Diffusional Effects in Nickel Oxide Reduction Kinetics

Nickel is widely used as an industrial catalyst and has recently been investigated as an oxygen carrier in chemical looping combustion (CLC), which is a novel approach for power generation that offers inherent CO2, capture. However, prior studies have reported a wide range of activation energy values for nickel formation from the reduction of NiO by hydrogen. In the present work, this reaction was investigated using the Friedman isoconversional method for a series of thermogravirnetric curves, which were obtained at varying heating rates. The intrinsic activation energy for the reduction of NiO was observed to be in the range of 91.8-94.5 kJ/mol, whereas values of 42.6 kJ/mol and similar to 0 kJ/mol were found at intermediate and high heating rates, respectively, which correspond to intermediate and high temperatures. This demonstrates the presence of internal and external mass-transfer limitations during the reaction, and it provides a likely explanation for the varying activation energy values reported in prior work. Therefore, these findings emphasize the importance of assessing diffusional effects in the determination of reaction kinetics.