Metal oxide nanoparticles embedded in porous carbon for sulfur absorption under hydrothermal conditions.

MO (M = Zn, Cu, Mn, Fe, Ce) nanoparticles (NPs) embedded in porous C with uniform diameter and dispersion were synthesized, with potential application as S-absorbents to protect catalysts from S-poisoning in catalytic hydrothermal gasification (cHTG) of biomass. S-absorption performance of MO/C was evaluated by reacting the materials with diethyl disulfide at HTG conditions (450 °C, 30 MPa, 15 min). Their S-absorption capacity followed the order CuO/C > CeO/C ≈ ZnO/C > MnO/C > FeO/C. S was absorbed in the first four through the formation of CuS, CeS, ZnS, and MnS, respectively, with a capacity of 0.17, 0.12, 0.11, and 0.09 mol mol. The structure of MO/C (M = Zn, Cu, Mn) evolved significantly during S-absorption reaction, with the formation of larger agglomerates and separation of MO particles from porous C. The formation of ZnS NPs and their aggregation in place of hexagonal ZnO crystals indicate a dissolution/precipitation mechanism. Note that aggregated ZnS NPs barely sinter under these conditions. Cu(0) showed a preferential sulfidation over CuO, the sulfidation of the latter seemingly following the same mechanism as for ZnO. In contrast, FeO/C and CeO/C showed remarkable structural stability with their NPs well-dispersed within the C matrix after reaction. MO dissolution in water (from liquid to supercritical state) was modeled and a correlation between solubility and particle growth was found, comforting the hypothesis of the importance of an Ostwald ripening mechanism. CeO/C with high structural stability and promising S-absorption capacity was suggested as a promising bulk absorbent for sulfides in cHTG of biomass.