Laboratory astrochemistry : catalytic reactions of organic molecules on olivine-type silicates and silicon carbide surfaces

Abstract

This thesis reports the study of heterogeneous catalytic reactions between small carbon containing molecules and solid grain surface. The time-of-flight-mass spectrometry technique has been used for detecting in situ the small to medium size gas phase molecules; the structure and physical properties of the catalysts, and solid deposition on the grain surface after reaction have been characterized using various surface techniques including: Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and thermogravimetric and differential thermogravimetric (TG/DTG) analysis. Three catalytic reaction systems have been studied, which include the carbon monoxide (CO) disproportionation reaction on olivine-type-silicates (OTS), methanol conversion reactions on OTS and the methanol reaction on silicon carbide (SiC). For the catalytic CO disproportionation reaction, gas-phase CO2 and solid carbon nanostructure deposition have been observed. The conversion reactions of methanol separately on a catalytic surface of OTS and also on SiC produce small organic and hydrocarbon molecules, and polycyclic aromatic hydrocarbon (PAHs) compounds in common, however, only the OTS is able to generate the gas-phase CO and CO2 products, but not the SiC catalyst. Reaction mechanisms have been proposed for the reactions studied. Recent observations of the circumstellar envelops (CSEs) of evolved stars indicate gas-phase molecules, high-temperature conditions and a high molecular density environment support rich and complex chemistry. The gas-phase molecules: CO and CH3OH, solid grains: OTS and SiC and product species: olefins, small hydrocarbon molecules, and PAHs have all been detected in the CSEs. The catalytic reactions studied in this thesis are of significance in astrochemistry and these reactions provide new relationships between small organic molecules and PAHs in astrophysical environments. More importantly, PAHs are the most common and abundant polyatomic molecules known in the visible universe, our study of heterogeneous catalytic reaction has important implication for new channels for the formation of PAHs.