Industrial & Engineering Chemistry Research, Vol.58, No.24, 10238-10248, 2019
Accumulation Dynamics as a New Tool for Catalyst Discrimination: An Example from Ammonia Decomposition
Transient kinetic experiments were used to investigate the microkinetic model of ammonia decomposition over iron, cobalt, and a bimetallic preparation of the two. Pulse response experiments of ammonia were performed using the temporal analysis of products (TAP) technique, and the Y-Procedure analytical method was used to calculate the time dependence of the transformation rates of NH3, H-2, and N-2. From this, a dynamic accumulation of surface species could be determined, and a region of coherent time behavior was identified for H- and N-storage on the surface. We found that the materials regulate surface species differently, with iron supporting hydrogenated species and materials with cobalt supporting dehydrogenation. These observations, together with the coherent time behavior, motivated a reduction of the kinetic model. The first reduction was based on the observation of hydrogen release behavior (both fast and slow processes in the case of cobalt-containing materials) from the surface, the second reduction was based on analysis of the H/N surface accumulation indicating negligible (quasi-steady-state species) versus dominant surface species. Simplified models are proposed such that each step can be verified by gas phase data. Together with the dynamic accumulation data, the rate coefficients of each reaction step were presented as a function of surface N species. This provides a unique window for comparing the intrinsic behaviors of materials as they regulate surface species and control chemical reactions. Through the example of ammonia decomposition, we present the observation of accumulation dynamics as a new method for comparing fundamental chemicokinetic behavior of complex catalytic surfaces.