NOx decomposition through seaweed biocarbon as biosourced catalyst: Experimental and density functional theory approaches

Understanding the NOx abatement and deactivation mechanisms is essential to improve the performance and regeneration of bio-based catalysts. In this study, biocarbon obtained from marine algae (SW), rich in nitrogen and minerals, was used as a biosourced catalyst for the decomposition of a flux of 1021 ppm of NO in argon (deNOx). Several parameters were studied, including pyrolysis temperature (650 and 800 °C) and deNOx operating temperature (200, 350, and 500 °C). Additionally, a theoretical study using density functional theory (DFT) calculations investigated the role of nitrogen and catalytic (Fe) or promoter (K) elements and functional groups (O-groups) in NO adsorption. The results indicated that the bio-based catalyst produced at 800 °C (SW800) achieved the highest conversion rate of 22.5 ± 1% at 350 °C compared to the catalyst produced at 650 °C (SW650), which achieved a conversion rate of 15.6 ± 1%. This result is attributed to the higher mineral content (52.99 wt%, db) of the SW800 catalyst rather than its specific surface area, which was not identified as a discriminating parameter under the studied conditions. Regarding the reaction mechanism, nitrogen and oxygen atomic balances confirm the multistep decomposition hypothesis. Furthermore, the slight increase in lactone and carbonyl groups after the deNOx process suggests that oxygen remains on the biocarbon. This contributes to reduce active sites and to deactivate the biocarbon catalyst, as observed. Coke deposition on active sites and metal sintering can also contribute to deactivation. Based on the calculations on representative nitrogen-rich biocarbon structures, the adsorption energies (E_ads) on pyridinic N (-71.10 kJ mol^-1) and pyrrolic N (-69.30 kJ mol^-1) sites are high, demonstrating their roles in NO abatement. However, the presence of an oxygen group close to a nitrogen site inhibits NO adsorption due to its high electronegativity. This information is in agreement with the deactivation of nitrogen sites by residual oxygen from NO decomposition, as observed experimentally.