Performance and mechanism of CuO/CuZnAl hydrotalcites-ZnO for photocatalytic selective oxidation of gaseous methanol to methyl formate at ambient temperature

Article abstract of DOI:10.1016/j.jcat.2016.03.033

The photocatalytic selective oxidation of methanol to methyl formate at 26-45°C under UV irradiation has been studied for the first time on the catalyst CuO supported on nanocomposites of CuZnAl hydrotalcites and ZnO. The catalyst exhibits high activity for this reaction, with a maximum methyl formate yield of 50% at 30°C and no by-product other than CO₂, although both ZnO and hydrotalcites are nonactive for the reaction. The interfaces between ZnO and CuZnAl hydrotalcites play an important role in the reaction, not only supplying necessary hydroxyls but also generating holes with redox potential positive enough for the reaction under UV irradiation due to the band bending at the interface. Copper oxide is photoreduced to metallic Cu during the reaction, which plays a role in trapping photoexcited electrons from the conduction band of the support as well as in dissociating chemisorbed oxygen.

Full text of DOI:10.1016/j.jcat.2016.03.033

Journal of Catalysis 339 (2016) 68–76
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Performance and mechanism of CuO/CuZnAl hydrotalcites-ZnO for
photocatalytic selective oxidation of gaseous methanol to methyl
formate at ambient temperature
a
a
a
a
Xiaoyuan Liang a, Xuzhuang Yang a, , Guanjun Gao , Changfu Li , Yuanyuan Li , Weida Zhang ,
⇑
Xuetao Chen a, Yanbing Zhang a, Bingbing Zhang a, Yanqiu Lei a, Quanquan Shi b
a School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot, Inner Mongolia 010021, People’s Republic of China
b State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
The photocatalytic selective oxidation of methanol to methyl formate at 26–45 °C under UV irradiation
has been studied for the first time on the catalyst CuO supported on nanocomposites of CuZnAl hydrotal-
cites and ZnO. The catalyst exhibits high activity for this reaction, with a maximum methyl formate yield
of 50% at 30 °C and no by-product other than CO2, although both ZnO and hydrotalcites are nonactive for
the reaction. The interfaces between ZnO and CuZnAl hydrotalcites play an important role in the reaction,
not only supplying necessary hydroxyls but also generating holes with redox potential positive enough
for the reaction under UV irradiation due to the band bending at the interface. Copper oxide is photore-
duced to metallic Cu during the reaction, which plays a role in trapping photoexcited electrons from the
conduction band of the support as well as in dissociating chemisorbed oxygen.
Received 20 February 2016
Revised 28 March 2016
Accepted 29 March 2016
Keywords:
Photocatalysis
Methanol
Methyl formate
Copper catalyst
Reaction mechanism
Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction
other than titania are seldom used in photocatalytic oxidation of
methanol to MF.
Methyl formate (MF) is an important industrial intermediate for
synthesizing formic acid, formamides, acetic acid, ethylene glycol,
etc. [1–3]. It is traditionally produced by dehydrogenation (direct
or oxidative) of methanol at elevated temperatures and/or pres-
sures [1–5], leading to energy waste, multiple by-products, and
low MF selectivity. Wittstock et al. reported selective oxidation
of methanol to MF with high MF selectivity on nanoporous gold
catalyst at low temperatures [6], which shed new light on green
MF production. In recent years, photocatalytic MF synthesis from
methanol has attracted great attention of researchers due to its
ambient reaction conditions and high MF yield [7–11], Liu et al.
reported photocatalytic methanol oxidation on MoO3/TiO2 and
TiO2 as early as 1985, MF being one of the multiple products
[12]. Kominami et al. studied this reaction on an anatase-type
TiO2 (ST-01) in 2010 [11]. We studied Au, Ag, Au–Ag alloy, and
Cu nanoparticles supported on TiO2 as photocatalysts for MF syn-
thesis from methanol at 15–45 °C under UV irradiation [7–9]. Col-
menares et al. reported that Pd–Au/TiO2 as a photocatalyst was
active in selective oxidation of methanol to MF [10]. Photocatalysts
The band gap energy of ZnO is close to that of TiO2, and the
valence band maximum and conduction band minimum of ZnO,
which determine the redox potential under irradiation, are also
similar to those of TiO2 [13]. Thus, ZnO exhibits photocatalytic per-
formance comparable or superior to that of TiO2 in many cases,
such as photodegradation of organics, water splitting, and photo-
catalytic conversion of CO2 and CH4 [14–20]. ZnO is becoming a
promising photocatalyst instead of TiO2 due to its low cost, high
reactivity, and easy production [14,16]. But unlike TiO2, ZnO exhib-
ited almost no any photocatalytic activity in the selective oxidation
of methanol to MF [7]. The mechanism of the reaction indicates
that hydroxyls on the surface of the catalyst are responsible for
the key step of forming methoxyl from methanol [3,7,8], but there
are much rarer surface hydroxyls on ZnO than on TiO2. It is still
challenging to use ZnO or modified ZnO as a photocatalyst in pho-
tocatalytic oxidation of methanol to MF.
Zn and Al or Cu, Zn, and Al can form hydrotalcites, which are
semiconductors with plenty of surface hydroxyls and have been
used in many photocatalytic processes [21–27], but the valence
band maximum of hydrotalcites is usually higher than that of
ZnO [28], leading to more negative redox potential of the photoex-
cited holes and thus lower oxidation capacity. Some researchers
⇑
Corresponding author. Fax: +86 471 4992981.
E-mail address: xzyang2007@yahoo.com (X. Yang).
http://dx.doi.org/10.1016/j.jcat.2016.03.033
0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

X. Liang et al. / Journal of Catalysis 339 (2016) 68–76
69
Fig. 1. Schematic diagram of the reaction process.
prepared semiconductors such as TiO2 [29–32] or CeO2 [33] – mod-
ified hydrotalcites for photocatalysis and achieved favorable
results. The heterojunction between the semiconductor and hydro-
talcites is considered to play a role in electron transfer and thus
promote photocatalysis [31]. So preparation of highly efficient
photocatalysts for selective oxidation of methanol to MF through
the combination of ZnO and CuZnAl hydrotalcites is anticipated.
The sites at the heterojunction close to the side of ZnO are expected
to provide photogenerated holes with redox potential high enough
for the reaction and those close to the side of hydrotalcites supply
hydroxyls for the reaction.
partial pressure, oxygen partial pressure, and light intensity on
photocatalysis, and the reaction mechanisms of the catalysts.
2. Experimental
2.1. Catalyst preparation
Typically, 7.425 g Zn(NO3)2ꢀ6H2O and a suitable amount of Al2(-
SO4)3ꢀ18H2O (molar ratio Al2O3/ZnO = 1%, 3%, 5%, 7%, 10%) were
dissolved in 250 mL deionized water followed by adding 2 mL Tri-
ton X-100 under ultrasonic stirring for 0.5 h.
A solution of
Copper oxide is also a semiconductor with a band gap of ca.
1.7 eV [34,35]. It has been used as a photocatalyst for H2 produc-
tion [36–39], CO2 reduction [40–45], organics degradation
[46,47], etc. under visible light irradiation in recent years. How-
ever, copper oxide is unstable under irradiation because the redox
potential for reduction and oxidation of monovalent copper oxide
lies within its band gap [48–50] and thus it is easily photoreduced
to metallic copper. This disadvantage of CuO turns into an advan-
tage when it is supported on titania for photocatalytic oxidation
of methanol to MF, because it can form an ohmic contact between
metallic copper and titania, as the work function of copper is
slightly lower than that of titania [8]. Such a structure facilitates
charge transfer from titania to Cu nanoparticles and oxygen disso-
ciation on Cu nanoparticles during the reaction. The work function
of ZnO (bulk) is ca. 5.3 eV [51], which is higher than that of Cu
(uCu = 4.60 eV [52,53]), and thus an ohmic contact might also be
formed between Cu nanoparticles and ZnO. Actually, the work
function of a metal or a semiconductor is not constant and changes
with its morphology and size. An electronic contact between metal
and semiconductor can form an ohmic contact or a Schottky bar-
rier. Both are beneficial to charge separation under irradiation. In
the case of ohmic contact, the photoexcited electrons are easy to
transfer to the Cu nanoparticles due to the lower resistance at
the interface between the Cu nanoparticle and the semiconductor,
while in the case of Schottky contact the photoexcited electrons
are trapped by Cu nanoparticles, which reduces the electron–hole
recombination [54–59]. In addition, the negatively charged sur-
faces of Cu nanoparticles are beneficial for the dissociative
chemisorption of oxygen molecules and thus conducive to the
reaction [7,8].
0.2 mol Lꢁ1 Na2CO3 was added dropwise under ultrasonic stirring
until the above solution reached pH 8. Ultrasonic stirring was con-
tinued for 2 h. The precipitate was recovered after centrifugation,
washing with deionized water and ethanol and drying in air. The
nanocomposites of H-ZnO-y(z) were then obtained after calcina-
tion at different temperatures (300, 400, 500, 600, 700, and
800 °C) for 3 h, where H denotes the hydrotalcites, y denotes the
Al loading, and z denotes the calcination temperature.
The as-prepared H-ZnO-y(z) nanocomposites of 0.2 g, where y
denotes the Al loading and z denotes the calcination temperature
of H-ZnO-y(z), were supersonically dispersed in a solution of
0.01 mol Lꢁ1 Cu(NO3)2 solution (molar ratio of Cu/Zn = 3%, 5%, 7%,
and 10%) and deionized water (volumetric ratio of water to Cu
(NO3)2 solution is 10) for 25 min followed by adding 5 mL aqueous
solution of NaBH4 (molar ratio of Cu/NaBH4 = 1/2.5) with vigorous
stirring for 24 h. The solid recovered was dried at 50 °C in air after
washing with deionized water and ethanol. The catalyst was
labeled xCu/H-ZnO-y(z), where x denotes the Cu loading.
2.2. Catalyst characterization
X-ray diffraction (XRD) measurements were performed using a
PANalytical B.V. Empyrean diffractometer with CuK
a radiation
operated at 40 kV and 40 mA. The scanning range (2h) was 10–
80°. The morphology of the samples was investigated by a FEI Tec-
nai S-Twin transmission electron microscopy (TEM). The light
absorbance was measured by a UVIKON/XL UV–vis diffuse reflec-
tance spectrometer (UV–vis) with a scanning range of 200–
800 nm. Temperature-programmed reduction with hydrogen (H2
TPR) was performed with a Micromeritics AutoChem 2910 ana-
lyzer. The gas mixture is 10% H2 balanced with N2 and the flow rate
is 50 mL/min. The heating rate is 8 °C/min. A 20 mg sample was
loaded into the quartz tube for each run. A Fourier transform infra-
red (FTIR) spectrum was recorded on a Bruker Vertex 70 FTIR spec-
trometer. A 20 mg sample was added into 200 mg KBr, followed by
tablet compressing. In order to remove water absorbed in the cat-
In this study, we have prepared nanocomposites of CuO sup-
ported on CuZnAl hydrotalcites and ZnO by a two-step wet chem-
ical method using simple precursors such as Zn(NO3)2ꢀ6H2O,
Al2(SO4)4ꢀ18H2O, and Cu(NO3)2 as an efficient photocatalyst for
selective oxidation of methanol to MF at 26–45 °C under UV irradi-
ation. The objectives of this study are to investigate the photocat-
alytic performance of the catalysts, the influence of methanol

70
X. Liang et al. / Journal of Catalysis 339 (2016) 68–76
Fig. 2. TEM images of catalysts 7Cu/H-ZnO-5(3) (a), 7Cu/H-ZnO-5(7) (b), and 7Cu/H-ZnO-5(8) (c). HRTEM of used 7Cu/H-ZnO-5(7) obtained within 2 min after catalysis (d).
alysts, all samples were dried at 120 °C for 24 h before testing. The
X-ray photoelectron spectra (XPS) of the catalysts were recorded
in batches and quantified online by
equipped with a FID detector.
a Shimadzu GC 2014C
by a Kratos Amicus spectrometer using an AlK
tion source. The binding energy (BE) was adjusted by the C1s tran-
sition at 284.6 eV.
a
(1486.6 eV) radia-
The methanol conversion was obtained, assuming the volumet-
ric flow rate was constant before and after the reaction due to the
low reactant content in the feed gas, by the equation
qM0
ꢁ
qM1
C ¼
ꢂ 100%;
ð1Þ
qM0
2.3. Photocatalytic reaction
where C is the methanol conversion, %; qM0 is the initial methanol
content, mg Lꢁ1; and qM1 is the methanol content in the gas after
reaction, mg Lꢁ1 . The selectivity of MF was calculated by the
equation
The photocatalytic activity of the catalyst was measured in a
continuous-flow aluminum alloy reactor with a rectangular quartz
window on top and a dividing-wall-type heat exchanger connected
to the back at the bottom of the reactor. The schematic diagram of
the reaction process can be seen in Fig. 1. Three pieces of rectangu-
lar glass coated with 0.02 g catalyst g on their top surfaces, which
were used as the catalyst holders, were installed in the bottom of
the reactor, with a thermocouple fixed in touch with the middle
catalyst holder. The dimensions of each of the glass holders were
26 ꢂ 80 ꢂ 1 mm. The whole surface coated by catalyst can be
irradiated by light. The cooling water flowed through the heat
exchanger to maintain a constant temperature. A 500 W high-
pressure mercury lamp (CEL-LAM500) with maximum light inten-
sity at wavelength 365 nm was positioned in a quartz cooling
jacket 2 cm over the quartz window of the reactor. The light inten-
sity was 25.3 mW/cm2. A gas mixture containing 1.0 vol.% metha-
nol and 0.5 vol.% O2 balanced with N2 was supplied at a flow rate of
20 mL/min into the reactor. The operating pressure was 1 atm.
Oxygen and nitrogen in the mixture were measured by mass
flowmeters. The reaction temperature was controlled at
26–45 °C. The products were qualified by a GC–MS and a LC–MS
2
qM0
qMF
qM1
S ¼
ꢂ 100%;
ð2Þ
ꢁ
where S is the MF selectivity, %, and
q
MF is the MF content in the gas
after reaction, mg Lꢁ1. The yield of MF was calculated by the
equation
Y ¼ C ꢂ S ꢂ 100%;
ð3Þ
where Y is the yield of MF, %; C is the methanol conversion, %; and S
is the MF selectivity, %.
3. Results and discussion
The TEM images of 7Cu/H-ZnO-5(3), 7Cu/H-ZnO-5(7), and 7Cu/
H-ZnO-5(8) are shown in Fig. 2a–c. The sheetlike structures are
CuZnAl hydrotalcites and the spherelike structures are ZnO
nanoparticles. Most of the ZnO nanoparticles are wrapped by the

X. Liang et al. / Journal of Catalysis 339 (2016) 68–76
71
Fig. 3. XRD profiles of catalysts: effect of calcination temperature (a), Al content (b), and Cu content (c). UV–vis spectra of catalysts (d).
sheets of hydrotalcites in the samples calcined at low temperatures
(Fig. 2a), and more and more of them are exposed in the samples
calcined at higher temperatures (Fig. 2b and c). The influence of
Al and Cu content on the morphology of the catalyst is shown in
Figs. S1–S7 in the Electronic Supplementary Information. The
amount of hydrotalcites in the catalyst increases with the increase
in Al content but with the decrease in Cu content. The inset of
Fig. 2b (image enlarged can also be seen in Fig. S8) shows the
HRTEM image of the as-prepared 7Cu/H-ZnO-5(7). The distances
between (100) facets of ZnO, between (012) facets of hydrotalcite,
and between (111) facets of CuO are clearly distinguished. The
crystal of ZnO is in close contact with that of hydrotalcite. The
HRTEM image of the used 7Cu/H-ZnO-5(7) obtained within 2 min
after catalysis is shown in Fig. 2d. The status of the species
observed in the image is considered to be similar to that in the cat-
alyst in the working state. The close contact interface formed by
the three phases of ZnO, hydrotalcite, and metallic Cu can be
observed clearly. Almost everywhere there are hydrotalcites there
are such interfaces, because hydrotalcites are developed on
Al-doped ZnO and CuO.
The XRD profiles of the catalysts are shown in Fig. 3a–c. The
diffractions from hexagonal ZnO (PDF code: 00-001-1136) and
rhombohedral hydrotalcites (PDF code: 00-037-0629 or 00-038-
0487) can be identified in most of the samples and the amount
of hydrotalcites in the catalyst decreases as the calcination temper-
ature is raised from 300 to 800 °C (Fig. 3a). Only a very small
amount of hydrotalcites exists in 7Cu/H-ZnO-5(7), and no hydro-
talcites, but ZnAl2O instead, are observed in 7Cu/H-ZnO-5(8). The
sample 7Cu/H-ZnO-5(8) was prepared using the same recipe under
the same condition as other 7Cu/H-ZnO-5(x) samples, except that
the support H-ZnO-5 was calcined at 800 °C. However, its mor-
phology (Fig. 2c) is quite different from that of other samples. No
sheetlike hydrotalcites can be observed. The XRD result provides
the same evidence that no hydrotalcites, but rather ZnAl2O, exist
in this sample. It indicates that the structure of ZnAl2O is stable,
and cannot react with copper species to give rise to hydrotalcites
at the second step of loading Cu species during the preparation.
At constant Cu and Zn content, the amount of hydrotalcites in
the catalyst increases with increasing Al content (Fig. 3b), but it
decreases with increasing Cu content at constant Al and Zn content
(Fig. 3c). This seemingly abnormal phenomenon might correlate
with thermodynamic equilibrium of the preparing system: copper
species tend to grow together in the case of high Cu content. Cop-
per species cannot be detected from the XRD patterns, and are
hardly observed from the HRTEM images except in the samples
with high Cu content. This is because some Cu is involved in the
formation of hydrotalcites.
Fig. 3d shows the UV–vis spectra of catalysts. The band gap of
ZnO is 3.16 eV and that of H-ZnO-5(7) is 2.99 eV, calculated by
the method in the literature [35,60]. The absorption band from
80 to 600 nm is attributed to the d–d transition of Cu2+ [61,62]
and that from 51 to 410 nm is attributed to charge transfer from
the valence band to the conduction band of Cu oxides. Combined
with the results of HRTEM and the following XPS, this indicates
that CuO exists in the as-prepared 7Cu/H-ZnO-5(7), as well as in
7Cu/H-ZnO-5(7) calcined at 350 °C. The absorption band centered
at 600 nm for the used 7Cu/H-ZnO-5(7), obtained within 2 min
after catalysis, is attributed to the local surface plasma resonance
effect (LSPR) of Cu [63], suggesting that CuO was reduced to metal-
lic Cu under UV irradiation during the reaction. However, in the

72
X. Liang et al. / Journal of Catalysis 339 (2016) 68–76
Fig. 4. H2 TRP profiles of catalysts: (a) 1. Used catalyst tested within 2 min after catalysis; 2. 7Cu/Al2O3; 3. 7Cu/ZnO; 4. 3Cu/H-ZnO-5(7); 5. 10Cu/H-ZnO-5(7); 6. 7Cu/H-ZnO-1
(7); 7. 7Cu/H-ZnO-10(7); 8. 7Cu/H-ZnO-5(3); 9. 7Cu/H-ZnO-5(7); 10. 7Cu/H-ZnO-5(8). FTIR spectra of catalysts calcined at different temperatures (b). XPS spectra of catalysts
(c). Activity of reference catalysts and blank experiment (d).
sample of used 7Cu/H-ZnO-5(7) 10 days after catalysis in air, the
characteristic absorbance of CuO is recovered instead of the LSPR
absorbance from metallic Cu. The color change from the original
light cyan of the as-prepared catalyst to the light slate gray of
the used catalyst indicates the variation of Cu species from another
angle.
hard to reduce decreases but that of CuO increases with increasing
calcination temperature due to the decrease of the hydrotalcite
content in catalysts (XRD profile in Fig. 3a). The catalyst 7Cu/H-
ZnO-5(8) only shows the reduction band of CuO due to the high
calcination temperature, while the used 7Cu/H-ZnO-5(7) (curve 1
in Fig. 4a) gives only the reduction band attributed to Cu species
in hydrotalcites, performed within 2 min after catalysis, suggesting
that CuO was reduced to metallic Cu but the Cu species in hydro-
talcites were preserved well during the reaction. This result is con-
sistent with that of UV–vis spectroscopy.
Fig. 4b shows the FTIR spectra of 7Cu/H-ZnO-5(z) and ZnO.
There are very few hydroxyls on the surface of ZnO. Most of the
hydroxyls on the catalyst come from hydrotalcites. The bands
ranging from 3700 to 2900 cmꢁ1 originate from the O–H stretching
vibrations, including those from absorbed water and hydroxyls on
the surfaces of hydrotalcites [23,31]. The bands around 1370 cmꢁ1
are ascribed to the stretching vibrations of COOꢁ and C–OH. Both of
these two bands decrease with increasing calcination temperature.
7Cu/H-ZnO-5(8) shows the smallest absorption from 3700 to
2900 cmꢁ1, indicating very few hydroxyls in this sample. The
absorption in this band is strengthened with increasing Al content,
but varies slightly with changing Cu content (Figs. S10 and S11,
Electronic Supplementary Information).
The variation of Cu species in catalysts can also be understood
from the H2 TPR profiles in Fig. 4a. The reduction bands at temper-
atures lower than 200 °C are ascribed to the reduction of CuO
nanoparticles [64,65]. The bands at temperatures higher than
200 °C are ascribed to the Cu species strongly interacting with
Al2O3 or to the CuZnAl hydrotalcites [66,67]. The reduction profile
of CuO on ZnO (curve 3 in Fig. 4a) indicates that CuO nanoparticles
are isolated and almost no interaction occurs between them. The
peak at the lower-temperature band is ascribed to the reduction
of highly dispersed CuO species and the shoulder at the higher-
temperature band is ascribed to that of larger CuO nanoparticles.
The reduction temperature of CuO on Al2O3 is 80 °C higher than
that on ZnO (curve 2 in Fig. 4a), indicating a strong interaction
between them. With increasing Cu loading at constant Al and Zn
content (curves 4, 9, and 5 in Fig. 4a), the reduction temperature
of Cu species gradually decreases due to more and more isolated
CuO nanoparticles being formed. With increasing Al loading at con-
stant Cu and Zn content (curves 6, 9, and 7 in Fig. 4a), the reduction
peaks attributed to CuO decrease but those attributed to Cu species
that are hard to reduce increase due to the formation of hydrotal-
cites (XRD profile in Fig. 3b). At constant Cu, Zn, and Al content
(curves 8, 9, and 10 in Fig. 4a), the amount of Cu species that are
Fig. 4c shows the core-level XPS spectra of Cu2p3/2 from 7Cu/
ZnO, 7Cu/Al2O3, and 7Cu/H-ZnO-5(7). The binding energy (BE)
peak (shoulders for 7Cu/Al2O3 and 7Cu/H-ZnO-5(7)) located at
933.5 eV from the three catalysts is ascribed to the Cu2p3/2 from
Cu2+ in CuO [68–70]. However, the peaks (shoulder for 7Cu/ZnO)

X. Liang et al. / Journal of Catalysis 339 (2016) 68–76
73
Fig. 5. Methanol conversion and MF selectivity of catalyst: 7Cu/H-ZnO-5(z) (a and b); 7Cu/H-ZnO-y(7) (c and d); xCu/H-ZnO-5(7) (e and f).
located at 935.0 eV are ascribed to Cu2+ in different Cu species,
from CuZnAl hydrotalcites for 7Cu/H-ZnO-5(7) but from CuO inter-
acting with hydroxyls for 7Cu/Al2O3 and 7Cu/ZnO [70]. The
hydroxyls originate from water solution during the loading of Cu.
The bands around 944 eV are ascribed to the shakeup satellite
peaks of Cu2+ related to an open 3d9 shell. The XPS results indicate
that less CuO exists in 7Cu/H-ZnO-5(7) and 7Cu/Al2O3 than in 7Cu/
ZnO, which is consistent with the results of H2 TPR investigation.
The MF yields on ZnO, Al2O3, H-ZnO-5(7), 7Cu/Al2O3, 7Cu/ZnO,
and 7Cu/H-ZnO-5(7) at 30 °C under or without UV irradiation are
shown in Fig. 4d. None of these catalysts showed any photocat-
alytic activity without UV irradiation. The bare ZnO and Al2O3 exhi-
bit negligible activity for this reaction under UV irradiation, while
H-ZnO-5(7) is slightly better than either ZnO of Al2O3. However,
the MF yields are remarkably enhanced after loading Cu, up to
5.4%, 3.6%, and 49.9% for 7Cu/Al2O3, 7Cu/ZnO, and 7Cu/H-ZnO-5
(7), respectively. The photocatalytic activities of catalysts prepared
Fig. 6. Influence of light intensity on photocatalytic performance.

74
X. Liang et al. / Journal of Catalysis 339 (2016) 68–76
Fig. 7. Influence of methanol partial pressure in raw flow gas (a and b); influence of oxygen partial pressure in raw flow gas (c and d).
under different conditions for selective oxidation of methanol to
MF under UV irradiation in the temperature range from 26 to
45 °C are shown in Fig. 5. No by-products other than CO2 can be
definitely detected for all of the catalysts. The methanol conversion
changes slightly, but the MF selectivity decreases gradually with
increasing reaction temperature from 26 to 45 °C for most of the
catalysts. The catalyst 7Cu/H-ZnO-5(7) exhibits the maximum MF
yield of 49.9% at methanol conversion of 84.5% and MF selectivity
The influence of methanol partial pressure and oxygen partial
pressure on the photocatalytic performance of 7Cu/H-ZnO-5(7) is
shown in Fig. 7. The methanol conversion decreases sharply from
ca. 100% to ca. 40% with increasing methanol concentration from
0.5 to 2.0 vol.% at oxygen concentration 0.5 vol.%. The MF selectiv-
ity changes slightly with the change in the methanol concentra-
tion. The methanol conversion increases greatly from ca. 70% to
ca. 100% with the increase of oxygen concentration from 0.2 to
4.5 vol.% at a methanol concentration of 1 vol.%, and the MF selec-
tivity decreases with increasing oxygen concentration. According
to the MF formation rate, the optimum ratio of methanol to oxygen
is 2:1 and the optimum methanol concentration is 1 vol.% for this
catalyst.
The bare ZnO and Cu/ZnO exhibit inferior photocatalytic activ-
ity for the selective oxidation of methanol to MF, although the
band gap energy, valence band maximum, and conduction band
minimum are almost similar to those of titania. This is because
there are few hydroxyls on the surface of ZnO, which are essential
to forming methoxy by reacting with methanol molecules. From
methanol to methoxy is the key step of the reaction [3,7,8]. From
the results of catalysis, the samples with high content of hydrotal-
cites such as 7Cu/H-ZnO-5(3) and 3Cu/H-ZnO-5(7) usually exhibit
poor photocatalytic performance, indicating that hydrotalcites are
not active for this reaction, although there are plenty of hydroxyls
on their surfaces. This is because the valence band maximum of
CuZnAl hydrotalcites is slightly higher than that of ZnO, and thus
the redox potential of the photoexcited holes on the surface of
hydrotalcites is slightly negative or similar to that of CH3Oꢁ/CH2-
Oꢁ, against further oxidation of methoxy to coordinated formalde-
hyde. On the other hand, catalysts without hydrotalcites, such as
7Cu/H-ZnO-5(8), exhibit the worst photocatalytic activity for this
reaction, while the best photocatalyst for selective oxidation of
methanol to MF is 7Cu/H-ZnO-5(7), a nanocomposite with CuO
of 59.1% at 30 °C. At
a constant molar ratio of Cu:Zn:Al
(Fig. 5a and b), the methanol conversion increases, from 35.5% to
84.5% at 30 °C with increasing calcination temperature from 300
to 700 °C, and the selectivity of the catalyst varies slightly. But
the methanol conversion and MF selectivity dramatically decrease
if the calcination temperature is at 800 °C, from 84.5% to 5.7% and
from 59.1% to 22.3%, respectively, at a reaction temperature of
30 °C. At a constant molar ratio of Cu to Zn and calcination temper-
ature (Fig. 5c and d), the methanol conversion increases from 26.3%
to 84.5% at 30 °C with the increase of Al content from 1% to 5%, but
the methanol conversion and MF selectivity decrease slightly at Al
content higher than 5%. The MF selectivity is parallel for the cata-
lyst with Al content higher than 5%. At constant molar ratio of Zn to
Al and calcination temperature (Fig. 5e and f), the methanol con-
version increases from 53.2% to 84.5% at 30 °C with the increase
of Cu loading from 3% to 7%, but it decreases from 84.5% to 75.3%
at 30 °C when the Cu content is up to 10%. The selectivity of the
catalyst with different Cu content varies slightly from 26 to
45 °C. But the catalysts with low Cu content exhibit slightly better
MF selectivity.
The influence of the light intensity on the catalytic performance
of 7Cu/H-ZnO-5(7) is shown in Fig. 6. The methanol conversion
increases sharply and almost linearly, but the MF selectivity
increases slightly with increased light intensity from 5.4 to
25.3 mW cmꢁ2
.

X. Liang et al. / Journal of Catalysis 339 (2016) 68–76
75
Scheme 1. Proposed reaction mechanism of selective oxidation of methanol to MF on the catalyst CuO supported on CuZnAl hydrotalcites and ZnO nanocomposite.
of 7% supported on ZnO and a suitable amount of hydrotalcites. So
we believe that the interface between ZnO and CuZnAl hydrotal-
cites must play an important role in the reaction.
the Al and Cu content. CuO is reduced to metallic Cu during the
reaction, which plays a role in the dissociation of oxygen. This
study provides new insights into the design of photocatalysts for
selective oxidation of methanol to MF.
Both ZnO and CuZnAl hydrotalcites are semiconductors. When
they electronically contact with each other the band bending/
heterojunction is formed at the interface between ZnO and CuZnAl
hydrotalcites in order to maintain aligned Femi level [13]. The
valence band maximum at the interface is higher than that of
ZnO but lower than that of hydrotalcites, leading to the redox
potential of the photogenerated holes at the heterojunction close
to the side of ZnO being more positive than that of CH3Oꢁ/CH2Oꢁ.
On the other hand, the outer surface around the interface has
plenty of hydroxyls from hydrotalcites. These two aspects make
the zone of the interface between ZnO and CuZnAl hydrotalcites
more effective for the selective oxidation of methanol to MF under
UV irradiation. The proposed mechanism of the reaction is shown
in Scheme 1.
Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (No. 21566026), a key project of the
Inner Mongolia Education Department, China (NJZZ13002), the
Natural Science Foundation of Inner Mongolia, China
(2014MS0207), a project of Grassland Talent of Inner Mongolia,
China and a project of the western light from the ODPC of Inner
Mongolia, China.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2016.03.033.
4. Conclusions
A novel catalyst of CuO supported on ZnO and CuZnAl hydrotal-
cites has been prepared using cheap raw materials and a simple
wet chemical method. The catalyst exhibits remarkable photocat-
alytic activity for selective oxidation of methanol to MF. The inter-
faces of ZnO, CuZnAl hydrotalcite, and Cu species in the catalyst
play an important role in the reaction. Hydrotalcites are not very
active in the reaction but supply hydroxyls in the high-efficiency
zone near the interface. The amount of hydrotalcites in catalysts
can be controlled by modulating the calcination temperature and
References
[1] S.P. Tonner, D.L. Trimm, M.S. Wainwright, N.W. Cant, Ind. Eng. Chem. Prod. Res.
Dev. 23 (1984) 384.
[2] E. Tronconi, A.S. Elmi, N. Ferlazzo, P. Forzatti, Ind. Eng. Chem. Res. 26 (1987)
1269.
[3] A.S. Elmi, E. Tronconi, C. Cristiani, J.P. Gomez Martin, P. Forzatti, Ind. Eng.
Chem. Res. 28 (1989) 387.
[4] M. Ai, J. Catal. 77 (1982) 279.
[5] H. Liu, E. Iglesia, J. Phys. Chem. B 109 (2005) 2155.

76
X. Liang et al. / Journal of Catalysis 339 (2016) 68–76
[6] A. Wittstock, V. Zielasek, J. Biener, C.M. Friend, M. Bäumer, Science 327 (2010)
[38] C. Ampelli, R. Passalacqua, C. Genovese, S. Perathoner, G. Centi, T. Montini, V.
Gombac, J.J. Delgado Jaen, P. Fornasiero, RSC Adv. 3 (2013) 21776.
[39] M. Hara, T. Kondo, M. Komoda, S. Ikeda, K. Shinohara, A. Tanaka, J.N. Kondoa, K.
Domen, Chem. Commun. 3 (1998) 357.
319.
[7] C. Han, X. Yang, G. Gao, J. Wang, H. Lu, J. Liu, M. Tong, X. Liang, Green Chem. 16
(2014) 3603.
[8] J. Liu, C. Han, X. Yang, G. Gao, Q. Shi, M. Tong, X. Liang, C. Li, J. Catal. 333 (2016)
162.
[9] X. Yang, A. Zhang, G. Gao, D. Han, C. Han, J. Wang, H. Lu, J. Liu, M. Tong, Catal.
Commun. 43 (2014) 192.
[40] X. An, K. Li, J. Tang, ChemSusChem 7 (2014) 1086.
[41] P.D. Tran, L.H. Wong, J. Barber, J.S.C. Loo, Energy Environ. Sci. 5 (2012) 5902.
[42] H. Li, Y. Lei, Y. Huang, Y. Fang, Y. Xu, L. Zhu, X. Li, J. Nat. Gas Chem. 20 (2011)
145.
[10] J.C. Colmenares, P. Lisowski, D. Łomot, O. Chernyayeva, D. Lisovytskiy,
ChemSusChem 8 (2015) 1676.
[43] X. Hua, O. Shuxin, L. Lequan, W. Defa, K. Tetsuya, Y. Jinhua, Nanotechnology 25
(2014) 165402.
[11] H. Kominami, H. Sugahara, K. Hashimoto, Catal. Commun. 11 (2010) 426.
[12] Y. Liu, G. Griffin, S. Chan, I. Wachs, J. Catal. 94 (1985) 108.
[13] A.L. Linsebigler, G. Lu, J.T. Yates Jr., Chem. Rev. 95 (1995) 735.
[14] C. Tian, Q. Zhang, A. Wu, M. Jiang, Z. Liang, B. Jiang, H. Fu, Chem. Commun. 48
(2012) 2858.
[44] X. Li, J. Wen, J. Low, Y. Fang, J. Yu, Sci. China Mater. 57 (2014) 70.
[45] A.D. Handoko, J. Tang, Int. J. Hydrogen Energy 38 (2013) 13017.
[46] Z. Xiong, L.L. Zhang, X.S. Zhao, Chemistry 17 (2011) 2428.
[47] H.-Y. Jing, T. Wen, C.-M. Fan, G.-Q. Gao, S.-L. Zhong, A.-W. Xu, J. Mater. Chem. A
2 (2014) 14563.
[15] C. Hariharan, Appl. Catal. A 304 (2006) 55.
[48] P.E. de Jongh, D. Vanmaekelbergh, J.J. Kelly, Chem. Commun. (1999) 1069.
[49] A.V. Walker, J.T. Yates, J. Phys. Chem. B 104 (2000) 9038.
[50] A. Paracchino, V. Laporte, K. Sivula, M. Grätzel, E. Thimsen, Nat. Mater. 10
(2011) 456.
[16] A. Akyol, H.C. Yatmaz, M. Bayramoglu, Appl. Catal. B: Environ. 54 (2004) 19.
[17] G. Mahmodi, S. Sharifnia, F. Rahimpour, S.N. Hosseini, Sol. Energy Mater.: Sol.
Cells 111 (2013) 31.
[18] R. Ullah, J. Dutta, J. Hazard. Mater. 156 (2008) 194.
[19] A.P. Bhirud, S.D. Sathaye, R.P. Waichal, L.K. Nikam, B.B. Kale, Green Chem. 14
(2012) 2790.
[20] M. Francavilla, A. Pineda, A.A. Romero, J.C. Colmenares, C. Vargas, M.
Monteleone, R. Luque, Green Chem. 16 (2014) 2876.
[21] N. Ahmed, Y. Shibata, T. Taniguchi, Y. Izumi, J. Catal. 279 (2011) 123.
[22] D. Carriazo, M. del Arco, E. García-López, G. Marcì, C. Martín, L. Palmisano, V.
Rives, J. Mol. Catal. A 342–343 (2011) 83.
[51] K. Jacobi, G. Zwicker, A. Gutmann, Surf. Sci. 141 (1984) 109.
[52] H.B. Michaelson, J. Appl. Phys. 48 (1977) 4729.
[53] V. Russier, J. Badiali, Phys. Rev. B 39 (1989) 13193.
[54] I. Paramasivam, J.M. Macak, P. Schmuki, Electrochem. Commun. 10 (2008) 71.
[55] V. Iliev, D. Tomova, L. Bilyarska, A. Eliyas, L. Petrov, Appl. Catal. B 63 (2006)
266.
[56] Z. Ji, M.N. Ismail, D.M. Callahan, E. Pandowo, Z. Cai, T.L. Goodrich, K.S. Ziemer, J.
Warzywoda, A. Sacco, Appl. Catal. B 102 (2011) 323.
[23] M.A. Larrubia Vargas, G. Busca, U. Costantino, F. Marmottini, T. Montanari, P.
Patrono, F. Pinzari, G. Ramis, J. Mol. Catal. A 266 (2007) 188.
[24] S. Nishimura, A. Takagaki, K. Ebitani, Green Chem. 15 (2013) 2026.
[25] P. Roy Chowdhury, K.G. Bhattacharyya, Dalton Trans. 44 (2015) 6809.
[26] Y. Zhi, Y. Li, Q. Zhang, H. Wang, Langmuir 26 (2010) 15546.
[27] E.M. Seftel, E. Popovici, M. Mertens, K.D. Witte, G.V. Tendeloo, P. Cool, E.F.
Vansant, Micropor. Mesopor. Mater. 113 (2008) 296.
[28] S.-M. Xu, T. Pan, Y.-B. Dou, H. Yan, S.-T. Zhang, F.-Y. Ning, W.-Y. Shi, M. Wei, J.
Phys. Chem. C 119 (2015) 18823.
[29] Y. Lee, J.H. Choi, H.J. Jeon, K.M. Choi, J.W. Lee, J.K. Kang, Energy Environ. Sci. 4
(2011) 914.
[30] R. Lu, X. Xu, J. Chang, Y. Zhu, S. Xu, F. Zhang, Appl. Catal. B 111–112 (2012) 389.
[31] G. Carja, A. Nakajima, S. Dranca, C. Dranca, K. Okada, J. Phys. Chem. C 114
(2010) 14722.
[32] E.M. Seftel, M. Mertens, P. Cool, Appl. Catal. B 134–135 (2013) 274.
[33] J.S. Valente, F. Tzompantzi, J. Prince, Appl. Catal. B 102 (2011) 276.
[34] F.P. Koffyberg, J. Appl. Phys. 53 (1982) 1173.
[57] A. Primo, A. Corma, H. Garcia, Phys. Chem. Chem. Phys. 13 (2011) 886.
[58] A. Dawson, P.V. Kamat, J. Phys. Chem. B 105 (2001) 960.
[59] M. Jakob, H. Levanon, P.V. Kamat, Nano Lett. 3 (2003) 353.
[60] S. Tsunekawa, T. Fukuda, A. Kasuya, J. Appl. Phys. 87 (2000) 1318.
[61] H. Irie, S. Miura, K. Kamiya, K. Hashimoto, Chem. Phys. Lett. 457 (2008) 202.
[62] X. Qiu, M. Miyauchi, K. Sunada, M. Minoshima, M. Liu, Y. Lu, D. Li, Y.
Shimodaira, Y. Hosogi, Y. Kuroda, ACS Nano 6 (2012) 1609.
[63] A. Marimuthu, J. Zhang, S. Linic, Science 339 (2013) 1590.
[64] G. Fierro, M. Lojacono, M. Inversi, P. Porta, R. Lavecchia, F. Cioci, J. Catal. 148
(1994) 709.
[65] G. Fierro, M. Lo Jacono, M. Inversi, P. Porta, F. Cioci, R. Lavecchia, Appl. Catal. A
137 (1996) 327.
[66] J.-P. Shen, C. Song, Catal. Today 77 (2002) 89.
[67] S. Velu, K. Suzuki, M. Okazaki, M.P. Kapoor, T. Osaki, F. Ohashi, J. Catal. 194
(2000) 373.
[68] F. Parmigiani, G. Pacchioni, F. Illas, P. Bagus, J. Electron Spectrosc. Relat.
Phenom. 59 (1992) 255.
[35] Y. Xu, M.A. Schoonen, Am. Mineral. 85 (2000) 543.
[36] D. Praveen Kumar, M.V. Shankar, M.M. Kumari, G. Sadanandam, B. Srinivas, V.
Durgakumari, Chem. Commun. 49 (2013) 9443.
[69] N. McIntyre, M. Cook, Anal. Chem. 47 (1975) 2208.
[70] S. Murcia-Mascarós, R.M. Navarro, L. Gómez-Sainero, U. Costantino, M.
Nocchetti, J.L.G. Fierro, J. Catal. 198 (2001) 338.
[37] Z. Wang, Y. Liu, D.J. Martin, W. Wang, J. Tang, W. Huang, Phys. Chem. Chem.
Phys. 15 (2013) 14956.