Water-soluble mononuclear cobalt complexes with organic ligands acting as precatalysts for efficient photocatalytic water oxidation

Hong, Dachao,Jung, Jieun,Park, Jiyun,Yamada, Yusuke,Suenobu, Tomoyoshi,Lee, Yong-Min,Nam, Wonwoo,Fukuzumi, Shunichi The Royal Society of Chemistry 2012 Energy & environmental science Vol.5 No.6

<P>The photocatalytic water oxidation to evolve O<SUB>2</SUB> was performed by photoirradiation (<I>λ</I> > 420 nm) of an aqueous solution containing [Ru(bpy)<SUB>3</SUB>]<SUP>2+</SUP> (bpy = 2,2′-bipyridine), Na<SUB>2</SUB>S<SUB>2</SUB>O<SUB>8</SUB> and water-soluble cobalt complexes with various organic ligands as precatalysts in the pH range of 6.0–10. The turnover numbers (TONs) based on the amount of Co for the photocatalytic O<SUB>2</SUB> evolution with [Co<SUP>II</SUP>(Me<SUB>6</SUB>tren)(OH<SUB>2</SUB>)]<SUP>2+</SUP> (<B>1</B>) and [Co<SUP>III</SUP>(Cp<SUP>*</SUP>)(bpy)(OH<SUB>2</SUB>)]<SUP>2+</SUP> (<B>2</B>) [Me<SUB>6</SUB>tren = tris(<I>N</I>,<I>N</I>′-dimethylaminoethyl)amine, Cp<SUP>*</SUP> = <I>η</I><SUP>5</SUP>-pentamethylcyclopentadienyl] at pH 9.0 reached 420 and 320, respectively. The evolved O<SUB>2</SUB> yield increased in proportion to concentrations of precatalysts <B>1</B> and <B>2</B> up to 0.10 mM. However, the O<SUB>2</SUB> yield dramatically decreased when the concentration of precatalysts <B>1</B> and <B>2</B> exceeded 0.10 mM. When the concentration of Na<SUB>2</SUB>S<SUB>2</SUB>O<SUB>8</SUB> was increased from 10 mM to 50 mM, CO<SUB>2</SUB> evolution was observed during the photocatalytic water oxidation. These results indicate that a part of the organic ligands of <B>1</B> and <B>2</B> were oxidized to evolve CO<SUB>2</SUB> during the photocatalytic reaction. The degradation of complex <B>2</B> under photocatalytic conditions and the oxidation of Me<SUB>6</SUB>tren ligand of <B>1</B> by [Ru(bpy)<SUB>3</SUB>]<SUP>3+</SUP> were confirmed by <SUP>1</SUP>H NMR measurements. Dynamic light scattering (DLS) experiments indicate the formation of particles with diameters of around 20 ± 10 nm and 200 ± 100 nm during the photocatalytic water oxidation with <B>1</B> and <B>2</B>, respectively. The particle sizes determined by DLS agreed with those of the secondary particles observed by TEM. The XPS measurements of the formed particles suggest that the surface of the particles is covered with cobalt hydroxides, which could be converted to active species containing high-valent cobalt ions during the photocatalytic water oxidation. The recovered nanoparticles produced from <B>1</B> act as a robust catalyst for the photocatalytic water oxidation.</P> <P>Graphic Abstract</P><P>Photocatalytic water oxidation was efficiently catalysed by Co(OH)<SUB><I>x</I></SUB> nanoparticles derived from mononuclear cobalt complexes with organic ligands during the reaction. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2ee21185h'> </P>

Efficient water oxidation by cerium ammonium nitrate with [Ir^III(Cp*)(4,4′-bishydroxy-2,2′-bipyridine)(H₂O)]²⁺as a precatalyst

Hong, Dachao,Murakami, Masato,Yamada, Yusuke,Fukuzumi, Shunichi The Royal Society of Chemistry 2012 ENERGY AND ENVIRONMENTAL SCIENCE Vol.5 No.2

<P>Water oxidation by cerium(<SMALL>IV</SMALL>) ammonium nitrate, CAN, with [Ir<SUP>III</SUP>(Cp*)(4,4′-R<SUB>2</SUB>-2,2′-bipyridine)(H<SUB>2</SUB>O)]<SUP>2+</SUP> (R = OH, OMe, Me or COOH) to evolve oxygen has been investigated together with the possible oxidation of the ligands by CAN. The apparent catalytic reactivity is highly dependent on the substituent R and the highest catalytic reactivity was obtained when R = OH. The apparent turnover frequency (TOF) of the catalytic water oxidation by CAN with [Ir<SUP>III</SUP>(Cp*){4,4′-(OH)<SUB>2</SUB>-2,2′-bipyridine}(H<SUB>2</SUB>O)]<SUP>2+</SUP>, which acts as a precatalyst, gradually increased during the reaction to reach the highest value among the Ir complexes. In the second run, the apparent TOF value was the highest from the beginning of the reaction. <SUP>1</SUP>H NMR and dynamic light scattering measurements for solutions after the first run indicated formation of insoluble nanoparticles, which exhibited a much higher catalytic reactivity as compared with iridium oxide prepared by a conventional method. The 4,4′-R<SUB>2</SUB>-2,2′-bipyridine ligand was also efficiently oxidized by CAN up to CO<SUB>2</SUB> only when R = OH. TG/DTA and XPS measurements of nanoparticles produced after the water oxidation suggested that the nanoparticles were composed of iridium hydroxide with a small amount of carbonaceous residue. Thus, iridium hydroxide nanoparticles act as an excellent catalyst for the water oxidation by CAN.</P> <P>Graphic Abstract</P><P>Water oxidation by Ce<SUP>IV</SUP> was efficiently catalyzed by Ir species derived from an iridium complex with 4,4′-dihydroxy-2,2′-bipyridine ligand during the reaction. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2ee02964b'> </P>

Water Oxidation Catalysis with Nonheme Iron Complexes under Acidic and Basic Conditions: Homogeneous or Heterogeneous?

Hong, Dachao,Mandal, Sukanta,Yamada, Yusuke,Lee, Yong-Min,Nam, Wonwoo,Llobet, Antoni,Fukuzumi, Shunichi American Chemical Society 2013 Inorganic chemistry Vol.52 No.16

<P>Thermal water oxidation by cerium(IV) ammonium nitrate (CAN) was catalyzed by nonheme iron complexes, such as Fe(BQEN)(OTf)<SUB>2</SUB> (<B>1</B>) and Fe(BQCN)(OTf)<SUB>2</SUB> (<B>2</B>) (BQEN = <I>N</I>,<I>N</I>′-dimethyl-<I>N</I>,<I>N</I>′-bis(8-quinolyl)ethane-1,2-diamine, BQCN = <I>N</I>,<I>N</I>′-dimethyl-<I>N</I>,<I>N</I>′-bis(8-quinolyl)cyclohexanediamine, OTf = CF<SUB>3</SUB>SO<SUB>3</SUB><SUP>–</SUP>) in a nonbuffered aqueous solution; turnover numbers of 80 ± 10 and 20 ± 5 were obtained in the O<SUB>2</SUB> evolution reaction by <B>1</B> and <B>2</B>, respectively. The ligand dissociation of the iron complexes was observed under acidic conditions, and the dissociated ligands were oxidized by CAN to yield CO<SUB>2</SUB>. We also observed that <B>1</B> was converted to an iron(IV)-oxo complex during the water oxidation in competition with the ligand oxidation. In addition, oxygen exchange between the iron(IV)-oxo complex and H<SUB>2</SUB><SUP>18</SUP>O was found to occur at a much faster rate than the oxygen evolution. These results indicate that the iron complexes act as the true homogeneous catalyst for water oxidation by CAN at low pHs. In contrast, light-driven water oxidation using [Ru(bpy)<SUB>3</SUB>]<SUP>2+</SUP> (bpy = 2,2′-bipyridine) as a photosensitizer and S<SUB>2</SUB>O<SUB>8</SUB><SUP>2–</SUP> as a sacrificial electron acceptor was catalyzed by iron hydroxide nanoparticles derived from the iron complexes under basic conditions as the result of the ligand dissociation. In a buffer solution (initial pH 9.0) formation of the iron hydroxide nanoparticles with a size of around 100 nm at the end of the reaction was monitored by dynamic light scattering (DLS) in situ and characterized by X-ray photoelectron spectra (XPS) and transmission electron microscope (TEM) measurements. We thus conclude that the water oxidation by CAN was catalyzed by short-lived homogeneous iron complexes under acidic conditions, whereas iron hydroxide nanoparticles derived from iron complexes act as a heterogeneous catalyst in the light-driven water oxidation reaction under basic conditions.</P><P>Water oxidation by Ce<SUP>IV</SUP> was catalyzed by nonheme iron complexes under acidic conditions. The ligand of the iron complexes dissociated and oxidized to yield CO<SUB>2</SUB>, resulting in limited reactivity. In contrast to the homogeneous catalysis under acidic conditions, light-driven water oxidation was catalyzed by iron hydroxide nanoparticles derived from the iron complexes as the result of the ligand dissociation under basic conditions.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/inocaj/2013/inocaj.2013.52.issue-16/ic401180r/production/images/medium/ic-2013-01180r_0014.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ic401180r'>ACS Electronic Supporting Info</A></P>

Catalysis of Nickel Ferrite for Photocatalytic Water Oxidation Using [Ru(bpy)₃]²⁺ and S₂O₈^2-

Hong, Dachao,Yamada, Yusuke,Nagatomi, Takaharu,Takai, Yoshizo,Fukuzumi, Shunichi American Chemical Society 2012 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.134 No.48

<P>Single or mixed oxides of iron and nickel have been examined as catalysts in photocatalytic water oxidation using [Ru(bpy)(3)](2+) as a photosensitizer and S(2)O(8)(2-) as a sacrificial oxidant. The catalytic activity of nickel ferrite (NiFe(2)O(4)) is comparable to that of a catalyst containing Ir, Ru, or Co in terms of O(2) yield and O(2) evolution rate under ambient reaction conditions. NiFe(2)O(4) also possesses robustness and ferromagnetic properties, which are beneficial for easy recovery from the solution after reaction. Water oxidation catalysis achieved by a composite of earth-abundant elements will contribute to a new approach to the design of catalysts for artificial photosynthesis.</P>

Graphene Oxide Nanosheet-Composited Poly(N-isopropylacrylamide) Hydrogel for Cell Sheet Recovery

Yongqing Xia,Han Wu,Dachao Tang,Shuai Gao,Binghe Chen,Zhujun Zeng,Shengjie Wang,Meiwen Cao,Dongxiang Li 한국고분자학회 2019 Macromolecular Research Vol.27 No.7

Cell sheet engineering technique has been applied to treat various tissues without the use of a traditional scaffold. To date, methods for the cell sheet harvesting depend mostly on grafted poly(N-isopropylacrylamide) (pNIPAAm) thin layers, while the native pNIPAAm hydrogel, which possibly presents the easiest way to prepare thermo-responsive materials, is not suitable for the cell sheet harvesting due to its low cell attachment. In this study, the graphene oxide (GO) nanosheet was utilized as an additive to enhance the bio-compatibility of the pNIPAAm hydrogel. Different concentrations of GO nanosheets were added to prepare GO/pNIPAAm composite hydrogels through the in-situ free radical polymerization with polyethylene glycol dimethacrylate (PEGDA) as a cross-linker. The results indicated that the physical properties of the composite hydrogels had little difference with that of the native pNIPAAm hydrogel. However, the cell attachment, proliferation and detachment behaviors on the composite hydrogel surface were greatly enhanced. Monkey fibroblast COS7 cells attached and proliferated better on the GO/pNIPAAm composite hydrogel, while intact COS7 cell sheets could be harvested from the composite hydrogels by simply lowering the temperature. In contrast, the cells appeared as clusters on the native pNIPAAm hydrogel. Furthermore, when HeLa and COS7 cells were seeded successively onto the micropatterned GO/pNIPAAm hydrogel, there could be the formation of a patterned HeLa/COS7 cell layer. The geometrically patterned GO/pNIPAAm hydrogel may provide an easy-to-prepare material for releasing patterned cell sheets compared to the specific cell-adhesive proteins reported to make patterned cell layers.

LaCoO₃ acting as an efficient and robust catalyst for photocatalytic water oxidation with persulfate

Yamada, Yusuke,Yano, Kentaro,Hong, Dachao,Fukuzumi, Shunichi The Royal Society of Chemistry 2012 Physical chemistry chemical physics Vol.14 No.16

<P>Cobalt-containing metal oxides [perovskites (LaCoO<SUB>3</SUB>, NdCoO<SUB>3</SUB>, YCoO<SUB>3</SUB>, La<SUB>0.7</SUB>Sr<SUB>0.3</SUB>CoO<SUB>3</SUB>), spinel (Co<SUB>3</SUB>O<SUB>4</SUB>) and wolframite (CoWO<SUB>4</SUB>)] have been examined as catalysts for photocatalytic water oxidation with Na<SUB>2</SUB>S<SUB>2</SUB>O<SUB>8</SUB> and [Ru(bpy)<SUB>3</SUB>]<SUP>2+</SUP> as an electron acceptor and a photosensitizer, respectively. Catalysts with the perovskite structure exhibited higher catalytic activity as compared with the catalysts with the spinel and wolframite structures. LaCoO<SUB>3</SUB>, which stabilizes Co(<SMALL>III</SMALL>) species in the perovskite structure, exhibited the highest catalytic activity in the photocatalytic water oxidation compared with CoWO<SUB>4</SUB>, Co<SUB>3</SUB>O<SUB>4</SUB> and La<SUB>0.7</SUB>Sr<SUB>0.3</SUB>CoO<SUB>3</SUB> which contain Co(<SMALL>II</SMALL>) or Co(<SMALL>IV</SMALL>) species in the matrices. The high catalytic reactivity of LaCoO<SUB>3</SUB> possessing perovskite structure was maintained in NdCoO<SUB>3</SUB> and YCoO<SUB>3</SUB> which exclusively contain Co(<SMALL>III</SMALL>) species. Thus, the catalytic activity of Co ions can be controlled by the additional metal ions, which leads to development of highly reactive and robust catalysts for the photocatalytic water oxidation.</P> <P>Graphic Abstract</P><P>Co-containing metal oxides were used as catalysts for photocatalytic water oxidation. Among them LaCoO<SUB>3</SUB> exhibited the highest catalytic activity, upon being used repeatedly without losing the catalytic activity. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2cp00022a'> </P>