Chemical Physics Impact (Dec 2022)
Importance of the dipole of interstitial H induced by Pauli - repulsion for the catalysis
Abstract
Recently we mentioned that the surface of solids often exerts Pauli repulsion on the interstitial/substitutional hydrogen, which markedly changes the properties of H, and increases its chemical activity, which is referred to as ‘Pauli activation of interstitial H’ for short. The purpose of this paper is to further confirm the reality of this activation mechanism at a quantitative level. We particularly mention the importance of the H-dipole of the interstitial hydrogen induced by Pauli activation for this catalysis, which is the key to understand the principle of this catalysis mechanism. Taking a simple diatomic molecule as an example, we first illustrate what are the prerequisites for forming a molecular covalent bond, when the two atoms are in close contact. Then we argue that, the Pauli-activated H at solid surface just meets these prerequisites due to the dual modulation effect of the induced H-dipole on both the energy and the spatial orientation of a foreign atom/molecule in the bonding process with H, thus accelerates the hydrogenation. Moreover, we emphasize that the size of the depression/hole at solid surface, where the interstitial H sited(simply referred to as ‘Pauli hole’), must match the radius of H to ensure the effectiveness of Pauli activation of H. Accordingly it provides a criteria for selecting a suitable catalysts for the specific chemical reaction, which would be useful in practical applications. Taking a common chemical reaction as an example, we make a calculation and quantitative analysis of the synthesis of MgH2, using Ni as the heterogeneous catalyst. Details of how to select a suitable catalyst are given in this reaction. And why and how the dual modulation effect of the induced H-dipole of interstitial H promotes the realization of the prerequisites, thus facilitates the formation of the covalent bond Mg-H. Therefore the rationality of the Pauli activation-catalysis is confirmed theoretically.
