Deciphering Mg-Surface Interactions with Unsaturated Hydrocarbons: An Integrated Experimental-Theoretical Study.

Abstract

Here, we used a combination of laser-induced experiments and density functional theory (DFT) calculations to study the mechanism of growth of carbonaceous species on the Mg surface. Experimental observations revealed that the carbon deposit forms upon laser illumination on the Mg surface, with the deposit being clearer and better structured in the presence of 1,3-butadiene (C4H6) compared to ethylene (C2H4) gas. DFT thermodynamic and kinetic calculations of C2-C4 hydrocarbons interaction on low-index Mg(0001) were used to explain this experimental observation. Our results on Mg(0001) showed that the cis isomer of C4H6 binds more strongly than its trans isomer via a [4+2] cycloaddition mechanism. We also investigated the adsorption of two units of C2H4 and C4H6 molecules, as well as the subsequent dehydrogenation stages that produce radical species responsible for chain growth mechanisms. The results showed that free energy of dehydrogenation of two units of cis-C4H6 [i. e. cis-C8H12] is lower than the dehydrogenation of trans conformer of C4H6 and C2H4 molecule, indicating that the dehydrogenation of two units of cis-C4H6 facilitates the initiation of growth of carbonaceous species on Mg surfaces. Therefore, the DFT calculations pinpoint the origin of the experimental observation of clearer carbon deposits on the Mg surface.