Surface properties and CO adsorption on zirconia polymorphs

Article abstract of DOI:10.1016/j.molcata.2004.10.017

FT-IR spectroscopy and TPD profiles were performed to determine and compare the surface properties of amorphous, monoclinic and tetragonal zirconia polymorphs. It was found that zirconia polymorphs exhibited different surface hydroxyl and acid-base proper

Full text of DOI:10.1016/j.molcata.2004.10.017

Journal of Molecular Catalysis A: Chemical 227 (2005) 119–124
Surface properties and CO adsorption on zirconia polymorphs
Zhong-Yi Ma, Cheng Yang, Wei Wei, Wen-Huai Li, Yu-Han Sun∗
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China
Received 2 June 2004; received in revised form 9 October 2004; accepted 13 October 2004
Available online 25 November 2004
Abstract
FT-IR spectroscopy and TPD profiles were performed to determine and compare the surface properties of amorphous, monoclinic and
tetragonal zirconia polymorphs. It was found that zirconia polymorphs exhibited different surface hydroxyl and acid–base properties. These
differences had great influence on the behavior of CO adsorption and reaction. Formate species could be formed at higher temperature via CO
reaction with hydroxyl groups on the surface of amorphous and monoclinic zirconia, while bicarbonate and carbonate species were detected
on tetragonal zirconia, whether hydrogen introduced or not.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Zirconia; Surface hydroxyl; Acidity and basicity; CO adsorption
1. Introduction
were sensitive towards the surface structure, which were con-
veniently studied by infrared spectroscopy [7–11]. But there
raised the arguments about the nature of hydroxyl which was
caused by zirconia polymorphs. Yamaguchi et al. [11] con-
sidered that two types of hydroxyl existed on zirconia sur-
face and the terminal hydroxyl was more active and liable
than that of bridged one. However, Guglielminotti [12] ar-
gued that there was no apparent difference in reactivity for
hydroxyl groups in the case of CO hydrogenation. Bachiller-
Baeza et al. [13] found that another hydroxyl band located at
about 3740 cm−1, which could be contributed to bi-bridged
hydroxyl, only existed on tetragonal zirconia surface. Thus,
the systematic research should be performed to determine
and compare the surface hydroxyl on zirconia polymorphs at
same treatment condition.
On the other hand, surface acid–base sites were associ-
ated with the CO and CO2 adsorption and then played an
important role in catalytic performance. These surface sites
were produced during thermal dehydroxylation of metal ox-
ides, depending on their co-ordination symmetry. Hence, the
acid–base properties of zirconia have been studied by the ad-
sorption of probe molecules such as CO, CO2, pyridine and
NH3 in the literatures [7,14–16]. Lewis acid sites were more
abundant on monoclinic zirconia than on tetragonal zirconia,
andtheformerbroughtaboutstrongersurfaceadsorptionsites
During the last two decades, zirconia has attracted much
attention due to its special surface chemical properties, i.e.
both redox and acid–base properties [1]. As a result, zirconia
acted not only as the supports but also an active component
in most reactions [2–4]. For instance, Cu/ZrO2 catalyst
showed high catalytic performance for alcohol synthesis
from CO2/CO hydrogenation [2], in which zirconia showed
splendid synergetic effect with Cu. Bianchi et al. [3] reported
that there existed the overflow of absorbed hydrogen and
carbon monoxide in the interface of zirconia and copper.
Fisher and Bell [4] proposed that methanol synthesis mainly
performed on zirconia support by studying the surface
intermediate species from CO/H2 reaction on Cu/Zr/SiO2.
Meanwhile, zirconia polymorphs had a great influence on
catalytic performance for many reactions, which might be
contributed to the different surface properties of various
zirconia polymorphs [5,6].
¨
Zirconia surface contained Bronsted acidic and basic
hydroxyl groups and co-ordinatively unsaturated Lewis
acid–base Zr4+–O2− pairs. These surface hydroxyl groups
∗
Corresponding author. Tel.: +86 351 4049612; fax: +86 351 4041153.
E-mail address: yhsun@sxicc.ac.cn (Y.-H. Sun).
1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.10.017

120
Z.-Y. Ma et al. / Journal of Molecular Catalysis A: Chemical 227 (2005) 119–124
concerning CO2 adsorption than the latter [13]. Two distinct
Lewis acid sites have been observed on the zirconia surface
revealed by FT-IR of CO adsorption, which were assigned to
two Zr4+ cations, co-ordinatively unsaturated, exposed either
at different crystallographic or with different co-ordinative
unsaturations [15]. While the weak acid sites of Zr3+ was
only detected on monoclinic zirconia by the vacuum thermal
condensation of surface hydroxyls [16].
self-supporting wafer and treated in a quartz cell with CaF2
windows at 573 K and 10−3 Pa for 2 h. A background spec-
trum was collected at room temperature before the pyridine
was introduced. Then the sample was evaporated at 423 K
for 30 min to remove physical pyridine, and another spec-
trum was collected at resolution of 4 cm−1 and 64 times. The
difference spectrum was obtained by subtraction with back-
ground spectrum.
In view of the importance of the surface properties of
zirconia, particularly in the field of CO hydrogenation, the
adsorption of CO, CO2, pyridine and NH3 over zirconia
polymorphs was systematically carried out in the present
work. The main goal was to detect the influence of zirconia
polymorphs on the surface acid–base properties, and the
relation between surface hydroxyl groups with acid–base
sites. Also, we considered, whether the differences in surface
properties had some contribution to CO adsorption and
reaction in the presence or absence of hydrogen.
CO and CO/H2 diffuse reflectance FT-IR (DRIFT) were
recorded using Nicolet Magna-II 550 FT-IR spectrometer
equipped with Spectra-Tech Diffuse Reflectance Accessory
and a high temperature in situ cell with ZnSe windows. A
KBr beam splitter had been used with a TGS detector. One
set of stainless steel lines was built and connected to the cell.
This allowed in situ measurement for adsorption in a flow of
gas. Powder sample was patched on the cell and then treated
at 573 K in an Argon flow for 4 h. CO (99.999%) or CO/H2
(1:2 molar ratio) was introduced at 623 K for 15 min. Af-
ter system cooling down to room temperature, the spectrum
was collected at resolution of 8 cm−1and 400 scans. Each
spectrum was then referenced to a spectrum of the catalyst
collected at the same temperature under Argon flow before
adsorption, as appropriate.
2. Experimental
2.1. Sample preparation
Amorphous zirconia (am-ZrO2) was prepared from a so-
lution of oxychloride zirconium co-precipitation with am-
monia. After aging in the mother liquid for 3 h, the gel was
washed and filtered with distilled water until chloride test
negative for AgNO3. Then the precipitate was dried at 393 K
for 12 h and calcined at 623 K for 3 h. The precipitate pre-
pared as above was aged in the mother liquid at 323 K for 5
days, and then filtered, dried and calcined at 823 K for 3 h,
the sample was characterized by XRD to prove as monoclinic
zirconia (m-ZrO2). The tetragonal zirconia (t-ZrO2) was pre-
pared by co-precipitating a solution of oxychloride zirconium
and sodium carbonate, followed similar filtering and drying
to am-ZrO2 and m-ZrO2, and then calcined at 823 K for 3 h.
XRD patterns showed that am-ZrO2 was only a broad peak
at 30◦ while the diffraction peaks of monoclinic and tetrag-
onal zirconia were predominant. The BET surface areas of
am-ZrO2, m-ZrO2 and t-ZrO2 determined by N2 adsorption
were 214.5, 41.46 and 26.91 m2 g−1, respectively.
3. Results and discussion
3.1. Surface properties
It was shown that on the surface of highly hydrated zir-
conia, co-ordinated molecular water was eliminated upon
evacuation at 450 K and various types of surface hydrox-
yls remained up to 873 K [17]. Generally, three types of hy-
droxyl groups could be observed with bands at the region
of 3770–3750 cm−1, 3750–3720 cm−1 and 3660–3680 cm−1
[6,11,18], which were ascribed to terminal, bi-bridged and
tri-bridged hydroxyl groups, respectively. It was clearly in-
dicated that there existed great difference in hydroxyl groups
on the surface of zirconia polymorphs (see Fig. 1). Besides
terminal and tri-bridged hydroxyls located at 3770 cm−1 and
2.2. Characterization methods
NH3-TPD and CO2-TPD were carried out on a U-shape
quartz reactor. 0.2 g sample was pre-treated in an Argon flow
at 573 K for 4 h. Aftercooling to certain temperature, the sam-
ple was saturated with NH3 (373 K) or CO2 (RT), and then
purged for 30 min to sweep the physical molecules. Here-
after, the sample was heated to 773 K in an Ar flow at rate of
10 K min−1. The desorbed NH3 and CO2 were monitored by
a Balzers Omnistar 200 mass spectroscopy.
Pyridine adsorption (Py-FT-IR) was performed on the
Nicolet Magna-II 550 FT-IR spectrometer equipped with
TGS detector. About 15 mg sample was pressed to a ꢀ15 mm
Fig. 1. Difference transmission infrared spectra of surface hydroxyls on
three zirconia polymorphs vacuum activated at 573 K for 2 h.

Z.-Y. Ma et al. / Journal of Molecular Catalysis A: Chemical 227 (2005) 119–124
121
3658 cm−1, respectively, a broad peak at 3500 cm−1 was ob-
served on the surface of am-ZrO2, which was attributed to
the surface hydroxyl perturbed by hydrogen bond [19]. Three
types of hydroxyl located at 3772, 3681 and 3660 cm−1 ex-
isted on the surface of m-ZrO2, which could be ascribed
to one type of terminal and two types of tri-bridge hy-
droxyls, respectively. The former tri-bridged hydroxyl might
be located at crystallographic defective configurations and
the latter belonged to the flat patches of low-index crystal
planes [20]. While in the case of t-ZrO2 terminal, bi-bridged
and tri-bridged hydroxyls were detected at 3765, 3723 and
3677 cm−1. It was remarked that no bi-bridged hydroxyl was
detected on monoclinic and amorphous zirconia polymorphs.
Cerrato et al. [20] considered that, in the (1 1 1) crystal plane
of monoclinic zirconia, the distance between cus O2− and cus
Zr4+ was only 0.2 nm and did not allowed the dissociation of a
water molecule to form the terminal and bi-bridged hydroxyl
groups. Therefore, the type of surface hydroxyl groups re-
maining upon evacuation depended strongly on the structural
modification.
Fig. 3. NH3-TPD profiles of three zirconia polymorphs.
the hydroxyl region (Fig. 1), m-ZrO2 showed strong terminal
hydroxyl, which might be responsible for the strong Bronsted
acid sites.
¨
The acid sites distributions of three zirconia polymorphs
were characterized by the NH3-TPD. Fig. 3 showed two NH3
desorption peaks located at about 480 K and 570 K on three
samples, corresponding to a weak acid sites I and a moderate
acid sites II, respectively. For the acid sites I, the intensity
was almost the same on three zirconia polymorphs, while
the acid sites II on am-ZrO2 was weaker than those on m-
ZrO2 and t-ZrO2. The amount of acid sites on am-ZrO2 was
higher than those on two others (see Table 1), and t-ZrO2 had
the smallest amount of acid sites, which was in line with the
result of Py adsorption. Though the present value was about
1.55 nm−2 NH3 adsorbed on t-ZrO2 and 1.88 nm−2 NH3 on
m-ZrO2, the tendency that more acid sites existed on m-ZrO2
was consistent with the literature [14].
The identification of acid type could be achieved from FT-
IR spectra after pyridine adsorption. Pyridi−ni1um ion (formed
¨
at Bronsted acid sites) displayed 1540 cm bands and co-
ordinately bonded pyridine (formed at Lewis sites) gave
1445 cm−1 bands [21]. Fig. 2 showed the difference FT-
IR spectra of pyridine adsorption on three zirconia poly-
morphs after evacuation at 423 K. The bands located at 1445,
1580 and 1606 cm−1, corresponding to Lewis acid sites,
were observed on three zirconia polymorphs, meanwhile,
the band intensity decreased in the order of am-ZrO2 > m-
ZrO2 > t-ZrO2. It was noted that there existed a weak band
at 1537 cm−1 only on m-ZrO2. These results reflected that
Lewis acid sites presented on three zirconia polymorphs and
the intensities were in the order of am-ZrO2 > m-ZrO2 > t-
Samples were further characterized by CO2-TPD to deter-
mine the base sites distribution (see Fig. 4). Two CO2 desorp-
¨
ZrO2, while Bronsted acid sites was only detected on the
Table 1
Acid sites density on three zirconia polymorphsa
¨
surface of m-ZrO2. Perhaps the Bronsted acid sites on the
surface of am-ZrO2 and t-ZrO2 were too weak to form pyri-
dinium ion. Yubao Zhao et al. [14] also detected the weak
Zirconia
Acidity sites I
Acidity sites II
Total acidity site
(nm−2
)
(nm−2
)
(nm−2
)
¨
Bronsted acid sites only on the surface of m-ZrO2 using pyri-
am-ZrO2
m-ZrO2
t-ZrO2
a
1.09
1.07
0.81
2.80
0.81
0.74
2.89
1.88
1.55
¨
dine as probe, but Hertl [7] observed Bronsted acid sites on
three zirconia polymorphs using NH3 as probe molecule. In
Calculated from NH3 desorption.
Fig. 2. Difference transmission infrared spectra of pyridine adsorbed on
three zirconia polymorphs after evacuation at 423 K for 30 min.
Fig. 4. CO2-TPD profiles of three zirconia polymorphs.

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Z.-Y. Ma et al. / Journal of Molecular Catalysis A: Chemical 227 (2005) 119–124
Table 2
Base sites density on three zirconia polymorphsa
Zirconia
Basic sites I (nm−2
)
Basic sites II (nm−2
)
Basic sites III (nm−2
)
Total basic sites (nm−2
)
am-ZrO2
m-ZrO2
t- ZrO2
a
0.65
0.37
0.64
1.08
0.56
0.49
–
–
2.10
1.73
0.93
3.23
Calculated from CO2 desorption.
at 2197 cm−1) were contributed to low regular and less
probable configurations, which could be exposed on rougher
(high index) planes. The low energetic sites (CO band at
2180 cm−1) were ascribable to the flat face (low index).
tion peaks located at low temperature and high temperature,
corresponding to a weak base sites I and a moderate base sites
II, were observed on am-ZrO2 and m-ZrO2, respectively. But
there existed another peak at higher temperature on t-ZrO2
indicating a strong base sites III. For weak base sites I, the
intensity was almost same for three zirconia polymorphs, but
for the base sites II, the intensity decreased in the order of
am-ZrO2 > t-ZrO2 > m-ZrO2. The total desorption CO2 on t-
ZrO2 was almost 4 times higher than that on m-ZrO2 (see
Table 2). As a result, t-ZrO2 showed much higher basicity
than the two others.
3.2. CO adsorption
The DRIFT spectra of CO adsorbed onto three zirconia
polymorphs were illustrated in Fig. 5. Two bands located
at 2136 cm−1 and 2197 cm−1 were detected when CO ad-
sorbed on am-ZrO2 at 623 K (see Fig. 5B). The difference
in wave number was as large as 60 cm−1, indicating that
two sites had large difference in electronic intensity. Bolis
et al. [15] considered that high ability of withdrawing elec-
tron of cus Zr4+ resulted in CO band shifting to higher wave
number. Thus, the band at 2197 cm−1 could be ascribed to
CO adsorption on cus Zr4+. On the other hand the band at
2136 cm−1 could not be explained by the polarization of the
carbon monoxide molecule in the electronic field of the sur-
face. And the band might be attributed to CO interacting with
surface hydroxyl groups via H-band [22], which was possibly
evidenced by the hydroxyl disturbance location at 3500 cm−1
(see Fig. 5A). When CO adsorbed on m-ZrO2 at 623 K, only
a band at 2109 cm−1 could be observed (see Fig. 5B), proba-
bly due to CO adsorption on cus Zr3+, proved by Morterra et
al. [16] using EPR spectroscopy. They also found other CO
peaks located above 2160 cm−1 on the surface of vacuum-
activated m-ZrO2. But in the present work, there was no
otherband could be seen except the 2109 cm−1 band. Vacuum
thermal activity of metal oxides could cause gradual surface
dehydration and, at higher temperatures, create surface co-
n+
ordinatively unsaturated cus Zr
ions and co-ordinatively
unsaturated anion–cation pairs. However, the sample was not
undergone vacuum thermal treatment in the present work,
which might result in the difference from the literature.
CO adsorption on t-ZrO2 showed a weak band at
2183 cm−1, with a difference of 14 cm−1 compared to
CO band at 2197 cm−1 on am-ZrO2 (see Fig. 5B). Bolis
[15] considered that two families of acid sites (Zr4+)
existed on zirconia surface, which exhibited different acidic
strength and stability. The high energetic sites (CO band
Fig. 5. Difference DRIFT spectra of CO adsorbed on three zirconia poly-
morphs. Section A: hydroxyl and νC–H region (4000–2600 cm−1); section
B: νCO region (2250–1900 cm−1); section C: surface intermediates region
(1800–1200 cm−1).

Z.-Y. Ma et al. / Journal of Molecular Catalysis A: Chemical 227 (2005) 119–124
123
Thus, the acid sites on t-ZrO2 were less energetic than
that on am-ZrO2, then leading to lower wave number. And
the low index sites possessed weak capacity of adsorption
CO, which could be proved by the intensity of CO band at
2183 cm−1 compared to the band at 2197 cm−1. On the other
hand, the intensity of CO band decreased in the order of am-
ZrO2 > m-ZrO2 > t-ZrO2, which was in accordance with the
intensity of surface Lewis sites determined by Py adsorption.
Intermediate species were observed in the region
of 1800–1200 cm−1 when CO adsorbed on zirconia poly-
morphs (see Fig. 5C). As CO adsorbed on am-ZrO2 at 623 K,
the formate species was well identified by the bands located
at 1570 (νasOCO), 1380(δCH) and 1361 cm−1(νsOCO), the
difference of 200 cm−1 between the symmetric (1361 cm−1
)
and anti-symmetric (1570 cm−1) stretching mode of OCO
tended to show that the formate was rather bidentate than
monodentate. In the case of m-ZrO2, the infrared spectra
were relatively complicated compared to am-ZrO2. Besides
the formate bands located at 1570, 1380 and 1361 cm−1, the
bands at 1469, 1303 cm−1 and 1442 cm−1 were detected.
The former could be assigned to the monodentate carbonate
and the latter was the bands of ion carbonate [8]. But for
t-ZrO2, no formate species but bicarbonate and bi-dented
carbonate with bands at 1647, 1614 and 1292 cm−1 formed
as CO adsorption at 623 K.
The mechanism of formate formation through CO reac-
tion with surface hydroxyls could be formulated as Scheme 1.
Hydroxyl and O2−, which had different families on zirconia
polymorphs, played an important role in the formation of in-
termediate species. CO reacted with hydroxyl to form the for-
mate species and consumed a certain hydroxyl group at high
temperature, showing negative peaks in the hydroxyl region
(Fig. 5A). On the other hand, bicarbonate and carbonate were
formed when CO contacted with surface O2−. In the case of
t-ZrO2, the surface basicity was strong and the surface oxy-
gen ions were abundant. The adsorption CO might quickly
react with O2− to form the carboxylate and further to bicar-
bonate and carbonate by reacting with hydroxyl or O2− (see
Scheme 2). This might be responsible for the complicated
carbonate as well as the absence of formate specie on t-ZrO2.
Scheme 2. Proposed reaction scheme for carbonate and bicarbonate forma-
tion on zirconia surface.
CO adsorption, formate species with bands at 2968, 2880,
1563, 1390 and 1360 cm−1 was detected (see Fig. 6A). In
addition, a weak band located at 3077 cm−1, attributable to
gas methane species [23], was observed. Hydrogen could be
hemolytically or heterolytically dissociated to form hydroxyl
or adsorbed hydrogen on zirconia [24], which facilitated the
hydrogenation formate to methoxide and then to methane.
On the other hand, according to the thermodynamics, the
3.3. CO + H2 adsorption
Fig. 6 showed the DRIFT spectra of CO/H2 adsorbed
on three zirconia polymorphs at 623 K. Similar to pure
Fig. 6. Difference DRIFT spectra of CO/H2 adsorption on three zirconia
polymorphs. Section A: hydroxyl and νC–H region (4000–2600 cm−1); sec-
tion B: surface intermediates region (1800–1200 cm−1).
Scheme 1. Proposed reaction scheme for formate formation on zirconia
surface.

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Z.-Y. Ma et al. / Journal of Molecular Catalysis A: Chemical 227 (2005) 119–124
formation of methane was 9 times higher than that of
methanol at ambient pressure under syngas atmosphere [25].
Hence, methane was formed from CO hydrogenation on
am-ZrO2 at 623 K. Similar phenomena were observed in the
case of CO/H2 adsorption onto m-ZrO2 (with the methane
band lowing to 3040 cm−1), and formate species also could
be detected at 623 K. Compared to pure CO adsorption
on m-ZrO2, the carbonate and ion carbonate species were
absent due to the presence of hydrogen.
In the case of CO/H2 adsorption on t-ZrO2, no formate
was produced even in the presence of hydrogen. Compared
to pure CO adsorption, the bidentate carbonate species lo-
cated at 1647 cm−1 disappeared while the bicarbonate was
also present, which might be the contribution of hydrogena-
tion. Other bands at 1523 and 1286 cm−1, ascribable to mon-
odentate carbonate species, were also detected. Onishi and
co-workers [26,27] concluded that the reaction of hydroxyl
groups with CO gave rise to bicarbonate, and the methoxide
species also could be obtained by hydrogenation of bicarbon-
ate with the formaldehyde as intermediate species. Methane
specieswithbandat3066 cm−1 wasformedont-ZrO2 though
no formate was appeared (see Fig. 6). But formaldehyde
species with bands at 2845 and 2790 cm−1 was detected. A
comparison of different species formation on zirconia poly-
morphs established the conclusion that the methane forma-
tion on am-ZrO2 and m-ZrO2 came from the hydrogenation
of formate but it came from bicarbonate hydrogenation on
t-ZrO2.
and carbonate were present on tetragonal zirconia, whether
hydrogen introduce or not. Though methane was formed on
three zirconia polymorphs, the intermediate species was dif-
ferent on three zirconia polygraphs.
Acknowledgments
This work was supported by State Key Foundation Project
for Development and Research of China (G1999022400)
and Innovation Project of Chinese Academy of Sciences
(KGCX2-302).
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4. Conclusions
Different zirconia polymorphs possessed varied surface
hydroxyl groups and acid–base properties. Bi-bridged hy-
droxyl only existed on tetragonal zirconia and amorphous
zirconia showed obvious H-band hydroxyl while two type of
tri-bridged hydroxyl appeared on monoclinic zirconia. These
differences effected surface acidity and basicity: amorphous
zirconia had strong Lewis acid sites and monoclinic zirconia
¨
had strong Bronsted acid sites, but for tetragonal zirconia, the
basicity was predominant. Subsequently, surface properties
greatly influenced the formation of surface intermediates as
CO or CO/H2 adsorption. Formate was formed on amorphous
and monoclinic zirconia while no formate but bicarbonate