Solar CO reduction using a molecular Re(I) catalyst grafted on SiO amide and alkyl amine linkages.

Heterogenized molecular catalysts have shown interesting activities in different chemical transformations. In our previous studies, a molecular catalyst, Re(bpy)(CO)Cl where bpy is 2,2'-bipyridine, was covalently attached to silica surfaces an amide linkage for use in photocatalytic CO reduction. Derivatizing the bpy ligand with electron-withdrawing amide groups led to detrimental effects on the catalytic activity of Re(bpy)(CO)Cl.

Well-defined surface catalytic sites for solar CO reduction: heterogenized molecular catalysts and single atom catalysts.

Exciting progress has been made in the area of solar fuel generation by CO reduction. New photocatalytic materials containing well-defined surface catalytic sites have emerged in recent years, including heterogenized molecular catalysts and single atom catalysts. This Feature Article summarizes our recent research in this area, together with brief discussions of relevant literature.

Probing active sites for carbon oxides hydrogenation on Cu/TiO using infrared spectroscopy.

The valorization of carbon oxides on metal/metal oxide catalysts has been extensively investigated because of its ecological and economical relevance. However, the ambiguity surrounding the active sites in such catalysts hampers their rational development. Here, in situ infrared spectroscopy in combination with isotope labeling revealed that CO molecules adsorbed on Ti and Cu interfacial sites in Cu/TiO gave two disparate carbonyl peaks.

Surface Basicity of Metal@TiO to Enhance Photocatalytic Efficiency for CO Reduction.

Photocatalytic reduction of CO to valuable chemical fuels is of broad interest, given its potential to activate stable greenhouse CO using renewable energy input. We report how to choose the right metal cocatalysts in combination with the surface basicity of TiO to enhance their photocatalytic efficiency for CO photoreduction. Uniform ligand-free metal nanoparticles (NPs) of Ag, Cu, Au, Pd, and Pt, supported on TiO, are active for CO photoreduction using water as an electron donor.

Photoelectrochemical NADH regeneration is highly sensitive to the nature of electrode surface.

(Photo)electrochemistry enables the synthesis of high-value fine chemicals and highly selective activation of molecules that are difficult to prepare using conventional chemical methods. In this work, light-driven NADH (reduced nicotinamide adenine dinucleotide) regeneration is achieved using a molecular Rh(III) mediator on Si photoelectrodes. This process is observed to be highly sensitive to the surface nature of Si photoelectrodes, exhibiting an overpotential reduction up to 600 mV on Si nanowires (SiNWs) as compared to planar Si.

Highly Crystalline Mesoporous Titania Loaded with Monodispersed Gold Nanoparticles: Controllable Metal-Support Interaction in Porous Materials.

We report the syntheses of mesoporous Au/TiO hybrid photocatalysts with ordered and crystalline frameworks using co-assembly of organosilane-containing colloidal amphiphile micelles (CAMs) and poly(ethylene oxide)-modified gold nanoparticles (AuNPs) as templates. The assembled CAMs can convert to inorganic silica during calcination at elevated temperatures, providing extraordinary thermal stability to preserve the porosity of TiO and the nanostructures of AuNPs. Well-defined AuNPs supported within mesoporous TiO (Au@mTiO) can be prepared using thermal annealing at temperatures up to 800 °C.

Selective CO Reduction Catalyzed by Single Cobalt Sites on Carbon Nitride under Visible-Light Irradiation.

Framework nitrogen atoms of carbon nitride (CN) can coordinate with and activate metal sites for catalysis. In this study, CN was employed to harvest visible light and activate Co sites, without the use of additional ligands, in photochemical CO reduction. Photocatalysts containing single Co sites on CN were prepared by a simple deposition method and demonstrated excellent activity and product selectivity toward CO formation.

Co-Template Directed Synthesis of Gold Nanoparticles in Mesoporous Titanium Dioxide.

A bottom-up synthetic methodology to encapsulate pre-synthesized, well-defined gold nanoparticles (AuNPs) into mesoporous titanium dioxide framework (Au@mTiO ) is reported. This method employs two structurally and chemically similar templates of amphiphilic block copolymers as well as poly(ethylene oxide)-tethered AuNPs, which showed excellent stability during sol-gel transition and thermal annealing at elevated temperatures. Such synthesis enabled precise control of sizes and loading of AuNPs within the mesoporous TiO framework.

Microwave-assisted deposition of a highly active cobalt catalyst on mesoporous silica for photochemical CO reduction.

Coupling with robust surfaces is a promising approach to improve the stability and recyclability of highly active molecular catalysts. This study builds on our prior success to deposit a Co(iii) cyclam complex, where cyclam is 1,4,8,11-tetraazacyclotetradecane, on mesoporous silica via a microwave-assisted process. The Co(iii) complex was successfully deposited on the silica surface through reacting with silanol groups in the silica mesopores.

CO reduction with Re(i)-NHC compounds: driving selective catalysis with a silicon nanowire photoelectrode.

The CO-reduction activity of two Re(i)-NHC complexes is investigated employing a silicon nanowire photoelectrode to drive catalysis. Photovoltages greater than 440 mV are observed along with excellent selectivity towards CO over H formation. The observed selectivity towards CO production correlates with strong adsorption of the catalysts on the photoelectrode surface.

Three-Dimensional Graphene-TiO Nanocomposite Photocatalyst Synthesized by Covalent Attachment.

We report the synthesis of a three-dimensional graphene (3DG)-TiO nanocomposite by covalently attaching P25 TiO nanoparticles onto pristine 3DG through a perfluorophenyl azide-mediated coupling reaction. The TiO nanoparticles were robustly attached on the 3DG surface, with minimal particle agglomeration. In photocatalytic CO reduction, the 3DG-TiO nanocomposite demonstrated excellent activity, about 11 times higher than that of the P25 TiO nanoparticles.

Photoelectrochemical CO2 Reduction by a Molecular Cobalt(II) Catalyst on Planar and Nanostructured Si Surfaces.

In the presence of a molecular Co(II) catalyst, CO2 reduction occurred at much less negative potentials on Si photoelectrodes than on an Au electrode. The addition of 1 % H2 O significantly improved the performance of the Co(II) catalyst. Photovoltages of 580 and 320 mV were obtained on Si nanowires and a planar Si photoelectrode, respectively.

Re(I) NHC Complexes for Electrocatalytic Conversion of CO2.

The modular construction of ligands around an N-heterocyclic carbene building block represents a flexible synthetic strategy for tuning the electronic properties of metal complexes. Herein, methylbenzimidazolium-pyridine and methylbenzimidazolium-pyrimidine proligands are constructed in high yield using recently established transition-metal-free techniques. Subsequent chelation to ReCl(CO)5 furnishes ReCl(N-methyl-N'-2-pyridylbenzimidazol-2-ylidine)(CO)3 and ReCl(N-methyl-N'-2-pyrimidylbenzimidazol-2-ylidine)(CO)3.

Photocatalytic CO2 reduction using a molecular cobalt complex deposited on TiO2 nanoparticles.

Hybrid photocatalysts were prepared by depositing a macrocyclic cobalt complex on TiO2 surfaces. Upon UV light irradiation, photoexcited electrons in TiO2 nanoparticles were transferred to the surface cobalt catalyst for CO2 reduction.

Adsorption and Photochemical Properties of a Molecular CO2 Reduction Catalyst in Hierarchical Mesoporous ZSM-5: An In Situ FTIR Study.

As part of our recent effort to attach well-defined molecular photocatalysts to solid-state surfaces, this present study investigates adsorption and photochemical properties of a tricarbonyl rhenium(I) compound, Re(bpy)(CO)3Cl (bpy = 2,2'-bipyridine), in hierarchical mesoporous ZSM-5. The molecular Re(I) catalyst, a Ru(bpy)3(2+) photosensitizer, and an amine-based electron donor were coadsorbed in the mesopores of the hierarchical ZSM-5 through simple liquid-phase adsorption. The functionalized ZSM-5 was then characterized with infrared and UV-visible spectroscopies and was tested in CO2 reduction photocatalysis at the gas-surface interface.

Reduction of CO2 on a tricarbonyl rhenium(I) complex: modeling a catalytic cycle.

Tricarbonyl rhenium(I) complexes, such as Re(bpy)(CO)(3)Cl where bpy = 2,2'-bipyridyl, have demonstrated superior activity in catalyzing CO(2) reduction in the presence of sacrificial electron donors. We have utilized density functional theory (DFT) to investigate a potential pathway for formate production via a rhenium-hydride insertion mechanism in the presence of triethylamine (TEA). On the basis of prior studies, we re-examined the role of TEA and studied a catalytic cycle for CO(2) reduction in which TEA functions as both the hydrogen atom and the electron donor for reducing CO(2) into formate.

Energy conversion in natural and artificial photosynthesis.

Modern civilization is dependent upon fossil fuels, a nonrenewable energy source originally provided by the storage of solar energy. Fossil-fuel dependence has severe consequences, including energy security issues and greenhouse gas emissions. The consequences of fossil-fuel dependence could be avoided by fuel-producing artificial systems that mimic natural photosynthesis, directly converting solar energy to fuel.

Reversible visible-light photooxidation of an oxomanganese water-oxidation catalyst covalently anchored to TiO2 nanoparticles.

Several polynuclear transition-metal complexes, including our own dinuclear di-μ-oxo manganese compound [H(2)O(terpy)Mn(III)(μ-O)(2)Mn(IV)(terpy)H(2)O](NO(3))(3) (1, terpy = 2,2':6',2''-terpyridine), have been reported to be homogeneous catalysts for water oxidation. This paper reports the covalent attachment of 1 onto nanoparticulate TiO(2) surfaces using a robust chromophoric linker L. L, a phenylterpy ligand attached to a 3-phenyl-acetylacetonate anchoring moiety via an amide bond, absorbs visible light and leads to photoinduced interfacial electron transfer into the TiO(2) conduction band.

Synergistic effect between anatase and rutile TiO2 nanoparticles in dye-sensitized solar cells.

A synergistic effect between anatase and rutile TiO2 is known, in which the addition of rutile can remarkably enhance the photocatalytic activity of anatase in the degradation of organic contaminants. In this study, mixed-phase TiO2 nanocomposites consisting of anatase and rutile nanoparticles (NPs) were prepared for use as photoanodes in dye-sensitized solar cells (DSSCs) and were characterized by using UV-vis spectroscopy, powder X-ray diffraction and scanning electron microscopy. The addition of 10-15% rutile significantly improved light harvesting and the overall solar conversion efficiency of anatase NPs in DSSCs.

Acetylacetonate anchors for robust functionalization of TiO2 nanoparticles with Mn(II)-terpyridine complexes.

A novel class of derivatized acetylacetonate (acac) linkers for robust functionalization of TiO2 nanoparticles (NPs) under aqueous and oxidative conditions is reported. The resulting surface adsorbate anchors are particularly relevant to engineering photocatalytic and photovoltaic devices since they can be applied to attach a broad range of photosensitizers and photocatalytic complexes and are not affected by humidity. Acac is easily modified by CuI-mediated coupling reactions to provide a variety of scaffolds, including substituted terpy complexes (terpy = 2,2':6,2''-terpyridine), assembled with ligands coordinated to transition-metal ions.

Photoreactive TiO2/carbon nanotube composites: synthesis and reactivity.

Electron-hole recombination limits the efficiency of TiO2 photocatalysis. We have investigated the efficacy with which anatase/carbon nanotube (CNT) composite materials reduce charge recombination and enhance reactivity. We synthesized nanostructured assemblies composed of different proportions of anatase (5 or 100 nm) and either single-or multi-walled CNTs.

Single-walled carbon nanotube-facilitated dispersion of particulate TiO2 on ZrO2 ceramic membrane filters.

We report that SWCNTs substantially improve the uniformity and coverage of TiO2 coatings on porous ZrO2 ceramic membrane filters. The ZrO2 filters were dip coated with 100 nm anatase TiO2, TiO2/SWCNT composites, a TiO2+SWCNT mixture, and a TiO2/MWCNT composite at pH 3, 5, and 8. Whereas the TiO2+SWCNT mixture and the TiO2/MWCNT composite promote better coverage and less clumping than TiO2 alone, the TiO2/SWCNT composite forms a complete uniform coating without cracking at pH 5 ( approximately 100% coverage).

The important role of tetrahedral Ti4+ sites in the phase transformation and photocatalytic activity of TiO2 nanocomposites.

Highly photoactive, tetrahedral Ti4+ sites can be created, other than in zeolite cavities and on silica substrate, in mixed-phase TiO2 nanocomposites. The tetrahedral Ti4+ species was shown to be an intermediate formed during the thermally driven phase transformation from anatase to rutile.

Development of improved materials for environmental applications: nanocrystalline NaY zeolites.

Two nanocrystalline NaY samples were synthesized with Si/Al ratios of 1.8 and crystal sizes of 23 and 50 nm, respectively. The synthesized NaY zeolites were characterized by powder X-ray diffraction, scanning electron microscopy, nitrogen adsorption isotherms, silicon solid-state magic angle spinning NMR and FTIR spectroscopy.