Showing papers on "NOx published in 2002"

A review of the selective reduction of NOx, with hydrocarbons under lean-burn conditions with non-zeolitic oxide and platinum group metal catalysts

Abstract: Research on the selective reduction of NOx with hydrocarbons under lean-burn conditions using non-zeolitic oxides and platinum group metal (PGM) catalysts has been critically reviewed. Alumina and silver-promoted alumina catalysts have been described in detail with particular emphasis on an analysis of the various reaction mechanisms that have been put forward in the literature. The influence of the nature of the reducing agent, and the preparation and structure of the catalysts have also been discussed and rationalised for several other oxide systems. It is concluded for non-zeolitic oxides that species that are strongly adsorbed on the surface, such as nitrates/nitrites and acetates, could be key intermediates in the formation of various reduced and oxidised species of nitrogen, the further reaction of which leads eventually to the formation of molecular nitrogen. For the platinum group metal catalysts, the different mechanisms that have been proposed in the literature have been critically assessed. It is concluded that although there is indirect, mainly spectroscopic, evidence for various reaction intermediates on the catalyst surface, it is difficult to confirm that any of these are involved in a critical mechanistic step because of a lack of a direct quantitative correlation between infrared and kinetic measurements. A simple mechanism which involves the dissociation of NO on a reduced metal surface to give N(ads) and O(ads), with subsequent desorption of N2 and N2O and removal of O(ads) by the reductant can explain many of the results with the platinum group metal catalysts, although an additional contribution from organo-nitro-type species may contribute to the overall NOx reduction activity with these catalysts. It is concluded, after the investigation of hundreds of catalyst formulations, that many of the fundamental questions relating to lean deNOx reactions have been addressed and the main boundary conditions have been established. It seems clear that catalysts with sufficient activity, selectivity or stability to satisfy the demanding conditions that appertain in automotive applications are still far away. The rapidly growing interest in NOx storage systems reflects this situation, and it now seems to be the case that acceptable direct NOx reduction catalysts may be very difficult to find for lean-burn applications.

Catalysts for combustion of methane and lower alkanes

Abstract: The important greenhouse effect of methane (more than an order of magnitude greater than CO2) makes it essential to eliminate/control the methane emission from natural gas engines/power plants and petroleum industries. Catalytic combustion of methane is favored over homogeneous combustion, because the former greatly facilitates the oxidative destruction of methane. Moreover, use of catalysts for methane combustion in gas-turbines affords lower working temperatures (as compared to gas-fired turbines) and thermodynamically limits NOx (which is an extremely harmful environmental pollutant) emission. A large amount of work has been undertaken to develop catalysts both for controlling methane emission as well as for generating power in high temperature natural gas-turbines. This review will address the different issues related to the variety of catalysts which have been employed for methane/lower alkane combustion. Although all the related important aspects of the combustion catalysts will be addressed, greater emphasis will be placed on recent work in this field.

Exhaust gas aftertreatment systems

Abstract: A method is presented for estimating an amount of ammonia stored in a urea-based SCR catalyst based on a dynamic model of the catalyst. The model takes into account chemical and physical properties of the catalyst, such as catalyst volume, the number of available ammonia storage cites, adsorption and desorption dynamics, as well as poisoning, thermal aging, and different catalyst operating temperatures, and generates the estimate based on a measured or estimated amount of NOx in an exhaust gas mixture upstream of the catalyst, an amount of reductant injected into the catalyst to facilitate NOx reduction, and on a measured value of NOx in an exhaust gas mixture downstream of the catalyst. The estimated ammonia storage amount is then used to control the amount of reductant injected into the catalyst to maintain desired ammonia storage amount such that maximum NOx conversion efficiency coupled with minimum ammonia slip are achieved.

Nitric Oxide in Biological Denitrification: Fe/Cu Metalloenzyme and Metal Complex NOx Redox Chemistry

Abstract: Department of Chemistry, The Johns Hopkins University, Charles and 34th Streets, Baltimore, Maryland 21218, Kluyver Laboratory for Biotechnology, Delft University of Technology, 2628 BC Delft, The Netherlands, Department of Biochemistry and Molecular Biology, OGI School of Science and Engineering at OHSU, Beaverton, Oregon 97006, and Department of Microbiology and Molecular Genetics, University of California, Los Angeles, California 90095

Ozone production rates as a function of NOx abundances and HOx production rates in the Nashville urban plume

Abstract: [1] Tropospheric O3 concentrations are functions of the chain lengths of NOx (NOx ≡ NO + NO2) and HOx (HOx ≡ OH + HO2 + RO2) radical catalytic cycles. For a fixed HOx source at low NOx concentrations, kinetic models indicate the rate of O3 production increases linearly with increases in NOx concentrations (NOx limited). At higher NOx concentrations, kinetic models predict ozone production rates decrease with increasing NOx (NOx saturated). We present observations of NO, NO2, O3, OH, HO2, H2CO, actinic flux, and temperature obtained during the 1999 Southern Oxidant Study from June 15 to July 15, 1999, at Cornelia Fort Airpark, Nashville, Tennessee. The observations are used to evaluate the instantaneous ozone production rate (PO3) as a function of NO abundances and the primary HOx production rate (PHOx). These observations provide quantitative evidence for the response of PO3 to NOx. For high PHOx (0.5 < PHOx < 0.7 ppt/s), O3 production at this site increases linearly with NO to ∼500 ppt. PO3 levels out in the range 500–1000 ppt NO and decreases for NO above 1000 ppt. An analysis along chemical coordinates indicates that models of chemistry controlling peroxy radical abundances, and consequently PO3, have a large error in the rate or product yield of the RO2 + HO2 reaction for the classes of RO2 that predominate in Nashville. Photochemical models and our measurements can be forced into agreement if the product of the branching ratio and rate constant for organic peroxide formation, via RO2 + HO2 → ROOH + O2, is reduced by a factor of 3–12. Alternatively, these peroxides could be rapidly photolyzed under atmospheric conditions making them at best a temporary HOx reservoir. This result implies that O3 production in or near urban areas with similar hydrocarbon reactivity and HOx production rates may be NOx saturated more often than current models suggest.

Ozone production rate and hydrocarbon reactivity in 5 urban areas: A cause of high ozone concentration in Houston

Abstract: [1] Observations of ozone (O3) and O3 precursors taken from aircraft flights over Houston, TX, Nashville, TN; New York, NY; Phoenix, AZ, and Philadelphia, PA show that high concentrations of reactive volatile organic compounds (VOCs) in the Houston atmosphere lead to calculated O3 production rates that are 2 to 5 times higher than in the other 4 cities even though NOx concentrations are comparable. Within the Houston metropolitan area, concentrations of VOCs and O3 production rates are highest in the Ship Channel region; the location of one of the largest petrochemical complexes in the world. As a consequence the concentration of O3 in the Houston metropolitan area has recently exceeded 250 ppb, the highest value observed in the U.S within the past 5 years.

Side Reactions in the Selective Catalytic Reduction of NOx with Various NO2 Fractions

Abstract: The main and side reactions of the three selective catalytic reduction (SCR) reactions with ammonia over TiO2−WO3−V2O5 catalysts have been investigated using synthetic gas mixtures matching the composition of diesel exhaust. The three SCR reactions are standard SCR with pure NO, fast SCR with an equimolar mixture of NO and NO2, and NO2 SCR with pure NO2. At high temperatures the selective catalytic oxidation (SCO) of ammonia and the formation of nitrous oxide compete with the SCR reactions. Water strongly inhibits the SCO of ammonia and the formation of nitrous oxide, thus increasing the selectivity of the SCR reactions. However, water also inhibits SCR activity, most pronounced at low temperatures. NO2 fractions exceeding 50% enhance the formation of nitrous oxide at low temperatures. If the feed of NOx consists of pure NO2, the formation of nitrous oxide may occur by two different reactions having different temperature regimes. The reaction responsible for N2O formation at low temperatures probably invo...

Modeling the effect of weekday-weekend differences in motor vehicle emissions on photochemical air pollution in central California.

Abstract: Ambient ozone concentrations vary by day of week in some locations, often with higher concentrations observed on weekends in urban and downwind areas. Emissions of ozone precursors appear to be lower on weekends, so the behavior of ozone concentrations on weekends may indicate the outcome of particular ozone control strategies. To examine the influence of day-of-week differences in motor vehicle emissions on ambient ozone concentrations, we combine a fuel-based motor vehicle emission inventory containing weekend-specific activity with an Eulerian photochemical airshed model applied to central California. Emissions of NOx on weekends are approximately 30% lower than on weekdays due to a large drop in heavy-duty diesel truck activity, and emissions of VOC are only slightly lower on weekends. In rural areas, passenger car traffic and the associated emissions are highest on Fridays and Sundays. The combination of VOC sensitivity and reduced emissions of NOx on weekends results in higher ozone concentrations on weekends. Changes in the timing of emissions also contribute to the weekend ozone effect, but sensitivity tests show that changes in emissions timing have a minor effect compared to changes in total mass of emissions on weekends. Even in situations where reductions in NOx emissions lead to higher ozone concentrations, NOx reductions may still be necessary for control of other air pollutants such as nitrogen dioxide, nitric acid, and aerosol nitrate.

Control system for mobile NOx SCR applications

Abstract: A diesel powered vehicle is provided with an SCR system which uses an external reducing reagent to convert NOx emissions in a manner which accounts for the effects of NOx transient emissions on the reducing catalyst. Actual NOx emissions produced by the engine are filtered using a variable NOx time constant in turn correlated to the reductant/NOx storage capacity of the reducing catalyst at its current temperature to account for changes in the SCR system attributed to NOx transient emissions. Catalyst temperature is filtered using a variable catalyst time constant corresponding to current space velocity of the exhaust gas to account for changes in the catalyst temperature attributed to NOx transient emissions. The reductant is metered on the basis of the filtered, corrected NOx concentration applied at a NSR ratio based, in turn, on the filtered, corrected reducing catalyst temperature.

Emission inventories of NOx from commercial energy consumption in China, 1995-1998.

Abstract: Emissions of nitrogen oxides (NOx) in China are of great concern because of their impact on local air pollution as well as on regional environmental risks such as acid rain, eutrophication, tropospheric ozone, fine particulate matter, and loss of biodiversity. In this paper, total anthropogenic emissions of NOx in China during the past two decades are estimated on the basis of commercial energy consumption and NOx emission factors of different sectors and fuel types. Specifically, emission inventories of NOx from 31 provinces, 7 economic sectors, and 11 fuel types from 1995 to 1998 are evaluated and analyzed in detail, and the distribution of provincial-based average NOx emission intensity in 1998 is presented. It can be seen that the calculated national total emissions of NOx have rapidly increased from 4.76 Mt in 1980 to a peak value of about 12.03 Mt in 1996, with an average annual growth rate of 6%. However, instead of increasing, NOx emissions in 1997 and 1998 decreased by 0.37 Mt and 0.85 Mt, respectively, as compared to the peak in 1996. In short, NOx emissions caused by energy use are mainly concentrated in the more populated and industrialized areas of China, i.e., the Eastern Central and Southeastern areas. These results imply that China's NOx emissions in the future may not become as high as previously expected which will be helpful for policy making to control local NOx pollution as well as to reduce acid rain and tropospheric ozone in East Asia.

OH-initiated oxidation of benzene

Abstract: The present work represents a continuation of part I of this series of papers, in which we investigated the phenol yields in the OH-initiated oxidation of benzene under conditions of low to moderate concentrations of NOx, to elevated NOx levels. The products of the OH-initiated oxidation of benzene in 700–760 Torr of N2/O2 diluent at 297 ± 4 K were investigated in 3 different photochemical reaction chambers. In situ spectroscopic techniques were employed for the detection of products, and the initial concentrations of benzene, NOx, and O2 were widely varied (by factors of 6300, 1500, and 13, respectively). In contrast to results from previous studies, a pronounced dependence of the product distribution on the NOx concentration was observed. The phenol yield decreases from approximately 50–60% in the presence of low concentrations ( 10 000 ppb) NOx concentrations. In the presence of high concentrations of NOx, the phenol yield increases with increasing O2 partial pressure. The rate constant of the reaction of hydroxycyclohexadienyl peroxyl radicals with NO was determined to be (1.7 ± 0.6) × 10−11 cm3 molecule−1 s−1. This reaction leads to the formation of E,E-2,4-hexadienedial as the main identifiable product (29 ± 16%). The reaction of the hydroxycyclohexadienyl radical with NO2 gave phenol (5.9 ± 3.4%) and E,E-2,4-hexadienedial (3.4 ± 1.9%), no other products could be identified. The residual FTIR product spectra indicate the formation of unknown nitrates or other nitrogen-containing species in high yield. The results from the present work also show that experimental studies aimed at establishing/verifying chemical mechanisms for aromatic hydrocarbons must be performed using NOx levels which are representative of those found in the atmosphere.

NO and NO2 Adsorption on Barium Oxide: Model Study of the Trapping Stage of NOx Conversion via Lean NOx Traps

Abstract: The use of NOx traps is one strategy being pursed to enable the implementation of more fuel-efficient lean-burn gasoline engines. Materials development to enhance NOx storage capacity and sulfur tolerance will be necessary for performance improvement. Progress in these areas will benefit from a more detailed understanding of the base metal oxide−precious metal surface chemistry involved in the trapping, release, and reduction of NOx. In this work, we have focused on the adsorption of NO and NO2 on in-situ evaporated thin films of barium oxide, the primary storage material in lean NOx traps, to accentuate the details of the trapping stage of NOx conversion using these systems. X-ray photoelectron spectroscopy has been used to identify the species formed and their relative abundance following room-temperature adsorption. Annealing experiments were performed to follow changes in adsorbed species with temperature. For NO, our results are consistent with nitrites forming as a result of molecular adsorption. In...

The Effect of an Oxidation Precatalyst on the NOx Reduction by Ammonia SCR

Abstract: The performances of oxidation and SCR catalysts were investigated individually and as a combined system for the removal of NOx by ammonia. Experiments were carried out with synthetic gas mixtures matching the composition of diesel exhaust. Introducing an oxidation catalyst upstream of the SCR catalyst enhances the removal of NOx at low temperatures. The beneficial effects of combining the two catalysts are the oxidation of unburned hydrocarbons that inhibit the SCR reaction and the conversion of NO to NO2 on the oxidation catalyst. NO2 fractions up to 50% of total NOx enhance the NOx conversion at temperatures below 300 °C. However, excess NO2 should be avoided because the reaction of NO2 with ammonia is slow, resulting in a decreased NOx conversion. Above 350 °C, the NOx conversion essentially does not depend on the ratio NO2/NOx or on the presence of hydrocarbons in the feed. A simple mathematical prediction of the NOx conversion for the combined system is also possible. The parameters used in this calc...

NOx CLEANING DEVICE FOR INTERNAL COMBUSTION ENGINE

Abstract: PROBLEM TO BE SOLVED: To maintain a high NOx cleaning factor by allowing the optimum addition of a reducer while detecting the occurrence of ammonia slip and the trouble of a system by utilizing errors included in information correlating to information for NOx in exhaust gas. SOLUTION: A reducer supply means 29 for supplying ammonia or urea water to a NOx catalyst 17 provided in an engine exhaust system 2 for adsorbing ammonia and selectively reducing NOx in the exhaust gas is controlled by a control means 45 so that the addition amount of the ammonia or the urea water is increased when an actual NOx cleaning factor is lower than a target NOx cleaning factor as a determination result that the target NOx cleaning factor Mη derived in accordance with an exhaust gas flow rate G and a catalyst temperature Tg of a NOx catalyst by a target NOx cleaning factor deriving means 43 is compared with the actual NOx cleaning factor ηof the NOx catalyst derived by an actual NOx cleaning factor deriving means 44 by a comparison means 46. In accordance with the actual NOx cleaning factor Hη after the control of the control means and the actual NOx cleaning factor η before the control, whether the NOx cleaning system is in trouble or not is determined by a determination means 47. COPYRIGHT: (C)2004,JPO

Ex Vivo Measurement of Brain Tissue Nitrite and Nitrate Accurately Reflects Nitric Oxide Synthase Activity In Vivo

Abstract: The ex vivo tissue concentration of nitrite and nitrate (NOx) was found to correlate closely with the activity of nitric oxide synthase (NOS; EC 1.14.13.39) in various brain regions. Systemic administration of the nonselective NOS inhibitor N omega-nitro-L-arginine (L-NA) at doses that completely inhibited both central and peripheral NOS, depleted whole-brain and CSF NOx by up to 75% but had no effect on plasma NOx. Selective inhibition of central NOS by intracerebroventricular administration of L-NA methyl ester produced similar decreases in levels of whole-brain NOx. A residual concentration of NOx of 10-15 microM remained in all brain regions even after complete inhibition of brain NOS. Brain NOx content decreased rapidly and in parallel with the inhibition of brain NOS. The ex vivo measurement of levels of brain NOx was found to reflect the in vivo efficacy of several different types of NOS inhibitor: L-NA, N omega-monomethyl-l-arginine, and 7-nitroindazole. Intraperitoneal administration of the NOS substrate L-arginine increased brain NOx concentrations by up to 150% of control values. These results demonstrate that the ex vivo measurement of levels of brain tissue NOx is a rapid, reliable, and straightforward technique to determine NOS activity in vivo. This method can be used to assess both the regional distribution and the degree of inhibition of NOS activity in vivo.

SOx TOLERANT NOx TRAP CATALYSTS AND METHODS OF MAKING AND USING THE SAME

Abstract: The present invention relates to an exhaust gas stream, in particular sulfur oxides sulfur allow useful for reducing contaminants in the gaseous stream containing (sulfur oxide) contaminant catalysts complexes. More particularly, the present invention relates to an improvement of the NOx trap catalyst for use in diesel engines and lean burn gasoline engines. Sulfur tolerant NOx trap catalyst complex comprises a NOx sorbent component prepared by the platinum component, the support and the NOx hot water (hydrothermal) synthesis. NOx adsorbent component comprises a first metal oxide and second metal oxide. First metallic metal in the oxide is selected from the group consisting of aluminum, titanium, zirconium, silicon and composites thereof, the metal in the second metal oxide is the IIA group metal, a Group III metal, a group IV metals, rare earth metals and transition It is selected from the group consisting of a metal. The metal in the first metal oxide is different than the metal in the second metal oxide. Sulfur tolerant NOx trap catalyst complexes are highly effective in sulfur-containing fuel by trapping the conventional sulfur oxide contaminants which tend to reduce the NOx trap catalytic activity (poisoning). Sulfur tolerant NOx trap catalyst complex is present, because the composite can be played on the middle temperature rather than pulses having a higher temperature is particularly suitable for diesel engines.

Predicting NOx emissions of a burner operated in flameless oxidation mode

Abstract: This paper deals with the technology of burning natural gas with combustion air preheated to 1300°C. The objective of this work is to assess the potential of several combustion models and NOx postprocessors for their abilities to predict NOx emissions. The steady-state computational results have been compared both with in-furnace measurements and with the measured furnace exit parameters. The following main conclusions have been drawn: (1) with the exception of the small region located within the natural gas jet, the computations resulted in good quality predictions: (2) the NOx has been formed in a thin elongated region (flamelet) located between the natural gas jet and the air jet: (3) the NO has been formed mainly by the thermal path, and the NO-reburning mechanism was of little importance: and (4) there are strong indications that the non-stationary behavior of the weak (fuel) jet must be accounted for if the predictions within the fuel jet were to be perfected.

A comparison between Pt and Pd in NOx storage catalysts

Abstract: The importance of Pt and Pd in noble metal-barium oxide type NOx storage catalysts was investigated. Model Pt/BaO/Al2O3 and Pd/BaO/Al2O3 catalysts were prepared and evaluated with respect to NOx storage capacity, activity towards NO reduction under lean conditions and NO oxidation capacity using synthetic lean burn exhausts containing NO, O2, C3H6 and N2. The study was carried out by performing static and transient flow reactor experiments and temperature-programmed desorption studies. At 300 degrees C, the Pd/BaO/Al2O3 sample shows a higher NOx storage capacity than Pt/BaO/Al2O3, i.e. more NOx is stored during the lean periods and almost all NOx is released and reduced during the subsequent rich periods. At this temperature(300 degrees C), the NO reduction is not complete during the rich phase for the Pt-based catalyst suggesting poisoning of Pt-sites by adsorbed species. At 400 degrees C, Pt/BaO/Al2O3 stores slightly more NOx than its Pd-based counterpart. XPS measurements on pre-treated catalysts, show some changes in oxidation state for Pd between the rich and lean phases. The oxidation of NO is much more limited on Pd based samples compared to Pt containing catalysts. The importance of NO2 as an intermediate in the storage of NOx as nitrate under lean conditions is confirmed in this study.

Excess air factor control of diesel engine

Abstract: A diesel engine (1) provided with a NOx catalyst (28A) and a diesel particulate filter (28B) performs lean burn operation during normal running, rich burn operation during regeneration of the NOx catalyst (28A), operation under the stoichiometric air-fuel ratio during desulphating of the NOx catalyst (28A), and operation under a slightly lean air-fuel ratio to regenerate the filter after desulphating of the NOx catalyst (28A). When the lean burn operation is applied, a controller (31) first controls the fuel injection amount, and controls an air supply amount based on the fuel injection amount. When rich burn operation is applied, the controller (31) first controls the air supply amount and controls the fuel injection amount based on the air supply amount. Due to this control, the response of the excess air factor control is enhanced while preventing torque fluctuation accompanying the variation of the target excess air factor.

Emissions of Aldehydes and Ketones from a Two-Stroke Engine Using Ethanol and Ethanol-Blended Gasoline as Fuel

Abstract: Besides aliphatic gasoline, ethanol-blended gasoline intended for use in small utility engines was recently introduced on the Swedish market. For small utility engines, little data is available showing the effects of these fuels on exhaust emissions, especially concerning aldehydes and ketones (carbonyls). The objective of the present investigation was to study carbonyl emissions and regulated emissions from a two-stroke chain saw engine using ethanol, gasoline, and ethanol-blended gasoline as fuel (0%, 15%, 50%, 85%, and 100% ethanol). The effects of the ethanol-blending level and mechanical changes of the relative air/fuel ratio, lambda, on exhaust emissions was investigated, both for aliphatic and regular gasoline. Formaldehyde, acetaldehyde, and aromatic aldehydes were the most abundant carbonyls in the exhaust. Acetaldehyde dominated for all ethanol-blended fuels (1.2-12 g/kWh, depending on the fuel and lambda), and formaldehyde dominated for gasoline (0.74-2.3 g/kWh, depending on the type of gasoline and lambda). The main effects of ethanol blending were increased acetaldehyde emissions (30-44 times for pure ethanol), reduced emissions of all other carbonyls exceptformaldehyde and acrolein (which showed a more complex relation to the ethanol content), reduced carbon monoxide (CO) and ntirogen oxide (NO) emissions, and increased hydrocarbon (HC) and nitrogen dixodie (NO2) emissions. The main effects of increasing lambda were increased emissions of carbonyls and nitrogen oxides (NOx) and reduced CO and HC emissions. When the two types of gasoline are considered, benzaldehyde and tolualdehyde could be directly related to the gasoline content of aromatics or olefins, but also acrolein, propanal, crotonaldehyde, and methyl ethyl ketone mainly originated from aromatics or olefins, while the main source for formaldehyde, acetaldehyde, acetone, methacrolein, and butanal was saturated aliphatic hydrocarbons.

Effects of biodiesel blends and Arco EC-diesel on emissions from light heavy-duty diesel vehicles.

Abstract: Chassis dynamometer tests were performed on 7 light heavy-duty diesel trucks comparing the emissions of a California diesel fuel with emissions from 4 other fuels: ARCO EC-diesel (EC-D) and three 20% biodiesel blends (1 yellow grease and 2 soy-based). The EC-D and the yellow grease biodiesel blend both showed significant reductions in THC and CO emissions over the test vehicle fleet. EC-D also showed reductions in PM emission rates. NOx emissions were comparable for the different fuel types over the range of vehicles tested. The soy-based biodiesel blends did not show significant or consistent emissions differences over all test vehicles. Total carbon accounted for more than 70% of the PM mass for 4 of the 5 sampled vehicles. Elemental and organic carbon ratios varied significantly from vehicle-to-vehicle but showed very little fuel dependence. Inorganic species represented a smaller portion of the composite total, ranging from 0.2 to 3.3% of the total PM. Total PAH emissions ranged from approximately 1.8 mg/mi to 67.8 mg/mi over the different vehicle/fuel combinations representing between 1.6 and 3.8% of the total PM mass.

Exhaust gas purifying apparatus for internal combustion engine

Abstract: An exhaust gas purifying apparatus including an HC absorbent located in an upstream region of a selective reduction type NOx catalyst for improving a NOx purification efficiency. In the exhaust gas purifying apparatus, an HC absorbing device is disposed upstream of a NOx catalyst converter arranged in an exhaust gas passageway. The selective reduction type NOx catalyst accommodated in the HC absorbing device uses a carrier, which is easy to absorb the HC when a temperature of the exhaust gas is lowered, and easy to desorb the HC when the temperature of the exhaust gas is raised, and a selective reduction type NOx catalyst accommodated in the NOx catalyst converter uses a carrier which has a strong absorption of HC for retention. In order to prevent a catalyst temperature of the NOx catalyst converter from being lowered to a temperature lower than a purification temperature window in an operating state of deceleration, an ECU corrects an opening degree of an EGR valve to be increased, and thereby a flow rate of the exhaust gas which flows into the NOx catalyst converter is decreased when an inlet gas temperature detected by an inlet gas temperature sensor is lowered to a temperature lower than a set temperature. Thus, lowering of the catalyst temperature is suppressed.