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

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 change 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.