Author:

Abstract:

Particulate matter (PM) and nitrogen oxides (NOx) emitted from power plant or mobile vehicles are responsible for air pollution and ever more stringent emission limits are proposed. Removal of particulate matter and NOx from diesel exhaust gases is currently achieved via a diesel particulate filter (DPF) and a NOx reduction catalyst (NOx storage and reduction catalyst (NSRC) or selectively catalytic reduction (SCR) catalyst). Particulate matter is trapped in the filter while the exhaust gases pass through the walls of the filter. Due to the increasing back pressure, particulate matter needs to be removed periodically by oxidizing trapped particles into CO2 and H2O. Nitrogen dioxide (NO2, “passive regeneration”) and O2 (“active regeneration”) are used as the oxidants for carbon (main component of particulate matter) oxidation during the filter regeneration. However, reactions of carbon with NO2 or O2 occur at high temperatures over 250 oC and 500 oC respectively. The decreasing trend of vehicle exhaust gas temperature (below 250 oC) makes these reactions unfavorable. Additional heating to increase the exhaust temperature is achieved by combustion of fuel injected in the exhaust gas, representing a fuel penalty. Nitric oxide (NO) is the main product after reaction of carbon with NO2 necessitating the implementation of a selective catalytic reduction catalyst after the filter for NO reduction to dinitrogen. Selective catalytic reduction uses a vanadium catalyst or iron/copper zeolite to reduce NOx in an oxidative atmosphere using ammonia as the reductant. In this work, photocatalysis which does not need thermal activation is investigated for simultaneous soot oxidation and NOx reduction in simulated exhaust gases. A homemade gas phase photoreactor was used to simulate the composition of exhaust gas and monitor the relevant components (NO, NO2, N2O, NH3, CO2, CO). First, the photocatalytic activity of titanium dioxide (TiO2) under UVA light has been investigated in carbon oxidation using nitric oxide as the only oxidant. Titanium dioxide is a widely investigated photocatalyst for water splitting, removal of organic compounds and self-cleaning materials. Both photocatalytic soot oxidation and NOx removal has been reported using titanium dioxide. However, photo-oxidation of carbon using nitrogen oxides has never been reported before. In this study, photocatalytic carbon oxidation with nitric oxide in the absence of oxygen was investigated on TiO2 surface at relatively low temperature (150 oC). In the absence of oxygen carbon was photocatalytically converted mainly to carbon dioxide, and nitric oxide primarily reduced into dinitrogen. Carbon oxidation rates and NO conversions under different reaction conditions were compared. The highest carbon oxidation rate of 2.0 μg carbon per hour per mg TiO2 with 98% NO conversion was achieved in the presence of 3,000 ppm NO. The effects of water vapor and NO concentration on carbon photo-oxidation rate was thoroughly studied. The addition of water enhances nitrate formation and decrease the NO reduction rate but shows little effect on CO2 formation. Under the investigated experimental conditions catalyst deactivation caused by nitrate formation is negligible. The main drawback is the low reaction rate, which required the experiments to be performed under static conditions rather than under flow through conditions. In the second part of this work, the focus shifted to the NO reduction effects. NO can be continuously reduced to N2 in the presence of solid carbon on TiO2 photocatalyst surface at low flow rate (5 ml/min) under UVA illumination. Batch mode experiments were also performed to investigate the nitrate formation on photocatalyst surface. Nitrate is formed on photocatalyst surface from the reaction of nitric oxide with photo-generated radicals. Samples after reaction were characterized by Fourier Transform Infrared spectroscopy (FTIR) and the nitrate formation on catalyst surface was quantified using a nitrate test kit. The reaction mechanisms of photo-SCR using soot and nitrate formation or decomposition are discussed. NO2, nitrous and nitric acid are assumed to be formed and transferred through the gas phase to carbon surface. These reactive species contribute to the carbon oxidation process. The temporary formation of N2O could be related to the reaction of NO with Ti3+ sites on photoactivated TiO2 before it reaches steady state saturation with nitrate. Formation and decomposition of nitrate can be a dynamic process. HNO3/NO3- can be photocatalytically transformed back to NO2 via the reaction of nitrate and NO in the presence of TiO2 under illumination. The TiO2/carbon sample before and after reaction were characterized by High-Resolution Scanning Electron Microscopy (HR SEM) providing a visual evidence of photocatalytic carbon oxidation. Photo-SCR of NOx using soot achieves the same target as NOx reburn via the reverse prompt NOx reaction in coal combustion. The concept of using soot for reducing NOx is an attractive concept, removing soot and NOx simultaneously. Further research is needed to increase the reaction rate for practical applications. In the last part of this work, the improvement of the carbon oxidation rate was further studied in the presence of oxygen. Photocatalytic oxidation of the most reactive carbon fraction proceeds equally well with O2 and NO. The combination of nitric oxide, oxygen and water was found to be effectively increasing the CO2 formation rate especially in the later phase of the reaction. Therefore a synergetic effect of NO and O2 on refractory carbon photocatalytic oxidation is observed. NO is more effective than O2 on photocatalytic oxidation of the more refractory part of the carbon. During the reaction, carbon is oxidized mainly to CO2 while NOx is mainly reduced to N2. Enhanced O2 and NO concentrations in the gas phase have a positive effect on the carbon oxidation rate. At optimum gas composition with 3,000 ppm NO and 13.3% O2 the highest carbon oxidation rate reaches 5.64 µgcarbon/mgTiO2 h, with formal electron/photon quantum efficiency of 0.052. Nitrate was formed during the reaction. A long term reaction provides the evidence that the catalyst deactivation caused by nitrates is limited. HR SEM was used to observe the carbon particles morphology change during carbon oxidation with oxygen as the only oxidant. Differences of carbon particles morphology change between the reactions with or without NOx are observed in the HR SEM measurements. The carbon particles were oxidized more significantly and uniformly in the presence of nitric oxide compared to the reactions using O2 as the only oxidant under identical reaction conditions. It is evident that the oxidizing species generated in the presence of nitric oxide can migrate longer distance than the hydroxyl radicals or oxygen related radicals which are formed in the presence of oxygen and water.