(130a) CO-Free Hydrogen From Formic Acid with Pt Ru Bi Ox/C Heterogeneous Catalyst for PEM Fuel Cells

CO-free
Hydrogen from Formic Acid with PtRuBiO_x/C Heterogeneous
Catalyst for PEM Fuel Cells

Kwong-Yu Chan¹, Siu Wa
Ting¹, Chaoquan HU¹, Jayasree PULLERI¹ and Jenkin
TSUI¹ (1)Department of Chemistry, The
University of Hong Kong, Pokfulam Road, Hong Kong.

Though hydrogen content in formic acid is
only 4.35% by mass, the convenient and complete release of hydrogen without CO is
still attractive for PEM fuel cells. Formic acid can be bio-derived and is a commonly
available chemical with known safety standards. High selectivity towards
dehydrogenation at low temperature has been observed recently in liquid phase
reactions [1-8]. Near ambient temperature hydrogen generation from dissolved formic
acid on a platinum-ruthenium-bismuth mixed metal/metal oxide supported catalyst
was reported [1] for a batch reactor at atmospheric pressure. The powering of a
PEM fuel cell by CO-free hydrogen generated from 15% formic acid at room
temperature can be demonstrated. The generation device is simple with reaction
rate moderated by liquid contact with the solid catalyst.

For supplying hydrogen to fuel-cell stacks,
post generation compression may be needed to overcome the flow resistance. To
keep a simple system design, it will be desirable to create moderate pressure
by a high gas generation rate, e.g. via reaction at higher temperature. Here,
liquid formic acid decomposition on PtRuBiOx/C
catalyst was investigated at temperatures ranging from 80 to 140^oC at
pressure up to 350 psi. [7] It was found that the selectivity towards
dehydrogenation reaction remained almost 100% and a complete conversion of
formic acid was achieved in a short period. The overall activation energy was
found to be 78 kJ/mol. This is higher compared to 37
kJ/mol reported earlier from initial rates in a reactor with gases exiting to atmosphere.
The increase in E_a was due
to different conditions for its determination. In the atmospheric pressure
reactor, E_a was determined
from initial rates and found to be dependent on fresh formic acid
concentration, as shown in Fig. 1. On the other hand, E_a was determined at 20% to 70% conversion, hence, lower
formic acid concentration and higher saturation of carbon dioxide. The
desorption energy of CO₂ required contributes to a large part the
increase E_a.

Investigations of rate laws, possible roles
of elements in PtRuBiO_x/C and reusability
of the catalyst were made.  Kinetics
measurements were also made in a continuous flow reactor[8].
In a batch reactor, the raw law based on initial rates was linear with respect
to either molecular formic acid or formate ion.  From steady-state rates in a continuous flow
reactor, however, the reaction was also linear with formate
ion but half-order with respect to molecular formic acid. A possible role of
bismuth oxide is reduce CO affinity. As shown in Table
1, CO chemisorptions measurements show a much lower CO adsorption on PtRuBiO_x on carbon versus PtRu
on carbon. The stability of the catalyst was confirmed by performing a series
of repeated runs. 

Fig.
1.  Activation energy of formic acid decomposition from semi-log plot
of initial rates vs 1/T in an atmospheric pressure
reactor.

Table 1 Carbon monoxide chemisorption measurements on catalysts with and without BiO_x.

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References

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