Mechanism of Reductive Fluorination by PTFE-Decomposition Fluorocarbon Gases for WO₃

Abstract

Reductive fluorination, which entails the substitution of O²⁻ from oxide compounds with F⁻ from fluoropolymers, is considered a practical approach for preparing transition-metal oxyfluorides. However, the current understanding of the fundamental reaction paths remains limited due to the analytical complexities posed by high-temperature reactions in glassware. Therefore, to expand this knowledgebase, this study investigates the reaction mechanisms behind the reductive fluorination of WO₃ using polytetrafluoroethylene (PTFE) in an Ni reactor. Here, we explore varied reaction conditions (temperature, duration, and F/W ratio) to suppress the formation of carbon byproducts, minimize the dissipation of fluorine-containing tungsten (VI) compounds, and achieve a high fluorine content. The gas–solid reaction paths are analyzed using infrared spectroscopy, which revealed tetrafluoroethylene (C₂F₄), hexafluoropropene (C₃F₆), and iso-octafluoroisobutene (i-C₄F₈) to be the reactive components in the PTFE-decomposition gas during the reactions with WO₃ at 500 °C. CO₂ and CO are further identified as gaseous byproducts of the reaction evincing that the reaction is prompted by difluorocarbene (:CF₂) formed after the cleavage of C═C bonds in i-C₄F₈, C₃F₆, and C₂F₄ upon contact with the WO₃ surface. The solid–solid reaction path is established through a reaction between WO₃ and WO₃–xFx where solid-state diffusion of O²⁻ and F⁻ is discerned at 500 °C.