Thursday, November 11, 2010: 1:54 PM
251 D Room (Salt Palace Convention Center)
Formic acid decomposition has been investigated over the decades using catalysts of metals, metal oxides, and metal supported on oxides. The reaction can also proceed without catalysts under hydrothermal conditions to temperatures of 280–330 °C and a pressure of 275 bar . Recent studies have been motivated by the interest of generating hydrogen to feed fuel cells and lowering the temperature of the water–gas-shift process. Selective dehydrogenation of aqueous formic acid at near ambient temperature was observed with soluble metal complexes of ruthenium [2-3] and rhodium . An inert atmosphere and the presence of an amine may be required. The recent report of Pd–Au and Pd–Ag catalysts shows good decomposition rates at near ambient temperature . It is desirable to have an insoluble catalyst for fixed bed flow-through operations and that excess water from aqueous formic acid feed can be removed without draining the catalyst. We report here a bismuth containing catalyst composited with platinum and ruthenium. This heterogeneous catalyst decomposes formic acid in liquid water at ambient temperature with high rates, yielding exclusively hydrogen and carbon dioxide with a very low activation energy. The catalysts were prepared by the citric acid gel method using metal precursors of RuCl3.xH2O, H2PtCl6.H2O, and Bi(NO3)3.5H2O dissolved in water and with citric acid added as a protecting agent. Vulcan carbon 72 XC was added to absorb the metal precursors. The water was evaporated and the powder was heated at 850°C in inert atmosphere to obtain the final product catalyst. The catalyst was characterized by Philips Tecnai G2 TEM and XPS. Decomposition of formic acid experiment was performed with premixed formic acid solution heated to the desired temperature and added to a stoppered Erlenmeyer flask containing the catalyst and immersed in a water bath at constant temperature. The gas product composition was analyzed by a GC from Perkin Elmer equipped with a thermal conductivity detector (TCD). Upon contacting the PtRuBiOx catalyst at room temperature, aqueous formic acid shows steady gas evolution. The product gas was continuously analyzed and contained equi-molar hydrogen and carbon dioxide without detection of carbon monoxide. A typical 7 h gas evolution from an 80 mL 15% v/v formic acid in water immersed with 400 mg catalyst of 30% metal loaded carbon is shown in Fig. 1. The high selectivity and high reaction rate of the PtRuBiOx catalyst at ambient conditions show promise for a convenient hydrogen generation device. Further increase in reaction rates is possible with a continuous flow-through reactor packed with the heterogeneous catalyst. The low activation energy of the reaction and the unique role of bismuth oxide suggest new perspectives in the mechanisms of formic acid dehydrogenation in the aqueous phase. References 1. S.W. Ting, S.A. Cheng, K.Y. Tsang, N. van der Laak, and K.Y. Chan, Chem. Commun., 2009, DOI: 10.1039/B916507J 2. C. Fellay, P. J. Dyson and G. Laurenczy, Angew. Chem., Int. Ed., 2008, 47, 3966 3. B. Loges, A. Boddien, H. Junge and M. Beller, Angew. Chem., Int. Ed., 2008, 47, 3962 4. S. Fukuzumi, T. Kobayashi and T. Suenobu, ChemSusChem, 2008, 1, 827 5. X. Zhou, Y. Huang, W. Xing, C. Liu, J. Liao and T. Lu, Chem. Commun., 2008, 3540.