Manganese oxide catalysts prepared by a precipitation method with various precipitants were investigated for the low temperature selective catalytic reduction (SCR) of NOx with NH 3 in the presence of excess O 2 . Various characterization methods such as N 2 adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA) and X-ray absorption near edge structure (XANES) were conducted to probe the physical and chemical properties of MnOx catalysts. The active MnOx catalysts, precipitated with sodium carbonate and calcined in air at moderate temperatures such as 523 K and 623 K, have the high surface area, the abundant Mn 4+ species, and the high concentration of surface oxygen on the surface. The amorphous Mn 3 O 4 and Mn 2 O 3 were mainly present in this active catalyst. The carbonate species appeared to help adsorb NH 3 on the catalyst surface, which resulted in the high catalytic activity at low temperatures.

Manganese oxide catalysts for NOx reduction with NH3 at low temperatures

A series of manganese oxides differing in the structure, composition, average manganese oxidation state and specific surface area have been used in the total oxidation of volatile organic compounds (VOC). Ethanol, ethyl acetate and toluene were chosen as models of VOC. Among the manganese oxides tested, cryptomelane (KMn8O16) was found to be very active in the oxidation of VOC. The performance of cryptomelane was significantly affected by the presence of other phases, namely, Mn2O3 and Mn3O4. Temperature-programmed experiments combined with X-ray photoelectron spectroscopy (XPS) show that the mobility and reactivity of the oxygen species were significantly affected, explaining the catalytic performances of those samples. Mn3O4 improves the catalytic performance due to the increase of the reactivity and mobility of lattice oxygen, while Mn2O3 has the opposite effect. These results show that there is a correlation between the redox properties and the catalytic performance of the manganese oxides. Temperature-programmed surface reactions (TPSR) after adsorption of toluene or ethanol, in addition to reactions performed without oxygen in the feed, show that lattice oxygen is involved in the VOC oxidation mechanism. The conversion level was found to be influenced by the type of VOC, the reactivity into CO2 increasing in the following order: Toluene

The role of lattice oxygen on the activity of manganese oxides towards the oxidation of volatile organic compounds

Nickel-doped Mn/TiO2 as an efficient catalyst for the low-temperature SCR of NO with NH3: Catalytic evaluation and characterizations

Mesoporous Co3O4–CeO2 and Pd/Co3O4–CeO2 catalysts: Synthesis, characterization and mechanistic study of their catalytic properties for low-temperature CO oxidation

Alternative energy technologies are greatly hindered by significant limitations in materials science. From low activity to poor stability, and from mineral scarcity to high cost, the current materials are not able to cope with the significant challenges of clean energy technologies. However, recent advances in the preparation of nanomaterials, porous solids, and nanostructured solids are providing hope in the race for a better, cleaner energy production. The present contribution critically reviews the development and role of mesoporosity in a wide range of technologies, as this provides for critical improvements in accessibility, the dispersion of the active phase and a higher surface area. Relevant examples of the development of mesoporosity by a wide range of techniques are provided, including the preparation of hierarchical structures with pore systems in different scale ranges. Mesoporosity plays a significant role in catalysis, especially in the most challenging processes where bulky molecules, like those obtained from biomass or highly unreactive species, such as CO2 should be transformed into most valuable products. Furthermore, mesoporous materials also play a significant role as electrodes in fuel and solar cells and in thermoelectric devices, technologies which are benefiting from improved accessibility and a better dispersion of materials with controlled porosity.

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Mesoporous materials for clean energy technologies

Manganese–lanthanum oxides modified with silver for the catalytic combustion of methane

Cryptomelane-type manganese oxides modified with silver were prepared by the redox precipitation technique. The catalysts were obtained by the direct introduction of silver to the synthesis mixture containing manganese and the impregnation technique. The catalysts showed high activity in the low-temperature CO oxidation reaction. Studies of the nitrogen adsorption/desorption, powder X-ray diffraction, Fourier transform infrared spectroscopy in the photoacoustic mode (FT-IR/PAS), and atomic force microscopy (AFM) evidenced strong influence of the preparation methods and the pre-treatment conditions on the structural and catalytic properties of the samples. It was found that the presence of silver in the synthesis mixture caused partial replacement of potassium ions in the tunnels, leading to the partial decomposition of the well ordered structure of the manganese oxides, increasing reducibility and activity of the catalyst. Structural changes were not observed in the samples prepared by the impregnation technique. Slight increase of activity of the impregnated samples was related to the increase of oxygen mobility.

The influence of silver on the structural, redox and catalytic properties of the cryptomelane-type manganese oxides in the low-temperature CO oxidation reaction

Ni/Al2O3 catalyst modified by small amounts of Mo show unusual properties in the steam reforming of hydrocarbons. There are no data about the effect of small amounts of molybdenum on reduction of the Ni-Mo supported catalysts. The properties of these very complex systems depend on the conditions of successive preparation stages (calcination, reduction) or the process conditions. A series of Ni/Al2O3 catalysts modified by Mo were prepared in order to investigate the influence of promoter amounts and preparation sequence on their properties. Temperature programmed reduction (TPR) has been employed to study the reducibility of Ni-Mo/Al2O3 catalysts. Catalysts were further characterized by BET area, H2 chemisorption and X-ray diffraction measurements. The TPR curves of Ni-Mo/Al2O3 catalysts are very complex. Mo addition leads to the decrease of catalysts reducibility. However, complete reduction of NiO and MoO3 can be achieved at 800 °C. The reduction course depends on the sequence of nickel and molybdenum addition into the support. Precise measurements of Ni peaks positions in the XRD pattern of Ni/Al2O3 and Ni-Mo/Al2O3 samples show the possibility of Ni-Mo solid solution formation.

Effects of small MoO3 additions on the properties of nickel catalysts for the steam reforming of hydrocarbons. III. Reduction of Ni-Mo/Al2O3 catalysts

Studies of methane decomposition were performed over nickel alumina catalysts modified with magnesia. The catalysts were obtained by the co-precipitation method. The properties of catalysts were investigated by the temperature programmed reduction (TPR) and hydrogen desorption (TPD) methods. Methane decomposition was studied by the thermogravimetric and transient temperature programmed reaction methods. The nature of carbon deposits formed under different reaction conditions was investigated by the temperature programmed oxidation method (TPO) and high resolution transmission electron microscopy (HRTEM). An introduction of magnesium to the preparation mixture of catalysts led to the formation of smaller nickel crystallites with stronger adsorption sites. It was found out that the activity of catalysts and the properties of carbon deposits were related to the catalysts composition and the reaction temperature. Carbon filaments with fishbone-like packed graphitic layers were formed at 500 °C, while the multi-walled nanotubes were observed after reaction performed at 700 °C. The increase of magnesium content led to the increase of the rate of methane decomposition at the initial stages of the reaction. Deactivation of the catalysts was related to the insufficient removal of carbon species from the surface of nickel crystallites and the formation of stable, graphitic carbon deposits covering the surface of metal.

Wojciech Gac

Papers

Manganese–lanthanum oxides modified with silver for the catalytic combustion of methane

The influence of silver on the structural, redox and catalytic properties of the cryptomelane-type manganese oxides in the low-temperature CO oxidation reaction

Effects of small MoO3 additions on the properties of nickel catalysts for the steam reforming of hydrocarbons. III. Reduction of Ni-Mo/Al2O3 catalysts

Methane decomposition over Ni–MgO–Al2O3 catalysts

Steam reforming of ethanol over Ni/support catalysts for generation of hydrogen for fuel cell applications