Design of a meso-structured Pd/NiO catalyst for highly efficient low temperature CO oxidation under ambient conditions

Article abstract of DOI:10.1039/c5ra05591a

A meso-structured Pd/NiO catalyst was successfully fabricated through a controlled pyrolysis and in situ reduction protocol. The resulting material possessed a relatively high surface area and highly dispersed palladium species. It showed much higher catalytic activity and stability for CO oxidation under ambient conditions. Complete CO conversion could be achieved at as low as -20 °C, when 1.2 vol% H₂O was introduced into the feed gas. The catalyst exhibited no detectable deactivation even after 100 hours of reaction. The extraordinary catalytic activity and durability were attributed to the promotion of the water molecules and the synergetic effect between the Pd nanoparticles and meso-structured NiO support.

Full text of DOI:10.1039/c5ra05591a

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Design of a meso-structured Pd/NiO catalyst for
highly efficient low temperature CO oxidation
under ambient conditions†
Cite this: RSC Adv., 2015, 5, 40352
a
a
b
ac
*
*
Gengnan Li, Liang Li, Dong Jiang and Jianlin Shi
A meso-structured Pd/NiO catalyst was successfully fabricated through a controlled pyrolysis and in situ
reduction protocol. The resulting material possessed a relatively high surface area and highly dispersed
palladium species. It showed much higher catalytic activity and stability for CO oxidation under ambient
conditions. Complete CO conversion could be achieved at as low as ꢀ20 ꢁC, when 1.2 vol% H2O was
introduced into the feed gas. The catalyst exhibited no detectable deactivation even after 100 hours of
reaction. The extraordinary catalytic activity and durability were attributed to the promotion of the water
molecules and the synergetic effect between the Pd nanoparticles and meso-structured NiO support.
Received 30th March 2015
Accepted 28th April 2015
DOI: 10.1039/c5ra05591a
www.rsc.org/advances
The most commonly used supported noble metal catalyst is
nano-sized gold supported on metal oxide. It is generally
Introduction
accepted that the gold nanoparticles dispersed on the reducible
oxides catalysts are extraordinarily active for CO oxidation and
have much stronger durability under moisture conditions.9–18
But there are still some unsolved deciencies when in practical
use, such as deactivation in storage and under indoor light,
over-high sensitiveness to halogen-including compound
et al.19–22 The alternatives are platinum group metal based
catalysts, which have much stronger anti-poison properties, and
are also promising catalysts for CO oxidation especially for that
supported on FeOx and Fe(OH)x. However, these reported
catalysts always need relatively high temperature for complete
CO conversion under moisture conditions.22–34 Low-
temperature CO oxidation catalyst for practical application
under ambient conditions is still expected and in challenge.
It is well known that synergy between active component and
support play a key role in determining the catalytic perfor-
mance.35 Beneted from the various oxidation states, charge-
variable transition metal oxides possess excellent performance
in oxygen storage and release which are also critical factors for
low-temperature CO oxidation. When appropriate charge-
variable transition metal oxide was selected and combined
with Pd nanoparticles, signicantly enhanced activity could be
expected. In this paper, the mesoporous nickel oxide was
chosen as support for Pd catalyst. The palladium component
was transferred into the porous structure through wet impreg-
nation and the noble metal nanoparticles were in situ produced
using hydrazine hydrate as reductant. The palladium nano-
particles could be homogeneously deposited in/on the NiO
supports, and thus possessed the much enhanced catalytic
activities. The CO oxidation reaction of the materials under
different conditions and the reaction mechanism were also
discussed in detail.
Carbon monoxide, one of the most common and widely
distributed air pollutants, and is harmful to the environment
and human health. It originates mainly from incomplete
combustion of carbonaceous materials, such as exhausts of
internal combustion engines and industrial processes, etc.
There is a general agreement that catalytic oxidation is the most
effective way to eliminate this diluted air pollutant, in which the
key issue is the availability of high performance catalysts under
ambient conditions.1 To date, many catalytic systems for low-
temperature CO oxidation have been well developed, including
supported noble metals and transition metal oxides.2–4 However,
most of them can only be used under dry conditions.5–8 Gener-
ally, moisture inevitably exists especially for practical applica-
tion of low-temperature CO oxidation processes and plays
completely different roles in the presence/absence of noble
metals. It could be a devastatingly poisonous species for tran-
sition metal oxides or a promoter for noble metals.2,4,9–16 Thus,
attention for low-temperature CO oxidation under ambient
conditions has been mainly focused on supported noble metal
catalysts.
aKey Laboratory for Ultrane Materials of Ministry of Education, School of Materials
Science and Engineering, East China University of Science and Technology,
Shanghai, 200237, China. E-mail: liliang@ecust.edu.cn; Fax: +86-021-64250740;
Tel: +86-021-64252599
bResearch Center of Analysis and Test, East China University of Science and
Technology, Shanghai, 200237, China
cState Key Laboratory of High Performance Ceramic and Superne Microstructure,
Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050,
China. E-mail: jlshi@mail.sic.ac.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra05591a
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gas contained 1.0 vol% CO balanced with air was introduced
into the reactor using mass-ow controllers at a ow rate of
50 ml minꢀ1, corresponding to a space velocity of 15 000
ml hꢀ1 gcatꢀ1. The conversion of CO was measured by online gas
chromatograph (GC) under steady-state condition. The activity
of the catalyst in the presence of moisture condition was tested
by passing the feed gas at a ow rate of 50 ml minꢀ1 through a
water vapor saturator. Changing the temperature of saturator
can adjust the water concentration in the feed gas.
Experimental section
Materials preparation
Mesoporous NiO support was prepared following a reported
approach of oxalate decomposition.36 Briey, 10 mmol
Ni(NO3)2$6H2O was dissolved into 100 ml aqueous solution, then
10 ml 1 M sodium oxalate was slowly added under vigorous stir-
ring. The precipitate was ltered, washed with deionized water
and dried at 60 ꢁC. The mesoporous NiO material was nally
obtained aer calcinated at 300 ꢁC for 2 h. The heating rateꢀd1uring
calcination should be controlled to be as low as 1 ꢁC min
.
In situ diffuse reectance infrared Fourier transform
A series of mesoporous NiO supported Pd catalysts were
synthesized through an improved wet impregnation procedure.
In a typical synthesis, 0.5 g mesoporous NiO material was added
in 25 ml aqueous solution containing calculated content of
Na2PdCl4, Aer stirred for 2 h, 0.1 M hydrazine hydrate
(NH2$NH2$H2O) was slowly added and stirred for 12 h succes-
sively. The resulting solid mass was collected through centri-
spectroscopy (DRIFTS) analysis
Infrared spectra were recorded by a FTIR spectrometer (Nicolet
iS10) equipped with a MCT detector. The sample cell was tted
with ZnSe windows. The sample was exposed to the corre-
sponding reaction stream (60 ml minꢀ1). Thꢀe1DRIFTS spectra
obtained at 25 ꢁC with a solution of 4 cm and 50 scans.
Typical gas mixture was 0.5 vol% CO, 20.0 vol% O2 balanced
with He. Water vapor was carried into the gas mixture by a
bubbler in a water bath at room temperature. Aer a designated
amount of time, the CO ow was switched to a humid stream
containing 1.5 vol% water.
ꢁ
fugation, washed with water and dried at 60 C for 24 h.
Characterization
Powder X-ray diffraction (XRD) patterns were recorded on a
Bruker D8 Focus powder diffract meter with graphite mono
chromatized Cu Ka radiation (l ¼ 0.15405 nm) operated at
40 kV. Thermo gravimetric analysis (TG-DSC) were carried out at
Results and discussion
a heating rate of 10 ꢁC minꢀ1 from ambient temperature to The mesoporous NiO support was obtained through direct
700 ꢁC with an air ow rate of 20 ml minꢀ1. Nitrogen adsorption pyrolysis of nickel oxalate and the palladium component was
and desorption isotherms were measured on a Micromeritics transferred into the porous structure through wet impregnation
ASAP 2020 M analyzer at liquid nitrogen temperature (77 K). and in situ reduction strategy. The actual contents of Pd in the
Prior to the measurements, the samples were degassed at 423 K meso-structured Pd/NiO materials were determined by the
in vacuum for 6 h. The specic surface area and pore size ICP-AES technique to be 1.0, 2.8, 5.3 and 7.3 wt%, respectively.
distribution were calculated using the Brunauer–Emmett–Teller The result shows that almost all of the palladium species in the
(BET) and Barrett–Joyner–Halenda (BJH) methods, respectively. aqueous solution have been deposited into/on the mesoporous
Transmission electron microscopy (TEM) observations were NiO support.
performed on a eld emission JEM-2100 (JEOL) electron micro-
scope operated at 300 kV equipped with a Gatan-666 electron
energy loss spectrometer and energy dispersive X-ray spectrom-
eter. XPS (X-ray photoelectron spectroscopy) signals were
collected on a VG Micro MK II instrument using monochromatic
Al Ka X-ray at 1486.6 eV operated at 200 W. All the elemental
binding energies were referenced to the C (1s) line situated at
284.6 eV. H2 temperature-programmed reduction (H2-TPR)
analysis was performed by using a Micromeritics Chemisorb
2750 apparatus. For each analysis, accurate amounts of calcined
sample (50 mg) were purged in a ow of pure argon at 200 ꢁC for
30 min to remove traces water (heating rate 10 ꢁC minꢀ1). Aer
cooling to room temperature, H2-TPR experiments were per-
formed using a 10 vol% H2–Ar mixture at a ow rate of
25 ml minꢀ1. The sample was heated from ambient temperature
to 500 ꢁC at heating rate of 10 ꢁC minꢀ1 and H2 consumption was
detected by a thermal conductivity detector (TCD).
X-ray powder diffraction
For supported catalysts, the crystalline structure and surface
chemical state of support and active components oen play
crucial roles in the performance of the catalysis. The crystalline
structure of the nickel oxalate precursor and resulting Pd/NiO
materials were rst characterized by X-ray diffraction (XRD)
analysis. The as-synthesized nickel oxalate is well crystallized
containing two coordinated water corresponding well with the
PDF card no. 10-0742 (Fig. 1A). Aer calcination, the nickel
oxalate hydrate was totally decomposed and converted to nickel
oxide (Fig. 1B). A series sharp and symmetrical Bragg diffraction
peak can be found in wide angle region which demonstrates its
well crystallized feature. All the peaks in the pattern can be well
assigned as the characteristic reection planes of NiO (PDF card
no. 73-1523). When loaded with palladium, the diffraction
peaks do not have any visible change. The (111) diffraction
peaks of face-centered cubic structure palladium are almost
unobservable at 2q ¼ 40ꢁ even at a high Pd loading of 7.3 wt%.
CO oxidation test
The catalytic test for CO oxidation was carried out in xed-bed The extremely weak characteristic XRD peak of metallic Pd
quartz tubular reactor (i.d. ¼ 6 mm) containing 200 mg of implies the highly dispersed amorphous or small palladium
catalyst samples without any pretreatment. A standard reaction particles.
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thermal treatment. It can be inferred that there must be
signicant porosity le within the NiO particles. Further
transmission electron microscopy (TEM) observations (Fig. 3)
prove our speculation. Pores are randomly but homogeneously
distributed within the whole particle, which are produced by a
great quantity of gases slowly released from the oxalate
precursor leaving behind numerous voids during thermal
decomposition process.36 Aer impregnation, the noble metal
ion species, PdCl42ꢀ, was transferred into the porous structure
of nickle oxide. The following in situ reduction results in Pd/NiO
material with Pd nanoparticles being homogeneously dispersed
in/on the mesoporous support. No large noble metal particle
can be found in the TEM images even Pd loading content
reached to 7.3 wt%. In addition, the selected area electron
diffraction (SAED) patterns from the same area are also pre-
sented in the TEM images. The well-dened diffraction rings for
NiO can be found in all the samples. However, the relative faint
diffraction ring of palladium indicates its much lower crystal-
linity. These results are in good agreement with above XRD
analysis. In addition to the crystalline structure, the
morphology and distribution of supported palladium species
have been examined by high resolution transmission electron
microscopy (HRTEM) (Fig. S3, ESI†). It clearly shows that all the
catalysts are composed of crystallized NiO and Pd nano-
particles. Nanocrystals with different lattice d-spacing values
can be found all over the images. Except for those with the
Fig. 1 XRD patterns of NiC2O4$2H2O precursor (A) and Pd/NiO
catalysts with different Pd loading contents (B): 0 wt% (a), 1.0 wt% (b),
2.8 wt% (c), 5.3 wt% (d) and 7.3 wt% (e).
X-ray photoelectron spectroscopy
To get the details of the chemical state of palladium and surface
NiO, the materials were then examined with X-ray photoelec-
tron spectroscopy (XPS), as shown in Fig. 2. For NiO support, the
Ni 2p region only comprises the typical NiO feature (853.0 eV,
854.6 eV). Aer loaded with Pd, the typical peak for hydroxylated
NiO emerged (855.2 eV). Therefore, the Ni 2p spectra of Pd/NiO
could be decomposed as the signal of pure NiO and hydroxyl-
ated NiO. With increase of Pd loading content, the intensity of
hydroxylated NiO increases. This phenomenon could be
assigned to the presence of signicant surface hydroxyl-
ation.37–42 In Pd 3d region, the typically binding energy of Pd0 at
335.3 and 340.5 eV for 3d5/3 and 3d3/2 can be observed. The
slight higher energy at 337.4 and 342.5 eV can be assigned to the
Pd2+. However, the peaks are very weak indicating the existence
of a small amount Pd2+. Element analysis indicated that the Pd0
in the catalysts are all above 85%. This clearly conrms that
most of the Pd2+ ions on the surface have been reduced to Pd0 by
hydrazine hydrate. With the increase of Pd loading amount, the
intensity of Pd spectra increases.
˚
d-spacing 2.2 A assignable to Pd crystallite, all of them can be
identied as NiO nanoparticles. The particle size distributions
of palladium species show that the highly dispersed Pd nano-
particles on the NiO support are mostly smaller than 5 nm
(Fig. S5, ESI†). The further energy dispersive X-ray spectroscopy
(EDS) mapping images conrm the enrichment of palladium
and its high dispersion on the NiO support. The impregnation
and in situ reduction method have been proved to be effective in
fabricating noble metal loaded NiO composite catalysts. The
noble metal nanoparticles could be homogeneously deposited
into/on the polycrystalline NiO support (Fig. S4, ESI†).
Texture properties
Scanning electron microscopic (SEM) analysis clearly indicates
that NiO support inherit the morphology of its corresponding
oxalate precursor (Fig. S2, ESI†). Except that some fragments
and broad cracks, limited size shrinkage can be found aer
Fig. 3 TEM images of NiO support (A) and Pd/NiO catalysts with
Fig. 2 XPS spectra of Ni 2p (A) and Pd 3d (B) for the NiO support (a) and different Pd loading contents: 1.0 wt% (B), 2.8 wt% (C), 5.3 wt% (D) and
Pd/NiO catalysts with different Pd loading contents: 1.0 wt% (b), 2.8 7.3 wt% (E) (the HRTEM images were obtained from the circular
wt% (c), 5.3 wt% (d) and 7.3 wt% (e).
region).
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Fig. 6 The catalytic performances of NiO support and Pd/NiO cata-
lysts with different Pd loading contents under dry (A) and moisture
conditions (B).
Fig. 4 N2 adsorption–desorption isotherms of NiO support (A) and
Pd/NiO catalysts with different Pd loading contents: 1.0 wt% (B), 2.8
wt% (C), 5.3 wt% (D) and 7.3 wt% (E).
the reduction peak shis to relative lower temperature region.
Such a shi is related with spillover effect involving either
hydrogen activated on the metal phase or mobile lattice oxygen
induced by intimate metal–support interactions.12 The signi-
cant low temperature reduction feature strongly suggests that
the nickel oxide substrate has been ‘activated’ to a large extent
in the Pd/NiO catalyst by Pd loading.
The texture properties of the resulting materials were further
investigated by measuring adsorption–desorption isotherms of
nitrogen at 77 K, as shown in Fig. 4. In all the cases, the typical
Langmuir IV isotherms suggest the mesoporous structure and
the appearance of H3 hysteresis loops indicate the formation of
slit-like meso-pore. The average pore size is about 2.1 nm and
the specic surface area is as high as 272 m2 gꢀ1. When loaded
with palladium, the specic surface area, pore size and pore
volume of the Pd/NiO materials only show slight decrease
(Table S1, ESI†). These results demonstrate that palladium
nanoparticles have been well dispersed into the pore channels
and/or on the surface of the support. The most of the pore
channels remain open, which is important to allow reactant
molecules to diffuse into the mesostructure and access the
catalyst nanoparticles.
Catalytic activities for CO oxidation
The catalytic performance of NiO and corresponding Pd
loaded samples for CO oxidation under dry conditions are
shown in Fig. 6. The NiO support does not show any obvious
ꢁ
catalytic activity at reaction temperatures lower than 100 C.
The Pd loaded material, 5.3 wt% Pd/NiO, shows the highest
catalytic activity. The complete CO conversion temperature is
as low as 50 ꢁC. Although this temperature is somewhat
higher than expected, it still exhibits excellent catalytic
performance of CO oxidation compared with the NiO sup-
ported Au catalysts.2 However, when water molecules were
introduced into the feed gas, the catalytic performance of
Pd/NiO catalyst is quite different. It shows a dramatic
enhancement (Fig. 6B). The 5.3 wt% Pd/NiO material still
demonstrates the highest catalytic activity. The 100%
conversion temperature decreased from 50 ꢁC to ꢀ20 ꢁC,
when 1.2% water was introduced into the feed gas. Continu-
ously increasing the Pd content, the catalytic performance
remains almost unchanged, which clearly indicates that not
only the content of active component (palladium), the
dispersion of palladium nanoparticles and the synergetic
effect between palladium and NiO may also play important
roles in CO oxidation process.
H2-TPR analysis
Fig. 5 shows the reduction proles of the catalysts, and for
comparison, the H2 temperature programmed reduction
(H2-TPR) prole of the parent NiO is also included. The reduc-
tion process of Ni2+ to Ni0 takes place at 350 ꢁC, which is proved
by the XRD analysis (Fig. S6, ESI†). Aer loaded with palladium,
Catalytic stability for CO oxidation
The catalytic stability of the 5.3 wt% Pd loaded catalyst was
also tested under the moisture conditions. Surprisingly, the
1.0 vol% CO was completely converted and the complete
conversion keeps unchanged even aer 100 h reaction at 25 ꢁC
(Fig. 7). This may be attributed to the very reactive absorbed
intermediate, COOH, which is formed from the H2O molec-
ular reaction with CO.23,43 The presence of water molecules
could lead to the decreased activation energy and thus
Fig. 5 H2-TPR of NiO support (a) and Pd/NiO catalysts with different
Pd loading contents: 1.0 wt% (b), 2.8 wt% (c), 5.3 wt% (d) and 7.3 wt%
(e).
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Fig. 9 In situ DRIFT spectra of 5.3 wt% Pd/NiO under 0.5 vol%
CO–20.0 vol% O2–He (A) and 0.5 vol% CO–20.0 vol% O2–1.5 vol%
H2O–He (B) at 25 ꢁC.
Fig. 7 The catalytic stability 5.3 wt% Pd loaded catalyst under moisture
conditions at 25 ꢁC.
promote the CO oxidation. Moreover, to gain insight into the
intrinsic activities of supported Pd catalysts under different
conditions, TOFs normalized by the number of the surface
noble atoms from differential reaction results were compared
(Table S2, ESI†). No matter under the dry or moisture condi-
tion, the TOFs of the 5.3 wt% Pd loaded catalyst all give the
maximum values (14.14 ꢂ 10ꢀ4 sꢀ1 under dry condition and
31.69 ꢂ 10ꢀ4 sꢀ1 under moisture condition, respectively)
among all samples synthesized. Aer reaction, the catalyst was
characterized by XRD, TEM and N2 adsorption–desorption
measurement, no structure collapse or phase segregation has
been observed, indicating the stable state of the catalyst
(Fig. S8–S10, ESI†).
Pd/NiO catalyst under different humidity conditions. According
to the literatures, the wide bands at 1360 and 1470 cmꢀ1 are
ascribed to ys(OCO) and d(OH) of [COOH]s, respectively.44–46 The
strong band at 1600–1700 cmꢀ1 can be assigned to the OH
groups,47,48 whereas, the weak band at 1810 cmꢀ1 is related to
bonded CO on metallic Pd.49,50 Fig. 9A shows the change in band
intensity of surface species over Pd/NiO catalyst under dry
conditions. Upon exposure to CO in dry stream, the bands at
1650 cmꢀ1 attributed to surface –OH group were observed. This
phenomenon is corresponding well with the above XPS analysis.
Interestingly, the band intensity gradually decreased with the
contact time, which means the surface –OH group participation
in the CO oxidation process. In contrast, under the moisture
conditions (Fig. 9B), due to the adequate supply from water, the
intensity of –OH group remains relatively constant during the
whole process. Besides, compared with that under dry stream,
Pd–CO band appear and become stable more rapidly. The
presence of water could not only be in favor of the formation of
carbonate species (COOH), but also promote the adsorption of
CO on the surface of metallic Pd. All these clearly indicate the
positive effect of water in CO oxidation on the surface of Pd/NiO
catalysts.
Effect of moisture
In order to explore the effect of H2O in CO oxidation process,
different amounts of H2O were rst introduced into the feed
gas, as shown in Fig. 8. The catalytic activity increases with the
increased water amount from 0.4 vol% to 1.2 vol%. Further
increasing the water content does not produce additional
enhancement of the catalytic activity. Under the relatively
higher humidity condition, H2O may capillary-condensed in the
mesopores of the NiO support, consequently preventing the
reactants from accessing to catalyst surface and reduced the
catalytic activity.
Conclusions
The meso-structured Pd/NiO catalysts have been successfully
prepared by a combined nickel oxalate decomposition and
in situ reduction protocol. The resulting composite material
possessed relatively high specic surface area and highly
dispersed Pd nanoparticles. Among all samples prepared,
5.3 wt% Pd loaded catalyst showed the highest reaction activity
in the low-temperature CO oxidation reaction, the complete
conversion could be achieved at as low as 50 ꢁC under dry
conditions, much lower than that of NiO supported Au catalysts.
More importantly, this material showed greatly enhanced
catalytic activity and stability under moisture conditions. The
complete CO conversion temperature can be decreased from
50 ꢁC to ꢀ20 ꢁC when 1.2 vol% H2O was introduced into the feed
gas. The catalytic activity remained un-deteriorated even aer
100 hours of reaction. The in situ DRIFTS analysis demonstrates
the promotion of H2O in CO oxidation process over Pd/NiO
catalyst.
In addition, in situ DRIFTS technology was also employed to
explore the difference of surface reaction of CO on 5.3 wt%
Fig. 8 The activity of 5.3 wt% Pd loaded catalyst under different
moisture conditions.
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Acknowledgements
This study was supported by National Basic Research Program
of China 2013CB933201.
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