Directed Assembly of Cuprous Oxide Nanocatalyst for CO₂ Reduction Coupled to Heterobinuclear ZrOCo^II Light Absorber in Mesoporous Silica

Hierarchical assembly of an oxo-bridged binuclear ZrOCo^II light absorber unit coupled to a cuprous oxide nanocluster catalyst for CO₂ reduction on mesoporous silica support is demonstrated. The proper positioning of the Cu oxide cluster was achieved by photodeposition of a [Cu­(NCCH₃)₄]²⁺precursor by visible light excitation of the ZrOCo charge transfer chromophore, followed by mild calcination at 350 C. Illumination of the Cu_<i>x</i>O_<i>y</i>-ZrOCo unit so formed in the presence of a diethylamine electron donor resulted in the reduction of surface Cu centers to Cu⁰ as demonstrated by the characteristic infrared band of adsorbed ¹³CO probe molecules at 2056 cm^–1. For analogous Cu_<i>x</i>O_<i>y</i>-TiOCo^II units, the oxidation state makeup of the surface Cu centers was dominated by Cu^I, and the Cu⁰, Cu^I, and Cu^II composition was found to depend on the wavelength of MMCT excitation. The observed strong dependence of the CO₂ photoreduction yield on the oxidation state of the surface Cu centers directly proves that CO₂ is reduced on the Cu_<i>x</i>O_<i>y</i> surface, thus establishing that the ZrOCo^II unit functions as light absorber, donating electrons to the Cu_<i>x</i>O_<i>y</i> catalyst on whose surface CO₂ is reduced

Light Induced Carbon Dioxide Reduction by Water at Binuclear ZrOCo^II Unit Coupled to Ir Oxide Nanocluster Catalyst

An all-inorganic polynuclear unit consisting of an oxo-bridged binuclear ZrOCo^II group coupled to an iridium oxide nanocluster (IrO_<i>x</i>) was assembled on an SBA-15 silica mesopore surface. A photodeposition method was developed that affords coupling of the IrO_<i>x</i> water oxidation catalyst with the Co donor center. The approach consists of excitation of the ZrOCo^II metal-to-metal charge-transfer (MMCT) chromophore with visible light in the presence of [Ir­(acac)₃] (acac: acetylacetonate) precursor followed by calcination under mild conditions, with each step monitored by optical and infrared spectroscopy. Illumination of the MMCT chromophore of the resulting ZrOCo^II–IrO_<i>x</i> units in the SBA-15 pores loaded with a mixture of ¹³CO₂ and H₂O vapor resulted in the formation of ¹³CO and O₂ monitored by FT-IR and mass spectroscopy, respectively. Use of ¹⁸O labeled water resulted in the formation of ¹⁸O₂ product. This is the first example of a closed photosynthetic cycle of carbon dioxide reduction by water using an all-inorganic polynuclear cluster featuring a molecularly defined light absorber. The observed activity implies successful competition of electron transfer between the IrO_<i>x</i> catalyst cluster and the transient oxidized Co donor center with back electron transfer of the ZrOCo light absorber, and is further aided by the instant desorption of the CO and O₂ product from the silica pores

Superior Electron Transport and Photocatalytic Abilities of Metal-Nanoparticle-Loaded TiO₂ Superstructures

Metal–semiconductor nanocomposites have been widely employed for designing efficient optoelectronic devices and catalysts. The performance of such nanocomposites is significantly influenced by both the method of preparation and the electronic and morphological structures of metals and semiconductors. Here, we have synthesized novel nanocomposites containing plate-like anatase TiO₂ mesocrystal superstructures and noble metal (Au, Pt) nanoparticles. These metal nanoparticles were preferentially photodeposited on the edge of TiO₂ mesocrystals. The electron transport and photocatalytic properties of the novel nanocomposites were subsequently studied. Single-molecule fluorescence spectroscopy measurements on a single particle directly revealed that most of the photogenerated electrons could migrate from the dominant surface to the edge of the TiO₂ mesocrystal with the reduction reactions mainly occurring at its lateral surfaces containing {101} facets. The loading of metal nanoparticles on the superstructure of TiO₂ was found to greatly improve the photogenerated charge separation efficiency allowing significant (more than 1 order of magnitude) enhancement of the photocatalytic reaction rate in organic degradation reactions. These outstanding features allowed significantly reduced consumption (ca. 10% of that of typical TiO₂ nanocrystal samples) of the Au or Pt loading on the TiO₂ mesocrystal while maintaining the same photocatalytic activity

Investigating the Unrevealed Photocatalytic Activity and Stability of Nanostructured Brookite TiO₂ Film as an Environmental Photocatalyst

Among three polymorphs of TiO₂, the brookite is the least known phase in many aspects of its properties and photoactivities (especially comparable to anatase and rutile) because it is the rarest phase to be synthesized in the standard environment among the TiO₂ polymorphs. In this study, we address the unrevealed photocatalytic properties of pure brookite TiO₂ film as an environmental photocatalyst. Highly crystalline brookite nanostructures were synthesized on titanium foil using a well-designed hydrothermal reaction, without harmful precursors and selective etching of anatase, to afford pure brookite. The photocatalytic degradation of rhodamine B, tetramethylammonium chloride, and 4-chlorophenol on UV-illuminated pure brookite were investigated and compared with those on anatase and rutile TiO₂. The present research explores the generation of OH radicals as main oxidants on brookite. In addition, tetramethylammonium, as a mobile OH radical indicator, was degraded over both pure anatase and brookite phases, but not rutile. The brookite phase showed much higher photoactivity among TiO₂ polymorphs, despite its smaller surface area compared with anatase. This result can be ascribed to the following properties of the brookite TiO₂ film: (i) the higher driving force with more negative flat-band potential, (ii) the efficient charge transfer kinetics with low resistance, and (iii) the generation of more hydroxyl radicals, including mobile OH radicals. The brookite-nanostructured TiO₂ electrode facilitates photocatalyst collection and recycling with excellent stability, and readily controls photocatalytic degradation rates with facile input of additional potential

Role of Interparticle Charge Transfers in Agglomerated Photocatalyst Nanoparticles: Demonstration in Aqueous Suspension of Dye-Sensitized TiO₂

The interparticle charge transfer within the agglomerates of TiO₂ nanoparticles in slurries markedly enhanced the dye-sensitized production of H₂ under visible light. By purposely decoupling the light absorbing part of Dye/TiO₂ from the active catalytic center of Pt/TiO₂, the role of bare TiO₂ nanoparticles working as a mediator that connects the above two parts in the agglomerates was investigated systematically. The presence of mediator in the agglomerate facilitated the charge separation and the electron transfer from Dye/TiO₂ to Pt/TiO₂ through multiple grain boundaries and subsequently produced more hydrogen. The dye-sensitized reduction of Cr­(VI) to Cr­(III) was also enhanced when Dye/TiO₂ nanoparticles were agglomerated with bare TiO₂ nanoparticles. The charge recombination between the oxidized dye and the injected electron was retarded in the presence of bare TiO₂ nanoparticles, and this retarded recombination on Dye/TiO₂ was confirmed by using transient laser spectroscopy. This phenomenon can be rationalized in terms of an interparticle Fermi level gradient within the agglomerates, which drives the charge separation

Carbon Dioxide Dimer Radical Anion as Surface Intermediate of Photoinduced CO₂ Reduction at Aqueous Cu and CdSe Nanoparticle Catalysts by Rapid-Scan FT-IR Spectroscopy

Monitoring of visible light sensitized reduction of CO₂ at Cu nanoparticles in aqueous solution by rapid-scan ATR FT-IR spectroscopy on the time scale of seconds allowed structural identification of a one-electron intermediate and demonstrated its kinetic relevancy for the first time. Isotopic labeling (¹²C: 1632, 1358, 1346 cm^–1; ¹³C: 1588, 1326, 1316 cm^–1) revealed a species of carbon dioxide dimer radical anion structure, most likely bound to the catalyst surface through carbon. Intermediacy of CuC­(O)­OCO₂^– surface species is in agreement with a recently proposed mechanism for electrocatalytic CO₂ reduction at Cu metal nanoparticles based on Tafel slope analysis. Spontaneous decrease of the intermediate after termination of the photosensitization pulse (Sn porphyrin excited at 405 nm) was accompanied by the growth of HCO₃^–. CO was produced as well, but sensitive detection required photolysis for tens of minutes. A direct kinetic link between a C₂O₄^– surface intermediate and the CO product was also demonstrated for photocatalyzed CO₂ reduction at aqueous CdSe nanoparticles, where first order growth of a CdC­(O)­OCO₂^– species was accompanied by rise of CO (monitored by a fast Ni complex trap) and HCO₃^– showing a distinct induction period. The detection of the one-electron surface intermediate and confirmation of its catalytic relevancy was enabled by the delivery of electrons one-by-one by the photosensitization method. The observation of carbon dioxide dimer radical anion points to approaches for rate enhancements of heterogeneous CO₂ reduction by creating catalytic environments that favor formation of this intermediate

Active {001} Facet Exposed TiO₂ Nanotubes Photocatalyst Filter for Volatile Organic Compounds Removal: From Material Development to Commercial Indoor Air Cleaner Application

TiO₂ nanotubes (TNT) have a highly ordered open structure that promotes the diffusion of dioxygen and substrates onto active sites and exhibit high durability against deactivation during the photocatalytic air purification. Herein, we synthesized {001} facet-exposed TiO₂ nanotubes (001-TNT) using a new and simple method that can be easily scaled up, and tested them for the photocatalytic removal of volatile organic compounds (VOCs) in both a laboratory reactor and a commercial air cleaner. While the surface of TNT is mainly composed of {101} facet anatase, 001-TNT’s outer surface was preferentially aligned with {001} facet anatase. The photocatalytic degradation activity of toluene on 001-TNT was at least twice as high as that of TNT. While the TNT experienced a gradual deactivation during successive cycles of photocatalytic degradation of toluene, the 001-TNT did not exhibit any sign of catalyst deactivation under the same test conditions. Under visible light irradiation, the 001-TNT showed degradation activity for acetaldehyde and formaldehyde, while the TNT did not exhibit any degradation activity for them. The 001-TNT filter was successfully scaled up and installed on a commercial air cleaner. The air cleaner equipped with the 001-TNT filters achieved an average VOCs removal efficiency of 72% (in 30 min of operation) in a 8-m³ test chamber, which satisfied the air cleaner standards protocol (Korea) to be the first photocatalytic air cleaner that passed this protocol

Mechanistic Investigations of Water Oxidation by a Molecular Cobalt Oxide Analogue: Evidence for a Highly Oxidized Intermediate and Exclusive Terminal Oxo Participation

Artificial photosynthesis (AP) promises to replace society’s dependence on fossil energy resources via conversion of sunlight into sustainable, carbon-neutral fuels. However, large-scale AP implementation remains impeded by a dearth of cheap, efficient catalysts for the oxygen evolution reaction (OER). Cobalt oxide materials can catalyze the OER and are potentially scalable due to the abundance of cobalt in the Earth’s crust; unfortunately, the activity of these materials is insufficient for practical AP implementation. Attempts to improve cobalt oxide’s activity have been stymied by limited mechanistic understanding that stems from the inherent difficulty of characterizing structure and reactivity at surfaces of heterogeneous materials. While previous studies on cobalt oxide revealed the intermediacy of the unusual Co­(IV) oxidation state, much remains unknown, including whether bridging or terminal oxo ligands form O₂ and what the relevant oxidation states are. We have addressed these issues by employing a homogeneous model for cobalt oxide, the [Co­(III)₄] cubane (Co₄O₄­(OAc)₄­py₄, py = pyridine, OAc = acetate), that can be oxidized to the [Co­(IV)­Co­(III)₃] state. Upon addition of 1 equiv of sodium hydroxide, the [Co­(III)₄] cubane is regenerated with stoichio­metric formation of O₂. Oxygen isotopic labeling experiments demonstrate that the cubane core remains intact during this stoichio­metric OER, implying that terminal oxo ligands are responsible for forming O₂. The OER is also examined with stopped-flow UV–visible spectroscopy, and its kinetic behavior is modeled, to surprisingly reveal that O₂ formation requires disproportionation of the [Co­(IV)­Co­(III)₃] state to generate an even higher oxidation state, formally [Co­(V)­Co­(III)₃] or [Co­(IV)₂­Co­(III)₂]. The mechanistic understanding provided by these results should accelerate the development of OER catalysts leading to increasingly efficient AP systems

Mechanistic Investigations of Water Oxidation by a Molecular Cobalt Oxide Analogue: Evidence for a Highly Oxidized Intermediate and Exclusive Terminal Oxo Participation

Artificial photosynthesis (AP) promises to replace society’s dependence on fossil energy resources via conversion of sunlight into sustainable, carbon-neutral fuels. However, large-scale AP implementation remains impeded by a dearth of cheap, efficient catalysts for the oxygen evolution reaction (OER). Cobalt oxide materials can catalyze the OER and are potentially scalable due to the abundance of cobalt in the Earth’s crust; unfortunately, the activity of these materials is insufficient for practical AP implementation. Attempts to improve cobalt oxide’s activity have been stymied by limited mechanistic understanding that stems from the inherent difficulty of characterizing structure and reactivity at surfaces of heterogeneous materials. While previous studies on cobalt oxide revealed the intermediacy of the unusual Co­(IV) oxidation state, much remains unknown, including whether bridging or terminal oxo ligands form O₂ and what the relevant oxidation states are. We have addressed these issues by employing a homogeneous model for cobalt oxide, the [Co­(III)₄] cubane (Co₄O₄­(OAc)₄­py₄, py = pyridine, OAc = acetate), that can be oxidized to the [Co­(IV)­Co­(III)₃] state. Upon addition of 1 equiv of sodium hydroxide, the [Co­(III)₄] cubane is regenerated with stoichio­metric formation of O₂. Oxygen isotopic labeling experiments demonstrate that the cubane core remains intact during this stoichio­metric OER, implying that terminal oxo ligands are responsible for forming O₂. The OER is also examined with stopped-flow UV–visible spectroscopy, and its kinetic behavior is modeled, to surprisingly reveal that O₂ formation requires disproportionation of the [Co­(IV)­Co­(III)₃] state to generate an even higher oxidation state, formally [Co­(V)­Co­(III)₃] or [Co­(IV)₂­Co­(III)₂]. The mechanistic understanding provided by these results should accelerate the development of OER catalysts leading to increasingly efficient AP systems

Mechanistic Investigations of Water Oxidation by a Molecular Cobalt Oxide Analogue: Evidence for a Highly Oxidized Intermediate and Exclusive Terminal Oxo Participation

Artificial photosynthesis (AP) promises to replace society’s dependence on fossil energy resources via conversion of sunlight into sustainable, carbon-neutral fuels. However, large-scale AP implementation remains impeded by a dearth of cheap, efficient catalysts for the oxygen evolution reaction (OER). Cobalt oxide materials can catalyze the OER and are potentially scalable due to the abundance of cobalt in the Earth’s crust; unfortunately, the activity of these materials is insufficient for practical AP implementation. Attempts to improve cobalt oxide’s activity have been stymied by limited mechanistic understanding that stems from the inherent difficulty of characterizing structure and reactivity at surfaces of heterogeneous materials. While previous studies on cobalt oxide revealed the intermediacy of the unusual Co­(IV) oxidation state, much remains unknown, including whether bridging or terminal oxo ligands form O₂ and what the relevant oxidation states are. We have addressed these issues by employing a homogeneous model for cobalt oxide, the [Co­(III)₄] cubane (Co₄O₄­(OAc)₄­py₄, py = pyridine, OAc = acetate), that can be oxidized to the [Co­(IV)­Co­(III)₃] state. Upon addition of 1 equiv of sodium hydroxide, the [Co­(III)₄] cubane is regenerated with stoichio­metric formation of O₂. Oxygen isotopic labeling experiments demonstrate that the cubane core remains intact during this stoichio­metric OER, implying that terminal oxo ligands are responsible for forming O₂. The OER is also examined with stopped-flow UV–visible spectroscopy, and its kinetic behavior is modeled, to surprisingly reveal that O₂ formation requires disproportionation of the [Co­(IV)­Co­(III)₃] state to generate an even higher oxidation state, formally [Co­(V)­Co­(III)₃] or [Co­(IV)₂­Co­(III)₂]. The mechanistic understanding provided by these results should accelerate the development of OER catalysts leading to increasingly efficient AP systems