Hydrogen catalyzed adsorption of alkenes on the diamond (001) surface

We present first-principles density-functional calculations for the $[2+2]$ cycloaddition reaction of ethylene on the (001) surface of diamond. For comparison we also study the same reaction of ethylene on the Si(001) surface as well as that of two ethylene molecules. Similar to the latter case, a concerted reaction on top of the C dimer is symmetry forbidden due to the symmetric dimer configuration of C(001). However, in both cases the symmetry-forbidden reaction can be avoided by allowing a rotation of the $\mathrm{C}---\mathrm{C}$ bond of ethylene. This asymmetric reaction path yields an energy barrier of 2.29 and 0.90 eV for the cases of two ethylene molecules and ${\mathrm{C}}_{2}{\mathrm{H}}_{4}/\mathrm{C}(001),$ respectively. In contrast, the reaction of ethylene on Si(001) takes place via an intermediate state, caused by the asymmetric Si dimer structure. The reaction energy barrier of ${\mathrm{C}}_{2}{\mathrm{H}}_{4}$ on C(001) is too high to attain direct adsorption. Here we propose that the $[2+2]$ reaction path is catalyzed by a few hydrogen impurities.