Imparting CO₂ reduction selectivity to ZnGa₂O₄ photocatalysts by crystallization from hetero nano assembly of amorphous-like metal hydroxides

Imparting an enhanced CO₂ reduction selectivity to ZnGa₂O₄ photocatalysts has been demonstrated by controlled crystallization from interdispersed nanoparticles of zinc and gallium hydroxides. The hydroxide precursor in which Zn(II) and Ga(III) are homogeneously interdispersed was prepared through an epoxide-driven sol–gel reaction. ZnGa₂O₄ obtained by a heat-treatment exhibits a higher surface basicity and an enhanced affinity for CO₂ molecules than previously-reported standard ZnGa₂O₄. The enhanced affinity for CO₂ molecules of the resultant ZnGa₂O₄ leads to highly-selective CO evolution in CO₂ photo-reduction with H₂O reductants. The present scheme is promising to achieve desirable surface chemistry on metal oxide photocatalysts

二酸化炭素の選択的変換を志向した活性部位設計

京都大学0048新制・課程博士博士(工学)甲第22467号工博第4728号新制||工||1738(附属図書館)京都大学大学院工学研究科分子工学専攻(主査)教授 田中 庸裕, 教授 江口 浩一, 教授 佐藤 啓文学位規則第4条第1項該当Doctor of Philosophy (Engineering)Kyoto UniversityDFA

Base Catalysis of Sodium Salts of [Ta6−xNbxO19]8− Mixed-Oxide Clusters

The solid base catalysis of sodium salts of Lindqvist-type metal oxide clusters was investigated using a Knoevenagel condensation reaction. We successfully synthesized the sodium salts of Ta and Nb mixed-oxide clusters Na8−nHn[(Ta6−xNbx)O19]·15H2O (Na-Ta6−xNbx, n = 0, 1, x = 0–6) and found them to exhibit activity for proton abstraction from nitrile substrates with a pKa value of 23.8, which is comparable to that of the conventional solid base MgO. The Ta-rich Na-Ta6 and Na-Ta4Nb2 exhibited high activity among Ta and Nb mixed-oxide clusters. Synchrotron X-ray diffraction (SXRD) measurements, Fourier-transform infrared (FT-IR) spectroscopy, and X-ray absorption spectroscopy (XAS) revealed the structure of Na-Ta6−xNbx: (1) The crystal structure changed from Na7H[M6O19]·15H2O to Na8[M6O19]·15H2O (M = Ta or Nb) by the anisotropic expansion of the unit cell with an increase in Ta content; (2) Highly symmetrical Lindqvist [Ta6−xNbxO19]8− was generated in Na-Ta4Nb2 and Na-Ta6 because of the symmetrical association of Na+ ions with [Ta6−xNbxO19]8− in the structure. DFT calculation revealed that the Lindqvist structures with high symmetry have large NBO charges on surface oxygen species, which are strongly related to base catalytic activity, whereas the composition hardly affects the NBO charges. The above results showed that the Brønsted base catalysis was sensitive to the symmetry of the Lindqvist [Ta6−xNbxO19]8− structure. These findings contribute to the design of solid base catalysts composed of anionic metal oxide clusters with alkaline-metal cations

Liquid amine–solid carbamic acid phase-separation system for direct capture of CO2 from air

The phase separation between a liquid amine and the solid carbamic acid exhibited >99% CO2 removal efficiency under a large-scale gas stream of 400 ppm CO2. Isophorone diamine [IPDA; 3-(aminomethyl)-3,5,5-trimethylcyclohexylamine] reacted with CO2 in the CO2/IPDA molar ratio of ≥ 1 even in H2O as a solvent. The captured CO2 was completely desorbed at 333 K because the disolved carbamate ion releases CO2 at low temperature. The reusability of IPDA under CO2 adsorption-and-desorption cycles without degradation, the >95% efficinecy kept for 100 hours under direct air capture condition, and high CO2 capture rate (214 mmol/h for 1 mol amine) suggest that the phase separation system using IPDA is robust and durable for practical use

Selective CO₂ Fixation to Styrene Oxide by Ta-Substitution of Lindqvist-Type [(Ta,Nb)₆O₁₉]⁸⁻ Clusters

Metal oxide clusters composed of group 5 metal ions, such as Nb and Ta, exhibit catalytic activities for CO2 fixation to styrene oxide (SO) due to the highly negative natural bonding charge of the terminal O atoms that could work as CO2 activation sites. In this study, tetrabutylammonium (TBA) salts of [TaxNb6−xO19]8− (TBA-TaxNb6−x, x = 0–6) were prepared and Ta-substitution effect on the catalytic properties of TBA-TaxNb6−x for CO2 fixation to SO was investigated. We found that TBA-Ta1Nb5 shows the highest styrene carbonate (SC) selectivity (95%) among TBA-TaxNb6−x, although the SO conversion monotonously increases with the incremental Ta substitution amount. The CO2 fixation to SO under various conditions and in situ X-ray absorption fine structure measurements reveal that CO2 is activated on both terminal O sites coordinated to the Ta (terminal OTa) and Nb (terminal ONb) sites, whereas the activation of SO proceeds on the terminal OTa and/or bridge O sites that are connected to Ta. Density functional theory (DFT) calculations reveal that the terminal OTa of TBA-Ta1Nb5 preferentially adsorbs CO2 compared with other ONb base sites. We conclude that the selective CO2 activation at terminal OTa of TBA-Ta1Nb5 without SO activation is a crucial factor for high SC selectivity in the CO2 fixation to SO

Small Copper Nanoclusters Synthesized through Solid-State Reduction inside a Ring-Shaped Polyoxometalate Nanoreactor

Cu nanoclusters exhibit distinctive physicochemical properties and hold significant potential for multifaceted applications. Although Cu nanoclusters are synthesized by reacting Cu ions and reducing agents by covering their surfaces using organic protecting ligands or supporting them inside porous materials, the synthesis of surface-exposed Cu nanoclusters with a controlled number of Cu atoms remains challenging. This study presents a solid-state reduction method for the synthesis of Cu nanoclusters employing a ring-shaped polyoxometalate (POM) as a structurally defined and rigid molecular nanoreactor. Through the reduction of Cu2+ incorporated within the cavity of a ring-shaped POM using H2 at 140 °C, spectroscopic studies and single-crystal X-ray diffraction analysis revealed the formation of surface-exposed Cu nanoclusters with a defined number of Cu atoms within the cavities of POMs. Furthermore, the Cu nanoclusters underwent a reversible redox transformation within the cavity upon alternating the gas atmosphere (i.e., H2 or O2). These Cu nanoclusters produced active hydrogen species that can efficiently hydrogenate various functional groups such as alkenes, alkynes, carbonyls, and nitro groups using H2 as a reductant. We expect that this synthesis approach will facilitate the development of a wide variety of metal nanoclusters with high reactivity and unexplored properties

Counteranion-induced structural isomerization of phosphine-protected PdAu8 and PtAu8 clusters

Abstract Controlling the geometric structures of metal clusters through structural isomerization allows for tuning of their electronic state. In this study, we successfully synthesized butterfly-motif [PdAu8(PPh3)8]2+ (PdAu8-B, B means butterfly-motif) and [PtAu8(PPh3)8]2+ (PtAu8-B) by the structural isomerization from crown-motif [PdAu8(PPh3)8]2+ (PdAu8-C, C means crown-motif) and [PtAu8(PPh3)8]2+ (PtAu8-C), induced by association with anionic polyoxometalate, [Mo6O19]2– (Mo6) respectively, whereas their structural isomerization was suppressed by the use of [NO3]– and [PMo12O40]3– as counter anions. DR-UV-vis-NIR and XAFS analyses and density functional theory calculations revealed that the synthesized [PdAu8(PPh3)8][Mo6O19] (PdAu8-Mo6) and [PtAu8(PPh3)8][Mo6O19] (PtAu8-Mo6) had PdAu8-B and PtAu8-B respectively because PdAu8-Mo6 and PtAu8-Mo6 had bands in optical absorption at the longer wavelength region and different structural parameters characteristic of the butterfly-motif structure obtained by XAFS analysis. Single-crystal and powder X-ray diffraction analyses revealed that PdAu8-B and PtAu8-B were surrounded by six Mo6 with rock salt-type packing, which stabilizes the semi-stable butterfly-motif structure to overcome high activation energy for structural isomerization

Ultra-stable and highly reactive colloidal gold nanoparticle catalysts protected using multi-dentate metal oxide nanoclusters

Abstract Owing to their remarkable properties, gold nanoparticles are applied in diverse fields, including catalysis, electronics, energy conversion and sensors. However, for catalytic applications of colloidal gold nanoparticles, the trade-off between their reactivity and stability is a significant concern. Here we report a universal approach for preparing stable and reactive colloidal small (~3 nm) gold nanoparticles by using multi-dentate polyoxometalates as protecting agents in non-polar solvents. These nanoparticles exhibit exceptional stability even under conditions of high concentration, long-term storage, heating and addition of bases. Moreover, they display excellent catalytic performance in various oxidation reactions of organic substrates using molecular oxygen as the sole oxidant. Our findings highlight the ability of inorganic multi-dentate ligands with structural stability and robust steric and electronic effects to confer stability and reactivity upon gold nanoparticles. This approach can be extended to prepare metal nanoparticles other than gold, enabling the design of novel nanomaterials with promising applications