Structure and acidic properties of phosphate-modified zirconia

Article abstract of DOI:10.1021/jp963785x

Phosphate-modified zirconia was prepared by impregnation of Zr(OH)₄ and of m-ZrO₂ with an aqueous solution of (NH₄)₂HPO₄. Similar to sulfate and tungstate, phosphate shows a strong stabilizing effect on the specific surface area and the tetragonal phase of zirconia when using Zr(OH)₄ as the precursor material. The adsorbed phosphate species were characterized by Raman, DRIFT, and ³¹P-MAS-NMR spectroscopies as pyrophosphates, chelate-bonded orthophosphates, and oligophosphates. Only a distortion of the geometry of the adsorbed phosphate species was found after dehydration of the materials in contrast to ZrO₂/SO₄. FT-IR spectroscopy with CO as a probe molecule was used to characterize the acidic properties of ZrO₂/PO₄. New hydroxyl groups, probably P-OH groups with enhanced protonic acidity as compared to pure zirconia but lower than that of ZrO₂/SO₄, were observed. In addition basic Zr-OH groups were observed. Furthermore, strong Lewis acidic centers (eus Zr⁴⁺), comparable to ZrO₂/SO₄, are formed by phosphate modification of zirconia.

Full text of DOI:10.1021/jp963785x

J. Phys. Chem. B 1997, 101, 4681-4688
4681
Structure and Acidic Properties of Phosphate-Modified Zirconia
D. Spielbauer, G. A. H. Mekhemer,† T. Riemer, M. I. Zaki,‡ and H. Kno1zinger*
Institut fu¨r Physikalische Chemie, UniVersita¨t Mu¨nchen, Sophienstrasse 11, 80333 Munich, Germany
ReceiVed: NoVember 13, 1996; In Final Form: March 24, 1997X
Phosphate-modified zirconia was prepared by impregnation of Zr(OH)4 and of m-ZrO2 with an aqueous solution
of (NH4)2HPO4. Similar to sulfate and tungstate, phosphate shows a strong stabilizing effect on the specific
surface area and the tetragonal phase of zirconia when using Zr(OH)4 as the precursor material. The adsorbed
phosphate species were characterized by Raman, DRIFT, and 31P-MAS-NMR spectroscopies as pyrophosphates,
chelate-bonded orthophosphates, and oligophosphates. Only a distortion of the geometry of the adsorbed
phosphate species was found after dehydration of the materials in contrast to ZrO2/SO4. FT-IR spectroscopy
with CO as a probe molecule was used to characterize the acidic properties of ZrO2/PO4. New hydroxyl
groups, probably P-OH groups with enhanced protonic acidity as compared to pure zirconia but lower than
that of ZrO2/SO4, were observed. In addition basic Zr-OH groups were observed. Furthermore, strong
Lewis acidic centers (cus Zr4+), comparable to ZrO2/SO4, are formed by phosphate modification of zirconia.
Introduction
wt % PO4. After calcination at T2 ) 773-1073 K, samples
were obtained, which are denoted Zr(393)P(T2). Additional
The modification of zirconia by sulfate and tungstate is
discussed in a number of recent publications, due to the strong
acidic character of these materials and their role as possible
catalysts, e.g., for the isomerization of n-alkanes and the
alkylation of alkanes with olefins.1 In the present paper the
effects of the modification of zirconia by the addition of
phosphate should be discussed. Whereas zirconium phosphates
and their catalytic properties are well-known in the literature,
phosphated zirconia was first investigated as a material for high-
performance liquid chromatography (HPLC).2 A most recent
work describes the preparation of zirconia-phosphate aerogels3,4
and their catalytic properties for the 1-butene isomerization
reaction.3 From their layered structure zirconium-phosphates
were first discussed as ion-exchanging materials.5 After cal-
cinination at 623-973 K an increasing Brønsted acidity was
obtained, which was attributed to the condensation reaction
between layers forming pyrophosphate ions.6 These materials
are able to catalyze the cis-trans isomerization of 2-butene or
the dehydration of isopropyl alcohol or butanols.6,7 Interesting
properties were also observed after partial exchange of H+ by
metall ions.7,8
samples were prepared in the same way by phosphation of a
monoclinic ZrO2, which was obtained by calcination of the
Zr(OH)4 at 873 K for 4 h and which had a specific surface area
of 35 m2/g. This dried material, with a nominal content of 6
wt % PO4, was calcined at T2 ) 773-973 K and denoted
Zr(873)P(T2).
The phosphate contents of the samples after calcination were
estimated from 31P-MAS-NMR spectra and from the intensity
of the PO bands in the Raman spectra. Whereas for the samples
Zr(393)P(T2) no loss of phosphate was found for any calcination
temperature T2, the phosphate content decreases from 6 wt %
PO4 to about 3.5-4 wt % for the samples Zr(873)P(T2) by
increasing the calcination temperature T2 from 773 to 873 K.
Surface Area Measurements and X-ray Diffraction. The
specific surface areas were measured by the BET method with
a Sorptomatic 1800 from Carlo Erba. X-ray diffraction was
performed by a Siemens D5000 Diffraktometer with Ni-filtered
Cu KR radiation under ambient conditions. The crystallographic
phase compositions were calculated by the method of Toraya
et al.10
Laser Raman Spectroscopy. For laser Raman spectroscopy
(LRS) an in situ Raman cell that was described previously11
was used. This cell allowed the heating of the samples at
temperatures up to 770 K in the presence of either static or
flowing gas atmospheres of any kind. Samples were pressed
into special stainless steel holders. Raman spectra were recorded
on a Dilor (OMARS 89) spectrometer which was equipped with
a Princeton Applied Instruments optical multichannel analyzer
(OMA). The Raman spectra were excited by the 488 nm line
of an Ar+-ion laser model series 2020 from Spectra Physics.
The laser power at the sample position was 30 mW. Spectra
were recorded using the scanning multichannel technique
(SMT).12
Infrared Spectroscopy. Transmission IR and DRIFT spectra
were recorded with a Bruker IFS-88 FT-IR spectrometer
equipped with a liquid nitrogen cooled MCT detector. For
spectra in transmission, a specially designed cell, which was
described in detail elsewhere,13 allows in situ pretreatment at
temperatures up to 773 K and permits recording of spectra
between 80 and 300 K in vacuo (e10-5 mbar) or in static gas
atmosphere. The samples were prepared as thin self-supporting
The aim of the present work was to investigate the change
of the structural and acidic properties of zirconia after adsorption
of phosphate on (i) monoclinic zirconia and (ii) zirconium
hydroxide with a subsequent calcination at 773-1073 K.
Furthermore, the results will be compared with the correspond-
ing sulfated materials.9
Experimental Section
Catalyst Preparation. Zr(OH)4 type XZO 632/3, dried at
393 K, was obtained from MEL, Manchester, England. Pure
zirconia as a reference was prepared by calcination at T1 ) 773-
973 K for 1 h. This materiel is denoted Zr(T1). Pure Zr(OH)4
was suspended in an aqueous solution of (NH4)2HPO4 (p.a.
Merck) for 1 h and subsequently dried by evaporation of the
water. The dried phosphated material contained nominally 6
* Author to whom correspondence should be addressed.
† On leave from Department of Chemistry, Faculty of Science, Minia
University, El-Minia, Egypt.
‡ Department of Chemistry, Kuwait University, Kuwait.
X Abstract published in AdVance ACS Abstracts, May 15, 1997.
S1089-5647(96)03785-6 CCC: $14.00 © 1997 American Chemical Society

4682 J. Phys. Chem. B, Vol. 101, No. 23, 1997
Spielbauer et al.
Figure 1. BET surface areas of Zr(T1) (+), Zr(393)P(T2) (∇), and Zr-
(873)P(T2) ()).
wafers (typically ca. 20 mg/cm2). These specimens were
pretreated in situ by calcination at 723 K in O2 for 1 h and
subsequent evacuation at 723 K for 1 h. The spectra were
recorded with a spectral resolution of 2 cm-1. CO (g99.997%,
Linde AG) was used as a probe molecule. CO was further
purified by passing it through an Oxisorb cartridge. Adsorption
of CO was carried out at 77 K by a stepwise increase of the
CO pressure from 0.5 to 40-55 mbar.
For DRIFT spectra a Spectratech cell was used, which allows
pretreatment in static or flowing gas atmospheres up to 693 K.
Solid State NMR. NMR measurements were performed on
a Bruker MSL 400 NMR spectrometer with a magnetic field
strength of 9.4 T and a 31P-NMR frequency of 161.96 MHz. A
sample of 85% H3PO4 was used as an external standard with
an accuracy of (0.2 ppm. The MAS rotation frequency was 8
kHz, and proton decoupling was applied. The samples were
pretreated in a quartz reactor at 723 K in O2 for 1 h and then
evacuated (<10-3 mbar) at 723 K for 1 h. The powder samples
were subsequently transferred into NMR tubes connected to the
reactor tube. These NMR tubes were sealed off without
exposure to the atmosphere.
Figure 2. Raman spectra of Zr(393)P(T2) at ambient conditions with
(a) T2 ) 773 K, (b) 873 K, (c) 973 K, and (d) 1073 K.
Zr(OH)4.14 The samples Zr(T1) and Zr(873)P(T2) show pre-
dominantly (>90%) the pattern of m-ZrO2.
Structure of Adsorbed Phosphate Species. Raman Spectra
of Hydrated Samples. Since the tetragonal and the monoclinic
phases of zirconia show intense bands of Zr-O vibrations up
to 650 cm-1 in the Raman spectra,15 the discussion must be
limited to P-O stretching vibrations greater than 750 cm-1
.
Consistent with the results from X-ray diffraction, the Raman
spectra of samples Zr(T1) and Zr(873)P(T2) predominantly show
the bands of the monoclinic phase, whereas for samples
Zr(393)P(T2) mostly bands of the tetragonal phase were
observed, besides a high background from amorphous parts
present at low calcination temperatures T2.
Because of this background and the intense Zr-O bands, the
spectral ranges of P-O vibrations shown in the figures are
corrected by background subtraction. Figure 2 shows the Raman
spectra of the samples Zr(393)P(T2) at ambient conditions.
Calcination at 773 K (spectrum a in Figure 2) causes a band at
995 cm-1 with shoulders at 1048 and 1133 cm-1 and an
additional weak band at 750 cm-1. Corresponding to literature
data of Na4P2O7 (see Table 1), these bands can be attributed to
pyrophosphates (P2O74-). Different frequencies were reported
Results and Discussion
BET Surfaces and X-ray Diffraction. Phosphate modifica-
tion of Zr(OH)4 for preparation of the samples Zr(393)P(T2)
increases the surface area as compared to the pure zirconia
samples Zr(T1) (see Figure 1). Increasing the calcination
temperature lowers the surface areas for pure and phosphate-
modified Zr(OH)4. No loss of surface area with calcination
temperature was observed for the samples Zr(873)P(T2), sug-
gesting that the adsorption of phosphate on ZrO2 suppresses
sintering and growth of the crystallites. A similar effect was
found for sulfated ZrO2.14
X-ray diffraction of the samples Zr(393)P(T2) showed pre-
dominantly (>90%) the pattern of tetragonal ZrO2, even at the
highest calcination temperature. At temperatures T2 up to 873
K additional broad reflections of amorphous zirconia were
observed, indicating a strong perturbation of the crystallization
of ZrO2 by the presence of phosphate. Because of the thermal
stability of phosphate the phase transition to m-ZrO2 was largely
suppressed even at this high temperature in contrast to sulfated
19
for Raman spectra of cubic ZrP2O7 with bands at 990, 1000,
.
1094, and 1200 cm-1 A weak band at 886 cm-1, which
increases with calcination temperature T2 (see spectra b-d in
Figure 2), suggests the presence of an additional chelated or
doubly bridged PO4 species (vide infra) (Chart 1), bearing an
isolated P-OH group. Raising the calcination temperature
decreases the intensity of the band at 995 cm-1 whereas the
band at 1048 cm-1 is simultaneously increased together with a
new shoulder at 1020 cm-1. As the intensity of the band at
750 cm-1 shows no change with T2, further condensation of
the pyrophosphates into oligophosphates can obviously be
excluded. Corresponding to this and the results of the 31P-MAS-
NMR spectra (vide infra), the observed changes in the Raman
spectra rather suggest a modification of the chemical environ-

Phosphate-Modified Zirconia
J. Phys. Chem. B, Vol. 101, No. 23, 1997 4683
TABLE 1: P-O Stretching Vibrations of Some Phosphorous-Oxy Compounds
reference compound
symmetry
Td
ref
16
Na2PO4
938, 1017
K2HPO4
OPF3
1084, 970 (PO3), 891 (P-OH)
16
16
6
C3V
1415
(Zr-O)3P-OH
(R-Zr(HPO4)2‚H2O)
KH2PO4
[Co(NH3)4PO4]‚2H2O
[Co(NH3)4(OH2)(OPO3H)]J‚2H2O
Na4P2O7
1140, 1105,
910 (P-OH)
1155, 1071 (PO2), 1027, 877 (P(OH)2)
1109, 1043, 918, 895
1090, 1044, 982, 878 (P-OH)
1120-1150 (PO3), 1020 (PO3), 915 (νas, P-O-P), 730 (νs, P-O-P)
C2V
16
17
17
18
C
2V (chelate)
Cs
C2V
Figure 4. DRIFT spectra of Zr(393)P(T2) at ambient conditions with
(a) T2 ) 773 K, (b) 873 K, (c) 973 K, and (d) 1073 K.
It should be noted that further statements about the coordina-
tion geometry of the pyrophosphate and oligophosphate species
are not possible due to the line width of the P-O bands in the
obtained spectra of the samples.
Figure 3. Raman spectra of Zr(873)P(T2) at ambient conditions with
(a) T2 ) 773 K, (b) 873 K, and (c) 973 K.
DRIFT Spectra of Hydrated Samples. The DRIFT spectrum
of Zr(393)P(773) (see spectrum a in Figure 4) shows bands at
1133 and 1095 cm-1. These frequencies are in agreement with
IR data of amorphous ZrP2O7,6,20 whereas an intense band at
760-780 cm-1, which could also be identified in the spectra
of pure ZrO2 samples, prevents the identification of P-O-P
bands at about 750 cm-1. Raising the calcination temperature
T2 increases the intensity of the band at 1095 cm-1, and new
bands at 1185, 948, and 918 cm-1 are observed (see spectra
b-d in Figure 4). The bands at 1185, 1095, and 948 cm-1 are
tentatively attributed to another kind of pyrophosphate species
with a different chemical environment. The band at 918 cm-1
is probably related to the band at 886 cm-1 observed in the
Raman spectra (see spectra b-d in Figure 2). These bands are
assigned respectively to the antisymmetric and symmetric
O-P-O stretching vibration of a doubly bridging (Chart 1a)
or chelating PO4 group (Chart 1b), as, e.g., in the complex
[Co(NH3)4PO4]‚2H2O with chelate type phosphate with bands
at 918 and 895 cm-1 (see Table 1). Chelate-bonded PO4 groups
CHART 1: Model of Adsorbed (a) Doubly Bridged and
(b) Chelating PO4 Groups
ment of the adsorbed pyrophosphate species, namely a more
covalent bonding character.
The P-O bands in the Raman spectra of the samples
Zr(873)P(T2) appear at higher wavenumbers (see Figure 3). The
sample calcined at 773 K (see spectrum a in Figure 3) shows a
band at 1035 cm-1 with additional shoulders at 965, 1080, and
1253 cm-1. Due to the results of the 31P-MAS-NMR spectra
(vide infra) the P-O bands in the Raman spectra of these
samples could also be attributed to pyrophosphates. The
characteristic stretching vibration of the P-O-P groups at about
750 cm-1 could not be identified for these samples, because of
the superposition of an overtone at about 760 cm-1 of Zr-O
fundamental vibrations of monoclinic ZrO2. Increasing the
calcination temperature to 873 K decreases the total intensity
of the P-O bands. The band at 1035 cm-1 and the shoulders
at 965 and 1253 cm-1 are less pronounced at higher tempera-
tures T2.
-
with PO2 groups were proposed by Boyse and Ko3 on the
surface of calcined ZrO2/PO4 gels.
The DRIFT spectra of the samples Zr(873)P(T2) show bands
at 1151, 1090, and 943 cm-1 (see Figure 5), which can be
attributed to pyrophosphate species.6,20 The intensity of the
bands decreasessconsistent with the Raman spectrasby raising
the calcination temperature T2 from 773 to 873 K (see spectra

4684 J. Phys. Chem. B, Vol. 101, No. 23, 1997
Spielbauer et al.
Figure 5. DRIFT spectra of Zr(873)P(T2) at ambient conditions with
(a) T2 ) 773 K, (b) 873 K, and (c) 973 K.
Figure 7. Raman spectra of Zr(873)P(T2) after calcination at 773 K
(1 h), measured at 773 K with (a) T2 ) 773 K, (b) 873 K, and (c) 973
K.
TABLE 2: Chemical Shifts δP of Some Crystalline
Na-Phosphates and Zirconium Phosphor-Oxy Compounds
reference compound
δP/ppm
ref
Na3PO4‚12H2O
Na2HPO4
NaH2PO4‚2H2O
Na4P2O7
Na3HP2O7‚H2O
Na2H2P2O7
Na5P3O10
6.9
-0.2
0.4
1.3, 2.9
-2.3, -5.0
-8.2
1.2, -7.4
-19.7, -22.4
-16.6
-21.7
-34 to -43
22
22
22
22
22
22
22
22
26
26
26
Na4P4O12‚4H2O
R-Zr(HPO4)2‚H2O
ꢀ-Zr(HPO4)2
a
ZrP2O7
a Prepared by evacuation of R-Zr(HPO4)2‚H2O or ꢀ-Zr(HPO4)2 at
1323 or 1473 K, respectively.
species on the zirconia surface, as observed for sulfated zirconia
by drastic changes in the Raman and IR spectra,14,21 can
probably be excluded. However, the small shifts of the
frequencies of the P-O stretching vibrations after dehydration
of the samples suggest gradual changes of the molecular
geometries and of the P-O bond orders. Samples calcined at
higher temperature (see spectra b-d in Figure 6) show weak
bands at 1380 and 1450 cm-1, possibly indicating the presence
of carbonates.
In the Raman spectra of the samples Zr(873)P(T2), measured
at 773 K (see Figure 7), similar bands at 940 (sh), 1028, and
1073 (sh) are obtained, whereas no bands between 1200 and
1600 cm-1 were observed.
Figure 6. Raman spectra of Zr(393)P(T2) after calcination at 773 K
(1 h), measured at 773 K with (a) T2 ) 773 K, (b) 873 K, (c) 973 K,
and (d) 1073 K.
a and b in Figures 3 and 5). No bands are observed for chelate
type PO4 species, which is again consistent with the Raman
spectra.
Raman Spectra of Dehydrated Samples. The Raman spectra
(recorded at 773 K) of dehydrated samples Zr(393)P(T2) showed
a shift of the low-frequency bands at 975 and 1020 cm-1 to
938 and 1015 cm-1, respectively, whereas the band at 1048
cm-1 is shifted to 1060 cm-1 (see Figure 6). The observed
frequency shifts are increased by about 10 cm-1 when cooling
the samples to room temperature (not shown), and the bands at
750 and 886 cm-1, which could not be identified in the spectra
measured at 773 K, were then detected. From these results, a
modification of the coordination symmetry of the phosphate
31P-MAS-NMR Spectra of Dehydrated Samples. As can be
seen in Table 2, which summarizes the chemical shifts δP of
some crystalline Na-phosphates,22 δP shifts to negative values
3-
4-
with increasing chain length (PO4 < P2O7 < P3O105-...)
and with increasing degree of protonation (P2O74- < HP2O7
3-
< H2P2O72-). No simple trends can be observed for the
presence of crystal water, since the crystalline structure also

Phosphate-Modified Zirconia
J. Phys. Chem. B, Vol. 101, No. 23, 1997 4685
TABLE 3: Chemical Shifts δP of Dehydrated ZrO2/PO4
Samples
sample
T2/K
δP/ppm
Zr(393)P(T2)
773
873
973
-6.9
-12 (sh, m) -17 (sh, w)
-6.9 (sh, m) -11.2
-6.9 (sh, w) -11.5
-12.8
-17 (sh, m)
-17 (sh, m)
1073
Zr(873)P(T2)
773 -18
873 -17
973 -16
-31
-31
-31
changes. More negative chemical shifts were found for cyclic
phosphates23 and branched phosphates23 as well as phosphate
in SiO2,23 AlPO4,24 and phosphate-modified USY zeolites25 with
a more covalent character of the P-O-Si or P-O-Al bonds.
However, for zirconium phosphates and zirconium pyrophos-
phate (ZrP2O7) with enhanced covalent character of the P-O-
Zr bonds, strong negative chemical shifts were observed26,27 (see
Table 2).
The results of the 31P-MAS-NMR spectra of the dehydrated
phosphated zirconia samples are presented in Table 3. Zr-
(393)P(773) shows a broad signal at -6.9 ppm with a shoulder
at about -12 ppm. Corresponding to the literature data of
crystalline phosphates (see Table 2) and the results of the Raman
and DRIFT spectra, the signal at -6.9 ppm is attributed to
predominant ionic, protonated pyrophosphate species. The
shoulder at -12 ppm suggests the presence of more highly
condensed phosphates and/or pyrophosphates with a more
covalent P-O-Zr bond. Raising the calcination temperature
T2 to 873 K increases the intensity of the signal at about -12
ppm, whereas the signal at ca. -7 ppm decreases and an
additional shoulder at -17 ppm could be observed. As the
intensity of the P-O-P stretching vibration at 750 cm-1 is
largely equal in the Raman spectra of all samples Zr(393)P-
(T2), the observed shift of the signals in 31P-MAS-NMR spectra
are probably not caused by the condensation to oligophosphates,
but rather by the transformation of ionic pyrophosphates to more
covalently bonded species, as observed for zirconium phos-
phates.2,26 Calcination at 973 and 1073 K decreases the signal
intensity at -7 ppm and increases that of the signal at -17
ppm further. For the sample Zr(393)P(1073) no signal of ionic
pyrophate at -7 ppm was observed.
Figure 8. FT-IR spectra of Zr(873)P(T2) with (a) T2 ) 773 K, (b) 873
K, (c) 973 K and (d) Zr(873) after calcination at 723 K (1 h), measured
at 77 K.
TABLE 4: Hydroxyl Stretching Frequencies of Acidic OH
9
Groups of ZrO2, ZrO2/PO4, and ZrO2/SO4 prior to and
after Addition of CO Under Saturation Conditions at 77 K
sample
T2/K
ν
OH/cm-1
ν
OH-CO/cm-1
∆νOH/cm-1
Zr(873)
3680
3664
3664
3664
3664
3664
3660
3640
3640
3610
3489
3498
-70
-175
-166
Zr(873)P(T2)
Zr(393)P(T2)
773
873
973
773
873
973
873
873
3533 (sh), 3502 -131, -162
∼3530
-134
∼3500
-164
∼3500
-160
a
ZrO2/SO4
ZrO2/SO4
3480, 3420
3480, 3420
-160, -220
-160, -220
b
In the spectra of the samples Zr(873)P(T2) two broad signals
at -18 to -16 ppm and at -31 ppm were obtained (see Table
3). The total intensity of the signals decreases by raising the
calcination temperature from 773 to 873 K. The low specific
surface area of the phosphate-free precursor material results in
an enhanced concentration of phosphate species at the surface,
which favors the condensation to oligophosphates. This con-
densation and the covalent character of the P-O-Zr bond
causes the shift of the signals in 31P-MAS-NMR spectra to more
negative values.
Acidity of ZrO2/PO4 FT-IR Spectra with Adsorbed
Carbon Monoxide. Hydroxyl Stretching Region (3300-3900
cm-1). After dehydration of pure zirconia Zr(873) two bands
at 3780 and 3680 cm-1 are observed in the IR spectrum (see
spectrum d in Figure 8), which are assigned to isolated and
bridging surface OH groups, respectively, of monoclinic zir-
conia.28,29 An additional weak band at 3740 cm-1, also found
in the spectra of the phosphated samples, is probably caused
by small amounts of silica,30 which is an impurity of the used
Zr(OH)4. Addition of CO causes a shift of the band of acidic
OH groups at 3680 to 3610 cm-1, whereas the other two bands
of basic OH groups are not affected (see Table 4).
a Prepared by calcination of sulfated Zr(OH)4 at 873 K (1 h).
b Prepared by calcination of sulfated Zr(873) at 873 K (1 h).
cm-1 prior to the adsorption of CO. A weak shoulder at ca.
3670 cm-1 can be clearly identified as an additional OH band
after the adsorption of CO (vide infra). Raising the calcination
temperature T2 causes an increased intensity of the band at 3770
cm-1 as compared to the band at 3664 cm-1. The stretching
frequencies of the OH bands of the phosphated samples may
suggest an assignment to Zr-OH groups with enhanced acidity
as compared to pure m-ZrO2 as indicated by the shift of the
bands by about -10 cm-1. However, (i) R-zirconium phosphate
shows a band at 3660-3670 cm-1 31
which was attributed to
,
POH groups; (ii) the small line width of the bands at 3770 and
3664 cm-1 as compared to pure zirconia (Zr(873)) or sulfated
zirconia9 under the same conditions suggests the presence of
isolated hydroxyl groups with OH bands, which are not
broadened by the inhomogenety of the surface; (iii) two bands
at 1200 and 1270 cm-1 (not shown), characteristic of P-O-H
(out-of-plane) deformation vibrations,31,32 suggest the formation
of POH groups.
Figure 9 shows the transmission IR spectra of the sample
Zr(873)P(873) after the exposure to increasing pressures of CO,
The IR spectra of phosphated m-ZrO2 (Zr(873)P(T2)) (see
spectra a-c in Figure 8) show bands at 3770, 3743, and 3664
which induces the formation of a new, broad band at 3498 cm-1
,

4686 J. Phys. Chem. B, Vol. 101, No. 23, 1997
Spielbauer et al.
Figure 10. FT-IR spectra of Zr(393)P(T2) with (a) T2 ) 773 K, (b)
873 K, and (c) 973 K after calcination at 723 K (1 h), measured at 77
K.
Figure 9. FT-IR spectra of the hydroxyl stretching region of Zr(873)P-
(873) after calcination at 723 K (1 h) after adsorption of (a) 0.0, (b)
0.5, (c) 1.0, (d) 3.0, (e) 5.0, (f) 10, (g) 20, (h) 30, (i) 40, and (j) 55
mbar CO, measured at 77 K.
Addition of CO causes a shift of the band of the acidic OH-
groups at 3664 cm-1 to 3530 cm-1 (T2 ) 773 K) and to 3500
cm-1 (T2 ) 873 and 973 K), respectively (see Table 4). The
increased negative shifts of the O-H stretching mode for
phosphate-modified materials relative to the Zr(873) reference
clearly indicate an enhanced Brønsted acid strength of the
former. The increase of the protonic acidity above a calcination
temperature of 773 K was also found by Boyse and Ko3 during
calcination of ZrO2/PO4 gels.
Comparing with sulfated zirconia (see Table 4), phosphate
modification induces lower protonic acidity on the surface of
zirconia; furthermore, basic hydroxyl groups were observed on
the phosphate-modified materials in contrast to ZrO2/SO4.9 It
should be noted that the line widths of the Zr-OH bands of
phosphate-modified zirconia are significantly decreased as
compared to pure zirconia, possibly due to an enhanced
separation of Zr-OH groups on ZrO2/PO4 or a reduction of
defects on the surface of zirconia by the modification with
phosphate.
whereas the band at 3664 cm-1 simultaneously decreases in
intensity. The enhanced shift of the OH band by 166 cm-1, as
compared to pure m-ZrO2 (see Table 4), indicates an increased
protonic acidity of zirconia by phosphate modification. It is
noteworthy that the band at 3664 cm-1 does not completely
disappear under saturation conditions of adsorbed CO (55 hPa),
but a residual band at 3670 cm-1, probably Zr-OH groups,
can be identified. Furthermore, the band at 3770 cm-1 is not
shifted to lower wavenumbers by the addition of CO and is
consequently attributed to Zr-OH groups, whereas the band at
3664 cm-1 is assigned to P-OH groups (vide supra). A similar
band of P-OH groups at 3670 cm-1 was observed in the IR
spectrum of phosphate-modified Aerosil silica surface.33 Ad-
sorption of benzene shows enhanced protonic acidity for these
groups, as compared to the pure support.
The IR spectra of the samples Zr(393)P(T2) (see Figure 10)
show bands of unperturbed OH groups at 3770, 3743, 3670 (sh),
and 3664 cm-1 with enhanced line widths as compared to the
samples Zr(873)P(T2) (see spectra a-c in Figure 8). The
intensity of the band at 3770 cm-1 increases as compared to
the band at 3664 cm-1, similar to the samples Zr(873)P(T2),
with increasing calcination temperature T2. Since the samples
Zr(393)P(T2) predominantly consist of t-ZrO2, which shows only
Carbonyl Stretching Region (2100-2250 cm-1). The spectra
in the CO stretching region were obtained by subtracting the
(background) spectra of the CO-free samples and the gas phase
spectra from the spectra of the samples with adsorbed CO. After
adsorption of 0.5 mbar CO on pure m-ZrO2 Zr(873) (not shown)
a band at 2188 cm-1 was observed, which is characteristic for
CO coordinated to Lewis acidic centers (Zr4+) via an interaction
of the nonbonding 5σ orbital of CO with the Zr4+ center (see
Table 5). At a pressure of 1.0 mbar CO, the band was shifted
to lower wavenumbers by an inductive effect caused by
neighboring CO molecules, which is characteristic of the
semiconducting oxides such as TiO2, ZrO2, and HfO2.34 An
additional band at 2177 cm-1, caused by CO adsorbed on OH
groups35,36 (see Table 4), is shifted to 2168 cm-1 at higher
pressures of CO. A band at 2152 cm-1 at highest pressures of
CO (40 mbar) is indicative of physisorbed CO.
a single OH band at 3682 cm-1 29
the previously suggested
,
assignment of the band at 3770 cm-1 to Zr-OH groups may
be uncertain. However, after evacuation of R-zirconium
phosphate at 820 K31 additional bands at 3700 and 3770 cm-1
were assigned to Zr-OH bands, besides the band of POH groups
at 3660-3670 cm-1. These results on R-zirconium phosphate
are possibly indicative for the formation of a kind of bulk
zirconium phosphate on the phosphate-modified zirconia. In
contrast to samples Zr(873)P(T2), only one band of P-O-H
(out-of-plane) deformation vibrations was found in the spectra
of Zr(393)P(T2) at 1240 (T2 ) 773 K) and at 1205 cm-1 (T2 )
873 and 973 K), respectively (not shown).
Figure 11 shows the IR spectra of the sample Zr(873)P(873).

Phosphate-Modified Zirconia
J. Phys. Chem. B, Vol. 101, No. 23, 1997 4687
TABLE 5: Carbonyl Stretching Frequencies of CO
9
Adsorbed to ZrO2, ZrO2/PO4, and ZrO2/SO4 at 77 K
ν
CO/cm-1
ν
CO/cm-1
OHb
ν
CO/cm-1
phys.b
sample
Zr(873)
Zr(873)P(T2)
T2/K
cus Zr4+ a
2188
2209
2209
2209
2204
2204
2207
2204
2204
2168
2169
2168
2169
2170
2169
2169
2166
2167
2152
2141
2142
2141
2146
2144
2144
2147
2149
773
873
973
773
873
973
873
873
Zr(393)P(T2)
c
ZrO2/SO4
ZrO2/SO4
d
a Adsorption of 55 mbar CO and subsequent evacuation at 77 K for
15 min. b Adsorption of 55 mbar CO (saturation conditions). c Prepared
by calcination of sulfated Zr(OH)4 at 873 K (1 h). d Prepared by
calcination of sulfated Zr(873) at 873 K (1 h).
Figure 12. FT-IR spectra of carbonyl stretching region of Zr(393)P-
(873) after calcination at 723 K (1 h) after adsorption of (a) 0.2, (b)
0.5, (c) 1.0, (d) 3.0, (e) 5.0, (f) 10, (g) 20, (h) 30, (i) 40, and (j) 55
mbar hPa CO, measured at 77 K.
to 973 K increases the amount of Lewis acid sites (not shown),
whereas their acid strength is not affected (see Table 5).
Samples Zr(393)P(T2) show slightly reduced Lewis acid
strength as compared to samples Zr(873)P(T2) (see Table 5).
Comparing the spectra of samples Zr(873)P(T2) and Zr(393)P-
(T2) (see Figures 11 and 12, respectively, for T2 ) 873 K) shows
that the amount of Lewis acid sites, as compared to the amount
of Brønsted acid sites, is higher for samples Zr(393)P(T2).
Increasing the pressure of CO causes a shift of the band for
CO coordinated to cus Zr4+ by -20 cm-1 (see Figure 12), as
observed for sample Zr(873)P(873).
Figure 11. FT-IR spectra of carbonyl stretching region of Zr(873)P-
(873) after calcination at 723 K (1 h) after adsorption of (a) 0.2, (b)
0.5, (c) 1.0, (d) 3.0, (e) 5.0, (f) 10, (g) 20, (h) 30, (i) 40, and (j) 55
mbar CO, measured at 77 K.
Concluding Remarks
Phosphate modification of Zr(OH)4 retards the crystallization
to ZrO2 and stabilizes the tetragonal phase of zirconia and the
specific surface area of the material. Due to the high thermal
stability of the adsorbed phosphate species, these effects are
more pronounced as compared to sulfated Zr(OH)4. The
adsorbed phosphate species, using Zr(OH)4 as the precursor
material, were identified by Raman, DRIFT, and 31P-MAS-NMR
spectroscopies as pyrophosphates and doubly bridging or
chelate-bonded orthophosphates. However, oligophosphates
were observed after phosphate adsorption on a well-crystallized
m-ZrO2. Dehydration of ZrO2/PO4 causes only a distortion of
the geometry of the adsorbed phosphate species, whereas
changes of the molecular symmetry of the sulfate species are
found for ZrO2/SO4, due to the formation of additional Zr-
O-S bonds or condensation of isolated sulfate molecules.
New hydroxyl groups, probably P-OH groups, were ob-
served in the FT-IR spectra of ZrO2/PO4. Adsorption of CO
shows enhanced protonic acidity as compared to pure zirconia,
but lower than that of ZrO2/SO4. Furthermore, basic Zr-OH
groups were found for ZrO2/PO4 prepared by phosphate
modification of Zr(OH)4 and m-ZrO2, whereas only acidic
At a pressure of 0.2 mbar CO (see spectrum a in Figure 11) an
asymmetric band at 2209 cm-1 indicates the presence of CO
coordinated to (different) cus Zr4+ centers. The shift of the
carbonyl stretching vibration by about 20 cm-1 suggests a
strongly enhanced Lewis acid strength as compared to phosphate-
free m-ZrO2. Increasing the pressure of CO causes a shift of
this band to 2189 cm-1 by inductive effects34 (see spectra b-j
in Figure 11). Comparing these results with those obtained for
sulfated zirconia9 suggests that phosphate modification induces
a slightly greater Lewis acid strength (see Table 5). However,
inductive effects between neighboring cus Zr4+ centers, causing
a shift of the carbonyl stretching vibration of CO coordinated
to cus Zr4+, are less pronounced for ZrO2/SO4 probably due to
a communication between adsorbed sulfate species and CO
coordinated to cus Zr4+ centers.9 Above 1 hPa CO, additional
bands at 2168 and 2142 cm-1 are obtained, which can be
attributed to CO H-bonded to hydroxyl groups and physisorbed
CO, respectively (see spectra d-j in Figure 11). Raising the
calcination temperature T2 for samples Zr(873)P(T2) from 773

4688 J. Phys. Chem. B, Vol. 101, No. 23, 1997
Spielbauer et al.
(11) Zeilinger, H. Doctoral Thesis, Universita¨t Mu¨nchen, 1991.
hydroxyl groups were identified for the corresponding sulfated
materials. While the Lewis acid strength of the observed cus
Zr4+ sites is comparable to that of ZrO2/SO4, inductive effects
between neighboring cus Zr4+ sites cause a more pronounced
low-frequency shift of the carbonyl stretching vibration of CO
coordinated to Zr4+ for phosphated zirconia, similar to the
unmodified oxide. These differences are probably due to the
absence of a communication between adsorbed phosphate
species and the Zr-CO adsorbates, an effect that was observed
on ZrO2/SO4.
(12) a)Knoll, P.; Singer, R.; Kiefer, W. Appl. Spectrosc. 1990, 44, 776.
b) Spielbauer, D. Appl. Spectrosc. 1995, 49, 650.
(13) Beutel, T. Dissertation Universita¨t Mu¨nchen, 1994.
(14) Spielbauer, D. Dissertation Universita¨t Mu¨nchen, 1995.
(15) Phillipi, C. M.; Mazdiyasni, K. S. J. Am. Ceram. Soc. 1971, 54,
254.
(16) Weidlein, J.; Mu¨ller, U.; Dehnicke, K. Schwingungsfrequenzen I,
Hauptgruppenelemente; Georg Thieme Verlag: Stuttgart, New York, 1981.
(17) Siebert, H. Z. Anorg. Allg. Chemie 1958, 296, 280.
(18) Simon, A.; Richter, H. Z. Anorg. Allg. Chemie 1959, 301, 154.
(19) Steger, E.; Leukroth, G. Z. Anorg. Allg. Chemie 1960, 303, 169.
(20) Horsley, S. E.; Nowell, D. V.; Stewart, D. T. Spectrochim. Acta
1974, 30A, 535.
Acknowledgment. This work was financially supported by
the Deutsche Forschungsgemeinschaft (SFB 338), the Bay-
erischer Forschungsverbund Katalyse FORKAT, and the Fond
der Chemischen Industrie. D.S. thanks the Studienstiftung des
deutschen Volkes for a grant. G.A.H.M. acknowledges the
Government of the Arab Republic of Egypt for a grant in the
framework of the Channel System.
(21) Morterra, C.; Cerrato, G.; Pinna, F.; Signoretto, M. J. Phys. Chem.
1994, 98, 12373.
(22) Griffiths, L.; Root, A.; Harris, R. K.; Packer, K. J.; Chippendale,
A. M.; Tromans, F. R. J. Chem. Soc., Dalton Trans. 1986, 2247.
(23) Duncan, T. M.; Douglass, D. C. Chem. Phys. 1984, 87, 339.
(24) Sanz, J.; Campelo, J. M.; Marinas, J. M. J. Catal. 1991, 130, 642.
(25) Corma, A.; Fornes, V.; Kolodziejski, W.; Matinez-Triguero, L. J.
J. Catal. 1994, 145, 27.
(26) Segawa, K.; Nakajima, Y.; Nakata, S.; Asaoka, S.; Takahashi, H.
J. Catal. 1986, 101, 81.
References and Notes
(27) Clayden, N. J. J. Chem. Soc., Dalton Trans. 1987, 1877.
(28) Agron, P. A.; Fuller, E. L.; Holmes, H. F. J. Colloid Interface Sci.
1975, 52, 553.
(1) Song, X.; Sayari, A. Catal. ReV.-Sci. Eng. 1996, 38, 329.
(2) Schafer, W. A.; Carr, P. W.; Funkenbusch, E. F.; Parson, K. A. J.
Chromatogr. 1991, 587, 137.
(3) Boyse, R. A.; Ko, E. I. Catal. Lett. 1996, 38, 225.
(4) Ciesla, U.; Schacht, S.; Stucky, G. D.; Unger, K. K.;. Schu¨th F.
Angew. Chem., Int. Ed. Engl. 1996, 35, 541.
(5) Kraus, K. A.; Phillips, H. O. J. Am. Chem. Soc. 1956, 78, 644.
(6) Segawa, K.; Kurusu, Y.; Nakajima, Y; Kinoshita, M. J. Catal. 1985,
94, 491.
(7) La Ginestra, A.; Patrono, P.; Bernadelli, M. L.; Galli, P.; Ferragina,
C.; Massucci, M. A. J. Catal. 1987, 103, 346.
(8) Clearfield, A.; Thakur, D. S. Appl. Catal. 1986, 26, 1.
(9) Spielbauer, D.; Mekhemer, G. A. H.; Zaki, M. I.; Kno¨zinger, H.
Catal. Lett. 1996, 40, 71.
(29) Fink, P.; Pohle, W.; Ko¨hler, A. Z. Chem. 1972, 12, 117.
(30) Yamaguchi, T.; Nakano, Y.; Tanabe, K. Bull. Chem. Soc. Jpn. 1978,
51, 2482.
(31) Beebe, T. P.; Gelin, P.; Yates, J. T. Surf. Sci. 1984, 148, 526.
(32) Busca, G.; Lorenzelli, V.; Galli, P.; La Ginestra, A.; Patrono, P. J.
Chem. Soc., Faraday Trans. 1 1987, 83, 853.
(33) Marchon, B.; Novak, A. J. Chem. Phys. 1983, 78, 2105.
(34) Bolis, V.; Morterra, C.; Fubini, B.; Ugliengo, P.; Garrone, E.
Langmuir 1993, 9, 1521.
(35) Zaki, M. I.; Kno¨zinger, H. J. Catal. 1989, 119, 311.
(36) Mirsojew, I.; Ernst. S.; Weitkamp, J.; Kno¨zinger, H. Catal. Lett.
1994, 24, 235.
(10) Toraya, H.; Yoshimura, M.; Somiya, S. Commun. Am. Ceram. Soc.
1984, C119.