Amorphousness matters: Its role on nonpolar gas diffusion at the nanoscale

Abstract

Amorphous carbon materials (ACM) produced using biomass waste feedstocks, offers an affordable, large-scale, and environmentally friendly production method. Their inherent porosity critically impacts gas diffusion and various applications, including gas separation, purification, and catalysis. This work determines, through molecular dynamics simulations, the effect of amorphicity on the diffusion of pure oxygen and methane, their 50% mixture and a 50% mixture of methane and carbon dioxide. For pure systems, we compare ACM with pores of similar size and regular shapes, such as slit pores and carbon nanotubes, with and without surface roughness. From the analysis of diffusion trajectories we find that molecular mobility decreases as the surface structural constrictions increase, configuring a more intricate molecular path for the gas to surface-diffuse. Specifically, constrictions imposed by the carbon surface structure increase in the next order: smooth surfaces, rough CNTs and ACMs. For ACM with a pore diameter of 1.5 nm, gas diffusion decreased by up to 95%. Notably, oxygen was trapped in surface defects (“pockets”) for both pure oxygen and its mixture with methane. The effects of pressure and temperature on the dynamic behavior of mixtures were also explored. For the 50% mixture of carbon dioxide and methane, we observed that in ACM with the smallest pore size, the relative diffusion is reduced by around 50% compared to bulk. Our findings suggest that amorphous materials could be preferred for devices that require high selectivity between gas mixtures due to their specific porous structure.