COx-free hydrogen production via decomposition of ammonia over Cu–Zn-based heterogeneous catalysts and their activity/stability

Abstract

Cu–Zn mixed metal oxides were synthesized with a modified citrate method, supported on an Al2O3 substrate, and tested for ammonia decomposition process to attain a high-purity hydrogen generation for fuel cells. Alumina-supported catalytic materials prepared by wet impregnation technique exhibited the highest surface copper species dispersion along with an increase in distributed acidic and basic sites. TPD studies using NH3 and CO2 revealed the presence of the Lewis and Brønsted acidity and basicity. SEM demonstrated a uniform particle distribution and morphology, regardless of Cu/Zn ratio. Cu/ZnO/Al2O3 exhibited a superior conversion activity when compared to neat Cu–Zn, which may be due to an improved Cu–Zn synergistic effect, a smaller average bimetallic nanoparticle size, and moderate acid–base characteristics. TEM micrographs confirmed the round-shaped metallic particles, ranging up to 7nm in diameter and comprising both active copper oxide phases (Cu1+ and Cu2+), further investigated by EELS spectroscopy, as well as by XPS analysis. With reactions resulting in an effective first-order turnover (rate-determining step limitation), the apparent activation energies of (50–80)±5kJmol−1 were estimated in between 450 and 600°C which is in agreement with other commercial catalysts. All fabricated bi-functional catalysts exhibited a high long-term stability and hydrogen productivity; nonetheless, comprised no critical scarce raw resources (e.g. platinum-group metals), which is appealing for emerging chemically-bonded H2 storage and release be it in relation to production only (electrolysis) or with consumption (regenerative fuel cells).