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

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.

Automated summary

Pd-Ag/Al2O3 catalysts were prepared using sequential impregnation with acidic and basic colloidal oxide solutions. The catalysts showed better performance in selective hydrogenation of acetylene to ethylene, with the basic mode exhibiting superior performance.