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Oxygen vacancy plays an important role in adsorption and activation of oxygen species and therefore promotes the catalytic performance of materials in heterogeneous oxidation reactions. Here, a series of K-doped ɑ-MnO2 materials with different K loadings were synthesized by a reproducible post processing process. Results show that the presence of K+ enhances the reducibility and oxygen vacancy concentration of ɑ-MnO2 due to the break of charge balance and the formation of low valence Mn species. 4-K/MnO2 material exhibits the highest toluene oxidation activity and satisfied long-term stability and water resistance owing to its superior reducibility and abundant surface absorbed oxygen (Oads). In situ DRIFTS demonstrate that Oads greatly accelerates toluene dehydrogenation rate and promotes benzoate formation, enhancing the activation and decomposition of toluene molecules. Moreover, the CC cleavage of benzene ring (forming maleic anhydride) is the rate-determining step of toluene oxidation, which can be easily occurred over 4-K/MnO2.

Two types spherical-like SiO2 with different specific surface area (SSA) and porosity were synthesized and used as support for Pd catalysts, which were prepared by various protocols (in situ synthesis (IS, Pd/S-1IS, and Pd/S-2IS), wet impregnation (WI, Pd/S-1WI, and Pd/S-2WI), and grafting (GA, Pd/S-1GA, and Pd/S-2GA)) and adopted in n-butylamine combustion. Results suggest that Pd dispersion is positive correlation with support SSA and GA method is the most favorable approach to obtain highly dispersed Pd active sites. Pd/S-1IS, Pd/S-1WI, and Pd/S-1GA catalysts with small SSA show inferior activity and higher NOx yield than those of Pd/S-2IS, Pd/S-2WI, and Pd/S-2GA with large SSA, irrelevant with the preparation methods. Amongst, Pd/S-2GA possesses the smallest Pd average diameter (ca. 1.72 nm) and highest activity with 90% of n-butylamine destructed at 234 °C; however, the yield of NOx over Pd/S-2GA is much higher than the other catalysts (except Pd/S-1GA) as the GA approach provides high concentration of active sites and the preparation procedure sacrifices the SSA and porosity of supports to some extent. In situ DRIFTS results reveal that the developed porosity of catalyst promotes nitrogen-containing by-products (NHx) transfer and diffusion and avoids their further oxidizing to NOx. The activity of samples prepared by the WI process is lower than that prepared by the IS or GA method due to limited active sites. Comparatively, the Pd/S-2IS prepared by the IS method has relative high activity (T90 of 240 °C) and low NOx yield (0.99% at T90) in n-butylamine oxidation among all materials, exhibiting an attractive prospective in nitrogen-containing VOC environment-friendly elimination.

Pt/Eu2O3-CeO2 materials with different Eu concentrations were prepared and applied to toluene destruction, and the remarkable promotion impact of EuOx on Pt/CeO2 can be observed. The characterization results reveal that the presence of EuOx significantly enhances the redox property, lattice O concentration, and Ce3+ ratio of the Pt/CeO2 material, which facilitates the dispersion and activity of Pt active sites and thus accelerates the decomposition process of toluene. Among all catalysts, a sample with an Eu content of 2.5 at.% (Pt/EC-2.5) possesses the best catalytic activity with 0.09 vol% of toluene completely destructed at 200 °C under a relatively high GHSV of 50000 h−1. The possible reaction pathway and mechanism of toluene combustion over Pt/Eu2O3-CeO2 samples are presented according to in-situ DRIFTS, which confirms that the toluene oxidation process obeys the Mars-van Krevelen mechanism with aldehydes and ketones as primary organic intermediates.

Shape effects of nanocrystal catalysts in different reactions have attracted remarkable attention. In the present work, three types of α-Fe2O3 oxides with different micromorphologies were rationally synthesized via a facile solvothermal method and adopted in deep oxidation of ethane. The physicochemical properties of prepared materials were characterized by XRD, N2 sorption, FE-SEM, HR-TEM, FTIR, in situ DRIFTS, XPS, Mössbauer spectroscopy, in situ Raman, electron energy loss spectroscopy, and H2-TPR. Moreover, the formation energy of oxygen vacancy and surface electronic structure on various crystal faces of α-Fe2O3 were explored by DFT calculations. It is shown that nanosphere-like α-Fe2O3 exhibits much higher ethane destruction activity and reaction stability than nanocube-like α-Fe2O3 and nanorod-like α-Fe2O3 due to larger amounts of oxygen vacancies and lattice defects, which greatly enhance the concentration of reactive oxygen species, oxygen transfer speed, and material redox property. In addition to this, DFT results reveal that nanosphere-like α-Fe2O3 has the lowest formation energy of oxygen vacancy on the (110) facet (Evo (110) = 1.97 eV) and the strongest adsorption energy for ethane (−0.26 eV) and O2 (−1.58 eV), which can accelerate the ethane oxidation process. This study has deepened the understanding of the face-dependent activities of α-Fe2O3 in alkane destruction.

MnOx oxides with different morphologies (nanowire (MnOx-W), nanocube (MnOx-C), nanorod (MnOx-R), and nanosphere (MnOx-S)) and exposed facets were synthesized via a solvothermal method. The catalytic performance of the synthesized MnOx materials for methyl ethyl ketone (MEK) destruction was investigated. Results show that the activity of MnOx-W with highly exposed {101} facets of Mn3O4 is superior to that of MnOx-C, MnOx-R, and MnOx-S exposing {321} facets of Mn2O3, {110} facets of MnO2, and {101} and {112} facets of Mn3O4, respectively. MEK can be completely mineralized into CO2 at 195 °C over MnOx-W under a relatively high gas hourly space velocity of 37 200 h−1, which is even better than some typical noble metal loaded catalysts. The lowest apparent activation energy of MnOx-W (27.7 kJ mol−1) for MEK destruction also confirms its excellent catalytic activity. Density functional theory (DFT) results reveal that the {101} facets of Mn3O4 have the highest MEK adsorption energy (0.79 eV), which indicates that MEK molecules have a high affinity to adsorb onto the MnOx-W surface, promoting the oxidation process of MEK. In situ DRIFTS and TPSR results indicate that the mineralization of MEK into CO2 over MnOx-W goes through an oxidation route with diacetyl as the primary intermediate. We found that the highly exposed {101} active facets, abundant oxygen vacancies, and excellent low-temperature reducibility are responsible for the superior oxidation performance of MnOx-W. This finding may bring new insights into the designing of highly effective catalysts and has implications for a wide range of reactions not limited to MEK oxidation.

A novel cathode of macroporous graphene aerogel (GA) with high specific surface area was proposed for the electro-Fenton (E-Fenton) system. The GA was prepared by reduction self-assembly method. The physicochemical properties were characterized in details. The GA displayed a low electrochemical resistance and exhibited an excellent electrocatalytic activity. Comparing with the traditional carbon fiber and graphite felt cathodes, the GA cathode showed more positive oxygen reduction potential of 0.07 V (versus saturated calomel electrode). In E-Fenton system, H2O2 could be in situ electro-generated efficiently and continuously via a two-electron oxygen reduction reaction on the GA cathode. The electron transfer number n was calculated to be 1.0–2.0 for GA. The production of H2O2 of 107.6 mg L−1 was obtained for GA in 90 min. Good performance was exhibited to degrade antibiotic ciprofloxacin. Results showed that nearly 100% ciprofloxacin degradation ratio and 91% TOC removal were achieved in 90 min and 120 min, respectively, which were much higher than control groups. The mineralization current efficiency was 12.75% in 30 min. It was attributed to the plenty of macro-pores of GA acted as reaction trap to accelerate electro-generated H2O2 decomposing by Fe2+ to form ·OH efficiently, which was verified by probe molecule trapping experiments and electron paramagnetic resonance analysis. Simultaneously, the strong charge transfer ability of GA was beneficial to the conversion of Fe3+/Fe2+. The GA also presented distinguished reusability and stability. Therefore, GA is a promising candidate material for E-Fenton cathode due to low cost, high efficient and corrosion resistance.

Developing economical and robust noble metal-free materials for obstinate light alkane low-temperature decomposition is of great significance. Here, birnessite-type short rod-like MnO2 was firstly prepared by facile mixing KMnO4 and ascorbic acid under ambient conditions and employed in propane catalytic decomposition. Experimental results revealed that abundant oxygen vacancies and high valent Mn4+ species, superior reducibility, and high surface oxygen mobility guarantee the remarkable low-temperature activity of MnO2-SR, over which 90% of propane can be oxidized (T90) at temperature as low as 225 °C, achieving a temperature reduction over 70 °C compared with that of δ-MnO2 (T90 = 300 °C) and α-MnO2 (T90 = 355 °C). In situ diffuse reflectance infrared Fourier transform spectroscopy revealed that the dissociation and activation of propane molecules is the decisive step in propane oxidation. Gaseous propane is slowly oxidized by adsorbed oxygen directly to the final mineralization products at low temperature region ( 200 °C) however, propane molecules are firstly dissociated and activated before oxidized to carbonate intermediate products over active oxygen, and then eventually oxidized to carbon dioxide and water.

Mn3O4, FeMnOx, and FeOx catalysts synthesized via a solvothermal method were employed for catalytic oxidation of methyl−ethyl−ketone (MEK) at low temperature. Mn3O4 with sphere-like morphology exhibited the highest activity for MEK oxidation, over which MEK was completely oxidized to CO2 at 200 °C, and this result can be comparable to typical noble metal loaded catalysts. The activation energy of MEK over Mn3O4 (30.8 kJ/mol) was much lower than that of FeMnOx (41.5 kJ/mol) and FeOx (47.8 kJ/mol). The dominant planes, surface manganese species ratio, surface-absorbed oxygen, and redox capability played important roles in the catalytic activities of catalysts, while no significant correlation was found between specific surface area and MEK removal efficiency. Mn3O4 showed the highest activity,\ud accounting for abundant oxygen vacancies, low content of surface Mn4+ and strong reducibility. The oxidation of MEK to CO2 via an intermediate of diacetyl is a reaction pathway on Mn3O4 catalyst. Due to high efficiency and low cost, sphere-shaped Mn3O4 is a promising catalyst for VOCs abatement.

Sulfur poisoning and regeneration behavior of In/H-BEA catalyst were carried out in NOx reduction by CH4. In/H-BEA catalyst exhibited a poor resistance to sulfur dioxide after addition of 200 ppm SO2 and 10 vol.% H2O into NO reduction with CH4 at 450 °C for 45 h. Sulfur poisoning of In/H-BEA was attributed to the inhibition of NOx adsorption on Bronsted acid sites, suppression of reaction intermediates generation on the active sites, and the formation of surface sulfate species. The formation of surface sulfate reduced the availability of surface active sites, blocked the pore structure and decreased the surface area of catalyst. These changes in chemical and textural properties resulted in a severe loss in the activity of sulfated In/H-BEA catalyst for NO reduction with CH4. H2 reduction is a promising technology for regeneration of In/H-BEA deactivated by SO2 for removing NOx from lean-burn and diesel exhausts. Indium sulfate could be reduced by H2 to InO+ with In2O3 and In(OH)2+ as the intermediates. The optimal parameters of H2 reduction was regeneration temperature of 400 °C and regeneration time of 60 min which completely recovered the catalytic activity of In/H-BEA.

Effects of propane/methane ratios on NO x reduction by mixtures of methane and propane over In/H-BEA catalyst were investigated at temperatures ranging from 250 to 550 °C. The higher catalytic activity of In/H-BEA was exhibited for CH 4 -SCR at high temperatures above 450 °C, while the higher NO x conversion was achieved in C 3 H 8 -SCR at below 425 °C. A broadened temperature window and enhanced CO 2 selectivity were achieved by combining of methane and propane as the co-reductant. The mixtures with propane/methane of 1/2 showed the most superior T 50 (325 °C) and T 90 (500 °C) temperatures for NO x reduction over In/H-BEA catalyst. For the promotion mechanism of propane on NO reduction by methane at low temperature, the formation of carbonaceous species (e.g. R-COOH) were enhanced by the activation of C 3 H 8 on Bronsted acid sites at low temperature, and further promoted the generation of NCO species, which was a crucial determining step for NO reduction.

Low-temperature degradation of short chain alkane is one of the greatest challenges of volatile organic compound purification. Here, rod-, sheet-, and cube-like Co3O4 (Co3O4-R, Co3O4-S, and Co3O4-C) with predominantly exposed (110), (111), and (100) facets respectively were fabricated. Co3O4-R presents excellent activity achieving 90 % of propane oxidized at just 195 °C owing to large amounts of lattice defects, oxygen vacancies and low coordinated Co atoms. Theoretical calculation reveals that Co3O4-R has the lowest formation energy of oxygen vacancy on (110) facet (Evo (110) =1.7 eV), which has a higher activation capacity for oxygen due to the largest O2 adsorption energy (−1.30 eV) and thus accelerates propane oxidation. Moreover, largest amount of lewis acid-base pairs existed in Co3O4-R polarizes substrate electron distribution and therefore accelerates the activation of C–H bonds. Electrophilic oxygen species (O22− or O−) caused the degradation of carbon skeleton and formed carboxylate intermediates before mineralized to CO2.