Bane and boon of oxygen mediating the performance of nickel catalysts in dry reforming of methane

A collaborative effort by scientists from the Departments of Inorganic Chemistry and Theory at the Fritz Haber Institute of the Max Planck Society in Berlin provided fundamental insights into the processes occurring at the catalyst surface and on how this modulates the catalytic performance during DRM.

In particular, they studied the role of different oxygen species on a Nickel catalyst during DRM by using a combination of experimental and computational science techniques, including operando scanning electron microscopy, near ambient pressure X-ray photoelectron spectroscopy, and computer vision. They highlighted the critical role of dissociative CO₂ adsorption in regulating the oxygen content of the catalyst and CH₄ activation. Furthermore, they discovered the presence of three metastable oxygen species at the catalyst: atomic surface oxygen, subsurface oxygen, and bulk NiO_x. Interestingly, these exhibited different catalytic properties and their interplay and transformation gave rise to oscillations in the surface states and in the catalytic function.

They observed that some of the surface oxygen leaked into the catalyst bulk, reducing the availability of the catalyst for CH₄ activation and favoring CO₂ and O diffusion instead. The extent of the leakage was further proved by X-ray spectroscopy and transmission electron microscopy, revealing the presence of oxygen several nanometers below the surface of the catalysts. Consequently, new metallic sites were exposed, thus leading to an increased rate of the oxygen uptake and to a decrease of the H₂/CO product ratio. Lastly, they understood that co-feeding of CO₂ is essential for CH₄ conversion, likely assisting its activation together with the presence of oxygenated species. "It was impressive to see how the metastability of the Ni-O system self-adjusts the catalytic performance and that one element from the reactants can direct the entire process which depends on its location and its chemistry. We hope that our findings can give new momentum in adjusting longevity and selectivity in catalysis,” says PD Dr. Thomas Lunkenbein, leader of the project and co-author of the study.

Understanding the metastability of the surfaces of catalysts, alongside with how to control them to stabilize the dynamical active state, holds important implications for the future of catalysis. In particular, it provides insights that can be transferred to the industrial level and the design of reactors where an active state with minimal energetic compromises is favored. This could be achieved  either by using more potent oxidants, such as water (H₂O) and nitrous oxide (N₂O), or by working on reducing the oxygen leakage into the bulk by means of nanoparticles or thin film technology. The development of catalysts based on tailor-made thin films is the focus of CatLab, a joint research platform between the FHI, Helmholtz Center of Berlin (HZB) and industrial partners, which is funded by the Federal Ministry of Education and Research (BMBF) intended as a bridge between research and industry.