Due to their high catalytic activity and low-cost Ni based catalysts are of special interest for the dry reforming of methane (DRM), however, they also suffer from heavy carbon deposition, which eventually results in catalyst deactivation. In this dissertation, the deactivation mechanism of Ni supported catalysts during DRM has been studied and highly active and stable Ni based catalysts have been developed.
To decouple active metal-support interactions, first Ni supported on SiO2 catalysts were studied. It was shown that carbon mostly forms on bigger Ni crystallites, hence, the sintering of Ni particles is one of the leading causes for catalyst deactivation. A novel synthesis method, referred to as “pressure dilution” was developed, which allowed the preparation of highly dispersed Ni/fumed SiO2 catalysts with increased stability.
In order, to prevent the sintering of Ni active sites, NiO-MgO solid solution-based catalysts with strong metal-support interactions were developed. Cellulose assisted solution combustion synthesis (CACS) method was used to synthesize Ni/MgO catalysts with record-high surface areas, which improved their catalytic activity and stability. The main criterions for the stability towards carbon formation were determined to be metal dispersion, strong metal-support interactions, and an increased amount of Ni3+ site defects.
To reveal the specifics of CACS method in obtaining high surface area materials, mechanistic studies have been performed (Chapter 5). It was found that combustion parameters, i.e., time-temperature profile, combustion front velocity, etc., have a direct impact on the microstructure (phase composition, crystallinity, morphology, etc.) of synthesized materials. It was demonstrated that cellulose degradation/decomposition itself is being catalyzed by metal ions and, depending on metal ions, the pyrolytic combustion reaction between metal nitrates and cellulose fibers proceeds with different mechanisms.
To better understand the reaction pathways of the DRM, the experimental data obtained for Ni/MgO catalyst was used to perform kinetic simulations. Chapter 6 presents DRM kinetic simulation results, which were obtained by the Monte-Carlo based “Kinetiscope” stochastic program. DRM reaction network was constructed and simulated based on Eley-Rideal and Langmuir Hinshelwood-Hougen Watson type models, using single and dual active site concepts.