Pyrene: hydrogenation, hydrogen evolution, and π-band model.

We present a theoretical investigation of the hydrogenation of pyrene and of the subsequent molecular hydrogen evolution. Using density functional theory (DFT) at the GGA-PBE level, the chemical binding of atomic hydrogen to pyrene is found to be exothermic by up to 1.6 eV with a strong site dependence. The edge C atoms are found most reactive. The barrier for the formation of the hydrogen-pyrene bond is small, down to 0.06 eV. A second hydrogen binds barrierless at many sites. The most stable structure of dihydrogenpyrene is more stable by 0.64 eV than pyrene plus a molecular hydrogen molecule and a large barrier of 3.7 eV for the molecular hydrogen evolution is found. Using a simple tight-binding model we demonstrate that the projected density of π-states can be used to predict the most stable binding sites for hydrogen atoms and the model is used to investigate the most favorable binding sites on more hydrogenated pyrene molecules and on coronene. Some of the DFT calculations were complemented with hybrid-DFT (PBE0) showing a general agreement between the DFT and hybrid-DFT results.