Aqueous phase reforming of xylose using bimetallic Pt₃-Re_x/SiO₂ catalysts for H₂ production: Experimental and computational study

In recent years, the aqueous phase reforming (APR) reaction using heterogeneous catalysts has been highlighted as one of possible approaches for converting nonedible biomass into hydrogen for fuel cell application. In this study, we have synthesized the rhenium (Re)-doped Pt/SiO₂ (Pt₃Re_x/SiO₂) (based on weight; x = 1, 3, 5) catalysts for enhancing the APR of organic liquid C5 sugar xylose. Our study showed that the activity of bimetallic Pt-Re catalysts toward the APR of xylose strongly depends on the level of Re dopant. In particular, the Pt₃-Re₁/SiO₂ catalyst exhibited the higher H₂ selectivity compared to the catalysts having the higher amount of Re dopant (Pt₃-Re₂/SiO₂ and Pt₃-Re₃/SiO₂). Furthermore, the oxygen state of ReO_x could be modified by changing reduction temperature. The measured hydrogen selectivity and reaction rate could be enhanced when the catalyst was reduced at higher temperature. Especially, the Pt₃-Re₁/SiO₂ catalyst reduced at 500°C under 10 vol% H₂ flow exhibited high catalytic performances close to the theoretical values in the initial stage of the reaction, but its activity was gradually reduced by re-oxidation of ReO_x. From the various analysis results, we can figure out that the presence of metallic state Pt and Re in Pt₃Re alloy phase and low oxidation state Re (Re³⁺) element is responsible for the improved H₂ selectivity in Pt₃-Re₁/SiO₂ catalyst reduced at 500°C. In addition, the density functional theory calculation also predicted that the surface exposure of Pt and Re atoms (rather than oxidized ReO_x) can boost the first dehydrogenation of xylose, which is in a good agreement with the experimental observation. This study can provide the optimal oxidation level of Re and Pt in the bimetallic Pt-Re catalyst to efficiently obtain hydrogen from xylose via APR reaction.