Alkali-modified non-precious metal 3D-NiCo2O4 nanosheets for efficient formaldehyde oxidation at low temperature

Article abstract of DOI:10.1039/c5ta09370h

Cost-effective catalysts for volatile organic compound (VOC) oxidation are critical to energy conversion and environmental protection. Herein, we developed new, low-cost and high-performance alkali-promoted 3D-NiCo₂O₄ nanosheet catalysts for HCHO oxidation at room temperature. Benefiting from the large surface area, high adsorption capacity and surface hydroxyls, the alkali-promoted 3D-NiCo₂O₄ nanosheet catalysts show substantially high catalytic activities for HCHO oxidation. The alkali-promoted 3D-NiCo₂O₄ nanosheets yield a remarkable HCHO conversion efficiency of 95.3% at room temperature, which is not achieved by any non-precious metal based catalysts at such low temperature. Additionally, the as-prepared alkali-promoted 3D-NiCo₂O₄ nanosheets retained excellent catalytic performance after 200 h, which can be applied to practical applications. This work provides a feasible approach to improve the efficiency of metal oxides for HCHO oxidation at low temperature.

Full text of DOI:10.1039/c5ta09370h

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Alkali-modified non-precious metal 3D-NiCo2O4
nanosheets for efficient formaldehyde oxidation at
low temperature†
Cite this: DOI: 10.1039/c5ta09370h
Received 19th November 2015
Accepted 12th January 2016
Yongchao Huang, Wenjie Fan, Bei Long, Haibo Li, Weitao Qiu, Fengyi Zhao,
*
*
Yexiang Tong and Hongbing Ji
DOI: 10.1039/c5ta09370h
www.rsc.org/MaterialsA
Cost-effective catalysts for volatile organic compound (VOC) oxida- signicantly promote the performance of HCHO oxidation by
tion are critical to energy conversion and environmental protection. enhancing the reaction between surface OHꢀ and formate
Herein, we developed new, low-cost and high-performance alkali- species at room temperature.13 Nie et al. found that surface
promoted 3D-NiCo2O4 nanosheet catalysts for HCHO oxidation at hydroxyls can remarkably enhance the catalytic performance of
room temperature. Benefiting from the large surface area, high Pt/TiO2 toward HCHO oxidation at room temperature.14
adsorption capacity and surface hydroxyls, the alkali-promoted 3D- Avgouropoulos et al.15 also reported alkali promoted Pt/Al2O3
NiCo2O4 nanosheet catalysts show substantially high catalytic activi- catalysts for the complete oxidation of ethanol. There is
ties for HCHO oxidation. The alkali-promoted 3D-NiCo2O4 nano- a similar case that alkali obviously enhanced the catalytic
sheets yield a remarkable HCHO conversion efficiency of 95.3% at performance of Ag/Co3O4 for HCHO oxidation due to more
room temperature, which is not achieved by any non-precious metal surface OHꢀ species on the catalyst surface.16 However, due to
based catalysts at such low temperature. Additionally, the as-prepared the scarcity and expensive nature of noble metals, it is exceed-
alkali-promoted 3D-NiCo2O4 nanosheets retained excellent catalytic ingly desirable to remove HCHO at low temperature using
performance after 200 h, which can be applied to practical applica- noble-metal-free catalysts.
tions. This work provides a feasible approach to improve the efficiency
Recently, transition metal oxides especially those having
large surface areas and high redox properties had been exten-
sively studied as promising materials for HCHO oxidation.17,18
Bai et al. reported that 3D-Co3O4 had much higher catalytic
activity for HCHO oxidation due to its large specic surface
area.19 Additionally, as a binary metal oxide, the spinel 3D-
Mn0.75Co2.25O4 possesses much higher activity because that the
coupling of two metal species could provide BMOs with rich
redox reactions, which are benecial to catalytic oxidation
applications.20 Compared with noble metal-based catalysts, the
activities of transition metal oxides are not satisfactory. There-
fore, it is a great challenge to develop transition metal oxide
catalysts with high efficiency for HCHO oxidation at low
temperature.
Spinel nickel cobaltite (NiCo2O4) has been conceived as
a promising cost effective and scalable alternative since it offers
many advantages such as low cost, abundant resources and
environmental friendliness. In its structure, the Co2+ along with
the Co3+ at the A-sites and Ni3+ along with the Ni2+ at the B-sites
provide a notable catalytic activity (Scheme 1). Herein, we
address novel alkali-promoted 3D-NiCo2O4 nanosheets for
HCHO oxidation. Signicantly, the catalyst could convert 95.3%
of HCHO at a low temperature of 25 ꢁC, which is not achieved by
any non-precious metal based catalysts at such low tempera-
ture. The large surface area, high adsorption capacity and
of metal oxides for HCHO oxidation at low temperature.
Introduction
Formaldehyde (HCHO) is one of the major volatile organic
compounds in indoor air, which signicantly impacts indoor
air quality thus inuencing human health.1–4 Long-term expo-
sure to HCHO will be detrimental to human health causing
headache, pneumonia, and lung cancer. Among the methods
employed in the removal of HCHO, catalytic oxidation had been
an outstanding approach to convert HCHO into nontoxic
products such as CO2 and H2O. Recently, noble metal-based
catalysts (Pt, Au, and Pd) had been validated to display excellent
catalytic performance for HCHO oxidation at room tempera-
ture.5–9 Moreover, superb catalytic activities were achieved by
surface modication with alkali.10–12 Zhang et al. demonstrated
that the addition of alkali-metal ions to the Pt/TiO2 catalyst can
MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of
Environment and Energy Chemistry, The Key Lab of Low-carbon Chem & Energy
Conservation of Guangdong Province, School of Chemistry and Chemical
Engineering, Sun Yat-Sen University, 135 Xingang West Road, Guangzhou 510275,
P. R. China. E-mail: jihb@mail.sysu.edu.cn; chedhx@mail.sysu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ta09370h
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TPR prole of the samples was recorded between 35 ꢁC and 600
ꢁC at a heating rate of 10 ꢁC minꢀ1 in 10% hydrogen in N2 with
a ow rate of 50 mL minꢀ1. Hydrogen uptake was monitored by
using a TCD detector. Specic surface areas (SBETs) of the
catalysts were calculated from a multipoint Brunauer–Emmett–
Teller (BET) analysis of the nitrogen adsorption and desorption
isotherms at 77 K recorded on an Autosorb-1 apparatus. The
Raman spectra were collected with a Laser Micro-Raman Spec-
trometer (Raman, Renishaw inVia). Fourier transform infrared
spectra were recorded with a Thermo Nicolet Nexus using the
KBr disk method. In situ diffuse reectance infrared Fourier
transformed spectra were measured on an EQVINOX-55 FFT
spectroscope apparatus (Bruker) equipped with a diffuse
reectance accessory and a MCT detector. The sample was
placed in a ceramic crucible in the chamber with O2 20 vol%,
about 35% relative humidity, He balance, and a total ow rate of
Scheme 1 Schematic unit cell structure of spinel-structured NiCo2O4.
surface hydroxyls are suggested to greatly inuence the catalytic
performance of alkali-promoted 3D-NiCo2O4 nanosheets.
Additionally, the surface OHꢀ plays the main role in the reac-
tion path of HCHO oxidation, which can directly react with
formate species to produce CO2 and H2O.
100 mL minꢀ1
.
Thermal catalytic reaction tests
Experimental section
Preparation of NiCo2O4 nanosheets
The HCHO oxidation was performed in a xed-bed reactor with
0.2 g catalyst. HCHO gas was generated and injected into an
incubator (lled with a solution of 37% HCHO) maintained at
In a typical process, Ni(NO3)2$6H2O (0.3 g), Co(NO3)$6H2O (0.6
g), and hexamethylenetetramine (0.8 g) were dissolved in an
ethanol/water solution (50 mL) with a volume ratio of 3 : 2. Aer
being magnetically stirred for 30 min, the as-obtained homo-
geneous solution was transferred into a 100 mL Teon-lined
stainless steel autoclave, sealed and maintained at 95 ꢁC for 8 h
to yield a Ni–Co precursor. Subsequently, the precursor was
collected by centrifugation and washed thoroughly with water
and ethyl alcohol several times and dried at 60 ꢁC for 12 h. And
then the precursor was calcined in an air atmosphere at 400 ꢁC
0 C, using a puried air ow (N2/O2 ¼ 4100 mL minꢀ1). A gas
ꢁ
hourly space velocity (GHSV) of 60 000 mL hꢀ1 was applied. The
products of the reaction were analyzed online by using an Agi-
lent 7890A gas chromatograph with a TCD detector and a Por-
apak-Q column. No other carbon-containing compounds except
CO2 in the products were detected for all the tested catalysts.
The HCHO conversion was calculated from the CO2 content as
follows:
HCHO conversion (%) ¼ [CO2]out/[HCHO]in ꢂ 100
ꢀ1
ꢁ
for 3 h with 2 C min heating rate to obtain NiCo2O4 nano-
sheets. The NiCo2O4 nanosheets (0.2 g) were immersed in an
aqueous solution of NaOH (KOH) (100 mL) with different
concentrations (0.1, 0.5, 1, 2, and 4 M) at 80 ꢁC. Aer stirring for
5 h, the catalysts were centrifuged and washed with water, and
then dried at 60 ꢁC. The resulting catalysts were denoted as
NiCo2O4-0.1, NiCo2O4-0.5, NiCo2O4-1, NiCo2O4-2, and NiCo2O4-
4 according to the concentration of NaOH.
[CO2]out and [HCHO]in in the formula are the CO2 concentration
in the products and the HCHO concentration in the reactor,
respectively.
Results and discussion
Synthesis and characterization of all the catalysts
3D-NiCo2O4 nanosheets were synthesized by a hydrothermal
method and a post-calcination treatment (details in the Exper-
Characterization
X-ray diffraction (XRD) measurements were performed using imental section). The structural information of the samples was
a Broker's D8 ADVANCE powder X-ray diffractometer with Cu K revealed by X-ray diffraction analysis (Fig. 1a), which veried
˚
radiation (l ¼ 1.5418 A). Transmission electron microscopy that the nal sample was spinel-structured NiCo2O4 (JCPDS 20-
(TEM) images were obtained with a JEM2010-HR electron 0781). NiCo2O4-X samples (X represents the concentration of
microscope. The morphology of the samples was investigated by NaOH) were obtained from the prepared NiCo2O4 by surface
using a eld emission scanning electron microscope. Atomic modication with different concentrations of alkali. Surpris-
force microscopy (AFM) images were obtained using a SPM- ingly, the phase was not changed and no residues or contami-
9500J3 microscope. X-ray photoelectron spectroscopy (XPS) nants had been detected, revealing the high purity of the
measurements were performed on an ESCALab250 XPS system samples (Fig. 1b). The broad diffraction peaks and the inten-
with an Al Ka source and a charge neutralizer, and all the sities are weak, which indicates the low degree of crystallization
binding energies were referenced to the C 1s peak at 284.8 eV of and the small crystallite size of all NiCo2O4 nanosheets. In order
the surface adventurous carbon. Temperature-programmed to obtain the detailed structural information of the samples,
reduction (TPR) analysis was conducted on a T-5080 Autochem FTIR and Raman spectroscopies were carried out. The Raman
analyzer. About 50 mg of the sample was loaded in a tube- spectra show three intense peaks at about 470, 550 and 643
shaped quartz cell above a small amount of quartz wool. The cmꢀ1, which are attributed to Ni–Co–O (Fig. 1c). The peaks of
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Fig. 1 (a) XRD spectra of the Ni–Co precursor and NiCo2O4. (b) XRD
spectra of all the samples. (c) Raman spectra and (d) FTIR spectra of all
the catalysts.
Fig. 2 (a) SEM image and (b) AFM images of NiCo2O4 nanosheets. (c)
SEM image and (d) AFM images of NiCo2O4-1 nanosheets.
NiCo2O4-X are much stronger than those of NiCo2O4 nano-
sheets, which may be due to the fact that the alkali can enhance
the interaction between Ni–Co–O. Aer NaOH treatment, the
peaks were shied. The amount of disorder in these materials,
both structural and electronic, contributed to frequency shis
and band broadening. Additionally, a new peak at around 330
cmꢀ1 was found for NiCo2O4-1, NiCo2O4-2, and NiCo2O4-4
samples, which has been assigned to the lattice mode involving
mostly the displacement of oxygen atoms at octahedral
sites.21–23 Moreover, from FTIR spectra of catalysts, the peaks at
around 662 and 565 cmꢀ1 are ascribed to the metal–oxygen
vibrations of the NiCo2O4 (Fig. 1d).24,25 Furthermore, the more
detailed elemental composition and the oxidation state of
NiCo2O4 and NiCo2O4-X were further characterized by X-ray
photoelectron (XPS) measurements. The surface spectra
collected from the samples suggest that there are only Ni, Co,
and O in the catalysts, revealing the high purity of the samples.
In the Ni 2p spectrum (Fig. S1b†), the tting peaks at 853.5 and
871 are indexed to Ni2+, while those at 855.5 and 873.8 belong to
Ni3+. The Co 2p emission spectrum (Fig. S1c†) was tted with
spin–orbit doublets, indexed to Co2+ and Co3+. These results
show that Co3+/Co2+ and Ni3+/Ni2+ were present in the NiCo2O4
nanosheets, which is consistent with the results in the literature
for NiCo2O4.26,27 Battle et al. found that the formula of NiCo2O4
can be considered to be Co(3ꢀd)+[Ni(2+d)+Co3+]O42ꢀ, which has
the redox couples Co3+/Co2+ and Ni3+/Ni2+ and provides
a notable catalytic activity.28,29 All the aforementioned results
unambiguously reveal that aer surface modication with
alkali the structure of NiCo2O4 is preserved.
nanosheets have a thickness around 3.6 nm, indicating their
ultrathin nature, as also evidenced by AFM (Fig. 2b). Surpris-
ingly, it can be seen that the thickness of the NiCo2O4-1 nano-
sheets increased to 6.5 nm aer the NaOH treatment. And the
surface of nanosheets turns rough (Fig. 2c and d). The high
resolution TEM image shows high crystallinity with an inter-
planar spacing of 0.468 nm, which is attributed to the d-spacing
of the (111) planes of NiCo2O4 (Fig. 2c). The selected-area elec-
tron diffraction pattern shows well-dened diffraction rings,
suggesting their polycrystalline characteristics. Signicantly, an
unexpected increase in thickness was found in NiCo2O4-X
nanosheets, as displayed in Fig. 3b and S2.† In comparison with
untreated NiCo2O4 nanosheets, the surface and inside of
NiCo2O4-1 become disordered, which is revealed in Fig. 3d.
The results of TEM-EDS mapping of NiCo2O4-1 nanosheets
reveal the homogeneous distribution of only Ni, Co, and O in
the whole selection area of porous architectures, also suggest-
ing the high purity of the samples (Fig. 3e). Moreover, the
nitrogen adsorption and desorption isotherms of catalysts are
shown in Fig. S3,† which belong to the IV isotherm with
a hysteresis loop, indicating the presence of a porous structure
The morphology of the catalysts was studied by scanning
electron microscopy (SEM), atomic force microscopy (AFM) and
transmission electron microscopy (TEM). The SEM image in
Fig. 2a suggests that the NiCo2O4 possesses porous architec-
tures that were assembled by many nanosheets. Such porous
structures afford very large active surface areas, facilitating the
diffusion of guest molecules. The NiCo2O4 nanosheets show
a folding silk-like morphology with transparent features and the
Fig. 3 TEM images of (a) untreated NiCo2O4 nanosheets and (b)
NiCo2O4-1 nanosheets. HRTEM images of (c) untreated NiCo2O4
nanosheets and (d) NiCo2O4-1 nanosheets. (e) EDS elemental mapping
of the same region, indicating spatial distribution of Co (red), Ni (green)
and O (yellow).
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in the catalysts. And the pore volume is about 0.164–0.265 cm3 untreated NiCo2O4 and NiCo2O4-X catalysts. There are three
gꢀ1. The results of the BET analysis reveal that the surface area peaks in the temperature range between 100 and 500 C. They
ꢁ
of untreated NiCo2O4 nanosheets (81.54 m2 gꢀ1) is greater than are distinguished corresponding to the consecutive stepwise
that of NiCo2O4-X nanosheets (71.69–48.19 m2 gꢀ1) (Table reduction of Ni2+, Co3+, and Co2+ cations, statistically distrib-
S1†).30,31 The large surface area of NiCo2O4-X nanosheets may be uted among tetrahedral and octahedral spinel positions.
one of the reasons why NiCo2O4-X nanosheets have such Importantly, a signicant enhancement in the reducibility was
excellent efficiency. The higher surface area could offer more observed for the NiCo2O4-1 nanosheets, which have a reduction
active catalytic sites for HCHO oxidation.32
band at ca. 195 ꢁC. It is reasonable to deduce that the peak at ca.
195 ꢁC is due to the presence of surface OHꢀ, leading to
enhanced catalytic activity in oxidation reactions. From the
above discussion we believe that the OHꢀ was bonded to the
surface of NiCo2O4 nanosheets rather than in the form of
a crystal in the bulk. The absorption capabilities of HCHO gas
were also studied to investigate the existence of surface OHꢀ on
the NiCo2O4-X nanosheet catalysts (Fig. 4c). The HCHO
adsorption capability over NiCo2O4 is only 9.2%, while the
NiCo2O4-1 nanosheets have the largest absorption capability
(15.5%) under the same conditions, indicating that the surface
OHꢀ could obviously improve the adsorption capability toward
HCHO. Combining with the XPS, TPR and absorption capability
measurements above, we can conclude that surface OHꢀs are
successfully introduced to the NiCo2O4-X nanosheets and the
surface OHꢀs are believed to be the primary reason for the
superior catalytic oxidation of HCHO.
Characterization of surface OHꢀ on NiCo2O4 nanosheets
To provide the ngerprint evidence for probing surface OHꢀ, X-
ray photoelectron spectroscopy (XPS) measurements were
carried out, in that the presence of surface OHꢀ causes distinct
coordinations of oxygen species. As shown in the O 1s XPS in
Fig. 4a, there is one peak at 529.5 eV in the O 1s region of
NiCo2O4 nanosheets, which is attributed to the typical metal–
oxygen bonds.33 However, a new peak at 531 eV appears for the
NiCo2O4-X nanosheets, which is associated with oxygen in OHꢀ.
In particular, the intensity of 531 eV for NiCo2O4-1 nanosheets
is much higher than that for other NiCo2O4-X nanosheets,
indicating that the surface of the NiCo2O4 catalyst is hydroxyl-
ated to some extent as a result of surface modication with
alkali. Moreover, the H2-temperature-programmed reduction
(TPR) measurements provided auxiliary evidence for the exis-
tence of surface OHꢀ. Fig. 4b shows the TPR spectra of the Thermal catalytic activity of all catalysts
In order to interpret the role of surface OHꢀ species in catalysts,
the catalytic activities of HCHO oxidation on the catalysts were
evaluated. Fig. 5a shows the catalytic oxidation of HCHO as
a function of temperature over different catalysts. The thermal
decomposition of HCHO is negligible under our experimental
conditions without catalysts. Signicantly, more and more
Fig. 5 (a) HCHO conversion over different catalysts as a function of
temperature (HCHO concentration ¼ 200 ppm, 25 vol% O2, N2 as
a balance gas, the hourly space velocity GSHV ¼ 60 000 mL hꢀ1). (b)
The comparison of HCHO conversion efficiencies of our NiCo2O4-1
nanosheets with the recently reported catalysts with the same GSHV.
(c) HCHO conversion over the NiCo2O4-1 nanosheet catalyst with
Fig. 4 (a) O 1s XPS spectra of the NiCo2O4 nanosheets, NiCo2O4-0.1 different masses as a function of temperature. (d) Catalytic perfor-
nanosheets, NiCo2O4-1 nanosheets and NiCo2O4-4 nanosheets. (b) mance of HCHO over NiCo2O4-1 and NiCo2O4 nanosheets as
H2-TPR profiles and (c) adsorption capacity of all the samples.
a function of time at 25 ꢁC and 80 ꢁC, respectively.
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HCHO is dramatically oxidized with increasing reaction
temperature for all the catalysts. The untreated NiCo2O4 nano-
sheets could convert 79% HCHO at a temperature of 100 ꢁC,
which may be because that the NiCo2O4 nanosheets have a large
specic surface area and a high redox of Co3+/Co2+ and Ni3+/
Ni2+. In stark contrast, all NiCo2O4-X nanosheet catalysts display
a higher conversion efficiency than the pristine NiCo2O4
nanosheets in the entire temperature windows, indicating that
surface OHꢀ has a positive inuence on HCHO oxidation.
Meanwhile, it is noteworthy that NiCo2O4-1 nanosheets
exhibit a remarkably higher HCHO conversion, which could
convert approximately 95% of HCHO at a low temperature of
Reaction mechanism
The reaction mechanism of HCHO oxidation over NiCo2O4-1
nanosheets has been studied by the in situ diffuse reectance
infrared Fourier transform (DRIFTS) experiments, which have
already proved to be a powerful technology for direct visuali-
zation of active species groups on the catalyst. In this regard, the
in situ DRIFTS spectra of NiCo2O4-1 nanosheets are shown in
Fig. 6. It is seen that aer exposing the sample to a ow of O2 +
HCHO (37%) + He at 30 ꢁC, the peaks appeared at 1360 and 1590
cmꢀ1 belonging to the symmetric stretch and asymmetric
stretch of COO; 2840 and 2950 cmꢀ1 belonging to the asym-
metric stretch and symmetric stretch of CH of formate,
respectively. Broad peaks between 3000 and 3500 cmꢀ1 can be
assigned to the OHꢀ species on the catalyst surface. Aer 60
min, the intensity of the OHꢀ peak at 3442 cmꢀ1 decreased,
indicating the reaction between formate and active OHꢀ species
on the NiCo2O4-1 nanosheet surface. It means that the pathway
of the reaction for HCHO oxidation on NiCo2O4-1 nanosheets
follows the HCHO / CHOOꢀ + OHꢀ / CO2 + H2O route, which
was also demonstrated in 2% Na-1% Pt/TiO2 (ref. 13) and K–Ag/
Co3O4.16 Certain conclusions can be made from these studies,
and the surface OHꢀ plays a pivotal role in proving the HCHO
oxidation of NiCo2O4-1 nanosheets at low temperature, which
could provide guidelines for understanding the importance of
the surface OHꢀ for HCHO oxidation. From the above experi-
mental results, the pronounced catalytic performance of
NiCo2O4-1 nanosheets is attributed to a synergistic effect of the
following factors. First, the 3D-NiCo2O4-1 nanosheets have
a porous structure and a large surface area, which are good for
the HCHO to diffuse and may offer more active sites for HCHO
conversion. Second, more surface OHꢀ groups on the NiCo2O4-1
nanosheets enhanced the adsorption capacity of HCHO. Some
results proposed that the adsorption over catalysts was the
crucial step for HCHO oxidation, which could enable the HCHO
to easily react with the catalysts. Third, the high redox of
NiCo2O4-1 nanosheets enhanced the performance. The activa-
tion and migration of oxygen species on catalysts are dependent
on the redox cycles of Co3+/Co2+ and Ni3+/Ni2+. The increase in
Co3+ and Ni2+ can enhance the redox cycles of NiCo2O4-1
ꢁ
20 C, whereas the NiCo2O4 nanosheets only achieve 5.2% at
the same temperature. It can be summarized that such
excellent performance indicates that the NiCo2O4-1 nano-
sheets are an outstanding active thermal catalyst at the low
temperature. More importantly, to the best of our knowledge,
such superb efficiency is extensively lower than the values
reported for any non-precious catalysts19,34–39 and many
precious catalysts.40–42 Fig. 5b compares the HCHO conversion
efficiencies of our NiCo2O4-1 nanosheets with other reported
catalysts at temperatures in the range of 20 to 100 ꢁC with the
same hourly space velocity (60 000 mL hꢀ1). Additionally, the
quality of catalysts is a very important factor for their
conversion efficiencies. Fig. 5c displays the catalytic perfor-
mance with different qualities of NiCo2O4-1 nanosheets.
Without doubt, with decreasing the quality of NiCo2O4-1
nanosheets, the efficiency declines. It is noteworthy that the
amount of NiCo2O4-1 nanosheets (0.05 g) could also convert
65% HCHO, further conrming the superior performance of
the NiCo2O4-1 nanosheets. Moreover, superb catalytic activi-
ties are also achieved by surface modication with KOH under
the same conditions, indicating that these superb catalytic
activities are due to the surface OHꢀ (Fig. S4†). No other
carbon-containing compounds except CO2 in the products are
detected for all the tested catalysts. Therefore, the above
results fully indicate that the NiCo2O4-1 sample is an excellent
thermal catalyst, and that surface OHꢀ plays an important role
in the catalytic activity.
To further evaluate the stabilities of the NiCo2O4 and
NiCo2O4-1 nanosheets, the catalytic performances on stream
at 80 ꢁC and 25 ꢁC for 200 h are shown in Fig. 5d, respectively.
It is worth noting that the NiCo2O4 nanosheets have ultra-
high stability with only 3% decrease in the HCHO conversion
efficiency aer 200 h at 80 ꢁC. In contrast, the NiCo2O4-1
nanosheets show much higher current decay over time than
the NiCo2O4 catalyst. However, NiCo2O4-1 nanosheets retain
84.1% of HCHO conversion efficiency aer 200 h, which
indicates that NiCo2O4-1 nanosheets have excellent perfor-
mance and good stability. Additionally, the morphology and
phase of NiCo2O4-1 nanosheets were retained aer testing for
200 h (Fig. S5†). In combination with the above experimental
results, we can unambiguously conclude that the NiCo2O4-1
nanosheets have outstanding performance and good
stability, which holds great signicance for practical
applications.
Fig. 6 In situ DRIFTS spectra of NiCo2O4-1 nanosheet catalysts at 30
ꢁC at 1 min and 60 min. Reaction conditions: HCHO ¼ 200 ppm, O2 20
vol%, about 35% relative humidity, He balance, and a total flow rate of
100 mL minꢀ1
.
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Acknowledgements
This work was preliminarily supported by the National Science
Fund for Distinguished Young Scholars (21425627), the Science
and Technology Plan Project (2013B090600036), the Natural
Science Foundation of China (21461162003 and 21476271),
and Natural Science Foundation (2014KTSCX004 and
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