The importance of the location between metal and support for CO2 methanation

Carbon dioxide (CO₂), known as a greenhouse gas, can also be used as a zero- or even negative-cost carbon feedstock in the production of valuable chemicals and fuels. The power-to-gas process is among the most attractive and promising technologies for transforming electrical energy into renewable chemical energy, which is able to convert CO₂ into methane (CH₄). This technology consists of an initial hydrogen production step via electrolysis of water, followed by the reduction of CO₂ via the Sabatier reaction (i.e., methanation). For the CO₂ methanation, the design and synthesis of highly active and selective Ni catalysts still remain big challenge.

Recently, researchers at the Taiyuan University of Technology, China, University of Toronto, Canada, and Karlsruhe Institute of Technology, Germany, employed 2D silicon surface chemistry to design catalysts consisting of Ni on siloxene nanosheets (SiXNS) in order to address the challenges posed by CO₂ methanation. In preparation of SiXNS supported Ni catalysts with impregnation method, hydride- and hydroxyl-terminated groups in layered silicon hydrolysis and condensation can be triggered in H₂O, resulted in the so-formed Ni nanoparticles being directed predominantly to the external surface of the layered siloxene host material. In contrast, these nanoparticles were mainly confined between siloxene layers when the reaction was performed using ethanol, EtOH (see Figure 1).

Figure 1. Electron tomography analysis of a nickel siloxene composite particle synthesized using ethanol as solvent.

It is neither the nickel nor the selectivity that matters most in this work. Rather, the innovation and most significant metric of our breakthrough is the discovery that the internal or external confinement of Ni, with respect to the multi-layered siloxene support, determines the reaction pathway, activity, selectivity, and stability in a manner distinct from anything known in this field. Notably, the internally confined nickel nanoparticles demonstrated significantly enhanced selectivity towards CH₄ over CO, relative to those located externally. This spatially confined Ni catalyst proved stable in the light and, over time, eventually reaching an impressive CH₄ production rate of ~100 mmol gNi⁻¹ h⁻¹ with ~90% selectivity.

Distinct reaction intermediates (i.e., CO and formate species) and reaction pathways were observed based on in situ diffuse reflectance infrared spectroscopy (DRIFTS) study in CO₂ hydrogenation reactions. It was demonstrated that a CO₂-dissociative pathway favored CO production by externally confined Ni nanoparticles, whereas a CO₂ formate associative pathway was preferred for Ni nanoparticles confined to the interior (see Figure 2).

Figure 2. Catalytic CO₂ reduction pathways for nickel siloxene samples made in H₂O and in EtOH showing key reaction intermediates detected by DRIFTS.

This work has been published in Nature Communications (Nickel@Siloxene catalytic nanosheets for high-performance CO₂ methanation, 2019, 10, 2608)。

https://www.nature.com/articles/s41467-019-10464-x

From College of Chemistry and Chemical Engineering