Cerium, manganese and cerium/manganese ceramic monolithic catalysts. Study of VOCs and PM removal

Modeling and experimental analysis of CO2 methanation reaction using Ni/CeO2 monolithic catalyst

In this study, the effect of the cell density of monolithic catalysts was investigated and further mathematically modeled on cordierite supports used in CO2 methanation. Commercial cordierite monoliths with 200, 400, and 500 cpsi cell densities were coated by immersion into an ethanolic suspension of Ni/CeO2 active phase. SEM-EDS analysis confirmed that, owing to the low porosity of cordierite (surface area < 1 m2 g-1), the Ni/CeO2 diffusion into the walls was limited, especially in the case of low and intermediate cell density monoliths; thus, active phase was predominantly loaded onto the channels' external surface. Nevertheless, despite the larger exposed surface area in the monolith with high cell density, which would allow for better distribution and accessibility of Ni/CeO2, its higher macro-pore volume resulted in some introduction of the active phase into the walls. As a result, the catalytic evaluation showed that it was more influenced by increments in volumetric flow rates. The low cell density monolith displayed diffusional control at flow rates below 500 mL min-1. In contrast, intermediate and high cell density monoliths presented this behavior up to 300 mL min-1. These findings suggest that the interaction reactants-catalyst is considerably more affected by a forced non-uniform flow when increasing the injection rate. This condition reduced the transport of reactants and products within the catalyst channels and, in turn, increased the minimum temperature required for the reaction. Moreover, a slight diminution of selectivity to CH4 was observed and ascribed to the possible formation of hot spots that activate the reverse water-gas shift reaction. Finally, a mathematical model based on fundamental momentum and mass transfer equations coupled with the kinetics of CO2 methanation was successfully derived and solved to analyze the fluid dynamics of the monolithic support. The results showed a radial profile with maximum fluid velocity located at the center of the channel. A reactive zone close to the inlet was obtained, and maximum methane production (4.5 mol m-3) throughout the monolith was attained at 350 °C. Then, linear streamlines of the chemical species were developed along the channel.

Reaction Kinetics and Mechanism of VOCs Combustion on Mn-Ce-SBA-15

A propane combustion catalyst based on Mn and Ce and supported by SBA-15 was prepared by the “two-solvents’’ method aiming at the possible application in catalytic converters for abatement of alkanes in waste (exhaust) gases. The catalyst characterization was carried out by SAXS, N2-physisorption, XRD, TEM, XPS, EPR and H2-TPR methods. The catalysts’ performance was evaluated by tests on the combustion of methane, propane and butane. The reaction kinetics investigation showed that the reaction orders towards propane and oxygen were 0.7 and 0.1, respectively. The negative reaction order towards the water (−0.3) shows an inhibiting effect on the water molecules. Based on the data from the instrumental methods, catalytic experiments and mathematic modeling of the reaction kinetics, one may conclude that the Mars–van Krevelen type of mechanism is the most probable for the reaction of complete propane oxidation over single Mn and bi-component Mn–Ce catalysts. The fine dispersion of manganese and cerium oxide and their strong interaction inside the channels of the SBA-15 molecular sieve leads to the formation of difficult to reduce oxide phases and consequently, to lower catalytic activity compared to the mono-component manganese oxide catalyst. It was confirmed that the meso-structure was not modified during the catalytic reaction, thus it can prevent the agglomeration of the oxide particles.

MnII/III and CeIII/IV Units Supported on an Octahedral Molecular Nanoparticle of CeO2

The preparation of three new heterometallic clusters [Ce₆Mn₁₂O₁₇(O₂CPh)₂₆] (1), [Ce₁₀Mn₁₄O₂₄(O₂CPh)₃₂] (2), and [Ce₂₃Mn₂₀O₄₈(OH)₂(tbb)₄₆(H₂O)₄](NO₃)₂ (3; tbb^- = 4-^tBu-benzoate) is reported. They all possess unprecedented structures with a common feature being the presence of an octahedral Ce^IV-oxo core: a Ce₆ in 1, two edge-fused Ce₆ giving a Ce₁₀ bioctahedron in 2, or a larger Ce₁₉ octahedron in 3. Complex 1 is the first Ce₆ cluster with a central μ₆-O^2-. 2 and the cation of 3 are molecular nanoparticles of CeO₂ (ceria) because they possess the fluorite structure of bulk ceria and are thus ultrasmall ceria nanoparticles in molecular form. The {Ce₁₉O₃₂} octahedral subunit of the cation of 3 had been predicted from density functional theory studies to be one of the stable fragments of the CeO₂ lattice, but has never been previously synthesized in molecular chemistry. Around the Ce/O core of 1-3 is an incomplete monolayer of Mn_n ions disposed as four Mn₃, two Mn₇, and four Mn₅ units, respectively. This represents a clear structural similarity with composite (phase-separated) CeO₂/MnO_x mixtures where at high Ce:Mn ratios the Mn atoms segregate on the surface of CeO₂ phases. Variable-temperature dc and ac magnetic susceptibility studies have revealed S = 2, S = ¹/₂, and S = ³/₂ ground states for 1-3, respectively. Fitting of the 5.0-300 K dc data for 1 to a two-J model for an asymmetrical V-shaped Mn₃ unit with no interaction between the end Mn^III ions gave an excellent fit with the following values: J₁ = 5.2(3) cm^-1, J₂ = -7.4(3) cm^-1, and g = 1.96(2).

The Formation of Mn-Ce Oxide Catalysts for CO Oxidation by Oxalate Route: The Role of Manganese Content

The Mn-Ce oxide catalysts active in the oxidation of CO were studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), transition electron microscopy (TEM), energy dispersive X-Ray (EDX), and a differential dissolution technique. The Mn-Ce catalysts were prepared by thermal decomposition of oxalates by varying the Mn:Ce ratio. The nanocrystalline oxides with a fluorite structure and particle sizes of 4–6 nm were formed. The introduction of manganese led to a reduction of the oxide particle size, a decrease in the surface area, and the formation of a Mn_yCe_1−yO_2−δ solid solution. An increase in the manganese content resulted in the formation of manganese oxides such as Mn₂O₃, Mn₃O₄, and Mn₅O₈. The catalytic activity as a function of the manganese content had a volcano-like shape. The best catalytic performance was exhibited by the catalyst containing ca. 50 at.% Mn due to the high specific surface area, the formation of the solid solution, and the maximum content of the solid solution.

Effect of ceria morphology on the performance of MnOx/CeO2 catalysts in catalytic combustion of N,N-dimethylformamide

MnO_x/CeO₂ catalysts were prepared by a deposition–precipitation method, through loading MnO_x into ceria supports with different morphologies (nanorods (NRs), nanocubes (NCs) and nano-octahedrons (NOs)).

Catalytic removal of toluene over manganese oxide-based catalysts: a review

It is necessary to control the emissions of toluene, which is hazardous to both human health and the atmosphere environment and has been classified as a priority pollutant. Manganese oxide-based (Mn-based) catalysts have received increased attention due to their high catalytic performance, good physicochemical characteristic, availability in various crystal structures and morphologies, and being environmentally friendly and low cost. These catalysts can be classified into five categories, namely single manganese oxide, Mn-based composite oxides, Mn-based special oxides, supported Mn-based oxides, and Mn-based monoliths. This review focused on the recent progress on the five types of Mn-based catalysts for catalytic removal of toluene at low temperature and further systematically summarized the strategies improving catalysts, including improving synthetic methods, incorporating MnO_x with other metal oxides, depositing Mn-based oxides on proper supports, and tuning the supports. Moreover, the effect of coexisting components, the reaction kinetics, and the oxidation mechanisms toward the removal of toluene were also discussed. Finally, the future research direction of this field was presented.