Synthesis of Pd-Ag/Al2O3 catalyst by colloidal oxide method for acetylene selective hydrogenation: a study on the sintering of PdO nanoparticles

摘要

Pd-Ag/Al2O3 catalysts (Pd = 0.03 wt% with Ag = 0.18 wt% in bimetallic samples) were prepared by sequential impregnation using acidic and basic colloidal oxide solutions. The catalysts were evaluated for selective hydrogenation of acetylene to ethylene (T = 60 degrees C, p = 10 bar, H-2/C2H2 = 1.5 mol/mol, GHSV = 4000 h(-1)). The colloidal solutions were characterized by HR-TEM, the fresh catalysts by CO chemisorption, CO-TPD and FE-SEM and the spent catalysts by TGA and FTIR techniques. The HR-TEM images showed fairly narrow size distributions of PdO nanoparticles (1.9-3.4 and 1.3-4.4 nm in acidic and basic solutions, respectively). However, the Pd particles on unpromoted catalysts were much larger (19-51 nm) indicating considerable sintering during the preparation steps due to low melting point of PdO and its weak interaction with the support. The FE-SEM results illustrated that the colloidal method is capable of producing egg-shell distribution of Pd with penetration depths (7-15 mu m) comparable to those achieved by conventional impregnation although due to a different mechanism. On the mono-metallic colloid-based samples, the acetylene conversions (96%) and ethylene selectivities (65-68%) were much larger than those of the sample prepared by conventional impregnation (90 and 24%, respectively). Upon Ag addition, the acetylene conversion was decreased to 40 and 56%, while ethylene selectivity was increased to 92 and 95% for acidic and basic samples, respectively. For the conventional bimetallic sample, the conversion and selectivity were increased to 95 and 86%, respectively. The results illustrated a better performance of the colloid-based catalysts with basic mode showing superior performance. The coke deposited on spent catalyst was found to be of soft type, that is, the one with high H/C ratio.