Characterization of sepiolite as a support of silver catalyst in soot combustion

Turkish sepiolite-zirconium oxide mixtures were applied as a support for the silver catalyst in a soot combustion. Sepiolite-Zr-K-Ag-O catalyst was characterized by XRD, N₂ adsorption, SEM, TPR-H₂ and EGA-MS. The combustion of soot was studied with a thermobalance (TG-DTA). The modification resulted in a partial degradation of the sepiolite structure, however, the morphology was preserved. The adsorption of N₂ of the modified sepiolite is a characteristic for mesoporous materials with a wide distribution of pores. The specific surface area S_BET equals 83 m²/g and the pores volume is 0.23 cm³/g. The basic character of the surface centers of sepiolite is indicated by CO₂ desorption (TPD-MS) at 170 °C and at about 620 °C due to a surface carbonates decomposition. The thermodesorption of oxygen at 650-850 °C indicates the decomposition of AgO_x phases at the surface. The presence of AgO_x phases is also confirmed by TPR-H₂ spectrum (low temperature reduction peak at 130 and 180 °C). The high-temperature reduction at about 570 °C is probably related to Ag-O-M phases on the support. The soot combustion takes place at T₅₀ = 575 °C. Without silver (sepiolite-Zr-K-O) T₅₀ = 560 °C but sepiolite modified with silver (sepiolite-Zr-K-Ag-O) undergoes the same process at T₅₀ = 490 °C.