Herein we report a detailed investigation of a highly robust hybrid system (sensitizer/TiO2/catalyst) for the visible-light reduction of CO2 to CO; the system comprises 5'-(4-[bis(4-methoxymethylphenyl)amino]phenyl-2,2'-dithiophen-5-yl)cyanoacrylic acid as the sensitizer and (4,4'-bis(methylphosphonic acid)-2,2'-bipyridine)Re(I)(CO)3Cl as the catalyst, both of which have been anchored on three different types of TiO2 particles (s-TiO2, h-TiO2, d-TiO2). It was found that remarkable enhancements in the CO2 conversion activity of the hybrid photocatalytic system can be achieved by addition of water or such other additives as Li(+), Na(+), and TEOA. The photocatalytic CO2 reduction efficiency was enhanced by approximately 300% upon addition of 3% (v/v) H2O, giving a turnover number of ≥570 for 30 h. A series of Mott-Schottky (MS) analyses on nanoparticle TiO2 films demonstrated that the flat-band potential (V(fb)) of TiO2 in dry DMF is substantially negative but positively shifts to considerable degrees in the presence of water or Li(+), indicating that the enhancement effects of the additives on the catalytic activity should mainly arise from optimal alignment of the TiO2 V(fb) with respect to the excited-state oxidation potential of the sensitizer and the reduction potential of the catalyst in our ternary system. The present results confirm that the TiO2 semiconductor in our heterogeneous hybrid system is an essential component that can effectively work as an electron reservoir and as an electron transporting mediator to play essential roles in the persistent photocatalysis activity of the hybrid system in the selective reduction of CO2 to CO.

Visible-light irradiation of a ternary hybrid catalyst prepared by grafting a dye, an H evolving Co catalyst and a CO-producing Re catalyst on TiO have been found to produce both H and CO (syngas) in CO -saturated N,N-dimethyl formamide (DMF)/water solution containing a 0.1 m sacrificial electron donor. The H /CO ratios are effectively controlled by changing either the water content of the solvent or the molar ratio of the Re and Co catalysts ranging from 1:2 to 15:1. The controlled syngas formation is discussed in terms of competitive electron flow from TiO to each of the CO -reduction and hydrogen-evolving sites depending on the efficiencies of the two catalytic reaction cycles under given reaction conditions.

A series of Zn–porphyrin dyes was prepared and anchored onto a TiO2 surface to complete a dye-sensitized photocatalyst system, Zn–porphyrin-|TiO2|-Cat, and tested as lower energy photosensitizers for photocatalytic CO2 reduction. Three major synthetic modifications were performed on the Zn–porphyrin dye to obtain a lower energy sensitization and improve the catalyst lifetime. We found that incorporating acetylene and linear hexyl groups into the Zn–porphyrin core allowed facile lower energy sensitization, and the addition of the cyanophosphonic acid as an anchoring group gave the long-term dye stability on the TiO2 surface. Under irradiation with red light of >550 nm and a light intensity of 207 mW/cm2, the hybrid ZnP CNPA catalyst showed a TONRe of ∼800 over an extended time period of 90 h. The photocatalytic activities of porphyrin hybrids differ greatly with the binding strength of the anchoring groups of dye and spectral range of the irradiated light and its intensity.

A series of cationic Ir(III) complexes ([Ir(btp)(bpy-X)] (Ir-X: btp = (2-pyridyl)benzo[b]thiophen-3-yl; bpy-X = 4,4'-X-2,2'-bipyridine (X = OMe, Bu, Me, H, and CN)) were applied as visible-light photosensitizer to the CO reduction to CO using a hybrid catalyst (TiO/ReP) prepared by anchoring of Re(4,4'-Y-bpy)(CO)Cl (ReP; Y = CHPO(OH)) on TiO particles. Irradiation of a solution containing Ir-X, TiO/ReP particles, and an electron donor (1,3-dimethyl-2-phenyl-1,3-dihydrobenzimidazole) in N,N-dimethylformamide at greater than 400 nm resulted in the reduction of CO to CO with efficiencies in the order X = OMe > Bu ≈ Me> H; Ir-CN has no photosensitization effect. A notable observation is that Ir-Bu and Ir-Me are less efficient than Ir-OMe at an early stage of the reaction but reveal persistent photosensitization behavior for a much longer period of time unlike the latter. Comparable experiments showed that (1) the Ir-X sensitizers are commonly superior compared to Ru(bpy), a widely used transition-metal photosensitizer, and (2) the system comprising Ir-OMe and TiO/ReP is much more efficient than a homogeneous-solution system using Ir-OMe and Re(4,4'-Y'-bpy)(CO)Cl (Y' = CHPO(OEt)). Implications of the present observations involving reaction mechanisms associated with the different behavior of the photosensitizers are discussed in detail.

Published Version

Conspectus During the last few decades, the design of catalytic systems for CO2 reduction has been extensively researched and generally involves (1) traditional approaches using molecular organic/organometallic materials and heterogeneous inorganic semiconductors and (2) combinatory approaches wherein these materials are combined as needed. Recently, we have devised a number of new TiO2-mediated multicomponent hybrid systems that synergistically integrate the intrinsic merits of various materials, namely, molecular photosensitizers/catalysts and n-type TiO2 semiconductors, and lower the energetic and kinetic barriers between components. We have termed such multicomponent hybrid systems assembled from the hybridization of various organic/inorganic/organometallic units in a single platform inorganometallic photocatalysts. The multicomponent inorganometallic (MIOM) hybrid system onto which the photosensitizer and catalyst are coadsorbed efficiently eliminates the need for bulk-phase diffusion of the components and avoids the accumulation of radical intermediates that invokes a degradation pathway, in contrast to the homogeneous system, in which the free reactive species are concentrated in a confined reaction space. In particular, in energetic terms, we discovered that in nonaqueous media, the conduction band (CB) levels of reduced TiO2 (TiO2(e–)) are positioned at a higher level (in the range −1.5 to −1.9 V vs SCE). This energetic benefit of reduced TiO2 allows smooth electron transfer (ET) from injected electrons (TiO2(e–)) to the coadsorbed CO2 reduction catalyst, which requires relatively high reducing power (at least more than −1.1 V vs SCE). On the other hand, the existence of various shallow surface trapping sites and surface bands, which are 0.3–1.0 eV below the CB of TiO2, efficiently facilitates electron injection from any photosensitizer (including dyes having low excited energy levels) to TiO2 without energetic limitation. This is contrasted with most photocatalytic systems, wherein successive absorption of single high-energy photons is required to produce excited states with enough energy to fulfill photocatalytic reaction, which may allow unwanted side reactions during photocatalysis. In this Account, we present our recent research efforts toward advancing these MIOM hybrid systems for photochemical CO2 reduction and discuss their working mechanisms in detail. Basic ET processes within the MIOM system, including intervalence ET in organic/organometallic redox systems, metal-to-ligand charge transfer of organometallic complexes, and interfacial/outer-sphere charge transfer between components, were investigated by conducting serial photophysical and electrochemical analyses. Because such ET events occur primarily at the interface between the components, the efficiency of interfacial ET between the molecular components (organic/organometallic photosensitizers and molecular reduction catalysts) and the bulk inorganic solid (mainly n-type TiO2 semiconductors) has a significant influence on the overall photo...

A Mn­(I)-based hybrid system (OrgD-|TiO2|-MnP) for photocatalytic CO2 reduction is designed to be a coassembly of Mn­(4,4′-Y2-bpy)­(CO)3Br (MnP; Y = CH2PO­(OH)2) and (E)-3-[5-(4-(diphenylamino)­phenyl)-2,2′-bithiophen-2′-yl]-2-cyanoacrylic acid (OrgD) on TiO2 semiconductor particles. The OrgD-|TiO2|-MnP hybrid reveals persistent photocatalytic behavior, giving high turnover numbers and good product selectivity (HCOO– versus CO). As a typical run, visible-light irradiation of the hybrid catalyst in the presence of 0.1 M electron donor (ED) and 0.001 M LiClO4 persistently produced HCOO– with a >99% selectivity accompanied by a trace amount of CO; the turnover number (TONformate) reached ∼250 after 23 h of irradiation. The product selectivity (HCOO–/CO) was found to be controlled by changing the loading amount of MnP on the TiO2 surface. In situ FTIR analysis of the hybrid during photocatalysis revealed that, at low Mn concentration, the Mn–H monomeric mechanism associated with HCOO– formation is dominant, whereas at high Mn concentration, CO is formed via a Mn–Mn dimer mechanism.

A series of dyes were synthesized to examine the roles of the hydrophilic characteristics of R in sensitized hydrogen generation by dye-grafted Pt/TiO(2) under visible light irradiation. The hydrogen-generation efficiencies and optimum amounts of the dyes grafted to Pt/TiO(2) were affected substantially by the hydrophilic and steric effects of R; moderately hydrophilic DEO1 and DEO2 showed higher sensitization activity at a lower loading than hydrophobic D-H.