Methane hydrogenation and confirmation of CHx intermediate species on NaY encapsulated cobalt clusters and Co/SiO2 catalysts: EXAFS, FTIR, UV characterization and catalytic performances

Article abstract of DOI:10.1039/a605124c

The adsorption of Co₂(CO)₈ onto dehydrated NaY powder under an N₂ atmosphere predominantly yielded supported Co₄(CO)₁₂. The molecular cobalt carbonyl clusters and their decarbonylated products have been structurally characterized by in situ IR, extended X-ray absorption fine structure (EXAFS) and diffuse reflectance spectroscopies. The IR spectrum assigned to the species Co₄(CO)₁₂/NaY is shifted significantly from that observed for externally supported analogues on NaY and for this cluster in solution, which indicates that the cobalt carbonyl clusters occurring on the NaY are similar to those occurring in weakly basic solution. EXAFS coordination numbers (N) show that the successively decarbonylated samples maintain small cluster sizes, which depend on the temperature-programmed oxidation of the precursor. Methane hydrogenation was carried out on intrazeolitic cobalt clusters using a two-step process. It showed a relatively higher activity and selectivity to C₂₊ hydrocarbons in comparison with Co/SiO₂ catalysts. On the basis of IR (ν_C-H 2960, 2880 cm^-1 and δ_C-H 1520, 1393 cm^-1) spectroscopy, mass spectrometry and reaction studies, one can conclude that CH_x (x = 0, 1 or 2) surface carbonaceous species were generated by CH₄ dissociation on the activated cobalt catalyst; the CH₂ species was quite reactive, and propagated higher hydrocarbons.

Full text of DOI:10.1039/a605124c

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Methane hydrogenation and conürmation of CH intermediate species
x
on NaY encapsulated cobalt clusters and Co/SiO catalysts: EXAFS,
2
FTIR, UV characterization and catalytic performances
Guo-Cheng Shena,b* and Masaru Ichikawaa
a Catalysis Research Center, Hokkaido University, Sapporo 060, Japan
b Department of Chemistry, University of T oronto, 80 St. George Street, T oronto, Ontario,
M5S 3H6 Canada
The adsorption of Co (CO) onto dehydrated NaY powder under an N atmosphere predominantly yielded supported
2
8
2
Co (CO) . The molecular cobalt carbonyl clusters and their decarbonylated products have been structurally characterized by in
4
12
situ IR, extended X-ray absorption Ðne structure (EXAFS) and di†use reÑectance spectroscopies. The IR spectrum assigned to the
species Co (CO) /NaY is shifted signiÐcantly from that observed for externally supported analogues on NaY and for this cluster
4
12
in solution, which indicates that the cobalt carbonyl clusters occurring on the NaY are similar to those occurring in weakly basic
solution. EXAFS coordination numbers (N) show that the successively decarbonylated samples maintain small cluster sizes,
which depend on the temperature-programmed oxidation of the precursor. Methane hydrogenation was carried out on intrazeoli-
tic cobalt clusters using a two-step process. It showed a relatively higher activity and selectivity to C hydrocarbons in compari-
2`
son with Co/SiO catalysts. On the basis of IR (l
2960, 2880 cm~1 and d
1520, 1393 cm~1) spectroscopy, mass
2
C~H
C~H
spectrometry and reaction studies, one can conclude that CH (x \ 0, 1 or 2) surface carbonaceous species were generated by
CH dissociation on the activated cobalt catalyst; the CH species was quite reactive, and propagated higher hydrocarbons.
x
4
2
Oxidation coupling of methane has been developed during the
last decade.1h5 Numerous basic metal oxides, mixed-metal
oxides and doped metal oxides have been observed to be
active catalysts, but co-reactants of oxygen/nitrous oxide and
high temperature are necessary in this reaction procedure.
These create a reaction environment conductive to deep oxi-
dation of CH and higher hydrocarbons to CO and CO .
Recently, Yang et al.6 reported the synthesis of benzene on a
Ni(111) surface by collision induced chemisorption of methane
using molecular beam techniques under ultrahigh vacuum
(UHV), and subsequent heating. Koerts and Van Santen7
stimulated much interest with their proposal that methane can
be converted into ethane and propane in a two-step procedure
by thermal dissociation of methane on a supported metal
catalyst, followed by hydrogenating the surface species depos-
ited on the catalyst. In these papers, the adsorbed surface
species seem to be key factors, which result in di†erent kinds
of hydrocarbon formation. In this regard, a high metallic
surface area per unit mass is necessary for the chemisorption
of methane and subsequent hydrogenation.
The well deÐned channel, pore microstructure, the large
internal surface area and the high thermal stability of zeolites
make them attractive supports for the preparation of
transition-metal catalysts with large metallic surface areas.
During the last decade, carbonyl clusters of Ru,8 Rh,9 Rd,10
Ir11 and Pt12 have been prepared in the cages of zeolites, by
“ship-in-a-bottleÏ synthesis techniques. The activation of metal
carbonyl clusters for methane conversion usually requires
their decarbonylation, to give nearly structurally uniform
materials that o†er the prospect of being size/shape selective
catalysts.13h15 However, during the decarbonylation, the
metal often migrates out of the zeolite pores to form crys-
tallites on the external surface.16 Preparation of metal carbon-
yl clusters with uniform nuclearities in zeolites has not been
well demonstrated and decarbonylated metal clusters, as yet,
are not well characterized.
and NO chemisorption; (ii) methane hydrogenation is per-
formed on NaY-encapsulated cobalt and Co/SiO catalysts;
2
(iii) the reactive intermediates are identiÐed by IR and MS
spectra.
Experimental
Sample preparation
4
2
An NaY zeolite-supported cobalt sample (10 wt.% Co
loading) was prepared by spontaneous monolayer dispersion
of Co (CO) (Strem Chemical, stabilized with 5È10% hexane)
2
8
with NaY zeolite powder (HSZ-320NAA, Lot no. D1-9915,
Si/Al \ 2.8, surface area 910 m2 g~1).17 The NaY zeolite
powder was dehydrated by evacuation at 623 K for 2 h, and
then was mixed mechanically with Co (CO) under an N
2
8
2
atmosphere at 300 K, followed by 1 h ageing at 300 K, to give
the brown sample, denoted as the precursor of the cobalt cata-
lyst.
Co/SiO (10 wt.% Co loading) was prepared by impregna-
2
tion of silica (Aerosil 300, surface area 300 m2 g~1, mesh 100)
with a solution of Co(NO ) É 6H O. After drying, the sample
3 2
2
was calcined at 673 K for 2 h, and was then reduced at 673 K
for 10 h under H Ñow, to give the Co/SiO catalyst.
2
2
The concentration of Co in the samples was determined by
using an inductively coupled plasma (ICP) atomic emission
spectrometer. Attempts to extract organometallic species from
the NaY supported samples were carried out with methanol.
Characterization of samples
IR spectroscopy. In situ IR spectra of the samples were
recorded with a Shimadzu FTIR 4200 spectrometer with
20È100 co-added scans at 2 cm~1 resolution. The precursor
was pressed into a self-supporting wafer (8 mg cm~2) in an N
2
atmosphere box and mounted in a quartz IR cell with CaF
2
windows that connected to a vacuum-closed circulating
Pyrex-glass line (10~5 Torr). The IR cell was equipped with an
This article reports the following: (i) intrazeolitic cobalt car-
bonyl clusters and their decarbonylated products are charac-
terized by IR, EXAFS and di†use reÑectance spectroscopies
electric heater and a liquid-N reservoir for high- and low-
2
temperature measurements. Before use, all gases were passed
through traps to remove water and/or oxygen.
J. Chem. Soc., Faraday T rans., 1997, 93(6), 1185È1193
1185

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EXAFS spectroscopy. EXAFS measurements were carried
out at the Photon Factory in the National Laboratory for
High Energy Physics (KEF-PF) using synchrotron radiation
with an electron energy of 2.5 GeV at currents of 200È350 mA.
The amount of sample in a wafer was calculated at the Co K
absorption edge, and the sample wafers were stored in spe-
cially designed Pyrex-glass cells with kapton Ðlm windows (50
nm thick). The spectra at the Co K edge (7.7 keV) were mea-
sured at 296 K under vacuum using a Si(111) double crystal
monochromator. The reference samples used were crystalline
Co (CO) , Co O and Co foil.
4
12
3 4
EXAFS data analysis. The EXAFS data were extracted
from the measured absorption spectra by standard methods.18
The normalization was done by dividing the absorption inten-
sities by the height of the absorption edge and subtracting the
background using cubic spline routines. The Ðnal EXAFS
function was obtained by averaging the individual
background-subtracted and normalized EXAFS data (two
scans for each sample). The main contributions to the spectra
were isolated by inverse Fourier transformation of the Ðnal
EXAFS function over a selected range in r-space. The analysis
was performed on these Fourier-Ðltered data. The parameters
charactering high-Z(Co) and low-Z(C, O) contributions were
reliably determined by multiple-shell Ðtting in k space (k is the
wavevector) and r space (r is the distance from the absorbing
atom) with application of K2 and K3 weighting of the Fourier
transform. The accuracy and limitation of parameters were
Fig. 1 PXRD patterns of (a) crystalline Co (CO) and (b) Co (CO)
2
8
2
8
mechanically mixed with NaY at 298 K under an N atmosphere
2
PXRD peaks for NaY. The dark-red Co (CO) changed
2
8
colour to brown in the precursor. This suggests that gaseous
Co (CO) is highly dispersed on the NaY framework, similar
2
8
to what has been reported previously for CuCl /NaY20 and
2
AuCl /NaY.21 Fig. 2(a) shows the IR spectrum of the precur-
3
sor. The bands at 2121w, 2078s, 2053 (sh) and 1816m cm~1
closely resembled those of Co (CO) in pentane solution
4
12
[2060s, 2035 (sh), 1870m cm~1; Fig. 2(c)] which has three
edge-bridging CO and nine terminal CO ligands.22 Co (CO)
4
12
appears to be formed from Co (CO) on NaY zeolite. On
2
8
other hand, the weak band at 1901 cm~1 in Fig. 2(a) can be
assigned directly to Co(CO) ~ species.23 Some CO (g) was
considered as: 0.02 \ p \ 0.08 and [10 \ *E \ 10, as well
4
0
released as shown by the band at 2153 cm~1 in Fig. 2(a). The
as the (calculated EXAFS contributions [ experimental
relative abundances of Co (CO ) and Co(CO) species of the
EXAFS contributions)/calculated EXAFS contributions of R
factor \ 15% for optimized coordination numbers (N) and
bond lengths.19 The di†erence Ðle technique with phase- and
amplitude-corrected Fourier transforms was applied to further
optimize the Ðt.19
4
12
4
precursor were calculated from the areas of the two CO
t
bands in Co (CO) and the CO in Co(CO) ~ species, where
12
4
t
4
Powder X-ray di†raction (PXRD) and di†use reÑectance.
The PXRD measurement was carried out using a MAC
Science di†ractometer with Cu-Ka radiation (j \ 1.5418 Ó).
Di†use reÑectance spectra were recorded on a Shimadzu
UV-2200 Series spectrometer with a photomultiplier detector.
BaSO was used as a reference. The samples were stored in
4
Pyrex-glass cells with Heraeus Amersil quartz windows. After
treatment, samples were measured at room temperature.
Methane hydrogenation
After reduction, the cobalt catalyst with adsorbed H was
2
Ñushed with He at 300 K for 30 min. The temperature of the
reaction procedure was raised to 723 K under Ñowing He, and
then switched to a pulsed gas containing dilute CH by He.
4
After methane dissociation, the cobalt catalyst was quickly
cooled to room temperature (r.t.) under He Ñowing to avoid
ageing of the surface carbonaceous species deposited on the
cobalt catalyst. The surface carbonaceous species were subse-
quently hydrogenated in an H Ñow of 20 cm~3 STP min~1
2
from 300 to 673 K. Before entering the catalyst bed, the gases
(He, CH and H ) were puriÐed with manganese oxide and
4
2
molecular sieves. The effluent gases from the reactor were
stored in the loops and were determined afterwards by Shi-
madzu GC-8A gas chromatography with a Ñame ionization
detector.
Results
Characterization of NaY-supported cobalt clusters
Precursor characterization by IR spectroscopy and powder
X-ray di†raction. The sample (10 wt.% Co loading) was char-
acterized by PXRD. The pattern shows reÑections at
2h \ 11.46, 16.64, 22.9, 29.78, 32.22, 32.80, 43.66 and 44.52¡
[Fig. 1(a)], ascribed to crystalline Co (CO) . In Fig. 1(b),
Fig. 2 IR spectra obtained for (a) Co (CO) adsorbed on dehydrated
2
8
NaY powder under N atmosphere, for (b) Co (CO) in pentane solu-
2
8
2
2
8
these reÑections completely disappeared, leaving only the
tion and for (c) Co (CO) in pentane solution
4
12
1186 J. Chem. Soc., Faraday T rans., 1997, V ol. 93

View Article Online
equal absorption coefficients of CO for Co (CO)
and
Ó, to 2.63 carbon ligands at an average CowC distance of
1.84 Ó and to 2.50 oxygen ligands at an average Co(wCw)O
distance of 2.90 Ó [crystalline Co (CO) in Table 1]. The
t
4
12
Co(CO) ~ species were assumed; the calculated abundance is
4
90% for Co (CO) and 10% for Co(CO) ~.
4
12
4
4
12
The Co (CO) in the precursor is identiÐed by principal
EXAFS results characterizing the precursor were in fair agree-
ment with the crystallographic data for Co (CO) (Table 1).
4
12
l
bands at 2121w, 2078s, 2053 (sh) and 1816m cm~1. The
2078 and 2053 cm~1 bands are assigned to the symmetric and
asymmetric stretching of the two CO ligands bound to each
Co atom of three out of four Co atoms in Co (CO) ; the
2121, 2078 and 2053 cm~1 bands are assigned to CO stretch-
ing of the three CO ligands bound to the fourth Co atom of
Co (CO) . The 1816 cm~1 band corresponds to the stretch-
ing of three edge-bridging CO ligands, each bound to two Co
atoms. The stretching frequency of the bridging CO bands
CO
4
12
The data indicated a CowCo coordination number of 2.6
with an average CowCo distance of 2.52 Ó, a CowC coordi-
nation number of 2.62 with an average CowC distance of 1.80
Ó and Co(wCw)O coordination number of 2.54 with an
average Co(wCw)O distance of 2.92 Ó for the sample. On the
other hand, the discrepancy between the CowCo coordi-
nation numbers of the precursor and crystalline Co (CO)
4
12
4
12
4
12
with an N(CowCo) ratio of 2.6 : 3.0, implies that about 90%
l
is red shifted by about 54 cm~1, while the terminal CO
of the Co atoms are present as Co (CO) clusters and about
10% of the Co atoms are present as mononuclear forms, such
COb
bands l
4
12
is blue shifted by 18 cm~1 with respect to those of
COb
crystalline Co (CO) 21 or crystalline Co (CO) deposited
12 12
on the NaY wafer from pentane solution.
as Co(CO) species, in the precursor. The calculation of the
4
4
4
coordination number for the precursor was in agreement with
the abundance of Co (CO) and Co(CO) ~ species calcu-
4
12
4
lated from the IR peak areas.
Precursor characterization by EXAFS spectroscopy. To
obtain more insight into the structure of cobalt carbonyl clus-
ters in the precursor, especially into the metal framework, the
EXAFS spectra of the Co K edge were analysed. The normal-
ized EXAFS function for the precursor was obtained from the
average of X-ray absorption spectra from two scans by a cubic
spline background subtraction. The EXAFS function was nor-
malized by division by the height of the absorption edge. The
raw EXAFS data of the precursor [Fig. 3(a)] shows oscil-
lations up to a value of k, the wavevector, of about 12.5 Ó~1,
indicating the presence of near-neighbour high-atomic-weight
backscatters, which are inferred to be Co. Since the IR spectra
indicate that carbonyl ligands were also present in the precur-
sor, the data were analysed for CowCo and CowCO inter-
actions. The raw EXAFS data were Fourier transformed with
a k2-weighting over the range 2.0 \ k \ 16.0 Ó~1. Fig. 3(b)
exhibits peaks in the range 1.3 \ R \ 2.8 Ó (where R is the
distance from the absorber Co atom), attributable to CowCo,
CowC and Co(wCw)O contributions. The Fourier-
transformed data were then inverse transformed in the range
1.3 \ R \ 2.8 Ó, to isolate the major contribution from low-
frequency and higher shell contributions. The results of the
curve-Ðtting analyses are shown in Fig. 3(c) and Table 1. The
EXAFS results provide the strongest evidence that the back-
scatters in the immediate vicinity of the Co absorber atoms
include Co and low-atomic-weight backscatters, identiÐed as
C and O. The relative contributions of these backscattering
atoms support the identiÐcation of the NaY-encapsulated
species as Co (CO) for the precursor. All the Co atoms in
IR and di†use reÑectance results characterizing the decarb-
onylated cobalt clusters. When the precursor wafer in the IR
cell was exposed to 200 Torr O at 298È313 K, more dramatic
2
e†ects were observed: the 1904 cm~1 band disappeared
rapidly, while the Co (CO) bands at 2078, 2053 and 1816
4
12
cm~1 were slightly diminished and the bridging band at 1816
cm~1 shifted down slightly [Fig. 4A(b), (c)]. As the tem-
perature reached 343 K, the bridging CO band at 1816 cm~1
and the terminal CO band at 2053 cm~1 completely disap-
peared, while the terminal CO band at 2078 cm~1 decreased
gradually in intensity [Fig. 4A(e)]. This indicated that some
CO ligands of the cobalt clusters vanished and the framework
of the cobalt cluster decomposed. Upon raising the tem-
perature to 353 K, the residual CO band eventually disap-
peared and the clusters were converted to cobalt oxide species
[Fig. 4A(f)]. The temperature-programmed oxidation of the
precursor was accompanied by the evolution of CO , and its
intense feature at 2354 cm~1 was observed.
As mentioned above, the precursor was stepwise oxidized in
200 Torr O at 298, 313, 353, 393, 423 and 473 K for 1 h, to
form the oxidized samples denoted as [Co ]
the subscript T refers to oxidization temperature) which were
then analysed by UV spectroscopy. The di†use reÑectance
2
2
/NaY, (where
x T, ox
spectrum of [Co ]
/NaY [Fig. 4B(b)] shows the absorp-
x 298, ox
tion band at 375 nm, ascribed to the characteristic tetrahedral
structure of Co (CO) ,24,25 still remained, as did that of Fig.
4
12
4B(a)]. In Fig. 4B(c), the 375 nm band has shifted down slight-
4
12
crystalline Co (CO) are stereochemically equivalent, with
12
each bonded to three Co atoms at an average distance of 2.50
ly and a new absorption band at 575 nm, due to the CoIIO
species,24 is observed. The spectra of [Co ]
4
/NaY,
x 353, ox
Table 1 Results of the curve-Ðtting analysis of Co-edge EXAFS data obtained at 296 K for samplesa
CowCo CowC Co(wCw)O or CowO
sample
precursor
N
R/Ó
*E /eV
p/Ó
N
R/Ó
*E /eV
p/Ó
N
R/Ó
*E /eV
p/Ó
0
0
0
2.6
2.52
[10.87
0.027
2.62
1.80
11.79
0.085
2.54
3.67
5.02
2.92
2.01
1.90
[6.93
0.064
0.066
0.039
[Co ]
/NaY
/NaY
4.16
x 353, ox
[Co ]
x 473, ox
2.90
2.46
1.12
8.83
2.86
3.36
2.56
2.50
[9.74
13.23
11.60
[0.04
0.023
0.034
0.037
0.029
4.13
[Co ]
[Co ]
ref. samples
Co foil
/NaY
/NaY
2.82
1.89
3.06
0.020
x 353, red
x 473, red
12
2.50
2.50
2.50
2.85
3.40
0.29
[10.09
[2.34
0.020
0.038
0.079
0.047
0.05
Co (CO)
3.0
3.8
4.0
9.3
2.63
2.80
1.84
1.82
12.14
8.98
0.075
0.067
2.50
2.63
5.4
2.90
2.92
1.90
[8.32
[2.15
11.19
0.061
0.067
0.051
4
12
16
Co (CO)
6
Co O
[11.67
[10.89
3
4
CoOb
8.0
2.13
a N \ coordination number for absorberÈbackscatter pair; R \ radial absorberÈbackscatter distance; p \ DebyeÈWaller factor and *E \
0
inner potential correction. b Ref. 24.
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
1187

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Fig. 4 A, In situ IR spectra characterizing oxidative treatment of the
precursor by 200 Torr of O at 298È353 K: (a) precursor; (b) 298 K, 1
2
h; (c) 313 K, 1 h; (d) 328 K, 1 h; (e) 343 K, 1 h; (f) 353 K, 1 h. B,
Di†use reÑectance spectra characterizing stepwise oxidation of the
precursor by 200 Torr of O at 298È473 K: (a) precursor; (b) 298 K, 1
2
h; (c) 313 K, 1 h; (d) 353 K, 1 h; (e) 393 K, 1 h; (f) 423 K, 1 h; (g) 473
K, 1 h.
characterized by EXAFS spectroscopy. Fig. 5B1 shows the
Fourier-transform function K3s(k) of [Co ]
/NaY. The
x 353, ox
EXAFS data show the breakage of direct CoÈCo bonds and
formation of a new species with a CoÈO bond at a distance of
2.01 Ó, with a coordination number, N(CowO), of 3.67. The
CowO distance is shorter than the 2.13 Ó distance observed
in crystalline CoO and longer than the CowO bond (1.90 Ó)
for crystalline Co O , as shown in Table 1. A plausible struc-
3
4
ture for [Co ]
could be a [CoO] , salt-like structure.24
x 353, ox
n
Fig. 5C1 shows the Fourier-transform function K3s(k) of
[Co ]
/NaY, from H treatment at a heating rate of 1 K
x 353, red
2
min~1 to 673 K, followed by 6 h at the Ðnal temperature. The
EXAFS data show the destruction of the CowO bond and
the formation of CowCo bonds with a bond length of 2.56 Ó.
The total N(CowCo ] CowO) of [Co ]
/NaY is much
x 353, red
smaller than those of Co foil or bulk CoO, and even smaller
than N(CowCo ] CowCO) of the precursor. This suggests
the [Co ]
/NaY sample retained small cobalt clusters,
x 353, red
highly dispersed on the NaY framework. No further aggre-
gation of cobalt clusters was observed even after high-
temperature reduction at 773 K. The zeolite channels, with
limited sizes, probably prevent cobalt clusters in NaY super-
cage from coalescing.
Fig. 3 Results of EXAFS with the best calculated coordination
parameters characterizing the precursor. (a) K2-weighted EXAFS
spectrum K2s(k) vs. K; (b) Fourier-transform of K2 weighted EXAFS
spectrum K2s(k) vs. R. (c) Inverse Fourier-transform of experimental
EXAFS in the range 1.3 \ R \ 0.28 Ó (È È È) and sum of the calcu-
lated CowCo ] CowC ] Co(wCw)O contributions (ÈÈ).
Reaction kinetics and product distribution
Fig. 6 shows temperature-programmed hydrogenation (TPH)
spectra of carbonaceous surface species produced by 0.02
mmol CH dissociation on [Co ] /NaY catalyst at 723 K.
[Co ]
x 393, ox
/NaY and [Co ]
x 423, ox
/NaY show the absorption
4
x red
band at 375 nm decreases and the band at 575 nm increases
with increasing oxidative temperature for the precursor [Fig.
4B(d)È(f)]. In Fig. 4B(g), the spectrum exhibits a dramatic
change: the absorption bands at 375 and 575 nm completely
disappeared, and the absorption bands at 420 and 700 nm due
to Co O species appeared,24 indicating that the CoO species
The favourable hydrogenation temperature varies with the
size of the hydrocarbon. The optimum hydrogenation tem-
peratures of C , C , C and C hydrocarbons were 333, 483,
2
3
4
5
423 and 453 K, respectively. The C , C and C hydrocarbon
3
4
5
fractions signiÐcantly displayed monotonous increases and
decreases when T was set at lower and higher values than
their optimum temperature, respectively. The selective cataly-
sis of the reaction is remarkable, with the production of a
large amount of C hydrocarbons but only a trace of C
3
4
converted to Co O at 473 K. Based on the results of IR and
3
4
di†use reÑectance, one can safely say that the tetrahedral
cobalt clusters species predominate in the sample at a low oxi-
dation temperature, whereas cobalt oxide species predominate
in the sample at a high oxidation temperature.
5
2
hydrocarbons at 453 K. At the optimum temperature for each
hydrocarbon, the selectivity towards C , C , C and C
2
3
4
5
reached 2.4, 17.7, 11.7 and 46.9%, respectively (Table 2). The
range of hydrogenation temperature remarkably a†ected the
EXAFS results characterizing the decarbonylated cobalt clus-
ters. To investigate the structure change of the clusters
resulting from a decarbonylated precursor, the oxidized states
yield of the C
2`
selectivity is [55%; for T \ 393 K, C
hydrocarbons: for T \ 393È543 K, C
2`
selectivity reaches
2`
10%, but also non-C and -C species were observed; for
of the precursor, [Co ]
/NaY and [Co ]
/NaY were
x 353, ox
x 473, ox
4
5
1188
J. Chem. Soc., Faraday T rans., 1997, V ol. 93

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Fig. 5 Fourier-transform of K3-weighted EXAFS spectrum K3s(k) vs. K characterizing the sample: A, precursor; B1, [Co ]
x 353, ox
/NaY from the reduction of
/NaY; B2,
[Co ]
x 473, ox
[Co ]
x 473, ox
/NaY; C1, [Co ]
x 353, red
/NaY from the reduction of [Co ]
x 353, ox
/NaY at 673 K; C2, [Co ]
x 473, red
/NaY at 673 K
Table 2 Kinetic parameters for methane hydrogenation on [Co ] /NaY and Co/SiO catalystsa
x red
2
[Co ] /NaYb
x red
Co/SiO c
2
hydrocarbon
optimum temp./K
selectivityd (%)
optimum temp./K
selectivityb (%)
C
2
333
483
423
453
2.4
17.7
11.7
46.9
543
543
È
3.7
6.6
È
C
3
C
4
C
È
È
5
a 100 mg [Co ] /NaY (10 wt.%) or Co/SiO (10 wt.%); step 1: exposure of the catalyst to 0.02 mmol CH at 723 K; step 2: hydrogenation of
x red
hydrocarbons.
2
4
adspecies by a Ñow of H (20 cm3 min~1, 1 atm). b CH /Co \ 0.11. c CH /Co \ 0.03. d Molar ratio of nC (n \ 2, 3, 4, or 5)/&nC (n \ 1È5)
2
4
4
n
n
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
1189

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was exposed to [Co ] /NaY and Co/SiO ; (ii) the molar
x red
2
amounts of C
hydrocarbons on [Co ] /NaY catalyst are
2`
x red
10 times those on the Co/SiO catalyst; (iii) the hydrocarbon
2
fractions on Co/SiO catalyst display a higher desorption
2
temperature than on [Co ] /NaY catalyst. The behaviour of
x red
[Co ] /NaY shows that the selectivity towards the heavier
x red
hydrocarbons was favoured by CH thermal dissociation and
subsequent hydrogenation at 393È543 K.
4
Characterization of carbonaceous adspecies by IR
spectroscopy
An IR wafer of [Co ] /NaY was brieÑy exposed to a Ñow of
x red
20 cm~3 min~1 of dilute CH (5% CH in He) at higher tem-
4
4
perature, quickly cooled to 298 K and the in situ IR spectra
recorded. As shown in Fig. 8A(a), the weak and medium
bands observed at 2960, 2880, 1520 and 1393 cm~1, respec-
Fig. 6 Temperature-programmed hydrogenation of the surface car-
bonaceous species created from 0.02 mmol CH dissociation on
4
[Co ] /NaY (10% wt. Co) catalyst at 723 K
x red
tively, are due to CH dissociation at 473 K. Georeg et al.
4
reported the analogous IR frequencies for ethylene and diazo-
methane thermal chemisorption on an Ru(001) surface,26 and
acetylene thermal chemisorption on W(110)26 and Ni(111) sur-
T [ 543 K, C
selectivity merely reaches 7%; when T is
increased beyond 673 K, the only product was methane.
2`
Moreover, it is interesting that the optimum C temperature is
3
higher than those of C and C (Fig. 6). This implies that a
4
5
part of C hydrocarbons would be created by hydrogenolysis
3
of C and/or C hydrocarbons.
4
5
Fig. 7 shows the hydrocarbon distribution as a function of
hydrogenation temperature on the Co/SiO catalyst. At an
2
optimum hydrogenation temperature of 543 K, the selectivity
of C and C only attain 3.7 and 6.6%, respectively (Table 2).
2
3
The selectivity towards the heavier hydrocarbons (C , C ) was
4
5
disfavoured even by the same operation on [Co ] /NaY.
Comparison of the kinetic parameters of the reaction
x red
between [Co ] /NaY and Co/SiO catalysts, summarized in
x red
2
Table 2, shows: (i) the concentration of carbonaceous
adspecies on [Co ] /NaY (CH /Co, molar ratio) is 3.6 times
x red
x
that on Co/SiO , even though an equal amount of methane
2
Fig. 8 In situ IR spectra of CH (A) and CD (B) dissociation and
4
4
Fig. 7 Temperature-programmed hydrogenation of the surface car-
subsequent hydrogenation or deuteriation on [Co ] /NaY catalyst.
x red
bonaceous species created from 0.02 mmol CH dissociation on
Decomposition at 473 (a), 573 (b) and 673 K (c); (d) hydrogenation/
4
Co/SiO (10% wt. Co) catalyst at 723 K
deuteriation at 393 K.
2
Table 3 Mode assignments for CH thermal dissociation on [Co ] /NaY compared to those of ethylene, diazomethane on Ru(001) and
4
x red
relevant organometallic complexes (in wavenumbers/cm~1)
mode
CH wag
1135
1450
2940
3050
26
1127
1350
2918
2981
26
961
1360
2935
2984
27b
1393
1520
2880
2960
this work
2
CH
2
l (CH )
s
2
l (CH )
a
2
ref.
a CH from both C H and CH N .
2
2
4
2 2
1190
J. Chem. Soc., Faraday T rans., 1997, V ol. 93

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Table 4 Deuterium distribution in methane obtained from D treat-
weakly basic c-Al O surface, however, the chemistry of metal
2
2
3
ment at 483 K of carbonaceous-deposited material on [Co ] /NaY
carbonyls di†ers, being similar to that occurring in Lewis-base
x red
and Co/SiO catalysts
solution. For example, Co (CO) reacts on the c-Al O
2
2
8
2 3
surface to give Co (CO) .24 The formation of neutral cobalt
methane (%)
4
12
carbonyl clusters reported here in the NaY zeolite is evidently
run
catalyst
CH
CH D
CH D
CHD
CD
similar to that occurring on the surface of c-Al O . Some
4
3
2
2
3
4
2
3
anionic cobalt carbonyl clusters would be oxidized to neutral
cobalt carbonyl clusters by Lewis-acid sites, such as Na` or
Al3` of the NaY zeolite; similarly anionic metal clusters were
1
2
[Co ] /NaY
0
0
0
0
34.3
0
40.0
44.7
25.7
55.3
x red
Co/SiO
2
oxidized by oxidizing agents such as FeCl .29 The result is
3
not surprising because NaY zeolite is less basic than NaX
faces.27a Oxton et al.27b showed IR bands of the wCH ligand
zeolite. These results give an indication of the possibilities for
modiÐcation of the reactivity in the solvent-like cages of the
zeolites by changing the Si : Al ratio.
2
of (k-CH )[(g-C H )Co(CO)
]
clusters at 2981, 2918, 1350
2
5
5
12 2
and 1127 cm~1 (Table 3). By comparing IR bands of Fig.
8A(a) with vibrational bands of modes involving the wCH
The shifts of the terminal and bridging CO bands observed
2
ligands (Table 3), one can reasonably suggest that the 2960
in Co (CO)
of the precursor are similar to the shifts
4
12
and 2880 cm~1 bands are assigned to CwH stretching of
observed for metal carbonyls in solutions containing Lewis
acids such as Al(C H ) , which have been examined exten-
CH
adspecies and the 1520, 1393 cm~1 bands arise from
1, 2
2
5 3
the deformation vibration of CH adspecies. Fig. 8A(b), (c)
show the same bands of CH
compared with that in Fig. 8A(a). This implies that the disso-
ciated fragments successively increased with increasing tem-
sively.30 Similar shifts have also been observed for
2
but a higher band intensity as
[Rh (CO) ] in NaY zeolite and attributed to the interaction
1, 2
6
16
of the oxygen atoms of the face-bridging CO ligands with the
Lewis-acid sites (Al3` ions).9c The basicity of the oxygen in
CO ligands of metal clusters depends on the CO coordination
geometry,31 as doubly bridging CO ligands are signiÐcantly
more basic than terminal CO ligands. This pattern is borne
out in the present results. The relatively large shift character-
istic of the bridging CO band of NaY-supported Co (CO)
perature. As H was admitted into the IR cell with heating,
2
the bands of the CH adspecies disappeared, and new bands
x
at 3024, 1310 cm~1 appeared, ascribed to physisorption of
gaseous CH [Fig. 8A(d)].
In an attempt to conÐrm that the carbonaceous species was
4
4
12
created from CH decomposition, the in situ IR spectra of
suggests a strong interaction of the oxygen in bridging CO
4
CD thermally dissociated on [Co ] /NaY was recorded.
ligands of the Co (CO) mainly with Na` and with possible
4
x red
4
12
Fig. 8B(a) shows CwD stretching and CwD deformation
Lewis-acid sites (Al3` ions) formed by dehydration of NaY.
Such an interaction is expected to result in a net electron
withdrawal from the clusters, decreasing the back-bonding to
the terminal CO ligands, strengthening the carbonÈoxygen
bond, and shifting the terminal CO bands to higher fre-
quencies.
2
bands at 2048 and 1478, 1433 cm~1, respectively. CD and
2
CD species cause overlapping of the CwD stretching vibra-
tion at 2048 cm~1.27a,28 The intensity of these bands
increased with temperature of CD decomposition [Fig. 8B(a),
4
(c)]. When CD adspecies were treated with deuterium at 393
x
K, the bands at 2048, 1478 and 1433 cm~1 disappeared, and a
One of the main issues complicating the synthesis and char-
acterization of metal clusters in zeolites is the possible simul-
taneous formation of metal clusters or crystallites on the
external surface of the zeolite. However, the results of the
work here suggest that virtually all the cobalt carbonyl clus-
ters were formed in and conÐned to the NaY cages. When
organometallic species from the sample were extracted by
methanol solution, the supernatant solution remained colour-
less and showed no IR absorption in the CO stretching
region. These results are consistent with a “ship-in-a-bottleÏ
synthesis of the cobalt carbonyl clusters: the precursor
Co (CO) and mobile nucleophilic Co(CO) ~ are presumed
new band emerged at 2272 cm~1, assigned to CD physisorp-
4
tion and created from the reaction of CD with D .
x
2
By contrast, using the same procedure of CH and CD
decomposition on the Co/SiO catalyst led to no carbon-
aceous species being determined by IR spectra.
4
4
2
Characterization of carbonaceous adspecies by mass
spectrometry
As mentioned above, the carbonaceous species were treated
with deuterium at 473 K, and the methane produced was
analysed by mass spectrometry (AQ-200, Anelva Co). Run 1 of
Table 4 clearly shows the presence of CD (26%), CHD
2
8
4
to be small enough to Ðt into the interior of the cages; the
4
3
charged environment, resulting from the large number of Na`
(40%) and CH D (34%). This suggests carbonaceous species
2
2
cations of a-cages,32 stabilizes the Co(CO) ~ within the a-
such as CH , CH and carbidic carbon were adsorbed on
4
2
cages; synthetic clusters, Co (CO) are small enough to Ðt in
[Co ] /NaY.
4
12
x red
the supercages of NaY but too large to di†use rapidly through
the apertures. Therefore, Co (CO) clusters, once formed,
Run 2 of Table 4 shows only CH (44.7%) and carbidic
carbon (55.3%) fragments were deposited on the Co/SiO
catalyst. From comparison of runs 1 and 2, it can be assumed
4
12
2
appear to be encapsulated in the a-cages.
that CH adspecies are characteristic of CH thermally
2
4
decomposed on the [Co ] /NaY catalyst which tends to pro-
Decarbonylation of intrazeolitic cobalt carbonyl clusters
x red
pagate higher hydrocarbons. These di†erences of the carbon-
The higher-shell CowCo neighbours cannot be seen in the
EXAFS r-space diagram of Fig. 5B1 or C1, which is consistent
with the inference that there was no cobalt oxide located on
the outside of NaY.33 This agrees with the claim that mild
annealing treatment results in the production of small-size
cobalt species inside the NaY cages, based on the NMR char-
acterization of size and location of cobalt clusters in NaY by
Sachtler and co-workers.34
aceous adspecies on [Co ] /NaY and Co/SiO catalysts is
responsible for the occurrence of high-molecular-weight
hydrocarbons on the former.
x red
2
Discussion
Reaction of cobalt carbonyl clusters in NaY zeolite cages
It has been shown that the supercages of NaX zeolite are suffi-
ciently basic to provide an efficient medium for the synthesis
In contrast, the Fourier transform function K3s(k) of
[Co ]
x 473, ox
K, resembles that of Co O according to the similarity of
/NaY from the oxidation of the precursor at 473
of anionic cobalt carbonyl clusters, such as Co(CO) ~.23 The
4
3
4
chemistry of anionic cobalt carbonyl clusters in the cages of
EXAFS characterization for a CowO bond at a distance of
NaX zeolite parallels its chemistry in basic solution. On the
1.90 Ó (coordination number of 5.02), a CowCo bond at a
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
1191

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distance of 2.86 Ó (N \ 2.90) and 3.36 Ó (N \ 2.46), as shown
CH ] ( ) H CH
] H
] H
(1)
(2)
(3)
(4)
in Fig. 5B2 and Table 1. However, the much smaller coord-
4
3 ads
2 ads
ads
ads
ination numbers for CowCo bonds of the [Co ]
x 473, ox
are in contrast with those of bulk spinel Co O (Table 1). This
/NaY,
CH
CH
H CH
3 ads
2 ads
3
4
H CH ] H
indicates a raft-like structure for [Co O ] on NaY.24
4 n
[Co ]
x 473, red
ads
ads
3
Upon
[Co ]
comparison
of
/NaY
with
CH H C ] H
ads
ads
ads
/NaY species, the CowCo coordination number
x 353, red
This mechanism indicates that the thermal decomposition
of methane on [Co ] /NaY results in H and CH adsorbed
N \ 8.83 for the former (Fig. 5C1 and Table 1) is larger than
N \ 1.12 for the latter (Fig. 5C2 and Table 1). This is an indi-
cation of the formation of cobalt particles larger than those of
x red
3
on the cobalt surface. Re-dissociation of the CH species into
3
H and methylene can then follow.
The bonding between adjacent hydrogen-deÐcient CH
groups and growing hydrocarbons chains are reasonably
assumed to explain our procedure according to:
[Co ]
/NaY. Moreover, the signiÐcant peaks at r-space
x 353, red
of 3.9 and 4.7 Ó of the Fourier transforms function (Fig. 5C2),
corresponding to higher-shell CowCo neighbours, is evidence
of the formation of large cobalt particles, possibly on the
external surface. However, EXAFS evidence is lacking for any
x
CH
2 ads
] CH
2 ads
H CH wCH
(5)
(6)
(7)
2
2 ads
ads
higher-shell Co atom in [Co ]
likely a consequence of the limited sensitivity of the EXAFS
/NaY sample, which is
x 473, red
CH
] CH H CH wCH
2 ads
ads
2
technique.35
CH
2 ads
] C H CHwC
ads
ads
The [Co ]
/NaY species obtained from mild oxidation
x 353, ox
of the precursor was very difficult to completely reduce using
and so on. The [CH ]
surface Ðlm was originated on the
x 2`
surface of Co clusters. The [CH ] is expected to be similar
to those obtained by chemisorption of unsaturated hydrocar-
the same reduction conditions as for [Co ] /NaY, even at
x 2`
x 473, ox
773 K, as evident from its CowO coordination number of
bons. Spontaneous desorption of unsaturated hydrocarbons is
not favoured in the temperature range that we have explored
here and over-heating could cause undesired cracking and
2.82 (Table 1).
The CowCo distance characteristic of [Co ]
/NaY is
x 353, red
2.56 Ó, 2% longer than that in Co foil and 1.5% longer than
that in the precursor. Tzou et al.36 reported that the RhwRh
bond of reduced Rh/NaY was about 1È2% shorter than that
in Rh foil. Since the Co sample was reduced incompletely, the
bonding of OÈCo may relax the CoÈCo bond.
coking. In our procedure, Ñushing with H is the simplest way
2
to release the [CH ]
unsaturated molecules. However,
x 2`
hydrogenolysis and inter-conversion of the oligomers may
unfortunately interfere with the hydrogenation. Occurrence of
these competing reaction would explain why the optimum
temperature for C is higher than those of C and C hydro-
Thus, one can conclude that the successively decarbonyl-
ated sample containing small cobalt clusters, depends on the
temperature-programmed oxidation of cobalt carbonyl clus-
ters: mild oxidation caused high dispersion of cobalt oxide,
which led to incomplete reduction, while high-temperature
oxidation caused larger clusters [Co O ] on NaY, allowing
3
4
5
carbons (Fig. 6).
As observed by Feibelman,40 the adsorption energy of
carbon fragments decreases with increasing d-valence
occupation. Yang et al.41 calculated that bond energies of
CH , CH and CH adsorbed on a Ni(111) surface are 1.7, 2.9
3
4 n
easy reduction. The Co (CO) was converted to [Co O ]
3
2
4
12
3 4 n
and 3.1 eV, respectively. A similar relative increase of bond
species through [CoO] species via a stepwise oxidation
n
energy of CH (x \ 0, 1, 2) adspecies on [Co ] /NaY is rea-
process.
x
x red
sonably expected. Highly hydrogenated CH (x \ 2) frag-
In order to ascertain the state of cobalt clusters in NaY
cages in the successively decarbonylated precursor, we
x
ments are thought to be mobile due to the decrease of
metalÈCH interaction.30 Therefore, the selectivity to heavier
exposed [Co ]
/NaY to NO at 298 K. After 20 min, IR
x
x 353, ox
hydrocarbons on [Co ] /NaY is proposed to be due to CH
bands at 1894 and 1814 cm~1 appeared and are ascribed,
x red
2
adspecies having a relatively low cobaltÈcarbon reactivity,
respectively, to the characteristic NwOd` antisymmetrical
being more mobile and more combinative to additional CH
groups. This enhances the chain-growth probability. In con-
and symmetrical stretches of [CoII(NO) ] (not shown). It was
x
2
interesting to Ðnd that the IR spectrum upon NO chemisorp-
trast, the distribution of C and C lighter hydrocarbons on
tion on [Co ]
/NaY exhibits signiÐcant di†erences from
2
3
x 353, ox
Co/SiO is ascribed to the lower mobility of carbidic carbon
that on a CoIIY sample prepared by ion exchange,37 apparent
from the 20 cm~1 red shift of the NwO band and by the
comparison of ONwCowNO bond angles.38 Naccache et
al.39 suggested that it is possible to distinguish between nitro-
syl complexes located in small cavities and those located in
large cavities by following the relative rates of reaction
between the complexed nitric oxide and molecular oxygen. It
is generally believed that NO is able to enter the sodalite
2
and CH fragments.
The combination of the catalysis of cobalt clusters with
steric constraints imposed by the NaY zeolite structure carries
great potential for activity and selectivity of methane hydro-
genation. The cages allow CH species to migrate on the
x
surface of the cobalt clusters but without di†usion, owing to
the constraints of the zeolite channels. On the other hand,
cobalt clusters inside supercages retain a small cluster size,
which results in the small [CH ]
x 2`
the desired C hydrocarbons. Such control of [CH ] Ðlm
2` x 2`
size would greatly reduce the formation of coke.
Based on the determination of carbonaceous adspecies and
the kinetic analyses, the reaction mechanism is most likely to
consist of: (i) dissociative chemisorption of CH , accompanied
cages (b-cages) at 298 K, but that O is not. In order to
2
size, as the precursors to
employ this potentially diagnostic technique, the
[CoII(NO) ]
/NaY was exposed to O at 298 K and the
2 353, ox
2
pair of NO bands of [CoII(NO) ]
/NaY at 1814 and 1894
2 353, ox
cm~1 rapidly decreased, suggesting the [Co ]
were still located in the a-cages of NaY, as Co (CO)
clusters
x 353, ox
.
4
12
4
Procedure for catalysis of methane hydrogenation
by H release, (ii) CwC bond formation and growth of hydro-
2
carbon chains and (iii) subsequent hydrogenation of [CH ]
x 2`
An important aspect of our procedure is that the
Ðlm on the cobalt catalyst. Step 2 seems to be the rate deter-
[Co ] /NaY was exposed to Ñowing CH ÈHe, so that H
produced was continuously removed from the surface. This
increased the total amount of adsorbed and its hydrogen deÐ-
x red
4
2
mining step.
Conclusions
ciency. Based on CH adspecies determined by IR and mass
x
NaY-encapsulated Co (CO) clusters were synthesized by the
spectra, one may propose that the processes following CH
chemisorption consists of the stepwise dehydrogenation of
4
12
4
adsorption of Co (CO) onto dehydrated NaY under an N
2
8
2
atmosphere. The clusters and their decarbonylated products
CH groups:
x
1192
J. Chem. Soc., Faraday T rans., 1997, V ol. 93

View Article Online
15 F-S. Xio, A. Fukuoka and M. Ichikawa, J. Catal., 1992, 138, 206.
16 P. A. Jacobs, in Metal Clusters in Catalysis, ed. B. C. Gates, L.
Guczi and H. Knozinger, Elsevier, Amsterdam, 1986, p. 357.
17 G-C. Shen, A-M. Liu, T. Shido and M. Ichikawa, T op. Catal.,
1995, 2, 141.
18 J. B. A. D. van Zon, D. C. Koningsberger, H. F. J. vanÏt Blik and
D. E. Sayers, J. Chem. Phys., 1985, 12, 5742.
have been structurally characterized by in situ IR, EXAFS,
di†use reÑectance spectroscopies and NO chemisorption.
EXAFS coordination numbers showed that the successively
decarbonylated sample maintained its small cluster size, which
depended on the temperature-programmed oxidation of the
precursor.
19 N. Kosugi and H. Kuroda, Program EXAFS 2, Research Center
for Spectrochemistry, University of Tokyo, 1988.
20 Y-C. Xie and Y-Q. Tang, Adv. Catal., 1990, 37, 1.
21 S. Qui, R. Ohnishi and M. Ichicawa, J. Chem. Soc., Chem.
Commun., 1992, 1425.
Methane was converted to heavier hydrocarbons by its che-
misorption on supported cobalt catalysts and subsequent
hydrogenation. NaY-encapsulated Co clusters showed
a
higher activity and selectivity of C hydrocarbons, in com-
2`
parison with a Co/SiO catalyst. The IR and mass spectra and
22 F. A. Cotton and R. R. Monchamp, J. Chem. Soc., 1960, 1882.
23 (a) R. L. Schneider, R. F. Howe and K. L. Watters, Inorg. Chem.,
1984, 23, 4600; (b) M. C. Connaway and B. E. Hanson, Inorg.
Chem., 1986, 25, 1445.
2
the reaction studies proved that CH (x \ 0, 1, 2) adspecies
x
were generated by methane thermal dissociation on the
[Co ] /NaY catalyst. The CH fragment is quite reactive
x red
2
24 Y. Iwasawa, M. Yamada, Y. Sato and H. Kuroda, J. Mol. Catal.,
1984, 23, 95.
enabling propagation of higher hydrocarbons.
25 R. A. Friedel, I. Wender, S. L. ShuÑer and H. W. Sternberg, J.
Am. Chem. Soc., 1995, 77, 3951.
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Paper 6/05124C; Received 23rd July, 1996
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