Effect of Soot During Operation of a Pt–K/Al2O3 LNT Catalyst

The effect of soot on the NO _x storage-reduction performances of a Pt–K/Al₂O₃ catalyst is analysed in this work by performing lean-rich cycles at constant temperature. The interaction between soot and the stored NO _x has been also investigated by temperature programmed methods. It has been found that the presence of soot reduces the NO _x storage capacity of the catalyst; besides the presence of soot favours the decomposition of the stored nitrates. A direct reaction between the stored nitrates and soot is suggested, that has been explained on the basis of the surface mobility of the adsorbed nitrates.     

Spectrokinetic Analysis of the NOx Storage Over a Pt–Ba/Al2O3 Lean NOx Trap Catalyst

In this work the kinetics of the (reactive) lean accumulation phase of the NO_x storage-reduction process is described through a detailed kinetic model, involving both the gas-phase molecules and the adsorbed species. Kinetic data have been collected following a novel approach based on simultaneous operando spectroscopic measurements and on-line pulse reactor effluent analysis. To our knowledge, this is the first time the temporal evolutions of the concentration of both the surface and the gas species are used jointly to describe the kinetic of a transient catalytic process.     

FTIR and Transient Reactivity Experiments of the Reduction by H2, CO and HCs of NO x Stored Over Pt–Ba/Al2O3 LNTs

The reduction of NO _x stored over a Pt–Ba/Al₂O₃ Lean NO _x Trap is analysed when H₂, CO or heptane are used as reductants. In all cases, the reduction proceeds via a Pt-catalyzed process involving the formation of intermediate species like ammonia and isocyanates in the case of H₂ and CO, respectively. No specific intermediates have been observed when heptane is used as reductant. It is claimed that the role of the reductant is to keep Pt in a reduced state; this favours nitrate decomposition and reduction over the Pt sites. The effect of water on the reaction is also investigated.     

Influence of Mn and Fe Addition on the NO x Storage–Reduction Properties and SO2 Poisoning of a Pt/Ba/Al2O3 Model Catalyst

This work deals with the effect of Mn or Fe addition on the NO _x storage–reduction properties of a Pt/Ba/Al₂O₃ model catalyst. NO _x storage capacity, SO₂ poisoning and regeneration and NO _x removal efficiency under rich/lean cycling conditions are studied. Fe addition to Pt/Ba/Al₂O₃ leads only to a small increase of NO _x storage capacity, and more interestingly, to a better sulfur removal due to the inhibition of bulk barium sulfate formation. Unfortunately, the NO _x storage property cannot be fully recovered. Moreover, Fe addition results in a decrease in the NO _x removal efficiency. Mn addition also improves the NO _x storage capacity, but no significant influence on the sulfur elimination is observed. Mn-doped catalyst does not improve the NO _x removal efficiency, but NH₃ selectivity is found to drastically decrease at 400 °C, from 20 to 3%. In addition, the NO _x conversion can be improved at higher H₂ concentration in the rich pulse, always keeping NH₃ selectivity at low level.     

Cyclic Lean Reduction of NO by CO in Excess H2O on Pt–Rh/Ba/Al2O3: Elucidating Mechanistic Features and Catalyst Performance

This study provides insight into the mechanistic and performance features of the cyclic reduction of NO_x by CO in the presence and absence of excess water on a Pt–Rh/Ba/Al₂O₃ NO_x storage and reduction catalyst. At low temperatures (150–200 °C), CO is ineffective in reducing NO_x due to self-inhibition while at temperatures exceeding 200 °C, CO effectively reduces NO_x to main product N₂ (selectivity >70 %) and byproduct N₂O. The addition of H₂O at these temperatures has a significant promoting effect on NO_x conversion while leading to a slight drop in the CO conversion, indicating a more efficient and selective lean reduction process. The appearance of NH₃ as a product is attributed either to isocyanate (NCO) hydrolysis and/or reduction of NO_x by H₂ formed by the water gas shift chemistry. After the switch from the rich to lean phase, second maxima are observed in the N₂O and CO₂ concentrations versus time, in addition to the maxima observed during the rich phase. These and other product evolution trends provide evidence for the involvement of NCOs as important intermediates, formed during the CO reduction of NO on the precious metal components, followed by their spillover to the storage component. The reversible storage of the NCOs on the Al₂O₃ and BaO and their reactivity appears to be an important pathway during cyclic operation on Pt–Rh/Ba/Al₂O₃ catalyst. In the absence of water the NCOs are not completely reacted away during the rich phase, which leads to their reaction with NO and O₂ upon switching to the subsequent lean phase, as evidenced by the evolution of N₂, N₂O and CO₂. In contrast, negligible product evolution is observed during the lean phase in the presence of water. This is consistent with a rapid hydrolysis of NCOs to NH₃, which results in a deeper regeneration of the catalyst due in part to the reaction of the NH₃ with stored NO_x. The data reveal more efficient utilization of CO for reducing NO_x in the presence of water which further underscores the NCO mechanism. Phenomenological pathways based on the data are proposed that describes the cyclic reduction of NO_x by CO under dry and wet conditions.     

Simultaneous Removal of NO x and Soot Over Pt–Ba/Al2O3 and Pt–K/Al2O3 DPNR Catalysts

The effect of NO _x storage on the soot combustion activity when alkaline- and alkaline/earth-containing model DPNR catalysts are used is investigated in this work. The influence of different experimental conditions (NO concentration, temperature, and particulate loading) is addressed and discussed in relation to the NO _x storage efficiency and soot oxidation activity as well.     

Reaction Pathways in the Reduction of NO x Species by CO over Pt–Ba/Al2O3: Lean NO x Trap Catalytic Systems

The reduction by CO of NO _x species stored over Pt–Ba/Al₂O₃ Lean NO _x Trap systems is analysed in this work. The reaction mechanisms and pathways leading to N₂ formation both under dry and wet conditions are investigated by complementary transient dynamic experiments and FTIR analyses.     

A Sol–Gel Derived CuOx/Al2O3–ZrO2 Catalyst for the Selective Reduction of NO by Propane in the Presence of Excess Oxygen

Copper catalysts supported on alumina-doped zirconia were prepared by sol–gel processing followed by supercritical drying or aging in the mother solution at 100°C. After drying and calcination, the catalyst supports were impregnated with a copper(II) nitrate aqueous solution by the incipient wetness method to achieve a Cu loading of about 2%. The samples showed 90% NO conversion at 350–400°C. The catalytic performance of these systems appears to be determined by the degree of clustering of copper cations as probed by FTIR spectroscopy of adsorbed CO.     

Rh–Sr/Al2O3 Catalyst for N2O Decomposition in the Presence of O2

The improving effect of Sr in the catalytic activity of Rh for N₂O decomposition has been studied under 1,000 ppm N₂O/He and 1,000 ppm N₂O/5% O₂/He (GHSV = 10,000 h^?1). Different techniques have been used for catalysts characterization: TEM, SEM-EDX, XRD, N₂ adsorption at ?196 °C and in situ XPS. Sr favours the Rh dispersion and reduction under reaction conditions, and allows the low temperature removal of N₂O in the presence of O₂ (100% decomposition at 350 °C).     

The NO x Reduction Mechanism by H2 under near Isothermal Conditions over Pt–K/Al2O3 Lean NO x Trap Systems

The reduction of NO _x stored over a Pt–K/Al₂O₃ Lean NO _x Trap is analyzed when H₂ is used as reductant. An in series 2-steps process is herein proposed, involving at first the formation of NH₃ upon reaction of nitrates with H₂ (step 1), followed by the reaction of NH₃ with residual nitrates to give N₂ (step 2).     

Kinetics of the Steady-State Acetylene Oxidation by Oxygen over a Pt/Rh/CeO2/γ-Al2O3 Three-Way Catalyst

The steady-state kinetics of acetylene oxidation has been studied in the framework of automotive exhaust gas catalysis over a commercially available three-way catalyst. Experiments under cold-start conditions have been carried out in a laboratory fixed-bed reactor, which can adequately be described by the developed elementary step model and rate parameters.     

Loss of NOx storage capacity of Pt–Ba/Al2O3 catalysts due to incorporation of phosphorous

This work aims at determining the effect of the incorporation of P on NO_x storage capacity by NO_x storage-reduction (NSR) catalysts. Different amounts of phosphorous were deposited on a Pt–Ba/Al₂O₃ catalyst (1 wt% Pt and 17 wt% Ba) by impregnation of a phosphate salt. Samples were calcined at 723 K and characterized using X-ray diffraction (XRD), N₂ adsorption isotherms and X-ray photoelectron spectroscopy (XPS). Their NO_x storage capacity was also measured. It was observed that storage capacity decreased almost linearly with the P/Ba ratio and besides at a phosphorous concentration P/Ba ratio of <0.1 deterioration was low. At higher P concentrations (P/Ba ratio = 0.7) the NO_x storage was significantly reduced. It is proposed that the cause of the decline in NO_x retention capacity would be the formation of Ba–P phases (very likely Ba phosphates) at the expense of Ba carbonate or Ba oxide. These new phases would be unable to exchange NO_x.     

Epoxidation of Propylene by Molecular Oxygen Over the Ag–Y2O3–K2O/α-Al2O3 Catalyst

Abstract The Ag/α-Al₂O₃ catalyst modified with rare earth metal oxide (Y₂O₃) and alkali metal oxide (K₂O) for the epoxidation of propylene by molecular oxygen were prepared and characterized by TG-DTA, XRD and XPS. The results show that a small quantity of Y₂O₃ added plays a role of electron and structure-type promoters, and can change the binding energies of Ag3d and restrain the sintering of Ag crystallites during catalyst preparation. The effects of promoters loading, Ag loading, reaction temperature, and calcination atmosphere on the performance of Ag catalyst were investigated. The results show that the loadings of K₂O, Y₂O₃ and Ag, and reaction temperature affect obviously the catalytic performance of Ag–Y₂O₃–K₂O/α-Al₂O₃ for the epoxidation of propylene to propylene oxide. Under the reaction conditions of 0.1 MPa, 245 °C, GHSV of 2000 h⁻¹ and the feed gas of 20%C₃H₆/8%O₂/N₂, the conversion of propylene of 4% and the selectivity to propylene oxide of 46.8% were achieved over the 20%Ag–0.1%Y₂O₃–0.1%K₂O/α-Al₂O₃ catalyst. Graphical Abstract Y₂O₃ plays a role of electron and structure-type promoters in the Ag–Y₂O₃–K₂O/α-Al₂O₃ catalyst, which can change the binding energies of Ag3d and restrain the sintering of Ag crystallites, resulting in obvious improvement of the catalytic performance of Ag–Y₂O₃–K₂O/α-Al₂O₃ for the epoxidation of propylene to propylene oxide by molecular oxygen.      

Role of Pt Oxidation State on the Activity and Selectivity for Crotonaldehyde Hydrogenation Over Pt–Sn/Al2O3–La and Pt–Pb/Al2O3–La Catalysts

Pt–Sn/ALa10 and Pt–Pb/ALa10 catalysts (10 wt% La₂O₃) were studied in the selective hydrogenation of crotonaldehyde. Oxidized Pt²⁺ and reduced Pt⁰ species were identified by XPS on the bimetallic catalysts. High selectivity to crotylalcohol was obtained on the Pt–Pb/ALa10 catalyst where an electron transfer effect from Pb to Pt was proposed. For the Pt–Sn/ALa10 catalyst the formation of Pt–SnO _x –La₂O₃ complexes showing low activity and low selectivity was inferred.     

Synergetic effect of combined use of Cu–ZnO–Al2O3 and Pt–Al2O3 for the steam reforming of methanol

Commercial Cu–ZnO–Al₂O₃ catalysts are used widely for steam reforming of methanol. However, the reforming reactions should be modified to avoid fuel cell catalyst poisoning originated from carbon monoxide. The modification was implemented by mixing the Cu–ZnO–Al₂O₃ catalyst with Pt–Al₂O₃ catalyst. The Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalyst mixture created a synergetic effect because the methanol decomposition and the water–gas shift reactions occurred simultaneously over nearby Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalysts in the mixture. A methanol conversion of 96.4% was obtained and carbon monoxide was not detected from the reforming reaction when the Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalyst mixture was used.     

Determining the Best Composition of a Ni–Fe/Al2O3 Catalyst used for the CO2 Hydrogenation Reaction by Applying Response Surface Methodology

Response surface methodology (RSM) with central composite design (CCD) was applied to determine the composition of an alumina-supported nickel-iron (Ni–Fe) catalyst that provided the highest CH₄ yield for the CO₂ hydrogenation reaction. This involved synthesis of alumina-supported Ni–Fe catalysts of compositions that were specified by CCD. The catalysts were then tested for the CO₂ hydrogenation reaction, and a model equation was developed that related the catalyst composition to the CH₄ yield. The model equation was validated by analysis of variance, and it was found to adequately represent the experimental data. The model equation predicted that the alumina-supported Ni–Fe catalyst containing 32.8% Ni and 7.7% Fe would provide the highest CH₄ yield. A catalyst with this specific composition and the same metal deposition method and two other catalysts of the same composition but different metal deposition metal were also synthesized, characterized, and tested for the CO₂ hydrogenation reaction. The three catalysts did show activities similar to those predicted by the model equation. Furthermore, characterization and reaction studies revealed that the three catalysts were similar, suggesting that the metal deposition methods do not have any effect on the catalytic activity.     

Reduction of NO by CO under oxidizing conditions over Pt and Rh supported on Al2O3–ZrO2 binary oxides

The platinum and rhodium particles supported on Al₂O₃–ZrO₂ binary oxides were prepared by adding the metal precursors to the binary gel. High specific surface areas (220–250 m²/g) and small metallic particle size (∼20 Å) were obtained. In the reduction of NO by CO without oxygen in the reactant flow high levels of N₂O were achieved, whereas in the presence of oxygen the formation of N₂O notably increases. This selectivity behavior was not observed in catalysts prepared by impregnation with the metallic precursors of Al₂O₃–ZrO₂ mixed oxide stabilized after calcination at 500 °C, since in these catalysts the selectivity to N₂O is the higher with or without oxygen. Thus, it is proposed that the metallic impregnation of gels strongly modifies the mechanism by which the NO reduction by CO occurs.     

Flame-Made Pt/K/Al2O3 for NO x Storage–Reduction (NSR) Catalysts

High surface area Pt/K/Al₂O₃ catalysts were prepared with a 2-nozzle flame spray method resulting in Pt clusters on γ-Al₂O₃ and amorphous K storage material as evidenced by Raman spectroscopy. The powders had a high NO _x storage capacity and were regenerated fast in a model exhaust gas environment. From 300 to 400 °C no excess NO _x was detected in the off gas during transition from fuel lean to fuel rich conditions, resulting in a highly effective NO _x removal performance. Above 500 °C, the NSR activity was lost and not recovered at lower temperatures as K-compounds were partially crystallized on the catalyst.