Oxygen Vacancies Stabilize Zr(III) in the Metal–Organic Framework UiO-66

Metal–organic frameworks with a photoaccessible d-manifold are potentially useful for redox catalysis. Yet, electronic structure calculations of materials composed of Zr(IV) preclude the photogeneration of Zr(III). This finding contrasts with a handful of studies that invoke a Zr(III) center in proposed catalytic cycles. Defects may be responsible for stabilizing Zr(III) centers. Here we examine the effects of hydrogenic and oxygenic defects on the position of the band edges in a representative Zr(IV)-containing framework, UiO-66. While Brønsted acid/base-type defects are generally favorable, none yield a Zr d-state below the computational hydrogen evolution reaction (HER). However, oxygenic vacancies may stabilize a Zr d-orbital ∼0.5 eV below the HER. The oxygenic defect occurs with a formation energy of +2.3 eV, and concentration of 1.50 × 10–7 defects/mol at 373 K of Zr(III), essentially preventing the defect without alternative stabilization. If such vacancies were installed, they may be the origin of Zr(III).