Thermal Methane Conversion to Formaldehyde Mediated by NiAlO in the Gas Phase.

Understanding the active sites and reaction mechanisms of Ni-based catalysts, such as Ni/AlO, toward methane is a prerequisite for improving their rational design. Here, the gas-phase reactivity of NiAlO cations toward CH is studied using mass spectrometry combined with density functional theory. Similar to our previous study on NiAlO, we find evidence for the formation of both the methyl radical (CH) and formaldehyde (CHO). The first step for methane activation is hydrogen atom abstraction by the terminal oxygen radical Ni(O)AlO from methane forming a [Ni(O)AlOH, CH] complex and leaving the Ni-oxidation state unchanged. The second C-H bond is subsequently activated by the association of a bridged Ni-O-Al. The oxidation state of the Ni atom is reduced from +3 to +1 during the formation of formaldehyde. Compared to AlO/CH and YAlO/CH systems, the Ni-atom substitution increases the overall reaction rate by roughly an order of magnitude and yields a CH/CHO 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/AlO catalysts.