Development of hydrotalcite-derived Ni catalysts for the dry reforming of methane at high temperatures

Abstract

Catalytic dry reforming of methane (DRM) is an attractive technology for industrial production of synthesis gas, an important feedstock for the production of many basic chemicals. The endothermic reaction operates at high temperatures above 640 °C. On nickel based catalysts high syngas yields are obtained. However, catalyst deactivation by coke formation over Ni based catalysts is still challenging. Deeper understanding of the structure-performance-relationships is needed to integrate the DRM in the well-established downstream syngas chemistry. This thesis presents a systematic study on the development of a long-term active and thermally stable Ni/MgAl oxide catalyst for the DRM reaction by understanding and optimization of the catalyst synthesis. Regarding the active catalyst, particular emphasis was laid on the understanding of the formation of carbon deposits. By comprehensive structural characterizations of the material in all stages of the preparation, a synthesis route via a Ni,Mg,Al hydrotalcite-like precursor was developed that leads to nanostructuring of the catalytic material. This procedure was successfully applied to Ni/MgAl oxide catalysts with various compositions. Upon high-temperature reduction the catalysts form Ni nanoparticles which are embedded in an oxide matrix and covered by an overlayer. The nature of the overgrowth was investigated applying surface sensitive methods, revealing the presence of predominantly oxidic species. Interestingly, the overgrowth was found to effectively attenuate the carbon formation. Despite coke formation and high Ni loading up to 55 wt.-%, the CH4 conversion in the DRM at 900 °C was stable over 100 hours. The thermal stability of the Ni nanoparticles is attributed to the embedding nature of the oxide matrix. This allows the high-temperature operation without losing substantial active Ni surface area. Furthermore, the DRM activity, as well as the carbon formation, was strongly depending on the Ni content. The incorporation of a higher amount of Ni was found to increase the activity as well as the coking propensity. By analysis of the spent catalysts thermal and compositional dependencies on the formed carbon species were found. The amount of filamentous carbon decreases with higher reaction temperature and lower Ni content. The carbon formation was found to be a continuous process over the investigated time and caused mainly by methane pyrolysis. From the overall gained insights it can be concluded, that a good catalyst have to make a compromise between activity and coke resistance, which can be controlled by an interplay of Ni dispersion, embedment and metal-support-interactions. This work demonstrates the relevance of a detailed characterization at all stages of the catalyst preparation, as well as after the reaction, to understand and improve the catalytic performance by rational approaches. The experimental findings give new insights into the current state of reforming knowledge and coke formation and will contribute to the development of advanced catalysts for DR