Catal Surv Asia (2016) 20:141–166
DOI 10.1007/s10563-016-9216-8

Recent Advances in the Preferential Thermal-/Photo-Oxidationof Carbon Monoxide: Noble Versus Inexpensive Metals and Their
Reaction Mechanisms
Yusuke Yoshida1,2 • Yasuo Izumi1

Published online: 6 July 2016
 Springer Science+Business Media New York 2016

Abstract Preferential oxidation (PROX) of CO is applicablebecause of its low cost, ease of implementation, and low lossof H2 during purification. Mo–SiO2 and Cr–SiO2 photocatalysts utilized charge separation at metal=O bonds, and the
PROX selectivity of CO was high. The CO PROX rates using
semiconductor-based photocatalysts were comparable tothose of photocatalysts (380 lmol h-1 g-1
cat ) and were also
selective (100 %). These photocatalysts are advantageous
because they do not activate H2 in comparison to noble metal
catalysts. Below 473 K using noble metals, Ru, Rh, Pt, or Au
supported on reducible metal oxides exhibited excellent CO
thermal-PROX rates of *4900 lmol h-1 g-1
cat ; however, theCO PROX selectivity of *48 % was insufficient because ofH2 activation on noble metals in nature. CO adsorbed onto
TiO2 and O2 was stabilized at the interface between a Ti site
and an Au atom. Weakly adsorbed water increased theeffective number of active sites by stabilizing Au–OOH or
Au–COOH. The PROX rates of CO using Au/TiO2-based
catalysts under dark conditions increased under UV–visible
light by the effect of charge separation and surface plasmon
resonance and the promoted electron transfer to the adsorbed
O2. In the case of CuO–CeO2 catalysts, CO adsorbed onto
CuO and reacted with lattice O atoms at the boundary between
CuO and CeO2 to form CO2 at an O vacancy, which was
subsequently filled with an O2 molecule. The combination of
Cu or Co with a reducible metal oxides also provided& Yasuo Izumi
yizumi@faculty.chiba-u.jp
1

Department of Chemistry, Graduate School of Science, Chiba
University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan

2

Present Address: Innovation Research Center of Fuel Cells,
The University of Electro-Communications, 1-5-1
Chofugaoka, Chofu, Tokyo 182-8585, Japan

performance comparable to or higher than that of CuO–CeO2
owing to adequate standard reduction potential for Cu2?,
Co3?, Mn3?/4?, and Ce4?. Finally, binary metal–organic
framework consisting of oxyhydroxide Ti clusters interlinked
by organic ligands and Cu oxyhydroxide ligands showedsuperior CO PROX performance (76 % conversion and 99 %selectivity) to that achieved using CuO–CeO2 owing toeffective dispersion of Cu–O–Ti connection in microporous
crystallites. Further progress is needed to alleviate the activity
loss in the presence of moisture and/or CO2 based on suggestions that steric hindrance of some types of microporous
crystallites would suppress the blocking of moisture or CO2.
Graphical Abstract

Keywords Preferential CO oxidation of CO  Photo
PROX  Copper catalyst  Cobalt catalyst  Binary metal–organic framework

1 Introduction
The production, storage, and transport of H2 are key
technologies for the emergence of a H2 energy society inthe near future [1]. Fuel cells (FCs) are the key device for

123

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Y. Yoshida, Y. Izumi

converting H2 into electricity for use in vehicles, mobile
phones, and residences and will be indispensable in a H2energy society [2]. H2 production from water utilizing
sustainable energy is ideal; however, these methods are
expensive, and a practical H2 source continues to be fossil
fuels [3]. A substantial part of the cost of FCs can be
attributed to the electrode catalysts, which are mostly
composed of Pt. Currently, most H2 is produced via thereforming of fossil fuels, and a few percent of CO inevitably remains because of the equilibrium limitation of thewater–gas shift reaction (WGSR):
H2 O þ CO  H2 þ CO2 :ð1ÞHere, electrode catalysts are easily deactivated if more than
10 parts per million (ppm), i.e., more than 1.0 Pa ofambient pressure, of CO is contained as an impurity in H2
because of the strong irreversible adsorption of CO onto
the Pt surface [4].
Well-known techniques for purifying H2 include pressure
swing adsorption (PSA) [5], separation using metal membranes [6], cryogenic separation [6], preferential oxidation(PROX) of CO [7–11], and methanation of CO [9]. PSA andmetal membrane separation are advantageous for achieving
higher H2 purity, whereas cryogenic separation recovers alarger amount of H2. The PSA apparatus incurs high initialinvestment costs and is therefore not appropriate for smallscale or personal use. PROX is potentially applicable since it
has the lowest cost and easiest implementation of theaforementioned methods, without the associated loss of H2during purification. 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. The methanation of CO using catalysts
generally requires reaction temperatures greater than 573 K.
Thus, to generate environmentally benign energy for smallscale or personal use, a new method for the purification of H2
that employs a cost-effective and sustainable membrane/catalyst is strongly demanded.
PROX catalysts based on noble metals [8–10] and transition/rare-earth metals [9, 11] were reviewed in 2008–2014;
however, inexpensive PROX photocatalysts assuming solar
light utilization have never been reviewed; in addition,
recently developed, inexpensive, and highly efficient Cu, Ni,
and Co PROX catalysts need to be reviewed based on theirefficiencies and plausible reaction mechanisms using stateof-the-art surface analytic techniques. This review covers
findings related to both photocatalysts and thermal catalysts
and the understanding of their mechanisms to examine themajor fundamental research trends of PROX catalysis
applicable for an H2 society. Because it is practically desirable to conduct PROX reactions between the temperaturesassociated with low-temperature WGSRs (473–523 K) andthe operation of polymer electrolyte (PE) FCs (353–373 K),

123

this review focuses on photo-PROX and photoassisted- or
thermal-PROX reactions in the temperature range from 353
to 523 K.
Various authors have noted that photocatalytic rates are
not always proportional to the amount of photocatalyst
charged as a suspension in a reaction solution [12] or as a filmin contact with the reaction gas [3, 13]; however, the photoPROX rates of CO using Cu–ZnO as fine powder were well
proportional to the amount of Cu–ZnO charged as a fine
powder in contact with the reaction gas (Fig. 1) [14]. Directcomparison between the photocatalytic and thermal PROX
rates of CO is difficult. Nonetheless, we herein introduce
catalysts as fine powders or molded and fractioned powders
by mesh and express the rates in units ofAmount of productsðmol h1 g1
cat Þ
Reaction duration time  Amount of catalyst
ð2Þ
to survey the major trends related to the photo/thermalPROX of CO.

2 Photocatalytic PROX Reactions of CO
2.1 Photocatalytic Water–Gas Shift Reaction
The WGSR (Eq. 1) involves neither the PROX reaction nor
the oxidation reaction of CO and apparently seems beyond

Fig. 1 a Dependence of time course of PROX reaction of CO in H2
on amount of Cu–spheroidal ZnO used (25–100 mg) under illumination with UV–visible light. b Dependence of transmitted UV–
visible light intensity through 30–120 mg of Cu–spheroidal ZnO in
5.3 cm2 area. Distance between bottom of reactor and lamp exit was
set to 24 mm [14]. Copyright (2012) Elsevier

Recent Advances in the Preferential Thermal-/Photo-Oxidation of Carbon Monoxide: Noble…

the scope of this review; however, the oxidation of CO by
O and/or OH groups photo-decomposed from water was
proposed in 1980 [15]. Under 3.2 kPa of water, H2 wasformed; however, the H2 formation rate was gradually
decreased using Pt(2 wt%)/TiO2 irradiated with a 200 W
high-pressure Hg lamp that was filtered through NiSO4
solution. Conversely, 80 Pa of 13CO and 3.2 kPa of water
were reacted using Pt/TiO2 at 273–298 K irradiated by thesame light source. The H2 formation rate became constantand essentially remained unchanged with decreasing rate ofCO, demonstrating that CO picked up O species derived
from water, thereby maintaining a clean surface. Based on
the band energy level of n-type TiO2 and Pt nanoparticles
connected by a Schottky barrier to trap electrons in Pt
nanoparticles, the following reaction steps were proposed:
TiO2 þ hm ! hþ þ e ;
þ

ð3Þ
þ

h ðTiO2 Þ þ H2 O ! OH þ H ;ð4Þ

CO þ OH ! CO2 þ H;

ð5Þ



ð6Þ

þ

e ðPtÞ þ H ! H;
and2H ! H2 :

ð7Þ

2.2 Photocatalytic Oxidation Using Metal OxideBased Catalysts
Before we discuss photocatalytic PROX reactions of CO,
we briefly introduce pioneering and recent works related tophotocatalytic simple oxidations of CO as the basis tounderstand the photocatalytic PROX reaction mechanisms.

e ðTiO2 Þ þ O2 ! O þ O ðvacancy filledÞ;

143

ð9Þ

and
COþ þ O ! CO2 :ð10Þ

2.2.2 PdPt/TiO2 Catalysts
Photooxidation of CO was recently reported using
bimetallic PdPt nanoparticles (mean size 1–2 nm) formedon TiO2 [17]; the authors discussed the effects of humidity.
The poisoning by moisture is one of the most serious
problems for PROX catalysts of CO. When 3.7 kPa ofmoisture was added at 313 K, the CO conversion using
0.3 wt% of Pt over TiO2 and Pd(0.3 wt%)/TiO2 decreasedby factors of 0.65–0.40 times: from 79 to 51 % and from84 to 34 %, respectively (Tables 1a, c, 2a, c). By contrast,
the CO conversion using bimetallic Pd0.5Pt0.5(in total
0.3 wt%)/TiO2 was higher as a consequence of thebimetallic effect in the absence of moisture (95 %) andremained high at 90 % in the presence of 3.7 kPa ofmoisture (Tables 1b, 2b). The ‘‘bimetallic effect’’ enabled
to design neighboring sites for CO and O2 adsorption.
Partial segregation of Pd to the corners, edges, and faces
was suggested based on simulated high-angle annular dark
field (HAADF) scanning transmission electron microscopy(STEM) images for Pd0.2Pt0.8 nanoparticles (Fig. 3).
However, a more random distribution of Pd and Pt wassuggested for Pd0.5Pt0.5 nanoparticles based on the limited
population of surface sites of higher surface energies, thefact that the samples were prepared at room temperature

2.2.1 TiO2 Catalysts
The feasibility of photocatalytic CO oxidation using TiO2
and the proposed reaction mechanism was reported in 1996[16]. Surface O vacancy sites were formed by annealing
rutile-type TiO2(110) crystal at temperatures greater than
400 K. Molecular O2 chemisorbed at O vacancy sites on
TiO2(110) at 105 K (Fig. 2) mainly desorbed as O2 andpartially oxidized coadsorbed CO molecules to produce
CO2 when irradiated by light with an energy greater than
3.1 eV. The decay times for CO2 formation and O2 desorption steps from relatively stable adsorbed O2 species
were essentially identical, suggesting common chemisorbed O2 species desorbed or oxidized CO molecules.
Because no CO2 desorption was monitored in the absenceof light [16], photoproduction of CO2 should be enabledenergetically via the hole oxidation of CO (Eq. 8) andelectron reduction of O2-derived species (Eq. 9).
hþ ðTiO2 Þ þ CO ! COþ ;ð8Þ

Fig. 2 Rutile-type TiO2(110) surface illustrating O vacancy sites
produced by annealing: (cross-hatched symbols) bridging O2overlayer ions, (open symbols) in-plane O2- ions, (solid symbols)
Ti4? or Ti3? ions, and (shaded symbols) bulk O2- ions [16].
Copyright (1996) American Chemical Society

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Y. Yoshida, Y. Izumi

Table 1 Comparison of rates of photocatalytic PROX or oxidation reactions of CO reported in literature
Entry

CatalystType

W (mg)a4

Pt (0.3 %)/TiO2

100

b4

Pd0.5Pt0.5(0.3 %)/
TiO2

c4

Pd (0.3 %)/TiO2

dNiO(0.5 %)–TiO2eMo(0.6 %)–SiO2

f

Mo(1.0 %)MCM-41g

Cr(0.7 %)MCM-41hCu(0.5 %)spheroidal ZnO

50

T (K)

Light

SystemReactants P

RCO?CO2(lmol h-1
1
g-1
cat )

CO
Conv.
(%)2

313

8 W Black
light

Flow
(250 mL min-1)

CO 25 Pa, O2
21.2 kPa

1200

79

1400

95

1300

84

38

89 @3 h

98
[27]
@3 h

95

*100
@3 h

99
[19]
@3 h

62

98 @3 h

98

[24]

380

*100
@2.5 h97[25]

CO 64 Pa, O2
150 Pa, H2
6.4 kPa

84

100
@3 h

94

[14]

CO 64 Pa, O2
76 Pa, H2
6.4 kPa56

91 @3 h99

293

100 W HP Hg,
H2O filter

Closedcirculation

CO 92 Pa, O2
180 Pa, H2
590 Pa20500 W Xe(k [ 420 nm)
50

\313

500 W Xe arc

iCO2
Selec.
(%)3

Ref.

[17]

j

Cu(0.5 %)-disklike ZnO59

91 @3 h99kCu(0.1 %)spheroidal ZnO

97

97 @3 h96lCu(0.5 %)spheroidal ZnO

82

97 @3 h99m

Cu(0.32 %)-rodlike ZnO

16

30 @3 h100

[28]

W sample weight charged, T reaction temperature, P pressure
1
RCO!CO2 : CO removal rate2

CO Conv.: CO conversion (%) = 100 9 (nCO,Start - nCO,End)/nCO,Start

3

CO2 Selec.: CO2 selectivity(% ) ¼ 100  nCO2 =ðnCO2 þ nH2 O Þ

4

Not PROX but simple oxidation

via liquid-phase reduction and were not calcined, andFourier transform infrared spectroscopy results. The surface Pt sites were suggested to adsorb O2 and water preferentially, leaving Pd sites free for CO adsorption.
In summary, supported PdPt photocatalysts facilitated
CO oxidation at rates of 1200–1400 lmol h-1 g-1
cat ; these
rates are similar to the photo-PROX rates obtained using
supported Au catalysts if the inhibiting effects of predominant H2 gas are neglected in the Au-catalyzed photoPROX of CO. Importantly, this study provides a solution
for one of the most serious problems for PROX: deactivation by humidity. Neighboring sites to CO adsorption
sites to preferentially adsorb water and thus to liberate COadsorption sites are required.

123

2.3 Photocatalytic PROX Using Inexpensive
Elements Supported on Metal Oxides
For H2 purification, the PROX reaction of CO using catalysts that do not contain noble metals and that can be
excited under natural light is ideal. In this category, twotypes of photocatalysts have been reported. One is metal
complex-like photocatalysts supported on metal oxides, inwhich charge separation occurs at the chemical bonds ofmolecule-like moieties upon irradiation. The other type issemiconductor photocatalysts, in which charge separation
occurs in the bulk [18]. The advantage of photocatalytic
PROX is selectivity toward CO PROX compared to H2oxidation
by-reaction
becausecomplex-likeorCatalyst

Pt(0.3 %)/TiO2

Pd0.5Pt0.5(0.3 %)/TiO2Pd(0.3 %)/TiO2

Pt/c-Al2O3

Au/a-Fe2O3

CuO–CeO2

Pt/c-Al2O3

Au/a-Fe2O3

CuO–CeO2

Cu-Fe/
K2x?yMnIIx MnIIIy
MnIV
8-x-yO16

CuO(17 %)–ZnO(46 %)–TiO2(37 %)Co3O4(20 %)/Ce0.85Zr0.15O2

Mn(2.6 %)–Co(11 %)–CeO2

Co(3.2 %)/SrCO3

Entry

a3

b3

c3

d

e

f

g

h

i

j

k

l

m

n

96

*100

2.1 mg mL-1
min

448

32

36

250 mL min-1
g-1

373

513

32

18 000 h-1

9695@353 K

20

63

97

5.0

84

95

79

No
addition38 (20 kPaCO2)

95 (20 kPa
CO2)*0 (10 kPa
CO2)

10 (15 kPa
CO2)

16 (10 kPa
CO2)

59 (15 kPa
CO2)

97 @368 K(15 kPa CO2)

17 (15 kPa
CO2)

23 (15 kPa
CO2)

79 (15 kPa
CO2)

3.3 (15 kPa
CO2)

CO2

CO Conv. (%)1

27

21

100

250

Flow rate(mL min-1)

100

363

373

313, 8 W
Black light

T (K)

4.3
(10 kPa
H2O)

14 (10 kPa
H2O)

34(3.7 kPa
H2O)

90(3.7 kPa
H2O)

51(3.7 kPa
H2O)

Moisture27 (18 kPa
CO2 ? 10 kPa H2O)

2.4 (10 kPa
CO2 ? 10 kPa H2O)

10 (10 kPa
CO2 ? 10 kPa H2O)

11 @393 K (15 kPa
CO2 ? 10 kPa H2O)

99 (15 kPa
CO2 ? 10 kPa H2O)

42 (15 kPa
CO2 ? 10 kPa H2O)

CO2 ? moisture

Table 2 Comparison of effects of CO2 and/or moisture in feed gas on PROX reactions of CO reported in literature

68

49

*100

*100

74 (20 kPa
CO2)

55 (20 kPa
CO2)

80 (10 kPa
CO2)

*100

91 (10 kPa
CO2)*100 (15 kPa
CO2)

*100

91

41 @368 K(15 kPa CO2)39@353 K

50 (15 kPa
CO2)*100 (15 kPa
CO2)

*100
44

45 (15 kPa
CO2)

38 @398 K(15 kPa CO2)

CO2

48

49
@398 KNo
addition

CO2 Selec. (%)2*100
(10 kPa
H2O)*100(10 kPa
H2O)

Moisture72 (18 kPa
CO2 ? 10 kPa
H2O)*100 (10 kPa
CO2 ? 10 kPa
H2O)

95 (10 kPa
CO2 ? 10 kPa
H2O)*100 (15 kPa
CO2 ? 10 kPa
H2O)

54 (15 kPa
CO2 ? 10 kPa
H2O)

42 (15 kPa
CO2 ? 10 kPa
H2O)

CO2 ? moisture

93

90

89

84

83

60

17

Ref.

Recent Advances in the Preferential Thermal-/Photo-Oxidation of Carbon Monoxide: Noble…
145

123

146

Y. Yoshida, Y. Izumi

69 (18 kPa
CO2 ? 10 kPa
H2O)

2.3.1 Charge Separation at Bonds
As an example of a metal complex-like photocatalyst
supported on SiO2, a Mo–SiO2 photocatalyst consisting of
0.6 wt% Mo (oxomolybdenum complex) performed well,with a CO conversion of *100 % at 3 h and a CO2
selectivity of 99 % under an initial reaction gas composition of CO (3.8 lmol) ? O2 (7.5 lmol) ? H2 (24.6 lmol)
and illumination by a high-pressure Hg lamp. The Mo–
SiO2 photocatalyst reduced CO to less than 10 ppm in H2,satisfying the requirement for the use of H2 in PEFCs(Table 1e) [19]. Based on Mo K-edge extended X-ray
absorption fine structure (EXAFS) curve-fit analysis[20], the predominant monomeric tetrahedral (O=)2Mo(–Osurface)2 species were proposed to be the active sites.
Therefore, charge separation at chemical bonds is likely
responsible for the photocatalysis. The charge-transfer triplet state at the Mo=O bond of [Mo6?=O2-]—specifically,[Mo5?–O-]à and/or a reduced state of Mo4?—both
induced by the irradiation of UV–visible light, were suggested to be responsible for CO PROX (Scheme 1).
The quenching rates of photoluminescence of Mo–SiO2
were evaluated by the addition of CO, O2, and H2 toevaluate each reactivity on the photocatalyst [21]. The data
were fitted to the Stern–Volmer equation [22, 23]:CO2

63 (18 kPa
CO2 ? 10 kPa H2O)

No
addition

123

CO Conv.: CO conversion (%) = 100 9 (nCO,Start - nCO,End)/nCO,Start

CO2 Selec.: CO2 selectivity(% ) ¼ 100  nCO2 =ðnCO2 þ nH2 O Þ

Not PROX but simple oxidation

2

3

T reaction temperatureCo(3.2 %)–Bi(0.3 %)/SrCO3oYield of photoluminescence
Yield of photoluminescencewith quencher¼ 1 þ s0 kquench ½quencher;

1

CO2
No
addition

Catalyst
Entry

Table 2 continued

T (K)Flow rate(mL min-1)

CO Conv. (%)1

Moisture

CO2 ? moisture

CO2 Selec. (%)2

Moisture

CO2 ? moisture

Ref.

semiconductor active sites basically activate polar molecules and does not activate H2 compared to facile H2 dissociative adsorption due to free electrons in metals andfollowing reaction on metal surface for thermal PROX
reaction.

ð11Þ

where s0 is the lifetime of the charge-transfer triplet excitedstate [Mo5?–O-]à in the absence of a quencher, and kquench
is the absolute quenching rate constant and thus a criterionof reactivity of CO, O2, and H2. The obtained kquench values
decreased in the order (Table 3)
CO [ O2 [ H2ðTable 3; MoSiO2 Þ;ð12Þ

demonstrating preferable interaction of CO with intermediate [Mo5?–O-]à species excited by light. Lowest interaction with H2 means lowest rate of unfavorable sidereaction of CO PROX:
2H2 þ O2 ! 2H2 O:

ð13Þ

Mo–MCM-41 exhibited CO photo-PROX performance(Table 1f) similar to that of Mo–SiO2, and the kquench
values decreased in the order
CO [ O2  H2

ðTable 3; Mo  MCM-41Þ;

ð14Þ

Recent Advances in the Preferential Thermal-/Photo-Oxidation of Carbon Monoxide: Noble…

Fig. 3 Simulation of a 1406-atom cuboctahedral Pd0.2Pt0.8 nanoparticle with Pd (red/darker spheres) surface segregation versus Pt(yellow/thinner spheres); b HAADF-STEM image of cuboctahedron;147

HAADF-STEM intensity profiles at c border (green/thinner line) andd core (red/darker line) of cuboctahedron [17]. Copyright (2015)
Elsevier (Color figure online)

Scheme 1 Photocatalytic
reaction cycle in photocatalytic
PROX of CO on tetrahedrally
coordinated Mo6?-oxide species[21]. Copyright (2007) Elsevier

123

148

Y. Yoshida, Y. Izumi

Table 3 Different rate constant kquench values (mol-1 s-1) for supported Mo and Cr photocatalysts depending on quenchers
Photocatalyst

Quencher

Ref.

CO
Mo–SiO2

H2

O2

2.23 9 1010

1.22 9 1010

1.19 9 109

[21]

10

10

Mo–MCM-41

2.36 9 101.33 9 107.73 9 108[24]

Cr–MCM-41

5.91 9 109

1.12 9 1010

8.63 9 105[25]

demonstrating a trend similar to that of Mo–SiO2 that leads
to preferable photo-PROX reaction of CO [24].
Under irradiation by visible light (k [ 420 nm),[Cr6?=O2-] species also exhibited charge transfer to an
excited triplet [Cr5?–O-]à in the case of Cr–MCM-41 [25].
The CO PROX reaction rate (380 lmol h-1 g-1
cat ; Table 1)
was superior to that achieved using Mo–MCM-41(62 lmol h-1 g-1
cat ) by a factor of 6.1 under similar reaction
conditions (Tables 1f, g). On the other hand, the absolute
quenching rate constant kquench values in Eq. 11 for photoluminescence decreased in the orderO2 [ CO  H2

ðTable 2; Cr  MCM-41Þ;

ð15Þ
5?

- à

demonstrating preferential reactivity of [Cr –O ] species with O2 rather than H2 and supporting better PROX
performance of CO. This order differs from that observedfor Mo catalysts, in which the values for CO were thegreatest (Table 3). This difference suggests the relative
importance of reductive O2 activation compared to oxidative CO activation on the supported metal (Mo or Cr)
complexes (Scheme 1). Importantly, the kquench (H2) value
for Cr–MCM-41 was smaller by an order of 3–4 comparedto those for Mo–MCM-41 and Mo–SiO2 (Table 3). Weaker
interaction of catalyst with predominate H2 may lead tohigher CO PROX reaction rates (Table 1). Furthermore,
photocatalysis using isolated Ti-, V-, and Cr-oxides withinzeolites has already been reviewed for CO PROX, CO2
photoreduction, and NO decomposition reactions [26].
In summary, metal complex-like supported inexpensive
Mo and Cr photocatalysts exhibited photo-PROX rates
ranging from 62 to 380 lmol h-1 g-1
cat . The rates were
substantially lower than those achieved using expensive Au
catalysts. By contrast, the CO PROX selectivity was consistently higher (97–99 %) than those achieved with Aubased catalysts due to the difference of polar metal complex and metal surface with free electrons.
2.3.2 Charge Separation in Semiconductors
Photo-PROX reactions of CO using semiconductors in thepresence of a NiO/TiO2 photocatalyst consisting of
0.5 wt% NiO have been reported to result in CO conversion of 89 % and CO2 selectivity of 98 % at 3 h for a123

reaction-gas mixture composed of CO (3.8 lmol) ? O2
(7.5 lmol) ? H2 (24.6 lmol) irradiated by UV light(Table 1d) [27]. The oxidation of CO and reduction reactions should take place over TiO2 and electron-trapping
NiO nanoparticles, respectively, based on Sect. 2.1. PolarTiO2 surface did not activate H2 and the Ni sites oxidized
rather than Ni0 metal did not dissociate H2, leading to highselectivity of CO PROX.
Detailed reaction mechanism recently reported for Cu–
ZnO photocatalysts [14, 28]. The semiconductor properties
of ZnO [14, 28] are similar to those of TiO2 [29–34].
Spheroidal ZnO nanocatalysts with an average major axisof 47 nm and an average minor axis of 22 nm and combined with 0.5 wt% of adsorbed Cu2? ions enabled 100 %CO conversion and 94 % CO2 selectivity in 3 h underirradiation by UV–visible light (Table 1h). The performance was comparable to those using metal complex-likephotocatalysts (see the Sect. 2.3.1) and the other semiconductor photocatalysts (this section). The rate corresponds to the preferential photooxidation of 9700 ppm of
CO to 0.35 ppm in 6.3 kPa of H2 in 5 h [14], satisfying therequirement for the use of H2 in PEFC.
The fresh Cu-spheroidal ZnO photocatalysts were
investigated by Cu K-edge X-ray absorption near-edge
structure (XANES) spectroscopy (Fig. 4A-a). The Cu
valence state was II in fresh Cu-spheroidal ZnO[14, 28, 35–39]. After illumination for 1 min under UV–
visible light, a weak shoulder peak appeared at 8983.1 eV(Fig. 4A-b); the intensity of this peak gradually increased
to 0.072 7 compared to normalized Cu K absorption edge
under UV–visible illumination for 3 h (Fig. 4A-d). One
minute after the UV–visible light was removed, the peakintensity decreased by 0.003 5 (Fig. 4A-e).
The background and peak intensities at 8983.1 eV for
CuII sites (100 %) and CuI sites (100 %) are indicated bydotted lines in Fig. 4B. The shoulder peak at 8983.1 eV
originates from CuI sites [36–40]. Therefore, the CuI-site
ratio in the sample was evaluated using the following
equation:
CuI site ratioPeak intensity@8983:1 eV  0:048ðfor CuII 100 %Þ
:
0:21ðfor CuI 100 %Þ  0:048ðfor CuII 100 %Þ
ð16Þ

Based on Eq. 16, the population of CuI sites gradually
increased to 15.6 % after 3 h of UV–visible illumination(Fig. 4B). The growth rate of CuI sites under UV–visible
illumination was 4.1 lmol-Cu h-1 g-1
cat , which is significantly lower than the photo-PROX rate of CO under similar reaction conditions (84 lmol-CO h-1 g-1
cat , Table 1h).
Thus, the accumulated electrons originally generated in thephotocatalyst by light and diffused to CuII sites should beRecent Advances in the Preferential Thermal-/Photo-Oxidation of Carbon Monoxide: Noble…

Fig. 4 A Normalized Cu K-edge XANES spectra for Cu-spheroidal
ZnO a before illumination and at b 1 min, c 80 min, and d 180 min
from beginning of illumination by UV–visible light with initialpressures of CO (135 Pa), O2 (270 Pa), and H2 (13.1 kPa), and e at
1 min after the illumination and for f Cu-spheroidal ZnO-reduced.

detected, whereas the electrons that were transferred fromthe Cu sites to O2-derived species for CO PROX reaction
should be subtracted [14]. Similarly, the reduction stepfrom CuII sites to CuI was also successfully monitored
based on the decrease in the peak intensities of the Cu 1s3d pre-edge peaks for layered double hydroxides during theCO2 photoreduction steps [41–43].
The anisotropic effects of nano-designed ZnO crystallites on CO PROX activity have also been reported [28].
The CO removal rates and conversions decreased in thefollowing order (Table 1j–m):
Cu(0:10 %Þ-spheroidal ZnO [ Cuð0:50 %Þ-spheroidal
ZnO [ Cu(0:50 %Þ-disk-like ZnO Cuð0:32 %Þ-rod-like ZnO:
ð17ÞThe CO photo-PROX selectivity was higher than 96 %for all the Cu–ZnO photocatalysts. Using XANES,
researchers compared the photoreduction rates of Cu sites
among Cu-disk-like ZnO, Cu-spheroidal ZnO, and Cu-rodlike ZnO irradiated by UV–visible light. During the irradiation, the intensity of a shoulder peak that occurred at8983.1 eV in the spectra of all of the Cu-ZnOs gradually
increased. The CuI site ratio was evaluated based on Eq. 16.
Assuming first-order reaction kinetics for the reduction½CuII  ¼ ½CuII initial ekCu reductiont ;ð18Þ

the rate constant kCu reduction was determined to be 0.0082,
0.0076, and 0.0032 min-1 for Cu-disk-like ZnO, Cuspheroidal ZnO, and Cu-rod-like ZnO, respectively (Fig. 5)[28]. These values were compared to CO PROX rates of59, 82, and 16 lmol-CO h-1 g-1
cat for first-order CO PROX
rate constants of 0.017, 0.025, and 0.0021 min-1,

149

B Intensity change of shoulder peak at 8983.1 eV during illumination
by UV–visible light and 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 [14]. Copyright(2012) Elsevier

Fig. 5 Intensity change in shoulder peak at 8983.1 eV in normalized
Cu K-edge XANES during irradiation by UV–visible light for Cudisk-like (filled circle), Cu-spheroidal (filled diamond), and Cu-rodlike ZnOs (filled square) [28]. Copyright (2015) American Chemical
Societyrespectively. Thus, the kCu reduction value for the photocatalyst was a general evaluation factor of CO PROX activity.
A photo-PROX reaction begins from a monodentate formate species A [44] (Scheme 2A1 ? 2; 4 ? 5) that isoxidized by photogenerated holes (Scheme 2A2 ? 3;
5 ? 6); surface Cu2? sites, which populate preferentially on
unsaturated ZnO{1011} faces, are reduced by photogenerated electrons (Scheme 2A2 ? 3; 5 ? 6) [28]; and O2 isreduced by electrons via Cu-ion trapping sites coupled with
protons of oxidized monodentate formate H?CO2 across the
crystalline face boundary (Scheme 2A3 ? 4 ? 5 ?
6 ? 7) to regenerate two surface hydroxy groups (Scheme 2A7 ? 1). In the case of Cu-spheroidal ZnO (see theGraphical abstract), the monodentate formate on the (0001)
face and the Cu sites on the {1011} face should be in close
proximity, enabling proton transfer (Scheme 2A3, A6) andhydroxy restoration (Scheme 2A7 ? 1). The proposedmechanism that includes proton transfer from formate species is closely related to the CO PROX mechanism on a Au/

123

150

Y. Yoshida, Y. Izumi

containing Mo or Cr. Both types of photocatalysts exhibited satisfactorily high CO photo-PROX selectivity of
96–100 %. The participation of hydroxy group was suggested for photocatalytic CO PROX on semiconductorbased catalysts. H2 is not activated on these photocatalysts
and enabled higher CO PROX selectivity in clear contrast
to thermal PROX using Au, Co, Cu, or Ni.

3 Thermal PROX Reactions of CO
3.1 Thermal PROX Using Noble Metals
3.1.1 Comparison of Noble Metals

Scheme 2 Proposed surface reaction steps for photo-PROX reactionof CO in H2 via monodentate formate A and route to accumulate
inactive bridging formate B for Cu–ZnOs [28]. Copyright (2015)
American Chemical Society

TiO2 catalyst, whereas proton transfer from carboxy intermediate on Au is proposed in Sect. 3.1 (see below).
In summary, semiconductor-type photocatalysts basedon typical semiconductors such as TiO2 or ZnO that consistof Ni or Cu ion sites enabled CO photo-PROX reactions at
rates of 16–97 lmol h-1 g-1
cat ; these rates are similar to
those achieved using molecule-like photocatalysts

123

Thermal oxidation of CO has been studied using supported
noble metals, e.g., Pt, Pd, Rh, and Au [9]. Pt, Pd, or Rh on
Al2O3 or Au on Fe oxide [8–11], as well as Ru/TiO2(Table 4A, entry d) [45], Ru/Al2O3 [46], and Ru/
SiO2 ? additives [47, 48], are known to be good catalysts.
Comparisons of Pt, Pd, Ru, and Rh supported on either
Al2O3 or SiO2 revealed that Ru/Al2O3 and Rh/Al2O3
exhibited the best performance with respect to CO PROX
activity and selectivity [9]. Furthermore, highly-dispersed
Au nanoparticles on TiO2 or on other reducible metal oxides
such as Co3O4 and CeO2 (Fig. 6), are highly active toward
CO oxidation at temperatures lower than those required forPt catalysts [9, 49]. Reducible metal oxides have been proposed to transfer electrons to Au nanoparticles leading to the
reduction reaction over the surface [50]. The CO oxidation
rate in balance N2 gas (not CO PROX) at a relatively suppressed flow rate of 17 mL min-1 in the presence of Au/
TiO2 was sufficiently fast: 72 000 lmol h-1 g-1
cat (Table 4A,
entry k) [51]. The CO PROX performance of Au catalysts on
reducible metal oxides, e.g., TiO2, Fe2O3, Co3O4, NiO,
MnO2, and CeO2, has been reviewed elsewhere [9, 10].
The higher reactivity of supported noble metals is great
advantage, however, as the nature of supported noble
metals, dissociative adsorption of H2 through free electronsof metals and the following reaction with O2 are inevitable,
especially at increased reaction temperature. The selectivity for CO PROX compared to H2 oxidation decreased
rapidly with increasing flow rates of the reactants and withincreasing reaction temperature [8–10]. At higher temperatures, weaker CO adsorption and/or a competing H2 oxidation reaction was suggested to cause the decrease in
PROX selectivity on Au catalysts. To clarify the dependence of CO PROX on the flow rates of the reactants andon the reaction temperature, we focus on the PROX
mechanism of CO on Au catalysts in the next section.
In summary, supported Ru, Rh, Pt, and Au catalysts,
especially those supported on reducible metal oxides, exhibit
excellent CO PROX rates of 1000–4900 lmol h-1 g-1
cat . The

Type

Catalyst
W (mg)

T (K)

Light

Au/a-Fe2O3

CuO–CeO2
Pt/c-Al2O3

Au/a-Fe2O3

CuO–CeO2

Au(2.0 %)/TiO2

m

n
o

p

q

r

s

Pt/c-Al2O3

Pt(1 %)/TiO2hlIr(1 %)/TiO2gAu(3.1 %)/TiO2

Ag(1 %)/TiO2

fk5

Pd(1 %)/TiO2

eAu(1 %)/TiO2

Ru(1 %)/TiO2

dPt(1.0 %)/TiO2

Cu(5 %)/TiO2

ci

Co(5 %)/TiO2bj5

Mn(5 %)/TiO2

a

200

50

50

200

323

373

300

353

4 W UV lamp 9 4

No(A) Comparison using noble metals versus inexpensive metals

Entry

Flow
(100 mL min-1)

Flow(21 mL min-1)

Flow(100 mL min-1)

Flow(17 mL min-1)

Flow(75 mL min-1)

SystemTable 4 Comparison of rates of thermal-PROX or oxidation reactions of CO reported in literature

H2 86 kPa

O2
0.5 kPa,CO
0.5 kPa,H2 51 kPa

O2
1.3 kPa,CO
1.0 kPa,

O2 21 kPa,
N2 78 kPaCO
1.0 kPa,H2 61 kPa

O2
1.0 kPa,CO
1.0 kPa,

Reactants
P

6.7
11
*0

[453 K[453 K[453 K

1000
*0

5000

4800

10,00010,00032,0002100

49,0002500

72,000

680

345

\303

367
444

310

434

235

96

95
@353 K

63
20

97

5.0

2.3

[453 K

630

339

*0

[453 K

*0
220

50

6.6

3.4

2.1

CO Conv.
(%)3

353

399

405

[453 K

T50%
(K)2

4700

620

320

200

RCO?CO21(lmol h-1 g-1
cat )

38

36

*100

39
@353 K

*100
44

48

49

–

78

36

92

63

*0

37

*100

90

81

CO2 Selec.
(%)4

[63]

[60]

[51]

[45]

Ref.

Recent Advances in the Preferential Thermal-/Photo-Oxidation of Carbon Monoxide: Noble…
151

123

123
293

W (mg)

CuO(5 %)–SiO2

CuO(5 %)–TiO2

CuO(5 %)–CeO2

CuO–CeO2

CnO–CeO2 (Cu/Ce = 1/1.7)

a

b

c

d

e
200

100

250

(B) Comparison using inexpensive transition metals

Type

Catalyst

Entry

333

373

343

363

513

Flow (30 mL min-1)

Flow (100 mL min-1)

Flow (40 mL min-1)

System

No
300 W Xe
(490 \ k\760 nm)

Au(0.5 %)/Al2O3

b
c
T (K)

300 W Xe
(490 \ k\760 nm)aNo

Au(0.5 %)/TiO2z

No
300 W Xe
(490 \ k\760 nm)

Au(1.0 %)–CuO(10 %)/TiO2xy

300 W Xe
(490 \ k\760 nm)

w

700

No

No

Au(1.0 %)-polyaniline(4.7 %)/
TiO2

298

Light

v

500

W (mg)T (K)300 W Xe
(490 \ k\760 nm)

Au(1.0 %)/TiO2

TypeCatalyst

u

t

Entry

Table 4 continued

N2 86 kPa

O2 10 kPa

CO 4.0 kPa

H2 51 kPa

O2 0.51 kPa,

CO 1.0 kPa,

N2 98 kPaO2 1.0 kPa

CO 1.0 kPa

Reactants P

Flow(100 mL min-1)

Flow(100 mL min-1)

System

1500

19,000

1700

2300

2200

RCO?CO21(lmol h-1 g-1
cat )H2 86 kPa

O2
0.3 kPa,CO
0.3 kPa,

H2 81 kPa

250
270

400

320

1000

820

770

690

360

360

T50%
(K)2

260

CO
0.3 kPa,O2
0.3 kPa,

RCO?CO21(lmol h-1 g-1
cat )Reactants
P

24
25

37

30

70

55

52

47

24

18

9.5

75

5.4

7.0

6.7

93
89

68

67

85

76

81

75

68

66

CO2 Selec.
(%)4

–

99

–

CO2 Selec.
(%)4

CO Conv.
(%)3

CO Conv.
(%)3

T50%
(K)2

[72]

[70]

[69]

Ref.

[67]

[66]

[65]

Ref.

152
Y. Yoshida, Y. Izumi

CuO(4 %)–CeO2

CuMn2O4

CuO–CeO2

Cu–Mn (1:2) oxide

l

m

n

o

200
100–200

Co–Mn (8:1) oxide nanosheet

Co–Mn (8:1) oxide/FeOx

Co3O4(20 %)–Ce0.85Zr0.15O2

Co(8.4 %)–CeO2

Mn(9.9 %)–Co(10 %)–CeO2

Mn(0.94 %)/CeO2

u

v

w

x

y

z

100

CuO(17 %)–ZnO(46 %)–TiO2(37 %)Cu-FeIV
K2x?yMnIIx MnIIIy Mn8-x-yO16

t

r
s

6

100

50

15–30

300

373

423

373

Flow (250 mL min-1 g-1)

Flow (2.1 mg mL-1 min)

373

451

377
H2 41 kPa

392

383

\303

\303

381

391

366

351

454

427

347

373

353

359

360
400

330

T50%
(K)2

O2 1.0 kPa,

6300

6300

5100
14,00016,0007100

19,000

9000

1300

5400

2700

4400

RCO?CO21(lmol h-1 g-1
cat )CO 1.0 kPa,

H2 51 kPaO2 1.0 kPa,

CO 1.0 kPa,

H2 51 kPa

CO 1.0 kPa,O2 1.0 kPa,

Flow (50 mL min-1)

H2 51 kPaO2 1.2 kPa,

CO 1.2 kPa,

H2 51 kPaO2 1.0 kPa,

CO 2.0 kPa,

H2 61 kPaO2 1.0 kPa,

CO 1.0 kPa,

H2 25 kPa

O2 0.51 kPa,

CO 0.51 kPa,

H2 50 kPa

O2 0.50 kPa,

CO 0.50 kPa,

H2 50 kPaO2 1.25 kPa,

CO 1.0 kPa,

N2 97 kPa

CO 1.0 kPa
O2 2.0 kPa

H2 50 kPaO2 1.25 kPa,

CO 1.0 kPa,

Reactants P

Flow (25 mL min-1)

Flow (18,000 h-1)

Flow (100 mL min-1)

Flow (50 mL min-1)

Flow (200 mL min-1)

Flow (600 mL min-1)

Flow (500 mL min-1 g-1
cat )Flow (2000 min-1)

Flow (1000 mL min-1
g-1
cat )

System

373

348

333

363

333
373

373

CuO–CeO2–NC

kHopcalite 8-14#

CuO–CeO2–NR

j

343

343

q6

CuO–CeO2–NS

i

400

323

333

CuO–CeO2–NR
CuO–CeO2–NSgh

W (mg)T (K)p

CuO–CeO2 (Cu/Ce = 1/4)

Type

Catalystf

Entry

Table 4 continued

2

41

22

36

*100

98

32

20
27

63

28

74

18

42

33.5

10

40

20

11
1.5

17

CO Conv.
(%)3

*100

*100

*100

*100

90

*100

91

91

84

100

83

80

100

–

48

CO2 Selec.
(%)4

[90]

[89]

[88]

[87]

[84]

[83]

[82]

[81]

[74]

[79]

[78]

[73]

Ref.

Recent Advances in the Preferential Thermal-/Photo-Oxidation of Carbon Monoxide: Noble…
153

123

123

Cu–OMS(Mn)

CuO–CeO2 (Cu:Ce = 1:5)

Cu–MIL125 (Cu:Ti = 1:2)

Au(5.0 %)/TiO2

CuO–CeO2 (Cu:Ce = 1:5)

Cu–MIL125 (Cu:Ti = 1:2)

h

i

j

k

l

323

293

323

473

RCO!CO2 : CO removal rate

T50%: Temperature at which the CO conversion reaches 50 %CO Conv.: CO conversion (%) = 100 9 (nCO,Start - nCO,End)/nCO,Start

CO2 Selec.: CO2 selectivity(% ) ¼ 100  nCO2 =ðnCO2 þ nH2 O Þ

Not PROX but simple oxidation

Hopcalite 8-14# was from GL Sciences Inc., Japan

OMS(Mn): Octahedral molecular sieve comprising Mn

2

3

4

5

6

7

Closed circulation

Closed circulation

Flow (20 mL min-1)

1

W sample weight charged, T reaction temperature, P pressure

50

10

100

100

513

Flow

NiFe2O4g120

H2 6.3 kPa

O2 150 Pa,

CO 63 Pa,

H2 6.3 kPa

CO 63 Pa,O2 150 Pa,

H2 50 kPa

O2 0.5 kPa,

CO 0.5 kPa,

H2 71 kPaO2 1.0 kPa,

CO 1.0 kPa,

H2 95 kPa

O2 0.51 kPa,

CO 1.0 kPa,

H2 5.1 kPa
Flow (32 mL min-1)

Co(3.2 %)–SrCO3

373

f

200

CO 0.51 kPa,

H2 25 kPa

CO 0.51 kPa,
O2 0.51 kPa,

H2 30 kPa

O2 0.51 kPa,

CO 1.0 kPa,

O2 0.51 kPa,

Flow (200 mL min-1)

4.3 mL min-1

Reactants P

Flow (50 mL min-1)

373

373

Systeme

Co(10 %)–ZrO2

d

30

40

W (mg)T (K)

Flow (25 mL min-1)

Co3O4

c

b

Co–OMS(Mn)

7

Type

Catalyst

a7

Entry

Table 4 continued

140

140

1200

1900

710

1700

1300

15,0006700

11,000

RCO?CO21(lmol h-1 g-1
cat )

368

356

398

T50%
(K)2

97 @1 h98 @1 h100 @1 h

76

28

9.1

96

53

81

17

76.4

91.1

CO Conv.
(%)3

98

90

93

99

96

68

68

87

91

81

*100

*100

CO2 Selec.
(%)4

[102]

[14]

[102]

[94]

[93]

[92]

[86]

[91]

Ref.

154
Y. Yoshida, Y. Izumi

Recent Advances in the Preferential Thermal-/Photo-Oxidation of Carbon Monoxide: Noble…

Fig. 6 Plausible models of supported Au nanoparticles designed to
explain why ordinarily inert Au turns into powerful catalysts whenshrunk to nanoparticle dimensions [7]. Copyright (2003) American
Association for the Advancement of Sciencedrawback of these catalysts is that the CO PROX selectivity
dramatically decreases with increasing flow rates of thereactants and increasing reaction temperature: 36–48 % atflow rates of 75–100 mL min-1 and at 353–373 K due toeffective activation of H2 on noble metal surfaces leading to
by-reaction of water formation.
3.1.2 CO PROX (or Oxidation) Mechanisms Using Au
Supported on Reducible Oxides
The reason that highly dispersed Au nanoparticles are good
catalysts for the CO PROX reaction has been studied bymany researchers [7] by designing controlled Au
nanoparticles through atom-by-atom architecture of theunderlying metal oxide surfaces, e.g., single-crystalline
surfaces. These previous studies have typically involved
mapping such controlled surfaces using scanning tunnelingmicroscopy and using density functional theory (DFT) tomodel how atoms and molecules react with Au nanoparticle surfaces and/or the underlying metal oxide surface via
a reaction route that minimizes the energy (Fig. 6).
A PROX mechanism was proposed in which CO adsorbs
onto metal nanoparticles (e.g., Au on TiO2) and reacts with
O2 at the boundary between the metal nanoparticles and the
support [52–56]. Lower temperature needs to be chosen to
avoid H2 dissociation on Au metal surface instead of CO toguarantee higher selectivity for CO PROX. In particular,
Au nanoparticles 2–8 nm in diameter, and most likely
3 nm in diameter, on TiO2 powder were carefully studied
in detail at 110–130 K using IR spectroscopy [52]. In thecase of TiO2 powder under 8.0 Pa of CO at 120 K, theresulting IR spectrum showed an intense, broad peak at
2102 cm-1 (chemisorbed CO on metallic Au0), an intense,155

sharp peak at 2179 cm-1 (adsorbed CO on TiO2), and asmall peak at 2341 cm-1 (chemisorbed CO2 on TiO2). The
CO2 was formed from CO by reaction with a trace amountof O2 that remained in the IR cell.
When 130 Pa of O2 was introduced at 120 K, theamount of CO on TiO2 gradually decreased, whereas theamount of CO2 on TiO2 increased over a period of 20 min(Scheme 3). In clear contrast, the amount of CO on Au0
decreased very slowly: only 12 % conversion in 120 min.
CO oxidation negligibly proceeded on TiO2 free from Au,
indicating the critical role of Au sites. The reaction was
first-order, and the apparent activation energy was evaluated to be 15 kJ mol-1 ± 1 kJ mol-1 at 110–130 K. The
rate dependence on O2 pressure was 0.5th order at 120 K
and suggested that the Au/TiO2 surface was saturated at an
O2 pressure of 330 Pa.
When the CO-adsorbed Au/TiO2 was heated to 215 K,
CO on TiO2 totally desorbed, whereas half of the CO on
Au0 remained. The sample was again cooled to 120 K andinteracted with 130 Pa of O2, but was inactive toward CO
oxidation. Although the reason for the increase in intensityof the weak, broad peak at 2353 cm-1 was not mentioned[52], CO on Au should migrate to TiO2 and be oxidized toCO2. In any event, it was observed that CO on TiO2 waslikely more reactive with O2 compared to that on Au0;
however, the oxidation required the participation of Au.
In this context, gradient-corrected DFT calculations
were performed for three layers of Au nanorods over arutile-type TiO2(110) surface, and the plausible reaction at
the interface was closely evaluated. CO adsorbed onto
TiO2, and O2 was stable in a di-r configuration with bonds

Scheme 3 Proposed mechanism of low-temperature CO oxidation
over Au/TiO2 catalyst at boundary zone between Au nanoparticle andTiO2. Experiments directly observing CO/TiO2 and CO/Au surface
species showed that processes 2 and 3 were fast compared to process
4 [52]. Copyright (2011) American Association for the Advancementof Science

123

156

Y. Yoshida, Y. Izumi

to both Ti5c (five-coordination to O atoms) sites and with
Au atoms at the interface (Scheme 3). The activation
barrier for diffusion of CO to the Ti5c site in proximity to
Au (Scheme 3`) was 25 kJ mol-1 and may be rate-determining as compared to the experimental apparent activation energy of 15 kJ mol-1. At temperatures greater than
110–130 K, CO diffusion from Au to the interface with
TiO2, for which the activation barrier is 48 kJ mol-1(Scheme 3ˆ), is also plausible.
This study enabled the adsorption and diffusion of CO
on TiO2 surface at low temperature (120 K) and the reaction with O2 at the interface sites with Au nanoparticle. The
proposed mechanism in most studies was confirmed at
higher than 120 K.
The O2 activation site at the interface between Au andits metal oxide support was further investigated based on
kinetic measurements utilizing isotope effects and DFT
calculations [53]. In cases in which the support material
lacked surface hydroxy groups, e.g., C and g-C3N4, thecomposites with Au were inactive [56, 57]. Furthermore, incontrast to the site-blocking effect of water on CO photooxidation catalysts (see the Sect. 2.2.2) [17] and Cu or
Co catalysts supported on reducible metal oxides (see theSects. 3.2.2, 3.2.3, 3.2.4 and Table 3), the presence ofwater enhanced the CO photooxidation rates by a factor of
6.8 [53] or 32 [58]. When the surface hydroxy group of the
Au/TiO2 catalyst was deuterated (surface O2H groups), theCO oxidation rate decreased by a factor of 0.56 comparedto that achieved using a non-deuterated Au/TiO2 catalyst
with surface O1H groups, suggesting a primary isotope
effect in which the O–1H (2H) bond cleavage was ratedetermining and the initial state zero-point energy affected
the activation efficiency.
When the Au/TiO2 catalyst was dried at 343 K for 16 h,
97 % of the area of the non-H-bonded hydroxy peak at
3741–3645 cm-1 remained, whereas the area of the water
peak in the IR spectrum substantially decreased. The COoxidation rates decreased by a factor of 0.094 as a consequence of the drying. Furthermore, the reaction order forthe moisture pressure was 0.32, which was substantiallygreater than that for CO (0.01) or O2 (0.1). The dependenceof the reaction rates on the partial pressures of O2 andmoisture was compared to the Michaelis–Menten kinetics
model [59]:
t1 ¼

Km
PO þ t1
max
tmax 2

ð19Þ

As PH2 O was increased from 1 Pa to 125 Pa, the variationof the value of Km/vmax was small whereas v-1
max substantially decreased (i.e., the reaction rate increased). Thus,
weakly-adsorbed water increased the effective number ofactive sites instead of modifying their inherent activity.
Rather than direct participation of hydroxy groups in the123

reaction mechanism, hydroxy groups were hypothesized toanchor water molecules near the Au nanoparticle andfacilitate the activation of water through H bonding. Furthermore, the reaction between O2 was stabilized by protontransfer (Au–OOH) from Au–CO to Au–O and Au–CO2H
through a low activation barrier (9.6 kJ mol-1; Fig. 7A`).
In summary, this paper successfully explained theplausible intermediates of promoted CO oxidation in thepresence of moisture. O–O bond scission was suggested by
the formation a CO–O2 complex at the interface between
Au nanoparticles and TiO2 according to the results of IR
and DFT studies. Furthermore, an H/D (2H) kinetic isotope
effect of nearly 2 suggested that the O–H(D) bond scission
step was rate-determining. The vmax value increased linearly with the amount of adsorbed water by changing thenumber of active sites and adsorbed water also stabilized
the intermediate species (Au–OOH, Au–COOH) to form
CO2.
3.1.3 Comparisons of Effects of CO2 and/or Moisture
in Reactant Gas
The T50% value is the temperature at which the CO conversion reaches 50 % and the criteria of higher CO PROX
activity as the value decreases. For practical applications,the effects of CO2 and moisture contained in real reactant
gas for CO PROX reactions are critical. The T50% values
for Au/a-Fe2O3 (310 K), CuO–CeO2 (367 K), and Pt/cAl2O3 (434 K) (Table 4A, entries l–n and Fig. 8) increased
to 350 K, 389 K, and 440 K, respectively, after the addition of 10 % of CO2 into the feed gas at a reactant flow rateof 100 mL min-1 (Table 2d–f; Fig. 8) [60]. More active
Au/a-Fe2O3 and CuO–CeO2 were dramatically affected by
the added CO2, whereas the effect of CO2 on relatively
inactive Pt/c-Al2O3 was marginal. The CO2 selectivity was
not substantially affected by the presence of CO2(Tables 2d–f, 4A-l–n) demonstrating that the mechanism
was not altered by CO2 but the frequency factor of reaction
was suppressed by CO2. At 373 K, Au/a-Fe2O3 was themost active, whereas CuO–CeO2 was the most selective to
CO2.
The T50% values for Au/a-Fe2O3 (301 K), CuO–CeO2(368 K), and Pt/c-Al2O3 (398 K) by the addition of 10 %of CO2 in the feed gas changed to 341 K, 414 K, and
377 K, respectively, by the addition of both 10 % of CO2
and 15 % of moisture in the feed gas at a flow rate of
21 mL min-1 (Tables 2g–i, 4A-o–q) [60]. A negative
effect of moisture was apparent for more active Au/aFe2O3 and CuO–CeO2, whereas promotion was observedfor Pt/c-Al2O3. Promoting effect of weakly adsorbed water
to stabilize Au–OOH and/or Au–COOH intermediate andenhance the frequency of simple CO oxidation in previous
section, but the promoting effect of moisture at much lower

Recent Advances in the Preferential Thermal-/Photo-Oxidation of Carbon Monoxide: Noble…

partial pressure of 1–700 Pa was observed for Au/TiO2 inreference 53 versus negative effect by the addition of muchhigher 10 kPa of moisture was confirmed for Au/a-Fe2O3
in reference 60. As the comparisons in reference 60 weremade below the onset temperature of the WGSR (418 K),
the promotion effect of CO oxidation by water using Pt/cAl2O3 was net and not due to the balance for rates of byreaction(s).
In summary, at practical temperature range for CO
PROX reactions (353–473 K), the addition of CO2 and/or
moisture in PROX gas of CO deactivated for both supported
noble metals (Au, Pt) and CuO–CeO2 catalysts by 6–46 K
of T50% except for the promotion of Pt/c-Al2O3 by moisture(-21 K of T50%). The Pt catalyst was quite inactive compared to the other two catalysts, marginal step at very low
temperature for Pt was improved and the mechanism would
be unimportant for highly active catalysts.
3.1.4 Photoassisted PROX Using Au Catalysts
Photocatalysis involving CO as product using noble metals
supported on TiO2 and other metal oxides is widely used
for, e.g., the photoconversion of CO2 into fuels [3, 13, 61].
The conversion of CO2 is endothermic [62], and one
plausible CO2 photoconversion reaction is the reverse
WGSR (reverse reaction of Eq. 1):
H2 þ CO2  H2 O þ CO:ð10 Þ

By contrast, CO PROX reactions are exothermic and often
proceed at 293–323 K in the presence of noble metals,
even without irradiation by light; indeed, the contributionof light energy for these noble metal catalysts is not always
clearly discussed in literature. Such cases are referred to as‘‘photoassisted PROX’’ and we try to separate the contributions of thermal and photocatalysis in this section.
Photoassisted PROX using Au/TiO2 and promoted byirradiation has been reported. Under a flow of 0.5 % of
CO ? 0.5 % of O2 ? 85 % of H2 ? balance gas and in
the absence of light, the CO PROX rate was quite high(4800 lmol h-1 g-1
cat ; Table 4A, entry r) using Au/TiO2
catalyst. Under irradiation by UV light at 323 K, the rateincreased by a factor of 1.04, although the PROX selectivity was low (36–38 %) at a reactant flow rate of
100 mL min-1 in both the presence and absence of light(Table 4A, entries r and s) [63]. Based on CO PROX
reaction mechanism proposed in Sect. 3.1.2 [52, 53], theelectrons separated from holes in TiO2 irradiated by UVlight would diffuse to Au and then to O2 adsorbed at theinterface between Au and TiO2 [41, 43]. Hot electrons
induced by surface plasmon resonance (SPR) [64] on Au
nanoparticles irradiated by visible light would also diffuse
to the O2 [41, 43]. However, the promotion of electron
injection into Au then to O2 by the effect of light was157

marginal (4 %) in the case of Au/TiO2 catalyst because thefree electrons of Au would be sufficient at the interface
between Au nanoparticles and TiO2. A lower reaction
temperature and/or lower flow rate was necessary toachieve better PROX selectivity.
In the case of an Au/TiO2 catalyst assembled withpolyaniline with the aim to respond to visible light, the CO
PROX rate increased by a factor of 2.1 (770 lmol h-1
g-1
cat ) under visible-light irradiation compared to that without polyaniline (Table 4A, entries u and w) [65]. The
electron transfer initially excited in polyaniline by visible
light toward TiO2 should be the reason for the observed
increase. The superior performance of polyaniline-assembled Au/TiO2 catalyst (690 lmol h-1 g-1
cat ) to Au/TiO2
under dark conditions (260 lmol h-1 g-1
cat ; Table 4A,
entries t and v) is attributable to charge transfer from thepolyaniline to the TiO2 rather than to charge separation by
light.
The PROX activity of Au/TiO2 modified with CuO wasalso tested in the presence/absence of visible light [66].
Because of the effects of CuO, which are likely electronic
effects, the rate achieved using Au-CuO/TiO2(820 lmol h-1 g-1
cat ) under dark conditions increased by afactor of 3.2 compared to that observed using Au/TiO2(260 lmol h-1 g-1
cat ) under the same conditions (Table 4A,
entries t and x). Both catalytic reactions were promoted to asimilar extent by irradiation with visible light: the rate in
the case of Au–CuO/TiO2 increased by a factor of 1.22
times (1000 lmol h-1 g-1
cat ), whereas that in the case of Au/
TiO2 increased by a factor of 1.38 (360 lmol h-1 g-1
cat ;Table 4A, entries t, u, x, and y). The photoassist mechanism is essentially the same as that for reference 63.
Separated electrons in TiO2 by UV light and hot electrons
by SPR at Au and/or CuO by visible light would transfer toO2 adsorbed at the interface between TiO2 and Au/CuO[41, 43] and accounted for 22–38 % increase of the CO
PROX rate.
The CO PROX performance in the case of Au/TiO2
was compared to that using Au/Al2O3 under visible-lightirradiation and otherwise similar reaction conditions [67].
The CO conversion using Au/TiO2 increased from 30 %in the dark to 37 % under irradiation (Table 4A, entries z
and a) similar to the mechanism for references 63 and 66.
In contrast, the CO conversion using Au/Al2O3 was substantially lower (24–25 %), and the effects of visible-lightirradiation were not observed because Al2O3 is an insulator and cannot induce charge separation (Table 4A,
entries b and c). For both catalysts, the CO PROX
selectivity negligibly changed under visible-light irradiation (from 67 to 68 % and from 89 to 93 %, respectively)
suggesting electron transfers to adsorbed O2 both thermal
and photocatalytic mechanism led to common reaction
paths.

123

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Y. Yoshida, Y. Izumi

Fig. 7 Proposed reaction mechanism of CO oxidation. Larger whiteball Ti atom, red/darker ball O atom, gray/darkest ball C atom, andsmaller white ball H atom. a Potential energy diagram. Bothpathways i, ii proceed through decomposition of COOH intermediate

and reaction between CO and OH (or O) intermediates. b Schematicof lower pathway i (green/thinner line) in a [53]. Copyright (2014)
American Association for the Advancement of Science (Color
figure online)

In summary, Au-supported photocatalysts exhibited CO
PROX at rates of 270–5000 lmol h-1 g-1
cat , and compara-1 -1
ble rates of 260–4800 lmol h gcat were achieved even in
the absence of light. Electron transfer via thermal process
to O2 at the interface between Au and TiO2 was dominant
and the photoassist via the transfer of photodissociatedelectrons in TiO2 to Au then to O2 at interface was marginal. The CO2 selectivity was generally low but ranged
from 36 to 93 %.

activate H2 in CO PROX reaction tests. The presence ofwater and CO2 in the feed gas retarded the PROX reaction
substantially [8].
Optimization of the performance of Ce-based catalysts
was already reported to explain why CuO–CeO2 catalysts
have been most intensively studied. The catalytic performance for CO PROX conversion and selectivity using
inexpensive transition metals—Cr, Co, Ni, Cu, or Zn—on
Ce0.63Zr0.37O2 decreased in the order [9, 68]
Cu [ Ni [ Co [ Zn [ Cr:ð21Þ

3.2 Thermal PROX Using Inexpensive Elements

In the case of Cu, the performance depending on support
material decreased in the order [9, 68]

3.2.1 Ce-Based Catalysts

CeO2 [ La2 O3 [ MgO [ SiO2 Al2 O3 :CeO2-based PROX catalysts of CO are known as some of thebest-performing PROX catalysts composed without noble
metal although Ce is a rare earth metal and not inexpensive;
they have been briefly reviewed and CuO–CeO2 is one of themost studied catalyst [8, 9, 11]. Furthermore, when incorporated as dopants, Fe, Ni, and Sn acted as promoters,
increasing the activity and selectivity of CuO–CeO2 catalysts. Pt increased the activity, but not the selectivity of CuO–CeO2 catalysts. The incorporation of 10 mol % of ZrO2 also
increased the activity of CuO–CeO2 catalysts [11]. Efficient
mixing of Cu2? and Ce4? sites, which enabled effective
utilization of the redox cycle, was established as the origin of
the high PROX activity toward CO.

The active sites of CuO–CeO2 catalysts were the wellmixed Cu and Ce oxides, ideally mixed on the atomic level
to enable Eq. 20, and excessive Cu in the form of segregated CuO species led to the inactivation [9].
In summary, CuO–CeO2 is systematically the best catalyst for CO PROX reactions owing to the redox of Eq. 20
and polar nature of metal oxide surface to activate CO andO2 rather than H2, free from expensive elements such as
Ru, Rh, Pt, and Au that would activate H2 above moderate
reaction temperature (*350 K).

CO þ Cu2þ OCe4þ ! CO2 þ Cuþ ½O vacancyCe3þ

Kinetics studies have revealed that CO oxidation proceeds
on CuO–CeO2 via the Mars van Krevelen mechanism (seethe Graphical abstract). It is specific mechanism to utilize
vacancy sites and worthy to be reviewed in relation to CO
PROX activity and selectivity. This mechanism includes a

ð20Þ
As described above for the PROX selectivity, metal oxides,
e.g., CuO and CeO2, are superior to noble metals not to

123

ð22Þ

3.2.2 Reaction Mechanism over CuO–CeO2

Recent Advances in the Preferential Thermal-/Photo-Oxidation of Carbon Monoxide: Noble…

159

Scheme 4 Proposed reaction mechanism of CO oxidation over CuO–CeO2 catalyst [72]. Copyright (2015) The Royal Society ofChemistryandO2 þ 2ðvacant latticeÞ  2ðlattice OÞ:

Fig. 8 Changes in CO and O2 conversion and in CO PROX
selectivity versus temperature at (catalyst weight)/(flowrate) = 0.030 g s mL-1 over Au/a-Fe2O3 (triangle), CuO–CeO2(circle), and Pt/c-Al2O3 (square) catalysts in absence of CO2 (filled
symbols) and in presence of 15 % CO2 in reactant feed (opensymbols) [60]. Copyright (2002) Elsevier

reaction step in which an adsorbed species reacts with lattice
O (Scheme 4) [69–71]: a CO molecule adsorbs onto CuO(Eq. 23) and reacts with a lattice O atom at the boundary
between CuO and CeO2 (Eq. 24), resulting in CO2 formationand leaving an O vacancy site on the CeO2 surface (Eq. 24).
An O2 molecule then fills the O vacancy (Eq. 25):
CO þ Cu  CuCO;

ð23Þ

CuCO þ ðlattice OÞ  Cu þ CO2 þ ðvacant latticeÞ;
ð24Þ

ð25Þ

The formation of O vacancies involves the reductions ofCeIV to CeIII and of CuII to CuI (Eq. 20 and Scheme 4), as
confirmed by X-ray photoelectron spectroscopy (XPS)[70, 72]. With respect to the reduction, the correlation
between the peak intensity of CuI–CO in diffuse-reflectance infrared Fourier transform (DRIFT) spectra and
CO PROX activity was confirmed. CO adsorbed onto CuI
sites, whereas H2 preferably adsorbed onto metallic Cu
sites. Therefore, the oxidation sites for H2 and CO wereindependent and enabled high CO PROX selectivity if theCu sites were kept as oxidized [71].
As described in Sect. 3.2.1, the reducibility of CuII to
CuI and of CeIV to CeIII was essential for CO PROX using
CuO–CeO2 catalysts [73–76]. The specific interface sites
between CuO and CeO2 were investigated by H2 temperature-programmed reaction (H2-TPR), CO-TPR, and temperature-programmed oxidation [74]. Reduction of CuO–CeO2 by H2 occurred at 433 K, 380 K lower than the H2
reduction temperature of CeO2 (813 K). The CuO sites at
the CuO–CeO2 interface were reduced by CO and reoxidized by O2 at 290–423 K in consistent with Scheme 4. At
temperatures above 423 K, CuII both at the interface sites
and at other sites began to be deeply reduced (namely toCu0) by H2. The deeply-reduced Cu sites were reoxidized
by O2; however, the catalysts exhibited only low CO
PROX selectivity.
The correlation between Cu valence states and CO
PROX performance was investigated using CuO–CeO2 byin situ XANES [73]. At temperatures below 390 K, the COoxidation reaction was predominant and the Cu oxidation
state was II. In the temperature range from 390 to 470 K,
H2 began to be oxidized, coupled with the partial reductionof CuII to CuI. At temperature above 470 K, H2 oxidation
became predominant and inactive Cu2O and metallic Cu
particles accumulated. Thus, redox between CuII and CuI
was suggested to participate in CO oxidation (Eq. 24) via
the Mars van Krevelen mechanism.

123

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Y. Yoshida, Y. Izumi

The preferable population of CuI sites on each crystal
face of CeO2 was investigated by first-principles calculations. The most stable and preferentially exposed crystalline face of CeO2 is its nonpolar {111} face, whereas its
polar {001} face is the least stable, as indicated by theorder of increasing surface energy density:
CeO2 f111gð0:69 J m2 Þ\CeO2 f110gð1:04 J m2 Þ\CeO2 f001gð1:41 J m2 Þ:

ð26Þ

The formation energies (enthalpy changes) of O vacancies
over each CeO2 crystalline face were also evaluated by
DFT calculations [77]:CeO2 f110gð1:99 eVÞ\CeO2 f001gð2:27 eVÞ\CeO2 f111gð2:60 eVÞ:ð27ÞThe order of the possibility of O vacancy formation (it ismost likely on CeO2{110}) in Eq. 27 correlates with thedensity of CuI sites if the CeO2 is combined with Cu
species. In the cases of nanospheres, nanorods, and nanocubes of CeO2, stable {111}, relatively unstable {110} andunstable {001}, and unstable {001} faces were exposed,
respectively [78–80]. DFT calculations revealed the orderof the CuI site density on each nano-CeO2:
nanorod(f 110g ,f 001g )[ nanocube(f 001g ) [ nanosphere(f 111g );ð28Þ

in accordance with the order in Eq. 27. In fact, CuO–CeO2
nanorods exhibited higher CO conversion by a factor of 7.3
times compared to CuO–CeO2 nanospheres in CO oxidation at 343 K (Table 4B, entries g and h) [78].
The activities of CO PROX were compared using CuO–CeO2 catalysts based on CeO2 nanospheres, nanorods, andnanocubes [79, 80]. The CO conversion decreased in theorder
CuOCeO2 nanorods [ CuOCeO2 nanospheres[ CuOCeO2 nanocubes

ð29Þ

at temperatures below 453 K (Table 4B, entries i–k),nearly in agreement with the trend in Eq. 28 [75].
The exchanges of multiple valence states of Cu and Ce
in CuO–CeO2 nanorods and their participation in CO
PROX were investigated by XAFS, X-ray diffraction(XRD), and DRIFT under CO and O2 [79]. At temperatures
below 373 K, CuI species were easily formed under CO by
the effects of CeO2, as observed by DRIFT. At temperatures above 413 K, further reduction of CuI to Cu0 was
confirmed by XANES and XRD. The dispersed CuI andCu0 species were quickly reoxidized in O2 at 393 K. The
reduction of CuI to Cu0 under CO was slower than the
reduction of CuII to CuI under CO, whereas the reoxidationof CuI and Cu0 to CuII was sufficiently fast not to be therate-determining step. Thus, the CuII/0 redox process was123

inert toward CO oxidation, whereas the CuII/I redox participated in the CO PROX cycle in consistent with reference 73.
However, the effects of Cu valence states are in controversy on CO PROX reactions. CO PROX tests were
performed using CuOx nanoparticles highly dispersed overCeO2 nanorods prepared by a deposition–precipitation
method [76]. An interatomic pair of Cu(–O–)Ce should beobserved if the majority of Cu sites interact with Ce oxides
to constitute a plausible interfacial active site for CO
PROX. The peak was clearly observed in the Fouriertransform of EXAFS spectra for CuOx–CeO2 nanorods in
O2 at 573 K. The peak negligibly varied under CO PROX
temperatures as high as 473 K. Conversely, the peak disappeared if the sample was treated in H2 at 573 K. Instead,an interatomic pair peak due to Cu–Cu emerged, indicating
reduction of CuII to Cu0 and deactivation of the catalyst.
Unexpectedly, the activity and selectivity of the CO PROX
reactions were improved by the H2 pretreatment. Such Cu0
sites were easily reoxidized to CuII under CO PROX
reaction conditions; however, the peak due to the Cu(–O–)Ce pair never recovered. Therefore, highly-dispersed
CuOx species were suggested to be alternative active sites
for CO PROX rather than Cu–O–Ce interface sites [76].
Ultimately, O vacancy at the interface between Cu andCeO2 and that on the highly dispersed CuOx may work
similarly based on the formation energy of O vacancy(Eq. 26).
TPR and DRIFT studies revealed that the reducibility of
CuO plays important roles in CO and H2 oxidation processes. The CuI–CO sites near the boundary between CuO
and CeO2 were believed to be the active sites for CO
oxidation, and the CuO sites reduced apart from CeO2 were
believed to act as H2 oxidation sites; however, thewavenumber of CuI–CO rather than the amount correlatedwith the catalytic activity when CeO2 nanorods, nanospheres, and nanocubes were used as supports for CuO. In
the DRIFT spectra, the CuI–CO peak intensity for CuO–CeO2 nanocubes was the strongest at 2115 cm-1 and2099 cm-1 and the peaks were red-shifted. The red shift of
the Cu I–CO peak indicated that the CO on the CuO–CeO2
nanocubes was thermally stable and that it was the cause of
the lowest activity of this catalyst at lower temperatures
among the three studied. By contrast, CuO–CeO2 nanocubes exhibited the highest activity and selectivity attemperatures above 453 K. DFT calculations revealed thatthe adhesion energy of CuO over the preferred {001} face
for CeO2 nanocubes was 1.5 times greater than that overthe {111} face. The strong interaction between CuO andthe CeO2 {001} face was suggested to maintain high CuO
dispersion and to suppress formation of Cu0, thus leading
to the highest activity and selectivity at temperatures above
453 K.

Recent Advances in the Preferential Thermal-/Photo-Oxidation of Carbon Monoxide: Noble…

In summary, the reaction mechanism over CuO–CeO2
was proposed to include the redox of interface sites with CO
and O2 (Eq. 20), whereas H2 oxidation was proposed tooccur on metallic Cu sites. The correlations between theinterface site number and the activity and also between thedensity of metal sites and the selectivity were confirmed
based on kinetics, thermal analyses, and spectroscopic
analyses. However, the dispersion of CuO and the nature of
the CeO2 surface were also critical to the CO PROX reaction.
Well-dispersed CuOx sites over CeO2 nanorods were also
reactive toward CO oxidation. Moreover, nanorod CeO2 and{110} faces placed important roles in stabilizing CuO particles over CeO2 and maintaining a good dispersion of CuO
particles under CO PROX conditions.
3.2.3 Cu-Based Catalysts Not Combined with CeO2
CuO–CeO2 is among the most investigated noble-metalfree CO PROX catalysts; however, various Cu catalysts
combined with reducible metal oxides other than CeO2
have been also reported for CO PROX reactions. Importantly, reducibility of support materials for Cu is essential
to reduce CO probably all via the Mars van Krevelen
mechanism. Practically, to choose inexpensive reducible
metal oxide is important rather than rare earth metal Ce
oxide as the support of active Cu species.
As a general trend, Cu-based catalysts exhibit highactivity utilizing the transformation between interfacial O
atom and the vacancy (Scheme 4); however, the active
temperature range (window) is relatively narrow and theselectivity is therefore not satisfactory primarily because
the effective redox is between Cu2? and Cu?, but unfavorable Cu0 is formed at higher temperature that would
proceed undesired by-reaction of H2 oxidation. The
CuMn2O4 catalyst is superior to CuO–CeO2 based on the
T50% values at 427 and 454 K, respectively (Table 4B,
entries m and n) [81]. Based on the standard reduction
potential [1.71 V for Ce4?/Ce3? and 1.51 V for Mn3?/
Mn2? vs. standard hydrogen electrode (SHE)], Ce4? andMn3? are comparable for the reducibility of neighboring O
site. CuMn2O4 is known as a mineral called Hopcalite thatremoves CO and/or N2O from dry air; it is used in air
filtration systems to purify breathing air for scuba diving
and firefighting. The reaction mechanism has been proposed as involving the reaction of CO with oxidized metal
ions and that of O2 with reduced metal ions. It is proposedthat the CO PROX reaction proceeds via the redox couplesof Cu?/Cu2? and Mn3?/Mn4? as follows [81]:CO þ Cu2þ ! OCþ Cuþ ;ð30Þ

CO þ Mn4þ ! OCþ Mn3þ ;ð31Þ

O2 þ Cu

þ2þ! O
2 Cu ;

ð32Þ4þ
O2 þ Mn3þ ! O
2 Mn ;

161

ð33Þ

and
OCþ ðreduced ionÞ þ O
2 ðoxidized ionÞ! O2 COðreduced ionÞ þ ðoxidized ionÞ! CO2 þ 2ðoxidized ionÞ;ð34Þ

but the reaction cycle would be essentially similar to theMars van Krevelen mechanism.
Cu–Mn oxides were tested for the CO PROX reaction,
as inferred from the properties of Hopcalite [82]. COconversion using Cu–Mn oxide (atomic ratio of
Cu:Mn = 1:2) was 74 % at 373 K (Table 4B, entry o),which was lower than that achieved using Au/a-Fe2O3 (97–95 %; Table 4A, entries m and p) but was comparable tothat achieved using CuO–CeO2 (63–96 %; Table 4A,
entries n and q) under similar reaction conditions (21–
100 mL min-1, 373 K). In fact, the T50% values were of
the same order as those of CO conversion:
CuOCeO2 ð345367 KÞ  CuMn(1:2) oxide (351 K)[ Au=a  Fe2 O3 ð310 KÞ:

ð35Þ

IV
Cu-Fe-K2x?yMnIIx MnIIIy Mn8-x-yO16 is based on the
porous octahedral structure of [Mn8O16]d- [83]. Fe wasincluded inadvertently, originating from the stainless steel
balls used for ball-milling during the catalyst preparation.
The CO PROX activity was higher than that of the Cu-freeIV
Fe-K2x?yMnIIx MnIIIy Mn8-x-yO16 (Table 4B, entry s), suggesting that Cu–O–Mn bridging sites play an essential
catalytic role, e.g., the redox between interface O andvacancy site (Scheme 4). The addition of CO2 and/or
moisture in the feed gas led to a decrease of the CO2
conversion by a factor of 0.52–0.59 (Table 2j), mainly as aconsequence of the blocking of CO adsorption sites and O
vacancy sites for the CO PROX reaction. However, theCO2 selectivity was not affected or was slightly improvedby the addition of CO2 and/or moisture, indicating that thereaction mechanism was not altered but the frequency was
reduced (Table 2j). The WGSR (Eq. 1) and the reverse
reaction (H2 ? CO2 ¢ H2O ? CO) would affect the
selectivity above a certain reaction temperature (*418 K).
Finally, CuO(17 wt%)–ZnO(46 wt%)–TiO2(37 wt%)
exhibited comparable CO PROX performances (CO conversion 32 % and PROX selectivity *100 %) (Table 4B,
entry t) to those using the other Cu catalysts in this section[84]. Similar to the other catalysts, in the presence of 15 %CO2 in the reactant gas, the CO conversion decreased by afactor of 0.31, whereas the PROX selectivity remained
constant at *100 % (Table 2k). In this system, TiO2
behaved as a reducible metal oxide support for CuO.
In summary, Cu catalysts, CuMn2O4, Cu–Mn (1:2)
IVoxide, and Cu-Fe-K2x?yMnIIx MnIIIy Mn8-x-yO16, exhibited
activity for CO PROX reactions via the Mars van Krevelen

123

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Y. Yoshida, Y. Izumi

mechanism similar to the activity of CuO–CeO2 at flow
rates of 50–600 mL min-1 and at 333–423 K because of
the effects of reducible Mn3? and/or Mn4? at a comparable
standard reduction potential to Ce4?.
3.2.4 Co- and Ni-Based Catalysts
Co is known as a good catalyst for CO oxidation [85] andworthy to be tested for CO PROX as a substitute for Cu or
noble metals. However, because CO methanation to consume H2
CO þ 3H2 ! CH4 þ H2 O

ð36Þwould also proceed at 473 K using Co [86], CO PROX
reactions using Co catalysts have been investigated atlower temperature than 473 K. The standard reduction
potential for Co3?/Co2? is 1.92 V @SHE and would be
advantageous compared to that for Cu2?/Cu? (0.159 V@SHE) if Co is included as Co3? in the catalysts.
A Co–Mn (atomic ratio 8:1) oxide nanosheet catalyst
was active (98 % conversion of CO) and selective (90 %)at 333 K (Table 4B, entry u) [87]. Under similar reaction
conditions but at 348 K, a similar component Co–Mn–O–
FeOx achieved *100 % conversion of CO (Table 4B,
entry v) [88]. The T50% values were less than 303 K for CoMn (8:1) oxide nanosheets and Co–Mn–O–FeOx(Table 4B, entries t–v), suggesting excellent performancecompared to noble-metal catalysts (T50%: 303–444 K)
under similar reaction conditions (Table 4A, entries l–q).
The Co3O4–CexZr1-xO2 catalyst was also active; however,
the T50% value (383 K) was apparently higher than that for
the Co–Mn oxide nanosheet or Co–Mn–O–FeOx [89]. As
the drawback, the poisoning effects of CO2 addition and/or
moisture addition were serious, especially with respect to
CO conversion (*0–2.4 %, compared to 36 % when CO2
and moisture were not included in the reactant using the
Co3O4–CexZr1-xO2 catalyst (Table 2l). A mixed spinel
phase was found in the Mn-Co-Ce oxide, and it exhibited
higher CO conversion (41 %) at 373 K compared to Mn–
CeO2 or Co–CeO2 (Table 4B, entries x–z) [90]. The Mn–
Co–Ce oxide was quite selective for CO PROX (*100 %)even in the presence of CO2 (Table 2m). The mixed spinel
phase enabled crystalline level of atomic dispersion of Co–
O–Mn and Co–O–Ce rather than the mixing as the interface between nanoparticles and surface for supported
catalysts.
Based on the same strategy as mixed spinel to maximize
the metal–O–(another metal) connection as crystallines, Co
and Cu ions were mixed in the octahedral molecular sieve(OMS) during the synthesis of OMS starting from KMnO4
and MnSO4 [91]. Their CO PROX activities and selectivities were quite high (Table 4B, entries a and b). Based on
XPS and titration experiments performed on Mn ions, the123formation of Co2?–O2-–Mn4? bridging sites was proposed, in which Co2?/? and Mn4?/3? redox would oxidize
O species and reduce O2, respectively, and the charge
would be transferred between the two metal ions. Furthermore, preliminary water tolerance was also demonstrated for Cu-OMS. The OMS crystalline structure may
reduce the blocking effect of water.
Co3O4 has been supported on metal wire mesh [86].
Higher dispersion and higher conversion of CO PROX
have been reported for the dispersion on metal wire
mesh in comparison to Co3O4–SiO2, Co3O4–Al2O3,
Co3O4–TiO2, Co3O4–ZrO2, Co3O4–CeO2, and Co3O4–
SrCO3. However, the low amount of catalyst charged(30 mg, Table 4B, entry c) is likely the reason for thehigher conversion. CoOx–ZrO2 was active at lower
temperatures among the Co-based catalysts, and the T50%
values were 356–368 K (Table 4B, entries d and e) [92].
TheCOconversion increased
from
53 to
81 lmol h-1 g-1
as
the
flow
rate
of
reactantdecreasedcatfrom 50 to 25 mL min-1, whereas the CO PROX
selectivity did not change substantially (87–91 %). As
most of the Co-catalysts in this paragraph are not supported on reducible oxides, Co–O–Co sites in well dispersed CoOx may work as the CO oxidation sites similar
to the proposed for CuOx introduced in Sect. 3.2.2 [76].
Co–SrCO3 exhibited poorer CO PROX activity, but the
CO conversion reached 96 % at 513 K (Table 4B, entryf) [93]. The addition of CO2 or both CO2 and moisture
reduced the CO conversion (to 38 and 27 %, respectively) compared to 96 % in the absence of CO2 andmoisture; however, the CO PROX selectivity did not
substantially change (68–72 %; Table 2n). The additionof 0.3 mol % of Bi alleviated the negative effect of CO2
and moisture (63 % CO conversion; Table 2o).
In comparison to Cu and Co, Ni was coupled with Fe
oxide as NiFe2O4; however, it was inactive even at 473 K(CO conversion 9.1 %, Table 4B, entry g) [94]. Although
Ni surface sometimes behaves similar to Co or Cu surface
in heterogeneous catalysis, Ni2? is difficult to be oneelectron reduced and the standard reduction potential for
Ni2?/Ni0 is quite negative: -0.257 V @SHE and would
not accept electron from neighboring O atom to make O
vacancy (Scheme 4).
In summary, when Co was combined with reducible
metal oxides, e.g., Mn, Fe, and Ce oxides or ZrO2, the CO
PROX conversion was 2–100 % with a selectivity of
81–100 % at reactant flow rates between 25 mL min-1 and
250 mL min-1 at 333–373 K. These results demonstratethat Co exhibits CO PROX activity similar to that of Cu in
the expectation of practical application of CO PROX. Co3?
would be favorable similar to Cu2? based on the standard
reduction potential for oxidation capacity, however Ni2?
should be disadvantageous.

Recent Advances in the Preferential Thermal-/Photo-Oxidation of Carbon Monoxide: Noble…

3.2.5 Binary Metal–Organic Framework (MOF) Catalysts
Based on Sects. 3.2.1, 3.2.2, 3.2.3, 3.2.4, Cu or Co combined with reducible metal oxides were excellent for CO
PROX, and more importantly in the practical point ofview, mixed spinel or OMS crystalline structure seemedadvantageous to inhibit the blocking effect of CO2 [90] ormoisture [91]. In this context, MOFs are widely investigated in chemical applications, and one of the mostwidely explored area of applications is catalysis, e.g.,
acid/base catalysis [95, 96], organic reactions [97, 98],
organic reactions catalyzed by metals (Pd, Au, Ru, Cu, Pt,
Ni, and Ag) embedded in MOFs [98, 99], asymmetriccatalysis [96, 100, 101], CO oxidation [98], photocatalysis[98], and hydrodesulfurization [98]. CO PROX tests
conducted under the conditions of CO 0.51 kPa, O20.51 kPa, H2 50.1 kPa, and balance N2 gas and 100 mg ofcatalyst using a flow reaction system at 323 K (Fig. 9) inthe presence of ‘‘binary MOF’’ catalysts synthesized fromgeneral MOF crystallites with second metal salts(Scheme 5) in the expectations of effective Cu–O–Ti
connection and to mitigate the blocking effect of CO2
and/or moisture [102].
MOFs are advantageous in comparison to classic bulk
materials in the applications of sorption and catalysis
because of their high surface areas, which result from their
porous structures (Scheme 5a, b), e.g., [Ti8O8(OH)4]12?
clusters are interlinked by organic ligands (terephthalates)
to form a porous crystal structure (MIL125) [103].
Apparently, [Ti8O8(OH)4]12? clusters in MIL125 are moredispersed than any type of TiO2 bulk material. Secondary
metal Cu oxyhydroxide linkers can replace some of theorganic linkers (yellow/thinner connectors in Scheme 5) inbinary MOFs (Scheme 5c). Binary MOFs are a crystal
model of supported catalysts (Scheme 5a) because the

Scheme 5 Binary MOF concept. a Traditional metal/metal oxide
supported catalysts in which support in contact with metal/metaloxide (blue/darker spheres) is active (red/darker faces); b general
MOF in which metal ions or metal oxyhydroxy clusters (gray/thinnerfaces) are linked by organics (yellow/thinner connectors); and c new

163

interface between the primary metal ion/cluster and secondary metal represents the ultimate dispersion of interface
sites of classic supported catalysts.
With the binary MOF Cu-MIL125, which was synthesized from MIL125 and Cu nitrate aqueous solution with a
Cu/Ti atomic ratio of 1/2 and heated at 613 K, the CO
PROX rate stabilized at 3 h from the start of reaction(Fig. 9A1), and the conversion to CO2 stabilized at
67–76 % at 3–24 h (Fig. 9A2 and Table 4B, entry i).
Water was detected until 4 h from the start of the reaction,
but disappeared at 4–24 h (Fig. 9A1). Accordingly, the CO
PROX selectivity was 98–99 % at 4–24 h (Fig. 9A2 andTable 4B, entry i), consistent with the 98 % selectivity
reported for the same catalyst at a lower pressure of
6.5 kPa (Table 4B, entry l) [102]. When the CO PROX
tests were repeated without contact with air, the initialwater formation was negligible. This result suggests thatthe water detected between 0 h and 4 h in Fig. 9A1
exclusively desorbed from the catalyst and was not a catalytic product derived from O2 reduction by H2.
In comparison, in the case of CuO–CeO2 as the catalyst,
the CO oxidation rate increased proportionally until 4 hfrom the start of the test and gradually increased for 24 h(Fig. 9B1). Correspondingly, the conversion to CO2 gradually increased from 16 % at 4 h to 28 % at 24 h(Fig. 9B2; Table 4B, entry h). Conversely, the CO PROX
selectivity stabilized at 93–96 % at 4–24 h (Fig. 9B2;
Table 4B, entry h), consistent with the 90 % selectivity
obtained in the lower-pressure test of the same catalyst at
6.5 kPa (Table 4B, entry k) [93].
In summary, under the reaction conditions listed inFig. 9, the conversion to CO2 using the binary MOF CuMIL125 was higher by a factor of 4.3–2.7 than the conversion obtained using CuO–CeO2 under the same conditions, demonstrating the practical importance of the binary

design of binary MOF in which metal ions or metal oxyhydroxy
clusters (red/darker faces) are linked by both organics (yellow/thinnerconnectors) and second metal hydroxides (blue/darker connectors)(Color figure online)

123

164

Fig. 9 Time courses of PROX tests in CO (0.5 %) ? O2
(0.5 %) ? H2 (49.5 %) ? N2 (49.5 %) at flow rate of 20 mL min-1
using a Cu-MIL125 and b CuO–CeO2. Catalyst amount 100 mg,
temperature 323 K. 1 Amounts of CO2, CO, and H2O in exit gas

Scheme 6 Proposed reaction sites for CO PROX reaction using CuMIL125-I catalysts [102]. Copyright (2015) Elsevier

metal (Ti, Cu) MIL125-derived catalyst. This is a crystal
model of Cu catalyst with reducible metal oxide to adsorb
CO and O2, respectively (Scheme 6). The binary MOF is
hopeful, however, it needs further development to alleviate
the blocking effect of CO2 and/or moisture [102].

4 Concluding Remarks
In the photocatalytic CO PROX reactions in predominant
H2, CO conversions of 89–100 % were achieved using
molecular (metal complex-like) photocatalysts (Mo–SiO2
and Cr–MCM-41) and semiconductor photocatalysts (NiO/

123

Y. Yoshida, Y. Izumi

sampled (40 mL) and 2 conversion to CO2 = 100 9 [CO2]exit gas/[CO]feed gas (filled square) and CO PROX selectivity = 100 9 [CO2]exit gas/([CO2]exit gas ? [H2O]exit gas) (filled triangle) [102]. Copyright (2015) Elsevier

TiO2 and Cu–ZnO) under UV and/or visible light, withhigh PROX selectivity (97–99 %). These photocatalysts do
not activate H2 via polar bond dissociation mechanism
whereas H2 is easily activated over noble metal full of freeelectrons especially at higher temperatures than 473 K
leading to lower selectivity of CO PROX reactions. CO andO2 reacted at the active site (Mo=O) over Mo–SiO2 activated by the charge separation at the bond due to theabsorption of photons. Photo-PROX was initiated by the
charge separation also for Cu–ZnO. However, O2 was
 } rather
reduced on photoreduced Cu sites over ZnO{ 1011
 tothan on the interface sites; CO adsorbed onto ZnO(0001)
form formate species combined with hydroxy groups andwas photooxidized to CO2. When irradiated by UV orvisible light, PROX reaction rates using typical thermal
PROX catalysts (Au/TiO2 and Au–CuO/TiO2) increased bya factor of 1.03–3.2 due to the SPR effect of Au and charge
separation in TiO2 and the subsequent electron transferfrom TiO2 to Au.
The CO PROX mechanism using CuO–CeO2 is considered to proceed via the Mars van Krevelen mechanism,
in which CO reacts with lattice O at the interface between
CuO and CeO2 and then forms an O vacancy that isRecent Advances in the Preferential Thermal-/Photo-Oxidation of Carbon Monoxide: Noble…

subsequently recovered by O2. Cu was also effective when
it was supported on reducible metal oxides other thanCeO2. CuMn2O4, Cu–Mn (1:2) oxide, and Cu-Fe-K2x?yIV
MnIIx MnIIIy Mn8-x-yO16 exhibited comparable activities for
CO PROX reactions to CuO–CeO2 at flow rates of
50–600 mL min-1 and at 333–423 K because of the effects
of reducible Mn. The CO PROX activity of Cu or Co on
reducible metal oxides would be rationalized based on thestandard reduction potential for Cu2?, Co3?, Ce4?, and
Mn3?/4?.
Modeling and extraction of active sites of classicalCuO–TiO2 for CO PROX were enabled using MOF(MIL125) as a Ti oxyhydroxide precursor reacted with Cu
hydroxides. The CO PROX conversion achieved using thebinary MOF at 323 K was 4.3 times higher than that
achieved using CuO–CeO2. For the fuels in a worldwide H2society unlimited by expensive elements, metal complexlike and semiconductor-based photocatalysts and thermal
catalysts consisting of Cu or Co and reducible metal oxides
are especially promising for small-scale/personal H2
purification. However, further progress is needed to alleviate the activity loss of CO PROX in the presence ofmoisture and/or CO2. Microporous crystallines, e.g., mixed
spinel, OMS, and MOF, are also expected to exhibit sterichindrance to suppress the blocking of moisture and/or CO2.
Acknowledgments The authors are grateful for the financial support
from the Grant-in-Aid for Scientific Research C (26410204,
22550117) and Research Fellowship for Young Scientists (25489;
Y.Y.) from the Japan Society for the Promotion of Science and forapproval of the Photon Factory Proposal Review Committee for X-ray
measurements (2014G631, 2013G159, 2011G083, 2009G552,
2009G596) and the Priority Program for Disaster-Affected Quantum
Beam Facilities (2011A1978, SPring-8 & KEK). The authors would
like to thank Enago (www.enago.jp) for the English language review.

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