Hydrogen Adsorption on Monoclinic ((1)over-bar11) and ((1)over-bar01) ZrO2 Surfaces: A Periodic ab Initio Study

The interaction between monoclinic zirconia and H-2 is studied by means of periodic density functional theory calculations. The ((1) over bar 11) and ((1) over bar 01) surfaces are represented by slab models. Molecular hydrogen is found to adsorb weakly on the slabs with exothermic energies of -0.10 eV. Heterolytic dissociative adsorption takes place, forming Zr-H and O-H pairs with endothermic energies up to 0.24 eV for the ((1) over bar 11) slab and -0.08 eV for the ((1) over bar 01) slab. Hydrogen adsorption induces important surface rearrangements. Three different Zr-H surface species are characterized: isolated Zr-H, Zr-H/OH pairs, and pseudobridging H-Zr center dot center dot center dot H. Experimental infrared bands at 1560 and 1370 cm(-1) can be attributed to isolated and paired Zr-H/OH groups, respectively, from an ab initio basis. Finally, dissociation is found to proceed via an activation barrier of 0.28-0.65 eV, in agreement with the experimental value of 0.34 eV, showing the catalytic properties of monoclinic zirconia toward hydrogen.