Study of hydrogen surface mobility and hydrogenation reactions over alumina-supported palladium catalysts

Alumina-supported Pd catalysts with different particle surface densities have been prepared using incipient wetness impregnation of aqueous solution of a palladium nitrite complex. Buta-1,3-diene and orthoxylene hydrogenation reactions were performed both in a batch and a fixed bed reactor. Hydrogen surface mobility was studied using H₂–D₂ isotopic exchange. The influence of (i) the particle surface density and (ii) the surface area of the support on the catalytic properties are discussed. The turnover frequency (TOF) of the but-1,3-diene hydrogenation was highly sensitive to the surface density of Pd particles (D_sp). Moreover, for a given surface density, TOF also depend on the nature of the alumina support. For a given support, modifications of the electronic properties of palladium can explain the increase of the reaction rate with D_sp while changes in the kinetics of hydrogen surface diffusion are proposed to explain the support effect.