Reduction of Different GeO2 Polymorphs

Abstract

A combination of volumetric adsorption, thermal desorption and structure-determining methods was used to study and compare the hydrogen reduction behavior of three different GeO2 polymorphs: tetragonal, water-free hexagonal, and water-containing (hydroxylated) commercial hexagonal GeO2. Marked differences in the onset and extent of reduction between the two water-free polymorphs have been observed. Tetragonal GeO2 adsorbs more hydrogen at temperatures T ≤ 673 K and tends to be more easily reducible at low temperatures, but extended Ge metal formation at elevated temperatures is rather suppressed compared to both hexagonal GeO2 phases. Temperature-programmed hydrogen desorption spectra indicate the reduction-induced formation of weakly bonded hydrogen adsorption sites both on hydroxylated and pure hexagonal GeO2. The existence of the most weakly bonded hydrogen is linked to the transformation of initially present hydroxylated species into a tetragonal structure fraction upon annealing at ∼500 K. Thus, analogous forms of hydrogen were not observed on any of the pure (dehydroxylated) structures.