Mechanisms of Ethylene Epoxidation over Silver from Machine Learning-Accelerated First-Principles Modeling and Microkinetic Simulations

Abstract

We employed machine learning-augmented density functional theory (DFT) thermodynamic calculations to assess the stability of different AgO x structures under catalytic ethylene epoxidation reaction conditions. We found that there are multiple AgO x surface motifs that could co-exist under the relevant conditions. These included Ag surface oxides (e.g., AgO_p(4 × 4) and Ag 1.83 O) and atomic oxygen-covered Ag(111) surfaces. Furthermore, we employed DFT calculations to evaluate the energetics of different reaction mechanisms by which ethylene and oxygen can react on these surfaces. These studies revealed several energetically viable reaction pathways for ethylene epoxidation. Microkinetic modeling analyses, based on the DFT-calculated reaction pathways, showed that ethylene epoxidation can proceed on all surfaces and that multiple pathways, including those involving Langmuir−Hinshelwood and Eley−Rideal mechanisms, could be involved in selective and unselective reactions. The diversity of mechanisms that we discovered in the context of the relatively simple ethylene epoxidation reaction on Ag suggests that the richness and complexity of surface chemistry are most likely a rule rather than an exception in heterogeneous catalytic chemical transformations on metal surfaces and that the concept of a single or even a dominant mechanism and reaction intermediates might need to be revisited for many reactions.

Non-Steady State Validation of Kinetic Models for Ethylene Epoxidation over Silver Catalysts

“…This work reinforces that even though the "ground truth" of ethylene epoxidation is very complicated, simpler, single site models can still recreate the majority of the trends in catalytic activity. Given the broad predictability of the HH model when lateral interactions are included, 27 and that the epoxidation of ethylene can occur on multiple different sites simultaneously, 9 it is most likely that all of these models contain some core truth within them, and that further transient experiments of ethylene epoxidation are required to identify the true reaction mechanism.…”

“…This has led to an open debate not only on the reaction mechanism, but also on the identity of the active site for this reaction. 2, [6][7][8][9] For ethylene epoxidation over silver there are three popular microkinetic models: The Linic-Barteau (LB) model was developed using Density Functional Theory (DFT) and surface science experiments to generate a simple 4-step, single site microkinetic model that was able to recreate the apparent activation energy of EO production. 3 The Huš-Hellman (HH) model used DFT to generate an 11-step, single site microkinetic model that was validated by comparing kinetic Monte Carlo simulations to experimentally measured turnover frequencies and selectivities.…”

In situ studies of reversible solid–gas reactions of ethylene responsive silver pyrazolates

“…Silver–ethylene complexes are of particular interest since silver is the metal of choice for partial oxidation of ethylene, which is a major industrial process. 16,17 They are challenging to stabilize and quite labile due to the relatively weak silver( i )–ethylene interactions. 18–24 Reversible binding of ethylene to silver, however, is valuable in applications such as the separation of ethylene from ethylene–ethane mixtures using silver complexes and silver-doped materials.…”

Revealing the Nature of Active Oxygen Species and Reaction Mechanism of Ethylene Epoxidation by Supported Ag/α-Al2O3 Catalysts

“…Many attempts have been made to determine the ethylene oxidation reaction mechanism on Ag/Al 2 O 3 by fitting rate expressions to steady-state experimental kinetics data; however, this has led to contradicting conclusions. For example, both one site Eley–Rideal − (E–R, recently also referred as Langmuir–Rideal (L–R)) and dual site Langmuir–Hinshelwood (L–H) reaction mechanisms have been postulated. − It has also been proposed that selective ethylene epoxidation proceeds through the E–R mechanism whereas the total combustion follows the L–H mechanism . Furthermore, the formation of selective and nonselective ethylene oxidation products under anaerobic conditions from the previous TPR studies with pre-oxidized Ag surfaces clearly suggests a possible limited contribution of the Mars–van–Krevelen (MvK) reaction mechanism that has not been systematically investigated …”

“…Many attempts have been made to determine the ethylene oxidation reaction mechanism on Ag/Al 2 O 3 by fitting rate expressions to steady-state experimental kinetics data; however, this has led to contradicting conclusions. For example, both one site Eley–Rideal 26 − 28 (E–R, recently also referred as Langmuir–Rideal (L–R)) 29 and dual site Langmuir–Hinshelwood (L–H) reaction mechanisms have been postulated. 30 − 32 It has also been proposed that selective ethylene epoxidation proceeds through the E–R mechanism whereas the total combustion follows the L–H mechanism.…”