Transient TPSR, DRIFTS-MS and TGA studies of a Pd/ceria-zirconia catalyst in CH4 and NO2 atmospheres

In this study, the parameters governing the activity of Pd/ceria-zirconia catalysts in the selective catalytic reduction (SCR) of NO_x assisted by methane are investigated using a combination of temperature-programmed spectroscopic and thermogravimetric techniques and transient SCR conditions. By DRIFTS of adsorbed CO, it is established that Pd species on Ce_0.2Zr_0.8O₂ are mainly present in cationic form but exhibit high reducibility. As found by temperature-programmed surface reaction (TPSR) in CH₄ + NO₂ atmosphere, the CH₄-SCR reaction is initiated at 280 °C on Pd/Ce_0.2Zr_0.8O₂ and yields almost 100% N₂ above 500 °C. DRIFTS-MS and TGA experiments performed under transient SCR conditions show that DeNO_x activity is due to a surface reaction between some methane oxidation products on reduced Pd sites with ad-N_xO_y species presumably located on the support. The detrimental effect of O₂ on DeNO_x is explained by the promotion of the total combustion of methane assisted by the ceria-zirconia component at the expense of the SCR reaction above 320 °C.     

Effects of Lean/Rich Timing and Nature of Reductant on the Performance of a NOx Trap Catalyst

A NO _x trap catalyst was studied in a laboratory reactor under simulated diesel passenger car conditions. The effects of lean/rich duration and the nature of reductant are investigated. At 300°C, the average NO _x conversion decreases with increasing lean duration; conversely the NO _x conversion increases with increasing rich duration. The NO _x conversion at this temperature was found to be a direct function of reaction stoichiometry. That is, the quantity of trapped NO _x under lean conditions must be balanced by the quantity of reductant during the rich trap regeneration step. At extreme temperatures, other factors, reaction kinetics (at lower temperatures) and NO _x storage capacity (at higher temperatures), dominate the NO _x conversion process. Overall, carbon monoxide was found to be the most effective reductant. Hydrocarbon, e.g., C₃H₆, is effective at higher temperatures (T>350°C), while H₂ is more efficient than other reductants at low temperatures (T<200°C). The individual steps of the NO _x conversion process are discussed.     

A reduced NOx reaction model for pulverized coal combustion under fuel-rich conditions

A reduced NO_x reaction model was developed for analysis of industrial pulverized coal firing boilers. The model was developed from experiments of laminar premixed combustion under a variety of stoichiometric ratios, burning temperatures, coal ranks (from sub-bituminous coal to anthracite) and particle diameters. Calculations agreed with experimental results for NO_x and nitrogen species (NH₃ and HCN), if the model assumed that the hydrocarbon radicals were formed not only from pyrolysis of volatile matter, but also from char oxidation and gasification. The presence of hydrogen in char at the final burnout stage supported this assumption. NO_x reduction by hydrocarbon radicals was the most important reaction in high temperature (>1500 K), fuel-rich, char combustion regions. NO_x reduction from nitrogen species was sensitive to peak NO_x concentration in volatile combustion regions, but NO_x emission downstream had little influence from the peak NO_x concentration. The heterogeneous reaction between char and NO_x was important for fuel-lean or low-temperature conditions.     

Catalytic reduction of NH4NO3 by NO: Effects of solid acids and implications for low temperature DeNOx processes

Ammonium nitrate is thermally stable below 250 °C and could potentially deactivate low temperature NO_x reduction catalysts by blocking active sites. It is shown that NO reduces neat NH₄NO₃ above its 170 °C melting point, while acidic solids catalyze this reaction even at temperatures below 100 °C. NO₂, a product of the reduction, can dimerize and then dissociate in molten NH₄NO₃ to NO⁺ + NO₃⁻, and may be stabilized within the melt as either an adduct or as HNO₂ formed from the hydrolysis of NO⁺ or N₂O₄. The other product of reduction, NH₄NO₂, readily decomposes at ≤100 °C to N₂ and H₂O, the desired end products of DeNO_x catalysis. A mechanism for the acid catalyzed reduction of NH₄NO₃ by NO is proposed, with HNO₃ as an intermediate. These findings indicate that the use of acidic catalysts or promoters in DeNO_x systems could help mitigate catalyst deactivation at low operating temperatures (<150 °C).     

Methanol interaction with NO2: An attempt to identify intermediate compounds in CH4-SCR of NO with Co/Pd-HFER catalyst

The objective of this work is the study of fundamental common aspects of NO_x catalytic reduction over a Co/Pd-HFER zeolite catalyst, using methanol or methane as reducing agent. Temperature Programmed Surface Reaction (TPSR) studies were performed with reactant mixtures comprising NO₂ and one of the reducing agents.The formation of formaldehyde was detected in both studied reactions (NO₂–CH₄ and NO₂–CH₃OH) in the temperature range between 100 and 220 °C. At higher temperature, when the NO_x reduction process effectively begins, formaldehyde starts to be consumed.Using methanol as reducing agent, nitromethane and nitrosomethane, are detected. At 300 °C these species are consumed and cyanides and iso-cyanides formation occurs. On the contrary, with methane, these last species were not detected; however, there are strong evidences for CH₃NO and CH₃NO₂ formation.Thus, using methanol or methane, similar phenomena were detected. In both cases, common intermediary species seem to play an important role in the NO_x reduction process to N₂.These results suggest that methanol can be considered as a reaction intermediate species in the mechanism of the reduction of NO₂ with methane, over cobalt/palladium-based ferrierite catalysts.     

Characterisation by TPR, XRD and NOx Storage Capacity Measurements of the Ageing by Thermal Treatment and SO2 Poisoning of a Pt/Ba/Al NOx-Trap Model Catalyst

The deactivation of a Pt/Ba/Al₂O₃ NO _x -trap model catalyst submitted to SO₂ treatment and/or thermal ageing at 800 °C was studied by H₂ temperature programmed reduction (TPR), X-ray diffraction (XRD) and NO _x storage capacity measurements.The X-ray diffractogram of the fresh sample exhibits peaks characteristic for barium carbonate. Thermal ageing leads to the decomposition of barium carbonate and to the formation of BaAl₂O₄. The TPR profile of the sulphated sample shows the presence of (i) surface aluminium sulphates, (ii) surface barium sulphates, (iii) bulk barium sulphates. The exposure to SO₂ after ageing leads to a small decrease of the surface barium-based sulphates, expected mainly as aluminate barium sulphates. This evolution can be attributed to a sintering of the storage material. TPR experiments also show that thermal treatment at 800 °C after the exposure to SO₂ involves the decomposition of aluminium surface sulphates to give mainly bulk barium sulphates, also pointed out by XRD. Thus, the thermal treatment at 800 °C leads to a stabilization of the sulphates.These results are in accordance with the NO _x storage capacity measurements. On non-sulphated catalysts, the treatment at 800 °C induces to a decrease of the NO _x storage capacity, showing that barium aluminate presents a lower NO _x storage capacity than barium carbonate. Sulphation strongly decreases the NO _x storage capacity of catalysts, whatever the initial thermal treatment, showing that barium sulphates inhibit the NO₂ adsorption. Moreover, the platinum activity for the NO to NO₂ oxidation is lowered by thermal treatments.     

Selective Catalytic Reduction of NOx over H-ZSM-5 Under Lean Conditions Using Transient NH3 Supply

The selective catalytic reduction (SCR) of NO _x over zeolite H-ZSM-5 with ammonia was investigated using in situ FTIR spectroscopy and flow reactor measurements. The adsorption of ammonia and the reaction between NO _x , O₂ and either pre-adsorbed ammonia or transiently supplied ammonia were investigated for either NO or equimolar amounts of NO and NO₂. With transient ammonia supply the total NO reduction increased and the selectivity to N₂O formation decreased compared to continuous supply. The FTIR experiments revealed that NO _x reacts with ammonia adsorbed on Brønsted acid sites as NH₄ ⁺ ions. These experiments further indicated that adsorbed -NO₂ ^– is formed during the SCR reaction over H-ZSM-5.     

Fundamental Investigations of Thermal Aging Phenomena of Model NOx Storage Systems

Investigations of the aging behavior induced by high temperatures coupled with oxidizing atmosphere of model NO _x storage systems Ba/Al₂O₃ and Ba/CeO₂ are reported in this paper. The samples were prepared, calcined and exposed to temperatures between 500 and 1000 °C in air for 12 h for thermal aging. Samples were characterized with XRD, HRSEM, DSC-TGA-MS and BET analyses. In XRD investigations of all model systems calcined at 500 °C for 2 h, the NO _x storage component was present in form of BaCO₃. The release of CO₂ as a result of the decarbonization of the NO _x storage component at increased temperatures was verified by thermogravimetric investigations. In the case of Ba/Al₂O₃, already during calcination a partial reaction of the NO _x storage component with Al₂O₃ resulting in the formation of barium aluminate was observed. In the model system Ba/CeO₂ the decomposition of the barium carbonate started above 780 °C and the formation of a barium cerium mixed oxide was observed. The presence of the barium containing NO _x storage component has a strong influence on the specific surface area of the model NO _x storage systems. The morphology and crystallite size of CeO₂ modified with the barium containing NO _x storage component exhibited distinct changes compared to the unmodified oxide. The NO _x storage efficiency determined by model gas tests of freshly prepared and engine aged model NO _x storage catalysts correlates well with the above described observations.     

Influence of low concentration of SO2 for selective reduction of NO by C3H8 in lean-exhaust conditions on the activity of Ag/Al2O3 catalyst

The effect of SO₂ for the selective reduction of NO by C₃H₈ on Ag/Al₂O₃ was investigated in the presence of excess oxygen and water vapor. The NO_x conversion decreased permanently even in the presence of a low concentration of SO₂ (0.5–10 ppm) at <773 K. The increase in SO₂ concentration resulted in a large decrease in NO_x conversion at 773 K. However, when the reaction temperature was more than 823 K, the activity of Ag/Al₂O₃ remained constant even in the presence of 10 ppm of SO₂. The sulfate species formed on the used Ag/Al₂O₃ were characterized by a temperature programmed desorption method. The sulfated species formed on silver should mainly decrease the deNO_x activity on the Ag/Al₂O₃. The sulfated Ag/Al₂O₃ was appreciably regenerated by thermal treatment in the deNO_x feed at 873 K. The moderate activity remains at 773 K in the presence of 1 ppm SO₂ for long time by the heat treatment at every 20 h intervals.     

Influence of the Storage Material on the Storage of NOx at Low Temperatures

The NO _x storage performance at low temperature (100–200 °C) has been studied for model NO _x storage catalysts. The catalysts were prepared by sequentially depositing support, metal oxide and platinum on ceramic monoliths. The support material consisted of acidic aluminium silicate, alumina or basic aluminium magnesium oxide, and the added metal oxide was either ceria or barium oxide. The NO _x conversion was evaluated under net-oxidising conditions with transients between lean and rich gas composition and the NO _x storage performance was studied by isothermal adsorption of NO₂ followed by temperature programmed desorption of adsorbed species. The maximum in NO _x storage capacity was observed at 100 °C for all samples studied. The Pt/BaO/Al₂O₃ catalyst stored about twice the amount of NO _x compared with the Pt/Al₂O₃ and Pt/CeO₂/Al₂O₃ samples. The storage capacity increased with increasing basicity of the support material, i.e. Pt/Al₂O₃·SiO₂ < Pt/Al₂O₃ < Pt/Al₂O₃ · MgO. Water did not significantly affect the NO _x storage performance for Pt/Al₂O₃ or Pt/BaO/Al₂O₃.     

Sorption–Desorption of NOx from a Lean Gas Mixture on H3PW12O40·6H2O Supported on Carbon Nanotubes

NO _x sorption capacities and efficiencies were measured on a new type of sorbent formed by 12-tungstophosphoric acid (HPW) supported on carbon nanotubes. On such a system, the sorption of both NO and NO₂ was observed but compared with HPW alone, a complementary sorption of NO _x is possible leading to a capacity of 25 mg/g_HPW at 300 °C with an efficiency of 50%. The sorption results from the formation of a [H⁺(NO₂ ^–,NO⁺)] complex on HPW and an additional mode of adsorption by a free-nitrate which was identified by the bands at 2261, 1384 and 1295 cm^–1 using infrared spectroscopy.     

NOx adsorption and diesel soot combustion over La2O3 supported catalysts containing K, Rh and Pt

In this work we report results of NOx adsorption and diesel soot combustion on a noble metal promoted K/La₂O₃ catalyst. The fresh-unpromoted solid is a complex mixture of hydroxide and carbonate compounds, but the addition of Rh favors the preferential formation of lanthanum oxycarbonate during the calcination step. K/La₂O₃ adsorbs NOx through the formation of La and K nitrate species when the solid is treated in NO + O₂ between 70 and 490 °C. Nitrates are stable in the same temperature range under helium flow. However, they become unstable at ca. 360 °C when either Rh and/or Pt are present, the effect of Rh being more pronounced. Nitrates decompose under different atmospheres: NO + O₂, He and H₂. The effect of Rh might be to form a thermally unstable complex (Rh–NO⁺) which takes part both in the formation of the nitrates when the catalyst is exposed to NOx and in the nitrates decomposition at higher temperatures. Regarding soot combustion, nitrates react with soot with a temperature of maximun reaction rate of ca. 370 °C, under tight contact conditions. This temperature is not affected by the presence of Rh, which indicates that the stability of nitrates has little effect on their reaction with soot.     

Selective catalytic reduction of NOx using propene and ethanol over catalysts of Ag/Al2O3 prepared by microemulsion and promotional effect of hydrogen

The present study explores the possibilities of catalysts of Ag/Al₂O₃, in which silver has been deposited using reverse microemulsions with the aim of getting maximum dispersion and homogeneity in the active superficial species, for the selective catalytic reduction of NO_x in excess of oxygen, using both propene and ethanol as reductants and in the scope of the control of the emissions produced by vehicles that operate in conditions of lean mixture like the diesel engine or those of gasoline direct injection. The promotional effect of the hydrogen presence in the reactive mixture has also been analyzed. For both reductants, when in presence of hydrogen, an important enhancement in NO_x conversion is produced, in particular for a catalyst with 3 wt.% silver. The production of acetaldehyde during the reaction employing ethanol is also analyzed and its role on the NO_x reduction process has been examined. The interpretation of catalytic properties has been complemented by means of in-situ DRIFTS.     

The Effect of Carbon Monoxide and Hydrocarbons on NOx Storage at Low Temperature

One possible way to reduce NO _x in lean exhausts is by using NO _x trap catalysts. This paper addresses storage of NO _x on such catalysts at temperatures below the catalyst light-off. Experiments carried out on commercial samples in synthetic exhausts revealed a large capacity for storage of NO _x when NO₂ was added at temperatures below 150°C. In contrast, when NO was added instead, no storage took place. CO was found to decrease the storage by reacting with NO₂ and forming NO and CO₂. Propene inhibited the reaction between NO₂ and CO and therefore gave rise to larger NO _x storage when CO was present. The paper concludes with a discussion of a possible mechanism for the storage of NO _x at low temperatures.     

Influence of the Type of Reducing Agent (H2, CO, C3H6 and C3H8) on the Reduction of Stored NOX in a Pt/BaO/Al2O3 Model Catalyst

In this investigation, a comparative study for a NO _X storage catalytic system was performed focusing on the parameters that affect the reduction by using different reductants (H₂, CO, C₃H₆ and C₃H₈) and different temperatures (350, 250 and 150 °C), for a Pt/BaO/Al₂O₃ catalyst. Transient experiments show that H₂ and CO are highly efficient reductants compared to C₃H₆ which is somewhat less efficient. H₂ shows a significant reduction effect at relatively low temperature (150 °C) but with a low storage capacity. We find that C₃H₈does not show any NO _X reduction ability for NO _X stored in Pt/BaO/Al₂O₃ at any of the temperatures. The formation of ammonia and nitrous oxide is also discussed.     

Kinetic modeling of storage and reduction with different reducing agents (CO, H2, and C2 H4) on a Pt–Ba/-Al2O3 catalyst in the presence of CO2 and H2O

In this paper a global reaction kinetic model is used to understand and describe the NO_x storage/reduction process in the presence of CO₂ and H₂O. Experiments have been performed in a packed bed reactor with a Pt–Ba/γ-Al₂O₃ powder catalyst (1 wt% Pt and 30 wt% Ba) with different lean/rich cycle timings at different temperatures (200, 250, and ) and using different reductants (H₂, CO, and C₂H₄). Model simulations and experimental results are compared. H₂O inhibits the NO oxidation capability of the catalyst and no NO₂ formation is observed. The rate of NO storage increases with temperature. The reduction of stored NO with H₂ is complete for all investigated temperatures. At temperatures above , the water gas shift (WGS) reaction takes place and H₂ acts as reductant instead of CO. At , CO and C₂H₄ are not able to completely regenerate the catalyst. At the higher temperatures, C₂H₄ is capable of reducing all the stored NO, although C₂H₄ poisons the Pt sites by carbon decomposition at . The model adequately describes the NO breakthrough profile during 100 min lean exposure as well as the subsequent release and reduction of the stored NO. Further, the model is capable of simulating transient reactor experiments with 240 s lean and 60 s rich cycle timings.     

Dynamics and selectivity of NOx reduction in NOx storage catalytic monolith

Several nitrogen compounds can be produced during the regeneration phase in periodically operated NO_x storage and reduction catalyst (NSRC) for conversion of automobile exhaust gases. Besides the main product N₂, also NO, N₂O, and NH₃ can be formed, depending on the regeneration phase length, temperature, and gas composition. This contribution focuses on experimental evaluation of the NO_x reduction dynamics and selectivity towards the main products (NO, N₂ and NH₃) within the short rich phase, and consequent development of the corresponding global reaction-kinetic model. An industrial NSRC monolith sample of PtRh/Ba/CeO₂/ -Al₂O₃ type is employed in nearly isothermal laboratory micro-reactor. The oxygen and NO_x storage/reduction experiments are performed in the temperature range 100–500 °C in the presence of CO₂ and H₂O, using H₂, CO and C₃H₆ as the reducing agents.The spatially distributed NSRC model developed earlier is extended by the following reactions: NH₃ is formed by the reaction of H₂ with NO_x and it can further react with oxygen and NO_x deposited on the catalyst surface, producing N₂. Considering this scheme with ammonia as an active intermediate of the NO_x reduction, a good agreement with experiments is obtained in terms of the NO_x reduction dynamics and selectivity. A reduction front travelling in the flow direction along the reactor is predicted, with the NH₃ maximum on the moving boundary. When the front reaches the reactor outlet, the NH₃ peak is observed in the exhaust gas. It is assumed that the ammonia formation during the NO_x reduction by CO and HCs at higher temperatures proceed via the water gas shift and steam reforming reactions producing hydrogen. It is further demonstrated that oxygen storage effects influence the dynamics of the stored NO_x reduction. The temperature dependences of the outlet ammonia peak delay and the selectivity towards NH₃ are correlated with the effective oxygen and NO_x storage capacity.     

Catalytic Properties of Ag/Al2O3 Catalysts for Lean NOx Reduction Processes and Characterisation of Active Silver Species

A study of the lean NO _x reduction activity employing different reductants over Ag/Al₂O₃ samples prepared from reverse microemulsions or impregnation with EDTA-complexes is presented. A multitechnique approach is employed for characterisation of the samples and/or processes taking place in the course of the NO _x -SCR reaction with propene and propane. Results by in situ-DRIFTS reveal that, for the propene reductant, silver provides a new path for hydrocarbon activation involving generation of adsorbed acrylate species as a partially oxidised active intermediate, in line with previous proposals for other non-noble metal systems. It is shown, mainly on the basis of XAFS studies, that active silver species are related to well dispersed silver aluminate-like phases with tetrahedral local symmetry and a relatively high disorder in the oxygen first shell.     

NOx storage behavior and sulfur-resisting performance of the third-generation NSR catalysts Pt/K/TiO2–ZrO2

The NO_x storage and reduction (NSR) catalysts Pt/K/TiO₂–ZrO₂ were prepared by an impregnation method. The techniques of XRD, NH₃-TPD, CO₂-TPD, H₂-TPR and in situDRIFTS were employed to investigate their NO_x storage behavior and sulfur-resisting performance. It is revealed that the storage capacity and sulfur-resisting ability of these catalysts depend strongly on the calcination temperature of the support. The catalyst with theist support calcined at 500 °C, exhibits the largest specific surface area but the lowest storage capacity. With increasing calcination temperature, the NO_x storage capacity of the catalyst improves greatly, but the sulfur-resisting ability of the catalyst decreases. In situ DRIFTS results show that free nitrate species and bulk sulfates are the main storage and sulfation species, respectively, for all the catalysts studied. The CO₂-TPD results indicate that the decomposition performance of K₂CO₃ is largely determined by the surface property of the TiO₂–ZrO₂ support. The interaction between the surface hydroxyl of the support and K₂CO₃ promotes the decomposition of K₂CO₃ to form –OK groups bound to the support, leading to low NO_x storage capacity but high sulfur-resisting ability, while the interaction between the highly dispersed K₂CO₃ species and Lewis acid sites gives rise to high NO_x storage capacity but decreased sulfur-resisting ability. The optimal calcination temperature of TiO₂–ZrO₂ support is 650 °C.