A redox pathway for selective NOx reduction: stationary and transient experiments performed on a supported Pt catalyst

Platinum-based catalysts can perform the selective reduction of NO_x under lean-burn conditions, though they function only over a narrow activity–temperature window and form undesirably high quantities of N₂O. With these stationary and transient experiments conducted using a Pt/SiO₂ catalyst, we attempted to better understand the mechanism of SCR NO when C₃H₆ is used as reductant. We propose a redox-type reaction pathway featuring NO adsorption as well as surface oxidation and reduction. To this end, a precise balance in hydrocarbon and oxygen surface coverage is required.     

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.     

A new NOx storage-reduction electrochemical catalyst

A new NO_x storage-reduction electrochemical catalyst has been prepared from a polycrystalline Pt film deposited on 8 mol% Y₂O₃-stabilized ZrO₂ (YSZ) solid electrolyte. BaO has been added onto the Pt film by impregnation method. The NO_x storage capacity of Pt-BaO/YSZ system was investigated at 350 °C and 400 °C under lean conditions. Results have shown that the electrochemical catalyst was effective for NO_x storage. When nitric oxides are fully stored, the catalyst potential is high and reaches its maximum. On the other hand, when a part of NO and also NO₂ desorb to the gas phase, the catalyst potential remarkably drops and finally stabilizes when no more NO_x storage occurs but only the reaction of NO oxidation into NO₂. Furthermore, the investigation has clearly demonstrated that the catalyst potential variation versus temperature or chemical composition is an effective indicator for in situ following the NO_x storage-reduction process, i.e. the storage as well as the regeneration phase. The catalyst potential variations during NO_x storage process was explained in terms of oxygen coverage modifications on the Pt.     

Microwave-assisted desulfurization of NOx storage-reduction catalyst

The activity of NO_x storage-reduction (NSR) catalysts is greatly reduced by sulfur poisoning, caused by the SO₂ present in the exhaust stream. Desorption of sulfur species from poisoned NSR catalysts occurs at temperatures in excess of 600 °C using reducing atmospheres and conventional heating. In this work, microwave (MW) heating has been used to promote desulfurization of poisoned NSR catalysts. The experiments were carried out by heating the catalyst with MW radiation and using hydrogen as the reducing gas. Desorption of H₂S at 200 °C was observed. Desorption at even lower temperatures (150 °C) was observed when water was introduced to the system. In the presence of water, sulfur species desorbed as both H₂S and SO₂. An overall reduction of sulfur species of about 60% was obtained. The use of MW heating proves to be an efficient way to achieve regeneration of poisoned NSR catalysts.     

Effect of basic metal additives on NOx reduction property of Pd-based three-way catalyst

The three-way catalytic (TWC) performance, especially CO and NO_x conversion, of a Pd-only catalyst was improved by the addition of basic elements, and the activity increased as the electronegativity of the additive cations decreased (Ba>Sr>La), indicating that the basicity or electron-donating ability of the additives played a significant role in the improvement of the TWC performance. Thermal decomposition temperature of PdO in promoted catalysts became higher with decreasing the ionic electronegativity of the promoters. XPS analysis proved that the electron density around Pd^(II) in the Ba-promoted catalyst was higher than that of Pd-only catalyst. The improvement of the TWC performance by the addition of basic promoters is discussed in terms of the stabilization of PdO, which may suppress the sintering, as well as of electron enrichment, which endows palladium with the Rh-like catalytic behavior.     

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.     

Reactivity of Mg-Ba catalyst in NOx storage/reduction

A new catalyst for NO_x storage/reduction was prepared to improve the activity of Ba-Pt/γ-Al₂O₃ by replacing Ba with a mixture of Ba and Mg. The catalyst was prepared by impregnating 1 wt.% Pt and then the alkaline-earth metals (Mg, Ba) on commercial γ-Al₂O₃. The tests have been carried out in a wide temperature range (ca. 200–400 °C) in order to understand the role of the mixture of alkaline-earth metals as a function of temperature. The behaviour of the two catalysts was different and indicated a synergetic effect between Mg and Ba.     

The transient response technique for the study of hydrocarbon oxidation on metal oxides: Simulation of the responses for various propene oxidation mechanisms

Partial differential equations, describing the consumption of reactants and the formation of products, have been solved numerically for a step change from inert to reactant feed considering different variants of propene oxidation mechanisms. The response features and their dependency on the rate limiting step have been rationalised in terms of conclusions, which can serve as general guidelines for mechanistic interpretation of experimental transients from hydrocarbon oxidation over metal oxides. Though the response pattern is characteristic of the rate limiting step, it is demonstrated that different variants of mechanisms can give identical response features. In that case complementary investigation with other techniques, or more detailed mathematical analysis is necessary.     

Sulphur dioxide interaction with NOx storage catalysts

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

Investigation of simultaneous adsorption of SO2 and NOx on Na-γ-alumina with transient techniques

The simultaneous adsorption of SO₂ and NO_x on Na-γ-alumina was studied by means of step experiments in a fixed bed plug flow reactor at 387 K and atmospheric pressure. Typically the molar composition of the feed gas was 1.5% SO₂, 1% O₂, 4000 ppm NO, 500 ppm NO₂, and Ar. First the adsorption behavior of the pure components was measured. SO₂ and NO₂ adsorb easily, whereas NO and O₂ do not adsorb. Moreover there is no influence of O₂ on the adsorption behavior of the pure components.

NO and O₂ adsorption require the simultaneous presence of SO₂, NO, and O₂. The NO and O₂ adsorption rate is enhanced by an increasing SO₂/NO ratio. The total amount of SO₂ adsorbed is not affected by the simultaneous adsorption of NO and O₂. However, NO₂ adsorption increases the SO₂ adsorption capacity. In the presence of NO₂ most of the adsorbed NO_x is released as NO.     

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.     

Immobilization of nanocatalysts on cordierite honeycomb monoliths for low temperature NOx reduction

Noble metal nanocatalysts such as Pd, Pt, and Au were strongly immobilized on the inside walls of monolithic honeycomb-structured cordierite, in which bi-functional molecules were used as linkers for anchoring noble metal nanoparticles (NPs) on the cordierite surface. The supported nanocatalysts were characterized by ICP-MS, TEM, and X-ray powder diffraction. The efficiencies of the immobilized nanocatalysts for the removal of harmful nitrogen oxides (NO_x) have been investigated by measuring the deNO_x capability as a function of temperature. The catalytic activities depend mainly on the compositions of the nanocatalysts. The Pd/Pt bi-metal catalyst anchored on the cordierite surface shows higher NO_x conversion and better activity than the commercial emission catalyst at low temperature region, which could be due to the large portion of active surface areas of the catalysts with nanometer scale.     

Mn/MFI catalyzed reduction of NOx with alkanes

Mn/MFI catalysts were prepared by different methods and probed as catalysts for the catalytic reduction of NO_x with CH₄ or iso-butane in a gas flow containing excess O₂. Mn/MFI with high manganese loading was obtained by solid state ion exchange (SSI). The intensity of an IR band at 957 cm⁻¹, which is due to the perturbation of zeolite lattice vibrations by Mn ions attached to cage walls is proportional to the Mn content of the catalysts. Conversely, the intensity of the 3610 cm⁻¹ band, assigned to Brønsted acid sites decreases linearly with the Mn loading. A catalyst obtained by exchanging Na/MFI with an aqueous solution of Mn acetate is found most active for NO_x reduction with methane. Transport by surface diffusion of Mn ions from MnI₂ to exchange positions in MFI is more efficient than their transport through the gas phase. High NO conversion over proton-free catalysts indicates that protons are not instrumental in NO_x reduction over Mn/MFI.     

Pt-Ba/alumina NOx storage-reduction catalysts: Influence of Ba loading on NOx storage behavior

The NO_x storage behavior of a series of Pt-Ba/Al₂O₃ catalysts, prepared by wet impregnation of Pt/Al₂O₃ with Ba(Ac)₂, has been investigated. The catalysts with Ba loadings in the range 4.5–28 wt.% were calcined at 500 °C in air and subsequently exposed to NO pulses in 5 vol.% O₂/He atmosphere. Catalysts were characterized by means of thermogravimetry (TG) combined with mass spectroscopy (MS) and XRD before and after exposure to NO pulses. Characterization of the calcined catalysts corroborated the existence of three Ba-containing phases which are discernible based on their different thermal stability: BaO, LT-BaCO₃ and HT-BaCO₃. Characterization after NO_x exposure showed that the different Ba-containing phases present in the catalysts possess different reactivity for barium nitrate formation, depending on their interfacial contact. The different Ba(NO₃)₂ species produced upon NO_x exposure could be distinguished based on their thermal stability. The study revealed that during the NO_x storage process a new thermally instable BaCO₃ phase formed by reaction of evolved CO₂ with active BaO. The fraction of Ba-containing species that were active in NO_x storage depended on the Ba loading, showing a maximum at a Ba loading of about 17 wt.%. Lower and higher Ba loading resulted in a significant loss of the overall efficiency of the Ba-containing species in the storage process. The loss in efficiency observed at higher loading is attributed to the lower reactivity of the HT-BaCO₃, which becomes dominant at higher loading, and the increased mass transfer resistance.     

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

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

Catalytic NOx reduction by carbon supporting metals

Catalytic NO_x reduction by carbon supporting transition metals (Fe, Co, Ni, Cu) and potassium has been studied. The effect of oxygen on the catalytic properties of the metals has been analyzed. Temperature-programmed reactions and isothermal reactions have been conducted in a fixed bed flow reactor. Temperature-programmed reduction in hydrogen, XRD and XPS have been used to characterize the catalysts. All the metals studied catalyze the NO_x reduction by carbon in the presence of oxygen, but also the O₂–carbon reaction. Metal catalytic activity is the result of two factors, the tendency of the metal to be oxidized by NO and the easiness of the resulting oxide to be reduced by carbon. Among the metals studied, nickel exhibits the highest selectivity for NO_x reduction.

The results of this study strengthen the possible benefit of the lack of a gaseous reducing agent (such as ammonia or hydrocarbons) since the reduction of NO_x is performed by the carbon support itself.