Controlled Synthesis of PtRe and NiRe Bimetallic Catalysts for Enhanced Biomass Hydrogenation

Bimetallic catalysts of the promoted transition metal catalysts have demonstrated excellent activity and selectivity for the hydrogenation of the range of biomass-derived oxygenates to targeted deoxygenated products. The formation of well-dispersed bimetallic interfaces in the form of either low-valent rhenium coordinated to the first metals (e.g., Pt-ReOx) or alloy nanoparticles (e.g., NiRe) is key to boost the hydrogenation activity for the carboxylic acids and phenols, respectively, and the synthesis method affects the formation of these active sites. Herein, the two representative Re-based bimetallic catalysts, i.e., Pt-ReOx/TiO2 and NiRe/SiO2, for biomass hydrogenation were synthesized through controlled surface reaction (CSR), and its efficacy toward the formation of bimetallic interfacial sites compared to the traditional synthesis method, i.e., incipient wet impregnation (IWI), was demonstrated. For the CSR, the first metal was impregnated into the supporting materials and then reduced under hydrogen flow. Subsequently, Re was selectively deposited onto the reduced metal surface by the reaction of metal hydride with NH4ReO4, denoted as ex situ CSR sample. The reduction of the first metal and rhenium deposition were also performed simultaneously under hydrogen flow, denoted as an in situ sample. The synthesized catalysts were analyzed using X-ray diffraction (XRD), H2-TPR, CO chemisorption, X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and high-resolution transmission electron microscopy (HR-TEM). The catalytic properties were probed using the selective hydrogenation of acetic acid to ethanol for Pt-ReOx/TiO2 and that of 2-methoxyphenol to cyclohexane for NiRe/SiO2. We found that the in situ CSR leads to more intimate contact of two metals, allowing for the formation of the desired Pt-ReOx and NiRe interfacial sites and thereby resulting in enhanced hydrogenation activity compared to the IWI method.