Insights into ammonia oxidation reforming over bimetallic Ni–Ru nanocatalysts for enhanced H2 production at low exhaust temperature of marine engines: Thermodynamic, experimental, and DFT investigation

Abstract

Addressing challenges associated with efficient H₂ generation from NH₃ at low temperature holds immense significance for ammonia application in marine engines. This work employs the thermodynamic model of NH₃ decomposition to elucidate chemical reaction equilibrium limitations across varying temperature and pressure conditions, defining optimal catalyst boundaries. Subsequently, the vacuum-impregnation-calcination method is utilized to synthesize a series of nanocatalysts loaded with Ni and Ru. The evaluation results reveal that Ru catalysts exhibit superior NH₃ decomposition performance compared to Ni catalysts at low temperature. Ammonia conversion (X_NH3) follows the trend of initially rising and subsequently declining with increasing Ru loading, attaining its optimum at 2 %. Notably, Ni–Ru bimetallic catalysts demonstrate excellent performance, and catalytic activities decrease in the order of 3Ni0.5Ru/Al₂O₃ > 6Ni0.5Ru/Al₂O₃ > 0.5Ru/Al₂O₃ > 6Ni/Al₂O₃ > 3Ni/Al₂O₃. For ammonia composite oxidative reforming reaction, DFT calculations and experimental results show that maintaining optimal A/O (NH₃ to O₂) ratio and GHSV under engine exhaust temperature conditions is essential for achieving preferable hydrogen yield (Y_H2). It should be emphasized that 3Ni0.5Ru/Al₂O₃ outperforms pure Ru catalysts in oxidative reforming process, achieving Y_H2 of 57.3 % at 10000 h⁻¹, 400 °C and A/O = 4. This is ascribed to the fact that Ni component can act as the additional effective active site in ammonia oxidation reaction, and the synergistic effect of Ni–Ru bimetal have the positive impact on H₂ production. Inspiringly, 3Ni0.5Ru/Al₂O₃ shows commendable stability within 50 h, with only slight Ni and Ru valence fluctuations. Moreover, bimetallic catalyst sustains substantial Y_H2 for oxidative reforming reaction at 300 °C.