Mesoporous CoO-supported palladium nanocatalysts with high performance for: O -xylene combustion

Article abstract of DOI:10.1039/c7cy02007d

The Pd-based catalysts have been widely studied for the combustion of volatile organic compounds, in which the roles of the Pd and the support are still not known clearly. In this study, mesoporous Co₃O₄ and CoO (i.e., meso-Co₃O₄ and meso-CoO) were prepared and used as support for the loading of Pd nanoparticles (NPs). The supported Pd samples performed much better than the supports for o-xylene combustion, in which the Pd/meso-CoO sample showed the best catalytic activity (T_90% = 173 °C), giving rise to reaction rates at 170 °C 3.5 and 84 times higher than those over the Pd/meso-Co₃O₄ and meso-CoO samples, respectively. It has been established that the meso-CoO sample possessed strong ability to activate oxygen molecules to the active oxygen species, the loaded Pd NPs in a metallic Pd phase was beneficial for o-xylene adsorption, and the adsorbed o-xylene species could immediately react with the active oxygen species at the interface between Pd NPs and meso-CoO. Therefore, we conclude that the excellent catalytic performance of the Pd/meso-CoO sample was associated with its highly active Pd-CoO interface.

Full text of DOI:10.1039/c7cy02007d

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Mesoporous CoO-supported palladium nanocatalysts with high
performance for o-xylene combustion
Shaohua Xiea, Yuxi Liua,*, Jiguang Denga,*, Jun Yanga, Xingtian Zhaoa, Zhuo Hana, Kunfeng
Zhanga, Yuan Wangb, Hamidreza Arandiyanb,*, Hongxing Daia,*
a
Beijing Key Laboratory for Green Catalysis and Separation, Key Laboratory of Beijing on
Regional Air Pollution Control, Key Laboratory of Advanced Functional Materials, Education
Ministry of China, and Laboratory of Catalysis Chemistry and Nanoscience, Department of
Chemistry and Chemical Engineering, College of Environmental and Energy Engineering,
Beijing University of Technology, Beijing 100124, China
b Particles and Catalysis Research Group, School of Chemical Engineering, The University of
New South Wales, Sydney, NSW 2052, Australia
ꢀ To whom correspondence should be addressed:
Dr. Yuxi Liu,
Eꢀmail address: yxliu@bjut.edu.cn
Tel. No.: +86ꢀ10ꢀ6739ꢀ6118; Fax: +86ꢀ10ꢀ6739ꢀ1983
Dr. Jiguang Deng,
Eꢀmail address: jgdeng@bjut.edu.cn
Tel. No.: +86ꢀ10ꢀ6739ꢀ6118; Fax: +86ꢀ10ꢀ6739ꢀ1983
1

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Dr. Hamidreza Arandiyan,
Eꢀmail address: h.arandiyan@unsw.edu.au;
Tel.: +61ꢀ2ꢀ9385ꢀ7994
Prof. Hongxing Dai,
Eꢀmail address: hxdai@bjut.edu.cn
Tel. No.: +86ꢀ10ꢀ6739ꢀ6118; Fax: +86ꢀ10ꢀ6739ꢀ1983
2

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Abstract:
The Pdꢀbased catalysts have been widely studied for the combustion of volatile organic
compounds, in which the roles of the Pd and the support are still not known clearly. In this
study, mesoporous Co3O4 and CoO (i.e., mesoꢀCo3O4 and mesoꢀCoO) were prepared and used
as support for the loading of Pd nanoparticles (NPs). The supported Pd samples performed
much better than the supports for oꢀxylene combustion, in which the Pd/mesoꢀCoO sample
showed the best catalytic activity (T90% = 173 °C), giving rise to the reaction rate at 170 °C of
3.5 and 84 times higher than those over the Pd/mesoꢀCo3O4 and mesoꢀCoO samples,
respectively. It has been established that the mesoꢀCoO sample possessed strong ability to
activate oxygen molecules to the active oxygen species, the loaded Pd NPs in a metallic Pd
phase was beneficial for oꢀxylene adsorption, and the adsorbed oꢀxylene species could
immediately react with the active oxygen species at the interface between Pd NPs and
mesoꢀCoO. Therefore, we conclude that the excellent catalytic performance of the
Pd/mesoꢀCoO sample was associated with its highly active Pd–CoO interface.
Key words: Mesoporous CoO; Supported Pd catalyst; Oꢀxylene combustion; Volatile organic
compound; Oꢀxylene adsorption.
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1. Introduction
Control of volatile organic compound (VOC) emissions is important since they are
hazardous to the environment and human health [1,2]. Catalytic oxidation is regarded as one
of the most promising pathways for VOC removal due to its facile application, high efficiency,
and low cost [3−8]. However, the key issue of such a technology is the availability of catalysts
with good lowꢀtemperature activity and stability. Among the catalysts investigated so far, the
supported noble metal catalysts are highly efficient for VOC removal at low temperatures, in
which the Pdꢀbased materials have attracted much attention [6−10].
Although the Pdꢀbased catalysts have been widely studied for the oxidation of VOCs, the
roles of the Pd and the support are still not known clearly. It is generally accepted that the
loading of noble metal nanoparticles (NPs) on an oxide support can greatly enhance the
oxygen activation on the support, thus improving the performance of the catalyst for VOC
oxidation. Usually, oxide supports exhibit weak ability for oxygen activation, especially at
low temperatures [11−14]. For example, loading of metallic Pd NPs on TiO2 increased the
ability of oxygen activation in the roomꢀtemperature oxidation of HCHO over Pd/TiO2 [12].
Ma et al. [14] reported that the active surface oxygen species were easily generated by the
active sites of nanosized gold on the 2.5 wt% Au/mesoporous Co3O4 sample, thus resulting in
o
an excellent catalytic activity (ethylene conversion = 76 %) at 0 C. Therefore, the choice of
an oxide support that possesses good oxygen activation ability at low temperatures would be
beneficial for improving the performance of the supported noble metal catalysts for the
oxidative removal of VOCs.
As one of the typical transitionꢀmetal oxides, Co3O4 is highly active for CO oxidation at
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low temperatures [15−18]. A number of works have reported that the surface Co3+ cation was
the active site for CO adsorption, whereas the Co2+ cation with a neighboring oxygen vacancy
was easily reꢀoxidized by a gasꢀphase O2 molecule to form the adsorbed oxygen species
[15,16]. For example, CoO nanorods with a higher surface area and a more amount of oxygen
vacancies exhibited superior catalytic activity and durability for the reduction or evolution of
oxygen [19]. Inspired by these results, we believe that the cobalt oxide rich of surface Co2+
cations and oxygen vacancies would have good oxygen activation ability at low temperatures.
It is known that increasing the surface area of a material can lead to a rise in amount of active
sites and favor the dispersion of noble metal NPs, hence enhancing its catalytic activity. A
facile route to obtain materials with high surface areas is to have them fabricated in
mesoporous structure [20−25]. Compared with bulk Co3O4, mesoporous Co3O4 with an
interconnected pore structure and a high surface area has been used as a highly active catalyst
and support for the total oxidation of CO, BTX, and methane [23,24]. Recently, we have
reported the successful synthesis of mesoꢀCoO from mesoꢀCo3O4 adopting a reduction
process with a glycerol solution, and found that the mesoꢀCoO was rich in Co2+ species and
showed a high catalytic activity for oꢀxylene oxidation [26]. We concluded that the Co2+
–
2–
species were the active sites that favored the formation of highly active O2 and O2
–
(especially O2 ) species. Therefore, the mesoꢀCoO with good oxygen activation ability at low
temperatures could be a promising support for the supported Pd catalysts and hence a suitable
support in determining the role of Pd on VOC oxidation.
Herein, we report the preparation of mesoꢀCoO (from the reduction of mesoꢀCo3O4 by
glycerol) and Pd/mesoꢀCoO and their catalytic properties for the complete oxidation of
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oꢀxylene. The asꢀobtained samples were characterized in detail, the effect of Pd loading on the
physicochemical property of mesoꢀCoO was probed, and the relationship between oxygen
activation ability or oꢀxylene adsorption ability and catalytic activity of the Pd/mesoꢀCoO
samples was established.
2. Experimental
2.1. Catalyst preparation
Mesoporous silica (KITꢀ6) was synthesized adopting the procedures (Supplementary
material) described in the literature [25]. Ordered mesoporous Co3O4 and CoO were
fabricated according to the strategy reported previously [26]. The typical fabrication
procedures are as follows: After 1.0 g of KITꢀ6 was suspended in 50 mL of toluene, an
ultrasonic irradiation (the 100 kHz ultrasonic wave was produced by the 300ꢀW output power)
was applied at room temperature (RT) for 0.5 h. After irradiation, the solution was stirred at
o
65 C for 0.5 h, then 2.0 g of Co(NO3)2·6H2O was added under vigorous stirring, and finally
o
dried at 50 C for 4 h, thus obtaining the pink powders. The pink powders were put in a
o
o
crucible and then calcined in a muffle furnace at a ramp of 1 C/min from RT to 600 C and
kept at this temperature for 6 h. The silica template was removed by twice etching with a hot
(80 oC) NaOH aqueous solution (2.0 mol/L), because NaOH aqueous solution was proven to
be effective in removing the silica template in literature [20,21]. The mesoꢀCo3O4 sample was
obtained after centrifugation, washing with deionized water and ethanol three times, and
o
drying at 80 C for 24 h. The ordered mesoporous CoO was prepared via a modified
mesoꢀCo3O4ꢀreduction procedure with glycerol. The reduction was carried out in a tubular
furnace, in which 1.0 g of mesoꢀCo3O4 was loaded on a quartz boat. Before the powders were
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o
o
heated at a ramp of 10 C/min from RT to 320 C, a nitrogen flow of 50 mL/min was
employed to remove the O2 in the system for 0.5 h. After cutting off the N2 flow, a glycerol
aqueous solution (50 wt%) was pumped into the reactor at a rate of 1.0 mL/h with a syringe
pump (Longer Pump LSPꢀ01ꢀ2A) and evaporated at the outlet of the capillary for 12 h.
Afterward, the reactor was kept at 320 oC for 1 h under a nitrogen flow of 50 mL/h to remove
the adsorbed species, the mesoꢀCoO sample was obtained by cooling down to RT at the same
atmosphere. From the previous work, we believe that there was no residual glycerol on the
sample surface, hence no significant influence on the combustion of oꢀxylene.
The mesoꢀCo3O4 and mesoꢀCoO supported Pd samples were prepared using the
PVAꢀprotected reduction method with PVA (MWaver. = 10000 g/mol) as protecting agent and
NaBH4 as reducing agent. The typical preparation procedures are as follows: A desired
amount of PVA (noble metal/PVA mass ratio = 1.0 : 1.2) was added to an aqueous solution of
PdCl2 in an ice bath under stirring. Then, a NaBH4 aqueous solution (2.0 g/L; noble
metal/NaBH4 molar ratio = 1.0 : 5.0) was rapidly injected to form a dark brown suspension,
thus obtaining the Pd NPs after stirring for 1 h. Finally, a desired amount of mesoꢀCo3O4 or
mesoꢀCoO was added into the noble metal sol (theoretical Pd loading was 1.0 wt%) under
stirring for 4 h. After that, the mixture was filtered and washed with hot deionized water and
o
ethanol to guarantee the total removal of residual ions and PVA. Drying at 80 C for 12 h and
o
calcining at 320 C for 1 h at a N2 flow of 100 mL/h could remove the adsorbed water, thus
obtaining the mesoꢀCo3O4ꢀ and mesoꢀCoOꢀsupported Pd catalysts. The results of inductively
coupled plasma atomic emission spectroscopic (ICP−AES) investigations reveal that the real
loading (0.94 or 0.96 wt%) of Pd was similar in each of the samples, and no residual SiO2 was
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detected.
All of the chemicals (A.R. in purity) were purchased from Beijing Chemical Reagents
Company and used without further purification.
2.2. Catalyst characterization
Physicochemical properties of mesoꢀCo3O4 and mesoꢀCoO, together with its supported Pd
samples were characterized by ICP–AES, Xꢀray diffraction (XRD), N2 adsorption–desorption
(BET), transmission electron microscopy (TEM), Xꢀray photoelectron spectroscopy (XPS),
carbon
monoxide
temperatureꢀprogrammed
reduction
(COꢀTPR),
oxygen
temperatureꢀprogrammed desorption (O2ꢀTPD), oꢀxylene temperatureꢀprogrammed
desorption (oꢀxyleneꢀTPD), and in situ diffuse reflectance Fourier transform infrared
spectroscopy (DRIFT). The detailed characterization procedures are described in the
Supplementary material.
2.3. Catalytic activity evaluation
Catalytic activities of the samples for oꢀxylene combustion were measured in a
continuous flow fixedꢀbed quartz tubular microreactor (i.d. = 6.0 mm). Before activity
measurement, all of the samples were pretreated in a N2 flow of 20 mL/min at 300 oC for 1 h.
To minimize the effect of hot spots, 50 mg of the sample (40–60 mesh) was diluted with 0.25
g of quartz sands (40–60 mesh). The reactant mixture was composed of 1000 ppm oꢀxylene +
20.0 vol% O2 + N2 (balance). The 1000ꢀppm oꢀxylene was generated by passing a N2 flow
o
through a pure oꢀxyleneꢀcontaining bottle that was chilled in an isothermal bath at 0 C. The
total flow was 16.6–100.0 mL/min, thus giving a space velocity (SV) of 20,000–120,000
mL/(g h), or a GHSV (gas hourly space velocity) of 19,100–191,000 h−1, respectively.
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Reactants and products were analyzed online by gas chromatography (GCꢀ14C, Shimadzu)
equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD),
using a stabilwax@ꢀDA column (30 m in length) and a Carboxen 1000 column (3 m in
length). The balance of carbon throughout the catalytic system was estimated to be 98.5 ±
1.5 %. In the water vapor tolerance experiment, 2.0 vol% H2O was introduced by passing a N2
o
flow of 13.08 mL/min through a water saturator at 34 C. Catalytic activities of the samples
were evaluated using the temperatures (T10%, T50%, and T90%) required for achieving oꢀxylene
conversions of 10, 50, and 90 %, respectively. The oꢀxylene conversion is defined as
(cinlet–coutlet)/cinlet × 100%, where the cinlet and coutlet were the inlet and outlet oꢀxylene
concentrations in the feed stream, respectively. The absence of mass transfer limitation under
reaction conditions was verified both experimentally and theoretically [27,28]. There was no
significant heat or mass transfer limitation under the adopted reaction conditions. The details
are described in the Supplementary material.
3. Results
3.1. Crystal phase composition, pore structure, and surface area
The smallꢀangle XRD pattern of the mesoꢀCo3O4 sample is shown in Fig. 1A. The
wellꢀresolved diffraction signals corresponding to the (100) and (200) crystal planes of Co3O4
were recorded, indicating that the mesoꢀCo3O4 sample possessed a highly ordered
mesoporous structure with a cubic Ia3d symmetry [29]. When the mesoꢀCo3O4 sample was
reduced by glycerol, the original mesoporous structure was basically maintained although the
smallꢀangle diffraction peak due to the (200) crystal plane of mesoꢀCoO disappeared (this
means a decrease in structural ordering). After the loading of Pd nanoparticles (NPs), there
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were no apparent changes in smallꢀangle diffraction pattern, indicating that Pd loading did not
induce a significant alteration in mesoporous architecture of the mesoꢀCoO support. The
wideꢀangle XRD patterns of the samples are shown in Fig. 1B. The mesoꢀCo3O4, mesoꢀCoO,
and their supported Pd samples displayed wellꢀresolved reflections that could be indexed to
the cubic Co3O4 (JCPDS PDF# 74ꢀ1657) and cubic CoO (JCPDS PDF # 48ꢀ1719) phases
[20,23], respectively. The grain sizes (Table 1) of Co3O4 and CoO in the samples were
calculated according to the Scherrer equation: D = 0.89
wavelength, is the full width at half maximum of the (311) plane of Co3O4 or the (200)
plane of CoO, and is the corresponding diffraction angle. It can be seen from Table 1 that
λ/(βcos(θ), where λ is the Xꢀray
β
θ
the calculated grain sizes of Co3O4 and CoO crystallites in the samples were 31−32 and 27−28
nm, respectively. No characteristic diffraction peaks assignable to the metallic Pd or oxidized
Pd phases were recorded in the Pd/mesoꢀCo3O4 and Pd/mesoꢀCoO samples, suggesting that
the Pd NPs were highly dispersed on the mesoꢀCo3O4 or mesoꢀCoO support.
The porous structures and morphologies of the Pd/mesoꢀCo3O4 and Pd/mesoꢀCoO samples
were further determined by the TEM technique, and the results are presented in Fig. 2. It is
clearly seen that a wellꢀordered mesoporous structure was generated in each of the two
samples, in good agreement with the smallꢀangle XRD results. The Pd NPs were well
dispersed on the support surface. As shown in Fig. 2c and f, the lattice spacing of 0.225 nm
corresponded to the Pd (110) crystal plane, whereas those of 0.404 and 0.466 nm could be
indexed to the Co3O4 (100) and CoO (111) crystal planes of the Pd/mesoꢀCo3O4 and
Pd/mesoꢀCoO samples, respectively. An estimation of Pd particle sizes in the TEM images
reveals that the mean Pd particle sizes in the Pd/mesoꢀCo3O4 and Pd/mesoꢀCoO samples were
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3.6 and 3.7 nm (Fig. 2 and Table 1), respectively. Fig. 3 shows the representative N2
adsorption–desorption isotherms and poreꢀsize distributions of the samples, and their textural
parameters are summarized in Table 1. A type IV sorption isotherm with a type H1 hysteresis
loop in the relative pressure (p/p0) range of 0.4–0.9 was observed for each of the samples (Fig.
3A), suggesting the presence of uniform mesopores [26,30]. This result was in good
agreement with those of the smallꢀangle XRD and TEM investigations. As shown in Fig. 3B,
the pore sizes of the samples were 3.8 and 10.5 nm, respectively. The BET surface areas and
pore volumes of the mesoꢀCo3O4 and Pd/mesoꢀCo3O4 samples were 93.6–95.1 m2/g and
0.20–0.23 cm3/g, whereas those of the mesoꢀCoO and Pd/mesoꢀCoO samples were 65.1–65.3
m2/g and 0.12–0.14 cm3/g, respectively. The drop in surface area and pore volume might be
due to collapsing of smaller pores (3.8 nm) in the reduced mesoꢀCoO and Pd/mesoꢀCoO
samples.
3.2. Catalytic performance
As a typical VOC, oꢀxylene was selected for evaluating catalytic activities of the samples.
It can be clearly observed from Fig. 4A that the mesoꢀCoO sample performed better than the
mesoꢀCo3O4 sample (the T50% over the former sample was 21 oC lower than that over the latter
one); furthermore, the loading of Pd significantly enhanced the lowꢀtemperature activity of
the mesoꢀCo3O4 and mesoꢀCoO samples, in which the Pd/mesoꢀCoO sample showed the
o
o
highest activity and the T50% (159 C) was 25 and 74 C lower than that over the
Pd/mesoꢀCo3O4 and mesoꢀCoO samples, respectively. It is known that a reaction takes place
more readily on the catalyst with lower apparent activation energy (Ea). Hence, the Ea is
calculated to compare the lowꢀtemperature activity of the asꢀobtained samples. It has been
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reported that oxidation of VOCs obeys a reaction mechanism under the condition of excessive
oxygen: the first order with respect to VOC concentration and zero order with respect to O2
concentration [31–33]. To avoid mass transfer limitations and consider the low oꢀxylene
o
concentration, the temperatures used for kinetic calculation were lower than 230 C, at which
the reactant conversions over the samples were below 10 %. Fig. 4B shows the Arrhenius
plots for oꢀxylene combustion over the samples at SV = 40,000 (mL/(g h)), and their Ea values
are summarized in Table 2. A highly linear relationship of lnk versus reverse reaction
temperature can be observed from Fig. 4B, confirming that oꢀxylene combustion over these
samples under the adopted reaction conditions followed a firstꢀorder reaction mechanism with
respect to oꢀxylene concentration. The Ea value (at a 95 % confidence level) decreased in the
order of mesoꢀCo3O4 (126.7 ± 4.5 kJ/mol)
> mesoꢀCoO (112.7 ± 5.1 kJ/mol) > Pd/mesoꢀCo3O4
(79.4 ± 3.7 kJ/mol) Pd/mesoꢀCoO (74.1 ± 3.4 kJ/mol). The loading of Pd on the surface of
>
mesoꢀCo3O4 or mesoꢀCoO resulted in a big decrease of Ea for oꢀxylene combustion. Therefore,
the catalytic performance decreased in the sequence of Pd/mesoꢀCoO > Pd/mesoꢀCo3O4 >
mesoꢀCoO > mesoꢀCo3O4.
We tentatively use the T90% to make a comparison on catalytic performance of the samples
o
(Table S1). One can clearly see that the T90% (173 C) over Pd/mesoꢀCoO at SV = 40,000
o
mL/(g h) was much lower than that (254 C) over 1 wt% Pd/Co3O4 (3DL) at SV = 60,000
mL/(g h) [8], that (204 oC) over 1 wt% Pd/Co3O4 (3D) at SV = 60,000 mL/(g h) [6], that (218
o
oC) over 0.8 wt% Pd/HFAU(17) at SV = 18,000 mL/(g h) [34], that (195 C) over 0.30 wt%
o
Pt/HBEA(50)ꢀreduced at SV = 18,000 mL/(g h) [5], and that (213 C) over 1.94 wt%
Au–Pd–0.22Fe/3DOM Mn2O3 at SV = 40,000 mL/(g h) [35]. Obviously, our Pd/mesoꢀCoO
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catalyst performed much better than most of the catalysts reported in the literature.
Fig. 5A shows the effect of SV on catalytic activity of the Pd/mesoꢀCoO sample. Oꢀxylene
conversion decreased with the rise in SV from 40,000 to 120,000 mL/(g h). It is generally
accepted that when the SV increases, the reactant conversion is influenced by two factors:
shortening of residence time and increasing of thermal power. In the present work, to
minimize the effect of hot spots, 50 mg of the sample (40–60 mesh) was diluted with 0.25 g
of quartz sands (40–60 mesh), and the concentration of oꢀxylene was low (1000 ppm). Hence,
with a rise in SV, the contact time was shortened, giving rise to a drop in oꢀxylene conversion.
To examine the catalytic stability, onꢀstream oꢀxylene combustion at 170 oC over
Pd/mesoꢀCoO was carried out (Fig. 5B). Apparently, no significant loss in activity was
detected over the Pd/mesoꢀCoO sample after 40 h of onꢀstream reaction, suggesting that the
Pd/mesoꢀCoO sample was catalytically stable under the adopted reaction conditions. In other
words, there was no carbonaceous deposition on Pd/mesoꢀCoO during the catalytic evaluation
processes. As pointed out in one of our previous works, the CoO phase in the mesoꢀCoO
sample could be oxidized into the Co3O4 phase in air above 270 oC according to the results of
in situ XRD characterization [26]. That is to say, the Pd/mesoꢀCoO sample was stable in air
o
below 270 C, otherwise Pd/mesoꢀCoO would transfer to Pd/mesoꢀCo3O4 at temperatures
o
above 270 C. Hence, it is easily understood that the good catalytic stability of the
Pd/mesoꢀCoO sample under the adopted reaction conditions was due to no significant changes
in its physicochemical property. It should be pointed out that the Pd/mesoꢀCoO sample could
not be used under the operating temperatures above 270 oC.
Considering the possible existence of carbon dioxide and water vapor in the feedstock, their
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effects on catalytic activity of the bestꢀperforming Pd/mesoꢀCoO sample were examined. We
o
detected oꢀxylene conversions over the Pd/mesoꢀCoO sample at 190 C and SV = 120,000
mL/(g h) in the presence of 10.0 vol% CO2 and/or 2.0 vol% water vapor, and their results are
shown in Fig. 6. It is observed that the oꢀxylene conversion slightly dropped (ca. 4 %) when
10.0 vol% CO2 was introduced, and it could be gradually restored to its original value when
10.0 vol% CO2 was cut off. This result indicates that deactivation due to CO2 introduction
was reversible. The slight negative effect of CO2 addition was due to the competitive
adsorption of CO2 and oꢀxylene as well as O2, rather than the carbonaceous deposition on the
Pd/mesoꢀCoO surface [7]. When 2.0 vol% water vapor or the mixture of 2.0 vol% water
vapor and 10.0 vol% CO2 was added to the feedstock, however, a big loss in catalytic activity
(10−12 %) was detected. Oꢀxylene conversion was hardly restored to its original value when
the H2O−CO2 gas mixture was cut off, suggesting that deactivation due to water vapor was
irreversible. After pretreated the sample in a N2 flow of 20 mL/min at 300 oC for 1 h, oꢀxylene
conversion was restored to its original value, demonstrating that the active sites were
recovered. XRD pattern (Fig. S1) of the used Pd/mesoꢀCoO sample for oꢀxylene combustion
o
at 190 C and SV = 120,000 mL/(g h) in the presence of 2.0 vol% water vapor was recorded.
No apparent change in CoO phase of the Pd/mesoꢀCoO sample was observed under the
present reaction conditions. It has been reported that water vapor was easily adsorbed on the
surface of Pd NPs, whereas PdO species were readily changed to Pd(OH)2 species via the
reaction of PdO + H2O → Pd(OH)2 in the presence of water vapor, thus leading to a decrease
in active site [36]. In addition, a competitive adsorption of H2O and oꢀxylene as well as O2
might also give rise to a decrease in catalytic performance for oꢀxylene combustion. In order
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to clarify the reasons of irreversible deactivation induced by water vapor, an in situ DRIFT
experiment was carried and the recorded spectra are shown in Fig. S2. It is obviously seen
that the strong OH groups appeared after exposure of the Pd/mesoꢀCoO sample to water vapor.
Such a strong adsorption of water vapor would lead to a loss in amount of active sites, thus
resulting in the irreversible deactivation of the Pd/mesoꢀCoO sample. Therefore, we conclude
that competitive adsorption of H2O and oꢀxylene as well as O2 and formation of Pd(OH)2
were responsible for the drop in oꢀxylene conversion in the presence of 2.0 vol% water vapor.
o
The lower reaction temperature (190 C) could not favor the decomposition of the formed
Pd(OH)2 and adsorbed OH groups, which reasonably explained why oꢀxylene conversion
could not be restored when cutting off the supply of the water vapor or the mixture of 2.0
vol% water vapor and 10.0 vol% CO2 in the feedstock. The pretreatment of the partially
deactivated sample at a higher temperature (300 oC) led to the decomposition of Pd(OH)2 and
adsorbed OH groups, and the recovery of active sites, thus restoring the catalytic activity of
Pd/mesoꢀCoO for oꢀxylene combustion.
3.3. Surface element composition, metal oxidation state, and oxygen species
Surface element compositions and surface species of the samples were determined by the
XPS technique. Fig. 7 illustrates the Co 2p3/2, Pd 3d, and O 1s XPS spectra of the samples,
and the related surface element compositions are inserted in Fig. 7. It can be seen from Fig.
7A that the Co 2p3/2 signal at binding energy (BE) = 779.8 eV of each of the samples was
assigned to the surface Co3+ species, whereas the Co 2p3/2 signal at BE = 781.8 eV together
with the shakeꢀup satellite at BE = 786.4 eV was ascribed to the surface Co2+ species [14,24].
By using the curveꢀfitting approach, one can decompose the asymmetrical Pd 3d XPS
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spectrum (Fig. 7B) of the Pd/mesoꢀCo3O4 sample into four components at BE = 336.1, 337.9,
341.3, and 343.1 eV, in which the two components at BE = 336.1 and 341.3 eV were
assignable to the surface Pd0 species, whereas the two components at BE = 337.9 and 343.1
eV were attributable to the surface Pd2+ species [12,35]. Only Pd0 species, however, were
detected on the surface of the Pd/mesoꢀCoO sample. It is generally believed that the interface
electron transfer from the noble metal NPs to the support can give rise to the rise in oxidized
noble metal concentration. In the present work, the interaction of Pd0 + 2Co3+ → Pd2+ + 2Co2+
would occur between the Pd NPs and the mesoꢀCo3O4, thus leading to coꢀexistence of the
surface Pd0 and Pd2+ species on the Pd/mesoꢀCo3O4 surface. Since there was a less amount of
Co3+ species on the mesoꢀCoO surface, only Pd0 species was detected on the surface of the
Pd/mesoꢀCoO sample. After deposition of Pd NPs on the mesoꢀCo3O4 surface, the surface
Co2+/Co3+ molar ratio increased from 0.54 to 0.66, indicating that the surface Co2+
concentration increased. This result reasonably explains existence of the surface Pd2+ species
on the surface of the Pd/mesoꢀCo3O4 sample. In other words, as compared to the mesoꢀCo3O4
support, the supported noble metal samples possessed a higher oxygen vacancy concentration
in the vicinity of the Co3O4 surface [33].
As shown in Fig. 7C, the asymmetrical O 1s spectrum of each of the samples could be
decomposed into three components at BE = 529.7, 531.4, and 533.1 eV, attributable to the
surface lattice oxygen (Olatt), adsorbed oxygen (Oads, e.g. O2−, O22− or O−), and adsorbed water
species [7,31], respectively. With the loading of Pd NPs, the Oads/Olatt molar ratio increased,
implying that the loading of Pd further enhanced the Oads species concentration on the sample
surface. Although the Oads/Olatt molar ratio on the mesoꢀCo3O4 surface increased from 0.47 to
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0.70 after loading of Pd NPs, their Oads/Olatt molar ratios were still lower than those
(0.86−1.12) on the surface of the mesoꢀCoO and Pd/mesoꢀCoO samples. It has been generally
established that the higher the surface oxygen vacancy density, the easier the activated
adsorption of O2 molecules, and hence the better is the performance of a catalyst for VOCs
combustion [37]. One of our recent works demonstrated that CoO possessed a higher oxygen
vacancy amount than Co3O4, showing better oxygen activation ability and favoring formation
–
2–
–
of the highly active O2 and O2 (especially O2 ) species [26]. A higher Oads species
concentration usually indicates a higher oxygen vacancy density on the sample surface. It is
understandable that the higher Oads species concentration on the surface of the mesoꢀCoO and
Pd/mesoꢀCoO samples implies that the two samples would exhibit better oxygen activation
ability than the mesoꢀCo3O4 and Pd/mesoꢀCo3O4 samples. To estimate the amounts of
different oxygen species, O2–TPD profiles of the samples were recorded, as shown in Fig. 8A.
There were three oxygen desorption peaks of the mesoꢀCo3O4 sample: the small peaks at 170
o
and 370 C were due to the surface adsorbed oxygen species (OWS) bound to the Co2+ and
o
Co3+ ions in different coordination environments, whereas the big peak at 795 C was
assignable to the desorption of lattice oxygen in mesoꢀCo3O4. Different from the mesoꢀCo3O4
sample, the mesoꢀCoO sample exhibited a strong desorption at 430 oC (assigned to the surface
adsorbed oxygen species (OSS) bound to the Co3+ ions) and a small peak at 525 oC (due to the
desorption of lattice oxygen from CoO) [14,38]. The oxygen desorption amounts of the
samples are summarized in Table 3. It is seen that after loading of Pd NPs, no apparent
influence on oxygen desorption was observed. Obviously, the oxygen desorption amounts
from the mesoꢀCoO and its supported Pd samples (171.3–179.1 ꢁmol/g) were over 20 times
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as much as those (6.1–7.6 ꢁmol/g) from the mesoꢀCo3O4 and its supported Pd samples. The
results of oxygen desorption experiments were in good agreement with those of the XPS
characterization, suggesting that the mesoꢀCoOꢀbased samples exhibited high oxygen
activation ability.
3.4. Reducibility
Fig. 8B illustrates the COꢀTPR profiles of the samples, and their quantitative analysis
results are listed in Table 3. There were three main reduction steps for the mesoꢀCo3O4
sample: The reduction bands below 260 oC were assigned to the removal of adsorbed oxygen
o
species [7], whereas the reduction bands above 260 C were ascribed to the reduction of
o
Co3O4. Specifically, the reduction band centered at 300 and 350 C were due to the reduction
of surface and bulk Co3+ to Co2+ species [24,39], respectively. For the mesoꢀCoO sample, the
broad reduction band above 300 oC was a sequential reduction of the Co2+ species by CO [38].
The peak corresponding to the reduction of adsorbed oxygen species slightly shifted to a
lower temperature after loading of Pd NPs on mesoꢀCo3O4. The CO consumption of the
o
samples below and above 260 C was in the ranges of 1.1–1.7 and 5.8–13.6 mmol/gcat,
respectively. The reduction profile of the Pd/mesoꢀCoO sample was rather similar to that of
the mesoꢀCoO sample, and no apparent changes in reducibility were observed in the
mesoꢀCoOꢀsupported Pd sample. It might be due to the weak electron transfer ability between
Pd and mesoꢀCoO, demonstrated by the XPS results that the Pd on mesoꢀCoO was metallic.
4. Discussion
It is generally accepted that the total oxidation of VOCs over the reducible oxide catalysts
follows a redox or Mars–van Krevelen (MVK) mechanism [40−43]. For instance, Wu et al.
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proposed a MVK mechanism for the total oxidation of toluene and pꢀxylene over the
Au/Co3O4 catalysts [41], the authors pointed out that the good activity of Au/Co3O4 was due
to its high concentration of superficial electrophilic oxygen species and oxygen vacancies
originated from a strong interaction in the Au/Co3O4 system. Solsona and coworkers used an
interesting transient reactor (TAP reactor) to carry out the study on the oxidation of propane
over Au/CoOx [42]. The positive role of gold in metallic Au0 form on the activity of cobalt
oxide was attributed to an increase of the exchange rate between lattice and surface oxygen
in/on Au/CoOx, and the reꢀoxidation of the partially reduced sample was associated with a
higher concentration of oxygen vacancies (i.e., higher oxygen storage capacity). Therefore,
the oxygen activation ability and active oxygen species of a catalyst have been considered as
the important factors influencing its catalytic performance for VOCs combustion.
The results of XPS and O2–TPD characterization demonstrated that mesoꢀCoO and
Pd/mesoꢀCoO possessed better oxygen activation ability than mesoꢀCo3O4 and
Pd/mesoꢀCo3O4, particularly the ability to form the surface adsorbed oxygen species (OSS)
bound to the Co3+ ions. However, the catalytic activity for oꢀxylene combustion over the
samples decreased according to the order of Pd/mesoꢀCoO > Pd/mesoꢀCo3O4 > mesoꢀCoO >
mesoꢀCo3O4. If the oxygen activation ability of the samples determined their catalytic
performance, the mesoꢀCoO sample would perform much better than the Pd/mesoꢀCo3O4
sample. In fact, Pd/mesoꢀCo3O4 exhibited higher activity than mesoꢀCoO. This result
obviously suggests that there might be other factors influencing the catalytic activity. In our
study, the sample with better oxygen activation ability performed better (e.g., Pd/mesoꢀCoO >
Pd/mesoꢀCo3O4; mesoꢀCoO > mesoꢀCo3O4). In order to identify the active oxygen species, in
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situ DRIFT spectra of the Pd/mesoꢀCo3O4 and Pd/mesoꢀCoO samples after exposure to an air
o
flow (10 mL/min) at 40 C for different time were recorded, shown in Fig. S3. When the
exposure time increased, the adsorbed oxygen molecules at 1245 cm–1 and peroxide species
(O22–) at 1068 cm–1 [26,44] on the samples were detected. There were the same active oxygen
2–
species (O22–) on the samples, but a big difference in O2 concentration after 120 min of
2–
oxygen adsorption (Fig. 9A). Obviously, the O2 concentration decreased in the sequence of
Pd/mesoꢀCoO > mesoꢀCoO > Pd/mesoꢀCo3O4 > mesoꢀCo3O4, which well agreed with the
results of XPS and O2–TPD investigations. The similar adsorbed oxygen species came from
the activation of gasꢀphase oxygen molecules on the samples. That is to say, the influence on
catalytic activity by the active oxygen species was negligible. As discussed above, the
sequence in catalytic performance of the samples did not match the order in their oxygen
activation ability (e.g., mesoꢀCoO), which clearly suggested that in addition to the effects of
oxygen activation ability and active oxygen species, there were still other important factors
influencing their catalytic activities of the asꢀobtained samples.
Since the activation of oxygen molecules was not the rateꢀdetermining step for oꢀxylene
complete oxidation, the oꢀxylene adsorption ability of the samples was evaluated. It is seen
from Fig. S4 that two signals of oꢀxylene desorption were detected on the samples in the
temperature range of 150–550 oC. The peaks centered at 200 and 400 oC were attributed to the
desorption of weakly and strongly chemically adsorbed oꢀxylene on the samples [45]. There
were no apparent oꢀxylene desorption peaks on the mesoꢀCo3O4 and mesoꢀCoO samples. The
loading of Pd significantly enhanced the oꢀxylene adsorption ability of the supports, in which
the Pd/mesoꢀCoO sample exhibited the largest oꢀxylene desorption. In other words, the
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Pd/mesoꢀCoO sample possessed the strongest ability of oꢀxylene adsorption. This result
suggests that the loading of Pd NPs on mesoꢀCo3O4 or mesoꢀCoO was beneficial for oꢀxylene
adsorption. To further determine the oꢀxylene adsorption ability, an in situ DRIFT experiment
o
was conducted on the samples in the presence of 1000 ppm oꢀxylene at 40 C. As shown in
Fig. S5, the intensity of absorption bands corresponding to oꢀxylene increased with the
elongation in exposure time. From Fig. 9B, one can observe the absorption bands at
2800–3100 and 740 cm−1 (the stretching vibration and outꢀplane bending vibration of C–H
bonds, respectively), the absorption bands at 1450 and 1630 cm−1 (the benzene ring inꢀplane
vibration and outꢀplane deformation vibration, respectively), and the absorption bands at 3570
and 3690 cm−1 (surface OH group of the samples interacting with the methyl group of
oꢀxylene) [46], and the results also confirm the adsorption of oꢀxylene on the samples.
Apparently, the signals due to oꢀxylene adsorption on the supported Pd samples were much
stronger in intensity than those on the supports, implying that the loading of Pd NPs
considerably enhanced the adsorption ability of oꢀxylene. Moreover, the presence of the
absorption bands at 1068 and 1245 cm−1 again indicates a good oxygen activation ability of
the samples [44]. Taking the good catalytic performance into account, one can easily
understand that the excellent catalytic activity of Pd/mesoꢀCoO was associated with its good
oꢀxylene adsorption and oxygen activation ability, particularly the oꢀxylene adsorption ability.
It has been reported that toluene and O2 were competitively adsorbed on the Pd surface
according to the reaction orders measured over the Pd/ZrO2 catalysts [47], which suggests that
VOCs could be preferentially adsorbed on the surface of Pd NPs. In the present work, the
supported Pd samples exhibited stronger ability of oꢀxylene adsorption than the supports,
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clearly supporting the conclusion that oꢀxylene molecules could be adsorbed on the surface of
Pd NPs, rather than the surface of the mesoꢀCo3O4 or mesoꢀCoO support. The phase of Pd in a
supported Pd catalyst has been generally believed to play a vital role in influencing its
catalytic performance for VOCs combustion. A number of researchers claimed the importance
of the Pd0 species for VOCs combustion [34,47,48]. For example, it was reported that the
intrinsic stability of Pd0 was the reason for high activity of Pd/ZrO2 in the total oxidation of
oꢀxylene [34] and toluene [47]. Some authors believed that the most active Pd phase was the
mixture of metallic Pd0 and oxidized Pd (PdO) under practical reaction conditions,
particularly for methane combustion [49]. It is well known that Pd and oxygen vacancies are
beneficial for oxygen activation. The generated active oxygen species could immediately
react with the adsorbed VOCs molecules on Pd NPs, hence exhibiting high catalytic activity
for VOCs combustion. In this work, there were only metallic Pd0 species on Pd/mesoꢀCoO,
but both metallic and oxidized Pd species on Pd/mesoꢀCo3O4. Since Pd/mesoꢀCoO possessed
higher oꢀxylene desorption amount than Pd/mesoꢀCo3O4 (the results of TPD and in situ
DRIFT characterization), we believe that the metallic Pd0 was beneficial for oꢀxylene
adsorption. For Pd/mesoꢀCo3O4, although Co3O4 exhibited lower oxygen activation ability,
the adsorbed oꢀxylene species on metallic Pd0 could quickly contact with the active oxygen
species generated on PdO or mesoꢀCo3O4, thus the combustion of oꢀxylene took place on the
Pd NPs and/or at the interface between Pd NPs and mesoꢀCo3O4. For Pd/mesoꢀCoO,
mesoꢀCoO possessed good oxygen activation ability to generate the active oxygen species,
and the metallic Pd0 NPs were beneficial for oꢀxylene adsorption. Therefore, the adsorbed
oꢀxylene species could immediately react with the active oxygen species at the interface
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between Pd NPs and mesoꢀCoO.
5. Conclusions
The mesoꢀCo3O4 sample was prepared via a nanocasting strategy, the mesoꢀCoO sample
was obtained from a reduction process of mesoꢀCo3O4 by glycerol, and the Pd NPs were
highly dispersed on the surface of mesoꢀCo3O4 and mesoꢀCoO via the PVAꢀprotected
reduction route. The supported Pd samples performed much better than their supports for
oꢀxylene combustion, in which the Pd/mesoꢀCoO sample exhibited the highest catalytic
activity (T90% = 173 °C), giving rise to the reaction rate at 170 °C of 3.5 and 84 times higher
than those over the Pd/mesoꢀCo3O4 and mesoꢀCoO samples, respectively. It is demonstrated
that mesoꢀCoO exhibited high ability of oxygen activation, and the loading of Pd NPs was
beneficial for oꢀxylene adsorption. Therefore, it is concluded that the excellent catalytic
performance of Pd/mesoꢀCoO was associated with its highly active Pd–CoO interface. Such a
finding may be useful for the design of novel and highꢀperformance catalysts for the
combustion of VOCs.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the NSF of China (21377008, 21477005, and 21677004),
National High Technology Research and Development Program of China (863 Program,
2015AA034603), Beijing Nova Program (Z141109001814106), the NSF of Beijing Municipal
Commission of Education (KM201410005008), and Scientific Research Base
Construction−Science and Technology Creation Platform−National Materials Research Base
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Construction.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at
doi:10.1039/c7cyXXXXXX
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Captions of Tables and Figures
Table 1. BET results, average Pd particle sizes, average crystallite sizes, and real Pd contents
of the samples.
Table 2. Catalytic performance and apparent activation energies (Ea) of the asꢀprepared
samples for oꢀxylene combustion.
Table 3. Oxygen desorption and H2 consumption of the samples.
Fig. 1. (A) Smallꢀangle and (B) wideꢀangle XRD patterns of the samples.
Fig. 2. TEM images of (a−c) Pd/mesoꢀCo3O4 and (d−f) Pd/mesoꢀCoO as well as the Pd
particleꢀsize distributions of the samples.
Fig. 3. (A) Nitrogen adsorption–desorption isotherms and (B) poreꢀsize distributions of the
samples.
Fig. 4. (A) Oꢀxylene conversion as a function of temperature and (B) Arrhenius plots for
oꢀxylene combustion over the samples.
Fig. 5. Oꢀxylene conversion over Pd/mesoꢀCoO (A) at different SVs and (B) at 170 oC within
40 h of onꢀstream oꢀxylene combustion.
Fig. 6. Effects of water vapor and carbon dioxide on oꢀxylene combustion over
Pd/mesoꢀCoO.
Fig. 7. (A) Co 2p3/2, (B) Pd 3d, and (C) O 1s XPS spectra as well as the surface elemental
compositions (insets) of the samples.
Fig. 8. (A) O2ꢀTPD and (B) COꢀTPR profiles of the samples.
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Page 29 of 42
Catalysis Science & Technology
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DOI: 10.1039/C7CY02007D
Fig. 9. In situ DRIFT spectra of the samples after exposure to (A) air flow (10 mL/min) and
(B) 1000 ppm oꢀxylene flow (10 mL/min) at 40 oC for 2 h.
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