Thermal Methane Conversion to Formaldehyde Mediated by NiAlO₃⁺ in the Gas Phase

Understanding the active sites and reaction mechanisms of Ni-based catalysts, such as Ni/Al₂O₃, toward methane is a prerequisite for improving their rational design. Here, the gas-phase reactivity of NiAlO₃⁺ cations toward CH₄ is studied using mass spectrometry combined with density functional theory. Similar to our previous study on NiAl₂O₄⁺, we find evidence for the formation of both the methyl radical (CH₃^•) and formaldehyde (CH₂O). The first step for methane activation is hydrogen atom abstraction by the terminal oxygen radical Ni(O)₂AlO^• from methane forming a [Ni(O)₂AlOH⁺, ^•CH₃] complex and leaving the Ni-oxidation state unchanged. The second C-H bond is subsequently activated by the association of a bridged Ni-O^2--Al. The oxidation state of the Ni atom is reduced from +3 to +1 during the formation of formaldehyde. Compared to Al₂O₃⁺/CH₄ and YAlO₃⁺/CH₄ systems, the Ni-atom substitution increases the overall reaction rate by roughly an order of magnitude and yields a CH₃^•/CH₂O branching ratio of 0.62/0.38. The present study provides molecular-level insights into the highly efficient gas-phase reaction mechanism contributing to an improved understanding of methane conversion by Ni/Al₂O₃ catalysts.