The selective reduction of NOx with propene on Pt-beta catalyst: A transient study

The mechanism of the NO/C₃H₆/O₂ reaction has been studied on a Pt-beta catalyst using transient analysis techniques. This work has been designed to provide answers to the volcano-type activity behaviour of the catalytic system, for that reason, steady state transient switch (C₃H₆/NO/O₂ → C₃H₆/Ar/O₂, C₃H₆/Ar/O₂ → C₃H₆/NO/O₂, C₃H₆/NO/O₂ → Ar/NO/O₂, Ar/NO/O₂ → C₃H₆/NO/O₂, C₃H₆/NO/O₂ → C₃H₆/NO/Ar and C₃H₆/NO/Ar → C₃H₆/NO/O₂) and thermal programmed desorption (TPD) experiments were conducted below and above the temperature of the maximum activity (T_max). Below T_max, at 200 °C, a high proportion of adsorbed hydrocarbon exists on the catalyst surface. There exists a direct competition between NO and O₂ for Pt free sites which is very much in favour of NO, and therefore, NO reduction selectively takes place over hydrocarbon combustion. NO and C₃H₆ are involved in the generation of partially oxidised hydrocarbon species. O₂ is essential for the oxidation of these intermediates closing the catalytic cycle. NO₂ is not observed in the gas phase. Above T_max, at 230 °C, C₃H_6 ads coverage is negligible and the surface is mainly covered by O_ads produced by the dissociative adsorption of O₂. NO₂ is observed in gas phase and carbon deposits are formed at the catalyst surface. From these results, the state of Pt-beta catalyst at T_max is inferred. The reaction proceeds through the formation of partially oxidised active intermediates (C_xH_yO_zN_w) from C₃H_6 ads and NO_ads. The combustion of the intermediates with O₂(g) frees the Pt active sites so the reaction can continue. Temperature has a positive effect on the surface reaction producing active intermediates. On the contrary, formation of NO_ads and C₃H_6 ads are not favoured by an increase in temperature. Temperature has also a positive effect on the dissociation of O₂ to form O_ads, consequently, the formation of NO₂ is favoured by temperature through the oxygen dissociation. NO₂ is very reactive and produces the propene combustion without NO reduction. These facts will determine the maximum concentration of active intermediates and consequently the maximum of activity.     

The origin of N2O formation in the selective catalytic reduction of NOx by NH3 in O2 rich atmosphere on Cu-faujasite catalysts

The selective catalytic reduction (SCR) of NO_x (NO + NO₂) by NH₃ in O₂ rich atmosphere has been studied on Cu-FAU catalysts with Cu nominal exchange degree from 25 to 195%. NO₂ promotes the NO conversion at NO/NO₂ = 1 and low Cu content. This is in agreement with next-nearest-neighbor (NNN) Cu ions as the most active sites and with N_xO_y adsorbed species formed between NO and NO₂ as a key intermediate. Special attention was paid to the origin of N₂O formation. CuO aggregates form 40–50% of N₂O at ca. 550 K and become inactive for the SCR above 650 K. NNN Cu ions located within the sodalite cages are active for N₂O formation above 600 K. This formation is greatly enhanced when NO₂ is present in the feed, and originated from the interaction between NO (or NO₂) and NH₃. The introduction of selected co-cations, e.g. Ba, reduces very significantly this N₂O formation.     

The low-temperature performance of NOx storage and reduction catalyst

The catalytic performance and the behavior of NO_x storage and reduction (NSR) over a model catalyst for lean-burn gasoline engines have been mainly investigated and be discussed based on the temperature and reducing agents use in this study. The experimental results have shown that the NO_x storage amount in the lean atmosphere was the same as the NO_x reduction amount from the subsequent rich spike (RS) above the temperature of 400 °C, while the former was greater than the latter below the temperature of 400 °C. This indicated that when the temperature was below 400 °C compared with the NO_x storage stage, the reduction of the stored NO_x is somehow restricted. We found that the reduction efficiencies with the reducing agents decrease in the order H₂ > CO > C₃H₆ below 400 °C, thus not all of the NO_x storage sites could be fully regenerated even using an excessive reducing agent of CO or C₃H₆, which was supplied to the NSR catalyst, while all the NO_x storage sites could be fully regenerated if an adequate amount of H₂ was supplied. We also verified that the H₂ generation more favorably occurred through the water gas shift reaction than through the steam reforming reaction. This difference in the H₂ generation could reasonably explain why CO was more efficient for the reduction of the stored NO_x than C₃H₆, and hinted as a promising approach to enhance the low-temperature performance of the current NSR catalysts though promoting the H₂ generation reaction.     

Impact of redox conditions on thermal deactivation of NOx traps for diesel

Performance of NO_x traps after high-temperature treatments in different redox environments was studied. Two types of treatments were considered: aging and pretreatment. Lean and rich agings were examined for a model NO_x trap, Pt–Ba/Al₂O₃. These were done at 950 °C for 3 h, in air and in 1% H₂/N₂, respectively. Lean aging had a severe impact on NO_x trap performance, including HC and CO oxidation, and NH₃ and N₂O formation. Rich aging had minimal impact on performance, compared to fresh/degreened performance. Deactivation from lean aging was essentially irreversible due to Pt sintering, but Pt remained dispersed with the rich aging. Pretreatments were examined for a commercially feasible fully formulated NO_x trap and two model NO_x traps, Pt–Ba/Al₂O₃ and Pt–Ba–Ce/Al₂O₃. Pretreatments were done at 600 °C for 10 min, and used feed gas that simulated diesel exhaust under several conditions. Lean pretreatment severely suppressed NO_x, HC, CO, NH₃ and N₂O activities for the ceria-containing NO_x traps, but had no impact on Pt–Ba/Al₂O₃. Subsequently, a relatively mild rich pretreatment reversed this deactivation, which appears to be due to a form of Pt–ceria interaction, an effect that is well known from early work on three-way catalysts. Practical applications of results of this work are discussed with respect to NO_x traps for light-duty diesel vehicles.     

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.     

Zeolite-mediated removal of NOx by NH3 from exhaust streams at low temperatures

NH₃ stored on zeolites in the form of NH₄⁺ ions easily reacts with NO to N₂ in the presence of O₂ at temperatures <373 K under dry conditions. Wet conditions require a modification of the catalyst system. It is shown that MnO₂ deposited on the external surface of zeolite Y by precipitation considerably enhances the NO_x conversion by zeolite fixed NH₄⁺ ions in the presence of water at 400–430 K. Particle-size analysis, temperature-programmed reduction, textural characterization, chemical analysis, ESR and XRD gave a subtle picture of the MnO₂ phase structure. The MnO₂ is a non-stoichiometric, amorphous phase that contains minor amounts of Mn²⁺ ions. It loses O₂ upon inert heating up to 873 K, but does not crystallize or sinter. The phase is reducible by H₂ in two stages via intermediate formation of Mn₃O₄. The manufacture of extrudates preserving stored NH₄⁺ ions for NO_x reduction is described. It was found that MnO₂ can oxidize NO by bulk oxygen. This enables the reduction of NO to N₂ by the zeolitic NH₄⁺ ions without gas-phase oxygen for limited time periods. The composite catalyst retains storage capacity for both, oxygen and NH₄⁺ ions despite the presence of moisture and allows short-term reduction of NO without gaseous O₂ or additional reductants. The catalyst is likewise suitable for steady-state DeNO_x operation at higher space velocities if gaseous NH₃ is permanently supplied.     

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.     

Transient analysis of the release and reduction of NOx using a Pt/Ba/Al2O3 catalyst

The release and reduction of NO_x in a NO_x storage-reduction (NSR) catalyst were studied with a transient reaction analysis in the millisecond range, which was made possible by the combination of pulsed injection of gases and time resolved time-of-flight mass spectrometry. After an O₂ pulse and a subsequent NO pulse were injected into a pellet of the Pt/Ba/Al₂O₃ catalyst, the time profiles of several gas products, NO, N₂, NH₃ and H₂O, were obtained as a result of the release and reduction of NO_x caused by H₂ injection. Comparing the time profiles in another analysis, which were obtained using a model catalyst consisting of a flat 5 nmPt/Ba(NO₃)₂/cordierite plate, the release and reduction of NO_x on Pt/Ba/Al₂O₃ catalyst that stored NO_x took the following two steps; in the first step NO molecules were released from Ba and in the second step the released NO was reduced into N₂ by H₂ pulse injection. When this H₂ pulse was injected in a large amount, NO was reduced to NH₃ instead of N₂.

A only small amount of H₂O was detected because of the strong affinity for alumina support. We can analyze the NO_x regeneration process to separate two steps of the NO_x release and reduction by a detailed analysis of the time profiles using a two-step reaction model. From the result of the analysis, it is found that the rate constant for NO_x release increased as temperature increase.     

SCR of NOx with CH3OH on H-mordenite: mechanism and reaction intermediates

The selective reduction of NO_x over H-mordenite (H-m) was studied using CH₃OH as reducing agent. Results are compared with those obtained with other conventional reducing agents (ethylene and methane), with gas-phase reactions, and with other metal-exchanged mordenites (Cu-mordenite (Cu-m) and Co-mordenite (Co-m)). H-m was found to be an effective catalyst for the SCR of NO_x with CH₃OH. When different reducing agents were compared over H-m, CH₃OH > C₂H₄ > CH₄ was the order according to the maximum NO conversion obtained using 1% of oxygen in the feed. Instead, if selectivity is considered, the order results CH₄ > CH₃OH > C₂H₄. In reaction experiments, two distinct zones defined by two maxima with NO to N₂ conversion are obtained at two different temperatures. A correlation exists between the said zones and the CO : CO₂ ratio. At low temperatures, CO prevails whereas at high temperatures CO₂ prevails. These results indicate that there exist different reaction intermediates. Evidence from reaction experiments, FTIR results, and transient experiments suggest that the reaction mechanism involves formaldehyde and dimethyl ether (DME) as intermediates in the 200–500°C temperature range. The surface interaction between CH₃OH (or its decomposition products) and NO is negligible if compared with NO₂, indicating that the oxidation of NO to NO₂ on acid sites is a fundamental path in this system. Different from other non-oxygenated reductants (methane and ethylene), a gas-phase NO_x initiation effect on hydrocarbon combustion was not observed.     

Influence of CO2 and H2O on NOx storage and reduction on a Pt–Ba/γ-Al2O3 catalyst

In this paper, the effect of CO₂ and H₂O on NO_x storage and reduction over a Pt–Ba/γ-Al₂O₃ (1 wt.% Pt and 30 wt.% Ba) catalyst is shown. The experimental results reveal that in the presence of CO₂ and H₂O, NO_x is stored on BaCO₃ sites only. Moreover, H₂O inhibits the NO oxidation capability of the catalyst and no NO₂ formation is observed. Only 16% of the total barium is utilized in NO storage. The rich phase shows 95% selectivity towards N₂ as well as complete regeneration of stored NO. In the presence of CO₂, NO is oxidized into NO₂ and more NO_x is stored as in the presence of H₂O, resulting in 30% barium utilization. Bulk barium sites are inactive in NO_x trapping in the presence of CO₂·NH₃ formation is seen in the rich phase and the selectivity towards N₂ is 83%. Ba(NO₃)₂ is always completely regenerated during the subsequent rich phase. In the absence of CO₂ and H₂O, both surface and bulk barium sites are active in NO_x storage. As lean/rich cycling proceeds, the selectivity towards N₂ in the rich phase decreases from 82% to 47% and the N balance for successive lean/rich cycles shows incomplete regeneration of the catalyst. This incomplete regeneration along with a 40% decrease in the Pt dispersion and BET surface area, explains the observed decrease in NO_x storage.     

Influence of NO2 on the selective catalytic reduction of NO with ammonia over Fe-ZSM5

The influence of NO₂ on the selective catalytic reduction (SCR) of NO with ammonia was studied over Fe-ZSM5 coated on cordierite monolith. NO₂ in the feed drastically enhanced the NO_x removal efficiency (DeNOx) up to 600 °C, whereas the promoting effect was most pronounced at the low temperature end. The maximum activity was found for NO₂/NO_x = 50%, which is explained by the stoichiometry of the actual SCR reaction over Fe-ZSM5, requiring a NH₃:NO:NO₂ ratio of 2:1:1. In this context, it is a special feature of Fe-ZSM5 to keep this activity level almost up to NO₂/NO_x = 100%. The addition of NO₂ to the feed gas was always accompanied by the production of N₂O at lower and intermediate temperatures. The absence of N₂O at the high temperature end is explained by the N₂O decomposition and N₂O-SCR reaction. Water and oxygen influence the SCR reaction indirectly. Oxygen enhances the oxidation of NO to NO₂ and water suppresses the oxidation of NO to NO₂, which is an essential preceding step of the actual SCR reaction for NO₂/NO_x < 50%. DRIFT spectra of the catalyst under different pre-treatment and operating conditions suggest a common intermediate, from which the main product N₂ is formed with NO and the side-product N₂O by reaction with gas phase NO₂.     

CO2, NOx and SO2 emissions from the combustion of coal with high oxygen concentration gases

The emissions of CO₂, NO_x and SO₂ from the combustion of a high-volatile coal with N₂- and CO₂-based, high O₂ concentration (20, 50, 80, 100%) inlet gases were investigated in an electrically heated up-flow-tube furnace at elevated gas temperatures (1123–1573 K). The fuel equivalence ratio, φ, was varied in the range of 0.4–1.6. Results showed that CO₂ concentrations in flue gas were higher than 95% for the processes with O₂ and CO₂-based inlet gases. NO_x emissions increased with φ under fuel-lean conditions, then declined dramatically after φ=0.8, and the peak values increased from about 1000 ppm for the air combustion process and 500 ppm for the O₂(20%)+CO₂(80%) inlet gas process to about 4500 ppm for the oxygen combustion process. When φ>1.4 the emissions decreased to the same level for different O₂ concentration inlet gas processes. On the other hand, NO_x emission indexes decreased monotonically with φ under both fuel-lean and fuel-rich combustion. SO₂ emissions increased with φ under fuel-lean conditions, then declined slightly after φ>1.2. Temperature has a large effect on the NO_x emission. Peak values of the NO_x emission increased by 50–70% for the N₂-based inlet gas processes and by 30–50% for the CO₂-based inlet gas process from 1123 to 1573 K. However, there was only a small effect of temperature on the SO₂ emission.     

On the dynamic behavior of “NOx-storage/reduction” Pt–Ba/Al2O3 catalyst

The NO_x storage and reduction functions of a Pt–Ba/Al₂O₃ “NO_x storage–reduction” catalyst has been investigated in the present work by applying the transient response and the temperature programmed reaction methods, by using propylene as the reducing agent. It is found that: (i) the storage of NO_x occurs first at BaO and then at BaCO₃, which are the most abundant sites following regeneration of catalyst with propylene; (ii) the overall storage process at BaCO₃ is slower than at BaO; (iii) CO₂ inhibits the NO_x storage at low temperatures; (iv) the amount of NO_x stored up to catalyst saturation at 350 °C corresponds to 17.6% of Ba; (v) the reduction of stored NO_x groups is fast and is limited by the concentration of propylene in the investigated T range (250–400 °C); (vi) selectivity to N₂ is almost complete at 400 °C but is significantly lower at 300 °C due to the formation of NO which can be tentatively ascribed to the presence of unselective Pt–O species.     

Co3O4 based catalysts for NO oxidation and NOx reduction in fast SCR process

Reaction activities of several developed catalysts for NO oxidation and NO_x (NO + NO₂) reduction have been determined in a fixed bed differential reactor. Among all the catalysts tested, Co₃O₄ based catalysts are the most active ones for both NO oxidation and NO_x reduction reactions even at high space velocity (SV) and low temperature in the fast selective catalytic reduction (SCR) process. Over Co₃O₄ catalyst, the effects of calcination temperatures, SO₂ concentration, optimum SV for 50% conversion of NO to NO₂ were determined. Also, Co₃O₄ based catalysts (Co₃O₄-WO₃) exhibit significantly higher conversion than all the developed DeNO_x catalysts (supported/unsupported) having maximum conversion of NO_x even at lower temperature and higher SV since the mixed oxide Co-W nanocomposite is formed. In case of the fast SCR, N₂O formation over Co₃O₄-WO₃ catalyst is far less than that over the other catalysts but the standard SCR produces high concentration of N₂O over all the catalysts. The effect of SO₂ concentration on NO_x reduction is found to be almost negligible may be due to the presence of WO₃ that resists SO₂ oxidation.     

Analysis of the thermodynamic feasibility of NOx decomposition catalysis to meet next generation vehicle NOx emissions standards

Free energy minimization calculations are used to determine the thermodynamic equilibrium concentrations of NO_x and other species in stoichiometric and lean gas mixtures over a range of temperatures and compositions. Under lean (excess N₂ and O₂) conditions, the NO decomposition (NO↔(1/2)N₂+(1/2)O₂) and NO oxidation (NO+(1/2)O₂↔NO₂) equilibria impose lower bounds on the NO_x concentrations achievable by thermodynamic equilibration or NO_x decomposition, and these equilibrium NO_x concentrations can be practically significant. Assuming a perfect isothermal catalyst acting on a representative diesel exhaust stream collected over the federal test procedure (FTP) cycle, equilibrium NO_x levels exceed upcoming California Low Emission Vehicle II (LEV-II) and Tier II NO_x emissions standards for automobiles and trucks at temperatures above approximately 800 K. Consideration of a perfect adiabatic catalyst acting on the same diesel exhaust shows that equilibrium NO_x values can fall below NO_x emissions standards at lower temperatures, but to achieve these low concentrations would require the catalyst to attain 100% approach to equilibrium at very low temperatures. It is concluded that NO_x removal based on a thermodynamic equilibrating catalyst under lean exhaust conditions is not practically viable for automotive application, and that to achieve upcoming NO_x standards will require selective NO_x catalysts that vigorously promote NO_x reactions with reductant and do not promote NO decomposition or oxidation. Finally, the ability of a selective NO_x catalyst system to reduce NO_x concentrations to or below thermodynamic equilibrium values is proposed as a useful measure for selective catalytic reduction (SCR) activity.     

High temperature catalytic combustion of methane and propane over hexaaluminate catalysts: NOx emission characteristics

Several hexaaluminate-related materials were prepared via hydrolysis of alkoxide and powder mixing method for high temperature combustion of CH₄ and C₃H₈, in order to investigate the effect of the concentration of the fuels, O₂ and H₂O on NO_x emission and combustion characteristics. Among the hexaaluminate catalysts, Sr_0.8La_0.2MnAl₁₁O₁₉₋ prepared by the alkoxide method exhibited the highest activity for methane combustion and low NO_x emission capability. NO_x emission at 1500 °C was increased linearly with O₂ concentration, whereas water vapor addition decreased NO_x emission in CH₄ combustion over the Sr_0.8La_0.2MnAl₁₁O₁₉₋ catalyst. In the catalytic combustion of C₃H₈ over the Sr_0.8La_0.2MnAl₁₁O₁₉₋ catalyst, the amount of NO_x emitted was raised in the temperature range between 1000 and 1500 °C when the C₃H₈ concentration increased from 1 to 2 vol.%. It was found that NO_x emission in this temperature range was reduced effectively by adding water vapor.     

NOx storage and reduction characteristics of Pd/MnOx–CeO2 at low temperature

The reaction between hydrogen and NO was studied over 1 wt.% Pd supported on NO_x-sorbing material, MnO_x–CeO₂, at low temperatures. The result of pulse mode reactions suggest that NO_x adsorbed as nitrate and/or nitrite on MnO_x–CeO₂ was reduced by hydrogen, which was spilt-over from Pd catalyst. The NO_x storage and reduction (NSR) cycles were carried out over Pd/MnO_x–CeO₂ in a conventional flow reactor at 150 °C. In a storage step, NO was removed by the oxidative adsorption from a stream of 0.04–0.08% NO, 5–10% O₂, and He balance. This was followed by a reducing step, where a stream of 1% H₂/He was supplied to ensure the conversion of nitrate/nitrite to N₂ and thus restore the adsorbability. It was revealed that the NSR cycle is much more suitable for the H₂–deNO_x process in excess O₂, compared to a conventional steady state reaction mode.     

Kinetic modelling of sulfur deactivation of Pt/BaO/Al2O3 and BaO/Al2O3 NOx storage catalysts

In this work, a kinetic model is constructed to simulate sulfur deactivation of the NO_x storage performance of BaO/Al₂O₃ and Pt/BaO/Al₂O₃ catalysts. The model is based on a previous model for NO_x storage under sulfur-free conditions. In the present model the storage of NO_x is allowed on two storage sites, one for complete NO_x uptake and one for a slower NO_x sorption. The adsorption of SO_x is allowed on both of these NO_x storage sites and on one additional site which represent bulk storage. The present model is built-up of six sub-models: (i) NO_x storage under sulfur-free conditions; (ii) SO₂ storage on NO_x storage sites; (iii) SO₂ oxidation; (iv) SO₃ storage on bulk sites; (v) SO₂ interaction with platinum in the presence of H₂; (vi) oxidation of accumulated sulfur compounds on platinum by NO₂. Data from flow reactor experiments are used in the implementation of the model. The model is tested for simulation of experiments for NO_x storage before exposure to sulfur and after pre-treatments either with SO₂ + O₂ or SO₂ + H₂. The simulations show that the model is able to describe the main features observed experimentally.     

Catalytic reduction of NOx with H2/CO/CH4 over PdMOR catalysts

Conversion of NO_x with reducing agents H₂, CO and CH₄, with and without O₂, H₂O, and CO₂ were studied with catalysts based on MOR zeolite loaded with palladium and cerium. The catalysts reached high NO_x to N₂ conversion with H₂ and CO (>90% conversion and N₂ selectivity) range under lean conditions. The formation of N₂O is absent in the presence of both H₂ and CO together with oxygen in the feed, which will be the case in lean engine exhaust. PdMOR shows synergic co-operation between H₂ and CO at 450–500 K. The positive effect of cerium is significant in the case of H₂ and CH₄ reducing agent but is less obvious with H₂/CO mixture and under lean conditions. Cerium lowers the reducibility of Pd species in the zeolite micropores. The catalysts showed excellent stability at temperatures up to 673 K in a feed with 2500 ppm CH₄, 500 ppm NO, 5% O₂, 10% H₂O (0–1% H₂), N₂ balance but deactivation is noticed at higher temperatures. Combining results of the present study with those of previous studies it shows that the PdMOR-based catalysts are good catalysts for NO_x reduction with H₂, CO, hydrocarbons, alcohols and aldehydes under lean conditions at temperatures up to 673 K.     

Selective catalytic reduction of N2O and NOx in a single reactor in the nitric acid industry

The nitric acid industry is a source of both NO_x and N₂O. The simultaneous selective catalytic reduction of both compounds using propane as a reductant has been investigated. A stacked catalyst bed with first a Co-ZSM-5 catalyst and second a Pd/Fe-ZSM-5 catalyst gives >80% conversion of N₂O and NO_x above 300 °C at atmospheric pressure. At 4 bar absolute pressure (bara) the Co-ZSM-5 DeNO_x catalyst shows higher NO_x and propane conversion. This leaves not enough propane for the Pd/Fe-ZSM-5 DeN₂O catalyst, which causes a ‘dip’ in N₂O conversion. Reducing the space velocity (SV) of the first catalyst bed secures high NO_x and N₂O conversions from 300 °C and up at 4 bara.