Highly dispersed ultra-small NiO nanoparticles on mesostructured silica as efficient catalysts for CO₂ methanation

Introduction

Due to the attention toward global warming, the current research is focusing on green fuels obtained by the reduction of captured CO₂ (e-fuels). A prominent example of these e-fuels is methane. Ni-based catalysts are among the most investigated systems for CO₂ methanation. Ni is often paired with a promoter, like CeO₂. In this work, composite catalysts consisting of a NiO and NiO/CeO₂ active phase dispersed on mesostructured silica (SBA-15) are presented, which, using a support, should allow us to reach a high activity with a reduced amount of the active phase.

Methods

To obtain NiO- and NiO/CeO₂-based nanocomposites, two different impregnation approaches were used: two-solvent (TS) impregnation and impregnation based on a self-combustion (SC) reaction. The NiO-based catalysts were obtained with a loading of 4.5%; the NiO/CeO₂-based catalysts were obtained using a 1:1 Ni:Ce molar ratio. To determine their structural and morphological properties, the catalysts were characterized with small-angle (SA-) and wide-angle (WA-) XRD (X-ray diffraction), TEM (transmission electron microscopy), and nitrogen physisorption and tested for CO₂ methanation.

Results and discussion

WA-XRD shows that the composites obtained with SC do not feature a diffraction peak, suggesting that NiO and CeO₂ are deposited into the mesopores as ultra-small nanoparticles. On the other hand, the composites obtained with TS impregnation show broad but visible crystal reflections attributed to both NiO and CeO₂, indicating that the active phase has crystallized, forming larger nanoparticles. This finding is also confirmed by the TEM micrographs, which for the SC composites do not show the presence of any visible particles outside the mesopores; the TS systems, conversely, show visibly darker nanoparticles dispersed over the support. The catalytic tests show a positive effect of CeO₂ on performance; furthermore, the catalysts obtained with SC impregnation show a higher CO₂ conversion, presumably due to the higher dispersion of the active phase obtained with this approach.