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

Selective oxidation of H 2 S to sulfur is an efficient process for industrial applications and environmental requirement to reduce the residual H 2 S 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 H 2 S and selectivity of S due to the inevitable overoxidation of H 2 S or S into SO 2 on such highly active catalysts. In this contribution, we achieve high selectivity of sulfur for the selective oxidation of H 2 S 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 (λ cat ) of 503 g sulfur ·kg cat –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, H 2 O (50 vol %), and impurity gas (i.e., 50 vol % CO 2 ), 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 −PO x . 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 O 2 enabled by pyridinic nitrogen sites and the P species interaction, which is evidenced by advanced characterization, kinetic analysis and density functional theory (DFT) simulation.