Microkinetic Modeling of CO2 Methanation on Nickel Surfaces

This PhD thesis advances the fundamental understanding of the structure-sensitive reaction mechanism of the Sabatier reaction, also known as the CO2 methanation reaction. By studying four nickel surface facets—Ni(111), Ni(100), Ni(110) and Ni(211)—this work explores both terrace and stepped surfaces of nickel metal nanoparticles. These investigations provide key insights into catalytic behavior and design. Chapter 1 introduces the societal and scientific urgency to reduce CO2 emissions and integrating renewable energy. The Sabatier reaction, which converts green hydrogen into storable and transportable methane, plays a crucial role in energy storage and infrastructure compatibility. This chapter also introduce the principles of heterogeneous catalysis and the use of first-principles calculations in catalyst design. Chapter 2 reviews the computational methodologies used in this research. Density Functional Theory (DFT) is used to calculate kinetic parameters for elementary reaction steps. Micro-Kinetics Modeling (MKM) simulates surface reactions and identifies rate-controlling steps. The synthesis of Wulff-constructed nanoparticles and the identification of their surface atoms is explored. These three methods are integrated to deepen the atomic-level understanding of catalytic mechanisms. Chapter 3 serves as an educational tool, introducing adsorption processes in heterogeneous catalysis to undergraduate students. Using the Sabatier reaction as a case study, it bridges Newtonian and Quantum mechanics to explain concepts like molecular collisions, energy conversion, sticking coefficients and adsorption behavior. Chapter 4 investigates the structure-sensitivity of the Sabatier reaction on nickel metal nanoparticles using DFT and MKM simulations. Ni(110) is identified as the most active surface, capable of supporting multiple reaction pathways for CO* activation, unlike other facets. The terrace facets Ni(111) and Ni(100) show minimal activity, with Ni(100) suffering from carbon poisoning, while Ni(211) shows moderate activity. These findings correlate with experimental observations, showing optimal activity for nickel nanoparticles around 2-3 nm. A Wulff-constructed nanoparticle analysis reveals the critical role of undercoordinated nickel atoms and identifies an ideal nanoparticle size of ~1.7 nm for CO2 methanation, with reduced activity for larger nanoparticles. Chapter 5 emphasizes the importance of sensitivity analysis and validation against experimental data in refining micro-kinetic models for CO2 methanation. Initial models based solely on DFT calculations showed limitations such as unrealistic 100 % surface coverage and excessively high apparent activation energies. Incorporating enthalpic corrections and lateral interaction penalties addressed these issues, aligning the refined models with experimental data. These models provide a robust framework for studying CO2 methanation on nickel surfaces and inform the design of more efficient catalysts. Chapter 6 explores the vibrational behavior of reaction intermediates during CO2 methanation, correlating calculated and experimental vibrational spectra on nickel facets. Various DFT methods and scaling procedures are evaluated to improve the accuracy of computed vibrational frequencies. The analysis highlights the effects of adsorption sites, back-donation and dipole coupling on CO* stretch frequencies and provides a detailed frequency dataset for intermediates on nickel facets, relevant for Ni-based catalysis in catalytic C1-reactions. These tabulated frequencies serve as a valuable resource for corroborating spectral features observed using infrared spectroscopy during catalytic processes such as CO2 methanation.