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Natural Gas Vehicle three-way catalyst(NGV-TWC)
Natural Gas Vehicle (NGV) is characterized by very low amount of PM emission comparted to gasoline and diesel vehicles. Nevertheless, the main drawback of NGV lies in the residual emission of methane which is characteristically refractory to break down due to the high bond energy of C-H (439.3 kJ mol-1).[B.C. Enger 2008] Pd-Rh three way catalysts and Pt-Pd oxidation catalysts have been widely applied to NGV emission control under stoichiometric(λ=1) and lean-burn condition (λ=1.3), respectively. It has been proved that compared to lean-burn condition, stoichiometric condition is more favorable to the improvement of emission abatement performance since the removal of NOx and methane is much easier under such condition. Moreover, the presence of water and low-concentration sulfur (<0.5 ppm) would lead to an enhanced resistance of the catalyst to poisoning. Leanburn condition has been attested to be advantageous to the combustion of methane at low temperature. A critical issue related to the NGV catalyst is the thermal stability of oxidic palladium which is susceptible to decomposition at⇄ high temperature. Hence, particular attention should be paid to the equilibrium of PdOx Pd0. The combination of Pd-Pt was suggested to be beneficial to the improvement of the thermal stability as well as the robustness against sulfur poisoning of the NGV catalyst. [P. Granger 2017]
Palladium has been considered as the benchmark catalyst for methane oxidation. Activation of methane is supposed to take place on site pairs composed of surface PdO and surface Pd (Fig. 2.11.). [P. Gélin 2002]High Pd loading (>200 g ft -3) is usually required for the sake of compensating the activity loss engendered by thermal sintering. [A. Raj 2016] Recent studies are mainly focused on threes aspects: (1) mechanism of thermal deactivation at high temperature (2) development of catalytic system with better resistance to sulfur poisoning (3) improvement of low-temperature activity during engine cold-start. [P. Gélin 2002]
Methane oxidation on a plasma-catalysis hybrid system was studied by H. Lee [H. Lee 2015]. Depending on the loading position of the catalyst, two different configurations of the hybrid system were proposed: in-plasma catalysis (wherein the catalyst is placed in the discharge zone) and post-plasma catalysis (wherein the catalyst is placed after the discharge zone) (Fig. 2.12.). The author claimed that a synergistic effect was found for the in-plasma configuration, demonstrating a quicker “light-off” of methane oxidation and a lower CO selectivity. Consequently, complete methane oxidation was achieved even at room temperature when 2 wt.% Pd/Al2O3 oxidation catalyst was employed. However, such synergy was not detected in the post-plasma configuration. In addition, electrochemical method has also introduced to assist the catalytic activity of methane oxidation. This method is based upon the concept of non-faradaic electrochemical promotion of catalysis (NEMCA) discovered by Stoukides [M. Stoukides 1981]. By applying a potential between the working electrode (consisting of the catalyst supported on a solid electrolyte) and the counter electrode (deposited on the same solid electrolyte), the metal-support interaction can be monitored.[P. Granger 2017] The effect of electrochemical promotion of catalysis on methane combustion was systematically studied by F. Matei [F. Matei 2013].It was established that impregnation of Pd on highly porous YSZ(Y2O3 -stabilized-ZrO2, an oxygen ion conductor) led to a much more active catalyst compared to that typically deposited on dense YSZ, with an increased catalytic rate as much as one order of magnitude.(Fig. 2.13.) The author attributed the improvement of catalytic performance to the much higher dispersion of palladium on porous electrolytes.
Fig. 2.13. Effect of the temperature on methane conversion, CO formation rate and= 1in.4-plane =4.5 2 4 resistance of the Pd catalyst deposited on dense and on porous YSZ. Conditions: , 2 , F = 200 ml/min. [Reproduced with permission from F. Matei 2013]
One of the major obstacles in methane conversion consists in the deactivation caused by coke deposition. [P. Granger 2017]. It is accepted that such problem can be bypassed with the help of application of materials with good Oxygen Storage Capacity (OSC) properties. Perovskites with superb OSC can be acquired via the stabilization of unusual oxidation states of transition metals and the creation of oxygen vacancies, which is usually achieved by means of substitution on A or B site. Meanwhile, previous studies have indicated the beneficial effect of perovskite-based material on stabilizing PdO. Eyssler asserted that depending on the calcination temperature and reaction temperature, a delicate equilibrium existed between the fraction of active PdO exposed on the LaFeO3 surface and the fraction of Pdn+ dissoluted in the Pd-LaFeO3 solid solution. [A. Eyssler 2011] The author further discovered that below 700 ℃, catalytic activity was in accordance with Pd2+ species present on the surface of LaFeO3. The higher the thermal treatment temperature, the lower the Pd dispersion, resulting in a diminished accessibility of PdO-surface and active centers. The author concluded that 2 wt.% Pd/LaFeO3 outperformed the conventional Pd/Al2O3 in terms of stabilizing active PdO at high temperature. [A. Eyssler 2011]. It was widely recognized that the self-regenerative mechanism of perovskites can protect noble metals (i.e. Pt, Pd, Rh) from sintering by inhibiting the particle growth. Yoon [D.Y. Yoon 2015] studied the thermal stability of Pd-containing LaAlO3 catalyst. A strong Pd-La interaction was clarified to induce the electron transfer from La to Pd. As a result, a higher electron density of Pd was identified, probably responsible for the enhancement of thermal stability of PdO when subjected to high temperature.
NGV is assumed to be an alternative for conventional gasoline or diesel vehicles to meet future harsh legislation requirements. The concept of NGV can be concretized through either CNG or LNG. Further amendments are still needed to cut the cost while improving the overall performance of the NGV prior to its large-scale application, in particular dealing with issues of deactivation caused by thermal ageing, water or poisoning (especially sulfur) as well as methane slip at low temperature. [A. Raj 2016]
Perovskite as modern Three Way Catalyst
Although three -way catalytic converters have been established as the most effective engine exhaust gas post-treatment system in the past decades, plenty of problems have emerged in the practical application, such as their high cost owing to the increasingly tight supply of precious metals, the deactivation issues as well as the difficulty and complexity of recycling process. More attention is being paid to the development of new materials as possible alternatives to precious-metal based TWC.
Perovskite is referred to oxides with the chemical formula of ABO3 or A2 BO4. In the typical ABO3 structure, A cations are 12-fold coordinated and B cations are 6-fold coordinated with nearby oxygen anions. The larger cations A and surrounding oxygens form a face-centered cubic lattice(FCC) in which the smaller cations B occupy the octahedral sites[Y.M. Chiang 1997]. A cations are usually alkaline elements, alkaline earth elements or lanthanide series whereas B cations can be transition metals with 3d, 4d or 5d electronic and > 0.051 for B cations, respectively. > 0.090 for A cations configuration[L.G. Tejuca 1993] The limit of cationic radii is Fig. 2.14. Schematic plot of ideal (cubic) structure of ABO3 perovskite a-lattice, b-polyhedrons(reproduced with permission of [H. Du 2015]).
Theoretically, perovskite structure could be maintained as long as the tolerance factor is kept within the range of . In the ideal cubic structure of highest symmetry, A oxygen anion have contact with each other. The ideal structure occurs only when the tolerance factor gets close to 1.0 at high temperature. A t value lower than 1.0 gives rise to a structure with less symmetry. When t<0.75 the crystal exists in the form of ilmenite. While when t >1.0 the crystal becomes hexagonal symmetry such as calcite or aragonite [M. A. Pena 2001]. In the majority of cases, deviations from the ideal structure are known, including orthorhombic, rhombohedral, tetragonal, monoclinic and triclinic(lowest symmetry) lattice systems.[C.P. Khattak 1979; J.B. Goodenough 1970, 1974] these distorted structures are present predominantly at room temperature but could transform to cubic structure at high temperature. According to Pena[M.A. Pena 2001], deviations from cubic structure may result from the distortion of cubic unit cell or an enlargement of the cubic unit cell or a combination of both.
An advantageous characteristic of perovskite oxides is their structural flexibility to accommodate a wide range of ions both on A and B site, allowing them to be chemically tailored with the aim to endow themselves with specific features. Cations both on the A site and B site could be substituted with various types of metal cations with same or different radii or valences in order to modify the physicochemical properties and subsequent catalytic performance of the material[R.J.H. Voorhoeve 1976, 1977; M.A. Pena 2001]. It is therefore possible to conduct partial substitution on A and B site on the premise that electroneutrality principle and geometrical requirement are met. Due to their good structure adaptability and tunability, multiple formulas of substituted perovskite could be formed, including simple compounds such as A1+B5+O3, A2+B4+O3, A3+B3+O3 and complex compounds such as A(B′1- yB″y)O3±δ, (A′1-xA″x)BO3±δ, (A′1-xA″x)(B′1-yB″y)O3±δ, etc.
Ruddlesden-Popper series of layered oxides
Certain metals (i.e. La2CuO4) only form Ruddlesden-Popper oxides(RP oxides) rather than the perovskite with typical ABO3 structure. The Ruddlesden-Popper phase (RP phase) is composed of n consecutive perovskite layers (ABO3) alternating with rock-salt layer (AO) along the crystallographic c -axis direction[Dongkyu Lee 2017]. A cation is located at the boundry between two types of layers with a coordination number of 9 while B cation is situated in the center of octahydron formed with nearby 6 oxygen anions. The general formula of RP oxides can be expressed as An+1BnO3n+1 (n≥1) or more specifically as (AO)•(ABO3) where n represents the number of connected layers of vertex-sharing BO6 octahedra[I.D. Brown 1992] When n=1, A2BO4 structure is formed, i.e. La2CuO4 ;When n>1 additional ABO3 blocks are introduced between two rock-salt layers to form complex oxides with multiple stacked octahedral layers[Dongkyu Lee 2017].
Related reaction mechanisms on perovskites:
Oxidation reactions for CO and HC:
Low-temperature oxidation: CO oxidation
It is established that CO oxidation follows a suprafacial mechanism that usually takes place at low temperature. Oxygen contained in the material is consumed constantly in the course of reaction, which should be replenished by the oxygen dissociated from gas phase so that the reaction could go on smoothly. [N. Guilhaume 1997]. Zhang-Steenwinkel [Y. Zhang-Steenwinkel 2004] studied the kinetics of CO oxidation on La0.8Ce0.2MnO3.
Perovskite for soot oxidation
The possibility of using perovskite for diesel soot oxidation has been investigated by many researchers.[S. Hernandez 2012; P. Ciambelli 2001; J.L. Hueso 2008; L.M. Petkovic 2011; R. Jiménez 2010; D. Fino 2003; N. Russo 2005; D. Hari Prasad 2012; X. Wang 2012] In summary, catalytic activity of perovskite for soot oxidation are related to several intrinsic properties: (ⅰ) surface and bulk oxygens mobility (ⅱ) activity at high temperature (ⅲ) high thermal stability (ⅳ) spillover mechanism involving the enrichment of suprafacial oxygen species (α-oxygen) with oxygenated species on the surface of soot particles[P. Ciambelli 2001; R. Jiménez 2010; D. Fino 2003].LaCoO3, LaNiO3, LaFeO3 have been revealed capable of oxidizing soot below 445℃[A. Mishra 2017]
Studies of L0.6Sr0.4BO3 (B=Fe, Mn, Ti) perovskite on soot oxidation were conducted by W.Y. Hernández [W.Y. Hernández 2015]. It was shown that L0.6Sr0.4FeO3 and L0.6 Sr0.4MnO3 presented better soot oxidation activity than L0.6Sr0.4TiO3. Furthermore, the author concluded that L0.6Sr0.4MnO3 was suitable to oxidize soot at low oxygen partial temperatures due to the abundant presence of Mn4+.[ W.Y. Hernández 2015]
Table of contents :
Abstract
Chapter I. Context and general introduction
Reference
Chapter II. Literature review
2.1. THREE-WAY CATALYST: AN ESTABLISHED TECHNOLOGY FOR EXHAUST POST TREATMENT
2.2. FOUR-WAY CATALYTIC SYSTEM: A SOLUTION TO MEET FUTURE EMISSION STANDARDS
2.3. CONSTRUCTION OF THE FOUR-WAY CATALYTIC SYSTEM:
2.3.1. GPF/TWC coupling
2.3.2. DPF-DOC-SCR integration
2.3.3. SCRF (SCR coated DPF)
2.3.4. Natural Gas Vehicle three-way catalyst(NGV-TWC)
2.4. PEROVSKITE AS MODERN THREE WAY CATALYST
2.4.1. Related reaction mechanisms on perovskites:
2.4.1.1. Oxidation reactions for CO and HC:
2.4.2. Perovskite for soot oxidation
2.5. OXYGEN STORAGE CAPACITY (OSC)
2.5.1. CeO2-based OSC materials
2.5.2. Perovskite-type OSC materials
2.6. METAL DISPERSION
2.7. SELF-REGENERATIVE MECHANISM OF PEROVSKITE-METAL
2.8. SYNTHESIS OF PEROVSKITE: FROM CONVENTIONAL METHODS TO MORE SOPHISTICATED TECHNOLOGIES WITH CONTROLLABLE MICROSTRUCTURE
2.8.1. Solid state process
2.8.1.1. Solid-solid method
2.8.1.2. Reactive grinding method
2.8.1.3. Microwave crystallization
2.8.2. Solution-based method
2.8.2.1. Sol-gel method
2.8.2.2. Mechanism of sol-gel method
2.8.2.3. Co-precipitation method
2.8.2.4. Auto-combustion method
2.8.2.5. Flame Spray Pyrolysis method
2.8.2.6. Solvothermal/Hydrothermal method
2.8.2.7. Spray/freeze drying method
Reference
Chapter III. Experimental methods and techniques
3.1. SYNTHESIS PROTOCOLS
3.1.1. Conventional Citric Method (CCM)
3.1.2. Macro-Structuring Method (MSM)
3.1.3. Incipient Wet Impregnation Method (IWIM)
3.2. PHYSICOCHEMICAL CHARACTERISATION OF SOLID CATALYST
3.2.1. Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES)
3.2.2. X-ray diffraction
3.2.3. H2-TPR
3.2.4. O2-TPD
3.2.5. N2-physisorption
3.2.6. Raman spectroscopy
3.2.7. X-ray Photoelectron Spectroscopy (XPS)
3.2.8. Scanning Electron Microscopy (SEM)/ Energy-dispersive X-ray spectroscopy (EDS)
3.2.9. Chemisorption
3.3. CATALYTIC ACTIVITY MEASUREMENTS
3.3.1. Experimental setup and protocols
3.3.2. Estimation of reaction rate and kinetic parameters
Chapter IV. PGM-doped reference Three-Way-Catalysts: Pd/CexZr1-xO2
4.1. STRUCTURAL PROPERTIES
4.1.1. XRD analysis
4.1.2. Raman spectroscopy
4.1.3. Scanning Electron Microscopy coupled with EDS
4.1.4. Reducibility
4.1.5. O2-Temperature-Programmed Desorption experiments
4.2. SURFACE PROPERTIES
4.2.1. Textural properties from N2-physisorption
4.2.2. Surface composition from XPS analysis
4.3. CATALYTIC ACTIVITIES OF BARE AND PD-DOPED CE0.5ZR0.5O2
4.3.1. Oxidative properties
4.3.2. Reductive properties
4.4. CONCLUSION
Chapter V. Stoichiometric and non-stoichiometric Ca and Cu doped La1-xFeO3±δ
5.1. STRUCTURAL PROPERTIES
5.1.1. XRD analysis
5.1.2. Mössbauer spectroscopy
5.1.3. SEM-EDS analysis
5.1.4. Reducibility
5.1.5. Oxygen mobility
5.2. SURFACE PROPERTIES
5.2.1. Nitrogen physisorption
5.2.2. XPS analysis
5.3. CATALYTIC MEASUREMENTS
5.3.1. Temperature-Programmed-Reaction
5.3.2. Kinetic analysis
5.3.2.1. STO1 experiments
5.3.2.2. STO2 experiments
5.3.3. Structure-reactivity relationship
5.4. CONCLUSION
Chapter VI. Stoichiometric and non-stoichiometric Ca and Mn doped La1-xFeO3±δ
6.1. STRUCTURAL PROPERTIES
6.1.1. XRD analysis
6.1.2. Mössbauer spectroscopy
6.1.3. SEM-EDS analysis
6.1.4. Reducibility
6.1.5. Oxygen mobility
6.2. SURFACE PROPERTIES
6.2.1. Nitrogen physisorption
6.2.2. XPS analysis
6.3. EVALUATION OF THE CATALYTIC PROPERTIES
6.3.1. Oxidative properties
6.3.2. Kinetics exploitation of TPR conversion curves for CO and propene oxidation
6.3.3. Reductive properties
6.3.4. Structure-reactivity relationship
6.4. CONCLUSION
Chapter VII. Palladium incorporation to stoichiometric and non-stoichiometric Ca and Cu doped La1-xFeO3±δ
7.1. REDUCIBILITY
7.2. OXYGEN MOBILITY
7.3. SURFACE ANALYSIS THROUGH XPS MEASUREMENTS
7.4. CATALYTIC PROPERTIES
7.4.1. Oxidative properties
7.4.2. Reductive properties
7.4.3. Kinetics of CO and propene oxidation
7.5. CONCLUSION
Chapter VIII. Rhodium incorporation to stoichiometric and non-stoichiometric Ca and Cu doped La1-xFeO3±δ
8.1. REDUCIBILITY
8.2. OXYGEN MOBILITY
8.3. SURFACE ANALYSIS THROUGH XPS MEASUREMENTS
8.4. CATALYTIC PROPERTIES
8.4.1. Oxidative properties
8.4.2. Reductive properties
8.4.3. Kinetics of CO and propene oxidation
8.5. CONCLUSION
Chapter IX. Rhodium incorporation to stoichiometric and non-stoichiometric Ca and Mn doped La1-xFeO3±δ Foreword
9.1. REDUCIBILITY
9.2. OXYGEN MOBILITY
9.3. SURFACE ANALYSIS
9.4. CATALYTIC PROPERTIES
9.4.1. Oxidative properties
9.4.2. Reductive properties
9.4.3. Kinetics of CO and propene oxidation
9.5. CONCLUSION
Reference
General conclusion and prospects
