Ground level ozone produced by the NOx emitted from lean burn and diesel engines is the main driving force for the active research in lean NOx reduction. In stoichiometric gasoline vehicles, the three-way catalytic converters efficiently convert the NOx. Accomplishing this reduction in compression ignition engines fueled by diesel is more difficult because of the presence of excess O2, which inhibits the reduction of NOx in the conventional three-way catalytic converter. Several NOx reduction technologies are under development such as selective catalytic reduction (SCR) using urea, NOx storage and reduction (NSR).
The SCR of NOx by nitrogen compounds such as ammonia or urea has been developed in 1970s and since been successfully used in stationary industrial applications as gas turbines, coal-fired cogeneration plants. In stationary source applications anhydrous ammonia (NH3) is injected into the flue gas containing NOx. However the application of SCR to mobile diesel engines requires several problems to be overcome like, storage of ammonia for injecting into the flue gas and also this process requires precise control of the ammonia injection rate in order to avoid ammonia slip. NSR process on the other hand is a two stage process taking place in the “lean NOx trap” (LNT) catalyst comprising a bifunctional monolithic catalyst of precious metals (Pt, Rh, Pd) used for NO oxidation and reduction, and an alkaline earth metal in the form of oxide, nitride or carbonate (Ba, K, Sr, etc.), used to store NOx. In the first stage of the NSR process, NO is oxidized on Pt to NO2 which then reacts with alkaline earth metal forming nitrites and nitrates. The second stage is commenced before NOx breaks through and this stage consists of a short purge of reductant (CO, H2, NH3, Hydrocarbons).
Considerable research is being done in developing technologies that combine both lean NOx traps (LNT) and SCR after-treatments systems. This type of configuration includes a diesel oxidation catalyst (DOC), followed by a LNT, a diesel particulate filter (DPF) and a SCR catalyst. The basic idea involved is that during lean exhaust conditions (lean storage cycle), the LNT stores NOx and during the rich exhaust conditions (purge cycle), the stored NOx is converted to N2 and NH3 is produced in addition to N2 and N2O. This NH3 is stored by the downstream SCR catalyst and is used to convert remaining NOx that did not react with the LNT. NOx reduction thus takes place in two stages: first by the LNT and second by the SCR catalyst, with NH3 supplied from the LNT system.
The focus of the current study is on the NSR process. Knowledge of the steady-state behavior of the NSR catalysts is essential for understanding the more complex two-step storage and reduction process. In particular knowledge of the pathways and kinetics of NH3 generation and consumption is critical in the design of NSR catalysts for conventional LNT or emerging LNT-SCR technology. The production of NH3 is known to occur by reaction between NO and H2 under the rich regeneration conditions on precious metals. A second pathway is by reaction between NO, CO, and H2O via a NCO-Pt or HNCO intermediate. The contribution of the second pathway is of significance because it proceeds without gas phase H2, which is present in lower concentrations in diesel exhaust.
To this end, comprehensive steady-state experiments of CO-NO, CO-NO-H2, CO-NO-H2-H2O reaction systems on Pt/BaO/Al2O3 monolithic catalyst have been carried out to evaluate the NOx conversion and product distribution features as a function of the feed composition and temperature. The reaction of NO and CO produces a mixture containing N2O, N2 and CO2, the composition of which is a function of the catalyst temperature and NO/CO ratio in the feed. Introduction of H2 and H2O into this system gives additional products which include NH3 and also effects the product distribution. Experiments were also conducted to study the effects of various factors like reaction temperature, reactant concentration and space velocity on the production of ammonia. The kinetic data obtained are interpreted through the expansion of existing literature models.