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 Ni-0 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 (k(0)) and activation energy (Ea) was observed and quantified. Moreover, La could partially lower the particle size of Ni-0 and reduce the stability of reaction intermediates, which could be the main reason for its positive influence on the specific activity of Ni. H-2 temperature-programmed desorption (TPD) and NH3 temperature-programmed surface reaction (TPSR) were used as supplements to study the influence of Ni-0 particle size and addition of La on the catalytic behavior. (c) 2005 Elsevier B.V. All rights reserved.