Oxidation of carbon monoxide on group 11 metal atoms: Matrix-isolation infrared spectroscopic and density functional theory study

Article abstract of DOI:10.1021/jp055155d

Laser-ablated Cu, Ag, and Au atoms react with CO and O₂ mixture in solid argon to produce carbonyl metal oxides, (O₂)Cu(CO) _n (n = 1, 2), (η¹-OO)MCO (M = Ag, Au), OCAuO ₂CO, and OAuCO, as well as grou

Full text of DOI:10.1021/jp055155d

J. Phys. Chem. A 2006, 110, 2655-2662
2655
Oxidation of Carbon Monoxide on Group 11 Metal Atoms: Matrix-Isolation Infrared
Spectroscopic and Density Functional Theory Study
Qiang Xu* and Ling Jiang
National Institute of AdVanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan, and
Graduate School of Science and Technology, Kobe UniVersity, Nada Ku, Kobe, Hyogo 657-8501, Japan
ReceiVed: September 12, 2005; In Final Form: December 14, 2005
Laser-ablated Cu, Ag, and Au atoms react with CO and O2 mixture in solid argon to produce carbonyl metal
oxides, (O2)Cu(CO)n (n ) 1, 2), (η1-OO)MCO (M ) Ag, Au), OCAuO2CO, and OAuCO, as well as group
11 metal carbonyls and oxides. These carbonyl metal oxides are characterized using infrared spectroscopy on
the basis of the results of the isotopic substitution and the CO concentration change. Density functional theory
(DFT) calculations have been performed on these molecules. The identifications of these carbonyl metal
oxides are confirmed by the good agreement between the experimental and calculated vibrational frequencies,
relative absorption intensities, and isotopic shifts. Carbon dioxide is eliminated from these carbonyl metal
oxides upon UV irradiation, providing the evidence for the oxidation of carbon monoxide on group 11 metal
atoms. The present experiments also reveal that the reactivity of copper toward CO is prior to O2, and the
reactivity of silver toward O2 is prior to CO, whereas the reactivity of gold toward CO is comparable to O2.
Introduction
CO2.8 Recent experimental and theoretical investigations reveal
that the WGS reaction can be promoted by preadsorbed oxygen
The oxidation of carbon monoxide is of considerable interest
from an academic and an industrial viewpoint due to its
importance in many practical applications such as combustion
exhaust, the development of CO detection devices, and improved
efficiency of CO2 lasers.1 Extensive research efforts have been
made both experimentally and theoretically to search for suitable
catalysts to enhance this oxidation process.1-7 It has been found
that gold becomes catalytically active when deposited on select
metal oxides as hemispherical ultrafine particles with diameters
smaller than 5 nm.1,2 The supported Au nanoparticles exhibit
remarkable catalytic activities and/or excellent selectivities in
a number of reactions such as low-temperature CO oxidation
and the reduction of nitrogen oxides.1,2 The reactivity of gold
cluster and/or anions toward CO and O2 has received much
attention in the recent past.1-7 The investigations of the
combustion of CO on supported, size-selected gold clusters Aun
(n e 20) have shown that the CO oxidation is size-dependent
in nature with Au8 being the smallest cluster to catalyze the
reaction.3 Mass-spectrometry-based studies of the coadsorption
of CO and O2 on small gold clusters have revealed that CO
and O2 adsorb cooperatively rather than competitively.5,6
by its acting either as a promoter or as a reaction intermediate.9
Recent studies have shown that, with the aid of isotopic
substitution, matrix-isolation infrared spectroscopy combined
with quantum chemical calculations is very powerful in
investigating the spectrum, structure, and bonding of novel
species and the related reaction mechanisms.10,11 In contrast to
the extensive experimental and theoretical studies of the
oxidation of carbon monoxide to carbon dioxide by gold clusters
or nanoparticles, however, to the best of our knowledge little
attention has been paid to systematic investigation of the
oxidation of carbon monoxide on group 11 metal atoms. Here
we report a study of the reactions of laser-ablated Cu, Ag, and
Au atoms with CO/O2 mixtures in argon matrixes. IR spectro-
scopy coupled with theoretical calculations provides evidence
for the oxidation of carbon monoxide to carbon dioxide on group
11 metal atoms.
Experimental and Theoretical Methods
The experiment for laser ablation and matrix-isolation infrared
spectroscopy is similar to those previously reported.12 Briefly,
the Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate
with 10 ns pulse width) was focused on the rotating Cu, Ag,
and Au targets. The laser-ablated metal atoms were co-deposited
with CO/O2 mixtures in excess argon onto a CsI window cooled
normally to 7 K by means of a closed-cycle helium refrigerator.
Typically, 1-15 mJ/pulse laser power was used. The CO
(99.95%), 13C16O (99%, 18O < 1%), 12C18O (99%), O2 (99.5%),
18O2 (99% 18O), scrambled O2 (16O2/16O18O/18O2 ) 1:2:1), and
mixed isotopic samples were used to prepare the CO/Ar
mixtures. In general, matrix samples were deposited for 1-2 h
with a typical rate of 2-4 mmol/h. After sample deposition,
IR spectra were recorded on a Bio-Rad FTS-6000e spectrometer
at 0.5 cm-1 resolution using a liquid nitrogen cooled HgCdTe
-
Furthermore, Au6 has been identified to be highly active for
the CO + O2 reaction.5 Interestingly, a combined photoelectron
spectroscopic and theoretical investigation of Au6(CO)n- (n )
0-3) has indicated that an electron shuttling from the outer
triangle to the inner triangle takes place upon CO chemisorption
in the negatively charged systems and that the neutral Au6(CO)n
complexes should be reactive toward O2 due to the destabiliza-
tion of the highest occupied molecular orbital of Au6 upon CO
chemisorption.7 On the other hand, the so-called “low-temper-
ature” Cu/ZnO catalyst is widely used to catalyze the water gas
shift (WGS) reaction (CO + H2O f CO2 + H2) and metallic
Cu provides the active site for catalysis.8 In the surface redox
mechanism, CO reacts with adsorbed oxygen atom to produce
(MCT) detector for the spectral range of 5000-400 cm-1
Samples were annealed at different temperatures and subjected
.
* To whom correspondence should be addressed. E-mail:
q.xu@aist.go.jp.
10.1021/jp055155d CCC: $33.50 © 2006 American Chemical Society
Published on Web 02/04/2006

2656 J. Phys. Chem. A, Vol. 110, No. 8, 2006
Xu and Jiang
Figure 1. Infrared spectra in the 2400-1800 and 1100-1020 cm-1 regions from co-deposition of laser-ablated Cu atoms with 0.5% CO + 0.25%
O2 in Ar: (a) 1 h of sample deposition at 7 K; (b) after annealing to 25 K; (c) after annealing to 30 K; (d) after 20 min of λ > 600 nm irradiation;
(e) after 20 min of λ > 340 nm irradiation; (f) after 20 min of broad-band irradiation; (g) after annealing to 34 K.
Figure 3. Infrared spectra in the 2400-1800 cm-1 region from co-
deposition of laser-ablated Cu atoms with 0.35% 12C16O + 0.35%
13C16O + 0.25% O2 in Ar: (a) 1 h of sample deposition at 7 K; (b)
Figure 2. Infrared spectra in the 2400-1800 cm-1 region for laser-
ablated Cu atoms co-deposited with different CO and O2 concentrations
after annealing to 25 K; (c) after annealing to 30 K.
in Ar after annealing to 30 K: (a) 0.5% CO + 0.25% O2; (b) 0.5% CO
+ 0.5% O2; (c) 0.5% CO + 5.0% O2.
are illustrated in Figures 1-9, and the absorption bands in
different isotopic experiments are listed in Table 1. Absorptions
common to these experiments such as metal carbonyls, metal
to UV-vis irradiation (λ > 250 nm) using a high-pressure
mercury arc lamp (Ushio, 100 W), and more spectra were
recorded.
oxides, O3, O4-, O4+, and O6+ have been reported previously17-20
and are not listed here. The stepwise annealing and photolysis
behavior of the product absorptions is also shown in the figures
and will be discussed below. Experiments have also been done
with doping CCl4 of different concentrations serving as an
electron scavenger in solid argon. It has been found that doping
with CCl4 has no effect on these bands, suggesting that the
products are neutral.21 Meanwhile, experiments have also been
done for the co-deposition of laser-ablated group 11 metal atoms
with separate CO and O2 samples to confirm the new absorp-
tions.
Quantum chemical calculations were performed to predict
the structures and vibrational frequencies of the observed
reaction products using the Gaussian 03 program.13 The B3LYP
density functional method was used.14 The 6-311+G(d) basis
set was used for C and O atoms,15 and the Los Alamos ECP
plus DZ (LANL2DZ) was used for Cu, Ag, and Au atoms.16
Geometries were fully optimized and vibrational frequencies
were calculated with analytical second derivatives. Additional
BP86 and BLYP14 calculations gave similar results and will
not be reported here.
Quantum chemical calculations have been carried out for the
possible isomers and electronic states of the potential product
molecules. Figure 10 shows the most stable structures of the
reaction products. The ground electronic states, point groups,
vibrational frequencies, and intensities are listed in Table 2.
Table 3 reports a comparison of the observed and calculated
isotopic frequency ratios for the C-O and O-O stretching
modes of the reaction products.
Results and Discussion
Experiments have been done with CO and O2 concentrations
ranging from 0.05% to 5.0% in excess argon. Typical infrared
spectra for the reactions of laser-ablated Cu, Ag, and Au atoms
with CO/O2 mixtures in excess argon in the selected regions

Oxidation of CO on Group 11 Metal Atoms
J. Phys. Chem. A, Vol. 110, No. 8, 2006 2657
The bands at 2174.7, 2114.2, and 1073.1 cm-1 have been
observed together after sample annealing. As shown in Figure
2, the experimental condition with lower CO/O2 ratio favors
the formation of (O2)CuCO with the 2128.3 cm-1 band (trace
c), whereas that of higher CO/O2 ratio (trace a) favors the
formation of the species with the 2114.2 cm-1 band, suggesting
that the species with the 2114.2 cm-1 band carries more CO
ligand than the (O2)CuCO species (2128.3 cm-1). The mixed
isotopic pattern (Figures 3 and 4) show that the absorptions at
2174.7, 2114.2, and 1073.1 cm-1 can be assigned to the
symmetric C-O, asymmetric C-O, and O-O stretching
vibrations of (O2)Cu(CO)2, respectively. It is noted that the
reaction of Cu with CO/O2 mixture is quite different from the
case of Cu reaction with CO2 in solid matrixes, which gave the
neutral CuCO2 complex22a and the insertion molecular anion
- 22b
OCuCO- and addition anion CuCO2
.
The density functional theory (DFT) calculations lend strong
support for the assignments. (O2)CuCO is predicted to have a
C2V symmetry (Figure 10) with a A2 ground state (Table 2),
2
Figure 4. Infrared spectra in the 1100-960 cm-1 region from co-
deposition of laser-ablated Cu atoms with isotopic CO/O2 mixtures in
Ar after annealing to 30 K: (a) 0.5% CO + 0.25% O2; (b) 0.5% CO
+ 0.2% 16O2 + 0.2% 18O2; (c) 0.5% CO + 0.2% 16O2 + 0.4% 16O18O
+ 0.2% 18O2; (d) 0.5% CO + 0.25% 18O2.
which lies 58.32 kcal/mol lower than the quartet one. The
calculated C-O and O-O stretching frequencies of the (O2)-
CuCO species are 2190.3 and 1134.1 cm-1 (Table 2), which
should be multiplied by 0.972 and 0.931 to fit the observed
frequencies, respectively. The calculated 12C16O/13C16O and
12C16O/12C18O isotopic frequency ratios of 1.0237 and 1.0234
(Table 3) are consistent with the experimental values, 1.0239
and 1.0228, respectively. The calculated 16O/18O isotopic O-O
stretching frequency ratio (1.0607) is also in accord with the
experiment (1.0593) (Table 3). Similarly, the good agreement
between the experimental and calculated vibrational frequencies,
relative absorption intensities, and isotopic shifts confirms the
identification of the (O2)Cu(CO)2 molecule (Tables 2 and 3,
Cu + CO + O2. New bands at 2128.3 and 1055.3 cm-1
(Table 1 and Figure 1) appeared together upon sample annealing.
They almost did not change after λ > 600 nm photolysis, but
sharply decreased on λ > 340 nm photolysis, disappeared after
broad-band irradiation, and did not recover upon further
annealing. For the 2128.3 cm-1 band, the mixed 12C16O +
13C16O + O2 isotopic spectra (Figure 3) only provide the sum
of pure isotopic bands, indicating that only one CO subunit is
involved in this mode. The lower mode (1055.3 cm-1) shifted
to 996.2 cm-1 with 18O2 and gave the 16O/18O isotopic ratio of
1.0593. The scrambled CO + 16O2 + 16O18O + 18O2 sample
(Figure 4, trace c) presented the oxygen isotopic triplet pattern
2
Figure 10). Briefly, (O2)Cu(CO)2 is predicted to have a A2
ground state with a C2V symmetry and CCuC of 132.3°, which
lies 23.02 kcal/mol lower than the quartet one.
with 1:2:1 intensity profile at 1055.3, 1025.9, and 996.2 cm-1
,
Interestingly, weak CO2 bands site split at 2344.8, 2339.0
cm-1 and 663.5, 661.9 cm-1 have been observed after sample
deposition, as shown in Figure 1. Annealing to 30 K and λ >
600 nm irradiation did not enhance these bands. However, the
CO2 bands sharply increased after λ > 340 nm irradiation and
visibly increased upon broad-band irradiation at the expense of
the (O2)CuCO and (O2)Cu(CO)2 species, suggesting that these
carbonyl copper oxides might act as the precursors for the
showing the intermediate isotopic component very near the
average of the pure isotopic bands, which suggests that two
equivalent oxygen atoms are involved in this vibration. Fur-
thermore, no intermediate isotopic band was observed in the
CO + 16O2 + 18O2 experiment (Figure 4, trace b). Accordingly,
the absorptions at 2128.3 and 1055.3 cm-1 are assigned to the
C-O and O-O stretching modes of (O2)CuCO, respectively.
Figure 5. Infrared spectra in the 2400-1800 and 1100-1000 cm-1 regions from co-deposition of laser-ablated Ag atoms with 0.25% CO + 0.5%
O2 in Ar: (a) 1 h of sample deposition at 7 K; (b) after annealing to 25 K; (c) after annealing to 30 K; (d) after 20 min of λ > 340 nm irradiation;
(e) after 20 min of broad-band irradiation; (f) after annealing to 34 K.

2658 J. Phys. Chem. A, Vol. 110, No. 8, 2006
Xu and Jiang
Figure 6. Infrared spectra in the 2400-1800 and 1150-800 cm-1 regions from co-deposition of laser-ablated Au atoms with 0.5% CO + 5.0%
O2 in Ar: (a) 1 h of sample deposition at 7 K; (b) after annealing to 25 K; (c) after annealing to 30 K; (d) after 20 min of λ > 340 nm irradiation;
(e) after 20 min of broad-band irradiation; (f) after annealing to 34 K.
Figure 8. Infrared spectra in the 2400-1800 cm-1 region from co-
deposition of laser-ablated Au atoms with 0.35% 12C16O + 0.35%
13C16O + 5.0% O2 in Ar: (a) 1 h of sample deposition at 7 K; (b) after
Figure 7. Infrared spectra in the 2400-1800 cm-1 region from co-
annealing to 25 K; (c) after annealing to 30 K; (d) after 20 min of λ >
deposition of laser-ablated Au atoms with 5.0% CO + 0.5% O2 in Ar:
340 nm irradiation; (e) after 20 min of broad-band irradiation; (f) after
(a) 1 h of sample deposition at 7 K; (b) after annealing to 25 K; and
annealing to 34 K.
(c) after annealing to 30 K.
pure isotopic bands. It can be inferred that the absorption at
2166.4 cm-1 belongs to the matrix trapping site of the 2155.7
cm-1 band on the basis of the similar growth/decay character-
istics as a function of changes of experimental conditions.
Accordingly, the 2166.4, 2155.7, and 1058.7 cm-1 bands should
be assigned to the C-O and O-O stretching modes of (η1-
OO)AgCO in different matrix sites, respectively, which is
oxidation of CO to CO2. The Cu(CO)n (n ) 1-3) molecules
survived during the UV irradiation and annealing.
Ag + CO + O2. Previous work has shown the formation of
(OC)Ag+,O2- 23 In this work, more detailed investigations with
.
complete isotopic substitution have been performed. Figure 5
illustrates infrared spectra for the reaction of laser-ablated Ag
atoms with CO/O2 mixture in excess argon after deposition,
ultraviolet photolysis, and annealing. The absorptions at 2166.4,
2155.7, and 1058.7 cm-1 have been observed upon sample
annealing. These bands decreased together after λ > 340 nm
and broad-band photolysis, and slightly recovered after further
annealing. For the C-O stretching mode, the mixed 12C16O +
13C16O + O2 and 12C16O + 12C18O + O2 isotopic spectra (not
shown here) only provide the sum of pure isotopic bands,
indicating that only one CO subunit is involved in this mode.
The scrambled CO + 16O2 + 16O18O + 18O2 sample gave the
oxygen isotopic quartet pattern with 1:1:1:1 intensity profile at
1058.7, 1029.3, 1027.4, and 998.9 cm-1, which suggests that
two inequivalent oxygen atoms are involved. Additionally, the
CO + 16O2 + 18O2 experiment only provided the sum of the
comparable with the absorptions at 2165 and 1110 cm-1 of the
- 23
previously reported (OC)Ag+,O2
.
The present DFT calculations predict that (η1-OO)AgCO has
a 2A′′ ground state with a Cs symmetry and OAgC of 177.3°
(Table 2, Figure 10), which lies 12.30 kcal/mol lower than the
quartet one. The calculated 12C16O/13C16O and 12C16O/12C18O
isotopic frequency ratios of 1.0231 and 1.0243 (Table 3) are
again in good agreement with the experimental values, 1.0225
and 1.0228, respectively. Also, the calculated 16O/18O isotopic
O-O stretching frequency ratio (1.0607) is consistent with the
experimental value (1.0597) (Table 3). Our DFT calculations
lead to the prediction that the oxygen isotopic quartet pattern
(i.e., CO + 16O2 + 16O18O + 18O2 spectra) has the approximate

Oxidation of CO on Group 11 Metal Atoms
J. Phys. Chem. A, Vol. 110, No. 8, 2006 2659
TABLE 1: IR Absorptions (in cm-1) Observed from Co-deposition of Laser-Ablated Cu, Ag, and Au Atoms with CO and O2
Mixtures in Excess Argon at 7 K
CO + O2
13CO + O2
C18O + O2
CO +18 O2
R(12CO/13CO)
R(C16O/C18O)
R(16O/18O)
assignment
Cu
2128.3
1055.3
2174.7
2114.2
1073.1
2078.6
1055.3
2126.3
2067.1
1073.1
2080.8
1055.2
2125.5
2064.8
1073.1
2128.1
996.2
2174.6
2114.1
1012.7
1.0239
1.0000
1.0228
1.0228
1.0000
1.0228
1.0001
1.0232
1.0239
1.0000
1.0001
1.0593
1.0000
1.0000
1.0596
(O2)CuCO
(O2)CuCO
(O2)Cu(CO)2
(O2)Cu(CO)2
(O2)Cu(CO)2
Ag
Au
2166.4
2155.7
1058.7
2118.7
2108.2
1058.7
2117.9
2107.6
1058.6
2166.4
2155.7
998.9
1.0225
1.0225
1.0000
1.0229
1.0228
1.0001
1.0000
1.0000
1.0597
site
(η1-OO)AgCO
(η1-OO)AgCO
2166.4
1126.8
2176.3
1825.6
1817.6
842.0
2116.1
1124.5
2128.0
1784.5
1776.6
825.4
2119.2
1126.0
2127.3
1786.1
1778.1
833.5
2165.8
1065.7
2176.3
1825.1
1817.1
829.3
1.0238
1.0020
1.0227
1.0230
1.0231
1.0201
1.0236
1.0223
1.0007
1.0230
1.0221
1.0222
1.0102
1.0220
1.0003
1.0573
1.0000
1.0003
1.0003
1.0153
1.0000
(η1-OO)AuCO
(η1-OO)AuCO
OCAuO2CO
OCAuO2CO
site
OCAuO2CO
OAuCO
2118.1
2069.2
2072.6
2118.0
1:1:1:1 intensity profile at 1141.3, 1110.0, 1108.3, and 1076.0
cm-1, which corroborates the (η1-OO)AgCO assignment.
It can be seen from Figure 5 (traces a-c) that weak bands of
CO2 appeared after sample deposition and almost did not
increase upon annealing to 30 K. The IR absorptions of CO2
observably increased after λ > 340 nm irradiation and remark-
ably increased upon broad-band irradiation at the expense of
the (η1-OO)AgCO species and Ag(CO)n (n ) 2, 3), exhibiting
the oxidation of CO to CO2 on Ag atom. After further annealing
to 34 K, the (η1-OO)AgCO molecule slightly recovered, while
the silver oxides disappeared and the silver carbonyls did not
recover.
Au + CO + O2. Previous work has exhibited the formation
of OCAuO2CO and OAuCO.24 In this work, we have carried
out more detailed experiments with complete isotopic substitu-
tion that confirm the formation of OCAuO2CO and OAuCO
and present a new species of (η1-OO)AuCO.
Figure 6 illustrates infrared spectra for the reaction of laser-
ablated Au atoms with CO/O2 mixture in excess argon after
deposition, ultraviolet photolysis, and annealing. New absorp-
tions at 2166.4 and 1126.8 cm-1 have been observed after
sample deposition. These bands slightly increased upon sample
annealing, almost did not change after λ > 340 nm, observably
decreased upon broad-band photolysis, and slightly recovered
after further annealing. For the C-O stretching mode, the mixed
12C16O + 13C16O + O2 isotopic spectra (Figure 8) only provide
the sum of pure isotopic bands, indicating that only one CO
subunit is involved. The CO + 16O2 + 18O2 experiment only
provided the sum of the pure isotopic bands (not shown here).
However, the scrambled CO + 16O2 + 16O18O + 18O2 sample
(Figure 9) gave the oxygen isotopic triplet pattern with 1:2:1
intensity profile at 1126.8, 1100.6, and 1065.7 cm-1, which
suggests that one OO subunit is involved in this molecule. The
attachment of OO to Au is uncertain because the intermediate
isotopic component (1100.6 cm-1) is near the strong Au(η1-
16O16O) absorption and cannot give obvious structural informa-
tion. Our DFT calculations suggest that (η1-OO)AuCO has end-
on coordination with a bent structure, which is analogous to
the (η1-OO)AgCO complex. Accordingly, the 2166.4 and 1126.8
cm-1 bands should be assigned to the C-O and O-O stretching
modes of (η1-OO)AuCO, respectively. Additionally, the absorp-
tion of another intermediate isotopic component of (η1-16O18O)-
AuCO may be overlapped by the band of Au(η1-16O16O) (Figure
9).
Figure 9. Infrared spectra in the 1150-960 cm-1 region from co-
deposition of laser-ablated Au atoms with 0.5% CO + 3.5% 16O2 +
7.0% 16O18O + 3.5% 18O2 in Ar: (a) 1 h of sample deposition at 7 K;
(b) after annealing to 25 K; (c) after annealing to 30 K; (d) after 20
min of λ > 340 nm irradiation; (e) after 20 min of broad-band
irradiation; (f) after annealing to 34 K.
Another group of absorptions at 2176.3 cm-1, 1825.6 cm-1
with a matrix trapping site at 1817.6 cm-1, and 842.0 cm-1
have also been observed after sample annealing. As shown in
Figure 7, these bands sharply increase together with the increase
of CO/O2 ratio. The mixed isotopic pattern (Figure 8) shows
that the absorptions at 2176.3, 1825.6, and 842.0 cm-1 can be
assigned to the C-O stretching vibrations of OCAuO2CO,
respectively, consistent with the previous report.24
Figure 10. Optimized structures (bond lengths in angstroms and bond
angles in degrees) of possible reaction products calculated at the
B3LYP/6-311+G(d)-LANL2DZ level.

2660 J. Phys. Chem. A, Vol. 110, No. 8, 2006
Xu and Jiang
TABLE 2: Ground Electronic States, Point Groups, Vibrational Frequencies (cm-1), and Intensities (km/mol) of Reaction
Products Calculated at the B3LYP/6-311+G(d)-LANL2DZ Level
species
electon. state point group
frequency (intensity, mode)
(O2)CuCO
2A2
C2V
C2V
2190.3 (798, A1), 1134.1 (105, A1), 205.9 (5, A1), 389.2 (2, B2), 351.1 (36, A1), 345.7 (0, B1), 193.1 (5, B2),
91.8 (10, B1), 75.5 (3, B2)
2215.0 (326, A1), 2181.0 (1058, B2), 1149.5 (202, A1), 375.1 (11, A1), 358.6 (17, B2), 351.1 (4, B1),
334.0 (0.1, A1), 317.6 (0, A2), 303.8 (14, A1), 272.8 (6, B2), 195.0 (0, B1), 104.9 (0, A2), 62.5 (3, B1),
44.3 (0.02, A1), 38.4 (3, B2)
2220.4 (683, A′), 1141.3 (210, A′), 434.9 (1, A′), 270.4 (1, A′), 255.5 (17, A′), 252.6 (1, A′′), 116.2 (28, A′),
54.3 (10, A′), 47.3 (4, A′′)
2220.4 (504, A′), 1145.7 (22, A′), 519.5 (7, A′), 421.6 (6, A′), 394.7 (6, A′′), 390.7 (5, A′), 187.3 (15, A′),
90.8 (4, A′′), 68.9 (3, A′)
2232.4 (409, A′), 1881.4 (353, A′), 1182.2 (38, A′), 841.6 (205, A′), 511.1 (38, A′), 506.6 (4, A′′), 453.5
(36, A′), 365.9 (2, A′), 341.9 (8, A′), 335.9 (2, A′′), 272.4 (3, A′), 141.4 (5, A′), 138.9 (1, A′′), 59.6 (1, A′′),
53.8 (0.4, A′)
(O2)Cu(CO)2
2A2
(η1-OO)AgCO
(η1-OO)AuCO
OCAuO2CO
2A′′
2A′′
2A′′
Cs
Cs
Cs
OAuCO
2Π
C∞V
2206.3 (603, σ), 547.2 (5, σ), 455.5 (2, π), 429.4 (10, σ), 423.2 (2, π), 109.2 (6, π), 107.7 (8, π)
TABLE 3: Comparison of Observed and Calculated IR Frequency Ratios for the Products
R(12CO/13CO) R(C16O/C18O)
R(16O/18O)
species
freq (cm-1
)
mode
obsd
calcd
obsd
calcd
obsd
calcd
(O2)CuCO
2128.3
1055.3
2174.7
2114.2
1073.1
2155.7
1058.7
2166.4
1126.8
2176.3
1825.6
842.0
νC-O
νO-O
νC-O
νC-O
νO-O
νC-O
νO-O
νC-O
νO-O
νC-O
νC-O
νC-O
νC-O
1.0239
1.0000
1.0228
1.0228
1.0000
1.0225
1.0000
1.0238
1.0020
1.0227
1.0230
1.0201
1.0236
1.0237
1.0000
1.0234
1.0231
1.0000
1.0231
1.0000
1.0238
1.0000
1.0234
1.0235
1.0237
1.0239
1.0228
1.0001
1.0232
1.0239
1.0000
1.0228
1.0001
1.0223
1.0007
1.0230
1.0221
1.0102
1.0220
1.0234
1.0000
1.0237
1.0242
1.0000
1.0243
1.0000
1.0232
1.0000
1.0238
1.0231
1.0102
1.0231
1.0001
1.0593
1.0000
1.0000
1.0596
1.0000
1.0597
1.0003
1.0573
1.0000
1.0003
1.0153
1.0000
1.0000
1.0607
1.0000
1.0000
1.0606
1.0000
1.0607
1.0000
1.0607
1.0000
1.0004
1.0126
1.0000
(O2)Cu(CO)2
(η1-OO)AgCO
(η1-OO)AuCO
OCAuO2CO
OAuCO
2118.1
The 2118.1 cm-1 band (Table 1 and Figure 6) is assigned to
OAuCO based on the isotopic substitution, whereas a weak,
broad band around 2190 cm-1 was assigned to this species in
the previous work.24 The OAuCO band appeared after sample
annealing; it greatly increased after λ > 340 nm and broad-
band irradiation at the expense of the OCAuO2CO species.
The present DFT calculations predict that (η1-OO)AuCO has
a 2A′′ ground state with a Cs symmetry and OAgC of 178.3°
(Table 2 and Figure 10), which lies 20.56 kcal/mol lower than
the quartet one. The calculated 12C16O/13C16O and 12C16O/12C18O
isotopic frequency ratios of 1.0238 and 1.0232 (Table 3) are in
excellent agreement with the experimental values, 1.0238 and
1.0223, respectively. Also, the calculated 16O/18O isotopic O-O
stretching frequency ratio (1.0607) is in accord with the
experiment (1.0573) (Table 3). OCAuO2CO is predicted to have
SCHEME 1: Reaction Pathways of Laser-Ablated Cu,
Ag, and Au Atoms with CO/O2 Mixtures in Solid Argon
1, 2) molecules are the primary products after sample deposition
and survive after photolysis, whereas Cu(O2) and CuOO slightly
appear after sample annealing, slightly increase after further
annealing, and disappear on photolysis (Figure 1), indicating
that Cu prefers the reaction with CO to the reaction with O2.
For Ag, silver oxides are the primary products after sample
deposition and survive after photolysis, whereas silver carbonyls
slightly appear after sample annealing, slightly increase after
further annealing, and disappear on photolysis (Figure 5),
implying that Ag prefers the reaction with O2 to the reaction
with CO. For Au, gold carbonyls and oxides appeared together
after sample deposition and (η1-OO)AuCO and OCAuO2CO
increased at the expense of these gold carbonyls and oxides
(Figure 6), indicating that the reactivity of Au to CO is
comparable to that of O2. As shown in Figures 1, 5, and 6,
traces of CO2 appeared after sample deposition and almost did
not increase upon annealing, indicating that they may be
produced by radiation from the ablation plume. After UV
irradiation, CO2 greatly increased at the expense of carbonyl
metal oxides; furthermore, silver carbonyls disappeared, but
copper and gold higher carbonyls increased, showing the
oxidation of CO to CO2 on Cu, Ag, and Au atoms. It is noted
that in all the cases only the isotopic form C16O18O has been
observed in the CO + 18O2 experiment, implying that one
oxygen atom in the CO2 production is from CO and another
one is from O2.
2
a A′′ ground state with a Cs symmetry, whereas OAuCO has
2
a Π ground state with a C∞V sysmetry (Table 2 and Figure
10). Similar good agreement between the experimental and
calculated vibrational frequencies, relative absorption intensities,
and isotopic shifts has also been obtained for OCAuO2CO and
OAuCO (Tables 2 and 3).
Similar to copper and silver, co-deposition of laser-ablated
Au atoms with CO/O2 mixture in the present experiments
produced weak CO2 bands (Figure 6). Annealing to 30 K also
did not enhance these CO2 bands. After UV irradiation, the
elimination of CO2 has been observed with the decrease of the
(η1-OO)AuCO and OCAuO2CO species, whereas the OAuCO
species increased and gold carbonyls and oxides survived,
indicating that these carbonyl gold oxides may be act as the
precursors for the oxidation of CO to CO2.
Mechanism of CO Oxidation. Under the present experi-
mental conditions, laser-ablated copper, silver, and gold atoms
react with CO and O2 mixture in the excess argon matrixes to
produce carbonyl metal oxide species as well as metal carbonyls
and metal oxides. In the case of Cu, mainly the Cu(CO)n (n )
Scheme 1 presents the possible reaction pathways for laser-
ablated group 11 metal atoms with CO and O2 mixture in argon

Oxidation of CO on Group 11 Metal Atoms
J. Phys. Chem. A, Vol. 110, No. 8, 2006 2661
References and Notes
matrixes. The reactivity toward CO and O2 is found to be
considerably different for Cu, Ag, and Au. Based on the above-
mentioned findings, it could be inferred that the reactivity of
copper toward CO is prior to O2, the reactivity of silver toward
O2 is prior to CO, and the reactivity of gold toward CO is
comparable to O2. In the reaction of Cu, Ag, and Au atoms
with CO/O2 mixture, Cu prefers pathway I and Ag prefers
pathway II, while pathways I and II are parallel for Au. On the
other hand, the effect of UV irradiation on M(O2) complexes
has been shown previously to induce insertion and formation
of metal oxide OMO.19 In the present experiments, copper
carbonyls, silver oxides, and gold carbonyls and oxides remain
after photolysis. Upon UV irradiation, it is possible that the
inserted OMO directly reacts with CO to produce CO2, and this
reaction may also contribute to some extent to the CO2
production.
The metal (n - 1)dπ, ns, np, and CO 2π* energy levels may
be helpful to understand this difference in the reactivity of group
11 metal atoms toward CO and O2. Taking the Cu-CO pair as
an example, the Cu 3dπ and CO π* levels are close (ca. 73.37
kcal/mol), but the Cu 4s and CO lone-pair levels are far apart
(ca. 96.85 kcal/mol),11d and thus the dπ f π* back-donation is
primarily responsible for the thermal stability of CuCO,
consistent with previous reports.18c As previously reported,18c
the absence of AgCO complex is attributed to the larger gap
between Ag 4d and CO π* levels relative to Cu. For the Au-
CO pair, the metal dπ and CO π* levels are further apart than
those in the Cu-CO case, but the metal s and the CO lone-pair
levels are closer. It has been found that the contribution of the
formation of AuCO is mainly from a larger σ-type dative
interaction. That is, the formation of AuCO is easier than that
of AgCO, but more difficult than that of CuCO. Consequently,
the reaction pathways of gold toward CO and O2 are comparable
when Au faces simultaneously the CO and O2 molecules; silver
forms silver oxides prior to the silver carbonyls, whereas copper
prefers the formation of copper carbonyls to that of copper
oxides.
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Conclusions
Reactions of laser-ablated Cu, Ag, and Au atoms with CO
and O2 mixture in solid argon have been studied using matrix-
isolation infrared spectroscopy. Besides the metal carbonyls and
oxides, the carbonyl metal oxides, (O2)Cu(CO)n (n ) 1, 2), (η1-
OO)MCO (M ) Ag, Au), OCAuO2CO, and OAuCO, are
formed on sample deposition or annealing and are characterized
using infrared spectroscopy on the basis of the results of the
isotopic substitution and the CO concentration change. UV
irradiation on these carbonyl metal oxides produced CO2,
indicating the oxidation of carbon monoxide to carbon dioxide.
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Acknowledgment. We thank the reviewer for valuable
suggestions. This work was supported by a Grant-in-Aid for
Scientific Research (B) (Grant 17350012) from the Ministry of
Education, Culture, Sports, Science and Technology (MEXT)
of Japan and by Marubun Research Promotion Foundation. L.J.
thanks the MEXT of Japan and Kobe University for an Honors
Scholarship.
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