Storage-reduction Catalyst Research Articles

Recent advances in automobile exhaust catalysts

Catalysts, which were recently developed by Toyota for the control of automobile exhaust, are reviewed. (1) For use in low emission vehicles, a CeO 2-ZrO 2 solid solution (CZ) with both high oxygen storage capacity and high heat resistance was developed as a support for a high performance three-way catalyst (TWC). (2) A novel three-way catalyst named the NO x storage-reduction catalyst (NSR) was developed for automotive lean-burn engines. The NSR catalyst can store NO x in an oxidizing atmosphere and then reduce stored NO x at stoichiometric or reducing conditions. Also, it has high tolerance to sulfur poisoning which is the most stringent problem for the NSR catalyst.

UV and Visible Raman Study of Thermal Deactivation in a NOxStorage Catalyst

In situ UV and visible Raman spectroscopy was used to characterize fresh and thermally aged NOx storage-reduction catalysts, Pt/Ba/Al2O3. From the presence, absence, and nature of certain features in the spectra depending on aging temperature, we conclude that sintering, oxide formation, and separation of components occurred in the thermally aged catalysts. As aging temperature increased, less atomic oxygen was generated on platinum but a high temperature form of oxide or more strongly bound oxygen species was formed. This coincided with a loss of oxidation activity with aging temperature. Under UV excitation, observation of the OH stretch of physisorbed H2O on aged Pt/Ba/Al2O3 indicated separation of Pt from γ-Al2O3, since this OH band was observed only on γ-Al2O3 (and Ba/Al2O3) but not on Pt/Al2O3. Nitrite/nitro species and NO (adsorbed on Pt) in aged Pt/Ba/Al2O3 indicate that Ba-containing particles are behaving somewhat independently from Pt and Al2O3, since these NOx species are observed only on Pt/Al2O3 but not on fresh Pt/Ba/Al2O3 under NO+O2 flow. Moreover, barium nitrate particles in aged Pt/Ba/Al2O3 are more crystalline, as shown by the intensity, width, and frequency of the nitrate peak, and possible photo-induced nitrite formation under UV excitation. Finally, a NOx species with a broad peak at ∼330 cm−1 appeared on fresh but not in aged Pt/Ba/Al2O3 (or γ-Al2O3, Pt/Al2O3, and Ba/Al2O3) which may indicate proximity of Pt and Ba-containing particles in fresh Pt/Ba/Al2O3.

Development of Catalysts for Diesel Particulate NOxReduction

While diesel vehicles feature high fuel economy with low CO2 emissions, further suppression of particulate matter (PM) and NOx in the exhaust stream is demanded worldwide. We have been working to develop a new diesel particulate-NOx reduction (DPNR) system to decrease both PM and NOx emissions by combining the NOx storage-reduction catalyst for direct injection gasoline engines with the most advanced engine control technologies. This paper describes the development of the DPNR system, a post-treatment technology for PM and NOx, which was achieved through a combination of catalysis and engine control technologies.

Advances in catalytic removal of NOx under leanburn conditions

The catalytic removal of NOx under lean conditions is one of the most important targets in catalysis research. The activities of metal oxides, zeolite-based catalysts and noble metal catalysts together with the factors are influencing the selective reduction of NOx with hydrocarbons are reviewed. The reaction mechanisms for the three types of catalysts are briefly discussed. Recent progress in combined catalyst and NOx storage reduction catalysts is also introduced. Finally, future research directions are forecasted.

排気管直挿型NOxセンサによるNOx吸蔵還元触媒の劣化検知(J25-2 交通機械と進化するエネルギー技術,J25 交通機械と進化するエネルギー技術)

排気管直挿型NOxセンサによるNOx吸蔵還元触媒の劣化検知(J25-2 交通機械と進化するエネルギー技術,J25 交通機械と進化するエネルギー技術)

Catalytic decomposition/regeneration of Pt/Ba(NO3)2 catalysts: NOx storage and reduction

The introduction of lean-burn engine technology has prompted the development of NOx storage-reduction (NSR) catalysts, which are currently based on a Pt/Ba(NO3)2/Al2O3 system. In this work a series of powdered catalysts based on this system were prepared and their reactivities examined with a pulsed-flow reactor, which was used to carry out temperature programmed desorption (TPD) experiments and to simulate the NSR reaction itself. TPD experiments reveal that Ba(NO3)2 is catalytically decomposed by the presence of Pt, at a significantly lower temperature than otherwise (by >200K), with the extent of the decomposition being dependent on the amount of Pt loaded. The products formed from the decomposition depend on the oxidation state of the Pt; firstly N2 and N2O are evolved (formed from cracking of the NO produced via the Ba(NO3)2 decomposition), but as the adsorbed oxygen resulting from their formation builds up, NO and O2 appear and N2 and N2O are lowered. NOx storage has been shown to occur even at room temperature following Ba(NO3)2 decomposition from a 0.5wt.% Pt/Al2O3 catalyst impregnated with 10wt.% Ba(NO3)2. These experiments can be carried out in a reversible way. Simulations of the NSR reaction, using H2 and CO as reductants, indicate a remarkable difference between the two; CO appears to facilitate Ba(NO3)2 decomposition, but not NOx reduction, whereas H2 enables both to take place, with excellent conversion to N2. The difference between the two reductants in these experiments reflects the difference in binding between the two adsorbates and consequent Pt surface poisoning by CO.

Formation of sulfur surface species on a commercial NOx-storage reduction catalyst

The influence of SO2 exposure under lean (oxidizing) and rich (reducing) reaction conditions on the storage and oxidation/reduction function of a commercial NOx storage-reduction catalyst was investigated by temperature-programmed uptake experiments and high temperature XRD. Both the storage capacity and the oxidation/reduction function of the catalyst were deactivated by SO2 exposure under lean and rich reaction conditions. The deactivation of the storage component, i.e. the loss of the NOx storage capacity, resulted mainly from the formation of Ba-sulfates accumulating in the bulk phase, which have a high thermal stability (>800°C) and, therefore, cannot be removed under the typical operation conditions of a NSR catalyst. For the oxidation function only a temporarily deactivation during lean reaction conditions was observed. Besides the formation of SO2-4 species on the storage component at the beginning of the SO2 exposure under rich conditions, an adsorption of SO2 on the noble metal component was observed resulting in the formation of sulfur deposits. The oxidation of these sulfur species with a subsequent spillover of SO2-4 species to the storage component during lean conditions could accelerate the deactivation of the storage capacity.

Elementary steps of NO x adsorption and surface reaction on a commercial storage–reduction catalyst

The surface species formed during adsorption of NO x on a commercial NSR catalyst (containing barium oxide, Pt, and alumina as the main components) were investigated by in situ IR spectroscopy. During adsorption of NO, mainly linear and bridged bonded nitrites of BaONOBa type were formed on Al and Ba oxide components. Nitrites were detected during the initial phase of the NO/O 2 and NO 2 adsorption, whereas with further exposure nitrates were the dominant surface species. Using the different surface species and reaction intermediates identified by IR spectroscopy a series of sequential reaction steps during the sorption of NO x on a NSR catalyst was derived. Initially, NO is stored in the form of nitrites on the storage component (Ba oxide). NO 2 formed by oxidation on the noble metal component (Pt) sorbs either molecularly by forming nitrate species or dissociatively by forming nitrites. After a certain concentration of NO x is adsorbed, the transformation and further oxidation of the surface nitrites into surface nitrates by NO 2 occur. The stability of the NO x surface species was found to increase in the order Al nitrites < Ba nitrites < Al nitrates < Ba nitrates.

524 排気管直挿型 NOx センサによる NOx 吸蔵還元触媒反応の解析

It is important to evaluate the performance of NO_x storage-reduction catalyst using with a gasoline lean burn engine under actual driving conditions. Then, we tried the measurement of NO_x in a lean burn exhaust gas by a ZrO2 NO_x sensor. As a result, we found that the NO_x sensor which can be directly installed in the exhaust stream has good sensitivity to NO_x, in addition, it is possible to evaluate performance of NO_x storage-reduction catalyst by using effectively air fuel ratio signals and NO_x concentration signals of two NO_x sensors placed on upstream and downstream positions of the catalytic converter.

Studies on the deactivation of NOx storage-reduction catalysts by sulfur dioxide

The interaction of sulfur dioxide with a commercial NOx storage-reduction catalyst (NSR) has been investigated using in situ IR and X-ray absorption spectroscopy. Two pathways of catalyst deactivation by SO2 were identified. Under lean conditions (exposure to SO2 and O2) at 350°C the storage component forms barium sulfates, which transform from surface to hardly reducible bulk sulfate species. The irreversible blocking of the Ba sites led to a decrease in NOx storage capacity. Under fuel rich conditions (SO2/C3H6) at 350–500°C evidence for the formation of sulfides on the oxidation/reduction component (Pt) of the catalyst was found, which blocks the metal surface and thus hinders the further reduction of the sulfides.

A study of the behaviour of Pt supported on CeO2–ZrO2/Al2O3–BaO as NOx storage–reduction catalyst for the treatment of lean burn engine emissions

The behaviour of a Pt(1wt.%) supported on CeO2–ZrO2(20wt.%)/Al2O3(64wt.%)–BaO(16wt.%) as a novel NOx storage–reduction catalyst is studied by reactivity tests and DRIFT experiments and compared with that of Pt(1%)–BaO(15wt.%) on alumina. The former catalyst, designed as a hydrothermally stable sample, is composed of an alumina modified with Ba ions and an overlayer of ceria-zirconia. The results pointed out that during the calcination barium ions migrates over the surface of the catalyst which thus show a good NOx storage–reduction behaviour comparable with that of Pt–BaO on alumina, although Ba ions result much better dispersed.

Novel low temperature NOx storage-reduction catalysts for diesel light-duty engine emissions based on hydrotalcite compounds

A series of Pt and Pt,Cu supported catalysts were prepared by wet impregnation of Mg–Al supports obtained from hydrotalcite-type (HT) precursor compounds. These novel NOx storage-reduction (NOxSR) catalysts show improved performances in NOx storage than Pt,Ba/alumina NOxSR catalysts at reaction temperatures lower than 200°C. These catalysts show also improved resistance to deactivation by SO2. The effect is attributed to the formation of well dispersed Mg(Al)O particles which show good NOx storage properties. The promoted low temperature activity is explained by the lower basicity of the Mg(Al)O mixed oxide in comparison to BaO, which induces on one hand a lower inhibition on Pt activity (NO to NO2 oxidation and/or hydrocarbon oxidation) due to electronic effect, and on the other hand a lower thermal stability of the stored NOx. The presence of Cu slightly inhibits activity at low temperature, although improves activity and resistance to deactivation at 300°C. On these catalysts FT-IR characterization evidences the formation of a Pt–Cu alloy after reduction.

Selective catalytic reduction of NO and NO 2 at low temperatures

The fast SCR reaction using equimolar amounts of NO and NO 2 is a powerful means to enhance the NO x conversion over a given SCR catalyst. NO 2 fractions in excess of 50% of total NO x should be avoided because the reaction with NO 2 only is slower than the standard SCR reaction. At temperatures below 200 °C, due to its negative temperature coefficient, the ammonium nitrate reaction gets increasingly important. Half of each NH 3 and NO 2 react to form dinitrogen and water in analogy to a typical SCR reaction. The other half of NH 3 and NO 2 form ammonium nitrate in close analogy to a NO x storage-reduction catalyst. Ammonium nitrate tends to deposit in solid or liquid form in the pores of the catalyst and this will lead to its temporary deactivation. The various reactions have been studied experimentally in the temperature range 150–450 °C for various NO 2/NO x ratios. The fate of the deposited ammonium nitrate during a later reheating of the catalyst has also been investigated. In the absence of NO, the thermal decomposition yields mainly ammonia and nitric acid. If NO is present, its reaction with nitric acid on the catalyst will cause the formation of NO 2.

NO x storage-reduction catalysts based on hydrotalcite : Effect of Cu in promoting resistance to deactivation

Novel NO x storage-reduction (NO x SR) catalysts prepared by Pt and/or Cu impregnation of Mg–Al (60:40) hydrotalcite (HT)-type compounds show better performances in NO x storage than Pt–Ba/Al 2O 3 Toyota-type NO x SR catalysts at reaction temperatures lower than 250 °C. The presence of Pt or Cu considerably enhances the activity, with the former more active. The nature of the HT source, however, also influences performance. The co-presence of Pt and Cu slightly worsens the low temperature activity, but considerably promotes the resistance to deactivation after severe hydrothermal treatment and in the presence of SO 2. This effect is attributed to both the possibility of formation of a Pt–Cu alloy after reduction, and the modification of the HT induced during the deposition of Cu. The overall Pt–Cu/HT performances are thus superior to those of the Pt–Ba/Al 2O 3 Toyota-type NO x SR catalysts.

A highly sulfur resistant Pt-Rh/TiO2/Al2O3 storage catalyst for NOx reduction under lean-rich cycles

Pt-Rh/TiO2/Al2O3 was investigated as a NOx storage-reduction catalyst using lean-rich cycles. It was found to be highly effective for NOx reduction and highly resistant to SO2 and H2O. In the presence of 100ppm SO2 and 2.3% H2O, approximately 90% of NOx conversion was obtained at 250°C and a GHSV of 30,000h−1 after 5h lean-rich cyclic runs. A model NOx trap catalyst, Pt-Rh/BaO/Al2O3, was used for comparison. In the absence of SO2 and H2O, nearly complete removal of NOx was achieved. NOx conversion dropped to only 30% after SO2/H2O were introduced into the reactant gases at 300°C. With Pt-Rh/TiO2/Al2O3, over 70% NO conversion was still obtained at 400°C. Although, the BaO containing catalyst has much higher capacity for NOx than that containing TiO2, it is much more susceptible to SOx poisoning. In situ FT-IR spectra of the titania catalyst showed that nitrate species was the final adspecies (i.e. at the end of the lean phase) on the storage catalyst and its intensity was only slightly weakened in the presence of SO2 and H2O.

Transient Surface Processes of Storage and Conversion of NO x Species on Pt–Me/Al2O3 Catalysts (Me = Ba, Ce, Cu)

The transient reactivity and surface phenomena of storage and conversion of NOx species on Pt(1%)–Me/Al2O3 catalysts, where Me = Ba, Ce and Cu, were studied by the RWF (rectangular wavefront) method. The Me component has a relevant influence on the processes of surface storage and transformation. The reduction of NOx by propene in the presence of oxygen is promoted by adding Cu to a Pt/Al2O3 catalyst, while cerium promotes transient conversion of NO in the absence of propene, but inhibits the reduction of NOx in the presence of propene. Copper is suggested to be a promising element to add together with Ba for new NOx storage-reduction catalysts due to its capacity to act both as a storage element and as promoter for NOx reduction.

Emission Control Management of Volkswagen FSI Fuel Stratified Injection Engines

Due to high lean NOx emissions of direct injection gasoline engines, NOx storage reduction catalysts are being implemented to meet european step 4 emission standards. In contrast to japanese applications, high sulphur concentrations in european fuel make means for active desulphation inevitable. Volkswagen has developed a new emission control management system which uses for the fIrst time NOx-sensor signals. The presentation gives a general overview of the Volkswagen emission control strategy of fuel stratified injection engines.

NOx storage-reduction catalyst for automotive exhaust with improved tolerance against sulfur poisoning

The main cause of deterioration for the NOx storage-reduction catalyst (NSR catalyst) is sulfur poisoning. On the basis of the thermogravimetric (TG) and FT-IR analyses of aged catalysts, we assumed sulfur poisoning of NSR catalyst to consist of two main factors. One is that sulfur dioxide in the exhaust gas is oxidized on precious metals and reacts with the support, forming aluminum sulfate. Another is that SOx reacts with the NOx storage components such as barium to form barium sulfate. These consequences lead to the concept that sulfur poisoning should be suppressed by the enhancement of sulfur desorption from the support and barium sulfate. Using a mixture of TiO2 and γ-Al2O3 as the support minimized the amount of SOx deposit on a catalyst after the sulfur poisoning test. As to γ-Al2O3, sulfur desorbed at a lower temperature from the catalyst with lithium doped γ-Al2O3 than the other alkaline or alkali-earth doped γ-Al2O3 after the sulfur poisoning test. It was effective for enhancing the desorption of sulfur from the aged catalyst. To make a uniform catalytic wash-coat thickness on the substrate, a hexagonal cell monolithic substrate was developed. Hydrogen was the most effective gas for enhancing the reduction of barium sulfate in the aged catalyst, and hydrogen generation on catalyst was enhanced by adding Rh/ZrO2 with high steam reforming reactivity. The catalyst developed by combining these technologies was subjected to on-vehicle durability testing simulating 50,000km of driving with 30ppm sulfur fuel to verify the improvement in the NOx purification performance of NSR catalyst.

Catalytic reduction of nitrogen oxides : In automotive exhaust containing excess oxygen by NOx storage-reduction catalyst

Automotive catalyst technology is now faced with very difficult problems. As a result of automakers' efforts to produce more efficient and lower-emission vehicles, lean-burn gasoline engines have been introduced into the market. While these are much more efficient than the conventional engines, the NOx removal has become significantly more difficult. After enormous efforts, we succeeded to solve the problem by inventing a new class of catalyst. Here, our challenge to develop the new catalyst that stores and then reduces NOx is described. The catalyst made it possible for emissions of lean-burn engines to meet the strict NOx regulations.

DeNO x catalyst for automotive lean-burn engine

The performance and durability of Cu-ZSM-5 were studied. Cu-ZSM-5 has fairly high NO x reduction activity but its durability is insufficient for practical use. We developed a new deNO x catalyst. In this catalyst, we call NO x storage reduction catalyst (NSR-catalyst), NO x emitted from an engine at lean A F operation is stored and this stored NO x is reduced at stoichiometric or rich operation. This catalyst has been used on the Toyota CARINA with a lean-burn engine in Japan since 1994. This report outlines the results of our study on Cu-ZSM-5 and NSR-catalyst.