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Layered δ-MnO2 catalysts were prepared using the one-step redox method in precursor solutions with five different pH values (pH = 7, 9, 11, 13, and 14). The effects of pH on the physical properties and catalytic activity of the catalyst were investigated through XRD, SEM, TEM, BET, XPS, H2-TPR, and HCHO degradation tests at room temperature. The results showed that the layer spacing, manganese vacancy content, Mn4+/Mn3+ ratio, and surface-reactive oxygen species content of MnO2 increased with the increase in pH value in the alkaline range. When the catalyst was prepared at pH = 13, the above characteristics of the catalyst reached the optimal value which contributed to the high catalytic activity. Combined with the related characterization results, it was proved that changing the pH can affect the degree of oxidation in the catalyst synthesis process, increase the number of active oxygen and the oxygen mobility of the catalyst, and effectively improve the catalytic activity of the manganese dioxide catalyst for HCHO. This work represents a giant step toward the preparation of an effective catalyst for practical applications of HCHO removal at room temperature at a low concentration and high velocity. [ABSTRACT FROM AUTHOR]

• Catalysts for the Knoevenagel are synthesized from different carbon sources such as melamine, dicyandiamide, chitosan, etc. • Co-N-C synthesized with melamine as a carbon source gave yields as high as 93.7% with excellent stability and recyclability. • Promotion of the Knoevenagel reaction by a solid frustrated Lewis pair (FLP) consisting of Coδ+ and Nδ- is discussed. Nitrogen-doped carbon confined cobalt nanoparticles (Co-N-C) were synthesized through multi-step pyrolysis method. The Co-N-C samples exhibit excellent catalytic performance in the Knoevenagel reaction of benzaldehyde and malononitrile, of which the yield can be as high as 93.7% within 2.5 min with outstanding stability and recyclability. The catalytic performance may be primarily ascribed to the richness in nitrogen and cobalt in the form of the solid Frustrated Lewis Pair (FLP), i.e. the Co ion acts as a Lewis acid site to activate the carbonyl oxygen of benzaldehyde molecules, and nitrogen in carbon acts as a Lewis base site to activate the methylene hydrogen of malononitrile. From the perspective of intrinsic catalytic activity, variations in catalytic properties are attributed to the electron density distribution on the surface of the catalyst. Nitrogen-doped carbon confined cobalt nanoparticles as the acid-base bifunctional catalysts exhibit excellent catalytic performance for the Knoevenagel reaction between benzaldehyde and malononitrile, which may be primarily ascribed to the highest contents of nitrogen and Cobalt in the form of the solid Frustrated Lewis Pair (FLP) from Coδ+ of Co nanoparticles and Nδ− of nitrogen-contained species. [Display omitted] [ABSTRACT FROM AUTHOR]

Addressing the problems of easy collapse, Ni sintering and low catalytic activity over the nanotubular nickel phyllosilicate catalyst, the Mg species was doped via a simple hydrothermal method to construct the nanotubular Ni-Mg bimetallic phyllosilicate solid solution. The high thermal stability, nonreducible property and strong basicity of magnesium species led to the enhancement of anti-collapse, Ni-sintering resistance and basicity. As a result, the fine Ni particles and enhanced H 2 and CO 2 chemisorption and activation properties resulted in boosting the catalytic activity of both CO 2 methanation and CH 4 -CO 2 reforming reactions. The Ni/Mg ratios and the reduction temperatures were detailed optimized, and the 600oC-reduced Ni 8 Mg 2 -NT catalyst with the Ni/Mg ratio of 8:2 was the optimal, which also exhibited high long-term stability. In short, the formed Ni-Mg-phyllosilicate solid solution structure and the special interactions of different species brought about high catalytic performance of the nanotubular Ni-Mg phyllosilicate catalyst. [Display omitted] • The nanotubular Ni-Mg bimetallic phyllosilicate solid solution was synthesized. • Nanotubular Ni-Mg phyllosilicate obtained high anti-collapse and Ni-sintering resistance properties. • The Ni/Mg ratio of 8:2 and the reduction temperature of 600 oC were the optimum. • This catalyst was versatile for both CO 2 methanation and CH 4 -CO 2 reforming reactions. [ABSTRACT FROM AUTHOR]

In this work, Mn(CeZr)O x was synthesized by using chelation-induced synergistic self-assembly strategy for the combustion of toluene. The physicochemical properties of the synthesized catalysts were characterized by XRD, ICP-MS, SEM, TEM, XPS and N 2 sorption. The Mn(CeZr)O x catalyst with T 90 = 225 °C exhibited improved catalytic performance than the original MnO x catalyst (T 90 = 260 °C) and had significant low-temperature activity. The relationship between catalyst activity and structure was analyzed. By substituting Ce and Zr elements into the hollow microspheres of MnO 2 , oxygen vacancies were produced. The main factors affecting the catalytic activity of the catalyst and the reason why it remained high catalytic activity after a long period of hydrothermal treatment were discussed. After hydrothermal aging, the original pore structure of Mn(CeZr)O x catalyst collapsed and the specific surface area decreased, but the overall crystallinity of the catalyst increased and the content of oxygen species in the lattice increased. The distribution of Mn and oxygen species on the catalyst surface changed significantly after hydrothermal treatment. The appropriate ratio of Mn4+ to Mn3+ and the ratio of lattice oxygen to adsorbed oxygen species are beneficial to the redox reaction cycle. [Display omitted] • A novel Mn(CeZr)O x was prepared by a chelation-induced self-assembly strategy. • The hydrothermal aging resistance of Mn(CeZr)O x was studied. • Synergistic effect of metal species significantly improved the toluene combustion. • The superior redox and structural properties made catalysts have high activity. [ABSTRACT FROM AUTHOR]