Controlled\nSynthesis of PtRe and NiRe Bimetallic Catalysts\nfor Enhanced Biomass Hydrogenation

Bimetallic catalysts of the promoted transition metal\ncatalysts\nhave demonstrated excellent activity and selectivity for the hydrogenation\nof the range of biomass-derived oxygenates to targeted deoxygenated\nproducts. The formation of well-dispersed bimetallic interfaces in\nthe form of either low-valent rhenium coordinated to the first metals\n(e.g., Pt-ReOx) or alloy nanoparticles (e.g., NiRe) is key to boost\nthe hydrogenation activity for the carboxylic acids and phenols, respectively,\nand the synthesis method affects the formation of these active sites.\nHerein, the two representative Re-based bimetallic catalysts, i.e.,\nPt-ReOx/TiO<sub>2</sub> and NiRe/SiO<sub>2</sub>, for biomass hydrogenation\nwere synthesized through controlled surface reaction (CSR), and its\nefficacy toward the formation of bimetallic interfacial sites compared\nto the traditional synthesis method, i.e., incipient wet impregnation\n(IWI), was demonstrated. For the CSR, the first metal was impregnated\ninto the supporting materials and then reduced under hydrogen flow.\nSubsequently, Re was selectively deposited onto the reduced metal\nsurface by the reaction of metal hydride with NH<sub>4</sub>ReO<sub>4</sub>, denoted as ex situ CSR sample. The reduction of the first\nmetal and rhenium deposition were also performed simultaneously under\nhydrogen flow, denoted as an in situ sample. The synthesized catalysts\nwere analyzed using X-ray diffraction (XRD), H<sub>2</sub>-TPR, CO\nchemisorption, X-ray photoelectron spectroscopy (XPS), X-ray absorption\nnear-edge structure (XANES), extended X-ray absorption fine structure\n(EXAFS), and high-resolution transmission electron microscopy (HR-TEM).\nThe catalytic properties were probed using the selective hydrogenation\nof acetic acid to ethanol for Pt-ReOx/TiO<sub>2</sub> and that of\n2-methoxyphenol to cyclohexane for NiRe/SiO<sub>2</sub>. We found\nthat the in situ CSR leads to more intimate contact of two metals,\nallowing for the formation of the desired Pt-ReOx and NiRe interfacial\nsites and thereby resulting in enhanced hydrogenation activity compared\nto the IWI method.