Catalysis-spillover-membrane. III: The effect of hydrogen spillover on the palladium membrane reactor in the steam reforming reactions

In this paper, the effect of varying the distance between a palladium membrane and the steam reforming catalyst in a packed bed catalytic membrane reactor is studied. A set of the specially structured palladium membrane tube is prepared and used for the steam reforming reaction of n-hexane and of methanol. This specially designed palladium membrane structure has a long tube at the upstream-end of membrane tube, and it is electroplated with thin nickel or palladium film. This nickel or palladium plated metal surface serves as a bridge for the surface diffusion of the nascent hydrogen atoms over a 10 cm distance to the membrane surface for permeation out of the system after they are formed on the catalyst site and spillover from the catalyst surface to the diffusion bridge. As a result, the conversion level of steam reforming reaction is increased while byproduct formation of methane is suppressed. Furthermore, it is believed that the direct migration of the nascent hydrogen atom from the catalyst site to the membrane surface via direct spillover or via surface diffusion brings about a stronger positive contribution to the steam reforming reaction than does the vapor phase diffusion of the molecular hydrogen on the reaction.This extension tube allows the membrane and the catalyst section to operate under mutually optimized temperatures in a single reaction chamber while preserving high membrane efficiency; thus the use of this new membrane structure makes design of a membrane reactor more flexible to maximize mutual efficiency of catalyst and the membrane.

Graphical abstractA set of the specially structured palladium membrane tube is prepared and used for the steam reforming reaction. This specially designed palladium membrane structure has an extension tube coated with thin nickel or palladium film to serve as an extended bridge of over 10 cm distance for the spillover of the nascent hydrogen atoms from the catalyst surface to the membrane surface for permeation out of the system. As a result, the conversion level of steam reforming reaction is increased while byproduct formation of methane is suppressed.Figure optionsDownload full-size imageDownload high-quality image (121 K)Download as PowerPoint slideResearch highlights▶ Long surface diffusion of H-atom optimizes reaction and permeation temperature. ▶ Surface diffusion of H-atom on Ni surface is faster than vapor phase diffusion of H2. ▶ Long diffusion of H-atom on Pd-surface results in over 40% hike of efficiency. ▶ Ni or Pd-coated surface for diffusion of H-atom is used to design HSP Pd-membrane.