Chapter 5 Nanostructured ZnO Nanorod@Cu Nanoparticle as Catalyst for
6.2 Structural Characterization of CuO Nanotip/ZnO nanorod Nanocomposites76
Our initial attempts employing a surfactant-free approach
(ammonia-evaporation-induced synthetic method) for synthesis of CuO
nanostructures[106] resulted in formation of radiating nanosheets as shown in
Figure 6.1a. The high-resolution transmission electron microscopy (HRTEM)
image in Figure 6.2 provides specific structural information about an individual
different morphologies of CuO were observed, ranging from nanosheet to NT
(Figure 6.1b). Figure 6.3 presents the TEM and HRTEM images of CuO NT/ZnO
NR catalyst precusors synthesized in this work, showing that NTs not only
adhere to but also directly conjoin with the NR. While detailed understanding of
the growth mechanism is essential, the nanoarchetecture provides opportunity for
advanced applications such as nano-catalysis.
In order to gain an insight into the changes in microstructure, electronic
configuration, and reducibility of various CuO NT/ZnO NR catalyst precusors,
X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray
absorption spectroscopy (XAS), N2O titration, micro-Raman spectroscopy, and
temperature-programmed reduction (TPR) investigations were performed. Figure
6.4a presents the XRD patterns of CuO NT/ZnO NR catalyst precusors after MW
irradiation and conventional thermal annealing. Noticeable shifts in the CuO
diffraction peaks are observed, especially after MW irradiation. This is due,
presumably, to CuO being partially dissolved in the ZnO lattice, leading to
creation of microstrain between NTs and NRs. Moreover, the CuO(111) peak
shows a significant up-shift after MW irradiation, implying that a specific MW
treatment can enhance the degree of microstructural disorder in CuO NTs. In
observations are well complemented by the micro-Raman results presented in
Figure 6.4b, in which a main peak at around 438 cm-1 corresponding to the
characteristic E2(high) vibrational mode of ZnO, and another major peak at about
286 cm-1 (Ag mode) attributed to the vibrations of oxygen atoms in CuO were
observed.[107,108]Significant variations in peak position as well as full width at
half maximum (FWHM) of both E2(high) and Ag modes occur as a result of the
formation of CuO/ZnO nanoarchitectures. These variations are believed to be
strongly related to the strain-induced shift and broadening of the phonon modes.
Moreover, higher FWHMs of E2(high) and Ag modes after MW treatment
suggest a significant change in band structure of NT/NR nanoarchitectures,
indicating that specific MW treatment can enhance the magnitude of lattice
distortions. Further evidence is obtained by HRTEM observations (presented in
Figure 6.5), that clearly shows a highly distorted lattice as a result of MW
irradiation.
Figure 6.4c presents the Cu 2p core level XPS spectra of NR-NT catalyst
precusors after MW irradiation and conventional thermal annealing. The position of
the main peak, corresponding to Cu 2p3/2 transition, shifts to higher binding energy
for CuO/ZnO catalyst precusors. This clearly indicates a modification of the electron
main Cu 2p3/2 peak continues to up-shift after MW irradiation. This up-shift results
from further distortions of the CuO lattice due to the presence of microstrain
between NTs and NRs. This behavior agrees well with the XRD and micro-Raman
results discussed above. TPR analysis was also carried out to determine the
reducibility of surface oxygen in CuO/ZnO catalyst precusors presented in Figure
6.4d. In contrast to CuO NDs, the CuO NT/ZnO NR catalyst precusors apparently
lower the reduction temperature of CuO. This is probably due to weakening of the
Cu-O bond due to the presence of strongly bound ZnO species. More surprisingly,
CuO NT-immobilized ZnO NR catalyst precusors exhibit a notable shift in the
reduction peak to the lowest temperature after MW irradiation, as compared to the
conventional thermal annealing. Furthermore, a narrower TPR peak is observed in
the MW-treated CuO NT/ZnO NR sample. They indicate far superior redox
properties for CuO NT/ZnO NR catalyst precusors, due to the SMSI effect caused
by MW irradiation.
6.3 Test of Methanol Reforming Reaction
Figure 6.6a and 6.6b compare the effects of MW and conventional thermal
treatments on the methanol conversion as well as hydrogen production rates.
After H2 pre-reduction of CuO NT/ZnO NR catalyst precursors, all experiments
(XANES) measurements (Figure 6.7a), showing all the CuO will turn to metallic
Cu (served as active species for MRR). Meanwhile, most experiments were
repeated for several catalysts. No significant deactivation of catalysts was noted
in these experiments. In addition, the structural factor such as morphology and
Cu surafce area, which would generally affect the activity of Cu-based catalysts
for MRR, have been completely excluded via N2O titration measurement,
showing no remarkable change of Cu surafce area (about 45.7 m2g-1) with or
without MW irradiation and conventional thermal annealing. The methanol
conversion and hydrogen production rates over the pristine Cu/ZnO working
catalysts were close to 70 % and 170 mmol gcat-1 h-1 at 290 oC, respectively.
Interestingly, after MW irradiation, the methanol conversion and hydrogen
production rates increased to 96 % and 230 mmol gcat-1 h-1 (at only 250 oC)
respectively, which are significantly higher than those obtained by the
conventional thermal annealing of the NT/NR catalysts. Furthermore, the
amounts of CO produced in the gas outlet is another indication of the activity.
The CO concentration of only 170-210 ppm at 250 oC produced from the
MW-treated Cu NT/ZnO NR catalyst is substantially lower that the typically 300
ppm CO produced from a conventional thermal annealed samples. Therefore, the
Previous investigations pertaining to heat treatment of the catalytic materials
using MW irradiation also revealed selective heating of specific catalytic sites and
led to “molecular hot spots” in the catalysts.[109] For the binary system comprising
CuO/ZnO catalyst precusors, only the CuO component is known to be strong MW
absorber.[110] Therefore, hot spot formation at CuO sites decorated on the ZnO
support is likely to result in pronounced microstructural rearrangement at the
NT/NR interface due to selective MW absorption by CuO. This may account for the
good correlation between the increase in defects in NTs and the enhancement in
catalytic performance after MW treatment. In this respect, MW processing of the
hybrid CuO NT/ZnO NR catalyst precusors appears to provide a unique opportunity
for generating significant microstructural deformation at the NT/NR interface due to
the formation of hot spots as a consequence of selective dielectric heating, thus
leading to the desired creation of highly strained CuO NTs. Last but not least, the
advantage of MW-treated NT/NR working catalysts is further exemplified by the
excellent stability (only 6 % reduction after 60 hours operation) in MRR as shown in
Figure 6.6c. The reason is the chemical state of metallic Cu (served as active species)
could be maintained after long-term operation in MRR as shown in Figure 6.7a. This
is significantly enhanced comparative to the 11.5% and 33.8% reduction after 36
Cu lattice after continuous 60 hours operation (Figure 6.7b). From extended X-ray
absorption fine structure (EXAFS) analyses as shown in Figure 6.7b, a apparent
decrease in the intensity of the fist-shell (Cu-Cu bonding) over MW-treated Cu
NT/ZnO NR working catalysts after long-term operation in MRR could be observed.
6.4 Summary
In summary, we have demonstrated an effective MW-treatment technique to
enhance the catalytic activity of CuO NT/ZnO NR nanostructures for MRR. Not
only higher conversion efficiency and lower operation temperature, but also lower
CO production and remarkably enhanced stability of the MW-treated catalysts have
been achieved. It’s believed that formation of defects, microstrains and SMSI due to
the selective heating by MW-treatment may have contributed to such enhancement
in catalyst performance. This finding offers an alternative route using MW-treatment
to enhance the activity of a wide range of catalysts.
Figure 6.1 (a) SEM images of CuO nanostructures (inset: high magnification). (b)
SEM images of CuO NT/ZnO NR catalyst precusors (inset: high magnification).
Figure 6.2 HRTEM image of CuO nanodandelions, showing specific structural
information about an individual nanosheet grown along the [111] direction.
(a)
(b)
CuO NT
(a)
(b) (a)
(b)
CuO NT
Figure 6.3 (a) TEM and (b) HRTEM images of CuO NT/ZnO NR catalyst precursors,
showing that NTs not only adhere to NR but also directly conjoin with the NR.
30 32 34 36 38 40
100 200 300 400 500
After thermal treatment
100 200 300 400 500
After thermal treatment
Figure 6.4 (a) X-ray diffraction, (b) micro-Raman, (c) X-ray photo-electron
spectroscopy, and (d) TPR spectra of the as-prepared, after thermal treatment, and
after MW treatment CuO NT/ZnO NR catalyst precursors.
Figure 6.5 HRTEM image of one single CuO NT, clearly showing a highly distorted
lattice (indicated by the circle) as a result of MW irradiation.
140 160 180 200 220 240 260 280 300
140 160 180 200 220 240 260 280 300 0
H2 production rate (mmol gcat -1 h-1 )
140 160 180 200 220 240 260 280 300 0
140 160 180 200 220 240 260 280 300 0
H2 production rate (mmol gcat -1 h-1 )
Figure 6.6 Methanol reforming reaction profiles : (a) Methanol conversion rate and
(b) hydrogen production rate for Cu NTs/ZnO NRs working catalysts : as-prepared
(■), after thermal treatment (◆), and after MW treatment (▲). Reaction conditions:
H2O/O2/MeOH = 1/0.125/1, W/F = 21 kgcat s mol-1methanol. (c) Stability test in MRR
over MW-treated Cu NT/ZnO NR working catalysts at H2O:O2:MeOH = 1:0.125:1,
Temperature = 250 oC, and W/F = 21 kg s mol-1 .
8960 8980 9000 9020 9040 9060 9080
Cu K-edge
Normalized absorbance (a.u.)
Photon energy (eV)
CuO NT/ZnO NR catalyst precursors Cu NT/ZnO NR working catalysts Working catalysts after MRR
(a) (b)
0 2 4 6 8
Cu K-edge
FT
(
χ(
κ)
κ3)
R ( )
Cu NT/ZnO NR working catalysts Working catalysts after MRR
Cu-Cu
0 2 4
Å
6 8Cu K-edge
FT
(
χ(
κ)
κ3)
R ( )
Cu NT/ZnO NR working catalysts Working catalysts after MRR
Cu-Cu
8960 8980 9000 9020 9040 9060 9080
Å
Cu K-edge
Normalized absorbance (a.u.)
Photon energy (eV)
CuO NT/ZnO NR catalyst precursors Cu NT/ZnO NR working catalysts Working catalysts after MRR
(a) (b)
0 2 4 6 8
Cu K-edge
FT
(
χ(
κ)
κ3)
R ( )
Cu NT/ZnO NR working catalysts Working catalysts after MRR
Cu-Cu
0 2 4
Å
6 8Cu K-edge
FT
(
χ(
κ)
κ3)
R ( )
Cu NT/ZnO NR working catalysts Working catalysts after MRR
Cu-Cu
Å
Figure 6.7 (a) Cu K-edge XANES and (b) Cu K-edge EXAFS of CuO NT/ZnO NR
catalyst precursors with MW irradiation, MW-treated Cu NT/ZnO NR working
catalysts, and MW-treated Cu NT/ZnO NR working catalysts after continuously
operating 60 hours for MRR.
Chapter 7
O
2Plasma-activated CuO-ZnO Inverse Opals as High-performance Methanol Microreformer
7.1 Introduction
Three-dimensionally ordered macroporous (3DOM) nanostructures (so-called
inverse opals) have attracted considerable attention in recent time owing to a large
variety of their potential applications such as photonic crystals, molecular sieves,
catalysts, antireflective coatings, sensors, as well as electrodes for fuel and solar
cells.[111] In addition, such well-ordered nanoarchitectures also exhibit several
advantages, such as quick transfer of heat and mass, low pressure drop, and high
contact area between the catalysts and reactants, that render them ideal for gas
catalyses.[112-114] With the recent upsurge in non-conventional energy generation,
and environmental and resource conservation research, there is a growing interest in
the development of novel catalysts that can effectively and selectively promote the
desired reactions related to the production of clean energy.[115] Hydrogen is an
efficient energy source that can combine with oxygen in a fuel cell to produce
microreactor is a promising approach for hydrogen generation for use in portable
power sources, which avoids the hydrogen storage and safety related issues.[35,116]
The typical oxidative steam reforming of methanol, which combines the
endothermic steam reforming reaction with the exothermic partial oxidation reaction
can be generalized as follows[35,116]
CH3OH + 1/2H2O + 1/4O2 → CO2 + 2.5H2
However, one of the most critical impediments that hinders the realization of
microreformers for catalyses is poor utilization and low efficiency of catalyst
layer deposited inside the microchannels, usually in the form of a dense film.[81]
Recent approaches for overcoming this issue typically involve a two-step process,
wherein active catalysts are prepared in the first step, followed by its
immobilization on the surface of the microchannels in the second step. Various
deposition techniques have also been reported for several heterogeneously
catalyzed processes in microreactors.[117] These methods, however, do not yield
sufficient effective surface required for the reactants to effectively access the
catalytically active sites. Therefore, evolution of 3DOM nanoarchitectures that
enable integration of direct synthesis of catalyst and its immobilization on the
surface is essential for efficient catalytic reforming of methanol at low
various Cu-based catalysts for high-performance methanol reformers or methanol
synthesis due to their low cost, environmental friendliness, and natural abundance as
compared to the conventional precious-metal catalysts (Pt, Pd).[14,15,17,77,118]
More recently, numerous studies have focused on gaining in-depth understanding of
the effect of oxide supports/promoters towards enhancement of catalytic
performance, including their electronic structure, nanostructure, morphology, and
surface defect characteristics.[78,90,92,97,100,119] It is particularly noteworthy that
native defects (oxygen vacancies; Vo) on the surface of oxide supports have been
shown to generate active entities that facilitate the reforming reaction as well as CO
oxidation, when doped with rare earth or transition-metal elements.[120-123]
Nevertheless, these doping approaches are unfavorable for the low cost objectives
and environmental benignity, especially from the point of view of the lack of “green
chemistry”. To address this problem, we have attempted here to employ an
extremely simple technique of plasma-treatment to create oxygen vacancies (Vo) on
the oxide surface to improve the performance of catalytic methanol reforming
reaction (MRR). In this communication, we report for the first time fabrication and
characterization of O2-plasma activated CuO-ZnO inverse opals as
high-performance microreformers for methanol. As can be seen from Figure 7.1, our
oxygen vacancies (Vo) through O2-plasma treatment to produce additional active
entities for MRR.
7.2 Structural Characterization of O
2Plasma-treated CuO-ZnO Inverse Opals
The CuO-ZnO inverse opals were synthesized directly inside the microreactor
by simple wet-chemical methods, using the colloidal crystal templating (CCT)
approach (Figure 7.2).[111] Polystyrene (PS) opals were first self-assembled
inside the microchannels of a microreactor, followed by infiltration with metal
precursors (Cu2+, Zn2+) within the voids, and final removal of the opal templates
by calcinations. After synthesis, the inverse opals were exposed to O2 plasma for
3, 5, 10, and 15 min. The microstructural characterization was carried out by
using several analytical techniques including electron microscopy (EM), X-ray
diffraction (XRD), X-ray photoelectron spectroscopy (XPS), micro-Raman
spectroscopy, X-ray absorption spectroscopy (XAS) and N2
adsorption/desorption. The catalytic performance of O2-plasma treated CuO-ZnO
3DOM nanoarchitectures in MRR was systematically evaluated.
Figure 7.3 shows the morphology and microstructure of hierarchical 3DOM
nanostructures. The scanning electron microscopy (SEM) image illustrates a
fabricated exhibit a uniform pore size of ~ 300 nm, interconnected by ~220 nm
windows between the adjacent pores. This represents a shrinkage of about 40%
during calcinations since PS beads used as sacrificial templates were of ~ 500 nm
size. One of the most significant advantages on the hierarchical 3DOM
nanoarchitectures, which are clearly distinct from the traditional catalysts, is the
high void fraction resulting in large surface area they offer for effective surface
contact between the reactants and catalysts. To further elucidate the inner
architectures of the inverse opals, the surface area and porosity were estimated
from the nitrogen adsorption-desorption isotherms (Figure 7.4). The
nanoarchitectures exhibited a typical type IV isotherm according to the IUPAC
classification. The Brunauer-Emmertt-Teller (BET) analysis of the isotherm
confirmed the presence of the mesoporous structure with a high specific surface
area of 281 m2 g-1, as well as a bimodal pore size distribution of ~3.1 and ~6 nm,
as determined through the Barrett-Joyner-Halenda (BJH) desorption pore
distribution method. As a result, this nanoarchitecture provides an isotropic
porous structure with easily accessible pore openings from any directions, which
is extremely desirable for various electrochemical and catalytic applications. The
crystal structure of these inverse opals was studied by XRD analysis, which
(NC) on the ZnO support shows that the surface of CuO NC with a particle size
of about 10 nm is partially covered by ZnO, preventing it from sintering during
MRR. Additionally, several disordered wormhole-like mesopores (indicated by
the circles) are also observed. This agrees very well with the BET results. A
higher-resolution TEM image (Figure 7.3b) reveals that the (111) plane of the
CuO NC is in contact with the (101) plane of ZnO support. Further investigations
with elemental mappings via energy-filtered TEM revealed the presence of
uniformly dispersed CuO NCs surrounded by ZnO supports inside the
hierarchically inverse opals (Figure 7.5).
In order to gain further insight into the evolution of defects during O2
plasma-exposure, micro-Raman analyses were performed. Figure 7.3c presents
the typical Raman spectra of CuO-ZnO 3DOM nanoarchitectures for different
durations of plasma-exposure. The spectra show several peaks characteristic of
various vibrational modes of ZnO and CuO. The presence of a sharp and
dominant E2 (high) mode at 438 cm-1, which is associated with the vibration of
oxygen atoms in ZnO, indicates the wurtzite nature of ZnO.[124] After plasma
treatment, this mode becomes weak and significantly broader, presumably due to
the plasma-induced lattice disorder and a change in band structure of ZnO.[124]
re-appearance of a strong E2 (high) mode. On the other hand, the broad peak at
500-700 cm-1 can be deconvoluted into two Lorentzian components viz, A1 (LO)
& E1 (LO) mode at 575 cm-1 (originating from the Vo of ZnO) and Bg mode at
630 cm-1 related to CuO, respectively.[124,125] Thus, the concentration of Vo
was calculated by integrating the intensity of the 575 cm-1 peak and taking its
ratio with the sum of the 575 and 438 cm-1 peaks. The concentration of Vo on
ZnO thus obtained, was plotted as a function of plasma-exposure time (Figure
7.6), which reaches a maximum value at 10 min of plasma-exposure, manifesting
the highest concentration of Vo produced by O2 plasma treatment. In contrast,
excess plasma-exposure may cause compensative effect leading to filling up of
the oxygen vacancies by O-doping.
In order to understand the modification of electronic structure in CuO-ZnO
inverse opals with O2-plasma treatment, XPS measurements were carried out, the
results of which are illustrated in Figure 7.7. The O 1s core level spectra of
CuO-ZnO 3DOM nanoarchitectures after plasma treatment in O2 in Figure 7.7A
display a slight shift of the main peak towards higher binding energy, likely as a
result of the modification in band bending.[126-128] Furthermore, to probe the
nature of charge-transfer in CuO-ZnO nanoarchitectures after O2-plasma treatment,
Zn 2p3/2 spectra after O2 plasma-treatment are likely due to the transfer of electron
from ZnO to CuO. This is consistent with the increase in density of Vo on ZnO
surface with plasma treatment, which involves loss of electrons from ZnO.
7.3 Test of Methanol Reforming Reaction
Figure 7.8 compares the effects of material architecture and O2-plasma
treatment on the methanol conversion and rates of hydrogen production during
MRR, respectively. After hydrogen pre-reduction, all experiments were run for
20 hours with working catalysts (Cu-ZnO inverse opals). The chemical state of
metallic Cu (served as active species) could be determined via XAS
measurements (Figure 7.9). Minor deviations were observed in repeated runs,
which are indicated by the small error bars in the figures. Notably, the methanol
conversion and hydrogen production rates in a microreactor with O2-plasma
treated Cu-ZnO inverse opals for plasma-exposure time of 10 min are nearly
100% and 300 mmol gcat-1 h-1 at a temperature as low as 230 ℃, respectively,
which are significantly higher than those obtained with the conventional
dense-film microstructures (non-inverse opals). SEM image of the Cu-ZnO
non-inverse opals clearly shows presence of randomly oriented microstructures
(Figure 7.10). The 3-fold enhancement in catalytic activity and consequently
treatment. Owing to special pore structure, the occurrence of fast gas-solid
reactions benefits significantly from proceeding inside the porous transport
pathways of 3DOM nanoarchitectures. In addition, O2-plasma treatment is shown
to increase the surface concentration of Vo, which provides more active sites for
MRR, based on the equation:
Vo․․ + 2e' + O＊ → Oox + ＊,
where Vo․․, Oox, and ＊ are oxygen vacancy, lattice oxygen, and surface
active site, respectively.[129] Accordingly, it is well recognized that both
hierarchical pore-channel networks as well as surface density of Vo are crucial to
the observed excellent catalytic activity. On the other hand, only minor amounts
of CO were detected by CO detector in the outlet gas stream. The CO
concentration of only 130-170 ppm at 230 ℃ was detected for the O2-plasma
treated 3DOM nanoarchitectures with plasma exposure time of 10 min. This
result can be attributed to a higher surface density of Vo as a result of the
O2-plasma treatment. It is believed that the Vo can generate lattice oxygen that, in
turn, catalyzes the CO oxidation reaction.[122] Previous investigations have
pointed out that the presence of Vo plays an important role in CO oxidation
pointed out that the presence of Vo plays an important role in CO oxidation