Commercial Three-way Catalyst Research Articles

Thermal deactivation mechanism of the commercial Pt/Pd/Rh/CeO2-ZrO2/Al2O3 catalysts aged under different conditions for the aftertreatment of CNG-fueled vehicle exhaust

Commercial three-way catalysts with high-activity and long-term durability are critical for the aftertreatment of compressed natural gas vehicle exhausts to meet increasingly stringent policy requirements, but high concentrations of CH4 and H2O in the exhaust gas make the catalyst susceptible to deactivation. In this work, a series of commercially deactivated and aged honeycomb Pt/Pd/Rh/CeO2-ZrO2/Al2O3 catalysts were obtained by dry-aging, hydrothermal-aging and engine bench-aging methods, and the effects of aging conditions on catalytic performance, surface composition and microstructure evolution were investigated. The combinations of H2-TPR, XRD, Raman, HRTEM, XPS and in situ DRIFTS of CO/CH4 characterizations confirmed that the stability of CeO2-ZrO2 solid solution structure is a key factor affecting the catalytic activity and durability. Hydrothermally-aged catalyst has a relatively stable CeO2-ZrO2 solid solution structure, which ensures a high dispersion of precious metal particles in the form of small particles on the catalyst surface. Dry-aged catalyst shows that more CeO2 crystals have been segregated from CeO2-ZrO2 solid solution, resulting in most precious metal particles in the form of agglomerates. For the deactivated catalysts after engine bench aging, the microstructure was significantly changed with a large number of CeO2 crystals detached from CeO2-ZrO2 solid solution, resulting in more active sites of Platinum Group Metal (PGM) being encapsulated. In situ DRIFTS of CH4 experiments showed that the process of generation and depletion of intermediate species (e.g., methoxy, formate and bicarbonate species) is a matching process. For the deactivated catalysts with fewer active sites, the subsequent reaction steps of these intermediate species to CO2 and H2O are hindered by the accumulation of stable formate and bicarbonate intermediate species.

Experimental and Theoretical Characterization of Rh Single Atoms Supported on γ-Al2O3 with Varying Hydroxyl Contents during NO Reduction by CO

Oxide-supported Rh catalysts are important components of commercial three-way catalysts for pollution abatement. Despite their universal application, many mysteries remain about the active structure of Rh on oxide supports as these materials often contain a mixture of nanoparticles and single-atom Rh species on the same support, even after aging. Probe molecule Fourier transform infrared (FTIR) spectroscopy in this work shows that atomically dispersed Rh on γ-Al2O3 prefer to strongly bind CO when exposed to NO and CO mixtures and that light-off of NO reduction occurs at temperatures similar to CO desorption, suggesting that the first and rate-determining step in NO–CO reactions may be the desorption of CO from single-atom Rh dicarbonyl complexes, Rh(CO)2. Two sets of symmetric and asymmetric stretching frequencies associated with distinct Rh(CO)2 species are observed in FTIR spectra at 2084/2010 and 2094/2020 cm–1. During temperature ramps, the latter pair of bands at 2094/2020 cm–1 converts to the 2084/2010 cm–1 bands at 463 K before all symmetric and asymmetric bands disappear at 573 K. Bands then appear in the range of 1975–1985 cm–1 associated with Rh monocarbonyl, Rh(CO), species upon the disappearance of the 2084/2010 cm–1 bands, suggesting that CO desorbs sequentially from Rh(CO)2 by forming Rh(CO) intermediates. Combined DFT and FTIR experiments suggest that local OH coverage on the γ-Al2O3 surface distinguishes the two Rh(CO)2 species: the higher frequency species resides on a less hydroxylated region and migrates to a more hydroxylated region at higher temperatures, causing the CO vibrational frequency to decrease by ∼10 cm–1. CO desorption occurs from this Rh(CO)2 structure with high local OH coverage, consistent with the DFT predicted trend of CO binding energies. Because of the coincidence of CO desorption with the light-off of NO reduction, local support hydroxylation of atomically dispersed Rh1/γ-Al2O3 catalysts likely affects both the Rh structure after CO desorption and the kinetics of NO reduction, studies of which are enabled by the Rh(CO)2 model developed here.

Control of the size of Pd nanoparticles by pre-precipitation for anti-sintering three-way catalysts

Restricting the Pd sintering during aging is crucial for the three-way catalysts (TWCs). Herein, following the Ostwald-ripening mechanism, Pd sintering in the commercial TWCs was greatly suppressed by increasing the initial size of the Pd NPs, through a pre-precipitation method. Our catalysts showed initially larger Pd NPs after calcination at 550 ℃ (3.3 vs. 1.8 nm) while smaller ones after aging at 1000 ℃ (35.0 vs. 70.1 nm) than the counterpart made by impregnation. The optimal aged catalyst showed much better performance than the counterpart: T50 was decreased by 40, 13 and 14 ℃ for CO, NO and C3H6 over the powder samples, and a higher oxygen storage capacity was observed (248 > 178 mg/L) over the monolith converters. The decreased size of the Pd NPs and the improved reducibility as well as the more concentrated adsorbed oxygen species for the optimal aged catalyst were responsible for the improved performance.

Relationship between design strategies of commercial three-way monolithic catalysts and their performances in realistic conditions

Monolith-shaped three-way catalysts (TWC) are always at the edge of science and technology as a response to the stringent updates on the emission standards worldwide. In spite of rapid research on TWC materials, the exact compositions adapted by major automotive companies are not fully known. This study focuses on comparing two fresh commercial monolithic catalysts for gasoline engines, namely, monolith-A and monolith-B, to highlight the significance of their manufacturing strategies by its characterization and catalytic evaluation under realistic lean/rich/stoichiometric regimes. Firstly, monolith-A demonstrates a zoning on its front and back side with high Pd and Rh loading to enhance CO/HC oxidation and NOx reduction, respectively, having higher amounts of trace dopants like La, Y, Pr and S. Whereas, monolith-B has a homogeneous composition with much lower noble metal/trace elements content without any Pr and S. Secondly, the double-layer washcoating is employed by following different routes with distinct compositions of alumina and ceria-zirconia. Monolith-B showed better low temperature CO oxidation performance while monolith-A performed better for NOx reduction owing to higher Rh loading. Overall, the zoning strategy of monolith-A raises question on its cost effectiveness in regards to its performance while poor reduction capabilities is exhibited by monolith-B. This study highlights that the latest TWC strategies have a scope of further development particularly with respect to the catalytic performance/expense ratio.

Characterization of the Evolution of Noble Metal Particles in a Commercial Three-Way Catalyst: Correlation between Real and Simulated Ageing

Ageing of automotive catalysts is associated to a loss of their functionality and ultimately to a waste of precious resources. For this reason, understanding catalyst ageing phenomena is necessary for the design of long lasting efficient catalysts. The present work has the purpose of studying in depth all the phenomena that occur during ageing, in terms of morphological modification and deactivation of the active materials: precious metal particles and oxidic support. The topic was deeply investigated using specific methodologies (FT-IR, CO chemisorption, FE-SEM) in order to understand the behavior of metals and support, in terms of their surface properties, morphology and dispersion in the washcoat material. A series of commercial catalysts, aged in different conditions, have been analyzed, in order to find correlations between real and simulated ageing conditions. The characterization highlights a series of phenomena linked to the deactivation of the catalysts. Pd nanoparticles undergo a rapid agglomeration, exhibiting a quick loss of dispersion and of active sites with an increase of particles size. The evolution of the support allows highlighting also the contribution of chemical ageing effects. These results were also correlated with performance tests executed on synthetic gas bench, underlining a good correspondence between vehicle and laboratory aged samples and the contribution of chemical poisoning to vehicle aged ones. The collected data are crucial for the development of accelerated laboratory ageing protocols, which are instrumental for the development and testing of long lasting abatement systems.

Thermal Ageing Effects in a Commercial Three-Way Catalyst: Physical Characterization of Washcoat and Active Metal Evolution

The main system to reduce engine emissions of gasoline-fueled vehicles is represented by the three-way catalyst (TWC). The main catalyst of this system usually works in extreme conditions which induce a phenomenon called thermal ageing, the major way of catalyst deactivation. Increasingly stringent emissions standards require not only high efficiency in emissions abatement, but also maintaining the performances at high vehicle mileage. Automotive manufacturers need to accelerate testing methods and that should be representative of the ageing on vehicle, at the minimum expense in terms or time and cost. In order to develop and validate such methods, a deep understanding of all the microscopic processes associated with ageing is necessary. In this work we discuss a methodology for the study of the static thermal ageing of a commercial TWC. A specific washcoat extraction method was developed, and the separated washcoat was analyzed using XRD, TEM microscopy, Raman Spectroscopy and BET. These techniques allow to follow the main morphological and chemical changes in the sample: the loss of porosity, the evolution of crystalline phases and metal particles sintering and oxidation. The potential of Raman spectroscopy for the spatially resolved analysis of ageing is also highlighted. The results allowed to understand the process that involve the washcoat at high temperature.

Reactivity of novel high-performance fuels on commercial three-way catalysts for control of emissions from spark-ignition engines

The Department of Energy “Co-Optimization of Fuels and Engines” initiative aims to simultaneously develop novel high-performance fuels with advanced engine designs to reduce petroleum consumption. To achieve commercialization, advanced engines running on alternative fuels still must meet emissions regulations. Warm three-way catalysts (TWC) are very effective at meeting the stringent emissions regulations on pollutants such as nitrogen oxides (NOx), non-methane organic gases (NMOG) and carbon monoxide (CO) from gasoline-fueled spark-ignition (SI) engines operating under stoichiometric conditions; thus, most SI engine emissions occur during cold-start, when the TWC has not yet achieved light-off. In the current study, the light-off behavior of novel high-performance fuel candidates has been investigated on a hydrothermally-aged commercial TWC using a synthetic engine-exhaust flow reactor system according to industry guidelines. Over 30 potential fuel components were examined in this study, including alkanes, alkenes, alcohols, ketones, esters, aromatic ethers, and non-oxygenated aromatic hydrocarbons. Short-chain acyclic oxygenates, including alcohols, ketones, and esters, tended to light off at relatively low temperatures, while alkenes, aromatics, and cyclic oxygenates tended to light off at relatively high temperatures. The light-off behavior of alkanes and alkenes depended strongly on their size and structure. In terms of the influence on CO light-off on the TWC, the fuels fell into two distinct categories: (i) non-inhibiting species including C2-C3 alcohols, alkanes, acyclic ketones, and esters; and (ii) inhibiting species including alkenes, aromatic hydrocarbons, cyclic oxygenates, and C4 alcohols.

Remarkable active-site dependent H2O promoting effect in CO oxidation

The interfacial sites of supported metal catalysts are often critical in determining their performance. Single-atom catalysts (SACs), with every atom contacted to the support, can maximize the number of interfacial sites. However, it is still an open question whether the single-atom sites possess similar catalytic properties to those of the interfacial sites of nanocatalysts. Herein, we report an active-site dependent catalytic performance on supported gold single atoms and nanoparticles (NPs), where CO oxidation on the single-atom sites is dramatically promoted by the presence of H2O whereas on NPs’ interfacial sites the promoting effect is much weaker. The remarkable H2O promoting effect makes the Au SAC two orders of magnitude more active than the commercial three-way catalyst. Theoretical studies reveal that the dramatic promoting effect of water on SACs originates from their unique local atomic structure and electronic properties that facilitate an efficient reaction channel of CO + OH.

Remarkable improvement in low temperature performance of model three-way catalysts through solution atomic layer deposition

The development of three-way catalysts with improved low temperature activity is essential for automotive catalysis. Here, we show that solution atomic layer deposition (SALD) of titania or zirconia promoters on alumina supports lowers the light-off temperatures of rhodium-based catalysts by 50–150 °C compared to a commercial benchmark three-way catalyst. X-ray diffraction, scanning transmission electron microscopy–electron energy loss spectroscopy, diffuse reflectance UV–visible spectroscopy and X-ray absorption near edge structure results indicate that titania incorporated by SALD at one monolayer loading is present primarily as atomically disperse 5-coordinate Ti4+ species. These species persist after exposure to steam and corrosive gases at temperatures up to 960 °C. Zirconia incorporated onto alumina by SALD is present as few-nanometre oxide particles and supports a three-way catalyst activity that is superior to that of Rh on either alumina or zirconia. Our results show that molecularly precise synthesis can lead to robust promotion of precious metal activity and provide a promising path towards reducing emissions from gasoline vehicles. Improving the performance of commercial three-way catalysts like rhodium on alumina is a major challenge considering the limited design space allowed for such systems. Now, solution atomic layer deposition is used to incorporate titania or zirconia promoters into this catalyst, leading to remarkable improvements in its overall performance.

Oxygen storage in three-way-catalysts is an equilibrium controlled process: Experimental investigation of the redox thermodynamics

A titration method is presented that measures the redox thermodynamics of oxygen storage in three way catalysts under typical operating conditions. The titration scheme starts with an oxygen pulse to completely oxidize the catalyst. The catalyst is then brought into equilibrium with an H2/H2O mixture and the resulting oxidation state is determined from the amount of H2 consumed to establish the equilibrium. Measurements were performed in the temperature range of 300°C–500°C for two commercial three-way catalysts containing a doped ceria-zirconia solid solution of the composition Ce0.404Zr0.552Nd0.044O1.978. The results confirm that under typical automotive operating conditions the oxygen available from the oxygen storage is limited by the equilibrium thermodynamics. Even under very rich conditions (8000ppm H2/10% H2O) at 500°C only about 20% of the theoretically available oxygen can be used. At the typical steady state operating point of a three-way catalyst (lambda sensor voltage of 650mV, 70ppm H2), at 500°C only about 6% of the ceria is reduced. Virtually identical oxygen storage performance is obtained for two catalysts with identical oxide composition but different precious metal loadings of 100g/ft3 and 10g/ft3.In a second part of the study, the dynamical oxygen breakthrough is studied experimentally in the H2/H2O/O2 system. First, the catalyst is brought into the typical steady state oxidation state and then it is exposed to an oxygen pulse of varying length.If the thermodynamic data is implemented in a kinetic model, the H2 and O2 signals during the dynamic oxygen breakthrough experiments are very well described. A sensitivity study shows that the H2 response to small O2 pulses, where no O2 breakthrough is observed yet, is completely controlled by the thermodynamics of the oxygen storage. As the O2 pulses get longer and O2 breakthrough is observed, besides thermodynamics also kinetics and internal as well as external mass transfer become important.

Photocatalytic treatment of automotive exhaust emissions

In the present work, the photocatalytic treatment of simulated automotive exhaust emissions was studied in an annular reactor operating in CSTR mode. Three feeds with different compositions were created, rich (λ=0.98), stoic (λ=1), and lean (λ=1.02) and five photocatalysts were tested: TiO2 isopropoxide, TiO2 P25 and TiO2-P25 with addition of 1% of Pt, Fe or Ce. Pt-TiO2 P25 showed the highest activity for NO and hydrocarbons (C3H6, C3H8) removal, with hydrocarbons oxidation being promoted in lean conditions of feed and NO reduction in rich condition of feed, behavior similar to commercial three-way catalysts. Additional work was carried out in order to study separately the main reactions that take place on the catalytic surface. For NO decomposition/oxidation reaction in the presence of water there was no NO2 formation observed and also no deactivation of the catalyst which indicates that decomposition reaction is the main reaction with good selectivity to N2 formation for these conditions. In C3H6 oxidation the increase of O2 inlet concentration leads to the increase of the reaction rate and in redox reaction of NO with C3H6 the results showed that a secondary reaction for C3H6 is required to explain the conversion values. The kinetic study of the reactions showed that Langmuir–Hinshelwood expression can interpret the experimental data assuming that NO and C3H6 are co-adsorbed on the same active sites, with dissociatively adsorption of C3H6 and O2 adsorption on different active sites.

Ultrastable Hydroxyapatite/Titanium-Dioxide-Supported Gold Nanocatalyst with Strong Metal-Support Interaction for Carbon Monoxide Oxidation.

Supported Au nanocatalysts have attracted intensive interest because of their unique catalytic properties. Their poor thermal stability, however, presents a major barrier to the practical applications. Here we report an ultrastable Au nanocatalyst by localizing the Au nanoparticles (NPs) in the interfacial regions between the TiO2 and hydroxyapatite. This unique configuration makes the Au NP surface partially encapsulated due to the strong metal-support interaction and partially exposed and accessible by the reaction molecules. The strong interaction helps stabilizing the Au NPs while the partially exposed Au NP surface provides the active sites for reactions. Such a catalyst not only demonstrated excellent sintering resistance with high activity after calcination at 800 °C but also showed excellent durability that outperforms a commercial three-way catalyst in a simulated practical testing, suggesting great potential for practical applications.

Integrated Performance Monitoring of Three‐Way Catalytic Converters by Self‐Regenerative and Adaptive High‐Temperature Catalyst and Sensors

This study reports the performance of a self‐regenerating perovskite, LaFeCoPdO3 as a three‐way catalyst (TWC) and its use for self‐diagnostic by means of integrated Al‐doped TiO2 and LaFeCoPdO3 sensor arrays, consisted of semiconducting oxides as sensing layers and LaFeCoPdO3 as catalytic filter. Although perovskite catalyst yields a reasonable NO conversion performance at lower temperatures, it cannot fully compete with a commercial TWC under the TWC relevant temperatures and conditions. On the other hand, as‐coated duplex layer exhibits reasonable sensor property toward NO2 at 600 °C but sensor response deteriorates in NO2 + CO mixed gas environment. Ageing tests in the harsh exhaust peripherals however yield that perovskite improves the duplex layer's response through crack network and by protecting semiconducting oxide.

Rh Supported on LaPO4/SiO2 Nanocomposites as Thermally Stable Catalysts for TWC Applications

Hydrothermally synthesized LaPO4/SiO2 nanocomposites were investigated as thermally stable support materials for Rh catalysts. The nanocomposites were composed of macroporous SiO2 and highly dispersed columnar LaPO4 crystallites of approximately 20 nm in thickness and 100 nm in length, the surface of which anchored Rh nanoparticles via Rh–O–P interfacial bonding. The microstructure was useful to prevent agglomeration and sintering of LaPO4 during high-temperature thermal aging under both oxidizing and reducing atmospheres. The Rh catalyst supported on the LaPO4/SiO2 composite exhibited higher catalytic activities for simulated NO–CO–C3H6–O2 reactions over a wide Rh loading range (0.01–0.4 wt%), compared to those of Rh catalysts supported on the individual components (LaPO4 and SiO2), and on ZrO2, which is widely used as a support for Rh in commercial three-way catalysts. Enhanced catalytic activities for elementary CO–O2, CO–H2O, and CO–NO reactions were achieved while maintaining the activity toward C3H6 as an advanced feature of phosphate supports.

Ammonia Generation over TWC for Passive SCR NOX Control for Lean Gasoline Engines

A commercial three-way catalyst (TWC) was evaluated for ammonia (NH3) generation on a 2.0-liter BMW lean burn gasoline direct injection engine as a component in a passive ammonia selective catalytic reduction (SCR) system. The passive NH3 SCR system is a potential low cost approach for controlling nitrogen oxides (NOX) emissions from lean burn gasoline engines. In this system, NH3 is generated over a close-coupled TWC during periodic slightly rich engine operation and subsequently stored on an underfloor SCR catalyst. Upon switching to lean, NOX passes through the TWC and is reduced by the stored NH3 on the SCR catalyst. NH3 generation was evaluated at different air-fuel equivalence ratios at multiple engine speed and load conditions. Near complete conversion of NOX to NH3 was achieved at =0.96 for nearly all conditions studied. At the =0.96 condition, HC emissions were relatively minimal, but CO emissions were significant. Operation at AFRs richer than =0.96 did not provide more NH3 yield and led to higher HC and CO emissions. Results of the reductant conversion and consumption processes were used to calculate a representative fuel consumption of the engine operating with an ideal passive SCR system. The results show a 1-7% fuel economy benefit at more » various steady-state engine speed and load points relative to a stoichiometric engine operation. « less

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 NOx 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.

Regeneration of Aged Commercial Three-Way Catalysts Using CI-Containing Reagents

The present work is focused on applying washing treatments using Cl-containing reagents, such as di-chloroacetic acid (DCA) for the first time, and 1,2-di-chloropropane, DCP (0.1 M) solutions in regaining the activity and oxygen storage capacity (OSC) of an aged commercial Pd-based TWC. Segments extracted from an aged (56,000 km) commercial Pd/Rh-based (9:1) TWC after washing with DCA resulted in 100 % removal of Pb and about 43 % of P, while washing with DCP solution was ineffective in removing these specific contaminants. DCA-washing regeneration method was found to increase Pd surface concentration by four times compared to the aged TWC, whereas DCP-washing was considerably less effective than DCA in recovering Pd active sites. The OSC of the aged catalyst was significantly improved by the use of DCA-washing, whereas no improvement was observed by DCP-washing. Nevertheless, DCP-washing was found to be more effective in improving the performance of aged TWC towards CO oxidation. The removal of Pb is likely a reason for the DCA-washing being more effective in improving C3H6 and NO conversions and OSC.

Activity function describing the effect of Pd loading on the catalytic performance of modern commercial TWC

A catalyst activity function for the commercial Pd-based three-way catalyst (TWC) has been developed to describe the effect of Pd loading ranging from 20 to 240g/ft.3 on its catalytic performance. Derived from the alteration of the Pd metallic surface area (MSA) due to the variation of the Pd loading, the activity function reasonably well captures the nonlinear dependence of catalytic activity on the Pd loading of the Pd-based TWCs. The overall reaction kinetics for the commercial TWC obtained from the combination of the catalyst activity function developed and the primary reaction kinetics over the 4k Pd240 reference catalyst has proven to be capable of well predicting the effect of Pd loading on the catalytic performance of the Pd-based TWCs. The accuracy of model predictions has been excellent for the oxidation of CO, C3H6 and H2, while leaving some room for improvement in the model accuracy for the reduction of NO and related byproducts.

Global Kinetics for Ammonia Formation and Oxidation Reactions in a Commercial Three-Way Catalyst

Lean-burn spark ignition direct injection (SIDI) engines offer significant potential for improving engine efficiency and reducing greenhouse gas emissions. However, NOx reduction in lean-burn SIDI engines presents significant challenges. One of the exhaust architectures that is currently being investigated and developed for lean-gasoline applications is the passive ammonia-SCR (three-way catalyst–selective catalyst reduction) system. It involves the use of the closed-coupled three-way catalyst (TWC) to generate NH3 (during fuel-rich conditions) for use by the downstream NH3–SCR for NOx reduction. However, the NH3 formed in TWC has to be controlled so that there is enough NH3 for NOx reduction in SCR, and at the same time, there is no NH3 slip in the tailpipe. A mathematical model for the TWC, which can predict the net NH3 coming out of the TWC, will be very useful for control algorithm development and for understanding and optimizing the exhaust architecture. The focus of this work is on the kinetic model...

Novel high-temperature reactors forin situstudies of three-way catalysts using turbo-XAS

Two novel high-temperature reactors for in situ X-ray absorption spectroscopy (XAS) measurements in fluorescence are presented, each of them being optimized for a particular purpose. The powerful combination of these reactors with the turbo-XAS technique used in a dispersive-XAS beamline permits the study of commercial three-way catalysts under realistic gas composition and temporal conditions.