Thermal Methane Conversion to Formaldehyde Mediated by NiAlO 3 + in the Gas Phase.

Understanding the active sites and reaction mechanisms of Ni-based catalysts, such as Ni/Al 2 O 3 , toward methane is a prerequisite for improving their rational design. Here, the gas-phase reactivity of NiAlO 3 + cations toward CH 4 is studied using mass spectrometry combined with density functional theory. Similar to our previous study on NiAl 2 O 4 + , we find evidence for the formation of both the methyl radical (CH 3 • ) and formaldehyde (CH 2 O). The first step for methane activation is hydrogen atom abstraction by the terminal oxygen radical Ni(O) 2 AlO • from methane forming a [Ni(O) 2 AlOH + , • CH 3 ] 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 Al 2 O 3 + /CH 4 and YAlO 3 + /CH 4 systems, the Ni-atom substitution increases the overall reaction rate by roughly an order of magnitude and yields a CH 3 • /CH 2 O 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/Al 2 O 3 catalysts.