Kinetic study of the catalytic reforming of CH4 with CO2 to syngas over Ni/α-Al2O3 catalyst: The effect of temperature on the reforming mechanism
The mechanism and the rate-determining steps (RDS) of CO2 reforming of CH4 were investigated over the typical Ni/α-Al2O3 catalyst in a wide temperature range of 550–750 °C using steady-state and transient kinetic methods. After elimination of the effects of side reactions, the reforming reaction was controlled by kinetics. The activation energies of the reforming reaction and the reaction orders of CH4, CO2, H2, and CO showed that the reforming reaction could be divided into three regions: 550–575 °C, 575–650 °C, and 650–750 °C. The reaction rate was constant in the low and high temperature ranges but was varied with temperature in the middle range. The CH4 dissociation reached equilibrium with Ni–H species above 650 °C. The surface oxygen species originating from CO2 became removable and reacted with CHx species above 575 °C. The reaction of CHx with CO2 was slower than that of CH4 dissociation above 650 °C, leading to the durative carbon deposition on the catalyst. CO competed with CH4 on the Ni active sites below 650 °C but was desorbed rapidly above 650 °C. The formation of hydrogen is a rapid or equilibrium step in the reforming reaction. The CH4 dissociation is the RDS and CO desorption also restrained the dry reforming in 550–575 °C. The reaction between CHx and CO2 became the RDS in 650–750 °C. And the restraining steps were switched from the former two steps to the latter step in 575–650 °C. The reaction temperature remarkably influences the reforming mechanism through altering the reaction steps.
Graphical abstractThe mechanism of the dry reforming over the Ni/α-Al2O3 catalyst remained constant in 550–575 °C and 650–750 °C but varied with temperature in 575–650 °C. CH4 dissociation was the rate-determining step (RDS) and CO desorption also restrained the dry reforming in 550–575 °C. The reaction between CHx and CO2 became the RDS in 650–750 °C.Figure optionsDownload full-size imageDownload as PowerPoint slide
Journal: Applied Catalysis A: General - Volume 318, 20 February 2007, Pages 79–88