The role of NO2 in the reduction of NO by hydrocarbon over Cu-ZrO2 and Cu-ZSM-5 catalysts

The reduction of NO _x with propene or propane in the presence of 1 or 4% O₂ was studied at low conversions over a 7.4 wt% Cu-ZrO₂ and a 3.2 wt% Cu-ZSM-5 catalyst. The rates of N₂ production were compared in experiments using only NO or a mixture of NO and NO₂ in the feed. They were also compared with the rates of NO₂ reduction to NO under the same conditions, and of NO oxidation to NO₂ in the absence of hydrocarbon. It was found that the reduction of NO₂ to NO was very fast, consistent with literature data. The data were best explained by a reaction scheme in which the hydrocarbon was activated primarily by reaction with adsorbed NO₂ to form an adsorbed oxidized N-containing hydrocarbon intermediate, the reaction of which with NO was the principal route to produce N₂ under lean NO _x conditions.On leave from State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China.     

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.     

Steam effect on NO x reduction over lean NO x trap Pt–BaO/Al2O3 model catalyst

The effect of steam on NO _x reduction over lean NO _x trap (LNT) Pt–Ba/Al₂O₃ and Pt/Al₂O₃ model catalysts was investigated with reaction protocols of rich steady-state followed by lean–rich cyclic operations using CO and C₃H₈ as reductants, respectively. Compared to dry atmosphere, steam promoted NO _x reduction; however, under rich conditions the primary reduction product was NH₃. The results of NO _x reduction and NH₃ selectivity versus temperature, combined with temperature programmed reduction of stored NO _x over Pt–BaO/Al₂O₃ suggest that steam causes NH₃ formation over Pt sites via reduction of NO _x by hydrogen that is generated via water gas shift for CO/steam, or via steam reforming for C₃H₈/steam. During the rich mode of lean–rich cyclic operation with lean–rich duration ratio of 60 /20 s, not only the feed NO, but also the stored NO _x contributed to NH₃ formation. The NH₃ formed under these conditions could be effectively trapped by a downstream bed of Co²⁺ exchanged Beta zeolite. When the cyclic operation was switched into lean mode at T < 450 °C, the trapped ammonia in turn participated in additional NO _x reduction, leading to improved NO _x storage efficiency.     

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.     

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 NO x reduction mechanism by H2 under near isothermal conditions over Pt–Ba/Al2O3 Lean NO x Trap systems

The study of the gas-phase NO reduction by H₂ and of the stability/reactivity of NO _x stored over Pt–Ba/Al₂O₃ Lean NO _x Trap systems allowed to propose the occurrence of a reduction process of the stored nitrates occurring via to a Pt-catalyzed surface reaction which does not involve, as a preliminary step, the thermal decomposition of the adsorbed NO _x species.     

Effect of Co and Rh promoter on NOx storage and reduction over Pt/BaO/Al2O3 catalyst

Effect of cobalt and rhodium promoter on NO_x storage and reduction (NSR) kinetics was investigated over Pt/BaO/Al₂O₃. Kinetics of 2% cobalt loading over Pt/BaO/Al₂O₃ demonstrated highest NO_x uptake during lean cycle, while reduction efficiency during rich cycle appeared most poor. In contrast to this, rhodium showed suppressing effect of NO_x uptake during lean cycle and demonstrated an enhanced effect for the higher efficiency of NO_x reduction during rich cycle. DRIFT study for NO_x uptake and regeneration confirmed formation of surface BaNO_x from the band at 1300 cm⁻¹ and formation of bulk BaNO_x from the band at 1330 cm⁻¹.     

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.     

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.     

NO x uptake mechanism on Pt/BaO/Al2O3 catalysts

The NO _x adsorption mechanism on Pt/BaO/Al₂O₃ catalysts was investigated by performing NO _x storage/reduction cycles, NO₂ adsorption and NO + O₂ adsorption on 2%Pt/(x)BaO/Al₂O₃ (x = 2, 8, and 20 wt%) catalysts. NO _x uptake profiles on 2%\Pt/20%BaO/Al₂O₃ at 523 K show complete uptake behavior for almost 5 min, and then the NO _x level starts gradually increasing with time and it reaches 75% of the inlet NO _x concentration after 30 min time-on-stream. Although this catalyst shows fairly high NO _x conversion at 523 K, only ~2.4 wt% out of 20 wt% BaO is converted to Ba(NO₃)₂. Adsorption studies by using NO₂ and NO + O₂ suggest two different NO _x adsorption mechanisms. The NO₂ uptake profile on 2%Pt/20%BaO/Al₂O₃ shows the absence of a complete NO _x uptake period at the beginning of adsorption and the overall NO _x uptake is controlled by the gas–solid equilibrium between NO₂ and BaO/Ba(NO₃)₂ phase. When we use NO + O₂, complete initial NO _x uptake occurs and the time it takes to convert ~4% of BaO to Ba(NO₃)₂ is independent of the NO concentration. These NO _x uptake characteristics suggest that the NO + O₂ reaction on the surface of Pt particles produces NO₂ that is subsequently transferred to the neighboring BaO phase by spill over. At the beginning of the NO _x uptake, this spill-over process is very fast and so it is able to provide complete NO _x storage. However, the NO _x uptake by this mechanism slows down as BaO in the vicinity of Pt particles are converted to Ba(NO₃)₂. The formation of Ba(NO₃)₂ around the Pt particles results in the development of a diffusion barrier for NO₂, and increases the probability of NO₂ desorption and consequently, the beginning of NO _x slip. As NO _x uptake by NO₂ spill-over mechanism slows down due to the diffusion barrier formation, the rate and extent of NO₂ uptake are determined by the diffusion rate of nitrate ions into the BaO bulk, which, in turn, is determined by the gas phase NO₂ concentration.     

Synergic effect of Pd/γ-alumina and Cu/ZSM-5 on the performance of NO x storage reduction catalyst

NO _x reduction with a combination of catalysts, Pd catalyst, NO _x storage reduction (NSR) catalyst and Cu/ZSM-5 in turn, was investigated to elucidate for the high NO _x reduction activity of this catalyst combination under oxidative atmosphere with periodic deep rich operation. The catalytic activity was evaluated using the simulated exhaust gases with periodically fluctuation between oxidative and reductive atmospheres, and it was found that the NO _x reduction activity with this catalyst combination was apparently higher than that of the solely accumulation of these individual activities, which was caused by the additional synergic effect by this combination. The Pd catalyst upstream of the NSR catalyst improved NO _x storage ability by NO₂ formation under oxidative atmosphere. The stored NO _x was reduced to NH₃ on the NSR catalyst, and the generated NH₃ was adsorbed on Cu/ZSM-5 downstream of the NSR catalyst under the reductive atmosphere, and subsequently reacted with NO _x on the Cu/ZSM-5 under the oxidative atmosphere.     

Comparative NOx Reduction Behavior of Pt, Pd, and Rh Supported Catalysts in Simulated Exhaust Gases as a Function of Oxygen Concentration

NO _x reduction activity on Pt and Pd catalysts had a maximum for S value as stoichiometry number at a fixed temperature, and the S value at the maximum NO _x conversion increased with decreasing temperature. NO _x conversion on Rh catalyst increased with decreasing S value, but independent of temperature. As for the effect of HC on NO _x reduction behavior, it was concluded that, for Pt and Pd catalysts, HC adsorbs strongly on the catalysts surface to cause the self-inhibition. Increasing O₂ concentration lead to oxidation of HC, but decreased the value of NO/O₂ ratio. The balance point of the two factors generated a maximum NO _x conversion. For Rh catalyst, the strongly adsorbed oxygen is more reactive with decreasing S value, and thus NO _x conversion is increased.     

NOx Storage and Reduction Catalysts for Automotive Lean-Burn Engines: Effect of Parameters and Storage Materials on NOx Conversion

The effect of various parameters on the NO _x conversion over NO _x storage and reduction catalysts supported on alumina was investigated. The Pt/BaO/Al₂O₃ catalyst exhibited a higher NO _x reduction activity than the Pt/Al₂O₃ catalyst under the static and cycling conditions. The activity of Pt/BaO/Al₂O₃ catalyst was improved in the cycled feedstream. The Pt/SrO/Al₂O₃ was found to have as high activity as Pt/BaO/Al₂O₃ for NO _x reduction. In order to achieve effective reduction of NO _x , NO _x storage in the form of Me(NO₃)₂ (Me = Ba or Sr) is more favorable than other nitrates and the rich condition should be chosen in such a way that the sorption capacity can be fully regenerated at a fast rate and the inhibition effect by strongly adsorbed molecules derived from C₃H₆ and CO can be minimized.     

The Catalytic Activity of Co/ZrO2 for NO Reduction with Propane in O2 Presence

The selective catalytic reduction (SCR) of NO by propane in the presence of excess oxygen was studied on a Co/ZrO₂ catalyst. This system is present as active for the NO reduction to N₂. It was found that the addition of Co could improve the activity and selectivity of propane towards NO_x reduction. The activity depends strongly on the space velocity (GHSV) when the system works with low oxygen concentration and it is independent of the space velocity when the system operates with excess oxygen. The water vapor present in the feed produces deactivation in the catalyst as well as in the support.     

Structural Evolution of Highly Active Ir-Based Catalysts for the Selective Reduction of NO with Reductants in Oxidizing Conditions

NO _x reduction over Ir-based catalysts in the presence of excess oxygen with hydrocarbon as a reductant was investigated in the focus on observing microstructure of Ir particle supported on various carriers and structural evolution of highly active Ir-based catalysts in the NO _x reduction. Characterization of Ir-based catalysts using SEM, TEM, XRD, CO chemisorption and XPS, and reaction studies on various Ir-based catalysts have proved that the formation of relatively large Ir metal particle with 40–60 nm of nanocrystal size carried on inert supports has been a prerequisite for the evolution of high activities in the NO _x reduction rather than the formation of Ir metal state on the catalyst. Furthermore, in inert gas conditions in a high temperature range of 850–950°C, Ir metal was easily formed by using the support such as TiO₂ and ZrO₂ that drastically decreased its specific surface area in the temperature range.     

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.     

Transient Surface Processes of Storage and Conversion of NOx Species on Pt–Me/Al2O3 Catalysts (Me = Ba,Ce, Cu)

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

Synergistic Effect in Selective Reduction of NO by Alkanes over Mechanically Mixed Oxide Catalysts

The process of selective catalytic reduction of nitrogen oxides by propane in the presence of O₂, as well as in the presence or absence of CO, was studied over series of commercial oxide catalysts used in petrochemical processes. For the first time synergistic effect was observed for catalytic systems consisting of mechanical mixtures of Cu–Zn–Ni–Al (catalyst I) + Fe–Cr (catalyst II) and Cu–Zn–Ni–Al (catalyst I) + Ni–Cr (catalyst III). The activity of these mixtures in nitrogen oxides reduction by propane was greater than that of individual components in each case. The worked-out catalytical systems showed high effectivity in the process of simultaneous removal of several toxic components: NO _x , CO, hydrocarbons – from model gas mixtures, as well as from real exhausts of automotive transport.     

Reduction of NOx Stored at Low Temperatures on a NOx Adsorbing Catalyst

Isothermal storage and reduction of NO₂ with CO, C₃H₆ and H₂ as reducing agents on a lean NO _x adsorber was investigated by temperature programmed desorption (TPD) and temperature programmed reduction (TPR) studies. The reduction of NO _x was clearly favoured with H₂ as reducing agent. Carbon monoxide and C₃H₆ showed fairly low reduction of NO _x . The NO _x reduction at low temperatures with H₂ as reducing agent was found to be effective, clearly much more effective than for CO.     

NO x -catalyzed partial oxidation of methane and ethane to formaldehyde by dioxygen

At 600 °C, NO_x catalyzes the partial oxidation of both methane and ethane by dioxygen to form formaldehyde. The yield of oxygenates from methane is over 11. The yield increases to over 16 when 0.7% of ethane is added to the gas mixture. The yield of oxygenates from ethane is over 24. A catalytic cycle involving NO₂ as the C–H activating species is proposed.