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

Abstract

Reductive fluorination, which entails the substitution of O 2− 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 WO 3 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 (C 2 F 4 ), hexafluoropropene (C 3 F 6 ), and iso-octafluoroisobutene (i-C 4 F 8 ) to be the reactive components in the PTFEdecomposition gas during the reactions with WO 3 at 500 °C. CO 2 and CO are further identified as gaseous byproducts of the reaction evincing that the reaction is prompted by difluorocarbene (:CF 2 ) formed after the cleavage of C�C bonds in i-C 4 F 8 , C 3 F 6 , and C 2 F 4 upon contact with the WO 3 surface. The solid−solid reaction path is established through a reaction between WO 3 and WO 3−x F x where solid-state diffusion of O 2− and F − is discerned at 500 °C.

Pure Fluorine Intercalation into Brownmillerite Oxide Thin Films by Using Ionic Liquid Gating

“…19 A significant issue with the reaction using fluoropolymers is that their carbon content extracts oxygens from precursors, leading to oxygen deficiencies. 20 A growing area of interest in recent years is the use of electrolyte gating including ionic liquid gating (ILG). Electricfield effects, originating from the electric double layer formed at the interface between films and electrolytes, offer a versatile platform for tuning the physical properties of strongly correlated oxide thin films, 21 induced superconductivity in electrostatically doped SrTiO 3 and KTaO 3 .…”

“…The F 2 gas treatments of both SrFeO 2 and SrFeO 2.5 result in the formation of SrFeO 2 F, − likely due to the stability of Fe 3+ . For cobalt, while SrCoO 2.5 F 0.5 is reported to be obtained from SrCoO 2.5 , using F 2 gas or XeF 2 , the detailed anion composition remains underexamined. , Although fluoropolymers such as polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE) are safer alternatives to corrosive F 2 gas, ,− they often lead to reduced products (e.g., SrCoO 2.5 → SrCoO 1.9(4) F 0.5(1) ) . A significant issue with the reaction using fluoropolymers is that their carbon content extracts oxygens from precursors, leading to oxygen deficiencies …”