A novel supported Cu catalyst with highly dispersed copper nanoparticles and its remarkable catalytic performance in methanol decomposition

Article abstract of DOI:10.1039/c4ra10101d

Using starch modified SiO₂ as the support, an efficient copper catalyst with superior catalytic performance for methanol decomposition can be obtained, suggesting the key role of the nature of the support in preparing Cu/SiO₂ catalysts with stable and highly dispersed copper nanoparticles using an impregnation method. This journal is

Full text of DOI:10.1039/c4ra10101d

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A novel supported Cu catalyst with highly dispersed
copper nanoparticles and its remarkable catalytic
performance in methanol decomposition†
Cite this: RSC Adv., 2014, 4, 52008
Shaojun Qing,‡a Xiaoning Hou,‡a Yajie Liu,a Hongjuan Xi,a Xiang Wang,b
Received 9th September 2014
Accepted 7th October 2014
c
d
a
*
Cheng-meng Chen, Zhiwei Wu and Zhixian Gao
DOI: 10.1039/c4ra10101d
www.rsc.org/advances
Using starch modified SiO2 as the support, an efficient copper catalyst method, and the catalyst prepared via the impregnation method
with superior catalytic performance for methanol decomposition can exhibit poor copper dispersion and inferior catalytic
be obtained, suggesting the key role of the nature of the support in activity.6,7,9a,9b
preparing Cu/SiO2 catalysts with stable and highly dispersed copper
The preparations of Cu supported on SiO2 by impregnation
method have been widely adopted due to its simplicity and the
easy control of conditions. However, good results have been
obtained only with M-Cu/SiO2 systems. For example, Akio
TADA12 prepared a catalyst with high selectivity and stability for
methanol decomposition by using nickel to modify Cu/SiO2.
Ching-Shiun Chen13 reported that the dispersion of copper
could be signicantly improved by modication with Fe, and
both the activity and stability were enhanced for the reverse
water gas shi reaction. Up till now, little work has been done
on enhancing catalytic performance of Cu/SiO2 via modifying
the SiO2 support. Our previous work14 showed that using the
starch modied SiO2 as support an efficient Cu/SiO2 for
preparing methyl formate by dehydrogenation of methanol at
200 ꢀC could be obtained by impregnation method, and the
results were attributed to the surface silanol (Si–OH) concen-
tration being modied by starch, which were benecial to the
dispersion of Cu species. In this contribution, the catalyst
system is further researched aiming at a better understanding
of the mechanism of starch modication and ꢀthe catalytic
performance for methanol decomposition at 300 C.
nanoparticles using an impregnation method.
Because of their low cost and good catalytic activity, Cu/SiO2
catalysts have been widely investigated in many catalytic reac-
tions, such as hydrogenolysis of esters to alcohols,1 dehydro-
genation of alcohols,2 decomposition of methanol,3 etc.
However, deactivation of Cu/SiO2 is unavoidable during the
reaction process. This can be attributed to the weak interaction
between copper and SiO2,4 and to the inferior thermostability of
5
¨
copper due to its low Huttig temperature, so copper metal
agglomerates and sinters easily, which shows a gradual
decrease of activity with time on stream.
Therefore, many research efforts toward stable Cu catalysts
have been carried out aiming at enhancing the stability of Cu/
SiO2 catalyst. The improvement has been achieved by using
novel preparation methods such as atomic layer epitaxy tech-
nique,6 electroless,7 deposition precipitation,8 ion-exchange9
and ammonia evaporation hydrothermal,10 and by modication
with a second element.11 These studies clearly demonstrate that
higher dispersion of copper helps delaying the agglomeration of
copper, giving a catalyst with enhanced catalytic performance.
The dispersion of copper is greatly affected by the preparation
The starch modied SiO2 support (denoted as SiO2-S) calci-
nated at 500 ꢀC was denoted as SiO2-S-500 (starch was
completely removed from the support, Fig. S1, ESI†). Aer
loading 22.5% of Cu via impregnation, the samples were sub-
jected to pre-calcination in different atmosphere at 300 ꢀC,
giving four catalysts for catalytic testing. (Details are listed in
ESI†).
aInstitute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China.
E-mail: gaozx@sxicc.ac.cn
bInstitute of Applied Chemistry, Department of Chemistry, Nanchang University,
Fig. 1 presented catalytic performance of the above four
catalysts for methanol decomposition, and among which Cu/
SiO2-S[300-Air] exhibit the best activity and stability. The initial
methanol conversion was 91.8% and decreased to 64.0% at 30
h, then levelled off up to 170 h. However, on Cu/SiO2[300-Air]
catalyst, the initial methanol conversion was only 45.7%, and
dropped drastically to 16.9% within 4.4 h. The data clearly
Nanchang 330031, China
cKey Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of
Sciences, Taiyuan 030001, China
dState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan 030001, China
† Electronic supplementary information (ESI) available: More experimental
details and results of catalyst characterization. See DOI: 10.1039/c4ra10101d
‡ These authors contributed equally.
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Fig. 2 XRD patterns of fresh catalysts (after calcination).
Fig. 1 The variations of methanol conversion with time on stream.
Reaction conditions: temperature ¼ 300 ꢀC, Weight Hourly Space
Velocity (WHSV) ¼ 1.58 hꢁ1, pressure ¼ 0.1 MPa.
indicated that the catalytic behaviour of Cu/SiO2 catalyst was
signicantly enhanced due to the pre-modication of support
by starch prior to Cu loading.
The evaluation data of Cu/SiO2-S[300-N2] showed an initial
methanol conversion of 77.0%, then gradually dropped to
44.9% within 153 h, with no stable platform appeared. In a
control experimental, when the support was pre-calcinated in
air to remove the starch, the obtained Cu/SiO2-S-500[300-Air]
revealed very rapid deterioration in catalytic activity, which is
similar to that of bare Cu/SiO2[300-Air]. These data demon-
strated that the effect of starch on improving the catalytic
performance occurred during the process of catalyst prepara-
tion and correlated well with the calcination atmosphere.
On the four catalysts, the major reaction of methanol was
decomposition, and the selectivity of H2 and CO was more
than 96%.
Fig. 3 TEM and HRTEM images of reduced catalysts. A and B:
Cu/SiO2[300-Air], C and D: Cu/SiO2-S[300-Air].
To understand the difference in catalytic performance, the
fresh catalysts (aer calcination) were characterized. With XRD
(Fig. 2), the typical lines assigned to CuO (JCPDS no. 45-1548)
were recorded except Cu/SiO2-S[300-N2]. Cu/SiO2-S[300-Air]
displayed broadened and decreased diffraction peaks as [300-Air] was hindered, producing smaller Cu particles, less
compared with both Cu/SiO2[300-Air] and Cu/SiO2-S-500[300- than 8 nm, and highly dispersed on the support (Fig. 3C). The
Air]. This indicated that CuO particles were smaller (Table S1 above results were further supported by characterizations
ESI†) with a higher dispersion in Cu/SiO2-S[300-Air], comparing with XRD (Fig. S5, ESI†) and dissociative N2O adsorption
with the other two catalysts.11b This was also demonstrated by (Table S1, ESI†). The used catalysts displayed similar XRD
the results of H2-TPR (Fig. S2, ESI†), XPS (Fig. S3 and Table S1, patterns (Fig. S5, ESI†) to those of reduced ones, but with
ESI†) and TEM-HRTEM (Fig. S4, ESI†).
increased intensities, indicating agglomeration of Cu metal
The Cu/SiO2[300-Air] and Cu/SiO2-S[300-Air] were chosen to during the reaction process. Considering the testing duration
be pre-reduced at 300 ꢀC by H2 for two hours, and then was 200 h for Cu/SiO2-S[300-Air], while only 4.4 h for Cu/
characterized by TEM and HRTEM (Fig. 3). In the HRTEM SiO2[300-Air], it could be inferred that the agglomeration rate
image, Cu(111) planes with d-spacing of 0.208 nm was iden- of Cu was much slower for the former. The XRD patterns of
tied, indicating the formation of metal Cu aer reduction.15 both reduced and used catalysts (Fig. S5, ESI†) revealed a
Obviously, Cu particles in the Cu/SiO2[300-Air] sintered seri- small portion of Cu2O phase, suggesting air oxidation of Cu at
ously (Fig. 3A), while the Cu agglomeration in the Cu/SiO2-S room temperature.16
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On the basis of above results, it was concluded that the high mechanism of starch modication is that the Cu(NO3)2
activity and good stability of Cu/SiO2-S[300-Air] catalyst corre- decomposition process is facilitated by the presence of starch in
late well with the highly dispersed small and stable copper the support, and further study is going on in the author's lab.
metal particles, which results from the using of starch modied The results strongly indicate that the property of support plays a
SiO2 as the support.
crucial role in preparing an efficient catalyst, which pave a way
In order to discuss the mechanism of starch modication, for stabilizing usually unstable metal species such as copper by
the precursors of catalysts (before calcination) were character- pre-modication of the support.
ized by TG-DTA (Fig. 4). The rate of weight loss was faster for
Cu(NO3)2/SiO2-S than that of Cu(NO3)2/SiO2 and Cu(NO3)2/SiO2-
S-500 (Fig. 4A). Simultaneously, the process was observed to be
Acknowledgements
exothermic for Cu(NO3)2/SiO2-S, while endothermic for both
The authors thank the nancial support from the Innovation
Cu(NO3)2/SiO2 and Cu(NO3)2/SiO2-S-500 (Fig. 4B). In the N2
Foundation of Institute of Coal Chemistry, Chinese Academy of
atmosphere, the heat release with Cu(NO3)2/SiO2-S was ascribed
Sciences (2011SQNRC03).
to the redox reaction between the starch and Cu(NO3)2, which
was conrmed by formation of Cu as identied by XRD (Fig. 2).
When air was used as the calcination atmosphere, the heat
release was stronger, which was attributed to the additional
Notes and references
starch oxidation by air. Consequently, decomposition of
Cu(NO3)2 started at lower temperatures with an accelerated rate,
thus resulting in the formation of highly dispersed CuO on the
support.
In conclusion, using starch modied SiO2 as the support, an
efficient supported copper based catalyst has been obtained.
Characterization data demonstrate that highly dispersed copper
species, CuO or Cu, exist as small particles, and their agglom-
eration and sintering is restrained, thus revealing high activity
and stability during the catalytic reaction. The suggested
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