Enhanced Replenishment of Active Lattice Oxygen Using Chiral Copper Oxide

Effective catalytic performance of the transition metal oxide is attributed to high specific surface areas, abundant surface oxygen atoms, and balanced valence ratios. Although the chirality of the transition metal has attracted attention, most studies have focused on optical application. A few chiral transition metal oxides were used as electrocatalysts and photocatalysts. The influence of the chiral catalysts on the thermal catalysis process has been less explored. In this study, Mn-loaded chiral (M/l-CuO and M/d-CuO) and achiral CuO (M/a-CuO) were synthesized and compared in the catalytic oxidization of toluene. Spectrally analyzed Mn was well-dispersed on both chiral and achiral CuO. l-CuO and d-CuO showed nanoflower-like chirality. The angles between each (001) plane of CuO were the source of chirality. The toluene turnover frequency (TOF) of the samples was in the order of Mn/d-CuO (5.6 × 10-5 s-1) > Mn/l-CuO (4.4 × 10-5 s-1) > Mn/a-CuO (3.2 × 10-5 s-1) at 240 °C, consistent with the order of the oxygen replenishment rate. The as-prepared catalysts had similar ratios of lattice oxygen/surface adsorbed oxygen, Mn3+/Mn4+, and Cu+/Cu2+. A higher TOF was attributed to chirality, which increased the lattice oxygen replenishment speed from the gaseous phase to the solid surface. Our study indicates gas-solid catalysis from a structure-activity viewpoint.

Exceptional Low-Temperature CO Oxidation over Noble-Metal-Free Iron-Doped Hollandites: An In-Depth Analysis of the Influence of the Defect Structure on Catalytic Performance

A family of iron-doped manganese-related hollandites, K_xMn_1–yFe_yO_2−δ (0 ≤ y ≤ 0.15), with high performance in CO oxidation have been prepared. Among them, the most active catalyst, K_0.11Mn_0.876Fe_0.123O_1.80(OH)_0.09, is able to oxidize more than 50% of CO at room temperature. Detailed compositional and structural characterization studies, using a wide battery of thermogravimetric, spectroscopic, and diffractometric techniques, both at macroscopic and microscopic levels, have provided essential information about this never-reported behavior, which relates to the oxidation state of manganese. Neutron diffraction studies evidence that the above compound stabilizes hydroxyl groups at the midpoints of the tunnel edges as in isostructural β-FeOOH. The presence of oxygen and hydroxyl species at the anion sublattice and Mn³⁺, confirmed by electron energy loss spectroscopy, appears to play a key role in the catalytic activity of this doped hollandite oxide. The analysis of these detailed structural features has allowed us to point out the key role of both OH groups and Mn³⁺ content in these materials, which are able to effectively transform CO without involving any critical, noble metal in the catalyst formulation.

Recent Manganese Oxide Octahedral Molecular Sieves (OMS–2) with Isomorphically Substituted Cationic Dopants and Their Catalytic Applications

The present report describes the structural and physical–chemical variations of the potassium manganese oxide mineral, α–MnO2, which is a specific manganese octahedral molecular sieve (OMS) named cryptomelane (K–OMS–2), with different transition metal cations. We will describe some frequently used synthesis methods to obtain isomorphic substituted materials [M]–K–OMS–2 by replacing the original manganese cationic species in a controlled way. It is important to note that one of the main effects of doping is related to electronic environmental changes, as well as to an increase of oxygen species mobility, which is ultimately related to the creation of new vacancies. Given the interest and the importance of these materials, here, we collect the most recent advances in [M]–K–OMS–2 oxides (M = Ag, Ce, Mo, V, Nb, W, In, Zr and Ru) that have appeared in the literature during the last ten years, leaving aside other metal–doped [M]–K–OMS–2 oxides that have already been treated in previous reviews. Besides showing the most important structural and physic-chemical features of these oxides, we will highlight their applications in the field of degradation of pollutants, fine chemistry and electrocatalysis, and will suggest potential alternative applications.

Mineral Manganese Oxides as Oxidation Catalysts: Capabilities in the CO-PROX Reaction

Cryptomelane is an abundant mineral manganese oxide with unique physicochemical features. This work investigates the real capabilities of cryptomelane as an oxidation catalyst. In particular, the preferential CO oxidation (CO-PROX), has been studied as a simple reaction model. When doped with copper, the cryptomelane-based material has revealed a great potential, displaying a comparable activity to the high-performance CuO/CeO₂. Despite stability concerns that compromise the primary catalyst reusability, CuO/cryptomelane is particularly robust in the presence of CO₂ and H₂O, typical components of realistic CO-PROX streams. The CO-PROX reaction mechanism has been assessed by means of isotopic oxygen pulse experiments. Altogether, CuO/CeO₂ shows a greater oxygen lability, which facilitates lattice oxygen enrolment in the CO-PROX mechanism. In the case of CuO/cryptomelane, in spite of its lower oxygen mobility, the intrinsic structural water co-assists as active oxygen species involved in CO-PROX. Thus, the presence of moisture in the reaction stream turns out to be beneficial for the stability of the cryptomelane structure, besides aiding into the active oxygen restitution in the catalyst. Overall, this study proves that CuO/cryptomelane is a promising competitor to CuO/CeO₂ in CO-PROX technology, whose implementation can bring the CO-PROX technology and H₂ purification processes a more sustainable nature.

Regeneration of Hopcalite used for the adsorption plasma catalytic removal of toluene by non-thermal plasma

A dielectric barrier discharge reactor packed with both Hopcalite & glass beads has been investigated for the total oxidation of toluene adsorbed on Hopcalite. The catalytic activity and selectivity through the possible formation of by-products during the NTP discharge for the abatement of irreversibly adsorbed toluene have been investigated by FT-IR and mass spectrometer. The regeneration of the used Hopcalite by NTP discharge has been established by (i) determining the amount of toluene adsorbed on NTP regenerated Hopcalite, (ii) investigating the catalytic activity of NTP regenerated Hopcalite and (iii) comparing the bulk and surface properties of the fresh calcined and NTP regenerated Hopcalite. The ratio of amount of irreversibly adsorbed toluene to that of the total amount of adsorbed toluene adsorbed is similar for the fresh calcined and NTP (I) regenerated Hopcalite. The catalytic activity of the NTP (I) regenerated Hopcalite is slightly enhanced when compared to that of the fresh calcined Hopcalite. Although the first NTP treatment induces partial transformation of Hopcalite into Mn₃O₄ with no detected related CuO_x and reduces specific surface area by a factor of 2, the toluene adsorption capacity remains less affected. A plausible reaction scheme for toluene decomposition in Hopcalite PBDBD reactor is proposed.