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

MO_x (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_x/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_x/C > CeO_x/C ≈ ZnO/C > MnO_x/C > FeO_x/C. S was absorbed in the first four through the formation of Cu_1.8S, Ce₂S₃, ZnS, and MnS, respectively, with a capacity of 0.17, 0.12, 0.11, and 0.09 mol_S mol_M^-1. The structure of MO_x/C (M = Zn, Cu, Mn) evolved significantly during S-absorption reaction, with the formation of larger agglomerates and separation of MO_x 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 Cu₂O, the sulfidation of the latter seemingly following the same mechanism as for ZnO. In contrast, FeO_x/C and CeO_x/C showed remarkable structural stability with their NPs well-dispersed within the C matrix after reaction. MO_x 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_x/C with high structural stability and promising S-absorption capacity was suggested as a promising bulk absorbent for sulfides in cHTG of biomass.