Abstract
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·gcat−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·gNi−1 at 30 L h−1·gcat−1 and 300 °C, respectively, with negligible deactivation even after 90 h of fluctuating operation.
Highlights
Converting anthropogenic CO2 into valuable fuels (e.g. CH4) using green hydrogen generated, for instance, from water electrolysis driven by renewable electricity is key to enable the energy transition of the chemical industry [1,2]
In the conversion of carbon dioxide to methane, large quantities of heat are released due to the exothermicity of the reaction (CO2 + 4 H2 → CH4 + 2 H2O, ΔH298K = –165 kJ/mol) and, in the absence of heat removal, the adiabatic temperature rise would be rather significant (773 K) [3]. When employing this technology in large-scale Power-to-Gas (P2G) processes, the conversion of CO2 can vary significantly due to the fluctuations in the production of renewable electricity that is used to generate the hydrogen required for the process. This results in large temperature swings as a function of time on stream [4]
The authors argue that coupling of thermo-/electrocatalytic processes with dynamic energy and feed supply will render additional complexities to the chemical industry as reactors are often operated within a narrow operational window for optimal performance
Summary
Converting anthropogenic CO2 into valuable fuels (e.g. CH4) using green hydrogen generated, for instance, from water electrolysis driven by renewable electricity is key to enable the energy transition of the chemical industry [1,2]. In the conversion of carbon dioxide to methane, large quantities of heat are released due to the exothermicity of the reaction (CO2 + 4 H2 → CH4 + 2 H2O, ΔH298K = –165 kJ/mol) and, in the absence of heat removal, the adiabatic temperature rise would be rather significant (773 K) [3] When employing this technology in large-scale Power-to-Gas (P2G) processes, the conversion of CO2 can vary significantly due to the fluctuations in the production of renewable electricity that is used to generate the hydrogen required for the process (for every mole of CO2 4 mol of H2 are needed). In order to compensate for the accelerated deactivation, one could either use an excess of catalyst, or use a fluidized reactor in which the catalyst is continuously replenished These strategies, could make the process economically unattractive at high catalyst consumption rates. New catalysts and reactor concepts are needed to facilitate the
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