Commercial Three-way Catalyst Research Articles

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 NOx storage component with cordierite support material.

Ageing and SO2 resistance of Pd containing perovskite-type oxides

The La0.91Mn0.85Ce0.24Pd0.05Oz and La1.034Mn0.966Pd0.05Oz perovskite-type oxides were prepared via a combination of the sol–gel and the combustion synthesis methods. They were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive X-ray, high resolution transmission electron microscopy and electron energy loss spectroscopy. The effect of ageing and SO2 on the catalytic effectiveness of the Pd/perovskite and of a Pd-based commercial three-way catalyst for stoichiometrically operating natural gas fuelled vehicles were investigated. The La1.034Mn0.966Pd0.05Oz catalyst exhibited the highest thermal stability and, in contrast to the La0.91Mn0.85Ce0.24Pd0.05Oz and the commercial catalyst, it appeared significantly activated in the presence of 8ppm SO2 in the reactant mixture. The formation of La–Pd mixed oxides or/and La2(Mn–Pd)O6, the enhanced surface Pd2+ composition and the low surface sulfur content were suggested as the possible reasons for the activation under sulfur environment.

Enhancement effect of water on oxidation reactions over commercial three-way catalyst

The light-off temperature of the oxidations of CO and C3H6 over a commercial three-way catalyst (TWC) was shifted to a lower temperature by the addition of water to the feed stream. From FTIR analysis, the formation of carboxylate and carbonate by a reaction between adsorbed CO and OH on the catalyst surface was observed during the course of the reactions. In addition, the CO desorption peak of CO-TPD shifted to a lower temperature by water, which may be directly indicative of the reduction of the adsorption strength of CO on the catalyst surface. Detailed reaction kinetic models have been developed in view of the concept of microkinetics to describe the effect of water, particularly on the oxidation activity over a commercial TWC.

The effect of Fe on the catalytic behavior of model Pd-Rh/CeO2-Al2O3 three-way catalyst

The present work attempts to address the issue whether iron (Fe) which is accumulated on the surface of “three-way” catalysts (TWCs) used in gasoline-driven cars is a true chemical poison of their catalytic activity. This important issue from a scientific and technological point of view is addressed via catalytic activity, temperature-programmed surface reaction (TPSR), and X-ray photoelectron spectroscopy (XPS) measurements over a model TWC (1wt% Pd–Rh/20wt% CeO2–Al2O3). It was found that deposition of Fe up to the level of 0.4wt% (an average concentration found in aged commercial TWCs) on the model TWC does not deteriorate its activity towards CO and C3H6 oxidation, and reduction of NO by H2. Instead it was found that iron improves significantly the T50 parameter in the activity versus temperature profile. Small Fe clusters in contact with the noble metal (Pd and Rh) particles due to the lower work function of Fe compared to Pd and Rh act likely as a source of electron flow towards the noble metals (as evidenced by XPS measurements), thus altering their surface work function and adsorption energetics of reaction intermediates. The latter have increased significantly the activity of the model TWC towards oxidation of CO and propylene, and to a lesser extent the activity towards the reduction of NO by H2. The presence of Fe on the surface of the model TWC provided and/or created also new active catalytic sites for the reactions investigated. According to previous work from this laboratory, iron up to the level of 0.4wt% was shown not to deteriorate the oxygen storage capacity (OSC) of the same model TWC used in the present work. Thus, it could be concluded that Fe when deposited on a commercial TWC at least up to the level of 0.4wt% acts likely as a promoter than a poison of its catalytic activity.

Efficient In-Situ Regeneration Method of the Catalytic Activity of Aged TWC

The expansion of durability of deactivated “three-way” catalysts (TWCs) used in gasoline-driven cars by applying efficient, economically viable and environmentally friendly methods for the in␣situ regeneration of their performance to acceptable levels was investigated. New experimental results on the use of a weak oxalic acid washing solution as a means of an efficient regeneration method of a severely aged (83,000 km mileage) commercial TWC are presented. Oxalic acid is shown to be the most efficient extracting agent of phosphorus, a severe poison of TWCs, among acetic acid, citric acid, NTA and EDTA investigated. X-ray diffraction studies provided strong evidence that washing of the aged TWC results in the removal of CePO4, AlPO4 and (Mg,Ca,Zn)3(PO4)2 type phosphates leading to a significant increase of BET area and pore volume, as well as of CO and NO conversions (catalytic activity tests). The latter is strongly related with the increase in the number of active catalytic sites, as illustrated by in␣situ DRIFTS studies, after opening closed pores and uncovering additional catalyst surface.

Effects of catalyst aging on the activity and selectivity of commercial three-way catalysts

A Pd catalyst is particularly effective for the oxidation of CO and C3H6 at low temperatures, while the Pt/Rh/Ce catalyst is active for NO reduction. The TWC activity of both catalysts generally decreased as the catalyst mileage increased. However, the NO reduction activity was less affected by catalyst aging compared to the oxidation reactions. The selectivity of the catalysts in favor of the CO–O2 reaction (vs. C3H6–O2 reaction) in the O2 partitioning experiments became less pronounced as the catalyst aged. The NO partitioning experiments reveal the superior capability of H2 in NO reduction to the other reductants (CO and C3H6) examined in the present study. The reactivities of NO with both H2 and CO were found to decrease upon catalyst aging, resulting in decreased overall NO removal activity.

Ir/ZSM-5/cordierite monolith for catalytic NOx reduction from automobile exhaust

Ir/ZSM-5/cordierite was studied as promising catalyst for the reduction of nitrogen oxides from automobile exhaust under both stoichiometric combustion and lean conditions. ZSM-5 was in situ synthesized on the surface of honeycomb cordierite substrate and iridium was introduced to ZSM-5/cordierite by wet impregnation. Under stoichiometric combustion, Ir/ZSM-5/cordierite exhibited good catalytic activity approaching commercial three-way catalysts, and the three main pollutants nitrogen oxides, carbon monoxide and unburned hydrocarbons could be purified simultaneously. Under lean conditions, Ir/ZSM-5/cordierite also exhibited quite good deNOx activities though excess oxygen showed inhibition effect on NOx reduction.

Detailed reaction kinetics over commercial three-way catalysts

A detailed reaction kinetic model to predict the activity of the commercial three-way catalyst (TWC), particularly 4k miles aged catalyst in an engine dynamometer, has been developed by considering all possible reactions in the catalytic converter based upon the Langmuir–Hinshelwood mechanism. The model predictions adequately describe the general trend of the measured conversions of CO, C 3 H 6 , NO and H 2 . NH 3 and N 2 O formed during the reaction can also be predicted by the model. Moreover, the decrease in activity of the 100k miles aged catalyst can be well described by the model developed in the present study.

Reactivation of severely aged commercial three-way catalysts by washing with weak EDTA and oxalic acid solutions

Ethylene diamine tetraacetic acid (EDTA) which is a well-known reagent for its metal extraction efficiency was studied for the first time towards the improvement of the catalytic activity and oxygen storage and release properties (OSC) of severely aged commercial three-way catalysts (TWC) on a laboratory scale. Optimization of the experimental parameters of EDTA-washing procedure of TWC was carried out by varying the washing time, volumetric flow rate, and temperature of EDTA solution. The EDTA-washing procedure of TWC was compared with that of oxalic acid-washing regarding their efficiency in removing P, Pb, S, Ca, Zn, Fe, Cu, Cr, Ni, and Mn, all known TWC contaminants that many of them cause severe deterioration of TWC's activity and oxygen storage and release properties. EDTA appears to be significantly efficient in removing Pb, Zn, Ca, Mn, Fe, Cu and Ni metal contaminants and sulfur but not of phosphorus (P). Phosphorus-containing species were found to be efficiently removed from the aged TWCs after oxalic acid washing. All regeneration procedures applied led to a significant partial recovery of catalytic activity of TWC (CO, CxHy and NOx conversions) under real exhaust gas conditions (dynamometer tests) due to the removal of large amounts of contaminants accumulated on the aged TWC. The washing procedures using oxalic acid alone or in combination with EDTA led to more significant improvements of both catalytic and OSC performance compared with those of EDTA washing alone. This was due to the ability of oxalic acid to remove P-containing compounds which appear to be one of the main causes of commercial three-way catalyst deactivation.

An investigation of a new regeneration method of commercial aged three-way catalysts

The ability to modify aged three-way catalysts (TWC) by regaining part of the fresh catalyst surface structure has been verified by both bulk and surface-sensitive characterisation techniques. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM/EDX) techniques were applied to fully evaluate the efficiency of a regeneration procedure of commercial three-way catalysts targeting the washcoat surface. The regeneration comprised combined thermal and liquid chlorine treatments. Structural changes of the washcoat were further investigated with nitrogen adsorption-desorption (BET) and Laser Ablation methods. The investigation showed that the regeneration treatments resulted in an enrichment of the washcoat surface with palladium, thereby increasing the number of catalytically active surface sites. Furthermore, the observed removal of phosphorous and sulphur contaminants resulted in an increase of the relative amount of small pores between 1 and 10nm and washcoat surface area. An increased catalytic activity regarding CO, NOx and HC emissions was observed after regeneration, providing proof of the proposed concept.

Reactivation of an Aged Commercial Three-Way Catalyst by Oxalic and Citric Acid Washing

The efficiency of dilute oxalic and citric acid solutions on improving the oxygen storage capacity (OSC) and catalytic activity of a severely aged (83,000 km) commercial three-way catalyst (TWC) has been investigated. Washing procedures applied after optimization of experimental parameters, namely, temperature, flow-rate, and concentration of acid solution, led to significant improvements of OSC and catalytic activity (based on dynamometer test measurements) of the aged TWC. The latterwas made possible due to the removal of significant amounts of various contaminants accumulated on the catalyst surface (e.g., P, S, Pb, Ca, Zn, Si, Fe, Cu, and Ni) during driving conditions, as revealed by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) and X-ray Photoelectron Spectroscopy (XPS) analyses. For the first time, it is demonstrated that dilute oxalic acid solution significantly improves the catalytic activity of an aged commercial TWC toward CO, Cx,Hy, and NOx conversions under real exhaust gas conditions (dynamometer tests) by two to eight times in the 250-450 degrees C range and the OSC quantity by up to 50%. Oxalic acid appears to be more efficient than citric acid in removing specifically P- and S-containing compounds from the catalyst surface, whereas citric acid in removing Pb- and Zn-containing compounds, thus uncovering surface active catalytic sites.

The effects of the use of weak organic acids on the improvement of oxygen storage and release properties of aged commercial three-way catalysts

The efficiency of weak organic acid solutions for improving the oxygen storage and release properties of aged commercial three-way catalysts (TWC) has been investigated. For the first time, three candidate leaching solutions, namely acetic acid (CH3COOH), oxalic acid ((COOH)2) and citric acid (C6H8O7) were tested for their efficiency in removing various contaminants, such as P, Fe, Zn, Cr, Pb, Ni and Cu that potentially reduce the oxygen storage capacity (OSC) of three-way catalysts. It was shown that over a severely aged (242,000km) commercial TWC, oxalic acid appears to be the most efficient in removing P and Cu, while citric acid in removing Zn, Fe, Cr, Pb and Ni. For all three weak organic acid solutions investigated, higher quantities of contaminants were removed with increasing aging of TWC. All regeneration procedures applied led to the partial recovery of the OSC of aged TWCs likely due to the removal of at least some of the contaminants mostly responsible for their negative effect on the processes of oxygen storage and release in CeO2 and CexZr1−xO2 “oxygen storage” materials present. It was found that oxalic acid leads to the largest improvement of OSC quantities, very likely because of its ability to remove P, the latter being known to cause severe reduction of OSC. Based on transient redox experiments in measuring the dynamic OSC of TWC (O2/He→He→CO/He), it was found that the peak maximum position of the transient response curve of CO2 obtained over the regenerated TWC samples shifts to higher reaction times compared to that of aged TWC. This behavior is very likely related to the increase of the rate constant (k) of the oxygen back-spillover step, based on mathematical simulations of the same transient experiment performed on a model Pd/CeO2 catalyst.

Effects of the CePO 4 on the oxygen storage and release properties of CeO 2 and Ce 0.8Zr 0.2O 2 solid solution

In an attempt to gain fundamental information about the role of P on the chemical deactivation of a three-way catalyst (TWC), P was deliberatively incorporated in CeO 2 and Ce 0.8Zr 0.2O 2 solid solution. XRD, Raman, XPS, and N 2 physisorption studies have indicated that monazite, a Ce(III) phosphate (CePO 4) solid phase, is located at the surface/subsurface region of both solids. Different experimental approaches were used to investigate the alteration by P incorporation of the redox chemistry of CeO 2 and Ce 0.8Zr 0.2O 2 solids that is relevant to the oxygen storage and release properties of the current commercial TWCs. In particular, H 2-TPR, dynamic OSC measurements using CO and H 2 pulses, 18O 2 temperature-programmed isotopic exchange (TPIE), and 18O 2 transient isothermal isotopic exchange have been used. These studies revealed that the amount of labile surface and bulk oxygen and the bulk oxygen diffusion rates are significantly reduced when the solids are contaminated with phosphorus. The monazite present in the surface and subsurface region of the solid is proposed to be responsible for these results. The reduction of Ce(IV) present in the CeO 2 and Ce 0.8Zr 0.2O 2 solids is also affected. Ce(III) in monazite is very stable and cannot be oxidized to Ce(IV). Therefore, it cannot participate in the redox Ce(IV)/Ce(III) couple needed for the reduction of Ce(IV). Moreover, the formation of bulk and surface O vacant sites needed for the O storage and release steps involved in the OSC process was also found to be affected. The results of the present work provide support to the hypothesis that formation of monazite is one of the main chemical deactivation mechanisms of real TWCs since CePO 4 was found in deactivated vehicle-aged TWCs.

Effects of calcination temperature on the stability of CePO4 detected in vehicle-aged commercial three-way catalysts

Samples extracted from a vehicle-aged commercial TWC monolith (the inlet part of the first cartridge) have been shown to contain CePO4. The effects of calcination temperature in the 773–1173K range on these samples have been explored aiming at studying the stability against oxidation of the detected CePO4. The recovery of the Oxygen Storage Capacity (OSC) and therefore of the catalytic properties of the aged TWC by means of decomposition of CePO4 has also been explored; the CePO4 is suspected to play a role in the chemical deactivation of TWC through OSC damaging. XRD studies evidenced that CePO4 was very stable against calcination even at the high temperature of 1173K. X-ray photoelectron spectroscopy (XPS) analysis confirmed the stability of the Ce(III) present in the aged sample. Tests with a simulated exhaust gas composition showed that the calcination procedures applied did not regenerate the catalytic activity of the aged TWC. In addition, temperature-programmed reduction (TPR) studies in H2/He flow and CO/O2 pulses have shown that the OSC of the aged sample was not recovered either. Although other effects can be responsible for TWC deactivation, the results of the present work support the view that CePO4 remains one of the reasons for the deactivation of TWC. It is proposed that the deactivation occurs through the formation of patches or islands of this solid phase around the Ce–Zr–O mixed oxide and CeO2 crystal particles, thus reducing significantly the rates of oxygen storage and back-spillover processes occurring through the participation of these oxides and the precious metal surface.

Mathematical modeling of the oxygen storage capacity phenomenon studied by CO pulse transient experiments over Pd/CeO 2 catalyst

A mathematical model has been developed for the first time to study the oxygen storage capacity (OSC) phenomenon by the CO pulse injection technique over a 1 wt% Pd/CeO 2 model catalyst in the 500–700 °C range. A two-step reaction path that involves the reaction of gaseous CO with the oxygen species of PdO (pre-oxidized supported palladium particles in the 500–700 °C range) and of the back-spillover of the oxygen process from ceria to the oxygen vacant sites of surface PdO has been proven to better describe the outlet CO pulse transient response and the experimentally measured quantity of OSC (μatoms of O/g) obtained in a CSTR microreactor. With the proposed mathematical model, the transient rates of the CO oxidation reaction and of the back-spillover of the oxygen process can be calculated. In the 500–700 °C range, the transient rate of CO oxidation was always greater than that of the back-spillover of oxygen. The ratio, ρ, of the maximum CO oxidation rate to the maximum back-spillover of the oxygen rate was found to decrease with increasing reaction temperature in the 500–700 °C range. In particular, at 500 and 700 °C the value of ρ was found to be 1.6 and 1.2, respectively. The present mathematical model allows also the calculation of the intrinsic rate constant k 1 (s −1) of the Eley–Rideal step for the reaction of gaseous CO with surface oxygen species of PdO to form CO 2. An activation energy of 9.2 kJ/mol was estimated for this reaction step. In addition, an apparent rate constant k 2 app (s −1) was estimated for the process of back-spillover of oxygen. The ratio of the two rate constants ( k 1/ k 2 app) was found to be greater than 100 in the 500–700 °C range. A Langmuir–Hinshelwood surface elementary reaction step of adsorbed CO with atomic oxygen of PdO failed to describe the experimental transient kinetics of CO oxidation in the 500–700 °C range. The results of the present work provide the means for a better understanding of the effects of various additives and contaminants present in a three-way commercial catalytic converter and other related model catalysts on their OSC kinetic behavior. In addition, intrinsic effects of a given regeneration method for a commercial three-way catalyst on the OSC phenomenon could better be studied by making use of the results of the present mathematical model.

Insight in the outside: New applications of low-energy ion scattering

Low energy ion scattering is used to selectively probe the outermost atomic layer of the surface. The development of double toroidal analyzers has improved the detection sensitivity of low energy ion scattering by orders of magnitude. The features of these analyzers are discussed. It is shown that the absence of matrix effects makes it possible to quantify the surface density of fluorine in polymers with a LiF(1 0 0) surface. The extreme surface sensitivity of LEIS also enables one to study intramolecular segregation processes. As an example the aging of a polypropylene surface that has been activated with atomic oxygen is described. As an example of the LEIS analysis of highly dispersed isolating surfaces, the formation of coke on a commercial three-way catalyst is discussed.

Noble Metal Segregation and Cluster Size of Pt/Rh/CeO2/γ-Al2O3 Automotive Three-Way Catalysts Studied with Low-Energy Ion Scattering

A method for determining the average cluster size of supported catalysts using low-energy ion scattering (LEIS) data is presented. The method is particularly suitable for determining the average cluster size of atomically dispersed metals and can be used for clusters as large as 10 nm. For Pt/γ-Al2O3 quantitative agreement is shown between the average cluster size as determined with LEIS and that determined with transmission electron microscopy (TEM). In the case of three-way catalysts, classic methods for determining the cluster size, such as TEM or CO chemisorption, often fail or produce ambiguous results because of the presence of ceria. Therefore, LEIS has been applied to determine the average noble metal cluster size of a commercial three-way catalyst. Moreover, it is shown by LEIS that the three-way catalyst contains mixed Pt/Rh clusters, which are strongly enriched in surface Pt after reduction.

Sol–Gel Pd Exhaust Catalysts and Hydrocarbon Speciation

Sol–gel Pd/alumina promoted by oxygen-storing CeO2 or CeO2–Tb4O7 catalysts has higher hydrocarbon oxidation activity than commercial three-way catalysts. It is active in hydrocarbon (and CO) oxidation even under net reducing conditions albeit over short periods of time. This observation is relevant to cold start conditions. Cryo-focussing GCMS and pulse RGA experiments suggest that mechanistically interesting by-product hydrocarbon species evolve from the catalysts under just these conditions, even when the fuel is LPG in type. Further catalyst developments and analytical refinements are likely to go hand in hand as the mode of operation of these environmental catalysts is better understood from their by-product fingerprint.

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/γ-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.

The effect of microwave energy on three-way automotive catalysts poisoned by SO2

Recent studies suggest that microwave energy can be employed in catalysis and that the results differ from “conventional” heating. The influence of microwave energy on automotive exhaust catalysis in the presence and absence of a catalyst poison (SO2) was studied. The conclusion is that microwave energy can induce carbon monoxide (CO) lightoff (the temperature where 50% of final conversion is achieved) more efficiently than conventional heating and can reverse the poisoning by SO2 for a commercial three-way catalyst.