Stability of zinc oxide samples prepared with sol- gel method in supercritical water

Abstract

Sol–gel preparation of heterogeneous solid catalysts has been reported by many research groups since the 1980s due to unique advantages such as the possibility of obtaining metastable materials, achieving superior purity and compositional homogeneity of the products at moderate temperatures with simple laboratory equipment. Therefore, ZnO nanocrystals with various shapes were synthesized via sol–gel reactions of zinc acetate dihydrate, ethylene glycol, n-propyl alcohol, and glycerol and calcined at 350 oC(ZnO-350) and 900 oC(ZnO-900). Stability of ZnO in the presence of supercritical water (SCW) was determined by various methods such as FTIR, XRD, TEM, SEM, BET, TPO, H2-TPR and zeta potential. The XRD results showed that ZnO samples calcined at 350 oC and 900 oC have a hexagonal crystal structure. After used in SCW, their XRD phase compositions were unchanged and the hexagonal phase composition was maintained. However, SEM images showed that the particle size of the ZnO samples exposed to SCW increased and partial agglomeration occurred. In addition, the surface area of ZnO samples exposed to SCW decreased due to agglomeration.. According to TGA results, ZnO calcined at 900 ° C showed high thermal stability. When ZnO samples were tested in the gasification of formaldehyde in SCW, the presence of both ZnO in the reaction medium decreased the percent distribution of CH4, CO2 and CO and increased H2 formation. The highest hydrogen formation and highest formaldehyde conversion is found on ZnO calcined at 900 oC.