Enhancing formaldehyde oxidation on iridium catalysts using hydrogenated TiO2 supports

Article abstract of DOI:10.1039/c8nj04472d

As promising catalyst supports, hydrogenated metal-oxides can afford engineered surface architectures and electronic configurations to promote catalytic turnover on loading metals. Herein, hydrogenated TiO₂ (H-TiO₂) fabricated via Na

Full text of DOI:10.1039/c8nj04472d

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Enhancing formaldehyde oxidation on iridium
catalysts using hydrogenated TiO2 supports†
Cite this: New J. Chem., 2018,
42, 18381
Hang Cheong Chan, Ting Chen, Lifang Xie, Yijin Shu and Qingsheng Gao
*
As promising catalyst supports, hydrogenated metal-oxides can afford engineered surface architectures
and electronic configurations to promote catalytic turnover on loading metals. Herein, hydrogenated
TiO2 (H-TiO2) fabricated via NaBH4 reduction is successfully introduced to boost the catalytic oxidation of
formaldehyde (HCHO), which is one of the most common and harmful indoor volatile organic
compounds. Distinct from the parent TiO2, H-TiO2 features enriched surface hydroxyls and enhanced
metal–support interactions upon loading Ir, which are responsible for remarkable promotion in HCHO
oxidation. In particular, the abundant hydroxyls on H-TiO2 promote the generation of HCOOÀ, which is
believed to be the rate-determining step in HCHO oxidation. This discovery is anticipated to pave the way
for exploring high-performance metal catalysts on oxide supports via surface and interfacial engineering.
Received 2nd September 2018,
Accepted 5th October 2018
DOI: 10.1039/c8nj04472d
rsc.li/njc
surface hydroxyl groups on TiO2 remarkably enhanced HCHO
adsorption via strong hydrogen-bond interactions.
Introduction
As one of the most common and harmful indoor volatile organic
Since the first discovery of its unexpected photocatalytic
compounds (VOCs), formaldehyde (HCHO) can cause health activity in 2011,28 hydrogenated TiO2 (H-TiO2) has attracted
problems such as headaches, skin/eye irritation, chest pain, nasal great attention in catalysis, as either an active component or a
tumors, etc.1,2 To satisfy stringent environmental regulations, support.29,30 Effective hydrogen doping into pristine TiO2 can
great efforts have been made to remove HCHO.3–6 Catalytic result in surprising structural and chemical alterations, such as
oxidation to harmless H2O and CO2 is an effective method, in abundant hydroxyls and oxygen vacancies on surface, and
which the key is the choice of catalytic materials.6 Catalysts used tunable band states with metallic features.31–34 Due to the fact
in HCHO oxidation are mainly noble metals supported by oxides.7 that surface hydroxyls can strengthen HCHO chemisorption,27,34
On one hand, Pt has been extensively investigated and demon- such surface variations on H-TiO2 are highly anticipated to
strated owing to its high performance related to the valance promote the catalytic oxidation of HCHO. Zeng et al. identified
states.8–13 As another important element in the Pt-group, Ir promoted HCHO oxidation on H-TiO2,35 which however was
is also active in oxidation reactions owing to its largely-expanded used as the active species and thereby delivered a relatively lower
d-bands with rich vacancies.14–17 However, extensive efforts are activity in comparison with supported noble-metals. Moreover,
still desired to uncover its catalytic application and mechanism H-TiO2 with enriched d band states around the Fermi level (EF)28
in formaldehyde oxidation. On the other hand, metal-oxide would even modify the reactivity of loaded metals through
(e.g., TiO2, Fe2O3, ZrO2, MnOx–CeO2, SnO2, etc.) supports can electronic perturbations,36 leading to a more efficient catalytic
promote the catalytic activity of loaded noble-metals.18–24 Besides turnover.
their large surface that affords highly dispersed noble-metal
Herein, we developed nanosized H-TiO2 as reactive supports
centers, metal oxides more importantly provide available surface/ to enhance the catalytic oxidation of HCHO on Ir nanocatalysts.
interfacial interactions to enhance HCHO chemisorption and Upon a solid-state reduction by NaBH4,37 the as-obtained
consequently the turnover.12,21,23,25 For example, Huang et al. H-TiO2 nanoparticles feature enriched surface hydroxyls and
reported on an efficient oxidation of HCHO on Pt/TiO2 enhanced electronic interactions upon loading Ir. As expected,
catalysts,26 and the activity at room temperature was further the Ir/H-TiO2 is more active than the Ir/TiO2 counterpart in
improved after NaOH modification,27 because the introduced HCHO oxidation to CO2 and H2O. It also affords a satisfactory
stability in a long-term continuous test. An in situ diffuse
Department of Chemistry, College of Chemistry and Materials Science,
reflectance infrared Fourier transform spectroscopy (DRIFTS)
study further reveals the generation of HCOOÀ on Ir/H-TiO2,
Jinan University, Guangzhou 510632, China. E-mail: tqsgao@jnu.edu.cn
† Electronic supplementary information (ESI) available: UV-vis DRS, FT-IR and N2
which is believed to be the rate-determining step in HCHO
sorption isotherms of H-TiO2 and TiO2, CO-DRIFTS and CO-TPD of Ir/H-TiO2 and
oxidation. The abundant hydroxyls on H-TiO2 can help enhance
HCHO chemisorption and thus promote formate generation.
Ir/TiO2, and catalytic performance of the counterparts of Ir and Pt catalysts. See
DOI: 10.1039/c8nj04472d
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At the same time, the electronic metal–support interactions arising higher BET surface area of H-TiO2 (42.4 m2 gÀ1) than that of
from H-TiO2 facilitate intermediate conversion. Thereby, Ir/H-TiO2 TiO2 (28.8 m2 gÀ1, Fig. S2, ESI†).
affords favored kinetics and improved catalytic activity. Elucidating
X-ray photoelectron spectroscopy (XPS) was further conducted
the clear promotion by hydrogenated metal oxide supports, this to understand the surface variation of TiO2 after NaBH4 reduction.
work will open up new opportunities to design high-performance The H-TiO2 (Fig. 1d) presents two peaks associated with Ti 2p3/2
heterogeneous catalysts via support engineering.
(458.3 eV) and Ti 2p1/2 (464.0 eV),39,40 which are slightly red-
shifted in comparison with those for TiO2 (458.6 and 464.3 eV).
This indicated that the surface Ti4+ species is partially reduced, in
good accordance with the previous reports.37 Meanwhile, the O 1s
profiles can be deconvoluted into three peaks (Fig. 1e). The peaks
Results and discussion
In comparison with the reduction by high-pressure H2 or high- at 530.0 and 531.0 eV are ascribed to the oxygen in the TiO2 lattice
temperature Al vapor,28,38 a recently developed route using (Ti–O–Ti) and the surface hydroxyls (Ti–OH),23,34,41 respectively,
NaBH4 is obviously facile and easy in operation.37 It is thus and the one at 532.8 eV is associated with the O 1s from the SiO2
adopted in this work to reduce white TiO2 to black H-TiO2, and impurity existing in the TiO2 precursor.42 SiO2 purity is inert
the residual NaBH4 and derivatives are removed after soaking in catalysis, and can be drastically removed in NaBH4 aqueous
in deionized water. The UV-vis diffuse reflection spectra (UV-vis solutions.43 Both of XPS (Fig. 1e) and X-ray fluorescence (Table S1,
DRS) accordingly show a broad absorption for the as-obtained ESI†) measurements show the obvious decrease of the SiO2
samples, starting at B400 nm and extending to the near content in H-TiO2. Thus, SiO2 impurity is ignored in the following
infrared region (Fig. S1, ESI†). This is consistent with the color discussion. Remarkably, the H-TiO2 presents the more obvious
change of the samples (insets of Fig. 1a and b), and suggests the signal for the O 1s of Ti–OH, indicating the enriched hydroxyls on
successful fabrication of H-TiO2. Fig. 1a and b display the the H-TiO2 surface. As determined by quantitative XPS analysis,
scanning electron microscopy (SEM) images of the parent the value of OTi–OH/Olattice increases from 0.21 on TiO2 to 0.38 on
TiO2 and the as-obtained H-TiO2, respectively, in which similar H-TiO2, agreeing with the observation in ref. 28.
nanoparticles with a size of tens of nanometers are observed. It
The above variation can be further confirmed by Fourier
is implied that the reduction by NaBH4 causes a negligible transform infrared (FT-IR) spectroscopy (Fig. S3, ESI†). The
alteration on the nanostructures. Meanwhile, the X-ray diffrac- visible band at 3700 cmÀ1, associated with the O–H stretching
tion (XRD) pattern of the H-TiO2 shows a rutile phase (JCPDS vibration, is observable for H-TiO2, but negligible for TiO2.
No. 12-1276), which is consistent with that of the TiO2 (Fig. 1c). Meanwhile, we performed TPO analysis on both TiO2 and H-TiO2
However, the relatively weaker diffraction peaks for the H-TiO2 (Fig. 1f), which confirms the above-mentioned variations on the
still indicate the degradation of the crystalline structures. The surface. The TiO2 presents a negligible signal in the range of
resulting structural defects will be responsible for the slightly 30–700 1C. By contrast, the H-TiO2 shows an obvious oxidation peak
Fig. 1 SEM images of (a) TiO2 and (b) H-TiO2 and (insets of a and b) the corresponding digital photos. (c) XRD patterns. (d) Ti 2p and (e) O 1s XPS profiles
of H-TiO2 and TiO2. (f) TPO profiles of H-TiO2 and TiO2.
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at 455 1C. And a negative peak at around 550 1C observed for H-TiO2
is possibly due to the dehydration from the surface hydroxyls.
Using a typical impregnation procedure, Ir nanoparticles were
successfully loaded onto TiO2 and H-TiO2. The transmission
electron microscopy (TEM) images of the Ir/TiO2 and Ir/H-TiO2
with a similar Ir loading (0.8 Æ 0.1 wt%) are displayed in Fig. 2a
and b, respectively. The Ir nanoparticles (as marked in the TEM
images) present a consistent distribution, with an average size
of 2.0 Æ 0.1 nm. Such highly dispersed metallic Ir is beneficial
for the catalytic performance. Meanwhile, the lattice fringes of
0.32 and 0.22 nm observed in their high-resolution TEM images
(HR-TEM, Fig. 2c and d) can be ascribed to rutile TiO2(110) and
Ir(111), respectively. Moreover, the above samples were analyzed
by means of energy dispersive spectroscopy (EDS) and elemental
mapping in conjunction with TEM. The EDS profiles clearly reveal
the presence of Ti, O, and Ir, and the signal for Cu should be
ascribed to the supporting grid used in TEM (Fig. S4, ESI†). The
elemental mapping images show the uniform distribution of Ti,
O, and Ir (Fig. 2e and f), confirming the good dispersion of Ir on
both TiO2 and H-TiO2 supports.
Fig. 3 (a) H2-TPR profiles of H2IrCl6/H-TiO2 and H2IrCl6/TiO2. (b) Ir 4f XPS
profiles for 0.9% Ir/TiO2 and 0.7% Ir/H-TiO2.
H2 temperature-programmed reduction (H2-TPR) was con-
ducted to investigate the metal–support interactions in Ir/TiO2
and Ir/H-TiO2. As shown in Fig. 3a, the H2IrCl6/TiO2 presents a
series of peaks at 162 1C, 197 1C, 246 1C and 418 1C. According consumption for Ir4+ reduction at 154 1C, 199 1C, and 370 1C,
to the previous reports, the reduction of Ir4+ species usually lower than those on TiO2. Such a relatively easier reduction at a
occurs below 300 1C.44,45 Thus, the H2 consumption at 418 1C is lower temperature indicates the enhanced metal–support inter-
possibly associated with the species stabilized by the TiO2 supports. actions arising from H-TiO2. In addition, a new peak is observed
In comparison, the H2IrCl6/H-TiO2 clearly presents H2 at 560 1C, which is possibly ascribed to the H2 spillover from the
in situ generated Ir to the defect-rich H-TiO2 surface.46
Metal–support interactions were further interpreted by XPS
and CO-DRIFTS. The XPS analysis shows the chemical state of
Ir affected by the H-TiO2 and TiO2 supports. As shown in
Fig. 3b, the Ir 4f profiles are deconvoluted into four peaks,
indicating the presence of two states of Ir on the surface. The
peaks located at 60.6 and 63.6 eV are ascribed to 4f7/2 and 4f5/2
of metallic Ir0, respectively, and those at 62.0 and 64.9 eV are
associated with the positively charged Ir (Ird+).47 According to
the XPS quantitative analysis, the Ir/H-TiO2 presents a slightly
higher Ir0/Ird+ ratio (1.71) than that of Ir/TiO2 (1.36). Previous
theoretical calculations have attested that the enriched band
states around the EF after introducing H into TiO2,28 and the
consequently down-shifted d-band center will be beneficial for
electron penetration into Ir.48 Wang and co-workers have dis-
covered strong interactions between single-atomic Au and
defective TiO2 nanosheets, which however promoted the for-
mation of Aud+ species.49 This different observation was possi-
bly associated with the single-atom nature. And in this work,
the Ir nanoparticles with a size of B2.0 nm suffer from charge
redistribution on the interfaces with H-TiO2, resulting in a
slightly increasing metallic Ir0 content. Moreover, the CO-DRIFTS
spectra present the consistent interactions on H-TiO2 (Fig. S5,
ESI†). The Ir/H-TiO2 presents two absorption bands at 1660
and 2000 cmÀ1, associated with the CQO stretching in the
Fig. 2 (a and b) TEM and (c and d) HR-TEM images of (a and c) 0.9% Ir/TiO2
and (b and d) 0.7% Ir/H-TiO2, and (e and f) the corresponding elemental
linear- and bridge-type CO on the Ir sites,50 respectively. These
bands visibly red-shift in comparison with those for Ir/TiO2
(1730 and 2040 cmÀ1). The higher Ir0/Ird+ on Ir/H-TiO2 determined
mapping (e: 0.9% Ir/TiO2; f: 0.7% Ir/H-TiO2). The Ir nanoparticles are marked
in the TEM images.
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Fig. 4 (a) Catalytic HCHO oxidation on Ir/TiO2, Ir/H-TiO2, TiO2 and H-TiO2 at different temperatures, and (b) the performance relying on the Ir loading
at 100 1C. (c) Arrhenius plots of 0.7% Ir/H-TiO2 and 0.9% Ir/TiO2. (d) HCHO oxidation on 0.7% Ir/H-TiO2 at 120 1C with time of stream.
by XPS indicates more electrons in the Ir sites, as compared with reaction rates against the reciprocal temperature (300–473 K), in
Ir/TiO2. Thereby, the improved electronic density on the Ir sites will which all the loaded Ir is assumed to contribute to the catalytic
promote electron feedback from dIr to 2p*CQO, which weakens the oxidation. As determined, the Ir/H-TiO2 shows a lower Ea of
CQO bond and contributes to its red-shift in the DRIFTS spectra.51 2.5 kJ molÀ1, in comparison with that of Ir/TiO2 (12.1 kJ molÀ1).
The catalytic HCHO oxidation was examined on TiO2, H-TiO2, This is in agreement with the high activity for HCHO oxidation
Ir/TiO2 and Ir/H-TiO2. Fig. 4a displays the HCHO conversion as a on the Ir/H-TiO2, and also illustrates the improved intrinsic
function of reaction temperature. Both of TiO2 and H-TiO2 present activity of Ir on the H-TiO2 supports.
a negligible activity in the temperature range of 50–250 1C. After
Moreover, the Ir/H-TiO2 also affords a satisfactory stability in
loading Ir, the catalysts show an obvious improvement in the HCHO oxidation (Fig. 4d). It shows negligible deactivation in a
HCHO conversion, confirming Ir to be the main active species. continuous test for more than 140 hours. The HCHO conver-
Only the oxidation products CO2 and H2O are detected in this sion is maintained above 95%. The Ir/H-TiO2 after reaction is
reaction. Remarkably, the Ir/H-TiO2 enables much higher HCHO further characterized (Fig. S7, ESI†). The negligible variation in
conversion than Ir/TiO2, which suggests the promotion due to the crystalline structure of H-TiO2 is confirmed by XRD analysis.
H-TiO2 that features abundant surface hydroxyls and enhanced And XPS analysis reveals the well-retained chemical states of the
metal–support interactions.
elements (Ti, O and Ir) after reactions, which contribute to the
The Ir loading on TiO2 and H-TiO2 was further monitored to high activity in the long-term test.
optimize their catalytic performance, in which their activity at
To understand the promotion by the H-TiO2 support, an
100 1C was taken for comparison. Visibly, there is a maximum in situ DRIFTS test was performed over Ir/TiO2 and Ir/H-TiO2
HCHO conversion on the Ir/H-TiO2 with an optimal Ir loading of (Fig. 5a and b). The catalysts are exposed to gaseous O2/N2
0.7%, reaching almost complete conversion at a space velocity of containing 48 ppm HCHO at 30 1C, and then DRIFTS spectra were
20 000 hÀ1. The above volcano plot is consistent with the quantitative recorded after different times. On the Ir/H-TiO2, the observable
CO desorption from Ir/H-TiO2 (Fig. S6a and b, ESI†), indicating band at 1530 cmÀ1 can be ascribed to the vibration of COOÀ in
that the high Ir dispersity resulting from the optimized metal– formate, indicating the HCHO oxidation via the formate
support interactions accounts for the efficient catalytic turn- intermediate.12,53 And the bands corresponding to the linear
over. By contrast, the Ir/TiO2 delivers the improved activity and bridged modes of CO chemisorption on Ir emerge with
with increasing Ir loading, which is always inferior to that of reaction time, which indicates the further reaction of formate
Ir/H-TiO2. This trend is consistent with the increasing CO into CO via dehydration.54 By contrast, the band for COOÀ is
desorption in the CO-TPD analysis (Fig. S6c and d, ESI†).
negligible on the Ir/TiO2 (Fig. 5b), implying a different reaction
Furthermore, the kinetics on Ir/TiO2 and Ir/H-TiO2 was pathway on Ir/H-TiO2. Along the reaction time, the bands for
studied with an assumption of pseudo-first order reaction in CQO adsorbed on the Ir surface are observed,55 which should be
HCHO oxidation.52 Fig. 4c shows the plots of the logarithm of generated from the direct dehydrogenation of HCHO on Ir.
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Fig. 5 In situ HCHO-DRIFTS of (a) 0.9% Ir/TiO2 and (b) 0.7% Ir/H-TiO2 after being exposed to a gas mixture of 48 ppm HCHO + N2 + O2. Schematic
illustrations of the reaction pathway for HCHO oxidation on (c) Ir/H-TiO2 and (d) Ir/TiO2.
For HCHO oxidation, the hydroxyl groups on the catalyst owing to the enhanced chemisorption of HCHO by the rich
surface play a key role in the generation of the formate intermediate surface hydroxyls on H-TiO2. Meanwhile, the Pt/H-TiO2 retains
that has been identified on Ir/H-TiO2 by DRIFTS. We, therefore, a higher Pt0/Ptd+ surface ratio (2.90) than Pt/TiO2 (2.65) due to
propose a reaction mechanism as depicted in Fig. 5c. The hydroxyls the enhanced metal–support interactions by H-TiO2. This also
on H-TiO2 will promote the chemisorption of HCHO via strong benefits the activation of O2.
hydrogen-bonding interactions, and the atomic O* on the Ir surface
from O2 dissociation will attack the chemisorbed HCHO,
subsequently producing the formate intermediate. After formate
Conclusions
dehydration, CO* is formed on the Ir surface, and is further oxidized
In summary, black H-TiO2 nanoparticles with rich surface hydroxyls
by the O* via the Mars–van Krevelen (MvK) mechanism.12,56
were fabricated and utilized as robust supports for Ir, accomplishing
Because of the formate intermediate, the HCHO oxidation
the obviously improved activity for HCHO oxidation. In comparison
kinetics is well promoted, as indicated by the relatively low Ea.
with the parent TiO2, the H-TiO2 features enriched surface hydroxyls
In comparison, Ir/TiO2 suffers from a disfavored kinetics due to
and enhanced metal–support interactions upon loading Ir, which
the absence of rich surface hydroxyls (Fig. 5d). In addition, the
are responsible for the remarkable improvement in the catalytic
metal–support interactions enhanced by H-TiO2 have enriched
performance. The abundant hydroxyls on H-TiO2 can enhance
main metallic species on Ir nanoparticles, which not only
HCHO chemisorption and formate formation, and the enhanced
promote the formation and dehydration of the formate inter-
metal–support interactions associated with H-TiO2 facilitate the
mediate, but also activate O2 to form highly active O*. This
further conversion of the intermediates. Thereby, the Ir/H-TiO2
feature is beneficial for the oxidation of HCHO.
affords favored kinetics and improved catalytic activity. In future
The promotion by surface hydroxyls in HCHO oxidation can
work, such innovation can be extended to mesoporous H-TiO2 with
be further confirmed by control experiments where NaOH
large surface areas and rich mesoporosity, accomplishing further
is used to modify TiO2 supports. As indicated by XPS analysis
improvement in catalysis. This work is anticipated to open up new
(Fig. S8, ESI†), such a modification results in enriched –OH on
opportunities for the exploration of high-performance materials in
the TiO2 surface. Accordingly, the Ir supported by NaOH-treated
catalysis and environmental science.
TiO2 enables an obviously improved HCHO conversion in
comparison with Ir/TiO2. This is in accordance with the pre-
viously reported Pt/TiO2 catalysts.27
As reactive supports, the H-TiO2 can also enhance HCHO
oxidation on other metals, e.g., Pt, which highlights its surface/
Experimental
Materials
interfacial functionalities. The Pt/H-TiO2 delivers superior All reagents were purchased from commercial sources and in
activity for HCHO oxidation to Pt/TiO2 (Fig. S9, ESI†). This is analytical or reagent grade when possible. TiO2 and H2IrCl6
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were purchased from Aladdin Industrial Corporation (Shanghai, Catalytic tests
China). NaBH4, ethanol and formaldehyde solution were pur-
The formaldehyde oxidation reaction was carried out in a fixed
bed quartz tube reactor (id = 5 mm) with 0.2 g of catalysts
(40–60 mesh). HCHO was produced by passing a stream of
99.999% N2 through a bubbler containing a 40 wt% HCHO
aqueous solution in a 25 1C water bath. The total flow of gas was
100 mL minÀ1, containing around 580 ppm of formaldehyde
balanced by 21% O2/N2. The space velocity was accordingly
20000 hÀ1. The conversion of HCHO was analysed using a gas
chromatograph (FL9790 II). For kinetics analysis, the apparent
activation energies (Ea) for oxidation over Ir/H-TiO2 and Ir/TiO2
were calculated by the Arrhenius equation, with an assumption
that all of the Ir contributes to the catalytic oxidation.
chased from Sinopharm Chemical Reagent Co. Ltd (Shanghai,
China). All aqueous solutions were prepared using ultrapure water
(418.2 MO).
Synthesis of the catalyst
The H-TiO2 nanoparticles were fabricated according to the
previous report.37 Typically, 1.0 g of TiO2 was mixed with
1.2 g of NaBH4, and then heated at 350 1C for 1 hour under
an Ar flow. NaBH4 is in excess to ensure the effective hydro-
genation of TiO2. The as-obtained black solid was soaked in
deionized water for 3 days. And then, H-TiO2 was obtained after
centrifugation. To fabricate Ir/H-TiO2, Ir/TiO2, Pt/H-TiO2 and
Pt/TiO2, the corresponding supports were impregnated with an
H2IrCl6 or H2PtCl6 aqueous solution and then stirred at 80 1C
for 4 hours. The samples were dried at 50 1C under vacuum
overnight, followed by reduction with a stream of H2/Ar
(5 vol%) at 300 1C for 2 hours.
In a control experiment using NaOH to modify TiO2, 1.0 g of
TiO2 was added into 100 mL of a NaOH solution (0.5 M) under
stirring for 30 min. And then, the solid was filtered and dried at
80 1C for 6 hours. To load Ir nanoparticles, the same procedure
as that used for Ir/H-TiO2 was adopted.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work is financially supported by the National Natural Science
Foundation of China (Grant No. 21773093 and 21433002), the
Guangdong Natural Science Funds for Distinguished Young
Scholar (Grant No. 2015A030306014), the Guangdong Program
for Support of Top-notch Young Professionals (Grant No.
2014TQ01N036), and the Science and Technology Program of
Guangzhou (Grant No. 201707010268).
Physical measurements
X-ray diffraction (XRD) measurements were performed on a
Bruker D8 powder X-ray diffractometer with Cu Ka radiation
(l = 0.15419 nm). Scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) investigations were
carried out using ZEISS ULTRA55 and JEOL JEM-2100F, respec-
tively. Energy dispersive spectroscopy (EDS) coupled with TEM
was carried out on a JEOL JEM-2100F. X-ray photoelectron
spectroscopy (XPS) analysis was performed on a Thermo
scientific Escalab 250Xi, with C 1s (284.6 eV) as a reference.
The UV-vis diffuse reflection spectra (UV-vis DRS) were recorded
on a Varian Cary 5000 at room temperature. The Brunauer–
Emmett–Teller (BET) specific surface areas were determined by
N2 adsorption–desorption measurements at liquid nitrogen
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