Browsing by Author "Xu, Bingjun"
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Item Catalytic Hydrodeoxygenation of High Carbon Furylmethanes to Renewable Jet-fuel Ranged Alkanes over a Rhenium Modified Iridium Catalyst(Wiley-Blackwell, 2017-07-07) Saha, Basudeb; Liu, Sibao; Dutta, Saikat; Zheng, Weiqing; Gould, Nicholas S.; Cheng, Ziwei; Xu, Bingjun; Vlachos, Dionisios G.; Sibao Liu, Saikat Dutta, Weiqing Zheng, Nicholas S. Gould, Ziwei Cheng, Bingjun Xu, Basudeb Saha, and Dionisios G. Vlachos; Saha, Basudeb; Liu, Sibao; Dutta, Saikat; Zheng, Weiqing; Gould, Nicholas S.; Cheng, Ziwei; Xu, Bingjun; Vlachos, Dionisios G.Renewable jet-fuel ranged alkanes are synthesized by hydrodeoxygenation of lignocellulose derived high carbon furylmethanes over ReOx modified Ir/SiO2 catalysts under mild reaction conditions. Ir-ReOx/SiO2 with a Re/Ir molar ratio of 2 exhibits the best performance, achieving a combined alkanes yield of 82-99% from C12-C15 furylmethanes. Catalyst can be regenerated in three consecutive cycles with only ~12% loss in the combined alkanes yield. Mechanistically, the furan moieties of furylmethanes undergo simultaneous ring saturation and ring opening to form a mixture of complex oxygenates consisting of saturated furan rings, mono-keto groups, and mono-hydroxy groups. Then, these oxygenates undergo a cascade of hydrogenolysis reactions to alkanes. The high yield of Ir-ReOx/SiO2 arises from a synergy between Ir and ReOx. The acidic sites of partially reduced ReOx activate the C-O bonds of the saturated furans and alcoholic groups, while the Ir sites are responsible for hydrogenation with H2.Item Correcting the Hydrogen Diffusion Limitation in Rotating Disk Electrode Measurements of Hydrogen Evolution Reaction Kinetics(The Electrochemical Society, 2015-10-10) Zheng, Jie; Yan, Yushan; Xu, Bingjun; Jie Zheng, Yushan Yan, and Bingjun Xu; Zheng, Jie; Yan, Yushan; Xu, BingjunRotating disk electrode (RDE) method is widely employed in studies on the hydrogen oxidation/evolution reaction (HOR/HER) owing to its well-defined mass transport behaviors. While it is accepted that the measured HOR current is controlled by both the electrode kinetics and the diffusion of H2, HER is typically assumed to be free of diffusion limitation. Here we demonstrate that HER could also be diffusion limited when the electrode kinetics is fast, as evidenced by the rotation speed dependent HER current on Pt in acid (pH = 1) and the overlap of the HER polarization curve with the concentration overpotential curve. The HER diffusion limitation originates from the insufficient mass transport of produced H2 from the electrode surface to the bulk electrolyte and the highly reversible nature of HOR/HER. Kinetic analyses based on HER polarization curves on Pt in acid without correcting for the diffusion limitation could lead to inaccurate Tafel slopes and mechanistic interpretations, and significantly underestimated HER activities. A general data analysis protocol based on the reversible Koutecky-Levich equation is developed to obtain accurate kinetic information of HOR/HER even when electrode kinetics is facile. This new method is compared with other existing methods on Pt disk electrodes at different pHs and thin-film electrodes with different Pt loadings.Item Insights into ring opening of biomass-derived furanics over Ru/C(Wiley Online, 2016-10-14) Gilkey, Matthew J.; Mironenko, Alexander V.; Yang, Leerang; Vlachos, Dionisios G.; Xu, Bingjun; Matthew J. Gilkey, Alexander V. Mironenko, Leerang Yang, Dionisios G. Vlachos, Bingjun Xu; Gilkey, Matthew J.; Mironenko, Alexander V.; Vlachos, Dionisios G.; Yang, Leerang; Xu, BingjunSelective ring opening of cellulose-derived furanic molecules is a promising pathway for the production of industrially relevant linear oxygenates, such as 1,6-hexanediol. 2,5-dimethylfuran (DMF) is employed as a model compound in combined experimental and computational investigations to provide insights into metal-catalyzed ring opening. Ring opening to 2-hexanol and 2-hexanone and ring saturation to 2,5-dimethyltetrahydrofuran (DMTHF) are identified as two main parallel pathways. Density functional theory calculations and microkinetic modeling indicate DMF adsorbs on Ru in an open-ring configuration, which is potentially a common surface intermediate leading to both ring opening and ring saturation products. While the activation barriers for the two pathways are comparable, formation of DMTHF is more thermodynamically favorable. In addition, steric interactions with co-adsorbed 2-propoxyl, derived from the solvent, and the oxophilic nature of Ru play key roles in determining the product distribution: the former favors less bulky, i.e., ring-closed, intermediates, and the latter retards O-H bond formation. Finally, we show that hydrodeoxygenation of oxygenated furanics, such as 5-methylfurfural and (5-methyl-2-furyl)methanol, on Ru preferentially occurs at oxygen-containing side groups to form DMF, followed by either ring opening or ring saturation.Item Poisoning of Ru/C by Homogeneous Brønsted Acids in Hydrodeoxygenation of 2,5-Dimethylfuran via Catalytic Transfer Hydrogenation(Elsevier, 2017) Gilkey, Matthew J.; Vlachos, Dionisios G.; Xu, Bingjun; Matthew J. Gilkey, Dionisios G. Vlachos, Bingjun Xu; Gilkey, Matthew J.; Vlachos, Dionisios G; Xu, BingjunIt has been proposed that the combination of metal and acid sites is critical for effective ring opening of biomass-derived furans to linear molecules, a reaction that holds promise for the production of renewable polymer precursors and alkanes. In this work, we use 2,5-dimethylfuran (DMF) as a model compound to investigate hydrogenolysis and hydrogenation pathways using a combination of H2SO4 and Ru-mediated catalytic transfer hydrogenation in 2-propanol. Acid-catalyzed hydrolytic ring opening of DMF to 2,5-hexanedione (HDN) occurs readily at 80 °C with a selectivity of 89% in 2-propanol. Over Ru/C, HDN is fully converted after only 2 h at 80 °C, forming a mixture of both ring-closed products (~68% total yield), i.e., 2,5-dimethyltetrahydrofuran (DMTHF) and 2,5-dimethyl-2,3-dihydrofuran (DMDHF), as well as ring opened products (~28% total yield), i.e., 2,5-hexanediol (2,5-HDL) and 2-hexanol (HOL). Rather than observing sequential hydrolysis/hydrogenation reactions, we observe severe suppression of metal chemistry when having both Ru/C and H2SO4 in the reaction system. While minor leaching of Ru occurs in the presence of mineral acids, X-ray photoelectron spectroscopy coupled with CO chemisorption studies suggest that the primary cause of the lack of Ru-mediated chemistry is poisoning by strongly adsorbed sulfate species. This hypothesis is supported by the observation of Ru-catalyzed chemistry when replacing H2SO4 with Nafion, a solid Brønsted acid, as sulfonic acid groups tethered to the polymer backbone cannot adsorb on the metal sites.Item Universal dependence of hydrogen oxidation and evolution reaction activity of platinum-group metals on pH and hydrogen binding energy(American Association for the Advancement of Science, 2016-01-14) Zheng, Jie; Sheng, Wenchao; Zhuang, Zhongbin; Xu, Bingjun; Yan, Yushan; Jie Zheng, Wenchao Sheng, Zhongbin Zhuang, Bingjun Xu, Yushan Yan; Zheng, Jie; Zhuang, Zhongbin; Xu, Bingjun; Yan, YushanUnderstanding how pH affects the activity of hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) is key to developing active, stable, and affordable HOR/HER catalysts for hydroxide exchange membrane fuel cells and electrolyzers. A common linear correlation between hydrogen binding energy (HBE) and pH is observed for four supported platinum-group metal catalysts (Pt/C, Ir/C, Pd/C, and Rh/C) over a broad pH range (0 to 13), suggesting that the pH dependence of HBE is metal-independent. A universal correlation between exchange current density and HBE is also observed on the four metals, indicating that they may share the same elementary steps and rate-determining steps and that the HBE is the dominant descriptor for HOR/HER activities. The onset potential of CO stripping on the four metals decreases with pH, indicating a stronger OH adsorption, which provides evidence against the promoting effect of adsorbed OH on HOR/HER.