Enhanced catalytic activity and stability of nanoshaped Ni/CeO₂ for CO₂ methanation in micro-monoliths

Coupling inherently fluctuating renewable feedstocks to highly exothermic catalytic processes, such as CO₂ methanation, is a major challenge as large thermal swings occurring during ON- and OFF- cycles can irreversible deactivate the catalyst via metal sintering and pore collapsing. Here, we report a highly stable and active Ni catalyst supported on CeO₂ nanorods that can outperform the commercial CeO₂ (octahedral) counterpart during CO₂ methanation at variable reaction conditions in both powdered and μ-monolith configurations. The long-term stability tests were carried out in the kinetic regime, at the temperature of maximal rate (300 °C) using fluctuating gas hourly space velocities that varied between 6 and 30 L h⁻¹·g_cat⁻¹. Detailed catalyst characterization by μ-XRF revealed that similar Ni loadings were achieved on nanorods and octahedral CeO₂ (c.a. 2.7 and 3.3 wt. %, respectively). Notably, XRD, SEM, and HR-TEM-EDX analysis indicated that on CeO₂ nanorods smaller Ni-Clusters with a narrow particle size distribution were obtained (∼ 7 ± 4 nm) when compared to octahedral CeO₂ (∼ 16 ± 13 nm). The fast deactivation observed on Ni loaded on commercial CeO₂ (octahedral) was prevented by structuring the reactor bed on μ-monoliths and supporting the Ni catalyst on CeO₂ nanorods. FeCrAlloy® sheets were used to manufacture a multichannel μ-monolith of 2 cm in length and 1.58 cm in diameter, with a cell density of 2004 cpsi. Detailed catalyst testing revealed that powdered and structured Ni/CeO₂ nanorods achieved the highest reaction rates, c.a. 5.5 and 6.2 mmol CO₂ min⁻¹·g_Ni⁻¹ at 30 L h⁻¹·g_cat⁻¹ and 300 °C, respectively, with negligible deactivation even after 90 h of fluctuating operation.