Carbon Nanotube-Supported RuFe Bimetallic Nanoparticles as Efficient and Robust Catalysts for Aqueous-Phase Selective Hydrogenolysis of Glycerol to Glycols

Carbon nanotube-supported RuFe bimetallic catalysts (RuFe/CNT) were prepared through a coimpregnation method for the selective hydrogenolysis of 20 wt % glycerol aqueous solution to produce glycols (1,2-propanediol and ethylene glycol). The Ru/CNT catalyst with smaller Ru nanoparticles (NPs) was significantly active for C-C bond cleavage, giving a considerable amount of CH4 in the hydrogenolysis product. The RuFe/CNT catalyst with bimetallic NPs having an average size similar to Ru/CNT was more efficient for C-O bond cleavage, affording higher selectivity to glycols. Almost 100% glycerol conversion and over 75% selectivity to glycol could be obtained using the optimized RuFe/CNT catalyst under relatively mild conditions. The bimetallic RuFe/CNT catalyst was structurally robust and showed excellent reusability. Transmission electron microscopic images revealed that, when an appropriate amount of Fe entity was added, the RuFe bimetallic NPs were uniformly dispersed on the CNT surfaces and had an average size of similar to 3 nm. X-ray photoelectron spectroscopy indicated that a portion of the Fe species were interacted with Ru moieties, forming Ru-Fe alloys on the Ru domain, whereas other Fe species were in the forms of iron oxides, likely FeO and FeO1+x (0 < x < 0.5), mostly presenting on the periphery of RuFe bimetallic NPs. The occurrence of iron oxide species is crucial for the stability of RuFe bimetallic NPs during catalytic runs; but excess iron oxides block the surfaces of RuFe bimetallic NPs, resulting in a decrease in catalytic activity. Higher performance of the RuFe/CNT catalyst is attributed to the synergistic effects of the formation of Ru-Fe alloys and the interactions between the RuFe bimetallic NPs and iron oxides on CNT surfaces.