The determining role of Tx species in the catalytic potential of MXenes: Water adsorption and dissociation on Mo2CTx

Density functional theory is used to investigate the origins of the excellent catalytic activity of the Mo2CTx MXene for the water gas shift reaction. By considering different possibilities for the MXene surface termination (Tx = none, O, F, or a mixture of O and F), we conclude that its ideal composition should contain both F and O adatoms, essential for controlling the exothermicity of the reaction and avoiding saturation by oxygenated species. More precisely, while Mo2CO2 and Mo2CF2 are too inert towards water adsorption and dissociation and the bare Mo2C MXene is inactivated upon coverage by oxygenated species, our calculations predict that regions near one or two O adatoms in the midst of F surface terminations should be the active catalytic sites. Indeed, in the vicinity of the O adatoms, water adsorbs with moderate strength, dissociates with a very low energy barrier (0.14-0.20 eV), and the dissociation is moderately exothermic.