Automotive Three-way Catalysts Research Articles

CO oxidation for the characterization of reducibility in oxygen storage components of three-way automotive catalysts

Steady-state, CO-oxidation kinetics at 515 K have been measured on model, Pd catalysts, prepared by vapor deposition of Pd onto either zirconia, praseodymia, ceria, or a ceria-zirconia mixed oxide. A second rate process (RE2), associated with both the metal and the oxide support and observed previously on ceria-supported catalysts in excess CO [7], was also found for Pd supported on ceria-zirconia, but neither zirconia nor praseodymia had any effect on CO oxidation under the conditions of our study. For ceria and ceria-zirconia, deactivation, through the loss of RE2, caused by high-temperature calcination, was examined, with the Pd added after calcination so that the metal particle size was not a factor in deactivation. For ceria, there was a strong dependence on calcination temperature, with almost complete loss of RE2 above 1170 K. XRD showed that the loss was accompanied by a large increase in the crystallite size. Results for ceria-zirconia showed that the loss in this case was more gradual, with CO oxidation activity due to RE2 maintained to much higher calcination temperatures. Taking the importance of RE2 as a measure of the ability of the catalyst to use oxygen from the oxide, the implications of these results with respect to oxide structure and the effect of aging on oxygen-storage properties of reducible oxides are discussed.

Partial regeneration of an aged commercial automotive catalyst

An aged commercial three-way automotive catalyst was examined with respect to regeneration procedures. Catalyst samples were leached with acetic acid to remove contaminants accumulated on the catalyst surface during operation. The optimum leaching conditions concerning acetic acid concentration, solution feed rate and temperature were investigated. The leached samples were examined for their catalytic activity (CO and C 2H 4 oxidation, NO reduction and N 2 production) in comparison with samples of fresh and aged catalysts. The leaching procedure improves considerably the catalytic activity, where in the case of C 2H 4 oxidation, NO reduction reaches the activity of the fresh catalyst.

Redox Behavior of High Surface Area Rh-Loaded Ce0.5Zr0.5O2Mixed Oxide

Cerium oxide is widely employed as a promoter in the automotive three-way catalysts (TWCs) due to its high oxygen storage/release capacity (OSC). The OSC acts as an oxygen pressure regulator in the converter which allows one to maintain a high efficiency for the TWC. This has stimulated a strong interest in the investigation of redox properties of CeO{sub 2}-based catalysts. Recently, the authors disclosed that a Ce{sub 0.5}Zr{sub 0.5}O{sub 2} mixed oxide with a surface area of 64 m{sup 2} g{sup -1} showed an unusual improvement of the redox behavior upon sintering induced by a repetitive reduction/oxidation of the solid solution. In the present paper, the redox behavior of the above Rh-loaded Ce{sub 0.5}Zr{sub 0.5}O{sub 2} is reported. The influence of the thermal/redox treatment on the H{sub 2} chemisorption is also addressed. Data obtained on a Rh/CeO{sub 2} catalyst are included to elucidate the role of ZrO{sub 2}. 16 refs., 2 figs., 3 tabs.

Redox Behavior Below 1000K of Pt-Impregnated CeO2-ZrO2Solid Solutions: An In-Situ Neutron Diffraction Study

ABSTRACTThe Ce3+↔ Ce4+redox process in automotive three-way catalysts such as Ce-ZrO2/Pt provides an essential mechanism to oxygen storage/release under dynamic air-to-fuel ratio cycling. Such a function requires a metal-support interaction which is not completely understood. We have carried out anin-situneutron powder diffraction study to monitor the crystal structures (a mixture of a major tetragonal and a minor monoclinic phase) of 10mol% Ce-doped ZrO2with and without Pt (1wt%) impregnation under oxidizing and reducing conditions over the temperature range of 25°-700°C. The samples were heated first in flowing 2%O2/Ar from room temperature to 400°C and then in 1%CO/Ar to about 700°C. A discontinued increase of the tetragonal unit-cell volume, a decrease of tetragonality (c/a), and a change of color from light yellow to gray when changing from oxidizing to reducing atmosphere were observed only in the sample containing Pt. This result supports the model which assumes the formation of oxygen vacancies initially near the Pt atoms. As more Ce ions are reduced from 4+ to 3+ oxidation states at high temperatures, oxygen vacancies migrate to the bulk of the oxide particles.

SIMS Investigation of Fresh and Aged Automotive Exhaust Catalysts

Secondary ion mass spectrometry (SIMS) has been employed to examine the surface composition of fresh and aged three-way automotive exhaust catalysts. The washcoat components and poison species could be monitored due to the unsurpassed sensitivity of this technique. The outermost 1-2 nm was found to be covered with species containing the elements P, Ca, Zn, Pb and S. These species are known to originate from fuel and lubricating oil additives and the amounts on the surface of catalysts increased with increasing distance travelled. The presence of calcium phosphate and lead sulphate was confirmed, and these species are known surface fouling agents. Evidence of Ce sintering on each of the aged catalysts was also found using this technique. Depth profile experiments showed that the poison species penetrated the washcoat layer between 0.5 and 4 μm, depending on the distance travelled. Imaging SIMS displayed the lateral position of the washcoat components and poison species.

FTIR Spectroscopy of Nitric Oxide Adsorption on Pd/Al2O3: Evidence of Metal-Support Interaction

Automotive three-way catalysts containing Pd as the only precious metal have certain advantages over current formulations consisting of various mixtures of Pt, Pd, and Rh. In this study, the adsorption of NO on 2 wt% Pd/Al2O3 was investigated using infrared spectroscopy. Bands for NO on Pd were assigned to linear (1753-1750 cm−1), twofold bridged (1615-1599 cm−1), and threefold bridged (1580-1572 cm−1) adsorption modes. For Pd supported on alumina, linear NO was favored on the reduced catalyst while two- and threefold bridged NO were favored on the oxidized catalyst. Upon heating, a decrease in the linear form was accompanied by increases in the two- and threefold bridged forms up to the highest temperature measured (300°C). This result may indicate a conversion of the linear to bridged adsorption modes upon heating. NO adsorption on pure alumina revealed a band near 1807 cm−1 attributable to weak, linear adsorption. This band increased drastically upon oxidation of the Pd/Al2O3 catalyst, indicating enhanced NO adsorption on alumina due to the presence of palladium. Conventional spillover from Pd to alumina was ruled out. The increased intensity is explained by adsorption of NO on alumina sites involved in a metal-support interaction brought about by catalyst oxidation. NO adsorption on La-promoted Pd/Al2O3 was less than on the unpromoted catalyst. Possible explanations are an increase in NO dissociation and coverage of Pd by the modifier.

Al2O3/CeO2 washcoats for three-way automotive emission catalysts

Platinum-rhodium based three-way-catalysts (TWC) are the primary catalytic system for control of hydrocarbon, CO, and NOx automotive emissions. Mixed Al2O3/CeO2 oxides are often dispersed on a cordierite honeycomb monolith as a washcoat and act as a high-surface-area carrier for the heavy metal catalyst clusters. There are both regulatory and marketplace demands for improved performance, lifetime, cost and reliability of these catalytic converters. The conversion efficiency and lifetime of a converter is determined by the microstructure of the washcoat/monolith and its evolution during high temperature exposure to the exhaust gas stream. Scanning electron microscopy (SEM), electron microprobe analysis, and analytical electron microscopy (AEM) were utilized to characterize these catalysts. Some AEM was performed on crushed washcoat specimens dispersed on holey carbon films. In addition, an innovative technique was developed for the preparation of transverse section specimens from the monolith, which preserves the spatial correlation of microstructure and elemental distribution with respect to position in the washcoat/cordierite.

Development of automotive palladium three-way catalysts

A rhodium-free automotive three-way catalyst was investigated for the purpose of reducing precious metal costs and for the enhancement of availability. The relative catalytic activities of platinum, rhodium and palladium were compared, and revealed that palladium could be a possible rhodium-free catalyst. From a practical viewpoint, the major problem was its low NO x conversion efficiency in a fuel-rich exhaust. In order to improve the NO x conversion activity of the palladium catalyst, the effects of additives were investigated. The addition of basic element (Cs, Ba and La) or transition metal oxide (Ni and Co) improved the activity, possibly due to reducing hydrocarbon poisoning by electrons being donated to the palladium. The performance of an improved palladium catalyst was as high as that of a conventional platinum-rhodium catalyst.

Why Rhodium in Automotive Three-Way Catalysts?

Abstract 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.

Diffusional Effects and Intrinsic Kinetics for NO Reduction by CO over Pt-Rh/.gamma.-Al2O3 Monolithic Catalysts

Automotive three-way catalysts consist mainly of platinum and rhodium. The reaction of nitric oxide (NO) with carbon monoxide (CO) is a removal pathway for nitrogen oxides and is enhanced by the existence of rhodium in catalytic converters. Experimental kinetic data have been obtained for the CO-NO reaction system over laboratory made platina-rhodium monolithic catalysts with different thicknesses of catalytic active substrate on a monolithic ceramic support. A synthetic gas mixture which consisted of CO, NO, and N[sub 2] was the feed of an integral tubular reactor between 200 and 350 C. A first-order kinetic rate expression with respect to NO concentration, which includes an inhibition term of second order, was found to fit the experimental data via a model which also accounts for internal transport resistances. The reaction rate constants, the activation energy of the reaction, the enthalpy of the adsorption process, and the effective diffusivity of NO were estimated. The obtained kinetic and diffusional data can be used in the design of a catalytic converter with improved efficiency.

Investigations on precious metal/alumina interactions on three-way automotive emission control catalysts, BMA catalysts and model samples

Combined XPS and SIMS measurements, partly supplemented by XRD investigations, were utilized to detect precious metal/support interactions on catalysts containing various precious metals. The Klemm-Bronger reaction between Pt and α-Al2O3 on methane/ammonia catalysts was used as a standard reference. Evidence was found for different stages of Pt/Al interactions and changing electronic properties of Pt on the surfaces of these Pt/α-Al2O3 catalysts by means of XPS and SIMS, and in the bulk material by XRD. The SIMS investigations on Pt and Rh on γ-Al2O3 showed that, even at temperatures around 500 °C, Rh/Al interactions can appear to a small extent in the topmost atomic layers, whereas the Pt/γ-Al2O3 specimens did not show any measurable effects. After on-road operation, enhanced SIMS cluster-ion signals as well as anomalous XPS signal contributions were measured on the surfaces of used three-way automotive emission control catalysts. These signals did not appear for the fresh catalysts. By analogy with the related results reported on the methane/ammonia catalysts, the ion signals were used as qualitative surface probes indicating the presence of precious metal/support interactions, especially between Rh and Al.

TEM measurement of surface area of automotive catalysts

The specific surface area of supported noble metal particles in an automotive catalyst is defined as the exposed surface area per unit mass of these particles. It is of great importance to know this parameter, since this is one of the major factors that determine the effectiveness of the catalyst. Commonly used methods for characterizing catalysts, such as X-ray diffraction and TEM, do not directly provide a measure of surface area, but, instead, provide a measure of the “average size” of supported particles. Moreover, the “average sizes” obtained from different experimental techniques are often not comparable. Furthermore, many previous electron microscopy catalyst studies measured only simple average particle size, and no detailed procedure for measuring area-weighted average size or surface area appear to have been reported.In the current study, a procedure for measuring surface area of supported particles by transmission electron microscopy(TEM) was developed, and applied to measure surface areas of various production three-way automotive catalysts.

Nitrous Oxide from Combustion and Industry: Chemistry, Emissions and Control

After an Introductory Part, presenting a survey of the present knowledge of nitrous oxide chemistry related to fossil fuel combustion and flue gas treatment, as a background for the understanding of emission factors, the paper deals successively with the average N2O emission factors from combustion and other industrial sources, and gives guidelines for appropriate N2O control technology ; with respect to the former item, some comments and criticisms on the 1991 OEDC/IPCC Report are formulated. As far as updated emission sources are concerned, emphasis is put on those which presently constitute issues : emissions from fluidized bed combustors, emissions caused by non catalytic selective NO reduction by ammonia and urea injection, N2O emissions caused by the use of automotive three-way catalysts as well as emissions from nitric acid and adipic acid manufacturing and from municipal wastes and sewage sludges incineration. Comments on the 1991 OEDC/IPCC Report mainly emphasize : (1) the surprising absence of emission factors from stationary combustion facilities and the inadequacy of some of the scarcely presented data, (2) the strange ignorance of the important effect of aging of three-way catalysts on the emission of N2O from gasoline vehicles. These omissions are the more surprising since reliable information in these two fields were already available at the period the OEDC Report was issued and or revised. For the assessment of adequate N2O control technologies, there is an urgent need for further R&D work. Presently existing understanding of homogeneous and heterogeneous N2O chemistry may provide interesting hints for N2O control, either by gas phase treatment or by catalytic reduction, depending on the concentration levels present in the off-gases to be treated.

Speciated Hydrocarbon Emissions from the Combustion of Single Component Fuels. II. Catalyst Effects

Six single-component fuels (isooctane, n-heptane, 1-hexene, cyclohexane, methyl-t-butyl ether (MTBE), and toluene) and a multicomponent tracer fuel were burned in a pulse flame combustor (PFC) and reacted over a three-way automotive catalyst. The composition of the raw, uncatalyzed PFC exhaust was characterized in Part I of this study. In Part II, we focus on the conversions of the individual exhaust HC species over the catalyst. In accord with previous studies, the order of reactivity observed for the various classes of HC species was: methane (least reactive) < saturated HC < aromatics < unsaturated HC (most reactive). These differences in catalytic reactivity led to increases in the relative concentrations of methane and some saturated hydrocarbons in the post catalyst exhaust, and corresponding decreases in the relative concentrations of aromatic and unsaturated hydrocarbons. Oxygenated organic compounds showed wide variability in catalytic reactivity depending on the specific compounds involved. Catalytic conversion of the air toxic, 1,3-butadiene, was essentially complete to within detection limits. Benzene and toluene appeared to have similar intrinsic catalytic reactivities. However, net conversion of benzene in most instances was significantly less than that of toluene owing to demethylation of toluene (to form benzene) occurring in parallel with benzene oxidation. Rich combustion of both isooctane and tracer fuel led to the production of methane by the catalyst, primarily from reactions of acetylene and small olefins.

Mechanisms of the various nitric oxide reduction reactions on a platinum-rhodium (100) alloy single crystal surface

The reduction of nitric oxide with hydrogen was studied over a Pt 0.25-Rh 0.75(100) alloy surface used as a model catalyst for the automotive three-way catalyst. This paper emphasizes the mechanisms of the different reactions leading to the products dinitrogen, ammonia and nitrous oxide. For this purpose the reaction was studied under various experimental conditions including reactivity measurements both in the 10 −7 mbar range under steady-state conditions and in the 10 mbar range with varying NO/H 2 ratio. In addition, the thermal decomposition of NO and the reactions of NO + NH 3 were investigated. 15NO and 15NH 3 were used in order to gather additional information concerning the mechanisms of the formation reactions of the various N-containing products. The surface was characterized by using low-energy electron diffraction. Auger electron spectroscopy and thermal desorption spectroscopy. The main conclusions emerging from these studies are: (a) N 2 can be formed by combination of 2 N adatoms in the whole temperature range used (350–1300 K) provided that sufficient N adatoms are available. (b) Below 600 K the main contribution to N 2 formation is via NO ads + N ads → N 2+ O ads . At higher temperatures the dominant mechanism is 2N ads → N 2. (c) N 2O and NH 3 are formed via N ads + NO ads → N 2 O, and N ads + 3 H ads → NH 3 the contributions of which respectively decre increase with increasing temperature, (d) The selectivities to N 2, NH 3 and N 2O are determined by the relative concentrations of NO ads, N ads and H ads which vary with the experimental conditions such as the temperature.

Identifying the functions of nickel in the attenuation of H 2S emissions from three-way automotive catalysts

The design of new automotive catalyst formulations requires development of laboratory tests which mimic the vehicle exhaust-gas fluctuations encountered by a three-way converter. Under specific vehicle operating conditions, an unmodified three-way catalyst (comprising precious metals on CeO 2-Al 2O 3) can emit a ‘spike’ of hydrogen sulphide. A laboratory test to emulate these spike conditions requires a gas-feed that contains SO 2, a sudden change in gas composition from fuel-lean (oxidising) to fuel-rich (reducing), and an operating temperature > ca.500°C. The emission of H 2S can be prevented by adding a nickel-containing precursor to the catalyst. Using a simplified gas mixture (H 2S/N 2) it can be shown that surface NiO is capable of acting as an ‘H 2S getter’ and that it can be reactivated by exposure to an oxidising atmosphere. However, tests in which spike conditions have been emulated do not show a simple correlation between attenuation and sulphur retention by NiO. The results are considered in the light of a recent computational study which indicated that NiO should form NiSO 4 under lean conditions, and then release oxygen (or consume hydrogen) when the gas-stream becomes reducing. We conclude that nickel-containing automotive catalysts can attenuate H 2S in the following ways: 1. by preventing its formation; 2. by trapping it once it appears in the gas-phase.

Mass spectrometric characterization of performance of a low-temperature oxidation catalyst

A low-temperature oxidation catalyst (monolithic Pt-TiO 2) has been characterized by studying the oxidation of dimethyl methylphosphonate (DMMP) at 350–523 K under low pressure conditions. The concentrations of reagents (DMMP and O 2) and products (CO, CO 2 and H 2O) were directly sampled and analyzed with a computer-controlled mass spectrometer during the course of reaction. The major oxidation products detected were CO 2 and H 2O. The reaction was studied as a function of temperature, pressure and DMMP/O 2 ratios. The apparent activation energies of formation of CO 2 and H 2O were found to be 3.6 and 4.2 kcal mol −1respectively. A comparison of performance between the monolithic catalyst and three-way pelleted automotive catalysts was made. It was found that the monolithic catalyst shows performance characteristics superior to three-way automotive catalysts in terms of operating temperature, DMMP destruction efficiency and reaction products. The difference in performance of these two catalysts may be due to the configuration of the substrate and Pt loading. Results of surface spectroscopic analysis (SAM and XPS) reveal the formation of P 2O 5 on catalyst surfaces.

Periodic operation effects in propane and propylene oxidation over noble metal catalysts

It had been observed that under cycling conditions, the catalytic activities of three-way automotive catalysts are superior to those under static conditions, and that catalyst performance depends on the cycling period and feedstream conditions. The oxidation of propylene and propane over platinum, palladium and rhodium catalyst under static and periodic conditions was therefore investigated. The results were as follows; (i) the order of catalytic acitivities was Pd>Pt>Rh for propylene—oxygen and Pt>Rh>Pd for propane-oxygen; (ii) the periodic operation effect is dependent on both catalyst species and reaction temperature; (iii) the optimum period for the maximum conversion decreases with increasing temperature; and (iv) from results of the partial reaction order and evolution pattern analysis, either hydrocarbons or oxygen self-poison the reaction on the catalysts. The order of sensitivity to self-poisoning corresponds to the order of the periodic operation effect.

Electron microscopy study of a rejuvenated vehicle-aged automotive exhaust catalyst

Electron probe microanalysis (EPMA) and analytical electron microscopy (AEM) were used to characterize the changes that occur on a vehicle-aged three-way automotive catalyst as a result of two rejuvenation treatments. One treatment involves washing a catalyst with an aqueous solution of oxalic acid followed by calcination. This treatment results in the redistribution of phosphorus- and lead-based poison deposits, but leaves silicon- and sulfur-containing species in place. Noble metal radial profiles within the catalyst appear to be unaffected. Some evidence was found that this washing treatment may reverse the segregation of Pd in multimetallic Pt/Pd particles. The second treatment involves the redispersion of the noble metals by a high-temperature chlorination. This process has little effect on poison or noble metal radial distributions within the catalyst pellet. However, substantial reduction in the noble metal particle size was observed; particle size distributions which are comparable to those observed on fresh catalysts were produced by this treatment. The redispersed noble metal particles were generally multimetallic, indicating little bulk separation of the noble metals from the multimetallic particles originally present on the aged catalyst.

Platinum-rhodium synergism in three-way automotive catalysts

The interactions between Pt and Rh in three-way automotive catalysts were investigated by conducting laboratory reactor experiments with both a Pt Rh bimetallic catalyst (prepared by stepwise metal impregnation) and a physical mixture of Pt and Rh monometallic catalysts while holding the absolute amount of each of the noble metals constant. Activity measurements in a CONOO 2 feedstream have shown that the CO oxidation activity of the bimetallic catalyst is substantially higher than that of the physical mixture over the concentration and temperature ranges characteristic of the converter warmup period. This synergistic enhancement of CO oxidation activity in the bimetallic catalyst can be rationalized by visualizing the catalyst surface structure as a statistical mixture of Pt and Rh sites. It was found that such a desirable surface structure (and the attendant synergistic activity enhancement) cannot be obtained by simultaneous impregnation of the noble metals because this catalyst preparation procedure leads to preferential Rh enrichment on the catalyst surface under the net-oxidizing reaction conditions considered in this study.