Adsorption and Dimerization of Late Transition Metal Atoms on the Regular and Defective Quartz (001) Surface

Using a density functional theory approach with dispersion corrections, we have studied the adsorption properties of Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au monomers and dimers on the hydroxylated surface of alpha-quartz. We also considered the interaction with Si-O-center dot paramagnetic centers (NBO centers) or with silanols ( Si-O-H) to form a Si-O-TMx (x = 1, 2) group and a 1/2 H-2 molecule. On the hydroxylated surface, the TM atoms bind relatively weakly, while strong adsorption energies of 4-5 eV are found on the NBO center where due to the coupling of the unpaired electron of the Si-O-center dot unit and the valence states of the metal atom. The TM atoms can react with the silanol groups releasing H-2 and forming a stable Si-O-TM complex with exothermic reactions. Dimers arriving from the gas-phase interact with the hydroxylated quartz surface with energies between 0.5 and 1.4 eV. If two TM atoms diffusing on the surface form a dimer, they release a large amount of energy which largely exceeds the dimer adsorption energy, leading to possible dimer desorption (with the exception of Pd). Also dimers bind strongly to NBO centers. Here the dimerization energies are considerably smaller than the desorption energy showing that the NBO sites act as effective nucleation centers. Differently from TM atoms, only some of the dimers react with Si-O-H groups.