Holli

Abundant and toxic hydrogen sulfide (H₂S) from industry and nature has been traditionally considered a liability. However, it represents a potential resource if valuable H₂ and elemental sulfur can be simultaneously extracted through a H₂S splitting reaction. Herein a photochemical-chemical loop linked by redox couples such as Fe²⁺/Fe³⁺ and I⁻/I₃⁻ for photoelectrochemical H₂ production and H₂S chemical absorption redox reactions are reported. Using functionalized Si as photoelectrodes, H₂S was successfully split into elemental sulfur and H₂ with high stability and selectivity under simulated solar light. This new conceptual design will not only provide a possible route for using solar energy to convert H₂S into valuable resources, but also sheds light on some challenging photochemical reactions such as CH₄ activation and CO₂ reduction.

Trash to Treasure: A photochemical–chemical loop for the overall H₂S splitting was developed by integrating photoelectrochemical H₂ production and H₂S chemical absorption redox reactions with the link of a redox couple (Fe²⁺/Fe³⁺ or I⁻/I₃⁻). Using functionalized silicon as photoelectrode, simultaneous extraction of H₂ and elemental sulfur from H₂S was successfully achieved (see picture).

In order to solve the problems of global warming and shortage of fossil fuels, researchers have been endeavoring to achieve artificial photosynthesis: splitting water into H₂ and O₂ under solar light illumination. Our group has recently invented a unique system that drives photoinduced water reduction through “Z-scheme” photosynthetic pathways. Nevertheless, that system still suffered from a low turnover number (TON) of the photocatalytic cycle (TON=4.1). We have now found and describe herein a new methodology to make significant improvements in the TON, up to around TON=14–27. For the new model systems reported herein, the quantum efficiency of the second photoinduced step in the Z-scheme photosynthesis is dramatically improved by introducing multiviologen tethers to temporarily collect the high-energy electron generated in the first photoinduced step. These are unique examples of “pigment–acceptor–catalyst triads”, which demonstrate a new effective type of artificial photosynthesis.

Electron harvesting: Photo-hydrogen-evolving molecular devices showing substantially improved turnover numbers have been developed by introducing multiviologen tethers into a [PtCl₂(2,2′-bipyridine)]-based moiety serving as a light-harvesting and H₂-evolving center (see scheme). The improved photocatalytic performance is attributed to the rapidly regenerating character of the pigment due to intramolecular electron transfer from the pigment to the electron reservoirs.

La_0.3(Ba_0.5Sr_0.5)_0.7Co_0.8Fe_0.2O_3−δ is a promising bifunctional perovskite catalyst for the oxygen reduction reaction and the oxygen evolution reaction. This catalyst has circa 10 nm-scale rhombohedral LaCoO₃ cobaltite particles distributed on the surface. The dynamic microstructure phenomena are attributed to the charge imbalance from the replacement of A-site cations with La³⁺ and local stress on Co-site sub-lattice with the cubic perovskite structure.

The perovskite La_0.3(Ba_0.5Sr_0.5)_0.7Co_0.8Fe_0.2O_3−δ (La_0.3-5582) is introduced as a bifunctional catalyst to compete with precious-metal-based catalysts. The newly introduced perovskite bears rhombohedral phase LaCoO_3−δ particles on the surface of the grains.

Environmentally friendly iron(II) catalysts for atom-transfer radical polymerization (ATRP) were synthesized by careful selection of the nitrogen substituents of N,N,N-trialkylated-1,4,9-triazacyclononane (R₃TACN) ligands. Two types of structures were confirmed by crystallography: “[(R₃TACN)FeX₂]” complexes with relatively small R groups have ionic and dinuclear structures including a [(R₃TACN)Fe(μ-X)₃Fe(R₃TACN)]⁺ moiety, whereas those with more bulky R groups are neutral and mononuclear. The twelve [(R₃TACN)FeX₂]_n complexes that were synthesized were subjected to bulk ATRP of styrene, methyl methacrylate (MMA), and butyl acrylate (BA). Among the iron complexes examined, [{(cyclopentyl)₃TACN}FeBr₂] (4 b) was the best catalyst for the well-controlled ATRP of all three monomers. This species allowed easy catalyst separation and recycling, a lowering of the catalyst concentration needed for the reaction, and the absence of additional reducing reagents. The lowest catalyst loading was accomplished in the ATRP of MMA with 4 b (59 ppm of Fe based on the charged monomer). Catalyst recycling in ATRP with low catalyst loadings was also successful. The ATRP of styrene with 4 b (117 ppm Fe atom) was followed by precipitation from methanol to give polystyrene that contained residual iron below the calculated detection limit (0.28 ppm). Mechanisms that involve equilibria between the multinuclear and mononuclear species were also examined.

It's easy being green: Structurally well-defined [(R₃TACN)FeX₂] complexes realized green atom-transfer radical polymerization by judicious choice of the R group on the N,N,N-trialkylated-1,4,9-triazacyclononane (R₃TACN) ligands (see scheme). [{(Cyclopentyl)₃TACN}FeBr₂] was the best catalyst for controlled polymerization of all three monomers.