Precious metal-support interaction in automotive exhaust catalysts

JOURNAL OF RARE EARTHS, Vol. 32, No. 2, Feb. 2014, P. 97

Precious metal-support interaction in automotive exhaust catalysts ZHENG Tingting (䚥။။)1, HE Junjun (ԩ֞֞)1, ZHAO Yunkun (䍉ѥᯚ)1,2,*, XIA Wenzheng (໣᭛ℷ)1, HE Jieli (ԩ⋕Б)1 (1. State-Local Joint Engineer Laboratory of Precious Metal Catalytic Technology and Application, Kunming Sino-platinum Metals Catalysts Co. Ltd., Kunming 650106, China; 2. State Key Laboratory of Advanced Technologies for Comprehensive Utilization of Platinum Metals, Kunming Institute of Precious Metals, Kunming 650106, China) Received 23 September 2013; revised 25 November 2013

Abstract: Precious metal-support interaction plays an important role in thermal stability and catalytic performance of the automotive exhaust catalysts. The support is not only a carrier for active compounds in catalysts but also can improve the dispersion of precious metals and suppress the sintering of precious metals at high temperature; meanwhile, noble metals can also enhance the redox performance and oxygen storage capacity of support. The mechanism of metal-support interactions mainly includes electronic interaction, formation of alloy and inward diffusion of metal into the support or covered by support. The form and degree of precious metal-support interaction depend on many factors, including the content of precious metal, the species of support and metal, and preparation methods. The research results about strong metal-support interaction (SMSI) gave a theory support for developing a kind of new catalyst with excellent performance. This paper reviewed the interaction phenomenon and mechanism of precious metals (Pt, Pd, Rh) and support such as Al2O3, CeO2, and CeO2-based oxides in automotive exhaust catalysts. The factors that affect SMSI and the catalysts developed by SMSI were also discussed. Keywords: strong metal-support interaction; automotive exhaust catalyst; precious metal; Al2O3; CeO2-based oxides; rare earths

Recently, three-way catalyst (TWC) was widely applied in catalytic removal technology due to its excellent conversion for CO, HC and NOx. The main components in TWCs are precious metals (PM) such as platinum (Pt), palladium (Pd), or rhodium (Rh) as the active component, and inorganic oxide such as -alumina (Al2O3), CeO2, and ceria-based composite oxides as the support. The support is not only a carrier for active compounds, but also can show catalytic activity and interact with active compositions, to influence the adsorption property and catalytic performance of catalysts. In 1987, Tauster et al.[1] found a surprisingly strong inhibition of CO and H2 chemisorption when the group VIII precious metals supported on TiO2 after reduction at 500 ºC in H2. The characterization results showed that the inhibition of CO and H2 chemisorption was not induced by the sintering of metals, encapsulation of metals and poisoning. Thus the strong metal-support interaction (SMSI) was proposed to explain this phenomenon. From then on, the SMSI was studied by many workers. Afterwards, the decreased chemisorption of CO and H2 on Pd/ CeO2[2] was also found after high temperature reduction. In this article, we discussed metal-support interactions between the precious metal (Pt, Pd, Rh) and the support such as Al2O3, CeO2, and CeO2-based composite oxides. The interactions appear to be different significantly in form and degree due to the kind of support and precious

metal. The mechanisms of metal-support interactions: electronic interaction, formation of alloy and inward diffusion of metal into the support or covered by support were discussed. In addition, we also reviewed the factors that affect SMSI and catalyst developed by strong metal-support interaction.

1 Role of SMSI in three-way catalyst Precious metal-support interaction plays an important role in catalytic performance of TWCs. The support can improve the dispersion of precious metals and suppress the sintering of noble metals at high temperature, and thus can enhance the catalytic performance and endurance ability. Farmer et al.[3] found when Ag nanoparticles supported on CeO2, smaller metal particles were maintained by strong metal-support bonding. The Pt–O–Ce bond acted as an anchor and inhibited the sintering of Pt particles on ceria-based oxide[4]. Under the simulated exhaust gas, the effects of SMSI between Pd and CeO2 in Pd/CeO2 on the catalytic activity were investigated by XRD and CO chemisorption. It was shown that high temperature reduction could generate the SMSI of Pd/CeO2 and the SMSI could sharply improve the catalytic activity for simulated exhaust gas, and the light off temperature of CO decreased from 520 to 350 ºC [2]. Noble metals can also enhance the redox performance

Foundation item: Project supported by National Science & Technology Pillar Program (2012BAE06B00) * Corresponding author: ZHAO Yunkun (E-mail: [email protected]; Tel.: +86-871-68393370) DOI: 10.1016/S1002-0721(14)60038-7

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and oxygen storage capacity of support[5–8]. For example, Pt, Pd, Rh promoted the reduction of surface oxygen of CeO2, especially Rh[9]. For Pt/Ce0.5Zr0.5O2, Pd/ Ce0.5Zr0.5O2 and Rh/Ce0.5Zr0.5O2, the surface area of Ce0.5Zr0.5O2 was strongly encumbered after treatments by Rh and the noble metal promoted the reduction of the support intensively compared with the metal-free Ce0.5Zr0.5O2[5]. After three cycles of Pt(1%)/Ce0.6Zr0.4O2 reduction[6], it was displayed that Pt favoured the structural reforming of ceria-zirconia into one cubic solid solution[8], and prevented CeAlO3 formation which is harmful to the catalytic performance. TEM investigated the redox cycle of Pt(1%)/Ce0.6Zr0.4O2 catalyst. It was shown that Pt particles (2–3 nm) strongly interacted with ceria, and it highly dispersed on ceria-zirconia grains with diameter between 10 and 35 nm. However, SMSI is not always good for the catalytic performance, too strong or too weak metal-support interaction was not favorable for the catalytic activity. Liotta et al.[8] reported that SMSI were detrimental for the catalytic activity when Pt(1%)/Ce0.6Zr0.4O2 was reduced at 1050 ºC. At this high temperature an inward diffusion of platinum into the ceria would induce the amount of exposed metal decrease noticeably. Kenevey et al.[10] found that the presence of noble metal accelerated the de-mixing process in Ce0.5Zr0.5O2 materials, but no effect of the metal was observed for the Ce0.68Zr0.32O2 system.

due to its large surface area and high thermal stability. Identifying metal-support interactions are essential because these interactions are known to affect catalytic structure and activity signicantly. To understand how metal particles bind and migrate on the surface is also important to develop accurate models to describe the processes such as metal growth as well as catalytic reactions involving both support and metal. In the earlier studies of Pt/-Al2O3 catalysts, it was found that after heating in H2 above 500 ºC, the hydrogen adsorption capacity on Pt decreased. The lost capacity can be recovered by calcination in O2 or air at 500 ºC and reduced at a lower temperature (300 ºC for example). It was supposed that the loss of adsorption capacity of Pt after reduction at 500 ºC was due to the reversible combination of Pt with Al to form a Pt-Al alloy which was inaccessible to H2. Kuczynski et al.[11] had studied the TPR of Pt/-Al2O3 catalysts and found no evidence of reduction in a -Al2O3 support even in the presence of Pt at temperature up to 750 ºC. In accordance with the results of Yao et al.[12], Ivanova et al.[13] found that two types of the Pt and Pd particles were typically present on the -Al2O3 surface, individual particles with dimensions of 1.5–3 nm and agglomerates about 100 nm in size. At low calcination temperature, Pt presented as metal cluster, such as Pt2, Pt3 and Pt4, some research also evidenced this result[14]. To better understand the interactions between Pt and -Al2O3 support, Deskins et al.[15] studied the adsorption and diffusion of a single Pt atom on -Al2O3 using density functional theory and found Pt binding to surface O atoms. The bonding is explained as being a combination of charge transfer between the surface and Pt atom, polarization of the metal atom, and some weak covalent bonding. At low temperature (<200 ºC), the Pt atom can be trapped at certain surface regions, which could explain why the sintering process is hindered at low temperature. But at higher temperature the Pt atom is much more mobile and can move across different regions of the surface. This trapping of the Pt at local regions on the surface would have a strong effect on the formation and clustering of larger metal particles. So according to recent reports, Pt generally existed as clustering of larger metal particles on -Al2O3 rather than be covered by -Al2O3. The interaction of Pd/-Al2O3 was different from Pt/ -Al2O3. According to the conclusion of Ivanova et al.[13], for Pd/Al2O3 catalyst, the palladium particles were almost completely decorated with a thin layer of an aluminate phase and led to the formation of a so-called “coreshell structure”, depending on the calcination temperature of catalyst in the range of 450–1000 ºC, the morphological form of active component was converted from the “core-shell” state to a state consisting of two phases, Pd0 and PdO. With a gradual decrease ratio of Pd0/PdO, the interaction of Pd with support became weak, after

2 Precious metal-support interaction The metal-support interaction was evidenced by many experiments. It depends on support and metal, different support and metal will lead to different interaction. The mainly metal-support interaction in automotive exhaust catalysts is shown in Fig. 1.

Fig. 1 Schematic diagram of metal-support interaction in automotive exhaust catalysts

2.1 Interaction of Pt, Pd, Rh with -Al2O3 -Al2O3 is applied in automotive catalysts as support

ZHENG Tingting et al., Precious metal-support interaction in automotive exhaust catalysts

catalyst calcinated at 1200 ºC, the palladium particles became much larger due to the loss interaction of the palladium with the support. The interaction of Rh/-Al2O3 depended on the prereduction procedure[16]. Under oxidation conditions, Rh is easily dispersed on -Al2O3 up to a saturation concentration of the support area (>10%). The interaction with the support is weak and both the dispersed and threedimensional phases are easily reduced at calcination temperature (<600 ºC in air. At a heat treatment temperature greater than 600 ºC, Rh oxides easily interact with -Al2O3, and diffuse into the subsurface region, then are covered[17]. This process can only be partially reversed by reduction in H2 (>550 ºC). Exposure of Rh/ Al2O3 catalysts to high temperature under oxidation conditions will cause loss of active area by both particle growth and Rh diffusion into the bulk of the support. Furthermore, Rh will react with -Al2O3 forming rhodium aluminate[18,19]. 2.2 Interaction of Pt, Pd, Rh with CeO2-based composite oxides The effect of CeO2-based composite oxides (CeO2, CeZr, CeZrM (M=rare earth) with precious metal can maintain the metal state through the reaction of Ce3+/Ce4+. Compared with -Al2O3, CeO2-based support can better restrain the sintering of precious metal and improve three-way catalytic performance[20]. For 1%Pt/CeO2/ Al2O3 and 1%Pt/Al2O3 [21], the former can be more easily reduced at low temperature, owing to the bond energy of Pt-O2 decreased by the interaction of Pt-CeO2. It was also found that heating in air at 700 ºC, Pt/Al2O3 is prone to very rapid sintering[22], but the presence of CeO2 as additives on -Al2O3 increases the interaction of noble metal oxide with support and restrains precious metal sintering. The enhanced metal oxide-support interaction is shown from the increased saturation of PtO2 in the dispersed phase and lower temperature reduction of CeO2 [23]. Nunan et al.[24] thought that CeO2 can be a far more effective support for Pt than -Al2O3, because direct interaction between Pt and CeO2 was shown to lead to large improvements in catalyst performance after activation in the synthetic exhaust gas. Compared with 0.5 wt.%Pt/ Ce20Al80 and 0.5 wt.%Pt/Ce50Al50, the ethane conversion was only 3% at 410 ºC of Pt/Al2O3 (900 ºC, 2 h), both the former had a conversion of ethane to methane at 410 ºC of 34% and 74%, respectively. Yasutaka et al.[25] investigated the behaviour of Pt/Al2O3 and Pt/CeZrY under oxidation condition at 800 ºC by TEM, XRD, and CO pulse adsorption. Pt supported on ceria-based oxide (CeZrY) did not sinter under oxidation condition at 800 ºC, whereas Pt atoms on Al2O3 sintered signicantly. Pt particles in the Pt/Al2O3 grew up to 23.6 nm (by the CO pulse method) after aging treatment. In contrast, Pt particles in the Pt/ CeZrY after aging continued to be highly dispersed with a

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diameter about 1 nm. As shown in Fig. 2, the sintering of Pt restrained on CeAl support compared to Al2O3, and after oxidation, the metal dispersed on the support again[26]. In the case of Pd, some author thought that compared with Al2O3, CeO2-based oxygen storage materials could improve the stability, oxygen storage capacity, catalytic activity and facilitate the transformation between Pd2+ and Pd0 due to the synergy between Pd and CeO2 [27–30]. Shen et al.[31] showed that Pd-(Ce,Zr)Ox interface promotes a higher degree of oxygen releasing than the Pd-(Al2O3) interface while maintaining the oxidation states of Pd. Colussi et al.[32] also found CeO2 could enhance the oxidation degree of Pd. Boronin et al.[33,34] showed that Pd-Ce interaction resulted in the formation of surface interaction phases as PdxCeO2 and small metal clusters (<1 nm), then reduced the light-off temperature of CO oxidation. Ranga and Fornasiero et al.[35,36] showed that ceria promotes NO and CO conversion through Pd-Ce interaction, oxygen vacancies on the Pd-Ce interaction sites favor NO dissociation and decrease its activation. On the contrary, Hu et al.[37] recently showed that ceria retards the reduction of Pd oxide through Pd-Ce interactions. They found that this effect was responsible for the low activity of Pd for NOx conversion while the inuence on CO conversion was negligible. Cordatos et al.[38] also found that Pd-Ce interactions inuence the adsorption of NO but have a negligible effect on CO adsorption. Those were in accordance with the conclusion of Shen et al.[31] that Pd-(Al2O3) is favored for NO and C3H8 conversion than Pd-(Ce,Zr)Ox. So there is no agreement with the effect of Pd-Ce interaction on catalytic performance. Maybe it relates to the component, the preparation[33] and the treatment atmosphere of catalysts[39]. Likewise, rhodium supported on pure alumina was found to sinter easily after heat treatment (>600 ºC). Rh diffused into the subsurface and bulk, and then was covered. At the high temperature, Rh would react with -Al2O3 to form an irreducible oxide phase[40], and the addition of Zr could retard this diffusion[41]. On the contrary, rhodium supported on Ce-Zr mixed oxide was observed to remain in a reduction state form after high temperature aging. It was also found that Rh supported on Ce0.82Zr0.18O2 was capable of restraining Rh particles sintering at elevated temperature, but for Rh/-Al2O3, the Rh dispersion decreased from 64% to 0.9%. So far, the mechanism of metal-support interaction mainly

Fig. 2 Image of changes of Pt state and nanostructure for Pt/CA and Pt/Al2O3 catalysts after oxidation, reduction and reoxidation treatments, CA: CeAl composite oxides

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include: (1) the formation of metal-support bonding, such as PM–O, PM–O–M (M=rare earth)[4,25,26,40,42–46]; (2) the formation of alloy[47–50]; (3) inward diffusion of metal into the support or be covered by support[17,40,51]. 2.2.1 Electronic interaction Murrell et al.[43,44] observed that Pt, Pd, Rh and Ir can form a surface phase precious metal oxide structure, M–O, which interacts strongly with the surface of bulk CeO2 by Laser Raman Spectroscopy in 1991. This strong oxide support interaction (SOSI) stabilized the structure of CeO2, and in turn maintained the high dispersion of precious metal. Yasutaka et al.[25] investigated the behaviour of Pt/ Al2O3 and Pt/CeZrY under oxidation condition at 800 ºC. According to the Fourier-transformed data of Pt L3-edge Extended X-ray Absorption Fine Structure (EXAFS) for supported Pt catalysts after 800 ºC aging in air (as shown in Fig. 3), for the aged Pt/Al2O3, only formed the Pt–Pt bond. But Pt atoms interacted strongly with the CeZrY support after aging and formed a Pt–O–Ce bond, the bonding energy was higher than

Fig. 3 Fourier-transformed (FT)-interatomic distance (R) curves of Pt L3-edge and Rh K-edge EXAFS for supported Pt and Rh catalysts after 800 ºC (Pt) or 1000 ºC (Rh) aging in air

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Pt–Pt, then suppressed the sintering of Pt. After the reduction treatment, the Pt–O–Ce bond broke and the Pt metal particles were highly dispersed on the CeZrY support[4,25,45]. Rh-based catalysts also have the same phenomenon[44], expecially with Nd2O3 [52]. As has been pointed out, the PM–O–M (where M is the cation in the support) bond was the key to the PM-oxide-support interaction, therefore, it is reasonable to suppose that the electron density of oxygen in the support oxide predominantly inuences the strength of Pt–O–M or Rh–O–M bond. As a result, the electron density of oxygen in the support would be expected to control the sintering of supported Pt particles. To verify this hypothesis, Yasutaka et al.[25] conducted a systematic investigation on various Pt-supported catalysts such as Pt/SiO2, Pt/TiO2. Fig. 4 shows the correlations among these factors on the various supported Pt catalysts. The oxidation state of Pt after aging increased with decreasing binding energy of the O1s electron. This indicates that the Pt-oxide-support interaction strengthened as the electron density of oxygen of support oxide increased. Subsequently, Pt particle size after aging decreased as the electron density of oxygen in the support increased. As a result, the precious metal oxides will be different on the different supports. The results of Miho[26] and Lin et al.[53] also evidenced this result. According to Miho’s results, for oxidized and re-oxidized Pt/CeAl catalysts, oxidized Pt+1.9eV was the dominant chemical species[26]. Oxidized Pt+1.9eV might form a Pt–O–Ce bond that inhibited Pt sintering. In contrast, Pt0 was the dominant species for all Pt/Al2O3 catalysts under heated oxidative conditions. The strength of PtOx-support and Pt-support interactions was investigated for PtOx/ SiO2, PtOx/Al2O3, and PtOx/CeO2 catalysts by Lin et al.[53]. The corresponding Raman spectra provided additional structural insights about the supported PtOx phases with crystalline -PtO2 and amorphous PtOx nano-particles on SiO2, amorphous PtOx nano-particles on Al2O3, and surface PtOx species on CeO2. As the Pt support interaction increases, the supported PtOx phase changed from crystalline PtO2 to amorphous PtO2, and then to surface PtOx species. The extent of the Pt support interaction also de-

Fig. 4 Pt-oxide-support interaction and its relation to Pt sintering in an oxidizing atmosphere

ZHENG Tingting et al., Precious metal-support interaction in automotive exhaust catalysts

termines the ease of reduction of the supported PtOx phase (SiO2~Al2O3>CeO2). Re-dispersion of the metallic supported Pt nano-particles to the supported PtOx phase also depends on the extent of the Pt support interaction (CeO2>Al2O3~SiO2). In addition, the precious metal state also depends on the treatment atmosphere[26,54]. Just as shown in Fig. 5, Pt0, oxidized Pt+1.0eV, and oxidized Pt+1.9eV are present in the reduced Pt/CeAl catalyst at abundance ratios of 42%, 41%, and 17%, respectively. The ratio of Pt0 decreased to zero when the Pt/CeAl catalyst was re-oxidized at above 400 ºC, the ratio of oxidized Pt+1.0eV gradually decreases with re-oxidizing temperature increasing. In contrast, the ratio of oxidized Pt+1.9eV monotonically increased with the treatment temperature. The formation of a Pt–O–Ce bond decreased the total energy of the whole Pt/CeAl catalyst system and stabilized Pt on the surface of CeAl support. According to above mentioned conclusion, it is not difficult to explain Fig. 6. Loading 0.1% Pt on Al2O3, ZrO2 and CeO2-based composite oxides, the last has the best catalytic performance[4]. 2.2.2 Encapsulation of metal and alloy formation It is generally accepted that minimization of surface energy is one of the main driving forces for encapsulation. As pointed out by Ertl et al.[55], oxides with a relative low surface energy (such as titania, ceria, and vanadia) prone to producing SMSI effect, while alumina and silica do not. Sun et al.[56] observed the interface of

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Pd/CeZrO (111) using HRTEM. A prominent morphological characterization of the reduced sample showed that ceria-zirconia support tends to cover the sublayer such as Pd particles. Occasionally, it was found that the Pd particles appeared to sink into the support after reduction, resulting in a curved interface. In this case, Pd (111) is no longer parallel to CeZrO (111), but tilted about 30º. The most interesting observation obtained from the cross-sectional TEM images displayed the strong tendency of ceria-zirconia migration and Pd was covered by support (as shown in Fig. 7). The increase in average cation radius will expand the lattice parameters and introduce a compressive stress on the top layer of the CeZrO surface. This stress could, in turn, provide the driving force for diffusion of oxide cations and anions along the CeZrO surface towards Pd particles, where eruption of the support material around the Pd particles, causing their partial encapsulation, would release the stress. While, it is worth mentioning that when yttria-stabilized zirconia (111) was used as support instead of CeZrO (111), it did not nd any tendency of Pd particle encapsulation, which conrmed that CeO2 is essential to SMSI in the Pd/CeZrO system. Bernal et al.[51] also observed that Rh was covered by CeO2 after reduction at 700 ºC in Rh/CeO2 catalyst using HRTEM. The metaloxide interaction thus probably depends on both properties of the oxide and the metal as well as their relative crystallographic orientations[57]. Compared with Pt, Pd and Rh were easily covered by support. Graham et al.[58] loaded the Pt, Pd, Rh on different supports which were

Fig. 5 Abundance of Pt in different oxidation states on a reduced Pt/CeAl catalyst before and after re-oxidation treatment at 400, 600, and 800 ºC

 Fig. 6 Effect of support kinds for catalytic activities at 0.1% Pt loading after 900 ºC ageing in a simulated exhaust gas

Fig. 7 TEM image of a Pd particle partially encapsulated by wetting material of the CeZrO support after the extended reduction treatment (a); HRTEM image of the Pd particle and its interface with the CeZrO substrate (b)

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made by different manufacturers and investigated the phenomenon of encapsulation. It is shown that as a consequence of the ceria-zirconia contraction, in the hightemperature partially reduced state, the encapsulation of Pd and Rh particles takes place, but not for Pt catalysts. This suggested that the stability of surface area, the nature of support, the composition of CeZr and the condition of aging would influence the extent of encapsulation. Generally speaking, the encapsulation of metal only happened under the high temperature reduction condition (>500 ºC) and the alloy formation happened at higher temperature (>900 ºC). Recently, many works reported the Pt-Ce alloy formation[47,48]. When Pt(4%)/CeO2 catalyst reduced at 900 ºC for 1 h, Bernal et al.[47] observed the CePt5 phase (Fig. 8). Pt particles deposited on CeO2 polycrystal thin lms can react with CeO2 and form PtCe and Pt3Ce compounds with a Cu3Au lattice structure[48]. Hardacre et al.[49] studied the oxidation of CO using the catalyst of Pt-Ce alloy (such as PtCe2, Pt3Ce7). The result displayed that Pt-Ce alloy treated with O2, N2O, or CO/H2 yield Pt/ceria. The most active species was Pt3Ce7 produced by N2O treatment at 447 ºC which is higher than the Pt/CeO2 made by chemical methods. But the interaction between Pd, Rh and CeO2 is different from Pt. So far, there were little reports about the Pd-Ce and Rh-Ce alloy formation[50]. However, when a single step solution combustion method was used to prepare Rh/ CeO2 and Pd/CeO2 catalysts, Gayen et al.[59] and Priolkar et al.[60] found the formation of Ce1–xRhxO2 configuration on the surface, and furthermore enhanced the catalytic activity.

3 Influencing factors of SMSI The form and degree of precious metal-support interaction depend on many factors, including the content of precious metal[13,43,61,62], the species of support[62], preparation methods[40,63] and so on. Albert Vannice et al.[64] found that the type of support influence the methanation behaviour of CO and CO2 by Pt catalyst, the activity of Pt/TiO2 was higher than that of Pt/Al2O3 and Pt/SiO2.

Fig. 8 HRTEM study of Pt(4%)/CeO2 catalyst with H2 treatment at 900 ºC for 1 h

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Papadakis et al.[65] studied several catalysts (Pt, Pd, Rh, loaded on different supports (TiO2, ZrO2, Al2O3, SiO2), and demonstrated that the effect of support on the performance of metal catalysts were very significant. According to Kohei’s result[66], the conversion of NO catalyzed by Pt, Pd supported catalysts was influenced by support. Pd/CeO2 and Pt/CeO2 showed the highest conversion of NO, followed by ZrO2, SiO2, Al2O3. Moreover, the stability and activity of Rh/ZrO2 was higher than Rh/Al2O3 and Rh/CeO2 for removing NO, furthermore, the tetragonal phase of ZrO2 for the support of Rh is more strong enough to withstand the aging than monoclinic phase[67,68]. Because the scarcity of natural resources and the increasing price of precious metals, it is a hot topic about improving the durability of TWCs and applying new insights to develop advanced TWCs with lower precious metal content, so the choice of support is very important. According to the conclusion of Yao et al.[12], the behavior of Pt oxide supported on -Al2O3 was found to depend on Pt loading, as well as reaction temperature and chemical atmosphere. Selective chemisorption of H2 and CO on Pt/-Al2O3 evidenced that Pt oxide concentration in the dispersed phase was found to increase proportionally with Pt loading until a saturation concentration of 2.2 mol/m2. Beyond this concentration, the excess Pt oxide aggregated to form a particulate phase. Under the suitable oxidation condition, the formation of Pt–O–Ce bond on the surface of Pt/Ce-based catalysts effectively retarded the sintering of Pt particles and made the aggregation particles disperse on the support again. After aging, the Pt particle size can recover to the level of fresh samples if loading the suitable Pt contents. Nagai et al.[69] also proved that a lower Pt loading brought higher stability of Pt against sintering using a time resolved X-ray absorption spectroscopy (XAS) technique, but this depends on the treatment temperature. For example, Pt supported on CeO2 shows a strong dependency of surface area stability on Pt content when calcined at 750 ºC for 2 h, however, at high temperature, the stability of surface area independent on the Pt content[44]. Fig. 9 shows the effect of Pt loading amount for sintering suppression, for the Pt/CeO2 (55%)-ZrO2 aged at 950 ºCfor 5 h in a simulated exhaust gas, we can conclude that Pt did not sinter when the Pt loading amount was less than 0.25%; while more than 0.25%, Pt sintered easily[61]. The degree of Pt-Ce interaction and catalytic activity could be controlled by controlling the CeO2 crystallite size. Decreasing the CeO2 crystallite size led to greater Pt/Ce interaction and higher activity. Different preparation methods can lead to different degrees of metal-support interaction. when prepared Pt/Ce-Zr by flame synthesis, the catalyst did not lose its low-temperature oxygen exchange capacity after exposure to high temperatures (1100 ºC). For flame-made Pd/Al2O3, Pd exsited as

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4 Catalyst development

Fig. 9 Effect of Pt loading amount for sintering suppression, Pt loaded on ceria based oxide was aged at 950 °C for 5 h in a simulated exhaust gas

PdO, which was very stable and hardly reduced into metallic Pd compared to Pd/Al2O3 prepared by other methods[63]. The encapsulation of Rh/CeZr and Rh/Ce-Zr-Al particles by -Al2O3 is related to the preparation method which can only be observed for the samples by pre-impregnation, but not by chemical impregnation and physical mixing method. Just as the support, precious metals have different interaction degree with support because of different nature. For instance, Rh can react with -Al2O3 easily, to be covered by -Al2O3 or form rhodium aluminate. Pd has the similar phenomenon with Rh, but for Pt there is no report so far. The afnity order among four kinds of major noble metals (Pt, Pd, Rh, and Ru) and CeO2 was investigated by XRD and XAFS techniques by Hosokawa et al.[62]. Rh and Pd retained their Rh–O–Ce and Pd–O–Ce bonds even after calcination at 800 ºC, and Pt kept the Pt–O–Ce bond below 500 ºC, while Ru/CeO2 formed bulk RuO2 after calcination at temperature as low as 500 ºC. The order of the afnity of noble metals with CeO2 was assumed to be Rh>Pd>Pt>Ru. The calcination atmosphere also influences the catalytic performance, such as Pt/-Al2O3 loses the activity of conversion of HC/CO/NO after aging in H2. For Rh/Al2O3 and Pt-Rh/-Al2O3, they have high activity for three-way conversion of HC/CO/NO. Under reduction condition, for Rh/-Al2O3, Rh was reduced to Rh0, Pt and Rh may react with each other to form Pt-Rh alloy in Pt-Rh/-Al2O3. But under oxidation condition, Rh deactivates readily by forming inert Rh-aluminate species at high temperature with -Al2O3, while for Pt/-Al2O3, Pt agglomerates to large Pt particles and loses its activity after high temperature exposure[70]. For Pd/CeZr catalysts, the presence of oxygen inhibits the sintering of palladium during air and cyclic ageing, but the inhibition effect does not work over the Pd/CeZr catalysts aged in reducing and inert atmospheres without the involvement of oxygen[71].

Since the strong metal support interaction plays a key role in the three way catalysts, one of the keys to develop advanced TWCs is to maintain the advantage of noble metal-support interaction even after high temperature aging. Many research papers reported that introducing the foreign ion into CeO2 can enhance the stability and oxygen storage capacity (OSC). So by choosing a suitable support and optimizing the assembly mode of support and metal, we can control the noble metal-suppurt interaction and develop a high performance catalyst. For example, Rh deposited on carefully prepared CeO2ZrO2[72] or La-ZrO2 with adequate Nd2O3 surface-enrichment amount[52,73] is expected to provide stable Rh-support interaction, which is responsible for good thermal stability and high TWC performance. Rohart et al.[74] developed a new generation of high thermo-stable hybrid zirconia, in a wide composition range, from ceria-rich to zirconia-rich and doped zirconia without ceria, which shows thermal stability at temperatures higher than 1100 ºC, these materials keep precious metal available even after very severe aging. New ceria rich oxides show improved light-off activity for Pt model catalysts. For Pd model catalysts, zirconia rich oxide with 20% ceria shows improved light-off activity. For Rh model catalysts, zirconia-rich oxide with 40% ceria shows the best trade-off between light-off and conversion at cross over point (COP) and the highest conversion at COP is observed for materials containing 60% and 40% ceria. Rh/ZrO2 shows outstanding NO and C3H6 light-off. On the basis of these results, full size vehicle tests and synthetic gas bench tests on fully formulated Pd/Rh and Pt/Rh technologies confirm the trends observed on model powder catalysts. It is known that ceria based oxide is suitable for Pt to achieve a good balance between catalytic activity and the sintering suppression. On the other hand, for Rh catalysts, zirconia based oxide is veried to meet the requirements for the similar mechanism. Rh is more stable in oxide state, and that makes stronger interaction with support oxides compared to Pt under oxidation condition. Therefore, zirconia based oxide with lower electron density than ceria based oxide is suitable for Rh[4,75]. Yoshida et al.[76] compared the catalytic performance of Pt/Rh/CeO2, Pt/CeO2+Rh/CeO2 and Pt/CeO2+Rh/ZrO2 aged at 1000 ºC in a simulated exhaust gas. The results showed that Pt/CeO2+Rh/ZrO2 has the lowest light-off temperature of HC, as shown in Fig. 10. A new TWC with a noble metal sintering suppression technology based on the support anchoring effect was designed[4], that is, Pt and Rh were separately and respectively loaded on ceria-based and zirconia-based oxides. Fig. 11 shows their catalytic activities upon engine test after ageing. Compared to the conventional catalyst, this

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plays a very important role in catalyst, a system with low Rh loading can readily be improved by increasing the Rh loading and there is a relatively large effect of doing this by a small amount. So in future, not only the choice of carrier, the configuration of noble metal and support, zone-coating, double-layers coating and the ratio of noble metals will play a significant role in design an advance catalyst.

5 Conclusions

Fig. 10 Effect of conguration with 0.2%Pt and 0.17%Rh in catalysts for catalytic activities after ageing test at 1000 ºC in a simulated exhaust gas

Now, one of the biggest issues for three-way catalysts is the degradation of catalytic activity caused by sintering of noble metals. The research results about SMSI gave a theory support for developing a kind of new catalyst with excellent performance. Choosing a suitable support (alumina or Ce-based oxide), finding an optimum assembly mode of support and metal, and controlling other influence factors such as precious metal loading content, preparation methods, a new catalyst which has good performance will be designed. In addition, along with the development of advanced characterization analysis technology, the study about SMSI would play a notable role in the development of catalysts.

References:

Fig. 11 Performance of the developed catalyst applying the Ptsupport-interaction concept in engine bench test after the engine ageing

developed catalyst exhibits a very high catalytic activity although it contains less precious metal than the conventional one. As a matter of fact, this catalyst has been used practically adopted in gasoline fueled engine automobiles from 2005[4]. With the vehicle emissions regulations become more and more stringent and restricted by nature resources, a three-way catalyst with excellent activity and low cost is the trend of future development. In response, the development of emissions reducing technologies has become an urgent task, so except the above mentioned anchoring effect, other ways must be considered. Zone coating of the catalyst layer that provides the optimal precious metals and materials on its front and rear portions can promote the HC and NOx conversion and OSC. Moreover, shifting the Rh catalyst layer to the rear portion and increasing the atmospheric moderation capability of the front portion can suppress the degradation of Rh, thus reducing the Rh content of the catalyst[73]. Optimizing OSC materials and palladium distribution in the surface and bottom washcoat layers can also reduce the total PGM content[75]. According to Cooper’s[77] result, Rh

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