Weakened Mn-O bond in Mn-Ce catalysts through K doping induced oxygen activation for boosting benzene oxidation at low temperatures

K-doped Mn-Ce solid solution catalysts were synthesized using a combination of coprecipitation and hydrothermal methods, demonstrating excellent performance in benzene oxidation. The catalyst K1Ce5Mn5 exhibited comparable activity to noble metal catalysts, achieving a 90 % benzene conversion at approximately 194 ℃. Durable tests under dry and moist conditions revealed that the catalyst could maintain its activity for 50 h at 218 ℃ and 236 ℃, respectively. Characterization results indicated that the catalyst's enhanced activity resulted from the weakened Mn-O bonding caused by the introduction of K+, facilitating the activation of oxygen and its involvement in the reaction. CeOx, the main crystalline phase of Mn-Ce solid solutions, provided abundant oxygen vacancies for capturing and activating oxygen molecules for the weakened Mn-O structures. This conclusion was further supported by partial density of state analysis from density functional theory computations, revealing that the introduction of K+ weakened the orbital hybridization of Mn3d and O2p. Finally, in situ diffuse reflectance infrared Fourier-transform spectroscopy (in situ DRIFTS) studies on Ce5Mn5 and K1Ce5Mn5 catalysts suggested that the introduction of K+ promoted the conversion of adsorbed benzene. Furthermore, intermediate products were transformed more rapidly for K1Ce5Mn5 compared to Ce5Mn5.

Evolution mechanism of transition metal in NH3-SCR reaction over Mn-based bimetallic oxide catalysts: Structure-activity relationships

The unexpected phenomenon in which different transition metals (Co, Ni and Cu) presented significant variation of participation levels as the auxiliaries in Mn-based bimetallic oxide catalysts were reported here. It is found that the Co element more easily to form Mn enriched surface bimetallic oxides with Mn than Ni and Cu, resulting in Co-MnO_x exhibited the best deNO_x activity and SO₂ tolerance, followed by Ni-MnO_x and Cu-MnO_x. The role of different transition metal and structure-activity relationships were systematically investigated by advanced techniques including Synchrotron XAFS and in situ DRIFTs analysis. The excellent activity of Co-MnO_x was related to its unique Mn-enriched surface (Co²⁺)_tet(Mn³⁺ Co³⁺)_octO₄ structure with Mn cations occupying the octahedral sites, which is superior to the Ni-MnO_x and Cu-MnO_x with Mn-lean surface. In addition, the reaction energy barrier of Co-MnO_x is weakened due to the lower electron cloud density around the Mn atom as compared to Ni-MnO_x and Cu-MnO_x. Moreover, Co-MnO_x benefiting from the rapid electron migration between Mn and Co, more active bidentate/bridged nitrates could react with adsorbed NH₃ in faster reaction rates following the L-H mechanism.

MnO2 -Based Materials for Environmental Applications

Manganese dioxide (MnO₂ ) is a promising photo-thermo-electric-responsive semiconductor material for environmental applications, owing to its various favorable properties. However, the unsatisfactory environmental purification efficiency of this material has limited its further applications. Fortunately, in the last few years, significant efforts have been undertaken for improving the environmental purification efficiency of this material and understanding its underlying mechanism. Here, the aim is to summarize the recent experimental and computational research progress in the modification of MnO₂ single species by morphology control, structure construction, facet engineering, and element doping. Moreover, the design and fabrication of MnO₂ -based composites via the construction of homojunctions and MnO₂ /semiconductor/conductor binary/ternary heterojunctions is discussed. Their applications in environmental purification systems, either as an adsorbent material for removing heavy metals, dyes, and microwave (MW) pollution, or as a thermal catalyst, photocatalyst, and electrocatalyst for the degradation of pollutants (water and gas, organic and inorganic) are also highlighted. Finally, the research gaps are summarized and a perspective on the challenges and the direction of future research in nanostructured MnO₂ -based materials in the field of environmental applications is presented. Therefore, basic guidance for rational design and fabrication of high-efficiency MnO₂ -based materials for comprehensive environmental applications is provided.

Mn2NiO4 spinel catalyst for high-efficiency selective catalytic reduction of nitrogen oxides with good resistance to H2O and SO2 at low temperature

Mn-Ni oxides with different compositions were prepared using standard co-precipitation (CP) and urea hydrolysis-precipitation (UH) methods and optimized for the selective catalytic reduction of nitrogen oxides (NO_x) by NH₃ at low temperature. Mn₍₂₎Ni₍₁₎O_x-CP and Mn₍₂₎Ni₍₁₎O_x-UH (with Mn:Ni molar ratio of 2:1) catalysts showed almost identical selective catalytic reduction (SCR) catalytic activity, with about 96% NO_x conversion at 75°C and ~99% in the temperature range from 100 to 250°C. X-ray diffraction (XRD) results showed that Mn₍₂₎Ni₍₁₎O_x-CP and Mn₍₂₎Ni₍₁₎O_x-UH catalysts crystallized in the form of Mn₂NiO₄ and MnO₂-Mn₂NiO₄ spinel, respectively. The latter gave relatively good selectivity to N₂, which might be due to the presence of the MnO₂ phase and high metal-O binding energy, resulting in low dehydrogenation ability. According to the results of various characterization methods, it was found that a high density of surface chemisorbed oxygen species and efficient electron transfer between Mn and Ni in the crystal structure of Mn₂NiO₄ spinel played important roles in the high-efficiency SCR activity of these catalysts. Mn₍₂₎Ni₍₁₎O_x catalysts presented good resistance to H₂O or/and SO₂ with stable activity, which benefited from the Mn₂NiO₄ spinel structure and Eley-Rideal mechanism, with only slight effects from SO₂.

Comparison of support synthesis methods for TiO2and the effects of surface sulfates on its activity toward NH3-SCR

Addition of SO₄²⁻inhibits the transformation of TiO₂from anatase to rutile and generates sulfate salts to increase the surface acidity.