(304c) Understanding NH3 Formation in Three Way Catalytic Converter Through Microkinetic Modeling: Effect of Co-Existence of Pt and Rh

The key reaction occurring in a three way catalytic converter (TWC) is reduction of NO with CO or HCs to give N₂. But most of the times traces of NH₃ are also found as a secondary pollutant in the converter outlet. NH₃ is involved in the atmospheric formation of secondary aerosols and hence is undesirable. Here, the indirect source of hydrogen is water or hydrocarbons. In this work we are exploring role of water in the converter chemistry as the automobile exhaust contains ~10% of water vapor. The traditional converter employs noble metals Platinum (Pt) and Rhodium (Rh) as the catalysts in different weight ratios. The noble metal catalysts present in the converter can catalyse water gas shift reaction (CO + H₂O ↔ H₂ + CO₂) to give hydrogen. Although both Pt and Rh are present in the TWC, it is believed that alloying doesn’t occur to a significant extent.

We have used microkinetic modeling as a tool to study NH₃formation process on Pt and Rh and their physical mixture with different weight ratios representing the TWC. The NO reduction reactions can be written as

2NO + 2CO → N₂ + 2CO₂

2NO + CO → N₂O + CO₂

2NO + 5H₂ → 2NH₃ + 2H₂O

The microkinetic model comprising of total 62 elementary steps for adsorption-desorption of various species, and surface reactions is developed. The pre-exponential factors, sticking coefficients and the activation energies are either taken from literature or calculated using semi-empirical Unity Bond Index-Quadratic Exponential Potential (UBI-QEP) method for Pt(111) and Rh(111) surfaces separately. The model thus developed is simulated using the software CHEMKIN PRO^®. The pre-exponential factors are optimized for relevant literature experimental data for NO-CO-H₂O system. The activation energies are not altered. While simulating for the TWC, both the mechanisms (Pt ans Rh) are fed to the simulator. The pre exponential factors are scaled as per the ratio of Pt and Rh.

If single metals Pt and Rh are evaluated then NO+CO reaction starts at ~ 180⁰C on Rh and 250⁰C on Pt while NO+H₂ and CO+H₂O reactions start above 300⁰C for both.

In the presence of both Pt and Rh, the steady state coverages of surface species change than those when single metal is present. This changes the rates of different elementary steps and overall activity and selectivity of the catalyst changes. The product distribution (N₂:N₂O:NH₃) depends upon the relative amounts of Pt and Rh. The simulations will be performed for various Pt:Rh to get optimum catalyst formulation such that N₂ formation is maximized.