Efficient and stable platinum nanocatalysts supported over Ca-doped ZnAl2O4 spinels for base-free selective oxidation of glycerol to glyceric acid

Article abstract of DOI:10.1016/j.mcat.2019.110559

In the work, highly dispersed platinum nanocatalysts supported over undoped and Ca-doped zinc aluminate spinels were developed and applied for aqueous-phase selective oxidation of glycerol to produce glyceric acid under base-free conditions. As-fabricated Pt-based catalyst with the incorporation of an appropriate Ca/(Ca + Zn) molar ratio of 0.1 into the spinel exhibited a higher catalytic activity, along with a selectivity to glyceric acid (>81%) and a high turnover frequency of 1160 h^?1, compared with other supported Pt-based ones over zinc aluminates, as well as most of supported Pt catalysts previously reported. The structural characterizations and catalytic experiments showed that surface synergy between highly dispersed metallic Pt⁰ species and medium-strength basic sites mainly contributed to its enhanced catalytic efficiency for base-free glycerol oxidation. Moreover, the present Pt catalyst also presented high structural stability and good reusability. The work opens an alternative approach for constructing highly efficient and stable metal-base bifunctional catalysts for a wide range of heterogeneous oxidation processes without the addition of liquid alkalis.

Full text of DOI:10.1016/j.mcat.2019.110559

MolecularCatalysis477(2019)110559
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Efficient and stable platinum nanocatalysts supported over Ca-doped
ZnAl2O4 spinels for base-free selective oxidation of glycerol to glyceric acid
Zhengya Hana,1, Renfeng Xieb,1, Yihui Songa, Guoli Fana, Lan Yanga, Feng Lia,⁎
a State Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical
Technology, Beijing, 100029, China
b Patent Examination Cooperation Sichuan Center, State Intellectual Property Office, Chengdu 610213, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
In the work, highly dispersed platinum nanocatalysts supported over undoped and Ca-doped zinc aluminate
spinels were developed and applied for aqueous-phase selective oxidation of glycerol to produce glyceric acid
under base-free conditions. As-fabricated Pt-based catalyst with the incorporation of an appropriate Ca/
(Ca + Zn) molar ratio of 0.1 into the spinel exhibited a higher catalytic activity, along with a selectivity to
glyceric acid (> 81%) and a high turnover frequency of 1160 h−1, compared with other supported Pt-based ones
over zinc aluminates, as well as most of supported Pt catalysts previously reported. The structural character-
izations and catalytic experiments showed that surface synergy between highly dispersed metallic Pt0 species
and medium-strength basic sites mainly contributed to its enhanced catalytic efficiency for base-free glycerol
oxidation. Moreover, the present Pt catalyst also presented high structural stability and good reusability. The
work opens an alternative approach for constructing highly efficient and stable metal-base bifunctional catalysts
for a wide range of heterogeneous oxidation processes without the addition of liquid alkalis.
High-surface-area spinel
Platinum nanocatalysts
Oxidation of glycerol
Strong metal-support interactions
Surface synergy
1. Introduction
task [9].
As we know, commonly, the catalytic activity of metal NPs is pro-
In order to make better use of biodiesel production, it is necessary to
convert abundant glycerol into high value-added chemicals in the fu-
ture. Commonly, glycerol can be selectively transformed into different
important chemicals of industrial relevance, such as acrolein, 1,2-pro-
panediol, glyceric acid, formic acid, and acrylic acid, through catalytic
dehydration, hydrogenolysis, oxidation, and oxidehydration processes
using some supported catalysts [1–5]. Among them, the oxidation of
glycerol has received considerable attention since the oxidation is a
facile method of activating molecules to synthesize chemical inter-
mediates. Taking advantage of the presence of several hydroxyl groups,
glycerol can be oxidized into a series of products including C1, C2, and
C3 products (e.g. glyceric acid, tartronic acid, dihydroxyacetone, oxalic
acid, glycolic acid, formic acid, and CO2) [6]. Interestingly, many at-
tractive noble metals (e.g. platinum, gold, and palladium) are active for
the oxidation of biomass. For example, supported Pt nanoparticle cat-
alysts have been applied for the oxidation of 5-hydroxymethylfurfural
[7,8]. And, Pd and Pt nanoparticles (NPs) as highly efficient catalysts
are explored in the oxidation of polyols using clean molecular oxygen as
oxidant; however, the control of product selectivities is a challenging
portional to the loading of active metals, inversely being proportional
to the particle size of NPs. In addition, the metal particle size also can
govern the product selectivities [10–13]. Usually, the reduced particle
size facilitates the increase in the metal particle edges, thus leading to
the exposure of more active sites and promoting the oxidation processes
[14]. Meanwhile, the basic solution medium (e.g. NaOH) promotes the
elimination of β-H in glycerol as the rate-determining step, finally being
in favor in the formation of the sodium salt of glyceric acid [15].
Moreover, the addition of homogenous bases can cause high cost of
production, difficult separation, and environmental pollution issues.
On the other side, the surface acid-base property of catalyst supports
can play a significant role in governing the catalytic performance of
catalysts [16]. For example, Su and co-workers found that different
functionalized carbon nanotubes and nanofibers supported Au catalysts
exhibited different catalytic performance in the glycerol oxidation [17],
and the enhanced basicity of the functionalized support was beneficial
to the glycerol conversion and the production of C3 products. In ad-
dition, different supports including carbon materials (active carbon and
graphite) [11,12] and metal oxides (e.g. TiO2, MgAl2O4) [18,19] are
⁎ Corresponding author.
E-mail address: lifeng@mail.buct.edu.cn (F. Li).
1 These authors contribute equally to this work.
https://doi.org/10.1016/j.mcat.2019.110559
Received 24 May 2019; Received in revised form 8 August 2019; Accepted 8 August 2019
Availableonline20August2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

Z. Han, et al.
MolecularCatalysis477(2019)110559
employed in the above reaction. In most cases, homogenous NaOH was
added into reaction systems with noble metal catalysts to promote the
deprotonation of the hydroxyl group in glycerol [14,20], together with
relatively high selectivity to glyceric acid (> 52%), although the pro-
ducts exist in the form of salts. Moreover, it was reported that supported
gold-based catalysts were inactive for base-free glycerol oxidation [21].
Up to now, there are a few reports on supported noble-metal-based
catalysts over basic supports applied in the base-free glycerol oxidation
[19,22–25]. Although some basic layered double hydroxides and MgO
were employed as supports for immobilizing noble metal NPs [26–28],
the leaching of supports was serious in the course of glycerol oxidation
under acidic reaction medium containing glyceric acid (pH could reach
about 2), thus easily leading to rapid deactivation of catalysts. In this
regard, it is highly desirable and a challenging task to develop efficient
and stable metal-base bifunctional catalysts for base-free glycerol oxi-
dation.
Recently, due to well-developed porous microstructure and large
surface area, porous metal oxides and complex metal oxides have been
widely employed as supports for metal-based catalysts in a variety of
catalytic processes [29–31], because their high surface areas facilitate
the active metal dispersion, thus promoting the occurrence of strong
metal-support interactions (SMSIs) and enhancing the stability of metal
NPs. Meanwhile, abundant pores of supports may be beneficial to the
diffusion and transfer of reactants in the course of reactions, thus being
in favor of the conversion of substrates. For instance, compared with
single metal oxides, spinel-type zinc aluminate (ZnAl2O4) possesses the
unique physicochemical properties including higher thermal/chemical
stability and mechanical resistance [32,33]. Correspondingly, ZnAl2O4
supported metal catalysts exhibited superior catalytic property to single
oxides supported ones [34,35].
were determined using inductively coupled plasma atomic emission
spectroscopy (ICP-AES, Shimadzu ICPS-7500). Transmission electron
microscopy (TEM) experiments were operated on JEOL 2100 instru-
ment. Micromeritics ASAP 2020 sorptometer was utilized to perform
low-temperature nitrogen adsorption-desorption experiments. VG
ESCALAB 2201 XL spectrometer (Al Kα X-ray radiation) was used to
obtain X-ray photoelectron spectra (XPS) of samples. CO2 temperature-
programmed desorption (CO2-TPD) was tested on ChemiSorb 2720 in-
strument. The sample (50 mg) was outgassed under a He flow of 40 ml/
min at 200 °C for 1 h. Subsequently, a CO2 flow (40 ml/min) was
switched for 1 h at room temperature, and then purged with a He flow
for 1 h. At last, the desorption of CO2 was performed with the increasing
temperature. FT-IR spectrophotometer (Thermo Nicolet 380) was ap-
plied to record Fourier transform infrared (FT-IR) spectra of CO2 ad-
sorbed over samples. The self-supporting sample wafer was heated at
70 °C under N2 flow for 1 h. After that, a CO2 flow was switched for 1 h
at room temperature and purged with a He flow for 1 h. Finally, FT-IR
spectra of samples were collected under vacuum.
2.3. Catalytic oxidation of glycerol
The base-free oxidation glycerol was conducted in a stainless-steel
reactor (100 ml), where glycerol (10 ml) and the catalyst (glycerol/Pt
molar ratio = 590: 1) were charged. When the temperature of the re-
actor reached a certain temperature, the oxygen was fed into the re-
actor at a certain partial pressure. The oxidation began under stirring
with a speed of 900 rpm. When the reaction finished, the reactor was
placed into an ice bath and the reactants were centrifuged. the products
were analyzed at 35 °C by a liquid chromatograph (Shimadzu LC-20A)
using HPX-87H column and 5 mM H2SO4 aqueous solution as mobile
phase with a flow rate of 0.6 ml/min with refractive index and ultra-
violet detector by the external standard method.
In this contribution, a series of highly dispersed Ca-doped zinc
aluminate spinels supported platinum catalysts were developed and
applied in the base-free glycerol oxidation. It was revealed that the
incorporation of a certain amount of Ca into the spinel structure could
improve surface basicity of supported catalysts and promote the for-
mation of SMSIs, thus synergistically enhancing catalytic performance
of catalysts. Moreover, the as-fabricated Pt catalyst showed high
structural stability and reusability with no significant loss of its activity
after six consecutive runs.
3. Results and discussion
3.1. Structural analysis of Pt-based samples
XRD patterns of a series of undoped and Ca-doped ZnAl2O4 spinel
supports and resulting supported Pt samples are shown in Fig. 1. No-
ticeably, in each case, spinel support shows several intensive (220),
(311), (400), (422), (511) and (440) diffraction planes for cubic
structure of zinc aluminate spinel phase (JCPDS No. 41-1745), re-
flecting the formation of the spinel structure with the good crystallinity.
In addition, as for Ca-doped three spinel supports, a small amount of
CaCO3 impurity (JCPDS No. 05-0586) can be observed and is gradually
enhanced in the intensity with the increasing content of ca. After
loading Pt, XRD partners of all supported Pt samples are almost iden-
tical to those of pristine supports. Interestingly, one cannot observe any
diffractions assignable to metallic Pt, probably because of the formation
of highly dispersed small-sized Pt particles, as well as the low content of
Pt (< 1.0 wt%) (Table 1).
As shown in Fig. 2, TEM images of representative two supported Pt
samples (Pt/ZCA-0 and Pt/ZCA-10) depict an irregular porous micro-
structure of spinel supports, suggestive of the large specific surface area
of samples. Meanwhile, it is noticed that many uniform small-sized
particles of about 2–4 nm in size are evenly distributed on the surface of
supports. For Pt/ZCA-10, a further HRTEM observation clearly reveals
the lattice fringes of black NP and the support matrix, which correspond
to the (111) and (311) planes of metallic Pt0 (JCPDS 04-0802) and
cubic ZnAl2O4 spinel phase (JCPDS 05-0669), with an interplanar dis-
tance of 0.225 nm and 0.245 nm, respectively. Furthermore, based on
the histogram of the narrow particle size distribution of Pt NPs on the
Pt/ZCA-10, the average Pt particle size is determined to be 3.13 nm.
Despite the composition of supports, small Pt NPs with a slight differ-
ence in the particle size (2.44–3.13 nm) and a 0.9 wt% Pt loading
amount can be obtained in all cases (Table 1), indicating that the
2. Experimental section
2.1. Catalyst preparation
Undoped and Ca-doped zinc aluminates synthesized by our pre-
viously reported solution-phase method [29]. Typically, Zn
(NO3)2·6H2O (4.8 mmol), Ca(NO3)2·4H2O (0.1 mmol), Al(NO3)3·9H2O
(10.0 mmol), and urea (50 mmol) were dispersed in a 100 ml of solvent
consisting of methanol and deionized water (v/v = 1:1). Subsequently,
the solution was aged at 180 °C overnight. Then, the resultant pre-
cipitate was washed with ethanol and deionized water several times. At
last, the solid was dried at 70 °C overnight and denoted as ZCA-x (x = 0,
5, 10 and 15; x is Ca/(Ca + Zn) molar ratio in initial synthesis mixture).
Spinel support (0.5 g) was dispersed into a 100 mL of deionized
water, and then 1.0 mL of H2PtCl6 solution (19.3 mM) was added with
stirring. Subsequently, 50 mL of NaBH4 solution (80 mM) was added
dropwise to the above suspension at room temperature, followed by
aging for 4 h. The obtained precipitate was washed and dried at 70 °C
for 12 h under vacuum. For comparison, other metal oxides supported
Pt-based catalysts were synthesized based on the above same synthesis
method.
2.2. Characterization
X-ray diffraction (XRD) patterns were gained on Shimadzu XRD-
6000 diffractometer using a Cu-Ka source. Metal contents in samples
2

Z. Han, et al.
MolecularCatalysis477(2019)110559
Fig. 1. XRD patterns of undoped and Ca-doped ZnAl2O4 spinels (A) and supported Pt-based samples (B).
[36,37]. No peak signal related to Pt(II) species can be observed at the
BE of about 317.4 eV. Meanwhile, it is interesting to note that a slightly
negative shift in BE of Pt 4d 5/2 (about 0.6 eV) from Pt/ZCA-5 to Pt/
ZCA-10 and Pt/ZCA-15, demonstrating the electron transfer from the
support to Pt species with the incorporation of a certain amount of Ca
cations and thus the formation of stronger metal-support interactions
and electron-rich metallic Pt species. It is speculated that the Ca-O-Al
structure should form in the tetrahedral interstitials of spinel, probably
promoting the electron transfer from the support to Pt species and thus
the formation of electron-rich metallic Pt species. In the case of Pt/ZCA-
15, however, more surface impure CaCO3 inversely inhibit electron
transfer between Pt species and the support to some extent.
CO2-TPD curves of supported Pt-based samples were analyzed to
reveal the effect of the incorporation of Ca into spinel structure on
surface basicity of samples (Fig. 5). Commonly, the desorption tem-
perature of CO2 adsorbed on the sample surface can be correlated with
the strength of surface basic sites. In all cases, there is a broad deso-
rption region for adsorbed CO2 between 50 and 700 °C. Further de-
convolution of the desorption can give five or six components [38,39].
The first peak below 175 °C is associated with chemically adsorbed CO2
on weak basic sites (WB.), as well as surface physically adsorbed CO2. In
the CO2-TPD curves, three or four desorption peaks in the range of 175
to 580 °C can be ascribed to adsorbed CO2 on medium-strength basic
sites (MSB.), which mainly result from surface hydroxyl groups, metal-
oxygen pairs in the supports or isolated Ca–O pairs [40,41], and co-
ordinatively unsaturated O2− ions, respectively. Meanwhile, the peak
at higher desorption temperature of about 650 °C reflects the desorption
of adsorbed CO2 on strong basic sites (SB.). Based on the integrated
desorption areas, one can note that among supported samples, the
surface density of total basic sites on the Pt/ZCA-10 sample, as well as
that of medium-strength sites, is largest (Table 1).
Table 1
Compositions and structural properties of Pt-based samples.
a
b
c
d
Samples
Pt
Caa
(wt%)
Dp
DisM
(nm)
DBA
(wt%)
(nm)
(mmol/g)
e
Pt/ZCA-0
Pt/ZCA-5
Pt/ZCA-10
Pt/ZCA-15
0.89
0.85
0.92
0.83
0
2.44
2.82
3.13
2.50
46.3
40.1
36.1
45.2
0.314 (0.269)
0.324 (0.297)
0.479 (0.431)
0.353 (0.296)
1.61
2.99
3.67
a
Obtained based on ICP-AES analysis.
Average size of Pt NPs obtained through TEM images.
Dispersion of Pt estimated based on the mean particle size of Pt from TEM
b
c
results.
d
The amount of total basic sites obtained from CO2-TPD curves.
Data in parentheses: the amount of medium-strength basic sites obtained
e
from CO2-TPD curves.
present ZnAl2O4 and Ca-doped ZnAl2O4 spinels are good supports for
immobilizing and dispersing Pt NPs, probably because of the occur-
rence of strong metal-support interactions (SMSIs).
In addition, as shown in Fig. 3A, low-temperature nitrogen ad-
sorption-desorption isotherms of samples are characteristic of I-type
with no hysteresis loop, mirroring the main microporous character of
all supported Pt samples. Such a porous structure can be further iden-
tified by pore size distribution curves (Fig. 3B), indicating that the pores
mainly exist in the form of micropores. It is noted from Table 2 that all
samples possess high BET surface areas (> 218 m2 g−1), where the
micropore surface area occupies more than about 38% in total surface
area. The above results clearly illustrate that well-developed inter-
connected porous network of spinels supported Pt catalysts can form,
mainly originating from the voids formed by the aggregation of spinel
particles.
As we know, surface strong basic sites mainly originate from co-
ordinatively unsaturated O2− ions of oxides [38,39]. With the in-
troduction of Ca2+ ions, the spinel structure of resulting ZCA-x supports
may slightly change. Correspondingly, in the case of Pt/ZCA-5, the
addition of a small amount of Ca2+ ions into the spinel structure
probably leads to the decrease in the surface concentration of co-
ordinatively unsaturated O2− ions, thus giving rise a decreased density
3.2. Surface property of Pt-based samples
XPS characterization was carried out to determine the surface
electronic states of Pt species in samples. As shown in Fig. 4, the binding
energy values (BEs) of Pt 4d5/2 for all supported samples lie in the range
of 314.2 to 314.8 eV, indicative of the character of metallic Pt(0) state
3

Z. Han, et al.
MolecularCatalysis477(2019)110559
Fig. 2. TEM and HRTEM images of representative Pt/ZCA-0 (a), and Pt/ZCA-10 (b,c) and the particle size distribution of Pt NPs (f) on the Pt/ZCA-10 sample.
of strong basic sites. With the increasing Ca content, inversely, the
density of strong basic sites is increased gradually. It implies that the
introduction of Ca2+ ions into the spinel structure of ZnAl2O4 can affect
surface heterogeneity of supports in varying degrees. Despite the varied
density of strong basic sites among Pt/ZCA-x samples, the density of
total basic sites or medium-strength basic sites increase gradually in the
following order: Pt/ZCA-0 < Pt/ZCA-5 < Pt/ZCA-10. For Pt/ZCA-15
(Table 1), however, the presence of more surface impure CaCO3 reduces
Fig. 3. Nitrogen adsorption-desorption isotherms (A) and pore size distributions (B) of Pt/ZCA-0 (a), Pt/ZCA-5 (b), Pt/ZCA-10 (c) and Pt/ZCA-15 (d).
4

Z. Han, et al.
MolecularCatalysis477(2019)110559
Table 2
According to the shape of the broad absorption band, eight components
can be fitted, indicative of the presence of surface diverse species ori-
ginating from absorbed CO2. Two intensive absorption bands at 1636/
1416 cm−1 for Pt/ZCA-0 and 1662/1416 cm−1 for Pt/ZCA-10 are re-
lated to the asymmetric and symmetric OeCOe stretching vibrations of
bridged carbonate (HCO3−) [42–44], indicative of the presence of
Brønsted basic sites associated with surface hydroxyl groups. While two
absorption bands at 1575/1302 cm−1 for Pt/ZCA-0 and 1623/
1291 cm−1 for Pt/ZCA-10 are due to the vibrations of bidentate car-
bonate (b-CO32−) generated on metal-oxygen sites. Meanwhile, it is
noted that two absorption bands at 1489/1375 cm−1 for Pt/ZCA-0 and
1490/1364 cm−1 for Pt/ZCA-10 are attributable to the vibrations of
unidentate carbonate (u-CO32−) generated by interacting isolated O2−
ions, mirroring the absorption of CO2 on medium-strength or strong
basic sites. Moreover, the other two bands at 1676/1535 for Pt/ZCA-0
and 1713/1538 cm−1 for Pt/ZCA-10 correspond to surface polydentate
carbonate species (p-CO32−). On the other side, one can evaluate the
strength of basic sites according to the splitting of asymmetric and
symmetric OeCOe vibration bands. A smaller value of band splitting
presents a stronger interaction between CO2 and basic sites. Corre-
Textual property of supported Pt-based samples.
a
b
c
d
e
Samples
SBET
Smicro
Vtotal
Vmicro
Dpore
(nm)
(m2/g)
(m2/g)
(cm3/g)
(cm3/g)
Pt/ZCA-0
Pt/ZCA-5
Pt/ZCA-10
Pt/ZCA-15
229.3
230.4
218.5
222.7
101.7
108.9
83.1
0.116
0.119
0.111
0.104
0.0412
0.0447
0.0408
0.0434
2.19
2.25
2.26
2.16
106.2
a
Specific surface areas calculated based on BET technique.
Micropore surface area determined based on t-plot model.
Determined from single-point total pore volume.
Micropore volume based on t-plot method applied to adsorption isotherm.
Determined based on BJH method.
b
c
d
e
2−
2−
spondingly, the relative fraction of u-CO3 and p-CO3 species with
smaller values of band splitting on the Pt/ZCA-10 determined by the
integrated areas of vibration bands is about 53.4%, which is higher than
that on the Pt/ZCA-0 (37.4%). It demonstrates that the incorporation of
a certain amount of Ca into the spinel structure can induce more surface
medium-strength or strong basic sites, well consistent with the results of
CO2-TPD experiments.
3.3. Catalytic performance of supported Pt-based catalysts
The reaction results in the oxidation of glycerol over different
supported Pt catalysts are summarized in Table 3 under the same re-
action conditions (60 °C, oxygen pressure of 0.5 MPa, and reaction time
of 5 h). In the present reaction, no glycerol conversion is achieved in the
absence of any catalysts. Significantly, the pure ZnAl2O4 supported Pt
catalyst (Pt/ZCA-0) affords a glycerol conversion of 50.8% with a gly-
ceric acid (GLYA) selectivity of 81%, besides the formation of other
deep oxidation products including tartronic acid (TARA), dihydrox-
yacetone (DHA), glycolic acid (GLYCOA), oxalic acid (OXA) and CO2
gas. Noticeably, in the case of Pt/ZCA-10, the incorporation of a Ca/
(Zn + Ca) molar ratio of 0.1 into the spinel leads to an obvious increase
in glycerol conversion (61.8%), despite an identical GLYA selectivity.
With the further introduction of Ca, the activity of Pt/ZCA-15 catalyst
inversely is slightly decreased. Furthermore, in all cases, the selectivity
to total C3 products can reach about 92.0%, reflecting the good cata-
lytic performance of the present supported Pt catalysts in base-free
glycerol oxidation. It is noted that compared with Pt/ZCA-0 with the
higher dispersion of Pt species and a similar density of surface total
basic sites, Pt/ZCA-5 shows a slightly reduced catalytic activity.
Meanwhile, despite the higher dispersion of Pt species in the case of Pt/
ZCA-15, the catalytic activity of Pt/ZCA-15 is lower than that of Pt/
ZCA-10 catalyst with the larger density of surface basic sites. As for Pt/
ZCA-10 with the highest density of surface basic sites, the highest ac-
tivity can be attained, despite the lowest dispersion of Pt species. The
above results demonstrate that the catalytic activity of Pt/ZCA-x cata-
lysts should be closely correlated with both the dispersion of active Pt
species and surface basicity. Interestingly, Pt/ZCA-x catalysts with
different Ca contents show very similar product selectivities, which
probably originates from the similar spinel structure of ZCA-x supports,
thereby leading to similar adsorption/activation behaviors for glycerol
and intermediate molecules on the surface of catalysts during the oxi-
dation process.
Fig. 4. XPS of Pt 4d region for Pt/ZCA-0 (a), Pt/ZCA-5 (b), Pt/ZCA-10 (c) and
Pt/ZCA-15 (d) samples.
Fig. 5. CO2-TPD curves of Pt/ZCA-0 (a), Pt/ZCA-5 (b), Pt/ZCA-10 (c) and Pt/
ZCA-15 (d) samples.
surface basicity of the sample. As a result, it can be concluded that the
introduction of appropriate amounts of Ca can improve surface basicity
of the present Pt/ZCA-x samples.
Furthermore, to identify the origin of surface basic sites on Pt-based
samples, IR spectra of adsorbed CO2 on two Pt/ZCA-0 and Pt/ZCA-10
samples were obtained in the range of 1200 to 1900 cm−1 (Fig. 6).
For comparison, several single oxides supported Pt catalysts with
the similar Pt loading amount to that of Pt/ZCA-10 (i.e. Pt/ZnO, Pt/
Al2O3, Pt/CaO, Pt/ZrO2, and Pt/CeO2) were prepared for the applica-
tion in the oxidation of glycerol. It is seen that these comparison
5

Z. Han, et al.
MolecularCatalysis477(2019)110559
Fig. 6. IR spectra of CO2 absorption on Pt/ZCA-0 (a) and Pt/ZCA-10(b).
Table 3
Catalytic reaction results over different Pt-based catalysts for the oxidation of
glycerol.a.
b
Entry Samples
Conv. Selectivity (%)
(%)
c
GLYA TARA DHA GLYCOA OXA Others
1
2
3
4
5
6
7
8
Pt/ZCA-0
Pt/ZCA-5
Pt/ZCA-10
Pt/ZCA-15
50.8
48.7
61.8
55.7
31.8
18.8
16.9
16.6
81.0
82.2
81.1
81.4
45.9
72.1
69.3
60.3
3.1
0.7
1.9
2.0
1.2
0
11.7 2.0
11.4 2.0
10.6 3.9
10.7 3.3
44.7 3.2
21.6 2.6
1.8
2.0
2.6
2.6
1.1
3.7
5.8
1.5
0.4
1.7
0
0
3.9
0
d
Pt/ZnO
Pt/Al2O3
Pt/CaO
d
d
0
0
0
6.2
18.7
8.3
Pt/ZnO-CaO-
28.7 1.2
e
Al2O3
9
10
Pt/CeO2
Pt/ZrO2
26.9
28.1
78.8
80.7
0.3
1.3
12.7 1.0
11.0 1.4
4.0
2.5
3.2
3.1
Fig. 7. Change in the conversion and product selectivities with the reaction
time over the Pt/ZCA-10 in the glycerol oxidation. Reaction conditions: 60 °C;
0.5 MPa oxygen partial pressure.
a
Reaction conditions: glycerol/Pt = 590/1; 60 °C; oxygen pressure of
0.5 MPa; 5 h.
b
Selectivity was determined as: mol of product formed × the number of
carbon atoms for product / (mol of glycerol converted × 3).
oxidation reaction, together with a remarkable decrease in the GLYA
selectivity at higher reaction temperatures of 80 and 100 °C because of
the deep oxidation of GLYA and the formation of more other by-pro-
ducts (GLYCOA, OXA, and C1 products). Meanwhile, noticeably, raising
oxygen pressure from 0.1 to 1.0 MPa promotes the conversion of gly-
cerol. Like the reaction temperature, the increase in oxygen pressure
leads to a slight decrease in GLYA selectivity. while that of total C2 and
C1 products increases with the raised reaction pressure. According to
above product distributions in the glycerol oxidation, it can be con-
cluded that in the Pt/ZCA catalyst system, glycerol can easily be oxi-
dized to produce GLYA, accompanied by a possible simultaneous con-
version of glycerol to DHA, and DHA and GLYA are separately formed
probably through two different pathways [45]. In our case, however,
glyceraldehyde cannot be detected, probably due to the fast oxidation
of glyceraldehyde as a reaction intermediate to generate GLYA (Scheme
1). The slightly decreased selectivities to GLYA and DHA with the
prolonged reaction time and the elevated oxygen pressure demonstrate
that the transformation modes of glyceraldehyde intermediate into two
main GLYA and DHA products probably keep unchanged in the course
of oxidation reaction, and the further conversion of GLYA and DHA into
other products does not easily take place at the low reaction tempera-
tures (< 80 °C).
c
Other deep oxidation C1 products mainly including formaldehyde, formic
acid and CO2.
d
1.0 wt % Pt loading amount.
Pt NPs supported over a physical mixture of three kinds of single metal
e
oxides.
catalysts afford much low glycerol conversions (< 32%), along with
changing selectivities to GLYA in the range of 45.9–80.7%. Meanwhile,
supported Pt one over the physical mixture of CaO, ZnO, and Al2O3
delivers a much lower glycerol conversion of 16.6%, indicative of a
poorer activity. The above results further confirm the significant pro-
motion of ZnAl2O4 spinel supports on the glycerol oxidation.
Fig. 7 shows the change in the conversion and product selectivities
with the reaction time over the Pt/ZCA-10. Notably, the GLYA se-
lectivity is slightly decreased by about 0.9% with the prolonged reac-
tion time from 0.5 to 6 h, together with a gradually increased conver-
sion from about 25.8% after a short reaction of 0.5 h up to 63.1% after
6 h. Selectivity to DHA shows a quite slight decrease from 11.9 to
10.6% as the reaction time is prolonged from 0.5 to 6 h. Meanwhile,
more C2 products (GLYCOA, OXA) are produced with the elongated
reaction time, despite relative smaller amounts of C2 products, due to
further scission of CCe bond in GLYA and DHA. Furthermore, the ef-
fects of reaction temperature and oxygen pressure on the oxidation of
glycerol over the Pt/ZCA-10 were evaluated (Fig. 8). Note that with the
reaction temperature from 40 to 100 °C, the rate of glycerol conversion
is enhanced progressively due to the endothermic character of
On the other side, to under the instinct catalytic activity of Pt-based
catalysts, the turnover frequency values (TOFs) of glycerol converted
also were calculated according to the conversion rate of glycerol per
surface metallic Pt sites per hour at the initial reaction stage of 15 min
6

Z. Han, et al.
MolecularCatalysis477(2019)110559
Fig. 8. Change in the conversion and product selectivities over the Pt/ZCA-10 in the glycerol oxidation with the reaction temperature (A) and the oxygen partial
pressure (B). Reaction conditions: (A) 0.5 MPa (O2) and 5 h; (B) 60 °C and 5 h.
(< 30% conversion of glycerol). It is interestingly noted from Fig. 9 that
Pt/ZCA-10 presents the largest TOF value of about 1160 h−1 among
supported Pt catalysts. In terms of TOF values, the instinct activity of
Pt/ZCA-10 is better than or comparable to those of the most mono-
metallic Pt catalysts previously reported using molecular oxygen for
base-free glycerol oxidation (Table S1) [23,46–52], further mirroring
its superior catalytic activity of Pt/ZCA-10 in base-free glycerol oxida-
tion. Previously, many researchers have proven that additional homo-
geneous base (e.g.NaOH) can facilitate the OeH bond activation via-
proton transfer, thus accelerating the oxidation of glycerol
[21,26,53,54]. Under base-free reaction conditions, it is difficult for
OH- to participate in the oxidation due to its quite low concentration. In
the present catalyst system, the catalytic activity of Pt/ZCA catalysts are
superior to those of other single metal oxides supported Pt ones.
Especially, the as-fabricated Pt/ZCA-10 catalyst with the highest TOF
value of 1160 h-1 shows the best catalytic performance. The structural
characterizations demonstrate that the Pt/ZCA-10 possesses the higher
surface density of total basic and medium-strength basic sites origi-
nating from surface uniformly dispersed MeOMe structure (M = Ca,
Zn or Al) in the spinel, probably promoting electron transfer from basic
sites (lattice oxygen species) to Pt NPs and thus the lower binding en-
ergy of Pt 4d5/2 region and the formation of SMSIs, as evidenced by the
above XPS results.
In order to reveal the adsorption mode of the hydroxyl group on
supported Pt-based catalysts, in situFT-IR spectra of glycerol adsorbed
over Pt/ZCA-0 and Pt/ZCA-10 catalysts were analyzed. The results are
shown in Fig. 10. Typically, glycerol can interact with a coordinatively
unsaturated metal site on the surface of metal oxide to form a poly-
dentate alkoxy species. Noticeably, when glycerol is adsorbed on the
surface of Pt/ZCA-0, two intensive peaks at 1088 and 1049 cm−1 can be
attributed to the CeO vibrations of primary alcohol groups of glycerol
adsorbed on the surface, due to Lewis acid/base interactions between
alcohol groups of glycerol and metal atom on the spinel surface, in
Fig. 9. Change in the initial TOF values over different supported Pt catalysts.
addition to two weak should peaks (1109 and 1014 cm−1) related to
physically adsorbed and bulk glycerol molecule [55], respectively. In
the case of Pt/ZCA-10, the absorption bands of the CeO vibration re-
lated to alcohol groups appears at about 1156 and 1076 cm−1, re-
spectively, which are assignable to hydrogen-bonding and Lewis acid/
base interaction between secondary or primary hydroxyl groups of
glycerol and surface metal atoms, indicating stronger surface adsorp-
tion of glycerol. Meanwhile, several characteristic absorption peaks
corresponding to the splitting of the CeH vibrations for Pt/ZCA-0 (2926
and 2858 cm−1) also shift to higher frequencies for Pt/ZCA-10 (2976
and 2889 cm−1), due to the difference in their electronic effects in-
duced by stronger surface adsorption of glycerol [56]. The above results
suggest that surface strong basicity can greatly promote the adsorption
and activation of hydroxyl groups in glycerol during glycerol oxidation.
According to the above comprehensive characterizations and
Scheme 1. Pathways of oxidation of glycerol over Pt/ZCA catalysts.
7

Z. Han, et al.
MolecularCatalysis477(2019)110559
GLYA product viathe following processes. Firstly, the water molecule
interacts rapidly with glyceraldehyde adsorbed on surface basic sites,
thus forming a surface intermediate [61]. Secondly, Pt species interact
with adsorbed intermediate to form a metal hydride viaa metal-hy-
drogen bond. Thirdly, two protons are abstracted from metal hydride
and further adsorbed on basic sites to generate GLYA product. At last,
adsorbed hydrogen was oxidized by active oxygen species on Pt sites to
release water.
Obviously, in the present catalyst system, undoped and Ca-doped
ZnAl2O4 supports can provide high surface areas and porous micro-
structure for dispersing and stabilizing active Pt species, while surface
basic sites mainly originating from MeO pairs (M = Ca, Zn or Al) in the
ZnAl2O4 spinel structure can facilitate the adsorption and activation of
alcohol hydroxyl groups in glycerol and greatly promote the abstraction
of protons in glycerol or glyceraldehyde intermediate from OeH and/or
CHe groups under base-free reaction conditions, like NaOH. In the case
of Pt/ZCA-10 catalyst, a larger surface density of basic sites may fa-
cilitate the activation of glycerol and further following elimination of β-
hydrogen in alkoxide. In addition, surface electron-rich Pt0 species can
promote the activation of molecular oxygen and CeH bond in reaction
intermediates. Thereupon, a surface synergy between highly dispersed
electron-rich Pt species and medium-strength basic sites mainly con-
tributes to its higher catalytic performance of the Pt/ZCA-10 catalyst.
At last, for as-fabricated Pt/ZCA-10 catalyst, the reusability and
stability were further examined. Note that the conversion and se-
lectivity to GLYA still keep stable after six consecutive runs (Fig. S1),
indicative of excellent structural stability of the Pt/ZCA-10 under actual
reaction conditions. The ICP-AES analysis of filtrates shows neglectable
leaching of Pt after six cycles, while the TEM observation of used cat-
alyst reveals no obvious aggregation and growth of Pt NPs (Fig. S2).
Such exceptional stability of the catalyst should be assigned to the
strong metal-basic sites interactions, as well as well-developed porous
framework, thereby tightly anchoring NPs on the support surface and
thus preventing the growth or aggregation of Pt particles during the
reaction process.
Fig. 10. In situFT-IR spectra of glycerol adsorption over Pt/ZCA-0 (a) and Pt/
ZCA-10 (b).
catalytic reaction results, obviously, a surface synergy between highly
dispersive Pt species and basic sites should contribute to the improved
catalytic performance of Pt/ZCA-10 catalyst. Here, we tentatively pro-
pose a mechanism for glycerol oxidation to produce GLYA on as-fab-
ricated Pt/ZCA catalysts (Scheme 2). Previously, it was reported that
the deprotonation of hydroxyl group with the help of base was one of
the rate-determining steps for the glycerol oxidation [14,20]. Further-
more, noble metals were responsible for the deprotonation of β-hy-
drogen in the oxidation of alcohols, which was another rate de-
termining step [57]. In our case, glyceraldehyde intermediate can first
form from glycerol oxidation [44,58,59]. Thereupon, two primary and
secondary hydroxyls in glycerol may be simultaneously adsorbed and
activated on basic sites (e.g. MOe pairs), adjacent to metallic Pt sites.
Subsequently, the β-hydrogen in the primary hydroxyl group adsorbed
on the surface can be abstracted by surface active O2− sites to form an
alkoxy compound intermediate. Secondly, the formed intermediate in-
teracts with adjacent Pt species to generate a metal-hydride through the
H-metal bond, while molecular oxygen can be activated on Pt atoms.
Here, the electron transfer from electron-rich Pt atoms to adsorbed
oxygen cannot only promote the activation of molecular oxygen but
also facilitate the formation of metal-hydride because of the nature of β-
hydrogen with partial negative charge in the alkoxy intermediate [60].
Then, further eliminating β-hydrogen in metal-hydride may produce
glyceraldehyde intermediate, while the eliminated hydrogen can be
captured by surface oxygen species. Finally, active oxygen species can
oxidize surface hydrogen adsorbed on basic sites to release water [43].
In our present Pt/ZCA-x catalyst system, surface basic sites are re-
sponsible for the first rate-determining step (the abstraction of α-hy-
drogen in terminal hydroxyl group of glycerol), while metallic Pt spe-
cies can assist in the second rate-determining step (the β-hydrogen
abstraction). Further, glyceraldehyde formed can be rapidly oxidized to
4. Conclusions
In summary, a series of supported Pt nanocatalysts over high-sur-
face-area and porous undoped and Ca-doped zinc aluminate spinels
were employed for base-free glycerol oxidation. The results showed that
Pt NPs could be evenly distributed and tightly immobilized on the
support surface, thus forming the strong interactions between Pt species
and the support. Comprehensive characterizations including XPS ana-
lysis, CO2-TPD, and FT-IR spectra of CO2 adsorption experiments re-
vealed that introduction of an appropriate amount of Ca into the
ZnAl2O4 resulted in the enhanced surface density of medium-strength
basic sites and the strengthened metal-support interactions. As-fabri-
cated Pt catalyst having a Ca/(Zn + Ca) molar ratio of 1:10 exhibited a
superior catalytic activity to other supported Pt ones, with a high TOF
value of 1160 h−1. Compared with pristine ZnAl2O4 supported Pt one,
Scheme 2. The proposed mechanism of base-free oxidation of glycerol to glyceric acid over Pt/ZCA catalysts.
8

Z. Han, et al.
MolecularCatalysis477(2019)110559
the instinct activity of Pt/ZCA-10 catalyst is nearly one time higher with
the introduction of ca. It was believed that a surface synergy between
highly dispersed electron-rich metallic Pt species and medium-strength
basic sites greatly promoted the hydroxyl group activation in glycerol,
as well as the activation of molecular oxygen, thus leading to its en-
hanced catalytic efficiency. Moreover, for the present supported Pt
catalysts, the several advantages of high surface area, acid-resistance of
the spinel supports, abundant surface basic sites, and SMSIs guaranteed
the effective stabilization and dispersion of Pt NPs and thus prevented
the aggregation of metal particles during actual reactions. The present
findings provide a new way to construct supported metal-base catalysts
applied in practical catalytic processes through the immobilization of
other noble metal or non-noble metal NPs on the as-formed high-sur-
face-area spinel-type Ca-doped zinc aluminate spinel supports with
appropriate surface basicity.
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