The Kinetic Study In The CH₄/CO₂ Reforming Reactions Over Ni-based Catalysts

The reforming reaction has received extensive attention. The reforming reaction can convert two greenhouse gases CH₄/CO₂ into syngas H₂/CO with low ratio, which are more advantageous for the succedent synthesis of alcohol and liquid fuels. The studies on the CH₄/CO₂ reforming reaction over Ni-based catalysts by the kinetic methods can further understand the kinetic characteristic and the mechanism. The reduced Ni-based catalysts can produce high temperature desorption hydrogen, which was originated from Ni-H species. The Ni-H species can increase the x value of CHx intermediate formed during the reforming reaction and improve the reaction between the CHx species and CO₂ with the decreased activation energy (Ea), enhancing the catalytic reforming activity.The turn-over frequencies (TOF) of CH₄ dissociation over per Ni active site on the Ni/α-Al₂O₃ catalyst are varied with the different Ni particle sizes and CH₄ dissociation is proven to be a structure sensitive reaction. The Ea for the CH₄ dissociation over Ni/MgAl₂O₄ is lower than that over Ni/α-Al₂O₃. The addition of promoter La₂O₃ increases the Ea value for the CH₄ dissociation. The Ea and pre-exponential factor (A) have remarkable compensation effect for the CH₄ dissociation.CH₄ and CO₂ are alternately pulsed through the Ni-based catalysts. The dissociation rates of CH₄ are higher than those between CHx and CO₂ over the Ni/α-Al₂O₃ catalyst, meaning the deposition of carbon species on the catalyst. The reaction rates for CHx and CO₂ are higher than those of CH₄ dissociation on the Ni/La₂O₃/α-Al₂O₃ catalyst, which can be due to that La₂O₃ tunes the Ea and A for the both reactions. The steady-state kinetic study exhibits no carbon species deposited on the Ni/La₂O₃/α-Al₂O₃ catalyst in the range 500-600 oC for the refroming reaction of CH₄/CO₂ with equal ratio.The steady kinetic study and the pulse results of CH₄/CO₂ reforming reaction overthe Ni/α-Al2O3 catalyst show that the reforming reaction can be divided into three temperature regions: the CH₄ dissociation and the desorption of CO are the rate-determining steps (RDS) in 550-575 oC; the RDS are the same as above and surface oxygen species also participates the reforming reaction in 575-650 oC; CH₄ dissociation and CO desorption reach equilibrium and the reaction between CH_x and CO₂ is RDS in 650-750 oC.The kinetic study on the CH₄/CO₂ reforming reaction over the Ni/La2O3/α-Al2O3 catalysts with different La2O3 loadings shows that the rates of the reforming reaction are decreased as the orders: 4%>6%>2%>1% (wt% La2O3). In 550-600 oC, CH₄ dissociation, which is a structure sensitive reaction, is a slow step in the reforming reaction, and the dissociation rates are depended on the Ni particle sizes on the La2O3-modified catalysts. In 600-750 oC, CH₄ dissociation and the Ni-H species reach equilibrium and the reaction between CH_x and CO₂ is a slow step in the reforming reaction. The amount and properity of the Ni-H species on the La2O3-modified catalysts determine the reformig reaction rates.