Interfacial Origin of Mn Promotion in Ni/AlO x Catalysts for Methane Pyrolysis and Carbon Nanotube Growth

Abstract Commercial deployment of methane pyrolysis (MP) requires H2 to be produced at a high rate while the solid carbon is recovered as a marketable product such as multiwalled carbon nanotubes (MWCNTs). Various promoters have been explored for Ni-based MP catalysts, including Mn, but the origin of its effect has remained elusive. Here, Ni/AlOx catalysts containing 0 to 5 wt % Mn were used to separate the two roles that Mn plays in activity and stability. The amount of Ni exposed at an oxide boundary correlated with the apparent CH4 reaction order across the series, identifying the Ni–O–Mn perimeter as the site at which CH4 activation is promoted. The same perimeter converted weakly bound NiO into strongly interacting species that resisted encapsulation, linking the rate and the lifetime of the catalyst to a single interface. At 1 wt %, Mn the apparent activation energy fell from 93.9 to 84.2 kJ mol–1, the CH4 reaction order rose from 0.46 to 0.84, and the turnover number increased from 375 to 456 mol C mol Ni–1. At 2 wt %, MnOx began to bury the Ni surface, and above that loading it segregated into a separate phase, so the promotion was lost. Mn also improved the graphitization and conductivity of the MWCNTs formed at the start of the reaction, but this benefit faded as disordered carbon accumulated. Co-feeding H2 suppressed this accumulation and gave MWCNTs with an electrical conductivity comparable to that of commercial material. The benefit of Mn is therefore set by where it resides rather than by how much is added, and both products of MP reach useful quality only when the promoter loading and the reaction conditions are optimized together.