Surface Reaction Kinetics for Oxidation and Reforming of H2, CO, and CH4 over Nickel-based Catalysts

The catalytic conversion of hydrocarbons for the production of hydrogen and syngas (H2/CO) is of great interest in research and technology. Detailed heterogeneous kinetics can also provide a better understanding of the reactions involved during the catalytic processes commonly used in the synthesis gas production. The developed models can predict conversion and selectivity at varying operation conditions and hence supply guidance to reactor and catalyst design. In this work, a hierarchical multistep surface reaction mechanism was developed for H2 and CO oxidation, water-gas shift (WGS), reverse water-gas shift (R-WGS), and the oxidation and reforming of methane over nickel-based catalysts as well as, with slight modifications, for CO methanation. The reaction mechanism consists of 52 reactions with 6 gas phase species and 13 surface species. Important intermediates such as adsorbed HCO and COOH species are included in the kinetic model. The surface reaction mechanism can be applied to the CH4/CO2/H2O/CO/O2/H2 systems operating in a wide range of external conditions. Models for gas-phase kinetics and flow fields are coupled with the surface mechanism to consider possible gas phase reactions at high pressures and temperatures. The overall thermodynamic consistency of the mechanism is ensured by a numerical approach in which surface reaction rate parameters are slightly modified to be thermodynamically consistent. Within this study, the kinetics of methane reforming and oxidation as well as systems H2/O2, CO/O2, CO/H2, CO/O2/H2, WGS, and R-WGS were investigated in different reactor configurations (plug-flow, fixed-bed, and stagnation-flow reactors) following a hierarchical approach for the development of a reliable mechanism. The product stream was analyzed by FT-IR and MS, which allow time-resolved monitoring. The mechanism was evaluated against experimental data at varying operating conditions performed in this study and also taken from literature. The model can be applied for industrial applications, quantitatively predicting the effect of inlet compositions, operating conditions. It can be extended to undesirable transient modifications of the active catalytic phase, e.g., by deactivation and coking, which are the main challenge in industrial catalytic reformers.