Intrinsic kinetics of low temperature catalytic methane-steam reforming and water-gas shift over Rh/Ce_αZr_(1-α)O2 catalyst

摘要

This paper presents the intrinsic kinetics of CH4 steam reforming developed over Rh/Ce_(0.6)Zr_(0.4)O2 catalyst in a relatively low temperature range of 475-575 °C and 1.5 bar pressure. The kinetic experiments are conducted in an integral fixed bed reactor with no mass and heat transport limitations and far from equilibrium conditions. Therefore, intrinsic reaction rate measurements are guaranteed. The model is based upon two-site adsorption surface hypothesis, and 14 elementary reaction steps are postulated. CH4 is dissociatively adsorbed onto the Rh active sites, and steam is dissociafively adsorbed on the ceria support active sites as an influential adsorption surface shown in the model. Therefore, no competition between CH4 and steam in adsorbing on the same site surface is observed. The kinetic rate expressions are derived according to the Langmuir-Hinshelwood formalism. The redox surface reactions between the carbon containing species and the lattice oxygen leading to CO and CO2 formation are considered as rate determining steps. The inhibitory effect of gaseous product species is also reflected in the kinetics. The model is found to be statistically accurate and thermodynamically consistent. The estimated activation energies and adsorption enthalpies are in agreement with literature for CH4 steam reforming reaction over Rh. The reaction kinetics is validated by steam reforming experiments at 550 °C and 1.5 bar using 150 mg catalyst in a diluted bed of 5 cm length. The kinetic model is implemented in a one-dimensional pseudo-homogenous plug flow reactor model and thus simulated at identical experimental conditions. The simulation results are in excellent agreement with the experimental values.