DFT study of Ni-CeO2 interaction: Adsorption and insertion

DFT-GGA calculations are used to investigate interaction of atomic nickel with ceria. Nickel adsorption on surfaces is compared with insertion into the bulk and subsurfaces using VASP calculations. The adsorption is considered upon the (111) and (110) surfaces of ceria since these surfaces are the most stable ones being formally generated from the least number of bond cleavages (one or two Ce-O bonds per Ce, respectively). When Ni atom is adsorbed on the (111) surface, it occupies a position atop whereas over the (110) surface it occupies a bridging position. The adsorption quenches the spin. Results for insertion both in the bulk material and in the (111) and (110) subsurfaces are presented. For the bulk, it is shown that an increase of Ni amount from 1/4 to 1 makes insertion more exothermic. The later is accompanied by a lattice expansion and a reduction of symmetry. For an amount 1/4, the Ni is inserted to a tetrahedral site. At larger concentration, it is in trigonal environment of three oxygen atoms, additional oxygen ligands being less tightly bound. For insertion in the sublayers of the (111) subsurface, the nickel atom occupies a similar position, also a ternary site. The interaction energy for nickel atom insertion is comparable to that for nickel adsorption, slightly larger for (111), slightly smaller for (110) surface. Diffusion into the bulk is thus likely. When inserted, the distance Ni-Ce is becomes short, 2.70A, in agreement with experiment and the system may evolve with the formation of a Ni2Ce alloy.