Qingsheng Gao

Porous nanoMoC@graphite shell derived from a MOFs-directed strategy: an efficient electrocatalyst for the hydrogen evolution reaction

The hydrogen evolution reaction using noble-metal free electrocatalysts has captured increasing attention due to its importance in renewable hydrogen production. Herein, a highly active and stable electrocatalyst of MoC encapsulated by graphitized carbon shells (nanoMoC@GS) has been developed via an in situ carburization of a Mo-based metal–organic framework (Mo-MOF) with the atomic periodic structure. The ultrafine MoC nanoparticles (∼3 nm) confined by 1–3 layered graphite shells significantly favor the efficient HER in both acidic and basic media. In particular, a low overpotential (η10 = 124 and 77 mV at a current density of −10 mA cm−2), a small Tafel slope (43 and 50 mV dec−1) and a high exchange current density (j0 = 0.015 and 0.212 mA cm−2) are achieved on nanoMoC@GS in 0.5 M H2SO4 and 1.0 M KOH, respectively. Such remarkable activity, outperforming most current noble-metal-free electrocatalysts, stems from the cooperative/synergistic effects of ultrafine MoC nanostructure, ultrathin and conductive graphitized carbon shells, and enriched porosity. This work demonstrates a feasible way to design high-performance electrocatalysts via converting “atomic contact” hybrid structures (e.g., MOFs), illustrating a new perspective for developing nanocatalysts in the energy chemistry field.

Phosphorus-Mo2C@carbon nanowires toward efficient electrochemical hydrogen evolution: composition, structural and electronic regulation

To explore high-performance electrocatalysts, electronic regulation on active sites is essentially demanded. Herein, we propose controlled phosphorus doping to effectively modify the electronic configuration of nanostructured Mo2C, accomplishing a benchmark performance of noble-metal-free electrocatalysts in the hydrogen evolution reaction (HER). Employing MoOx–phytic acid–polyaniline hybrids with tunable composition as precursors, a series of hierarchical nanowires composed of phosphorus-doped Mo2C nanoparticles evenly integrated within conducting carbon (denoted as P-Mo2C@C) are successfully obtained via facile pyrolysis under inert flow. Remarkably, P-doping into Mo2C can increase the electron density around the Fermi level of Mo2C, leading to weakened Mo–H bonding toward promoted HER kinetics. Further density functional theory calculations show that the negative hydrogen-binding free energy (ΔGH*) on pristine Mo2C gradually increases with P-doping due to electron transfer and steric hindrance by P on the Mo2C surface, indicating the effectively weakened strength of Mo–H. With optimal doping, a ΔGH* approaching 0 eV suggests a good balance between the Volmer and Heyrovsky/Tafel steps in HER kinetics. As expected, the P-Mo2C@C nanowires with controlled P-doping (P: 2.9 wt%) deliver a low overpotential of 89 mV at a current density of −10 mA cm−2 and striking kinetic metrics (onset overpotential: 35 mV, Tafel slope: 42 mV dec−1) in acidic electrolytes, outperforming most of the current noble-metal-free electrocatalysts. Elucidating feasible electronic regulation and the remarkably enhanced catalysis associated with controlled P-doping, our work will pave the way for developing efficient noble-metal-free catalysts via rational surface engineering.

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

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

Microwave-Assisted Reactant-Protecting Strategy toward Efficient MoS2 Electrocatalysts in Hydrogen Evolution Reaction

The exposure of rich active sites is crucial for MoS2 nanocatalysts in efficient hydrogen evolution reaction (HER). However, the active (010) and (100) planes tend to vanish during preparation because of their high surface energy. Employing the protection by thiourea (TU) reactant, a microwave-assisted reactant-protecting strategy is successfully introduced to fabricate active-site-rich MoS2 (AS-rich MoS2). The bifunctionality of TU, as both a reactant and a capping agent, ensures rich interactions for the effective protection and easy exposure of active sites in MoS2, avoiding the complicated control and fussy procedure related to additional surfactants and templates. The as-obtained AS-rich MoS2 presents the superior HER activity characterized by its high current density (j = 68 mA cm(-2) at -300 mV vs RHE), low Tafel slope (53.5 mV dec(-1)) and low onset overpotential (180 mV), which stems from the rich catalytic sites and the promoted conductivity. This work elucidates a feasible way toward high performance catalysts via interface engineering, shedding some light on the development of emerging nanocatalysts.

Mesoporous Mo2C/N-doped carbon heteronanowires as high-rate and long-life anode materials for Li-ion batteries

Transition metal carbides are an emerging class of anode materials for Li-ion batteries (LIBs), which have recently drawn attention because of their good conductivity and high capacity after rational nano-engineering. In this work, we have developed Mo2C/N-doped carbon mesoporous heteronanowires (Mo2C/N–C MHNWs) with enhanced capacitive behaviour as high-performance anode materials for LIBs. With the heterostructure, the Mo2C nanocrystallites offer short paths for Li+ diffusion, while the N-doped carbon matrix facilitates fast electron transportation and buffers the volume change of Mo2C during the discharge/charge cycles. When evaluated as anodes for LIBs, the Mo2C/N–C MHNWs exhibited high capacity and high rate capability, as well as a long-term cycle life. In particular, a reversible capacity of 744.6 mA h g−1 was achieved in the first cycle, and 732.9 mA h g−1 was preserved after 700 cycles at a current density of 2 A g−1. The outstanding performance stems from fast kinetics enhanced by the pseudocapacitive effect, which was evidenced in the further analysis based on electrochemical impedance spectra and cyclic voltammetry. Our results elucidate the attractive Li+ storage performance of Mo2C-based nanocomposites, which may shed some light on the development of high-performance materials for energy storage and utilization.

Preparation of organic–inorganic hybrid Fe–MoOx/polyaniline nanorods as efficient catalysts for alkene epoxidation

Novel Fe-MoO(x)/polyaniline nanorods were fabricated via in situ polymerization of Mo(3)O(10)(C(6)H(5)NH(3))(2)·2H(2)O nanowires, in which interface reactions remarkably influenced the morphology of products; and the nanorods showed high performance in cyclooctene epoxidation due to the organic-inorganic hybrid structure and Fe(3+) additive.

Hierarchical MoO2/Mo2C/C Hybrid Nanowires as High-Rate and Long-Life Anodes for Lithium-Ion Batteries

Hierarchical MoO2/Mo2C/C hybrid nanowires (MoO2/Mo2C/C HNWs) have been fabricated through facile calcination of Mo3O10(C6H5NH3)2·2H2O nanowires which serve as both precursors and self-templates. In the MoO2/Mo2C/C HNWs, nanoparticles dispersed in the nanowires are beneficial for Li(+) transportation due to the decreased diffusion paths. Moreover, hybridization with Mo2C and carbon facilitates the electron transfer and increases the structural stability without sacrifice of capacity. As anode materials for lithium-ion batteries, the MoO2/Mo2C/C HNWs exhibit a reversible capacity of 950 mA h g(-1) after 320 cycles at a current density of 200 mA g(-1). Even when cycled at 2000 mA g(-1), they maintained a reversible capacity of 602 mA h g(-1) after 500 cycles. By incorporation of Mo2C and C with MoO2, the MoO2/Mo2C/C HNWs show high-rate capability and long cycle life and can be a promising candidate for lithium-ion battery anodes.

Electrospinning Hetero-Nanofibers of Fe3C-Mo2C/Nitrogen-Doped-Carbon as Efficient Electrocatalysts for Hydrogen Evolution

Heterostructured electrocatalysts with multiple active components are expected to synchronously address the two elementary steps in the hydrogen evolution reaction (HER), which require varied hydrogen-binding strength on the catalyst surface. Herein, electrospinning followed by a pyrolysis is introduced to design Fe3 C-Mo2 C/nitrogen-doped carbon (Fe3 C-Mo2 C/NC) hetero-nanofibers (HNFs) with tunable composition, leading to abundant Fe3 C-Mo2 C hetero-interfaces for synergy in electrocatalysis. Owing to the strong hydrogen binding on Mo2 C and the relatively weak one on Fe3 C, the hetero-interfaces of Fe3 C-Mo2 C remarkably promote HER kinetics and intrinsic activity. Additionally, the loose and porous N-doped carbon matrix, as a result of Fe-catalyzed carbonization, ensures the fast transport of electrolytes and electrons, thus minimizing diffusion limitation. As expected, the optimized Fe3 C-Mo2 C/NC HNFs afforded a low overpotential of 116 mV at a current density of -10 mA cm-2 and striking kinetics metrics (onset overpotential: 42 mV, Tafel slope: 43 mV dec-1 ) in 0.5 m H2 SO4 , outperforming most recently reported noble-metal-free electrocatalysts.

Enhancing Metal-Support Interactions by Molybdenum Carbide: An Efficient Strategy toward the Chemoselective Hydrogenation of α,β-Unsaturated Aldehydes.

Metal-support interactions are desired to optimize the catalytic turnover on metals. Herein, the enhanced interactions by using a Mo2C nanowires support were utilized to modify the charge density of an Ir surface, accomplishing the selective hydrogenation of α,β-unsaturated aldehydes on negatively charged Ir(δ-) species. The combined experimental and theoretical investigations showed that the Ir(δ-) species derive from the higher work function of Ir (vs. Mo2C) and the consequently electron transfer. In crotonaldehyde hydrogenation, Ir/Mo2C delivered a crotyl alcohol selectivity as high as 80%, outperforming those of counterparts (<30%) on silica. Moreover, such electronic metal-support interactions were also confirmed for Pt and Au, as compared with which, Ir/Mo2C was highlighted by its higher selectivity as well as the better activity. Additionally, the efficacy for various substrates further verified our Ir/Mo2C system to be competitive for chemoselective hydrogenation.

Controlled nitridation of tantalum (oxy)nitride nanoparticles towards optimized metal-support interactions with gold nanocatalysts

The electron regulation on supports can vary metal-support interactions with loaded metals in heterogeneous catalysis. In this paper, a facile Sr2+-mediated ionothermal route was introduced to control the nitridation degree in tantalum (oxy)nitrides, resulting in varied electronic properties and optimized interactions with gold nanocatalysts. A new mechanism was proposed that the formation of SrTa4O11 intermediates facilitated the replacement of O by N in controlled nitridation, and more importantly avoided undesired over-nitridation. As expected, the TaON support with defined nitridation promoted electronic metal-support interactions to generate Auδ− species, which was highly active for the thermal hydrogenation of nitrobenzene due to the moderated adsorption and effective activation on Auδ− in Au/TaON. This work elucidated the optimized metal-support interactions achieved on controllably nitridated supports, opening up new opportunities for the development of efficient nanocatalysts.