Studies on the reduction of Nitrous Oxide formation in NOx-trap Catalysts

Abstract

Current society has become more concerned about being environmentally friendly. Catalytic gas after treatment is one of the solutions adopted to reduce pollutant emissions from a combustion engine. A Three Way NOx Storage Catalytic Converter (TWNSC) is a new development of Daimler AG together with Umicore AG [1]. It consists of a Catalyst with some of the main properties of a Three Way Catalyst (TWC) together with NOx storage capacity (lean-NOx trap). This catalyst is used in Otto direct-injection engines with lean/rich operation mode. This technology can reduce fuel consumption in a range of 10%. During lean engine operation time, high quantities of Nitrogen Oxides (NOx) are generated. In presence of a TWNSC, this NOx can be stored. When the engine changes to rich operating mode, the amount of NOx in exhaust gases decreases become rich of unburned hydrocarbons (HC), hydrogen (H2) and carbon monoxide (CO) that can reduce the NOx stored. However, during NOx reduction, formation of undesired byproducts occur. That is the case of nitrous oxide (N2O) and ammonia (NH3). In this Master thesis, studies on the reduction of nitrous oxide formation in Three Way NOx Storage Catalytic Converter are performed. Studies on N2O formation during catalyst performance have not been widely studied and published. In this master thesis, lean/rich experimentations on two new TWNSC (catalyst A and B) are performed to find conditions in which N2O formation can be reduced. Experiments are performed in a test bench where lean gases are provided by a 1- cylinder-engine and rich gases from synthetic gas mixtures. At the beginning of the master thesis, two preliminary investigations are performed. The first consists of the calculation of Oxygen Storage Capacity (OSC) of a cylindrical sample (25 mm diameter, 30 mm length) of catalyst A and B. The results of the experiment show that catalyst B has less Oxygen Storage Capacity. The experiment consisted on applying a flow of 12,5 l/min of Oxygen (O2) in nitrogen (N2) (0,4% by volume) through the previously reduced sample. An average of 0,3 g./l.cat. less oxygen is stored in catalyst B for temperatures of 300, 350 and 400 ºC. At 300ºC, catalyst A stores 1,44 g/l.cat. compared to the 0,93 g/l.cat. in catalyst B. The second preliminary investigation consists of determining the temperature in which the Diesel Oxidation Catalyst (DOC) in reactor 1 has to operate. The objective of this DOC is to oxidize the HC and maintain the original NO2/NOx ratio from the engine exhaust gases. During lean mode, gases from a 1-cylinder-engine (Hatz-motor [2]) are used. NOx and HC concentrations are analyzed for a range of temperatures from 150 to 650 ºC. It is concluded that a temperature of 620 ºC has to be reached in reactor one to get rid of HC and maintain the NO2/NOx ratio of the bypass exhaust gases (2% of NO2 in NOx). After the preliminary investigations, the first objective is getting to know the basic performance of the two different TWNSC. Lean/rich experimentations are performed on both samples A and B at the range of temperatures from 150ºC to 450ºC. Lean/rich timing is set on 120/15 seconds respectively. In addition, three different rich gas mixtures (lambdas 0,95, 0,9 and 0,82) have been used for the rich mode. Results show that for lambda 0,95 less N2O is generated (0,06 g/l.cat. at 300ºC in catalyst A). The minimum N2O detected is at catalyst B at temperatures of 400 and 450ºC (0,01 and 0,00 g/l.cat.). The main part of the Master Thesis consists of four different experimentations that have the objective to find any reduction in N2O formation: 1. N2O formation studies with lean/rich experimentations at modified TWNSC catalysts. Instead of the 30 mm sample previously used, two 15 mm samples are used together. Modifications are applied on the first 15 mm sample and consist on five perforations (2 mm diameter) and the introduction of an uncoated central part section. These modifications try to increase reductants velocity during rich mode. Results show a decrease in N2O formation in the experiment with 15 mm uncoated catalyst A together with another 15 mm catalyst A. An average of 2,8 g/l.cat. of N2O reduction is obtained at temperature of 300ºC. In addition, an increase of NOx conversion efficiency has been detected: for the same sample and temperature an average increase of 20% NOx performance 2. N2O formation studies with lean/rich experimentations at a combination of catalysts A and B together. It is concluded that the combination of catalyst A and B does not have a beneficial effect on N2O formation. 3. N2O formation studies with lean/rich experimentations with variation of rich time period. The objective is to see if the reduction of rich time period has an effect on N2O formation. 4. Lean/rich experimentations with variation of the lambda during rich period. The objective is to see if a reduction in N2O is obtained with these variations. For low temperatures (150ºC and 200ºC) a diminution in N2O formation is appreciated (0,05 g/l.cat to 0,04 g./l.cat at 150ºC for 30 mm TWNSCA with uncoated section). This Master Thesis represents a base line study for further investigations on N2O formation on TWNSC. Catalyst modifications are a feasible solution for N2O diminution as well as NOx conversion efficiency. These results encourage further experimentations with these current and other new catalyst modifications. Variation of lambda during rich period and variation of the rich time period are variables that can have a relevant role.