Characterization and catalytic functionalities of copper oxide catalysts supported on zirconia

Article abstract of DOI:10.1021/jp063335x

A series of zirconia supported copper oxide catalysts with varying copper loadings (1.2-19.1 wt %) were prepared by impregnation method. The catalysts were characterized by X-ray diffraction, UV-visible diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), and temperature-programmed desorption of CO₂. Copper dispersion and metal area were determined by N₂O decomposition method. X-ray diffraction patterns indicate the presence of crystalline CuO phase beyond 2.7 wt % of Cu on zirconia. UV-visible diffuse reflectance spectra suggest the presence of two types of copper species on the ZrO₂ support. XPS peaks intensity ratio of Cu 2p_3/2 and Zr 3d _5/2 was compared with Cu dispersion calculated from N₂O decomposition. TPR patterns reveal the presence of highly dispersed copper oxide at lower temperatures and bulk CuO at higher temperatures. The basicity of the catalysts was found to increase with Cu loading, and the activity of the catalysts was also found to increase with the increase in Cu loading up to 2.7 wt % Cu loading. The catalytic properties were evaluated for the dehydrogenation of cyclohexanol to cyclohexanone and were related to surface properties of the copper species supported on zirconia.

Full text of DOI:10.1021/jp063335x

J. Phys. Chem. B 2007, 111, 543-550
543
Characterization and Catalytic Functionalities of Copper Oxide Catalysts Supported on
Zirconia
Komandur V. R. Chary,* Guggilla Vidya Sagar, Chakravarthula S. Srikanth, and
Vattikonda Venkat Rao
Catalysis DiVision, Indian Institute of Chemical Technology, Hyderabad - 500 007, India
ReceiVed: May 30, 2006; In Final Form: August 23, 2006
A series of zirconia supported copper oxide catalysts with varying copper loadings (1.2-19.1 wt %) were
prepared by impregnation method. The catalysts were characterized by X-ray diffraction, UV-visible diffuse
reflectance spectroscopy, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR),
and temperature-programmed desorption of CO . Copper dispersion and metal area were determined by N O
2 2
decomposition method. X-ray diffraction patterns indicate the presence of crystalline CuO phase beyond 2.7
wt % of Cu on zirconia. UV-visible diffuse reflectance spectra suggest the presence of two types of copper
species on the ZrO
calculated from N
2
support. XPS peaks intensity ratio of Cu 2p3/2 and Zr 3d5/2 was compared with Cu dispersion
O decomposition. TPR patterns reveal the presence of highly dispersed copper oxide at
2
lower temperatures and bulk CuO at higher temperatures. The basicity of the catalysts was found to increase
with Cu loading, and the activity of the catalysts was also found to increase with the increase in Cu loading
up to 2.7 wt % Cu loading. The catalytic properties were evaluated for the dehydrogenation of cyclohexanol
to cyclohexanone and were related to surface properties of the copper species supported on zirconia.
1
. Introduction
copper oxide supported on zirconia for the steam reforming of
methanol was improved when compared to Cu/SiO2.
Catalysts with copper oxide as an active component have been
The study of determination of dispersion of the active phase
in supported Cu catalysts is an interesting topic of research in
recent years for understanding the nature of active phase on
the catalytic properties. A fundamental understanding of the
structure-activity relationship in heterogeneous catalytic de-
hydrogenation is of basic importance for the development of
new catalytic materials and for improving the performance of
existing catalysts. For this, N2O decomposition was chosen to
find active phase dispersion in supported copper catalysts.31
extensively employed in the recent past for the selective catalytic
reduction of NOx and synthesis and steam reforming of
methanol.1-5 Copper catalysts are very selective in hydrogena-
tion-dehydrogenation reactions such as conversion of benz-
6
7
aldehyde to benzyl alcohol, furfural to furfuryl alcohol, or the
transformation of alcohols into their corresponding aldehydes
or ketones.8
-10
The catalytic properties of the active copper
oxide phase can be greatly influenced by the nature of the
supported oxide and the dispersion of active component.
Zirconia based materials have attracted considerable interest in
In the present investigation we report the characterization of
3
,11-14
CuO/ZrO catalysts by powder X-ray diffraction (XRD), UV-
recent years for their potential use as catalyst supports.
2
vis diffuse reflectance spectroscopy (UV-DRS), X-ray photo-
ZrO2 presents special characteristics such as high thermal
stability, extreme hardness, and stability under reducing condi-
tions. Further the possession of both acid and base functions
electron spectroscopy (XPS), temperature-programmed reduction
(TPR), and temperature-programmed desorption (TPD) of CO2.
Cu dispersion and metal area were determined by N2O
decomposition. The catalytic properties were evaluated for the
vapor phase dehydrogenation of cyclohexanol to cyclohexanone.
The purpose of this work is to estimate the dispersion of CuO
supported on zirconia as a function of copper loading, in order
to identify changes in the structure of the CuO phase with
loading to understand the relation between the dispersion of
Cu and basicity of catalyst for dehydrogenation of cyclohexanol
and also to compare with other supported copper catalysts.
make it very appealing as a carrier for several catalytic
applications.14 In the catalytic sense, these have some advantages
1
5
over traditional catalyst supports, such as SiO2 and Al2O3, in
the areas of practical application. Zirconia is often included in
heterogeneous systems, either as a promoter or more generally
as a support. In CO hydrogenation for instance, zirconia addition
1
6,17
to metal supported catalysts induces a long-term stability
18-20
and also enhances both the activity
and the selectivity
It has also been employed in many
2
1,22
towards alcohol.
2
3
industrially important reactions such as hydroprocessing,
oxidation of alcohols,24-26 dehydrogenation of cyclohexanol,
27
2. Experimental Section
and synthesis of methanol and higher alcohols.28 Amenomiya29
reported that the catalytic activity of CuO/ZrO2 is better than
those of CuO/Al2O3, CuO/SiO2, CuO/MgO, and CuO/TiO2
Zirconia support was prepared from saturated aqueous
zirconyl nitrate hydrate ZrO(NO3)2‚xH2O (Fluka), with the
addition of aqueous ammonia till pH reaches to 9. The resulting
precipitate was washed repeatedly with portions of deionized
water until the precipitate is free from the base. The precipitate
was dried at 383 K for 12 h, and the resulting hydroxide was
calcined in air at 773 K for 5 h. A series of copper catalysts
catalytic systems for the methanol synthesis from hydrogenation
of CO2. Takezawa et al.30 revealed that the catalytic activity of
*
Corresponding author. Fax: +91-40-27160921. Tel: +91-40-27193162.
E-mail: kvrchary@iict.res.in.
1
0.1021/jp063335x CCC: $37.00 © 2007 American Chemical Society
Published on Web 12/29/2006

5
44 J. Phys. Chem. B, Vol. 111, No. 3, 2007
Chary et al.
with Cu loadings varying from 1.2 to 19.1 wt % were prepared
saturated with CO2 in a flow of 10% CO2-He mixture at 303
K with a flow rate of 50 mL/min and was subsequently flushed
at 378 K for 1 h to remove physisorbed CO2. TPD analysis
was carried out from ambient temperature to 1073 K at a heating
rate of 10 K/min. The amount of CO2 desorbed was also
calculated using GRAMS/32 software.
by impregnation with requisite amount of Cu (NO3)2‚3H2O
(Fluka) on ZrO2 support. The samples were dried at 383 K for
1
6 h and subsequently calcined at 773 K for 5 h in air. The
same set of catalysts were used for all characterization and
evaluation studies.
The copper content was determined by atomic absorption
spectroscopy using a Perkin-Elmer Analyst 300 double beam
spectrometer. The powders were first dissolved in acidic solution
Copper surface area, dispersion, and crystallite size were
calculated by N2O decomposition method conducted on a Auto
Chem 2910 (Micromeritics, USA). This method consists of two
0
(40% HF) and diluted to concentrations within the detection
steps (i) oxidation of Cu to Cu2O by N2O and (ii) H2
range of the instrument.
temperature-programmed-reduction of the formed Cu2O surface
2
2
species (s-TPR). Before analysis, in-situ prereduction of the
X-ray powder diffraction patterns were obtained with a
Siemens D5000 diffractometer, using Cu KR radiation (1.5406
Å) at 40 kV and 30 mA and a secondary graphite monochro-
mator. The measurements were recorded in steps of 0.045° with
a count time of 0.5 s in the 2θ range of 2-65°.
The specific surface areas of the catalyst samples were
calculated from N2 adsorption-desorption data acquired on a
single point Pulse Chemisorb 2700 instrument (Micromeritics,
USA) at liquid N2 temperature. The powders were first
outgassed at 423 K to ensure a clean surface prior to construction
0
CuO phase to Cu was performed in flow mixture of 5% H2-
Ar raising the temperature at a heating rate of 10 K/min up to
6
23 K. The catalyst was purged and cooled to 333 K in Ar and
0
Cu to Cu2O oxidation was performed by adsorptive decom-
position of N2O at 333 K flowing N2O (50 mL/min) for 45
min. In sequence, the catalyst was again purged in Ar flow and
cooled to ambient temperature. After this, s-TPR was carried
out similarly as in TPR, raising the temperature up to 723 K on
the freshly oxidized Cu2O surface in order to reduce Cu2O to
0
2
Cu . H2 uptake was calculated using same GRAMS/32 software.
of adsorption isotherm. A cross-sectional area of 0.164 nm of
A down flow fixed bed reactor made of Pyrex glass was used
to test the catalysts for the dehydrogenation of cyclohexanol to
cyclohexanone at atmospheric pressure. About 500 mg of the
catalyst diluted with an equal amount of quartz grains was
charged into the reactor and was supported on a quartz wool
bed. Prior to introducing cyclohexanol with a syringe pump,
the catalyst was reduced at 523 K for 3 h, in purified hydrogen
flow. After prereduction, the reactor was fed with cyclohexanol
the N2 molecule was assumed in the calculations of the specific
surface areas using the method of Brunauer, Emmet, and Teller
(BET). Pore size distribution (PSD) measurements were per-
formed on Auto Pore III (Micromeritics) by mercury penetration
method.
UV-visible spectra were recorded in air at room temperature
using a GBC UV-visible Cintra 10e spectrometer with a diffuse
reflectance accessory, in the 200-900 nm wavelength range.
The ZrO2 support was used as reference. The Kubelka-Munk
function F(R) was plotted against the wavelength (in nm).
X-ray photoelectron spectroscopy was used to study the
chemical composition and oxidation state of catalyst surfaces.
The XPS spectra of the catalysts were measured on a XPS
spectrometer (Kratos-Axis 165) with Mg KR radiation (hν
(6 mL/h) at 523 K in N2 (flow rate 50 mL/min), which is used
as a carrier gas. The liquid products, mainly cyclohexanone and
cyclohexene, were analyzed by a Hewlett-Packard 6890 gas
chromatograph equipped with a flame ionization detector using
a HP-5 capillary column. The products were also identified using
a HP 5973 quadrupole GC-MSD system using a HP-1MS
capillary column.
)1253.6 eV) at 75 W. The Cu 2p and Zr 3d core-level spectra
were recorded and the corresponding binding energies were
referenced to the C 1s line at 284.6 eV (accuracy within (0.2
eV). The background pressure during the data acquisition was
3
. Results and Discussion
The X-ray diffraction patterns of pure ZrO2 and calcined CuO/
-
10
kept below 10
bar.
ZrO2 catalysts are shown in Figure 1. ZrO2 exists in three
crystallographic polymorphs viz., monoclinic, tetragonal, and
cubic. The zirconia synthesized in the present study shows
the presence of mixed phases of monoclinic and tetragonal
(Figure 1). In all the samples, XRD peaks due to ZrO were
observed at d ) 3.676, 3.16, 2.842, and 1.65 Å (2θ ) 24.2,
28.2, 31.5, and 55.65°) which are due to the monoclinic phase
of zirconia and the peaks at d ) 2.96, 2.60, 2.54, and 1.81 Å
(2θ ) 30.16, 34.46, 35.30, and 50.37°) are corresponding to
tetragonal zirconia. At higher Cu loadings (above 2.7 wt % Cu,
Figure 1), XRD peaks due to the crystalline CuO phase are
noticed at d ) 2.52 Å (2θ ) 35.5°) and 2.38 Å (2θ ) 38.7°),
in addition to the characteristic peaks of zirconia. The intensity
of these peaks were found to increase with Cu loading. However,
at low Cu loadings the absence of crystalline CuO peaks cannot
be ruled out as they might be less than 40 Å in size, which is
beyond the detection capacity of the XRD technique. XRD
results also suggest that no new crystalline oxide phase
formation between CuO and ZrO2. Normally, the monoclinic
phase is stable below 1273 K and the tetragonal phase of ZrO2
Temperature-programmed reduction studies were carried out
on a Auto Chem 2910 (Micromeritics, USA) instrument to study
the copper dispersion and reducibility. In a typical experiment,
Ca. 250 mg of oven-dried sample (dried at 383 K for 15 h) was
taken in a U-shaped quartz sample tube. The catalyst was
mounted on a quartz wool plug. Prior to TPR studies, argon
gas was passed with a flow of 50 mL/min at 393 K for 2 h to
pretreat the catalyst sample. After pretreatment, the sample was
cooled to ambient temperature and TPR analysis was carried
out in a flow of 5% H2-Ar mixture (50 mL/min) from ambient
temperature to 673 K at a heating rate of 10 K/min. H2
consumption and Tmax positions are calculated using GRAMS/
3
2
2
3
2 software.
Temperature-programmed desorption (TPD) studies were also
conducted on same instrument. In a typical experiment for TPD
studies ca. 200 mg of oven dried sample (dried at 383 K for
overnight) was taken in an U-shaped quartz sample tube. Prior
to TPD studies, the catalyst sample was pretreated at 473 K for
3
0 min by passing very pure helium (99.999%, 50 mL/min).
After pretreatment of the sample, it was reduced at 523 K for
h by passing very pure hydrogen (99.99%, 50 mL/min) and
33
34
appears above 1400 K. Rijnten and Afanaiev et al. observed
the tetragonal to monoclinic phase transformation occurs with
the impregnation of molybdenum oxide. Maity et al.35 have also
observed phase transformation of zirconia in their study of
2
subsequently flushed with pure helium (50 mL/min) for 30 min
to ensure a clean surface. After reducing the sample, it was

Copper Oxide Catalysts Supported on Zirconia
J. Phys. Chem. B, Vol. 111, No. 3, 2007 545
Figure 2. UV-visible diffuse reflectance spectra of various CuO/
ZrO catalysts.
2
2
Figure 1. X-ray diffraction patterns of various CuO/ZrO catalysts
(9 due to monoclinic zirconia, 2 due to tetragonal zirconia, b due to
CuO).
The UV-visible spectra of calcined CuO/ZrO2 samples are
shown in Figure 2. For comparison, the diffuse reflectance (DR)
spectrum of pure ZrO2 sample was also recorded. The DR
spectrum of pure ZrO2 exhibits a characteristic band around
TABLE 1: BET Surface Area and Pore Size Distribution of
Various CuO/ZrO Catalysts
2
BET
total
pore
volume
(mL/g)
total
pore
area
2
-
4+
36
2
20 nm due to the O2 f Zr charge-transfer transition.
surface
area
wt %
of Cu
The DR spectrum of CuO/ZrO2 catalysts show strong absorption
bands at 350 nm and a large absorption band at 600-900 nm.
The charge-transfer (CT) bands give an idea of the environment
2
2
sample
(m /g)
(m / g)
1
2
3
4
5
6
7
0.0
1.2
2.7
5.3
9.5
13.9
19.1
74
54
55
51
49
47
45
0.37
0.32
-
0.27
-
0.25
-
74
63
-
57
-
53
-
2
+
in the neighborhood of Cu
reports,
ions. According to earlier
3
7,38
the band at 350 nm indicates the ligand-to-metal
charge-transfer (LMCT) transition (O f Cu ), where the
2-
2+
Cu ions occupy isolated sites over the support. The large
2
2
absorption band at 600-900 nm is assigned to Eg f T2g
2+
transitions of Cu situated in more or less tetragonally distorted
3
7
zirconia supported hydrotreating catalysts. However, in the
present study we did not observe any such phase transition after
impregnation with copper nitrate when the samples were
calcined at 773 K.
The BET surface area determined by nitrogen physisorption
of all the catalysts are presented in Table 1. The specific surface
octahedral environment with an distorted Oh symmetry.
3
9
Iwamoto and his group have reported that the peaks at 320
and 440 nm in the DR spectra of Cu-ZSM-5 zeolites were
identified as a O f Cu and (Cu-O-Cu) , respectively.
Mendes et al. have observed that the broad absorption band
between 600 and 900 nm and the band at 350 nm indicate the
formation of copper clusters different from those of bulk CuO.
The O f Cu charge-transfer band at 350 nm is shifted to
higher wavelength range and an increase in intensity of the broad
absorption band between 600 and 900 nm is found with an
increase of Cu loading. It clearly suggests the increase in
crystallinity and formation of bulk CuO at higher loadings. This
is also in good agreement with XRD results observed wherein
the intensities of the CuO peaks increased with the increase of
Cu loading.
2
-
2+
2+
4
0
2
area of the pure ZrO2 support was found to be 74 m /g. The
2
-
2+
BET surface area decreases as a function of copper content on
ZrO2, and it might be due to surface hydroxyl groups (OHsurf)
of the support consumed by reaction with the active phase
precursor. Such a surface reaction may have caused the de-
crease of available surface area of the support, probably by
closure of the pores as evidenced by PSD. Total pore volume
and total pore area of samples measured by mercury penetrating
porosimeter have also been reported in Table 1. The total pore
volume and total pore area are found to decrease with increase
of copper loading in the similar lines of surface area of the
catalysts.
The catalyst surface composition and oxidation state were
investigated by XPS. Figures 3 and 4 show Zr 3d and Cu 2p
XPS of various CuO/ZrO2 calcined catalysts, respectively. The

546 J. Phys. Chem. B, Vol. 111, No. 3, 2007
Chary et al.
2
Catalysts
TABLE 2: XPS Results of Various CuO/ZrO
position
and FWHM
of Cu 2p3/2
position
and FWHM
of Zr 3d5/2
XPS
intensity
Cu 2p/Zr 3d
Cu
(wt %)
sample
1
2
3
4
5
1.2
2.7
5.3
9.5
13.9
933.2 (1.9)
933.2 (2.3)
933.5 (1.9)
933.0 (1.7)
933.0 (1.7)
181.7 (1.35)
181.8 (1.33)
181.6 (1.33)
181.6 (1.35)
181.8 (1.38)
0.17
0.37
0.26
0.29
0.24
The binding energy and FWHM values of Cu 2p3/2 peaks are
listed in Table 2 for different copper loadings. Generally, the
lower binding energy (932.2-933.1 eV) and absence of shakeup
1
+
42
peaks are characteristics of Cu ion. The results indicate that
2+
copper species are present as Cu in all the samples as indicated
by shakeup peaks in Figure 4. The binding energy of the Cu
2
p3/2 transition for the various calcined CuO/ZrO2 catalysts is
found to be lower than the bulk CuO. Although CuO reduces
readily to Cu2O, the possibility of CuO reduction to Cu2O cannot
4
3
be ruled out during the XPS analysis. The constant FWHM
values are around 1.9, implying that only one type of doublet
is present. This provides an evidence for the presence of a single
type of copper oxide with an oxidation state of 2+. The intensity
of Cu 2p core level spectra increases with the increase of Cu
loading and levels off at higher loadings. This is also in good
agreement with the previously reported XRD results, wherein
the intensities of the CuO peaks increased with increase of Cu
loading.
Figure 3. X-ray photoelectron spectra of Zr 3d spectra of various CuO/
ZrO catalysts.
2
The XPS intensity ratio of Cu 2p/Zr 3d values for various
CuO/ZrO2 catalysts are reported in Table 2, which reflect the
copper dispersion on zirconia support. The XPS intensity ratio
of Cu 2p/Zr 3d increases with the increase of copper loading
with a maxima at 2.7 wt % and then decreases with further
increase of Cu loading. This could be probably due to the
formation of large CuO crystallites. The XRD also further
confirm detection of bulk CuO in the sample with 5.3 wt %
and above copper loadings. The activity of the catalysts in the
dehydrogenation of cyclohexanol to cyclohexanone was also
found to increase up to 2.7 wt % Cu loading and decreases
with further increase of Cu loading. A low dispersion of copper
oxide is also noticed at higher Cu loadings on ZrO2 by the N2O
decomposition method described later in this section. Thus, the
present XPS results are in good agreement with the dispersion
of copper determined by N2O decomposition method.
Temperature-programmed reduction (TPR) has been exten-
sively applied in recent years for characterizing reducible
catalysts including metal and metal oxide systems. This
technique, therefore, allows us to obtain a profile or ‘finger print’
of catalyst reduction. This is eminently suitable for studying
low loading and highly dispersed systems whose characteristics
are beyond the limits of detectability by other direct structural
analysis methods (eg. X-ray diffraction). The TPR profiles of
CuO/ZrO2 catalysts are shown in Figure 5. All the samples
exhibit two reduction profiles (R, â) during TPR in the
temperature range of 454-567 K but for the samples containing
low Cu loadings (1.2 and 2.7 wt %). The hydrogen consumption
values and Tmax during the TPR are reported in Table 3. The
Tmax position of R peak is shifting to the low-temperature region
with increase of copper loading. When the copper loading is
above 1.2 wt %, the R peak splits into two peaks (R1, R2). A â
peak was observed at high-temperature region, when the copper
loading is higher than 2.7 wt %, and its intensity is increased
with increase of copper loading. The low-temperature TPR peak
is attributed to the highly dispersed surface CuO species, and
high-temperature TPR peak is attributed to the bulk CuO. Such
Figure 4. X-ray photoelectron spectra of Cu 2p spectra of various
CuO/ZrO catalysts.
2
Zr 3d5/2 and Zr 3d3/2 binding energy values are in the range of
1
3
81.8 and 184.5 eV, respectively. The binding energies of Zr
d5/2 and its full width at half maximum (FWHM) values are
reported in Table 2. The constant FWHM values are found to
be around 1.3, implying that only one type of doublet is present.
This provides an evidence for the presence of a single type of
zirconium oxide with an oxidation state of 4+. The intensity
of Zr 3d core level spectra does not change much with increase
of copper loading.
Figure 4 shows Cu 2p XPS of various CuO/ZrO2 catalysts.
The binding energy of the Cu 2p3/2 peak at 934.2 eV together
with FWHM of 3.8 eV and the characteristic shakeup feature
at a binding energy of 944 eV are indicative for Cu2 species.
+
41

Copper Oxide Catalysts Supported on Zirconia
J. Phys. Chem. B, Vol. 111, No. 3, 2007 547
TABLE 3: Temperature-Programmed Reduction Results of Various CuO/ZrO
2
Catalysts
1
consumption1
2
consumption2
total H
2
consumption1+2
Cu
T
max
H
2
T
max
H
2
sample
(wt %)
(K)
(µmol/g)
(K)
(µmol/g)
(µmol/g)
1
2
3
4
5
6
1.2
2.7
5.3
9.5
13.9
19.1
454
450
448
442
435
436
96
315
310
258
236
207
-
-
531
538
543
567
-
-
96
315
673
1475
2275
3044
363
1217
2039
2837
a low-temperature peak of TPR was also noticed by Shimokawabe
et al.44 and Dow et al.45 They have reported that the low-
TABLE 4: Dispersion, Copper Metal Area, and Average
Particle Size of Various CuO/ZrO
2
Catalysts
temperature peaks were due to highly dispersed CuO with an
Cu
metal
area
average
particle
size
2+
46
octahedral environment of Cu ions. Robertson et al. and
Vander Grift et al.47 have also investigated the reduction
Cu
(wt %)
dispersion
2
sample
(%)
(m /gcu
)
(nm)
behavior of CuO/SiO2 catalysts and concluded that the highly
dispersed copper oxide species were easily reducible when
compared to bulk CuO. An explanation for the supported copper
species for its easier reduction over the reference sample (bulk
CuO) could be a dispersion effect, but it could also be an effect
1
2
3
4
5
6
1.2
2.7
5.3
9.5
13.9
19.1
73
88
56
27
18
13
470
568
364
173
116
81
1.4
1.2
1.8
3.8
5.8
8.3
4
8,49
of interaction between the support and the copper species.
2+
The highly dispersed copper species include both isolated Cu
ions that strongly and weakly interact with the support (the
cupric ions have close contact with each other) and also a small
two-dimensional clusters. The TPR peak at higher temperatures
is attributed to large three-dimensional clusters. As the loading
increases, the TPR peak becomes broad and shifts to high
temperature. The broadening of the peak and shifting of the
Tmax toward higher temperatures might be due to increase in
crystallinity of CuO with increase of Cu loading as evidenced
from XRD results. As can be seen from Table 3, the H2
consumption of the R peak is found to be the highest for 2.7 wt
that the dispersion is maximum for 2.7 wt % of Cu on ZrO2
and decreases with further increase of copper loading.
From the results of TPR and XRD, it is clear that larger â
peak area of TPR (Figure 5) and larger signal intensity of CuO
peaks in XRD (Figure 1) are observed with higher loadings of
copper. There is a maximum area of R peak when the loading
of Cu is 2.7 wt %. This further suggests that the â peak in TPR
is due to copper oxide species belonging to the bulk CuO and
the R peak to the dispersed CuO.
Copper dispersion (D ), defined as the ratio of Cu exposed
Cu
%
of Cu loading among all the samples. This clearly suggests
at the surface to total Cu present, was calculated from the
amount of H2 consumed in the s-TPR analysis. H2 consumption
(from s-TPR) can also be used to calculate the copper metal
surface area (SCu) and the average particle size (φav) by means
of the following assumptions and equations:
The area per copper surface atom in (100), (110), and (111)
2
50
planes are 0.065, 0.092, and 0.056 nm , respectively. An equal
abundance of these three planes give an average copper surface
2
19
atom area of 0.071 nm , equivalent to 1.4 × 10 copper atoms
per square meter. By assuming a spherical shape of the copper
metal particles, SCu and φav can be expressed as eqs 1 and 2,
respectively,
2
-1
4
SCu (m g
) ) Mol ‚SF‚N /(10 ‚C ‚W )
(1)
(2)
Cu
H2
A
M
Cu
φ (nm) ) 6000/(S ‚F )
av
Cu Cu
where MolH2, SF, NA, CM, WCu, and FCu are moles of hydrogen
experimentally consumed per unit mass of catalyst (µmolH2
-
1
g
1
cat), stoichiometric factor (2), Avogadro’s number (6.022 ×
02 mol ), number of surface Cu atoms per unit surface area,
Cu content (wt %), and the density of copper (8.92 g cm ).
3
-1
-
3 50
The Cu percentage dispersion, metal surface area, and average
particle size are given in Table 4. It was observed that the
dispersion and metal area of copper are increased and average
particle size is decreased up to 2.7 wt % of Cu on ZrO2. This
might be due to the maximum number of dispersed copper sites
that are available on the catalyst surface. However, the disper-
sion and metal area are decreased, and average particle size is
increased beyond this loading due to formation of CuO
crystallites. This is also in good agreement with XRD and TPR.
The basicity measurements have been carried out by tem-
perature-programmed desorption of CO2 method. TPD of CO2
Figure 5. Temperature-programmed reduction profiles of various CuO/
ZrO catalysts.
2

548 J. Phys. Chem. B, Vol. 111, No. 3, 2007
Chary et al.
Figure 7. Dependence of dehydrogenation of cyclohexanol over
various Cu/ZrO
2
catalysts.
indicates that moderate to strong basic sites are responsible for
5
4
dehydrogenation of cyclohexanol. The TPD results also
suggest that the strength of basic sites plays a crucial role in
determining the catalytic activity for dehydrogenation of cyclo-
hexanol.
Figure 7 shows the dependence of activity and selectivity on
copper loading for dehydrogenation of cyclohexanol to cyclo-
hexanone at 523 K. The cyclohexanol conversion was found to
increase with increase of Cu loading up to 2.7 wt % and decrease
with further increase of copper loading on ZrO2. The decrease
in the catalytic activity of the catalysts beyond 2.7 wt % of Cu
is due to increase of crystallinity of copper on the ZrO2 support.
The conversion of cyclohexanol for 1.2 wt % Cu loading catalyst
was 38%, and it is increased to 54% when the copper loading
is increased to 2.7 wt %. The medium and strong basic sites of
the catalysts were also found to increase with copper loading
up to 2.7 wt % and levels off at higher copper loadings. The
selectivity toward cyclohexanone was independent of Cu
loading. However, the selectivity toward cyclohexanone forma-
tion is slightly decreased at higher copper loadings. This might
be due to increase in the crystallinity of Cu that makes more
support surface for dehydration of cyclohexanol, resulting in
the formation of cyclohexene due to weak acidic sites of the
support. The bare support (pure ZrO2) was found to be inactive
for the cyclohexanone formation under the similar experimental
conditions.
A comparison of the surface characterization and catalytic
activity of 2.5 wt % of Cu supported on alumina, titania, niobia,
and zirconia catalysts are reported in Table 6. The catalytic
experiments were carried out under similar conditions for all
the samples. The aim of this study is to assess the effect of the
support on the catalytic properties in relation to surface
properties such as dispersion, metal area, and basicity at
approximately the same loading of copper. The results clearly
show that copper is well dispersed on ZrO2 support and also
has higher copper metal area. The conversion of cyclohexanol
is found to be higher in the case of the CuO/Al2O3 catalysts
compared to those of the CuO/ZrO2, CuO/Nb2O5, and CuO/
TiO2 catalysts. However, the selectivity towards cyclohexanone
is found to be higher for CuO/ZrO2 catalyst, when compared
to other supported copper catalysts. This might be due to
presence of more moderate and strong basic sites on CuO/ZrO2
catalyst leading to dehydrogenation functionality during the
vapor-phase dehydrogenation of cyclohexanol. The basic sites
of all the samples have been measured by TPD of CO2 and are
reported in Table 6. Cyclohexene is the only byproduct formed
Figure 6. Temperature-programmed desorption of CO
various Cu/ZrO catalysts.
2
profiles of
2
TABLE 5: Temperature-Programmed Desorption of CO
2
of
Various Cu/ZrO Catalysts
2
a
CO
2
uptake (µmol/g)
Cu loading
(wt %)
total CO
2
uptake (µmol/g)
a
sample
A
B
C
1
2
3
4
5
6
0.0
1.2
2.7
5.3
9.5
136
161
166
225
127
121
247
253
517
260
262
252
-
74
88
77
78
68
383
488
771
562
467
441
13.9
a
2
Calculated from temperature-programmed desorption of CO . A
)
due to weak basic sites. B ) due to moderate basic sites. C ) due
to strong basic sites.
profiles of pure ZrO2 and various Cu/ZrO2 catalysts are shown
in Figure 6. The desorbed peak of CO2 was deconvoluted into
three temperature regions, i.e., 330-500, 500-700, and >700
K correspond to weak, moderate, and strong basic sites,
respectively.51 The three peak areas were integrated and basic
strengths are given in Table 5. In the case of pure ZrO2 support,
there are only two peaks corresponding to weak and moderate
basic sites. However, the possibility that the desorption sites
show continuous changes in desorption energy with the change
in degree of surface coverage cannot be excluded. TPD of CO2
results suggests that pure ZrO2 is less basic when compared to
Cu/ZrO2 catalysts. Impregnation of CuO to ZrO2 support
facilitated the formation of strong basic sites that desorb CO2
at higher temperatures. Such strong basic sites corresponding
to desorption of CO2 at higher temperatures were also mentioned
5
2,53
over MgO-ZrO2 and alkali modified CaO catalysts.
The
number of basic sites with moderate and strong basic strengths
was found to increase with Cu loading up to 2.7 wt % on
zirconia and decreased with further loadings. The decrease in
basicity at higher copper loadings might be due to the ag-
glomeration of copper crystallites. This behavior is in good
agreement with the catalytic activity, which decreases with
increase of Cu loading beyond 2.7 wt % on zirconia. This clearly

Copper Oxide Catalysts Supported on Zirconia
J. Phys. Chem. B, Vol. 111, No. 3, 2007 549
TABLE 6: Dispersion, Copper Metal Area, BET Surface Area, TPD of CO
Supported Copper Catalysts
2
, and Dehydrogenation Activity Results of Various
a
dispersionb
(%)
Cu metal
b
CO
2
uptake (µmol/g)
conversion of
cyclohexanol (%)
selectivity of
cyclohexanone (%)
2
catalyst
area (m /gcu
)
A
B
C
2
2
2
2
.7% Cu/ZrO
.5% Cu/Al O
2 3
2
88
67
22
63
568
433
140
408
166
407
222
190
517
882
80
88
-
65
-
54
58
24
06
100
03
65
.5% Cu/Nb
2
O
5
.5% Cu/TiO
2
108
49
a
Calculated from temperature-programmed desorption of CO
2
. A ) due to weak basic sites. B ) due to moderate basic sites. C ) due to strong
b
2
basic sites. Calculated from N O decomposition method.
with increase of copper loading and decreases at higher loadings.
The activity of the catalysts was found to increase up to 2.7 wt
%
and decreases at higher loadings in similar lines to copper
dispersion and basicity measurements. CuO/ZrO2 catalyst is
found to be better than other supported copper catalysts under
similar conditions.
Acknowledgment. G.V.S. thanks the Council of Scientific
and Industrial Research (CSIR) for the award of a Senior
Research Fellowship (SRF).
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