A comparative study on Pt/CeO2 and Pt/ZrO2 catalysts for crotonaldehyde hydrogenation

Article abstract of DOI:10.1016/j.molcata.2012.05.002

Vapor-phase hydrogenation of crotonaldehyde was carried out over Pt/CeO₂ and Pt/ZrO₂ catalysts. It was found that both catalysts suffered deactivation, with the conversion of crotonaldehyde decreasing from 31 to 6% over the Pt/CeO₂ catalyst and 20 to 7% over the Pt/ZrO₂ catalyst, which was due to the formation of organic compounds on the catalyst surface as revealed by temperature programmed oxidation technique on the spent catalysts, and the poisoning effect of CO chemisorptions on Pt atoms via decarbonylation reaction. For the Pt/ZrO ₂ catalyst, selectivity to crotyl alcohol reached 48% and kept stable during the reaction, while for the Pt/CeO₂ catalyst, the selectivity to crotyl alcohol decreased dramatically from 51 to 33%. The decrease of selectivity for the Pt/CeO₂ catalyst was attributed to carbon deposit formed on the catalyst surface by the reaction between CO and reduced Ce ³⁺ ion in CeO₂. However, for the Pt/ZrO₂, no carbon deposit was formed on the catalyst surface, which could account for the stable selectivity during the reaction process.

Full text of DOI:10.1016/j.molcata.2012.05.002

Journal of Molecular Catalysis A: Chemical 361–362 (2012) 52–57
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
A comparative study on Pt/CeO and Pt/ZrO catalysts for crotonaldehyde
2
2
hydrogenation
∗
Lin Zhu, Ji-Qing Lu, Ping Chen, Xiao Hong, Guan-Qun Xie, Geng-Shen Hu, Meng-Fei Luo
Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Vapor-phase hydrogenation of crotonaldehyde was carried out over Pt/CeO2 and Pt/ZrO2 catalysts. It was
found that both catalysts suffered deactivation, with the conversion of crotonaldehyde decreasing from
Received 23 December 2011
Received in revised form 22 March 2012
Accepted 3 May 2012
3
1 to 6% over the Pt/CeO2 catalyst and 20 to 7% over the Pt/ZrO2 catalyst, which was due to the formation of
organic compounds on the catalyst surface as revealed by temperature programmed oxidation technique
on the spent catalysts, and the poisoning effect of CO chemisorptions on Pt atoms via decarbonylation
reaction. For the Pt/ZrO2 catalyst, selectivity to crotyl alcohol reached 48% and kept stable during the
reaction, while for the Pt/CeO2 catalyst, the selectivity to crotyl alcohol decreased dramatically from 51
to 33%. The decrease of selectivity for the Pt/CeO2 catalyst was attributed to carbon deposit formed on the
Available online 10 May 2012
Keywords:
Pt/CeO2
Pt/ZrO2
Crotonaldehyde
Hydrogenation
Crotyl alcohol
3
+
catalyst surface by the reaction between CO and reduced Ce ion in CeO2. However, for the Pt/ZrO2, no
carbon deposit was formed on the catalyst surface, which could account for the stable selectivity during
the reaction process.
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
alcohol could reach a value as high as 75–80% because of the
alloying and chlorine effect [9]. In our recent work, it was sug-
gested that Lewis acidic sites generated by chlorine species may
be the key factor to the high crotyl alcohol selectivity [10]. For the
Pt/Cr/ZnO catalyst, a high crotyl alcohol selectivity of 80% could be
obtained and maintained during the hydrogenation reaction, due
to the change of electronic structure of the active Pt metal [11]. For
Selective hydrogenation of ␣-, ␤-unsaturated aldehyde to ␣-
␤-unsaturated alcohol is an important reaction in industrial
,
production, because ␣-, ␤-unsaturated alcohols are vital inter-
mediates in pharmaceutical, fragrance and fine chemicals [1–4].
As a typical type of ␣-, ␤-unsaturated aldehydes, crotonaldehyde
(
CH3 CH CH CHO) is often used in model reaction of selective
Pt/ZnCl /SiO2 catalyst, the selectivity to crotyl alcohol was above
2
hydrogenation. Containing the conjugated C O bond and C
C
68%, but the catalytic activity did not change significantly with the
change of reduction temperature [12].
bond, the selective hydrogenation of crotonaldehyde usually pro-
duces three major products, namely, the partial hydrogenation as
crotyl alcohol (C O bond) and butanal (C C bond), and the deep
hydrogenation as butanol (C C and C O bond). Generally, hydro-
genation of the C C bond is much easier than that of the C O bond
in respect of thermodynamic and kinetic factors, so it is difficult for
selective hydrogenation of the carbonyl bond while keeping the
It can be summarized from the above findings that the catalytic
behaviors of the Pt catalysts are quite different during the reaction.
For example, some supported Pt catalysts can give steady activ-
ity and selectivity, while some catalysts suffer severe deactivation
and loss of selectivity. To better understand the causes of catalyst
deactivation and the factors governing the selectivity, we designed
C
C double bond intact in the hydrogenation of crotonaldehyde.
Supported Pt [5–7] catalysts have been frequently used in the
two supported Pt catalysts, Pt/CeO2 and Pt/ZrO , and applied them
2
to the gas phase selective hydrogenation of crotonaldehyde. The
different catalytic behaviors of these two catalysts relating to the
catalyst property were compared and discussed.
selective hydrogenation of crotonaldehyde to crotyl alcohol in
recent years. The catalytic activity of Pt/SnO2 catalyst deactivated
quickly in the hydrogenation of crotonaldehyde, while an increas-
ing selectivity to crotyl alcohol was observed due to the formation
of PtSn alloys [8]. A Pt/ZnO catalyst also gave a rapid deactiva-
tion during the hydrogenation progress, but its selectivity to crotyl
2. Experimental
2.1. Catalyst preparation
CeO2 and ZrO2 supports were obtained by thermal decom-
◦
∗ Corresponding author. Tel.: +86 579 82283910; fax: +86 579 82282595.
E-mail address: mengfeiluo@zjnu.cn (M.-F. Luo).
position of Ce(NO ) ·6H O and Zr(NO ) ·5H O in air at 400 C
3
3
2
3
4
2
2
2
for 4 h, with a specific surface area of 66 m /g and 73 m /g,
1
381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.05.002

L. Zhu et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 52–57
53
Table 1
respectively. Both the Pt/CeO and Pt/ZrO catalysts were prepared
2
2
Physical properties of Pt/CeO2 and Pt/ZrO2 catalysts.
by an impregnation method. The support was impregnated with
H PtCl aqueous solution with a nominal Pt content of 3 wt%. Excess
Catalysta
Pt content
wt%)
Particle size
(nm)
Surface area
2
6
a
2
(
(m /g)
water solution was removed by a mild evaporation. Finally, the
◦
resulting solid was dried overnight at 120 C, and was denoted as
Pt/CeO2
Pt/ZrO2
2.90
2.84
4.4
3.1
58
62
Pt/CeO2 and Pt/ZrO2 catalysts.
a
◦
Sample reduced at 500 C.
b
Determined by CO adsorption.
2.2. Catalyst characterizations
The specific surface areas of catalysts were determined by
◦
nitrogen adsorption at −196 C on a Quantachrome Autosorb-1
apparatus. X-ray diffraction (XRD) patterns were recorded using
a PANalytic X’Pert PW3040 diffractiometer with Cu K␣ radiation
operating at 40 kV and 40 mA. The patterns were collected in a 2Â
range from 20 to 110 , with a scanning step of 0.15 s . Particle
sizes of Pt in the Pt/CeO2 and Pt/ZrO2 catalysts were measured by
◦
◦
◦
−1
Pt/CeO
2
CO chemisorption on a Chembet 3000 instrument. The sample was
◦
reduced in a H2 N2 mixture gas (5 vol% H , 30 ml/min) at 500 C
CeO
2
2
◦
for 1 h and cooled down to 30 C under pure He flow (30 ml/min),
then pulse of CO was injected. Pt dispersion was evaluated from
the consumption of CO, assuming that CO: Pt = 1. The Pt particle size
was estimated using the cubic Pt particle model with the calculated
Pt metal dispersion. The reduction properties of the samples were
measured by hydrogen temperature-programmed reduction (H2-
TPR) experiments. The sample was placed in a quartz reactor, and
Pt/ZrO
2
ZrO
2
◦
◦
then heated from room temperature to 550 C at a rate of 10 C/min
in a H2 N2 gas (5 vol% H ; 30 ml/min). The hydrogen consumption
2
during the reduction was determined by a gas chromatograph with
a thermal conductivity detector. Raman spectra were obtained on a
Renishaw RM1000 confocal microscope with exciting wavelength
of 514.5 nm and scanning range of 200–2000 cm under an ambi-
ent condition. In order to study the carbon deposit, in situ Raman
spectroscopy of CO adsorption was carried in a Renishaw RM1000
confocal microscope with an excitation wavelength of 514.5 nm
20
40
60
80
100
ο
2
θ /
−
1
Fig. 1. XRD patterns of Pt/CeO2 and Pt/ZrO2 catalysts reduced at 500 ◦C.
3
. Results and discussion
−
1
and a scanning range of 200–2000 cm . The sample was reduced
◦
3.1. Physical properties of catalysts
in a H2 N2 mixture (5 vol% H , 30 ml/min) stream at 500 C for
h and cooled down to 25 C in a He flow (30 ml/min). Then the
2
◦
1
◦
Table 1 lists the physical properties of the Pt/CeO2 and Pt/ZrO2
catalysts reduced at 500 C. The actual Pt contents in the Pt/CeO2
and Pt/ZrO catalysts are 2.90 and 2.84 wt%, respectively, which are
close to the nominal values. By CO chemisorption measurement,
the Pt particle sizes in the Pt/CeO2 and Pt/ZrO2 are 4.4 and 3.1 nm,
respectively. The specific surface areas of the Pt/ZrO and Pt/CeO2
catalysts are 62 and 58 m /g, respectively. Fig. 1 shows the XRD
patterns of Pt/CeO2 and Pt/ZrO2 catalysts reduced at 500 C. For
both catalysts, only diffraction peaks of the support (CeO and ZrO )
are observed. Diffraction peaks of PtO or Pt are not detected, which
indicates that the Pt species are highly dispersed on the catalyst
surface.
Fig. 2 shows the H -TPR profiles of Pt/ZrO2 and Pt/CeO2 cata-
lysts and their corresponding supports. The ZrO2 support shows
no reduction peak up to 550 C [13], while the pure CeO shows
one reduction peak at around 470 C, which can be assigned to
the reduction of surface ceria [14]. As for the supported catalysts,
a reduction peak at 200 C is observed for the Pt/ZrO catalyst,
sample was exposed CO N2 (1 vol% CO) 30 min at 50 C. For the
temperature-programmed oxidation (TPO) study, 50 mg of the used
catalyst was placed in the middle of a quartz microreactor with a
inner diameter of 6 mm. The outlet was analyzed on-line by mass
spectrometry (Qic-20 Benchtop, Hiden Analytical). Then the sample
◦
2
2
was subsequently flushed by O (20 ml/min) from room tempera-
2
2
◦
◦
ture to 600 C at a rate of 10 C/min. The mass numbers of 44 and 18
were selected to monitor the desorption of CO and H O fragments,
◦
2
2
2
2
respectively.
.3. Catalytic test
The gas phase crotonaldehyde hydrogenation was performed
2
2
2
in a fixed bed reaction system at atmospheric pressure, using a
quartz tube (8 mm i.d.) reactor. 100 mg of catalyst was loaded in
the reactor with a thermal couple placed in the middle of the cat-
alyst bed to monitor the reaction temperature. Before running the
◦
2
◦
◦
2
◦
catalytic test, the catalyst was reduced at 500 C for 1 h in ultra-
◦
and two reduction peaks appear at 180 (␣) and 410 C (␤) for the
Pt/CeO2 catalyst.
◦
pure H2 (20 ml/min) and then it was cooled down to 50 C. The
crotonaldehyde was introduced in a trap set before the reactor
Assuming that the initial Pt oxidation state in the catalysts is
◦
tube and maintained at 0 C to achieve a constant crotonalde-
Pt4+ (PtO ), the nominal hydrogen consumptions of the Pt/ZrO and
2
2
hyde partial pressure (1.06 kPa), therefore, aldehyde at constant
−1
Pt/CeO2 catalysts are 291.2 and 297.3 ␮molH2 gcat , respectively.
According to the TPR profiles in Fig. 2, the actual hydrogena-
tion consumptions (␣ peak) calibrated by the known amount of
partial pressure was carried over the catalyst by hydrogen flow
◦
(
26 ml/min). The gas line was kept at about 50 C to avoid any con-
densation. The reaction products and reactant were analyzed on
line using a gas chromatography (Shimazu GC-2014) equipped with
a flame ionization detector (FID) and a DB-Wax capillary column
CuO powder for the Pt/ZrO2 and Pt/CeO2 catalysts are 288.8 and
−1
6
08.0 ␮molH2 gcat , respectively. For the Pt/ZrO catalyst, it can be
2
seen that the nominal hydrogen consumption is very close to the
(
30 m × 0.25 mm × 0.25 ␮m).
◦
actual consumption, suggesting that the reduction peak at 200 C

5
4
L. Zhu et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 52–57
◦
0
the catalysts after reduction 500 C are metallic Pt , together with
α
partial reduction of Ce4+ to Ce
3+
.
Actual Hydrogen consumption
-
1
(
mmolH gcat )
2
3.2. Catalytic performance for the hydrogenation of
crotonaldehyde
β
Fig. 3 shows the catalytic behaviors of the Pt/ZrO2 and Pt/CeO2
catalysts. It should be noted that the initial reactivity of the Pt/CeO2
catalyst (about 31%) is much higher than that of the Pt/ZrO2 (about
20.0%), indicating the vital role of support in the reaction. In
addition, the conversion of crotonaldehyde decreases with reac-
tion time for both catalysts. Concerning the selectivity, the main
products are crotyl alcohol (hydrogenation of C O bond), butanal
6
08.0
Pt/CeO
2
2
88.8
Pt/ZrO
2
CeO
2
(hydrogenation of C C bond) and butanol (complete hydrogena-
tion). Typical side reactions in the gas phase hydrogenation of
crotonaldehyde are polymerization resulting in the formation of
heavy compounds product and decarbonylation resulting in the
formation of C3 (propane and propylene) hydrocarbons and car-
bon monoxide [16,17]. For the desired crotyl alcohol, the selectivity
keeps a stable level around 48% on the Pt/ZrO2 catalyst, while
its selectivity decreases from 51.6 to 31.9% after 350 min on the
ZrO
2
100
200
300
400
500
o
Temperature / C
Fig. 2. H2-TPR profiles of Pt/CeO2 and Pt/ZrO2 catalysts and their corresponding
supports.
Pt/CeO catalyst. However, the selectivity to butanal increases with
2
reaction time for both catalysts. Detailed catalytic results based
on Fig. 3 are also listed in Table 2. It is found that the turnover
frequencies (TOFs) are similar for the two catalysts after 300 min
is probably due to the reduction of PtO . On the contrary, for the
2
Pt/CeO2 catalyst, it is found that the actual hydrogen consumption
reaction, however, with the Pt/CeO catalyst having higher TOF
2
−1
−3 −1
−3 −1
s ) at the ini-
(
608.0 ␮molH2 gcat ) is significantly higher than the nominal value
(9.7 × 10
s
) compared to the Pt/ZrO (6.1 × 10
2
−1
(
297.3 ␮molH2 gcat ), which is due to the reduction of the CeO2
tial stage. As for the bare supports, it is found that the pure CeO or
2
support. This indicates that the presence of Pt promotes reduction
of ceria via spillover of hydrogen from platinum to the support [15],
which remarkably shifts the surface ceria reduction downwards in
ZrO2 is inactive for the reaction.
It is obviously that these two catalysts suffer deactivation during
the reaction. The cause of deactivation over Pt catalysts has been
extensively investigated and it was generally recognized that the
temperature. The H -TPR results also imply that the Pt species in
2
Table 2
a
The crotonaldehyde hydrogenation performance of Pt/CeO2 and Pt/ZrO2 catalysts.
TOFb (×10−3
s
−1
)
Selectivity (%)
Catalyst
Conv. (%)
Crotyl alcohol
Butanal
Butanol
Pt/CeO2
Pt/ZrO2
CeO2
6.4 (18.4)
8.5 (15.8)
0
0
3.4 (9.7)
3.3 (6.1)
0
0
34.3 (55.4)
48.1 (50.3)
–
–
56.1 (26.6)
43.5 (35.5)
–
–
8.3 (17.1)
8.5 (13.5)
–
–
ZrO2
a
Experimental data were taken at 300 min or 60 min.
Calculated based on Pt dispersion by CO chemisorption results.
b
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
1.8
70
60
50
40
30
20
10
0
1.8
1.2
0.6
0.0
Pt/CeO2
Pt/ZrO2
1
0
0
.2
.6
.0
conv.
crotyl alcohol
butanal
butanol
C3
conv.
crotyl alcohol
butanal
butanol
C3
0
50 100 150 200 250 300 350
0
50 100 150 200 250 300 350
Time on stream / min
Time on stream / min
Fig. 3. Crotonaldehyde hydrogenation over Pt/CeO2 and Pt/ZrO2 catalysts.

L. Zhu et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 52–57
55
3
2
1
0
0
0
0
Used Pt/CeO
Used Pt/ZrO
2
2
B
A
CO
2
H
2
O
O
H
2
CO
2
0
5
10
15
20
25
1
00
200
300
400
500
O
Time / h
Temperature / C
Fig. 6. Catalytic performance of Pt/ZrO2 catalyst after N2 purging and calcination in
Fig. 4. TPO spectra of used-Pt/CeO2 and Pt/ZrO2 catalysts at O2 atmosphere.
◦
air atmosphere (A: purge with N2 (24 ml/min) for 0.5 h at 100 C; B: calcination at
◦
4
00 C in air for 1 h).
formation of heavy organic compounds on catalyst surface via poly-
merization could block the active sites [10]. To further check the
surface species on the catalysts, we performed TPO examinations
of the used Pt/CeO2 and Pt/ZrO2 catalysts. As shown in Fig. 4, for
the Pt/CeO and Pt/ZrO catalysts, there is one H O desorption peak
In order to further confirm whether the deactivation is caused by
CO poisoning, 1 ml CO was injected into reactor before the reaction.
Fig. 5 shows the effect of CO poisoning on the catalytic behavior of
the Pt/CeO2 and Pt/ZrO2 catalysts. It can be seen that the croton-
aldehyde conversion of both catalysts have a decline at the initial
stage after CO adsorption, especially for the Pt/CeO2 catalyst (from
31 to 8%), and the reactivity is obviously suppressed by the poison-
ing during the whole reaction. Therefore, it can be concluded that
the CO chemisorption on the surface Pt atoms may also be respon-
sible for the deactivation of the catalysts. Concerning the selectivity
to the desired crotyl alcohol, no apparent changes are observed for
both catalysts, indicating that the CO poisoning does not influence
the selectivity of target product. Therefore, the deactivation of the
catalysts during the reaction was caused by two factors. One is the
deposition of organic compounds on the catalyst surface, as evi-
denced by TPO results (Fig. 4); the other is the strong CO adsorption
on the Pt atoms via decarbonylation reaction, as evidenced by the
CO poisoning experiments (Fig. 5).
2
◦
2
2
around 70 C, respectively, and no CO2 were detected in this tem-
perature range, indicating that this signal from the desorption of the
adsorbed H O on the surface of the catalyst. With increasing tem-
2
perature, simultaneous desorption of H O and CO2 are detected,
2
suggesting the oxidation of surface organic compounds at high
temperature. This result indicates that organic compounds were
deposited on the catalyst surface during the reaction, which may
block the active sites and thus suppress the reactivity. Additionally,
the ratio of desorption area of CO2 between Pt/CeO2 and Pt/ZrO2 is
1
.6/1, implying more deposition of the organic compounds on the
Pt/CeO2 compared to Pt/ZrO , which can be explained the rapid
2
deactivation and lower activity at steady state.
In Fig. 3, small amount of C3 hydrocarbons are produced during
the reaction, indicating the occurrence of decarbonylation reaction
and the simultaneous formation of CO [18]. It was reported that
CO molecules could be strongly adsorbed on the Pt surface and the
active sites are blocked, which led to a partial loss of active sites [19].
In order to further verify the reasons of catalyst deactivation, a
series of experiments were conducted, as shown in Fig. 6. It was
Pt/CeO
2
Pt/ZrO
2
60
40
20
0
60
40
20
0
Sel.
Sel.
Conv.
Conv.
0
100
200
300
400
0
100
200
300
400
Time on stream / min
Time on stream / min
Fig. 5. Effect of CO on the crotonaldehyde hydrogenation performance of Pt/CeO2 and Pt/ZrO2 catalysts (ꢀ, ᭹-fresh catalyst; ♦, ꢀ-catalyst pretreated with CO).

5
6
L. Zhu et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 52–57
found that the fresh catalyst deactivated rapidly in 5 h reaction.
When the spent catalyst was in situ purged with N2 (26 ml/min)
◦
at 100 C for 30 min and exposed to reactant mixtures (point A),
the conversion of crotonaldehyde was recovered to some extent
(
but less than the initial conversion). After that, the catalyst was
◦
in situ calcined in air at 400 C for 1 h and subsequently reduced in
H2 (26 ml/min) at 500 C for 30 min. (point B). When the treated
◦
Used Pt/CeO2
sample was exposed to the reactant mixture, it was found that
the initial conversion of crotonaldehyde could be fully recov-
ered. The recovery of reactivity after N2 purge (point A) could
be attributed to the removal of chemisorbed CO on the catalyst
surface, and the recovery of reactivity after calcination (point B)
could be attributed to the combustion of surface organic com-
pounds. Therefore, it could be validated that the deactivation of
the catalyst is due to the CO chemisorption (mainly on Pt) and the
deposition of organic compounds on the catalyst. It also could be
concluded that the CO chemisorption on the surface Pt atoms is
probably reversible as the CO molecules could be removed by N2
purging.
Used Pt/ZrO
2
800
1000 1200 1400 1600 1800
It is interesting that the selectivities of crotyl alcohol on these
-1
Raman shift / cm
two catalysts are different. In the case of the Pt/CeO , the selectiv-
2
ity decreases with reaction time, while it keeps stable in the case
Fig. 7. Raman spectra of used Pt/CeO2 and Pt/ZrO2 catalysts.
of Pt/ZrO . Chen [20] reported that carbon deposition formed on
2
the catalyst surface during the hydrogenaton process may account
for the decrease in selectivity for crotyl alcohol. In order to further
verify this point, Raman spectroscopic measurements are carried
out. Fig. 7 shows the Raman spectra of the used catalysts. For the
In order to further explain the effects of the generation of car-
bon on the catalysts, in situ Raman spectroscopy of CO adsorption
was performed on the reduced catalyst, as shown in Fig. 8. For the
Pt/ZrO2 catalyst, the Raman spectrum remains unchanged before
−
1
used Pt/CeO2 catalyst, two Raman bands at 1350 and 1600 cm
are observed, which are assigned to the carbon deposit [21,22].
However, for the used Pt/ZrO2 catalyst, no Raman peak associ-
ated to carbon deposit is observed. The deposition of carbon on
the Pt/CeO2 surface could result in the formation of Pt C interface.
It was reported that very low selectivity to crotyl alcohol (<10%)
was obtained over Pt catalysts supported on activated carbon and
carbon blacks, and it was concluded that Pt/C catalysts preferred
the hydrogenation of C C bond to C O bond [23,24]. These find-
ings indicate that the presence of carbon on the catalyst surface
may suppress the selectivity. This point is also supported by the
fact that the selectivity to butanal increases with reaction time. On
the other hand, no peaks due to carbon deposit are observed on
and after CO adsorption. For the Pt/CeO catalyst, the characteristic
2
−
1
band at 460 cm assigned to F mode vibration of CeO2 shifts to
2
1
g
−
lower wave number at 445 cm for the reduced catalyst compared
to the as prepared catalyst. The shift is caused by lattice expan-
sion of CeO2 due to the presence of Ce3+ cations formed during the
3
+
reduction treatment. Similar result has also been reported in a La
-
doped CeO2 material, in which replacement of Ce4+ by La led to
3+
−
1
a lattice expansion and consequently a red shift of the 460 cm
Raman band [25]. After the CO adsorption, the Raman band at
−
1
−1
445 cm returns to its initial position (460 cm ), indicating the
oxidation of Ce3+ by reacting with CO. Meanwhile, two intensive
−
1
the surface of the Pt/ZrO catalyst in the reaction, which could well
bands at 1340 and 1600 cm are observed on the Pt/CeO catalyst,
2
2
explain the stable selectivity on this catalyst.
which could be assigned to the carbon deposit [21,22]. The results
Pt/CeO
2
Pt/ZrO
2
C
B
A
A
C
4
50
500
550
C
B
A
B
3
00 600 900 1200 1500 1800
300
600
900 1200 1500 1800
-1
-
1
Raman shift (cm )
Raman shift (cm )
◦
Fig. 8. In situ Raman spectra of Pt/CeO2 and Pt/ZrO2 catalysts at 50 C (A) as prepared catalysts; (B) reduced catalysts; (C) after CO adsorption on catalysts.

L. Zhu et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 52–57
57
4. Conclusions
Catalytic behaviors of the Pt/CeO and Pt/ZrO catalysts are com-
2
2
pared for selective hydrogenation of crotonaldehyde. It is found
that both catalysts suffer deactivation due to the formation of
organic compounds on the catalyst surface, and the poisoning effect
of CO chemisorptions on Pt atoms. It is also found that the selectiv-
ity to crotyl alcohol decreases with reaction time for the Pt/CeO2
catalyst, while it remains stable for the Pt/ZrO catalyst. The decline
2
of the selectivity on the Pt/CeO2 catalyst could be attributed to the
formation of carbon deposit on the catalyst surface. In contrast, no
carbon deposits are formed on the surface of Pt/ZrO catalyst in the
2
reaction process, which could explain the stable selectivity.
Scheme 1. Proposed adsorption model of crotonaldehyde over Pt/CeO2 and Pt/ZrO2
catalysts.
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CO and Ce species on the catalyst surface (Ce O + CO 2CeO + C).
2
3
2
1
3+
It was reported that the reduced Ce cations are beneficial to the
selectivity of crotyl alcohol [26]. However, the reaction between
CO and Ce could results in carbon deposits and deactivate such
Ce sites, and consequently the Pt/CeO2 catalyst turns unselective
due to a poisoning of support sites, which could explain the loss of
selectivity over the Pt/CeO2 catalyst.
[
3
+
[
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3+
[7] F. Ammari, J. Lamotte, R. Touroude, J. Catal. 221 (2004) 32–42.
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[
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1
In summary, the comparison of these two catalysts suggests that
[
[
[
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the decline of crotyl alcohol selectivity on the Pt/CeO catalyst may
2
result from the formation of carbon species on the catalyst surface.
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3.3. Adsorption model
[13] J.C. Serrano-Ruiz, J. Luettich, A. Sepúlveda-Escribano, F. Rodríguez-Reinoso, J.
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[
[
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Combined with the Raman and TPO results, adsorption mod-
els of crotonaldehyde over the Pt/CeO2 and Pt/ZrO2 catalysts are
proposed in Scheme 1. CxHyOz stands for organic compounds such
as C8 condensation product generated during the reaction on the
catalysts surfaces. These organic compounds will cover the active
sites of the catalyst, which consequently suppresses the reactiv-
ity. For the Pt/CeO2 catalyst, the carbon deposits formed by CO
reacting with Ce3+ cover the catalyst surface and produce Pt/C inter-
faces, which tends to C C hydrogenation [19] and thus inhibits
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[20] P. Chen, G.Q. Xie, H.Y. Zheng, L. Zhu, M.F. Luo, Chin. J. Catal. 32 (2011) 513–519.
[
[
21] T.W. Zerda, W. Xu, A. Zerda, Y. Zhao, R.B. Von Dreele, Carbon 38 (2000) 355–361.
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[
C
O hydrogenation. For the Pt/ZrO2 catalyst, considering that no
24] F. Coloma, J. Narciso-Romero, A. Sepfilveda-Escribano, F. Rodríguez-Reinoso,
Carbon 36 (1998) 1011.
carbon deposit is formed on catalyst surface and the support is
not reducible, the selectivity to crotyl alcohol remains unchanged
during the reaction.
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26] B. Campo, M. Volpe, S. Ivananova, R. Touroude, J. Catal. 242 (2006) 162–171.
[