Showing papers on "NOx published in 2012"

A superior Ce-W-Ti mixed oxide catalyst for the selective catalytic reduction of NOx with NH3

Abstract: A superior Ce-W-Ti mixed oxide catalyst prepared by a facile homogeneous precipitation method showed excellent NH3-SCR activity and 100% N2 selectivity with broad operation temperature window and extremely high resistance to space velocity, which is a very promising catalyst for NOx abatement from diesel engine exhaust. The excellent catalytic performance is associated with the highly dispersed active Ce and promotive W species on TiO2. The introduction of W species could increase the amount of active sites, oxygen vacancies, and Bronsted and Lewis acid sites over the catalyst, which is also beneficial to improve the low temperature activity by facilitating “fast SCR” reaction and enhance both of the high temperature activity and N2 selectivity simultaneously by inhibiting the unselective oxidation of NH3 at high temperatures.

Selective photoreduction of nitric oxide to nitrogen by nanostructured TiO2 photocatalysts: role of oxygen vacancies and iron dopant.

Abstract: Conventional TiO2-based photocatalysts oxidize NOx to nitrate species, which do not spontaneously desorb and therefore deactivate the catalyst. We show that the selectivity of this reaction can be changed by creating a large concentration of oxygen vacancies in TiO2 nanoparticles through thermal reduction in a reducing atmosphere. This results in the photoreduction of nitric oxide (NO) to N2 and O2, species which spontaneously desorb at room temperature. The activity of the photoreduction reaction can be greatly enhanced by doping the TiO2 nanoparticles with Fe3+, an acceptor-type dopant that stabilizes the oxygen vacancies. Moreover, the photoinduced reduction of Fe3+ to Fe2+ provides a recombination pathway that almost completely suppresses the formation of NO2 and thus enhances the selectivity of the reaction for N2 formation. Gas chromatography confirms that N2 and O2 are formed in a stoichiometric ratio, and the activity for NO decomposition is found to be limited by the concentration of oxygen vacan...

Low-Temperature Selective Catalytic Reduction of NOx with NH3 over Fe–Mn Mixed-Oxide Catalysts Containing Fe3Mn3O8 Phase

Abstract: Novel Fe–Mn mixed-oxide catalysts were prepared for the low-temperature selective catalytic reduction (SCR) of NOx with ammonia in the presence of excess oxygen. It was found that Fe(0.4)–MnOx catalyst showed the highest activity, yielding 98.8% NOx conversion and 100% selectivity of N2 at 120 °C at a space velocity of 30 000 h–1. XRD results suggested that a new crystal phase of Fe3Mn3O8 was formed in the Fe–MnOx catalysts. TPR and Raman data showed that there was a strong interaction between the iron oxide and manganese oxide, which is responsible for the formation of the active center―Fe3Mn3O8. Intensive analysis of fresh, used, and regenerated catalysts by XPS revealed that electron transfer between Fen+ and Mnn+ ions in Fe3Mn3O8 may account for the long lifetime of the Fe(0.4)–MnOx catalyst. In addition, the SCR activity was suppressed a little in the presence of SO2 and H2O, but it was reversible after their removal.

Influence of fuel properties and composition on NOx emissions from biodiesel powered diesel engines: A review

Abstract: Biodiesel has proved to be an environment friendly alternative fuel for diesel engine because it can alleviate regulated and unregulated exhaust emissions. However, most researchers have observed a significant increase in NO x emissions with biodiesel when compared to petrodiesel. The exact cause of this increase is still unclear; however, researchers believe that the fuel properties have been shown to effect the emissions of NO x . The present work reviews the effect of fuel properties and composition on NO x emissions from biodiesel fuelled engines. The paper is organised in three sections. The first section deals with the NO x formation mechanisms. In the following section, the reasons for increased NO x emissions of biodiesel fuel are discussed. After this, the influence of composition and fuel properties on NO x emissions from biodiesel fuelled engines has been reviewed. Finally, some general conclusions concerning this problem are summarised and further researches are pointed out.

Enhancement of Catalytic Activity Over the Iron-Modified Ce/TiO2 Catalyst for Selective Catalytic Reduction of NOx with Ammonia

Abstract: A series of iron-modified Ce/TiO2 catalysts with different Fe/Ti molar ratios were prepared by an impregnation method and used for selective catalytic reaction (SCR) of NOx with NH3. The Fe–Ce/TiO2 catalyst with a Fe/Ti molar ratio of 0.2 had good low-temperature activity and sulfur-poisoning resistance compared with the Ce/TiO2 catalyst. The introduction of Fe could increase the amount of Ce3+ and chemisorbed oxygen species on the catalyst surface and thereafter generate more ionic NH4+ and in situ formed NO2, respectively. In addition, the dispersion of cerium oxide could be improved by the addition of iron, and no visible phase of iron oxide could be observed at low Fe/Ti molar ratios (≤0.2). All of these factors played significant roles in the enhanced catalytic activity, especially the low-temperature activity. Furthermore, mechanisms of the SCR reaction and the SO2 poisoning of the Fe(0.2)–Ce/TiO2 catalyst were studied using in situ diffuse reflectance infrared Fourier transform spectroscopy. Coordi...

The Effect of Copper Loading on the Selective Catalytic Reduction of Nitric Oxide by Ammonia Over Cu-SSZ-13

Abstract: The effect of Cu loading on the selective catalytic reduction of NOx by NH3 was examined over a series of Cu ion-exchanged (20–80%) SSZ-13 zeolite catalysts. High NO reduction efficiencies (80–95%) were obtained over all catalyst samples between 250 and 500 °C, and at the gas hourly space velocity of 200,000 h−1. Both NO reduction and NH3 oxidation activities under these conditions were found to increase slightly with increasing Cu loading at low temperatures. However, NO reduction activity was suppressed with increasing Cu loadings at high temperatures (>500 °C) due to excess NH3 oxidation. The optimum Cu ion exchange level appears to be ~40–60% since higher than 80% NO reduction efficiency was obtained over 50% Cu ion-exchanged SSZ-13 up to 600 °C. The NO oxidation activity of Cu-SSZ-13 was found to be low regardless of Cu loading, although it was somewhat improved with increasing Cu ion exchange level at high temperatures. During the “fast” SCR (i.e., NO/NO2 = 1), only a slight improvement in NOx reduction activity was obtained for Cu-SSZ-13. Regardless of Cu loading, near 100% selectivity to N2 was observed; only a very small amount of N2O was produced even in the presence of NO2. The apparent activation energies for NO oxidation and NO SCR were estimated to be ~58 and ~41 kJ/mol, respectively. .

Atmospheric reactive nitrogen in China: sources, recent trends, and damage costs.

Abstract: Human activities have intensely altered the global nitrogen cycle and produced nitrogenous gases of environmental significance, especially in China where the most serious atmospheric nitrogen pollution worldwide exists. We present a comprehensive assessment of ammonia (NH3), nitrogen oxides (NOx), and nitrous oxide (N2O) emissions in China based on a full cycle analysis. Total reactive nitrogen (Nr) emission more than doubled over the past three decades, during which the trend of increase slowed for NH3 emissions after 2000, while the trend of increase continued to accelerate for NOx and N2O emissions. Several hotspots were identified, and their Nr emissions were about 10 times higher than others. Agricultural sources take 95% of total NH3 emission; fossil fuel combustion accounts for 96% of total NOx emission; agricultural (51%) and natural sources (forest and surface water, 39%) both contribute to the N2O emission in China. Total atmospheric Nr emissions related health damage in 2008 in China reached US...

Pt-Pd Diesel Oxidation Catalyst with CO/HC Light-off and HC Storage Function

Abstract: The present invention is directed to a diesel oxidation catalyst for the treatment of exhaust gas emissions, such as the oxidation of unburned hydrocarbons (HC), and carbon monoxide (CO) and the reduction of nitrogen oxides (NOx). More particularly, the present invention is directed to a novel washcoat composition comprising two distinct washcoat layers containing two distinctly different ratios of Pt:Pd.

Recent advances in the selective catalytic reduction of NOx by hydrogen in the presence of oxygen

Abstract: Selective catalytic reduction of NOx by hydrogen (H2-SCR) in the presence of oxygen has received much attention as a potential technology for reducing NOx emissions. A lot of research has been done in order to understand the reaction mechanism of H2-SCR and some possible mechanisms have been proposed. These mechanisms can be classified into two categories: NO adsorption/dissociation mechanisms and oxidation–reduction mechanisms. Based on the discussion of the reaction mechanism, the influence of the nature of the noble metal, catalyst support, catalyst preparation method, promoters and reaction conditions (including the presence of H2 and O2, water, sulfur, CO and CO2) on the catalytic performance of some H2-SCR catalysts has been discussed. Finally, future research directions in the area of H2-SCR have been proposed.

Impacts of midlatitude precursor emissions and local photochemistry on ozone abundances in the Arctic

Abstract: [1] We assess the impact of transport of pollution from midlatitudes on the abundance of ozone in the Arctic in summer 2006 using the GEOS-Chem global chemical transport model and its adjoint. We find that although the impact of midlatitude emissions on ozone abundances in the Arctic is at a maximum in fall and winter, in July transport from North America, Asia, and Europe together contributed about 25% of surface ozone abundances in the Arctic. Throughout the summer, the dominant source of ozone in the Arctic troposphere was photochemical production within the Arctic, which accounted for more than 50% of the ozone in the Arctic boundary layer and as much as 30%–40% of the ozone in the middle troposphere. An adjoint sensitivity analysis of the impact of NOx emissions on ozone at Alert shows that on synoptic time scales in both the lower and middle troposphere, ozone abundances are more sensitive to emissions between 50°N and 70°N, with important influences from anthropogenic, biomass burning, soil, and lightning sources. Although local surface NOx emissions contribute to ozone formation, transport of NOx in the form of peroxyacetyl nitrate (PAN) from outside the Arctic and from the upper troposphere also contributed to ozone production in the lower troposphere. We find that in late May and June the release of NOx from PAN decomposition accounted for 93% and 55% of ozone production at the Arctic surface, respectively.

Growth in NO x emissions from power plants in China: bottom-up estimates and satellite observations

Abstract: . Using OMI (Ozone Monitoring Instrument) tropospheric NO2 columns and a nested-grid 3-D global chemical transport model (GEOS-Chem), we investigated the growth in NOx emissions from coal-fired power plants and their contributions to the growth in NO2 columns in 2005–2007 in China. We first developed a unit-based power plant NOx emission inventory for 2005–2007 to support this investigation. The total capacities of coal-fired power generation have increased by 48.8% in 2005–2007, with 92.2% of the total capacity additions coming from generator units with size ≥300 MW. The annual NOx emissions from coal-fired power plants were estimated to be 8.11 Tg NO2 for 2005 and 9.58 Tg NO2 for 2007, respectively. The modeled summer average tropospheric NO2 columns were highly correlated (R2 = 0.79–0.82) with OMI measurements over grids dominated by power plant emissions, with only 7–14% low bias, lending support to the high accuracy of the unit-based power plant NOx emission inventory. The ratios of OMI-derived annual and summer average tropospheric NO2 columns between 2007 and 2005 indicated that most of the grids with significant NO2 increases were related to power plant construction activities. OMI had the capability to trace the changes of NOx emissions from individual large power plants in cases where there is less interference from other NOx sources. Scenario runs from GEOS-Chem model suggested that the new power plants contributed 18.5% and 10% to the annual average NO2 columns in 2007 in Inner Mongolia and North China, respectively. The massive new power plant NOx emissions significantly changed the local NO2 profiles, especially in less polluted areas. A sensitivity study found that changes of NO2 shape factors due to including new power plant emissions increased the summer average OMI tropospheric NO2 columns by 3.8–17.2% for six selected locations, indicating that the updated emission information could help to improve the satellite retrievals.

Selective catalytic reduction of NOx on combined Fe- and Cu-zeolite monolithic catalysts: Sequential and dual layer configurations

Abstract: Iron and copper-based zeolites are effective catalysts for the lean selective catalytic reduction (SCR) of NOx with NH3. Cu-zeolites are more active at lower temperatures (≤350 °C) while Fe-zeolites are more active at higher temperatures (≥400 °C). The effectiveness of a catalytic system comprising Fe- and Cu-based zeolites was examined for the standard (NO + O2 + NH3) and fast (NO + NO2 + NH3) SCR reactions. Experiments carried out with in-house and commercial Fe- and Cu-zeolite monoliths of varying lengths quantified their relative SCR activities. The commercial Cu-zeolite achieved complete NOx conversion for the standard SCR at 250 °C while the commercial Fe-zeolite achieved high NOx conversion at higher temperatures (≥400 °C) where it out-performed the Cu-zeolite. Subsequently, three configurations of combined Fe and Cu-zeolite catalysts were compared: • “Sequential brick” catalyst comprising Fe-zeolite and Cu-zeolite monolith. • “Mixed washcoat” catalyst comprising a washcoat layer having equal mass fractions of Fe- and Cu-zeolites. • “Dual layer” catalyst comprising monolith coated with individual layers of Fe- and Cu-zeolites of different thicknesses and mass fractions. The sequential brick design with Fe-zeolite brick followed by a Cu-zeolite brick gave a higher conversion than the Cu/Fe sequence of equal loadings with the Fe(33%)/Cu(67%) achieving the highest NOx conversion over a wide range of temperatures. The mixed washcoat catalyst achieved NOx conversion that was nearly an average of the individual Fe-only and Cu-only catalysts. The dual layer catalyst with a thin Fe-zeolite (33% of the total washcoat loading) layer on top of a thicker Cu-zeolite layer (67%) resulted in very high NOx removal efficiencies over a wide temperature range for both the standard and fast SCR reactions. The performance of this dual-layer system was comparable to the series arrangement of Fe and Cu-bricks. The Cu-zeolite on Fe-zeolite dual layer catalyst was not nearly as effective for the same loadings. The Fe/Cu dual layer catalyst also exhibited superior performance for the fast SCR reaction. The results are interpreted in terms of the activities of each catalyst for SCR and ammonia oxidation. An assessment of the extent of washcoat diffusion limitations shows that the dual layer configuration is superior to the sequential brick configuration. The existence of an optimal loading distribution of the Fe- and Cu-zeolite catalysts as well as other intangible benefits of the dual layer SCR catalyst are discussed.