Early transition metal nano-carbides and nano-hydrides from solid-state metathesis initiated at room temperature

Abstract

Transition metal carbides (TMCs) have attracted great interest due to their mechanical and catalytic properties but their syntheses generally require energy-consuming processes with temperatures above 800 °C. We report here a solid-state metathesis reaction between metal chlorides (ZrCl₄, NbCl₅, MoCl₃, MoCl₅, HfCl₄, TaCl₅, WCl₆) and potassium dispersed in carbon (graphite or acetylene black). The reaction is initiated at room temperature or with gentle heating below 75 °C. Within seconds, it produces carbon-supported carbide or metallic nanoparticles of diameter below 50 nm. The phase speciation depends on the metal and different process parameters. In some cases (ZrCl₄, NbCl₅, HfCl₄, TaCl₅), the metal is further converted into a hydride upon neutralization of the reaction crude with ethanol. The products were characterized by powder X–ray diffraction (PXRD) and Transmission Electron Microscopy (TEM). Based on thermodynamic and kinetic considerations, we propose a mechanism explaining the coexistence of several phases (metal, carbide, hydride) and their occurrence at each step of the reaction.