Steric and Orbital Effects Induced by Isovalent Dopants on the Surface Chemistry of ZrO2

Abstract

Sometimes, dopants in oxide surfaces are referred to as single-atom catalysts, at least when these species are incorporated in the supporting lattice. Usually, single atom catalysts are transition metal atoms stabilized on an oxide surface, and the activity is due to the valence electrons of these species. However, the surface chemistry can be modified also by the presence of isovalent heteroatoms, where the total number of valence electrons of the active site is the same as for the regular surface. The effect of isovalent dopants on the chemical reactivity of tetragonal ZrO2 has been studied with first principles calculations. Zr ions in the bulk, subsurface, and surface sites have been replaced with Si, Ge, Sn, Pb, Ti, Hf, and Ce ions. Surface or subsurface sites are clearly preferred. The dopants modify the local structure of the surface and introduce new empty states in the band gap, thus affecting the Lewis acid properties of the surface. We studied the effect of the dopants on the decomposition of HCOOH. This can follow four paths with desorption of (a) H2, (b) CO, (c) H2O, or (d) CO2. On pure ZrO2 reaction (a) dehydrogenation is preferred followed by decarbonylation (b). Ti, Hf, and Ce have some effect on the decomposition but do not change the order of reactivity. On the contrary, in the presence of Si, decarbonylation becomes the preferred path. If Ge occupies surface sites, reaction (d) loss of CO2 is by far more favorable. With Sn, dehydrogenation remains energetically preferred but the ordering of the other reactions changes, while Pb makes CO2 desorption slightly preferred over release of H2. These effects virtually disappear when the dopants occupy subsurface sites. The study shows that "steric"and/or "orbital"effects of isovalent dopants on a catalyst surface are sufficient to change the reaction products compared to the undoped system.