Abstract :
Microcalorimetric adsorption and FTIR were used to study the surface species and energy of surface bonding for ethylene adsorption on Ni/SiO2 and NiBi/SiO2 catalysts. Microcalorimetric adsorption of H2 was employed to titrate the surface sites of Ni in the catalysts. It was found that addition of a small amount of Bi (Ni/Bi = 64 and 16) significantly decreased the number of Ni surface sites for H2 adsorption. The adsorption of ethylene at room temperature on the Ni/SiO2 and Ni64Bi/SiO2 produced mainly ethylidyne species on the surface plus adsorbed hydrogen atoms, generating the initial adsorption heats of 170 and 160 kJ/mol, respectively. In contrast, the adsorption of ethylene on the Ni16Bi/SiO2 at room temperature produced mainly molecularly adsorbed surface species (di-σ or π-bonded species) with much lower adsorption heat. The enrichment of Bi on the surface may change the ensemble size of Ni for the adsorption of ethylene, thereby changing the adsorption state and energy of ethylene on the surface of Ni.
