Regeneration of Aged Commercial Three-Way Catalytic Converters

The present work demonstrates for the first time that rejuvenation of the catalytic activity of full-scale aged commercial three-way catalytic (TWC) converters is feasible after treatment with weak oxalic acid solution (0.1 M). Up to 50% reduction in CO, 75% in NO _x and 32% in HC emissions was attained under warm engine driving conditions following regeneration of three aged TWC converters with 42,500, 156,000 and 165,000 km mileage. Efforts to expand the durability of aged commercial TWC converters installed in gasoline-driven cars after applying specific treatments with weak oxalic acid-based solutions appear to be a very promising starting point in recovering to a very large degree their performance.     

Hydrothermal Aging-Induced Changes in Washcoats of Commercial Three-Way Catalysts

In order to quantify hydrothermal aging effects on the mono- and bi-metallic Pd and Rh supported on cerium-zirconium promoted alumina commercial three-way catalysts (TWCs), the catalysts were tested both fresh and after accelerated hydrothermal aging. The catalytic tests showed clear deactivation of the aged samples and influence on TWC’s property, such as: light-off temperature, specific surface area (BET), dispersion of noble metals, oxygen storage capacity, oxygen storage capacity complete and labile oxygen storage capacity. Water inhibition have been investigated and confirmed for the performance of NO_x reduction of the fresh catalyst. X-ray photoelectron spectroscopy was used to study changes in the oxidation state of the Pd and Rh present in the washcoat of the catalyst, before and after hydrothermal aging. Element maps by SEM-EDX analysis was perform in order to characterize the catalysts morphology, the surface’s composition and element distribution.     

Deactivation and Regeneration of Spent Three-Way Automotive Exhaust Gas Catalysts (TWC)

The effect of oxidation, oxy-chlorination and reduction treatments at elevated temperatures on the dispersion of palladium (Pd) and rhodium (Rh) for commercially aged three-way automotive exhaust gas catalysts (TWC) has been investigated. The catalytic activity of treated samples was compared with a reference sample, which was taken from the corresponding aged TWC and tested using a mini-cuts reactor simulating real driving conditions. In the case of oxygen, the improvement of the noble metal dispersion on the catalysts was dependent on the noble metal loading and the degree of metal sintering. Adding chlorine to the oxygen atmosphere facilitates the restructuring of the metals with an improved increase in the noble metal dispersion. The temperature and the composition of the gas used during these thermal treatments proved to be of importance not only to increase the metal dispersion, but also to prevent possible losses of noble metals, in the form of volatile MO _x Cl _y compounds. TEM-EDS techniques indicated changes in the size of the largest noble metal agglomerates of up to 100 nm in size after thermal gas treatment. BET porosity and XRD analyses were employed to investigate restructuring of the washcoat and showed a decrease in pore size distribution and an increase in surface area.     

Influence of Ageing Conditions on the Performance and Structure of Commercial Three-Way Catalysts

The ageing behaviour of engine test bench and road aged commercial three-way catalysts (TWCs) is compared. Their catalytic performances are tested and the findings are correlated to structural investigations. The TWCs were tested on a chassis dynamometer using the New European Driving Cycle and the catalytic activities were determined by measurements of the oxygen storage capacity and the light-off (LO) temperature. Selected catalysts were characterised by X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray analysing system, nitrogen adsorption (BET) and X-ray fluorescence spectroscopy. A linear increase of the emissions with duration for the HC and CO components and of the LO temperature was observed. A surface area decrease and a growth of Pd particles after ageing could be detected. Also a crystalline BaAl₂Si₂O₈ phase was identified, which can be attributed to a reaction of NO _x storage component with cordierite support material.     

贵金属三效催化剂的研究进展

概述了国内外贵金属三效催化剂的催化性能和发展现状,并对其制备、催化特性及催化作用进行了分析,介绍了以铈氧化物为主的固溶体在贵金属三效催化剂中的应用及其发展前景。     

A Novel Preparation Route of Three-Way Catalysts

A novel preparation route of TWC, which is combined with the synthesis of nanosized (Ce–Zr–M)O₂ (M = La, Pr) solid solution by modified sol–gel method, is introduced. The TWC precursor shows high thermal stability and possesses high OSC. High thermally stable TWCs with this kind of precursors are also exhibited. The main reason is discussed.     

工业应用失活与再生加氢催化剂研究

对不同类型工业应用失活和再生后的加氢催化剂研究结果表明,经使用失活及再生后的催化剂在物化性质、孔结构、活性及选择性方面均有不同程度的改变。抚顺石油化工研究院（FRIPP）开发的各种加氢催化剂稳定性强,再生后的催化剂活性能够恢复到较高的水平。     

Why Rhodium in Automotive Three-Way Catalysts?

The catalytic treatment of motor vehicle exhaust gas has been a feature of all passenger cars in the USA since the 1975 model year. It has since been adopted in all the developed countries as the principal means of con-trolling the emissions from vehicles powered by Otto-cycle internal com-bustion engines. These include passenger cars, light trucks, and heavy-duty trucks. Catalysts have been developed for motorcycles and even smaller power plants. The so-called three-way catalyst (TWC) has been in use since the 1979 model year. The name reflects the simultaneous treatment by this catalyst of the two reducing pollutants, carbon monoxide and uncombusted hydrocarbons, and the oxidizing pollutant, oxides of nitrogen.     

Oxygen Storage and Mobility on Model Three-Way Catalysts

Oxygen storage capacity (OSC) of TW catalysts has a strong impact on their performances under oscillatory conditions. This paper reviews the different methods developed for OSC measurements, the effects of catalyst composition on OSC as well as the impact of OSC on TW catalysis, concentrating on the results obtained in our laboratory. Use of OSC measurements for catalysts control (on-board diagnostic) is also examined. The role of superoxide species as intermediates in the oxygen storage process will be emphasized. Finally, we shall review the recent developments on CeZr-based oxides.     

Interaction of Perchloroethylene with Cerium Oxide in Three-Way Catalysts

The role of cerium oxide on direct oxidation of perchloroethylene (PCE) by a three-way catalyst was explored. In the absence of an external oxidizing agent, PCE was oxidized over an alumina supported Pt/Rh three-way catalyst. We hypothesize that the chlorine atoms in the adsorbed PCE interact with oxygen in CeO(2), reducing the cerium to create CeCl(3).     

Coke Deposition on Automotive Three-Way Catalysts Studied with LEIS

LEIS measurements have been performed on fresh and used commercial three-way catalysts (Pt/Rh/CeO₂/-Al₂O₃). The catalysts showed a significant decrease in the platinum surface concentration of approximately 50% after use in hydrocarbon oxidation at cold-start conditions, and could be completely regenerated. The selectivity towards the outermost atomic layer of LEIS allowed a one to one correlation between the surface Pt concentration as detected with LEIS and that obtained by kinetic modelling. This supports recent assumptions on selective deactivation. Conventional surface science techniques such as XPS, SIMS or AES would have yielded ambiguous results since their probing depths are not limited to the outermost atomic layer.     

Characterisation of Three-Way Automotive Aftertreatment Catalysts and Related Model Systems

The techniques applied to the characterisation of three-way automotive aftertreatment catalysts, and some of the most relevant results obtained from those, are briefly overviewed. Studies dealing with both, commercial converters and NM/CeO₂ (CeO₂-ZrO₂) model systems are considered. The analytical and structural characterisation studies on commercial TWCs reported here are mainly devoted to the identification of some of the most relevant factors causing their deactivation. Regarding the reviewed literature on model systems, some more fundamental aspects are considered. Among them, the use of HRTEM, spectroscopic, and chemical techniques in studies on metal sintering and re-dispersion, metal/support interaction phenomena, and redox characterisation of ceria-based catalytic materials are discussed.     

From Light-Off Curves to Kinetic Rate Expressions for Three-Way Catalysts

A new approach for obtaining reliable kinetic data from light-off curves is proposed. By fitting laboratory and simulated light-off curves with a simple 1-D model, apparent activation energies and frequency factors are adjusted using the Simplex method. The originality of the method is that kinetic parameters are obtained sequentially using simple mixtures. The whole set of parameters is then checked against experimental results involving all the reactants. Simulation results are satisfactory despite the empirical nature of the rate laws.     

Observation of Aluminate Whiskers and Nanotubes in Dynamometer-Aged Three-Way Automotive Catalysts

Short nanotubes and whiskers of aluminates of different compositions, Ni _x Al₂O_3+x , SrAl₁₂O₁₉ and (Nd,La)AlO₃, with a noble metal particle attached to one end, are observed in dynamometer-aged automotive three-way catalysts (TWCs). The samples were from different production formulations for the US market that were aged for 120 h to simulate 100 000 miles of driving with a peak temperature of 1000-1050 °C in the middle of the catalyst monolith. Ni _x Al₂O_3+x appears as nanotubes in both Pt/Rh- and Pd-based catalysts with NiO additions. The nanotubes suspend the precious metal catalyst particles in the gas stream. Solid whiskers of SrAl₁₂O₁₉ have been found in a Pd-based catalyst with SrO additions in both pores and within alumina agglomerates. (Nd,La)AlO₃ whiskers have thus far been found only within alumina agglomerates in a Pd-based catalyst. The aspect ratio of the whiskers and nanotubes observed after aging is 4:1 or more. The solid noble metal particles are cuboctohedral in shape and average 90-150 nm in diameter, depending on the sample. Catalyst particle size controls the nanotube and whisker diameter, although some conical whiskers have also been observed.     

Cex Zr1-xO2固溶体三效催化剂研究进展

重点介绍了具有良好储氧能力的铈氧化物及其复合氧化物固溶体作为助剂在汽车尾气净化三效催化剂中的作用原理,阐述了制备方法对催化剂结构与性能的影响,综述了CeO2、CexZr1_xO2(CZ)和REyCexZr1-x-yO2(RECZ,RE=Rare Earths)等相关氧化物的研究进展,提出了该领域今后的研究重点和发展趋势.     

Evolution of Structural Properties of Three-Way Catalysts According to the Ageing Procedure

The conversions of CO, NO and HC of TWC submitted to either cycling ageing (CA) or thermal ageing (TA) were compared. The results show a strong deactivation with TA catalysts, whereas the loss of activity is very moderate when using CA ageing. To understand these differences, the catalysts were analysed with different physical techniques (microscopy, TPR, TPD, infrared). These techniques have put in evidence different compositions (bulk and surface) between fresh, CA and TA catalysts. Indeed, TPR has allowed the distinction between two bulk phases on the fresh catalyst while only one phase remains on the CA catalyst. Equally, TPD and FTIR led to a partial elucidation of the surface composition and a good distinction of the different phenomena occurring during thermal ageing or oscillating ageing allowing the proposition scheme to represent ageing of a TWC.     

Study of the Oxygen Diffusion on Three-Way Catalysts: A Kinetic Model

A computer model was developed to take into account all the phenomena that can occur in ¹⁸O/¹⁶O isotopic exchange over Pt/CeZrO _x materials: adsorption/desorption on the metal, surface and bulk O diffusion. Discriminating each step of the exchange process is no longer necessary: kinetic parameters and O diffusivity can be calculated in a single experiment.     

仲丁醇脱氢催化剂的再生

简单地介绍了仲丁醇脱氢反应的机理,催化剂的特点和催化剂的再生步骤,通过操作看出较低的再生温度有利于保护催化剂。     

Testing Commercial Catalysts in Recycle Reactors

Recycle reactors offer some interesting possibilities for catalyst testing. To make full use of the advantages, and to avoid the difficulties associated with recycle reactors, one has to understand their performance characteristics and how this is related to the performance of commercial reactors.