Journal of Catalysis 287 (2012) 190–202

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Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat

Preferential oxidation of carbon monoxide in hydrogen using zinc oxide
photocatalysts promoted and tuned by adsorbed copper ions
Yusuke Yoshida a, Yu Mitani a, Takaomi Itoi b, Yasuo Izumi a,⇑ab

Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan
Department of Mechanical Engineering, Graduate School of Engineering, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japana r t i c l e

i n f oArticle history:
Received 9 September 2011
Revised 9 December 2011
Accepted 23 December 2011
Available online 28 January 2012
Keywords:
Hydrogen
PROX
Photocatalyst
Zinc oxide
Coppera b s t r a c t
Preferential oxidation of CO (63 Pa) and O2 (76 Pa) in H2 (6.3 kPa) using spheroidal ZnO nanoparticles(22 nm  47 nm) converted 52% of CO into CO2, and selectivity to CO2 was 92 mol% under UV–visible
light for 5 h. When 0.5 wt.% of Cu2+ was adsorbed on ZnO, 91% of CO was converted into CO2 with a selectivity of 99 mol% under UV–visible light for 3 h. CO (63 Pa) was photocatalytically decreased to 2.3 mPa(0.35 ppm) in O2 (150 Pa) and H2 (6.3 kPa) for 5 h with a selectivity of 94 mol%. As evident from a XANES
peak at 8983.1 eV, the surface CuII sites trapped photogenerated electrons. Furthermore, O2-derived species were reduced by accepting electrons from CuI and protons from the neighboring formate species, as
indicated by the FT-IR peaks at 2985, 2879, 1627, 1587, and 1297 cm1.
Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction
The production, storage, and transportation of hydrogen are key
technologies for the emergence of a hydrogen energy society in thenear future [1]. Fuel cells (FCs) as the key device to convert hydrogen into electricity for use in vehicles, mobile phones, and at home
are indispensable for a hydrogen energy society. A signiﬁcant partof the cost of FCs can be attributed to the electrode catalysts, which
are mostly composed of Pt. However, electrode catalysts are easily
deactivated if more than 10 ppm (under 1.0 Pa of ambient pressure) of CO is contained as an impurity in hydrogen because of
the strong irreversible adsorption of CO on the Pt surface [2].
Well-known techniques for purifying hydrogen include pressure swing adsorption (PSA) [3], separation using metal membranes [4], cryogenic separation [4], and preferential oxidation(PROX) of CO [5–8]. PSA and metal membrane separation are
advantageous for achieving higher hydrogen purity, whereas cryogenic separation recovers a larger amount of hydrogen gas. The
apparatus of PSA incurs high initial investment costs and therefore
is not appropriate for small-scale/personal use. Membrane separation typically uses Pd metal, whereas the PROX reaction of CO often
utilizes expensive metals such as Au and Pt as the supported nanoparticle metal. To generate environmentally benign energy forsmall-scale/personal use, a new method for the puriﬁcation of⇑ Corresponding author. Fax: +81 43 290 2783.
E-mail address: yizumi@faculty.chiba-u.jp (Y. Izumi).
0021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2011.12.022

hydrogen that employs a cheap and sustainable membrane/catalyst is demanded.
For this purpose, the PROX reaction of CO using catalysts that do
not contain noble metals and that can be excited under naturallight is ideal. Such photo-PROX reactions of CO have been reported
using an NiO/TiO2 photocatalyst consisting of 0.5 wt.% NiO with a
CO conversion of 89%, deﬁned as the CO decrease ratio at 3 h,and a CO2 selectivity of 98%, deﬁned as the CO2 formation divided
by the H2 decrease + CO2 formation at 3 h [9], as well as a Mo/SiO2
photocatalyst consisting of 0.6 wt.% Mo with a CO conversion of
100% and a CO2 selectivity of 99%. Both photocatalysts were
tested in CO (3.8 lmol) + O2 (7.5 lmol) + H2 (24.6 lmol) illuminated by a high-pressure mercury lamp [10]. In particular, theMo/SiO2 photocatalyst reduced CO to less than 10 ppm in H2. However, the mechanism of the reaction including the effects of thepartial pressures of H2 and O2 and the factors that determine theselectivity of the photo-PROX reactions of CO are still debatable.
In this study, photo-PROX reactions were investigated using
zinc oxide-based photocatalysts at 290 K. Nanoparticles/nanorodsof ZnO with a narrow size distribution were synthesized, and theiradditive effects on the PROX reactions of CO were examined. The
Cu–ZnO photocatalysts reduced 9700 ppm of CO to as small as
0.35 ppm in 5 h under UV–visible light. The effects of adsorbed
Cu2+ were investigated using nitrogen gas adsorption, UV–visible
absorption, X-ray diffraction (XRD), scanning electron microscopy(SEM), high-resolution transmission electron microscopy (TEM),
Fourier-transform infrared absorption (FT-IR), and X-ray absorption ﬁne structure (XAFS). The photo-PROX reaction mechanism
is discussed herein.

Y. Yoshida et al. / Journal of Catalysis 287 (2012) 190–202

2. Experimental
2.1. Syntheses of ZnO
Spheroidal ZnO nanoparticles were synthesized according to areported procedure [11]. Zn acetate dihydrate (11.6 g, 53 mmol;
99%, Wako Pure Chemical) was suspended in ethanol (110 mL).
A 0.50 M sodium hydroxide ethanol solution (50 mL, 25 mmol;
97%, Kanto Chemical) was added to the Zn2+ solution over
20 min and was agitated using ultrasound (85 W, 28 kHz). The
mixed suspension was reﬂuxed at 353 K for 3 days. The obtained
white precipitate was ﬁltered using a polytetraﬂuoroethane-based
membrane ﬁlter (Omnipore JGWP04700, Millipore) with a poresize of 0.2 lm, washed with deionized water, and dried at 333 K
for 12 h.
Rod-like ZnO nanoparticles were synthesized according to a reported procedure [12]. A 0.20 M Zn sulfate heptahydrate aqueous
solution (30 mL, 6.0 mmol; 99.5%, Kanto Chemical) was added to1.33 M NaOH (90 mL, 120 mmol) over 6 min. The suspension wasmaintained at 333 K for 2 h. The white precipitate obtained was ﬁltered using Omnipore JGWP04700, washed with deionized water,and dried at 333 K for 12 h. Commercial ZnO (FZO-50, ZnO
97.4 wt.%) was kindly provided by Ishihara Sangyo Co.

2.2. Syntheses of doped ZnO and Au–TiO2
A 0.40 mM Cu nitrate trihydrate aqueous solution (30 mL;
99.9%, Wako Pure Chemical) was added to spheroidal ZnO(150 mg), which was synthesized as described in the previous section, and magnetically stirred at 290 K for 30 min. The obtained
light blue powder was ﬁltered using Omnipore JGWP04700,
washed with deionized water, and dried at 333 K for 8 h. This compound is hereafter denoted as Cu–spheroidal ZnO. Most or all Cu
ions introduced were adsorbed on ZnO, as conﬁrmed by X-ray
absorption spectra (see Section 2.3), and the Cu content was typically 0.5 wt.% and varied between 0.01 and 1.0 wt.%.
Furthermore, a 0.40 mM Cu(NO3)23H2O aqueous solution(30 mL) was mixed with spheroidal ZnO (150 mg) and ethanol(1 mL) as a reductant. The suspension was stirred using a magnetic
stirrer under the illumination of UV–visible light from a xenon arc
lamp (Model UXL-500D-O, Ushio) set at 300 W for 1 h. The distance between the light exit window and the ﬂask was 181 mm.
The color of the suspended powder changed from light blue to yellow, suggesting the reduction of CuII sites to CuI during illumination. The yellow powder obtained was ﬁltered using Omnipore
JGWP04700 and treated via the same procedure used for Cu–spheroidal ZnO. This compound is hereafter denoted as Cu–spheroidal
ZnO-reduced.
Spheroidal ZnO (150 mg) was suspended in a cerium nitratehexahydrate aqueous solution (30 mL, 0.18 mM; >98%, Wako Pure
Chemical) and stirred at 290 K for 30 min. The pale yellow powder
was ﬁltered using Omnipore JGWP04700, washed with deionizedwater (200 mL), and dried at 333 K for 8 h. This compound ishenceforth denoted as Ce–spheroidal ZnO. Most or all Ce ionsintroduced were adsorbed on ZnO, as conﬁrmed by X-ray absorption spectra, and the Ce content was 0.5 wt.%.
Zn acetate dihydrate (0.859 g, 3.91 mmol) was suspended inethanol (10 mL) using ultrasound (85 W, 28 kHz) for 1 h. Ethanethiol (0.10 mL, 1.4 mmol; >98%, Wako Pure Chemical) was added tothe suspension. The mixture was introduced into an autoclave(TVS-1-30, Taiatsu Techno) and maintained at 433 K for 24 h. The
pale yellow powder obtained was ﬁltered using Omnipore
JGWP04700 and washed with ethanol (200 mL), deionized water(200 mL), and acetone (50 mL). The maximum S-content shouldbe 12.4 wt.%, but the actual value was smaller because a notable

191

smell originating from S remained in the initial portions of the solvents used for washing.
A reference Au–TiO2 catalyst was prepared via the deposition–
precipitation method [13,14]. 2.5 mM hydrogen tetrachloroaurate(III) tetrahydrate (100 mL; 99%, Wako Pure Chemical) was
added over 1 h to TiO2 (0.95 g; P25, Degussa; anatase/rutile
phase = 7/3, speciﬁc surface area = 60 m2 g1) and was suspended
in a minimum amount of deionized water. Then, a 25 mM urea
solution (100 mL) was added to the suspension over 90 min. The
mixture was stirred at 353 K for 21 h in the absence of light. The
powder obtained was ﬁltered using a 1G glass ﬁlter (Shibata Scientiﬁc Technol.), washed with deionized water (total 150 mL), anddried at 353 K for 18 h. The light yellow-green powder obtained
was calcined in air with an elevation rate of 4 K min1 and wasmaintained at 573 K for 4 h. The Au content in the purple powder
ﬁnally obtained was 5.0 wt.%.

2.3. Characterization
N2 adsorption isotherm measurements were performed at 77 K
in the pressure range of 1.0–90 kPa in a vacuum system connected
to diffusion and rotary pumps (106 Pa) and equipped with acapacitance manometer (Models CCMT-1000A and GM-2001, ULVAC). The Brunauer–Emmett–Teller (BET) surface area (SBET) was
calculated using eight-point measurements between 10 and
46 kPa (P/P0 = 0.10–0.45) in the adsorption isotherm. The samples(100–200 mg) were evacuated, and the temperature was elevated
at a rate of 5 K min1 to 393 K and maintained at 393 K for 2 h before the measurements.
XRD data for the sample powders were obtained using a MiniFlex diffractometer (Rigaku) at a Bragg angle (hB) of 2hB = 10–80°
with a scan step of 0.01° and a scan rate of 7 s/step. The measurements were performed at 30 kV and 15 mA using Cu Ka emission
and a nickel ﬁlter. SEM measurements were performed using Hitachi Model S-80U. The samples were mounted on a conducting carbon tape. The electron accelerating voltage was 3.0 kV, and themagniﬁcation was between 2000 and 200,000 times. In addition,
SEM images were observed using JEOL Model JSM-6510A at theChemical Analysis Center, Chiba University. The electron accelerating voltage was in the range of 15–30 kV, and the magniﬁcation
ranged from 2000 to 80,000 times. The samples were mounted
on a conducting carbon tape (JEOL, DTM9101) and coated withgold.
High-resolution TEM measurements were performed using TEM
equipment (JEOL, Model JEM-2010F) with an accelerating voltage
of 200 kV. Samples were dispersed in ethanol (>99.5%) andmounted on amorphous C-coated Cu mesh (CU150 Mesh, JEOL).
Optical spectroscopic measurements were performed using a
UV–visible spectrophotometer (JASCO, Model V-650). D2 and halogen lamps were used for wavelengths below and above 340 nm,
respectively, and an integrating sphere (JASCO, Model ISV-469)
was used for diffuse reﬂectance measurements. Measurements
were performed at 290 K in the wavelength range of 200–800 nm
using fresh samples (100–200 mg). Diffuse reﬂectance spectra
were converted to absorption spectra using the Kubelka–Munkequation [15,16]. The bandgap value was calculated by the simple
extrapolation of the absorption edge or by ﬁtting to the equation of
Davis and Mott [15,16]:a  hm / ðhm  Eg Þn ;where a, h, and m are the absorption coefﬁcient, Planck’s constant,and the wavenumber, respectively, and n is 1/2, 3/2, 2, and 3 for allowed direct, forbidden direct, allowed indirect, and forbidden indirect transitions, respectively.

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Y. Yoshida et al. / Journal of Catalysis 287 (2012) 190–202

FT-IR spectra were measured using a single-beam model FT/IR4200 spectrophotometer (JASCO) in the wavenumber region between 4000 and 650 cm1. A MCT-M detector was used, and theenergy resolution was set to 1–4 cm1. The sample was set as adisk (/ = 20 mm, 80–150 mg) in a quartz reactor equipped with
NaCl windows on both sides and connected to a vacuum reaction
system (106 Pa).
Cu K-edge XAFS spectra were measured at 290 K on a beamline
9C and 7C in the transmission mode in the Photon Factory at theHigh Energy Accelerator Research Organization (Tsukuba). The
storage ring energy was 2.5 GeV, and the ring current was
450 mA. A Si(1 1 1) double-crystal monochromator was inserted
in the X-ray beam path. The X-ray intensity was maintained at
65% of the maximum ﬂux using a Piezo translator set on the crystal. The slit opening size in front of the I0 ionization chamber was
1 mm (vertical)  (1–2) mm (horizontal). The I0 and Itransmit ionization chambers were purged with N2 and Ar gases, respectively. Cu
K-edge XAFS spectra were also measured at 290 K on a beamline
01B1 at SPring-8. The storage ring energy was 8 GeV, and the ring
current was 100 mA. A Si(1 1 1) double-crystal monochromator andan Rh-coated double-mirror were inserted in the X-ray beam path.
The incident angle of the X-ray to the mirror was set to 3.0 mrad.
The sample disk was set in a Pyrex glass cell equipped with a
25-lm-thick polyethylene naphthalate window (Q51-25, Teijin).
The sample was vertically positioned at the center between theexit window of the I0 ionization chamber and the entrance windows of the Itransmit ionization chamber and tilted to make the Xray incident angle to the sample surface equal to 45°. The sample
was illuminated with an Xe arc lamp (500 W, UXL-500D-O) placed
perpendicular to the X-ray beam path in the same horizontal planewith the initial pressures of CO (135 Pa), O2 (270 Pa), and H2(13.1 kPa). The distance between the center of the sample andthe exit window of the lamp house (Model UI-502Q) was
262 mm. The scan steps were 9, 0.3, and 2.5 eV in the preedge, edge, and post-edge regions, respectively. The data accumulation time was 0.5–1.0 s for each data point. The Cu K-edge
absorption energy value was calibrated to 8980.3 eV for the spectrum of Cu metal [17,18]. The energy position of the monochromator was reproduced within ±0.1 eV.
The XAFS data were analyzed using an XDAP package [19]. The
pre-edge background was approximated by the modiﬁed Victoreen
function C2/E2 + C1/E + C0. The background of post-edge oscillation
was approximated by a smoothing spline function calculated using
the following equation for the number of data points, where k is
the wavenumber of photoelectrons.
DataX
Pointsðlxi  BGi Þ2

i¼1

expð0:075ki Þ

2

6 smoothing factor

wavelength of 555 nm was 42 mW cm2 at the center of the sample cell and 28 mW cm2 at the periphery of the bottom plate of
the sample cell. Note that the intensity was measured at 555 nm,
but the lamp irradiated in the wide spectrum of 200–1100 nm.
During illumination, the temperature was 305 K at the catalyst position [20]. A sharp cutoff ﬁlter L-42 (>420 nm) was placed at thelight exit to examine the photo-PROX reactions under visible lightalone.
The products and reactants were analyzed using columns ofMolecular Sieve 13X-S and polyethylene glycol (PEG)-6000 supported on Flusin P (GL Sciences) set in the online GC equipped witha thermal conductivity detector (Shimadzu, Model GC-8A). Thechanges in CO pressure during the photo-PROX reactions could
be ﬁt to the following ﬁrst-order reaction equation:

dPCO;t¼ kPCO;tdt

PCO;t ¼ PCO;initial ekt
Here, PCO,initial is the initial CO pressure and k is the rate constant. In
some tests, the CO pressure converged to a certain value not equal
to zero (PCO,ﬁnal) owing to the total consumption of O2 gas during
the tests, according to the following equation.

dPCO;t¼ kðPCO;t  PCO;final Þdt

PCO;t ¼ PCO;final þ ðPCO;initial  PCO;final Þekt
The changes in the CO pressure data during the photo-PROX reactions were also ﬁt to the following 3/2 order reaction, but did not
ﬁt well for any test data.

3dPCO;t¼ kðPCO;t Þ2
dt

1
PCO;t ¼ 
2
1
pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
ﬃ þ 2k t
PCO;initial

Several blank tests were performed, including reaction tests underillumination with UV–visible or visible light alone in the absenceof photocatalysts and reaction tests using Cu–spheroidal ZnO in
the absence of reactants or in the absence of any light at 290 or
327 K. The reactor was in water bath controlled at 327 K in the latter test. For the test in the absence of light, the reactor was completely covered with a 20-lm-thick aluminum foil.
3. Results

2.4. PROX reaction of CO

3.1. Nitrogen adsorption, XRD, SEM, TEM, and UV–visible and FT-IR
spectraThe photo-PROX reactions of CO were performed in a closed circulating system (total volume: 143 mL) using a photoreaction
quartz cell [20–22]. The catalyst (typically 50 mg, varied between
25 and 100 mg) was homogenously spread in the quartz reaction
cell (bottom ﬂat plate area: 23.8 cm2) connected to the circulation
loop and was evacuated (106 Pa) at 290 K for 2 h until the desorbed gas was detected by an online gas chromatograph (GC). H2(0.60–63 kPa, 0.035–3.7 mmol), CO (63 Pa, 3.7 lmol), and O2 (76–150 Pa, 4.5–9.0 lmol) were introduced. In addition, a CO (63 Pa)
oxidation test (O2 150 Pa) in the absence of H2 was performed.
The catalyst in the quartz reactor was illuminated for 3–5 hwith UV–visible light from the 500-W Xe arc lamp placed beneaththe ﬂat bottom. The distance between the bottom of the reactor
and the lamp exit was set to 24 mm. The light intensity at the

The SBET value was measured on the basis of the N2 adsorptionof ZnO and the modiﬁed ZnO samples. The SBET value for spheroidal
ZnO was 35 m2 g1 and the value decreased to 57% for rod-like ZnO(Table 1a and b). The SBET values for Cu–spheroidal ZnO and Ce–
spheroidal ZnO increased by 11% and 26%, respectively, comparedwith that for spheroidal ZnO (Table 1c and d). Under the condition
that the average particle size of 34–48 nm for the synthesized samples estimated on the basis of the XRD peak width was greater than
21 nm for FZO-50, the SBET values for the synthesized ZnO and the
doped derivatives (20–44 m2 g1) were consistent with the valueof 50 m2 g1 for commercial FZO-50 (Table 1).
The XRD patterns measured for the pristine and modiﬁed ZnO
samples are depicted in Fig. 1. All samples exhibited peaks derived
from a wurtzite crystal structure [23–25], and the peaks were as-193

Y. Yoshida et al. / Journal of Catalysis 287 (2012) 190–202
Table 1
Physicochemical characterization of spheroidal ZnO, rod-like ZnO, Cu–spheroidal ZnO (0.5 wt.% Cu), Ce–spheroidal ZnO, S–ZnO, and commercial ZnO (FZO–50).
SBET (m2 g1)

Entry

Eg (eV)
Extrapolated

a
b
c
c0
d
e
f

Spheroidal ZnO
Rod-like ZnO
Cu–spheroidal ZnO
Cu–spheroidal ZnO-reduced
Ce–spheroidal ZnO
S–ZnO
FZO-50

35
20
39
–
44
26
50

3.21
3.21
3.19
3.17
3.19
3.22
3.24

Unit cell of wurtziten

Fit to a  hm / (hm–Eg)
n = 1/2

3/2

2

3

3.24
3.25
3.22
3.21
3.24
3.27
3.27

3.18
3.17
3.16
3.13
3.16
3.18
3.21

3.16
3.13
3.14
3.09
3.13
3.15
3.18

3.07
3.06
3.09
3.03
3.07
3.09
3.14

b

Average particle sizeaa (nm)c (nm)t (nm)ctmajor (nm)dtminor (nm)d0.32540.32490.3255–0.32510.3254–

0.52110.5204
0.5211–0.52050.5211
–

39
48
34
–
34
38
21

46.9

22.2

35.1
–
33.0

23.5
–
18.4

 0) X-ray diffraction peak position.
Based on (1 0 1
Based on (0 0 0 2) X-ray diffraction peak position.
c
0:9k
Based on Scherrer equation: t = Peak width.
cos hBdBased on SEM photographs.
abFig. 1. XRD patterns of fresh samples of spheroidal ZnO (a), rod-like ZnO (b), Cu–
spheroidal ZnO (0.5 wt.% Cu; c), Ce–spheroidal ZnO (0.5 wt.% Ce; d), S–ZnO (e), andcommercial ZnO (FZO-50, Ishihara Sangyo; (f)).

signed as in Fig. 1. Peaks due to impurities were not detected in all 0) and (0 0 0 2)
samples. Based on the diffraction angles for the (1 0 1
peaks, the lengths a and c of the unit cell were found to be nearly
constant at 0.3249–0.3255 nm and 0.5204–0.5211 nm (Table 1),
respectively, which are similar to the values for the wurtzite ZnO
crystal (a = 0.325 nm, c = 0.5207 nm) [23].
Whereas for S–ZnO and FZO-50, the intensity ratio of the three 0):(0 0 0 2):(1 0 1
 1) =
peaks in the range of 2hB = 30–40° was (1 0 1
52–57:48–53:100 (Fig. 1e and f), the intensity ratio of the spheroidal and rod-like ZnO and Ce–spheroidal ZnO peaks was 53–55:62–
64:100 (Fig. 1a, b, and d). The signiﬁcantly greater intensity of the(0 0 0 2) peak for the spheroidal and rod-like ZnO and Ce–spheroidal ZnO suggested preferable crystalline growth along the c-axis
direction. For Cu–spheroidal ZnO, the intensity ratio of the three 0):(0 0 0 2):(1 0 1
 1) = 52:50:100, which was nearlypeaks was (1 0 1equivalent to that for S–ZnO (52:48:100). The typical trend of thegreater (0 0 0 2) peak intensity for spheroidal ZnO was suspended
when the Cu ions were adsorbed probably because of partial dissolution and recrystallization during ion exchange (Fig. 1c).
A representative SEM image for spheroidal ZnO is depicted inFig. 2A1 and B1. The size distribution along the minor axis was narrow and centered at 22.2 nm (Fig. 2C1), whereas that along the major axis was larger and centered at 46.9 nm (Table 1a), suggestingthat the growth of ZnO proceeds along the direction of the c-axis inethanol. The preferential growth along the c-direction was inagreement with the interpretation based on XRD (Fig. 1a).

The size distribution along the minor axis in the representative
SEM image for Cu–spheroidal ZnO (Fig. 2A2 and B2) was centeredat 23.5 nm, indicating a negligible change compared with that for
spheroidal ZnO. In contrast, the size distribution along the major
axis became larger compared with that for spheroidal ZnO andthe center decreased to 35.1 nm (Fig. 2C2 and Table 1c). This trendof size distribution along the major axis was also evident for Ce–
spheroidal ZnO, which was centered at 33.0 nm (Fig. 2A3, B3, andC3). In addition, the size distribution along the minor axis wasclearly smaller and centered at 18.4 nm compared with 22.2–
23.5 nm for spheroidal ZnO and Cu–spheroidal ZnO.
A representative SEM image for rod-like ZnO is depicted inFig. 2A4 and B4. The distribution of the length and diameter isshown in Fig. 2C4. The average size of 79.8 nm  443 nm combinedwith the observation of the relatively larger (0 0 0 2) XRD peak(Fig. 1b) suggested preferable crystalline growth along the cdirection.
In the high-resolution TEM images for spheroidal ZnO, hexagonal shapes were clearly observed (Fig. 3a1 and a3) viewed from
[0 0 0 1] direction, in addition to spheroids (a1, a3, a5) that were already observed in SEM (Fig. 2B1). In the image a4 of Fig. 3, the 0) lattice pattern with a spacing of 0.273 nm was observed(1 0 1pﬃﬃ
in agreement with 23 a (=0.281 nm) for the wurtzite crystal struc 0) lattice pattern, the (1 1 2
 0)
ture. Rotated by 30° from the (1 0 1
lattice pattern with a spacing of 0.158 nm was also observed inagreement with 2a (=0.163 nm). The crystal growth direction in ethanol was [0001] direction as observed in images a6 and a7 in which
the (0 0 0 2) lattice pattern with a spacing of 0.251 nm was observed in agreement with 2c (=0.2604 nm). In the high-resolution
TEM images b5 and b7 of Fig. 3 for rod-like ZnO, the lattice patternwith a spacing of 0.275–0.276 nm was parallel to the major axis of 0) lattice pattern viewed from theZnO rods. This is the (1 0 1
pﬃﬃ
(1 2 1 0) direction in agreement with 23 a (=0.281 nm).
The UV–visible spectra for the pristine and modiﬁed ZnO samples are summarized in Fig. 4. The absorption edge was extrapolated, and the intersection with the x-axis (Fig. 4, inset) was
estimated for each bandgap value (Table 1). The bandgap reducedin the order of FZO-50 (f, 3.24 eV) > S–TiO2 (e, 3.22 eV) > rod-likeZnO (b, 3.21 eV)  spheroidal ZnO (a, 3.21 eV) > Ce–spheroidal
ZnO (d, 3.19 eV)  Cu–spheroidal ZnO (c, 3.19 eV) > Cu–spheroidal
ZnO-reduced (c0 , 3.17 eV). The foot of the absorption edge wasclearly extended to the visible light region for Cu–spheroidal
ZnO-reduced (Fig. 4c0 ).
For the three ZnO samples, the Davis–Mott equation was ﬁt to
the UV–visible absorption data with n values between 1/2 and 3(Table 1a, b, and f). The ﬁtting provided Eg values of 3.17–3.27 eV
with n values between 1/2 and 3/2, similar to those estimated fromthe simple extrapolation of the absorption edge (3.21–3.24 eV).

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Y. Yoshida et al. / Journal of Catalysis 287 (2012) 190–202

Fig. 2. SEM images for spheroidal ZnO (A1 and B1), Cu–spheroidal ZnO (A2 and B2), Ce–spheroidal ZnO (A3 and B3), and rod-like ZnO (A4 and B4), and the size distribution
along the major and minor axes for the projected ellipses (C1–3) and the length and diameter of nanorods (C4).

Fig. 3. High-resolution TEM images for spheroidal ZnO (a1–a7) and rod-like ZnO (b1–b7) and a model of ZnO spheroidal crystal.

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Y. Yoshida et al. / Journal of Catalysis 287 (2012) 190–202

Fig. 4. Diffuse reﬂectance UV–visible absorption spectra of fresh samples ofspheroidal ZnO (a), rod-like ZnO (b), Cu–spheroidal ZnO (0.5 wt.% Cu; (c)), c was
reduced with ethanol under UV–visible light (c0 ), Ce–spheroidal ZnO (0.5 wt.% Ce;(d)), S–ZnO (e), and commercial ZnO (FZO-50; (f)). (Inset) Enlarged view of theabsorption edge between 350 and 420 nm.

The nature of electronic transition did not change when Cu2+ or
Ce was adsorbed on spheroidal ZnO or when S was doped in ZnO.
The Eg values estimated on the basis of the simple extrapolation
were 3.19–3.22 eV, similar to values given by ﬁtting to the Davis–Mott equation with n values between 1/2 and 3/2 (Table 1c–e).
The FT-IR spectra were measured for Cu–spheroidal ZnO. Afterthe as-prepared sample was held in a vacuum for 2 h, a shoulder
peak and an accompanying broad peak appeared centered at
3666 cm1 and 3310 cm1, respectively. When CO (46 Pa) was
introduced, two new peaks were observed at 1630 cm1 and1339 cm1. At 46 Pa of CO and 46 Pa of O2 under UV–visible light,
peaks at 2985, 2879, 1627, 1587, and 1297 cm1 were observed.
On the other hand, peaks appeared at 2952, 2877, 2746, 1645,
1620, and 1320 cm1 when formic acid (46 Pa) was introduced toCu–spheroidal ZnO.
3+

3.2. Cu K-edge XANESThe Cu K-edge X-ray absorption near-edge structure (XANES)
spectrum for the fresh Cu–spheroidal ZnO catalyst is shown in
Fig. 5A–a. The Cu valence state was II in fresh Cu–spheroidal
ZnO. A tiny 1s–3d pre-edge peak was observed at 8979 eV [26].
After illumination for 1 min under UV–visible light, a weak shoulder peak began to appear at 8983.1 eV (Fig. 5A–b). The shoulder
peak intensity grew gradually to 0.0727 under UV–visible illumination for 3 h (Fig. 5A–d). One minute after the UV–visible light

was removed, the peak intensity at 8983.1 eV reduced by 0.0035(Fig. 5A–e).
The background and peak intensities at 8983.1 eV for as-synthesized Cu–spheroidal ZnO (CuII sites only) and Cu–spheroidal
ZnO-reduced (CuI sites only) are indicated by dotted lines inFig. 5B. The Cu–spheroidal ZnO-reduced sample was prepared by
UV–visible illumination for 1 h and the UV–visible (Fig. 4c0 ) and
X-ray (Fig. 5A–f) absorption spectra changed negligibly following
UV–visible illumination for 1–3 h. The weak 1s–3d pre-edge peakat 8979 eV for as-synthesized Cu–spheroidal ZnO diminished for
Cu–spheroidal ZnO-reduced. The shoulder peak at 8983.1 eV canbe understood to originate from CuI sites because a similar shoulder peak was reported for Cu2O at a distinctly lower energy than
the absorption edge for CuO [27,28]. The Cu K-absorption edge energy negligibly changed at 8992.9–8993.1 eV when the CuII sites
were reduced to CuI sites [29]. Therefore, the CuI-site ratio in thesample was approximated using the following equation:II

CuI -site ratio Peak intensity@8983:1 eV  0:048 ðfor Cu 100%Þ
III

0:21ðfor Cu 100%Þ  0:048 ðfor Cu 100%Þ

Based on this approximation, 3.9% of CuI sites were formed in
the initial 1 min, and this population gradually increased to15.6% after 3 h of UV–visible illumination. When the UV–visible
illumination was removed, the CuI population dropped to 13.4%.
CuII and CuI sites were found to be in a dynamic equilibrium in a
steady-state population of transmitted electrons that were photogenerated and did not recombine with holes during the diffusion tosurface, as given by the following equation:

K¼h½CuI 
i:CuII ediffused to surface

3.3. Photo-PROX reactions of COFirst, CO photo-PROX reaction tests were performed using synthesized and commercial ZnO samples, beginning with the initialpressures of CO (63 Pa), O2 (76 Pa), and H2 (6.3 kPa) under UV–visible light for 5 h. The initial formation rates of CO2 within 1 h were1
in the following order: spheroidal ZnO (17 lmol h g1cat ) > FZO-50
1 1
1 1
(13 lmol h gcat )  rod-like ZnO (0:42 lmol h gcat ) (Table 2A, B,
and P, Fig. 6A, B, and I). The rates were not correlated to each speciﬁc surface area (35, 50, and 20 m2 g1, respectively; Table 1), suggesting that the catalytic reactivity critically depended on the
exposed crystal face. Selectivity to CO2 was 92–89 mol% for spheroidal ZnO and FZO-50, whereas it was quite poor (53 mol%) for
rod-like ZnO (Table 2A, B, and P).

Fig. 5. (A) Normalized Cu K-edge XANES spectra for Cu–spheroidal ZnO before illumination (a), at 1 (b), 80 (c), and 180 min (d) from the start of illumination of UV–visible
light (500-W Xe arc lamp) with the initial pressures of CO (135 Pa), O2 (270 Pa), and H2 (13 100 Pa), and at 1 min after the illumination (e) for Cu–spheroidal ZnO-reduced (f).
(B) The intensity change of a shoulder peak at 8983.1 eV during the illumination of UV–visible light and the intensity for as-prepared Cu–spheroidal ZnO (CuII 100%) and Cu–
spheroidal ZnO-reduced (CuI 100%). (Inset) Enlarged pre-edge region of spectra in panel A.

:

196

Table 2Preferential oxidation rates of CO in H2 and O2 using the synthesized ZnO samples: Cu–spheroidal ZnO (0.5 wt.% Cu), Ce–spheroidal ZnO, S–ZnO, commercial ZnO (FZO–50), and reference catalysts.a
Entry

A
B
C
C0

Spheroidal ZnO
Rod-like ZnO
Cu (0.5 wt%)–spheroidal ZnO
Cu (0.5 wt%)–spheroidal ZnOreduced
Cu (0.01 wt%)–spheroidal ZnO
Cu (0. 1 wt%)–spheroidal ZnO
Cu (1.0 wt%)–spheroidal ZnO
Cu (0.5 wt%)–spheroidal ZnO

K0

a

L
M
N
O
P
Q
R
S

Cu (0.5 wt%)–spheroidal ZnOg
Ce (0.5 wt%)–spheroidal ZnO
S–ZnO

T
U

Reactor only

FZO-50NiO/TiO2
Mo/SiO2
Au–TiO2

Light

Initial gas (Pa)

Time
(h)

1st order rate constant k(min1)

Initial formation rates of CO2

Conversion to CO2(%)c

Selectivity to CO2(mol%)d,e

Reference

76
76
76
76

5
5
3
3

0.01620.00010.02490.0242

17
0.42
56
51

52
3.4
91
86

92d
53d
99d
98

This work
This work
This work
This work

6300
6300
6300
6300
0
600
3000
63,000
0
6300
6300

76
76
76
150
150
150
150
150
0
150
150

3
3
3
3
1.5
2
3
3
3
3
3

0.02040.03050.01810.03410.08290.04300.03420.00709–
0.00050.0004

27
77
40
84
94
61
70
18
6.2
1.5
0.42

69
100
91
100
100
62
69
66
–
6.5
0.86

97d
96d
96d
94d
97d
99d
98d
79d
–
86d
100d

This work
This work
This work
This work
This work
This work
This work
This work
This work
This work
This work

63

6300

150

3

0.0007

2.2

5.1

79d

This work

63
63
63
63
63
92
92
63

6300
6300
6300
6300
6300
590
590
6300

150
150
76
76
76
180
180
150

3
3
3
3
5
3
3
1

0.00775
0.01720.00090.00070.0157
–
–
0.141

1.5
44
2.4
0.88
13
38
95
1200

43
51
4.2
2.2
42
81
100
100

99.6d
99d
92d
90d
89d
98e
99e
93d

This work
This work
This work
This work
This work[9][10]This work

63
63

6300
6300

150
150

3
3

0.00010.0004

–
–

2.9
11

77d
90d

This work
This work

CO

H2

O2

UV–visible
UV–visible
UV–visible
UV–visible

63
63
63
63

6300
6300
6300
6300

UV–visible
UV–visible
UV–visible
UV–visible
UV–visible
UV–visible
UV–visible
UV–visible
UV–visible
Visiblef
No light(290 K)No light(327 K)UV–visible
UV–visible
UV–visible
Visiblef
UV–visible
UV–visible
UV–visible
No light(290 K)UV–visible
Visiblef

63
63
63
63
63
63
63
63
0
63
63

(lmolhCatalyst 50 mg except for Au/TiO2 (11 mg).
In initial 0.25, 0.5, or 1 h.
(Molar amount of CO2 formed at the end of reaction test)/{initial molar amount of CO (3.7 lmol)}  100.
d
(Molar amount of CO2 formed)/(molar amounts of CO2 + H2O formed)  100.
e
(Molar amount of CO2 formed)/(initial molar amount of H2  ﬁnal molar amount of H2 + molar amounts of CO2 formed)  100.
f
>420 nm using L-42 ﬁlter.
g
Preheated in air at 573 K for 2 h.
bc

1 1 bgcat )

Y. Yoshida et al. / Journal of Catalysis 287 (2012) 190–202

C00
C00 0
C00 00
D
E
F
G
H
I
J
K

Catalyst

Y. Yoshida et al. / Journal of Catalysis 287 (2012) 190–202

197

Fig. 6. Time course of the PROX reaction of CO in H2 using spheroidal ZnO (A), rod-like ZnO (B), Cu–spheroidal ZnO (C–F), Cu–spheroidal ZnO-reduced (C0 ), Ce–spheroidal ZnO(G), S–ZnO (H), FZO-50 (I), and Au–TiO2 (J) illuminated with UV–visible light except for E, BL2 (visible light), and F, J (no light). No catalyst charged for control tests (BL1, BL2).
Reaction gases: CO 3.7 lmol (63 Pa), H2 370 lmol (6.3 kPa), and O2 4.5 lmol (76 Pa) (A–C, C0 , H, I) or O2 9.0 lmol (150 Pa) (D–G, J, BL1, BL2).

Next, the most active spheroidal ZnO among the tested ZnO
samples was adsorbed with Cu ions or doped with S. The photoPROX tests of CO using adsorbed/doped catalysts were conductedunder the same conditions as for undoped ZnO samples, but for
3 h. The initial CO2 formation rate for Cu–spheroidal ZnO increased
by a factor of 3.3, while the rate for S–TiO2 decreased by a factor of
0.14, both compared with that for spheroidal ZnO (Fig. 6A, C, and H,
Table 2C and N). Selectivity to CO2 was improved from 92 mol% to
99 mol% by the addition of Cu2+ to spheroidal ZnO. The initial 9800
ppm concentration of CO was decreased to 280 ppm following the
3-h photoreaction (Fig. 6C). All the supplied O2 was consumed
within 2 h, and further removal of CO did not proceed. Furthermore, Cu–spheroidal ZnO-reduced was tested under photocatalytic
reaction conditions (Fig. 6C0 ). The result was essentially identical tothat observed for Cu–spheroidal ZnO (Table 2C and C0 ).
The dependence of CO photo-PROX reactivity on the content of
Cu was also tested for Cu–spheroidal ZnO between 0.01 and
1.0 wt.% of Cu (Table 2C00 , C00 0 , C, C0000 ). Initial formation rates of
CO2 and the conversion to CO2 were greatest in the case of
1
0.1 wt.% Cu (77 lmol h g1
cat and 100%, respectively), whereas
the selectivity to CO2 was best in the case of 0.5 wt.% Cu (99 mol%).
Next, the partial pressure of O2 was increased from 76 Pa to150 Pa (63 Pa of CO and 6.3 kPa of H2). The initial formation rateof CO2 increased by a factor of 1.5, and selectivity to CO2 was
94 mol% (Table 2D). In this test, the initial 9 700 ppm concentrationof CO was decreased to 21 ppm in 3 h and 0.35 ppm in 5 h (Fig. 6D).
The adsorbed Ce ions also promoted the photo-PROX reaction of
spheroidal ZnO (Fig. 6G and Table 2M). However, the initial CO2
formation rate was 52% compared with that for Cu–spheroidal
ZnO (Table 2D). As-prepared Cu–spheroidal ZnO powder was calcined at 573 K for 2 h, and the photo-PROX test was performed(Table 2L). While selectivity to CO2 remained at 99.6 mol%, the initial CO2 formation rate decreased to 1.9% of that for as-prepared
Cu–spheroidal ZnO.
The photo-PROX tests under visible light alone (>420 nm) wereconducted for Cu–spheroidal ZnO and S–ZnO. With the initial pressures of CO (63 Pa), O2 (150 Pa), and H2 (6.3 kPa), the initial CO2
formation rate for Cu–spheroidal ZnO decreased to as low as 1.8%
compared with that for Cu–spheroidal ZnO excited under UV–visible light (Table 2D and J, Fig. 6D and E). S–ZnO was found to beinactive (Table 2O).
For Cu–spheroidal ZnO, by setting the initial pressures of CO at
63 Pa and O2 at 150 Pa, the dependence of the reaction on the H2pressure was examined between 0 kPa and 63 kPa (Table 2D–H).
As the partial pressure of H2 increased, the initial CO2 formation
1
1 1
rates decreased from 94 lmol h g1
gcat . Selectivcat to 18 lmol hity to CO2 was excellent (99–97 mol%) when the H2 pressure was
0–3.0 kPa, but decreased to 94 mol% at an H2 pressure of 6.3 kPa
and then to 79 mol% at an H2 pressure of 63 kPa.
The amount of Cu–spheroidal ZnO photocatalyst was varied between 25 and 100 mg for photo-PROX reaction tests of CO (Fig. 7A).
The (pseudo) ﬁrst-order reaction rate constants k based on thedecreasing rates of CO (Section 2.4) linearly increased as a functionof catalyst amount charged. Further, transmitted UV–visible light
linearly decreased when the homogeneously spread catalystamount increased up to 0.5 g (Fig. 7B). Thus, absorbed photon
numbers by photocatalysts (typically 50 mg) were proportional
to the amount of photocatalysts in this study, and the catalytic
rates (Table 2) were expressed per catalyst amount (gcat).
In the absence of a catalyst, the photoreaction proceeded underUV–visible light, but CO conversion to CO2 was only 2.9% after 3 h(Fig. 6BL1 and Table 2T). Surprisingly, the photoreaction performed

198

Y. Yoshida et al. / Journal of Catalysis 287 (2012) 190–202

Fig. 7. (A) Dependence of time course of the PROX reaction of CO in H2 on theamount of Cu–spheroidal ZnO used (25–100 mg) illuminated with UV–visible light.
Reaction gases: CO 3.7 lmol (63 Pa), H2 370 lmol (6.3 kPa), and O2 4.5 lmol
(76 Pa). (B) Dependence of transmitted UV–visible light intensity through the 30–
120 mg of Cu–spheroidal ZnO in the area of 5.3 cm2. The distance between thebottom of the reactor and the lamp exit was set to 24 mm.

under visible light alone converted a greater amount of CO to CO2
after 3 h (11%) (Fig. 6BL2 and Table 2U). The photoreaction under
UV light appears to be unselective, and some amount of the O2
gas was consumed through reaction with H2. On the other hand,the photoreaction under visible light was more selective(90 mol% compared to 77 mol% selectivity under UV–visible light)
and formed a larger amount of CO2.
In the absence of light at 290–327 K, Cu–spheroidal ZnO converted 0.86–5.1% of CO to CO2 after 3 h (Table 2K, K0 and Fig. 6F).
In the photocatalytic tests in Table 2D, the temperature was
305 K at the catalyst position. Further, the obtained ﬁrst-order rate
constant at 290–327 K (0.0004–0.0007 min1) under dark was only
1.2–2.1% of that under visible light. Thus, thermal PROX reactionsof CO were negligible using Cu–spheroidal ZnO. The small but
1
non-zero initial CO2 formation rate (6:2 lmol h g1cat ) for Cu–spheroidal ZnO under UV–visible light in the absence of reaction gases(Table 2I) may be because of the impurity carbonates pre-adsorbedon the ZnO surface.
For comparison with the photo-PROX reactions, the PROX reaction of CO in the absence of light at 290 K using an Au–TiO2 catalyst
was examined starting with initial pressures of CO (63 Pa), O2(150 Pa), and H2 (6.3 kPa) (Fig. 6J and Table 2S). The initial CO2 formation rate was 14 times greater compared with that for Cu–spheroidal ZnO under UV–visible light (Table 2D). Furthermore,
selectivity to CO2 was comparable (93–94 mol%). In addition, thephoto-PROX test of CO was performed using Au–TiO2 under UV–
visible light; however, except for an increase in water formation
by a factor of 1.2, the effect of light was negligible.
4. Discussion
4.1. Syntheses and properties of ZnO-based photocatalysts
Spheroidal and rod-like ZnO particles were synthesized. The
spheroidal and rod-like particle shape in the SEM images(Fig. 2B1 and B4) and the greater intensity of the (0 0 0 2) peak in 0) and (1 0 1
 1) peaks
the XRD patterns compared with the (1 0 1(Fig. 1a and b) indicate that the particles grew anisotropically along
the c-axis direction [30] to preferentially expose the polar (0 0 0 1)
 0),face [27]. High-resolution TEM images demonstrated the (1 0 1
 0), and (0 0 0 2) lattice pattern and crystal growth direction(1 1 2of [0 0 0 1] for spheroidal ZnO (Fig. 3a1–7). For the spheroids, a 0), (0 0 0 1),
 andmodel of spheroid consisted of (0 0 0 1), (1 0 1
(1 0 1 n) was proposed (Fig. 3, model on left). The SBET value forspheroidal ZnO (35 m2 g1) was an intermediate between the values for FZO-50 (50 m2 g1) and rod-like ZnO (20 m2 g1; Table 1),
and those for Cu–spheroidal ZnO and Ce–spheroidal ZnO (39–44 m2 g1) were greater compared with spheroidal ZnO. A reason
for this was the partial dissolution and recrystallization of spheroidal ZnO during the Cu/Ce ion exchange in the metal nitrate solution. Based on the relative intensity of the (0 0 0 2) XRD peaks, forCe–spheroidal ZnO, the anisotropic particle shape was preserved(Fig. 1d, Fig. 2A3 and B3), whereas for Cu–spheroidal ZnO, the particle shape became similar to that of general ZnO crystals (Figs. 1cand 2 A2 and B2).
The Eg values were estimated on the basis of the UV–visible
absorption spectra for the ZnO samples and the heteroatom adsorbed/doped ZnO samples (Fig. 4). The Eg values were evaluatedby the extrapolation of the absorption edge to the x-axis. The value
for FZO-50 (3.24 eV) was 0.03 eV greater than those for spheroidal
and rod-like ZnO (3.21 eV; Table 1a, b, and f) probably because of
the nanosize effect [24] of the smaller particles of FZO-50(21 nm, average) compared with those of spheroidal and rod-likeZnO (average 39–48 nm based on XRD, Table 1).
The adsorption of Cu2+ or Ce3+ on spheroidal ZnO decreased theEg values by 0.02 eV [25]. The Eg value further decreased by 0.02 eV
when Cu2+ was reduced to Cu+ under UV–visible light in the presence of ethanol (Table 1c0 ). It is known that the electronic transition of ZnO is direct [31]. For all ZnO-based samples synthesizedin this study, the Eg values estimated by the simple absorption
edge extrapolation were in agreement with those given by ﬁtting
to the Davis–Mott equation with n values between 1/2 and 3/2 (Table 1). Thus, the electronic transition was direct from 2p of O to thefollowing: 4s or 4p of Zn; 3d, 4s, or 4p of Cu; and 6s or 4f of Ce.
4.2. Optimization of photo-PROX of CO
Among the ZnO samples tested for photo-PROX reactions, spheroidal ZnO exhibited the highest CO2 formation rates (Table 2A, B,
and P). The initial CO2 formation rate was enhanced by a factor of
3.3, and selectivity to CO2 was improved to 99 mol% upon theadsorption of Cu2+ on spheroidal ZnO (Table 2C). The CO2 formation rate positively depended on the partial pressure of O2. Under
63 Pa of CO, 150 Pa of O2, and 6.3 kPa of H2, the initial 9 700
ppm concentration of CO was photocatalytically decreased to
21 ppm within 3 h and to 0.35 ppm within 5 h (Table 2D and
Fig. 6D), suggesting potential application to purify H2 to the polymer electrolyte FCs.
In contrast to the promoting effects of adsorbed Cu2+ or Ce3+
(Table 2C and M), S–ZnO was fairly inactive (Table 2N). The valenceof S in the S–ZnO should be negative because ethanethiol was usedin the solvothermal synthesis at 433 K and obtained powder was
not calcined. The anionic S atoms may block the surface active sites
of Zn for the photo-PROX reaction of CO [15].
The initial CO2 formation rate of Cu–spheroidal ZnO photocatalyst decreased to only 1.9% of the incipient one when it was calcined at 573 K (Table 2L). We suspect that adsorbed Cu2+ sites forincipient Cu–spheroidal ZnO became doped in/on ZnO and/or
aggregated at 573 K and became less reactive based on the changeof coordination number of Cu–O from 6 to 4 by Cu K-edge EXAFS
analyses. It is also plausible that the surface hydroxy groups decreased by the heating at 573 K and the catalytic step(s) involvinghydroxy group became quite slower.

Y. Yoshida et al. / Journal of Catalysis 287 (2012) 190–202

UV light illumination was essential for the photo-PROX activityof Cu–spheroidal ZnO because the initial CO2 formation rate wasonly 1.8% under visible light alone (>420 nm; Table 2J and D). This
dramatic decrease in reactivity was in accordance with the UV–visible spectrum (Fig. 4c); the absorption tail negligibly extended tomore than 420 nm.
In addition, the photo-PROX activity of the Cu–spheroidal ZnO
photocatalyst depended on the partial pressure of H2 between
0 kPa and 63 kPa (Table 2D–H). As the partial pressure of H2 increased, the initial CO2 formation rate and CO2 selectivity gradually
1
1 1
decreased from 94 lmol h g1
gcat and from 99–
cat to 18 lmol h
97 mol% to 79 mol%, respectively. The effect of the presence/absence of H2 has previously been reported for the PROX reactionof the Au–TiO2 catalyst [32]. The reaction order of PCO clearly increased from 0.34 to 0.82 owing to the presence of H2, that is, poor
PROX activity for low concentration of CO in predominant H2.
Considering the dependence on the partial pressure of H2, thephoto-PROX results for Cu–spheroidal ZnO at an H2 pressure of
600 Pa were compared with literature data for NiO/TiO2 and Mo/
SiO2 tested with an H2 pressure of 590 Pa [9,10] (Table 2F, Q, and
R). The initial CO2 formation rates were in the order of Mo/SiO2 > Cu–spheroidal ZnO > NiO/TiO2. Furthermore, selectivity to CO2 wasin the order of Cu–spheroidal ZnO  Mo/SiO2 > NiO/TiO2.
In the absence of photocatalysts, 2.9–11% of CO was converted
to CO2 in 3 h, whereas in the presence of Cu–spheroidal ZnO,
100% of CO was converted to CO2 (Table 2D, T, and U). The contribution of the photoreaction was small under UV–visible light. Under visible light, the conversion to CO2 was 11% in the absence of
catalyst (Table 2U, Fig. 6BL2), while the conversion was only 6.5%
in the presence of Cu–spheroidal ZnO (Table 2J, Fig. 6E). Unfortunately, Cu–spheroidal ZnO was poorer than photochemical reaction under visible light and a part of low concentration of CO2
formed would be adsorbed on Cu–spheroidal ZnO, as shown in
Fig. 6E. The relatively greater amount of O2 decreased, as shownin Fig. 6BL1 and BL2, compared to formed amount of CO2. Thiswas due to water formation especially for BL1, but the low concentrations of water adsorbed in the reaction glass tube and were notdetected in the time course. Quantitative (but still small amountof) water was detected when the reaction gas was collected in liquid nitrogen trap after the reaction tests (Table 2T and U). In
the absence of light, Cu–spheroidal ZnO converted only 0.86–
5.1% of CO to CO2 at 290–327 K in 3 h (Table 2K and K0 ). Spheroidal
ZnO and Cu–spheroidal ZnO were excellent catalysts for the photoPROX of CO but poor catalysts for the PROX of CO at 290–327 K.
4.3. Reaction mechanism of photo-PROX of CO using Cu–spheroidal
ZnO
During the photo-PROX reaction, the initial rates of decrease in
CO and O2 gases were essentially equivalent for spheroidal ZnO,
Ce–spheroidal ZnO, and FZO-50 on a molar amount basis(Fig. 6A, G, and I). In contrast, CO initially decreased more rapidly
than O2 for Cu–spheroidal ZnO (Fig. 6C and D), which corresponds
better to the stoichiometric ratio of the reaction 2CO + O2 = 2CO2.
In contrast, the molar amount of O2 decreased more rapidly thanthat of CO for rod-like ZnO, S–ZnO, and Au–TiO2 (Fig. 6B, H, and J).
The ratio of CO2 formed versus CO diminished suggested differences in the catalytic mechanism. The ratio was close to unity forspheroidal ZnO (89%), rod-like ZnO (99%), Cu–spheroidal ZnO(93–100%), and Au–TiO2 (100%), whereas it was lower for FZO-50(69%), Ce–spheroidal ZnO (54%), and S–ZnO (43%) (Fig. 6I, G, and
H). The catalysts that do not remain CO-derived species on the surface exhibited higher CO2 formation rates.
During the photo-PROX reaction, the change in the valence stateof Cu adsorbed on spheroidal ZnO was monitored using Cu K-edge
XANES (Fig. 5). The ratio for CuI sites was 3.9% in the initial 1 min

199

and progressively increased to 15.6% over the 3-h photo-PROX
reaction, maintaining dynamic equilibrium. The stabilization of
CuI over basic ZnO was suggested [33]. The growing rate of CuI sites
1
under UV–visible illumination was 4:1 lmol  Cu h g1
cat in 135 Pa
of CO, 270 Pa of O2, and 13.1 kPa of H2. The photo-PROX reaction
rate to form CO2 under these reaction conditions was1
29:3 lmol  CO2 h g1cat (kinetic data not shown). Therefore, thereduction of CuII to CuI sites detected by Cu K-edge XANES was
not a direct step incorporated in the photo-PROX reaction mechanism of CO, but non-catalytic step to accumulate CuI sites.
The CO photo-PROX results were optimum when the Cu content
was 0.1–0.5 wt.% (Table 2C00 , C00 0 , C, C0000 ). Small amount of CuII sites
should be enough to accept photogenerated and diffused electronsin ZnO under the reaction conditions and excessive amount of CuII
sites than 0.5 wt.% even blocked active sites over ZnO.
When the photo-PROX reaction was performed using Cu–spheroidal ZnO-reduced in which the CuII sites were reduced to CuI, theresults (Table 2C0 , Fig. 6C0 ) were essentially identical to those obtained using Cu–spheroidal ZnO in which the initial Cu valence
was II (Table 2C, Fig. 6C). This can be explained by the fact that CuII
sites accept diffused photoelectrons in ZnO or prereduced CuI sites
were set and rapidly transfer them to O2-derived species; however,these quick steps themselves were not detected by XANES (Fig. 5).
In the photodecomposition of sucrose, Cu2+ ions in the aqueous
solution increased the photodecomposition rates, and it is proposed that the photogenerated electrons were transferred from
TiO2 to the Cu2+ ions in the aqueous solution and then transmitted
to oxygen molecules [34]. It is believed that the hole diffusion
length is 10 nm versus the electron diffusion length is 10 lm
in TiO2 [35]. In ZnO, the hole and electron diffusion lengths are suggested to be 240–440 nm [36,37] and 50–140 lm [38], respectively. Furthermore, direct photoelectronic transition takes placefrom 2p of O to relatively diffused 4s or 4p of Zn and 3d, 4s, or
4p of Cu in Cu–spheroidal ZnO. Thus, photogenerated electrons
could diffuse in entire spheroidal ZnO (35.1 nm  23.5 nm in average, Fig. 2C2) probably following in the proximity to Zn2+ ions tosurface Cu2+ sites before they recombined with holes. The indirect
orbital overlap between diffused 4s or 4p of Zn and 3d, 4s, or 4p of
Cu was also suggested at surface [39].
The proposed photo-PROX reaction mechanism is illustrated in
Scheme 1. The two routes are not similar to the PROX reaction
mechanism where CO adsorption on the metal particle surface
and the activation of O2 at the interface between metal particles
and support oxide [32,40] is proposed. The CO molecules reactwith surface hydroxy to form formates [41] as monitored in FTIR in Route A. Oxidation of CO coupled with oxygen species atthe surface and surface reoxidation using O2 were proposed for
CO oxidation of Co3O4 nanorods, predominantly on the (1 1 0) plane[42].
The rod-like ZnO was fairly inactive compared to spheroidal
ZnO (Table 2A and B, Fig. 6A and B), and thus, the crystal faces parallel to [0 0 0 1] axis should be inactive. The reactivity difference of
crystal faces may be exaggerated because electron–hole pairs
photogenerated in rod-like ZnO (443 nm  79.8 nm in average,
Fig. 2C4) mostly diffused to nearer side faces of nanorods and face at the end of nanorods.
rarely reached to relatively far (0 0 0 1)
 face terminates O atoms and/or hydroxy group.
The polar (0 0 0 1)
Because the ion-exchanged Cu2+ sites for reduction reaction andformate species starting from CO and hydroxy can be formed in face of Cu–spheroidal ZnO (Fig. 3, model on
vicinity on (0 0 0 1)
left), the reduction and oxidation would occur preferably on face compared to (1 0 10)
 and ð1 1 20)
 face.
(0 0 0 1)
In the FT-IR spectrum for ZnO, the absorption peaks of formate
species were reported to be due to the antisymmetric (mas(OCO))
and symmetric stretching vibration of the O–C–O group (ms(OCO))
at 1580 cm1 and 1365 cm1, respectively, the coupling mode of

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Y. Yoshida et al. / Journal of Catalysis 287 (2012) 190–202

Scheme 1. Proposed mechanism of the photocatalytic PROX reaction of CO using spheroidal ZnO promoted by Cu2+ ions. Route A is via the coupling of adsorbed O2 withformate species, whereas Route B is via the coupling of adsorbed O2 with surface hydroxy to form a hydroxy radical.

mas(OCO) and the bending vibration of C–H (d(CH)) at 2970 cm1,the stretching vibration of C–H (m(CH)) at 2880 cm1, the coupling
mode of ms(OCO) and d(CH) at 2740 cm1, and d(CH) at 1383 cm1

HCO2 H ! CO2 þ 2Hþ þ 2e ; Eo ¼ 0:199 V[43,44]. For Cu–spheroidal ZnO, the peaks at 1630 cm1 and1339 cm1 were observed at 46 Pa of CO and can be assigned tomas(OCO) and ms(OCO) peaks of the formate species, respectively(Scheme 1A–b). Furthermore, under these conditions or at 46 Pa
of CO and 46 Pa of O2 under UV–visible light, peaks at 2985 cm1and 2879 cm1 were observed due to mas(OCO) + d(CH) and m(CH),respectively. The ﬁve peaks (2985, 2879, 1627, 1587, and
1297 cm1) under CO + O2 illuminated with UV–visible light were
well compared to ﬁve peaks (2952, 2877, 1645, 1620, and1320 cm1) given at 46 Pa of formic acid for Cu–spheroidal ZnO
sample. The peak due to ms(OCO) + d(CH) was not clearly resolved
because of the low signal/background ratio.
For Cu–spheroidal ZnO, the adsorbed Cu ions clearly exhibited a
promoting effect to enhance CO2 formation by a factor of 3.3(Fig. 6A, C) and therefore should be incorporated in the promoted
photo-PROX reaction mechanism. The redox potential values for
the Cu ions and the reaction steps in the photo-PROX reaction
are as follows:

We propose that the electrons produced in ZnO under UV–visible
light diffused to CuII sites on the surface, especially effectively forshorter spheroidal ZnO (35.1 nm in average) than for longer rod-likeZnO (443 nm in average) before they recombined with holes during
the diffusion, and the trapped electrons as CuI state were transferred to O2 (Scheme 1A–c). The energetic is depicted in Scheme 2
including the electron ﬂow from valence band to conduction band,to Cu ions (or directly from VB to Cu ions), and then to O2 species.
Based on the comparison of the reduction potential values, steps b
to c can not only be considered as the diffusion of photogenerated
electrons to CuII sites and holes to the formate species but also as
the electron transfer from formate to CuII. Route A assumes effective
H+ transfer from the formate species to the neighboring reduced O2derived species on the Cu ion sites.
On the other hand, Route B assumes H+ transfer from the surface hydroxy species to the reduced O2-derived species on the Cusites (Scheme 1B–c ? d). A shoulder peak at 3666 cm1 and abroad peak centered at 3310 cm1 were observed for as-prepared
Cu–spheroidal ZnO and are assigned to isolated hydroxy andhydrogen-bonded hydroxy species, respectively. In both Routes A
and B, hydroxy radicals are formed by the photoreduction of O2
and promote the decomposition of formates (Scheme 1Af ? b)

Cu2þ þ e ¢ Cuþ ; Eo ¼ 0:159 V
O2 þ 2Hþ þ 2e ! H2 O2 ; Eo ¼ 0:695 V

Y. Yoshida et al. / Journal of Catalysis 287 (2012) 190–202

201

Scheme 2. Energy levels at the bandgap of ZnO and Cu2+/+ ions versus the reaction steps of photo-PROX of CO.

[45] or directly react with CO to form CO2 and H+(Scheme 1Bg ? a).
In both Routes A and B, if hydrogen peroxide was reduced
through coupling with H+, water was produced as a byproduct. In
the photo-PROX reactions, the initial CO2 formation rates decreased
1 1from 94 lmol h1 g1gcat under
cat in the absence of H2 to 18 lmol h
63 kPa of H2 (Table 2D–H). In contrast, water formation rates were1
1 1
nearly constant between 2:9 lmol h g1
gcat
cat and 4:3 lmol hunder these photocatalytic reaction conditions. Hydrogen did not
simply affect the CO oxidation reaction by enhancing the by-reaction to form water. Instead, it inhibited the adsorption sites of CO,
for example, by the decomposition of formates (Scheme 1A–b).

5. Conclusions
In this study, spheroidal and rod-like ZnO semiconductors weresynthesized. Based on the XRD peak intensity ratio and SEM andhigh-resolution TEM images, both samples were found topreferentially and anisotropically grow in the c-axis direction(22 nm  47 nm and 80 nm  443 nm, respectively). The speciﬁc
surface area of spheroidal ZnO (35 m2 g1) was 1.75 times greaterthan that of rod-like ZnO, and their semiconductivity was similar(Eg = 3.21 eV). However, the initial CO2 formation rate in the
1photo-PROX of CO using spheroidal ZnO (17 lmol h g1cat ) was
40.5 times greater than that using rod-like ZnO, strongly suggesting critically dependent photocatalytic activity on the exposed
crystal face and effective electron/hole diffusion length to eachcrystal face before charge recombination takes place.
The adsorption of 0.5 wt.% of Cu2+ ions was found to be effective
for promoting and tuning the photo-PROX reactivity of CO using
spheroidal ZnO. The initial CO2 formation rate using Cu–spheroidal
ZnO was further increased by a factor of 3.3 compared with that for
spheroidal ZnO. When the initial pressures were 63 Pa of CO,
150 Pa of O2, and 6.3 kPa of H2, CO was decreased to 0.35 ppm in
5 h under UV–visible light. The comparison of photo-PROX performance under visible light indicated that photoreactivity was
mostly due to UV light illumination (<420 nm).
The photogenerated electron trap at the CuII sites was demonstrated as the increase in the shoulder peak at 8983.1 eV, speciﬁcally for CuI sites below the Cu K-absorption edge. Under photoPROX reaction conditions, the increase in the rate was
1I
4:1 lmol h g1
cat and 15.6% of Cu sites were reduced to Cu in
3 h. In the FT-IR spectra under photo-PROX reaction conditions,the peaks due to the hydroxy group were observed at 3666 cm1and 3310 cm1 and the peaks due formate species were observed
at 2985 cm1, 2879 cm1, 1627 cm1, 1587 cm1, and 1297 cm1.

The reaction of CO with the hydroxy group produced formate species, and the O2-derived species would be reduced by the CuI sites
and would accept protons from the neighboring formate species.
Acknowledgments
The authors are grateful for the ﬁnancial support from theGrant-in-Aid for Scientiﬁc Research C (2255 0117) from MEXT
(2010–2011). The X-ray absorption experiments were performed
with the approval of the Photon Factory Proposal Review Committee (No. 2011G083, 2009G552, and 2009G596) and the grant of thePriority Program for Disaster-Affected Quantum Beam Facilities(2011A1978, SPring-8 & KEK). The authors thank Prof. Yoshitakefor the Xe arc lamp and Prof. Kaneko and Prof. Kanoh for the
XRD apparatus.
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