Elementary reaction pathway study and a deduced microkinetic model for the unified understanding of Ni-catalyzed steam methane reforming

Significance 

Steam methane reforming (SMR) reaction has been an area of interest in energy and chemical engineering. Among the available catalysts, nickel is widely used in these reactions. Thus, a thorough understanding of Ni-catalyzed steam methane reforming reactions is a prerequisite in the design and optimization of operating conditions for various applications as well as exploration of other hydrocarbon reforming processes. Unfortunately, the kinetics of the reaction on Ni-catalyzed steam methane reforming have not been fully understood due to inadequate and confusing data. Microkinetic modeling has been identified as a promising approach for understanding steam methane reforming kinetics and its application in various technological fields.

In a recent paper published in the Reaction Chemistry and Engineering journal, Dr. Changming Ke and Professor Zijing Lin from the University of Science and Technology of China at Department of Physics studied the elementary reaction pathway using a microkinetic model to understand the seemingly contradicting data on the Ni-catalyzed steam methane reforming. Specifically, they performed a density functional theory computation in conjunction with transition state analysis to investigate 80 elementary reactions steps involved in steam methane reforming over the Ni (111) surface. These computation and analysis findings were further used in the construction of a microkinetic model that was used mainly in the investigation of the reaction pathways.

The microkinetic model was validated through experimental data. Based on the results, the predicted reaction rates using this model showed good agreement with experimental data obtained under different conditions. For instance, when compared to the experiments of Nakagawa and colleagues, similar variation behaviors for the steam methane reforming rate were observed. Furthermore, the dominant reaction pathways predicted by the microkinetic model were used to develop an analytical macrokinetic model. The simplified analytical macrokinetic expression was easy to use and proved to accurately predict the microkinetic model and experimental data for different steam methane reforming operating conditions. Additionally, it unified the rate equations proposed in previous studies, and thus it provided clarification on the contradicting experimental kinetic data on the reaction orders of hydrogen, methane, water, and steam methane reforming activation energy. Unlike methane, the reaction orders of water and hydrogen are more dependent on the operating conditions.

According to the authors, the obtained reaction mechanisms are very useful in the understanding of the steam reforming kinetics of varying types of hydrocarbon molecules. In particular, the macrokinetic model can be used in a wide range of chemical engineering applications for optimizing steam methane reforming operating conditions.

In conclusion, study successfully clarified a rather incompatible experimental data on the reaction orders of methane, hydrogen, and water as well as the activation energy of the steam methane reforming. Through studies on the elementary reaction pathways, microkinetic and macrokinetic models that showed good consistency with the experimental data were constructed. In a statement to Advances in Engineering, Professor Zijing Lin noted that the study insights will be of great importance in the design and optimization of steam methane reforming operating conditions which is helpful in developing different chemical engineering applications.

Reference

Ke, C., & Lin, Z. (2020). Elementary reaction pathway study and a deduced macrokinetic model for the unified understanding of Ni-catalyzed steam methane reforming. Reaction Chemistry & Engineering, 5(5), 873-885.

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