279100 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 PtRuBiOx/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  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 140oC at pressure up to 350 psi.  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 Ea was due to different conditions for its determination. In the atmospheric pressure reactor, Ea was determined from initial rates and found to be dependent on fresh formic acid concentration, as shown in Fig. 1. On the other hand, Ea 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 Ea.
Investigations of rate laws, possible roles of elements in PtRuBiOx/C and reusability of the catalyst were made. Kinetics measurements were also made in a continuous flow reactor. 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 PtRuBiOx 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 BiOx.
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