Turning a Nonreducible into a Reducible Oxide via Nanostructuring: Opposite Behavior of Bulk ZrO2 and ZrO2 Nanoparticles Toward H2 Adsorption

We show that hydrogen adsorption leads to completely different dissociation mechanisms on extended zirconia surfaces or on zirconia nanoparticles. The reaction of molecular hydrogen with various forms of tetragonal ZrO2 (bulk, regular (101), and stepped (156) surfaces and nanoparticles of 0.9-1.9 nm size) has been studied by means of density functional theory calculations. The heterolytic H-2 splitting, in which Zr-H- and O-H+ bonds are formed, is energetically favorable on the extended surfaces. This mechanism does not lead to a chemical reduction of the oxide. Incorporation of the H species from the surface into the bulk results in the formation of a proton and an electron trapped at a Zr ion (Zr3+) but is energetically very unfavorable. Completely different is the behavior of zirconia nanoparticles where the reductive homolytic splitting, in which two OH bonds are formed and two electrons are transferred to Zr3+ sites, is the preferred adsorption mechanism (exothermic reaction). This shows that under-coordination and nanostructuring may have dramatic effects on the reactivity of this oxide toward hydrogen, turning the nonreducible bulk zirconia into a reducible material when prepared in form of nanoparticles.