Non-thermal plasma-assisted catalytic NOx storage over Pt/Ba/Al2O3 at low temperatures

NO_x storage performances have been investigated on a Pt/Ba/Al₂O₃ catalyst by comparison using two types of non-thermal plasma (NTP) reactor: the “PDC system” reactor and the “PFC system” reactor. In the PDC system, the catalyst was placed in the discharge space and was activated by the plasma directly, whereas in the PFC system, the plasma reactor was followed by the catalyst. The results showed that the NO_x storage capacity (NSC) of the Pt/Ba/Al₂O₃ catalyst was significantly enhanced by the non-thermal plasma in the PDC and PFC system, and the PDC system exhibited better promotional effect than the PFC system in the temperature range of 100–300 °C. The NSC of the catalyst was increased with the increase of the input energy density both in the PDC and PFC system due to the higher NO oxidation at higher input energy density. It was also found that the ionic wind induced by plasma in the PDC system enhanced the quantity of the NO adsorbed onto the catalyst surface and therefore could react with the O-radical to form more NO₂, and thus promote the formation of nitrate on the catalyst.     

Mean field modelling of NOx storage on Pt/BaO/Al2O3

A mean field model, for storage and desorption of NO_x in a Pt/BaO/Al₂O₃ catalyst is developed using data from flow reactor experiments. This relatively complex system is divided into five smaller sub-systems and the model is divided into the following steps: (i) NO oxidation on Pt/Al₂O₃; (ii) NO oxidation on Pt/BaO/Al₂O₃; (iii) NO_x storage on BaO/Al₂O₃; (iv) NO_x storage on Pt/BaO/Al₂O₃ with thermal regeneration and (v) NO_x storage on Pt/BaO/Al₂O₃ with regeneration using C₃H₆. In this paper, we focus on the last sub-system. The kinetic model for NO_x storage on Pt/BaO/Al₂O₃ was constructed with kinetic parameters obtained from the NO oxidation model together with a NO_x storage model on BaO/Al₂O₃. This model was not sufficient to describe the NO_x storage experiments for the Pt/BaO/Al₂O₃, because the NO_x desorption in TPD experiments was larger for Pt/BaO/Al₂O₃, compared to BaO/Al₂O₃. The model was therefore modified by adding a reversible spill-over step. Further, the model was validated with additional experiments, which showed that NO significantly promoted desorption of NO_x from Pt/BaO/Al₂O₃. To this NO_x storage model, additional steps were added to describe the reduction by hydrocarbon in experiments with NO₂ and C₃H₆. The main reactions for continuous reduction of NO_x occurs on Pt by reactions between hydrocarbon species and NO in the model. The model is also able to describe the reduction phase, the storage and NO breakthrough peaks, observed in experiments.     

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.     

Steady State Isotopic Transient Kinetic Analysis of Flameless Methane Combustion over Pd/Al2O3 and Pt/Al2O3 Catalysts

The SSITKA measurements were performed in the steady state of complete methane oxidation on the Pd/Al₂O₃ and Pt/Al₂O₃ catalysts. It was found that the number of intermediates and their average life-time on the catalyst surface changes with the increase of reaction temperature. On the Pd/Al₂O₃ catalyst there is larger number of active centres than on Pt/Al₂O₃ catalyst which permits the course of methane oxidation at lower temperatures.     

Structure sensitivity of dimethylamine deep oxidation over Pt/Al2O3 catalysts

The deep oxidation of dimethylamine (DMA) was studied over Pt/Al₂O₃ catalysts with small (1 nm) and large (7.8–15.5 nm) Pt crystallite sizes. The turnover frequency (TOF) was higher for the large than for the small Pt crystallites, indicating that the reaction is structure sensitive. Two kinetic models were used to interpret the obtained results, i.e., the Mars van Krevelen and a mechanism based on the adsorption of oxygen and adsorption of dimethylamine on different active sites were employed. Both models showed that the activation energy for the oxygen chemisorption rate constant (k_o) decreased with increasing of Pt crystallite size and that the activation energy for the surface reaction rate constant (k_i) was independent of the Pt crystallite size. The structure sensitivity may be explained by differences in the reactivity of the oxygen adsorbed on these Pt crystallites.The Mars van Krevelen model fits the TOF values very well at concentrations of DMA higher than 1500 ppm, while in the lower concentrations region, the model under predicts the experimental data. The model based on the adsorption of oxygen and DMA on different active sites fits the experimental data quite well over the whole temperature and concentration range. The fitted values of the Henry adsorption constant are independent of the Pt crystallite size.     

Novel promoting effect of SO2 on the selective catalytic reduction of NOx by ammonia over Co3O4 catalyst

Spinel nano-Co₃O₄ was prepared by solid-state reaction at room temperature and investigated for selective catalytic reduction of NO_x by NH₃ (NH₃-SCR). Although suffering from pore filling and plugging, treatment of this catalyst by SO₂ showed novel promoting effect on NH₃-SCR above 250 °C. Bulk cobalt sulfate was observed over the sulfated Co₃O₄ with XRD, which would be an active component for NH₃-SCR. The sulphated Co₃O₄ catalyst exhibited good resistance to SO₂ (500 ppm, 100 ppm) and 10% H₂O at a space velocity of about 25 000 h⁻¹ at 300 °C, as tested for 12 h.     

Reactor and in situ FTIR studies of Pt/BaO/Al2O3 and Pd/BaO/Al2O3 NOx storage and reduction (NSR) catalysts

The reduction of NO under cyclic “lean”/“rich” conditions was examined over two model 1 wt.% Pt/20 wt.% BaO/Al₂O₃ and 1 wt.% Pd/20 wt.% BaO/Al₂O₃ NO_x storage reduction (NSR) catalysts. At temperatures between 250 and 350 °C, the Pd/BaO/Al₂O₃ catalyst exhibits higher overall NO_x reduction activity. Limited amounts of N₂O were formed over both catalysts. Identical cyclic studies conducted with non-BaO-containing 1 wt.% Pt/Al₂O₃ and Pd/Al₂O₃ catalysts demonstrate that under these conditions Pd exhibits a higher activity for the oxidation of both propylene and NO. Furthermore, in situ FTIR studies conducted under identical conditions suggest the formation of higher amounts of surface nitrite species on Pd/BaO/Al₂O₃. The IR results indicate that this species is substantially more active towards reaction with propylene. Moreover, its formation and reduction appear to represent the main pathway for the storage and reduction of NO under the conditions examined. Consequently, the higher activity of Pd can be attributed to its higher oxidation activity, leading both to a higher storage capacity (i.e., higher concentration of surface nitrites under “lean” conditions) and a higher reduction activity (i.e., higher concentration of partially oxidized active propylene species under “rich” conditions). The performance of Pt and Pd is nearly identical at temperatures above 375 °C.     

Selective synthesis of mixed alcohols from syngas over catalyst Fe2O3/Al2O3 in slurry reactor

The Fe₂O₃/Al₂O₃ catalyst was studied to selectively synthesize mixed alcohols from syngas in a continuously stirred slurry reactor with the oxygenated solvent Polyethylene Glycol-400 (PEG-400). The selectivity of mixed alcohols in the products reached as high as 95 wt.% and the C₂₊ alcohols (mainly ethanol) was more than 40 wt.% in the total alcohol products at the reaction conditions of 250 °C, 3.0 MPa, H₂/CO = 2 and space velocity = 360 ml/g_cat h. The hydrogen temperature programmed reduction (H₂-TPR) and X-ray photoelectron spectroscopy (XPS) measurements of the catalyst confirmed that the FeO phase was responsible for the high selectivity to mixed alcohols in the process. And the oxygenated solvent PEG-400 was also necessary for the selective synthesis of mixed alcohols in the reaction system.     

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.     

Improvement of coke resistance of Ni/Al2O3 catalyst in CH4/CO2 reforming by ZrO2 addition

This work investigates the improvement of Ni/Al₂O₃ catalyst stability by ZrO₂ addition for H₂ gas production from CH₄/CO₂ reforming reactions. The initial effect of Ni addition was followed by the effect of increasing operating temperature to 500–700 °C as well as the effect of ZrO₂ loading and the promoted catalyst preparation methods by using a feed gas mixture at a CH₄:CO₂ ratio of 1:1.25. The experimental results showed that a high reaction temperature of 700 °C was favored by an endothermic dry reforming reaction. In this reaction the deactivation of Ni/Al₂O₃ was mainly due to coke deposition. This deactivation was evidently inhibited by ZrO₂, as it enhances dissociation of CO₂ forming oxygen intermediates near the contact between ZrO₂ and nickel where the deposited coke is gasified afterwards. The texture of the catalyst or BET surface area was affected by the catalyst preparation method. The change of the catalyst texture resulted from the formation of ZrO₂–Al₂O₃ composite and the plugging of Al₂O₃ pore by ZrO₂. The 15% Ni/10% ZrO₂/Al₂O₃ co-impregnated catalyst showed a higher BET surface area and catalytic activity than the sequentially impregnated catalyst whereas coke inhibition capability of the promoted catalysts prepared by either method was comparable. Further study on long-term catalyst stability should be made.     

The Effect of a Changing Lean Gas Composition on the Ability of NO2 Formation and NOx Reduction over Supported Pt Catalysts

Three model catalysts (Pt/Al₂O₃, Pt/TiO₂, Pt/V₂O₅/TiO₂) were examined in regard to their NO₂ formation ability under a changing lean gas composition. The results show that the NO to NO₂ oxidation function as well as the NO _x reduction under lean gas conditions is affected by a change in the lean gas atmosphere. The NO oxidation activity also decreased with time, for Pt/Al₂O₃ and Pt/TiO₂, and a possible explanation may be platinum oxide formation. This deactivation was not observed for Pt/V₂O₅/TiO₂.     

Phase diagram of the Al2O3-HfO2-Y2O3 system

The phase diagram of the Al₂O₃-HfO₂-Y₂O₃ system was first constructed in the temperature range 1200-2800 °C. The phase transformations in the system are completed in eutectic reactions. No ternary compounds or regions of appreciable solid solution were found in the components or binaries in this system. Four new ternary and three new quasibinary eutectics were found. The minimum melting temperature is 1755 °C and it corresponds to the ternary eutectic Al₂O₃ + HfO₂ + Y₃Al₅O₁₂. The solidus surface projection, the schematic of the alloy crystallization path and the vertical sections present the complete phase diagram of the Al₂O₃-HfO₂-Y₂O₃ system.     

Production of hydrogen by steam reforming of bio-oil over Ni/Al2O3 catalysts: Effect of addition of promoter and preparation procedure

Catalytic steam reforming of bio-oil was investigated in a fixed bed tubular reactor for production of hydrogen. Two series of nickel/alumina (Ni/Al₂O₃) supported catalysts promoted with ruthenium (Ru) and magnesium (Mg) were prepared. Each catalyst of the first series (Ru–Ni/Al₂O₃) was prepared by co-impregnation of nickel and ruthenium on alumina. They were examined to investigate the effect of adding ruthenium on the performance of the catalysts for hydrogen production. The effect of the temperature, the most effective parameter in the steam reforming of bio-oil, on the activity of the catalysts was also investigated. Each catalyst of the second series (Ni–MgO/Al₂O₃) was prepared by consecutive impregnation using various preparation procedures. They were tested to determine the effect of adding magnesium as well as the effect of the preparation procedure on the outlet gas concentrations. It was shown that in both series, the catalysts were more efficient in hydrogen production as well as carbon conversion than Ni/Al₂O₃ catalysts. The highest hydrogen yield was 85% which was achieved over Ru–Ni/Al₂O₃ at 950 °C. It was also found that the effect of adding a small amount of ruthenium was superior to that of nickel on the yield of hydrogen when the nickel content was equal to or greater than 10.7%.     

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.     

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.     

Laser machining of Al2O3-ZrO2 (3%Y2O3) eutectic composite

In this work we present the study of the interaction between NIR pulsed laser and Al₂O₃-ZrO₂ (3%Y₂O₃) eutectic composite. The effect produced by modifying the reference position as well as the working conditions and laser beam features has been studied when the samples are processed by means of pulse bursts.The samples were obtained by the laser floating zone technique using a CO₂ laser system. The laser machining was carried out with a Q-switched Nd:YAG laser at its fundamental wavelength of 1064 nm with pulse-widths in the nanosecond range.Geometric dimensions, i.e. ablated depth, machined width and removed volume as well as ablation yield of the resulting holes have been studied. We have described and discussed the morphology, composition and microstructure of the processed samples.     

Support modification to improve the sulphur tolerance of Ag/Al2O3 for SCR of NOx with propene under lean-burn conditions

Ag/Al₂O₃ catalysts with 1 wt% SiO₂ or TiO₂ doping in alumina support have been prepared by wet impregnation method and tested for sulphur tolerance during the selective catalytic reduction (SCR) of NO_x using propene under lean conditions. Ag/Al₂O₃ showed 44% NO_x conversion at 623 K, which was drastically reduced to 21% when exposed to 20 ppm SO₂. When Al₂O₃ support in Ag/Al₂O₃ was doped with 1 wt% SiO₂ or TiO₂ the NO_x conversion remained constant in presence of SO₂ showing the improved sulphur tolerance of these catalysts. Subsequent water addition does not induce significant deactivation. On the contrary, a slight promotional effect on the activity of NO conversion to nitrogen is observed after Si and Ti incorporation. FTIR study showed the sulphation of silver and aluminum sites of Ag/Al₂O₃ catalysts resulting in the decrease in the formation of reactive intermediate species such as –NCO, which in turn decreases NO_x conversion to N₂. In the case of Ag/Al₂O₃ doped with SiO₂ or TiO₂, formation of silver sulphate and aluminum sulphate was drastically reduced, which was evident in FTIR resulting in remarkable improvement in the sulphur tolerance of Ag/Al₂O₃ catalyst. These catalysts before and after the reaction have been characterized with various techniques (XRD, BET surface area, transmittance FTIR and pyridine adsorption) for physico-chemical properties.