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Structure sensitivity of ammonia decomposition over Ni catalysts: A computational and experimental study

Duan, Xuezhi, Qian, Gang, Liu, Yan, Ji, Jian, Zhou, Xinggui, Chen, De, Yuan, Weikang
Fuel processing technology 2013 v.108 pp. 112-117
activation energy, adsorption, ammonia, catalysts, dehydrogenation, desorption, nickel, particle size
Ni catalysts are widely used to catalyze ammonia decomposition. In this study, first-principles calculations are performed to investigate the preferred adsorption sites and the adsorption energies of NHₓ (x=0–3) and H, and the transition states, activation energies and rate constants of NH₃ stepwise dehydrogenation and associative desorption of N on the stepped Ni(211) and the close-packed Ni(111) surfaces. The results show that the barrier of associative desorption of N, which is higher than those of NH₃ stepwise dehydrogenation, on Ni(211) are 1.10eV higher than that on Ni(111), indicating that the associative desorption of N on Ni(211) is difficult to take place. Hence the strongly adsorbed N atoms would block some of the step sites of Ni(211). The N-blocking Ni(211) surface is constructed, and then NH₃ decomposition on 2N–Ni(211) is investigated. The results indicate that Ni catalysts with too many or too few step sites, which can be associated with Ni particle sizes, show low ammonia decomposition activities. Therefore, different sized Ni-based catalysts are employed to catalyze ammonia decomposition, and TOFH₂ as a function of Ni particle size is correlated. Finally, the structure sensitivity of ammonia decomposition over Ni catalysts is rationally interpreted.