Monometallic and bimetallic catalysis for C1-C4 alkane and propylene total oxidation
Date
2017
Authors
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Journal ISSN
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Publisher
University of Delaware
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.