Laboratory astrochemistry: catalytic conversion of acetylene to polycyclic aromatic hydrocarbons over SiC grains

Abstract

Catalytic conversion reactions of acetylene on a solid SiC grain surface lead to the formation of polycyclic aromatic hydrocarbons (PAHs) and are expected to mimic chemical processes in certain astrophysical environments. Gas-phase PAHs and intermediates were detected in situ using time-of-flight mass spectrometry, and their formation was confirmed using GC-MS in a separate experiment by flowing acetylene gas through a fixed-bed reactor. Activation of acetylene correlated closely with the dangling bonds on the SiC surface which interact with and break the C-C π bond. The addition of acetylene to the resulting radical site forms a surface ring structure which desorbs from the surface. The results of HRTEM and TG indicate that soot and graphene formation on the SiC surface depends strongly on reaction temperature. We propose that PAHs as seen through the 'UIR' emission bands can be formed through decomposition of a graphene-like material, formed on the surface of SiC grains in carbon-rich circumstellar envelopes.

Catalytic conversion of methanol to larger organic molecules over crystalline forsterite: Laboratory study and astrophysical implications

“…30 nm (Fig. 4b and 4c), similar to the observation of acetylene conversion to PAHs over SiC grains (Zhao et al 2016). As reported in our earlier publication (Tian et al 2013) Figure 4d shows the thermal gravimetric (TG) profiles of the carbon deposit.…”

“…The time-of-flight mass-spectrometer (TOF-MS) used in this work has been discussed in detail in earlier publications (Tian et al 2012(Tian et al , 2013Zhao et al 2016) so only a brief description of the equipment and experimental conditions is given here. This home-built instrument consists of a source chamber with a quartz tube fixed-bed reactor, an ionization chamber, a flight tube and a detection chamber.…”

“…Kress & Tielens (2001), using kinetic model calculations, confirmed that Fischer-Tropsch catalysis plays an important role in solar nebula chemistry by converting CO to less volatile hydrocarbons. Our own earlier laboratory studies (Tian et al 2012(Tian et al , 2013Zhao et al 2016) have shown that olivine-type silicates, pyroxene, alumina and SiC particles can promote the conversion of acetylene gas to polycyclic aromatic hydrocarbons (PAHs).…”

VUV spectroscopy of an electron irradiated benzene : carbon dioxide interstellar ice analogue

“…5 Formation routes for PAHs in the diffuse ISM include the top-down processes of SiC dust grain destruction from the intense UV radiation eld and sputtering from shockwaves. 6,7 Alternative bottom-up processes include the formation of PAHs from benzene derivatives or radicals in the gas- 8 or solid-phase which are more likely in low temperature, molecular cloud environments. Within our own Solar System, PAHs and benzene derivatives have been detected on some Saturnian moons, 9 interplanetary dust grains 10 and meteorites.…”

“…The spectra are normalised to the integrated area of the overlapping 1 E 1u ) 1 A 1g and 1 B 1u ) 1 A 1g transitions of the 1 : 100 C 6 H 6 : CO 2 mixture deposited at 20 K. Table 2 Band assignments and positions of the CO A 1 P ) X 1 P + transition formed from a 1 : 100 C 6 H 6 : CO 2 mixture irradiated by 1 keV electrons for 180 min and compared with pure CO taken from Mason et al 25 and CO formed from VUV photons in a CO 2 matrix. 31 Band splitting is indicated by numbers in parenthesis the (7,0) band no longer visible and the (6,0) band less well dened. Like the unirradiated sample discussed in Section 3.1.2 the benzene transition bands are present with a lower intensity for both electronic transitions and the vibronic structure of the 1 B 1u ) 1 A 1g transition are less well dened compared to that of spectrum taken at 20 K. At 90 K CO 2 and CO have desorbed and interestingly the benzene band has increased in intensity.…”