Ammonia decomposition kinetics over Ni-Pt/Al2O3 for PEM fuel cell applications

Due to the potential employment of ammonia as a hydrogen source for proton exchange membrane (PEM) fuel cell applications, an experimental study has been undertaken to determine the ammonia decomposition rates over a Ni-Pt/Al2O3 catalyst under conditions likely to be employed in such an application. Using differential analysis of integral data, intrinsic rate data have been obtained between temperatures of 520 and 690 degreesC and at ammonia pressures between 50 and 780 Torr. It is shown that a first-order rate expression provides an adequate fit of the experimental data over the entire range of pressures and temperatures, although hydrogen inhibition effects may be significant at the lowest temperature. The activation energy was found to be on the order of 50 kcal/mol as opposed to the very low values (5-10 kcal/mol) obtained previously by others. The data are analyzed in the context of previous studies that were conducted under markedly different reaction conditions and differences are attributed to the types of catalysts employed and the fact that previous studies have primarily dealt with very low ammonia concentrations. Consequently, it does not appear as though N-H bond cleavage is the rate determining step under these conditions. It is also shown that the rate expression is capable of predicting, to within 5%, the high ammonia conversions required for fuel cell applications. (C) 2002 Elsevier Science B.V. All rights reserved.