Influence of zirconia raw materials on the development of DeNOx monolithic catalysts

Pd-zirconia-based monolithic catalysts were prepared with various commercial zirconia raw materials and a natural magnesium silicate binder, sepiolite, for the selective catalytic reduction (SCR) of NO with CH₄ in oxygen excess. The different textural properties, metastable tetragonal zirconia phase stability, surface acidity, Pd dispersion and catalytic properties of these monoliths were compared to select the most suitable structured catalyst for NO_x control in natural gas-fired power plants. The influence of operating temperature in the two reactions, NO reduction and CH₄ combustion, with the monolithic catalysts was determined. A 0.4 wt.% Pd-zirconia catalyst, manufactured from a sulphated zirconium hydroxide raw material, was selected as the most appropriate in the reaction under study, reaching a maximum NO conversion at 400 °C.     

Sulphur dioxide interaction with NOx storage catalysts

The effect of SO₂ on the NO_x storage capacity and oxidation and reduction activities of a model Pt/Rh/BaO/Al₂O₃ NO_x storage catalyst was investigated. Addition of 2.5, 7.5 or 25 vol. ppm SO₂ to a synthetic lean exhaust gas caused deactivation of the NO_x storage function, the oxidation activity and the reduction activity of the catalyst. The degree of deactivation of the NO_x storage capacity was found to be proportional to the total SO₂ dose that the catalyst had been exposed to. SO₂ was found to be accumulated in the catalyst as sulphate.     

Low-temperature SCR of NOx with NH3 over carbon–ceramic cellular monolith-supported manganese oxides

This work describes the development and use of carbon–ceramic cellular monoliths as catalyst supports for the low-temperature selective catalytic reduction (SCR) of NO_x with ammonia. Manganese oxide was selected as catalyst and deposited over the support, which was obtained by coating the cellular ceramics with a polymeric film. The coated material was cured, carbonised and activated prior to impregnation of the active phase. The produced catalysts showed a good NO_x reduction (in the range 34–73%) at 150°C for a space velocity of 4000 h⁻¹. Gasification of the support was negligible at the mentioned conditions.     

NOx storage/reduction over lean-burn automotive catalysts

This paper shows the behavior of a Pt/Ba/γ–Al₂O₃ automotive catalyst in a fixed bed reactor during cyclic operation at lean and rich gas phase conditions at short (seconds) and long (hours) cycling times at different temperatures. Reactor exit gas phase concentrations have been measured and catalyst properties have been determined before and after selective cycling experiments. The experimental results indicate that: (i) Upon 9 h lean and 15 h rich cycling, the NO oxidation efficiency of the catalyst decreases with time while incomplete regeneration is seen, even after 15 h rich exposure with H₂. The cyclic steady state is reached after 3 lean/rich cycles, at which only 60% of the available barium is involved in the NO_x storage/reduction. (ii) The BET surface area, pore volume, and Pt dispersion decrease by approximately 40%, which may be a result of masking of Pt sites or blocking of pores of the barium clusters as BaCO₃ becomes Ba(NO₃)₂. Experiments with catalyst pellet sizes of 180 and 280 μm along with XPS measurements show that blocking of catalyst pellet pores is not taking place. (iii) When applying lean/rich cycling in the order of seconds, it appears that catalyst history and lean/rich timing affect the number of cycles required to arrive at a closed N balance. XRD results after lean exposure confirm the formation of barium nitrate in the bulk of the barium cluster.     

Bench-scale demonstration of an integrated deSoot–deNOx system

A catalytic deSoot–deNO_x system, comprising Pt and Ce fuel additives, a Pt-impregnated wall-flow monolith soot filter and a vanadia-type monolithic NH₃-SCR catalyst, was tested with a two-cylinder DI diesel engine. The soot removal efficiency of the filter was 98–99 mass% with a balance temperature (stationary pressure drop) of 315 °C at an engine load of 55%. The NO_x conversion ranged from 40 to 73%, at a NH₃/NO_x molar ratio of 0.9. Both systems were measured at a GHSV of 52 000 l/(l h). The maximum NO_x conversion was obtained at 400 °C. The reason for the moderate deNO_x performance is discussed. No deactivation was observed after 380 h time on stream. The NO_x emission at high engine loads is around 15% lower than that of engines running without fuel additives.     

Transient behaviour of catalytic monolith with NOx storage capacity

Transient behaviour of catalytic monolith converter with NO_x storage is studied under conditions typical for automobiles with lean-burn engines (i.e., diesel and advanced gasoline ones). Periodical alternation of inlet concentrations is applied—NO_x are adsorbed on the catalyst surface during a long reductant-lean phase (2–3 min) and then reduced to N₂ within a short reductant-rich phase (2–6 s). Samples of industrial NO_x storage and reduction catalyst of NM/Ba/CeO₂/γ-Al₂O₃ type (NM = noble metal), washcoated on 400 cpsi cordierite substrate, are used in the study. Effects of the rich-phase length and composition on the overall NO_x conversions are examined experimentally. Reduction of NO_x by CO, H₂ and unburned hydrocarbons (represented by C₃H₆) in the presence of CO₂ and H₂O is considered.

Effective, spatially 1D, heterogeneous mathematical model of catalytic monolith with NO_x and oxygen storage capacity is described. The minimum set of experiments needed for the evaluation of relevant reaction kinetic parameters is discussed: (i) CO, H₂ and HC oxidation light-off under both lean and rich conditions, including inhibition effects, (ii) NO/NO₂ transformation, (iii) NO_x storage, including temperature dependence of effective NO_x storage capacity, (iv) water gas shift and steam reforming under rich conditions, i.e., in situ production of hydrogen, (v) oxygen storage and reduction, including temperature dependence of effective oxygen storage capacity, and (vi) NO_x desorption and reduction under rich conditions. The experimental data are compared with the simulation results.     

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 NO_x storage-reduction (NO_xSR) catalysts show improved performances in NO_x storage than Pt,Ba/alumina NO_xSR 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 NO_x 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 NO_x. 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.     

Regeneration of NOx trap catalysts

We have investigated the regeneration of a nitrated or sulphated model Pt/Ba-based NO_x trap catalyst using different reductants. H₂ was found to be more effective at regenerating the NO_x storage activity especially at lower temperature, but more importantly over the entire temperature window after catalyst ageing. When the model NO_x storage catalyst is sulphated in SO₂ under lean conditions at 650 °C almost complete deactivation can be seen. Complete regeneration was not achieved, even under rich conditions at 800 °C in 10% H₂/He. Barium sulphate formed after the high temperature ageing was partly converted to barium sulphide on reduction. However, if the H₂ reduced sample was exposed to a rich condition in a gas mixture containing CO₂ at 650 °C, the storage activity can be recovered. Under these rich conditions the S²⁻ species becomes less stable than the CO₃²⁻, which is active for storing NO_x. Samples which were lean aged in air containing 60 ppm SO₂ at <600 °C, after regeneration at λ=0.95 at 650 °C, have a similar activity window to a fresh catalyst. It is, therefore, important that CO₂ is present during the rich regenerations of the sulphated model samples (as of course it would be under real conditions), as suppression of carbonate formation can lead to sulphide formation which is inactive for NO_x storage.     

Application of numerical modeling of selective NOx reduction by hydrocarbon under diesel transient conditions in consideration of hydrocarbon adsorption and desorption process

A model of NO_x selective reduction by hydrocarbon (HC) was developed, which takes into account the adsorption and desorption of HC. The model was applied for predicting the performance of a De–NO_x catalytic reactor, working under transient conditions such as a legislative driving cycle. Diesel fuel was used as a supplemental reductant.

The behavior of HC and NO_x reactions and HC adsorption and desorption has been simulated successfully by our numerical approach under the transient conditions of the simulated Japanese 10–15 driving cycle. Our model is expected to optimize the design of selective diesel NO_x reduction systems using a diesel fuel as a supplemental reductant.     

Kinetic experiments and modeling of NO oxidation and SCR of NOx with decane over Cu- and Fe-MFI catalysts

The kinetic model of the reduction of NO to N₂ with decane, developed based on the experimental data over Fe-MFI catalyst, has been applied for the oxidation of NO to NO₂ and reduction of NO₂ to N₂ with decane over Cu-MFI catalyst. The model fits well the experimental data of oxidation of NO as well as reduction of NO to N₂. Remarkable differences have been found in performance of Cu-MFI and Fe-MFI catalysts. While Fe-MFI is more active in oxidation of NO to NO₂, Cu-MFI exhibits much higher activity in the reduction of NO with decane. The kinetic model indicates that the significantly lower activity of Fe-MFI in comparison with Cu-MFI in transformation of NO_x to nitrogen is due to higher rate of transformation of NO₂, formed in the first step by the oxidation of NO, back to NO instead to molecular nitrogen.     

A novel laboratory bench for practical evaluation of catalysts useful for simultaneous conversion of NOx and soot in diesel exhaust

This study deals with the development of a laboratory bench for the practical evaluation of catalysts that are useful for the direct conversion of NO_x and soot in the exhaust of diesel engines. The employed model exhaust is generated by using a diffusion burner with additionally dosing some gaseous components to the burner gas to obtain a realistic feed composition. The produced soot is extensively characterized by employing thermogravimetry, transmission electron microscopy, N₂ physisorption and temperature programmed techniques. The results of the different characterization methods show that the present soot is suitable for the intended catalytic investigations. The simultaneous conversion of NO_x and soot is examined like in practice, i.e. the soot is separated from the tail gas by a diesel particulate filter (DPF) that is coated with the catalyst. The deposited soot is then catalytically converted by NO_x and O₂ to form N₂ and CO₂. The conversions of NO_x and soot are measured by exclusively applying gas analysers, whereby a special experimental procedure is developed to determine the soot removal. Hence, additional soot related analytics are not required. To show the suitability of the constructed bench a Pt/Fe₂O₃/β-zeolite sample is taken as test catalyst that is reported to be very active in NO_x/soot reaction. The measurements performed with and without catalyst clearly show the effect of the used sample in simultaneous NO_x/soot conversion. We therefore consider the constructed laboratory bench to be a useful tool for testing and ranking catalytic materials.     

Pt dispersion effects during NOx storage and reduction on Pt/BaO/Al2O3 catalysts

This study provides insight into the effect of Pt dispersion on the overall rate and product distribution during NOx storage and reduction. The storage and reduction performance of Pt/BaO/A₂O₃ monoliths with varied Pt dispersion (3%, 8%, and 50%) and fixed Pt (2.48 wt.%) and BaO (13.0 wt.%) loadings is reported. At low temperature (<200 °C), the differences in storage and reduction activity were the largest between the three catalysts. The amount of NOx stored increased with increased dispersion, as did the amount of stored NOx that was reduced. These trends are attributed to larger Pt surface area and Pt–BaO interfacial perimeter, the latter of which enhances the spillover of surface species between the precious metal and storage components. At high temperature (370 °C), the stored NOx was almost completely regenerated for the three catalysts. However, the regeneration of the 3% dispersion catalyst was much slower, suggesting a rate limitation involving the reverse spillover of stored NOx to Pt and/or of adsorbed hydrogen from Pt to BaO. The results indicate that the catalyst dispersion and operating conditions may be tuned to achieve the desired ammonia selectivity. For the aerobic regeneration feed, the most (net) NH₃ was generated by the 50% dispersion catalyst at the lowest temperature (125 °C), by the 3% dispersion catalyst at the highest temperature (340 °C), and by the 8% dispersion catalyst at the intermediate temperatures (170–290 °C). Similar trends were observed for the net production of NH₃ with an anaerobic regeneration feed. A phenomenological picture is proposed that describes the effects of Pt dispersion consistent with the established spatio-temporal behavior of the lean NOx trap.     

Abatement of diesel-exhaust pollutants: NOx storage and soot combustion on K/La2O3 catalysts

Potassium-loaded lanthana is a promising catalyst to be used for the simultaneous abatement of soot and NO_x, which are the main diesel-exhaust pollutants. With potassium loadings between 4.5 and 10 wt.% and calcination temperatures between 400 and 700 °C, this catalyst mixed with soot gave maximum combustion rates between 350 and 400 °C in TPO experiments, showing a good hydrothermal stability. There was no difference in activity when it was either mixed by grinding in an agate mortar or mixed by shaking in a sample bottle (tight and loose conditions, respectively). Moreover, when the K-loaded La₂O₃ is used as washcoat for a cordierite monolith, there were found no significant differences in the catalytic behaviour of the system, which implies its potentiality for practical purposes.

The influence of poisons as water and SO₂ was investigated. While water does not affect the soot combustion activity, SO₂ slightly shift the TPO peak to higher temperature. Surface basicity, which is a key factor, was analysed by measuring the interactions of the catalytic surface with CO₂ using the high frequency CO₂ pulses technique, which proved to be very sensitive, detecting minor changes by modifications in the dynamics of the CO₂ adsorption–desorption process. Water diminishes the interaction with CO₂, probably as a consequence of an adsorption competition. The SO₂ treated catalyst is equilibrated with the CO₂ atmosphere more rapidly if compared with the untreated one, also showing a lower interaction. The lower the interaction with the CO₂, the lower the activity.

Differential scanning calorimetric (DSC) results indicate that the soot combustion reaction coexists with the thermal decomposition of hydroxide and carbonate species, occurring in the same temperature range (350–460 °C). The presence of potassium increases surface basicity shifting the endothermic decomposition signal to higher temperatures.

We also found that NO₂ strongly interacts with both La₂O₃ and K/La₂O₃ solids, probably through the formation of monodentate nitrate species which are stable under He atmosphere until 490 °C. These nitrate species further react with the solid to form bulk nitrate compounds. The addition of Cobalt decreases the nitrates stability and catalyses the NO_x to N₂ reduction under a reducing atmosphere, which is a necessary step for a working NO_x catalytic trap. Preliminary studies performed in this work demonstrated the feasibility of using these catalysts to simultaneously remove NO_x and soot particles from diesel exhausts. The nitrate formation is still observed during the catalytic combustion of soot in the presence of NO_x, making our K/La₂O₃ a very interesting system for practical applications in simultaneous soot combustion and NO_x storage in diesel exhausts.     

NOx sorption–desorption study: application to diesel and lean-burn exhaust gas (selective NOx recirculation technique)

NO_x adsorption/desorption capacities of barium aluminates and BaSnO₃ were measured under representative exhaust gas mixture at temperatures below 550°C and compared to those of bulk BaO. The capacities are high and the test of sorption–desorption is reproducible on barium aluminate and BaSnO₃, while this is not the case on BaO. The difference is due to the electronic environment of barium oxide. If BaO is not engaged in a chemical bond, progressive formation of high stability carbonates is observed. This is not the case with barium aluminate and BaSnO₃, where carbonation does not take place because the competition between nitrate and carbonate formation is in favour of the nitrate due to its chemical nature. An N-bounded nitrate, with IR frequencies at 1360 and 1415 cm⁻¹, is formed on barium aluminate and BaSnO₃ and not on bulk BaO.     

Synergistic catalysis of carbon black oxidation by Pt with MoO3 or V2O5

A series of SiO₂-supported MoO₃, V₂O₅, and Pt catalysts were prepared for the study of model soot oxidation with simulated diesel exhaust gas. Composite samples of Pt with the metal oxides demonstrated higher oxidation activities than the single-component SiO₂-supported MoO₃, V₂O₅ or Pt catalysts in the absence of SO₂ in the reactant gas. Based on the effects of NO₂ on carbon oxidation, a synergistic reaction mechanism was suggested to explain the effects of combining Pt with the oxides: Pt catalyzes the oxidation of NO with gas phase O₂ to NO₂, while MoO₃ and V₂O₅ catalyze the oxidation of carbon with NO₂. Finally, the effects of SO₂ on the carbon oxidation reaction were examined and discussed.     

Role of zeolite structure on reduction of NOx with methane over In- and Pd-based catalysts

Catalytic performances of ZSM-5 based catalysts containing indium or palladium were examined for NO reduction with CH₄ and NO_x chemisorption. The amounts of NO_x chemisorbed on In/H-ZSM-5 were well proportional to the catalytic activities for NO_x reduction. Pd/H-ZSM-5, on the other hand, hardly chemisorbed NO₂, while the catalytic activity for NO₂ reduction with CH₄ is very high. Furthermore, Pd loaded on SiO₂ showed comparably high catalytic activity for NO₂ reduction with CH₄ at 400°C in the absence of oxygen as Pd/H-ZSM-5. CH₄ combustion during NO_x reduction with CH₄ in the presence of oxygen significantly occurred over PdO on SiO₂, while less over Pd/H-ZSM-5. The role of zeolite might be slightly different between In/H-ZSM-5 and Pd/H-ZSM-5: the zeolitic porous structure is needed for In/H-ZSM-5 in order to concentrate NO₂ adspecies on InO⁺ sites, which is important for NO reduction with CH₄ on In/H-ZSM-5 based catalysts, while the ion-exchangeable ability of zeolite is needed for Pd/H-ZSM-5 in order to make Pd²⁺ located in a highly dispersed state, on which NO is strongly chemisorbed.     

A comparative study of the water-gas shift activity of Pt catalysts supported on single (MOx) and composite (MOx/Al2O3, MOx/TiO2) metal oxide carriers

The water-gas shift (WGS) activity of platinum catalysts dispersed on a variety of single metal oxides as well as on composite MO_x/Al₂O₃ and MO_x/TiO₂ supports (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, La, Ce, Nd, Sm, Eu, Gd, Ho, Er, Tm) has been investigated in the temperature range of 150–500 °C, using a feed composition consisting of 3% CO an 10% H₂O. For Pt catalysts supported on single metal oxides, it has been found that both the apparent activation energy of the reaction and the intrinsic rate depend strongly on the nature of the support. In particular, specific activity of Pt at 250 °C is 1–2 orders of magnitude higher when supported on “reducible” compared to “irreducible” metal oxides. For composite Pt/MO_x/Al₂O₃ and Pt/MO_x/TiO₂ catalysts, it is shown that the presence of MO_x results in a shift of the CO conversion curve toward lower reaction temperatures, compared to that obtained for Pt/Al₂O₃ or Pt/TiO₂, respectively. The specific reaction rate is in most cases higher for composite catalysts and varies in a manner which depends on the nature, loading, and primary crystallite size of dispersed MO_x. Results are explained by considering that reducibility of small oxide particles increases with decreasing crystallite size, thereby resulting in enhanced WGS activity. Therefore, evidence is provided that the metal oxide support is directly involved in the WGS reaction mechanism and determines to a significant extent the catalytic performance of supported noble metal catalysts. Results of catalytic performance tests obtained under realistic feed composition, consisting of 3% CO, 10% H₂O, 20% H₂ and 6% CO₂, showed that certain composite Pt/MO_x/Al₂O₃ and Pt/MO_x/TiO₂ catalysts are promising candidates for the development of active WGS catalysts suitable for fuel cell applications.     

Urea-SCR: a promising technique to reduce NOx emissions from automotive diesel engines

Urea-SCR, the selective catalytic reduction using urea as reducing agent, has been investigated for about 10 years in detail and today is a well established technique for DeNO_x of stationary diesel engines. It is presently also considered as the most promising way to diminish NO_x emissions originating from heavy duty vehicles, especially trucks.

The paper discusses the fundamental problems and challenges if urea-SCR is extended to mobile applications. The major goal is the reduction of the required catalyst volume while still maintaining a high selectivity for the SCR reaction over a wide temperature range. The much shorter residence time of the exhaust gas in the catalyst will lead to higher secondary emissions of ammonia and isocyanic acid originating from the reducing agent. Additional problems include the control strategy for urea dosing, the high freezing point of urea, and the long term stability of the catalyst.     

Fundamental study on the NOx direct decomposition catalysts

We are developing direct decomposition catalysts to decompose the NO_x involved in high temperature exhaust gases to N₂ and O₂ without any reductants such as urea and plan to bring this technology into practice in the 21st century. We expect to create very simple deNO_x systems using direct decomposition catalysts applicable to a wide range of fields (co-generation, boilers, automobiles and so on) after overcoming the technical difficulties. Perovskite catalyst and zeolite catalyst are the most promising materials for direct decomposition catalysts. This study focuses on seeking and designing novel NO_x direct decomposition catalysts having high activities through theoretical studies using computational chemistry and experimental studies using surface-science techniques.