Experimental and numerical analysis of the effects of thermal degradation on carbon monoxide oxidation characteristics of a three-way catalyst

This work investigates oxygen-storage capacity (OSC) changes during thermal degradation in modern three-way catalysts. Two experiments are performed using catalysts with different degradation degrees to evaluate OSC and reaction rates. The CO2 production test, where CO and O2 are supplied at a constant temperature, shows decreased CO2 production with more degraded catalysts and reduced purification. The CO2 production test is conducted using transient temperature increases, showing that the maximum CO2 production temperature increases with catalyst degradation. The results reveal an increase in activation energy in the oxygen desorption reaction caused by thermal degradation progresses and a decrease in OSC, resulting in temperature increases in the oxygen storage reaction. In the surface reaction and mass transport model considering the 30 elementary reactions, the predicted results are well-validated for CO2 production, enabling good oxygen storage predictions based on actual data. These results can be used to predict OSC by formulating the changes in active site density and activation energy due to degradation.

Bioleaching of the α-alumina layer of spent three-way catalysts as a pretreatment for the recovery of platinum group metals

Acid bioleaching of Al by Acidithiobacillus thiooxidans has been explored as an environmentally friendly pretreatment to facilitate the extraction of platinum group metals from spent three-way catalysts (TWC). Biogenic sulfur obtained from desulfurization bioreactors improved the production of acid by A. thiooxidans compared to commercially available elemental sulfur. The lixiviation abilities of bacteria-free biogenic acid and biogenic acid with exponential or stationary phase bacteria were compared against a control batch produced by commercial H2SO4. The maximum Al leaching percentage (54.5%) was achieved using biogenic acids with stationary-phase bacteria at a TWC pulp density of 5% w/v whereas bacteria-free biogenic acid (23.4%), biogenic acid with exponential phase bacteria (21.7%) and commercial H2SO4 (24.7%) showed lower leaching abilities. The effect of different pulp densities of ground TWC (5, 30, and 60% w/v) on Al leaching and bacterial growth was determined. While greater Al leaching yields were obtained at lower TWC pulp density solutions (54.5% at 5% w/v and 2.5% at 60% w/v), higher pulp densities enhanced microbial growth (2.3 × 109 cells/mL at 5% w/v and 9.5 × 1010 cells/mL at 60% w/v). The dissolution of the metal from the solid into the liquid phase triggered the production of biological polymeric substances that were able to absorb traces of both Al (up to 24.80% at 5% w/v) and Pt (up to 0.40% at 60% w/v).

Sintering-resistant, highly thermally stable and well-dispersed Pd@CeO2/halloysite as an advanced three-way catalyst

The high thermal stability of halloysite (H)-supported core-shell Pd@CeO₂ endowed it with promising catalytic performance and superior sintering resistance as a three-way catalyst. In this work, the synthesis of Pd@CeO₂ nanoparticles with various shell thicknesses was performed, and the properties of the shell and support were examined. From the results, the Pd@6CeO₂/H catalyst (Ce/Pd = 6) without any pretreatment or activation was achieved with a well-dispersed and optimal shell thickness of Pd@6CeO₂ nanoparticles to inhibit sintering and aggregation via electrostatic attractions with halloysite. Moreover, the halloysite support imparted thermal stability for enhanced catalytic stability under long-term and high-temperature reaction conditions compared with Pd@6CZ/H (cerium-zirconium shell) and Pd@6CeO₂/Al₂O₃ catalysts. To further ascertain the electronic effect on halloysite, Pd@6CeO₂/H-12 (halloysite solution at pH = 12) was prepared. The results showed that Pd@6CeO₂/H-12 enhanced the catalytic activity and decreased the light-off temperature compared with the other studied catalysts, and these results were attributed to the high content of Ce³⁺ and oxygen vacancies and the strong interaction between Pd@6CeO₂ and halloysite, making it a promising three-way catalyst.

Key role of NO + C3H8 reaction for the elimination of NO in automobile exhaust by three-way catalyst

Pd-only three-way catalysts with improved catalytic activity for NO elimination were prepared. In order to explore the catalytic reaction rules of NO reduction under a three-way catalytic system, a series of single reactions related to NO reduction were evaluated. It was found that the reaction temperatures of NO + H₂ or NO + CO or NO + C₃H₆ reactions were below 250 °C, while that of NO + C₃H₈ was up to 350 °C. Thus, the reaction NO + C₃H₈ served as the key reaction in determining the purification efficiency of NO at the high-temperature stage. By in situ FTIR, we proposed that three possible steps were involved in NO + C₃H₈ reaction. The first step was the oxidation of C₃H₈ and NO to acetone and nitrate species by active oxygen species, respectively (C₃H₈ + O* → C₃H₆O, NO + O* → NO₃^-). XPS results revealed that the amount of active oxygen species in Pd/CeO₂-ZrO₂-Al₂O₃ (Pd/CZA, 73.7%) was much higher than that in Pd/Ce_xZr_1-xO₂+Al₂O₃ (Pd/CZ+A, 64.1%). This was in line with the higher reaction efficiency of the first step over Pd/CZA. Then the NO + C₃H₈ reaction was accelerated by the first step, which consequently contributed to the higher NO elimination efficiency of Pd/CZA.

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.