Monometallic and bimetallic catalysis for C1-C4 alkane and propylene total oxidation
| Author(s) | Lee, Garam | |
| Date Accessioned | 2017-04-17T11:41:13Z | |
| Date Available | 2017-04-17T11:41:13Z | |
| Publication Date | 2017 | |
| Abstract | In this work, we used small alkanes (methane, ethane, propane, and isobutene), as probe molecules for larger alkanes, and propylene, as a representative of alkene, in total oxidation. The catalytic activity of seven monometallic (Pt, Pd, Rh, Ag, Ni, Cu, and Co/Al2O3) and three compositions (1:3, 3:1, and 1:10) of Ag-Pd/Al2O3 bimetallic catalysts is evaluated for the total oxidation of C1-C4 alkanes in the temperature range of 280-400 °C. In oxygen rich conditions (equivalence ratio of 0.5), volcano-type dependences of the turnover frequency (TOF) on the C and O binding energy are observed for all small alkanes tested; Pt/Al2O3 is the most active among monometallic catalysts, and 1:3 Ag-Pd bimetallic exhibits superior activity compare to the most active Pt/Al2O3 catalyst in the oxidation of ethane and higher alkanes, while Pt shows superior activity in methane oxidation. For fuel rich conditions (equivalence ratio of 2), volcano-type relation of the TOF on the C and O binding energy for methane is different from C2-C4 alkanes; among monometallic catalysts, Pd/Al2O3 is the most active in methane oxidation, while Pt/Al2O3 is the most active in C2-C4 alkane oxidation. We also show that in propane oxidation over Pt/Al2O3, Pd/Al2O3, and Ag- Pd/Al2O3, there are three distinct kinetic regimes in which the TOF is (1) first order, (2) negative order, and (3) zero order with respect to oxygen concentration. Hysteresis with respect to oxygen concentration is also observed. ☐ In order to understand the reaction order shift in methane oxidation under different reaction conditions, the kinetics of methane oxidation over Pt/Al2O3, Pd/Al2O3, and Ag-Pd/Al2O3 is investigated as a function of O2/CH4 ratio at 340 ˚C. We show that as the oxygen fraction increases from low values, three distinct kinetic regimes exist that are (1) nearly first order, (2) negative order, and (3) zero order with respect to the oxygen concentration. At O2/CH4 < 3, Pd and Ag-Pd are superior catalysts, while at higher oxygen concentrations, Pt is superior. Hysteresis with bistability is demonstrated over all three catalysts with two distinct activity regimes at identical feed compositions: higher activity upon increasing oxygen concentration from low values and lower activity upon decreasing oxygen concentration from high values. In situ X-ray absorption spectroscopy (XAS) over Ag-Pd catalyst suggests that the bistability stems from different oxidation states of the catalyst as well as structural changes from the core-shell structure in the reduced state. ☐ Finally, the activities of seven monometallic catalysts under oxygen rich conditions in the temperature range of 90-170 °C and the kinetics of propylene oxidation over Pt/Al2O3, Pd/Al2O3 as a function of O2/C3H6 are investigated. We observed the same volcano-type relation of the TOF on the C and O binding energy as in propane total oxidation. However, the kinetic study shows that propylene chemistry is different from propane chemistry. Specifically, there are three kinetic regimes but no hysteresis as the oxygen concentration varies. | en_US |
| Advisor | Vlachos, Dionisios G. | |
| Degree | M.Ch.E. | |
| Department | University of Delaware, Department of Chemical and Biomolecular Engineering | |
| Unique Identifier | 982481773 | |
| URL | http://udspace.udel.edu/handle/19716/21225 | |
| Publisher | University of Delaware | en_US |
| URI | https://search.proquest.com/docview/1884695844?accountid=10457 | |
| dc.subject.lcsh | Metal catalysts. | |
| dc.subject.lcsh | Alkanes. | |
| dc.subject.lcsh | Propene. | |
| dc.subject.lcsh | Oxidation. | |
| dc.subject.lcsh | Methane. | |
| dc.subject.lcsh | Hysteresis.006m o d | |
| Title | Monometallic and bimetallic catalysis for C1-C4 alkane and propylene total oxidation | en_US |
| Type | Thesis | en_US |