汽车尾气净化催化剂用Ce_xZr_（1-x）O_2固溶体的研究进展

综述了CeO2和CexZr1-xZr1-xO2固溶体在汽车尾气净化催化剂重要作用、铈锆氧化物的体相结构及影响铈锆氧化物固溶体储养能力（OSC）和织构热稳定性的因素,并对其在催化剂中的应用作了简要陈述。     

Towards the comprehension of oxygen storage processes on model three-way catalysts

CeO₂- and Ce_0.63Zr_0.37O₂-supported noble metal catalysts were studied. Samples were fully characterized using TEM, XRD, N₂ adsorption and H₂ chemisorption. The oxygen storage process was investigated focusing on the evolution as a function of temperature of both the oxygen storage capacity (OSC) and the oxygen storage complete capacity (OSCC). Aging effect on OSC was also examined in details in the case of Rh catalysts. Finally, the major role of oxygen diffusion, partly influenced by the metal/support interface quality, was confirmed.     

Effect of specific surface area on oxygen storage capacity (OSC) and methane steam reforming reactivity of CeO<Subscript>2</Subscript>

It was found from the work that the specific surface area of ceria presents an important role on the oxygen storage capacity (OSC), the reactivity toward methane steam reforming, and the resistance toward carbon formation of this material. After calcination at 900°C, ceria prepared by surfactant-assisted method (SF) was observed from the present work to have significantly higher surface area than those prepared by templating (TP) and precipitation (PP) methods; this material showed strong OSC with good reforming reactivity in terms of thermal stability and resistance toward carbon formation compared to others. In detail, the degree of OSC was measured by the number of hydrogen uptake from the temperature programmed reduction (TPR). It was found that the value of hydrogen uptake from the TPR-1 of ceria prepared by SF was 2084 mmol g⁻¹, whereas those of ceria prepared by TP and PP were 1724 and 781 mmol g⁻¹, respectively. In addition, it was also proven in the present work that the OSC of these materials are reversible, according to the temperature programmed oxidation (TPO) and the second time temperature programmed reduction (TPR-2) results. According to the reactivity toward methane steam reforming, after purging in 3 kPa methane and 9 kPa steam at 900°C for 8 h, the methane conversion at steady state of ceria prepared by SF was approximately 38% with very low amount of carbon formed on the surface (0.16 mmol g⁻¹), whereas those of ceria prepared by TP and PP were 22% (with the amount of carbon formation of 0.30 mmol g⁻¹) and 13% (with the amount of carbon formation of 0.33 mmol g⁻¹), respectively.     

Dynamics of oxygen storage and release on commercial aged Pd-Rh three-way catalysts and their characterization by transient experiments

The present work has provided new fundamental information on the effects of aging (mileage, km) of a commercial Pd-Rh (9:1, w/w) three-way catalyst (TWC) on (a) the H₂ chemisorption, (b) the redox properties of washcoat material, and (c) the dynamic oxygen storage and release properties of TWC. Hydrogen chemisorption in the 25–400 °C range was found to decrease with increasing aging of TWC. On the other hand, H₂ chemisorption performed at 200 and 400 °C resulted in a significantly larger amount of chemisorbed hydrogen (exceeding the monolayer value based on the noble metals) compared to that obtained after chemisorption at 25 °C. This is due to the onset of a hydrogen spill over process in the 200–400 °C range. The features of H₂-TPD spectra were found to strongly depend on the aging of TWC. Hydrogen chemisorption at 25 °C followed by TPD after a given pre-treatment of the catalyst surface might be considered as a procedure for a good estimate of the metal dispersion of a commercial Pd-Rh TWC. The redox properties of the oxygen storage components of the washcoat of a commercial Pd-Rh TWC were found to drastically change with increasing aging of TWC. It was found that the oxygen storage capacity (OSC) of the TWC investigated decreases significantly with catalyst aging in the 0–56,000 km range and in the temperature range of 500–750 °C. In the case of fresh TWC, the presence of 20 ppm SO₂ or 10% CO₂ in a 1.5% O₂/He gas mixture used for oxygen storage (oxidizing gas) was found to result in a large decrease in the amount of dynamic OSC measured by alternating switches between oxidizing and reducing feed gas compositions. The shift in time of the peak maximum of the transient response of CO₂ obtained during the switch O₂/He → He → CO/He (t) can be ascribed to the alteration of the kinetics of the oxygen back-spillover process and not to the kinetics of CO oxidation on the metal surface.     

Investigation on properties of a novel ceria-zirconia-praseodymia solid solution and its application in Pd-only three-way catalyst for gasoline engine emission control

Different contents of praseodymia were introduced to modify Ce_0.2Zr_0.8O₂ (CZ) and the supported Pd-only three-way catalysts before and after aging were also prepared. The influence of praseodymia doping on the structural/textural properties of CZ and the effect on the three-way catalytic activity were investigated. Structural and textural characterizations reveal that the addition of praseodymia results in the formation of Ce-Zr-Pr ternary solid solution (CZP) with higher specific surface area, better thermal stability and larger oxygen storage capacity (OSC) than that of CZ. It is more credible that Zr is replaced by Pr during the formation of CZP. The modified Pd-only three-way catalysts present relatively higher catalytic activity to the main target pollutants in gasoline engine exhaust and exhibit wider air/fuel operation window due to the improved properties of CZP.     

Transient analysis of oxygen storage capacity of Pt/CeO2–ZrO2 materials with millisecond- and second-time resolution

A simple method for determining the speed of oxygen release from Pt/CeO₂–ZrO₂ materials is proposed. We determined the speed of oxygen release from three differently prepared Pt(1 wt%)/CeO–ZrO₂ catalysts by two methods: the method employing temporal analysis of products (TAP) reactor with CO, O₂, and CO₂ pulsed gases, and a conventional method with CO and O₂ switch gases. Strong CO₂ adsorption precluded correct analysis of oxygen release speed, when it was determined from the amount of CO₂ formed. Therefore, instead of the amount of CO₂ formed, we suggested the t_max value, defined as the position of maximal intensity of CO₂ transient responses obtained upon low-intensity (10¹⁵–10¹⁶ molecules) CO pulsing in the TAP reactor. Shorter the t_max value, higher the rate of oxygen release, because the temperature dependence of the t_max determined in the present study resembles the previous results of oxygen release analysis [T. Tanabe, A. Suda, C. Descorme, D. Duprez, H. Shinjoh, M. Sugiura, Stud. Surf. Sci. Catal. 138 (2001) 135].     

Aluminum-doped ceria-zirconia solid solutions with enhanced thermal stability and high oxygen storage capacity

A facile solvothermal method to synthesize aluminum-doped ceria-zirconia (Ce_0.5Zr_0.5-xAl_xO_2-x/2, x = 0.1 to 0.4) solid solutions was carried out using Ce(NH₄)₂(NO₃)₆, Zr(NO₃)₃·2H₂O Al(NO₃)₃·9H₂O, and NH₄OH as the starting materials at 200°C for 24 h. The obtained solid solutions from the solvothermal reaction were calcined at 1,000°C for 20 h in air atmosphere to evaluate the thermal stability. The synthesized Ce_0.5Zr_0.3Al_0.2O_1.9 particle was characterized for the oxygen storage capacity (OSC) in automotive catalysis. For the characterization, X-ray diffraction, transmission electron microscopy, and the Brunauer-Emmet-Teller (BET) technique were employed. The OSC values of all samples were measured at 600°C using thermogravimetric-differential thermal analysis. Ce_0.5Zr_0.3Al_0.2O_1.9 solid solutions calcined at 1,000°C for 20 h with a BET surface area of 18 m² g⁻¹ exhibited a considerably high OSC of 427 μmol-O g⁻¹ and good OSC performance stability. The same synthesis route was employed for the preparation of the CeO₂ and Ce_0.5Zr_0.5O₂. The incorporation of aluminum ion in the lattice of ceria-based catalyst greatly enhanced the thermal stability and OSC.     

Investigation of the oxygen storage process on ceria- and ceria–zirconia-supported catalysts

A total of 10 noble metal (Rh, Pt, Pd, Ru and Ir) catalysts, either supported on CeO₂ or Ce_0.63Zr_0.37O₂, were prepared. Catalysts were fully characterized using XRD, N₂ adsorption at −196 °C, TEM and H₂ chemisorption. Oxygen storage processes were carefully investigated. The influence of temperature was checked and a key role of oxygen diffusion was further demonstrated. A review of the reactions involved in the CO transient oxidation reaction is finally proposed.     

The use of temperature-programmed and dynamic/transient methods in catalysis: characterization of ceria-based, model three-way catalysts

Temperature-programmed (TP) methods have been increasingly used in recent years for the characterization of catalytic materials under conditions similar to those encountered in commercial applications. A large variety of complementary TP techniques can be used with minimum variation of experimental conditions, thus allowing great characterization potential in a single apparatus. Modern analytical and numerical tools allow accurate analysis and modeling of TP profiles, to obtain kinetic and other reaction parameters. Here, we will briefly review the experimental TP methods used for the characterization of the reduction features and the dynamic behavior of oxygen-storage/redox components of auto exhaust catalysts, based mainly on CeO₂ and ceria-zirconia.     

Effect of SO2 on the oxygen storage capacity of ceria-based catalysts

We have examined the effect of SO₂ poisoning on a series of catalysts having Pd supported on ceria, alumina, and ceria–zirconia. For pre-exposure of 20 ppm SO₂ at 673 K, we observed no changes in the light-off curves for CO oxidation on Pd/alumina. This pre-exposure of SO₂ to Pd/ceria resulted in a significant upward shift in the light-off curve, so that the poisoned Pd/ceria catalyst exhibited similar rates to that of Pd/alumina. Similar upward shifts were observed for the water–gas-shift reaction upon exposure of Pd/ceria or Pd/ceria–zirconia samples to SO₂. However, pulse-reactor data with alternating CO and O₂ pulses showed that SO₂ poisoning actually increased the amount of oxygen that could be transferred to and from the catalyst over the entire temperature range that was examined. The implication of these results for understanding the effect of SO₂ poisoning and the measurement of OSC are discussed.     

Cerium–terbium mixed oxides as alternative components for three-way catalysts: a comparative study of Pt/CeTbOx and Pt/CeO2 model systems

By using a combination of oxygen buffering capacity (OBC) and oxygen storage capacity (OSC) measurements, the redox behaviour of a Pt/CeTbO_x catalyst is compared to that of a classic model TWC system: Pt/CeO₂. The results reported here show that the redox efficiency of the Pt/CeTbO_x catalyst is much better, especially at low temperature operation conditions such as those occurring during the cold start of engines. The catalyst containing terbium also shows lower ‘light-off’ temperatures for both methane and carbon monoxide oxidation.     

Insights into the dynamics of oxygen storage/release phenomena in model ceria–zirconia catalysts as inferred from transient studies using H2, CO and soot as reductants

Samples of ceria–zirconia pre-treated under various conditions have been used as catalysts in CO and soot oxidation under stationary and transient conditions, in the presence and in the absence of oxygen. Their behaviour has been compared with that observed under redox conditions in the presence of hydrogen (oxygen storage activity). All the catalysts are active in CO and soot oxidation. Under stationary conditions, the activity in CO oxidation depends on the amount of Ce present, with little contribution from the redox capacity of the support and is strongly influenced by surface area. When the reaction is carried out under transient conditions, especially with low-surface area samples, the performances of ceria–zirconia are higher than those of ceria, with a maximum in the middle composition range. Interestingly, a similar behaviour is observed in soot combustion, where the activity for low-surface area sample is dependent on composition. This suggests that oxygen from the support plays a key role also in the oxidation of large carbon particles under a fully oxidizing mixture.     

Preparation of Pt-Co catalysts on mesoporous carbon and effect of alloying on catalytic activity in oxygen electro-reduction

Mesoporous carbon (MC)-supported PtCo catalysts were prepared by a sodium borohydride (NaBH₄) reduction method. To increase the alloy degree of PtCo catalyst, the heat treatment was carried out at various temperatures (300–700°C). The heat-treated PtCo catalysts (PtCo/MC-x) had the higher degrees of Pt-Co alloy than that of as-synthesized PtCo catalyst (PtCo/MC). MC supported-PtCo catalyst (PtCo/MC-500) that was treated at 500°C, had the highest activity and lowest overpotential in oxygen electro-reduction (ORR) among the prepared PtCo catalysts. The high alloy degree and favorable chemical states of PtCo/MC-500 are believed to be responsible for the superior activity in oxygen electroreduction compared to the other PtCo catalysts. This paper was presented at the 11th Korea-Japan Symposium on Catatysis held at Seoul, Korea, May 21–24, 2007.     

Effect of oxygen deficiency reduction in Mg-doped Mn-spinel on its cell storage performance at high temperature

The effect of doping foreign metal ions into the LiMn₂O₄ spinel structure on the oxygen deficiency by high-temperature firing has been investigated. The degree of the oxygen deficiency in the Mg- and/or Al-doped spinel is very small even when fired at 1000 °C, and then the oxygen deficiency nearly disappeared after low temperature annealing. Mg-doped Mn-spinels (Li_1.05Mn_1.94−xMg_xCa_0.01O_4−z, 0 ≤ x ≤ 0.15) obtained at 1000 °C contained a small oxygen deficiency amount when the x-value reached 0.03 or more. The annealing at 800 °C then reduced the oxygen defect content in the Mg-doped spinels. The crystallinity of the spinels is improved by doping with a small amount of Ti. The storage performance of the coin-type cell at 60 °C is improved about 1.6 times using both Mg- and Ti-doping, which would be due to the oxygen stoichiometric spinels with a low solubility by firing at high temperatures and/or by Ti-doping to increase the crystallinity.     

Axial characterization of oxygen storage capacity in close-coupled lightoff and underfloor catalytic converters and impact of sulfur

The oxygen storage capacity of a 56,000 mile aged warmup and underfloor converter system was characterized as a function of axial location along the converters and compared with fresh samples having the same formulation. Measurements of oxygen storage were made using a titration technique and at conditions expected to be commonly encountered during OBD-II diagnosis of catalyst performance. Vehicle aging resulted in a dramatic loss of oxygen storage in the warmup converter presumably due to the severe thermal sintering, but the significant amount of phosphorus (P) and zinc (Zn) poison accumulation on this converter was found to impact oxygen storage minimally. This is in contrast to the measured impact of P and Zn deposition on warmed-up hydrocarbon conversion, which was found to be significant relative to the impact of thermal sintering. The underfloor converter was found to have retained nearly all of its original oxygen storage after vehicle aging, consistent with operation of this converter at moderate temperatures which do no result in severe thermal sintering of the noble metals and the ceria.

The impact of sulfur on the oxygen storage of both warmup and underfloor converter sections was dramatic. Sections in the forward part of the warmup converter and in the front brick of the underfloor converter had relatively modest oxygen storage capacity which was almost completely blocked as the sulfur concentration reached 75–150 ppm (equivalent in gasoline). Other sections such as the rear of the warmup converter and the rear monolith of the underfloor converter had more oxygen storage capacity, which was significantly decreased as the sulfur concentration reached 150 ppm equivalent in fuel, and was approached complete loss near 500 ppm sulfur equivalent in fuel.     

Oxygen partial pressure-dependent behavior of various catalysts for the total oxidation of VOCs using cycled system of adsorption and oxygen plasma

In this work, comprehensive investigation was done on the oxygen partial pressure-dependent behavior of the various catalysts using a flow-type plasma-driven catalyst (PDC) reactor. These data provide a useful guideline for the optimization of the cycled system using adsorption and the O₂ plasma-driven catalysis of adsorbed volatile organic compounds (VOCs). The potentials of the tested catalysts for the cycled system were evaluated based on the enhancement factor and the adsorption capability. All the tested materials (TiO₂, γ-Al₂O₃, zeolites) exhibited positive enhancement factor, while negative values with the dielectric-barrier discharge (DBD) plasma alone. TiO₂ catalysts showed the highest enhancement factor of about 100 regardless of the type of metal catalysts and their supporting amount. Based on the experimental findings in this study and the literature information, a plausible mechanism of plasma-driven catalysis of VOCs was suggested.     

Preparation and oxygen reduction activity of stable RuSex/C catalyst with pyrite structure

Carbon supported RuSe_x (x = 0.35-2) catalysts of controlled stoichiometry and phase are synthesized via precipitating ruthenium nanoparticles on Vulcan XC-72R, then selenizing ruthenium with hydrogen annealing. The competition for Se between solid-state Ru selenization and volatile selenium hydride formation results in RuSe_x nanoparticles with the pyrite structure, the Ru hcp structure, or a mixture of the two. These catalysts are methanol tolerant, and catalytically active toward oxygen reduction reaction (ORR). RuSe_x/C (x ≅ 2) with a pyrite structure, produced at 400 °C, exhibits catalytic activity and stability superior to that of RuSe_x/C with a ruthenium hcp structure or a mixed phase. And this pyrite RuSe_x/C (x ≅ 2) catalyst yields H₂O₂ less than 1.5% in the technically pertinent potential range of 0.4-0.6 V (vs. Ag/AgCl). Its stability is verified in 100 CV cycles, which shows that the catalyst annealed at 400 °C is more enduring in potential cycling over 0.65 V (vs. Ag/AgCl), compared with the RuSe_x/C catalyst annealed at 300 °C and the RuSe_cluster/C reference catalyst prepared by thermolysis of Ru₃(CO)₁₂. After CV cycles, the 400 °C-annealed catalyst still exhibits a higher ORR activity than the other two catalysts.     

An XPS study of metallic three-way catalysts: The effect of additives on platinum, rhodium, and cerium

The effect of Zr, Si, La, and Ba on the thermal aging of Pt, Rh, and Ce in automotive exhaust gas catalysts supported on metal foils was studied with X-ray photoelectron spectroscopy (XPS). Platinum was in metallic state under all circumstances studied in this work. Rhodium was partially (about 30%) oxidized already at room temperature. The decreases in the binding energies of the reduced noble metals due to aging was explained in terms of particle growth. The binding energy of rhodium oxide increased during aging indicating rhodium diffusion into alumina. Cerium was oxidized during aging. It was found that in the lanthanum containing catalysts the binding energies of platinum and rhodium were below the values for bulk metals, and the degree of oxidation of cerium was lower than in the other samples. It was concluded that the presence of lanthanum facilitated the reduction of the noble metals and cerium.     

Nucleation of isolated PO4 units on CeO2 driven by high temperatures and the effect on its oxygen storage and release properties

The chemical nature of P-containing species of varying concentration present in CeO₂ after impregnation with (NH₄)₂HPO₄ and calcination at 1273 K, and their effects on the oxygen storage and release (OSR) properties of ceria are reported for the first time. The samples were characterized by different techniques and the results were compared with those recently reported on the same samples but calcined at 873 K. When P-containing ceria solids were calcined at 1273 K, CePO₄ (monazite) was the predominant P-containing species on the surface of ceria, confirming previous studies that showed that monazite exists for those samples in which the surface P loading (P atoms per nm⁻²) is larger than 5.5. For lower surface P concentrations, isolated orthophosphate units are present at the surface and within the subsurface region of the solid. Severe sintering of CeO₂ after calcination at 1273 K resulted in P concentrations >5.5 P atoms·nm⁻² in all samples. Isolated PO₄ units that could initially be present in the samples calcined at 873 K nucleated and CePO₄ was formed when samples were calcined at 1273 K. OSR properties of CeO₂ deteriorated progressively when P loading increased due to the presence of larger crystals of the very stable Ce(III) phase of CePO₄ at the surface of the P-containing ceria solids.