(5a) Microkinetic Investigation of the Transient CO2 Methanation on Ni Catalysts in a Berty Reactor

Dynamic operation of reactors becomes increasingly important to couple fluctuating renewable energy with chemical reactions, such as the production of CH₄ from CO₂ via power-to-gas technology. Consequently, microkinetics are required that can accurately quantify transient phenomena to design efficient and safe reactors. This study combines transient methanation experiments with automated mechanism generation considering DFT-based uncertainties to develop a microkinetic model for the transient CO₂ methanation.

Experiments were performed with a Ni/SiO₂ catalyst in a Berty reactor, which can be modeled as an ideal CSTR using the Cantera framework. Periodic concentration forcing was applied to the system by alternating between H₂ and CO₂. 5000 microkinetic models for the CO₂ methanation on Ni(111) were taken from a previous study [1] that embedded the Reaction Mechanism Generator into a global uncertainty assessment (GUA) considering DFT-based uncertainties [1]. Adsorbate-adsorbate interactions were included in the model.

The experiment in Figure 1a shows a build-up behavior after the feed is initially switched from H₂ to CO₂, which converges to a limit cycle and ends with a small overshot for CH₄ after the last change to H2. Moreover, the CO profile shows a doubling in the frequency response. These complex phenomena can be investigated and understood through microkinetic modeling. The GUA creates a broad spread of possible results considering the uncertainty in the DFT-derived databases, as seen in Figures 1b, c. A feasible set of energetic parameters can be identified from this set of mechanisms, which is in quantitative agreement with all experiments as displayed for CH₄ and CO. The identified best match microkinetic model provides insights into the CO₂ methanation mechanism under transient conditions, which can be used to screen for optimal reactor operation conditions and process intensification through a forced periodic operation.

[1] Kreitz, B., et al. JACS Au. DOI: 10.1021/jacsau.1c00276