Metal Oxide Materials for Automotive Catalysts

Abstract

Nanostructured metals and metal oxides are combined to produce advanced automobile catalysts for exhaust pollutant control. Catalytic emissions control was introduced in the form of noble metal-based three catalysts for the removal of exhaust gas pollutants of hydrocarbons (HC), carbon monoxide, and nitrogen oxides (NOx). Alumina as wash coat components provides a high and stable surface area for dispersion of the precious metals. Cerium oxides (ceria, CeO₂) and ceria-zirconia (CeO₂–ZrO₂) as oxygen storage capacity components are typical non-metallic functional materials in the automotive catalysts. The catalysts component layer is some hundreds of micrometers thick and loaded on the substrate, usually made from cordierite ceramic and metallic alloys, which is called coat layer with alumina-based and precious metal and ceria-based ceramic composite. This section deals with developed metal oxide materials controlled with nanometer scale, their structures, and some current advances including the author’s achievement.

1 Catalysts and Metal Oxide Nanomaterials

Automotive catalysts for the exhaust depollution were first applied to US and Japan vehicles manufacture industry in 1975 [1,2,3,4,5,6]. They are the principal emission control tools and typical model as an application of environmental materials to devices, proving their usefulness on environmental improvement. The environmentally functional materials have made catalytic devices which contain practical nanomaterials, and the catalytic methods for exhaust gas treatment have established now and most of the automobiles have equipped the catalysts for emission control. Thus, these environmental materials have both large industrial market and requirement of improvement for pollution control around citizen life. Figure 24.1 shows an example of a catalyst converter part which is usually both attached near engine exhaust manifold and underbody in a car.

Fig. 24.1

Reproduced from [4] with permission from Elsevier Science

Schematic catalytic monolith with carious composition-containing coat layer for exhaust treatment.

When a driver first starts the automobile both the engine and the catalyst are cold, and it reaches a temperature high enough to initiate the catalytic reactions after the exhaust gradually warms. This is referred to as the light-off performance of catalyst and it depends on the nanomaterials combination of catalyst and its chemistry since all the transport reactions must be fast and complete. The three-way catalysts (TWCs) in gasoline engine are the most widely used and effective system for the exhaust gas pollutants including hydrocarbons (HC), carbon monoxide, and nitrogen oxides (NOx). The chemically pollutant-purifying reaction rates are enhanced through pore diffusion and/or bulk mass transfer controlling the overall conversion in honeycomb substrate with capillary pores, followed by coating catalytic wash coat layer (Fig. 24.2). The TWC is the fundamental, however, leading technology, consisting of precious nanometals (Pt, Rh, and Pd) dispersed on an alumina support coated on cordierite monolith, oxygen storage catalyst such as ceria or ceria-zirconia, and other functional promoters. Such inexpensive materials have been examined through more advanced technology, and their effective application has resulted in both better properties and lower cost level for advanced automotive catalyst converters including gasoline, diesel and hybrid engine systems.

Fig. 24.2

Reproduced from [5] with permission from Elsevier Science

Examples of composite coat layers of Pd–Rh–CeO₂–ZrO₂–Al₂O₃ catalysts for TWC performance.

Regarding materials design in the practical use of catalysts, essential factors is the control in the state of catalysts from the aspect of nanometer-scaled composite of several catalytic compositions. Also, fabrication of catalysts was required with nanometer and micron scale, for example, bimetallic combination between precious metals and their controlled interaction between ceria and alumina as the supporting phase in uniform wash coat layer on the substrate. This chapter touches on the development and scientific and technological effort about catalytic materials, especially metal oxides nanoparticulate compounds such as alumina and ceria-zirconia.

2 Alumina Support and Its Modification

Nanoparticle alumina (Al₂O₃) as wash coat components on honeycombs provides a dispersion of the precious metals due to its high and stable surface area even at high temperatures [6]. Since the temperature in the catalyst can rise to over 1000 °C in engine combustion, the thermal stabilization of catalysts is important. In general, the surface area of starting alumina support with ca. 100 m² g⁻¹ decreases to below 30 m² g⁻¹ after heat treatment over 1100 °C, because phase transition in metastable gamma alumina induces large sintering with the formation of alpha alumina. The performance of surface area stabilization is strongly influenced by the purity and morphology of alumina, as well as additive elements modification and their optimization in processing. Lanthanum (La) is actually the most industrial modifier to alumina supports, which are prepared by several methods, in automotive catalysts. The additive of La species greatly improves the thermal stability to inhibit the sintering and phase transformations of alumina [7,8,9], and the method of adding La is important to the practical fabrication of catalysts. Besides the direct fabrication of alumina support in industrial large scale, both surface and bulk modification using some precipitation agent can be possibly applied to prepare catalytic alumina. The content of a modifier should be selected if the surface area versus concentration is optimized for heat condition used. The relationship of surface area versus modifier content after heat treatment at 1200 °C for comparison of several high surface area alumina powders shows that any alumina has the optimum content with relatively low concentration if they are used at 1000–1200 °C. The other factor is the interaction between nanoparticles in agglomeration, as well as particles and water, which is the same as in oxides, such as silica with hydroxyl group on the surface. The surface coverage and/or bulk doping of lanthanum oxide as a final form affect the surface area of alumina and other stabilizing factors. Figure 24.3 shows an example of surface area stabilization in developed alumina with La, where the best performance appears in limited dopant contents depending starting alumina purities.

Fig. 24.3

Rreproduced from [9] with permission from Elsevier Science

Relationship of surface area versus La content for several alumina catalytic support (powder) after heat treatment at 1200 °C.

The author has provided a new concept of the nanocomposite of complex oxide/alumina system [10]. Figure 24.4 shows an example of the TEM image, where a 10 nm-size LaAlO₃ (as a dark-contrast particle) has nucleated in alumina nanoparticulate aggregate matrix when overloaded La content exits. In the simple impregnation, the La and nitrate species both remains on alumina surface during the drying process, so that an attractive force by the bridging La species should be active, leading the hard agglomerates of alumina. Such starting state of agglomeration should induce the difference about sintering, phase transformation and solid-state reactions in heat treatment. Although LaAlO₃ is believed as one of the stabilizing form on alumina with mono-unit layer on the surface [7], it is generally difficult to detect such crystallites in practical materials. In industrial-stabilized alumina products, La species is combined with metastable alumina in nanoscaled dispersion state.

Fig. 24.4

Electron microscopic image of 5 mol% La-modified alumina support heated at 1200 ℃: secondary (upper) and reflection (lower) electron microscopy image in the same area

Concerning morphology stabilization of alumina coat layer in practical catalytic converters, an aspect of ceramics is important to make a thermal stable wash coat layer. The sintering of catalyst layers was related to neck growth, grain growth and phase evolution in alumina support on the substrate. The so-called “sintering” of ceramics, in this case, has finally resulted in the fracture of wash coat layers as well as grain growth in catalysts. A larger shrinkage of porous alumina than that of the honeycomb substrate in the automotive catalyst is induced by high-temperature exhaust, leading to a local stress to form cracks. Figure 24.5 shows the scanning electron microscopy (SEM) image of compared pure Al₂O₃ and La-stabilized Al₂O₃, which were coated on an Fe–Cr–Al foil (general metal substrate in TWC), followed by heating at 1100 °C for 3 h in air [11]. No critical fracture was found when alumina has been stabilized with La, although some fair cracks are observed. In practical fabrication processing, the rheology of alumina suspension must be controlled during the coating process of the porous, stable and homogeneous thick coat layer in the capillary pore of alloy or ceramic substrates. After then, the La-modified composite alumina will become stable support (as wash coat layer) on an automotive honeycomb bed even subjected to high heat environment. The strength of agglomerates in powders often prepared by a chemical route is controlled by the extent of particle–particle interaction.

Fig. 24.5

See Ref. [11] in details, reproduced from [11] with permission from Elsevier Science

Surface morphologies of alumina coat layer on FeCrAl alloy substrate, heated at 1100 °C for 3 h in air. a γ-Al₂O₃ and b La-stabilized γ-Al₂O₃.

3 Ceria-Zirconia for Oxygen Storage Capacity (OSC)

The TWCs has characteristic properties that the operation under a certain air/fuel ratio (A/F) of around 14.5 (stoichiometric point, λ = 1) results in highest performance about removal efficiencies [1,2,3,4,5]. The performance of TWC rapidly decreases as illustrated in Fig. 24.6 in both conditions out of λ = 1. In a series of reactions, CO, H₂, and HC are oxidized and NOx are reduced simultaneously into CO₂, H₂O, and N₂. At stoichiometric condition, the right balance of CO, H₂, and HC to reduce NOx and O₂ can be achieved. In general, A/F occasionally fluctuates in actual vehicle engine operation. For example, NOx emissions increase during acceleration in actual driving conditions, since the A/F fluctuates out of the stoichiometric point to lean (excess oxygen) condition.

Fig. 24.6

Hydrocarbons (HC), CO, and NOx conversions versus excess oxygen condition in TWC. Oxygen sensor output is also illustrated

Oxygen storage capacity (OSC) has been a key function in automotive catalysts for precisely controlling the variation of A/F in exhaust [1,2,3,4,5, 14]. The optimization of atmosphere in nanometer-sized space around catalyst particles leads better efficiencies to remove CO, HC, and NOx. Also, the oxidation performance may possibly be used for the design of the advanced catalyst for combustion, for example, cold starting condition. Now, ceria-zirconia system has been a standard composition of OSC in high-performance automotive catalyst including TWC and diesel exhaust systems. The composite (solid solution) subcatalyst is widely applied to practical TWCs that must have excellent catalytic OSC performance, high durability, and thermal stability. Such ceria-zirconia (CeO₂–ZrO₂) catalytic components for TWCs was invented by Ozawa et al. in 1987, and during the 1990s, the OSC material has been one of the main targets as improvement method of a series of automotive catalysts [12,13,14,15,16], and then this material has recently been applied to the research and development of various catalysis including other air pollution catalysts, hydrogen production, reforming, and so on. The fundamental dynamic performance of Pt catalyst was given by comparing the model examples of TWCs; Pt/Al₂O₃ and Pt/CeO₂–ZrO₂/Al₂O₃ [17]. The light-off TWC activity of catalysts was tested during temperature-arising condition by using the simulated mixture of gases (CO, NO, C₃H₆, CO₂, H₂, H₂O, O₂, N₂ balance) with the variation of O₂/CO ratio. These observations (Figs. 24.7 and 24.8) indicated that the activities of Pt catalyst under the condition corresponding to A/F modulation (CO–O₂) were enhanced by CeO₂–ZrO₂. The experiment of the model catalysts directly shows the effect of the mixed oxides themselves on the activities under A/F modulation. The behavior of oxygen evolution and/or uptake originates from the nonstoichiometry and oxygen diffusion at the surface and in lattice in Ce_1−xZr_xO₂. The OSC promoter should satisfy both factors; the wide-range operation for redox between Ce³⁺ and Ce⁴⁺ in the reducing and oxidizing atmosphere, and the essentially high reaction rate for oxygen evolution and storage over the modified catalysts. Catalytic reaction for OSC is essentially the phenomenon between a reactant such as gaseous hydrogen and CO and oxygen at a surface active site of CeO₂ based materials. The enhanced oxygen diffusion is an important factor to improve and find OSC promoters for TWCs.

Fig. 24.7

Reproduced from [16] with permission from Elsevier Science

Light-off three-way catalytic performance of 0.5 wt% Pt/Al₂O₃ (a), and 0.5 wt% Pt/Ce_0.8Zr_0.2O₂/Al₂O₃ (b) after aging test at 1000 C. Symbols: ■: CO, ●: C₃H₆, □: NO.

Fig. 24.8

Reproduced from [16] with permission from Elsevier Science

Rich-lean transient conversion behavior of CO and NOx concentrations at 600 °C over 0.5 wt% Pt/Al₂O₃ (······), 0.5 wt% Pt/CeO₂/Al₂O₃ (- - - -), and 0.5 wt% Pt/Ce_0.8Zr_0.2O₂/Al₂O₃ (–). Full scales are 400 ppm for NO and 800 ppm for CO.

There appears the cubic solid solution of Ce_1−xZr_xO₂ in Ce-rich region, while tetragonal and monoclinic solid solutions form in the tight regions of Zr-rich side. In the central region, there are several structures such as Ce₂Zr₂O₇ as well as another metastable mixed oxide [15]. However, the compounds or mixed oxides in CeO₂–ZrO₂ are often obtained as their metastable states, which are also useful for the catalytic application. Also, a strong requirement for OSC materials is their thermal stability and durability for hot exhaust at around or above 1000 °C. Usual CeO₂ powder easily sinters at elevated temperatures, although it is one of the good refractory oxides with high melting point. The addition of zirconium, especially the formation of CeO₂–ZrO₂ solid solution is effective to the inhibition of the sintering of ceria. The simple experiments indicated that the Zr modification of CeO₂ powder, followed by solid-state reactions, had the effects to the improvement of the thermal stability of CeO₂ promoter [13]. The chemical synthesis processes, as well as an impregnation are expected to lead to more excellent inhibition to the thermal deactivation (sintering) of CeO₂. The system of CeO₂–ZrO₂/Al₂O₃ (ACZ) composite powder brings a larger effect on surface area stabilization compared with ordinary CeO₂–ZrO₂ subcatalyst. Since the influence of stabilized nanoparticulate alumina is the same as the figure effect, the interaction between CeO₂, ZrO₂ and Al₂O₃ is found to has specially induced inhibition behavior on sintering (Fig. 24.9). There is often the trade-off relationship between an atomically structured crystal and practical nanoparticulate heterogeneous catalyst with high surface area.

Fig. 24.9

Transmission electron microscopy images of Al₂O₃/CeO₂–ZrO₂ (ACZ) OSC subcatalyst

As a matter of fact, the state of precious metals are very important to design overall three-way removal performance as well as OSC. Thus, the interaction of precious metals with CeO₂–ZrO₂ subcatalysts and Al₂O₃ support should be examined more with respects to SMSI (strong metal support interaction) in oxidizing and reducing atmosphere [17,18,19,20,21]. Recent technologies provide a strong method to examine the interaction by using X-ray and electrons, for example XAFS and environmental electron microscopy. Figure 24.10 shows a transmission electron microscopy (TEM) image observed in order to reveal the morphology and state of palladium species in Pd/CeO₂–ZrO₂ catalysts [19]. In general, the XRD peaks assigned as Pd or PdO were not identified due to the strong diffraction of support, and TEM image suggests an amorphous state of Pd or PdO with two or three atomic layers on CeO₂–ZrO₂. Thus, it is confirmed that the palladium species were highly dispersed on OSC support, and the strong interaction between palladium and CeO₂–ZrO₂ seems to inhibit the particle growth of palladium.

Fig. 24.10

Reproduced from [15] with permission from Elsevier Science

Transmission electron microscopy images of 1 wt% Pd/CeO₂–ZrO₂ catalyst. Allows indicate PdOx nanophase.

4 Summary

Catalytic nanomaterials are considered to be important inorganic such as catalytic aluminas, ceria, and zirconia or ceramic products. They are very much in demand, because they are widely used as components in environmental pollution control and automotive exhaust treatment. Concerning surface area and morphology stabilization of alumina coat layer, an aspect of nanocomposite is important to make a thermal stable and smooth wash coat. CeO₂–ZrO₂ and modified CeO₂ promoters like ACZ have been applied in practice so far, and there are extensive researches and development of oxides themselves as catalytic materials. Research using more advanced fabrication techniques should be enhanced toward nanoparticles which are well dispersed and with high surface area as the original single and/or mixed oxides. An advanced approach to such catalytic nanomaterials is believed to play a more important role toward future development of novel ecosystem. Of course, the detailed studies of catalytic reactions and mechanism of catalysis regarding with chemistry will be strongly required, and the author suggests that the materials research and development to find novel environmental catalyst is promising to research and industrial field for various depollution requirements.