Thesis (Ph.D.(Chemistry)) -- University of Limpopo, 2020 / The interest on the use of proton exchange membrane (PEM) fuel cells for vehicle
application has increase due to its efficiency, high power density and rapid start up.
The on-board reforming process is used to generate hydrogen; however, this process
simultaneously produces 1% CO which poisons Pt-based anode catalyst. Previous
studies have shown that supported Pd-based catalysts have very good stability on
preferential oxidation (PROX) of CO, but these catalysts suffer from lower selectivity.
Metal oxides such as Co3O4 and CeO2 are known to have high oxygen vacancy which
promotes CO oxidation. Furthermore, the pre-treatment of the catalysts by hydrazine
as well as the addition of MnOx species have been shown to improve the surface
properties of metal/metal oxides catalysts. The study envisages that the modification
of PROX catalysts will improve the CO conversion and its selectivity while maintaining
higher stability.
In this work, as-prepared (Co3O4) and hydrazine treated cobalt (Co3O4(H)) based
catalysts were prepared by precipitation method and investigated at temperature
range of 40-220 oC for preferential oxidation (PROX) of CO in excess hydrogen. The
FTIR and XPS data of hydrazine treated Co3O4 does not show peak ratio differences,
indicating that usual amounts of Co3+ and Co2+ were formed. An improved surface
reducibility with smaller crystallite size was noted on Co3O4(H) catalyst, which indicate
some surface transformation. Interestingly, the in-situ treatment of standalone
Co3O4(H) decreased the maximum CO conversion temperature (T100%) from 160 oC
(over Co3O4) to 100 oC. The Co3O4(H) catalyst showed good stability, with
approximately 85% CO conversion at 100 oC for 21 h, as compared to fast deactivation
of the Co3O4 catalyst. However, the Co3O4(H) catalyst was unstable in both CO2 and
the moisture environment. Based on the spent hydrazine treated (CoO(H)) cobalt
catalyst, the high PROX is associated with the formation of Co3+ species as confirmed
by XRD, XPS, and TPR data.
The Pd species was incorporated on different Co3O4 by improved wet impregnation
method and this has improved the surface area of the overall catalysts. However, the
presence of Pd species on Co3O4(H) catalyst decreased the CO conversion due to
formation of moisture. Although, the Pd on Co3O4(H) had lower activity, the catalyst
showed better stability under both moisture and CO2 conditions at 100 oC for 21 h.
vi
The 2wt.% metal oxides (MnO2, CeO2, Cr3O4, TiO2, MgO) on cobalt, and Pd on CeO2-
Co3O4 and MnO2-Co3O4 were prepared by co-precipitation method and the structural
composition was confirmed by XRD, FTIR, XPS and TPR data. Although, 2wt.%MnO2
on Co3O4(H) showed higher activity at 80 oC, both MnO2 and CeO2 improved the
activity of Co3O4(H) at 100 oC. The higher activity of MnO2 is attributed to the higher
surface area of the composite catalyst, in relation to ceria composite catalyst. Although
the MnO2 species transformed the structure of Co3O4 by lowering the oxidation state
to Co2+, the spent catalyst showed transformation from Co2+ to Co3+ during PROX, as
confirmed by TPR data.
Studies on the effects of CeO2 loading on Co3O4 catalysts, showed an optimum activity
over 2wt.%CeO2-Co3O4 as compared to other ceria loadings (i.e., 3, 5, 8, 10, 15,
30wt.%CeO2). However, upon addition of 0.5wt.%Pd species on 2wt.%CeO2-
Co3O4(H) composite, the activity of the catalyst decreased slightly at 100 oC, which
could be due to a decreased surface area. Although its activity is lower, the catalyst
has shown good stability in dry, moisture and CO2 conditions at 100 oC for 21 h.
In addition, studies were also undertaken on the effect of MnO2 concentration on
Co3O4 catalysts. The data shows that 7wt.%MnO2 species improved the activity of
Co3O4 catalyst at 60 oC, however, the catalyst could not improve the activities at higher
temperatures. This low activity is associated with a decrease in surface area as
concentration increases. The presence of 0.5wt.%Pd species on 7wt.%MnO2-Co3O4
increased the activity at 60 and 80 oC, which could be due to reduction of Co3+ to Co2+
in the presence of Pd, as confirmed by XPS data. The catalyst has shown good
stability in dry, moisture, and CO2 conditions at 100 oC for 21 h. The hydrazine treated
cobalt-based catalysts in the presence of palladium and manganese oxide is the
promising catalysts for proton exchange membrane fuel cells technology. / National Research Foundation (NRF) , Faculty of Science and Agriculture University of Limpopo and School of Physical and Mineral Sciences
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ul/oai:ulspace.ul.ac.za:10386/3365 |
| Date | January 2020 |
| Creators | Mhlaba, Reineck |
| Contributors | Magadzu, T., Mosuang, T. E. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | xx, 158 leaves |
| Relation |
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