Effect of sol–gel conditions on BET surface area, pore volume, mean pore radius, palladium dispersion, palladium particle size, and catalytic CO oxidation activity of Pd/Al₂O₃ cryogels

Abstract

Porous and homogeneous palladium-alumina cryogels were synthesized from aluminium sec-butoxide (ASB) and palladium nitrate through one-pot sol–gel processing in an aqueous system and subsequent freeze drying. In order to optimize the sol–gel conditions so that higher catalytic oxidation activity could be acquired after high temperatures heating, effects of H₂O/ASB, HNO₃/ASB, ethylenediamine/Pd, and urea/ASB ratios on BET surface area, pore volume, mean pore radius, Pd dispersion, Pd diameter, and catalytic CO oxidation activity of the Pd/Al₂O₃ cryogels were examined. It was revealed that optimized molar ratios were ASB:H₂O:HNO₃:urea = 1:76:0.26:0.29 and ethylenediamine:Pd = 3.4:1, at the ratios of which higher Pd dispersion and more superior catalytic activity were obtained. It was suggested that maintenance of as large BET surface area and pore volume as possible even after the heating was important to obtain high Pd dispersion, which consequently brought about superior catalytic activity. It was also suggested that porosity of the cryogel also played an important role in suppressing the sintering of palladium. The palladium-alumina cryogel prepared under the optimized sol–gel conditions was compared with corresponding commercial catalyst in regard to Pd particle size, Pd dispersion, PdO reducibility, catalytic CO oxidation activity. It was shown, after heating the cryogel at 800 °C for 5 h, that finer palladium particles with ca. 3.5 nm diameter were dispersed throughout alumina support with higher dispersion (ca. 32%), which was primarily responsible for higher catalytic oxidation activity on the cryogel.