Biomimetic Oxygen Activation by MoS2/Ta3N5 Nanocomposites for Selective Aerobic Oxidation

Abstract

The selective oxidation of petroleum-based feedstocks to useful functionalized chemicals is an important family of chemical transformations. Of these transformations, the selective oxidation of alcohols, alkenes, amines, and sulfides are among the most challenging reactions in green chemistry. There is significant interest in the design of new, costeffective, and environmentally friendly heterogeneous catalysts that use molecular oxygen (O2) under mild conditions, to avoid the use of a large excess of toxic and expensive stoichiometric metal oxidants. Although a number of catalysts based on novel metals and transition-metal oxides have been introduced, the precise design of catalysts with well-defined behaviors that depend on surface properties and electron features is still desired. Such catalysts are significant not only for use with multifunctional substrates, but also for insightful studies of catalytic mechanisms. These challenges are expected to be met through facet engineering and component control at the catalyst surface and in the active sites on the level of nanochemistry. Crystal-facet engineering has been successfully introduced to exploit novel metal nanocatalysts with high-surface-energy planes. This approach has led to high activity and selectivity in oxidation catalysis. However, it is difficult to control facet growth in metal-oxide catalysts with lowsymmetry crystal structures owing to the complexity of their structures. On the other hand, the ability to effectively vary the surface properties and electronic features of metal oxides by doping with other elements of different electronegativity, such as N, P, and S, enables new strategies for catalyst design. For example, the introduction of N into metal oxides can increase the energy of the HOMO orbital and narrow the band gap to thus enhance the catalytic activity, although controlled nitridation is difficult by current synthetic strategies. Recently, we proposed Caand SiO2-assisted urea methods for the controlled nitridation of transition metals. Remarkably, we discovered tunable oxidation ability associated with tailored nitridation, namely, improved activity and tunable selectivity for alkene epoxidation on TaON and Ta3N5 nanoparticles (NPs) with H2O2. This discovery opens up opportunities to develop superior tantalum-based catalysts with well-defined properties, especially for reactions involving cheap O2 as the oxidant. Access to such catalysts is needed to enable the important factors for catalytic turnover and selectivity to be uncovered. However, the absence of O2 activation in such (oxy)nitrides synthesized so far seriously limits further exploration. Biomimetic studies point to a new way to develop catalysts by learning from nature. In nature, the active center of nitrogenase enzymes contains metal atoms usually bound to sulfur, such as active Mo S and Fe S clusters. In nitrogen fixation, Mo S and Fe S sites activate inert N2 to react with H, with the generation of NH3 and H2. [11,12] This process inspired the use of MoSx for electroand photoelectrocatalytic H2 evolution based on electron transfer from MoS2 to H . The close energy potentials of E(H/H2)= 0 V and E(O2/CO2)= 0.16 V versus the normal hydrogen electrode suggest that MoSx could be used as a biomimetic O2-activation reagent to exploit bifunctional tantalum-based nanocatalysts for aerobic oxidation reactions. Herein, we describe the development of a new MoS2/ Ta3N5 catalyst in which Ta3N5 NPs are integrated with ultrathin MoS2 layers on the nanoscale by a hydrothermal method. The MoS2 nanolayers act as a biomimetic O2activation reagent in the MoS2/Ta3N5 NPs, which showed high activity and selectivity in the aerobic oxidation of alcohols as a result of the synergistic effect betweenMoS2 and Ta3N5. The MoS2/Ta3N5 NPs were also active in the aerobic oxidation of alkenes, amines, and sulfides. The different activities observed for these different substrates imply the potential use of this catalyst with multifunctional substrates. For example, high selectivity for hydroxy-group oxidation (> 90%) was observed in the oxidation of unsaturated alcohols. Well-defined Ta3N5 NPs of approximately 20 nm in diameter were prepared by our previously reported SiO2assisted urea method (see Figure S1 in the Supporting Information). Hydrothermal treatment of the Ta3N5 NPs with varying amounts of ammonium heptamolybdate (AHM) and thiourea at 180 8C for 20 h (see the Supporting Information) gave a series of MoS2/Ta3N5 nanocomposites that varied in their MoS2 content. The color of the composites changed from red to black as the MoS2 content increased (Figure 1a; see also Figure S2 in the Supporting Information). Inductively coupled plasma analysis and CHNS elemental analysis were used to determine the Mo and S content, respectively. The [*] Dr. Q. S. Gao, Dr. C. Giordano, Prof. Dr. M. Antonietti Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, Research Campus Golm 14424 Potsdam (Germany) E-mail: qingsheng.gao@mpikg.mpg.de