Electron delocation mechanism – a new reaction mechanism of heterogeneous catalysis

Since Taylor proposed the concept of heterogeneous catalysis “active bit” in 1925, most reaction mechanisms have been localized reaction mechanisms based on active sites. Recently, the team of Professor Tang Fuxing of Fudan University, the team of Professor Li Jun of Southern University of Science and Technology, and Dr. Tang Daiming of the National Institute of Materials Science of Japan have cooperated to propose a new mechanism of heterogeneous catalytic reaction for the first time – “electron delocalization mechanism”.

On August 3, 2022, the study was published in the journal Chem under the title “Interplay between remote single-atom active sites triggers speedy catalytic oxidation”. By accurately designing the model catalyst, this study proposes a widespread electron delocation mechanism that exists in heterogeneous catalytic reactions, as shown in Figure 1. The first authors of the paper are Huang Zhiwei, Liang Jinxia, Tang Daiming and Dr. Chen Yaxin, and the corresponding authors are Professor Tang Fuxing and Professor Li Jun.

Figure 1: Schematic diagram of the concept of “electron delocalization mechanism”.

Essentially, a chemical reaction is the recombination of chemical bonds, a process that is often accompanied by electron transfer, and catalysts play an important role in the electron transfer process. Most reaction mechanisms are based on the Local Reaction Mechanism (LRM) that occurs on a single active site, as shown in Figure 2. In fact, the active site is not isolated and stationary, and there will be an interaction between the active sites that are conducive to the reaction, such as the migration of the atom in the center of the active site, and the migration of the reactive species (or intermediates) between the active sites. Because the mass of electrons is very small, the energy required for electron migration is generally much lower than the migration of atoms or reactive species in the center of the active site, so electron migration is both easy to ignore and more common in catalytic reactions. This catalytic mechanism for promoting chemical reactions based on the migration of electrons between active sites is defined as the Electron Delocalized Mechanism (EDM), as shown in Figure 2. However, because it is almost impossible to extract the contribution of two active sites to total activity from a single catalytic system, it is extremely challenging to prove that EDM is extremely challenging.

Figure 2: Schematic of the EDM and LRM models on individual metal particles.

To explore the interaction between active sites, the authors designed two single-atom active site catalysts to explore the EDM and LRM reaction mechanisms. The author synthesized a silver chain in the pore channel using MnO2 with a one-dimensional aperture as the carrier. When the silver chain fills the pores, the active bit pairs of exposed silver monoatoms at both ends (named Ag1+1) can be obtainedMnO2)。 When the silver load is controlled so that the single-atom silver chain does not fill the full pores, one end of the exposed silver single-atom active site (named Ag1) can be obtainedMnO2)。 By STEM, EXAFS characterized the structure of the active bit pairs of exposed silver single atoms at both ends, with the single atomic silver chain in a straight line state and the average coordination number of silver being 2 (Figure 3).

Figure 3: Ag1+1Structure of mnO2 single-atom active bit pairs.

The authors measured Ag1+1 assembled with a single atomic activity pairThe MnO2 bar verifies Ag1+1 by current-to-voltage (I-V) curves in the low bias rangeThe conductivity of the silver chain in the MnO2 rod. As shown in Figure 4, Ag1+1MnO2 exhibits metallic conductive behavior along the silver chain. The pure MnO2 rod has insulating characteristics and makes Ag1+1MnO2 is not electrically conductive in the direction perpendicular to the silver chain. To elucidate the conduction channels of electron transfer, the authors performed density functional theory (DFT) calculations on the state density of silver wires, and the results showed that electrons are conducted along the silver chain through the s and d bands around the Fermi energy level.

Figure 4: Ag1+1Electron conduction between the active sites of two end silver atoms in a metal silver chain in MnO2.

Due to 1+1 per AgMnO2 rods all have nearly identical silver chain arrays, and the single-atom active sites exposed at each end of the silver chain are connected by electrically conductive metal wires, so the macroscopic average reaction rate and kinetic properties reflect the ability of each pair of active sites at the atomic scale. Therefore, the author chose Ag1+1MnO2 to test the EDM model. Ag1MnO2 is prepared in the same way, but the silver loading is lower, and its active site has a role similar to Ag1+1MnO2 has the same geometric and electronic structure, but the EDX element distribution confirms, Ag1Only one end of the silver chain on MnO2 is exposed to the catalyst surface due to the lack of a complete metal silver chain that connects the two active sites, Ag1MnO2 acts as a model catalyst to test the LRM model. The authors evaluated the activity of two single-atom active-site catalysts by using low-temperature CO oxidation. As shown in Figure 5, within the range of reaction kinetics, Ag1+1MnO2 shows inherent catalytic activity in the temperature range of -80 ~ -40 °C, while Ag1MnO2 and MnO2 do not have catalytic activity under the same conditions, which means that electrons play a key role in the transfer between active sites (Figure 5B). The authors measured Ag1 in the relatively high temperature rangeCarbon monoxide conversion on MnO2 and then Ag1+1 is calculated within the range of reaction kineticsMnO2 and Ag1Apparent activation energy (Ea) of MnO2. In Figure 5C, Ag1+1The Ea value of MnO2 is ~23 kJ mol-1, which is significantly lower than Ag1Ea value of MnO2 (~60 kJ mol-1). Therefore, the huge Ea difference between the two catalysts indicates that the reaction mechanism of the two catalysts is different. To investigate the difference between EDM and conventional LRM, the authors prepared a single-atom catalyst (SAC), Ag1/MnO2, with silver single atoms loaded on the mnO2(100) side. Ag1/MnO2 is a typical catalyst for studying LRM models. Ea values for Ag1/MnO2 (~62 kJ mol-1) and Ag1The Ea value of MnO2 (~60 kJ mol-1) anastomoses well, indicating that it is in Ag1CO oxidation on MnO2 follows LRM (Figure 2B).

By comparing AgNP/MnO2 and Ag1+1Structural characteristics and reaction kinetics of MnO2, the authors found that the most active interface atoms of silver particles on AgNP/MnO2 have a catalytic behavior similar to Ag1+1MnO2。 The Ea value of AgNP/MnO2 is approximately 23 kJ mol-1 (Figure 5C), with Ag1+1MnO2 is exactly equal. AgNP/MnO2 and Ag1+1MnO2 has similarities in Ag-O structure and Ag-Ag electron transport, and the authors found interactions at the active sites on individual silver metal particles of AgNP/MnO2 in CO oxidation. Thus, similar electron conduction characteristics and equal Ea values prove AgNP/MnO2 and Ag1+1MnO2 follows EDM in both CO oxidation processes.

Figure 5: Differences in catalytic performance and reaction activation energy of EDM and LRM model catalysts.

The electron delocation mechanism shows that under certain conditions, compared with the traditional localization mechanism, the catalytic reaction is preferentially carried out in accordance with the electron delocation mechanism, that is, the migration of electrons between the active sites can effectively reduce the reaction activation energy and accelerate the reaction rate. The concept of electron delocation mechanism may be conducive to reasonable explanation of some common important catalytic phenomena, such as metal nano effect can be explained as: during the process of metal particles becoming smaller, the activity begins to rise due to the increase in the number of active digits, when the metal particles undergo a transition from metallicity to non-metallicity, the reaction mechanism reason electron delocation mechanism is converted to a localization mechanism, resulting in a decrease in activity; For another example, why the activity of metal catalysts is superior to their oxide catalysts can be reasonably explained from the fact that the conductivity of metals is better than their oxides. At the same time, the concept of electron delocation mechanism also enriches the basic theory of catalytic reaction mechanism, and provides a certain scientific basis and theoretical support for the design and development of efficient catalysts.

The work was supported by the National Key Research Program (2021YFB3500601), the National Natural Science Foundation of China (21777030, 91645203, 22076051, 21590792 and 21976037), the Kakenhi Foundation of the Japan Association for the Promotion of Science (JP20K05281 and JP25820336), the Guangdong Provincial Key Laboratory of Catalysis Fund (2020B121201002). Supported by the Australian Research Council Laureate Scholarship (FL160100089). (Source: Science Network)

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