New adsorptive, catalytic and photocatalytic approaches for elimination of NOx and NH3 from a gas stream

Internal combustion engines are important sources of nitrogen oxides, carbon monoxide, unburned hydrocarbons and particulate matter in the troposphere. Environmental concern is leading to ever more stringent emission legislation and innovation in catalytic combustion aftertreatment systems to reduce tailpipe emissions. Modern fuel efficient lean burn gasoline and diesel engines produce oxygen rich exhaust gases. This makes the reduction of nitrogen oxides chemically very demanding. Two NOx elimination strategies have been developed to deal with this problem. The NOx storage and reduction (NSR) concept uses supported BaO as adsorbent to store nitrogen oxides temporary as nitrates. After saturation of the adsorbent the engine operation is changed to rich conditions and reductants are generated. Stored nitrates are reduced into harmless N2 during these short rich excursions of the engine. Drawbacks of the NSR technique are the gradual degradation of the NOx uptake ability of the barium based adsorbent due to reaction of BaO with sulfur compounds present in the exhaust gas. The regeneration process involving reductants also leads to enhanced fuel consumption. A second widespread NOx elimination strategy is Selective Catalytic Reduction (SCR). Ammonia is used to reduce nitrogen oxides into N2 in the oxygen rich exhaust gases. For safety reasons aqueous urea solution instead of ammonia is being used as a source of ammonia in automotive applications. The emission of unreacted NH3 has to be avoided and therefore SCR units are operated using substoichiometric NH3/NO ratios. This limits the total NOx removal efficiency. In this work the potential of heteropolyacid adsorbents, NH3 oxidation catalysts and photocatalysts in exhaust gas treatment has been investigated.An experimental set-up was built simulating the composition of exhaust gas and enables on-line monitoring of relevant components (NO, NO2, NH3, N2O, CO, CO2, SO2). In a first part of the experimental work the potential of heteropolyacids (HPAs) for adsorbing NOx was explored. Keggin type heteropolyacids consist of a central tetrahedral atom (P, Si), surrounded by octahedrally coordinated metal oxygen clusters. The metal ion is usually Mo or W. The negative charge of the Keggin ions is balanced with protons, creating a 3D crystalline network. Out of a variety of heteropolyacids tungsten containing compounds were identified as reversible NOx adsorbents. The phosphotungstic acid (H3PW12O40) was especially performing well. Co-adsorption of NO and NO2 was observed in the temperature window 120-275°C. Large amounts of nitrogen oxides (ca. 4 wt%) could be reversibly stored by substitution of crystal water. Via thermogravimetric analysis it was shown crystal water is more strongly retained on the tungsten containing HPAs compared to molybdenum based HPAs. This explains why the latter heteropolyacids do not adsorb NOx. The mechanism of NOx adsorption on the H3PW12O40 heteropolyacid was identified with a variety of experimental and theoretical techniques. Via FT-IR analysis adsorbed NOx was found to be the nitrosonium ion (NO+). The structure of heteropolyacids loaded with nitrogen oxides was investigated with in-situ synchrotron X-ray diffraction and neutron diffraction. Structure refinement of the diffraction patterns confirmed NO+ species are trapped between the different Keggin ions. Density Functional Theory served as a tool to show NOx adsorption is entropy driven. HPAs might be used as NOx adsorbents in NOx abatement technologies requiring performant NOx adsorbents like Selective NOx recirculation (SNR).In a second part of this work catalysts for the selective catalytic oxidation of ammonia (NH3-SCO) were evaluated. Ruthenium oxide supported on Na-Y zeolite was identified as highly active and selective catalyst for this reaction. RuO2 supported on Na-Y is capable of converting trace amounts of NH3 in the presence of O2, CO2 and H2O selectively into N2 and H2O at temperatures as low as 175°C. The complete conversion of ammonia into N2 at low reaction temperatures is important because NH3-SCO catalysts are used at the end of the exhaust gas purification system. The peculiar reactivity of this catalyst is attributed to the presence of dispersed RuO2 at the outer surfaces of the zeolite and the presence of Brønsted acid sites in the zeolite pores. These sites can act as NH3 trap at low reaction temperatures.When the active RuO2 phase was dispersed on supports lacking strong Brønsted acid sites the activity at low temperature was substantially lower. This type of catalyst could find application as clean-up catalyst downstream of SCR units where unreacted NH3 must be converted into N2 and H2O. NH3-SCO catalysts are also used in various applications requiring removal of ppm levels of ammonia in waste gases.In a last part the potential of photocatalysts in exhaust gas purification reactions was explored. As the average exhaust gas temperature in automotive applications in new generations of car engines tends to decrease (e.g. due to the use of turbocharging) photocatalysts might offer superior low temperature activity compared to conventional thermal catalysts. Commercial TiO2 based catalysts were evaluated in several exhaust gas treatment reactions: oxidation of nitrogen monoxide, selective catalytic oxidation of NH3 and selective catalytic reduction of NO with NH3. Oxidation of only small amounts of NO into NO2 was observed over the TiO2 based catalysts. However, photo-assisted NH3-SCO was found to be very promising as complete NH3 conversion and high N2 selectivities (>90%) were observed at 150°C. The high NH3 oxidation activity limits the potential of these TiO2 based materials in photo-assisted SCR reactions. However, the activity at low temperature in NH3-SCO reactions makes photocatalysts interesting alternatives to thermal NH3-SCO catalysts.

Jaar van publicatie:2012

Toegankelijkheid:Closed