Dry Reforming of Methane and Light Alkanes over Nickel-based Catalysts

Abstract

This thesis addresses catalyst deactivation in the dry reforming of methane (DRM) and the related dry reforming of ethane (DRE) and propane (DRP) over Ni-based catalysts. DRM converts two major greenhouse gases, CH₄ and CO₂, into synthesis gas, offering a sustainable alternative to fossil-fuel-based routes such as steam reforming. Its industrial application remains limited by rapid deactivation through carbon deposition and metal sintering. Ni catalysts supported on MgAl₂O₄ spinel are particularly promising owing to their thermal stability, moderate basicity and strong metal–support interactions, yet their synthesis–structure–composition–performance relationships and deactivation mechanisms remain poorly understood. Using a combination of advanced characterization and operando spectroscopy, this work systematically examines how the synthesis method, preparation parameters and support composition govern catalyst structure and stability. Chapter 2 shows that the synthesis method determines Ni dispersion and phase distribution in Ni/MgAl₂O₄. Co-precipitation gives high initial activity but deactivates through Ni exsolution onto alumina-rich domains, colloidal deposition provides excellent coking resistance, and hydrotalcite impregnation offers a practical compromise. Calcination at 900 °C produces the most active catalyst despite the lowest reducibility. This paradox is explained by controlled partial NiAl₂O₄ formation, which creates interfacial Ni–spinel domains that anchor metallic nanoparticles against sintering and stabilize Ni⁰ against CO₂ re-oxidation. Optimal performance therefore requires balanced Ni incorporation rather than maximal metallic Ni content. Chapter 3 uses operando full-field transmission X-ray micro-imaging (FF-TXM) to map the Ni oxidation state along the catalyst bed with 15 μm spatial and ~10 s temporal resolution. Increasing the Mg content strengthens metal–support interactions, Ni/MgAl₂O₄ balances Ni⁰ formation and oxidation resistance, whereas Ni/MgO stays oxidized through NiO–MgO solid-solution formation. Distinct reduction fronts emerge: under H₂, reduction propagates from the hottest zone, whereas under DRM it starts at the outlet and advances toward the inlet, driven by in situ H₂ production. These oxidation-state gradients persist even after 30 minutes, demonstrating that single-spot measurements miss critical heterogeneities and that spatially resolved operando spectroscopy is essential. Chapter 4 extends the study to DRE and DRP. Unlike DRM, which is stable near equilibrium, ethane and propane cause rapid deactivation whose onset scales with hydrocarbon chain length, temperature and CO₂ content. Deactivation coincides with a mechanistic shift from C–C to C–H bond cleavage, consistent with selective blocking of Ni step-edge sites by disordered amorphous carbon. Higher temperatures reduce total carbon but favour graphitic deposits, while longer chains produce more deactivating amorphous carbon, and higher CO₂ content delays deactivation. In mixed feeds, methane has a protective effect at moderate higher-alkane fractions (up to 25%), which is lost at 50% substitution. Together, these findings reframe the metal–support interface as a controllable structural feature, establish that the catalyst redox state is inherently heterogeneous along the bed, and clarify the distinct coking mechanisms of higher alkanes, providing a basis for the rational design of more stable, feedstock-flexible Ni-based dry reforming catalysts.