Pingwu Du

Catalysts made of earth-abundant elements (Co, Ni, Fe) for water splitting: Recent progress and future challenges

This article reviews recent significant advances in the field of water splitting. Catalysts play very important roles in two half reactions of water splitting - water reduction and water oxidation. Considering potential future applications, catalysts made of cheap and earth abundant element(s) are especially important for economically viable energy conversion. This article focuses only on catalysts made of cobalt (Co), nickel (Ni) and iron (Fe) elements for water reduction and water oxidation. Different series of catalysts that can be applied in electrocatalytic and photocatalytic water spitting are discussed in detail and their catalytic mechanisms are introduced. Finally, the future outlook and perspective of catalysts made of earth abundant elements will be discussed.

Making Hydrogen from Water Using a Homogeneous System Without Noble Metals

A photocatalytic noble metal-free system for the generation of hydrogen has been constructed using Eosin Y (1) as a photosensitizer, the complex [Co(dmgH)(2)pyCl](2+) (5, dmgH = dimethylglyoximate, py = pyridine) as a molecular catalyst, and triethanolamine (TEOA) as a sacrificial reducing agent. The system produces H(2) with an initial rate of approximately 100 turnovers per hour upon irradiation with visible light (lambda > 450 nm). Addition of free dmgH(2) greatly increases the durability of the system addition of 12 equiv of dmgH(2) (vs cobalt) to the system produces approximately 900 turnovers of H(2) after 14 h of irradiation. The rate of H(2) evolution is maximum at pH = 7 and decreases sharply at more acidic or basic pH. Spectroscopic study of photolysis solutions suggests that hydrogen production occurs through protonation of a Co(I) species to give a Co(III) hydride, which then reacts further by reduction and protolysis to give Co(II) and molecular hydrogen.

A Homogeneous System for the Photogeneration of Hydrogen from Water Based on a Platinum(II) Terpyridyl Acetylide Chromophore and a Molecular Cobalt Catalyst

The complex [Co(dmgH)2pyCl]2+ (1, dmgH = dimethylglyoximate, py = pridine) has been used as a molecular catalyst for visible light driven hydrogen production in the presence of [Pt(tolylterpyridine)(phenylacetylide)]+ (3) as a photosensitizer and triethanolamine (TEOA) as a sacrificial reducing agent. Complex 3 is quenched oxidatively by [Co(dmgH)pyCl]2+ (1) with a rate constant kq of 1.27 x 10(9) M(-1) s(-1). Photogeneration of H2 is only seen when 1 + 3 + TEOA are all present. H2 production is maximized for this system at pH 8.5 and declines to very low levels at pH < 7 and pH > 12. Irradiation of the reaction solution initially containing 1.61 x 10(-2) M TEOA, 1.11 x 10(-5) M of 3, and 1.99 x 10(-4) M of Co catalyst 1 in MeCN/water (3:2 v/v) at pH = 8.5 for 10 h with lambda > 410 nm yields 400 turnovers of H2. When TEOA is 0.27 M, approximately 1000 turnovers are obtained after 10 h of irradiation. Spectroscopic study of the photolyses solutions suggests that H2 formation proceeds via Co(I) and protonation to form Co(III) hydride species.

Extraordinarily efficient photocatalytic hydrogen evolution in water using semiconductor nanorods integrated with crystalline Ni2P cocatalysts

Photocatalytic hydrogen evolution via water splitting is an attractive scientific and technological goal to address the increasing global demand for clean energy and to reduce the climate change impact of CO2 emission. Although tremendous efforts have been made, hydrogen production by a robust and highly efficient system driven by visible light still remains a significant challenge. Herein we report that nickel phosphide, as a cocatalyst to form a well-designed integrated photocatalyst with one-dimensional semiconductor nanorods, highly improves the efficiency and durability for photogeneration of hydrogen in water. The highest rate for hydrogen production reached ∼1200 μmol h−1 mg−1 based on the photocatalyst. The turnover number (TON) reached ∼3 270 000 in 90 hours with a turnover frequency (TOF) of 36 400 for Ni2P, and the apparent quantum yield was ∼41% at 450 nm. The photoinduced charge transfer process was further confirmed by steady-state photoluminescence spectra and time-resolved photoluminescence spectra. Such extraordinary performance of a noble-metal-free artificial photosynthetic hydrogen production system has, to our knowledge, not been reported to date.

Photodriven Charge Separation Dynamics in CdSe/ZnS Core/Shell Quantum Dot/Cobaloxime Hybrid for Efficient Hydrogen Production

Photodriven charge-transfer dynamics and catalytic properties have been investigated for a hybrid system containing CdSe/ZnS core/shell quantum dots (QDs) and surface-bound molecular cobaloxime catalysts. The electron transfer from light-excited QDs to cobaloxime, revealed by optical transient absorption spectroscopy, takes place with an average time constant of 105 ps, followed a much slower charge recombination process with a time constant of ≫3 ns. More interestingly, we also observed photocatalytic hydrogen generation by this QD/cobaloxime hybrid system, with >10,000 turnovers of H(2) per QD in 10 h, using triethanolamine as a sacrificial electron donor. These results suggest that QD/cobaloxime hybrids succeed in coupling single-photon events with multielectron redox catalytic reactions, and such systems could have potential applications in long-lived artificial photosynthetic devices for fuel generation from sunlight.

Catalytic water oxidation at single metal sites

Nature utilizes solar energy to extract electrons and release protons from water, a process called photosynthetic water oxidation or oxygen evolution. This sunlight-driven reaction is vital to the planet because it directly produces dioxygen and couples with photosystem I to generate the reducing equivalents for the reduction of carbon dioxide to carbohydrates (also known as CO2 fixation). Inspired by this natural process, people are intensely interested in water splitting using sunlight to convert and store solar energy into chemical energy, which is believed to be able to ultimately solve the energy problem that we are facing. Water splitting can be separated into two half reactions, namely water oxidation and water reduction, and they can be studied individually. Catalysts are very helpful in both reactions. Recent progress in finding new highly efficient water oxidation catalysts (WOCs) has shed light on this complicated four-electron/four-proton reaction and made it possible to catalyze water oxidation using mononuclear metal complexes. This article focuses on molecular catalysts that are able to perform catalytic water oxidation at single metal sites. Different series of catalysts (or precatalysts) made of ruthenium, iridium and earth abundant elements (iron, cobalt, and manganese) that can be applied in chemical, electrochemical and photochemical (light-driven) water oxidation are summarized, and their catalytic mechanisms are discussed in detail. Finally, the future outlook and perspective to design and develop catalysts that are efficient, cheap and stable are presented.

A Highly Selective Turn-On Colorimetric, Red Fluorescent Sensor for Detecting Mobile Zinc in Living Cells

We describe ZRL1, a turn-on colorimetric and red fluorescent zinc ion sensor. The Zn(2+)-promoted ring opening of the rhodamine spirolactam ring in ZRL1 evokes a 220-fold fluorescence turn-on response. In aqueous media, ZRL1 turn-on luminescence is highly selective for Zn(2+) ions, with no significant response to other competitive cations, including Na(+), K(+), Ca(2+), Mg(2+), Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), Cd(2+), or Hg(2+). In addition to these characteristics, preliminary results indicate that ZRL1 can be delivered to living cells and can be used to monitor changes in intracellular Zn(2+) levels.

Cobalt complexes as artificial hydrogenases for the reductive side of water splitting

The generation of H2 from protons and electrons by complexes of cobalt has an extensive history. During the past decade, interest in this subject has increased as a result of developments in hydrogen generation that are driven electrochemically or photochemically. This article reviews the subject of hydrogen generation using Co complexes as catalysts and discusses the mechanistic implications of the systems studied for making H2. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.

Black Phosphorus Revisited: A Missing Metal-Free Elemental Photocatalyst for Visible Light Hydrogen Evolution.

Metal-free elemental photocatalysts for hydrogen (H2 ) evolution are more advantageous than the traditional metal-based inorganic photocatalysts since the nonmetal elements are generally cheaper, more earth-abundant, and environmentally friendly. Black phosphorus (BP) has been attracting increasing attention in recent years based on its anisotropic 2D layered structure with tunable bandgap in the range of 0.3-2.0 eV; however, the application of BP for photocatalytic H2 evolution has been scarcely reported experimentally although being theoretically predicted. Herein, for the first time, the visible light photocatalytic H2 evolution of BP nanosheets prepared via a facile solid-state mechanochemical method by ball-milling bulk BP is reported. Without using any noble metal cocatalyst, the visible light photocatalytic hydrogen evolution rate of BP nanosheets reaches 512 µmol h-1 g-1 , which is ≈18 times higher than that of the bulk BP, and is comparable or even higher than that of graphitic carbon nitrides (g-C3 N4 ).

Energy upconversion sensitized by a platinum(II) terpyridyl acetylide complex

The platinum(II) terpyridine acetylide complex, [Pt(ttpy)(CCPh)]ClO4 (1, where ttpy = 4′-p-tolylterpyridine), sensitizes the 3π–π* excited state of 9,10-diphenylanthracene (DPA) that in turn produces upconverted fluorescence via triplet–triplet annihilation (TTA). The photoluminescence of 1 is readily quenched by DPA with a rate constant close to the diffusion limit via energy transfer. Selective excitation of 1 leads to the upconverted fluorescence with a near quadratic dependence of the DPA fluorescence intensity on incident light power. This is the first time that the 3MLCT charge transfer excited state of a platinum polypyridine complex has been used to promote photon upconversion.