Carbon-Supported Platinum Catalysts for Hydrogen Production and Butyric Acid Decarboxylation.
Zugic, Branko.
2013
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Abstract: The growing
global concerns over energy security and sustainability have resulted in the study of a
number of technologies as alternatives to fossil fuels. Among those considered viable,
hydrogen and biofuel based technologies are at the forefront. However, considerable
progress must be made to advance our fundamental understanding of these processes in
order to actively bring these ... read morefuels into the scope of our energy economy. The water-gas
shift reaction is an important component of hydrogen fuel purification for fuel cell
applications. The current catalyst standards for the water-gas shift reaction are based
on copper-zinc oxide and iron-chromium oxides. These are not suitable for use in fuel
cell systems due to their toxicity, pyrophoricity, and susceptibility to deactivation
under ambient start-up and shut-down conditions. Due to these deficiencies, a new class
of water-gas shift catalysts based on active noble metals has emerged in the research
arena. Among the most widely studied are platinum-based catalysts. Ceria-supported
platinum catalysts have been widely studied due to their high activity. It is thought
that the defect concentration of the support is responsible for the performance of this
catalyst formulation. The active site on these materials has been identified as ionic
species of the form Pt-O-Ce. Recent studies have shown that the addition of sodium to
platinum-based catalysts on a number of metal oxide supports results in the
stabilization of similar species of the form Pt-Na-OHx. This work deals with an effort
to evaluate carbon as a potential support for water-gas shift catalysts and to better
characterize the active site on these materials. It is of particular interest to study
the ability of sodium to stabilize such a complex in the absence of surface and bulk
support oxygen. Multi-walled carbon nanotubes were used as the support for platinum
catalysts. Various surface modifications of the carbon were carried out in an effort to
understand the effects on the water-gas shift activity of the corresponding catalysts.
The oxidation of the carbon nanotubes by nitric acid was used to introduce surface
oxygen groups of various acidity and function onto the surface. In order to selectively
remove groups, a heat treatment was applied. The ability of carboxylic anhydride groups
to stabilize ionic Pt and activate water was thus demonstrated. While this provides
active sites for the water-gas shift reaction, the rates are considerably lower than
those obtained by the ion exchange of sodium onto the surface prior to platinum
addition. This is due to the ability of sodium to activate water to a greater degree.
Interestingly, the presence of highly electronegative groups, such as carboxylic acids,
on the surface while useful for dispersing Pt were not highly active for the WGS
reaction. The ability of sodium to stabilize platinum was also shown by a simple
co-impregnation technique. Various characterization approaches were taken to understand
these effects in detail such as XPS, XANES, EXAFS, and HAADF-STEM. The preparation of a
highly annealed, oxygen-free carbon nanotube support allowed for the comparison of Pt
and PtNa systems under reaction conditions using atmospheric pressure X-ray
photoelectron spectroscopy. These findings indicate that while platinum can be finely
dispersed on a carbon nanotube support, it is not active for the WGS reaction due to the
absence of water activation sites. When sodium is added to the surface with platinum, a
highly active catalyst is prepared. The AP-XPS study indicates that Pt-OHx is present on
the Na-promoted sample to a high degree. Furthermore, this surface was found to be
extremely responsive to changes in the reaction conditions. Removal of water from a
product-free reaction gas results in the increase of bridge CO and CO bound to
low-coordinated Pt sites, in line with previous studies. The introduction of hydrogen to
this environment resulted in an apparent local reduction, causing a decrease in -OH
content and an increase in CO binding. Determinations of reaction orders on the
MWNT-supported Na-promoted catalyst showed that water activation seems to be somewhat
suppressed on the carbon support compared to metal oxides. This is thought to be related
to the ability of carbon nanotubes to adsorb hydrogen by spillover from activation sites
on the surface. Therefore a competition for these sites is established in the presence
of hydrogen and water. A process by which butyric acid is derived from biomass through a
fermentation step is also proposed as a means of the sustainable production of propane
through decarboxylation. Platinum and palladium catalysts were evaluated for the
decarboxylation of butyric acid and it was found that both suffer from severe
deactivation due to coking. Efforts to control the extent of deactivation by modifying
surface functionality of carbons were unsuccessful. The use of bimetallic formulations
appeared to show some promise for improved stability and selectivity to propylene, a
commodity chemical. Furthermore, the use of zeolites was found to be of interest for
this reaction, although energetically unfavorable conditions are necessary to achieve
high yields.
Thesis (Ph.D.)--Tufts University, 2013.
Submitted to the Dept. of Chemical and Biological Engineering.
Advisor: Maria Flytzani-Stephanopoulos.
Committee: Matthew Panzer, Terry Haas, and Gary Haller.
Keyword: Engineering.read less - ID:
- bg257s11p
- Component ID:
- tufts:22056
- To Cite:
- TARC Citation Guide EndNote