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.     

NOx storage on barium-containing three-way catalyst in the presence of CO2

NO_x trapping capability of NO_x storage–reduction commercial catalysts (4–9 wt% Ba-containing three-way catalysts) was compared to that of bulk barium carbonate and alumina-supported barium carbonate from Rhodia (9 wt% Ba). These samples were characterized by infrared spectroscopy, X-ray diffraction, HRTEM and EDX. It was shown that bulk barium carbonate was partially converted to barium nitrate in flowing NO/O₂ mixture without CO₂. Thermodynamic calculation showed that bulk barium nitrate could not form in the presence of CO₂-containing gas exhausts. Using HRTEM and EDX, it was evidenced that barium was engaged as either large barium carbonate crystals or highly dispersed barium species on the alumina support. NO_x storage experiments using gas mixtures containing or not O₂ or CO₂, confirmed firstly that NO was stored on barium trap only via NO₂ and secondly that NO₂ and CO₂ are competing for the same barium trapping sites. The fact that no significant amount of stored NO_x could be evidenced in the bulk barium carbonate, suggested that over the catalytic surface, the well dispersed barium phase can play an important role in the NO_x trapping properties of these catalysts.     

Modelling of NOx adsorption over NOx adsorbers

Modelling of the phenomena involved during the adsorption of NO_x on NO_x trap catalysts was developed. The aim of the model is the prediction of the quantity of stocked barium nitrate as well as the emissions of NO and NO₂, as a function of time and temperature. The mechanism of the process is sounded on the adsorption of gas species (NO, NO₂, O₂) on platinum sites, equilibrium reaction between adsorbed species followed by the formation of Ba(NO₃)₂. This formation of barium nitrate is limited by the thermal decomposition reaction which liberates NO in the gas phase. The kinetic constant of decomposition of barium nitrate was determined by temperature programmed thermogravimetry on pure Ba(NO₃)₂, using the method of Freeman and Carroll. Other kinetic constants bound to the mechanism were estimated by fitting the results of the model to experimental results.The mechanism was validated for various values of the molar fraction of O₂, the molar fraction of NO and various values of the NO/NO₂ ratio in the gas entering the reactor. It was also tested with different catalyst compositions (variation of the platinum and BaO concentrations). The importance of oxygen in the process was clearly demonstrated as well as the promoting role of NO₂.     

Combined XPS and TPR study of sulfur removal from a Pt/BaO/Al2O3 NO x storage reduction catalyst

Pt/Al₂O₃ and Pt/BaO/Al₂O₃ catalysts (1 wt% Pt, 10 wt%BaO) were sulfated under conditions simulating a real NSR catalyst operation. Comparative TPR and XPS studies of sulfur removal from Pt/Al₂O₃ and Pt/BaO/Al₂O₃ catalysts indicate that the sulfur removal from Al₂O₃ surface precedes reductive decomposition of BaSO₄ (250–400 °C). Barium sulfate decomposition started with further increase in desulfation temperature at the point of surface atomic ratio Ba:S = 1 (~450^o). Simultaneously, an intensive formation of sulfide species on the catalyst surface was observed. Thermodynamic analysis of the desulfation process allows us to hypothesize that barium sulfide formation may hinder sulfur removal under reducing conditions.     

Thermal decomposition of pure and rhodium impregnated cerium(III) carbonate hydrate in different atmospheres

The thermal decomposition of pure and rhodium impregnated cerium(III) carbonate hydrate in oxidising, reducing and inert atmospheres has been studied using combined thermogravimetry/mass spectrometry (TG/MS) and X-ray photoelectron spectroscopy (XPS). In oxygen, the decomposition of the pure carbonate proceeds in two steps, yielding H₂O,CO₂, and cerium(IV) oxide. In inert or reducing atmospheres, the second decomposition step is shifted towards higher temperatures and is divided into two parts. In the second part, CO₂ evolved is partly reduced by Ce(III) to CO and to elemental carbon, and non-stoichiometric CeO_2–x is formed as the solid product. In the presence of rhodium as a reduction catalyst, decomposition in helium yields hydrogen and less carbon monoxide than that of the pure carbonate, due to the water-gas shift activity of the solid. In hydrogen, quantitative reduction of the carbon dioxide evolved to methane and water is observed when rhodium is present.     

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.     

Reactions Between Alkaline-Earth Sulfates and Cirstobalite

The decomposition of magnesium and calcium sulfates was measured preliminary to measuring the reactions between alkaline-earth sulfates and cristobalite. Since gaseous sulfur oxides were evolved in the processes, a starch-iodide titration was used to determine the amounts of decomposition and reaction with time. Magnesium sulfate decomposed at temperatures above 900°C, before reaction with cristobalite. Calcium sulfate was found to react with cristobalite previous to thermal decomposition with an accompanying evolution of sulfur dioxide. Strontium and barium sulfates reacted with cristobalite at progressively higher temperatures. The applications of the data to the sulfur problems in the production of clay products are discussed.     

Role of Support in Lean DeNOx Catalysis

FTIR and pulse thermal analysis were applied to investigate catalysts containing Pt (1 wt%)/Ba (17 wt%) supported on -Al₂O₃, SiO₂ and ZrO₂. The aim was to learn how the support material affects the thermal stability of barium carbonate and its activity in the reaction to bulk Ba(NO₃)₂. The lower thermal stability of BaCO₃ in alumina supported samples was found to influence the formation of barium nitrate during the NO _x storage process. Quantification of Ba(NO₃)₂ formed during NO _x storage indicated that for alumina supported catalysts only ca. 30% of barium present in the sample is involved in the storage process. The low thermal stability found for alumina supported barium nitrite excludes its role in the formation of barium nitrate during interaction of NO _x with the catalyst at 300 °C. The studies indicate that -Al₂O₃ plays a major role in influencing the thermal stability of BaCO₃ and Ba(NO₃)₂. This finding seems to be relevant for the higher activity of -Al₂O₃-supported catalysts in NO _x storage reduction reactions.     

An XRD, FTIR and TPD Investigation of NO2 Surface Adsorption Sites of δ, γ Al2O3 and Barium Supported δ, γ Al2O3

In this paper an XRD, FTIR and TPD investigation of NO₂ surface adsorption sites of , Al₂O₃ and barium supported , Al₂O₃ is reported. Aim of this study is to bring additional light on the surface structures involved in NOx adsorption. Two samples of barium supported aluminas have been prepared and aged at 800 °C. These samples were characterised in comparison with the relative alumina support. The XRD characterisation of these samples shows the presence of barium carbonate and barium aluminate supported on alumina. The comparison of the FTIR spectra, before and after NO₂ adsorption, has revealed the formation, upon NO₂ contact, of a complex variety of nitrate and nitrite groups. The thermal desorption of nitrate and nitrite species has been simultaneously studied by means of FTIR spectroscopy and by TPD technique. By comparing the structural, adsorptive and spectroscopic results obtained on alumina and on barium supported alumina samples, a hypothesis on the basic sites active in NO₂ adsorption and of the possible decomposition paths induced by thermal heating are proposed.     

DRIFTS study of the catalytic N2O reduction by SO2 on FeZSM-5

SO₂ has been recognized as an effective reducing agent for N₂O over iron-containing zeolite catalysts, lowering the operation temperature up to 100 K with respect to the direct N₂O decomposition. This unique behavior contrasts with the common poisoning effect of SO₂ over other active de-N₂O metals (e.g. Co, Cu, Rh, and Ru). The formation of surface sulfates has been generally posed as the main cause for catalyst deactivation by SO₂. Through the use of in situ infrared spectroscopy (DRIFTS), we show that steam-activated FeZSM-5 indeed builds up stable sulfate species during the N₂O + SO₂ reaction. Significant amounts of sulfur were detected in the used catalyst by elemental analysis and X-ray photoelectron spectroscopy. However, the enhanced N₂O conversion is remarkably stable, indicating that the reducing action by SO₂ and the sulfation of the surface are decoupled. The resulting sulfate species are thus spectators in the catalytic process and do not block or alter the structure of the active sites for N₂O reduction and decomposition.     

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.     

NOx Abatement by Catalytic Traps: On the Mechanism of NOx Trapping Under Automotive Conditions

NO _x adsorption was measured with a barium based NO_x storage catalyst at an engine bench equipped with a lean burn gasoline direct injection engine (GDI). In order to study the influence of gas phase NO₂ on the NO_x storage efficiency two different pre-catalysts were used: One with excellent NO oxidation activity to produce a high NO₂ concentration and another pre-catalyst without NO oxidation activity and therefore high NO concentration at the NO _x storage catalyst inlet. Both pre-catalyst had excellent HC and CO conversion efficiency and therefore the CO and HC concentration at the NO _x storage catalyst inlet was practically zero. No lean NO _x reduction was observed. Under that conditions, experiments with NO _x storage catalysts of different length show that a high NO₂ inlet concentration did not enhance the NO _x storage efficiency. Moreover, we observed reduction of NO₂ to NO over the NO_x storage catalyst. However, in presence of a high NO inlet concentration NO₂ formation was observed which may proceed parallel to NO _x storage.     

Reduction of NOx over a NOx-Trap Catalyst and the Regeneration Behaviour of Adsorbed SO2

A conventional NO _x -trap catalyst containing platinum, rhodium, barium and lanthanum was conditioned with oxygen at 500°C, preloaded with NO under standard oxidising conditions and then subjected to regeneration with the reductants H₂, CO and C₃H₆, either alone or as a mixture. Hydrogen is the most efficient reductant in terms of NO _x conversion efficiency and reductant usage efficiency. There is a temperature optimum for CO between 300 and 400°C and a catalyst loading optimum (mols reductant added)/(mols NO _x adsorbed) between 1.5 and 3.0. The behaviour of the catalyst towards sulphur poisoning was examined in supplementary trials with the adsorption of SO₂ in the presence or absence of water vapour. When water is not present in both adsorption and reduction steps, very stable sulphates are formed, unattacked by reductants even at 1000°C. Sulfates are more easily reduced when water is present in the reductant mixture.     

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.     

Thermal and kinetic study of manganese dithionate from absorbing or leaching solution via TG/XRD/IC analysis

ABSTRACT

Mangano-manganic oxide can be prepared through thermal decomposition of manganese sulfate from the absorption or leaching solution, so desulfurization by pyrolusite or leaching pyrolusite with sulfur dioxide should be fully exploited for the recovery of manganese salt. However, upon preparing MnSO₄ using above techniques, manganese dithionate is an inevitable by-product, which lowers the purity of the industrial raw material MnSO₄ and exerts negative influences on pyrolysis technology. Information regarding thermal decomposition of solid-state manganese dithionate is scarce. To recycle manganese dithionate efficiently, pyrolysis mechanism and kinetics were systematically investigated by thermal decomposition method. The characteristics of thermal decomposition products were determined by thermogravimetric analysis techniques (TG), X-ray diffraction (XRD), and Ion chromatography (IC). The experimental results revealed that both desulfurization and following dehydration of MnS₂O₆·H₂O were a one-step process, and the desulfurization occurred at about 150°C lower than the dehydration at 230°C. As increasing pyrolysis temperature to 900°C, the manganese sulfate was firstly formed, and to 1100°C, Mangano-manganic oxide was obtained by losing sulfur oxides. Consequently, the kinetic parameters for each decomposition steps of manganese dithionate were determined by the Coasts and Redfern (CR) integral method. The as-obtained experimental and kinetic results may provide theoretical guides for recycling manganese dithionate.     

Simultaneous removal of SO2 and NOx by microwave with potassium permanganate over zeolite

Simultaneous sulfur dioxide (SO₂) and nitrogen oxides (NO_x) removal from flue gas can be achieved with high efficiency by microwave with potassium permanganate (KMnO₄) over zeolite. The experimental results showed that the microwave reactor could be used to oxidation of SO₂ to sulfate with the best desulfurization efficiency of 96.8% and oxidize NO_x to nitrates with the best NO_x removal efficiency of 98.4%. Microwave accentuates catalytic oxidation treatment, and microwave addition can increase the SO₂ and NO_x removal efficiency by 7.2% and 12.2% separately. The addition of zeolite to microwave potassium permanganate increases from 16.5% to 43.5% the microwave removal efficiency for SO₂, and the NO_x removal efficiency from 85.6% to 98.2%. The additional use of potassium permanganate to the microwave zeolite leads to the enhancement of SO₂ removal efficiency up from 53.9% to 95%, and denitrification efficiency up from 85.6% to 98.2%. The optimal microwave power and empty bed residence time (EBRT) on simultaneous desulfurization and denitrification are 259 W and 0.357 s, respectively. SO₂ and NO_x were rapidly oxidized in microwave induced catalytic oxidation reaction using potassium permanganate with zeolite being the catalyst and microwave absorbent.     

Removal of nitrogen oxides from air by chemicals-impregnated carbons

Fixation of nitrogen oxides (NO_x) in air onto granular activated carbon impregnated with chemicals was attempted to improve removal efficiency of NO_x by activated carbon adsorption. Nitric oxide (NO) and nitrogen dioxide (NO₂), were tried to remove by a flow test. Fixed-bed adsorption breakthrough curves were obtained when some kinds of carbon were used. The amount adsorbed of NO₂ changed with the amount and kinds of metallic salts impregnated. Chemicals-impregnated carbons were prepared from a commercial activated carbon. Among obtained carbons, the one which showed the highest selectivity for NO_x was chosen, and its performance with the change in humidity was determined. Removal mechanism of NO₂ was estimated, and the carbon impregnated with potassium hydroxide was found to be superior to any other carbon tested. The amount of the adsorbed NO and that produced by the reduction of NO₂ were determined from the breakthrough curves.     

Fundamental studies of NOx storage at low temperatures

A commercial NO_x storage catalyst (Pt, BaO and alumina containing) was investigated by temperature programmed desorption (TPD) experiments in the temperature range 100–400 °C. The catalyst stored a substantial amount of NO_x at 100 °C using NO + O₂. Nitrites or loosely bound NO species are suggested for this storage, since no NO was oxidised at this low temperature. In addition, the released NO_x during the temperature ramp consisted of mainly NO and at lower temperatures the NO₂ dissociation is limited. Water and CO₂ was found to decrease the storage substantially, 92% for the NO + O₂ adsorption at 100 °C. The total storage for 60 min using NO₂ + O₂ at 200 °C was similar when introducing CO₂ and H₂O. However, the initial total uptake of NO_x was decreased. Initially we probably formed loosely bound NO_x species, which likely are strongly influenced by water and CO₂. After longer time periods are barium nitrates probably formed and they can remove the carbonates by forming stable nitrates, thus resulting in the same total uptake of NO_x.     

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.     

Low-temperature catalytic decomposition of hydrogen sulfide on metal catalysts under layer of solvent

When hydrogen sulfide decomposition {2 H₂S ? 2 H₂?+?S₂^(gas)} is carried out in the flow regime at room temperature on metal catalysts placed in a liquid capable of dissolving H₂S and sulfur, the reaction equilibrium can be significantly (up to 100%) shifted to the right yielding the desired product – hydrogen. The process efficiency was demonstrated using aqueous solutions of monoethanolamine (MEA), sodium carbonate, which is widely used in industry for H₂S absorption from tail gases, and aqueous hydrazine as examples. IR and Raman spectroscopy data demonstrated that sulfur obtained in the solutions is in the form of diatomic molecules. DFT calculations showed that diatomic sulfur forms weakly bound coordinative complexes with solvent molecules. Some problems related to sulfur accumulation and recovery from the solvents are discussed.