(50h) Unraveling the Influence of Water on Hydrocarbon Conversion to Produce Hydrogen and Solid Carbon: A DFT Study

The conversion of hydrocarbons into carbon nanotubes (CNTs) and hydrogen (H₂) is a rapidly expanding area of research, requiring a thorough investigation into the influence of co-feedstocks on the process. Experimental results demonstrated that increasing H₂O concentration along with methane (CH₄) leads to reduced yields of both H₂ and CNTs, whereas injecting H₂O following methane decomposition enhances production rates. In this study, Density Functional Theory (DFT) calculations were performed to investigate H₂O dissociation on Ni (111) with various co-adsorbates to probe its influence on CH₄ decomposition and the mechanism underlying carbon removal from the surface. The results revealed that H₂O dissociates easily on Ni surfaces, thereby reducing H₂ production through competition for active sites. Furthermore, the oxygen species generated may oxidize the catalyst surface, while the repulsion between surface oxygen (O*) and methyl (CH₃*) groups increases the activation barrier for CH₄ dissociation. In contrast, in the presence of surface carbon deposition, H₂O exhibits a positive effect by directly reacting with carbon, releasing CO, and restoring active sites for CH₄ decomposition. This reaction route is both thermodynamically and kinetically preferable compared to the alternative pathway where H₂O dissociates initially, followed by the interaction of C* and O*, which exhibits a notably high activation barrier. These results are in agreement with experimentally determined barriers over a Ni based catalyst. This work thus shows that adjusting the operation condition and controlling population of the surface species could be valuable for hydrogen generation and production of carbon nanotubes.