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Determining the effect of crystal facets on reaction performance is essential for designing an efficient propane oxidation catalyst. Herein, α-MnO2 nanowires with exposed (110), (211), (310) and (200) facets were proposed, and their crystal facet-dependent reactivity to propane oxidation was investigated. The resultant α-MnO2 nanowires with predominantly exposed (110) facets showed the maximum propane oxidation activity (T90 = 262.1 °C), oxidation rate, and turnover frequency (at 230 °C), and high stability during the oxidation of propane. In situ diffuse reflectance infrared Fourier transform spectroscopy revealed that the oxidation to form carboxylates is the decisive step; therefore, the production of reactive oxygen species is very important. The results from density of states computations revealed that the lowest electron energy was present in the (110) facet, which helps in the generation of reactive oxygen species and the regulation of the surface valence of Mn. The characterization and calculation results indicated that the (110) facets are conducive to forming oxygen vacancies, which allowed the activation and adsorption of O2 and propane, resulting in significantly enhanced propane oxidation activity. Apart from offering new insights about the structure–activity relationship of α-MnO2 with different facets exposed for short-chain volatile oxidation reactions, this study proposes a technique that can be used for improving other manganese-based catalyst oxidation systems. [ABSTRACT FROM AUTHOR]

[Display omitted] • Highly dispersed Pd-Mn 3 O 4 catalyst was constructed by limited pyrolysis oxidation strategy. • Strong metal-support interaction effect between Pd and Mn 3 O 4 make the surface electron rich. • Strong metal-support interaction effect was conducive to catalyse propane degradation. Adjusting the interaction between dual active components for enhancing volatile organic compounds (VOCs) degradation is an effective but still challenging means of air pollution control. Herein, a limited pyrolysis oxidation strategy was adopted to prepare Pd-Mn 3 O 4 spinel catalysts with uniform morphology and active component dispersion. Among these, 1.08Pd-Mn 3 O 4 presented the highest catalytic efficiency with a T 90 value of 240 °C, which was 94 °C lower than that of Mn 3 O 4. Characterization and density functional theory (DFT) calculation results revealed that the strong metal-support interaction (SMSI) effect between Pd and Mn 3 O 4 promoted the redistribution of surface charges, thus strengthening the oxidation-reduction ability of the active sites. Moreover, the SMSI effect led to a better migration of surface oxygen species, and boosted the generation of active surface oxygen species. Simultaneously, the Pd catalyst further reduced the energy barrier in the initial stage of the dehydrogenation of propane. Overall, this study provided a novel design strategy for dual active components catalysts with SMSI effect and extended the application of these catalysts in the important field of VOCs elimination. [ABSTRACT FROM AUTHOR]

[Display omitted] • MnO 2 treated by urea-H 2 O 2 exhibits great photocatalytic propane oxidation activity. • H 2 O 2 treatment increase active sites to thermal catalytic performance. • The photocatalytic performance improvement is related to photogenerated electrons. • A new photothermal synergetic catalysis mechanism was proposed. Photothermal catalysis has an advantage in effective and economical elimination technology of volatile organic compounds (VOCs) in the ascendant. Herein, various surface defect engineering routes were adopted to enhance the low-temperature propane oxidation of δ-MnO 2. Compared to reducing etchants urea and vitamin C, δ-MnO 2 treated with urea - H 2 O 2 exhibited an excellent thermal (T 90 = 240 ℃) and photothermal (T 90 = 196 ℃) activities of propane oxidation. Urea - H 2 O 2 treatment provided high concentration of Mn4+ and surface-active oxygen (Mn4+-O sur) species as surface-active sites, and produced numerous oxygen vacancies to improve charge separation and superoxide species generation capacity. Thus, the photothermal conversion efficiency and low-temperature reducibility were remarkably enhanced. Furthermore, the photothermal synergistic catalytic mechanism was proposed based on in-situ diffuse reflectance infrared Fourier transform spectroscopy and control experiments. The strategy here offered insight into the rational design of efficient transition catalysts, and in-depth understanding of the photothermal catalytic VOCs removal mechanism. [ABSTRACT FROM AUTHOR]

The construction of isotype homojunctions composed of the same semiconductor can improve the photoelectrochemical properties, adsorption and catalytic properties of the catalysts, which is important for the enhancement effect in photothermal synergistic catalytic oxidation (PTSCO) system. Herein, a series of homojunctions with δ- and α-phase MnO 2 catalysts were designed and synthesized for propane oxidation in PTSCO system. The 4-δ/α exhibited outstanding propane catalytic oxidation performance (T 90 = 198 ℃, Ea = 20.56 kJ·mol−1, remarkable water resistance, long-time stability and recycling performance) due to the rich oxygen vacancy, higher low-temperature redox, increased surface acidity, fast transfer and effective separation of photogenerated electron-holes. Additionally, a photothermal synergistic mechanism was proposed based on reaction kinetics, active species detection, density functional theory (DFT) and in-situ DRIFTs. This work presents a novel method for the design and preparation of homojunction catalysts. Furthermore, it explains the synergistic mechanism of the PTSCO system. [Display omitted] • 4-δ/α with p-p isotype homojunction was synthesized by hydrothermal-stirring method. • Z-type charge transfer ably promotes the separation of photogenerated carriers. • The water vapor resistance of 4-δ/α homojunction catalyst was enhanced. • A multi-pathway enhancement mechanism in PTSCO was proposed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Pd/Mn 3 O 4 heterojunction catalyst was constructed by impregnation - pyrolysis - oxidation strategy. • Strong metal-support interaction effect between Pd and Mn 3 O 4 make the surface electron rich. • Formation of p-n junction between Pd and Mn 3 O 4 promoted the charge separation and the migration of carriers. Photothermal synergistic low-temperature catalytic volatile organic compounds (VOCs) degradation has an advantage on energy efficiency and reduction of carbon emissions. Herein, an impregnation-pyrolysis-oxidation strategy was adopted to synthesize Pd/Mn 3 O 4 heterojunction catalyst. Under photothermal synergistic conditions, 0.60Pd/Mn 3 O 4 catalyst exhibited an excellent activity for propane total oxidation, with a T 90 value of 219.1 ℃, which was 115.3 ℃ lower than that of Mn 3 O 4. Enhancement effect of the heterojunction structure on photothermal synergy was detected by various characterization techniques and density functional theory calculation. The presence of Pd species enhanced the low-temperature reducibility of Mn 3 O 4. Meanwhile, O 2 molecules were more easily activated at the Pd sites and overflow to the surface of Mn 3 O 4 for reaction. Besides, the formation of p-n junction between Pd species and Mn 3 O 4 in heterojunction structure promoted the charge separation and the migration of carriers, and boosted the generation of oxygen free radical, resulting in a significant improvement in photothermal catalytic activity. Overall, this study provided new strategy and deep insight for constructing heterojunction to enhance photothermal catalytic performance of VOC elimination. [ABSTRACT FROM AUTHOR]

• In-situ regulation of oxygen vacancies controlled by the crystal planes. • Oxygen vacancies promoted the valid separation of photogenerated electrons and holes. • Oxygen vacancies enhanced the adsorption and activation of O 2 and propane. • A multi-pathway enhancement mechanism in PTSCO was proposed. Crystallographic effects in photothermal synergistic catalytic oxidation (PTSCO) system for VOCs removal are essential but still lack in-depth systematic studies. Therefore, key factors and synergistic mechanism of propane removal by MnO 2 with different crystal planes in PTSCO system were investigated. The results showed that 160-M−300 with the mainly exposed (1 1 0) crystal plane displayed the optimal activity (T 90 = 235℃, TOF = 4.90 × 10-7·s−1) and long-term stability (40 h) due to the abundant oxygen vacancies (OVs) controlled by the crystal planes, outstanding low temperature reduction ability and higher oxygen mobility. OVs substantially enhanced the adsorption and activation of propane and O 2 , and accelerated the efficient separation of photogenerated electrons and holes by capturing the electrons. Correspondingly, density functional theory (DFT) calculations confirmed that (1 1 0) crystal facet had the lowest OVs generation energy and work function, based on which, a multi-pathway enhancement mechanism of PTSCO was proposed. This study clarified the crystal-PTSCO activity dependence, provided theoretical support for the efficient and energy-saving removal of low-carbon alkanes. [ABSTRACT FROM AUTHOR]

The exploration of enhancing the low-temperature oxidation performance of catalysts by element doping is desirable but still challenging. Herein, a series of Cr-doping Mn 3 O 4 spinel catalysts (Mn x Cr 3−x O 4) were prepared through MOF-74 as precursors and tested for propane total oxidation. Due to the synergy between Cr and Mn, Mn x Cr 3−x O 4 with rich surface-active Mn4+, notable low-temperature reducibility and excellent oxygen migration capability displays notable catalytic activity for propane oxidation. The constructed Mn 2.70 Cr 0.30 O 4 exhibited superior propane catalytic activity and notable thermal stability. The T 90 of Mn 2.70 Cr 0.30 O 4 is 264 ℃ under humid condition, that 51 ℃ lower than that of Mn 3 O 4. Meanwhile, in-situ spectroscopy and DFT calculations revealed that the addition of Cr can convert propane into propylene in the initial stage thus to reduce the reaction energy barrier. This work provides an element doping strategy to effectively enhance spinel catalysts performance, and can be provided with bright prospect for efficient VOCs catalysts. [Display omitted] • Cr was successfully doped at B position of Mn 3 O 4 spinel via MOF-74 as precursor. • Cr-dopant enhanced the catalytic performance during propane oxidation. • Cr-dopant promoted the oxygen mobility by forming oxygen vacancy. • Cr-dopant could catalyze propane to propene, reducing initial reaction barrier. [ABSTRACT FROM AUTHOR]

• The active sites of α-MnO 2 are the Mn4+-O sur -Mn3+ structures on the surface. • The Langmuir-Hinshelwood mechanism with competitive adsorption plays a major role at low temperature. • There is synergism between Langmuir-Hinshelwood and Mars-van-Krevelen mechanisms at medium and high temperature. • A complete propane conversion path on MnO 2 surface is proposed. Mn-based oxides exhibit attractive catalytic activity for VOCs oxidation, but the catalytic mechanism and kinetics are still significant challenges and rarely mentioned. Therefore, we prepared OMS-2 manganese oxide octahedral molecular sieve with α-MnO 2 crystal phase as model compound. By analyzing the properties of the surface chemical states of the catalysts in different reaction stages, combined with the control experiments, the coordination of Mn4+-O sur -Mn3+ was confirmed as the active site at low temperature. Meanwhile, the reaction path of propane catalytic oxidation on MnO 2 surface based on condensation mechanism was proved by in-situ spectroscopy and density functional theory calculation. And kinetic studies revealed that the catalytic combustion of propane processed through Langmuir-Hinshelwood mechanism at low temperature. All these systematic study results supplied a well-defined understanding for the propane combustion over manganese oxide. [ABSTRACT FROM AUTHOR]

[Display omitted] • Dopant of Ag on MnCeO x promotes the catalytic activity for propane oxidation. • Ag dopant increases Mn4+ and Ce3+ concentrations. • Ag dopant enhances surface oxygen migration capability. • Ag dopant leads to a novel carboxylate formation path. The development of highly active catalysts for eliminating volatile organic compounds (VOCs) at low temperature is desirable but remains challenging. In this study, a series of Ag-loaded MnCeO x catalysts were fabricated. Catalysts with an Ag loading of 5 wt% exhibited the highest catalytic activity (T 90 = 242 °C) and excellent stability in H 2 O, NO 2 , and CO atmosphere. Characterization results revealed that the introduction of Ag increased Mn4+ and Ce3+ concentrations and enhanced surface oxygen migration capability, which is conducive to the adsorption and dissociation of propane and O 2. Combining in-situ diffuse reflectance infrared Fourier transform and discrete Fourier transform calculation revealed a novel propane oxidation pathway on MnCeO x and Ag/MnCeO x. The addition of Ag species can convert propane into propylene in the initial stage and reduce the activation energy. This study supplied a novel insight into Ag-doped catalysts catalytic mechanism for VOCs oxidation reaction. [ABSTRACT FROM AUTHOR]

The low defect content and poor oxygen mobility of perovskite catalysts limit its application in VOC elimination. Herein, we report a strategy involving defect engineering route following an easy urea treatment method to enhance the propane oxidation performance of perovskite catalysts. The constructed LaCoO 3 -D43 exhibits superior catalytic activity (T 90 = 309.3 °C), the T 90 value is 150 °C lower than that of LaCoO 3 , and excellent thermal stability against CO 2 and H 2 O. Experimental results revealed that the urea pyrolysis resulted in the generation of La and O defects and rich surface-active Co species in high-valence states, increasing the utilization of Co active sites. DFT calculations show that the exposed Co surface is conducive to the adsorption and dissociation of oxygen and propane. This work provides a defect engineering strategy to effectively activate perovskite catalysts performance, and can be generalized for the fabrication of other types of perovskite catalysts. [Display omitted] • Urea treatment for the fabrication of defect engineering-modified LaCoO 3 perovskite surface. • Defect engineering improved the catalytic activity during propane oxidation. • Defect engineering improved the extent of utilization of Co active sites. • Defect engineering promoted the generation of active oxygen species. [ABSTRACT FROM AUTHOR]

[Display omitted] • Sphere-structured MnO 2 samples were synthesized through various methods. • MnO 2 -SS displays excellent propane oxidation catalytic activity. • MnO 2 -SS possesses a large specific surface area. • MnO 2 -SS exhibits abundant high valence Mn4+ as active sites. • MnO 2 -SS promoted the generation of active oxygen species. Through morphology control and structure construction of manganese catalyst to improve the conversion of VOCs is still a significant challenge in the field of environmental catalysis. Herein, secondary sphere structured δ-type manganese oxide (MnO 2 -SS) was synthesized via a simple room temperature two-step method. Compared with other sphere-type MnO 2 samples, MnO 2 -SS exhibited a higher propane oxidation activity (T 90 = 238 ℃), turnover frequency (14.54 × 10−7 s−1 at 240 ℃), and stability in H 2 O, NO 2 , and CO atmospheres. Experimental results indicated that MnO 2 -SS exhibited an ultrahigh specific surface area, a high surface concentration of active Mn4+ species and superior surface oxygen mobility. In-situ DRIFTs revealed that active oxygen species exhibit a key role in formation and dissociation of intermediate carboxylates. Electron paramagnetic resonance, O K-edge results and density functional theory calculation revealed that numerous oxygen vacancies existed on MnO 2 -SS. These vacancies can facilitate adsorption and activation of O 2 to generate active reactive oxygen species, which lead to superior propane and intermediate carboxylates oxidation activity. This work provides new strategy to enhance the surface oxygen activity of manganese oxide. [ABSTRACT FROM AUTHOR]