Catalytic oxidation of toluene by manganese oxides: Effect of K+ doping on oxygen vacancy

Synergistic engineering of oxygen vacancies, cation vacancies, and surface acidity in MOFs-derived Co-Mn spinel oxides via acid etching: A pathway to enhanced toluene oxidation performance

MOFs-derived spinel oxides hold great potential for the catalytic oxidation of VOCs, but their low intrinsic activity and unmanageable surface defects limit practical applications. Herein, this study proposes a method for oxalic acid etching MIL-101(CoMn) to synergistically regulate oxygen vacancies, cation vacancies, and surface acidity for enhanced toluene oxidation. Oxalic acid selectively dissolves Co2 +, leading to the generation of cobalt vacancies. While cobalt vacancies can facilitate the migration or detachment of oxygen from the lattice, thus contributing to the formation of oxygen vacancies. The coexistence of oxygen vacancies and cobalt vacancies, with exposed surface atoms and unsaturated metal centers, enhances surface acidity. Importantly, mild oxalic acid converts strong acid sites into medium-strength ones, improving toluene adsorption and reducing carbon deposition. Meanwhile, acid etching reconfigures surface morphology to transform spherical into lamellar structures with higher specific surface area and edge atomic density. DFT calculations confirm that oxygen vacancies and cobalt vacancies optimize electronic structures of Co and Mn, boosting electron exchange and redox properties. In-situ DRIFTS reveals that this synergistic modulation improves the generation and consumption of intermediates. Hence, MIL-101(CoMn)/O shows higher activity and stability with 90 % toluene conversion at 223 °C, lower than CoMn (293 °C) and MIL-101(CoMn) (267 °C). This work offers insights for designing efficient MOFs-derived spinel oxides in environmental remediation.

Constructing oxygen vacancies in Cu-doped MnO2 by a quenching strategy for boosting the catalytic oxidation of toluene

Here, a quenching strategy was developed to create oxygen vacancies in Cu doped α-MnO2. The evolutions of oxygen vacancies were directly followed by means of XRD refinement, EPR and XPS. In combination with DFT calculations and detailed characterizations, evidence is captured that oxygen vacancies not only act as direct sites for the adsorption and activation of gaseous oxygen and toluene, but also accelerate the consumption and replenishment cycle of lattice oxygen species by weakening the strength of metal-oxygen bonds. In situ DRIFTS study reveals that both adsorbed oxygen and lattice oxygen species directly participate in the oxidative decomposition of toluene, where adsorbed oxygen species play pivotal roles in the initial oxidation of toluene to benzoate, whereas the process of ring opening of benzoate relies on the activation of lattice oxygen. Benefiting from crucial contribution of oxygen vacancies in activating oxygen species, α-CuMnO2-500-Q obtained by the quenching method is capable of fully catalyzing the oxidation of toluene at 240 ℃, representing a reduction of about 80 ℃ compared to pristine α-CuMnO2-500. Furthermore, the toluene oxidation mechanism was proposed as well via in situ DRIFT spectra.

Extraordinary synergy on 3D hierarchical porous Co-Cu nanocomposite for catalytic elimination of VOCs at low temperature and high space velocity

It is still a challenge to develop hierarchically nanostructured catalysts with simple approaches to enhance the low-temperature catalytic activity. Herein, a set of mesoporous Co-Cu binary metal oxides with different morphologies were successfully prepared via a facile ammonium bicarbonate precipitation method without any templates or surfactants, which were further applied for catalytic removal of carcinogenic toluene. Among the catalysts with different ratios, the CoCu0.2 composite oxide presented the best performance, where the temperature required for 90% conversion of toluene was only 237°C at the high weight hour space velocity (WHSV) of 240,000 mL/(gcat·hr). Meanwhile, compared to the related Co-Cu composite oxides prepared by using different precipitants (NaOH and H2C2O4), the NH4HCO3-derived CoCu0.2 sample exhibited better catalytic efficiency in toluene oxidation, while the T90 were 22 and 28°C lower than those samples prepared by NaOH and H2C2O4 routes, respectively. Based on various characterizations, it could be deduced that the excellent performance was related to the small crystal size (6.7 nm), large specific surface area (77.0 m2/g), hollow hierarchical nanostructure with abundant high valence Co ions and adsorbed oxygen species. In situ DRIFTS further revealed that the possible reaction pathway for the toluene oxidation over CoCu0.2 catalyst followed the route of absorbed toluene → benzyl alcohol → benzaldehyde → benzoic acid → carbonate → CO2 and H2O. In addition, CoCu0.2 sample could keep stable with long-time operation and occur little inactivation under humid condition (5 vol.% water), which revealed that the NH4HCO3-derived CoCu0.2 nanocatalyst possessed great potential in industrial applications for VOCs abatement.

Inhibition Effect of H2O on the Heterogeneous Reaction between Isoprene and Fe-Substituted Cryptomelane

The transportation and transformation of biogenic isoprene are vital for the organic carbon cycle in the troposphere. As a typical mineral with high oxidation potential, Fe-substituted cryptomelane oxidizes the surface monolayer of isoprene into formic and acetic acids, and simultaneously, the Mn4+ ions in the structure are reduced to Mn3+ and Mn2+. The flow of H2O in isoprene decreases the adsorption and oxidation of isoprene significantly, even at low relative humidity (10%). As physisorbed H2O retains Fe-substituted cryptomelane's crystal structure and oxidation ability, the adsorption and oxidation capacity recovers when H2O is absent in the isoprene flow. Theoretical calculations on (001) surfaces show that isoprene prefers to be adsorbed by the Fe3+ site and H2O tends to form hydrogen bonds. Due to the decrease in total adsorption energy of H2O and isoprene, Fe-substituted cryptomelane favors the adsorption of H2O in the flow of humid isoprene. The low oxidation performance at ambient relative humidity suggests that direct oxidation by aerosols of mineral dust might not be the transformation pathway of biogenic isoprene at night.

Engineering oxygen vacancies in acid-etched MgMn2O4 for efficiently catalytic benzene combustion: Synergistic activation of gaseous oxygen and surface lattice oxygen

The synergistic activation of gaseous oxygen and surface lattice oxygen is essential for designing highly efficient catalysts to eliminate VOCs. Herein, an effective acid treatment was carried out to create more oxygen vacancies by modulating the electronic structure of MgMn2O4 spinels and MgMnOx mixed oxides. The acid-treated MgMn2O4 exhibited outstanding catalytic performance, with the reaction rate of benzene rising by 8.55 times at 200 °C. After acid treatment, MgMn2O4 partially retained its spinel structure, while Mn2O3 in situ grew on the surface due to the selective removal of Mg2+. The transformation of Mn-O-Mg into Mn-O weakened the strength of adjacent Mn-O bonds, thereby promoting the release of surface lattice oxygen and the regeneration of oxygen vacancies. In addition, acid-treated MgMn2O4 facilitated the adsorption and activation of gaseous oxygen. In situ DRIFTS analysis proved that the synergistic activation of gaseous oxygen and surface lattice oxygen accelerated the conversion of intermediates, thus contributing to the efficient degradation of benzene.

Revealing the key role of interfacial oxygen activation over CoMn2O4@MnO2 in the catalytic oxidation of acetone

The accumulation of intermediate products on the catalyst surface caused by insufficient oxygen activity is an important reason for the poor activity of catalysts towards oxygenated volatile organic compounds (OVOCs). CoMn2O4@MnO2 heterogeneous catalysts were fabricated to decipher the interfacial oxygen activation mechanism for efficient acetone oxidation. Experimental and theoretical explorations revealed that oxygen vacancies were easily formed at the interface. Gaseous oxygen tended to adsorb on the interfacial vacancies while bonding with adjacent Mn sites, resulting in the stretching of O-O bonds. Rapid electron transfer at the interface led to the charge accumulation on the two oxygen atoms inducing electrostatic repulsion. These factors are conducive to the O-O bond breaking and gaseous oxygen activation. The obtained CoMn2O4@0.8MnO2 exhibited excellent catalytic performance with 90 % of acetone conversion at 159 °C, better than CoMn2O4 and MnO2. The acetone oxidation on CoMn2O4@0.8MnO2 not only avoided the accumulation of aldehydes, but also realized the rapid degradation of acetate into formate, achieving the shortest degradation pathway due to the rapid interfacial oxygen activation. CoMn2O4@0.8MnO2 also exhibited better catalytic activity for other OVOCs (ethyl acetate, ethylene oxide, methanol). This work provides new insights for the mechanism of interfacial oxygen activation and the design of heterogeneous catalyst for efficient OVOC oxidation.

Regulating the mobility of lattice oxygen on hollow cobalt-manganese sub-nanospheres for enhanced catalytic oxidation of toluene and o-xylene

Promoting lattice oxygen mobility of Co-based catalysts is crucial to making progress in catalytic oxidation technology. The addition of manganese, a transition metal with similar ionic radius to cobalt and variable valence, was supposed to enhance the mobility of lattice oxygen species of Co-based oxide. A range of hollow CoMnaOx sub-nanosphere catalysts with different Mn/Co ratios was synthesized via a template-sacrificed method, and the effects of different Mn/Co ratios on the structural properties of the catalysts and their catalytic performance for benzene series volatile organic compounds (VOCs) oxidation were investigated. Hollow CoMn2Ox sub-nanosphere exhibited good catalytic activity for oxidation of toluene (T90 = 265 °C) and o-xylene (T90 = 297 °C), as well as excellent recycling ability and water resistance. By adjusting the Mn/Co ratio, metal ions enter into the different tetrahedral or octahedral active sites. Compared with Co3O4, the desorption temperature of surface lattice oxygen on CoMn2Ox decreased by 110 °C. These results demonstrate that the addition of manganese can encourage the electron transfer on CoMnaOx, indicating that the introduction of the appropriate amount of manganese accelerates the activation of gas O2 and mobility of surface lattice oxygen species, thereby expediting the oxidation of benzene series VOCs.

Ce-based three-dimensional mesoporous microspheres with Mn homogeneous incorporation for toluene oxidation

Ce-based three-dimensional (3D) mesoporous microspheres with Mn homogeneous incorporation were synthesized. The CeMn-0.4, characterized by a Ce/Mn molar ratio of 6:4, demonstrated exceptional catalytic activity and stability. The formation of CeMn solid solution strengthened the Ce-Mn interaction, yielding higher concentrations of Ce3+ and Mn4+. Mn4+ initiated toluene preliminary activation owing to its robust oxidative properties, while Ce3+ contributed to oxygen vacancy generation, enhancing the activation of gaseous oxygen and lattice oxygen mobility. Integrating experiments and Density Functional Theory (DFT) calculations elucidated the oxygen reaction mechanisms. A portion of oxygen was converted into surface reactive oxygen species (Oads) that directly oxidized toluene. Additionally, the presence of oxygen vacancies promoted the participation of oxygen in toluene oxidation by converting it into lattice oxygen, which was crucial for the deep oxidation of toluene. Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFTS) indicated the accumulation of benzene-ring intermediates on the catalyst surface hindered continuous toluene oxidation. Thus, the abundant oxygen vacancies in CeMn-0.4 played a pivotal role in sustaining the oxidation process by bolstering the activation of gaseous oxygen and the mobility of lattice oxygen.

A facile method for preparing the CeMnO3 catalyst with high activity and stability of toluene oxidation: The critical role of small crystal size and Mn3+-Ov-Ce4+ sites

Volatile organic compounds (VOCs) cause severe environmental pollution and are potentially toxic to humans who have no defense against exposure. Catalytic oxidation of these compounds has thus become an interesting research topic. In this study, microcrystalline CeMnO3 catalysts were prepared by a precipitant-concentration-induced strategy and evaluated for the catalytic oxidation of toluene/benzene. The effect of crystal size on catalytic performance was confirmed by XRD, TEM, N2 adsorption-desorption, XPS, Raman, H2-TPR, and TPSR. The CeMnO3 catalyst with more Mn3+-Ov-Ce4+ active sites exhibited enhanced VOCs catalytic oxidation performance, lowest active energy, and highest turnover frequency, which was attributed to its larger surface area, lower crystal size, higher low-temperature reducibility, and presence of more oxygen defects. In-situ FTIR results suggested more oxygen vacancies can profoundly promote the conversion of benzoate to maleate species, the rate-determining step of toluene oxidation. The work provides a convenient and efficient strategy to prepare single-metal or multi-metal oxide catalysts with smaller crystal sizes for VOC oxidation or other oxidation reactions.