Journal of Catalysis 332 (2015) 1–12

Contents lists available at ScienceDirect

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

Binary metal (Ti, Cu) oxyhydroxy–organic (terephthalate) framework:
An interface model nanocatalyst for hydrogen purification
Yusuke Yoshida, Yasuo Izumi ⇑Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japana r t i c l e

i n f oArticle history:
Received 11 June 2015
Revised 22 August 2015
Accepted 7 September 2015

Keywords:
CopperTitanium oxide
Metal–support interface
Metal–organic frameworks
EXAFS spectroscopy
Rietveld refinement
Preferential oxidationa b s t r a c t
The importance of metal–support interfaces is widely known in commercial and fundamental
heterogeneous catalysis; however, it is difficult to characterize the active interface sites. In this study,
we synthesize a new class of compound comprising tetragonal [Ti8O8(OH)4]12+ clusters interlinked byterephthalates (bdc) and [Cu2(OH)6]2 linkers {Ti8O8(OH)4(bdc)2[Cu2(OH)6]4}. The crystalline structure
was refined for X-ray diffraction and direct links between [Cu2(OH)6]2 and [Ti8O8(OH)4]12+ are confirmedby extended X-ray absorption fine structure. This compound functions very well (k = 0.117 min1 in CO
63 Pa + O2 76 Pa at 323 K) as a catalytic model of interface Cu sites on ultra-dispersed Ti [hydro]oxide forpreferential oxidation of CO in predominantly H2 gas, that is important for the purification of hydrogenused in fuel cells. In comparison, mean 1.7-nm CuO nanoparticles embedded inside the pores of MIL125
were inert (k = 0.0035 min1) because of the absence of links between Cu and [Ti8O8(OH)4]12+ clusters. In
CO 0.51 kPa + O2 0.51 kPa at 323 K, the conversion to CO2 and CO PROX selectivity using
Ti8O8(OH)4(bdc)2[Cu2(OH)6]4 (76% and 99%) was significantly higher than that using CuO/CeO2 (28%
and 96%, respectively) for 24 h.
Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction
Oxyhydroxy metal clusters, e.g., [Zr6O4(OH)4]12+ [1], [Al12O(OH)18]16+ [2], and [Ti8O8(OH)4]12+, [3] were linked using organic
molecules containing more than two anionic functional groups,e.g., terephthalate (1,4-benzene dicarboxylate; bdc), 1,3,5-benzene
tricarboxylate (btc), and bipyridine [4], to form metal–organic
frameworks (MOFs). Cu oxides supported on TiO2 catalyzed CO
oxidation [5,6], methane combustion [7], NOx decomposition [8],and photocatalytic water splitting [9].
For heterogeneous catalysis, metal–support interfaces havefrequently been considered as active sites [10–13]. The interface
between the nanoparticles and the metal oxide support is believedto play a crucial role in the catalysis. The examples of metal–
support interfaces are copper nanoparticles on zinc oxide formethanol synthesis [10], rhodium nanoparticles on titanium oxide
for hydrogenations [11], ruthenium nanoparticles promoted withalkali metal on carbon for ammonia synthesis [12], and gold
nanoparticles on TiO2 for preferential oxidation (PROX) of CO[13]. However, in nature, the interfaces have been difficult tocharacterize by spectroscopic or physicochemical methods. In this
⇑ Corresponding author.
E-mail addresses: aata3481@chiba-u.jp (Y. Yoshida), yizumi@faculty.chiba-u.jp(Y. Izumi).
http://dx.doi.org/10.1016/j.jcat.2015.09.007
0021-9517/Ó 2015 Elsevier Inc. All rights reserved.

study, exact model composite consisting of [Ti8O8(OH)4]12+ clusters
and [Cu2(OH)6]2 linkers was prepared as a molecular model ofCu oxide/hydroxide on Ti oxide/hydroxide support (supported
catalysts).
Recently, nanosized metal oxide/sulfide and layered doublehydroxide (LDH) have been synthesized utilizing MOF as a replica
for the crystalline morphology [14] and MOF as a core template followed by the elimination of MOF to form hollow LDH, respectively[15]. The synthesis of core–shell nanocomposites of Cu3(btc)2 andanatase TiO2 (210-nm thickness) has also been reported [16]. In
this study, [Ti8O8(OH)4]12+ cluster, tetragonally linked by bdc
ligands (MIL125), was chosen as a model of TiO2 support. Theinterface between [Ti8O8(OH)4]12+ and added Cu species duringcatalyst synthesis was characterized and evaluated for CO PROX
in predominantly H2 gas.

2. Experimental
2.1. Synthesis of model catalysts
2.1.1. MIL125
MIL125 with a formula of Ti8O8(OH)4(bdc)6 was synthesized
via the reported procedure [3]. In brief, 3.75 mmol of Ti(IV) tetrabutoxide (>95%, Wako Pure Chemical) was mixed with 45 mL of N,

2

Y. Yoshida, Y. Izumi / Journal of Catalysis 332 (2015) 1–12

N-dimethylformamide (DMF; >99.5%, Super dehydrated grade,
Wako Pure Chemical) and 5.0 mL of methanol (>99.8%, Super dehydrated grade, Wako Pure Chemical). Terephthalic acid (15 mmol,>95%, Wako Pure Chemical) was added to the solution and dissolved. The mixed solution was magnetically stirred for 5 min at
300 rotations per minute (rpm) at 290 K. Subsequently, the solution was transferred into a Teflon-lined autoclave (TVS-N2-100,
Taiatsu Techno; the inner volume 100 mL) and heated at 423 K
for 20 h. The white precipitate thus obtained was filtered using a
0.2-lm-pore polytetrafluoroethylene membrane filter (Omnipore
JGWP04700, Millipore) and washed several times with DMF (total
60 mL) and acetone (total 90 mL; >99.5%, Wako Pure Chemical).
The obtained powder was dried in air at 353 K for 12 h and heatedin vacuum at 473 K for 6 h.
2.1.2. Solid-state embedding of Cu
Cu was embedded in MIL125 using the following procedure.
Using a mortar and pestle, 200 mg of MIL125 and 102 mg of Cu(II) acetate monohydrate (>99.8%, Wako Pure Chemical) were well
mixed for 10 min. The mixture was treated in vacuum (106 Pa) at
290 K for 1 h. The resultant powder was heated at 473 K under
25 kPa of H2 for 3 h. This reduction step was aimed for the growthof metallic Cu nanoparticles inside the nanopores of MIL125(Chart 1A). The color of the powder changed from light blue tored, indicating the formation of metallic Cu nanoparticles in thesample. The sample was calcined in air at 573 K for 2 h. If the
reduction step in H2 was skipped, the crystalline of MIL125 wascritically decomposed and 15 nm of CuO particles significantly
greater than the nanopores (1.7 nm) of MIL125 was formed basedon the X-ray diffraction (XRD) analysis. The obtained lime green
powder was denoted as Cu/MIL125-S-ratio, in which the ratio
was the atomic ratio of Cu/Ti. The atomic ratio of Cu/Ti was fixed
at 1/2 (Table 1). During the solid-state embedding procedures,the framework of MIL125 was intact (see Section 3.1).
2.1.3. Cu impregnation
0.4 mL of Cu nitrate (0.5 mmol; >99.9%, Wako Pure Chemical)
aqueous solution was prepared and 200 mg of MIL125 was added.
The mixture was magnetically stirred at 300 rpm and 290 K for
20 min and the water was evaporated at 353 K. The atomic ratioof Cu/Ti was basically set to 1/2, but varied between 0 and 1(0/1, 1/4, 1/2, 3/4, and 1/1) by incorporating 0.4 mL of Cu nitrate(0–1.0 mmol) for comparative purposes (Table 1). The powder
was calcined in air at 553–613 K for 2 h. The obtained green
samples by impregnation method prepared via impregnation of
MIL125 with Cu nitrate aqueous solution and calcination were
denoted as Cu/MIL125-I-T-ratio, in which T was the calcination
temperature (K) and the ratio was the atomic ratio of Cu/Ti (1/4,
1/2, 3/4, and 1/1). MIL125-I-573-0/1 was a control sample thatwas obtained from the impregnation procedure without Cu salt.
The reaction of Cu ions with [Ti8O8(OH)4]12+ cluster occurred andthe framework of MIL125 was lost if the solution was used. Therefore, reduction step in H2 employed for Cu/MIL125-S-1/2 aiming togrow metallic Cu nanoparticles inside the nanopores of MIL125
was not followed for impregnated samples.
2.1.4. Reference catalysts
JRC-TIO-6 [rutile-type TiO2, mean particle size 15 nm, the Bru
nauer–Emmett–Teller surface area (SBET) 100 m2 g1; Catalysis
Society of Japan (CSJ)], JRC-TIO-7 (anatase-type TiO2, mean particle
size 8 nm, SBET 270 m2 g1; CSJ), and P25 (TiO2, anatase/rutile
phase = 8/2, SBET 60 m2 g1; Degussa) [17] were used as references.
Cu was embedded via solid-state reaction with or impregnated by
the incipient wetness method with these TiO2 samples andcalcined at 573 K. These samples were denoted as Cu/brand of
TiO2-S (via solid-state reaction) and Cu/brand of TiO2-I (by the
incipient wetness method; Table 1).
CuO/CeO2 catalyst was prepared via co-precipitation method[18]. Aqueous solution of Cu nitrate (0.15 M, 10 mL) and diammonium Ce(IV) nitrate (0.075 M, 100 mL; >95.0%, Kanto Chemical)
was prepared, and sodium carbonate aqueous solution (24 mL,
7 wt%) was added to the mixed solution in 1 h at 290 K, and thenset still at 290 K for 30 min. The precipitate was filtrated using a
JGWP04700 filter and washed several times with deionized water(<0.05 lS cm1; total 400 mL). The obtained green powder wasdried at 383 K for 18 h, followed by calcination at 573 or 923 K
for 4 h. These samples were denoted as CuO/CeO2-T in which T
was the calcination temperature (K; Table 1).
2.2. Characterizations
N2-adsorption measurements were performed at 77 K in thepressure range 1.0–40 kPa in a vacuum system (106 Pa). The samples (10–100 mg) were evacuated and heated from 290 K to 423 K

Chart 1. Proposed structure for Cu/MIL125-S (A) and Cu/MIL125-I (B: side and top views), in which gray sticks/circles indicate [Ti8O8(OH)4]12+ clusters, dashed lines indicate
terephthalates, and green lines/wedges indicate [Cu2(OH)6(OH2)n]2 (n = 0–4) linkers. The Model B0 is based on Model B0 in Chart 3. Hydrogen atoms are abbreviated forclarity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3

Y. Yoshida, Y. Izumi / Journal of Catalysis 332 (2015) 1–12
Table 1
List of catalyst samples and the explanation of preparation method.
Catalyst

Synthesis in medium
Precursorsa

Gas treatment
Atomic ratio of metals

a

Ref.

Solvent used

a

T (K)

T (K)

Gas

DMF

423

353
473

Air
Vacuum[3]

MIL125Ti(OC4H9)4, Terephthalic acid

Cu/MIL125-S-1/2

MIL125, Cu(O2CCH3)2H2O

Cu/Ti = 1/2

None

290

473
573

H2
Air

This work

MIL125-I-573-0/1
Cu/MIL125-I-573-1/4
Cu/MIL125-I-573-1/2
Cu/MIL125-I-573-3/4
Cu/MIL125-I-573-1/1
Cu/MIL125-I-613-1/2
Cu/MIL125-I-613-1/1

MIL125, Cu(NO3)2

Cu/Ti = 0/1
Cu/Ti = 1/4
Cu/Ti = 1/2
Cu/Ti = 3/4
Cu/Ti = 1/1
Cu/Ti = 1/2
Cu/Ti = 1/1

Water

290

573
573
573
573
573
613
613

Air

This work

Cu/P25-S (13.5 wt% Cu)

P25, Cu(O2CCH3)2H2O

Cu/Ti = 1/5

None

290

P25, Cu(NO3)2
JRC-TIO-6, Cu(NO3)2
JRC-TIO-7, Cu(NO3)2(NH4)2Ce(NO3)6, Cu(NO3)2

Water
Water

290
290

H2
Air
Air
Air

This work

Cu/P25-I (13.5 wt% Cu)
Cu/JRC-TIO-6-I (rutile; 13.5 wt% Cu)
Cu/JRC-TIO-7-I (anatase; 13.5 wt% Cu)
CuO/CeO2-573
CuO/CeO2-923

473
573
573
573

Cu/Ce = 1/5

Water

290

573
923

Air

This work

Au/TiO2

P25, HAuCl44H2O

Au/Ti = 1/48

Water

353

573

Air

[31]

This work

Temperature.

at a ramping rate of 4 K min1 and maintained at 423 K for 2 h as
pretreatment. The SBET value was calculated using eight points inthe adsorption isotherm.
The XRD pattern was observed using a D8 ADVANCE diffractometer (Bruker) at the Center for Analytical Instrumentation,
Chiba University at a Bragg angle (hB) of 2hB = 5.0–60° with a scanstep of 0.02° and a scan rate of 3 s per step. The measurements
were performed at 40 kV and 40 mA using Cu Ka emission(k = 0.15419 nm) [19,20] and a nickel filter. The sizes of crystallines(t) were estimated based on the Scherrer equation:

t¼

0:9k
:
Peak width  cos hB

ð1Þ

High-resolution transmission electron microscopy (HR-TEM)
images were obtained using a Model H-7650 (Hitachi). The samples were mounted on Cu mesh (150 mesh per inch) coated witha copolymer film of polyvinyl alcohol and formaldehyde (Formvar,
Monsanto) and deposited carbon. A tungsten filament was used in
the electron gun and the electron accelerating voltage was 100 kV.
The magnification was between 60 000 and 600 000 times.
Fourier transform infrared (FTIR) spectra were measured using
a Model FT/IR-4200 spectrophotometer equipped with a mercury–cadmium–tellurium-M detector (JASCO) in the wave number
region between 4000 and 650 cm1. The energy resolution was set
to 2 cm1. A sample disk (/ = 20 mm, 4 mg diluted in KBr) was
positioned perpendicular to the IR beam.
Ti K and Cu K-edge X-ray absorption fine structure (XAFS) spectra were measured at 290 K in a transmission mode on a beamline
9C in the Photon Factory and NW10A in the Photon Factory
Advanced Ring at the High Energy Accelerator Research Organization (Tsukuba). The storage ring energy was 2.5 GeV and the ring
current was 450 mA for Ti K-edge. The values were 6.5 GeV and
37–54 mA for Cu K-edge. A Si(1 1 1) and Si(3 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 frontof the I0 ionization chamber was 1.0 mm (vertical)  2.0 mm(horizontal). The I0 and Itransmit ionization chambers were purged
with a mixed gas of N2 (70%) + He (30%) (I0) and N2 (Itransmit) for
Ti K-edge and N2 (I0) and a mixed gas of N2 (85%) + Ar (15%)(Itransmit) for Cu K-edge. The Ti K and Cu K-edge absorption energy

values were calibrated to 4964.5 and 8980.3 eV, respectively, forthe spectra of Ti and Cu metal foils [19,20]. The energy positionof the monochromator was reproduced within ±0.1 eV.
The XAFS data were analyzed using an XDAP package [21]. The
analytical procedure has been described previously [22–24].
Multiple shell curve fit (CF) analyses were performed for the
Fourier-filtered k3-weighted extended XAFS (EXAFS) data in kand R-space using empirical amplitude and phase-shift parameters
extracted from the EXAFS data of JRC-TIO7 for TiAO and Ti(AOA)Ti
interatomic pairs, CuO powder for CuAO and Cu(AOA)Cu
interatomic pairs, and Cu metal for CuACu interatomic pair. Theinteratomic distance (R) and its associated coordination number(N) values for the TiAO, Ti(AOA)Ti, CuAO, Cu(AOA)Cu, and CuACu
interatomic pairs were set to an R value of 0.1949 nm with an N
value of 6, an R value of 0.3039 nm with an N value of 4, an R valueof 0.1956 nm with an N value of 4, an R value of 0.3028 nm with an
N value of 10, and an R value of 0.2553 nm with an N value of 12,
respectively, on the basis of their crystal structures [25]. The
Debye–Waller factor (r2) value was with reference to that for themodel parameter (Dr2).
2.3. CO PROX tests
Most of the CO PROX reaction tests in predominant H2 were
performed as follows: 50 mg of catalyst was placed in a Pyrex reactor connected to a closed circulating system (total volume 0.142 L)
and evacuated (106 Pa) for 2 h at 290 K. Then, the sample washeated to 323 K and reaction gas (H2 370 lmol, CO 3.7 lmol, O2
4.5 lmol) was introduced to the system. The gas flow rate was
0.33 L min1. The products and reactants were analyzed using columns of Molecular Sieve 13X-S and polyethylene glycol-6000 on
Flusin P (GL Sciences) set in an online gas chromatograph equippedwith a thermal conductivity detector (GC-TCD; Shimadzu, Model
GC-8A). Independently, 19 lmol of CO2 and/or moisture was mixed
to above reaction gas (H2 + CO + O2) and the effects on CO PROX
reaction were tested at 323 K.
To evaluate the dependence of reaction system and/or reactant
pressure, CO PROX tests in predominant H2 were also performed
using a gas flow system for representative catalysts. 100 mg of catalyst was placed in a Pyrex reactor (internal diameter 4 mm) and
101 kPa of mixed gas (O2 1%, N2 99%) was flowed at a rate of

4

Y. Yoshida, Y. Izumi / Journal of Catalysis 332 (2015) 1–12

10 mL min1 for 1 h at 323 K. Then, the gas was switched to
101 kPa of reaction gas (CO 0.5%, O2 0.5%, H2 49.5%, N2 49.5%)
and the flow rate was 20 mL min1. The products and reactantsin the exit gas (40 mL time1) were analyzed using GC-TCD in asimilar way to that for closed circulating system.
3. Results and discussion
3.1. Cu/MIL125 synthesized via solid-state method
Major XRD peaks for Cu/MIL125-S-1/2 synthesized via thesolid-state method, i.e., physical mixing of Cu salt with MIL125
at an atomic ratio of Cu/Ti of 1/2 and heating in H2 at 473 K(Fig. 1A-b), appeared at 6.89°, 9.58°, 9.83°, and 11.80°, similar to
those for MIL125 (6.94°, 9.61°, 9.90°, and 11.82°; Fig. 1A-a). XRD
peak intensity decreased to 26–44% of the corresponding
peaks for MIL125. The SBET value and mean crystalline size basedon Eq. (1) consistently decreased from 852 m2 g1 and 94 nm for
MIL125 to 349 m2 g1 and 25 nm for Cu/MIL125-S-1/2(Table 2a and b). The SBET and mean crystalline size for MIL125
were in consistent with reference 3. MIL125 is ultimately dispersed
and all the constituent atoms are in contact with the nanopores(Chart 1A). Thus, contribution of the external surface area should
be small and smaller mean crystalline size (25 nm) does notnecessarily correspond to higher SBET value (349 m2 g1) for
Cu/MIL125-S-1/2. One of the major reasons of SBET decrease wasthat the nanopores of MIL125 were blocked by Cu species(Chart 1A). Furthermore, partial decomposition of MIL125 toless nanoporous structure was also possible for Cu/MIL125-S-1/2.
The Ti K-edge XANES (Fig. S1D-b) of the Cu/MIL125-S-1/2 pattern was very similar to that of MIL125 (spectrum a). Ti K-edge
EXAFS peaks for Cu/MIL125-S-1/2 resembled those of MIL125(Fig. 2b and a). The N value for the Ti(AOA)Ti pair at 0.270 nm
remained at 0.4, while the N value for the Ti(AOA)Ti pair at
0.314 nm increased from 1.6 to 2.2 (Table 3b and a) probably
due to the local distortion of the framework. Thus, the frameworkof MIL125 was basically retained in Cu/MIL125-S-1/2.
From Cu K-edge EXAFS CF analysis, formation of mean 1.7-nm
metallic Cu nanoparticles was suggested on the basis of the N valueof a CuACu interatomic pair (9.1) [26] after solid-state mixing andheating at 473 K in H2. Due to the high Cu content (13.5 wt%) in thesample, the Cu0 nanoparticles should ideally occur in the pores of
MIL125 (Chart 1A). The lattice parameters (a = 1.85 nm;
c = 1.80 nm) based on 0 1 1 and 0 0 2 diffraction for Cu/MIL125-S1/2 (Fig. 1A-b) are negligibly different from the values of MIL125(a = 1.86 nm; c = 1.81 nm). As the mean size for metallic Cu wasalmost the upper limit of the pore space of MIL125 (Chart 1A),
on the basis of the N value (2.2) for Cu(AOA)Cu interatomic pairs
obtained from Cu K-edge EXAFS (Fig. 2d and Table 3d), the mean
size of Cu oxides formed by calcination at 573 K in Cu/MIL125-S1/2 would be 1.7 nm. The 1.7-nm CuO particles were not detected
by XRD above the detection limit. 5.9-nm mean size CuO particles
were observed in HR-TEM attached to the MIL125 crystallines’
external surfaces (Fig. 3B); however, the number was very few
and corresponded to approximately 8 atomic%. The population of
CuO nanoparticles for CuO (13.5 wt%)/SiO2 was used as a standard
in HR-TEM image. As the result, CuO seen for Cu/MIL125-S was 8%of that for CuO (13.5 wt%)/SiO2.
3.2. Cu/MIL125 synthesized via impregnation method
The framework structure for Cu/MIL125-I-573-1/2 was alteredfrom that of MIL125, as demonstrated by XRD. The peak pattern
for MIL125 (Fig. 1A-a) changed to totally different one forFig. 1. (A and B) XRD patterns for MIL125 (a), Cu/MIL125-S-1/2 (b), MIL125-I-573-0/1 (c), Cu/MIL125-I-573-1/4 (c0 ), Cu/MIL125-I-573-1/2 (c00 ), Cu/MIL125-I-573-3/4 (c00 0 ), Cu/
MIL125-I-573-1/1 (c00 00 ), Cu/MIL125-I-613-1/2 (d), Cu/MIL125-I-613-1/1 (d0 ), and Cu/MIL125-I: model B0 (Chart 1 and see later description in text) (e and f). Patterns e (panelA) and f (panel B) were theoretically generated using Diamond 3.2 [27] and refined by Rietveld algorithm using TOPAS 4.2, respectively. The residue was between pattern c00
and f (panel B). (C) FTIR spectra for MIL125 (a), Cu/MIL125-I-553-1/2 (b), Cu/MIL125-I-573-1/2 (c), and Cu/MIL125-I-613-1/2 (d).

5

Y. Yoshida, Y. Izumi / Journal of Catalysis 332 (2015) 1–12

Table 2
Particle sizes of MIL125, compound 1, and CuO based on XRD peak widtha, specific surface area, and CO PROX reaction performances in H2b using MIL125, Cu/MIL125, Cu/TiO2,
CuO/CeO2, and Au/TiO2.
Entry

Catalyst

a
b
c
d
e
e0
f
g
h
h0
i
j
k
l
m
n
o
p

MIL125
Cu/MIL125-S-1/2f
MIL125-I-573-0/1
Cu/MIL125-I-573-1/4
Cu/MIL125-I-573-1/2
Cu/MIL125-I-573-1/2g
Cu/MIL125-I-573-3/4
Cu/MIL125-I-573-1/1
Cu/MIL125-I-613-1/2
Cu/MIL125-I-613-1/2g
Cu/MIL125-I-613-1/1
Cu/P25-S (13.5 wt% Cu)
Cu/P25-I (13.5 wt% Cu)
Cu/JRC-TIO-6-I (rutile; 13.5 wt% Cu)
Cu/JRC-TIO-7-I (anatase; 13.5 wt% Cu)
CuO/CeO2-573
CuO/CeO2-923
Au/TiO2h

Mean crystalline size (nm)a
MIL125Compound 1

Time (h)cRate const. k (103 min1)

Conv. (%)dSelec. (%)e

852
349

3
3
3
3
2

0.6
3.5
0.8
0.5
45

4.9
42
9.9
8.4
99

94
98
25
79
98

2
2
1

47
69
117

99
100
97

93
100
98

2
3
3
3
3
2
1
1

50
9.2
3.4
0.3
0.9
30
85
141

100
73
39
3.8
14
95
98
100

100
94
58
53
80
93
90
93

CuO

94
25
68
92
106
92
51
68
101
90

SBET (m2 g1)

9.2
8.6
7.2
13
19
23
37
<4

165
320
238
250
215
337
332
185
58

 1 1Þ + (0 0 2) reflection peaks.
Based on Scherrer equation (Eq. (1)) for MIL125 (011), Compound 1 (0 1 1), and CuO ð1
Initial gases: CO 3.7 lmol, O2 4.5 lmol, H2 370 lmol. At 323 K.
c
Reaction period.
d[CO]/[CO]0min  100.
e[CO2]/([CO2] + [H2O])  100.
f
Calcined sample.
gAfter CO PROX test of entry e or h.
hSample 10 mg, reaction temperature 293 K; Ref. [31].
abCu/MIL125-I-573-1/2 (Fig. 1A-c00 ). Main peaks appeared at
8.38° and 9.22°. The peaks at 6.94° and 9.61° for tetragonal
MIL125 were assigned to the 0 1 1 and 0 2 0 diffractions [3].
Assuming that the tetragonal and body-centered framework for
MIL125 (a = 1.86 nm; c = 1.81 nm) was preserved in this unknown
structure for Cu/MIL125-I-573-1/2, the two peaks can be ascribedto the 0 1 1 and 0 2 0 planes, respectively, with lattice parameters
a = 1.936 nm and c = 1.277 nm (see also Rietveld refinement
below).
This assumption of tetragonal body-centered structure was justified based on the theoretical XRD pattern using Diamond version
3.2 obtained by calculating crystal structure factors (Fig. 1A-e)[27]. The angle and the intensity ratio for major peaks at 8.38°,
9.22°, 12.42°, and 16.72°, assignable to the 0 1 1, 0 2 0, 1 2 1, and
0 2 2 diffractions (Fig. 1A-c00 ), were exactly reproduced in the theoretical pattern (8.29°, 9.21°, 12.40°, and 16.62°, respectively) calculated for the model structure containing MIL125 sheets and[Cu2(OH)6]2 linkers (Chart 1B0 ; vide infra). Furthermore, themodel structure was refined via the Rietveld algorithm using
TOPAS 4.2 (Bruker). The refined pattern resembled pattern e given
by Diamond 3.2; however, if the anisotropy in the direction of 0 1 0
and 1 2 1 was also considered in the refinement, the theoretical
pattern (8.37°, 9.21°, 12.43°, 13.96°, and 16.72°) assignable to the0 1 1, 0 2 0, 1 2 1, 0 0 2, and 0 2 2 diffractions was nearly completely
reproduced (Fig. 1A-f).
A weak peak in the theoretical pattern f, the 0 3 1 plane at
15.47°, did not clearly appear in experimental pattern c00 , andthe 0 4 0 peak at 18.41° in pattern c00 was weak in pattern f probably because of imperfect crystallinity, anisotropy, and/or thedetection limit of the apparatus. Moreover, the weak experimental peak at 8.93° and 17.83 (pattern c00 ) was absent in pattern f,presumably due to formation of a different very minor crystal
lattice.
In the Cu/MIL125-I-573-1/2 HR-TEM images, fringes with aninterval of 4.1 nm were observed that crossed the other fringes
perpendicularly with the same interval (Fig. 3C, inset). Considering
the spatial resolution (2–3 nm) of the HR-TEM apparatus, three 0 planes would be
repetitions of perpendicular 1 1 0 and 1 1
observed with intervals of 1.37  3 nm (a = 1.936 nm).
In the FTIR spectra for Cu/MIL125-I-1/2 (Fig. 1C), the peak intensity centered at 1546, 1397, and 1019 cm1 because of coupling
modes of m(C@C) + b(CCH) + m(CH), m(C@C) + d(C@CAC), and c(CCC) + b(CCH) due to terephthalate [28] decreased to two-thirds,
one-half, and one-third by calcination at 553, 573, and 613 K,
respectively (Fig. 1C-b–d), compared with intact MIL125 (a).
For determining the crystal structure of Cu/MIL125-I, thecompressed c length by a factor of 0.71 needs to be explained ascompared with MIL125, while a length remained constant (1.04
times). We concluded that one-third of terephthalates in theab-plane for
MIL125 [Ti8O8(OH)4(bdc)6]
remained for
Cu/MIL125-I-613-1/2; those connecting [Ti8O8(OH)4]12+ clusters inneighboring [Ti8O8(OH)4(bdc)2]8+ sheets were replaced by shorter[Cux(OH)2x+2(OH2)y]2 (x = 2, 3, or 4) linkers. In this model, thenearest interatomic distance between Ti atoms in neighboring[Ti8O8(OH)4(bdc)2]8+ sheets [the length (L) of green sticks in
Chart 1B] was 0.874 nm.

sﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
2  2ac
pﬃﬃﬃ  2r þ
L¼
2
2
sﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
2

2 
1:936
1:277
pﬃﬃﬃ  2  0:386 þ¼ 0:874 nm:
¼
2
2

ð2Þ

in which r is the distance between the center of Ti8 octagon and the
middle point of two neighboring Ti atoms (Chart 2).
Based on the CuAO and TiAO interatomic distances determinedby EXAFS for Cu/MIL125-I-573-1/2 (both 0.195 nm; Table 3c and e)
and assuming complete tetrahedral (Td) or octahedral (Oh) coordination around Cu, three plausible Cu linker models were drawn(Chart 3).
The first model is [Cu2(OH)6]2, in which the Cu sites are Td
coordinated (Chart 3A). The second and third are[Cu2(OH)6(OH2)4]2 and [Cu4(OH)10(OH2)6]2, in which the Cu sites
are Oh coordinated (Chart 3B and C, respectively).

6

Y. Yoshida, Y. Izumi / Journal of Catalysis 332 (2015) 1–12

Fig. 2. Ti K-edge (a–c) and Cu K-edge (d and e) EXAFS for MIL125 (a), Cu/MIL125-S-1/2 (b and d), and Cu/MIL125-I-573-1/2 (c and e). The k3-weighted EXAFS v-function (1),
its associated Fourier transform (3), and the best CF for Fourier-filtered data in k-space (2) and R-space (4).

In models A, B, and C, the nearest distances between Ti atoms in
the neighboring [Ti8O8(OH)4(bdc)2]8+ sheets (green bar length inCharts 1B and 2) were calculated as 0.838, 0.936, and 1.100 nm,
respectively (Chart 3). Thus, model C was rejected as the linker
in comparison with the experimental value: 0.874 nm (Eq. (2)). If
the OACuAO angles for central Cu2O2 core in model A were slightly
distorted from 109.5° (Td) to 96.1° (model A0 , Chart 3) or if thedihedral angle of two CuO4 planes was distorted from 180° to
132° (model B0 ), the Cu linker length L became 0.874 nm, in agreement with XRD.
Based on the Ti K-edge EXAFS CF analyses for Cu/MIL125-I-5731/2 (Table 3c and Fig. 2c), the Ti(AOA)Ti interatomic distancechanged from 0.305 nm (mean) for MIL125 (Table 3a) to
0.276 nm. Its associated N value increased from 2.0 to 3.8. In addition, a new farther peak appeared at 0.33 nm (phase shift uncorrected; Fig. 2c3), that can be ascribed to a Ti(AOA)Cu
interatomic pair fitted with the R value 0.380 nm and N value 2.6(Table 3c), strongly suggesting that new Ti(AOA)Cu interatomic
links formed around the [Ti8O8(OH)4]12+ clusters. The N value

was consistent with models A0 or B0 for Cu/MIL125-I (Chart 3)
within variation of ±0.6 (Table 3c), because the models assumed
that two-thirds of terephthalates coordinated to the Ti atoms in
MIL125 were replaced by [Cu2(OH)6]2 linkers; thus, the N[Ti
(AOA)Cu] value was two. The distance (0.380 nm; Table 3c) wasin agreement with the Ti  Cu interatomic distance in model B0(0.373 nm; Chart 3) rather than 0.355 nm in model A0 . The formula
would be Ti8O8(OH)4(bdc)2[Cu2(OH)6]4 (Compound 1) for
Cu/MIL125-I-613-1/2. Model B0 in Chart 3 is also drawn in Chart 1B0 .
The
connection
between[Cu2(OH)6]2
linkersand[Ti8O8(OH)4]12+ clusters was investigated by Cu K-edge EXAFS.
The N value for CuAO pairs (3.6; Table 3e and Fig. 2e) supported
the coordination of four O atoms in both models A0 and B0 (Chart 3)
within the variation of ±0.6 (Table 3e). The N value for Cu(AOA)M(M = Cu or Ti) was shown to be 2.8 (Table 3e), consistent withcoordination to one Cu and two Ti (Chart 3A0 and B0 ). Theinteratomic distance for Cu(AOA)M (0.310 nm; Table 3e) was
consistent with model A0 : 2/3  0.355 + 1/3  0.261 = 0.324 nm
and model B0 : 2/3  0.373 + 1/3  0.259 = 0.335 nm.

7

Y. Yoshida, Y. Izumi / Journal of Catalysis 332 (2015) 1–12
Table 3Best-fit results of Ti and Cu K-edge EXAFS analyses for MIL125 and CuO supported on/in MIL125 and TiO2.a
Catalyst

Ti(AOA)Cu

CuAO

Ti(AOA)TiNN

R (nm)N

R (nm)

3.2(±0.6)

0.1956(±0.0008)

2.2(±0.6)

0.307(±0.003)

3.6(±0.6)

0.195(±0.001)

2.8(±2.2)

0.310(±0.006)(f) Cu/P25-I

3.2(±1.5)

0.196(±0.002)9.2(±1.2)

0.303(±0.008)(g) Cu/JRC-TIO-7-I

3.1(±0.6)

0.194(±0.001)

4.9(±2.7)

0.308(±0.004)N

R (nm)N

R (nm)(a) MIL125

4.4(±1.5)
1.6(±1.8)

0.175(±0.003)0.2174(±0.0008)

0.4(±0.3)
1.6(±0.3)

0.271(±0.001)
0.314(±0.003)(b and d) Cu/MIL125-S-1/2

2.6(±1.2)
2.4(±0.6)
0.9(±0.3)

0.169(±0.003)
0.192(±0.004)
0.220(±0.002)0.4(±0.9)
2.2(±0.6)

0.270(±0.02)0.314(±0.003)(c and e) Cu/MIL125-I-573-1/2

5.2(±1.2)

0.1945(±0.0004)

3.8(±0.3)

0.276(±0.002)(a0 ) MIL125 (X-ray
crystallography)3
a

TiAO

2
1
3

0.18940.1941
0.19821
1

2.6(±0.6)R (nm)

0.3797(±0.0008)

Cu(AOA)M (M = Ti,Cu)

0.27610.3543The values in parenthesis are fit errors.

Fig. 3. HR-TEM images for MIL125 (A), Cu/MIL125-S-1/2 (B), and Cu/MIL125-I-573-1/2 (C).

3.3. CO PROX tests
CO PROX activity was tested, and the rates were evaluatedassuming first-order kinetics of CO pressure (PCO):

dPCO;t =dt ¼ kPCO;t :Chart 2. The diagram for length of Cu linker in the crystal structure of Chart 1B.

The theoretical XRD pattern (Fig. 1A-e and B-f) calculated formodel B0 (Chart 1) was similar to experimental one (Fig. 1A-c00 ),as discussed above. The theoretical Ti and Cu K-edge X-ray absorption near-edge structure (XANES) pattern generated using ab initio
multiple scattering calculation code, FEFF8.4 [29] and the greaterCu 1s-3d pre-edge peak intensity [30] directly supported model
B0 as linkers for Cu/MIL125-I. However, model A0 may be also
acceptable (Fig. S1B and C; Supplementary material).

ð3Þ

We performed three CO PROX tests under the same conditions
and the data (rate constant k, conversion, and selectivity) variation
was within 5%. The data for all the catalysts were nicely fit to theequation, suggesting stability of these catalysts (Fig. 4A). The kvalue was 0.0006 min1 for MIL125 (Table 2a). Introducing Cu species in MIL125 by the impregnation method improved the activity
by a factor of 200 to 0.117 min1 (Cu/MIL125-I-613-1/2; Table 2hand Fig. 4A-e). The performance was due to the formation of Compound 1 and maximal SBET value (Table 2h) due to the desorption of
extra terephthalates (Eq. (5); see discussion below). This value wassuperior to 0.085 and 0.030 min1 using CuO/CeO2-923 and CuO/
CeO2-573, respectively (Table 2o and n and Fig. 4A-h) and comparable to 0.141 min1 using Au/TiO2 [31] (Table 2p) if the difference
in reaction conditions (50 versus 10 mg of catalyst, 323 versus
293 K, respectively) was not considered.
The mass was nicely balanced for the reaction 2CO + O2 ? 2CO2
for Cu/MIL125-S-1/2, Cu/MIL125-I, and Cu/P25-I (Fig. 4A-a–g and8

Y. Yoshida, Y. Izumi / Journal of Catalysis 332 (2015) 1–12

Chart 3. Plausible Cu hydroxide linker models. The CuAO and TiAO interatomic distances were fixed at 0.195 nm based on EXAFS. (A, A0 , and B0 ) [Cu2(OH)6]2, (B)[Cu2(OH)6(OH2)4]2, and (C) [Cu4(OH)10(OH2)6]2. The Cu site symmetry was Td (A and A0 ), planar (B0 ), and Oh (B and C).

B) whereas CO decrease was greater than the stoichiometry for Cu/
MIL125-I-573-1/1 (Fig. 4A-d) and O2 decrease was smaller than the
stoichiometry for Cu/MIL125-I-613-1/1 (Fig. 4A-f). We suspect thatextra adsorption of CO or the consumption of surface O species
occurred for the catalysts because the population of Compound 1
was quite low for these samples of Cu/Ti ratio = 1/1 as demonstrated by XRD (Fig. 1A-c0000 and d0 ). CO2 formation was smaller than
the stoichiometry for CuO/CeO2-923 (Fig. 4A-h) because CeO2
should adsorb significantly CO and O2 especially at the initial stageof reaction test. Preferable adsorption of CO on CuI sites [32,33] andCuO nanoclusters [34] for CuO/CeO2 catalysts and the adsorption of
O2 on defect sites of CeO2 were reported [35,36].
For Cu/MIL125-I-573-1/2, the k value was smaller by a factor of
0.38 (0.045 min1; Table 2e and Fig. 4A-b) compared with thatusing Cu/MIL125-I-613-1/2. The XRD pattern (Fig. 1A-c00 , and d),
crystalline size (106–101 nm; Table 2e and h), and SBET values(320–337 m2 g1) were comparable for the two catalysts. We consider the formation reactions for Compound 1 as follows:

Ti8 O8 ðOHÞ4  ðbdcÞ6 þ 4Cu2þ
! Ti8 O8 ðOHÞ4  ðbdcÞ2 layer þ 4CuðbdcÞ ðaq:; 290 KÞð4Þ

! 0:5Ti8 O8 ðOHÞ4  ðbdcÞ2  ½Cu2 ðOHÞ6 4 þ 0:5Ti8 O8 ðOHÞ12 ðbdcÞ2ðvacuum; 613 KÞð5ÞThus, Cu(bdc) remaining until the heating at 613 K was inactive for
the CO PROX reaction and Compound 1 should be the active species.
The terephthalates coordinated to Cu2+ ions progressively desorbed
as the temperature rise in vacuum (Fig. 1B), and resultant

Cu hydroxides would link [Ti8O8(OH)4]12+ clusters as in
Charts 1 and 3B0 .
The stability of Cu/MIL125-I-573-1/2 and Cu/MIL125-I-613-1/2
catalysts was checked by XRD and SBET after CO PROX tests(Table 2e, e0 ; h, h0 and Fig. 5A). Mean crystalline size based on
Eq. (1) of Compound 1 after PROX was 87–90% of that before PROX.
Conversely, the changes of SBET values and the amount of terephthalate critically depended on samples: 74 versus 99% (SBET) and
80 versus 96% (the amount of bdc; Fig. 5B-a–d), respectively. Thus,
Eq. (5) was almost completed and the structure was stabilized for
Cu/MIL125-I-613-1/2 whereas Compound 1 was gradually formed
during CO PROX test for Cu/MIL125-I-573-1/2. Accordingly, thePROX activity of Cu/MIL125-I-613-1/2 was completely preserved
for six cycles (2 h cycle1; Fig. 4B).
The effects of CO2 and/or moisture, that are plausible impurity
gases for CO PROX prior to hydrogen supply to fuel cells, on the
PROX activity using Cu/MIL125-I-573-1/2 were also checked.
19 lmol of CO2 and 19 lmol of moisture mixed to reaction gas
reduced the rate constant to 38% and 44% (Table 4b and c), respectively, of that free from CO2 and moisture (Table 4a). Moreover, if
both 19 lmol of CO2 and 19 lmol of moisture mixed, the rate constant further reduced to 24% of that free from CO2 and moisture(Table 4c). The trend suggested these polar molecules would blocka part of the Cu linker sites for CO activation (Scheme 1).
Next, CO PROX activity was compared by varying the Cu/Ti
atomic ratio between 0 and 1 for Cu/MIL125-I-573 (Table 1). The
framework of MIL125-I-573-0/1, i.e., the control sample free fromCu, was the same as that of MIL125 (Fig. 1A-a and c). The control
sample was inert (k = 0.0008 min1; Table 2c), demonstrating that

Y. Yoshida, Y. Izumi / Journal of Catalysis 332 (2015) 1–12

9Fig. 4. (A) Time course of PROX tests in CO (63 Pa) + O2 (76 Pa) + H2 (6.3 kPa) using Cu/MIL125-S-1/2 (a), Cu/MIL125-I-573-1/2 (b), Cu/MIL125-I-573-3/4 (c), Cu/MIL125-I573-1/1 (d), Cu/MIL125-I-613-1/2 (e), Cu/MIL125-I-613-1/1 (f), Cu/P25-I (g), and CuO/CeO2-923 (h). Catalyst amount 50 mg, temperature 323 K. (B) Durability test of CO
PROX using Cu/MIL125-I-613-1/2. The reaction conditions were similar to A except for the catalyst amount (20 mg). The gases were removed in vacuum from the reactor after
each cycle before introducing new gases.

Cu was indispensable for CO PROX. Notably, other than Compound
1 for Cu, no other diffraction peaks were detected for Cu/MIL125-I573 series (Fig. 1A-c–c0000 ).
The Compound 1 crystalline phase determined for Cu/MIL125-I573 was more stable when the atomic ratio of Cu/Ti was 1/2 rather
than 1/4, 3/4, or 1, based on 0 1 1 diffraction intensity. The diffraction intensity decreased to 47% (Cu/Ti = ¼), 32% (Cu/Ti = 3/4), and
18% (Cu/Ti = 1) of that for a Cu/Ti ratio of 1/2, respectively(Fig. 1A-c0 –c00 00 ). The SBET value was maximal when the atomic ratio
was 1/2 (320 m2 g1; Table 2d–g). The k values exponentially
increased as the Cu/Ti atomic ratio increased to 1/2 (Table 2c–e
and Fig. 4A-b). This trend can be explained that remained inert
Cu(bdc) (Eq. (4)) was significant if the Cu/Ti ratio was less than
1/2 and the sample was heated below 573 K (Fig. 1B).
When the atomic ratio of Cu/Ti was 1, the initial reaction inaqueous solution for the catalyst synthesis should be10

Y. Yoshida, Y. Izumi / Journal of Catalysis 332 (2015) 1–12

Scheme 1. Proposed reaction sites for CO PROX reaction using Cu/MIL125-I
catalysts.

Fig. 5. (A) XRD patterns for Cu/MIL125-I-573-1/2 before (a) and after PROX test (b)and Cu/MIL125-I-613-1/2 before (c) and after PROX test (d). (B) FTIR spectra for Cu/
MIL125-I-573-1/2 before (a) and after PROX test (b) and Cu/MIL125-I-613-1/2
before (c) and after PROX test (d).

Ti8 O8 ðOHÞ4  ðbdcÞ6 þ 8Cu2þ
! Ti8 O8 ðOHÞ4  ðbdcÞ2 layer þ 4CuðbdcÞ þ 4Cu2þ ðaq:; 290 KÞ: ð6Þ
Thus, extra Cu2+ ions more than the Cu/Ti ratio of 1/2 would not bein close contact with Ti and/or bdc species and thus did not increase
the crystallinity of Compound 1 (Fig. 1A-c00 , c000 , and c0000 ). Conversely,
extra Cu2+ ions in Eq. (6) contributed to add smaller crystallines ofCompound 1 and accordingly to reduce the SBET values from 320 to
250 and then to 215 m2 g1 (Table 2e–g). As the result, the effective
interface of Cu linker with [Ti8O8(OH)4]12+ cluster (Scheme 1)
increased moderately and the k values for CO PROX gradually
increased from 0.045 to 0.046 and then to 0.069 min1 startingfrom Cu/Ti ratio of 1/2 to 1/1 (Table 2e–g and Fig. 4A-b, c, and d)).
In these comparisons, the conversion (99–100%) and selectivity(93–100%) for 1–2 h were quite excellent for these catalysts(Table 2e–g).
TiO2 surface is weakly acidic. The specific surface area should
correlate positively with the number of the weak acid sites. However, rate constants in Table 2 basically unrelated to SBET values. If
the atomic ratio of Cu/Ti was fixed to 1/2 (entries b, e, and h), the
SBET values were nearly the same (349, 320, and 337 m2 g1),whereas the rate constants changed by a factor of 33 (0.0035,
0.045, and 0.117 min1, respectively). Thus, the concentration ofacid sites was not influential factor for the PROX activity using
composite catalysts of Cu and MIL125.
The k value was the maximum for Cu/MIL125-I-613-1/2(0.117 min1; Table 2h and Fig. 4e). Compared to the k value for
Cu/MIL125-I-573-1/2 (0.045 min1; Table 2e and Fig. 4b), heating
effect to remove Cu(bdc) on the activity was apparent. In thesecomparisons, the conversion (97–99%) and selectivity (98%) for
1–2 h were quite excellent for the two catalysts (Table 2e and h).
Conversely, the XRD peak intensity did not change significantly
between these Cu/MIL125 samples (Fig. 1A-c00 and d).
The k value decreased for Cu/MIL125-I-613-1/1 as compared to
Cu/MIL125-I-613-1/2 (Table 2h and i and Fig. 4A-e and f). The
active sites at the boundary between [Cu2(OH)6]2 and[Ti8O8(OH)4]12+ (Scheme 1) formed via Eq. (5) would be partiallydecomposed by heating for Cu/MIL125-I-613-1/1 because the

XRD peaks due to Compound 1 almost disappeared (Fig. 1A-d0 ).
Compound 1 was favorably formed for Cu/MIL125-I-1/2 (Fig. 1Ac00 and d) rather than Cu/MIL125-I-1/1 (c0000 , d0 ) probably because
extra amorphous phase, e.g. Ti8O8(OH)12(bdc)2 or TiO2, stabilized
Compound 1.
The N value for Cu(AOA)M (M = Ti, Cu) interatomic pair wasquite small for Cu/MIL125-I-573-1/2 (2.8; Table 3e and Fig. 2e).
The mean size of CuO in Cu/JRC-TIO-7-I was less than 4 nm basedon XRD peak (Table 2m), and the N value for Cu(AOA)M was 4.9(Table 3g and Fig. S2c) by far smaller than 9.2 for Cu/P25-I(Table 3f and Fig. S2d). The significantly smaller N[Cu(AOA)M]
value for Cu/MIL125-I-573 and no diffraction peaks other thanCompound 1 for Cu (Fig. 1A) supported that Cu2 linker structure(Model B0 or A0 , Chart 2) was predominant in Cu/MIL125-I
catalysts.
The activity of Cu/MIL125-S-1/2 was 7.8% of Cu/MIL125-I-5731/2 (Table 2b and Fig. 4A-a). For Cu/MIL125-S-1/2, direct links
between Cu sites of mean 1.7-nm CuO nanoparticles and [Ti8O8(OH)4]12+ clusters were not detected by EXAFS (Fig. 2b, d andTable 3b, d). Various reference Cu/TiO2 catalysts tested by varying
the catalyst preparation (solid state embedding/impregnation) andcrystalline phase of TiO2 (anatase and/or rutile) showed small kvalues: 0.0003–0.0092 min1 (Table 2j–m and Fig. 4A-g).
For CO oxidation using Cu/TiO2 catalysts, CO is proposed toadsorb on smaller CuO, while O2 preferably dissociates on rutiletype TiO2 [6]. The relatively greater k value (0.0092 min1) using
Cu/P25-S, in which TiO2 is 80% of the anatase phase and 20% of
the rutile phase [17], would result from relatively smaller CuO(mean 19 nm; Table 2j) on the anatase for CO adsorption and effective contact of the smaller CuO with rutile for O2 dissociation.
Atomically dispersed [Cu2(OH)6]2 should be advantageous for
CO adsorption and [Ti8O8(OH)4]12+ would be more effective thanrutile TiO2 for O2 dissociation using Compound 1 (Scheme 1).
Finally, in comparison with lower-pressure CO PROX tests (CO0.063 kPa, O2 0.076 kPa, H2 6.3 kPa; Table 2 and Fig. 4), relatively
high-pressure CO PROX tests (CO 0.51 kPa, O2 0.51 kPa, H2
50.1 kPa) were also performed using a flow reaction system atthe same reaction temperature (323 K; Fig. 6). The amount of catalyst was increased from 50 mg to 100 mg.
Using Cu/MIL125-I-613-1/2, the best catalyst among Cu/MIL125
and Cu/TiO2 catalysts in lower-pressure tests (Table 2), CO oxidation rate became stabilized at 3 h from the start of reaction(Fig. 6A1) and the conversion to CO2 was stabilized at 67–76% at
3–24 h (Fig. 6A2). Water was detected until 4 h from the start ofreaction, but the amount was negligible at 4–24 h (Fig. 6A1).

Table 4Comparisons of CO PROX reaction performances in H2a at 323 K using Cu/MIL125-I-573-1/2 in the presence/absence of CO2 and/or H2O.
Entry

Catalyst

CO (lmol)

O2 (lmol)

H2 (lmol)abcdCu/MIL125-I-573-1/2

3.7
3.7
3.7
3.7

4.5
4.5
4.5
4.5

370
370
370
370

Initial gases: CO 3.7 lmol, O2 4.5 lmol, H2 370 lmol. At 323 K.
Reaction period.
c[CO]/[CO]0min  100.
d[CO2]/([CO2] + [H2O])  100.
ab

CO2 (lmol)

H2O (lmol)

Time (h)bRate const. k (103 min1)

Conv. (%)c

Selec. (%)d

45
17
20
11

99
80
86
63

98
97

19
19

2
2
2
2

19
19

Y. Yoshida, Y. Izumi / Journal of Catalysis 332 (2015) 1–12

11

Fig. 6. (A) Time course of PROX tests in CO (0.5%) + O2 (0.5%) + H2 (49.5%) + N2 (49.5%) at a flow rate of 20 mL min1 using Cu/MIL125-I-613-1/2 (A) and CuO/CeO2-923 (B).
Catalyst amount 100 mg, temperature 323 K. (1) The amounts of CO2, CO, and H2O in exit gas (40 mL) and (2) the conversion to CO2 = 100  [CO2]exit gas/[CO]feed gas (j, square)
and CO PROX selectivity = 100  [CO2]exit gas/([CO2]exit gas + [H2O]exit gas) (N, triangle).

Accordingly, CO PROX selectivity was 98–99% at 4–24 h (Fig. 6A2)
in consistent with 98% for lower-pressure test for the catalyst(Table 2h). When the CO PROX test was repeated for used catalyst
without the contact with air, initial water formation was negligible. This fact suggested that the water in 0–4 h in Fig. 6A1 exclusively desorbed from catalyst and was not catalytic productderived from H2 and O2 gas.
In contrast, using CuO/CeO2-923 (Table 2o and Fig. 4A-h), CO
oxidation rate increased proportionally until 4 h from the start of
the test and gradually increase continued for 24 h (Fig. 6B1). Correspondingly, the conversion to CO2 gradually increased from 16%(@4 h) to 28% (@24 h; Fig. 6B2). Conversely, the CO PROX selectivity was stabilized at 93–96% at 4–24 h (Fig. 6B2) nearly in consistent with 90% for lower-pressure test for the catalyst (Table 2o).
In summary, under the reaction conditions of Fig. 6, the conversion to CO2 using Cu/MIL125-I-613-1/2 was higher by 4.3–2.7
times than that using CuO/CeO2-923, demonstrating practicalimportance of binary metal (Ti, Cu) MIL125-derived catalyst.
The binary MOF Ti8O8(OH)4(bdc)2[Cu2(OH)6]4 synthesized inthis study is a crystalline model of active site at the metal–support
interface, and can pave the way for various binary MOF model catalysts of Zr, Al, and Ti clusters linked by V, Fe, Co, Ni, Cu, Ru, Rh, Ag,
Pt, and Au (hydro)oxides.
4. Conclusions
Solid-state embedding of Cu(II) and Cu(II) impregnation with
MIL125 [Ti8O8(OH)4(bdc)6] was investigated in this study as theinterface model of Cu sites on TiO2. Mean 1.7-nm CuO nanoparticles were formed embedded inside the pores of MIL125 via

solid-state embedding. No CuTi interatomic pair was detectedby EXAFS, demonstrating no direct links between Cu sites and [Ti8O8(OH)4]12+ clusters. The sample was inert for CO PROX reaction in
predominant H2 (k = 0.0035 min1 in CO 63 Pa + O2 76 Pa at 323 K)
due to the lack of interface sites. In clear contrast, Cu(II)impregnated samples did not retain the framework of MIL125,
but transformed to new tetragonal binary MOF, Ti8O8(OH)4(bdc)2
[Cu2(OH)6]4 (a = 1.936 nm, c = 1.277 nm), based on XRD. The direct
links between Cu sites and [Ti8O8(OH)4]12+ clusters were demonstrated by the appearance of Ti(AOA)Cu peak at 0.380 nm andthe increase of coordination number by one for Cu(AOA)M(M = Ti, Cu) peak by Ti and Cu K-edge EXAFS. Due to the direct links
between Cu sites and [Ti8O8(OH)4]12+ clusters, the impregnatedsamples functioned far better in CO PROX reaction in predominant
H2(k = 0.117 min1)
thanreference
Cu/TiO2
catalysts(k = 0.0003–0.0092 min1) and equivalent to CuO/CeO2-923 andAu/TiO2. Furthermore, in CO 0.51 kPa + O2 0.51 kPa at 323 K, theconversion to CO2 and the CO PROX selectivity using Ti8O8(OH)4(bdc)2[Cu2(OH)6]4 (67–76% and 98–99%) were significantly higher
than that using CuO/CeO2 (16–28% and 93–96%, respectively) for
24 h. Extra terephthalates (bdc) than the formula Ti8O8(OH)4
(bdc)2[Cu2(OH)6]4 blocked the active Cu sites below 613 K as sample pretreatment, whereas best atomic ratio of Cu/Ti was 1/2 for CO
PROX activity presumably because extra TiIV species stabilized thecatalytically active Ti8O8(OH)4(bdc)2[Cu2(OH)6]4 crystallines.
Acknowledgments
The authors are grateful for the financial supports from theGrant-in-Aid for Scientific Research C (26410204, 22550117) and12

Y. Yoshida, Y. Izumi / Journal of Catalysis 332 (2015) 1–12

Research Fellowship for Young Scientists (25489; Y.Y.) from Japan
Society for the Promotion of Science and approval of the Photon
Factory Proposal Review Committee for X-ray measurements(2013G159, 2011G083).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcat.2015.09.007.
References[1] S. Chavan, J.G. Vitillo, D. Gianolio, O. Zavorotynska, B. Civalleri, S. Jakobsen, M.
H. Nilsen, L. Valenzano, C. Lamberti, K.P. Lillerud, S. Bordiga, Phys. Chem. Chem.
Phys. 14 (2012) 1614.
[2] T. Loiseau, L. Lecroq, C. Volkringer, J. Marrot, G. Férey, M. Haouas, F. Taulelle, S.
Bourrelly, P.L. Llewellyn, M. Latroche, J. Am. Chem. Soc. 128 (2006) 10223.
[3] M. Dan-Hardi, C. Serre, T. Frot, L. Rozes, G. Maurin, C. Sanchez, G. Férey, J. Am.
Chem. Soc. 131 (2009) 10857.
[4] M.P. Suh, H.J. Park, T.K. Prasad, D.W. Lim, Chem. Rev. 112 (2012) 782.
[5] J.Q. Lu, C.X. Sun, N. Li, A.P. Jia, M.F. Luo, Appl. Surf. Sci. 287 (2013) 124.
[6] M.Y. Kang, H.J. Yun, S. Yu, W. Kim, N.D. Kim, J. Yi, J. Mol. Catal. A 268 (369)(2013) 72.
[7] Z. Rui, Y. Huang, Y. Zheng, H. Ji, X. Yu, J. Mol. Catal. A 372 (2013) 128.
[8] S. Bennici, A. Gervasini, Appl. Catal. B 62 (2006) 336.
[9] D.P. Kumar, M.V. Shankar, M.M. Kumari, G. Sadanandam, B. Srinivas, V.
Durgakumari, Chem. Commun. 49 (2013) 9443.
[10] K. Klier, Adv. Catal. 31 (1982) 243.
[11] S.H. Chien, B.N. Shelimov, D.E. Resasco, E.H. Lee, G.L. Haller, J. Catal. 77 (1982)
301.
[12] K. Aika, K. Tamaru, in: A. Nielsen (Ed.), Ammonia – Catalysis and Manufacture,
Springer-Verlag, Heidelberg, 1995, p. 103.
[13] A. Cho, Science 299 (2003) 1684.

[14] M.Y. Masoomi, A. Morsali, Coord. Chem. Rev. 256 (2012) 2921.
[15] Z. Jiang, Z. Li, Z. Qin, H. Sun, X. Jiao, D. Chen, Nanoscale 5 (2013) 11770.
[16] R. Li, J. Hu, M. Deng, H. Wang, X. Wang, Y. Hu, H.L. Jiang, J. Jiang, Q. Zhang, Y.
Xie, Y. Xiong, Adv. Mater. 26 (2014) 4783.
[17] B. Ohtani, O.O. Prieto-Mahaney, D. Li, R. Abe, J. Photochem. Photobiol. A 216(2010) 179.
[18] C.S. Polster, H. Nair, C.D. Baetsch, J. Catal. 266 (2009) 308.
[19] J.A. Bearden, Rev. Mod. Phys. 39 (1967) 78.
[20] G. Zschornack, Handbook of X-ray Data, Springer, Berlin/Heidelberg, 2007. p.
233.
[21] M. Vaarkamp, H. Linders, D. Koningsberger, XDAP Version 2.2.7, XAFS Services
International, Woudenberg, The Netherlands, 2006.
[22] Y. Fujishima, S. Okamoto, M. Yoshiba, T. Itoi, S. Kawamura, Y. Yoshida, Y.
Ogura, Y. Izumi, J. Mater. Chem. A 3 (2015) 8389.
[23] Y. Izumi, T. Itoi, S. Peng, K. Oka, Y. Shibata, J. Phys. Chem. C 113 (2009) 6706.
[24] Y. Izumi, H. Nagamori, F. Kiyotaki, D. Masih, T. Minato, E. Roisin, J.P. Candy, H.
Tanida, T. Uruga, Anal. Chem. 77 (2005) 6969.
[25] B.G. Hyde, S. Andersson, Inorganic Crystal Structures, John Wiley & Sons, New
York, 1989. p. 12.
[26] A. Jentys, Phys. Chem. Chem. Phys. 1 (1999) 4059.
[27] Diamond – Crystal and Molecular Structure Visualization – Version 3.2, K.
Brandenburg, H. Putz (Eds.), Crystal Impact, Bonn, Germany.
[28] C.A. Téllez, E. Hollauer, M.A. Mondragon, V.M. Castaño, Spectrochim. Acta 57A(2001) 993.
[29] A.L. Ankudinov, B. Ravel, J.J. Rehr, S.D. Condradson, Phys. Rev. B 58 (1998)7565.
[30] J.L. Dubois, P. Mukherjee, T.D.P. Stack, B. Hedman, E.I. Solomon, K.O. Hodgson,
J. Am. Chem. Soc. 122 (2000) 5775.
[31] Y. Yoshida, Y. Mitani, T. Itoi, Y. Izumi, J. Catal. 287 (2012) 190.
[32] A.N. Il’Ichev, V.A. Matyshak, V.N. Korchak, Kinet. Catal. 56 (2015) 115.
[33] H.C. Lee, D.H. Kim, Catal. Today 132 (2008) 109.
[34] G. Avgouropoulos, T. Ioannides, J. Mol. Catal. A 296 (2008) 47.
[35] Z. Wu, M. Li, J. Howe, H.M. Meyer, S.H. Overbury, Langmuir 26 (2010) 16595.
[36] Y.M. Choi, H. Abernathy, H.T. Chen, M.C. Lin, M. Liu, ChemPhysChem 7 (2006)
1957.