Start
May 08, 2014 - 17:00
End
May 08, 2014 - 18:00
Venue
Room 118 Chemical Engg.
Event Type
Speaker
Rucha Paranjpe PhD Student Chemical Engg. IITB
Title
Detailed Modeling of Catalytic Reduction.
Abstract Selective reduction of NO_X to N_2 is the most important reaction occurring in automobile emission control devices. The three-way catalytic converter (TWC) is one such extensively used device in automobiles to treat exhaust gas emissions. The main chemical reactions occurring in the TWC are the oxidation of carbon monoxide and hydrocarbons to CO_2 and H_2 O; and the reduction of nitrogen oxides to N_2 . TWC employs metals Pt and Rh in various ratios as the catalyst. The other specific aspect of TWC is that these catalyst metals are present as pure metals and alloy formation is avoided. It is reported that such a physical mixture of Pt and Rh exhibits synergistic NO reduction activity. Though this system is widely studied in literature the exact reason behind this synergism is not explicitly known. In this work we have explored this aspect with the help of microkinetic model simulations and few experiments. The NO reduction system is complicated due to multiple reducing agents and catalysts working simultaneously. The reactions NO+CO NO+H_2 and NO+CO+H_2 on single metals Pt and Rh and bimetallic Pt+Rh catalysts in the form of physical mixtures are studied by developing microkinetic models from literature and CHEMKIN PRO^® simulations for Perfectly Stirred Reactor (PSR) model. A few fixed bed experiments on NO+CO/ Pt+Rh system are also performed.The some of the important finding are as follows: The activity of the catalyst depends upon two aspects the activation energy required for NO dissociation and for the reaction between reducing agent and adsorbed oxygen. From the microkinetic model for NO+CO reaction it is observed that CO_2 formation has almost same activation energy on both Pt and Rh (~ 23 kcal/mol) but NO dissociation activation energy is almost half on Rh (6kcal/mol) of that on Pt (13 kcal/mol). This explains why Rh is more active than Pt for NO+CO reaction. When a physical mixture of Pt and Rh is used the overall activity is dominated by Rh. Similarly for NO+H_2 reaction from the microkinetic model it is observed that activation energy for OH_S formation (precursor for water) is low on Pt (8 kcal/mol) than on Rh (14 kcal/mol) and hence Pt is more active than Rh for NO+H_2 reaction. When a physical mixture of Pt and Rh is used the mixture activity is dominated by Pt. When both CO and H_2 are present then the H_2 acts as the preferred reducing agent.