Indian Journal of Chemical Technology
Vol. 16, May 2009, pp. 272-277
Notes
Influence of preparation method on the
performance of Pd/ ZrO2-Al2O3 catalysts
for HDS
Sufen Chen, Laitao Luo*& Xinsun Cheng
Department of Chemistry, Nanchang University,
Nanchang 330031, P.R. China
Email: [email protected]
Received 11 September 2008; revised 18 February 2009
ZrO2-Al2O3 (I) and ZrO2-Al2O3(S) mixed supports were
prepared using impregnation and sol-gel methods, separately.
Influences of preparation methods on the properties of Pd based
catalysts for hydrodesulphurization (HDS) of thiophene were
studied. The prepared samples were characterized by means of
XRD, H2-TPD, H2-TPR, XPS, NH3-TPD, H2 chemisorption and
BET surface area. Pd/ZrO2-Al2O3(S) catalyst supported on ZrO2Al2O3 (S) exhibited much higher HDS activity than that of the
Pd/ZrO2-Al2O3 (I) catalyst supported on ZrO2-Al2O3 (I). As
compared with Pd/ZrO2-Al2O3 (I) catalyst, Pd/ZrO2-Al2O3(S)
catalyst has higher dispersion, more acid sites and stronger
interaction between palladium and mixed support, which
presented an increasing driving-force toward the catalyzed
conversion of thiophene.
metals to transformation into inactive sulfides. Their
sulphur resistance is further improved by the use of an
acidic support and by alloying3. There are various
mixed oxide supports, for instance, TiO2–ZnO,
TiO2–SiO2, SiO2–Al2O3, ZrO2–Al2O3 and zeolites etc.
Among the mixed oxides used as HDS catalytic
supports, ZrO2–Al2O3 has received greater interest
due to the unique properties of ZrO2 to sustain acidic
and basic sites, and to possess high thermal stability4.
The ZrO2-Al2O3 oxide composites that were often
composed of a heterogeneous mixture of the two
oxides provided much higher specific surface areas
and revealed a positive effect on conversion of
tetralin5. To find new suitable systems for HDS
reactions, Pd catalysts supported on ZrO2-Al2O3
mixed oxides, which were prepared by impregnation
(I) and sol-gel (S) methods separately were studied in
this work. The catalysts were characterized by several
methods, such as nitrogen adsorption, XRD, TPD and
TPR. The influences of preparation methods of
ZrO2-Al2O3 mixed supports on the performance of
Pd/ZrO2-Al2O3 catalysts for HDS were also studied.
Keywords: Hydrodesulphurization, Thiophene, Catalyst
Experimental Procedure
At the 2007 horizon, more-stringent regulations on
the aromatics and sulphur levels of diesel fuels will
further affect the refining industry. To meet these
strict standards for diesel fuels, oil refineries must
make use of new catalytic systems and
hydrotreatment processes. Essentially the approaches
that can be explored are (a) new supports; (b) noble
metal catalysts; (c) zeolite-containing combinations;
(d) new compositions1. Palladium is a well-known
hydrogenation catalyst, but only recently has it been
shown that Pd can directly activate the thiophene
decomposition, resulting in the deposition of sulphur
and the formation of C4 species on the surface2.
Metals are much better hydrogenation catalysts than
metal sulphides and might be well suited as catalysts
for
deep
hydrodesulphurization
(HDS).
Unfortunately, metal particles may transform into
metal sulphide particles in the presence of sulphurcontaining molecules and H2S. The noble metals on
the right-hand side of the Periodic Table are less
sensitive to sulphur, and several investigations have
shown that Pt and Pd are less susceptible than other
Preparation of ZrO2-Al2O3 supports
Sol-gel method
Aqueous solutions of ZrOCl2.8H2O and
Al(NO3)3.9H2O were mixed in a total concentration of
0.5 mol·L-1 using Zr/Al = 0.125 molar ratios. This
solution was, drop by drop, added into a 350 mL
vessel that contained a solution of 5 g polyethylene
glycol 20000 (PEG-20000) as molecular template.
The aqueous solution was stirred at 353 K until the
solution reached a gelatinous state. The gel was aged
at room temperature for 48 h, then filtered and
washed with deionized water thoroughly to remove
extraneous impurities. The filtered cake was dried at
393 K for 12 h and calcined in air at 773 K for 4 h.
The prepared sample was designated asZrO2-Al2O3 (S).
Impregnation method
5.000 g alumina (203.1 m2·g-1) was impregnated
with an aqueous solution of ZrOCl2.8H2O
(0.207 mol·L-1) for 24 h, the sample was dried in oven
at 393 K for 12 h and calcined in air at 773 K for 4 h.
The prepared sample was designated as ZrO2-Al2O3 (I).
NOTES
Preparation of catalysts
Pd/ZrO2-Al2O3 catalysts were prepared by incipient
wetness impregnation.1 g ZrO2-Al2O3 (S) or
ZrO2-Al2O3 (I) supports were impregnated
to aqueous solutions of PdCl2 (0.156 mol·L-1) for
24 h.
The samples were dried at 393 K for 12 h and
calcined at 773 K for 4 h in air. The prepared samples
were designated as Pd/ZrO2-Al2O3 (S) and Pd/ZrO2Al2O3 (I), respectively. In both catalysts, the loading
of Pd was 2.0 wt%.
Catalytic activity measurements
The HDS of thiophene was performed in
continuous micro-reactor. First, the catalysts (100 mg)
were reduced in situ in H2 (30 mL·min-1) at 623 K for
2 h and cooled to the reaction temperature. Then,
thiophene–H2 mixed gas (mol ratio = 0.0289:1) was
introduced into the reactor with the operation
conditions of 2.0×105 Pa, 543-663 K reaction
temperature and 15.46 m mol·h-1·g-1 of LHSV. The
reaction products were analyzed by on-line gas
chromatography using a FID detector and a PEG20000 column. The chromatogram contained peaks
corresponding to the C4 products and unreacted
thiophene. The different components of C4 were not
separated. Therefore, only the total C4 signal was
considered in activity calculations. The conversion
was calculated from the ratio of the peak areas of
products over the sum of the peak areas of products
and thiophene. The catalytic activities of the catalysts
were expressed by the rate constants Ka. The HDS of
thiophene was a typical stair reaction6, its rate
constant was calculated according to the formula
K=- Fln(1-X)/W, where K is rate constant of the
reaction (mmol·g-1·h-1), F flow rate of thiophene gas
(mmol·h-1), W weight of catalyst (g), X conversion
rate (%). According to Arrhenius equation
k=Aexp (-Ea / RT), apparent activation energy Ea
can be calculated from slope of straight line of ln Ka
versus 1/T.
Characterization of catalysts
X-ray diffraction measurements were carried
out with German Bruker-AXS Corporation D8
Advance diffractometer equipped with a rotating
anode, Cu Kα radiation. Operating voltage was 40 kV
and current 30 mA, with a scanning rate of 1°/min
from 2θ =30° to 50°. The crystal size was estimated
using the Scherrer formula
273
d = k λ / B12 cos θ ,
where K is Scherrer constant (0.89); B1/2
integral half high width (expressed with radian);
θ diffraction angle (o) λ X-ray wave length
(0.154056 nm).
BET surface area was evaluated by N2 adsorption
isotherms obtained at 77 K, using an ASAP 2020
(Micrometrics) equipment. Before each measurement,
the samples were degassed at 623 K in vacuum (≈1
µm Hg) for 1 h.
Temperature-programmed reduction (TPR) was
carried out in-house apparatus over 0.1 g catalysts.
The samples were first heated from room temperature
to 1073 K in N2 (40 mL/min) at a rate of 10 K /min in
order to remove possible impurities contained in the
samples. After being cooled to room temperature in
N2, a gas mixture consisting of H2 (10% by volume)
N2 (90% by volume) was introduced into the system,
finally heated at a rate of 10 K /min for recording
TPR spectra.
Temperature-programmed desorption (NH3-TPD
or H2-TPD) were carried out in the same apparatus
as TPR. Before the measurements, the samples
were firstly heated from room temperature to
673 K at a rate of 5 K/min and kept at 573 K for 1 h
in H2. After cooling the samples to 398 K
(or 298 K) in Ar (40 mL/min), ammonia (or H2)
was pulsed until adsorption was saturated.
Then, the samples were heated to 1073 K at a rate of
10 K/min for recording the NH3-TPD (or H2-TPD)
spectra.
H2 chemisorption was used to measure the
dispersion. After reduction at 573 K, the catalysts
(0.1 g) were out gassed under vacuum for an hour
then cooled down to 343 K in N2 flow, the amount of
chemisorpted H2 was calculated by pulse injection
method. The dispersion of Pd was expressed by H to
Pd atomic ratio, as described earlier7.
The X-ray photoelectron spectroscopy (XPS)
analysis were performed with a KRATOS XSAM800,
equipped with a dual Mg/K anode. The binding
energy (BE) was corrected using the 285.0 eV signal
of C 1s.
Results and Discussion
XRD
To determine the structure of the catalysts in
detail, the samples were investigated with XRD. The
XRD patterns of Pd/ZrO2-Al2O3(S) and Pd/ZrO2-
274
INDIAN J. CHEM. TECHNOL., MAY 2009
Al2O3 (I) catalysts are shown in Fig. 1. No obvious
diffraction peak of palladium oxide was observed in
the XRD of Pd/ZrO2-Al2O3 (S) and Pd/ZrO2-Al2O3 (I)
catalysts at a loading 2.0 wt.% of Pd, suggesting that
the loaded PdO had been homogeneously dispersed
on the ZrO2-Al2O3. A peak at 2θ = 30.2°over
Pd/ZrO2-Al2O3(S) was obviously the trait of
tetragonal zirconia. No peak identifying monoclinic
zirconia (2θ = 28.1° and 31.4°) in the pattern of
Pd/ZrO2-Al2O3(S) was observed, namely, ZrO2
component in Pd/ZrO2-Al2O3(S) sample was only
tetragonal phase. On the other hand, the XRD pattern
of the Pd/ZrO2-Al2O3 (I) catalyst revealed that the
sample was chiefly composed of monoclinic zirconia,
with a small quantity of tetragonal zirconia. The
appearance of a monoclinic phase of zirconia was
possibly due to the non-homogeneous mixing of Al
and Zr species during the preparation process8. It
should be noted that the average crystal size of
tetragonal zirconia of Pd/ZrO2-Al2O3 (I) catalyst was
approximately 8.7 nm, and that of Pd/ZrO2-Al2O3 (S)
catalyst was approximately 5.7 nm. This indicated
that the grain size of ZrO2 in the ZrO2-Al2O3 (I)
exceeded the critical size causing a phase
transformation from the tetragonal to the monoclinic
phase8.
ZrO2-Al2O3 (I) mixed oxide provided a lower SBET
than ZrO2-Al2O3 (S), possibly, which was due to the
different particle size of ZrO2 in the pores of Al2O3.
The component of ZrO2–Al2O3 (S) mixed oxide
prepared by sol-gel method was dispersed in a
homogenous way, in particular, their particles were of
nanometric size, which led to a significant change of
the textural properties of the oxide precursor. It was
postulated that the zirconia phase dispersed into the
alumina phase in a homogenous way lead to structural
stabilization and strengthened electron-donated effect
of Zr9,10, which was beneficial to the dispersion of Pd
particle in the ZrO2-Al2O3(S) mixed oxide11.
Meanwhile, surface analyses of the ZrO2-Al2O3
mixed oxide prepared by different methods were
performed by XPS, as shown in Fig. 2. Binding
energy detected around 199 eV in Fig. 2 (a) of ZrO2Al2O3 (I) mixed oxide corresponded to Cl 3p,
indicating the existence of chlorine residues. Different
with ZrO2-Al2O3 (I), no chlorine residues were clearly
observed on XPS pattern of ZrO2-Al2O3 (S), presence
of chlorine ions lead to the lower dispersion of Pd on
ZrO2-Al2O3 (I) mixed oxide.
Hydrogen chemisorption
H2 chemisorption is an efficient means for
determining metal dispersion over the catalyst. The
results of hydrogen adsorption over the catalysts were
summarized in Table 1. The H2 adsorption over the
Pd/ZrO2-Al2O3 catalysts was performed at 423 K in
order to avoid Pd hydride formation, as suggested in
the literature7. The metal palladium was the
adsorptive sites of hydrogen, the activity for HDS was
influenced by the adsorption and activation of
hydrogen. Compared with Pd/ZrO2-Al2O3(I) catalyst,
H2 absorptive amounts of Pd/ZrO2-Al2O3 (S) catalyst
were a higher, which indicated that the ZrO2-Al2O3
(S) mixed oxide had a higher Pd dispersion than that
of ZrO2-Al2O3(I). The BET surface areas (SBET) of
ZrO2-Al2O3 (I) and ZrO2-Al2O3 (S) were 181.0 m2·g-1
and 195.0 m2·g-1, respectively, the larger surface area
was good to the higher dispersion of PdO, the
dispersion of Pd/ZrO2-Al2O3(S) and Pd/ZrO2-Al2O3(I)
catalysts was 58% and 76%, respectively, a higher
dispersion lead to the more activity sites. The SBET of
ZrO2-Al2O3 mixed oxides was smaller than that of
Al2O3 (203.1 m2·g-1) due to plugging of the pores of
Al2O3 with zirconium oxide species. However,
Fig. 1—XRD patterns of catalysts (a) Pd/ZrO2-Al2O3 (I)
(b) Pd/ZrO2-Al2O3 (S)
Table 1—Amount of H2 adsorbed, dispersion and activation
energy (Ea) of the catalysts
Catalysts
Amount of
H2 adsorbed
Pd
dispersion
Activation
energy
(10-5molH2·g-1)
(%)
（kJ·mol-1）
Pd/ZrO2-Al2O3（I）
5.94
58
55.7
Pd/ZrO2-Al2O3（S）
7.78
76
52.9
NOTES
275
Fig. 4—H2-TPR profiles of catalysts (a) Pd/ZrO2-Al2O3 (I)
(b) Pd/ZrO2-Al2O3 (S)
Fig. 2—XPS spectra of ZrO2-Al2O3
(a) ZrO2-Al2O3 (S) ( b) ZrO2-Al2O3 (I)
mixed
oxides
lattice defects in the structure was suggested to
mainly associate to the incorporation of the cationic
surfactant in the solid during the synthesis by strong
interactions between the deprotonated hydroxyl
groups and positively charged surfactant headgroups.
At a proper calcination temperature, these
incorporated surfactant species were combusted,
generating some lattice defects in the corresponding
locations, where the defects probably served as
acceptors for storage of hydrogen species.
H2-TPR
Fig. 3—H2-TPD profiles of catalysts (a) Pd/ZrO2-Al2O3 (I)
(b) Pd/ZrO2-Al2O3 (S)
H2-TPD
The H2-TPD spectras of the Pd/ZrO2-Al2O3
catalysts are given in Fig. 3. The H2-TPD profiles
gave evidence in the presence of at least two types of
hydrogen species adsorbed on the surface of reduced
Pd catalysts. The low-temperature peak might be
related to the adsorption of hydrogen on superficial
Pd centers, while high temperature peak could be
attributed to the adsorption of hydrogen on cationic
lattice defects12. The low and high temperature peak
areas of the Pd/ZrO2-Al2O3(S) catalyst were higher
than those of Pd/ZrO2-Al2O3(I) catalyst, which
suggested that Pd/ZrO2-Al2O3(S) catalyst had higher
dispersion of Pd and more lattice defects in the
structure of the catalyst. Comparing with Pd/ZrO2Al2O3 (I) catalyst, the higher desorption temperature
of Pd/ZrO2-Al2O3(S) catalyst was due to a stronger
interaction between metal and support. Creation of the
The TPR can provide information concerning the
reducibility of the surface reduction properties of PdO
presented in the catalyst, as well as the degree of
interaction between metal and support. The TPR
profiles of Pd/ZrO2-Al2O3 catalysts calcined at 773 K
are showed in Fig. 4. The low temperature peak (316
or 331 K) could be attributed to the reduction of
superficial PdO in Pd/ZrO2-Al2O3 (I) and Pd/ZrO2Al2O3 (S) catalysts. The peak at 383 K for Pd/ZrO2Al2O3 (I) sample was attributed to the reduction of the
bulk PdO phase. The low temperature peak area of the
Pd/ZrO2-Al2O3(S) catalyst was higher than that of
Pd/ZrO2-Al2O3 (I) catalyst, which resulted from more
amount of superficial PdO in Pd/ZrO2-Al2O3 (S)
catalyst. The superficial PdO in Pd/ZrO2-Al2O3 (S)
catalyst was reduced at higher temperature, indicating
that PdO in the catalyst synthesized by sol-gel method
had a stronger interaction between metal and support,
as reported by Shen et al.13, PdO particles interacted
strongly with the support were more resistant to
reduction. A broad negative consumption peak in the
temperature range between 400 and 600 K was
276
INDIAN J. CHEM. TECHNOL., MAY 2009
observed for Pd/ZrO2-Al2O3 (I) and Pd/ZrO2-Al2O3 (S)
catalysts. This was likely a consequence of desorption
of hydrogen, which adsorbed on cationic defects in
the crystalline structure13,14. Hydrogen species
adsorbed dissociatively on Pd crystals might spill over
to ZrO2-Al2O3 support and stored in ZrO2-Al2O3
lattice defects. The high temperature reduction peak
of Pd/ZrO2-Al2O3 catalyst could be attributed to the
reduction of ZrO2 species at the interface between
metal and support. Compared with reduction
temperature of pure nano ZrO2 (1000 K)15, the
reduction temperature of ZrO2 species at the interface
of Pd/ZrO2-Al2O3 catalyst decreased, which might be
caused by the high dispersion of ZrO2 in the catalysts
and the effect of H2 spillover16.
Fig. 5—NH3-TPD profiles of the catalysts (a) Pd/ZrO2-Al2O3 (I)
(b) PdO/ZrO2-Al2O3 (S)
NH3-TPD
Acidity plays an important role on noble metal
catalysts of HDS, to which hydroxyl is the adsorptive
and active site of thiophene17. NH3- TPD was used to
determine the surface acidity of the reduced catalysts.
The desorption temperature is a measure of the
strength of the corresponding acid sites, while the
desorption peak area showed the number of acid sites
at the surface. The NH3-TPD profiles for the Pd/ZrO2Al2O3 catalysts prepared with two different methods
were shown in Fig. 5. The NH3 -TPD profile for the
Pd/ZrO2-Al2O3 (S) catalyst exhibited two desorption
peaks, a maximum desorption peak at 450 K and a
shoulder at about 650 K, while the NH3 -TPD
spectrum for the Pd/ZrO2-Al2O3 (I) catalyst consisted
of a single peak with a maximum desorption at about
460 K. The amount of total acid sites of the catalysts
was increased in the following order: Pd/ZrO2-Al2O3
(S) > Pd/ZrO2-Al2O3 (I). It was postulated that the
increase in the acid sites for the Pd/ZrO2-Al2O3 (S)
was due to the high dispersion of ZrO2 in Al2O3,
which created more interface between ZrO2 and
Al2O3, leading to the local charge imbalance
associated with the zirconia chemically mixing with
the alumina matrix18. In accordance with the
desorption temperature of NH3, the acid sites were
classified into weak (350–550 K) and medium strong
adsorption (over 550 K). It was reported that the
desorption peak at low temperature was not ammonia
directly adsorbed on acid sites, and this ammonia was
attributed to hydrogen bonding to NH4+ adsorbed on
acid sites. The acidities of catalyst could only be
evaluated by the medium tempenture peak19. The acid
sites of medium strength of Pd/ZrO2-Al2O3 (S) were
more than those of Pd/ZrO2-Al2O3 (I). It had been
Fig. 6—Effect of preparation methods of supports on activity of
Pd/ZrO2-Al2O3 catalysts for HDS
reported that there was correlation between medium
strength acid sites and the catalytic activities of Pd
based catalyst for HDS. The more the acid sites of
medium strength are, higher catalytic activities for
HDS20 will be.
HDS activities
Activities of Pd/ZrO2-Al2O3 are shown as Fig. 6. It
indicated that the activities of Pd/ZrO2-Al2O3 catalysts
increase as the reaction temperature increases. The
HDS rate of the catalysts at 663 K was 10 times faster
than that of the catalysts at 543 K. The reason for
increase in the activity for HDS with increasing
reaction temperature has been observed earlier21. The
order of catalytic activity was Pd/ZrO2-Al2O3 (S)
> Pd/ZrO2-Al2O3 (I). The apparent activation energy
of Pd/ZrO2-Al2O3 (S) was lower than those of
Pd/ZrO2-Al2O3 (I) (Table 1). The HDS reaction of
thiophene was consistent with a first-order reaction, in
which hydrogen was adsorbed and activated
at Pd particle spilled over to thiophene adsorbed and
activated on acid sites. HDS activity was influenced
by the adsorption of thiophene and activation of
NOTES
hydrogen. Thus, two kinds of active sites, i.e. the
adsorptive site of thiophene and active centres of
hydrogen are postulated. The acid sites are the
adsorptive and active sites of thiophene,
while the metal Pd was the absorptive and
active sites of hydrogen22. It may be proposed
that the existence of more acid sites and higher
dispersion of Pd in the Pd/ZrO2-Al2O3 (S) catalyst
are favourable to the enhancement of catalyst
activity.
Conclusion
The results of the study showed that the Pd/ZrO2Al2O3 (S) catalyst had higher activity for HDS of
thiophene than that of Pd/ZrO2-Al2O3 (I) catalyst.
Compared with Pd/ZrO2-Al2O3 (I) catalyst, Pd/ZrO2Al2O3 (S) catalyst had stronger interaction between Pd
and ZrO2-Al2O3 (S) support, larger dispersion, more
acid sites and lower apparent activation energy.
Therefore, the catalytic activities of Pd/ZrO2-Al2O3
(S) catalyst for HDS of thiophene were significantly
improved.
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