Dopant driven tuning of the hydrogen oxidation mechanism at the pore/nickel/zirconia triple phase boundary

The effects of cation dopants in zirconia on the H2 oxidation mechanism at the pore/nickel/zirconia triple phase boundary (TPB) were theoretically examined. Y, Sc, Al, Ce, and Ca were considered as dopants, and on-boundary, O-migration, and H-migration reaction mechanisms were examined. Based on density functional theory calculations, Y as a dopant favored the on-boundary mechanism with water molecule formation within the immediate proximity of the TPB. The corresponding rate-limiting step is H transfer from the nickel surface to the boundary. In contrast, the on-boundary mechanism is not completed with the Al-, Sc-, and Ca-doped systems, due to the dissociation of water molecules at the boundary. In the Al-doped system, the O-migration mechanism is the major reaction pathway due to a low barrier for the rate-limiting step that corresponds to O transfer from zirconia to the nickel surface. The H-migration mechanism, which implies water molecule formation on the zirconia surface at a position distant from the boundary, should dominate at the Sc-, Ca-, and Ce-doped TPBs, with the lowest activation barrier at the Sc-doped TPB. The reasons for the switching of the reaction mechanisms depending on the dopant species are analyzed.