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.