Systematic screening and modification of Ni based catalysts for hydrogen generation from supercritical water gasification of lignin

In this study, systematic screening of 29 catalysts synthesised using five supports: Al2O3, activated carbon (AC), TiO2, ZrO2, MgO, and three promoters: Co, Cu, Ce (with two different Ce precursors) was performed. Then, the optimization of the Ni-Ce/Al2O3 catalyst was carried out by varying Ni loading (four levels) and Ce/Ni molar ratio (two levels). All these catalysts were screened for supercritical water gasification (SCWG) of lignin under the operating conditions of 650 °C, 26 MPa and water to biomass mass ratio of 5 in a tubular batch reactor. Various characterization techniques were used to investigate the properties of these catalysts further. As the result, using different supports, the activity of Ni-based catalysts for SCWG of lignin was found in the order of Al2O3 > TiO2 > AC > ZrO2 > MgO. As the best promoter, Ce enhanced the hydrogen selectivity by promoting the Ni dispersion and weakening the Ni-Al2O3 interaction. Use of different precursors for the Ce promoter (CeCl3·6H2O or Ce(NO3)3·6H2O) showed no effect on the hydrogen yield. For a fixed Ni loading, hydrogen yield increased with an increase in Ce/Ni molar ratio. The 20Ni-0.36Ce/Al2O3 catalyst showed highest hydrogen yield of 2.15 mmol/g, which is 35% higher than that for non-catalytic run. The better performance of Ni-Ce/Al2O3 catalyst can be attributed to enhanced coke resistance as well as increased reducibility for Ni by addition of Ce promoter.