DTA—TG.
Finally the exotherm at 1180 °C was attributed to a-
alumina formation.
The mixed metal alcogel DTA—TG results were
similar to those for alumina. It was found that with
increasing titanium content the changes in rate of
weight loss for the TG curves became less discrete.
Also, thermodynamic events associated with the
formation of alumina decreased in magnitude and
some events attributable to the formation of titanium
oxide species became more evident. For example, the
magnitude of the alumina exotherm at &500 °C de-
creased with increasing titanium content, and was not
detected for the 75 : 25 sample. Thermal events asso-
ciated with titania were more easily identified in the
DTA—TG plot for the 75 : 25 sample, Fig. 1d. These
events were a large exotherm at 270 °C, attributed to
the formation of a titanium oxide network, and a small
exotherm at 815 °C, assigned to rutile formation. The
temperature of formation of a-alumina decreased with
increasing titanium-content. The exotherm at 1035 °C
with a shoulder at 950 °C was assigned to the formation
of a-alumina (cf. 1180 °C for alumina).
The powder samples were put into a dish-shaped
holder and lightly pressed to give a compacted smooth
surface. A Kratos XSAM 800 photoelectron spec-
trometer was used for the analysis which consisted of
scans of the O 1s, C 1s, Ti 2p and Al 2p spectral re-
gions. Primary excitation was achieved with MgKa
X-rays (120 W) and a multichannel detector was used
to detect the emitted photoelectrons. The spectro-
meter was run in the Fixed Analyser Transmission
(
(
FAT) mode and at the highest available resolution
1.2 eV). Data processing was performed using the
Kratos DS800 software. The spectra for alumina and
the mixed metal oxides were referenced to the O 1s
position for alumina, 532.7 eV, and the titania spectra
were referenced to an O 1s position of 530.7 eV. Pre-
vious work has shown that charge referencing using
the C 1s peak is unreliable for some metal oxide and
mixed metal oxide systems, and that the O 1s peak is
a suitable internal reference [17, 18].
3
. Results and discussion
The DTA—TG results are given in Table I, and Fig. 1
shows the DTA—TG plots for selected gels. For the
titanium alcogel, Fig. 1a, two endotherms were detec-
ted with associated weight loss, and this was assigned
to loss of water. Towards the end of the second en-
dotherm there was a large exotherm with little change
in sample weight, corresponding to a phase change.
After the exotherm there was a small broad en-
dotherm with associated weight loss, followed by an-
other sharp exotherm corresponding to a further
phase change. There was no further weight loss after
the second exotherm and the total weight loss for the
sample was 22.3%. At 530 and 830 °C there were two
thermal events with no corresponding change in
sample weight. These were further phase transforma-
tions which would have evolved heat; however, be-
cause the exotherms were so small, it was difficult to
differentiate them from the change in the DTA base-
line. The product after DTA—TG was a yellow pow-
der. The stoichiometry of the gel determined from the
sample weight loss was TiO ·1.25 H O. Although the
ꢁ
ꢁ
gel is expressed as an oxide, it is more likely that it was
a hydrated oxyhydroxide.
TABLE I DTA-TG results for aluminium—titanium alcogels: exo, exotherm; endo, endotherm; sh, shoulder
Al O : TiO DTA—TG results (°C), weight loss in parentheses
ꢁ
ꢂ
ꢁ
1
00: 0
80 endo
9%)
80 endo
11%)
80 endo
14%)
95 endo
9%)
70 endo
15%)
100 endo
8%)
215 exo
(3%)
225 exo
(4%)
200 endo
(6%)
200 exo
(6%)
220sh; 270 exo
(20%)
215 endo; 260 exo
(10%)
430 endo; 510 exo
(11%)
440 endo
1180 exo
1170 exo
1170 exo
1130 exo
(
9
9
8
7
5 : 5
(
(8%)
0 : 10
0 : 20
5 : 25
450 endo; 510 vw exo
(9%)
350 exo; 490 exo
(9%)
(
(
815 exo
830 exo
950 sh
1035 exo
(
0
: 100
340 endo; 430 exo
(4.3%)
(
Assignment:
dehydration
alcogel to oxide transformation
a-Al O formation
ꢁ
ꢂ
5585

Figure 1 DTA—TG plots for alumina—titania cogels: (a) 0 : 100, (b) 100 : 0, (c) 80 :20, (d) 75 : 25.
Fig. 2 summarizes the XRD phase analysis. The
titanium alcogel was found to be amorphous, but
calcination of gel produced anatase at 400 °C, a mix-
ture of anatase and rutile at 600 °C and was finally
converted to rutile by 1000 °C. All the samples were
slightly off-white powders. The yellow powder col-
lected after DTA—TG analysis of the titanium alcogel
was also rutile. Combining these findings with the
DTA—TG results, the titanium alcogel transformed
upon heating to an oxide by loss of water (endotherms
at 100 and 215 °C) accompanied by the formation of
a hydrated anatase phase (exotherm at 260 °C). This
was followed by a further loss of water (340 °C) and
another phase transformation (exotherm at 430 °C)
forming anatase. It is expected that the phase change
detected at 430 °C by DTA was fully completed after
calcining the gel at 400 °C for 6 h. The onset of rutile
formation was detected by DTA at 530 °C and the
thermal event at 830 °C may have been caused by loss
of lattice oxygen. The latter assignment is supported
by the observation that titania slightly deficient in
Figure 2 Phases identified by powder XRD for Al O : TiO sam-
ꢁ
ꢂ
ꢁ
ples calcined at different temperatures. a/p"uncalcined samples
as-prepared).
(
patterns, the crystallinity of the products decreasing
with increasing titanium content. The incorporation
of Tiꢆ> has disrupted the alumina phase, producing
a more X-ray amorphous phase. The products formed
at 800 °C were also c-alumina, but were more crystal-
line than those calcined at lower temperature. Once
again, crystallinity decreased with increasing titanium
content. The only exception was the 75 : 25 sample
which also gave a rutile XRD pattern. This supports
the assignment of the exotherm reported at 815 °C
being due to rutile formation.
The materials produced after calcination at 1000 °C
were all multi-phase oxides, Fig. 2, the 95 : 5 sample
was the only mixed metal oxide to have no titania
phase detected. The alumina phases produced were
dependent on the titanium content; as this increased,
oxygen (TiO
) is yellow in colour [22], as
—
ꢀ
ꢃꢄꢄꢄ ꢀꢃꢄꢄꢅ
was the DTA—TG product. Also, the transformation
of rutile from a white to a yellow solid has been
detected in air at 900 °C [23].
Powder XRD of the aluminium and mixed metal
oxyhydroxide cogels confirmed that they had a boeh-
mite structure. The XRD lines were quite broad and
the crystallinity decreased with increasing titanium
content. Titania was not detected for any of the mixed
metal oxides formed at 600 °C. XRD of the mixed
oxides and pure alumina gave diffuse c-alumina
5
586

so the a-alumina patterns became stronger and the
diffraction lines narrower. It would appear that the
presence of Tiꢆ> reduced the temperature for the
alumina phase changes. The crystallinity of the boeh-
mite can affect the phase transformations, with some
less-crystalline boehmites not forming d-alumina [19].
The increasing amorphous nature of the precursor ma-
terials with titanium content indicates that the alumina
matrix was disrupted by the inclusion of titanium. It is
postulated that this prevented the c, d and h phases
from forming properly, and so decreased the activation
energy for the alumina transformations.
The XRD results show that the mixed metal oxy-
hydroxide cogels consisted of titanium cations incor-
porated in a boehmite matrix. When calcined, these
cogels formed a single-phase solid solution with tita-
nium incorporated in an alumina matrix. The solid
solutions showed no sign of phase separation up to
with increasing calcination temperature indicating
loss of porosity.
Titania formed at 400 °C had low surface area,
50 mꢁ g\ꢀ, and little porosity, 0.1 cmꢂ g\ꢀ. Alumina
and the mixed metal oxides produced after calcination
at 600 °C had surface areas of &400 mꢁ g\ꢀ and por-
osities of &1.5 cmꢂ g\ꢀ. Many alumina—titania sam-
ples reported in the literature [6—8, 11] prepared via
the alkoxide route typically have surface areas and
porosities of 200—260 mꢁ g\ꢀ and 1.0 cmꢂ g\ꢀ, respec-
tively. Compared with these, the materials discussed
here had much higher surface areas and greater poros-
ity. The enhanced textural properties are, in part, due
to the method of drying. It was found that samples
dried at room temperature by slow evaporation had
lower surface areas (&300 mꢁ g\ꢀ) than those dried
under reduced pressure using a rotary evaporator
(bath temperature &30 °C). Even the lower surface-
area oxides prepared had significantly greater areas
than other preparations reported in the literature. The
most likely reason for this is the amount of water used
during hydrolysis. The preparations described here
used a 50% excess water; however, many literature
preparations used up to a 30-fold excess of water. The
preparation conditions affect the porosity and surface
area of the gels [25—28] and it is reported that water
added to undried alumina/silica gels can enhance the
porosity of the gel, but that a large amount of water
greatly reduced its porosity [16]. We suggest that
a large excess of water during hydrolysis has a detri-
mental effect on the textural properties of the solids
produced.
8
00°C except for the 75 : 25 sample, which was a mix-
ture of rutile and a c-alumina-based phase after cal-
cination at 800 °C. Lower titanium content solid
solutions underwent separation at higher temper-
atures and the 95 : 5 sample showed no sign of phase
separation, even after calcination at 1000 °C.
The adsorption isotherms for alumina and titania
are given in Fig. 3. The hysteresis loop for the alumina
isotherm is characteristic of a substrate with cylin-
drical pores, while that for the titania isotherm is
indicative of either ink-bottle pores or voids between
close-packed spherical particles [24]. Titania had little
porosity, Table II, and so the sample was most likely
a non-porous powder consisting of spherical particles.
The isotherms for the mixed oxides were similar to
that for alumina. It was found that the uptake of
nitrogen at a relative pressure of &0.85 decreased
Comparing the samples calcined at 600 °C, the
mixed metal oxides had somewhat larger surface areas
than pure alumina, Fig. 4 and Table II. Calcination at
8
00°C reduced the surface areas for all the samples,
but the 75 : 25 mixed metal oxide had sintered to
a much greater extent than the other samples
(
180 mꢁ g\ꢀ compared to 250 mꢁ g\ꢀ for alumina).
After calcination at 1000 °C the mixed metal oxides
were all lower in surface area than alumina. It can be
seen from Fig. 4 that the surface area decreases with
increasing titanium content and that for titanium
Figure 3 Nitrogen adsorption isotherms for (a) alumina calcined at
Figure 4 Surface area measured for alumina : titania samples cal-
cined at 600, 800 and 1000 °C.
6
00°C and (b) titania calcined at 400 °C.
5587

TABLE II Surface area and porosity for alumina—titania mixed oxides
Sample Al O : TiO
Calcination temp. ( °C)
Surface area (mꢁ g\ꢀ)
Pore vol. (cmꢂg\ꢀ)
Pore diameter (nm)
ꢁ
ꢂ
ꢁ
1
00: 0
600
370
250
125
1.6
1.2
0.7
13, 40—100
15,'50
21
8
00
1
1
1
1
1
000
9
9
8
7
5 : 5
600
395
290
105
1.8
1.5
0.7
17, 30—100
18, 40—50
30—100
8
00
000
0 : 10
0 : 20
5 : 25
600
400
275
75
1.5
1.4
0.5
10, 20—100
20, 50 (sh)
60—100
8
00
000
600
415
275
35
1.4
1.0
0.1
65, 20—100
10—100
'40
8
00
000
600
385
180
20
1.2
0.7
0.1
7, 30—100
11, 20—100
'50
8
00
000
0
: 100
400
50
0.1
5
contents of 20% or more, the products were of low
surface area (i.e. (40 mꢁ g\ꢀ).
Similar trends were also found for the pore volumes
of the samples, Table II. For the mixed metal oxides,
the pore volume decreased with increasing titanium
content and it was also found that the porosity de-
creased with increasing calcination temperature. The
most marked loss of porosity was after calcination at
1
000 °C. The higher titanium content samples had
negligible porosity and the pore volume for the other
oxides was half that of the same cogel calcined at
8
7
00°C. It is interesting to note that, excluding the
5 : 25 sample, the mixed oxides had comparable or
slightly greater pore volumes than alumina, except
when calcined at 1000 °C where only the 95 : 5 sample
had similar pore volume.
Figure 6 Pore-size distribution for alumina: titania : : 95 : 5 calcined
at (——) 600, (— ——) 800 and (— · —) 1000 °C.
The pore-size distribution was also affected by tita-
nium content. The predominant pore size decreased
with increasing titania for samples calcined at 600 °C.
When the samples were calcined at higher temper-
atures, there was a loss of mesoporosity (2—20 nm). It
can be seen from Fig. 5 that mesoporosity was lost
with higher calcination temperatures leading to peaks
in the plots at 13, 15 and 21 nm after calcinations at
6
00, 800 and 1000 °C, respectively. However, there was
very little change in the volume of pores with diameter
greater than 20 nm (macropores). For the 95:5 sample,
Fig. 6, it can be seen that as well as a similar loss of
mesoporosity with increasing calcination temper-
ature, there was also fewer 20—30 nm pores at 1000 °C.
The 90 : 10 sample was similar but the two samples
with greatest titanium content were greatly sintered by
1
000 °C, leaving behind negligible porosity. The loss of
surface area and porosity of the products coincides
with an increase in crystallinity, samples with a well-
formed a-alumina phase having the lowest surface
area and porosity.
Many acidity measurements in the literature use
pyridine as the probe molecule; however, pyridine is
a weak base and will only interact with strong acid
sites. Lahousse et al. found that using 2,6-dimethyl-
pyridine, which is a stronger base than pyridine, they
were able to detect by IR, Brønsted acidity at the
surface of alumina—titania mixed oxides but only
Lewis acidity when using pyridine [11]. However, 2,6-
dimethylpyridine cannot coordinate to Lewis sites
because of steric hindrance. The use of bases with
Figure 5 Pore-size distribution for alumina calcined at (——) 600,
(— — —) 800 and (— · —) 1000°C.
differing pK values to separately characterize Lewis
!
5
588

and Brønsted sites is less than satisfactory because one
would be measuring one type of acidity to higher
values of pK than the other. Thus, we have chosen to
!
use ammonia as the probe molecule as it is a strong
base, able to dissociate protons from weaker Brønsted
sites and will coordinate to Lewis sites. In order to
fully characterize surface acidity, two techniques have
been employed, FT—IR to measure the ratio of Lewis
to Brønsted sites, and TPD to determine the acid site
density and indicate the strength of these sites.
A problem sometimes encountered is the reaction of
the probe molecule whilst adsorbed to the substrate.
Pyridine has been found to form dipyridyl radicals
[
29, 30] and pyridine carbanions [29] over basic ma-
terials. The anion adsorbs at 1546 cm\ꢀ which is
coincidental with pyridinium IR bands [29]. In the
literature, there is evidence that hydrogen abstraction
of ammonia forming amides occurs at elevated tem-
peratures over basic sites [29, 31—33]. IR bands due
to these surface amide species are found at
1
550—1500 cm\ꢀ. Whichever probe is used, careful
analysis of the spectra is required to determine if
hydrogen abstraction has occurred.
The acid-site density for the oxides calcined at
6
00°C was measured by TPD of ammonia and the
Figure 7 Temperature programmed desorption of ammonia from
alumina: titania samples calcined in situ at 600 °C. Sample composi-
tion is given on the right-hand side of the figure.
results are given in Table III. It was found that titania
had a higher density of acid sites than alumina, but
because the titania had a much lower surface area
than alumina it had fewer acid sites per gram of oxide
(6.9;10\ꢈ compared to 1.5;10\ꢆ mol g\ꢀ). This is
reflected in the TPD profiles for the two oxides, Fig. 7,
which have been normalized to 1 g of sample. The
TPD profile for titania was a very broad peak at
the 75 : 25 sample the TPD profile consists of a large
peak at 240 °C, the rate of desorption slowly decreas-
ing with increasing temperature and levelling off at
500°C.
3
50°C, ammonia started to desorb at 150 °C and
finished by 550°C. More ammonia was desorbed from
alumina at lower temperatures than titania, forming
a TPD peak at 230 °C. Ammonia continued to be
desorbed at higher temperatures up to 580 °C, forming
a long tail in the TPD plot.
For the mixed metal oxides, the amount of ammo-
nia desorbed increased with increasing titanium con-
tent, with the 75 : 25 sample desorbing a similar
amount of ammonia to that from titania. The TPD
profile for the 95 : 5 sample starts off similar to that for
alumina but after the peak at 230 °C there are shoul-
ders at 345 and 500 °C. For the other mixed metal
oxides, the amount of ammonia desorbed at higher
temperatures increased with increasing titanium con-
tent, with ammonia still being desorbed at 600 °C. For
The IR results for ammonia adsorbed on to the
oxides are also given in Table III. Ammonia adsorbed
on titania had IR bands at 1594 and 1187 cm\ꢀ,
indicative of ammonia coordinated to a Lewis acid
site [34], see Fig. 8. The spectrum of ammonia adsor-
bed on alumina had bands at 1608, 1585 (sh) and
1258 cm\ꢀ corresponding to Lewis sites and absorp-
tions at 1680, 1470 (sh), 1459 and 1395 cm\ꢀ from the
ammonium ion (i.e. Brønsted sites) [31, 34]. Ammo-
nia adsorbed on the mixed oxides gave IR bands in
the 1615 and 1470 cm\ꢀ regions. The band at
&1615 cm\ꢀ appears to be comprised of two peaks,
one at &1610 and the other at &1620 cm\ꢀ, super-
imposing to form a broad peak. There were two peaks
n the other region, 1480 and 1455 cm\ꢀ, indicative of
TABLE III TPD and in situ FT—IR surface acidity results for Al O : TiO samples calcined at 600 °C.
ꢁ
ꢂ
ꢁ
Sample Al O : TiO
NH desorbed (10\ꢇ mol m\ꢁ)
Coordinated ammonia (cm\ꢀ)
Lewis: Brønsted
ꢁ
ꢂ
ꢁ
ꢂ
m (NH )
m (NH>)
ꢀ
ꢂ
!ꢀ
ꢆ
1
00: 0
5 : 5
0 : 10
0 : 20
5 : 25
3.6
4.2
6.1
6.6
11
1608, 1585(sh)
1624!
1618!
1480 (sh), 1459
1483, 1457
1475, 1460
—
1461!
—
2: 1
3: 2
1: 1
—
2: 1
Lewis
9
9
8
7
—
1626!
1594
0
: 100
14
!
wave number corresponds to the absorption maxima of a composite band.
5589

increasing titania content for alumina samples im-
pregnated with titania and the strength of the acid
sites was unaffected by the presence of titania [4]. The
acidity of materials obtained via coprecipitations is
dependent on preparation conditions [10]. It was
found that copreciptations using ammonia yielded
mixed oxides with the same acidity as alumina, but
when urea was used for the precipitation, some of the
mixed oxides had a greater number of acid sites than
alumina and some sites of higher acid strength. How-
ever, there was no real trend between titania content
and acidity. In contrast, for the single-phased
alumina—titania materials reported here, a clear rela-
tionship was found between titanium content and
both acid strength and density. Both increased with
increasing titanium content.
The XPS technique can, in favourable circumstan-
ces, give information about chemical composition and
environment. The Al 2p peak position for alumina
(
[
Table IV) was in agreement with literature values
36] (75.8 eV) and there was little change detected in
the Al 2p position for the mixed metal oxides,
5.6$0.2 eV. A single Ti 2p peak was detected for the
7
mixed metal oxides and, compared to pure titania was
shifted to higher binding energy by 0.8$0.1 eV. The
O 1s signal for the alumina—titania samples was
a single peak, no titania O 1s signal was detected for
any of the mixed metal samples. The shift in the Ti 2p
peak position indicates that the titanium was in
a modified environment, suggesting that the cations
were homogeneously dispersed throughout an
alumina matrix as opposed to a heterogeneous mix of
alumina and titania. This is also supported by the O 1s
spectra which showed no separate titania features for
the mixed metal oxides. These findings corroborate
the conclusions from the XRD data that the mixed
oxides calcined at 600 °C were single phase.
Figure 8 IR spectra of ammonia adsorbed on alumina : titania sam-
ples calcined at 600 °C. The spectra for the oxides have been sub-
tracted. Sample composition is given on the left-hand side of the
figure.
Brønsted acidity. However, the former band was only
a shoulder in the spectrum for the 75 : 25 sample.
Amide bands (&1530 cm\ꢀ) were not observed in any
of the IR spectra, indicating that ammonia had not
dissociated on the surface of the samples.
Using the extinction coefficients for the Lewis and
Brønsted IR absorptions (2.6 and 2.0 cm lmol\ꢀ, re-
spectively [35]) the ratio of the acid sites can be
calculated from the peak areas. The calculated Lewis
Brønsted ratios are given in Table III. Both alumina
and 75 : 25 sample had twice as many Lewis acid sites
as Brønsted, whereas the other two mixed oxides had
a slightly higher proportion of Brønsted sites, with the
The Ti 2p shift to higher binding energy indicates
that the titanium cations were in an electron-deficient
environment compared to that for pure titania, and
will therefore be better electron acceptors, i.e. stronger
acid centres. This is in agreement with the TPD results
which showed that the mixed metal oxides had stron-
ger acid sites than pure titania. The reason for the
titanium cations being more electron deficient could
be due to the charge imbalance resulting from a tita-
nium (IV) cation located in an aluminium (III) site, or
arise from the differing electronegativities of the two
cations [37].
9
0 : 10 sample having an equal number of Lewis and
Brønsted sites.
From the ammonia adsorption experiments, it has
been found that titania has Lewis surface acidity and
that these acid sites are of moderate strength (desorp-
tion at 350 °C). Alumina had both Lewis and Brønsted
acid sites in a 2 : 1 ratio, and there were both weaker
and moderate strength sites. The alumina—titania
samples had greater acid-site density than pure
alumina and the mixed oxides had stronger acid sites
than the pure oxides. The acid-site density and num-
ber of stronger acid sites increased with increasing
titanium content.
It is difficult directly to compare these results with
those in the literature because of the different tech-
niques used to measure acidity, giving relative rather
than absolute measurements of acidity. It has been
found that the number of acid sites decreased with
TABLE IV XPS results for Al O :TiO samples calcined at
ꢁ
ꢂ
ꢁ
6
00°C.
Sample
Al2p BE!
(eV)
Ti 2p BE"
(eV)
Surface composition
Al O : TiO
Al O :TiO
ꢁ
ꢂ
ꢁ
ꢁ
ꢂ
ꢁ
1
9
00: 0
5 : 5
90 : 10
0 : 20
5 : 25
75.6
75.5
75.6
75.7
75.8
—
—
—
460.2
460.2
460.3
460.4
459.5
95: 5
91: 9
75: 25
50: 50
—
8
7
0
: 100
!
Referenced to alumina O 1s (532.7 eV).
"As !except pure titania referenced to O 1s BE"530.7 eV.
5
590

normally reported in the literature. Furthermore,
these properties were essentially retained at higher
temperatures while the materials remained single
phase. The enhanced textural properties were in part
attributed to the limited amount of water used in the
preparation and the removal of the solvent using a ro-
tary evaporator. For lower calcination temperatures,
the level of crystallinity of the oxides decreased with
increasing titania content. In contrast, the crystallinity
and the amount of a-alumina formed after calcination
at 1000 °C increased with increasing titanium content.
The textural properties of the mixed oxides were
affected by both composition and calcination temper-
ature. At lower calcination temperatures, the mixed
oxides had enhanced textural properties compared to
alumina. Pore size was related to the composition of
the cogel, i.e. the greater the titanium content the
smaller were the pores. However, the smaller pores
were lost as a result of sintering processes at higher
temperatures, so that by 1000 °C only macroporosity
remained. This resulted in the oxides with greater
titanium content (and smaller pores) losing a large
proportion of their surface area by 1000 °C.
XPS showed that the titanium ion was in a modified
state in the mixed oxides; the titanium cations were in
an electron-deficient environment producing en-
hanced surface acidity. These conclusions were sup-
ported by the ammonia TPD experiments which
showed that the incorporation of titanium into an
alumina matrix produced sites with stronger acidity
than the pure oxides. The mixed oxides were found to
have greater density of acid sites than alumina and
both density and strength of the acid sites were found
to increase with titanium content. The Lewis sites
density was found to increase with titanium content
and the density of Brønsted sites was found to reach
a maximum by 10% titanium content. This work has
shown that single-phase alumina—titania solid acids
have both stronger acid sites and greater acid-site
density. This, coupled with the their high surface area,
has produced materials with an even greater number
of acid sites per gram, making them useful solid acids
materials.
Figure 9 Acid site density versus surface composition for
alumina—titania samples calcined at 600°C: (᭿) Lewis, (᭹)
Brønsted.
From the surface composition determined by XPS,
Table IV, the oxides with up to 20% titania appear to
be homogeneous and exhibit no surface separation.
Surface enrichment of titania was detected for the
sample with a bulk titania content of 25%. The surface
enrichment may have been due to ion migration dur-
ing calcination or to the preparative conditions. The
homogeneity of cogels is sensitive to preparative con-
ditions (e.g. mixing and hydrolysis rate) and this may
have resulted in the surface enrichment observed.
Comparing surface composition with the acidity
results (Table III) shows that the density of acid sites
increased with titania content. Lewis and Brønsted
acidity is plotted against surface composition in Fig. 9.
This plot shows that the Lewis acid-site density
increased almost linearly with titania content, whereas
the number of Brønsted sites reached a maximum by
1
0% titania content. A dotted line was used in the
Brønsted plot to indicate that the density would not
necessarily decrease after 50% surface titania content.
Lewis acid density was a function of titanium content,
i.e. the substitution of titanium cations into an
alumina lattice created Lewis acid sites. However, for
Brønsted acidity although there was an initial increase
in density with increasing titanium content, a maxi-
mum was reached by 10% titania inclusion. The
density of Brønsted sites was not related to surface
titanium content alone. A Brønsted site also requires
a hydroxyl group, and it is postulated that the number
of sites was limited by the availability of hydroxyl
species. Based on this postulation it is expected that
Brønsted site density would remain at a constant level
until the surface characteristics became more like
titania, i.e. Lewis acidity only, whereupon Brønsted
density would decrease with titania content.
Acknowledgements
The authors thank Dr David R. Pyke for the helpful
discussions and advice given. We also acknowledge
the assistance given by other members of the group,
C.R. Werrett and R.C. Reynolds for running the XPS
analysis, D. Croci for carrying out the surface area
and porosity measurements and K.K. Mallick for
DTA—TG and XRD experiments.
4
. Conclusion
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5
592