Assessing the Impact of Adlayer Description Fidelity on Theoretical Predictions of Coking on Ni(111) at Steam Reforming Conditions

Methane steam reforming is an important industrial process for hydrogen production, employing Ni as a low-cost, highly active catalyst, which, however, suffers from coking due to methane cracking. Coking is the accumulation of a stable poison over time, occurring at high temperatures; thus, to a first approximation, it can be treated as a thermodynamic problem. In this work, we developed an Ab initio kinetic Monte Carlo (KMC) model for methane cracking on Ni(111) at steam reforming conditions. The model captures C-H activation kinetics in detail, while graphene sheet formation is described at the level of thermodynamics, to obtain insights into the terminal (poisoned) state of graphene/coke within reasonable computational times. We used cluster expansions (CEs) of progressively higher fidelity to systematically assess the influence of effective cluster interactions between adsorbed or covalently bonded C and CH species on the terminal state morphology. Moreover, we compared the predictions of KMC models incorporating these CEs into mean-field microkinetic models in a consistent manner. The models show that the terminal state changes significantly with the level of fidelity of the CEs. Furthermore, high-fidelity simulations predict C-CH island/rings that are largely disconnected at low temperatures but completely encapsulate the Ni(111) surface at high temperatures.

Automated summary

Methane steam reforming is an important industrial process for hydrogen production, but suffers from coking due to methane cracking. In this study, an Ab initio KMC model was developed to understand the terminal state of graphene/coke during methane cracking on Ni(111). By systematically assessing the influence of effective cluster interactions, the model predicts significant changes in the terminal state morphology with increasing fidelity of the cluster expansions. High-fidelity simulations reveal C-CH island/rings that are disconnected at low temperatures but encapsulate the Ni(111) surface at high temperatures.