Direct evidence for the interfacial oxidation of CO with hydroxyls catalyzed by Pt/oxide nanocatalysts

Article abstract of DOI:10.1021/ja908081s

(Graph Presented) By rational design of the FeO(111)/Pt(111) inverse model catalyst and the control experiments, we report for the first time direct experimental evidence for the interfacial CO_ads + OH_ads reaction to produce CO₂

Full text of DOI:10.1021/ja908081s

Published on Web 10/27/2009
Direct Evidence for the Interfacial Oxidation of CO with Hydroxyls Catalyzed
by Pt/Oxide Nanocatalysts
†,‡,§
§
†,‡,§
†
,†,‡,§
Lingshun Xu,
Yunsheng Ma, Yulin Zhang,
Zhiquan Jiang, and Weixin Huang*
Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Materials for Energy
ConVersion, and Department of Chemical Physics, UniVersity of Science and Technology of China, Hefei 230026, China
Received September 22, 2009; E-mail: huangwx@ustc.edu.cn
Understanding the catalytic reaction mechanism at the molecular
level is very important for the design of an efficient catalyst but
still remains a great challenge. Oxide-supported platinum nano-
particles are among the promising commercial catalysts for the
1
-3
water-gas shift (WGS) reaction (CO + H
the preferential oxidation (PROX) of CO in excess H
2
O f CO
2
+ H
2
2
)
and
at low
4
-6
temperatures, which serve as practical strategies for reducing the
CO concentration in the hydrogen stream produced from the steam
reforming of various hydrocarbons in order to supply clean
hydrogen fuel for polymer-electrolyte-membrane fuel cells (PEM-
7
FCs). In the low-temperature WGS and PROX reactions catalyzed
by Pt/oxide nanocatalysts, CO molecules are generally believed to
chemisorb onto the Pt surface, but there is no agreement on the
oxidation mechanism of COads, a key elementary step. An associa-
tive mechanism involving the interfacial reaction of COads on Pt
with hydroxyls on the oxide surface has long been proposed to
Figure 1. (a) O 1s XPS spectra of the clean FeO(111)/Pt(111) inverse
model catalyst surface. (b) The surface exposed to 10 L of D at 120 K. (c)
The surface exposed to 10 L of D at 120 K and then flashed to 600 K.
produce CO
late in the low-temperature WGS reaction.
mechanism was also proposed to occur in the low-temperature
2
via surface intermediates such as formate or carboxy-
8
-10
Recently, a similar
(Figure S2) are located at 709.8 and 529.5 eV, respectively, also
1
3
in agreement with those of FeO.
It has been reported that CO and H2 cannot chemisorb on
the stoichiometric monolayer FeO(111) film, and therefore, selective
1
1,12
14
15
PROX reaction.
However, because of the complex nature of
heterogeneous catalytic reaction systems, unambiguous experimental
evidence for the above interfacial CO oxidation mechanism is still
lacking. Well-defined model catalysts have been proven to be very
useful in fundamental studies of heterogeneous catalytic reactions.
In this work, we fabricated monolayer FeO(111) islands dispersed
on a Pt(111) substrate as an inverse model catalyst. Surface
hydroxyls were successfully prepared on FeO(111) and found to
react readily with COads on Pt(111) to form CO at low temperatures,
2
providing for the first time direct experimental evidence for the
interfacial CO oxidation mechanism.
chemisorption of CO and D on Pt(111) was employed to titrate
2
the fraction of bare Pt(111) surface on FeO(111)/Pt(111). The CO
TDS spectrum (Figure S3A) shows that in addition to the desorption
trace of CO from Pt(111), a new desorption peak appeared at ∼230
2
+
K that likely arose from CO chemisorbed on Fe exposed at the
1
6
Pt-FeO interface. Desorption of CO
onstrating that COads cannot react with oxygen in FeO(111). Only
the desorption trace of D from Dads on Pt(111) appeared in the D
TDS spectrum (Figure S3B). By comparing the TDS peak areas of
CO and D desorbed from bare Pt(111) on FeO(111)/Pt(111) with
2
was not observed, dem-
2
2
The experiments described below were carried out in an ultrahigh
2
-10
those from clean Pt(111), we estimated the fraction of bare Pt(111)
surface on the FeO(111)/Pt(111) inverse model catalyst surface to
be 32% (Figure S3C).
vacuum (UHV) chamber with a base pressure of 1-2 × 10 mbar
that was equipped with capabilities for X-ray photoelectron
spectroscopy (XPS), low-energy electron diffraction (LEED), and
differentially pumped thermal desorption spectroscopy (TDS). The
UHV chamber was also equipped with a QUAD-EV-S mini
electron-beam evaporator and an MGC75 thermal gas cracker. The
well-established receipt of the growth of an O-terminated epitaxial
We found that the exposure of gas-phase atomic deuterium (D)
at 120 K could form ODads on FeO(111) of FeO(111)/Pt(111).
Figure 1b shows the O 1s XPS spectrum after exposure to 10 L of
D at 120 K. The formation of ODads on FeO(111) is evidenced by
1
5
1
3
the O 1s component with binding energy at 531.4 eV. ODads
disappeared after the surface was flashed to 600 K. The corre-
sponding D2 TDS spectrum shows that in addition to the D2
monolayer FeO(111) film on Pt(111) was adopted to fabricate
the FeO(111)/Pt(111) inverse model catalyst by oxidizing a clean
-
6
Pt(111) surface partially covered by Fe in oxygen (PO2 ) 1 × 10
desorption trace from Pt(111) (Figure 2A), additional tiny D
desorption peaks between 450 and 600 K (Figure 2B) were
observed. Meanwhile, a D
O desorption peak was observed at ∼235
K (Figure 2C). It has been reported that OH on the monolayer
2
mbar) at 850 K. The LEED pattern of the inverse model catalyst
surface (Figure S1A in the Supporting Information) was the same
as that of a monolayer FeO(111) film epitaxially grown on
Pt(111), proving the formation of a monolayer FeO(111) structure.
The Fe 2p3/2 and O 1s binding energies of the inverse model catalyst
2
13
ads
1
5
FeO(111) film reacts to form H
we attributed the D desorption peaks between 450 and 600 K to
the ODads + ODads reaction on FeO(111) and the D O desorption
peak to the interfacial ODads + Dads reaction at the Pt-FeO interface.
The interfacial ODads + Dads reaction to produce D might also occur
2 2
and H O above 300 K. Thus,
2
2
†
Hefei National Laboratory for Physical Sciences at the Microscale.
‡
CAS Key Laboratory of Materials for Energy Conversion.
§
Department of Chemical Physics.
2
1
6366 9 J. AM. CHEM. SOC. 2009, 131, 16366–16367
10.1021/ja908081s CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
D
ads on Pt(111) result in a D
TDS spectra (Figure S4). The D
interfacial ODads + Dads reaction appears at 230 K (Figure S5).
2
desorption peak at ∼145 K in the D
2
2
O desorption peak resulting from the
The redox mechanism and the associative mechanism are two
popular mechanisms proposed for the low-temperature WGS reaction
3
catalyzed by Pt-based catalysts. COads on Pt reacts with oxygen in
17
the oxide support in the redox mechanism but with hydroxyls on
8-10
the oxide support in the associative mechanism.
Our results clearly
vindicate the associative mechanism for the low-temperature WGS
reaction. Meanwhile, the interfacial COads + OHads reaction to produce
2 a
CO at low temperatures is not affected much by the precovered D
Figure 2. (A) D
and 600 K, and (C) D
inverse model catalyst exposed to 10 L of D at 120 K. The dashed lines
indicate the baselines.
2
TDS spectra, (B) zoom-in D
2
TDS spectra between 420
on Pt(111), strongly implying the involvement of this interfacial
reaction in the low-temperature PROX reactions catalyzed by Pt/oxide
nanocatalysts. The interfacial COads + OHads reaction previously had
not been considered in the low-temperature PROX reactions catalyzed
2
O TDS spectra of Pt(111) and FeO(111)/Pt(111)
6
11
by Pt/oxide nanocatalysts until Fukuoka et al. recently attributed
the excellent performance of Pt nanoparticles in mesoporous silica in
PROX below 353 K to the attack of CO on Pt by the OH groups on
mesoporous silica. The activity of Pt/oxide nanocatalysts in the PROX
reaction at low temperatures could be enhanced by Fe or FeO
x
18-20
12
additives,
and a recent in situ DRIFT study indicated that the
enhancement effect resulted from the oxidation of CO by hydroxyls.
In summary, the results of control experiments on the FeO(111)/
Pt(111) inverse model catalyst directly prove that the interfacial
2
COads + OHads reaction to produce CO occurs facilely at the
Pt-oxide interface at low temperatures, providing deep insights
into the reaction mechanism and active site of the important low-
temperature WGS and PROX reactions catalyzed by Pt/oxide
nanocatalysts at the molecular level.
Figure 3. CO
after (a) exposure to 2 L of CO, (b) 2 L of CO followed by 10 L of D, and
0 L of D followed by 2 L of CO at 120 K.
2
TDS spectra of the FeO(111)/Pt(111) inverse model catalyst
Acknowledgment. This work was supported by the National
Natural Science Foundation of China (20503027, 20773113,
20803072), the Ministry of Science and Technology of China, the
Solar Energy Project of the Chinese Academy of Sciences, and
the MPG-CAS Partner Group Program.
1
but could not be identified in our experiments. After the TDS
experiment, the monolayer FeO(111) structure of FeO(111)/Pt(111)
remained but was less ordered than the original one (Figure S1B).
The successful preparation of ODads on FeO(111) enabled us to
further investigate the interfacial COads + ODads reaction on the
FeO(111)/Pt(111) inverse model catalyst. The FeO(111)/Pt(111)
surface was exposed to 2 L of CO followed by 10 L of D at 120
Supporting Information Available: Figures S1-S6. This material
is available free of charge via the Internet at http://pubs.acs.org.
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JA908081S
J. AM. CHEM. SOC. 9 VOL. 131, NO. 45, 2009 16367