Three-way Catalytic Reaction Research Articles

The structure–activity relationships of Rh/CeO2–ZrO2 catalysts based on Rh metal size effect in the three-way catalytic reactions

The structure–activity relationships of Rh/CeO2–ZrO2 catalysts based on Rh metal size effect in the three-way catalytic reactions

Selective Formation of Cu Active Sites with Different Coordination States on Pseudospinel CuAl2O4 and Their NO Reduction Catalysis.

In the spinel framework, copper (Cu) in two distinct coordination states exhibits catalytic activity for NO reduction through different mechanisms. However, detailed exploration of their respective catalytic properties, such as the redox behavior of Cu and substrate molecule adsorption, has been challenging due to difficulties in their separate formation. In this study, we present the controlled formation of pseudospinel CuAl2O4, containing exclusively tetrahedrally or octahedrally coordinated Cu, achieved by manipulating aging temperature and O2 concentration. Through these materials, we observed that in the CO-NO reaction, the step primarily determining the rate differs: NO reduction dominates with octahedrally coordinated Cu, whereas carbon monoxide (CO) oxidation is prominent with tetrahedrally coordinated Cu. The lower coordination number of Cu significantly benefits NO reduction but negatively impacts the CO-NO reaction, albeit positively influencing NO reduction in three-way catalytic reactions.

Study on the effect of hydrocarbon structure on the reactivity of the three-way catalytic converter

The effect of hydrocarbon structures on reduction of nitric oxide and oxidation of carbon monoxide and hydrocarbons on the palladium-rhodium catalyst supported on aluminum-cerium-zirconium oxide was examined with fixed-bed flow reactor using synthetic gas. As regards olefinic hydrocarbons, the lower the number of carbon atoms, the higher the rate of reduction of nitric oxide, the lower the number of carbon atoms, the greater the rate of oxidation of hydrocarbons and carbon monoxide. Olefinic hydrocarbons have a π bond and are highly adsorbed on catalytic sites. Consequently, it may be hypothesized that if the number of carbon atoms increases, the adsorption ability becomes too high and the reactivity of the hydrocarbons decreases. On the other hand, for paraffinic hydrocarbons, the greater the number of carbon atoms, the greater the rate of reduction of nitric oxide, and the more carbon atoms there are, the higher the oxidation rate of the hydrocarbon. In the case of paraffinic hydrocarbons, there are only σ bonds and less adsorption on the catalyst site, it is therefore assumed that the greater the number of carbon atoms, the greater the reactivity of the hydrocarbon. The results of the effects of these hydrocarbon structures on the overall three-way catalytic reaction activity provide an indicator of the development of new catalysts.

Promoting effect of basic metal additives on DeNOx reactions over Pt-based three-way catalysts

Pt has been economically favored over Rh and Pd. However, Pt-based three-way catalysts (TWCs) have been studied less than Pd- and Rh-based TWCs; therefore, their properties have not yet been sufficiently evaluated, especially under conditions relevant to modern three-way catalysis operation, wherein deviations from stoichiometric conditions are often encountered. In this study, we examined the promotional effect of basic metal additives in Pt/M/Al2O3 (M = Ba, La, Sr) on the efficiency of three-way catalytic reactions. The catalytic activities of the Pt/M/Al2O3 catalysts for NO, CO, and total hydrocarbons were superior to those of Pt/Al2O3; moreover, Ba enhanced the catalytic activity of Pt-based catalysts more efficiently than La and Sr. The best-performing catalyst, Pt/Ba/Al2O3, was subjected to kinetic studies and various in situ/operando spectroscopic experiments, namely, X-ray absorption spectroscopy, infrared spectroscopy, and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), to investigate the role of Ba in the NO reduction reaction. The Pt0 species loaded on Ba/Al2O3 were more electron-rich than those loaded on Al2O3, thereby promoting NO dissociation into N and O atoms, as revealed by the AP-XPS results. Moreover, the efficient formation of intermediate surface NOx species, including nitrites and nitrates, and their reactivities toward reductant gases, such as H2 or CO, were critical for promoting the effect of Ba on the NO reduction reaction.

Reaction gas-induced partial exsolution of Pd from PdCeMnO for methane combustion

Besides the Pd/Al2O3, the Pd-CeO2 catalysts are the indispensable and promising candidates for methane combustion. The Pd-CeO2 composites with intimate interaction realized by elaborate methods are regarded as the most active formulations. However, more facile and cost-efficient methods are needed for industrial application. Herein, the Pd was incorporated into the lattice of the Mn-doped CeO2 by an optimized co-precipitation method to synthesize the PdCeMnO-CP catalyst. The PdCeMnO-CP showed much higher catalytic activity with the T10 decreased by 70 °C than that of the Pd/CeMnO-Imp prepared with impregnation. The partial exsolution of the initially doped Pd species and the lowered valence of Pd (from Pd4+ to Pdδ+, δ: 2 ∼ 4) during the reaction were confirmed by the low-temperature CO-DRIFTs, CO-pulse chemisorption, XRD and XPS, which were responsible for the activity enhancement. The undercoordinated Ce4+ species were generated during the reaction, thus improving the mobility of the lattice oxygen which is vital for the methane combustion following the MvK mechanism. The PdCeMnO-CP also showed higher activity for the complex methane-containing three-way catalytic reactions than the Pd/CeMnO-Imp, demonstrating its potential for industrial application.

Insight into the roles of Pd state and CeO2 property in C3H8 catalytic oxidation on Pd/CeO2

Propane oxidation is a key reaction in the three-way catalytic reaction. The dispersion state of Pd species and property of CeO2 have significant impacts on the C3H8 oxidation over Pd/CeO2 catalyst via altering the activation pathways of C3H8. Herein, Pd/CeO2 catalysts with highly dispersed Pd species (Pd/CeO2-IWI) and uniform PdO nanoparticles (4.3 ± 0.3 nm) (Pd/CeO2-EG) were prepared, respectively. The highly dispersed Pd species on Pd/CeO2-IWI promoted Pd entering the lattice to form PdxCe1-xO2-σ solid solution through strong Pd-CeO2 interaction, while PdO nanoparticles were the main existing form on Pd/CeO2-EG. The results showed that C3H8 was adsorbed and activated to form acrylate on PdxCe1-xO2-σ, while C3H8 preferred to generate acetone on PdO. However, the main surface intermediates were converted to acetate and formate at the elevated reaction temperature. Compared with PdxCe1-xO2-σ, PdO was more favorable for the rapid oxidative decomposition of acetate to formate, which facilitated the C3H8 oxidation activity. Meanwhile, the property of CeO2 could also influence the C3H8 oxidation activity through the accumulation of formate and carbonate. In which, Pd/CeO2(1000)-EG with the most PdO species and the lowest accumulation of surface formate/carbonate performed the highest C3H8 oxidation activity.

Ce-modified Rh overlayer for a three-way catalytic converter with oxygen storage/release capability

A nanometric Rh overlayer exhibits excellent three-way catalytic activity under stoichiometric conditions. However, dynamic changes in the air-to-fuel (A/F) ratio under experimental conditions deteriorates the catalytic activity caused by the lack of oxygen storage/release capability of Rh. In this study, we first report that the surface modification of the Rh overlayer by a small amount of Ce using a sequential metal deposition technique based on the pulsed arc-plasma method enhances its three-way catalytic activity under A/F perturbation conditions. X-ray photoelectron spectroscopy and X-ray absorption fine structure measurements revealed that the redox behavior of the surface Ce contributed to an oxygen storage/release and buffered the A/F change. The Ce-modified Rh overlayer with a metal honeycomb structure exhibited high activity for the three-way catalytic reaction even under A/F perturbation conditions at high space velocity. This novel catalytic converter is expected to be significantly process and energy saving compared to the conventional catalytic converter. Thus, the oxygen storage/release performance, heat resistance, and low-temperature activity can be improved by controlling the surface composition.

Influence of the strategy for mixing Pd/La-Al2O3 and Pd/Ce0.5Zr0.5O2 on the activity of three-way catalysts

As the two main components in typical three-way catalysts, Pd-Al2O3 and Pd-Ce0.5Zr0.5O2 usually show different Pd-support interaction strength. Thus, Pd could re-distribute between Al2O3 and Ce0.5Zr0.5O2 upon heating. Here the strategy to mix Pd/La-Al2O3 and Pd/Ce0.5Zr0.5O2 (before or after calcination, or after aging) was researched. The optimal choice was “mixing-calcination-aging”, offering the highest oxygen storage capacity, Pd dispersion and catalytic activity for CO and C3H6 elimination during the three-way catalytic reaction. Thus, the “mixing-calcination-aging” strategy holds promise in industrial applications, as demonstrated at the monolith converter level. The transient drop in conversion with increasing temperature (“kink”) appeared simultaneously for CO, NO and C3H6 over the “mixing-calcination-aging” monolith, but did not over the calcined counterpart “mixing-calcination”. The kinks disappeared for the used “mixing-calcination-aging”, but could be restored upon re-oxidizing. By virtue of the in situ Raman characterization, the reduction of PdO under the reaction conditions was found to cause the kinks.

Slow Synthesis Methodology-Directed Immiscible Octahedral Pdx Rh1-x Dual-Atom-Site Catalysts for Superior Three-Way Catalytic Activities over Rh.

This study provided an effective strategy to construct dual-atom sites by solid-solution alloying. A slow synthesis methodology was established for the solid-solution preparations as dual-atom-site catalysts. The atomic-level homogeneous Pdx Rh1-x dual-atom-site catalysts were successfully synthesized over the whole composition range, as evidenced by X-ray powder diffraction and scanning transmission electron microscope energy-dispersive X-ray spectroscopy mapping measurements. The challenging morphology formation in the immiscible alloys was achieved by an energy-controlling process as the octahedral Rh-rich alloys. The Pd0.3 Rh0.7 dual-atom-site catalyst had unique surface states to activate the key reactants of CO and NO in the complex three-way catalytic reactions, and it performed significantly better than pure Rh.

Research on ammonia emissions from three-way catalytic converters based on small sample test and vehicle test

In this paper, a combination of catalyst sample evaluation and vehicle test is used to deeply study the formation mechanism of ammonia in the process of three-way catalytic reaction, and further explore the influence of catalyst formulation and aging on ammonia emissions. The catalytic sample test shows that CO reacts with terminal hydroxyl and bridging hydroxyl on the surface of the catalyst to generate H2 at low temperature, which then reduces NO to generate NH3. At high temperatures, CO reacts with water to generate H2, or hydrocarbon compounds in exhaust react with steam to generate hydrogen, and then H2 reacts with nitrogen oxides to generate NH3. On the one hand, the presence of water vapor can be prompted catalytic hydroxylation of materials and promote the reaction of the hydroxyl and bridging hydroxyl to improve the selectivity of NH3, on the other hand, as the competitive adsorption of H2O molecules and NO on the catalyst surface inhibits the reduction reaction between NH3 and NO, the consumption of NH3 molecules is reduced, and more NH3 vaporizes from the catalyst surface to the gas phase. The combination of Pd/Rh can effectively reduce the NH3 generation compared with the single Pd formulation. Ammonia emission can be effectively reduced by precisely controlling the air-fuel ratio of the engine and combining it with the catalytic converter which optimizes the ratio of precious metals.

Oxygen Release and Storage Property of Fe-Al Spinel Compounds: A Three-Way Catalytic Reaction over a Supported Rh Catalyst.

We evaluated the catalytic performance of the Rh-Fe/Al2O3 catalyst during a three-way catalytic reaction and found, by chance, that a part of Fe species was dissolved into the γ-Al2O3 support and worked as an oxygen storage material, which adjusts the oxygen concentration around the catalytically active sites to a suitable level for three-way catalysis. In this study, we demonstrated that the Fe-doped γ-Al2O3 can reversibly store and release oxygen by the redox of Fe2+/Fe3+ at the tetrahedral (Td) site of the spinel structure without its structure deformation. The finding that a spinel-structured metal oxide, Fe-doped γ-Al2O3, could work as an oxygen storage material suggested a new opportunity for the development of oxygen storage materials without rare metals.

Understanding the three-way catalytic reaction on Pd/CeO2 by tuning the chemical state of Pd

To investigate the correlation of activity and chemical state of Pd species in the three-way catalytic reaction, the supported Pd catalysts were prepared by using CeO2 calcined at different temperatures as supports. It turned out that the chemical state of Pd could severely affect the catalytic activity of Pd/CeO2, which depended on the CeO2 properties and the interaction between Pd and CeO2. With increasing CeO2 calcination temperature from 500 to 1200 °C, the state of Pd gradually transferred from Pd4+ as PdxCe1-xO2-σ to Pd2+ as PdO due to the decreased interaction between Pd and CeO2. Meanwhile, the TOFs of C3H8 oxidation linearly increased and the TOFs of CO oxidation showed a reversed trend, demonstrating the active states of Pd in C3H8 and CO oxidation were different. NO conversion was correlated to the activity of CO oxidation in low-temperature zone and C3H8 oxidation in high-temperature zone during the three-way catalytic reaction. Among them, Pd/CeO2(500) showed the highest CO oxidation activity and low-temperature NO conversion due to the enhanced adsorption ability of CO and NO. The existence of PdO on the surface of high-temperature calcined CeO2 facilitated C3H8 activation and reaction with surface nitrate/nitrite, which promoted NO conversion in high temperature.

Self-Regeneration Process of Ni-Cu Alloy Catalysts during a Three-Way Catalytic Reaction-An Operando Study.

It is important to understand the reduction processes of mixed metal oxides or metal oxide interfaces in three-way catalytic reactions toward replacing the currently used high-cost Pt group metal catalysts. The redox behavior of simple Ni-Cu alloy catalysts, which exhibit high catalytic activity and durability during a three-way catalytic reaction, was studied by operando X-ray absorption spectroscopy (XAS). The operando XAS analyses revealed that Ni-Cu species changed from the NiO-Cu2O to Ni-Cu alloy and vice versa under reductive and oxidative conditions, respectively. The real-time monitoring of the oxidation states of Ni and Cu species indicated that the Cu species assisted the reduction of Ni species, in agreement with the density functional theory-based study of NiO reduction by carbon monoxide in the presence of metallic Cu nanoparticles.

Single-atom site catalysts for environmental catalysis

In recent decades, the environmental protection and long-term sustainability have become the focus of attention due to the increasing pollution generated by the intense industrialization. To overcome these issues, environmental catalysis has increasingly been used to solve the negative impact of pollutants emission on the global environment and human health. Supported platinum-metal-group (PGM) materials are commonly utilized as the state-of-the-art catalysts to eliminate gaseous pollutants but large quantities of PGMs are required. By comparison, single-atom site catalysts (SACs) have attracted much attention in catalysis owing to their 100% atom efficiency and unique catalytic performances towards various reactions. Over the past decade, we have witnessed burgeoning interests of SACs in heterogeneous catalysis. However, to the best of our knowledge, the systematic summary and analysis of SACs in catalytic elimination of environmental pollutants has not yet been reported. In this paper, we summarize and discuss the environmental catalysis applications of SACs. Particular focus was paid to automotive and stationary emission control, including model reaction (CO oxidation, NO reduction and hydrocarbon oxidation), overall reaction (three-way catalytic and diesel oxidation reaction), elimination of volatile organic compounds (formaldehyde, benzene, and toluene), and removal/decomposition of other pollutants (Hg0 and SO3). Perspectives related to further challenges, directions and design strategies of single-atom site catalysts in environmental catalysis were also provided.

Synthesis of high stability nanosized Rh/CeO2–ZrO2 three-way automotive catalysts by Rh chemical state regulation

The chemical state of precious metals plays an important role in redox reaction. In order to maintain rhodium (Rh) at metallic state, which is recognized as reactive center in three-way catalytic reactions, we synthesized Rh/CeO2–ZrO2 (Rh/CZ) catalyst by liquid phase reduction adsorption with glycerol as reductant (Rh/CZ-g). Detailed characterizations were conducted to investigate the chemical state of Rh species, and the CO-FTIR and XPS results indicated that Rh species mainly existed in metallic state for Rh/CZ-g. Through CO chemisorption and H2-TPR, it’s proved that the liquid phase reduction treatment would weaken the interaction between Rh species and CeO2–ZrO2 support, avoiding the supply of oxygen species from CeO2–ZrO2 to Rh in some degree. Compared with Rh/CeO2–ZrO2 catalyst prepared by incipient wetness impregnation, Rh/CZ-g exactly had superior activity in three-way catalytic process. Combined with in situ DRIFTS, some important intermediate species, such as Rh–CN, Rh-(NO)2, formates and acetates, preferred to adsorb and transform on Rh/CZ-g with larger proportion of metallic Rh, which determined Rh/CZ-g was more favorable for three-way catalytic reactions, even after aged at 950 °C for 5 h. The purpose of this work is to inspire researchers use simple and valid methods to synthesize highly efficient three-way catalysts.

Highly durable metal ensemble catalysts with full dispersion for automotive applications beyond single-atom catalysts

Reducing the size of metal nanoparticles down to the single-atom level has been actively pursued to maximize the use of precious metals. Recently, single-atom catalysts, in which all the metal atoms are isolated on a support with 100% dispersion, have received much attention. However, the lack of ensemble sites prevents valuable surface reactions that require metal proximity to occur. Here, we present metal (Pt, Pd and Rh) ensemble catalysts with 100% dispersion and a reduced metallic surface state. More specifically, nanoceria particles were anchored on Al3+penta sites of activated γ-alumina, and then metal was deposited and reduced. The ensemble catalysts are highly durable: their structure was maintained even after hydrothermal ageing at 900 °C for 24 h or after long-term reaction. These catalysts have superior activity and durability for three-way catalytic reactions and can provide insights beyond single-atom catalysts for heterogeneous catalysis. Supported single atoms can minimize metal utilization in catalysis, although reactivity restrictions exist. Here, fully exposed Pt, Pd and Rh ensembles localized on CeO2 islands anchored onto partially reduced γ-Al2O3 are introduced as a superior and durable alternative for three-way catalysis.

Comparative Study of Moisture-Treated Pd@CeO2/Al2O3 and Pd/CeO2/Al2O3 Catalysts for Automobile Exhaust Emission Reactions: Effect of Core-Shell Interface.

In this article, moisture-treated Pd@CeO2/Al2O3 and Pd/CeO2/Al2O3 catalysts were synthesized and applied in automotive three-way catalytic (TWC) reactions. Compared to the Pd/CeO2/Al2O3 catalyst, the Pd@CeO2/Al2O3 core-shell catalyst had better TWC activities. Transmission electron microscopy (TEM) images and X-ray photoelectron spectra (XPS) showed excess PdO2 on the Pd and CeO2 interface of Pd@CeO2 nanoparticles. Fourier transform infrared (FT-IR) spectra analysis demonstrated the generation of the hydroperoxyl (*OOH) groups on the surface of the Pd@CeO2 nanoparticle. CO-diffuse reflectance Fourier transform (DRIFT) measurement suggested that the CO adsorbed on *OOH species contributed to the formation of CO2 and intermediate *COOH. NO-DRIFT results showed that more *NO2 species appeared on the moisture-treated Pd@CeO2 nanoparticle, which was the main active site in the automobile TWC reaction. These were the main factors contributing to the moisture-treated Pd@CeO2/Al2O3 catalyst's high catalytic activities. The collected data revealed the crucial role of the co-promoting effect of moisture and core-shell interface on TWC reactions over the Pd@CeO2/Al2O3 catalyst, which could be applied to other catalytic reactions.

Core-shell design and well-dispersed Pd particles for three-way catalysis: Effect of halloysite nanotubes functionalized with Schiff base

In this study, we have described the synthesis of core@shell three-way catalyst with well-dispersed Pd nanoparticles which were intercalated into halloysite nanotubes (HNTs) material via ligand assistance. The prepared parameters of Pd@HNTs catalyst included amine source, the molar ratio of amine and aldehyde, and the addition of CeO2 promoter. As a result, Pd@HNTs performed a good dispersion of Pd particles and high stability, which is attributed to the strong interaction between Pd and HNTs with Schiff base ligands and the high thermal resistance of HNTs as a sintering barrier. Moreover, the results of various characteristic analyses revealed that Pd@HNT-E12 (ethylenediamine: salicylaldehyde in a molar ratio of 1:2) exhibited the highest gases conversion to the others, which has excellent redox ability. Furthermore, the addition of CeO2, which acted as both a promoter and a protector, could provide more oxygen vacancies for promoting NO reduction and CO and C3H8 oxidation at gradually elevated temperatures. Such core-shell catalyst Ce@Pd@HNT-E12 could avoid excess CeO2 penetrating into the pore volume of halloysite support and facilitate the three-way catalytic reaction.

Self-regeneration of a Ni-Cu alloy catalyst during a three-way catalytic reaction.

Ni-Cu alloy supported on γ-Al2O3 catalysts prepared by high-temperature hydrogen reduction exhibit high catalytic activity and durability for a three-way catalytic reaction under both oxidative and reductive conditions because of their self-regenerating feature. DFT calculations showed that Ni-oxide was reduced to Ni metal by CO in the presence of Cu metal because of the Ni-Cu alloy effect but was not in the absence of Cu metal.

Dynamic Behavior of Rh Species in Rh/Al2O3 Model Catalyst during Three-Way Catalytic Reaction: An Operando X-ray Absorption Spectroscopy Study.

The dynamic behavior of Rh species in 1 wt% Rh/Al2O3 catalyst during the three-way catalytic reaction was examined using a micro gas chromatograph, a NOx meter, a quadrupole mass spectrometer, and time-resolved quick X-ray absorption spectroscopy (XAS) measurements at a public beamline for XAS, BL01B1 at SPring-8, operando. The combined data suggest different surface rearrangement behavior, random reduction processes, and autocatalytic oxidation processes of Rh species when the gas is switched from a reductive to an oxidative atmosphere and vice versa. This study demonstrates an implementation of a powerful operando XAS system for heterogeneous catalytic reactions and its importance for understanding the dynamic behavior of active metal species of catalysts.