Abstract
Carbon is found in many forms in extraterrestrial environments; however, there is a discrepancy where the measured abundance of carbon in space is significantly less than that predicted by theoretical models. It is thought that a significant proportion of interstellar carbon could be in the form of polycyclic aromatic hydrocarbon (PAH) molecules which, due to their size and high symmetry that result in no dipole moment, cannot be detected. The focus of the present study is to determine whether mineral surfaces such as forsterite, a type of olivine, can lead to hydrocarbon polymerisation and subsequent PAH growth. Specifically, the plausibility of a proposed pathway from literature through the adsorption of the smallest hydrocarbon, acetylene, on olivine surfaces. Subsequent addition reactions follow and as well as a cyclisation to form the PAH building block, benzene. Here, a combination of computational and spectroscopic techniques have been utilised to investigate this chemistry.
The computational work began with a thorough benchmarking of methods and the study of acetylene on forsterite.
A proposed pathway from literature between acetylene to benzene via addition reactions was chosen for investigation. It was split into two competing pathways and individually investigated to identify the energies of minima and transition states. In Pathway 1, the formation of diacetylene was studied, where a large energetic barrier for the first addition reaction of two acetylene molecules to form a C4H4 intermediate was found, suggesting it did not form in a facile manner. Pathway 2, the formation of benzene (and a competing pathway of a C6H6 intermediate), was then investigated. Here, it was found that the formation of benzene was barrierless, making the intermediate formation step of C4H4 the most crucial barrier in the overall mechanism, meaning that the proposed reaction mechanism is not plausible.
The laboratory component aimed to observe the formation of PAHs from C2H2 by Infrared (IR) spectroscopy. Here, the experimental work used a more complex olivine that contained iron. Various experimental parameters, including olivine disks, reaction temperature, and time were investigated. It was found that the system needed to be heated at high temperatures for an extended period of time for any reaction to occur. Although it was determined by the appearance of peaks characteristic of adsorbed acetylene, that acetylene did bind to the surface and no benzene or cyclic products were formed, providing experimental evidence that PAH formation did not occur as proposed.
A final series of calculations were completed to investigate how the energetics of the reaction were affected by different pressures and temperatures and in particular, the temperature and pressure of Titan. It was found that high pressures give more favourable overall energies and a similar trend is seen for the overall energy for low temperatures. However, neither factor significantly reduced the large barrier for the formation of C4H4; therefore, the overall proposed reaction scheme remains unlikely.