Elucidating the role of potassium in methane steam reforming using first-principles-based kinetic Monte Carlo simulations

Abstract

In the chemical industry, potassium is commonly employed as a promoter to reduce coke formation on the Ni catalyst surface and has been demonstrated to significantly improve the productivity of the methane steam reforming (MSR) reaction. Despite numerous studies, a detailed understanding of the potassium effect at steam reforming conditions is lacking. In this contribution, we have developed a first-principles-based KMC model of MSR on Ni(1 1 1) and potassium-doped Ni(1 1 1) surfaces. The cluster expansion (CE) methodology was employed to systematically capture adsorbate–adsorbate interactions between MSR species and their effects on reaction rates. We performed KMC simulations with different loadings of potassium (0.5–3 %) on Ni(1 1 1) to understand the effect on MSR net turnover rates and macroscopic coverages. At high temperatures, we found that potassium strongly promotes the oxidation of CH and carbon adsorbates. For instance, at 1273.0 K, we observe that the MSR net turnover rate on the potassium-doped Ni(1 1 1) system (K-Ni(1 1 1)-2.8 %) is around 13.6 times higher than on Ni(1 1 1). The KMC process statistics analysis reveals that key oxidation events such as the formation of CHO, CHOH, COH and CO occur significantly faster on the potassium sites compared to Ni sites. Furthermore, on K-Ni(1 1 1)-2.8 % at 1273.0 K, the CO pathway on the potassium sites, which entails complete dehydrogenation of methane to C and oxidation of the latter to CO, makes the highest contribution (around 57 %) to the MSR net turnover rate. Our KMC simulations provide a deeper mechanistic-level understanding of the role of potassium in MSR and can potentially aid in the design of next-generation Ni-based catalysts that exhibit high activity and stability at steam reforming conditions.