Glycerol Hydrogenolysis to 1,3-Propanediol on Tungstate/Zirconia-Supported Platinum: Hydrogen Spillover Facilitated by Pt(1 1 1) Formation

Article abstract of DOI:10.1002/cctc.201600981

In glycerol hydrogenolysis, WO_x and ReO_x catalysts have been investigated widely because of their importance for the selectivity of 1,3-propanediol, but few studies have focused on hydrogen spillover. In this work, the hydrogen spillover effect on the catalytic performance was investigated for a series of Pt/WO₃/ZrO₂ nanocatalysts. The spillover capacities were tuned by changing the tetragonal/monoclinic ZrO₂ composition and thermal treatment conditions of the Pt precursor. H₂/O₂ titration and X-ray photoelectron spectroscopy (XPS) confirm that all of the catalysts present a uniform Pt dispersion and a similar surface electron environment. Diffuse reflectance infrared Fourier transform spectra of adsorbed CO and H₂ chemisorption reveal the strong correlation between the amount of Pt(1 1 1) terraces and the spillover capacities. We demonstrate that the reaction rate is influenced by hydrogen spillover, but not the acid amount or acid strength of the catalysts in this case. The important roles of Pt(1 1 1) and WO₃ in the spillover process are discussed.

Full text of DOI:10.1002/cctc.201600981

Accepted Article
Title: Glycerol Hydrogenolysis to 1,3-Propanediol on Pt/WO3/ZrO2:
Hydrogen Spillover Facilitated by Pt(111) Formation
Authors: Wei Zhou, Yujun Zhao, Yue Wang, Shengping Wang, and
Xinbin Ma
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Glycerol Hydrogenolysis to 1,3-Propanediol on Pt/WO3/ZrO2:
Hydrogen Spillover Facilitated by Pt(111) Formation
Wei Zhou, Yujun Zhao, Yue Wang, Shengping Wang, and Xinbin Ma*[a]
Abstract: In glycerol hydogenolysis, the WOx or ReOx species of
catalysts have been widely concerned because of their importance
for the selectivity of 1,3-propanediol, but few studies are focused on
the hydrogen spillover. In this work, the hydrogen spillover effect on
Extensive research efforts have been devoted to understanding
Re- and W-based catalysts. In the case of Re catalysts, Nak-
agawa et al.[3] conducted a comprehensive investigation and
proposed that a Ren+ coated metal structure is responsible for
the efficient catalytic performance. In a recent study,[4] Re spe-
cies were proved to be the source of acid site.
the catalytic performance was investigated on
a series of
Pt/WO3/ZrO2 nano-catalysts. The spillover capacities were tuned by
changing tetragonal/monoclinic ZrO2 composition and thermal treat-
ment conditions of the Pt precursor. H2-O2 titration and XPS confirm
that all of the catalysts present a uniform Pt dispersion and a similar
surface electron environment. DRIFT spectra of adsorbed CO and
H2 chemisorption reveal the strong correlation between the amount
of Pt(111) terraces and spillover capacities. We demonstrate that the
reaction rate is influenced by hydrogen spillover, but not the acid
amount or acid strength of the catalysts in this case. The important
roles of Pt(111) and WO3 in the spillover process are discussed.
With respect to W-containing catalyst systems, Chaminand et
al.[5] reported that tungstic acid improved the 1,3-PDO selectivity
on Rh-based catalysts. Dam et al.[6] screened four commercial
catalysts and four acidic additives for glycerol hydrogenolysis,
and they achieved 28% 1,3-PDO selectivity with 49% conversion
on Pt/Al2O3 with silicotungstic acid as an additive. However,
these additives are usually soluble in water and difficult to recy-
cle. As a consequence, many researchers are engaged in the
development of W-loaded solid catalysts. Zhu et al.[7] carried out
a series of studies on metal–heteropolytungstic acid supported
catalysts and observed that the Brønsted acid site is important
for the formation of 1,3-PDO. For a better understanding of the
catalytic roles of W species, WOx is loaded onto supports in-
stead of heteropolyacids. Qin et al.[8] achieved 1,3-PDO yield as
high as 32.0% with 70.2% conversion by tuning the tungsten
loading and calcination temperature of Pt/WO3/ZrO2 catalysts.
Zhang et al.[9] fabricated mesoporous Ti–W oxides with tunable
W/Ti ratios in a wide range for glycerol hydrogenolysis, and they
concluded that the presence of Brønsted acid site and the syn-
ergy between the well-isolated WOx species and the highly
dispersed Pt nanoparticles play important roles in the hydrogen-
olysis. Arundhathi et al.[10] obtained a high 1,3-PDO selectivity of
66% at 100% glycerol conversion using a boehmite-supported
platinum/tungsten catalyst, and they attributed the superior
performance to the Al-OH structure on the catalyst surface.
Fernández et al.[11] studied Pt/WOx/Al2O3 catalysts and observed
that the interaction between Pt and WOx is also a key factor in
addition to the Brønsted acid sites.
Introduction
As fossil resources are decreasing continuously, extensive
efforts are focused on the conversion of biomass. Glycerol is a
renewable resource that is readily available because of the rapid
increase in biodiesel production. The conversion of glycerol to
highly valuable products can expand the industrial chain of
biomass and ease the pressure of the fossil resource crisis.
Many processes have been reported so far such as glycerol
esterification, etherification, reforming, dehydration and selective
oxidation, among which propanediols (PDOs) production from
glycerol hydrogenolysis is gaining more and more attention.[1]
The production of 1,2-PDO by glycerol hydrogenolysis has
been extensively studied, and high yields more than 90% on Cu-
based catalysts have been reported by many groups.[2] However,
the hydrogenolysis of glycerol to produce 1,3-PDO, which is a
much more valuable commercial product than 1,2-PDO, remains
a challenge. Generally, 1,3-PDO formation is based on bifunc-
tional catalysts with both acid sites and metal sites. The acid
sites enable the adsorption and dehydration of glycerol, and the
metal sites provide active hydrogen for the hydrogenation step.
Regarding catalyst materials, both Re- and W-based catalysts
can be classified as supported noble metal–oxophilic metal
promoter model, the acid strength and bifunctional catalytic
behavior of which have been
systematically
studied
by
Hibbitts et al. using density functional theory (DFT)
calculations.[12] In the hydrogenolysis of glycerol, the oxophilic
metal promoter provides the site to activate C-O bonds of glyc-
erol, while the noble metal is responsible for dissociating the
hydrogen molecule. Most of the researchers have focused on
the former site in attempts to obtain highly dispersed oxophilic
sites or Brønsted acid sites by optimizing the promoter loading,
heat treatment conditions or the structure of the support.[8-9, 13]
By contrast, few reports have been focused on the hydrogen
activation.
[a]
W. Zhou, Dr. Y. Zhao, Y. Wang, Prof. Dr. S. Wang, Prof. Dr. X. Ma
Key Laboratory for Green Chemical Technology of Ministry of
Education
Collaborative Innovation Centre of Chemical Science and
Engineering
School of Chemical Engineering and Technology
Tianjin University
Tianjin 300072 (P.R. China)
E-mail: xbma@tju.edu.cn
The dissociative adsorption of hydrogen usually occurs at the
metal site and is known to be especially facile on a noble metal.
However, an important phenomenon known as hydrogen spillo-
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ver could affect the hydrogenation system. Hydrogen spillover
was first reported by Khoobiar et al. and then it was enforced
with the work of Pajonk and Teichner.[14] This concept is fully
developed and gained application in the field of hydrogen stor-
age. Pevzner et al.[15] studied solid-phase hydrogenation kinetics
on different carbonaceous allotrope spillover agents, and they
found a dimensionality effect of the support on the hydrogen
transport rate. Wang et al.[16] prepared a series of Pt-doped
IRMOF-8 samples via organometallic chemical-vapor deposition
and studied their hydrogen storage capacities. They claimed that
the Pt catalyst size is a crucial factor in hydrogen spillover. Thus,
hydrogen spillover has been proved to be influenced by the
properties of both metal and support.
Although there are still arguments on this phenomenon, many
excellent works have been carried out to demonstrate the im-
portance of hydrogen spillover in catalysis. Taniya et al.[17] de-
signed a Sn-modified SiO2-coated Pt catalyst and applied it in
crotonaldehyde hydrogenation. In the absence of crotonalde-
hyde–metal contact, they achieved a crotyl alcohol selectivity of
100% through hydrogen spillover. Im et al.[18] revealed the im-
portant catalytic functions of hydrogen spillover using a Pt en-
capsulated model catalyst. Lee et al.[19] investigated the effects
of physical diluents and concluded that Lewis acid sites on the
aluminosilicate surface can enhance hydrogen spillover. In fact,
hydrogen spillover has also been proposed in glycerol hydro-
genolysis by many investigators, but direct evidences of its
influence on catalytic activity have not yet been reported.[8, 10, 13,
20]
In this work, WO3/ZrO2, a type of ceramic acid, was chosen as
the support in view of its good stability in hydrothermal environ-
ment. Because of the low surface area of ZrO2, a solvothermal
method was applied to prepare a nano-ZrO2 precursor, on which
WO3 could be loaded as much as possible while maintaining a
high dispersion. To reveal the spillover effect of the catalysts on
glycerol hydrogenolysis, the monoclinic/tetragonal crystalline
composition of ZrO2 and thermal treatment conditions of Pt
precursor were changed, from the respects of support and metal
sites respectively mentioned above. Finally, a mechanism of
hydrogen spillover facilitated by Pt(111) formation was proposed
for the Pt/WO3/ZrO2 catalyst system.
Figure 1. XRD patterns of the (a) supports and (b) reduced catalysts.
Results and Discussion
Characterization and Catalytic performance
The crystalline structures of the supports and reduced catalysts
are presented in Figure 1(a) and (b), respectively. For compari-
son, m-ZrO2 and t-ZrO2 without WO3 were also prepared by
calcination of the precursors at 700 C in air and nitrogen, re-
spectively. As shown in Figure 1(a), pure phases of m-ZrO2 and
t-ZrO2 can be prepared by the solvothermal method, and the
particle sizes of both samples are approximately 12 nm, as
calculated by the Scherrer equation. Moreover, the crystalline
phases of ZrO2 were remained after WO3 species were loaded.
When the ZrO2 precursors were physically mixed before impreg-
nation, WO3/ZrO2 with heterogeneous crystalline phases was al-
Figure 2. Pore distribution of various Pt/ZrW catalysts.
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eter. In addition, the thermal pretreatment has little
effect on pore distributions of the catalysts. According
to the H2-O2 titration results, the dispersion of Pt is
slightly higher on the ZrW support with larger specific
surface area. Moreover, the Pt dispersion was further
improved when Pt/ZrW catalysts were calcined before
the reduction. But in fact, it can be observed from
Table 1 that average Pt particle sizes for all the cata-
lysts ranged from 3.0 to 3.6 nm. Direct reduction or
thermal treatment at a low temperature does not obvi-
ously affect the Pt particle size.
Table 1. Physicochemical properties of various Pt/ZrW catalysts.
[a]
[a]
[a]
[b]
SBET
Dpore
[nm]
Vpore
Dispersion[b]
[%]
dPt
H/Pt[c]
[m2 g-1]
[cm3 g-1]
[nm]
Catalyst
Pt/ZrW(m)
65.3
63.4
49.6
66.2
71.6
54.2
15.7
11.5
6.2
0.27
0.21
0.10
0.25
0.21
0.10
35.6
34.6
31.5
36.0
36.4
38.1
3.2
3.3
3.6
3.2
3.1
3.0
1.02
1.35
1.11
1.97
2.00
2.03
Pt/ZrW(mt)
Pt/ZrW(t)
Pt/ZrW(m-350)
Pt/ZrW(mt-350)
Pt/ZrW(t-350)
12.7
12.1
6.0
The acid strength distribution of the ZrW supports
was obtained by NH3-TPD. The profiles were normal-
ized by sample weight for comparison with each other.
It is known that NH3-TPD peaks at <200 °C,
200−350 °C, and 350−450 C represent the weak,
[a] Obtained by N2 adsorption/desorption analysis. [b] Calculated from H2-O2 titration. [c]
Calculated from H2 chemisorption.
so obtained successfully. Using a previously reported method,[21]
medium and strong acid sites, respectively.[23] As shown in Fig-
ure 3, ZrW(t) exhibits the most intense TCD signal among the
samples, indicating that this support contains the most acidic
sites. Additionally, some strong acid sites were observed on
ZrW(t) and ZrW(mt), while ZrW(m) contained the fewest acid
sites which are mainly presented as weak acids, with a small
amount of medium-strength sites. Therefore, the presence of t-
ZrO2 promotes the acid properties of the supports.
the volume ratio of m/t phases in this sample was calculated to
be 1.2, which is similar to the designed value of 1.0. In addition,
the diffraction peaks of WO3 at 2 = 23.5° and 33.3° are not
observed in the XRD patterns of all the samples, indicating that
WO3 is highly dispersed on these ZrO2 supports. Meanwhile, the
particle sizes of ZrO2 decreased from 12 to 7 nm after the load-
ing of WO3, regardless of the crystalline structures. The addition
of WO3 apparently prevented the aggregation of ZrO2 nanoparti-
cles by reacting with the surface hydroxyl groups during calcina-
tion, consistent with the observations of Song et al.[22] Figure
1(b) shows that the diffraction peaks of reduced catalysts are
nearly the same as those of WO3/ZrO2 samples, except for a
weak peak characteristic of Pt(111) at 40° in the case of
Pt/ZrW(t). This result indicates that Pt addition doesn’t change
the crystalline structures of the supports.
Table 1 shows the textural properties of the reduced catalysts.
The specific surface area of the catalysts is increased with the
increment of m-ZrO2 in the supports, which is in agreement with
the pore volume trend. Pore distributions of the catalysts are
presented in Figure 2. Interestingly, t-ZrO2 based catalysts pos-
sess pores of 3.5 and 6 nm in diameter, whereas m-ZrO2 based
catalysts exhibit only one type of pores, which is 19 nm in diam-
The surface chemical states of various catalysts were identi-
fied by XPS and Pt 4f, Zr 3d and W 4f lines are shown in Figure
4. For comparison, Pt/ZrO2(m-350) was also analyzed. Two
peaks at 74.5 eV and 71.0 eV are observed in the spectrum of
Pt/ZrO2 (Figure 4(a)), which are assigned to Pt 4f5/2 and Pt 4f7/2
signals of Pt0 species, respectively.[24] When W species were
introduced into the support, the binding energies of Pt0 shifted to
lower values, demonstrating an electron-rich Pt state and a less
interaction between Pt and ZrO2.[25] This effect was accompa-
nied by a broadened FWHM and a slight shift of Zr4+ peaks
toward higher binding energies (Figure 4(b)). As Jacome et al.[26]
analyzed, these changes may be caused by a strong interaction
between Zr and W atoms and the formation of a few deformed
Zr4+ cations on the surface. In Figure 4(c), the two peaks at 37.3
Table 2. The deconvolution results of elemental peaks in the band region of
CO linear adsorption.
[a]
Catalyst
Peak
Height
FWHM
Area
A2088/Atotal
position
[cm-1]
2074
2088
2072
2087
2072
2088
2074
2090
2074
2089
2076
2091
0.15
0.03
0.11
0.03
0.07
0.06
0.03
0.05
0.05
0.07
0.04
0.05
24
15
22
12
17
13
15
15
17
18
11
13
3.74
0.48
2.69
0.35
1.52
0.85
0.65
0.96
1.02
1.50
0.52
0.77
Pt/ZrW(m)
0.038
0.037
0.172
0.344
0.361
0.310
Pt/ZrW(t)
Pt/ZrW(mt)
Pt/ZrW(m-350)
Pt/ZrW(t-350)
Pt/ZrW(mt-350)
[a] Atotal is the total area of IR peaks of CO chemisorption on Pt.
Figure 3. NH3-TPD curves of ZrW supports.
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Figure 5. (a) DRIFT spectra of CO adsorbed on reduced catalysts and (b) the
deconvoluted band region of CO linear adsorption.
constant, implying similar electron environments on these cata-
lysts.
Figure 4. XPS of the reduced catalysts: (a) Pt 4f, (b)Zr 3d, (c)W 4f.
H2 chemisorption is an important method to reveal the effect of
hydrogen spillover, and the H/Pt ratio is often introduced to
evaluate the spillover capacity of materials.[27] Because one H
atom is adsorbed onto one Pt atom, the H/Pt value is usually
higher than the Pt dispersion when hydrogen spillover occurs.
As listed in Table 1, all the catalysts exhibit a spillover capacity
eV and 35.3 eV are assigned to W 4f5/2 and W 4f7/2 signals of
W6+species. But the FWHM values are wider than the reported
ones,[26] which indicates the co-existence of the W5+ species. For
all of the samples containing WO3, it should be noticed from
Figure 4 that the binding energies of the three elements remain
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Pt(111) terraces for the catalysts, and the effect of the
later method is stronger.
Table 3. Catalytic performance of glycerol hydrogenolysis on Pt/ZrW catalysts.[a]
Table 3 lists the reaction results of the glycerol hydro-
genolysis on these catalysts. Compared with the perfor-
mances of Pt/ZrW(m) and Pt/ZrW(t), the catalyst with
heterogeneous ZrO2 as support exhibited a better activity,
and the conversion of glycerol reached 15.2%. Moreover,
when a thermal treatment was conducted on the three
catalysts, further increases of the glycerol conversion
were obtained, with the highest conversion being 21.9%.
At high glycerol conversion of 77.7% on Pt/ZrW(t-350),
the selectivity of 1,3-PDO was decreased and more 1-
PO was formed because of deep hydrogenolysis.
Mass balance was well achieved for all of the reactions
within a deviation of ±2%, thus very few gaseous or coke
products were formed. The main products were 1,3-PDO,
1,2-PDO and 1-PO, with a small amount of 2-PO. The
product distribution appears to have no clear relationship
with the catalysts because of the complicated reaction
system, which involves both parallel reactions and deep
hydrogenolysis. Although the selectivity of 1,3-PDO is
greater than that of 1,2-PDO (except in the case of
Pt/ZrW(m-350)), the advantage is not significant com-
pared with the reported results.[11, 13] Even at low conver-
sion, the formation of 1-PO is large, consistent with the
results reported by Li et al.[8]
Catalyst
Conversion
[%]
Selectivity[b] [%]
1,3-PDO
32.2
1,2-PDO
1-PO
35.8
36.1
27.9
40.3
42.4
42.9
44.8
48.1
2-PO
7.7
others
0.1
Pt/ZrW(m)
6.7
24.2
21.1
30.6
25.4
12.7
12.4
14.6
10.6
Pt/ZrW(mt)
15.2
7.0
32.5
8.3
2.0
Pt/ZrW(t)
34.1
7.4
0.0
Pt/ZrW(m-350)
Pt/ZrW(mt-350)
Pt/ZrW(t-350)
Pt/ZrW(t)[c]
20.4
19.3
21.9
53.4
77.7
23.8
10.4
9.9
0.1
30.7
4.3
35.1
9.6
0.0
36.0
3.3
1.3
Pt/ZrW(t-350)[d]
29.7
10.4
1.2
[a] Reaction conditions: 4.0 g glycerol, 36.0 g water, 1.0 g catalyst, 180 C, 8 MPa,
1000 rpm, 24 h. [b] 1,3-PDO: 1,3-propanediol; 1,2-PDO: 1,2-propanediol; 1-PO: 1-
propanol; 2-PO: 2-propanol; others include ethylene glycol, ethanol and methanol. [c]
The input glycerol is 2.0 g, catalyst 1.25 g and reaction time is 78 h. [d] The input
glycerol is 2.0 g and reaction time is 50 h.
The effect of hydrogen spillover on the catalytic
activities
to some extent, as H/Pt is much higher than Pt dispersion. Cal-
cination of the Pt precursor and the m/t-phase mixture of ZrO2
facilitate the hydrogen spillover on the catalysts, and the effect
of the former is more significant. In addition, H/Pt values are
nearly the same for the pure m-ZrO2 and t-ZrO2 based catalysts
when the same preparation conditions are applied.
As it is sensitive to the noble metal particle size, crystal plane,
and electronic states, CO chemisorbed DRIFT spectra was
performed for a better understanding of Pt states of the catalysts.
The results are shown in Figure 5(a). All of the catalysts present
an absorption band corresponding to linear adsorption of CO on
Pt (CO = 2070–2094 cm-1) and a low frequency band at about
1850 cm-1 corresponding to bridge adsorption of CO on Pt.[28] In
the spectra of Pt/ZrW(m) and Pt/ZrW(t), an obvious shoulder
peak is observed at approximately 2045 cm-1, which represents
the linear adsorption of CO on Pt0 of smaller size.[29]
Additionally, the band in the linear adsorption region is divided
into two peaks, as clearly observed in the cases of Pt/ZrW(mt),
Pt/ZrW(m-350), Pt/ZrW(mt-350) and Pt/ZrW(t-350). For a better
analysis, deconvolution of the absorption band in this region was
taken and the results are shown in Figure 5(b). As previously
reported,[30] the elemental band at 2073 cm-1 can be assigned to
less packed terraces or step sites such as Pt(100), and the
elemental band at about 2088 cm-1 represents close packed
terraces such as Pt(111). In this way, a semi-quantitative analy-
sis of Pt(100) and Pt(111) can be carried out on the basis of the
intensity ratio between these two bands. Table 2 lists the decon-
volution data and area ratio of elemental bands, and it is indicat-
ed that using heterogeneous ZrO2 phase based support and
calcination of Pt precursor before reduction can lead to more
To elucidate the influence of hydrogen spillover, some factors
should be excluded first. In our case, a large change in Pt size
was avoided by a relatively mild thermal treatment during the Pt
loading process, and Pt particle sizes only ranged from 3.0 to
3.6 nm (Table 1). In addition, a clear difference was observed in
glycerol conversion between Pt/ZrW(m) and Pt/ZrW(m-350),
even though they had similar Pt particle sizes. Therefore, the
size effect is not the dominant factor in the catalytic activity.
As mentioned in the introduction, acid properties of the sup-
ports play an important role in glycerol hydrogenolysis, and a
better catalytic activity can usually be ascribed to more acid sites
especially strong acid sites. However, NH3-TPD results (Figure
3) show that although the acid amount and strength of ZrW(t)
exceed those of ZrW(m), the corresponding catalysts perform a
similar conversion (Table 3). Therefore, glycerol activation on
the catalysts should not be the main reason for the different
catalytic performances, and other factors should be highlighted.
We thus focus on the activation of hydrogen. H2 chemisorption
demonstrates that hydrogen spillover occurs on all of the cata-
lysts. As shown in Table 1, the spillover effect is the most signifi-
cant on Pt/ZrW(t-350), on which the conversion of glycerol is
also the highest (Table 3). A further revelation is presented in
Figure 6. It is clear that H/Pt has a liner relationship with glycerol
conversion, which illustrates that hydrogen spillover plays an
important role in the catalytic activity.
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spillover capacity ensures the hydrogenation of adsorbed glyc-
erol far away from Pt particles, thus increased the conversion.
It should be pointed out that by a simple impregnation method,
the composition of WO3 could reach 38 wt.% on the nano-ZrO2
support, which is higher than those reported for many other
10-11, 13, 25]
catalysts.[8,
A novel two-phase interface hydrolysis
method was used by Song et al. to prepare WOx/ZrO2 nano-
sized catalysts with a similar WO3 loading, but their process was
complicated.[22] As mentioned in the introduction, WO3 plays an
important role on the formation of acid sites and the activation of
glycerol. But in our case, the conversion of glycerol is not affect-
ed by the acid amount or strength, no matter the hydrogen spill-
over capacity is high or low. So we propose that the acid amount
and acid strength of the catalysts have nothing to do with the
activation step of glycerol, and other factors should be investi-
gated.
Figure 6. The correlation between H/Pt and glycerol conversion.
The role of Pt(111) structure and WO3 on the hydrogen
spillover
In the hydrogenolysis of other reactants, the roles of spillover
hydrogen and the reaction models were discussed. Shin et al.[31]
studied the gas phase hydrodechlorination of chlorobenzene
and 3-chlorophenol on Ni/SiO2 catalysts. They found that the
hydrodechlorination proceeds via the electrophilic attack of the
adsorbed aromatic by surface dissociated hydrogen, possibly
spillover hydronium ions. And the Kinetic parameters reflect that
spillover hydrogen and the adsorbed aromatic do not compete
for the same adsorption site on the catalyst surface. This model
is consistent with the hydrodesulfurization reactions.[32] In the
case of hydrodeoxygenation, Ranade et al.[33] investigated the
hydrogenolysis of benzylic alcohols on rhodium catalysts, and
they concluded that direct hydrogenolysis occurs on the carbon
support because of the formation of a better leaving group (OH2)
from the benzylic hydroxy group and its subsequent substitution
by spillover hydrogen. However, Thakar et al.[34] argued that the
model depends on the nature of the support. They found that
both hydrogenolysis and the dehydration–hydrogenation route
were observed over Pd/C, which was attributed to the acidic
nature of the carbon support. But hydrogen spillover was not
involved in the direct hydrogenolysis of the C–O bond on
Pd/SiO2 because the reactant was only adsorbed on Pd metal in
this case, which was proved by the same initial reaction rate on
Pd black and Pd/SiO2. These reports demonstrate that hydrogen
spillover is important when the adsorption of the organic reactant
mainly occurs on the support.
Using heterogeneous ZrO2 as support and thermally treating Pt
precursor, hydrogen spillover and glycerol conversion were both
improved (Table 1 and 3). Interestingly, the signal changes in
the CO chemisorbed DRIFT spectra appear to be consistent
with such improvements (Figure 5 and Table 2). To better illus-
trate it, the relationship between A2088/Atotal and H/Pt is given in
Figure 7. H/Pt is clearly proportional to the A2088/Atotal value,
which indicates that the formation of Pt(111) terraces can facili-
tate hydrogen spillover on the catalysts, thereby increasing their
catalytic activity.
Based on these researches, we also proposed here the role of
spillover hydrogen in the glycerol hydrogenolysis. Generally,
dehydration is a reversible process and hydrogenation of the
adsorbed intermediate species plays a role in breaking this
balance in aqueous system. In this way, spillover effect is the
key for the activated hydrogen atoms to reach the sites where
glycerol is adsorbed. However, tremendous physical/energy
barriers impede the surface diffusion of hydrogen atoms from
one material to another,[35] and inter-particle H spillover is only
enhanced in some cases.[19] This fact implies that in the case of
the nano-ZrO2 based Pt/ZrW catalysts, the enhanced hydrogen
Figure 7. The relationship between A2088/Atotal and H/Pt.
To further confirm the importance of heterogeneous ZrO2 and
Pt precursor pretreatment for the Pt(111) formation, Pt/ZrO2 (m,
mt and m-350) samples were prepared by the same method
without W loading. CO chemisorbed DRIFT spectra of the sam-
ples are presented in Figure 8. Compared with the Pt/ZrW cata-
lysts, the spectra differ substantially. The band of bridge CO
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10.1002/cctc.201600981
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FULL PAPER
species is strengthened and red shifted from 1850 cm-1 to 1837
cm-1. This change also occurs in the case of the band of CO
adsorbed on smaller sized Pt at 2037 cm-1. The bands of CO
linear adsorption on Pt(100) and Pt(111) are blue shifted to 2077
cm-1 and 2093 cm-1, respectively. This shift is ascribed to a
decrease of Pt electron density caused by the interaction be-
tween Pt and ZrO2,[36] which is consistent with the XPS results in
Figure 4. In addition, a weak band at 2129 cm-1 corresponding to
CO adsorption on Pt+ is observed in the spectra of Pt/ZrO2(m)
and Pt/ZrO2(mt), further confirming the interaction between Pt
species and ZrO2.
Table 4. H/Pt values of Pt/ZrO2 samples and the corresponding WO3-
containing catalysts
Catalyst
H/Pt
0.34
0.38
0.53
Catalyst
H/Pt
1.02
1.35
1.97
Pt/ZrO2(m)
Pt/ZrW(m)
Pt/ZrO2(mt)
Pt/ZrO2(m-350)
Pt/ZrW(mt)
Pt/ZrW(m-350)
Pt/ZrO2, the proton−electron migration may be limited to the
interface between the Pt particle and the metal oxide particle
because of the difference in crystal lattices and the low reducibil-
ity and conductibility of ZrO2 compared to those of WO3.
Finally, how Pt(111) terraces can enhance hydrogen spillover
is an open question. Gee et al.[37] studied the dynamics of hy-
drogen dissociation on Pt(533) by temperature programmed
desorption and time-of-flight technique. They observed from H2-
TPD that the associative desorption of H2 on Pt(100) step sites
occurs at 375 K, while H2 desorption from Pt(111) terraces takes
place below 300 K with a broad peak. These results indicate that
H2 desorption from Pt(111) terraces is easier than that from
Pt(100) steps. In addition, they compared two channels for H2
dissociation on Pt and found that Pt(100) step sites are respon-
sible for the low energy component to dissociation on Pt(533),
and the high energy component is a result of a direct dissocia-
tion channel on Pt(111) terraces. This was also proved by van
Duin et al.[38] using molecular dynamics and the reactive force
field ReaxFF.
Considering that high pressure and high concentration condi-
tions are generally applied in industry. Under such conditions,
the indirect channel is significantly diminished because the Pt
surface is often littered with adatoms.[38-39] Therefore, we pro-
pose that the direct H2 dissociation channel on Pt(111) terraces
is dominant on the Pt/WO3/ZrO2 catalysts in glycerol hydrogen-
olysis, and the advantages of dissociation and desorption of H2
on Pt(111) both enhance the hydrogen spillover capacity.
Figure 8. DRIFT spectra of CO adsorbed on reduced Pt/ZrO2 samples.
Because of the interference of the broad band at 2037 cm-1,
the deconvolution of the region corresponding to CO linear
adsorption and semiquantitative analysis of Pt(100) and Pt(111)
are difficult. However, as evident in Figure 8, the bands at 2093
cm-1 for Pt/ZrO2(mt) and Pt/ZrO2(m-350) are both distinguished
compared with those for Pt/ZrO2(m). Therefore, the heterogenei-
ty of ZrO2 crystalline structure and Pt precursor thermal treat-
ment can provide more Pt(111) sites on the catalysts, regardless
of the presence of WO3.
H2 chemisorption was also carried out for Pt/ZrO2 catalysts,
and H/Pt values are given in Table 4. The H/Pt values of
Pt/ZrO2(mt) and Pt/ZrO2(m-350) are higher than that of
Pt/ZrO2(m), consistent with the trend of the intensity of the
DRIFT band at 2093 cm-1 (Figure 8). This result again demon-
strates that Pt(111) formation can facilitate the spillover of hy-
drogen on catalysts.
Notably, as shown in Table 4, the H/Pt values are significantly
lower for Pt/ZrO2 catalysts than for W-containing catalysts,[28]
which imply much weaker spillover capacities. Thus, WO3 must
play a certain role in hydrogen spillover. As Prins reviewed,[14]
for Pt/WO3 and Pt/WO3−ZrO2 catalysts, hydrogen atoms on Pt
move to the WO3 surface and become a combination of protons
and electrons. Then the electrons reduce WO3 and migrate by
W5+−W6+ exchange, while the protons are solvated in water and
transport rapidly via hydrogen bonding. However, in the case of
Conclusions
In this study, Pt/ZrW nano-catalysts with highly dispersed WO3
and uniform Pt dispersion were used in glycerol hydrogenolysis,
and the hydrogen spillover capacity was tuned by changing
nano-ZrO2 crystalline structures and thermal treatment condi-
tions of Pt precursor. A linear relationship was observed be-
tween H/Pt and glycerol conversion, which confirms that hydro-
gen spillover is important for glycerol hydrogenolysis on these
WO3 rich nano-catalysts. The acid strength and total amount of
acid sites of supports seem to have no influence on the glycerol
conversion, so further works should be done to reveal the es-
sential catalyst properties for the glycerol activation step.
When a heterogeneous ZrO2 phase based support was used
or a thermal treatment of Pt precursor was performed before
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10.1002/cctc.201600981
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conversion (%) = (moles of converted glycerol) / (moles of inlet glycerol)
selectivity (%) = (moles of glycerol required for specific product) / (moles
of converted glycerol)
reduction, the amount of Pt(111) terraces was increased and the
effect of hydrogen spillover was also enhanced. The important
role of Pt(111) terraces on hydrogen spillover is proposed from
two aspects. One aspect is that H2 dissociation preferentially
occurs on Pt(111) terraces via the direct dissociation channel
during glycerol hydrogenolysis. The other aspect is that H2 de-
sorption is easier on close packed Pt(111) terraces than on less
packed sites such as Pt(100) steps, which ensures a faster
spillover pathway from the Pt particle to the support.
Catalyst characterization
Nitrogen adsorption/desorption isotherms were obtained at -196 °C on a
Micromeritics Tristar II 3000 analyzer. The X-ray powder diffraction
(XRD) patterns of the catalysts were recorded on a Rigaku D/MAX-2500
diffractometer equipped with a Cu K radiation source (λ = 1.5406 Å).
The scanning angle (2) ranged from 20° to 90°, and the scanning speed
was 10°/min. X-ray photoelectron spectroscopy (XPS) was performed on
a PHI 1600 ESCA spectrometer (PE Company) equipped with an Al Kα
X-ray radiation source (h = 1486.6 eV). The powder was pressed into a
self-supporting wafer and reduced in a quartz tube. It was then preserved
in a centrifuge tube filled with Ar and introduced into the testing chamber.
The measurement was taken at 10-8 Torr, and the binding energy values
were calibrated by the C 1s peak at 284.6 eV within an error of ±0.2 eV.
Experimental Section
Chemicals
Zirconyl nitrate (ZrO(NO3)2·2H2O, >45.0% ZrO2) was purchased from
Tianjin Guangfu Fine Chemical Research Institute. Urea (CO(NH2)2,
>99.0%) and methanol (CH3OH, >99.0%) were obtained from Tianjin
Kemiou Chemical Reagent Co., Ltd. Ammonium metatungstate
((NH4)6(H2W12O40)·nH2O, >99.9%) was provided by J&K Chemical Corp.
Chloroplatinic acid (H2PtCl6·6H2O, >37.0% Pt) was purchased from
Real&Lead Co., Ltd.
H2-O2 titrations were carried out on a Micromeritics Autochem II 2920
chemisorption analyzer to measure the dispersion of Pt0 on the reduced
catalysts.[41] Briefly, the sample (about 100 mg) was put into a U-shaped
quartz tube and reduced in 10% H2/Ar mixed gas under the aforemen-
tioned reduction condition, and then purged with He. After the tempera-
ture was decreased to 45 °C, the catalyst was oxidized by 10%(vol.)
O2/He (0.5082 mL) pulse injection until the TCD peaks became stable. H2
titrations were subsequently performed under the same condition. The
adsorption stoichiometry factor of Pt/H2 was 2/3. H2 chemisorption was
measured to evaluate the levels of hydrogen spillover on the catalysts.
The sample was reduced and then purged with He at 300 C to remove
chemisorbed hydrogen. After cooling to 45 °C, H2 chemisorption uptakes
were measured by pulses of 10% H2/Ar. The H/Pt value is defined as the
moles of adsorbed H atoms per mole Pt on the catalyst. Ammonia tem-
perature-programmed desorption (NH3-TPD) of the acid support was also
carried out using the same instrument. The sample was pretreated in He
flow at 300 C for 1 h and cooled to 50 C. Then it was treated in NH3
flow for 0.5 h. After adsorption, the sample was purged in He until the
baseline was stable. Subsequently, it was heated to 800 C at a ramping
rate of 10 C/min. The desorbed NH3 was monitored by a TCD detector.
Catalyst preparation
Monoclinic (m) and tetragonal (t) ZrO2 precursors were prepared by the
solvothermal method of Li et al.[40] A mixture phase of the ZrO2 precur-
sors was also prepared via physical mixing of the two aforementioned
pure precursors, such that the m/t ZrO2 ratio was 1:1 after calcination.
Pt/WO3/ZrO2 catalysts were prepared by the sequential impregnation
method. Briefly, the ZrO2 precursors were impregnated with a suitable
amount of ammonium metatungstate dissolved in deionized water, ensur-
ing a WO3 loading of 38 wt.%. The samples were dried at 100 C over-
night and then calcined at 700 C for 4 h, with a heating rate of 2 C/min.
After cooling to room temperature, the white samples were impregnated
with an aqueous solution of H2PtCl6·6H2O (corresponding to 2 wt.% Pt
loading in the catalysts) and dried at room temperature. Before the
reaction, the powder was directly reduced or reduced after calcination at
350 C for 2 h with a ramping rate of 2 C/min. The obtained catalysts
were labelled as Pt/ZrW(m), Pt/ZrW(t), Pt/ZrW(mt), Pt/ZrW(m-350),
Pt/ZrW(t-350) and Pt/ZrW(mt-350), where the information in parentheses
represents the ZrO2 crystalline state and the calcination temperature for
Pt precursor after drying.
Diffuse reflectance infrared Fourier transform (DRIFT) spectra of ad-
sorbed CO were collected on a Nicolet 6700 spectrometer equipped with
an MCT/A detector. The powder sample was put into a high temperature
cell, reduced in situ by flowing H2, and then purged with He for 30 min.
After the sample cooled to 30 C, the background spectrum was record-
ed and CO chemisorption was subsequently carried out for 30 min. Then
the sample was purged again and desorption spectra were obtained with
the background subtracted. The result was acquired when the spectra
were stable.
Catalytic reactions
Typically, the catalysts were reduced at 150 C for 2 h under pure H2.
The reduction temperature was increased to 250 C when the calcined
catalysts were used. All of the reduction conditions were identical, unless
noted otherwise. The reduced catalyst (1.0 g) was introduced into 10
wt.% glycerol aqueous solution (40 mL) in a stainless steel autoclave
(100 mL). The system was purged five times with H2, and the pressure
was increased to 5.5 MPa. The reactor was then heated to 180 C and
maintained at this temperature for 24 h, during which the pressure of the
reaction system was maintained at 8 MPa by pulsing H2. After the reac-
tion, the products were cooled and liquid samples were analyzed using a
gas chromatograph (Agilent Micro GC 4890) equipped with a flame
ionization detector and an HP-INNOWAX capillary column (Hewlett–
Packard Company, 30 m  0.32 mm  0.50 m). n-Butanol and 1,4-
butanediol were added as internal standards. The conversion of the
glycerol and the selectivity toward each product are defined as follows:
Acknowledgements
Financial support from the National Natural Science Foundation
of China (21276186, 21325626, 91434127, U1510203) is grate-
fully acknowledged.
Keywords: glycerol • heterogeneous catalysis • hydrogenolysis
• hydrogen spillover • platinum
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Wei Zhou, Yujun Zhao, Yue Wang,
Shengping Wang, and Xinbin Ma *
Page 1. – Page 10.
Glycerol Hydrogenolysis to 1,3-
Propanediol on Pt/WO3/ZrO2: Hydro-
gen Spillover Facilitated by Pt(111)
Formation
Structure-spillover relationship: The spillover capacities on Pt/ZrW catalysts
were tuned and they were investigated in glycerol hydrogenolysis. The formation of
Pt(111) terraces facilitates both H2 dissociation and desorption steps, thus enhanc-
es the capacity of H2 spillover and the conversion of glycerol.
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