Evolution and enhancement of the oxygen cycle in the catalytic performance of total toluene oxidation over manganese-based catalysts

Coupling effect of reaction conditions on the catalytic activity of Cu-Mn composite oxide catalysts for toluene

Volatile organic compounds (VOCs) are one of the major components of air pollution. Catalytic combustion is a promising technology for the treatment of VOCs and at its center is the preparation of efficient and cheap catalysts. In this study, by loading copper (Cu) and manganese (Mn) on Santa Barbara Amorphous-15 (SBA-15) molecular sieve, the Cux-Mny/SBA-15 (x = 1, 2; y = 1, 2) composite metal oxide catalyst was prepared using the equal volume impregnation method. Their performance in the toluene catalytic combustion reaction was investigated by adjusting the molar ratio (x : y), and the loading of Cu and Mn. The results of the Brunner-Emmett-Teller (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) analyses show that the CuMnO spinel phase can be detected in the Cu-Mn composite metal oxide catalyst doped with a low concentration of Cu. The overall rod-like structure of the fibrous network structure provides a large specific surface area, and the particle crystallinity is low and the dispersion is good. Due to the synergistic effect of Cu and Mn, the greater the amount of Mn3+ and adsorbed oxygen species (Oads) that are available, and the higher the turnover frequency (TOF) value, the better and more superior catalytic performance and excellent stability is obtained, when compared with the single-component oxides used in toluene catalytic combustion. After a continuous catalytic reaction for 12 h, the toluene conversion rate remained above 95%. The coupling effect of the catalytic reaction temperature and concentration of oxygen on the catalytic combustion of toluene was also studied. At a low reaction temperature (<250 °C), the increase of the concentration of oxygen played a superior role in promoting the conversion of toluene. The kinetic analysis of the toluene catalytic combustion process showed that the catalytic combustion of toluene by Cu-Mn/SBA-15 followed both the Mars-Van Krevelen (MVK) and Langmuir-Hinshelwood (L-H) reaction mechanisms. With the increase of the Oads amount caused by the decrease of the Cu ratio, the proportion of the L-H reaction mechanism increases.

Mesoporous molecular sieve-based materials for catalytic oxidation of VOC: A review

As the main contributor of the formation of particulate matter as well as ozone, volatile organic compounds (VOCs) greatly affect human health and the environmental quality. Catalytic combustion/oxidation has been viewed as an efficient, economically feasible and environmentally friendly way for the elimination of VOCs. Supported metal catalyst is the preferred type of catalysts applied for VOCs catalytic combustion because of the synergy between active components and support as well as its flexibility in the composition. The presence of support not only plays the role of keeping the catalyst with good stability and mechanical strength, but also provides a large specific surface for the good dispersion of active components, which could effectively improve the performance of catalyst as well as decrease the usage of active components, especially the noble metal amount. Mesoporous molecular sieves, owing to their large surface area, unique porous structures, large pore size as well as uniform pore-size distribution, were viewed as superior support for dispersing active components. This review focuses on the recent development of mesoporous molecular sieve supported metal catalysts and their application in catalytic oxidation of VOCs. The effect of active component types, support structure, preparation method, precursors, etc. on the valence state, dispersion as well as the loading of active species were also discussed and summarized. Moreover, the corresponding conversion route of VOCs was also addressed. This review aims to provide some enlightment for designing the supported metal catalysts with superior activity and stability for VOCs removal.

Inexpensive and easily replicable precipitation of CuO nanoparticles for low temperature carbon monoxide and toluene catalytic oxidation

Herein CuO nanoparticles (NPs) with nanostructures were prepared by precipitation method using hydrate copper sulfate (CuSO₄.5H₂O) and sodium hydroxide followed by heat treatment at 400 °C. The as-prepared CuO NPs with nanostructures were investigated using X-ray diffraction (XRD), Fourier Transformed Infra-red spectroscopy (FTIR), Raman spectroscopy, Scanning electron microscopy (SEM), X-ray photochemical spectroscopy (XPS), Energy dispersive spectroscopy (EDS), and Ultra-violet-visible (UV-vis) spectroscopy. In order to evaluate the reducibility, temperature programmed reduction (H₂-TPR) was applied. More importantly, CuO NPs was successfully tested as catalyst towards the total conversion of carbon monoxide (CO) and toluene (C₇H₈). Both XRD and Raman analysis as well as FTIR show that the sample exhibited a monoclinic spinel structure. SEM images indicate that CuO NPs are well-covered by grains size exhibiting homogeneous morphology composed of very fine interconnected particles with an apparent porosity. The sample was made up of Cu and O, according to the XPS and EDS measurements. The band gap energy obtained from optical property analysis is ∼2.65 eV. The catalytic performances of CuO NPs can be assigned to the combined effects of crystal structure, morphology, surface oxygen mobility, redox property and the higher specific surface area (∼87 m²/g). More precisely XPS and H₂-TPR data suggests that, the conversion of CO and C₇H₈ over CuO NPs follows a Mars-van Krevelen type mechanism. More importantly CuO NPs catalysts is reusable and exhibited good stability in the prolonged isothermal test. Thus, CuO NPs is confirmed as an efficient and inexpensive catalysts for CO and C₇H₈ conversion at low temperatures.

Exploring the roles of oxygen species in H2 oxidation at β-MnO2 surfaces using operando DRIFTS-MS

Understanding of the roles of oxygen species at reducible metal oxide surfaces under real oxidation conditions is important to improve the performance of these catalysts. The present study addresses this issue by applying a combination of operando diffuse reflectance infrared Fourier transform spectroscopy with a temperature-programmed reaction cell and mass spectrometry to explore the behaviors of oxygen species during H2 oxidation in a temperature range of 25-400 °C at β-MnO2 surfaces. It is revealed that O2 is dissociated simultaneously into terminal-type oxygen (M2+-O2-) and bridge-type oxygen (M+-O2--M+) via adsorption at the Mn cation with an oxygen vacancy. O2 adsorption is inhibited if the Mn cation is covered with terminal-adsorbed species (O, OH, or H2O). In a temperature range of 110-150 °C, OH at Mn cation becomes reactive and its reaction product (H2O) can desorb from the Mn cation, resulting in the formation of bare Mn cation for O2 adsorption and dissociation. At a temperature above 150 °C, OH is reactive enough to leave bare Mn cation for O2 adsorption and dissociation. These results suggest that bare metal cations with oxygen vacancies are important to improve the performance of reducible metal oxide catalysts.

Constructing a Pt/YMn2O5 Interface to Form Multiple Active Centers to Improve the Hydrothermal Stability of NO Oxidation

The hydrothermal stability of NO oxidation is the key to the practical application of diesel oxidation catalysts in diesel engines, which in the laboratory requires that NO activity does not decrease after aging for 10 h with 10% H₂O/air at 800 °C. On the one hand, the construction of a metal/oxide interface can lead to abundant oxygen vacancies (O_v), which compensate for the loss of activity caused by the aggregation of Pt particles after aging. On the other hand, YMn₂O₅ (YMO) has excellent thermal stability and NO oxidation capacity. Therefore, a Pt/YMn₂O₅-La-Al₂O₃ (Pt/YMO-LA) catalyst was prepared by the impregnation method. The support of the catalyst, YMn₂O₅-La-Al₂O₃ (YMO-LA), was obtained by mixing high specific surface LA and YMO ball-milling. Under laboratory-simulated diesel exhaust flow, the NO oxidation performance of Pt/YMO-LA did not decrease after hydrothermal aging. Combining high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and oxygen temperature-programmed desorption (O₂-TPD), the Pt/YMn₂O₅ interface was formed after hydrothermal aging, and the increased O_v can provide reactive oxygen to Pt and YMO. The cooperative catalysis of multiple active centers composed of Pt, YMO, and O_v is the crucial factor to maintain the NO oxidation performance. In addition, in situ diffuse reflectance infrared Fourier transform spectra (DRIFTs) show that an increase in O_v is beneficial to the adsorption and desorption of more nitrate and nitrite intermediates, thus achieving the hydrothermal stability of NO oxidation.

Insight into the effects of oxygen vacancy on the toluene oxidation over α-MnO2 catalyst

In order to clarify the role of oxygen vacancy (OV), five α-MnO₂ catalysts with abundant OVs are fabricated via a novel and facile redox-precipitation approach and employed to the toluene oxidation in air. The concentration of OVs in α-MnO₂ catalysts is regulated via the alkyl chain length of alcohols, and its correlation with catalytic performances is scientifically investigated based on various characterization technologies and density functional theory (DFT) calculation. The α-MnO₂-C2 catalyst exhibits excellent catalytic activity (T₉₀ = 217 °C), stability, and water resistance for toluene oxidation in air. The OVs can induce the new bandgap states (BGS), which upshift the antibonding orbitals relative to the Fermi level (E_f), eventually favoring the formation of adsorbed active oxygen species. Furthermore, the OVs cause an increase in the amount of Mn³⁺, resulting in the elongated Mn-O bonds due to the strong Jahn-Teller effect of Mn³⁺. Therefore, the synergistic effects of OVs benefit toluene oxidation through L-H and MvK mechanisms over the prepared α-MnO₂-Cx catalysts. This work reveals the important role of OVs in the promotion of toluene catalytic oxidation activity and also may provide new insights for the design of high-performance VOCs oxidation elimination catalyst.

High ratio of Ce3+/(Ce3++Ce4+) enhanced the plasma catalytic degradation of n-undecane on CeO2/γ-Al2O3

n-Undecane (C11) is the main component of volatile organic compounds (VOCs) emitted from the printing industry, and its emission to the atmosphere should be controlled. In this study, a dielectric barrier discharge reactor coupled with CeO₂/γ-Al₂O₃ catalysts was used to degrade C11. The effect of the chemical state of CeO₂ on C11 degradation was evaluated by varying the CeO₂ loading on γ-Al₂O₃. The C11 conversion and COx selectivity were as high as 92% and 80%, respectively, under mild reaction conditions of energy density 34 J/L and 423 K to degrade 134 mg/m³ C11 in a simulated air using 10 wt%CeO₂ impregnated on γ-Al₂O₃. After analyses using in-situ plasma diffuse reflectance Fourier transform infrared spectroscopy and gas chromatography-mass spectrometry, it was found that most of C11 were degraded to CO₂, and the main by-products on catalyst surfaces were alcohols and ketones. It was concluded from X-ray photoemission spectroscopy that the good performance of the 10 wt%CeO₂/γ-Al₂O₃ catalyst was due to its high Ce³⁺/(Ce³⁺+Ce⁴⁺) ratio as well as the oxygen vacancies. The Ce³⁺/(Ce³⁺+Ce⁴⁺) ratio of CeO₂ on γ-Al₂O₃ is crucial for the degradation of C11, providing a further roadmap for the plasma catalytic oxidation of alkanes.

Modulate the metal support interactions to optimize the surface-interface features of Pt/CeO2 catalysts for enhancing the toluene oxidation

Metal support interactions modulation is one of the effective strategies to enhance the catalytic performance. Herein, we reported that modulating metal support interactions by switching the strength (CO, H₂, NH₃) and temperature (200, 300, 400 °C) of reducing gases is a facile way to improve the catalytic performance of Pt/CeO₂ for toluene oxidation. The distinct reduction treatments will stepwise enhance the reducibility, ratio of Pt⁰ and oxygen vacancy concentration, which dominated the activity. The metal support interactions modulation can significantly affect toluene deep oxidation (from benzoate to formate or monodentate carbonate) via enhancing the mobility of surface/lattice oxygen and activation ability towards O₂ molecules, since the main activation sites for O₂ molecules expand from Pt⁰ sites to oxygen vacancies and Pt⁰ sites with temperature increasing.

Boosting Toluene Combustion by Tuning Electronic Metal-Support Interactions in In Situ Grown Pt@Co3O4 Catalysts

Electronic metal-support interaction (EMSI) has attracted great attention in volatile organic compound (VOC) abatement. Herein, Pt@Co₃O₄ catalysts were prepared via a metal-organic framework (MOF) in situ growth approach to boost toluene degradation. The partial electron transfer from Co₃O₄ to Pt species was induced by the EMSI effect to generate the electron-rich Pt and Co³⁺ species. The electrophilic O₂ molecules could be activated by picking up the electrons from electron-rich Pt species to form nucleophilic oxygen species, which is conducive to attack C-H bonds in toluene. The redox ability and surface oxygen species activity of catalysts were improved due to strong EMSI. As expected, the excellent toluene activity was achieved, meanwhile exhibiting satisfactory water resistance and long-term stability for toluene combustion. In situ diffuse reflectance infrared Fourier transform spectroscopy results elucidated that surface lattice oxygen species should deeply participate in toluene degradation, which could be efficiently replenished by gaseous oxygen. This work may provide a new idea for exploring the relationship between the electron transfer effect and efficient catalytic performance of VOCs.

Uniform platinum nanoparticles loaded on Universitetet i Oslo-66 (UiO-66): Active and stable catalysts for gas toluene combustion

Highly dispersed Pt nanoparticles supported UiO-66 catalysts were successfully prepared by the incipient wetness impregnation method. Their thermal catalytic performances were evaluated by toluene degradation. The physicochemical properties of the samples were characterized using a series of characterization methods. The catalytic activity of catalysts remained essentially unchanged in the high weight hourly space velocity, stability and water resistance test, which also indicated good catalytic performance. In the reusability test, the catalytic performance was found to be enhanced after the reaction, because of the catalyst might follow a Pt⁰-PtO synergistic catalytic mechanism (similar to Mars-van Krevelen mechanism) and there was a phase transition between Pt⁰ and PtO during the reaction. Firstly, the toluene adsorbed on the catalyst surface was oxidized by the activated lattice oxygen of the PtO. Then, consumption of oxygen atoms led to formation of oxygen vacancies, and finally the molecular oxygen adsorbed by Pt⁰ was activated and passed to the PtO to supplement the oxygen vacancies, forming a redox cycle. In addition, the possible catalytic oxidation mechanism of toluene was also revealed.

Low Temperature Combustion of VOCs with Enhanced Catalytic Activity Over MnO2 Nanotubes Loaded with Pt and Ni-Fe Spinel

MnO₂ nanotubes loaded with Pt and Ni-Fe spinel were synthesized using simple hydrothermal and sol-gel techniques. After loading with Ni-Fe spinel, the specific surface area of the material increases 3-fold. This change helped to provide more active sites and facilitated the association between the catalyst and volatile organic compounds (VOCs). X-ray photoelectron spectroscopy determined that the adsorbed oxygen concentrations were all greatly increased after Pt loading, indicating that Pt promoted the adsorption of oxygen and so accelerated the combustion process. The performance of the catalyst after loading with 2 wt % Pt was greatly improved, such that the T₉₀ for benzene decomposition was decreased to 113 °C. In addition, the 2% Pt/2Mn@NFO exhibited excellent low-temperature catalytic activity when reacting with low concentrations of toluene and ethyl acetate. This work therefore demonstrates a viable new approach to the development of Mn-based catalysts for the low temperature catalytic remediation of VOCs.

Optimized Synthesis Routes of MnOx-ZrO2 Hybrid Catalysts for Improved Toluene Combustion

In this contribution, the three Mn-Zr catalysts with MnxZr1−xO2 hybrid phase were synthesized by two-step precipitation route (TP), conventional coprecipitation method (CP) and ball milling process (MP). The components, textural and redox properties of the Mn-Zr hybrid catalysts were studied via XRD, BET, XPS, HR-TEM, H2-TPR. Regarding the variation of synthesis routes, the TP and CP routes offer a more obvious advantage in the adjustment of the concentration of MnxZr1−xO2 solid solution compared to the MP process, which directly commands the content of Mn4+ and oxygen vacancy and lattice oxygen, and thereby leads to the enhanced mobility of reactive oxygen species and catalytic activity for toluene combustion. Moreover, the TP-Mn2Zr3 catalyst with the enriched exposure content of 51.4% for the defective (111) lattice plane of MnxZr1−xO2 exhibited higher catalytic activity and thermal stability for toluene oxidation than that of the CP-Mn2Zr3 sample with a value of 49.3%. This new observation will provide a new perspective on the design of bimetal catalysts with a higher VOCs combustion abatement.

Oxygen vacancy induced MnO2 catalysts for efficient toluene catalytic oxidation

The α-MnO2, with abundant oxygen vacancies, facilitates the adsorption and activation of O2 to produce active adsorbed oxygen species and weakens lattice oxygens species. These oxygen species can significantly improve toluene catalytic oxidation.

Nanocage-Shaped Co3- x Zrx O4 Solid-Solution Supports Loaded with Pt Nanoparticles as Effective Catalysts for the Enhancement of Toluene Oxidation

Nanocage-shaped Co_{3- x} Zr_x O₄ solid-solution supports and the corresponding platinum loaded nanocomposites, yPt/Co_{3- x} Zr_x O₄ (x =0.27, 0.50, 0.69; y = 0.5, 1.0, 2.0 wt.%), are successfully fabricated via a Cu₂ O nanocube hard template method and a glycol reduction method, respectively. The hollow nanocage structures obviously improve surface areas; moreover, the Zr doping forms the Co_{3- x} Zr_x O₄ solid-solution supports, and the corresponding yPt/Co_{3- x} Zr_x O₄ catalysts promote the enhancement of catalytic performance. Catalytic activity toward toluene combustion is enhanced for the 2.0 wt% Pt/Co_2.73 Zr_0.27 O₄ catalyst. The catalysts are characterized using multiple techniques. Pt nanoparticles are uniformly dispersed across the Co_2.73 Zr_0.27 O₄ nanocage surface. The 2.0 wt% Pt/Co_2.73 Zr_0.27 O₄ catalyst exhibits the highest catalytic activity among all the samples and demonstrates good stability, with 90% toluene conversion obtained at a temperature of 165 °C. The same catalyst accomplishes full toluene oxidation at 180 °C, at a weight hourly space velocity of 36 000 mL h^-1 g^-1 . The apparent activation energy (E_a ) over the yPt/Co_2.73 Zr_0.27 O₄ samples are significantly lower than those over the Co_{3- x} Zr_x O₄ supports, with the 2.0 wt% Pt/Co_2.73 Zr_0.27 O₄ catalyst exhibiting the lowest E_a value. These findings demonstrate the potential of the 2.0 wt% Pt/Co_2.73 Zr_0.27 O₄ catalyst as a promising catalyst toward atmospheric toluene removal.