Microkinetic Modeling of Ammonia Synthesis and Decomposition on Ruthenium and Microreactor Design for Hydrogen Production A detailed reaction mechanism for ammonia decomposition on Ru has been developed. Specifically, the multistep methodology described in Aghalayam et al. 4 is employed to construct a surface reaction mechanism for ammonia decomposition on Ru. In particular, 6 reversible elementary steps for ammonia decomposition are laid down, which include adsorption/desorption of NH 3 , N 2 , and H 2 , and hydrogen abstraction steps from NH x intermediates and their reverse. Order of magnitude estimates of pre-exponential factors are determined using Transition State Theory (TST) 5 , whereas activation energies are calculated using the semi-empirical Bond Order Conservation (BOC) technique 6,7 with input from quantum mechanical density functional theory (DFT) as well as surface science experiments. Adsorbate-adsorbate interactions, shown in Table 1, are also taken into account through the BOC framework in desorption and surface reactions. The mechanism, shown in Table 2, is enthalpically consistent at 300 K. The entire semi-empirical framework has been integrated with surface CHEMKIN to enable transparent, easy use in computing the reaction rates. Simulations have then been carried out and compared to targeted experiments in order to slightly optimize the reaction mechanism against uncertainties. The reaction mechanism is able to predict well atmospheric ammonia decomposition and high-pressure ammonia synthesis data, along with various ultra-high vacuum temperature programmed desorption (TPD) and reaction (TPR) data. Soumitra R. Deshmukh, Ashish B. Mhadeshwar and Dionisios G. Vlachos Department of Chemical Engineering and Center for Catalytic Science and Technology (CCST) University of Delaware, Newark, DE 19716-3110 Introduction The overall objective of this work is to develop design rules for production of hydrogen from ammonia at the small scale. While ammonia synthesis is one of the most extensively studied processes in catalysis, due to its use in the manufacturing of fertilizers, ammonia decomposition has only recently become of interest due to its potential use in CO-free hydrogen production for fuel cells. Ruthenium is one the better catalysts for the latter but the kinetics of decomposition is not as well understood. Noting the shortcomings of the literature models, we develop a complete microkinetic model to describe the ammonia synthesis as well as decomposition on Ru. An easy to implement reduced model is developed using a novel computer-aided reduction methodology. Computational fluid dynamics (CFD) simulations are then performed in a microreactor. Optimization of a microreactor for hydrogen production for fuel cell applications is also discussed. Table 1. Heat of Chemisorption Data. θ indicates coverage of surface species. Chemistry Modeling Species Heat of chemisorption (kcal/mole) References N* 135-35θ N 8, 9 H* 63 10, 11 NH 3 * 18.23 10, 11 NH* 86.79 Our BOC calcns. NH 2 * 59.95 Our BOC calcns. Although ammonia synthesis ( 2 ) is one of the better-studied reactions in the literature, decomposition kinetics on Ru is not as well established. Partial mechanisms and reduced rate expressions have been proposed 2 2 3 3H N NH + ↔ 1,2 to qualitatively describe experimental trends. These reduced rate expressions have been derived based on the traditional Langmuir-Hinshelwood approximations where a posteriori validation has occasionally been done. As seen in Fig. 1, these models fail to capture the experimental data in a microreactor. Table 2: Screening surface reaction mechanism of ammonia decomposition on Ru at 300 K. 0 20 40 60 80 100 600 650 700 750 800 850 900 950 1000 % NH 3 conversion Temperature [K] reduced model full model exptl. data Ganley et al. models from Bradford et al. (1997) model from Tsai and Weinberg (1987) The activation energies are in kcal/mol, calculated in the zero coverage limit (θ * =1). All activation energies are coverage dependent as determined from the BOC framework (see Table 1). No. Reaction Sticking coefficient or pre-exponential [s -1 ] Activation energy at θ * =1 1 * 2 2* H 2 H → + 1 1.9 2 * 2 H 2H* 2 + → 1.0 ×10 13 23.7 3 * N 2 * 2 N 2 → + + 1 6.2 4 * 2 N 2N* 2 + → 1.0 ×10 13 50.3 5 * N * H * * NH + → + 1.0 ×10 11 5.8 6 * * NH N* * H + → + 1.0 ×10 11 37.2 7 * * 2 NH * H * NH + → + 1.0 ×10 11 19.1 8 * NH NH * H 2 + → + * * 1.0 ×10 11 17.4 9 * * 2 3 NH * H * NH + → + 1.0 ×10 11 17.5 10 * NH NH * H 3 2 + → + * * 1.0 ×10 11 13.2 11 * 3 3 NH * NH → + 1 0 12 * NH NH 3 3 + → * 1.0 ×10 13 18.2 Figure 1. PFR simulations with full and reduced reaction models compared to the experimental data of Ganley et al. 3 The reaction model can describe faithfully the experimental data without any adjustment in rate parameters. The reduced reaction model is in very good agreement with the full model and the experimental data. Literature, reduced reaction models are not as accurate. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2003, 48(2), 936