(542g) Kinetic Modelling of Hydrogen Production By Catalytic Ammonia Decomposition

Hydrogen production by ammonia decomposition is attracting attention recently as single-step hydrogen generation process. Currently, the technical difficulties involved in the use of hydrogen as an energy carrier are mainly related to transportation and storage, which make the incorporation of this technology slower than initially expected. Among the main competitors of hydrogen as an energy carrier, methane/natural gas and methanol offer clear advantages in terms of energy capacity and distribution infrastructure, but suffer from their intrinsic carbon content causing end-user CO₂ emissions, since in-situ capture is not feasible. However, ammonia combines an easy way to store hydrogen and supply carbon-free stream to the fuel cell stack. In addition, ammonia has other desirable characteristics that make it potentially attractive. It is non-flammable and non-explosive, it can be liquefied under mild conditions, and it has a large weight hydrogen fraction (17.65% of the mass of ammonia). In order to developing efficient reactors for ammonia decomposition for fuel cell applications it is necessary to have realistic kinetic models based on the reaction mechanism.

The ultimate goal of this work is to develop consistent kinetic models able to predict the performance of reactors, like the monolithic reactors, where the NH₃ decomposition is almost complete attaining high H₂ concentrations at the exit. Under these conditions, in addition to the high temperatures needed to attain such large conversions, the gas composition change dramatically along the reactor. Several reaction mechanisms have been suggested, but there is still no consensus about the rate determinant step (RDS), or about the most abundant reactive intermediate (MARI). In spite of the large research effort developed over decades into this reaction, there is no general agreement about which step(s) are the RDSs, or even whether there is a single RDS, and which surface species are kinetically relevant.

The kinetic models developed in this contribution are based on the Langmuir hypothesis considering that all the adsorbed species can be kinetically relevant, that the slow step or steps can be partially reversible, and that the surface can be considered as energetically uniform, i.e. ideal. The results obtained here indicate that the variable values of the kinetic orders and of the apparent activation energies frequently reported in the literature can be direct consequences of the methodology used in the data analysis and can therefore also be explained without considering any change in the controlling step with the reaction temperature or in the hydrogen or ammonia concentration.