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 1.8 S, Ce 2 S 3 , 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 Cu 2 O, 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.