Kinetic study of the catalytic reforming of CH4 with CO2 to syngas over Ni/alpha-Al2O3 catalyst: The effect of temperature on the reforming mechanism

摘要

The mechanism and the rate-determining steps (RDS) Of CO2 reforming of CH4 were investigated over the typical Ni/alpha-Al2O3 catalyst in a wide temperature range of 550-750 degrees 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, H-2, and CO showed that the reforming reaction could be divided into three regions: 550-575 degrees C, 575-650 degrees C, and 650-750 degrees 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 degrees C. The surface oxygen species originating from CO2 became removable and reacted with CH, species above 575 degrees C. The reaction of CH, with CO2 was slower than that of CH4 dissociation above 650 degrees C, leading to the durative carbon deposition on the catalyst. CO competed with CH4 on the Ni active sites below 650 degrees C but was desorbed rapidly above 650 degrees 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 degrees C. The reaction between CHx and CO2 became the RDS in 650-750 degrees C. And the restraining steps were switched from the former two steps to the latter step in 575-650 degrees C. The reaction temperature remarkably influences the reforming mechanism through altering the reaction steps. (c) 2006 Elsevier B.V. All rights reserved.