Yuan-Yuan Ma

Assembly of new organic–inorganic hybrids based on copper-bis(triazole) complexes and Keggin-type polyoxometalates with different negative charges

Three new organic–inorganic hybrid compounds based on copper-bis(triazole) complexes and Keggin-type polyoxometalates (POMs) with different negative charges, [Cu(I)BBTZ]3[PMo12O40] (1), [Cu(I)BBTZ][Cu(I)(HBBTZ)2][SiMo12O40]·2.5H2O (2) and [Cu(I)BBTZ]6[SiWV2WVI10O40]·2H2O (3) (BBTZ = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene), have been hydrothermally prepared and characterized by elemental analyses, IR spectra, TG analyses, XPS spectra, powder X-ray diffraction and single-crystal X-ray diffraction analyses. Compounds 1 and 2 exhibit the POM-templated 3D host–guest supramolecular frameworks. Compound 3 displays a POM-supported 3D open framework. In these three hybrid compounds, the structural transformation from host–guest framework to open framework were modulated by the introduction of Keggin-type polyoxoanions with different negative charges. Moreover, the electrochemical properties of compounds 1–3 were investigated and all these compounds show the electrocatalytic activities towards the reduction of nitrite.

MoP/Mo2C@C: A New Combination of Electrocatalysts for Highly Efficient Hydrogen Evolution over the Entire pH Range

During the exploration of highly efficient noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER), a promising and challenging strategy is to fabricate composite nanocatalysts by finely tuning metal and/or nonmetal element components. Herein, we report a new HER electrocatalyst, which is composed of molybdenum phosphide and molybdenum carbide composite nanoparticles (NPs) coated by few-layer N-doped graphitic carbon shells (denoted as MoP/Mo2C@C). Such a new combination mode of electrocatalysts is realized by a one-step annealing route with the mixture of a Mo/P-based polyoxometalate (POM) and dicyandiamide. On the basis of this method, the simultaneous phosphorization and carbonization in a nanoscale confined space can be easily achieved by the use of POM as the molecular-element-regulating platform. MoP/Mo2C@C exhibits more remarkable HER performance over the whole pH range than those of MoP, Mo2C, and the physical mixture of MoP and Mo2C. The low overpotentials of 89, 136, and 75 mV were obtained at a current density of 10 mA cm-2 in the media of pH = 0, 7, and 14, respectively. Furthermore, MoP/Mo2C@C shows a long-term durability for 14 h over the entire pH range (0-14). Because of the protection of carbon shells, such composite electrocatalyst also possesses better transition-metal tolerance exemplified by Fe2+, Co2+, and Ni2+ than that of 20% commercial Pt/C. This work demonstrates the advantage of POM precursors in adjusting the component and properties of nanoscale composite electrocatalysts for HER, which may suggest new options for the fabrication of highly efficient composite electrocatalysts.

New entangled coordination networks based on charge-tunable keggin-type polyoxometalates.

Investigation into a hydrothermal reaction system with transition-metal (TM) ions, 1,4-bis(1,2,4-triazol-1-lmethyl)benzene (BBTZ) and various charge-tunable Keggin-type polyoxometalates (POMs) led to the preparation of four new entangled coordination networks, [Co(II) (HBBTZ)(BBTZ)2.5 ][PMo12 O40 ] (1), [Cu(I) (BBTZ)]5 [BW12 O40 ]⋅H2 O (2), [Cu(II) (BBTZ)]3 [AsW(V) 3 W(VI) 9 O40 ]⋅10 H2 O (3), and [Cu(II) 5 (BBTZ)7 (H2 O)6 ][P2 W22 Cu2 O77 (OH)2 ]⋅6 H2 O (4). All compounds were characterized by using elemental analysis, IR spectroscopy, thermogravimetric analysis, powder X-ray diffraction, and single-crystal X-ray diffraction. The mixed valence of W centers in compound 3 was further confirmed by using XPS spectroscopy and bond-valence sum calculations. In the structural analysis, the entangled networks of 1-4 demonstrate zipper-closing packing, 3D polythreading, 3D polycatenation, and 3D self-penetration, respectively. Moreover, with the enhancement of POM negative charges and the use of different TM types, the number of nodes in the coordination networks of 1-4 increased and the basic metal-organic building motifs changed from a 1D zipper-type chain (in 1) to a 2D pseudorotaxane layer (in 2) to a 3D diamond-like framework (in 3) and finally to a 3D self-penetrating framework (in 4). The photocatalytic properties of compounds 1-4 for the degradation of methylene blue under UV light were also investigated; all compounds showed good catalytic activity and the photocatalytic activity order of Keggin-type species was initially found to be {XMo12 O40 }>{XW12 O40 }>{XW12-n TMn O40 }.

Polyoxometalate-based metal–organic coordination networks for heterogeneous catalytic desulfurization

Three new polyoxometalate (POM)-based metal–organic coordination networks (MOCNs) with chemical formulae of [Co(BBTZ)1.5(HBBTZ)(H2O)2][PW12O40]·H2O (1), [Co2.5(BBTZ)4(H2O)2] [BW12O40]·4H2O (2) and [Cu(BBTZ)2]5[BW12O40]2·4H2O (3) (BBTZ = 1,4-bis-(1,2,4-triazol-1-ylmethyl)benzene) were hydrothermally synthesized in a reaction system containing Keggin-type POMs, transition metal salts (cobalt salts and copper salts) and BBTZ ligands. All compounds were characterized by elemental analyses, IR, powder X-ray diffraction, TG analyses and single-crystal X-ray diffraction analyses. Compound 1 exhibits a POM-encapsulated 3-D supramolecular network, while compounds 2 and 3 display POM-supported 3-D coordination networks. Using the oxidative desulfurization of dibenzothiophene (DBT) as the model, the catalytic activities of compounds 1–3 are investigated. All three compounds show efficient catalytic activity for the oxidation of DBT with the order of 2 > 3 > 1. It is found that the POM species of compounds 1–3 play the main role in the catalytic oxidative desulfurization process, while the TM ions, the loading amounts of POMs, and the structural features of these POM-based MOCNs are also necessary factors that affect the catalytic activities. Furthermore, a surfactant-assisted hydrothermal synthesis method has been developed to prepare nanocrystal 2. SEM reveals that the as-synthesized nanocrystalline 2 is about 245 nm in diameter. The catalytic oxidative desulfurization experiments show that nanocrystal 2 possesses much higher catalytic activities than those of the large single-crystal products of 2.

Isopolymolybdate-induced organic–inorganic hybrid assemblies with copper ions and bichelate-bridging ligands

The heptamolybdate [NH4]6[Mo7O24], as a common polyoxometalate (POM) precursor with dynamic configuration transformation in aqueous solution by changing the pH, has been introduced into the hydrothermal reaction system containing copper ions and bichelate-bridging ligands, leading to the isolation of seven new organic–inorganic hybrid compounds [CuI2Lo2][Mo6O19] (1), [CuII2Lo2][β-Mo8O26]·2H2O (2), [CuII4Lo2(pypz)4][α-Mo8O26]·3H2O (3), [CuII4Lp4(H2O)4][α-Mo8O26][β-Mo8O26]·4H2O (4), [CuI4Lp3][β-Mo8O26]·H2O (5), [CuI4Lp4][α-Mo8O26] (6), and [CuI2Lp2(Emim)2][β-Mo8O26] (7) (bis(3-(2-pyridyl)pyrazole-1-ylmethyl)benzene = bppmb, Lo = 1,2-bppmb, Lp = 1,4-bppmb, pypz = 3-(2-pyridyl)pyrazole, Emim = 1-ethyl-3-methylimidazolium). All compounds were characterized by elemental analysis, IR spectroscopy, TG analysis, powder X-ray diffraction and single-crystal X-ray diffraction analysis. The structural analyses show that the isopolymolybdate clusters act as a type of “molecular adaptor”, which modulate the coordination mode of metal–organic subunits, leading to a series of new metal–organic secondary building units, such as the “Mobius-Ring”-type binuclear unit, “S”-type binuclear unit, butterfly-type unit, helical chain and mesomeric helical chain. As a result, the POM anions and the metal–organic subunits have suitable matching modes to form new hybrid self-assemblies. The photocatalytic properties of compounds 1–7 for the degradation of Rhodamine-B (RhB) under UV irradiation were investigated. All compounds showed good catalytic activity. Furthermore, the luminescent properties of compounds 1–7 have also been explored, indicating an organic ligand-based emission mode for all compounds.

Ag/AgxH3–xPMo12O40 Nanowires with Enhanced Visible-Light-Driven Photocatalytic Performance

Photocatalysis, a promising technology platform to address the environmental problems, has been attracting considerable attention. In this paper, Ag/AgxH3-xPMo12O40 (simplified as Ag/AgHPMo12) nanowires have been synthesized by a facile solid reaction route and in situ photodeposited method. The results of SEM and TEM indicate that the diameters of AgHPMo12 nanowires are about 45 ± 10 nm, and Ag nanoparticles with diameters in the range of 5-15 nm are uniformly anchored on the surface of AgHPMo12 nanowires. The Ag content in the Ag/AgHPMo12 composite was manipulated by the light irradiation time (Ag/AgHPMo12-x; x stands for the irradiation time; x = 2, 4, 6, 8 h, respectively). With increasing irradiation time, the light absorption of as-synthesized samples in the visible region was gradually enhanced. The Ag/AgHPMo12-4 exhibits the best photocatalytic performance for the degradation of methyl orange and reduction of Cr2O72- under visible-light (λ > 420 nm) irradiation. The study of the photocatalytic mechanism reveals that both Ag and AgHPMo12 can be excited by visible light. The photoinduced electrons were transferred from AgHPMo12 to metallic Ag, and combined with the Ag plasmonic holes. The Ag plasmonic electrons were trapped by O2 to form ·O2-, or directly reduced Cr2O72- to Cr3+. Meanwhile, the ·O2- species and the photogenerated holes of AgHPMo12 were used to oxidize MO or i-PrOH; thus, they showed highly efficient and recyclable photocatalytic performance for removing the organic and inorganic pollutants.

Polyoxometalate-Based Metal–Organic Frameworks for Selective Oxidation of Aryl Alkenes to Aldehydes

Polyoxometalates (POMs) show considerable catalytic performance toward the selective oxidation of alkenes to aldehydes, which is commercially valuable for the production of pharmaceuticals, dyes, perfumes, and fine chemicals. However, the low specific surface area of POMs as heterogeneous catalysts and poor recyclability as homogeneous catalysts have hindered their wide application. Dispersing POMs into metal-organic frameworks (MOFs) for the construction of POM-based MOFs (POMOFs) suggests a promising strategy to realize the homogeneity of heterogeneous catalysis. Herein, we report two new POMOFs with chemical formulas of [Co(BBTZ)2][H3BW12O40]·10H2O (1) and [Co3(H2O)6(BBTZ)4][BW12O40]·NO3·4H2O (2) (BBTZ = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene) for the selective oxidation of alkenes to aldehydes. Compound 1 possesses a non-interpenetrated three-dimensional (3D) cds-type open framework with a 3D channel system. Compound 2 displays a 3D polyrotaxane framework with one-dimensional channels along the [100] direction. In the selective oxidation of styrene into benzaldehyde, compound 1 can achieve a 100% conversion in 4 h with 96% selectivity toward benzaldehyde, which is superior to that of compound 2. A series of control experiments reveal that the co-role of [BW12O40]5- and Co2+ active center as well as a more open framework feature co-promote the catalytic property of the POMOFs in this case. This work may suggest a new option for the development of POMOF catalysts in the selective oxidation of alkenes.

A Co2P/WC Nano‐Heterojunction Covered with N‐Doped Carbon as Highly Efficient Electrocatalyst for Hydrogen Evolution Reaction

The hydrogen evolution reaction (HER) produces clean hydrogen through an electrochemical process. However, new nonprecious-metal electrocatalysts for the HER are required to reduce the consumption of energy. Herein, we report a new Co2 P/WC nano-heterojunction that consists of Co2 P and WC composite phases coated with a few-layer N-doped graphitic carbon shells (Co2 P/WC@NC). The composite was prepared by a one-step annealing of the polyoxometalate Na9 (NH4 )5 [{(B-α-PW9 O34 )Co3 (OH)(H2 O)2 (Ale)}2 Co]⋅35 H2 O (Co7 P6 W18 ) and dicyandiamide (DCA). The preparation method consisted of the simultaneous phosphorization of Co and carbonization of W in a confined space to isolate a Co2 P/WC nano-heterojunction phase for the first time. Co2 P/WC@NC facilitated the generation of hydrogen in the electrolysis process, which had an overpotential of only 91 mV at a current density of 10 mA cm-2 in the acid solution; an excellent HER performance (2 H+ +2 e- →H2 ) and Tafel slope (40 mV dec-1 ) as well as durability over a period of 50 h were achieved. Theoretical calculations showed that the Co2 P, WC, and Npyridinic C dopants in the material synergistically promoted the HER activity rather than the individual constituents. Furthermore, Co2 P/WC@NC nano-heterojunctions showed good HER performance in the whole pH range of electrolytes and considerable durability in acidic media containing transition metal ions, which may attract more attention for the exploration and optimization of nano-heterojunction catalysts for the HER.

Two new Keggin-type polyoxometalate-based organic-inorganic hybrid compounds with metal ions and chelate ligands

Abstract Two new hybrid compounds with chemical formulas [Co(L1)2(H2O)2]2[SiW12O40]·6H2O (1) and [Cu2(C9H22 N)2(L2)2][SiW12O40]·2H2O (2) (L1 = 3,3′-diamino-5,5′-bis(1H-1,2,4-triazole), L2 = 1,2-bis(3-(2-pyridyl)pyrazole-1-ylmethyl)benzene, C9H22 N = tripropylamine) were prepared in a hydrothermal reaction system containing Keggin-type polyoxometalates (POMs), transition metal (TM) salts (cobalt salts and copper salts), and chelate L1/L2 ligands. X-ray structural analysis reveals that 1 and 2 possess 3-D supramolecular host frameworks with large cavities and the polyoxoanions SiW12 reside in these cavities as the guest templates. The photocatalytic activities of 1 and 2 were investigated. Both compounds show efficient catalytic activity for degradation of RhB with 1 > 2. The loading amount of the POM units and the structural features of these POM-based organic-inorganic hybrid compounds may be necessary factors that affect the activity of catalysts in the photocatalytic degradation.

Electrocatalytic performance of ultrasmall Mo2C affected by different transition metal dopants in hydrogen evolution reaction

Molybdenum carbides are considered as one type of privileged noble-metal-free electrocatalysts for hydrogen evolution reactions (HER) due to their d-band electron structure, which is similar to Pt. Especially, the electronic structure of such materials can be further adjusted by elemental doping to improve their electrocatalytic activity. Herein, we selected the Anderson-type polyoxometalates (POMs) (NH4)n[TMMo6O24H6]·5H2O (TM = Ni2+, Co2+, n = 4; TM = Fe3+, Cr3+, n = 3) as precursors to prepare new transition-metal-doped Mo2C materials. When these POMs were mixed with dicyandiamide (DCA) by solid grinding, and carbonized at a high temperature, a series of Ni-, Co-, Fe-, and Cr-doped Mo2C composite nanoparticles covered by few-layer graphitic carbon shells (abbr. TM-Mo2C@C) were obtained. All these nanoparticles possess a similar size, morphology, and TM/Mo component ratio, and thus it is feasible to systematically investigate the influence of different TM dopants on the electrocatalytic activity of Mo2C for HER. Both electrocatalytic experiments and DFT calculations reveal that TM dopants have a significant effect on the hydrogen binding energy (ΔGH*) and the catalytic activity of Mo2C. The sequence of HER electrocatalytic activity is as follows: Ni-Mo2C > Co-Mo2C > Fe-Mo2C > Cr-Mo2C. As a result, Ni-Mo2C@C possesses the best HER performance, which required an overpotential of 72 mV at a current density of 10 mA cm-2 and the Tafel slope is 65.8 mV dec-1. This work suggests a shortcut to reasonably investigate the effects of elemental doping on molybdenum carbides and explore new high-efficient and low-cost electrocatalysts for HER.