(676f) The Fe Catalyzed Boudouard Reaction: Mechanism, Rate Limiting Steps, and Structural Evolution of the Fe Catalyst

The Boudouard reaction – the reaction of 2 carbon monoxide (CO) molecules to form carbon dioxide (CO₂) and elemental carbon (C) – is an important chemistry that can impact chemical conversion processes involving CO, the main constituent of syngas. Carbon deposition in particular can serve as a virtue or vice: uncontrolled C-deposition causes metal dusting that embrittles metal reactors and cokes catalysts leading to deactivation, while controlled deposition may serve as a means to sequester solid carbon and synthesize functional carbon nanomaterials. Early transition metals such as iron (Fe) are known to catalyze the Boudouard reaction, however systematic kinetic investigations and mechanistic insights are lacking.

Through continuous plug flow reactor experiments conducted under conditions of strict kinetic control coupled with materials characterization we report on the mechanism and dynamic structural evolution of Fe catalyst species for the Boudouard reaction. XRD characterization shows that metallic Fe is rapidly converted to a bulk iron carbide (Fe₃C) phase under reaction conditions. At low temperatures (T < 450°C), the Fe catalyst is stable and can sustain C-deposition to C:Fe molar ratios of > 45. At higher temperatures (T > 450°C), the catalyst exhibits dynamic time dependent reactivity consisting of initial increases in rate followed by a gradual decline that ultimately results in a fully deactivated catalyst. TEM shows that the temporal evolution is aligned with periods of surface carbon nucleation, filament growth, and finally encapsulation. Kinetic measurements in these two regimes are consistent with CO dissociation being kinetically relevant (rate-limiting) at low temperatures and mass transport of carbon diffusion through the catalyst particle being rate limiting at high temperatures.

These findings suggest a balance between controlled carbon nucleation and catalyst longevity and provide important insights into the rates, mechanisms, and Fe catalyst evolution, which may be useful in the design of hydrocarbon conversion chemistries.