Heteroatom-Doped Monolithic Carbocatalysts with Improved Sulfur Selectivity and Impurity Tolerance for H₂S Selective Oxidation

Abstract

Selective oxidation of H2S to sulfur is an efficient process for industrial applications and environmental requirement to reduce the residual H2S in the Claus technology to an ultralow content (<0.1 ppm) before releasing the off-gas into the atmosphere. Recently, numerous research studies have been devoted to the development of highly active, selective and stable catalysts for such fields of application. However, there is usually a trade-off between conversion of H2S and selectivity of S due to the inevitable overoxidation of H2S or S into SO2 on such highly active catalysts. In this contribution, we achieve high selectivity of sulfur for the selective oxidation of H2S without losing conversion by phosphate-modified N-doped three-dimensional (3D) mesoporous carbon/carbon nanotube (N-C/CNT) monolith carbocatalysts. The as-synthesized P-modified N-C/CNT monolith (N-C/CNT-6%P) presents a high sulfur selectivity of 91.3% with an excellent normalized sulfur formation rate (λcat) of 503 gsulfur·kgcat–1·h–1, which is comparable with the most active carbon-based and metal-oxide catalysts ever reported. Notably, the P-modified N-C/CNT monolith exhibits extremely high stability even under severe reaction environments with a high partial pressure of oxygen, H2O (50 vol %), and impurity gas (i.e., 50 vol % CO2), indicating the promising potential for the practical application. An in-depth investigation including structure evolution, performance variation and promotion mechanism is conducted from the perspective of interaction between the carbon matrix and −POx. The results indicate that the surface properties of P-modified N-C/CNT are regulated by the interaction between the phenol, pyrrolic, pyridinic N groups and P species after the phosphate modification. The improved selectivity with nearly unchanged conversion could be attributed to the moderate adsorption and activation of O2 enabled by pyridinic nitrogen sites and the P species interaction, which is evidenced by advanced characterization, kinetic analysis and density functional theory (DFT) simulation.