Shifa Ullah Khan

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.

Assembly of hybrids based on polyoxotungstates and Co-tris(imidazolyl) complexes with bifunctional electrocatalytic activities

Three new organic–inorganic hybrid compounds based on the Keggin polyoxometalates, [Co2(tib)2(H2O)4(α-HBW12O40)]·6H2O (1), [Co2(tib)2(H2O)4(α-SiW12O40)]·4H2O (2) and [Co(H2tib)4(α-BW12O40)2]·8H2O (3) (tib = 1,3,5-tris(1-imidazoly)benzene), were hydrothermally synthesized by utilizing a pH-dependent approach in the same POM/Co/tib reaction systems. X-ray structural analyses reveal that isostructural compounds 1 and 2, obtained at pH 3.6, have a (3,4)-connected 2D net with the (42 6)(42 63 8) topology; compound 3, isolated at pH 4.8, is a “butterfly-like” structure stemming from α-BW12 polyoxoanions and Co–tib motifs. In addition, the electrocatalytic properties of compounds 1–3 were studied by cyclic voltammetry. Compounds 1–3 displayed bifunctional electrocatalytic activities toward reduction of nitrite and oxidation of ascorbic acid.

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.

Polyoxometalate and Resin‐Derived P‐Doped Mo2C@N‐Doped Carbon as a Highly Efficient Hydrogen‐Evolution Reaction Catalyst at All pH Values

A new type of P-doped Mo2 C coated by N-doped carbon (P-Mo2 C@NC) has been successfully prepared by calcining a mixture of H3 [PMo12 O40 ] polyoxometalates (POMs) and urea-formaldehyde resin under an N2 atmosphere. Urea-formaldehyde resin not only serves as the carbon source to ensure carbonization but also facilitates the uniform distribution of POM precursors, which efficiently avoid the aggregation of Mo2 C particles at high temperatures. TEM analysis revealed that the average diameter of the Mo2 C particles was about 10 nm, which is coated by a few-layer N-doped carbon sheet. The as-prepared P-Mo2 C@NC displayed excellent hydrogen-evolution reaction (HER) performance and long-term stability in all pH environments. To reach a current density of 10 mA cm-2 , only 109, 159, and 83 mV were needed for P-Mo2 C@NC in 0.5 m H2 SO4 (pH 0), 0.1 m phosphate buffer (pH 7), and 1 m KOH (pH 14), respectively. This could provide a high-yield and low-cost method to prepare uniform nanosized molybdenum carbides with highly efficient and stable HER performance.

Ultrafine cable-like WC/W2C heterojunction nanowires covered by graphitic carbon towards highly efficient electrocatalytic hydrogen evolution

Developing highly active, durable and economical noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER) plays a crucial role in the future hydrogen economy. Herein, ultrafine cable-like WC/W2C heterojunction nanowires (NWs) covered by few-layer N-doped graphite-like carbon (labeled WC/W2C@C NWs) have been firstly accomplished via a simple two-step pyrolysis treatment of W18O49 NWs precursor, exhibiting tunable phase composition, excellent HER activity, fast kinetic metrics and remarkable stability along the entire pH range. With the optimized phase composition, WC/W2C@C NWs (WC : W2C ≈ 1 : 1.3) display the low overpotentials of 69 mV and 56 mV at the cathodic current density of 10 mA cm−2 and small Tafel slopes of 52 and 59 mV dec−1 in 0.5 M H2SO4 and 1 M KOH, respectively. DFT calculations and experimental results demonstrate that such prominent performance towards the HER was mainly attributed to abundant interfaces between WC and W2C phases in the cable-like heterojunction nanowires which may exert both the high HER activity of W2C and the excellent electrochemical stability and conductivity of WC. Furthermore, the one-dimensional (1D) nanowire morphology and graphite-like carbon shells also promote the conductivity of the whole electrocatalytic material. This work presents a new route to accomplish the morphology control, composition and electronic regulation so as to boost the HER performance of tungsten-carbide-based electrocatalysts, providing new guidance for exploring efficient noble-metal-free electrocatalysts.

Polyoxometalates decorated with metal-organic moieties as new molecular photo- and electro-catalysts

Abstract Three new polyoxometalate-based organic-inorganic hybrid compounds (POIHCs), [Cu(PBI)2(H2O)][{Cu(PBI)(OH)(H2O)}{PW12O40}]·5H2O (1), [Cu(PBI)2(H2O)][{Cu(PBI)2(H2O)}{SiW12O40}]·4H2O (2) and [Cu4(HPO4)(PO4)(H2O)2(PBI)4][PMo12O40]·H2O (3) (PBI =2-(pyridin-2-yl)-1H-benzo[d]imidazole), have been synthesized under hydrothermal conditions. All three compounds were characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, powder X-ray diffraction and single-crystal X-ray diffraction. In 1 and 2, the Keggin-type polyoxometalate (POM) units are decorated by the {Cu/PBI} metal-organic fragments, together with additional {Cu/PBI} isolated cations, which further stack into 3-D supramolecular frameworks with porous structural features. Compound 3 consists of a rare cationic cluster [Cu4(HPO4)(PO4)(H2O)2(PBI)4]3+ and the Keggin-type POM anion, forming a 3-D supramolecular framework with limited interspaces for the entry of guest molecules. Comparable photocatalytic properties of 1–3 were investigated by the use of methylene blue (MB) and Rhodamine B (RhB) degradation under UV light. All compounds showed good catalytic activity for the degradation of MB in contrast to RhB. The possible catalytic mechanism was discussed based on the size of dye molecules, the low adsorption ability and crystal packing mode of the as-synthesized compounds. Additionally, the electrochemical properties of 1–3 have also been investigated, exhibiting good electrocatalytic performance for nitrite reduction. Graphical Abstract