Hydrogen production using catalytic decomposition of ammonia over supported Ru-based catalysts

Abstract

Due to their superior intrinsic activity compared to PGM-based, Ru-supported catalysts have been receiving extensive attention for the NH3 cracking reaction. In this study, Ru-based catalysts were synthesized using three different supports. The experimental results indicate that Ru loaded on CeO2 exhibits higher catalytic activity than the used comprehensively supports such as Al2O3 and SiO2. Despite their wide specific surface area, SiO2 and Al2O3 exhibited lower Ru particle size and dispersion compared to the Ru/CeO2 catalyst. In the Ru/CeO2 catalyst, Ru demonstrates a small particle size of 5.3 nm and about 20% dispersion based on CO-chemisorption results. Additionally, the highest electron density was confirmed in XPS, aligning well with the variety of CO adsorption behaviors observed in CO-DRIFTS. In situ CO-DRIFTS results showed that on the Ru/CeO2 catalyst, strong adsorption peaks such as bridged Ru-CO were detected not observed in other catalysts. Additionally, the high electron density facilitated electron transfer to the adsorbed CO, resulting in rapid desorption as the temperature increased. The three supports with distinct characteristics were systematically analyzed through in-situ DRIFTS study to investigate the interactions between Ru and the supports as the affecting factors in ammonia decomposition reactions. In situ NH3-DIRFTS results for Ru/CeO2 revealed the formation of intermediate species at low temperatures, which rapidly transitioned to N2 and H2 with increasing temperature. Furthermore, the rapid desorption of produced N2 indicated the swift occurrence of N-H cleavage and N2 desorption steps, crucial in ammonia decomposition reactions. In contrast, while the Ru/Al2O3 catalyst does produce N2 and H2 through N-H cleavage, the appearance of strong adsorption peaks MS/EN 20221079 Bogyung Kim (김보경) Hydrogen production using catalytic decomposition of ammonia over supported Ru-based catalysts (루테늄 촉매 기반 암모니아 분해를 통한 수소 생산). School of Earth Sciences and Environmental Engineering. 2024. 70P. Prof. Sung Bong Kang ii for N2 even at high temperatures suggests that the N2 desorption step is not occurring efficiently, which may be one of the reasons for its relatively lower catalytic activity. Additionally, the Ru/SiO2 catalyst exhibits limited adsorption of NH3 and CO gases, suggesting a reduced presence of exposed Ru as active sites on the catalyst surface. As a result, the Ru/CeO2 catalyst with high electron density possesses numerous Ru sites for reactant adsorption and it was observed that ammonia adsorbed at low temperatures rapidly decomposes into intermediate species. The facile electron transfer in Ru/CeO2 accelerates the desorption rates of adsorbed species, consequently contributing to the enhancement of the catalytic activity. This study offers valuable insights for research into application of support properties.