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

Abstract

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 Soot Combustion─General Concepts and Alkali Promotion

“…Detailed data used for the compilation of Figure are reported in the Table , Table , and Table S1 in the Supporting Information. Depending on their chemical composition and morphology, the catalysts were divided into six groups: platinum metal group, ,− ceria–zirconia-based, − ,,,,,,,,,,− alkali-nanostructured, ,,,,− ,,,,,,,,− ,− spinel structure, ,,,,− ,,,− perovskite structure, ,,,,,,,,,…”

“…For application purposes, the soot combustion catalysts are fabricated in the form of monoliths, such as catalytic diesel particulate filters, CDPF (Figure a). The active phases promoted by alkali dispersed in the channels of the ceramic monoliths (mainly cordierite) include ceria-based catalytic materials, , perovskites, spinels, , other mixed oxides, , and glasses. ,, Alkali-modified metallic foams, as well as metallic foams coated with other potassium-containing active phases, were also investigated. A separate class constitutes an alkali-modified ceramic paper that can be used to form structured catalysts. , Metal oxides are deposited on cordierite monoliths by sequential impregnation, ,,, sol–gel method, or rarely by molten salt synthesis .…”

“…In a typical experiment, the filter is placed in the reactor, and the compositions of inlet and outlet reaction gases are analyzed. , Accumulation of soot particles in the monolithic channels leads to pressure buildup, and the activity of the catalyst can easily be assessed by monitoring the pressure drop profile in the filter during the regeneration step (Figure b) . Consecutive activity tests were also applied to assess the stability of the investigated materials under the operation conditions. ,, Usually an improvement in activity was observed after the addition of alkali regardless of the type of transition metal oxide active phase. ,,,, However, for some systems, a decrease in activity has been observed after a few cycles due to alkali loss when the catalyst was overheated to high temperatures (800 °C). , In terms of stability, glass-based monolithic catalysts seem to be an interesting solution. Although in such systems potassium ions can migrate between the bulk and the surface stimulating the catalytic activity, their loss due to volatilization is rather small. , It should be mentioned, however, that for applications of structured and monolithic catalysts including ceramic and metal foams, apart from the catalyst integrity, resistance to contaminants, such as SO 2 and water vapor, defined the urgent direction for the development of robust CDPF exhaust treatment systems …”

“…For application purposes, the soot combustion catalysts are fabricated in the form of monoliths, such as catalytic diesel particulate filters, CDPF (Figure 17a). The active phases promoted by alkali dispersed in the channels of the ceramic monoliths (mainly cordierite) include ceria-based catalytic materials, 57,248 perovskites, 195 spinels, 38,185 other mixed oxides, 136,182 and glasses. 228,229,249 Alkali-modified metallic foams, 250 as well as metallic foams coated with other potassium-containing active phases, 168 were also investigated.…”

“…A separate class constitutes an alkali-modified ceramic paper that can be used to form structured catalysts. 156,232 Metal oxides are deposited on cordierite monoliths by sequential impregnation, 57,136,195,248 sol−gel method, 185 or rarely by molten salt synthesis. 182 In the case of the glass active phases, the sol−gel method is preferred.…”