CO2 reduction using oxynitrides and nitrides under visible light

Charge Transport Approaches in Photocatalytic Supramolecular Systems Composing of Semiconductor and Molecular Metal Complex for CO2 Reduction

The design of photocatalytic supramolecular systems composing of semiconductors and molecular metal complexes for CO2 reduction has attracted increasing attention. The supramolecular system combines the structural merits of semiconductors and metal complexes, where the semiconductor harvests light and undertakes the oxidative site, while the metal complex provides activity for CO2 reduction. The intermolecular charge transfer plays crucial role in ensuring photocatalytic performance. Here, we review the progress of photocatalytic supramolecular systems in reduction of CO2 and highlight the interfacial charge transfer pathways, as well as their state-of-the-art characterization methods. The remaining challenges and prospects for further design of supramolecular photocatalysts are also presented.

CO2 Photoreduction Activity Enhancement and Unexpected Observation of Carbon Monoxide Adsorbates on the Surface of TiO2 Nanotube Arrays Synthesized in Formamide Electrolytes

TiO2 nanotube arrays grown through electrochemical anodization in a formamide-based electrolyte (TNTA-FA) exhibited a whole host of unusual properties compared to nanotubes grown in the conventional ethylene glycol-based electrolyte (TNTA-EG). TNTA-FA exhibited shorter phonon lifetimes, lower lattice strain, more visible light absorption, lower work function, and a highly unusual adsorbate structure consisting of physisorbed and chemisorbed CO along with linearly adsorbed CO2 and various monodentate and bidentate carbonate species. The observation of adsorbed CO in the dark is highly unusual and indicates spontaneous deoxygenation of CO2 on the surface of TNTA-FA. The significance of this finding is that the formation of CO2•- is no longer the rate-limiting bottleneck for the reduction of CO2 on TNTA-FA surfaces as it is for all TiO2 surfaces. TNTA-FA samples are strongly colored (inclusive of a fluorescent green color) and consist of rounded, vertically oriented hollow cylinders as opposed to the honeycomb-like morphology of TNTA-EG arranged in an approximate triangular lattice. The photocatalytic activity was tested through the CO2 photoreduction and dye degradation tests. Formamide-based nanotubes outperformed the EG-based nanotubes by almost 1.7 and 2 times, respectively, in CO2 reduction and dye degradation tests done on methylene blue, brilliant green, and rhodamine B dyes. These results are attributed to stronger surface band bending in TNTA-FA which facilitates more efficient separation of photogenerated electron-hole pairs.

Effects of Nitrogen/Fluorine Codoping on Photocatalytic Rutile TiO2 Crystal Studied by First-Principles Calculations

Nitrogen/fluorine codoping of rutile TiO₂ was recently reported to be effective for introducing visible-light absorption, and the resultant TiO₂:N,F worked efficiently as an O₂ evolution photocatalyst in a Z-scheme water-splitting system. Although an increase in the amount of nitrogen doped into rutile TiO₂ lattice in the presence of fluorine was experimentally demonstrated, the role of fluorine in the system remained unclear. Here, we report a computational study on TiO₂:N,F through the construction of supercell models with substitutional defects to reveal the atomic arrangement of the material and the electronic band structure. Calculations for all possible structures of nitrogen/fluorine and nitrogen/oxygen-vacancy relative positions revealed that the defect complexes were preferentially located on the (110) plane and that the distance between defects did not have a strong correlation with the formation energy. The present work also showed that although fluorine did not directly contribute to the narrowing of the band gap of TiO₂:N,F, the fluorine activity of the synthetic atmosphere promotes the formation of substitutional defect complexes of nitrogen/fluorine for anion sites. This eventually increases the amount of nitrogen incorporated into the rutile TiO₂ lattice and also results in reduction of the amount of oxygen vacancy, which is in qualitative agreement with our previous result of transient absorption measurement for rutile TiO₂:N,F. The role of fluorine in TiO₂:N,F is thus clarified through our systematic first-principles calculations.

Metal-Complex/Semiconductor Hybrid Photocatalysts and Photoelectrodes for CO2 Reduction Driven by Visible Light

CO₂ reduction to carbon feedstocks using heterogeneous photocatalysts is an attractive means of addressing both climate change and the depletion of fossil fuels. Of particular importance is the development of a photosystem capable of functioning in response to visible light, which accounts for the majority of the solar spectrum, representing a kind of artificial photosynthesis. Hybrid systems comprising a metal complex and a semiconductor are promising because of the excellent electrochemical (and/or photocatalytic) activity of metal complexes during CO₂ reduction and the ability of semiconductors to efficiently oxidize water to molecular O₂ . Here, the development of hybrid photocatalysts and photoelectrodes for CO₂ reduction in combination with water oxidation is described.

Mechanisms of catalytic reduction of CO2 with heme and nonheme metal complexes

The catalytic conversion of CO2 into valuable chemicals and fuels has attracted increasing attention, providing a promising route for mitigating the greenhouse effect of CO2 and also meeting the global energy demand. Among many homogeneous and heterogeneous catalysts for CO2 reduction, this mini-review is focused on heme and nonheme metal complexes that act as effective catalysts for the electrocatalytic and photocatalytic reduction of CO2. Because metalloporphyrinoids show strong absorption in the visible region, which is sensitive to the oxidation states of the metals and ligands, they are suited for the detection of reactive intermediates in the catalytic CO2 reduction cycle by electronic absorption spectroscopy. The first part of this review deals with the catalytic mechanism for the one-electron reduction of CO2 to oxalic acid with heme and nonheme metal complexes, with an emphasis on how the formation of highly energetic CO2˙ is avoided. Then, the catalytic mechanism of two-electron reduction of CO2 to produce CO and H2O is compared with that to produce HCOOH. The effect of metals and ligands of the heme and nonheme complexes on the CO or HCOOH product selectivity is also discussed. The catalytic mechanisms of multi-electron reduction of CO2 to methanol (six-electron reduced product) and methane (eight-electron reduced product) are also discussed for both electrocatalytic and photocatalytic systems.