Polycyclic aromatic hydrocarbons (PAHs) are known to be present in many astrophysical objects and environments, but our understanding of their formation mechanism(s) is far from satisfactory. In this paper, we describe an investigation of the catalytic conversion reaction of acetylene gas to PAHs over pyroxene and alumina. Crystalline silicates such as pyroxenes (with general formula [Mg, Fe]SiO3) and alumina (Al2O3) are observed astrophysically through their infrared spectra and are likely to promote grain surface chemical reactions. In the experiments reported here, gas-phase PAHs were produced by the catalytic reaction of acetylene over crystalline silicates and alumina using a pulsed jet expansion technique and the gaseous products detected using time-of-flight mass spectrometry. In a separate experiment, the catalytic formation of PAHs from acetylene was further confirmed with acetylene gas at atmospheric pressure flowing continuously through a fixed-bed reactor. The gas effluent and carbonaceous compounds deposited on the catalysts were dissolved separately in dichloromethane and analysed using gas chromatography–mass spectrometry. Among the samples studied, alumina showed higher activity than the pyroxene-type grains for the acetylene reaction. It is proposed that formation of the PAHs relies on the Mg2+ ions in the pyroxenes and Al3+ ions in alumina, where these ions act as Lewis acid sites. X-ray diffraction, Fourier transform infrared and high-resolution transmission electron microscopy techniques were used to characterize the structure and physical properties of the pyroxene and alumina samples.
Polycyclic aromatic hydrocarbons (PAHs) have been identified through their infrared emission signatures in many astrophysical objects and environments, including ionized (HII) regions, reflection nebulae, young stellar objects, planetary and protoplanetary nebulae, post-asymptotic giant branch (AGB) objects, galactic nuclei and ultraluminous infrared galaxies [1,2]. However, the formation of PAHs remains one of the longstanding enigmas of interstellar chemistry [3,4]. Only a few studies have been conducted to date, most of which concern possible routes for PAH formation in the outflows of carbon-rich AGB and post-AGB objects [5–7]. Frenklach & Feigelson  proposed a hydrogen abstraction and acetylene addition (HACA) radical-based mechanism, in which C2H2 can add to radical sites on an aromatic structure and form a new aromatic ring in a temperature window of 900–1100 K . Woods et al. [10,11] and Carelli et al.  have discussed ion-molecule chemistry involving C2H2 to form benzene. The above-mentioned reaction conditions are similar to those of flame chemistry using C2H2 as the carbon source in which PAHs are considered to play a key role in the chemistry and growth process of soot particles [13,14]. However, the mechanism for activation and cyclization of C2H2 is still a largely unresolved question.
Oxygen-containing grains are abundant in many astronomical environments and contribute to both interstellar extinction and emitted radiation at infrared and millimetre wavelengths . The majority of these are amorphous and crystalline silicates which have emission features at wavelengths beyond approximately 15 μm [16,17]. One of the crystalline silicates identified is pyroxene which has the general formula MgxFe1−xSiO3 where x lies between 0 and 1. The Fe-free end member of this group is enstatite. The characteristic peaks from pyroxenes fall at 29.5, 33.0, 36.22, 40.56 and possibly at 43.99 μm , whereas those of enstatite lie in the range of 8–16 μm, the strongest being around 9–12 μm . As well as silicates, other oxide minerals have recently been observed in some evolved stars [20–22], among which small refractory grains such as Al2O3 are considered to serve as nucleation sites for the subsequent condensation of silicate grains [23,24]. According to Stroud et al. , the broad asymmetric feature peaking near 12 μm is attributable to amorphous Al2O3, whereas crystalline corundum (α-Al2O3) appears to be a likely contributor to the 13 μm feature . From a catalytic point of view, these dust grains can provide surface sites on which gas-phase species such as C2H2 could be activated and dimerized leading to the formation of PAHs. Grain surface chemistry in astronomical environments has been discussed by a number of authors [25–28], but, to date, the possible role of oxide dust grains in the formation of PAHs has not been widely explored.
In previous work, we reported the aromatization reaction of C2H2 over olivine-type silicates . In this study, we have investigated the synthesis of PAHs over pyroxene-type silicate and alumina nanoparticles by the catalytic conversion of acetylene, which is common in various astrophysical environments . Here, the focus is their catalytic role in the formation of PAHs. The relative conversion efficiency of pyroxenes and alumina is discussed.
2. Experimental conditions
(a) Characterization of pyroxene, enstatite and alumina samples
The pyroxene sample was kindly provided by the Geological Museum of the University of Hong Kong, and the enstatite and aluminium oxide (Al2O3) samples were purchased from Sigma-Aldrich. The samples used in the experiments were characterized using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) and Fourier transform infrared (FT-IR) spectroscopic techniques. Powder XRD patterns of all three samples were recorded with a Bruker AXS D8 advance powder X-ray diffractometer, using CuKα non-monochromatic radiation (λ=0.1541 nm, 40 kV, 40 mA). The phases of the samples were identified using the powder diffraction files-ICCD PDF-2 database. HR-TEM images were acquired on a TECNAI G2 F20 electron microscope instrument at 200 kV to characterize the microstructure of the pyroxene. An energy dispersive X-ray (EDX) technique was used to analyse the compositions of the pyroxene and enstatite samples only; pure Al2O3 was purchased directly, and no composition analysis was needed in this case. FT-IR spectra of acetylene adsorption were used to investigate the surface properties of the three samples. Prior to the measurement, the samples were pre-treated in vacuo for 3 h at 450°C. C2H2 was then adsorbed on the samples with a flow rate of 20 ml min−1 at room temperature (RT). After gas-phase C2H2 was purged in argon (20 ml min−1), FT-IR spectra were acquired using an FT-IR 8300 spectrophotometer (Shimadzu).
(b) Apparatus and activity measurement
The acetylene conversion reactions were carried out using the apparatus shown schematically in figure 1 which comprises a pulsed nozzle source, a heated reaction chamber, a laser-induced ionization region and a homebuilt time-of-flight mass spectrometer (TOF-MS). In the source chamber, the acetylene gas was released by an electronically controlled pulsed valve (diameter: 0.15 mm; backing pressure: approx. 2×105 Pa), and entered the reaction chamber where the catalyst was located. A skimmer with a 2 mm diameter aperture, situated between the reaction and laser ionization chambers, allowed a very small amount of gas to pass into the ionization chamber, thus allowing a high vacuum (less than 2.7×10−4 Pa) to be maintained. The neutral gaseous products were ionized by a pulsed ArF excimer laser (193 nm; 10 Hz repetition; 5–6 mJ and pulse width 8 ns) and detected with a microchannel plate (MCP) detector. Typically, 100 mg of solid catalyst was loaded between two quartz wool plugs in the quartz tube (i.d. 4 mm; length 12.5 cm). A thermocouple placed at the centre of the quartz reactor was used to monitor the reaction temperature. Prior to carrying out the C2H2 aromatization reactions, the solid samples were heated at 700°C for 30 min in argon and then returned to RT. Acetylene was then allowed to pass through the catalyst samples with a pulse duration time of 220–250 μs at a repetition rate of 10 Hz. The reaction bed temperature was increased from 50 to 600°C in steps of 50°C, being held at each temperature for 20 min. The gas effluent after reaction was directed into the ionization chamber through the skimmer and ionized by 193 nm radiation from the ArF laser. The ionized species were accelerated by the TOF lens assembly and into a standard TOF field-free region. The signals detected by an MCP detector were collected and analysed with a 300 MHz digital storage oscilloscope.
In order to further confirm the formation of PAHs in the TOF-MS experiment, the acetylene aromatization reaction with enstatite was also investigated in a separate experiment using a fixed-bed reaction system at atmospheric pressure. The catalyst was pre-treated in argon at 600°C for 1 h prior to this reaction. Pure C2H2 gas was then allowed to pass through 100 mg of catalyst at 600°C with a flow rate of 20 ml min−1 for 3 h. The effluent gas was trapped on a cold surface cooled with a mixture of liquid nitrogen and ethanol. After this deposition, 2.5 ml of dichloromethane (DCM) solvent (solution A) was used to dissolve the trapped solid products. The enstatite solid catalyst was washed with another small volume of DCM solvent (solution B) to collect compounds deposited on the surface of the catalyst. Both the solutions (A and B) were then analysed with an offline gas chromatography–mass spectrometry (GC–MS) instrument (Agilent Technologies 5975C) equipped with a flame ionization detector.
3. Results and discussion
(a) Surface properties, structure and composition analysis of pyroxene, enstatite and alumina samples
Figure 2 shows the XRD patterns of pyroxene, enstatite and aluminium oxide samples. For the pyroxene (figure 2a), the characteristic peaks of pigeonite ([Ca, Mg, Fe][Mg, Fe]Si2O6; JCPDS no. 83-0100) are accepted as a monoclinic system structure. It is well known that the generic formula of pyroxene is (X2+, Y2+)Si2O6 in which X and Y can be iron, magnesium and calcium. Our result is in agreement with the description of pyroxene by Deer et al. . The HR-TEM image of pyroxene in figure 3 presented ordered morphology indicating that crystalline pyroxene has single chains of silica tetrahedra in a monoclinic system (shown in the inset of the lower-left of figure 3). The selected area electron diffraction (SAED) pattern of pyroxene (inset of top-left corner of figure 3) revealed that the distances between the two bright spots are 0.309 and 0.645 nm, respectively, corresponding to diffraction of the (220) and (110) crystal planes. In contrast to the pyroxene sample, enstatite appears to have an amorphous structure as shown in figure 2b, which is in line with the SAED result (inset of top-right corner of figure 3). Owing to the difference in structure of the samples used, we have not considered the size effect in this work. Prior to the Infrared Space Observatory, it was reported that silicates formed as disordered or amorphous grains in the outflows of O-rich stars at various stages of their evolution [32,33] and very likely that they are a precursor to the crystalline silicates . Therefore, it is of interest to investigate the catalytic conversion of C2H2 on the surface of amorphous enstatite grains. For the Al2O3 sample, the characteristic peaks corresponding to the γ-Al2O3 (2θ=45.7° and 67.2°; JCPDS no. 01-1308) and α-Al2O3 (corundum) phases (JCPDS no. 01-071-1123) were observed in figure 2c, suggesting that the Al2O3 sample existed as a mixture of the γ-Al2O3 and α-Al2O3 (corundum) phases . The composition of pyroxene and enstatite grains analysed by EDX is summarized in table 1. For the pyroxene sample, Mg, Si, O and Fe are present with a small amount of Ca, which was consistent with our result obtained by XRD. In contrast to the pyroxene, there are present only Mg, Si and O with a little amount of Ca in the enstatite sample. These metal ions such as Mg2+, Fe2+ and Al3+ can act as active sites to react with acetylene and promote the formation of PAHs. Dust grains with different compositions and structure may exhibit distinct behaviour in the dimerization and cyclization reaction of acetylene. After the acetylene aromatization reaction, no significant change was detected except for the appearance of a black colour layer on the surface of the grains owing to carbon and PAH deposition.
Figure 4 shows the FT-IR characterization results for the pyroxene, enstatite and aluminium oxide grains. For the pyroxene sample, the bands between 1100 and 800 cm−1 can be attributed to the asymmetric and symmetric stretching vibrations of the SiO4 tetrahedron. The peak at 646 cm−1 corresponds to the bending vibration of Si−O bonds owing to the interaction with metal cations. The feature beyond 480 cm−1 is caused by bending vibrations of the Si−O−Si bonds [15,36,37]. Compared with the pyroxene sample, the peak at 1076 cm−1 for enstatite exhibited no splitting, which is similar to the results reported by Jäger et al. [37,38]. The band at 796 cm−1 can be ascribed to a frequency shift of the in-plane bending vibrations from amorphous SiO2 . The peak at 646 cm−1 shifted to a higher wavenumber (664 cm−1) for the enstatite sample compared with that for pyroxene is due to the iron in our pyroxene sample (EDX characterization) . Unlike the IR spectrum of pyroxene and enstatite silicates, the aluminium oxide sample presented three bands at 827, 739 and 572 cm−1, corresponding to amorphous Al2O3 and crystalline α-Al2O3 (corundum) [23,39–41]. This result indicates that our Al2O3 sample was in a state between amorphous Al2O3 and crystalline α-Al2O3. Consequently, we consider the Al2O3 sample to be a mixture of γ-Al2O3 and α-Al2O3, which is supported by our XRD characterization results. The absorption peaks around 2366–2372 cm−1 originate mainly from the adsorption of CO2 owing to imperfect background subtraction for a single beam instrument because of the possible variation in the environments, especially for Al2O3 pre-treated in vacuum (figure 4a). After the adsorption of C2H2 over pyroxene at RT, two new peaks at 1927 and 672 cm−1 appeared. According to the literature , the peak at 1927 cm−1 can be assigned to the stretching vibration of the triple bond of the acetylene molecule and the peak at 672 cm−1 to the Si−C stretching vibration. This result suggests that pyroxene can adsorb C2H2 at RT via the interaction of Lewis acid sites (such as Fe2+ or Al3+) with the triple bond of acetylene as reported in the literature . The oxygen-containing active sites (Bronsted acid) on the surface of pyroxene or Al2O3 nanoparticles reacted with hydrogen in acetylene to form water. We observed that after the aromatization of C2H2 over pyroxene or Al2O3 for 4 h, the intensity of both peaks at 3472 and 1647 (or 1640) cm−1 reduced significantly owing to the removal of water. After the aromatization of C2H2 over the pyroxene sample for 4 h, the intensities of both these peaks increased indicating that, even at 600°C, C2H2 could not desorb completely from the surface of pyroxene. For the enstatite sample, there remained strong peaks that appear to be related to OH/H2O at 3472 and 1641 cm−1 owing to stable surface hydroxide groups for amorphous silicates; no new peaks were observed after the adsorption of C2H2 and the aromatization reaction. Unlike the FT-IR spectrum of pyroxene sample recorded after the adsorption of acetylene, a peak at 2963 cm−1 appeared which can be assigned to C−H stretching vibrations in the di- and tetra-vinyl metals . This result indicates that C2H2 can be adsorbed and dimerized to di- and tetra-vinyl species on the surface of Al2O3 at RT. It is notable that a weak and broad band centred at 1200 cm−1 was observed after the acetylene aromatization reaction. It is known that one of the characteristic astrophysical features falls near 7.7 μm in emission [1,44]. Consequently, it is plausible that this broad band results from a C−C vibrational mode in a PAH molecule. This result strongly suggests that the activation and aromatization of C2H2 occurs over Al2O3 leading to PAH deposition covering the surface of Al2O3. This may be the reason why the absorption between 1000 and 500 cm−1 became weaker than that for fresh Al2O3.
(b) Catalytic conversion of acetylene to polycyclic aromatic hydrocarbons over pyroxene, enstatite and alumina
Figure 5 shows the TOF-MS signals arising from the acetylene aromatization reaction over pyroxene. When the reaction vessel was at a temperature higher than 400°C, mass peaks that can be assigned to naphthalene, anthracene/phenanthrene and pyrene were observed. Their intensity increased with increase in reaction temperature and reached a maximum at 550°C. This is a much lower temperature than holds in the gas phase . The polyatomic ions C3H+, , and (in addition to ) accompanied PAH formation. It is plausible that the C3Hx species were formed by the decomposition of C4H4 species over the surface of the pyroxene. The main products are summarized in table 2.
Compared with pyroxene-type grains, alumina (Al2O3) exhibited a much higher activity and peaks below m/z=140 were observed even at 150°C (figure 6). Stronger mass peaks appeared with an increase in vessel temperature and these reached a maximum at 350°C. Our FT-IR results demonstrate that C2H2 can be adsorbed and dimerized to di- and tetra-vinyl species over Al2O3 at RT, accompanied by a large amount of PAH deposition on the surface of the Al2O3. Clearly, Al2O3 has a far higher ability to adsorb and activate acetylene than pyroxene-type silicates at low temperature.
In order to further confirm the formation of PAHs in the acetylene aromatization reactions, we performed a separate experiment with a continuous flow of acetylene over heated enstatite grains at atmospheric pressure. Figure 7 shows the GC–MS results of the acetylene aromatization reaction over enstatite at 600°C at atmospheric pressure. PAHs, including benzene, indene, naphthalene, 3-ethynylindene, fluorene, anthracene or phenanthrene, pyrene, 11H-benzo[a]fluorene, benzoanthrathene, benzopyrene, were detected by GC–MS; this is consistent with the results obtained under low-pressure conditions by TOF-MS. For solution A (see §2) for gas-phase-trapped species, the more intense GC peaks were for the smaller PAHs such as indene and naphthalene, with the relatively heavy PAHs being much lower in abundance. However, analysis of solution B obtained from deposits on the surface of the catalyst showed a different pattern. In this case, the more intense GC peaks were those of heavier PAHs (with mass greater than 150). It is inferred that desorption of smaller PAHs from the enstatite surface is easier, with most of the heavier PAHs remaining on the surface of the enstatite catalyst. The numbered peaks in figure 7 correspond to the identified PAH compounds analysed from the MS.
The TOF-MS and GC–MS results indicate that pyroxene-type and aluminium oxide grains promote acetylene aromatization reactions so that this reaction can occur at lower temperatures than that without using catalysts. Moreover, Al2O3 showed higher activity for C2H2 aromatization reaction than pyroxene-type grains.
(c) Reaction mechanisms and pathways
It is generally accepted that closure of the first single aromatic ring (C6H6) is the rate-determining step for the formation of PAHs [45–47]. Our TOF-MS results show that formation of PAHs from acetylene can occur over pyroxene, enstatite and aluminium oxide grains at a lower temperature than that so far reported in the literature when no grain particles are used. Evidently, the existence of active surface sites of these grains promotes C6H6 production, thus allowing PAHs to form at lower temperature. Based on the literature on the formation of benzene [3,48,49] and the intermediates observed in our TOF-MS measurements, we speculate that the reaction paths for benzene generation are as shown in schemes 1 and 2 with the recombination of propargyl radicals (C3H3) leading to formation of benzene or the phenyl radical and a hydrogen atom, as shown in scheme 2. Owing to the presence of vinyl-acetylene (C4H4), two other pathways to benzene closure involving the reaction of 1-buten-3-ynyl (1-C4H3) with acetylene and 1,3-butadienyl (1-C4H5) with acetylene [49,50] cannot be excluded. After the formation of benzene, the first regular mass number sequence of 24 in the aromatic compounds appears at 78, 102, 126, 150 through the HACA mechanism mentioned earlier (scheme 3). 1 2 3Other HACA products were observed at m/z=152, 176 and 202 which are readily assigned to acenaphthylene, pyracylene and pyrene starting from naphthalene and phenanthrene, respectively, as demonstrated in schemes 4 and 5. 4 5Another regular sequence with an interval of 50 in mass number is m/z=78, 128 and 178 which is obtained by the addition of diacetylene to benzene (78) passing through naphthalene (128; scheme 6). 6It is interesting to note that the PAHs with an odd number of carbon atoms such as the indenyl radical (115), indenylacetylene (139), fluorenyl (165), fluorenylacetylene (189) and fluorenyldiethynyl (213) were also observed in TOF-MS signal of products, as shown in figures 5– 7. Obviously, a different mechanism from that for PAHs with an even number of carbon atoms was responsible for their formation. Owing to the detection of the C3H3 intermediates, we propose the following mechanism (schemes 7–10) for the formation of these PAHs: 78910Based on the mechanism proposed earlier, one can clearly find that for PAHs without the substitution of ethynyl radical, such as 78, 115, 128, 165, 178, 215, the growth of PAHs follows two regular sequences with intervals of 24 (HACA) and 50 (C4H2), whereas for phenylacetylene (102), 1,3-benzenediethynyl (126), indenylacetylene (139), indenyldiethynyl (163), fluorenylacetylene (189) and fluorenyldiethynyl (213), only the substitution of ethynyl radical occurred by the HACA mechanism. Besides the formation of aromatic compounds, we also observed chain hydrocarbons in the TOF mass spectrum. These polyacetylenes could be generated by the reaction of C4H2 with acetylene. Consequently, these species could follow the following reaction pathways (scheme 11): 11Chain hydrocarbons with an odd number of carbon atoms could form starting with the C3H3 species and acetylene with a mass number sequence of 26, as shown in scheme 12. 12As well as the products aforementioned, another mass number sequence by 26 (not observed for pyroxene and enstatite) was 53, 79, 105, 131 and is assigned to chain hydrocarbons when Al2O3 is used. Scheme 13 shows the reaction pathways for these species. 13
It is of interest to compare the results obtained here with those obtained for acetylene conversion into PAHs in the gas phase. Formation of PAHs from acetylene in combustion chemistry occurs at temperatures higher than 1000°C through the HACA mechanism . However, no PAHs except for the related C4H4 species were observed when the temperature was lower than 900°C. Our TOF-MS results indicate that the oxide nanoparticles indeed play an important role in the formation of PAHs, enabling the reactions to proceed at much lower temperatures. In our experiments, no aromatic compounds were detected at temperatures below 700°C in the absence of the catalyst.
4. Summary and astrophysical context
The formation of PAHs in catalytic reactions of acetylene over pyroxene-type and alumina samples was studied in a pulsed jet-expansion experiment, and the results were further confirmed in a continuous flow experiment with product analysis by GC–MS. It was found that the alumina sample has a much higher activity than pyroxene-type grains or silicate grains more generally  in promoting the acetylene conversion reaction. Reaction mechanisms and pathways for the formation of PAHs over pyroxene-type and alumina samples are proposed. The results indicate that Mg2+ ions in the pyroxene-type grains and Al3+ ions in alumina are active sites and play a critical role in the formation of PAHs.
PAHs are detected in a wide variety of astrophysical environments and it is entirely possible that more than one formation mechanism exists; these may include neutral–neutral and ion-molecule gas-phase reactions [8–12]. However, of most significance in the context of the experimental results presented here are warm ‘mixed-chemistry’ circumstellar and nebular environments. One such example is the Red Rectangle which comprises a binary star with a circumbinary disc formed during an oxygen-rich mass-loss phase and an extended carbon-rich nebula resulting from later loss of stellar carbon-rich material . Infrared spectra of the Red Rectangle show that the disc contains crystalline silicate grains including pyroxenes, as studied here, whereas the extended nebula exhibits a very strong display of infrared emission bands from PAHs . This reflects the chemical history of the star, having passed through an oxygen-rich phase to form a thick dusty disc, followed by carbon-rich material flowing out through the silicate-containing disc. Hence, the Red Rectangle has a near-star environment of warm oxygen-rich grains that can provide catalytic surfaces for conversion of gaseous carbon molecules into PAHs. The grain temperature from spectroscopy and modelling is estimated to be 100–1000 K [51–53] depending on the proximity of the dust to the star. Given that astrophysical mixed-chemistry environments are quite common and not restricted to the later stages of stellar evolution, it is concluded that the catalytic reactions examined here could play an important role in PAH formation more generally. Further experimental and theoretical studies are required to explore these reactions in more detail, particularly under ultrahigh-vacuum conditions, including reaction rate measurements and studies of the effect of particle size.
This work was supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, PR China (project no. 701309). M.H. thanks the University of Nottingham for the award of a Universitas 21 travel scholarship and EPSRC for a studentship.
One contribution of 11 to a Theme Issue ‘Surface science in the interstellar medium’.
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