A First-Principles Microkinetics for Homogeneous–Heterogeneous Reactions: Application to Oxidative Coupling of Methane Catalyzed by Magnesium Oxide

Abstract

The oxidative coupling of methane (OCM), a major catalytic reaction in natural gas utilization, was examined by first-principles density functional theory (DFT) calculations combined with microkinetic and reactor simulations. We investigated magnesium oxide (MgO), which is a standard catalyst for the OCM reaction. We used the stepped MgO as the catalyst model, as this surface is a strong candidate for the active site for the CH3 generation from CH4. Previous experimental and kinetic simulations have shown that CH3 generated by the surface reactions couples into C2H6 in the gas phase, and thus the OCM is the homogeneous–heterogeneous process. Based on this, we conducted a theoretical study using DFT-based microkinetics by including both gas-phase and surface reactions. The calculations provide theoretical CH4 conversion and C2 selectivity without using any kinetic experimental parameters, which are in reasonable agreement with the experiments. The dependence of these quantities on the reaction condition such as the temperature or inlet gas component is also analyzed, and it has been shown that the CH4 conversion increases at higher temperature; on the other hand, the C2 selectivity decreases. The opposite trend was observed with respect to the partial pressure ratio of CH4 and O2, as the C2 selectivity increases when the partial pressure of CH4 is high. These tendencies are in agreement with the experiments. Consequently, our calculation supports the experimental suggestion that the stepped MgO is the active site for the OCM. We also carried out the analysis on the reaction network, indicating that a large portion of deep oxidation occurs via dehydrogenation of C2 compounds.

Synthesis of Heterogeneous Catalysts in Catalyst Informatics to Bridge Experiment and High-Throughput Calculation

“…It has been demonstrated that microkinetic analysis combined with first-principles calculations is able to represent surface reactions in heterogeneous catalysis. − In general, a more detailed analysis of surface reactions is investigated using first-principles calculations. However, if a large amount of catalyst data is generated via first-principles calculations, it could be possible to uncover key knowledge and patterns for designing catalysts from a bird’s-eye point of view using catalyst informatics.…”

Heterogeneous catalyst design by generative adversarial network and first-principles based microkinetics

“…Therefore, it is often more accurate than the kinetic analysis based on the global rate expression 11 . Currently, DFT-based microkinetics is widely used in catalysis research because it is a powerful tool that allows the calculation of kinetic information, such as the reaction energy and activation barrier, can be calculated by DFT 11 – 15 . Considering this, a combination of DFT calculations, microkinetics, and catalytic material generation from generative models could be a promising approach for the rational design of catalysts.…”

Noncatalytic Oxidative Coupling of Methane (OCM): Gas-Phase Reactions in a Jet Stirred Reactor (JSR)

“…In fact, since surface species are difficult to observe or identify, OCM surface kinetics are indirectly investigated experimentally by extrapolating conversion rates and selectivity at zero methane conversion for initiation steps 8 , 12 , 37 − 39 or by applying isotopic techniques to identify the pathways of products. 8 , 17 , 40 − 42 Other than these, the parameters of surface elementary reactions (sticking coefficient and activation energy) are estimated mostly via density functional theory (DFT) calculations 43 − 54 or Polanyi relationships. 20 , 21 , 23 , 25 , 26 It is challenging to precisely predict the entire surface reaction mechanism for OCM, which indicates the critical role of an accurate and reliable gas-phase model over the entire mechanism.…”

“…Ishioka et al also used a machine learning technique to better understand the gas-phase performances against operating conditions from the high-throughput experimental data. In fact, since surface species are difficult to observe or identify, OCM surface kinetics are indirectly investigated experimentally by extrapolating conversion rates and selectivity at zero methane conversion for initiation steps ,,− or by applying isotopic techniques to identify the pathways of products. ,,− Other than these, the parameters of surface elementary reactions (sticking coefficient and activation energy) are estimated mostly via density functional theory (DFT) calculations − or Polanyi relationships. ,,,, It is challenging to precisely predict the entire surface reaction mechanism for OCM, which indicates the critical role of an accurate and reliable gas-phase model over the entire mechanism. To fulfill this requirement, gas-phase reaction models should accurately describe well-known homogeneous processes (oxidation, pyrolysis, etc.…”