Kinetic Analysis of Devolatilized Diesel-Soot Oxidation Catalyzed by Ag/Al2O3 and Ag/CeO2 Using Isoconversional and Master-Plots Techniques

This work presented the kinetic analysis of devolatilized diesel-soot combustion accelerated by Ag/Al2O3 and Ag/CeO2 catalysts. Isoconversional and master-plots techniques were employed to estimate activation energy and identify the reaction model. The apparent activation energy of uncatalyzed soot oxidation was 101.85 kJ/mol, and it was reduced to 61.85 and 82.78 kJ/mol for the combustion catalyzed by Ag/Al2O3 and Ag/CeO2, respectively. The reaction-order model, f(α) = (1- α)n, with n of 1.4, 1, and 1 showed the best fit for the uncatalyzed soot oxidation and soot oxidation catalyzed by Ag/Al2O3 and Ag/CeO2, respectively. The proposed single-step reaction models were quite capable of reproducing experiments for the uncatalyzed soot oxidation and soot oxidation catalyzed by Ag/CeO2. In the presence of Ag/Al2O3, the oxidation rate at the first 20% of conversion was faster than the 1st-order reaction reflecting that the soot was rapidly oxidized by highly active species generated by Ag/Al2O3. The oxidation of the remaining soot closely followed the 1st-order reaction mechanism.

Preparation of Cordierite Monolith Catalysts with the Coating of K-Modified Spinel MnCo2O4 Oxide and Their Catalytic Performances for Soot Combustion

Diesel engines are important for heavy-duty vehicles. However, particulate matter (PM) released from diesel exhaust should be eliminated. Nowadays, catalytic diesel particulate filters (CDPF) are recognized as a promising technology. In this work, a series of monolith Mn1−nKnCo2O4 catalysts were prepared by the simple citric acid method. The as-prepared catalysts displayed good catalytic performance for soot combustion and the Mn0.7K0.3Co2O4 catalyst gave the best catalytic performance among all the prepared samples. The T10 and Tm of Mn0.7K0.3Co2O4-HC catalyst for soot combustion are 310 and 439 °C, respectively. The physical and chemical properties of catalysts were characterized by means of SEM, XPS, H2-TPR, Raman and other techniques. The characterization results indicate that K substitution is favorable for the formation of oxygen vacancies, enhancing the mobility of active oxygen species, and improving the redox properties and so on. In-situ Raman results prove that the strength of Co-O bonds in the catalysts became weak during the reaction at high temperatures. In addition, SEM and ultrasonic test results show that the peeling rate of the coat-layer is less than 5%. The as-prepared catalysts can be taken as one kind of candidate catalyst for promising application in soot combustion because of its facile synthesis, low cost and high catalytic activity.

Catalytic Materials for Gasoline Particulate Filters Soot Oxidation

The energy efficiency of Gasoline Direct Injection (GDI) engines is leading to a continuous increase in GDI engine vehicle population. Consequently, their particulate matter (soot) emissions are also becoming a matter of concern. As required for diesel engines, to meet the limits set by regulations, catalyzed particulate filters are considered as an effective solution through which soot could be trapped and burnt out. However, in contrast to diesel application, the regeneration of gasoline particulate filters (GPF) is critical, as it occurs with almost an absence of NOx and under oxygen deficiency. Therefore, in the recent years it was of scientific interest to develop efficient soot oxidation catalysts that fit such particular gasoline operating conditions. Among them ceria- and perovskite-based formulations are emerging as the most promising materials. This overview summarizes the very recent academic contributions focusing on soot oxidation materials for GDI, in order to point out the most promising directions in this research area.

In situ identification of the metallic state of Ag nanoclusters in oxidative dispersion

Oxidative dispersion has been widely used in regeneration of sintered metal catalysts and fabrication of single atom catalysts, which is attributed to an oxidation-induced dispersion mechanism. However, the interplay of gas-metal-support interaction in the dispersion processes, especially the gas-metal interaction has not been well illustrated. Here, we show dynamic dispersion of silver nanostructures on silicon nitride surface under reducing/oxidizing conditions and during carbon monoxide oxidation reaction. Utilizing environmental scanning (transmission) electron microscopy and near-ambient pressure photoelectron spectroscopy/photoemission electron microscopy, we unravel a new adsorption-induced dispersion mechanism in such a typical oxidative dispersion process. The strong gas-metal interaction achieved by chemisorption of oxygen on nearly-metallic silver nanoclusters is the internal driving force for dispersion. In situ observations show that the dispersed nearly-metallic silver nanoclusters are oxidized upon cooling in oxygen atmosphere, which could mislead to the understanding of oxidation-induced dispersion. We further understand the oxidative dispersion mechanism from the view of dynamic equilibrium taking temperature and gas pressure into account, which should be applied to many other metals such as gold, copper, palladium, etc. and other reaction conditions.

Designing ultra small metal nanoclusters or single atoms with metallic state is a challenge. Here, the authors demonstrate the stabilization of ultra small silver clusters in the nearly-metallic state by oxygen adsorption at high temperature, using in situ spectroscopy and microscope technologies.

Developments and advances in in situ transmission electron microscopy for catalysis research

Recent developments and advances in in situ TEM have raised the possibility to study every step during the catalysts' lifecycle. This review discusses the current state, opportunities and challenges of in situ TEM in the realm of catalysis.

Kinetic analysis of Ag particle redispersion into ZSM-5 in the presence of coke using in situ XAFS

In the present study, the redispersion behavior of Ag particles on ZSM-5 in the presence of coke was observed using in situ X-ray absorption fine structure (XAFS) spectroscopy.

Resolving the puzzle of single-atom silver dispersion on nanosized γ-Al2O3 surface for high catalytic performance

Ag/γ-Al2O3 is widely used for catalyzing various reactions, and its performance depends on the valence state, morphology and dispersion of Ag species. However, detailed anchoring mechanism of Ag species on γ-Al2O3 remains largely unknown. Herein, we reveal that the terminal hydroxyls on γ-Al2O3 are responsible for anchoring Ag species. The abundant terminal hydroxyls existed on nanosized γ-Al2O3 can lead to single-atom silver dispersion, thereby resulting in markedly enhanced performance than the Ag cluster on microsized γ-Al2O3. Density-functional-theory calculations confirm that Ag atom is mainly anchored by the terminal hydroxyls on (100) surface, forming a staple-like local structure with each Ag atom bonded with two or three terminal hydroxyls. Our finding resolves the puzzle on why the single-atom silver dispersion can be spontaneously achieved only on nanosized γ-Al2O3, but not on microsized γ-Al2O3. The obtained insight into the Ag species dispersion will benefit future design of more efficient supported Ag catalysts.

Real-Time Observation of Carbon Oxidation by Driven Motion of Catalytic Ceria Nanoparticles within Low Pressure Oxygen

Carbon particulate matter (PM) is an undesirable aerosol pollutant formed from combustors such as power plants, refineries, and engines. The most common and effective method of mitigating PM emission is the capture of particulates using a filter, before particles are released into the atmosphere. In order to develop and improve advanced filtering materials, a better understanding is required of their chemical and mechanical behavior. We report on a novel phenomenon on the mobility and oxidation behavior of catalytic iron doped ceria nanoparticles in contact with mobile carbon black nanoparticles. The process is recorded by real time imaging within an environmental transmission electron microscope. In contrast to observations in previous studies, the separated ceria nanoparticles are found to actively move on the substrate and consume the connecting carbon particles one-by-one. The velocity of particle motion is correlated to the reaction temperature and oxygen pressure, both determining the reaction rate. Modeling using the Density Functional Theory suggests this motion is driven by the chemical bonding between the surface oxygen of the catalyst and the graphite layers of carbon black, initiated through the Van der Waals force between two types of nanoparticles.

Promotion effect of water in catalytic fireplace soot oxidation over silver and platinum

The activity of the catalysts in the fireplace soot oxidation depends on water content in the gas feed. Water is partially dissociated with formation of hydroxyls over silver and platinum, which promote soot oxidation.