Kinetic study of NH3 decomposition over Ni nanoparticles: The role of La promoter, structure sensitivity and compensation effect

Some nickel nanoparticles supported on alumina were synthesized and tested for ammonia decomposition. Steady-state reaction kinetic results reveal that the turnover frequency (TOF) increases firstly with decreasing particle size, passes through a maximum in the range of mean size of Ni0 1.8-2.9 nm and then decreases. A structure sensitivity of NH3 decomposition over Ni catalysts is strongly suggested. An excellent fit of experimental data by the Temkin-Pyzhev equation was obtained. A compensation effect between the pre-exponential factor (k0) and activation energy (Ea) was observed and quantified. Moreover, La could partially lower the particle size of Ni0 and reduce the stability of reaction intermediates, which could be the main reason for its positive influence on the specific activity of Ni. H2 temperature-programmed desorption (TPD) and NH3 temperature-programmed surface reaction (TPSR) were used as supplements to study the influence of Ni0 particle size and addition of La on the catalytic behavior.