Chemical reactivity of solid [PtnC60] towards carbon monoxide

Article abstract of DOI:10.1016/S0022-3697(03)00284-1

Evidence of chemical reactivity of solid platinum-fullerene [Pt _nC₆₀] compounds towards carbon monoxide is presented. The interaction was systematically studied by means of infrared spectroscopy, X-ray powder diffraction and thermogravimetric analysis. The interaction of carbon monoxide, even under low pressure, is confirmed by the appearance of infrared absorption bands in the CO stretching region at 2064, 2014 and 1991 cm ^-1 for the carbonylation products. The exceptions were those products with low Pt:C₆₀ ratios, which also displayed bands at 1870 and 1830 cm^-1. The data suggest that the CO coordination depends on the specific morphology of the solids, the original Pt:C₆₀ ratio, and the carbon monoxide nominal pressure. Therefore, these results indicate the formation of [(CO)_xPt]_m species supported in a fullerene matrix mixed with [Pt_n-mC₆₀] compounds. As there is a competition between carbon monoxide and fullerene molecules for the electronic density at the platinum centers, the nature of the CO interaction with [Pt _nC₆₀] was found to be destructive, leading to the displacement of the latter. Nevertheless, the platinum-carbonyl species formed presents relatively high stability, as shown by desorption tests.

Full text of DOI:10.1016/S0022-3697(03)00284-1

Journal of Physics and Chemistry of Solids 64 (2003) 2423–2428
www.elsevier.com/locate/jpcs
Chemical reactivity of solid ½Pt C Š towards carbon monoxide
n 60
*
Marcelo Hawrylak Herbst , Gilson Herbert Magalh a˜ es Dias
Instituto de Qu ´ı mica, Universidade Estadual de Campinas, CP 6154, Campinas SP 13873-970, Brazil
Received 23 October 2002; revised 15 July 2003; accepted 22 July 2003
Abstract
Evidence of chemical reactivity of solid platinum-fullerene ½Pt C Š compounds towards carbon monoxide is presented. The
n
60
interaction was systematically studied by means of infrared spectroscopy, X-ray powder diffraction and thermogravimetric
analysis. The interaction of carbon monoxide, even under low pressure, is confirmed by the appearance of infrared absorption
2
1
bands in the CO stretching region at 2064, 2014 and 1991 cm for the carbonylation products. The exceptions were those
2
1
products with low Pt:C60 ratios, which also displayed bands at 1870 and 1830 cm . The data suggest that the CO coordination
depends on the specific morphology of the solids, the original Pt:C60 ratio, and the carbon monoxide nominal pressure.
Therefore, these results indicate the formation of ½ðCOÞ PtŠ species supported in a fullerene matrix mixed with ½Pt C Š
x
m
n2m 60
compounds. As there is a competition between carbon monoxide and fullerene molecules for the electronic density at the
platinum centers, the nature of the CO interaction with ½Pt C Š was found to be destructive, leading to the displacement of the
n
60
latter. Nevertheless, the platinum-carbonyl species formed presents relatively high stability, as shown by desorption tests.
q 2003 Elsevier Ltd. All rights reserved.
Keywords: A. Fullerenes; A. Amorphous materials; C. Infrared spectroscopy; B. Chemical synthesis
1
. Introduction
order to obtain new and suitable materials for catalytic and
gas sensor applications, mainly transition metal fullerene
compounds [5a–c].
The diffusion of gaseous molecules into the interstitial
cavities of solid fullerenes is well documented in the
literature [1a–d]. Moreover, the adsorption of nitrogen and
argon at 77 K on solid fullerenes has been reported to be less
effective than in graphite [2]. Attempts to employ fullerenes
as support of metallic particles in heterogeneous catalytic
processes have not attracted much attention when compared
to the most common supports, as for example amorphous
carbon [3]. These results could be due to the low superficial
The coordination of fullerenes to electron rich transition
metals such as palladium, platinum and ruthenium leads to
loss of crystallinity, originating insoluble solids whose
catalytic properties are similar or even superior to the
metallic dispersions over amorphous activated carbons
[
6a–c]. In comparison to pure fullerene, those novel
materials have each metal atom linked to two adjacent
fullerene molecules without forming clusters, and super-
ficial area ca. 20-fold higher, affording atomic dispersion
properties [7a–c].
2
area (ca. 5 m /g) and low sublimation temperature (ca.
7
00 K) of fullerenes [4]. Nevertheless, a combination of the
Herein we report chemical reactivity evidences for solid
platinum-fullerene compounds towards carbon monoxide.
The interaction was systematically studied by means of
infrared spectroscopy, and the products were also analyzed
by X-ray powder diffraction and thermogravimetric analysis
properties of conventional supports, affording high super-
ficial areas, such as silica and fullerenes, led to more fruitful
applications in catalytic processes [3]. Therefore, structural
and electronic properties of fullerenes can be investigated in
(
TGA). The absorptions in the CO stretching region in the
*
Corresponding author. Address: Instituto de F ´ı sica, UFRJ, Rio
infrared spectra show that under carbon monoxide pressure
there occurs the formation of carbonyl species containing
½Pt C Š: Moreover, the carbon monoxide molecules coexist
de Janeiro, RJ CP 68528, 21945-970, Brazil. Tel.: þ55-21-2562-
7
341; fax: þ55-21-2562-7368.
E-mail address: herbst@if.ufrj.br (M.H. Herbst).
n
60
0022-3697/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0022-3697(03)00284-1

2424
M.H. Herbst, G.H.M. Dias / Journal of Physics and Chemistry of Solids 64 (2003) 2423–2428
in different coordination sites in the carbonylated products
prepared under high pressure, even after heating treatment
under reduced pressure, suggesting a strengthening of the
Pt–CO bond. The displacement of fullerene molecules from
the platinum coordination sphere was also observed under
high pressure conditions with a reduction in the formation of
carbonyl species.
Table 1
Wavenumbers of infrared absorptions in the CO stretching region
observed for ½Pt C60Š exposed to 1 bar of carbon monoxide for 1 h
n
21
Pt:C60 ratio nCO (cm
)
Terminal
Bridging
1
1
.0; 1.2
.5; 2.0
2064 (S), 2014 (s), 1991 (s)
2066 (S), 2014 (w), 1990 (w)
1871 (w), 1830 (w)
–
Intensities: S ¼ strong; s ¼ shouder; w ¼ weak.
2
. Experimental
Platinum-fullerene ½Pt C Š compounds were prepared
n
60
for 1 h. They clearly form two sets, which are related by the
amount of platinum.
as described elsewhere from the reaction between Pt(cod)
2
(
cod ¼ 1,5-cyclooctadiene) and C60 [6b], and dried under
The observation of more than one CO stretching
absorption band in the infrared spectrum may be ascribed
to the existence of different chemisorption sites. Moreover,
the simultaneous observation of typical absorption bands for
terminal and bridging carbonyl species with different
intensities suggests that the surface is heterogeneous to a
large extent. It should be noted, however, that the bands
assigned to bridging carbonyls are observed only in the
spectra of the two compounds whose platinum content is
close to one. On the other hand, the other two compounds
with Pt:C60 ratios greater than 1.5:1 do not show such
carbonyl species. This behavior could be attributed to a
facile saturation of platinum coordination sites in the first
two compounds, which present less available coordination
sites than their higher platinum content counterparts.
The thermal stability of the platinum-fullerene com-
pounds was also investigated by a thermogravimetric study
under synthetic air before and after CO chemisorption, in
the temperature range between 25 and 600 8C. The results,
listed in Table 2, suggest that the formation of carbonyl
species implies less thermal stability for the two compounds
whose Pt:C60 ratio is close to one, presumably due to the
displacement of fullerene from the platinum coordination
with the possible formation of unstable platinum carbonyl
species, which decompose more rapidly under the air flow
when compared to the parent platinum-fullerene com-
pounds. Such observation suggests that in these compounds
fullerene exclusion from platinum coordination is more
reduced pressure at room temperature for removing all
volatile species. Products with four different morphologies
were obtained when the platinum to C60 ratios varied in the
range from 1 to 2 [7c], and were shown to be structurally
similar, according to infrared absorption and Raman spectra,
previously reported by us [6b]. Carbon monoxide (Air
Liquide) and KBr (Aldrich) were grade products. The
carbon monoxide chemisorption/desorption measurements
over KBr pellets with about 5% ½Pt C Š were carried out by
n
60
infrared absorption spectroscopy (Bomem MB, FT-IR).
Samples were prepared by admitting carbon monoxide up to
1
bar into a Schlenk tube containing the samples. The high-
pressure experiments were carried out in a stainless steel
reactor fitted with Teflon. The same procedure, but in
absence of KBr, was used for X-ray powder diffraction
(Shimadzu XRD-6000, Cu Ka) and TGA analysis (Shi-
madzu TA-50) of the samples.
3
. Results and discussion
The carbon monoxide chemisorption is commonly
studied in solids by means of infrared spectroscopy,
allowing the identification of the several ways of interaction.
However, while the changes in the spectrum of the adsorbed
species, compared to the free molecule, are readily assigned,
bands originated from the interaction between the adsorbent
and adsorbate are not at all assigned. Moreover, the infrared
technique allows a distinction between chemisorption and
physical adsorptions: whereas the absence of new bands in
the spectrum after an interaction does not necessarily mean a
physical adsorption, their observation can be unequivocally
associated to a chemisorption [8]. As mentioned above, the
infrared spectra of pristine ½Pt C Š have no absorption
Table 2
Decomposition temperatures of pristine ½Pt C60Š and of their
n
carbonylated products as measured by TGA analysis
Pt:C60 ratio
Tdec: (K)
Pristine
CO exposed
n
60
2
1
bands in the region between 2100 and 1750 cm . There-
fore, when a band is observed in this region after exposing
the samples to carbon monoxide it will be characteristic of
the latter. In Table 1 are listed the wavenumbers of the
absorptions observed in the infrared spectra of the platinum-
fullerene compounds subjected to 1 bar of carbon monoxide
1
1
1
2
.0
.2
.5
.0
596
582
536
543
515
526
538
548
Defined as the temperature of maximum weight loss.

M.H. Herbst, G.H.M. Dias / Journal of Physics and Chemistry of Solids 64 (2003) 2423–2428
2425
severe in comparison with their high platinum content
analogues.
the fullerene molecule, until here treated as a mere support,
a further experiment was carried out. The infrared spectra of
the samples exposed to CO were measured in the range from
Analysis of the solid residues after heating at 600 8C of
all four compounds by X-ray powder diffraction showed that
only metallic platinum was formed. It is worth of mention
that the conditions employed in the TGA experiments (air
flow 10 ml/min, heating rate 5 8C/min) afford data that are
unable to ascribe to different contents of CO, and that
fullerene solids deposited onto cool parts of the thermo-
balance were not detected.
2
1
2500 to 400 cm . It was noted after CO exposure that the
spectra of the platinum-fullerene compounds displayed in
addition to the usual strong carbonyl bands a weak
2
1
absorption at 476 cm
assigned to a Pt–CO vibration
mode [12]. Other four sharp and intense absorption bands,
2
1
observed at 526, 577, 1182 and 1428 cm , are character-
istic of the uncoordinated fullerene molecule. All these new
bands are superimposed on the spectra of the platinum-
fullerene compounds, suggesting that the fullerene mol-
ecules were partially displaced from the platinum coordi-
nation. The spectra of the platinum-fullerene compounds
and of the carbonylated products are shown in Fig. 1.
It should be mentioned that the same behavior was
observed in the presence of solvent. When a suspension of
Complementary studies were carried out to investigate
the stability and the reversibility of the interactions of these
novel platinum-fullerene compounds with carbon monox-
ide. Although the compounds have been maintained under
reduced pressure for 15 h, no noticeable change was
observed in the infrared spectra. This result also confirms
the carbon monoxide chemisorption over ½Pt C Š; with the
n
60
formation of relatively stable carbonyl species, instead of a
mere physical adsorption. Nevertheless, the products were
heated at 373 K under vacuum for about 5 h in another
experiment with the loss of bridging carbonyls and an
absorption band shift of the remaining terminal carbonyl
½
Pt C Š in toluene is treated with a gentle carbon monoxide
n
60
stream, a magenta solution is formed in a few minutes. After
solvent removal under vacuum at low temperature, a
brownish solid is recovered. The infrared spectra of this
solid either in KBr pellet or in CH Cl solution show
2
2
1
2
from 2064 to 2059 cm . The small shift of the latter band
when compared to the non-treated sample can be attributed
to a decrease in the concentration of chemisorbed terminal
CO molecules, in terms of a lower dipolar interaction among
chemisorbed adjacent CO molecules. The reversibility of
the CO chemisorption was also tested by submitting the
heated pellets to 1 bar of carbon monoxide pressure for 1 h,
absorption bands in the CO stretching region, at 2058, 2016
21
and 1978 cm . However, the solid decomposes rapidly
under normal conditions, and the infrared absorption bands
are no longer observed.
As mentioned earlier, platinum-fullerene compounds
under ordinary CO pressures behave distinctively with the
platinum content. In order to verify this behavior, high-
pressure experiments were performed by submitting the
compounds to carbon monoxide pressures from 3 to 25 bar.
Whereas a nominal pressure of 3 bar increased only the
intensities of the absorption bands, the pressure of 25 bar
2
1
but only the 2059 cm band was observed without
significant changes in intensity.
It is interesting to note that similar results were reported
in the literature, particularly in investigations on oxide-
supported platinum [9]. Moreover, it was also observed that
the nature of the support has strong influence on the
stretching frequency of metallic carbonyls. Generally, the
higher the acidity of the support, the higher the stretching
frequencies of the chemisorbed carbon monoxide [10]. This
effect is commonly explained by the electronic density
donation from the metal to the support. In terms of HOMO
and LUMO frontier orbitals in the MO approach, the
electronic states of the surfaces near the Fermi level in solid
materials are equivalent to the HOMO. The synergistic
effects of CO chemisorption manifest themselves in a
lowering of the CO bonding stretching frequency, due to the
favoring of the p back-donation from the filled metal d
2
1
induced new bands at 2013 and 1989 cm in the infrared
spectra. A substantial increase in the intensity of the whole
spectrum was also noted, as shown in Fig. 2.
The absorption bands related to the infrared spectrum of
pure C60 shown in Fig. 2 have their intensities enhanced
with an increase in the carbon monoxide pressure,
suggesting that the pressure promotes the removal of
fullerene molecules from the platinum coordination. One
can argue that under these conditions there is formation of
½
ðCOÞ PtŠ species mixed with ½Pt C Š compounds,
x
m
n2m 60
where n is the original platinum fullerene ratio, m is the
amount of platinum excluded from fullerene coordination
and x denotes the nuclearity of the carbonyl species.
p
orbitals to the empty 2p molecular orbitals of carbon
monoxide.
Concerning the stability of the carbonyl species formed
under high-pressure conditions, a desorption test was
conducted by maintaining the samples overnight under
vacuum. Since no noticeable change was observed in the
infrared spectra, the result reinforced the previous con-
clusion about a relative high stability for the carbonyl
species. However, there was a shift of the band assigned to
the terminal carbonyl species formed at a pressure of 25 bar
Concerning the influence of the nature of the support in
the stretching frequency of the CO vibration, fullerenes
behave like an electron deficient polyolefin with p-acid
characteristics [11]. Therefore, some competition is
expected between C60 and CO for the electronic density at
the platinum atom, as was indeed suggested by the strong
shift in the CO stretching frequency from the free CO and
the chemisorbed species. In order to investigate the role of
2
1
from 2064 to 2052 cm , and the absence of the bands at

2426
M.H. Herbst, G.H.M. Dias / Journal of Physics and Chemistry of Solids 64 (2003) 2423–2428
Fig. 1. Transmitance IR spectra of pristine ½Pt C60Š (A) and after exposition to CO at 1 bar (B). The fullerene absorptions are marked.
n
2
1
2
013, 1989, 1879 and 476 cm for the samples heated to
73 K for 1 h in vacuum, as shown in Fig. 3.
presumably of crystalline fullerene (peak near 2u ¼ 30;
corresponding to the fcc C60 [311] diffraction). On the other
hand, the two solids of higher platinum content exposed to
carbon monoxide did not present distinct peaks in the region
between 10 # 2u # 50: Despite the high noise levels of
these diffractograms, such distinctive profiles further
support our initial assumption of different degrees of
saturation upon low pressures of carbon monoxide.
3
It is interesting to note that the X-ray powder
diffractograms of the solids exposed to CO show distinctive
profiles with respect to the platinum content. The solids with
lowest platinum content display broad and weak peaks
assigned to the formation of metallic platinum (prominent
peak near corresponding to the Pt [111] diffraction) and
Fig. 2. Absorbance IR spectra of ½Pt C60Š submitted to CO at 3 bar (A) and at 25 bar (B).
n

M.H. Herbst, G.H.M. Dias / Journal of Physics and Chemistry of Solids 64 (2003) 2423–2428
2427
Fig. 3. Absorbance IR spectra of ½Pt C60Š submitted to CO pressure of 25 bar (A) and after heating at 373 K under vacuum for 1 h (B).
n
4
. Conclusion
Acknowledgements
Authors wish to thank FAPESP for financial support
and Professor O.L. Alves (IQ-UNICAMP) for kindly
permitting to use the TGA apparatus. We are indebted to
Professor C.A.L. Filgueiras (IQ-UFRJ) for revising the
manuscript.
Infrared spectroscopy, TGA and X-ray powder
diffraction suggest the formation of {ðCOÞ Pt C }
x
n2m 60
species due to chemical interaction between CO and ½
Pt C Š: These platinum-carbonyl species supported in a
n
60
fullerene matrix are mixed with ½Pt C Š compounds,
n2m 60
where the value of ðn 2 mÞ depends on several aspects of
the original Pt:C60 ratio and the carbon monoxide
nominal pressure. The results also point to a distinctive
behavior upon CO coordination, which depends on the
specific morphology of the solids. The nature of the CO
interaction over ½Pt C Š is found to be destructive, that
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