Three-way Catalytic Reaction Research Articles

Three-way catalytic reactions on Rh-based catalyst: Effect of Rh/ceria interfaces

Rh-based catalysts were prepared by various methods and it was found that preparation methods play an important role in metal-support interaction (MSI) control which affects the catalytic performance of catalyst. The results suggest that the catalytic reduction of NO is mainly achieved by C3H6 in exhaust and H2 generated from the water-gas shift reaction as well as the steam reforming of C3H8 and CH4. Concentration of water in reaction stream has a significant influence on the water-gas shift and steam reforming reactions. The removal of C3H6 is accomplished by oxygen-induced oxidation instead of steam reforming reaction. In addition, Rh/ceria interactions promote the formation of active oxygen species and surface oxygen vacancy that respectively favors CO oxidation and NO reduction with a high N2 selectivity. Rh@CeO2 system shows high thermal stability due to Rh/ceria interaction.

Three-way catalytic converter reactions aspects at near-ambient temperatures on modified Pd-surfaces

The dissolution of oxygen in palladium plays an important role in palladium catalysis. The present study shows that the surface modification (SM) due to the dissolution of atomic oxygen into the subsurfaces of palladium can be used as a control to tune its catalytic activity. CO oxidation and NO + H 2 + O 2 reaction was separately carried out on metallic Pd and on surface modified Pd using a molecular beam instrument and the results were compared. The metallic Pd does not show activity below 400 K for both reactions, whereas the SM-Pd shows activity at near-ambient temperatures. The electronic change due to SM was investigated using ambient pressure photoelectron spectroscopy, and the investigation clearly shows the effect of subsurface oxygen in the ambient temperature activity of palladium.

NiFe2O4 as an active component of a platinum group metal-free automotive three-way catalyst

To develop a platinum group metal (PGM)-free automotive three-way catalyst (TWC), we investigated the structure–activity relationship for Fe–Ni oxide catalysts and found NiFe2O4 with a spinel structure is an active component of the three-way catalytic reaction (NO–C3H6–CO–O2).

Preparation of Ir@Pt Core–Shell Nanoparticles and Application in Three-Way Catalysts

The Ir@Pt core–shell spherical nanoparticles with Pt deposited on Ir cores were synthesized for the first time using successive reduction method based on epitaxial growth. The Ir@Pt/SiO2 core–shell structure catalysts were used in the three-way catalysts of controlling automobile exhaust emission, which exhibited high catalytic activity of NO reduction in three-way catalytic reaction.

Steam effects over Pd/Ce0.67Zr0.33O2-Al2O3 three-way catalyst

Ceria-zirconia-alumina (CZA) solid solution was prepared by sol-gel method in the present study. 0.5 wt.% Pd supported on CZA was prepared by incipient wetness impregnation. The steam effects for CO and C3H8 oxidation, three-way catalytic activity and stoichiometric window property were studied. The light-off temperature of the CO oxidation reaction shifted to a lower temperature due to the water-gas shift (WGS) reactions. The oxidation of C3H8 was enhanced due to the steam reforming (SR) reactions. The steam promoted the C3H8 oxidation and NO reduction in three-way catalytic reaction. The amplitude of stoichiometric window was amplified by the addition of water to the feed stream.

Operando X-ray absorption spectroscopy study of supported Pt catalysts during NO reduction by hydrocarbons

To investigate the support effect on the hydrocarbon-NO-O2 reaction in automotive three-way catalytic reactions, operando X-ray absorption near edge structure (XANES) experiments were performed for reactions of C3H6 (or C3H8)-NO-O2 over Pt supported catalysts under reducing conditions. γ-Al2O3, ZrO2, CeO2-ZrO2 and La2O3 were used as the support oxides in this study. After oxidation pre-treatment, the oxidation state of the supported Pt was monitored by in situ Pt L3-edge XANES spectra while the temperature in the reaction mixture was ramped up and the catalytic reaction was followed by measuring the concentrations of C3H6 or C3H8, NO and O2. The operando experimental results were analyzed in terms of the catalytic reaction start-up temperature (20% HC conversion temperature) and the 50% Pt reduction temperature. A V-shape correlation was found between these two temperatures showing that an appropriate Pt reduction temperature should exist to lower the catalytic reaction start-up temperature. This result suggests that the creation of active metallic Pt sites together with the self-poisoning effect of adsorbed carbonaceous species on metallic Pt should determine the catalytic reaction start-up behavior of supported Pt catalysts. Besides the suitable Pt reduction temperature, the oxygen reactivity in the support oxide is also important for lowering the catalytic start-up temperature. The control of the Pt–support interactions to provide suitable Pt reducibility and also oxygen reactivity in oxide support are both important to achieve a lower catalytic reaction start-up temperature.

Pd-Supported Interaction-Defined Selective Redox Activities in Pd−Ce0.7Zr0.3O2−Al2O3 Model Three-Way Catalysts

Effects of Pd-supported interactions toward redox behaviors concerning three-way catalytic reactions and oxygen-buffering effects are investigated through stepwise changing Pd-loading locations over ceria−zirconia and alumina. Through light-off tests and kinetics, texture, and surface studies, discrepant but redistributable Pd−A2O3 and Pd−Ce0.7Zr0.3Ox interfaces are defined and analyzed. Pd species are inclined to promote the transformation of Ce4+ to Ce3+ and maintain themselves as fine particles on ceria−zirconia surfaces. Oxygen spillover promoted by Pdn+/Pd0−Ce4+/Ce3+ redox couples benefits the oxygen-buffering effect, but is limited by the increase of reaction temperatures and ceria−zirconia reducibility. This strong oxidative interaction overcomes the possibility of ceria-related anionic vacancies in facilitating NO dissociation and, thus, improves CO conversion only. Stochiometric light-off tests show higher activities for NO reduction and C3H8 oxidation on a Pd−Al2O3 interface, where the different morphologies and redox states of Pd-supported interfaces should be the main contributing factors for efficient molecular bond dissociation.

A New Approach in the Kinetic Modelling of Three-Way Catalytic Reactions

Until now, the flexibility of a global kinetic approach in the modelling of three-way catalysis, for redox reactions, has met with little success. It is an approach, which is poorly adapted when considering all the physical and chemical phenomena involved in a catalytic cycle. To this end, the work presented herein describes a novel approach in the kinetic modelling which includes a comprehensive understanding of catalytic cycles. From this basis, we have established detailed reaction rate equations that successfully model three-way catalysts behaviour under different test reaction conditions.

Synergistic effect of vanadium and zirconium oxides in the Pd-only three-way catalysts synthesized by sol–gel method

Various Pd-only three-way catalysts containing vanadium and zirconium oxides were prepared by the sol–gel method and investigated with XRD, XPS, 51V NMR, 27Al MAS NMR, Raman spectrometry, BET analysis and three-way catalytic reaction. In the Pd-only three-way catalyst prepared by the sol–gel method, vanadium and zirconium were good promoters to improve the resistance against SO 2 poisoning and to enhance the thermal stability, respectively. Excellent properties such as high activity at low-temperature, improved durability against SO 2 gas and enhanced thermal stability at high temperature were found only on the catalyst containing the specific mole ratio of V/Zr=0.36, Pd (1 wt.%)-V (2 wt.%)-Zr (10 wt.%)-Al 2O 3 catalyst, which was ascribed to some synergism between V and Zr like a formation of V–O–Zr bonds. Because of the synergism on the catalyst, the advantageous textural forms such as highly dispersed Pd clusters, Zr–O–X bond (X=V and Al), γ-Al 2O 3 and amorphous tetrahedrally coordinated (M–O) 3VO were not deteriorated significantly to the disadvantageous textural forms such as large crystalline Pd clusters, monoclinic ZrO 2, κ-Al 2O 3 and θ-Al 2O 3, and microcrystalline octahedrally coordinated V 2O 5 species, in spite of thermal aging at 1000°C.

Effect of pH in a sol–gel synthesis on the physicochemical properties of Pd–alumina three-way catalyst

The effect of pH during sol–gel synthesis on the structural and physicochemical properties of a Pd–Al2O3 three-way catalyst (TWC) prepared by the sol–gel method was investigated by using BET, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and solid state 27Al MAS NMR. The Pd–Al2O3 catalyst prepared at pH=10 (Pd–Al2O3–B) showed a high activity in three-way catalytic reaction, a high dispersion of Pd, and large surface area and pore volume. A basic condition (pH=10) in the sol–gel process was essential for the preparation of highly dispersed palladium clusters on alumina gel. The formation of highly stable palladium oxide species in Pd–Al2O3–B that were not completely reduced at 423K was ascribed to the strong interaction between Pd and oxygen in alumina texture, resulting in the formation of –Al–O–Pd bond.

Influence of Oxygen on the Performance of a Three-Way Catalyst

During experiments designed to simulate the oscillations of composition observed upon the catalyst in a vehicle exhaust device, a Pt-Rh bimetallic catalyst supported on alumina was studied for three-way catalytic reactions under both stationary and cycling conditions, The results were compared with those obtained from employing platinum or rhodium metals supported separately upon alumina and indicated that during the operation of the oscillatory regime on the bimetallic catalyst there is a deactivation/activation mode associated with a fast redox process of the rhodium component which becomes partially oxidised to its inactive form, Rh 2O 3. Deactivation was not observed with platinum alone nor under steady feed conditions in the bimetallic case. The effect of this cycling deactivation is examined with respect to the temperature required to affect the bulk of the metals, its consequential reorganisation of the surface, and subsequent interaction with the oxygen.

The effects of SO 2 on the performance of noble metal catalysts in automobile exhaust

SO 2 inhibits important three-way or stoichiometric catalytic reactions. Of the noble metals studied (Pt, Pd, Rh, Ir), Rh is particularly resistant to SO 2 for NO removal while Pt is strongly inhibited by SO 2. This is an important reason why Rh is useful for effective NO control. Ir is particularly effective for converting NO under strongly oxidizing conditions in the presence of SO 2. NH 3 formation is inhibited by SO 2 over the metals studied. Similarly under reducing conditions, propylene conversion is inhibited by SO 2 over the metals studied.