Support effect on Ni-based catalysts using methane decomposition analysed from in-situ TEM images.

Abstract

As the world is transitioning to green energy sources, we are still dependent on the energy that fossil sources such as methane (CH4) provide us. Making use of methane decomposition over Ni-based catalysts, we can decompose CH4 into carbon nanofibers (CNF) and hydrogen (H2) which are of great interest as high-tensile strength material and CO2-free energy source, respectively. Prior research has shown that the choice of support materials greatly influences the performances of Ni-based catalysts. However, these works were dependent on bulk data which relies on a combination of factors influencing catalytic results, making direct support and particle size comparisons difficult. In this work, the particle size and support effect during methane decomposition were investigated using gas-cell in-situ transmission electron microscopy (TEM). Using incipient wetness impregnation Ni particles were deposited on different support materials: carbon (GNP), oxidised carbon (GNP-ox), SiO2, and TiO2. Catalytic tests were performed (550 ◦C, 75/25% CH4/H2, 1 bar) on these catalysts and image series were obtained using in-situ TEM. An approach was presented to analyse the CNF growth from these images while accounting for the errors this introduces. This study found that the support material influences the carbon growth rate and deactivation rate of individual particles. Ni supported on GNP and GNP-ox saw fast deactivation of small particles (< 10 nm) and slow deactivation on larger ones (> 30 nm), while Ni supported on SiO2 and TiO2 saw no significant particle size dependency. Ni/SiO2 showed significantly higher carbon growth rates compared to other catalysts. We postulate that this is due to stronger metal-support interactions, which stimulate carbon accumulation at active metal sites. Furthermore, two growth modes were identified and characterised: stuttering CNF growth on Ni/GNP and Ni/GNP-ox and continuous growth on Ni/SiO2 and Ni/TiO2. Additionally, regeneration experiments performed by brief exposure of H2 to the deactivating catalyst proved to be a promising method to regenerate catalytic activity.