The adaption of the sol-gel autocombustion method to the Cu/ZrO₂ system opens new pathways for the specific optimisation of the activity, long-term stability and CO₂ selectivity of methanol steam reforming (MSR) catalysts. Calcination of the same post-combustion precursor at 400 °C, 600 °C or 800 °C allows accessing Cu/ZrO₂ interfaces of metallic Cu with either amorphous, tetragonal or monoclinic ZrO₂, influencing the CO₂ selectivity and the MSR activity distinctly different. While the CO₂ selectivity is less affected, the impact of the post-combustion calcination temperature on the Cu and ZrO₂ catalyst morphology is more pronounced. A porous and largely amorphous ZrO₂ structure in the sample, characteristic for sol-gel autocombustion processes, is obtained at 400 °C. This directly translates into superior activity and long-term stability in MSR compared to Cu/tetragonal ZrO₂ and Cu/monoclinic ZrO₂ obtained by calcination at 600 °C and 800 °C. The morphology of the latter Cu/ZrO₂ catalysts consists of much larger, agglomerated and non-porous crystalline particles. Based on aberration-corrected electron microscopy, we attribute the beneficial catalytic properties of the Cu/amorphous ZrO₂ material partially to the enhanced sintering resistance of copper particles provided by the porous support morphology.

We focus on the stability and bulk/surface structural properties of the Ruddlesden-Popper phase La2NiO4 and their consequences for dry reforming of methane (DRM) activity. Fuelled by the appearance as a crucial intermediate during in situ decomposition of highly DRM-active LaNiO3 perovskite structures, we show that La2NiO4 can be equally in situ decomposed into a Ni/La2O3 phase offering CO2 capture and release necessary for DRM activity, albeit at much higher temperatures compared to LaNiO3. Decomposition in hydrogen also leads to an active Ni/La2O3 phase. In situ X-ray diffraction during DRM operation reveals considerable coking and encapsulation of exsolved Ni, yielding much smaller Ni crystallites compared to on LaNiO3, where coking is virtually absent. Generalizing the importance of intermediate Ruddlesden-Popper phases, the in situ decomposition of La-based perovskite structures yields several obstacles due to the high stability of both the parent perovskite and the Ruddlesden-Popper structures and the occurrence of parasitic structures.

To elucidate the role of earth alkaline doping in perovskite-based dry reforming of methane (DRM) catalysts, we embarked on a comparative and exemplary study of a Ni-based Sm perovskite with and without Sr doping. While the Sr-doped material appears as a structure-pure Sm_1.5Sr_0.5NiO₄ Ruddlesden Popper structure, the undoped material is a NiO/monoclinic Sm₂O₃ composite. Hydrogen pre-reduction or direct activation in the DRM mixture in all cases yields either active Ni/Sm₂O₃ or Ni/Sm₂O₃/SrCO₃ materials, with albeit different short-term stability and deactivation behavior. The much smaller Ni particle size after hydrogen reduction of Sm_1.5Sr_0.5NiO₄, and of generally all undoped materials stabilizes the short and long-term DRM activity. Carbon dioxide reactivity manifests itself in the direct formation of SrCO₃ in the case of Sm_1.5Sr_0.5NiO₄, which is dominant at high temperatures. For Sm_1.5Sr_0.5NiO₄, the CO : H₂ ratio exceeds 1 at these temperatures, which is attributed to faster direct carbon dioxide conversion to SrCO₃ without catalytic DRM reactivity. As no Sm₂O₂CO₃ surface or bulk phase as a result of carbon dioxide activation was observed for any material - in contrast to La₂O₂CO₃ - we suggest that oxy-carbonate formation plays only a minor role for DRM reactivity. Rather, we identify surface graphitic carbon as the potentially reactive intermediate. Graphitic carbon has already been shown as a crucial reaction intermediate in metal-oxide DRM catalysts and appears both for Sm_1.5Sr_0.5NiO₄ and NiO/monoclinic Sm₂O₃ after reaction as crystalline structure. It is significantly more pronounced for the latter due to the higher amount of oxygen-deficient monoclinic Sm₂O₃ facilitating carbon dioxide activation. Despite the often reported beneficial role of earth alkaline dopants in DRM catalysis, we show that the situation is more complex. In our studies, the detrimental role of earth alkaline doping manifests itself in the exclusive formation of the sole stable carbonated species and a general destabilization of the Ni/monoclinic Sm₂O₃ interface by favoring Ni particle sintering.