The model and image of the in situ formed [Ni₁O₄Ni₄] active structure. Credit: DICP.

An international team of researchers have uncovered reconstructed active motifs for efficient methane partial oxidation.

Partial oxidation of methane (POM) offers a promising industrial route for syngas production and metallic nickel (Ni) nanoparticles have long been considered the catalytic active centres for this reaction.

However, the metallic Ni detected after reaction might simply be a product of nickel oxide reduction by syngas at high temperatures, rather than the intrinsic active species.

Under high-temperature redox conditions, Ni species undergo dynamic valence-state changes and structural reconstruction.

However, these evolutionary processes have remained largely unclear, hindering identification of the true active site structure for POM.

In a recent study, researchers revealed that active motifs can be generated in situ through the reconstruction of NiO surface during POM.

These findings identify the atomic-scale origin of catalytic activity and demonstrate the importance of capturing dynamic structural evolution under realistic operating conditions.

The research was led by Professors Zhang Tao, Wang Aiqin and Liu Xiaoyan from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS), together with Professor Liu Wei from DICP, Professor Yang Tao from Xi’an Jiaotong University, and Professor Graham J Hutchings from Cardiff University.

Microemulsion method

The researchers prepared a low-loading Ni/Al₂O₃ catalyst containing 0.8 wt% Ni using a microemulsion method.

The catalyst exhibited high POM activity, achieving 92% methane conversion, with CO and H₂ selectivities of 87.0% and a stable H₂/CO molar ratio of about 2.0.

Notably, almost no metallic Ni species were detected in the spent catalyst.

Its overall performance was comparable to that of a high-loading 8.0 wt% Ni/Al₂O₃ catalyst prepared by impregnation and substantially higher than that of a low-loading 0.8 wt% counterpart prepared by the same method, which showed only methane combustion activity under identical conditions.

The researchers found that metallic Ni nanoparticles present at the initial stage of the reaction were rapidly oxidised to the NiO phase under POM conditions.

However, a pre-formed pure-phase NiO catalyst showed no POM activity, catalysing only the complete oxidation of methane.

Further investigation captured the in situ reconstruction of a [Ni₁O₄Ni₄] active structural unit on the NiO(100) surface.

DFT calculations showed that this reconstructed motif facilitates C–H bond cleavage in methane, with an activation barrier of only 12.5 kcal·mol^-1.

This is substantially lower than the barriers on the intact NiO(100) surface (38.5 kcal·mol^-1) and the metallic Ni(111) surface (15.7 kcal·mol^-1), clearly demonstrating the kinetic advantage of the reconstructed structure for methane activation. 

Together, the experimental and theoretical results identify this reconstructed motif as the true active centre for POM.

Professor Liu said: “Our study highlights the critical role of in situ characterisation in identifying dynamic active structures under reaction conditions.

“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.”

The study is published in Nature Catalysis.