Promoting effect of ceria on supported noble metal catalysts

Full text: Alumina supported precious metal catalysts are well known and widely used for hydrogenation and oxidation duties in various industrial processes. Because of the high cost and non availability, one approach to reduce the active noble metal concentration is to keep it in high degree of dispersion by adding suitable promoters. It is believed that the incorporation of rare earth oxides imparts extra stability and activity to the catalysts. In view of limiting the noble metal concentration, an attempt has been made to investigate the effect of ceria addition on supported catalysts for the oxidation of CO/H 2 and the hydrogenation of benzene and -methylstyrene. A series of noble metal catalysts were prepared on -Al2O3 and -Al2O3 /CeO2 supports. -Al 2O3 /CeO2 was prepared by impregnation of -Al2O3 extrusions (1.2mm dia) with cerous nitrate solution to get 2% by weight of CeO2. Pd and Pt catalysts were prepared on these supports by impregnation from respective aqueous chloride solutions. The metal loading on Al2O3 support was maintained at 0.3% and on Al2O3 -CeO2 supports, it was varied from 0.2-0.3% for a series of preparations. Supported Pd catalysts were evaluated for oxidation of CO/H2 and hydrogenation of methylstyrene to cumene whereas Pt based catalyst was evaluated for hydrogenation of benzene to cyclohexane. The Pd/Al2O3 catalyst was reduced and tested at 175 deg C for CO oxidation activity using a gas mixture(96% N2 +3%O2 +1% CO) at a space velocity 30000h -1. The oxidation of H2 was carried out at ambient temperature using a mixture of 92% H2 and 8% O2 at a space velocity of 60000 h-1. Pressure for these oxidation reactions was maintained at 1 kg/cm2. Hydrogenation of -methylstyrene to cumene was evaluated at 100-105 deg C and at 4 LHSV of a feed containing 10% -methylstyrene and 90% cumene and H2 /feed ratio was maintained at 1.5. Hydrogenation of benzene to cyclohexane was carried out at 180 deg C, LHSV 8 and H2 /C6H6 around 5.6. Pressure was maintained at 8 kg/cm2 for these hydrogenation reactions. An enhancement of lined out activity was observed for the catalysts prepared on ceria incorporated catalysts. The percentage conversions for the oxidation/hydrogenation reactions on these catalysts of relatively low metal loadings (0.2 to 0.25%) is almost comparable to those obtained with non-ceria catalysts of higher metal loadings (0.3%). Comparative CO chemisorption studies indicated higher metal dispersions for the ceria incorporated catalysts. Thus, the noble metal content of the catalysts could be reduced without affecting catalytic activity by the addition of CeO2 as a promoter to keep the active phase at a high degree of dispersion