Microscale mechanism study of pressure effects on interfacial heterogeneous catalysis of silica-based thermal protective materials

Reactive molecular dynamics simulation provides an important approach to elucidate the microscale heating mechanisms involved in high-temperature wall effects and to deepen our understanding of non-equilibrium aerothermodynamics of high-speed aircraft. However, microscale simulations that enhance the computational efficiency by artificially increasing the gaseous pressure often lead to discrepancies in reaction pathways and rate coefficients, thereby affecting the aerothermodynamics and causing the misunderstanding of reaction mechanisms. Using a molecular dynamics approach based on the ReaxFF force field, this investigation addresses the catalytic recombination reaction system of dissociated oxygen atoms on the surface of silica-based thermal protective materials. The primary objectives involve the computation and analysis of elementary reaction rates, surface coverages, and recombination coefficients under various gaseous pressure conditions. The purpose is to establish a quantitative correlation between the rate constants of elementary reactions and pressure, thereby elucidating the upper bounds of computational efficiency enhancement through pressure augmentation. The results indicate that pressure augmentation leads to a transition in the dominant reaction pathway from adsorbate-adsorbate interactions to gas-adsorbate interactions. Furthermore, it causes a deviation in the relationship between the rate constants of elementary reactions and pressure from the patterns observed under experimental/flight conditions. At 1200 K, within the pressure range associated with single-atom collisions, a consistent decrease in the rate constants of individual elementary reaction steps is observed as the pressure decreases. Among them, the rate constants of ER1～ER3 recombination exhibit a linear relationship with pressure. Specifically, the rate constants can be expressed as a power law function of pressure, with exponents of 1.10179, 1.01686 and 0.91654 respectively. The rate constants of LH1～LH3 recombination reactions display a logarithmic dependence on pressure, with significantly smaller pre-logarithmic factors compared to those observed in the non-single collision region. The rate constant of thermal desorption is exponentially related to pressure. Based on the microscale mechanism of catalytic reaction influenced by gaseous pressure, and with the stable relationship between elementary reaction rate constants and pressure, a constraint upper limit for artificially increasing pressure is proposed. The Knudsen number with the system height as the characteristic length should be greater than the magnitude of 10^2 to ensure the gas-solid single collision. This investigation provides support for molecular simulation methods of gas-solid interface reactions and the accumulation of microscale catalytic data for thermal protective materials.