Interface effect of mixed phase Pt/ZrO2 catalysts for HCHO oxidation at ambient temperature

Article abstract of DOI:10.1039/c7ta03888g

A series of high-efficiency Pt/ZrO₂ catalysts were successfully prepared by simple methods on the basis of a ZrO₂ support with a mixed monoclinic/tetragonal phase structure. The activity test results showed that the mixed phase catalysts exhibited higher catalytic activity than the pure monoclinic phase, and HCHO can be completely oxidized into CO₂ and H₂O at near ambient temperature. XRD, Raman and HRTEM results demonstrated that the monoclinic-tetragonal phase interface with abundant defects was formed due to the introduction of the tetragonal phase. According to the results of TEM, XPS and H₂-TPR, the mixed phase interfacial structure can induce the formation of the active oxygen species, ionic Pt^δ+ species, strong metal-support interaction and low-temperature reducibility, which was vital for the significant improvement of the catalytic activity. Furthermore, the specific HCHO reaction rate of the catalysts at 55 °C increased from 0.8 × 10^-3 to 10.4 × 10^-3 mmol h^-1 m^-2 and the activation energy decreased remarkably from 213.5 to 24.7 kJ mol^-1 with the increase of the biphase interface content. In situ DRIFTS spectra showed that the special interfacial structure can change the reaction pathway of HCHO oxidation and inhibit the formation of inert carbonate species, thus greatly enhancing the HCHO oxidation activity.

Full text of DOI:10.1039/c7ta03888g

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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Interface Effect of Mixed Phase Pt/ZrO2 Catalyst for HCHO
Oxidation at Ambient Temperature
Xueqin Yang,†, ‡ Xiaolin Yu,*,† Mengya Lin,†, ‡ Maofa Ge,*,†, ‡,§ Yao Zhao,# Fuyi Wang‡,#
Received 00th January 20xx,
Accepted 00th January 20xx
A series of high-efficiency Pt/ZrO2 catalysts were successfully prepared by simple methods on the basis of a ZrO2 support
with a mixed monoclinic/tetragonal phase structure. The activity test results showed that the mixed phase catalysts
exhibited higher catalytic activity than pure monoclinic phase, and HCHO can be completely oxidized into CO2 and H2O at
near ambient temperature. XRD, Raman and HRTEM results demonstrated that the monoclinic-tetragonal phase interface
with abundant defects were formed due to the introduction of tetragonal phase. According to the results of TEM, XPS and
H2-TPR, the mixed phase interfacial structure can induce the formation of the active oxygen species, ionic Ptδ+ species,
strong metal-support interaction and low-temperature reducibility, which was vital for the significant improvement of the
catalytic activity. Furthermore, the specific HCHO reaction rate of catalysts at 55 °C increased from 0.8×10-3 to 10.4 ×10-3
mmol h-1 m-2 and the activation energy decreased remarkably from 213.5 to 24.7 kJ mol-1 with the increase of the biphase
interface content. In situ DRIFTS spectra showed that the special interfacial structure can change the reaction pathway of
HCHO oxidation and inhibit the formation of inert carbonate species, thus greatly enhacing the HCHO oxitdation activity.
DOI: 10.1039/x0xx00000x
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polymorphism and amphoteric nature.9, 12, 13 Zirconia contains
three crystal phases: monoclinic (m-ZrO2), tetragonal (t-ZrO2)
and cubic (c-ZrO2). In general, m-ZrO2 is stable below 1000 °C,
1. Introduction
Formaldehyde (HCHO) is a common toxic indoor air pollutant.
Long-term exposure to this air pollutant, even at levels of a
few ppm, may cause serious health problems such as skin
irritation and nasal tumors.1, 2 The low temperature catalytic
oxidation of HCHO into CO2 and H2O is regarded as the most
promising HCHO removal technology, because this process is
energy-saving, high efficient and environmental friendly.3, 4 To
date, a series of transition metal oxides and supported noble
metals have been extensively studied for HCHO oxidation. In
general, the transition metal oxides exhibit low catalytic
and t-ZrO2 and c-ZrO2 structurally stabilize at high temperature
or can be stable at room temperature by adding dopants, such
as Y, La, Ce, etc..9, 14 However, ZrO2-based catalysts for HCHO
oxidation frequently present low catalytic activity and require
a high conversion temperature. For example, Huang et al.
investigated the influence of different supports loaded Pt (i.e.,
TiO2, Al2O3, CeO2, ZrO2, and MgO) on total HCHO oxidation,
and found that Pt/ZrO2 catalyst showed the lowest Pt
dispersion and the turnover frequency (TOF).15 Zhang et al.
successfully synthesized a novel mesoporous ZrO2 support
with high specific surface area, and the lowest temperature for
complete oxidation of HCHO by these ZrO2-based Au catalysts
was 157 °C, which is much higher than ambient temperature.16
It is well-known that the catalytic performance of a catalyst
is often closely related to its polymorphism.12, 17 The crystal
structure and corresponding structure transformation have a
dramatically influence on the physicochemical and catalytic
properties of samples.17, 18 Benefited from the understanding
of crystal structure, some groups attempted to improve the
catalytic performance of samples through changing crystal
structure or introducing mixed phase structure. For instance,
Zhang et al. investigated the HCHO catalytic activities over
several manganese oxides with different crystal structures,
and found that the δ-MnO2 exhibited the best activity.19
Hurum et al. reported that the mixed-phase titania displayed
high photocatalytic efficiency due to the coexistence of
anatase and rutile phase structure.20 Further studies have
shown that the photocatalytic improvement of mixed phase
activity and high reaction temperature,5, while the noble
6
metal catalysts possess high specific activity and excellent low-
temperature oxidation performance, which are very desirable
for indoor HCHO elimination.7, 8
Zirconia (ZrO2) has been extensively applied as supports in
various fields, such as VOC oxidation,9 methane combustion,10
and hydrodeoxygenation of phenol,11 because of its thermal
stability, chemical resistance, redox surface property,
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system is probably attributed to the presence of the 2.3 Catalysts characterization
rutile/anatase interface and the rapid electron transfer
Brunauer-Emmett-Teller (BET) surface areas of the samples
were measured by N2 adsorption-desorption isotherms at 77K
using an autosorb-iQ instrument. X-ray powder diffraction
(XRD) patterns were obtained by a D/max 2500 X-ray powder
diffractometer with Cu Kα radiation at a scan rate (2θ) of 2°
min-1. The tetragonal phase zirconia (t-ZrO2) content in the
mixed phase samples was calculated by X’Pert HighScore Plus
software. Raman spectra were recorded with a LabRAM
Raman spectrometer using a 532 nm laser. Transmission
electron microscopy (TEM) and high-resolution transmission
electron microscopy (HRTEM) were conducted using a JEM-
2011, HT7700 and JEM-2100F. X-ray photoelectron
spectroscopy (XPS) was performed on an ESCALAB250XI with
Al Kα radiation. The samples were freshly prepared and dried
in vacuum oven before the XPS measurement. Inductively
coupled plasma mass spectrometry (Agilent 7700 Series ICP-
MS) was carried out to determine the actual loading of
platinum in the samples.
through transition stages of the mixed phase system.20-22 Up to
the present, however, little attention has been devoted to the
relationship between HCHO catalytic oxidation and interfacial
structure of the mixed phase ZrO2 catalysts.
Herein, we prepared a series of Pt/ZrO2 catalysts with mixed
phase structure and investigated the interface effect on HCHO
catalytic oxidation. It was proved that the catalytic
performance of catalysts could be vastly improved by the
special monoclinic-tetragonal (m-t) phase interface of
catalysts, and 100 ppm HCHO could be completely oxidized at
near ambient temperature by the Pt/ZrO2 catalyst with the
maximum interfacial structure content. The reasons of the
promotion effects by the mixed phase structure were studied
profoundly by various characterizations, and the possible
mechanism was also clearly elucidated.
2. Experimental
H2 temperature-programmed reduction (H2-TPR) and pulse
CO chemisorption were measured by an AutoChem II 2920
apparatus. For H2-TPR, the samples were first pretreated in an
Ar flow of 20 cm3 min-1 at 200 °C for 30 min. After that,
reduction profiles were obtained by passing a flow with 10%
H2/Ar (50 cm3 min-1) through the samples and the temperature
was increased from 50 to 600 °C at a heating rate of 10 °C
/min. For CO chemisorption, the samples were pretreated at
200 °C for 1 h in 10% H2/Ar (50 cm3 min-1) and then switched
to an Ar flow (50 cm3 min-1) for 40 min-1. Next, 10% CO-He (50
cm3 min-1) was introduced and the samples were measured at
50 °C.
2.1 Preparation of ZrO2 supports
Mixed phase ZrO2 supports were prepared using different
precipitation reactions. Briefly, ZrOCl2·8H2O (32 g) and urea (6
g) powders were dissolved in 70 mL of distilled water and
vigorously stirred for 30 min. After that, the solution was
transferred to a 100 mL autoclave with a Teflon liner at 150 °C
for 12 h. The resulting precipitate was washed with deionized
water until it reached pH 7 and dried in a vacuum oven at 60
°C for 12 h, which was denoted as ZrO2-U. The other two
supports were prepared by dropwise addition of an aqueous
solution (50 mL) of ZrOCl2·8H2O (20 g) to 25 % ammonium
hydroxide (50 mL) and a 1.8 mol/L potassium hydroxide
solution (100 mL) at room temperature. The pure monoclinic
phase ZrO2 were synthesized according to the experimental
procedure as reported by Witoon et al.23 In detail, ZrOCl2·8H2O
(9.7 g) powders were dissolved in 60 mL of distilled water, and
then 25 % ammonium hydroxide was added dropwise until pH
1.5. After that, the mixture was transferred to a 100 mL
autoclave with a Teflon liner at 140 °C for 24 h. Thereafter, the
three white precipitates were washed with deionized water
until they reached pH 7 and dried in a vacuum oven at 60 °C
for 12 h. The obtained precipitates were denoted as ZrO2-N,
ZrO2-K and ZrO2-M, respectively. All supports were ground and
calcined at 500 ° C in the air for 4 h before using.
The in situ diffuse reflectance FTIR spectroscopy (DRIFTS)
was recorded in Nicolet iS50. Prior to the measurements, all
samples were pretreated at room temperature in a gas flow of
N2 for 5 h to remove any adsorbed impurities. The all spectra
scanned from 1000 to 4000 cm−1 with 32 scans at a resolution
of 4 cm−1 and the background spectrum was subtracted from
each spectrum, respectively. Samples were tested at room
temperature under the following conditions: 100 ppm HCHO,
20 % O2 and 74 % N2 (balance). The total flow rate was 100 mL
min−1.
2.4 Activity test
The activity tests for the catalytic oxidation of HCHO over the
samples (100 mg) were performed in a continuous flow fixed-
bed quartz tubular reactor (i.d. = 4 mm). Gaseous HCHO was
generated by flowing nitrogen through the paraformaldehyde
container in a water bath kept at 35 °C. The feed gas
composition was 100 ppm HCHO and 20 % O2 balanced by N2.
The total flow rate was 100 mL min−1, corresponding to a
2.2 Preparation of Pt/ZrO2 catalysts
The proper amount of the ZrO2 support was uniformly
dispersed into the PtCl4 solution (1 wt
% Pt). After
impregnation for 30 min, NaBH4 was added into the
suspension as a reducing agent (NaBH4 / Pt = 10, molar ratio)
under vigorous stirring at room temperature for 3 h. Finally,
the samples were centrifuged and dried in a vacuum oven at
60 °C for 12 h. The obtained samples were denoted as Pt/ZrO2-
M, Pt/ZrO2-U, Pt/ZrO2-N and Pt/ZrO2-K, in which the actual
loading of platinum was 0.84, 0.89, 0.79, 0.88 wt %.
-1
weight hourly space velocity (WHSV) of 60 000 mL gcat h-1.
The concentration of HCHO in the inlet and outlet gas was
measured by an online gas chromatograph of Agilent 7890B.
The performance of the catalysts is presented in terms of
24
X
HCHO conversion ( HCHO ) as follows:
2 | J. Name., 2012, 00, 1-3
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CO
[
]
2
outlet
XHCHO
=
×100%
HCHO
[
]
inlet
HCHO
CO
[
]
[
]
2
inlet
outlet
where
and
are the HCHO concentration in the
inlet gas and the CO2 concentration in the outlet gas,
respectively.
2.5 Kinetic test
Kinetic data were measured by separate experiments, and the
HCHO conversion was kept below 10 % to keep the reaction in
the kinetic regime. That is, the experimental conditions were
425 ppm HCHO, 20 % O2, 62 % N2 (balance) and WHSV 600 000
-1
mL gcat h-1. The total flow rate was 100 mL min−1. The specific
surface area and activation energy of catalysts were calculated
from kinetic data.
Fig. 2 Raman spectra of as-synthesized Pt/ZrO2 catalysts.
The reaction rates normalized by the specific surface area (
R
s ) of the Pt/ZrO2 catalyst were calculated using the following
expression:25
increased in the order of ZrO2-M, ZrO2-U, ZrO2-N and ZrO2-K
supports, implying the increase of mixed-phase interfacial
amount.
Rs = X HCHO QC f W SBET
After Pt supported on the ZrO2, no Pt species could be
detected, and the diffraction intensity of the tetragonal ZrO2
decreased dramatically (Fig. 1b). This phenomenon was
probably attributed to the low Pt contents, small Pt particles
-1
where is the volumetric flow rate (mL h ) and is the inlet
C
Q
f
-1
W
concentration of HCHO (mol mL ). is the mass of the catalyst
employed (g) and SBET is the BET surface area of the catalyst (m2
g-1). The activation energy (Ea) was calculated from the slope
of the Arrhenius-type plot of the HCHO oxidation rate.
28
and/or special mixed phase structure of the catalysts.27,
Compared with the pure monoclinic phase Pt/ZrO2-M catalyst,
the presence of abundant defects on other samples might be
27
beneficial to adsorb and stabilize platinum species.15,
Moreover, the geometrical similarity between the Pt cubic
structure and ZrO2 tetragonal structure also favored the
formation of strong interactions and the stabilization of the Pt
species on t-ZrO2 or m-t phase interface.12 The strong
interactions might induce the distortion of the t-ZrO2 crystal
structure and then lead to a dramatic decrease of the
diffraction intensity of t-ZrO2 in the samples.
3. Results and discussion
3.1 Catalyst characterization
Fig.
1 shows the XRD patterns of ZrO2 supports and
corresponding Pt/ZrO2 catalysts. As shown in Fig. 1a, it was
observed that all the diffraction peaks in the ZrO2-M could be
assigned to monoclinic zirconia (JCPDS PDF No. 37-1484). No
diffraction peaks of tetragonal zirconia (JCPDS PDF No. 50-
1089) were detected, indicating that ZrO2-M consisted of pure
monoclinic zirconia. For the ZrO2-U, ZrO2-N and ZrO2-K
supports, distinct diffraction peaks of tetragonal zirconia
appeared in addition to those of the monoclinic zirconia,
implying the formation of the monoclinic-tetragonal (m-t)
mixed phase. It should be pointed out that the special biphase
structure might lead to the formation of more lattice disorder,
oxygen vacancies or structure defects, especially around the
transition stages of the mixed phase.20, 26 As shown in Table 1,
it was calculated that the amount of the tetragonal phase
It is generally believed that Raman spectroscopy is sensitive
to crystal defects such as lattice disorder and oxygen
30
Raman
vacancies, which are not detectable by XRD.29,
spectra of the Pt/ZrO2 catalysts and corresponding supports
are displayed in Fig. 2 and S1. For all Pt/ZrO2 samples, the
major bands located at ~179, 334, 476, 623 cm-1 and the band
at 476 cm-1 were stronger than that at 623 cm-1, which is a
feature of m-ZrO2.31, 32 As shown in the inset of Fig. 2, the
Raman peak at 470 cm-1 shifted to a higher frequency (477 cm-
1) with the introduction of tetragonal phase. The positive shift
confirmed the presence of abundant defects in samples, and
the defects were probably attributed to the special interfacial
structure of the catalysts.33, 34
Nitrogen adsorption-desorption isotherms and pore size
distribution patterns of Pt/ZrO2 catalysts and ZrO2 supports are
displayed in Fig. S2 and S3, and the corresponding parameters
are listed in Table 1 and S1. Type IV isotherms (according to
the IUPAC classification) were observed in all the samples,
demonstrating the presence of mesoporous structure.35 There
was a big pore around 17.5 nm besides 3.4 nm for ZrO2-M
sample as shown in Table S1, which might facilitate the
dispersion of Pt species.3 The incorporation of Pt species
further resulted in the disappearance of big pore in Pt/ZrO2-M.
Fig. 1 XRD patterns of as-synthesized ZrO2 supports (a) and corresponding
Pt/ZrO2 catalysts (b).
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Table 1 Physical-chemical and HCHO catalytic properties of as-synthesized Pt/ZrO2 catalysts
samples
Surface
area
(m2 g-1)
Total pore
volume (cm3
g-1)
Pore
diameter
(nm)
Pt content a Pt dispersion b
Tetragonal phase
content c (%)
Rs d
Ea e
(kJ mol-1)
(%)
(mmol h-1 m-2)
(wt %)
catalysts
supports
Pt/ZrO2-M
Pt/ ZrO2-U
Pt/ ZrO2-N
Pt/ ZrO2-K
78.6
54.4
90.2
70.3
0.32
0.16
0.22
0.12
3.4
7.8
6.5
4.9
0.84
0.89
0.79
0.88
58.5
9.00
31.1
51.3
0
0
0.0008
0.0041
0.0056
0.0104
213.5
61.3
38.2
24.7
6
7
9
7
22
30
a Determined by ICP-MS. b Pt dispersion calculated by CO chemical adsorptions assuming a CO:Pt stoichiometry of 1:1. c The tetragonal phase content of samples were
calculated by X’Pert HighScorePlus software. d Specific HCHO reaction rate of samples estimated at 55 °C under a kinetically controlled regime. The detail experimental
e
conditions were shown in kinetic test.
The activation energy calculated from the slope of the Arrhenius-type plot of the HCHO oxidation rate.
Due to the surface cover and pore blockage by Pt species, the energies and the percentages of XPS peaks calculated are
samples supported Pt exhibited the slightly decrease of surface summarized in Table 2 and S1. It is reported that the Zr 3d5/2
areas and pore diameters compared with the supports.15, 36
binding energy of stoichiometric zirconia is in the range of
However, total pore volume somewhat increased. This could 182.2-183.3 eV.38 Table 2 shows that the Zr 3d5/2 binding
be attributed to the generated H2 and the increase of pH energy of all Pt/ZrO2 catalysts ranged from 181.9 to 182.0 eV,
during the preparation process.37
indicating that the supports were a non-stoichiometric oxide
The morphology and microstructure of Pt/ZrO2 catalysts are and that defects existed in the catalysts, in good agreement
further elucidated by transmission electron microscopy (Fig. with the Raman results. As shown in Fig. 4b, three typical
3). TEM analysis confirmed that small Pt species (ca. 3 nm) peaks were observed in the O 1s XPS spectra of the Pt/ZrO2
were uniformly dispersed on the surface or in the pores of the catalysts. The binding energies at 529.6-530.1, 531.7-532.1
supports for all Pt/ZrO2 samples, consistent with the measured and 533.1-533.5 eV were assigned to the lattice oxygen (Olatt),
Pt dispersion (Table 1). For the pure monoclinic phase Pt/ZrO2- adsorbed
M catalyst, the high dispersion might be due to the high (hydro)carbons, respectively.39, 40
surface area and big pore size. However, the special m-t Fig. 4c shows that all Pt/ZrO2 catalysts displayed two Pt 4f7/2
oxygen
(Oads),
and
oxygen-containing
interfacial structure in the mixed phase Pt/ZrO2 samples peaks centered at 70.8-71.3 and 72.4-73.1 eV, which were
should be beneficial to the stabilization of Pt species, resulting assigned to metallic Pt0 and ionic Pt2+ species, respectively.3, 41
from abundant defects, strong interactions and geometrical From Table 2, the Ptδ+/(Pt0+Ptδ+) ratio of samples ranked in the
similarity.
following order: Pt/ZrO2-M < Pt/ZrO2-U < Pt/ZrO2-N < Pt/ZrO2-
The chemical states of elements in the samples are K, which coincided with the tetragonal phase content in the
investigated by X-ray photoelectron spectroscopy (XPS). The samples. The mixed phase structure might induce the partial
XPS spectra are shown in Fig. 4 and S4, and the binding
flow of the 5d electrons of Pt into the 4d of Zr by the strong
interactions, leading to the formation of cationic Ptδ+
species.42, 43
Considering that abundant defects in the samples were
mainly caused by the lattice mismatch around the biphase
interface, it was deduced that strong interactions probably
only existed in the m-t phase interface and could affect the
electronic structure of Pt adjacent to the special structure.
Therefore, Ptδ+ cation species were dominant, and the ratio
increased in the order of Pt/ZrO2-U, Pt/ZrO2-N and Pt/ZrO2-K.
Nevertheless, there were some ionic Ptδ+ species in Pt/ZrO2-M,
Fig. 3 TEM images of as-synthesized Pt/ZrO2 catalysts. (a) Pt/ZrO2-M, (b)
Pt/ZrO2-U, (c) Pt/ZrO2-N, and (d) Pt/ZrO2-K.
Fig. 4 XPS spectra of Pt/ZrO2 catalysts: (a) Zr 3d, (b) O 1s, (c) Pt 4f.
4 | J. Name., 2012, 00, 1-3
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Table 2 Surface chemical composition and element molar ratios of Pt/ZrO2 catalysts
samples
Zr 3d5/2 (eV)
Oads/Olatt
Ptδ+/(Pt0+Ptδ+) (%)
Pt/ZrO2-M
Pt/ZrO2-U
181.9
181.9
0.98
1.41
26.5
76.9
Pt/ZrO2-N
Pt/ZrO2-K
181.9
182.0
1.93
1.79
82.0
84.7
though ZrO2-M is pure monoclinic zirconia. This is because Pt
nanoparticles exposed on the surface of the support could be
easily oxidized. On the basis of the above-mentioned results,
we can speculate that Pt species in close proximity to the
biphase interface are in the cation state and those around
pure monoclinic or tetragonal phases are in the metallic state.
To visualize the lattice mismatch around the m-t phase
interface and the relationship between the mixed phase
structure and Pt species, the Pt/ZrO2-K catalyst with the
maximum tetragonal phase content was investigated by
HRTEM. Fig. 5a shows that there were abundant defects
around the transition stages of the two phases caused by the
lattice mismatch, and the measured lattice spacing of the Pt
nanoparticles adjacent to the interface structure was ca. 2.51
Å, consistent with the (200) plane of cubic platinum oxide
(Ptδ+). As shown in Fig. 5b and 5c, the measured lattice fringes
of the Pt nanoparticles around pure monoclinic or tetragonal
phase were ca. 1.98 Å, ascribed to the (200) plane of cubic
platinum (Pt0). For the pure monoclinic phase Pt/ZrO2-M
catalyst, however, Pt nanoparticles mainly presented their
metallic state (Fig. S5), in accordance with the XPS results.
Accordingly, the mixed phase structure might induce the
formation of Ptδ+ cation species at the interface, which cannot
be caused by the pure phase structure. Moreover, the obscure
boundary between the Pt nanoparticles and the support
demonstrated the presence of a strong interaction.44 More
importantly, the strong interaction was essential for anchoring
metal nanoparticles, weakening the Zr-O bond, and
transferring interfacial electrons or oxygen atoms, which is
Fig. 6 H2-TPR profiles of the Pt/ZrO2-M, Pt/ZrO2-U, Pt/ZrO2-N and Pt/ZrO2-K catalysts.
indispensable for high-efficiency catalysts.28, 45
To gain the new insights into the interface effect on
samples, H2-TPR was performed and the results are displayed
in Fig. 6. The mixed phase samples showed the three reduction
peaks, assigned to the reduction of surface active oxygen
interacting with Pt adjacent to the mixed phase interface (peak
3), the pure monoclinic or tetragonal phases (peak 2) and bulk
oxygen in zirconia (peak 1), respectively.28, 38 It was noted that
increasing the tetragonal phase content made the
corresponding reduction peak shift to a lower temperature in
the sequence of Pt/ZrO2-U, Pt/ZrO2-N and Pt/ZrO2-K, which
indicated the enhancement of the low-temperature
reducibility. This result was consistent with previous reports
that noble metal and mixed phase structures could accelerate
the reduction of surface oxygen species and facilitate the
activation or transfer of oxygen species.20, 26 For the Pt/ZrO2-M
sample, the reduction peaks might be related to the reduction
of surface active oxygen around small Pt species (peak 3), large
Pt species (peak 2) and bulk oxygen (peak 1), resulting from
the weak crystallinity and small crystallite size of Pt/ZrO2-M
sample (see Fig. 1b).34
3.2 Catalyst Activity
The catalytic activities of the Pt/ZrO2 catalysts and the
corresponding supports toward HCHO oxidation are displayed
in Fig. 7a and S6. All supports were inactive for HCHO
decomposition at low temperature, and catalytic performance
was improved significantly after Pt loading. It could be seen
that the pure monoclinic phase Pt/ZrO2-M catalyst had the
poor catalytic activity with HCHO complete convention
temperature at 85 °C, while the as-prepared mixed phase
Pt/ZrO2 catalysts showed high catalytic activity, and HCHO
could be completely oxidized into CO2 and H2O by Pt/ZrO2-K at
30 °C, as well as Pt/ZrO2-N and Pt/ZrO2-U at 35 °C. Obviously,
the Pt/ZrO2-M catalyst with relatively high surface area was
even inferior to the as-prepared mixed phase Pt/ZrO2-U
catalyst with lowest surface area for HCHO oxidation, which
meant that the effect of surface area could be excluded as the
main responsibility for the higher catalytic activity. However,
the presence of the mixed phase could distinctly enhance the
Fig. 5 HRTEM images for the Pt/ZrO2-K catalyst (a) Pt adjacent to the m-t phase
interface, (b) Pt around the pure tetragonal phase, and (c) Pt around the pure
monoclinic phase.
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surface areas.47 It was apparent from Table 1 that Pt/ZrO2-M
with relatively high surface area showed the lowest specific
HCHO reaction rate (0.8×10-3 mmol h-1 m-2 ), indicating that the
surface area was not key factor for the higher catalytic activity.
As seen in Fig. 7d and Table 1, the activation energies
decreased remarkably from 213.5 to 24.7 kJ/mol in the
sequence of Pt/ZrO2-M, Pt/ZrO2-U, Pt/ZrO2-N and Pt/ZrO2-K,
consistent with the tetragonal phase content. This result
demonstrated that the higher the tetragonal phase content,
the easier the HCHO complete oxidation, thus achieving a
superior performance. Moreover, it is generally agreed that
the activation energy is related to the reaction mechanism.35,
48
Therefore, we speculate that the mixed phase structure of
Pt/ZrO2 catalysts could significantly enhance the intrinsic
catalytic efficiency and induce a new reaction pathway. This is
different behavior compared to that of the pure monoclinic
phase Pt/ZrO2-M catalyst and results in an enormous decrease
of activation energy. That is, there was an intrinsic relation
between the mixed phase structure and high catalytic
efficiency of the samples, thus further verifying the
aforementioned hypothesis.
Fig. 7 (a) HCHO conversion over Pt/ZrO2 catalysts. Reaction conditions: 100 ppm HCHO,
-1
20% O2, WHSV = 60,000 mL gcat h-1; (b) The relationship between the catalytic activity
of Pt/ZrO2 catalysts at 25 oC and the tetragonal phase content in supports; (c) The
stability test of Pt/ZrO2-K catalyst at 30 °C. Reaction conditions: 100 ppm HCHO, 20%
-1
O2, WHSV = 60,000 mL gcat h-1. (d) Arrhenius plots of reaction rates obtained from
kinetic test over as-prepared Pt/ZrO2 catalysts.
low temperature catalytic performance of the samples. As
shown in Fig. 7b, such an increasing tendency of catalytic
activity for Pt/ZrO2 catalysts was directly related to the
tetragonal phase content in the supports. Moreover, the
Ptδ+/(Pt0+Ptδ+) ratio of samples and corresponding catalytic
activity increased dramatically in the sequence of Pt/ZrO2-M,
Pt/ZrO2-U, Pt/ZrO2-N and Pt/ZrO2-K, probably attributed to the
formation of the biphase structure in the samples, because the
high surface area of ZrO2-M did not result in the high Ptδ+
content. The HRTEM image of the Pt/ZrO2-K catalyst confirmed
the aforementioned hypothesis, and the defects caused by the
lattice mismatch around the m-t phase interface were
visualized. Combined with the XRD, Raman and XPS results, it
was presumed that the mixed phase structure might induce
interfacial oxygen or electron transfer among the Pt-biphase
interface perimeters and anchor low coordinated Pt atoms in
the form of Ptδ+ species by means of strong interactions.40
Consequently, we speculate that Ptδ+ species adjacent to the
biphase interface are more active than Pt0 species around the
pure phase and main active sites of HCHO catalytic oxidation
for mixed phase Pt/ZrO2 catalysts. It must also be mentioned
that the catalytic performance of the catalysts was closely
associated with the low-temperature reducibility.46 The peak
shift to a lower temperature with the increase of the
tetragonal phase content (see Fig. 6) verified the speculation
above. In addition, the stability of Pt/ZrO2-K sample with the
best catalytic performance was also checked by a long-term
test at 30 °C. Figure 7c shows that the Pt/ZrO2-K catalyst still
exhibited 88% HCHO conversion after a 60 h long test in the
same test conditions.
3.3 In-situ DRIFT test
In situ DRIFTS spectra of the Pt/ZrO2-K and Pt/ZrO2-M catalysts
exposed to HCHO/O2 were performed (Fig. 8) to detect the
intermediates and investigate the catalytic mechanism of
HCHO oxidation. As shown in Fig. 8a, formate species (1582,
1379 cm-1 for υ(COO-) and 2986, 2862, 2765 cm-1 for υ(CH))49,
50
were formed and dominant. Bands also appeared at 1479,
1324, 1180 and 1117 cm-1, which were ascribed to the CH and
CO vibrations of dioxymethylene species.36, 50 Therefore, it can
be concluded that dioxymethylene and formate species were
formed and accumulated during HCHO oxidation over Pt/ZrO2-
K. Additionally, there was a negative band at approximately
3689 cm-1 assigned to surface hydroxyl groups (-OH), indicating
that the formation of dioxymethylene and formate species
consumed -OH groups24, 51. However, the DRIFTS spectrum of
Pt/ZrO2-M (Fig. 8b) was obviously different from that of
Pt/ZrO2-K. Formate species (1630, 1521 cm-1 for υ(COO-) and
2938, 2895, 2831 cm-1 for υ(CH))49,
and dioxymethylene
50
(1456, 1409, 1337,1307 cm-1 for CH vibrations and 1128,1096
cm-1 for CO vibrations)36, 50 became weak. The bands of the
hydroxyl groups appeared at 3764 and 3676 cm-1.24,
52
Interestingly, two new bands at approximately 1667 and 1178
cm-1 appeared, which were assigned to υ(C=O) and υas(COO) of
To further clarify the origin of this high efficiency, the
reaction kinetics of the Pt/ZrO2 catalysts were measured. The
specific HCHO reaction rates (per unit surface area of catalyst),
which represents the intrinsic catalytic efficiency of catalysts,
were calculated to clarify whether the significant
enhancement of the catalytic activity arised from the different
Fig. 8 Dynamic changes of in situ DRIFTS of (a) Pt/ZrO2-K and (b) Pt/ZrO2-M
catalysts as a function of time in a flow of O2 + HCHO + N2 at room temperature.
Reaction conditions: 100 ppm HCHO, 20 % O2 and 74 % N2 (balance). The total
flow rate was 100 mL min−1
.
6 | J. Name., 2012, 00, 1-3
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and the formation of inert carbonate species in HCHO
oxidation.
4. Conclusions
In this study, a series of mixed phase Pt/ZrO2 catalysts were
prepared by simple methods. Compared with
a pure
monoclinic phase Pt/ZrO2-M catalyst, the catalytic activity of
the mixed phase system was enhanced significantly, and HCHO
can be completely oxidized into CO2 and H2O at near ambient
temperature. XRD, Raman, TEM, XPS, HRTEM, and H2-TPR
characterizations revealed that the m-t phase interface can
induce the formation of more defects and cationic Ptδ+ species.
It was also proved that the Ptδ+ adjacent to the interfacial
structure was more active than the Pt0 around the pure
monoclinic or tetragonal phases for HCHO oxidation.
Moreover, the DRIFTS results of the Pt/ZrO2-K and Pt/ZrO2-M
catalysts demonstrated that the special biphase structure can
change the reaction pathway and inhibit the formation of inert
carbonate species during HCHO oxidation. Thus, the enormous
increase in the catalytic activity of the mixed phase system can
be attributed to the presence of the m-t phase interface. Our
findings shed light on the interface effect of the mixed phase
Pt/ZrO2 catalyst, which may afford guidance for developing
high-efficiency catalysts to eliminate indoor HCHO by
designing mixed phase systems.
Scheme 1 The proposed catalytic mechanism of the Pt/ZrO2-K catalyst for HCHO
catalytic oxidation
carbonate species, respectively.51
Herein, a possible mechanism for HCHO catalytic oxidation
over Pt/ZrO2-K and Pt/ZrO2-M was proposed. From the
aforementioned characterizations, there were two active sites
(metallic and ionic Pt species) and Ptδ+ species adjacent to the
interfacial structure were the main active sites for the Pt/ZrO2-
K catalyst. However, for the Pt/ZrO2-M catalyst, Pt0 species
were the active sites and the activity of Ptδ+ species was
negligible, as reported in previous studies.3, 15, 37 According to
the DRIFTS results of Pt/ZrO2-K, adsorbed formate and
dioxymethylene (DOM) species were abundant and carbonate
species were absent on the Pt/ZrO2-K catalyst. As shown in
Scheme 1, when HCHO molecules were adsorbed on the
surface of Pt/ZrO2-K, DOM rapidly formed by the nucleophilic
attack of the oxygen atom derived from PtO adjacent to the m-
t interface of the support, accompanying the formation of Pt0.
Then, the DOM was easily converted into formate species
(HCOO-). Adsorbed oxygen species on the support of Pt/ZrO2-K
sample could be transferred rapidly to the perimeter of Pt
nanoparticles in close proximity to the biphase interface,
leading to the formation of active oxygen and Ptδ+ species. This
oxygen species could completely oxidize the adsorbed formate
species into CO2 and H2O. However, the incomplete oxidation
of HCOO- might occur for Pt/ZrO2-M in the form of inert
carbonate species according to the DRIFTS results, which did
not further decompose into CO2 and H2O (Scheme S1). More
importantly, carbonate species probably occupied the active
sites, resulting in low catalytic performance of Pt/ZrO2-M.53 By
this cycle, HCHO was converted into CO2 and H2O for Pt/ZrO2-K
and inert carbonate species, CO2 and H2O for Pt/ZrO2-M.
Considering the coexistence of Ptδ+ and Pt0 species on the
Pt/ZrO2-K catalyst, there was another pathway for HCHO
oxidation catalyzed by Pt0 species, which is similar to the
pathway above. However, it was believed that this pathway is
not the dominant process due to the low content and catalytic
activity of Pt0 species. Therefore, the Ptδ+ species in close
proximity to the mixed phase interface in the Pt/ZrO2-K
catalyst dominated the efficiency of the HCHO conversion into
CO2 and H2O. In addition, the low catalytic activity of Pt/ZrO2-
M was attributed to the absence of the interfacial structure
Acknowledgements
This project was supported by the National Key Research and
Development Program of China (2016YFC0202202), and the
National Natural Science Foundation of China (21407152,
21590811)
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High-efficiency Pt/ZrO2 catalysts with mixed phase were successfully prepared.
The catalysts exhibited high activity and interfacial structure can change reaction
pathway.