Effect of Confinement in Nanopores of Carbon On Reaction Kinetics, Catalyst Activity and Selectivity

Polymer-derived nanoporous carbon materials with high thermal stability, high surface area and narrow pore size distribution have been widely used as catalyst supports. Catalyst performance (activity and selectivity) is significantly affected by the pore size and porosity of the carbon support. Mass transfer limitation inside the pores and steric hindrance imposed by the pore walls can enhance catalyst selectivity and alter reaction kinetics. In this study, platinum embedded in nanoporous carbon spheres was used as a shape selective catalyst for liquid phase hydrogenation of alkenes. Experimental data obtained for different synthesized catalysts with different porosity and pore sizes were used to estimate kinetic parameters (Langmuir-Hinshelwood) and diffusion coefficients for different reactants and products inside the pores, using numerical solution of reaction-diffusion equation. It was shown that adsorption equilibrium constants and reaction rate constants are larger inside the nanopores of embedded catalyst than those on the surface of impregnated catalyst. Diffusion coefficients were estimated to be in the range of 10^-14 to 10^-16  m²/sec and hence observed reaction rate was limited by diffusion process. As the pore size and pore accessibility was increased by applying different approaches, adsorption and reaction rate constants increased due to decreased steric hindrance. Combined experimental and theoretical study in this work, confirms the effect of confinement on kinetics, mass transfer and overall catalyst performance (activity and selectivity).