Partial oxidation of methane (POM) is considered a promising industrial method for producing syngas, a mixture commonly used to make fuels and chemicals. For years, scientists have assumed that metallic nickel (Ni) nanoparticles serve as the main active centers that drive this reaction. However, there has been an important unresolved question. The metallic Ni observed after a reaction may simply form when nickel oxide is reduced by syngas at high temperatures, rather than representing the species that actually performs the catalysis.
Nickel can change both its oxidation state and its atomic arrangement under the high-temperature redox conditions involved in POM. Until now, these changes have been difficult to track in detail, making it challenging to determine the true structure responsible for the reaction.
A Hidden Active Structure Forms During the Reaction
In a recent study published in Nature Catalysis, researchers found that highly active structures can form in situ when the surface of NiO reconstructs during POM. The results reveal the atomic-scale source of the catalytic activity and show why it is important to observe catalysts while they are operating under realistic reaction conditions.
The research was led by Profs. Tao Zhang, Aiqin Wang and Xiaoyan Liu from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS), together with Prof. Wei Liu from DICP, Prof. Tao Yang from Xi’an Jiaotong University, and Prof. Graham J. Hutchings from Cardiff University.
To investigate the process, the team created a Ni/Al2O3 catalyst containing only 0.8 wt% Ni using a microemulsion method. Despite its relatively low nickel content, the catalyst performed strongly during POM. It converted 92% of the methane, while CO and H2 selectivities reached 87.0%, with the H2/CO molar ratio remaining stable at about 2.0.
High Performance With Much Less Nickel
One of the most striking findings was that almost no metallic Ni could be detected in the catalyst after the reaction. Even so, its overall performance was comparable to that of an 8.0 wt% Ni/Al2O3 catalyst produced through impregnation, despite containing only one tenth as much nickel.
The low-loading catalyst also performed far better than another 0.8 wt% Ni/Al2O3 material prepared using the same impregnation method. Under identical conditions, that catalyst did not effectively carry out POM and instead showed only methane combustion activity.
The researchers also observed that metallic Ni nanoparticles present at the beginning of the reaction were quickly oxidized into the NiO phase under POM conditions. Yet NiO alone was not enough. A pre-formed pure-phase NiO catalyst showed no POM activity and instead catalyzed only the complete oxidation of methane.
Atomic Reconstruction Reveals the True Active Site
A closer examination revealed what was actually happening on the catalyst surface. During the reaction, the researchers captured the in situ formation of a reconstructed [Ni1O4Ni4] structural unit on the NiO(100) surface.
DFT calculations indicated that this newly formed motif makes it much easier to break the C-H bonds in methane, one of the key steps in activating the molecule. The calculated activation barrier was only 12.5 kcal·mol-1.
That barrier was far lower than the value calculated for the intact NiO(100) surface (38.5 kcal·mol-1) and also below the barrier for the metallic Ni(111) surface (15.7 kcal·mol-1). This substantial kinetic advantage strongly supports the conclusion that the reconstructed structure is the true active center responsible for POM.
Together, the experimental observations and theoretical calculations show that catalytic activity does not come simply from metallic nickel or ordinary nickel oxide. Instead, it emerges from a specific atomic structure that forms dynamically while the reaction is taking place.
“Our study highlights the critical role of in situ characterization in identifying dynamic active structures under reaction conditions,” said Prof. Liu. “Dynamic reconstruction enables low-loading catalysts to achieve high performance, offering new opportunities for the rational design of efficient catalysts while reducing reliance on high metal loadings.”
