Zn Redistribution and Volatility in ZnZrO<sub><i>x</i></sub> Catalysts for CO<sub>2</sub> Hydrogenation
E. Redekop, Tomás Cordero‐Lanzac, Davide Salusso, Anuj Pokle, Sigurd Øien‐Ødegaard, Martin F. Sunding, Spyros Diplas, Chiara Negri, Elisa Borfecchia, Silvia Bordiga, Unni Olsbye
Abstract
High Resolution Image Download MS PowerPoint Slide ZnO–ZrO 2 mixed oxide (ZnZrO x ) catalysts are widely studied as selective catalysts for CO 2 hydrogenation into methanol at high-temperature conditions (300–350 °C) that are preferred for the subsequent in situ zeolite-catalyzed conversion of methanol into hydrocarbons in a tandem process. Zn, a key ingredient of these mixed oxide catalysts, is known to volatilize from ZnO under high-temperature conditions, but little is known about Zn mobility and volatility in mixed oxides. Here, an array of ex situ and in situ characterization techniques (scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), Infrared (IR)) was used to reveal that Zn 2+ species are mobile between the solid solution phase with ZrO 2 and segregated and/or embedded ZnO clusters. Upon reductive heat treatments, partially reversible ZnO cluster growth was observed above 250 °C and eventual Zn evaporation above 550 °C. Extensive Zn evaporation leads to catalyst deactivation and methanol selectivity decline in CO 2 hydrogenation. These findings extend the fundamental knowledge of Zn-containing mixed oxide catalysts and are highly relevant for the CO 2 -to-hydrocarbon process optimization.