Ammonia decomposition activity over Ni/SiO2 catalysts with different pore diameters

Ammonia decomposition over Ni-loaded SiO2 catalysts (Ni/SiO2) was observed in a fixedbed reactor at different temperatures (ranging from 773 to 973 K) and ammonia feeding rates (ranging from 1200 to 18,000 h1). As support materials, several porous and inert SiO2 particles with different mean pore diameters ðdÞ ranging from 7.7 to 34.8 nm were used to clarify the effect of pore diameter on the kinetic parameters for catalytic ammonia decomposition. The Ni/SiO2 catalyst with the smallest pores, d ¼ 7.7 nm, showed the highest activity at temperatures below 923 K, while the activity of this catalyst at 973 K was lower than that of catalysts with larger pores. Kinetic analysis indicated that the activation energy for d ¼ 7.7 nm was significantly decreased at higher temperatures, suggesting the occurrence of strong diffusion resistance of ammonia molecules in the pores. Our experiments also confirmed that almost complete decomposition of ammonia could be achieved over Ni/SiO2 with d ¼ 26.7 nm at 973 K and a gas hourly space velocity as high as 42,000 h1.