Adsorption research, a piece of "Science"!

Adsorption is an important initial step in all heterogeneous chemical processes.
The adsorption of molecules on the metal surface will trigger many chemical reactions
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However, the exact way it occurs is difficult to study
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Adsorption involves molecules colliding on the solid surface, and through a dynamic way to reach equilibrium and lose their incident energy
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The interactions that cause energy loss usually include chemical bond formation (chemical adsorption) and non-bonded interactions (physical adsorption)
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Although adsorption is the core of surface chemical kinetics, it is quite difficult to predict and detect adsorption pathways
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The electronic structure theory not only challenges the precise and simultaneous description of covalent and non-covalent interactions, but so far, there has not been any experimental report on the direct tracking of microscopic adsorption pathways through chemical adsorption and physical adsorption pores
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? [Problems] Attenuating the vibration of the adsorbate on the metal surface provides an effective solution for the experimental and theoretical study of surface dynamics
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For the CO/Au(111) adsorption system, density functional theory (DFT) calculations show that when CO is chemically adsorbed, CO binds in a fixed (OC-Au) direction and at a specific location
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But when CO is physically adsorbed, CO behaves as a free rotor and is weakly bound at every surface part
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According to common sense, when molecules collide in random directions at different surface positions, physical adsorption is advantageous
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But this situation ignores that the equilibrium rate depends to a large extent on the nature of the interaction
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For example, the vibrational relaxation time of chemically adsorbed CO to Cu is about 2 ps, while that of CO physically adsorbed to Au is 49 ps
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Therefore, if the process and transition of physical and chemical adsorption can be tracked quantitatively, and the effect of equilibrium rate on adsorption can be considered, it can provide a new theoretical basis for the reaction kinetics of the metal surface
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? In order to solve this problem, Borodin et al.
from the University of Göttingen in Germany used vibrationally excited carbon monoxide molecules to be captured on the Au (111) surface, and the CO molecules adsorbed on the Au surface only have the characteristics of vibration freedom, reducing Other factors interfere with the experiment
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Based on the molecular beam experiment and the theoretical model of the prototype system, the author revealed the complex interaction between the physical adsorption and chemical adsorption states
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As the experimental temperature increased from 0K to 1250K, the contribution of chemical adsorption to the thermal adhesion coefficient decreased from 50% to 20% and then increased to 40%; while the trend of physical adsorption contribution was the opposite
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By controlling the residence time of the molecules on the surface, the author can make the temperature-independent vibration relaxation life can be used as an internal clock to detect different adsorption types
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It reveals the quantitative energy profile and the microscopic pathways for molecules to balance with the surface in the prototype system
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The results show that the vibration relaxation time can be used as an internal clock to track the microscopic path of adsorption and equilibrium on the surface
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The adsorption energy of chemical adsorption is 38 meV higher than that of physical adsorption, but the energy barrier for conversion from chemical adsorption to physical adsorption is only 13 meV
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This makes the main body in the adsorption transition from chemical adsorption to physical adsorption
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The detailed equilibrium principle can use the information obtained in the molecular beam experiment to explore the way of thermal adsorption
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? The research is published in the latest issue of "Science" with a paper entitled "Following the microscopic pathway to adsorption through chemisorption and physisorption wells"
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??? Although the physical adsorption state has the smallest free energy, when an excited molecule collides with the surface, it first falls into a metastable chemical adsorption state, in which it quickly loses its translational energy
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Analysis shows that at all temperatures, thermal adsorption involves chemical adsorption and physical adsorption
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The first step in achieving equilibrium involves capturing CO (v = 2) into a chemical adsorption well
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This state can then undergo vibration relaxation, thermal desorption or thermal conversion to a physical adsorption state
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For the desorption process, vibration relaxation is dominant at low temperatures because it does not require thermal activation
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At high temperatures, it quickly transforms into a physically adsorbed substance, followed by desorption or vibrational relaxation
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At moderate temperatures, the physical adsorption state can even be transferred back to the chemical adsorption well
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? Channel flux for adsorption and desorption? At low temperatures, the two states of adsorption are equally important
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At moderate temperatures, the higher entropy physical adsorption state is relatively important
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However, at the highest surface temperature, the translation and rotation entropy of chemically adsorbed CO is close to that of physically adsorbed CO, and the importance of chemically adsorbed CO increases again
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This increased entropy is due to more sampling of higher energy chemisorption states at different binding sites on the surface
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? Thermal adsorption to chemical adsorption and physical adsorption state
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? [Future prospects] Industrial catalytic oxidation is an important type of catalytic reaction, which is initiated by triggering the conversion of oxygen molecules into excited oxygen molecules on a variety of metals.
The excited oxygen molecules can be physically adsorbed or chemically adsorbed
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Therefore, the catalytic activation of oxygen is a complex thermal rate process network, including the adsorption, desorption and mutual conversion of physical and chemical adsorption molecular states
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Based on the experimental results, a model that can accurately describe this kinetic adsorption network is constructed
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And test the accuracy of the theoretical description of the complex equilibrium rate process
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Through the results given here, the rate constant of the thermal adsorption network was successfully determined, and the basic energy and entropy characteristics of adsorption and desorption were revealed
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This result strengthens the basis for the development of surface chemistry and heterogeneous catalysis prediction theory
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It provides ideas for further extensive research on the adsorption of different gases and metal molecules
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Future chemical engineers can rely on the obtained kinetic parameters to better design different catalytic reactions
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