Acetylene oligomerization on the surface of TiO2: A step forward in the in situ synthesis of nanostructured carbonaceous structures on the surface of photoactive oxides

Article abstract of DOI:10.1039/c4ta01581a

The interaction of acetylene with TiO₂ (Evonik P25) was studied as a function of gas pressure and contact time by in situ vibrational (infrared and Raman) and electronic (UV-vis) spectroscopy and by gas chromatography-mass spectrometry analysis (GC-MS) of the reaction products. At low pressure the reaction proceeds through surface adsorption of acetylene and cyclotrimerization to benzene. At higher pressure (P > 100 mbar) the adsorption step is followed by the progressive formation of more complex oligomerization products containing an increasing number of conjugated double bonds. The final products obtained after mild heating at 373 K in acetylene excess absorb in the visible spectral region and confer to the system a strong blue color. These colored species are stable for many days in pure oxygen or air and cannot be extracted with common solvents, appearing strongly anchored to the TiO₂ surface. The formation of saturated -CH₃ and -CH₂ groups during the oligomerization process and the spectroscopic and GC-MS results show that the colored species consist of polycyclic aromatic hydrocarbons containing a considerable number of condensed rings. Following these results, the controlled oligomerization of acetylene can represent a route for direct production of graphene-like species tightly anchored to the TiO₂ surface, i.e. of composite materials of potential interest in photocatalysis or photovoltaic applications. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ta01581a

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Materials Chemistry A
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PAPER
Acetylene oligomerization on the surface of TiO2: a
step forward in the in situ synthesis of
nanostructured carbonaceous structures on the
surface of photoactive oxides†
Cite this: J. Mater. Chem. A, 2014, 2,
12247
Sagar Motilal Jain, Jakub Jan Biedrzycki, Valter Maurino, Adriano Zecchina,
*
*
Lorenzo Mino and Giuseppe Spoto
The interaction of acetylene with TiO2 (Evonik P25) was studied as a function of gas pressure and contact
time by in situ vibrational (infrared and Raman) and electronic (UV-vis) spectroscopy and by gas
chromatography-mass spectrometry analysis (GC-MS) of the reaction products. At low pressure the
reaction proceeds through surface adsorption of acetylene and cyclotrimerization to benzene. At higher
pressure (P > 100 mbar) the adsorption step is followed by the progressive formation of more complex
oligomerization products containing an increasing number of conjugated double bonds. The final
products obtained after mild heating at 373 K in acetylene excess absorb in the visible spectral region
and confer to the system a strong blue color. These colored species are stable for many days in pure
oxygen or air and cannot be extracted with common solvents, appearing strongly anchored to the TiO2
surface. The formation of saturated –CH3 and –CH2 groups during the oligomerization process and the
spectroscopic and GC-MS results show that the colored species consist of polycyclic aromatic
hydrocarbons containing a considerable number of condensed rings. Following these results, the
controlled oligomerization of acetylene can represent a route for direct production of graphene-like
species tightly anchored to the TiO2 surface, i.e. of composite materials of potential interest in
photocatalysis or photovoltaic applications.
Received 1st April 2014
Accepted 4th June 2014
DOI: 10.1039/c4ta01581a
www.rsc.org/MaterialsA
TiO2 with narrow band gap semiconductors.9 All these methods
aim to reduce the extension of the TiO2 band gap either by
Introduction
creating inter-band (defective) states or by band bending.
An alternative method is sensitization of TiO2 by anchoring
appropriate dye molecules (sensitizer) on its surface.10 In this
case, the key steps to increase the photo-efficiency are excitation
of the sensitizer by visible light and subsequent charge transfer
from the excited dye to the active solid. Presently Ru-based
complexes are the most widely employed dyes for photocatalytic
applications11,12 as well as for photovoltaic conversion using
dye-sensitized solar cells (DSSC),13 notwithstanding consider-
able drawbacks represented by their cost and low chemical
stability.
In connection with the sensitization route, an interesting
observation comes from the work of Rives-Arnou and Shep-
pard14 dating back to 1980 concerning a Raman study of the
interaction of acetylene with the TiO2 rutile polymorph. It was
found that under laser irradiation acetylene polymerizes on the
oxide surface to give adsorbed coloured trans-polyacetylene
chains.14 In the context sketched above this result can be
considered a promising starting point to develop a method to
produce in situ a dye molecule directly on the surface of a
photoactive solid. For this reason, we decided to investigate the
Efficient conversion of solar energy into electrical power by
photovoltaic devices1 or into fuels by articial photosynthesis2,3
is among the most challenging goals in modern science. The
strategies aimed at these purposes include the use of photo-
active materials based on semiconductors (see Kubacka et al.4
and references therein). Among them, titanium dioxide
emerges as one of the most promising candidates because of
favourable properties like photo-stability, affordable cost and
low toxicity. The main drawback of TiO2 is its large band gap
(3.2 eV for the anatase polymorph),5 which restrains absorption
of light to the ultraviolet region, i.e. in a small fraction only
(about 5%) of the whole solar spectrum. Much effort has been
devoted to increasing signicantly the conversion efficiency of
TiO2 by extending the response to the more favourable visible
region. These include doping with metal and non-metal impu-
rities,5–7 synthesis of oxygen-decient forms,8 and coupling of
Department of Chemistry and NIS Centre of Excellence, University of Turin, via P.
Giuria 7, 10125 Torino, Italy. E-mail: lorenzo.mino@unito.it; giuseppe.spoto@
unito.it
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ta01581a
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reactivity of acetylene on TiO2 P25, a material prevalently con- Raman and UV-vis spectroscopic results and gas chromatog-
taining the anatase polymorph (85%) and which is oen raphy-mass spectrometry analysis.
considered a benchmark for photovoltaic and photocatalytic
applications.
Experimental
For the scope of this paper it is also worth recalling that,
whatever is the excitation frequency, limits to the efficiency of TiO2 used in these experiments was commercial Degussa
solar energy conversion by using semiconductor-based systems (Evonik) P25, which is considered a benchmark in the eld of
can also derive from fast charge recombination between the photocatalysis owing to its outstanding activity. P25 is prepared
photo-generated electrons and holes. The strategies proposed to by ame hydrolysis of TiCl4 and contains a mixture of about
enhance charge separation on TiO2 include the modication of 85% anatase and 15% rutile. The P25 surface area is about
the surface and bulk structure as well as the preparation of 60 m2 gꢀ1
.
20,21
nanostructures where a second phase can preferentially trap
electrons or holes.15
Before acetylene dosage the P25 samples underwent the
following consecutive treatments: (i) outgassing at 773 K for 2
In this respect, much interest has been recently shown hours under high vacuum (residual pressure < 10ꢀ4 mbar), (ii)
towards composites of TiO2 with carbonaceous materials char- oxidation at 623 K with O2 (15 mbar for 20 minutes, repeated
acterized by unusual structural and electronic properties, like twice), and (iii) cooling down to 293 K in an O2 atmosphere and
carbon nanotubes, fullerenes and graphene nanosheets.16 In outgassing of the gas phase at the same temperature. The
fact these carbon structures combine high affinity for electrons treatment with O2 was performed to ensure the full stoichi-
with favourable characteristics like chemical inertness, ometry of the resulting TiO2 phases, designated as activated
stability, and tunable textural and chemical properties.16–18 The TiO2 or, alternatively, activated P25.
methodologies for preparing TiO2/carbon composites include
All the above treatments were performed in the same cells
the synthesis of the photoactive solid from suitable precursors used for the spectroscopic measurements which also allowed
in the presence of the carbon material, solution mixing, dosage of acetylene from the gas phase. Before use, acetylene
hydrothermal and/or solvothermal methods, CVD, etc.16,18
was distilled by repeated freeze/thaw cycles to avoid the pres-
In particular, as far as the preparation of metal oxide/gra- ence of any gaseous impurities. The TiO2 samples were in the
phene composites is concerned, the most widely used tech- form of self-supporting pellets.
nique is the chemical reduction of graphene oxide (GO). Indeed,
Infrared spectra were recorded on a Bruker IFS 66 FTIR
using solution mixing or hydrothermal methods, semi- spectrometer equipped with an MCT cryogenic detector; 64
conductor nanoparticles or their precursors are usually loaded interferograms (recorded at 2 cmꢀ1 resolution) were typically
on graphene oxide sheets which are then reduced to graphene.17 averaged for each spectrum.
However, it is expected that signicant improvements of
Raman spectra were recorded by using a Renishaw Raman
performances could be achieved by more efficient synthetic Microscope spectrometer. An Ar+ laser emitting at 514 nm was
routes ensuring a better contact between the carbonaceous used and the output power was limited to 1% (100% power ¼
phase and the semiconductor surface. A closer contact could in 8.2 mW at the sample) in order to avoid sample damage.
fact improve the efficiency of the electron transfer step from the
Diffuse reectance UV-visible spectra were recorded on a
scaffold to graphene and hence the efficiency of the whole Cary 5000 Varian spectrophotometer equipped with a reec-
photoactive system.17,18 In this regard, it is conceivable that in tance sphere on thick self-supported pellets.
situ synthesis of the carbonaceous phase directly on the TiO2
Gas chromatography-mass spectrometry (GC-MS) were per-
surface could satisfy the close-contact requisite better than the formed with an Agilent gas chromatograph mod 6890 plus
preparation routes cited above (where one phase is deposited equipped with a Varian VA-5MS column (30 m length, 0.25 mm
onto the other). An example of in situ synthesis of graphene/ ID, 0.25 mm phase thickness) and an Agilent mass spectrometer
TiO2 composites is reported by Kamegawa et al.19 with a system mod 5973 N. Analytical conditions were as follows: an injector
where TiO2 anatase is supported on MCM-41 mesoporous silica. in pulsed splitless mode (20 PSI) at 573 K; sample volume 1 mL; a
The graphene layer was in fact obtained by graing 2,3-dihy- column oven at 308 K for 6 minutes, rst temperature ramp at
droxynaphthalene on the TiO2 surface and subsequent 10 K minꢀ1 up to 453 K, second temperature ramp at 4 K minꢀ1
carbonization at 1073 K, i.e. at a temperature where anatase is up to 573 K; carrier: He at 1 mL minꢀ1 constant ow.
known to undergo phase transformation into a less photoactive
rutile polymorph.
In this paper we show that polycyclic condensed hydrocar-
bons, possibly representing intermediate steps toward the in
Results and discussion
(1) FTIR spectroscopy
situ synthesis of graphene layers in close contact with the As shown below, the composition of the surface products
surface, can be obtained on TiO2 by acetylene oligomerization formed by the interaction of acetylene with TiO2 P25 depends
under very mild conditions (room temperature and sub-atmo- on the pressure of the gas phase as well as on the contact time.
spheric C2H2 pressure), which avoid the anatase to rutile For this reason, two series of FTIR experiments are illustrated
transformation.
and discussed here: in the rst series, a collection of trans-
The reactivity of the acetylene/TiO2 system and the possible mission spectra was obtained by gradually increasing the acet-
structure of the products are discussed on the basis of FTIR, ylene equilibrium pressure from 0.5 up to 10 mbar (low pressure
12248 | J. Mater. Chem. A, 2014, 2, 12247–12254
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experiment: Fig. 1); in the second the spectral sequence was region22,23 centered at 3225 cmꢀ1, accompanied by an intense
generated by contacting the activated oxide with a single dose of shoulder at 3200 cmꢀ1 and weaker features at ca. 3336, 3248 and
acetylene at an initial pressure of 120 mbar and then collecting 3160 cmꢀ1; (ii) a weak doublet in the n(C^C) stretching
spectra at increasing contact time (high pressure experiment: region22,23 with a main component at ca. 1950 cmꢀ1 and a less
Fig. 2). In the high pressure experiment the subsequent effect intense satellite at ca. 1945 cmꢀ1; (iii) a weak and complex
on the reaction products formed aer 30 minutes of contact by absorption in the 1400–1300 cmꢀ1 region; (iv) weak features at
increasing the temperature up to 373 K in the presence of ca. 1690 and 1618 cmꢀ1
acetylene gas was also investigated (Fig. 2).
The assignment of the bands in the 3250–3160 cmꢀ1 interval
.
(a) Low C2H2 pressure experiment. The results of a low to the stretching modes of C2H2 molecules perturbed by inter-
pressure experiment are reported in Fig. 1. The spectrum action with the TiO2 surface is straightforward and supported
obtained aer the rst small dose of C2H2 (red curve in Fig. 1) by the literature concerning the adsorption of acetylene on
shows: (i) a main complex absorption in the n(^C–H) stretching other oxides24 and zeolites.25,26 The interaction with the surface
of the solid is also responsible for the activation of the n(C^C)
vibration giving rise to a weak (and possibly complex) absorp-
tion at 1954 cmꢀ1, which is red-shied 20 cmꢀ1 compared with
the gas phase value, and for the presence of weak absorptions
centered at ca. 1360 cmꢀ1, close to the frequency of the
combination between the symmetric and anti-symmetric HCC
bending modes (dsym + dasym), measured at 1328 cmꢀ1 in
gaseous C2H2.27
The formation of the bands due to adsorbed acetylene is
apparently accompanied by the erosion of the manifestations
due to the TiO2 residual surface OH groups in the 3750–
3600 cmꢀ1 region20,28 and the appearance of a broad and
complex absorption in the 3600–3350 cmꢀ1 interval (with
maxima becoming more evident on increasing the acetylene
coverage at ca. 3550, 3500 and 3450 cmꢀ1). Although these
Fig. 1 Acetylene/TiO2 low pressure FTIR experiment: the effect of
increasing the acetylene pressure. Red curve: after a dosage of
0.5 mbar of C2H2 on activated TiO2 P25. Black curve: after a dosage of
10 mbar of acetylene. Grey curves: intermediate pressures. All the
spectra are background subtracted and therefore negative bands
correspond to species which are consumed upon C2H2 dosage, while
the positive bands correspond to species which are formed.
phenomena certainly suggest the formation of families of OH/
C2H2 hydrogen bonded surface adducts, this interaction
mechanism alone cannot completely explain the complex
manifestations of the adsorbed acetylene molecules in Fig. 1.
Indeed, similar interactions on the strongly acidic OH Brønsted
sites of zeolites, expected to perturb the adsorbed C2H2 mole-
cules strongly than the weak acidic OH groups of TiO2, result in
a shi of the n(C–H) mode of only ꢀ37 cmꢀ1 while the shis
observed here are as large as 60 cmꢀ1 and higher (up to ꢀ127
cmꢀ1). Also, the shi of the n(C^C) mode actually found here
on TiO2 (ca. ꢀ20 cmꢀ1) appears to be too large when compared
with that observed on the strongly acidic OH groups of zeolites
(ꢀ24 cmꢀ1).
On the basis of these considerations, we can infer that
adsorption of acetylene on TiO2 occurs also on OH-free regions
of the surface and involves coordinatively unsaturated surface
sites or Ti–O acid–base pairs located on different exposed faces,
corners or steps.29
Concerning the doublet of weak bands at ca. 1690 and
1618 cmꢀ1 (Fig. 1), they are in a region where conjugated double
bonds are expected to fall, although they do not nd exact
correspondence with those of the polyacetylenic species repor-
ted by Rives-Arnou and Sheppard14 on TiO2 or by some of us in
zeolites (where they have a carbocationic character).25
Coming to the evolution of the spectra upon increasing the
acetylene pressure, it can be seen from Fig. 1 that the n(^CH)
component at 3225 cmꢀ1 grows preferentially in intensity (while
slightly downward shiing to 3220 cmꢀ1) with respect to the
satellites at ca. 3330, 3250 and 3160 cmꢀ1 suggesting that this
band could be associated with acetylene molecules adsorbed on
Fig. 2 Acetylene/TiO2 high pressure FTIR experiment: the effect of
increasing the contact time and the temperature after a dosage of
120 mbar of C2H2 on activated TiO2 P25. Red curve: immediately after
the dosage of 120 mbar of C2H2. Black curve: after 30 minutes at the IR
beam temperature. Grey curves: intermediate contact time (about 3
minutes for each spectrum). Blue curve: after heating in an acetylene
atmosphere at 373 K for 30 minutes (the spectrum has been vertically
shifted for the sake of clarity). As in Fig. 1, all the spectra are back-
ground subtracted.
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abundant surface sites, possibly like those on extended faces. complete erosion of those due to residual OH groups at 3722,
The contemporary growth of the band of the n(C^C) mode at 3690 and 3670 cmꢀ1. This observation further conrms that
1950 cmꢀ1 is noticeable.
acetylene adsorption occurs also on OH-free regions of the TiO2
In parallel, the OH bands due to residual hydroxyl groups at surface and involves coordinatively unsaturated surface sites on
3722, 3690 and 3670 cmꢀ1 are completely eroded in favor of the the extended faces and on steps and defects.
broad and complex band in the 3600–3000 cmꢀ1 region due to
From the red curve in Fig. 2 and by comparison with the
hydrogen bonded species. Although we cannot exclude the spectra of Fig. 1 it is also evident that the vanishing intensity of
contribution to the phenomenon of acetylene adsorption on the bands at 3090, 3069, 3037, 1970, 1825 and 1478 cmꢀ1 is due
the most acidic residual –OH groups, we will be able to see in to adsorbed benzene formed by acetylene cyclotrimerization.
the following that a hydrogen bonding interaction is also The formation of this reaction product, which is predominant
conceivable with the reaction products itself.
under low pressure conditions, is therefore depressed at higher
Finally, the 1690 and 1618 cmꢀ1 doublet as well grows in acetylene pressure in favor of new reaction products which are
intensity. As it will be clear in the following these bands will progressively formed on increasing the contact time.
become the predominant features in the high pressure spectra.
Among them, the rst to be formed immediately aer acet-
Besides the features discussed above, by increasing the ylene dosage is responsible for a complex spectrum consisting
acetylene pressure, we can note a new triplet of absorptions at of at least a triplet of bands at 1698, 1368 (superimposed to the
3090, 3069 and 3037 cmꢀ1, in the region typical of aromatic C–H R branch of the rotational contour centered at 1328 cmꢀ1ꢀo1f the
stretching vibrations, and of a peak at 1478 cmꢀ1, in the region dsym + dasym mode of gaseous acetylene27) and 1238 cm and
where aromatic ring deformation modes can be expected. These possibly of weaker features at 1422, 1198, 1138, and 1092 cmꢀ1
bands grow along with the acetylene pressure (Fig. 1) and (red curve in Fig. 2). Some of these bands were already present
readily disappear at the same rate upon outgassing the C2H2/ in the low pressure spectra, but with much lower intensity. The
TiO2 system at room temperature (results not shown), which complexity of this spectrum is a clear indication that in acety-
allows us to conclude that they belong to the same surface lene excess we can immediately observe the formation of reac-
species. Based on previous literature data23,29–31 these bands can tion products which already have a complex structure involving
be assigned to adsorbed benzene molecules formed by acety- more than one acetylene molecule.
lene cyclotrimerisation. Following this assignment the doublet
A complex evolution of the spectrum was then observed on
of bands also growing in intensity with the gas pressure at 1970 increasing the TiO2/acetylene contact time at the IR beam
and 1825 cmꢀ1 are attributed to benzene overtones.
temperature (grey and black curves in Fig. 2):
On the basis of the above discussion we can therefore
(i) the bands due to adsorbed acetylene at 3214 and
conclude that under low pressure conditions (0.5–10 mbar) the 1954 cmꢀ1 decrease gradually while those of the primary
interaction of acetylene with TiO2 P25 mainly results in physical products at 1698, 1368, 1238, 1422, 1198, 1138, and 1092 cmꢀ1
adsorption on surface sites and in the formation of oligomeri- grow in intensity in parallel. This demonstrates that the
zation products prevalently consisting of benzene. As it will be adsorbed surface complexes are transformed into products
shown in the next paragraph, by increasing the gas pressure through an activated process;
(100 mbar and above) a more complex surface chemistry
prevails, leading to larger reaction products and revealing an gradually develop at 1665 (sh), 1630, 1615, and 1590 cmꢀ1. It is
unexpected surface activity of TiO2 towards acetylene. noteworthy that the relative intensity of these absorptions
(ii) new bands associated with conjugated double bonds14
(b) High C2H2 pressure experiment. The results of a typical changes with the contact time since those at lower frequencies
high pressure FTIR experiment are reported in Fig. 2. The rst become predominant with longer contact times. This progres-
spectrum (red curve) was obtained immediately aer the dosage sive downward shi of the n(C]C) frequency is an indication
of 120 mbar of acetylene (this pressure was chosen to avoid that more and more complex oligomerization products con-
overwhelming of the manifestations of the reaction products by taining an increasing number of conjugated double bonds are
those of the gas phase). The grey spectra were recorded in progressively formed by acetylene insertion. These products are
sequence at an interval of about 3 minutes up to a nal contact also responsible for the growth of other weak or very weak
time of ca. 30 minutes (black curve). Finally, the blue curve features at 1478 (very narrow), 1449, 1279 and 1164 cmꢀ1
(shied upward for clarity) shows the effect on the spectrum of
(iii) the evolution of the spectra in the 2000–1000 cmꢀ1
;
the products formed aer 30 minutes of contact aer further region is accompanied by the growth of a quartet of bands at
heating the TiO2/acetylene system at 373 K (in the presence of 2970, 2937, 2920, and 2875 (sh) cmꢀ1 undoubtedly associated
acetylene excess in the gas phase).
with the stretching vibrations of saturated –CH3 and –CH2
In the rst spectrum (red curve in Fig. 2), the main bands groups;22
due to the n(C^C) and n(^CH) modes of adsorbed acetylene
(iv) the growth of the manifestations discussed in (ii) and (iii)
can still be recognized, respectively, at 1954 and 3214 cmꢀ1. The is accompanied by further progressive erosion of those due to
latter is superimposed to the R and P branches of the anti- the residual –OH groups of activated TiO2 in the 3750–3600
symmetric C–H vibration of the gaseous phase centered at ca. cmꢀ1 (negative bands in Fig. 2) and by the parallel formation of
3290 cmꢀ1 and 6 cmꢀ1 downward shied with respect to the low a broad absorption covering the entire 3600–2750 cmꢀ1 range.
pressure experiment. It is noteworthy that the formation of This behavior indicates the occurrence of hydrogen bonding
these bands at the maximum intensity is not accompanied by interactions20 which involve not only the acetylene molecules
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(as already discussed in the previous parts) but also the reaction the UV-vis spectrum is basically not changed by the dosage of
products. C2H2 and only a very weak absorption appears in the visible
Together with the formation of OH groups evidenced by the region of the spectrum. Conversely, under high pressure
IR spectra in Fig. 2, the hypothesis that the oligomerization conditions (Fig. 3b), shortly aer acetylene dosage, a remark-
reaction is initiated by heterolytic splitting of the acetylene able absorption in the visible region has been developed
molecule by reaction with a Ti4+O2ꢀ acid/base pair is supported centered at ca. 470 nm. On increasing the contact time at room
by the observations that it is greatly slowed down when the less temperature this absorption grows in intensity while progres-
Bronsted acidic methyl-acetylene molecule is used instead of sively shiing to ca. 550 nm (black curves in Fig. 3b). A further
acetylene and does not occur at all with dimethyl-acetylene.
redshi down to the limit of the visible region and increase in
It is worth underlining that outgassing the sample at room the intensity are then observed by heating at 373 K in an
temperature under high vacuum, while causing immediate acetylene atmosphere (blue curve in Fig. 3b).
disappearance of the bands due to the adsorbed acetylene,
As shown in the right part of Fig. 3, the evolution of these
affects very little those related to the reaction products (see spectra reects a change in the sample color which turns from
Fig. S1 in the ESI†), as expected for irreversible species chemi- white (activated TiO2) to blue (aer heating at 373 K in acetylene
cally adsorbed (anchored) on the surface of the TiO2 crystals. No excess). The UV-vis spectrum is very little affected upon out-
signicant changes are observed as well if the system is heated gassing the TiO2/C2H2 system under high vacuum (results not
up to 373 K in the absence of acetylene in the gas phase (see shown). This result conrms that we are dealing with irrevers-
Fig. S1 in the ESI†).
ible reaction products tightly anchored to the surface of the
In contrast, heating at this temperature in the presence of TiO2 nanocrystals. Moreover, only small changes are observed
acetylene excess leads to the formation of more complex prod- when exposing the blue samples for some hours to pure O2 or to
ucts containing a higher number of conjugated double bonds. air. This observation suggests that the blue color is not basically
In particular, the intensity of the main peak centered at associated with the presence of Ti3+ species which are known to
1694 cmꢀ1 is further reduced in favor of the components at give rise to a similar blue color.32,33
1630, 1615, and 1590 cmꢀ1, which collapse to a strong absorp-
The gradual redshi of the absorption band with the contact
tion centered at 1623 cmꢀ1 showing an intense shoulder at time supports the hypothesis already advanced on the basis of
1605 cmꢀ1 and a weak satellite at 1539 cmꢀ1 (see blue curve in the FTIR results of the occurrence of surface reactions leading
Fig. 2). Further insertion of acetylene molecules is also to acetylene oligomers containing an increasing number of
conrmed by the complete consumption of the bands by the conjugated double bonds. Indeed, in the case of linear polyenes
adsorbed acetylene at 3214 and 1954 cmꢀ1
.
a relation has been established between the number of C]C
Heating at 373 K in an acetylene atmosphere also induces bonds and the position of the absorption in the electronic
some changes in the absolute and relative intensity of the bands
due to the saturated C–H groups: in particular, the couple of
bands initially observed at 2970 and 2920 cmꢀ1 preferentially
grow in intensity and downward shi to 2960 and 2910 cmꢀ1
becoming predominant with respect to the doublet at 2937 and
2875 cmꢀ1 whose position remains nearly unchanged. The
assignment of the doublet at 2960–2910 cmꢀ1 to aliphatic –CH3
groups is quite straightforward,22 while that of the couple at
2937 and 2875 cmꢀ1 is more uncertain. It is in fact compatible
with –CH2 groups (although 2875 cmꢀ1 appears as an unusually
high frequency for the symmetric methylene vibration) as well
as with aromatic –CH3 methyl groups. However, it is important
to stress that the reaction of acetylene with the TiO2 surface
results in the formation of hydrogen-rich saturated species.
Since our reactant (C2H2) has a ratio C/H ¼ 1, the formation of
the hydrogen-rich –CH2 and/or –CH3 moieties requires parallel
Fig. 3 UV-vis spectra of the acetylene/TiO2 system. (a) Effect of
production of an highly (or even completely) dehydrogenated increasing the acetylene pressure (0.5–10 mbar range) under the same
conditions used for the acetylene/TiO2 low pressure FTIR experiment
counterpart characterized by the presence of a network of
conjugated double bonds, as suggested by the IR spectra and
further conrmed by the UV-vis and Raman spectra discussed
in the next section.
(Fig. 1). The inset shows magnification (100ꢁ) of the very small
absorption which appears in the visible region. (b) Effect of contact
time and thermal treatment at 373 K under the same conditions used
for the acetylene/TiO2 high pressure FTIR experiment (Fig. 2). Red
curve: the spectrum of pure activated TiO2 P25. Black curves (from the
bottom): spectra immediately after dosage of C2H2 (initial pressure 120
mbar) and at increasing contact times up to 30 minutes. Blue curve:
after heating in an acetylene atmosphere at 373 K for 30 minutes. The
pictures in the right part of the figure illustrate the color change of the
sample passing from pure activated TiO2 to the final TiO2/C2H2 system
(2) UV-vis and Raman spectroscopy
The dependence of the UV-vis spectrum of the TiO2/C2H2
system in the low and high pressure conditions is shown in
Fig. 3. We can see that under low pressure conditions (Fig. 3a) heated in an acetylene atmosphere for 30 minutes at 373 K.
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spectra, which further depend on the charge of the oligomers
(see ref. 14 and 25, and references therein).
This hypothesis seems to be supported by the Raman spec-
trum shown in Fig. 4 of the C2H2/TiO2 system aer 30 min of
On the basis of this relationship, the rst spectra obtained contact, where two main bands are observed at 1590 and
aer acetylene dosage could be compatible with linear oligo- 1277 cmꢀ1. The former, which is known as the graphitic (G)
mers already containing 5–10 conjugated double bonds.14 band, is the only feature observed in the rst order Raman
However, in our case, linear polyenes are not probably the most spectrum of highly ordered, crystalline graphite. The second
abundant products owing to the absence in the FTIR spectra of band (D band) appears in disordered samples of graphite or in
the typical bands in the 3100–2990 cmꢀ1 associated with the amorphous carbon materials containing sp2 graphitic islands.34
stretching vibrations of olenic ]CH2 and/or ]CH-groups.22 Interestingly, also the Raman spectra of polycyclic aromatic
This observation, together with the FTIR evidence given above hydrocarbons (PAHs) are characterized by bands in approxi-
of the presence of a relevant number of –CH3 and/or –CH2 mately the same G and D regions.35
aliphatic groups, suggests that the nal products obtained by
The two smaller bands, centered at about 1487 and
the reaction of acetylene on the TiO2 surface mainly consist of 1150 cmꢀ1, are compatible with the Raman spectra of polyenes
polycyclic aromatic hydrocarbons terminated with methyl and adsorbed on TiO2.14
methylene groups (see Fig. 5). In these products, in fact,
extended double bond conjugation (as required by both the
FTIR and UV-vis evidence) is combined with the highly dehy-
(3) Gas chromatography-mass spectrometry analysis of the
products
drogenated region (as required due to the presence of methyl
To achieve deeper insights about the nature of the products
and aliphatic methylenic groups).
formed upon C2H2 interaction with TiO2, we tried to remove
them from the oxide surface using different solvents, including
dichloromethane, toluene, hexane, and acetonitrile. None of
these solvents was able to extract all the reaction products since
the sample did not regain its white colour aer the extraction
process. This nding suggests that the organic compounds,
especially the heavier fraction, are tightly bound to the surface.
The products extracted with dichloromethane were analysed
using gas chromatography-mass spectrometry (GC-MS). The
most relevant compounds that were found are benzene,
toluene, dimethylbenzene, trimethylbenezene, styrene, diphe-
nylmethane, biphenyl, and naphthalene.
The compound extracted with the highest yield was benzene,
which was clearly observed also in the FTIR spectra. The
formation of toluene, dimethylbenzene and trimethylbenezene
is in agreement with the observation of –CH3 groups in the FTIR
spectra. The presence of styrene supports the possibility of the
acetylene molecule addition to the aromatic ring which can be
the rst step in the formation of a second aromatic ring36
Fig. 4 Raman spectra, recorded with a 514 nm laser line, of activated
TiO2 P25 30 minutes after dosage of 120 mbar of C2H2 at room
temperature.
present in biphenyl, diphenylmethane and naphthalene. The
Fig. 5 Schematic representation of the reaction of acetylene on TiO2 P25 showing the first steps of the reaction (adsorption and dissociation of
C2H2 on the oxide surface) and the main reaction products highlighted by the spectroscopic results and by the GC-MS analysis.
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observation of these molecules further conrms that the reac-
tion of acetylene on the TiO2 surface can lead to polyaromatic
compounds as already discussed in the previous sections. As
suggested by the FTIR, UV-vis and Raman spectroscopic results,
more complex molecules containing a higher number of
aromatic rings are also formed, particularly when the temper-
ature and the acetylene pressure are increased (see Fig. 5).
Indeed, traces of phenanthrene and pyrene have been observed
using GC-MS for samples treated at 373 K. Unfortunately, it was
not possible to extract the heavier products from the TiO2
surface, therefore we can only propose a general structure for
these polycyclic aromatic hydrocarbons terminated with methyl
and methylene groups, which is reported in the right part of
Fig. 5. A forthcoming more detailed investigation, by dissolving
the TiO2 matrix with HF and using LC-MS with ESI, APCI and
APPI ion sources, could provide further insight on the reaction
products.
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In this contribution we showed that in situ acetylene oligo-
merization is an effective way to obtain polycyclic condensed
hydrocarbons in close contact with the TiO2 surface. These
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The strong absorption in the visible region as shown by
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Dr A. Damin is acknowledged for the acquisition of the Raman
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through the FIRB Project RBAP115AYN “Oxides at the nano-
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