Knowledge of the catalytic performances of different crystalline phases of a material is vital for the development of superior catalysts. In this study, different phases of MnO2 (α, β, γ, and δ) have been prepared by the oxidation of Mn2+, and their catalytic performances were evaluated using the aerobic oxidation of benzyl alcohol to benzaldehyde as a model reaction. α-MnO2 promoted the reaction to the highest yield. However, when the yields were normalized by the corresponding surface areas, δ-MnO2 exhibited the highest specific activity and α-MnO2 the lowest, indicating that the diverse microstructures resulting from the crystalline phase have a profound effect on catalytic performance. α-MnO2 showed the highest catalytic stability, resulting from its unchanged composition and morphology after use. Informed by the experimental results, a possible reaction mechanism involving the Mars-van Krevelen process was proposed. This work provides useful information for the development of effective catalysts for aerobic oxidation.研究同种物质不同晶相材料的催化性能对于遴选最佳的催化剂十分重要. 本文利用Mn2+的氧化反应制备了高纯度的不同晶相MnO2 (α, β, γ, δ相), 通过苯甲醇耗氧氧化成苯甲醛的反应对其催化性能进行了评估. 结果显示, 在相同条件下, α-MnO2催化该反应的产率最高.如果用表面积归一化产率, 则δ-MnO2具有最高的面积比活性, 而α-MnO2的却最低. 不同晶相导致的不同微观结构决定了它们的催化性能. 经过多次催化反应后, 只有α-MnO2的成分和形貌均未改变, 显示出最高的催化稳定性. 因此提出了一种基于Mars-van Krevelen过程的可能的反应机理. 此工作可为发展有效的耗氧氧化反应催化剂提供有利支持.

Controlling the size of SBA-15 can be beneficial for exploiting CMK-3, which has excellent structural parameters, for better performance in adsorption and/or catalytic processes. In this study, the width of freestanding SBA-15 rods was readily and successfully regulated by simply altering the stirring power during the synthesis. A higher stirring rate produced SBA-15 rods with larger width. Then, the size of the CMK-3 rods was adjusted by duplication of the different-sized SBA-15. The results show that the larger sized CMK-3 has higher specific surface area and pore volume, which led to a higher adsorption capacity and a faster adsorption rate. It is believed that the synthetic method reported here is powerful for developing better mesoporous carbon for application in water purification and catalysis.

We report that 5-fold twinned nanowires and single twinned right bipyramids of Pd with high yields can be selectively synthesized in a hydrophilic system with the assistance of acetonitrile and ethanol, respectively. The controlled synthesis is based on an idea that small organic molecules (SOMs) that can attract halide ions via electrostatic interactions of different strengths could well adjust their activity to tune the etching degree of O2/halides for protecting the twinned Pd crystal nucleus. We consider that relatively stronger interaction between acetonitrile and halide ions for the formation of nanowires is due to the existence of three C–Hδ+ bonds induced by the electron-withdrawing CN group of CH3CN, which is confirmed by an as-called iodine starch test, 1H nuclear magnetic resonance spectra, and theoretical calculations. On the basis of this finding, we then have successfully expanded SOMs to other molecules, including acetone, 1,4-dioxane, and 1,3,5-trioxane, which have a function similar to that of acetonitrile for the production of Pd nanowires, and 2-propanol, which has a function similar to that of ethanol for the fabrication of right bipyramids. Although nanowires and bipyramids are both mainly bound by {001} planes, nanowires show better catalytic performance toward the reduction of 4-nitrophenol, indicating that more twin boundaries could offer more active catalytic sites. This work not only provides new information to decrease the degree of etching of O2/halides for controlling twin structure of noble metals but also supports the idea that creating a twin structure is good for enhancing catalytic activity.

Heterogeneous catalysts are promising candidates for use in organic reactions due to their advantages in separation, recovery, and environment compatibility. In this work, an active porous catalyst denoted as Pd embedded in porous carbon (Pd@CMK-3) has been prepared by a strategy involving immersion, ammoniahydrolysis, and heating procedures. Detailed characterization of the catalyst revealed that Pd(0) and Pd(II) species co-exist and were embedded in the matrix of the porous carbon (CMK-3). The as-prepared catalyst has shown high activity toward Suzuki reactions. Importantly, if the reaction mixture was homogenized by two minutes of ultrasonication rather than magnetic stirring before heating, the resistance to mass transfer in the pore channels was significantly reduced. As a result, the reactions proceeded more rapidly and a four-fold increase in the turnover frequency (TOF) could be obtained. When the ultrasonication was employed throughout the entire reaction process, the conversion could also exceed 90% even without the protection of inert gas, and although the reaction temperature was lowered to 30 °C. This work provides a method for fabricating highly active porous carbon encapsulated Pd catalysts for Suzuki reactions and proves that the problem of mass transfer in porous catalysts can be conveniently resolved by ultrasonication without any chemical modification being necessary.

Due to its intrinsic structure and characteristics, small size and monodispersity, control of single-crystalline Cu2O polyhedra in aqueous media is a challenge, which is important to overcome to achieve enhanced photocatalytic activity. Here, we use heterogeneous nucleation, rather than homogeneous nucleation, of Cu2O with gold nanorods as seeds to realize subsequent uniform crystal growth. We obtained nearly monodisperse octahedral Au@Cu2O nanocrystals with single-crystalline shells, which are distinct from the pentagonal column-shaped structures previously described. Due to the fact that one Au@Cu2O holds only one Au nanorod, two formulas were deduced for convenient size control of the Cu2O shell. The formulas were calculated by adjusting the amount of Au rods that are relatively quantified. The formula also allows the size of the final product to be predicted when a given amount of gold seeds are employed. The experimental results agree well with the calculated data. The result of larger surface area and improved charge separation from core-shell interaction, made five samples of different sizes exhibit excellent photocatalytic activity toward MO degradation. The synthetic strategy reported here provides a clue to monodispersity and size control of core-shell nanocrystals, which is useful in developing new catalysts with better performance that are urgently needed in the fields of both science and technology.

Due to its intrinsic structure and characteristics, small size and monodispersity, control of single-crystalline Cu2O polyhedra in aqueous media is a challenge, which is important to overcome to achieve enhanced photocatalytic activity. Here, we use heterogeneous nucleation, rather than homogeneous nucleation, of Cu2O with gold nanorods as seeds to realize subsequent uniform crystal growth. We obtained nearly monodisperse octahedral Au@Cu2O nanocrystals with single-crystalline shells, which are distinct from the pentagonal column-shaped structures previously described. Due to the fact that one Au@Cu2O holds only one Au nanorod, two formulas were deduced for convenient size control of the Cu2O shell. The formulas were calculated by adjusting the amount of Au rods that are relatively quantified. The formula also allows the size of the final product to be predicted when a given amount of gold seeds are employed. The experimental results agree well with the calculated data. The result of larger surface area and improved charge separation from core-shell interaction, made five samples of different sizes exhibit excellent photocatalytic activity toward MO degradation. The synthetic strategy reported here provides a clue to monodispersity and size control of core-shell nanocrystals, which is useful in developing new catalysts with better performance that are urgently needed in the fields of both science and technology.