Automotive Exhaust Gas Catalysts Research Articles

Molecular Structures, Tribological Properties, and Working Mechanisms of Sulfur- and Phosphorus-Free Organic Molybdenum as Additives in Lubricants: A Short Review

Abstract Traditional oil-soluble organic molybdenum (OM) as friction modifiers (FMs) in engine oils contain sulfur and/or phosphorus. Both sulfur and phosphorus are detrimental to the automotive exhaust gas catalysts. Consequently, sulfur and phosphorus in commercial engine oils are limited seriously by laws. Recently, oil-soluble sulfur- and phosphorus-free organic molybdenum (SPFMo) has been developed and measured intensively. This article reviews the molecular structures, tribological properties, and working mechanisms of SPFMo as FM in oils. Some bottlenecks that constrain the wide applications of SPFMo in engine oils are also summarized. In addition, some routes for overcoming the bottlenecks are suggested. Finally, some potential developments of SPFMo in the future are proposed. This review will provide a comprehensive understanding of SPFMo to the researchers in the field of oil additives.

In Situ Small-Angle Neutron Scattering Analysis of Water Evaporation from Porous Exhaust-Gas-Catalyst Supports.

Porous media as catalyst supports are key to developing automotive exhaust purification systems. In particular, the water content of these porous media is attracting research attention because catalyst supports containing condensed water vapor at the early stage of cold start require a longer warm-up period. In this regard, water isotherms and evaporation in porous Al2O3 were investigated in this study using in situ small-angle neutron scattering (SANS) experiments. Unlike conventional evaluation methods, such as weighing and X-ray tomography, SANS distinguishes water in the primary and secondary pores using a contrast-matching method. Time-resolved measurements showed that water started to evaporate from the secondary pores in tens of seconds and subsequently from the primary pores in a hundred seconds. Exhaustive experiments conducted using nine alumina-based samples revealed that the drying rate depended on the secondary pore size of the porous Al2O3. The proposed approach can enable the evaluation of controlling factors to additionally optimize the performance of automotive exhaust gas catalysts, especially during cold start.

A Joint Theoretical and Experimental Study of Phase Equilibria and Evolution in Pt-Doped Calcium Titanate under Redox Conditions

Pseudoternary phase diagrams of the PtOx–CaO–TiOx system were constructed using first-principles theoretical methods to interpret experimental observations of phase formation and evolution in Pt-doped CaTiO3 made by aberration-corrected scanning transmission electron microscopy. In this system, promoted as a self-regenerative automotive exhaust-gas catalyst, Pt precipitates as metallic particles from the doped perovskite under reducing conditions and tends to redissolve under oxidizing conditions. Due to the likely influence of kinetic limitations on experimental observations, we focus on a restricted (metastable) phase diagram in which single and other unobserved oxides of Pt are intentionally excluded. Various titanate phases appear, however, and their presence helps inform the calculation of the phase diagrams, which in turn confirm that the self-regenerative effect is a consequence of thermodynamic phase stability under the specific redox environments examined.

Developing a Technology for Recycling Automotive Exhaust-Gas Catalysts

Methods of recycling platinum-group metal (PGMs) and recovering them from spent automobile catalysts are described in detail. The results of an analysis are used to recommend a method that entails pyrolytic concentration in an electric furnace and subsequent electrochemical separation. Test specimens of metal and slag were studied by x-ray fluorescence analysis (XRF), atomic-absorption spectroscopy (AAS), and inductively-coupled plasma mass spectrometry (ICP-MS). XRF is a quick and inexpensive method of determining PGMs in catalysts only when calibrated specimens are available, while AAS is suited only for determining rhodium and palladium in specimens of catalysts and metals. The ICP-MS method is well-suited for determining PGMs in catalysts and products obtained from their recycling and is indispensible for analyzing slags. A series of heats was performed in an ac furnace to find the most effective method of introducing a collector-metal to precipitate PGMs. Analysis of the heats showed that briquetting reduces the amount of metal recovered somewhat due to an increase in the temperature of the melt. The addition of a collector-metal to the briquettes decreases the volume of dust-gas emissions and increases PGM yield. The largest amount of PGMs is recovered when iron-ore concentrate and a reducing agent are used as the collector-metal.

Self-Regeneration of Pd–LaFeO3 Catalysts: New Insight from Atomic-Resolution Electron Microscopy

Aberration-corrected transmission electron microscopy was used to study atomic-scale processes in Pd-LaFeO(3) catalysts. Clear evidence for diffusion of Pd into LaFeO(3) and out of LaFe(0.95)Pd(0.05)O(3-δ) under high-temperature oxidizing and reducing conditions, respectively, was found, but the extent to which these processes occurred was quite limited. These observations cast doubt that such phenomena play a significant role in a postulated mechanism of self-regeneration of this system as an automotive exhaust-gas catalyst.

Catalysts as Sensors—A Promising Novel Approach in Automotive Exhaust Gas Aftertreatment

Sensors that detect directly and in situ the status of automotive exhaust gas catalysts by monitoring the electrical properties of the catalyst coating itself are overviewed. Examples included in this review are the in-situ determination of the electrical impedance of three-way catalysts based on ceria-zirconia solutions and of lean NOx traps of earth-alkaline based coatings, as well as approaches to determine the ammonia loading in Fe-SCR-zeolites with electrical ac measurements. Even more sophisticated approaches based on interactions with electromagnetic waves are also reviewed. For that purpose, metallic stick-like antennas are inserted into the exhaust pipe. The catalyst properties are measured in a contactless manner, directly indicating the catalyst status. The radio frequency probes gauge the oxygen loading degree of three-way catalysts, the NOx-loading of lean NOx traps, and the soot loading of Diesel particulate filters.

Influence of aging effects on the conversion efficiency of automotive exhaust gas catalysts

The influence of chemical and thermal aging effects on catalytic activity was analyzed for a vehicle (bifuel) aged pre- and underfloor three-way catalyst (p-/u-TWC) as well as an engine aged diesel oxidation catalyst (DOC). The conversion results were compared with the respective new catalysts. Although the vehicle aged p-TWC showed massive thermal aging, its CO conversion rate was comparable to that of the vehicle aged u-TWC. On the other hand, the conversion rate for methane abatement was significantly affected above 325 °C. Methane conversion appeared as the parameter mostly affected by (thermal) aging. The DOC showed massive chemical aging with typical engine oil based contaminants like P and Zn but also with Si. The contamination is likely responsible for the shift of propene and NO light-off curves to higher temperatures by about 70 °C.

Methodology development for laboratory-scale exhaust gas catalyst studies on phosphorus poisoning

Deactivation of powdery automotive exhaust gas catalyst components (Rh or Pt loaded cerium–zirconium mixed oxides) was studied in laboratory-scale by developing two different chemical ageing procedures: incipient wetness impregnation with an aqueous salt solution of (NH4)2HPO4 and the adding of the (NH4)2HPO4 salt solution into the samples from a gaseous phase. Vehicle-aged commercial exhaust gas catalysts were used as a reference material for phosphorus poisoning. Phosphorus contamination in the poisoned samples was detected with X-ray fluorescence spectroscopy (XRF). Scanning electron microscope equipped with energy dispersive spectrometer (SEM–EDS) as well as XRF were used to analyze contaminants in the reference catalysts. Poison compounds formed on the catalytic surface of both laboratory-scale aged samples and vehicle-aged reference catalysts were identified with X-ray diffraction spectroscopy (XRD). The poisoning induced changes in the catalytic activities and characteristic surface areas of the samples were studied with activity tests and BET measurements, respectively. By both ageing methods, phosphorus could be added to the samples repeatably, time-savingly, affordably, and safely, and the same phosphorus compounds formed under real driving conditions were detected. These methods are therefore considered to be potential ageing procedures when studying accelerated chemical poisoning of powdery automotive exhaust catalyst components in laboratory-scale.

Partial encapsulation of Pd particles by reduced ceria-zirconia

Direct observation of metal-oxide interfaces with atomic resolution can be achieved by cross-sectional high-resolution transmission electron microscopy (HRTEM). Using this approach to study the response of a model, single-crystal thin film automotive exhaust-gas catalyst, Pd particles supported on the (111) ceria-zirconia (CZO) surface, to a redox cycle, we have found two distinct processes for the partial encapsulation of the Pd particles by the reduced CZO surface that depend on their relative crystallographic orientations. In the case of the preferred orientation found for Pd particles on CZO, Pd(111)[110]∕∕CZO(111)[110], a flat and sharp metal/oxide interface was maintained upon reduction, while ceria-zirconia from the adjacent surface tended to accumulate on and around the Pd particle. In rare cases, Pd particles with other orientations tended to sink into the oxide support upon reduction. Possible mechanisms for these encapsulation processes are proposed.

Principal component analysis of Raman spectra from phosphorus-poisoned automotive exhaust-gas catalysts

Ramanspectrafromreal-world,phosphorus-poisonedautomotivecatalystscanberemarkablycomplicated, owing to the fact that factors such as variable driving conditions, catalyst formulations, and oil and fuel additives can contribute to the formation of a huge variety of compounds. To classify the Raman spectra from many catalysts without necessarily identifying all phosphorus contaminants, principal component analysis was performed. Examination of the scores in the PC1-PC2 plane showed that samples with similar spectra are grouped together while dissimilar ones are spaced apart from one another. The score plot also confirmed that the strongest peaks of orthophosphates generally lie close to one another. Examination of the loadings revealed that CePO4, and possibly Ca10(OH)2(PO4)6and BaSO4 ,a re among the major components present in the spectra. CePO4 is readily observed in many of the spectra, but Ca10(OH)2(PO4)6 is not, showing that principal component analysis was useful in uncovering possible spectral constituents difficult to unambiguously assign from the original spectra. The spectra can be adequately reproduced using eight principal components, which means that a much smaller set of numbers can be used to represent the spectra instead of typical wavenumber-intensity data. Copyright  2005 John Wiley & Sons, Ltd.

Thermal gas treatment to regenerate spent automotive three-way exhaust gas catalysts (TWC)

The effect of regeneration on the metal dispersion and catalyst activity of commercial automotive three-way catalysts (TWC) in oxygen, hydrogen and oxy-chlorine at elevated temperatures has been investigated. In addition, characterisation of the catalysts has been performed using XRD, TEM, TPR, CO chemisorption and BET surface area measurements. Activity tests on monolith fragments were conducted in a micro-reactor supplied with real exhaust gases.An increase in activity was observed with oxygen treatment on catalysts showing heavy sintering as well as a high noble metal loading. Hydrogen proved to be an ineffective treatment procedure, whereas the addition of chlorine to the oxygen stream resulted in an activity increase even on catalysts less sintered and with lower noble metal contents. A comparison of the XRD and BET profiles showed no or small changes in the washcoat structure compared to fresh catalysts after successful regeneration. The TEM, TPR and CO chemisorption measurements suggest a decrease in the size of the largest noble metal agglomerates on the catalyst surface, as well as some Pd and RhO restructuring. The oxy-chlorine regeneration procedure is shown to be the most efficient, both regarding the catalyst activity and the metal dispersion. An increase in the amount of noble metal particles with a size of 20–70nm, coupled with a decrease in size of larger clusters up to 100nm in size, was observed for this treatment.

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 MOxCly 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.

Effect of ageing atmosphere on the deactivation of Pd/Rh automotive exhaust gas catalysts: catalytic activity and XPS studies

In the present work, the catalytic activity and thermal stability of metallic Pd and Rh catalysts have been examined under different ageing atmospheres. It was shown that thermal treatment under the reducing and oxidising ageing atmospheres strongly affected catalyst stability in terms of surface areas and the changes observed in the chemical states of active metals. This was also related to catalytic activities that remained higher if laboratory scale ageing was carried out in the reducing ageing atmosphere. It was also found that engine bench and vehicle ageings correlated with the reducing laboratory scale ageing. Thermal deactivation of the aged catalysts is due to the collapse in surface area and the sintering of the Rh metal particles. Active metal particles may also be encapsulated in the sintered washcoat that is observed as a decrease in catalytic activity.

Pd encapsulation in automotive exhaust-gas catalysts

Raman spectroscopy was used to detect strained PdO in fully formulated Pd-based automotive exhaust-gas catalysts that had been subjected to high-temperature accelerated aging treatments. The presence of strained PdO provides evidence of Pd encapsulation by ceria-zirconia, as in a previous study of a model powder catalyst. The relationship between strained PdO and Pd encapsulation was further examined for other model catalysts, and major factors influencing the application of this detection method were identified.

XPS characterization of adsorbed reaction intermediates on automotive exhaust gas catalysts: NO and CO + NO interactions with Pd

AbstractNitrous oxide (NO) adsorption and the CO + NO reaction have been investigated on bulk and alumina‐supported Pd catalysts by XPS at various temperatures between 25 and 300°C with PNO in the 0.5–5.0 (×10−2) atm range and PCO = 5 × 10−3 atm. A catalytic reactor settled in the preparation chamber coupled to the spectrometer allows surface changes occurring under catalytic conditions on these catalysts to be characterized. Various nitrogen‐containing species have been detected, which depend on the oxidation state of Pd. The results have been explained tentatively in the light of a mechanism proposed for NO transformation. Copyright © 2002 John Wiley & Sons, Ltd.

Acetylene and carbon monoxide oxidation over a Pt/Rh/CeO 2/ γ-Al 2O 3 automotive exhaust gas catalyst: kinetic modelling of transient experiments

The transient kinetics of acetylene (C 2H 2) conversion by oxygen over a commercial Pt/Rh/CeO 2/ γ-Al 2O 3 three-way catalyst have been modelled. Experiments to validate the model were carried out in a fixed-bed reactor with two separate inlets, enabling alternate feeding of acetylene and oxygen. Frequencies of feed composition cycling up to 1/6 Hz were applied. The experimental conditions resemble the cold-start period of an Otto engine. Two types of adsorbed acetylene species seem to exist. A selective catalyst deactivation for oxygen adsorption, due to deposition of carbonaceous deposits, was found. Ceria proved to have a significant influence on the acetylene oxidation. A kinetic model was developed for the conversion of acetylene to carbon monoxide, based on elementary reaction steps. This model was combined with the published kinetics for transient carbon monoxide oxidation to carbon dioxide over the same catalyst (Nibbelke, R. H., Chapman, M. A. J., Hoebink, J. H. B. J., & Marin, G. B. (1997). Kinetic study of the CO oxidation over Pt/V-Al 2O 3 and Pt/Rh/ CeO 2/ γ−Al 2O 3 in the presence of H 2O and CO 2. Journal of Catalysis, 171, 358–373.), in order to describe the total oxidation of acetylene quantitatively. The combined model is able to describe the results of the transient experiments on simultaneous acetylene and carbon monoxide oxidation. During a transient, both acetylene and carbon monoxide react mainly in a front moving through the reactor, carbon monoxide hardly influencing the acetylene oxidation.

NO reduction by CO over automotive exhaust gas catalysts in the presence of O2

The reduction of NO by CO in absence and presence of O2 has been investigated by transient experiments at automotive cold-start conditions over Pt/Rh/CeO2/γ-Al2O3, and derived model catalysts. A high-resolution magnetic sector mass spectrometer was used for distinguishing CO/N2 and CO2/N2O. Mechanistic comparisons are made between the catalyst formulations. A kinetic scheme of elementary reaction steps is proposed, which highlights the various contributions of the catalyst constituents.

Transient Kinetic Modeling of the Ethylene and Carbon Monoxide Oxidation over a Commercial Automotive Exhaust Gas Catalyst

The transient kinetics of ethylene oxidation by oxygen over a commercial Pt/Rh/CeO2/γ-Al2O3 three-way catalyst were modeled. Experiments were carried out in a fixed-bed microreactor with two separate inlets, enabling alternate feeding of ethylene and oxygen with frequencies up to 1/4 Hz. The experimental conditions resemble the cold-start period of an Otto engine in a car. Two types of adsorbed ethylene species seem to exist. A selective catalyst deactivation for oxygen adsorption, due to deposition of carbonaceous species, was found. A kinetic model was developed, based on elementary reaction steps, that allows one to describe the experiments quantitatively. Furthermore, this model was combined with the published model for transient carbon monoxide oxidation over the same catalyst,1 which enables one to predict the results of simultaneous ethylene and carbon monoxide oxidation. Both components react in rather distinct zones, with ethylene being converted only when carbon monoxide oxidation is almost comp...

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.

Properties of Rapidly Solidified FeCrAl Ribbons for the Use as Automotive Exhaust Gas Catalyst Substrates

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