Unravelling the pH-dependence of a molecular photocatalytic system for hydrogen production

Photocatalytic systems for the reduction of aqueous protons are strongly pH-dependent, but the origin of this dependency is still not fully understood. We have studied the effect of different degrees of acidity on the electron transfer dynamics and catalysis taking place in a homogeneous photocatalytic system composed of a phosphonated ruthenium tris(bipyridine) dye (RuP) and a nickel bis(diphosphine) electrocatalyst (NiP) in an aqueous ascorbic acid solution. Our approach is based on transient absorption spectroscopy studies of the efficiency of photo-reduction of RuP and NiP correlated with pH-dependent photocatalytic H₂ production and the degree of catalyst protonation. The influence of these factors results in an observed optimum photoactivity at pH 4.5 for the RuP–NiP system. The electron transfer from photo-reduced RuP to NiP is efficient and independent of the pH value of the medium. At pH <4.5, the efficiency of the system is limited by the yield of RuP photo-reduction by the sacrificial electron donor, ascorbic acid. At pH >4.5, the efficiency of the system is limited by the poor protonation of NiP, which inhibits its ability to reduce protons to hydrogen. We have therefore developed a rational strategy utilising transient absorption spectroscopy combined with bulk pH titration, electrocatalytic and photocatalytic experiments to disentangle the complex pH-dependent activity of the homogenous RuP–NiP photocatalytic system, which can be widely applied to other photocatalytic systems.     

Tuning of redox potentials for the design of ruthenium anticancer drugs -- an electrochemical study of [trans-RuCl(4)L(DMSO)](-) and [trans-RuCl(4)L(2)](-) complexes, where L = imidazole, 1,2,4-triazole, indazole

The electrochemical behavior of [trans-RuCl(4)L(DMSO)](-) (A) and [trans-RuCl(4)L(2)](-) (B) [L = imidazole (Him), 1,2,4-triazole (Htrz), and indazole (Hind)] complexes has been studied in DMF, DMSO, and aqueous media by cyclic voltammetry and controlled potential electrolysis. They exhibit one single-electron Ru(III)/Ru(II) reduction involving, at a sufficiently long time scale, metal dechlorination on solvolysis, as well as, in organic media, one single-electron reversible Ru(III)/Ru(IV) oxidation. The redox potential values are interpreted on the basis of the Lever's parametrization method, and particular forms of this linear expression (that relates the redox potential with the ligand E(L) parameter) are proposed, for the first time, for negatively (1-) charged complexes with the Ru(III/II) redox couple center in aqueous phosphate buffer (pH 7) medium and for complexes with the Ru(III/IV) couple in organic media. The E(L) parameter was estimated for indazole showing that this ligand behaves as a weaker net electron donor than imidazole or triazole. The kinetics of the reductively induced stepwise replacement of chloride by DMF were studied by digital simulation of the cyclic voltammograms, and the obtained rate constants were shown to increase with the net electron donor character (decrease of E(L)) of the neutral ligands (DMSO < indazole < triazole < imidazole) and with the basicity of the ligated azole, factors that destabilize the Ru(II) relative to the Ru(III) form of the complexes. The synthesis and characterization of some novel complexes of the A and B series are also reported, including the X-ray structural analyses of (Ph(3)PCH(2)Ph)[trans-RuCl(4)(Htrz)(DMSO)], [(Ph(3)P)(2)N][trans-RuCl(4)(Htrz)(DMSO)], (H(2)ind)[trans-RuCl(4)(Hind)(DMSO)], and [(Hind)(2)H][trans-RuCl(4)(Hind)(2)].     

ZnSe Nanorods as Visible‐Light Absorbers for Photocatalytic and Photoelectrochemical H2 Evolution in Water

A precious‐metal‐ and Cd‐free photocatalyst system for efficient H₂ evolution from aqueous protons with a performance comparable to Cd‐based quantum dots is presented. Rod‐shaped ZnSe nanocrystals (nanorods, NRs) with a Ni(BF₄)₂ co‐catalyst suspended in aqueous ascorbic acid evolve H₂ with an activity up to 54±2 mmol g_ZnSe^?1 h^?1 and a quantum yield of 50±4 % (λ=400 nm) under visible light illumination (AM 1.5G, 100 mW cm^?2, λ>400 nm). Under simulated full‐spectrum solar irradiation (AM 1.5G, 100 mW cm^?2), up to 149±22 mmol g_ZnSe^?1 h^?1 is generated. Significant photocorrosion was not noticeable within 40 h and activity was even observed without an added co‐catalyst. The ZnSe NRs can also be used to construct an inexpensive delafossite CuCrO₂ photocathode, which does not rely on a sacrificial electron donor. Immobilized ZnSe NRs on CuCrO₂ generate photocurrents of around ?10 μA cm^?2 in an aqueous electrolyte solution (pH 5.5) with a photocurrent onset potential of approximately +0.75 V vs. RHE. This work establishes ZnSe as a state‐of‐the‐art light absorber for photocatalytic and photoelectrochemical H₂ generation.     

Shorter Alkyl Chains Enhance Molecular Diffusion and Electron Transfer Kinetics between Photosensitisers and Catalysts in CO2-Reducing Photocatalytic Liposomes

Covalent functionalisation with alkyl tails is a common method for supporting molecular catalysts and photosensitisers onto lipid bilayers, but the influence of the alkyl chain length on the photocatalytic performances of the resulting liposomes is not well understood. In this work, we first prepared a series of rhenium-based CO₂-reduction catalysts [Re(4,4’-(C_nH_2n+1)₂-bpy)(CO)₃Cl] ( ReC_n ; 4,4’-(C_nH_2n+1)₂-bpy=4,4’-dialkyl-2,2’-bipyridine) and ruthenium-based photosensitisers [Ru(bpy)₂(4,4’-(C_nH_2n+1)₂-bpy)](PF₆)₂ ( RuC_n ) with different alkyl chain lengths (n=0, 9, 12, 15, 17, and 19). We then prepared a series of PEGylated DPPC liposomes containing RuC_n and ReC_n , hereafter noted C_n , to perform photocatalytic CO₂ reduction in the presence of sodium ascorbate. The photocatalytic performance of the C_n liposomes was found to depend on the alkyl tail length, as the turnover number for CO (TON) was inversely correlated to the alkyl chain length, with a more than fivefold higher CO production (TON=14.5) for the C₉ liposomes, compared to C₁₉ (TON=2.8). Based on immobilisation efficiency quantification, diffusion kinetics, and time-resolved spectroscopy, we identified the main reason for this trend: two types of membrane-bound RuC_n species can be found in the membrane, either deeply buried in the bilayer and diffusing slowly, or less buried with much faster diffusion kinetics. Our data suggest that the higher photocatalytic performance of the C₉ system is due to the higher fraction of the more mobile and less buried molecular species, which leads to enhanced electron transfer kinetics between RuC₉ and ReC₉ .     

Denaturation mapping of Saccharomyces cerevisiae

Optical mapping of DNA provides large-scale genomic information that can be used to assemble contigs from next-generation sequencing, and to detect rearrangements between single cells. A recent optical mapping technique called denaturation mapping has the advantage of using physical principles rather than the action of enzymes to probe genomic structure. Denaturation mapping uses fluorescence microscopy to image the pattern of partial melting along a DNA molecule extended in a channel of cross-section 120 nm at the heart of a nanofluidic device. We used denaturation mapping to locate single DNA molecules on the yeast genome (12.1 Mbp) by comparing images to a computationally predicted map for the entire genome sequence. By locating 84 molecules we assembled an optical map of the yeast genome with > 50% coverage.     

Mechanical Properties of Dense Zeolitic Imidazolate Frameworks (ZIFs): A High‐Pressure X‐ray Diffraction,Nanoindentation and Computational Study of the Zinc Framework Zn(Im)2, and its Lithium?Boron Analogue,LiB(Im)4

The dense, anhydrous zeolitic imidazolate frameworks (ZIFs), Zn(Im)₂ ( 1 ) and LiB(Im)₄ ( 2 ), adopt the same zni topology and differ only in terms of the inorganic species present in their structures. Their mechanical properties (specifically the Young’s and bulk moduli, along with the hardness) have been elucidated by using high pressure, synchrotron X‐ray diffraction, density functional calculations and nanoindentation studies. Under hydrostatic pressure, framework 2 undergoes a phase transition at 1.69 GPa, which is somewhat higher than the transition previously reported in 1 . The Young’s modulus (E) and hardness (H) of 1 (E≈8.5, H≈1 GPa) is substantially higher than that of 2 (E≈3, H≈0.1 GPa), whilst its bulk modulus is relatively lower (≈14 GPa cf. ≈16.6 GPa). The heavier, zinc‐containing material was also found to be significantly harder than its light analogue. The differential behaviour of the two materials is discussed in terms of the smaller pore volume of 2 and the greater flexibility of the LiN₄ tetrathedron compared with the ZnN₄ and BN₄ units.     

Vibration-rotation and deperturbation analysis of A2Π-X2Σ+ and B2Σ+-X2Σ+ systems of the CaI molecule

Doppler-limited laser excitation spectroscopy employing narrow-band fluorescence detection was used to obtain a rotational and vibrational analysis in the (0, 0) and (1, 1) bands of the A²Π-X²Σ⁺ system and the (4, 2) (3, 1), (0, 0), (0, 1), (1, 2), (2, 3), and (3, 4) bands of the B²Σ⁺-X²Σ⁺ system of CaI. The A and B states are deperturbed to obtain spectroscopic constants and Franck-Condon factors. Deperturbation was necessary because of the small separation of the A and B states relative to the A ～ B interaction strength and the A²Π spin-orbit splitting. The main deperturbed constants (in cm^?1) are

     

78.

Linear time approximation of 3D convex polytopes     

Mario A. Lopez  Shlomo Reisner   《Computational Geometry》2002,23(3):291-301

We develop algorithms for the approximation of a convex polytope in by polytopes that are either contained in it or containing it, and that have fewer vertices or facets, respectively. The approximating polytopes achieve the best possible general order of precision in the sense of volume-difference. The running time is linear in the number of vertices or facets.     

79.

Inside Back Cover: Selective Reduction of Aqueous Protons to Hydrogen with a Synthetic Cobaloxime Catalyst in the Presence of Atmospheric Oxygen (Angew. Chem. Int. Ed. 37/2012)     

Dr. Nicoleta M. Muresan  Dr. Erwin Reisner 《Angewandte Chemie (International ed. in English)》2012,51(37):9453-9453

     

80.

Improving the Photocatalytic Reduction of CO2 to CO through Immobilisation of a Molecular Re Catalyst on TiO2          下载免费PDF全文

Dr. Christopher D. Windle  Ernest Pastor  Dr. Anna Reynal  Dr. Adrian C. Whitwood  Dr. Yana Vaynzof  Prof. James R. Durrant  Prof. Robin N. Perutz  Dr. Erwin Reisner 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(9):3746-3754

The photocatalytic activity of phosphonated Re complexes, [Re(2,2′‐bipyridine‐4,4′‐bisphosphonic acid) (CO)₃(L)] (ReP; L=3‐picoline or bromide) immobilised on TiO₂ nanoparticles is reported. The heterogenised Re catalyst on the semiconductor, ReP–TiO₂ hybrid, displays an improvement in CO₂ reduction photocatalysis. A high turnover number (TON) of 48 mol_CO mol_Re^?1 is observed in DMF with the electron donor triethanolamine at λ>420 nm. ReP–TiO₂ compares favourably to previously reported homogeneous systems and is the highest TON reported to date for a CO₂‐reducing Re photocatalyst under visible light irradiation. Photocatalytic CO₂ reduction is even observed with ReP–TiO₂ at wavelengths of λ>495 nm. Infrared and X‐ray photoelectron spectroscopies confirm that an intact ReP catalyst is present on the TiO₂ surface before and during catalysis. Transient absorption spectroscopy suggests that the high activity upon heterogenisation is due to an increase in the lifetime of the immobilised anionic Re intermediate (t_{50 %}>1 s for ReP–TiO₂ compared with t_{50 %}=60 ms for ReP in solution) and immobilisation might also reduce the formation of inactive Re dimers. This study demonstrates that the activity of a homogeneous photocatalyst can be improved through immobilisation on a metal oxide surface by favourably modifying its photochemical kinetics.     

[首页] « 上一页 [3] [4] [5] [6] [7] 8 [9] [10] [11] [12] [13] 下一页 » 末  页»

	$\text{X}{}^2\text{Σ}{}^{\mathrm{+}}$	$\text{A}{}^2\text{Π}$	$\text{B}{}^2\text{Σ}{}^{\mathrm{+}}$
$\text{T}{}_{\mathrm{e}}$	0	15 624.67(5)	15 700.52(12)
$\text{ω}{}_{\mathrm{e}}$	238.7496(33)	241.19(7)	242.63(17)
$\text{ω}{}_{\mathrm{e}}\text{χ}{}_{\mathrm{e}}$	0.62789(64)	0.53(5) (Pekeris)	1.17(12) (Pekeris)
$\text{B}{}_{\mathrm{e}}$	0.0693254(84)	0.070460(14)	0.071572(22)
$\text{α}{}_{\mathrm{e}}\text{× 10}{}^4$	2.640(35)	2.15(10)	3.95(2)
$\text{A}{}_{\mathrm{e}}$	—	45.8968(52)	—
$\text{R}{}_{\mathrm{e}}\text{(}\text{A}\text{?}\text{)}$	2.8286(2)	2.8057(3)	2.7839(4)

Linear time approximation of 3D convex polytopes

We develop algorithms for the approximation of a convex polytope in by polytopes that are either contained in it or containing it, and that have fewer vertices or facets, respectively. The approximating polytopes achieve the best possible general order of precision in the sense of volume-difference. The running time is linear in the number of vertices or facets.     

Improving the Photocatalytic Reduction of CO2 to CO through Immobilisation of a Molecular Re Catalyst on TiO2

The photocatalytic activity of phosphonated Re complexes, [Re(2,2′‐bipyridine‐4,4′‐bisphosphonic acid) (CO)₃(L)] (ReP; L=3‐picoline or bromide) immobilised on TiO₂ nanoparticles is reported. The heterogenised Re catalyst on the semiconductor, ReP–TiO₂ hybrid, displays an improvement in CO₂ reduction photocatalysis. A high turnover number (TON) of 48 mol_CO mol_Re^?1 is observed in DMF with the electron donor triethanolamine at λ>420 nm. ReP–TiO₂ compares favourably to previously reported homogeneous systems and is the highest TON reported to date for a CO₂‐reducing Re photocatalyst under visible light irradiation. Photocatalytic CO₂ reduction is even observed with ReP–TiO₂ at wavelengths of λ>495 nm. Infrared and X‐ray photoelectron spectroscopies confirm that an intact ReP catalyst is present on the TiO₂ surface before and during catalysis. Transient absorption spectroscopy suggests that the high activity upon heterogenisation is due to an increase in the lifetime of the immobilised anionic Re intermediate (t_{50 %}>1 s for ReP–TiO₂ compared with t_{50 %}=60 ms for ReP in solution) and immobilisation might also reduce the formation of inactive Re dimers. This study demonstrates that the activity of a homogeneous photocatalyst can be improved through immobilisation on a metal oxide surface by favourably modifying its photochemical kinetics.