ABSTRACT:
Catalytic ammonia decomposition is considered as an attractive method for onset hydrogen production but is limited by the lack of efficient none-noble catalysts. Nickel based catalysts is found to be the most active among all the none-noble metals. However, its catalytic activity is hardly satisfactory at mid-temperatures. In this work, we propose several bimetallic M₅Ni₅/Al₂O₃ (M=Co, Fe, Cu) as catalysts to decompose ammonia at 700 ^oC and the gas hourly space velocity of 30 000 ml/(g_cat.h). Co₅Ni₅/Al₂O₃ shows nearly 100% NH₃ conversion and the hydrogen generation rate of 1947.9 mmol/(g_cat.h). The conversion is ~20.8% higher than that of the monometallic Ni₁₀/Al₂O₃ catalyst. DFT calculations imply that the recombinative desorption of N is the rate-limiting step of ammonia decomposition over Ni-based catalysts and the addition of Co can decrease the energy barriers for this step. NH₃-TPD manifests that bimetallic Co₅Ni₅/Al₂O₃ displays the minimum acidity sites on the surface. The acidity sites have been reported to be detrimental to the electron conduction. Therefore, the high catalytic activity of Co₅Ni₅/Al₂O₃ can be explained by the synergy between Ni and Co to decrease the surface acidity sites, thus reducing the rate-limiting N-N recombinative desorption step. This work provides a feasible method to design efficient none-noble catalysts to decompose ammonia for onset hydrogen generation.