Revealing the effects of pore hierarchy and surface hydrophilic oxygen group on nano-confined mass transfer in heterogeneous catalytic desulfurization by porous carbon: A multiscale modelling study

Efficient mass transfer of reactant or product is an essential process for heterogeneous catalysis cycle in porous catalyst, which ensures timely recovery of reaction vessel and long-term stability. Taking SO2 catalytic removal by porous carbon as an example, the removal efficiency and capacity is limited by the mass transfer behavior of product H2SO4 in nanometer-sized porous channel. Herein, based on multi-scale modelling combining density functional theory (DFT), grand canonical Monte Carlo (GCMC) and molecular dynamic (MD) simulation, we found that coupling of surface hydrophilic oxygen group with hierarchical pore can efficiently enhance the mass transfer behavior of product H2SO4, thereby improving overall catalytic process. For the hierarchical pore configuration with 0.7 nm confined space matched to nanometer-sized slit pore, the hydrophilic oxygen group in 0.7 nm reaction vessel is essential for H2O local enrichment to induce H2SO4 mass transfer from surface active site to confined space; the coupling between hydrophilic oxygen group and slit pores with sizes in 1.0-1.5 nm range can provide mobile H2O to further carry out product H2SO4 away from 0.7 nm reaction vessel, increasing H2SO4 self-diffusion coefficient by nearly one order of magnitude compared to that in sub-nanometer confined space. By comparing H2SO4 mass transfer in different types of hierarchical pore, hierarchical structure with 0.7 nm reaction vessel matched to 1.0 nm-sized slit pore that contains 3.9 at% hydrophilic oxygen group is found to own the strongest mass transfer ability for H2SO4, and the calculated H2SO4 self-diffusion coefficient is as high as 3.9 x 10-6 cm2 s- 1 . Above observed the enhancement of H2SO4 mass transfer was further validated by desulfurization experiment using model porous carbon. This work for the first time reveals coupling effects of surface oxygen group and pore configuration on product mass transfer during SO2 catalytic removal by porous carbon, and obtained conclusions about confined mass transfer can also provide guidelines for heterogeneous catalysis in other porous catalysts.