Efficiency of Solar-Light-Driven TiO₂ Photocatalysis at Different Latitudes and Seasons. Where and When Does TiO₂ Really Work?

Efficiency of Solar-Light-Driven TiO₂ Photocatalysis at Different Latitudes and Seasons. Where and When Does TiO₂ Really Work

Understanding the Coordination Modes of [Cu(acac)₂(imidazole)_<i>n</i>=1,2] Adducts by EPR, ENDOR, HYSCORE, and DFT Analysis

The interaction of imidazole with a [Cu­(acac)₂] complex was studied using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), hyperfine sublevel correlation spectroscopy (HYSCORE), and density functional theory (DFT). At low Im ratios (Cu:Im 1:10), a 5-coordinate [Cu­(acac)₂Im_<i>n</i>=1] monoadduct is formed in frozen solution with the spin Hamiltonian parameters <i>g</i>₁ = 2.063, <i>g</i>₂ = 2.063, <i>g</i>₃ = 2.307, <i>A</i>₁ = 26, <i>A</i>₂ = 15, and <i>A</i>₃ = 472 MHz with Im coordinating along the axial direction. At higher Im concentrations (Cu:Im 1:50), a 6-coordinate [Cu­(acac)₂Im_<i>n</i>=2] bis-adduct is formed with the spin Hamiltonian parameters <i>g</i>₁ = 2.059, <i>g</i>₂ = 2.059, <i>g</i>₃ = 2.288, <i>A</i>₁ = 30, <i>A</i>₂ = 30, and <i>A</i>₃ = 498 MHz with a poorly resolved ¹⁴N superhyperfine pattern. The isotropic EPR spectra revealed a distribution of species ([Cu­(acac)₂], [Cu­(acac)₂Im_<i>n</i>=1], and [Cu­(acac)₂Im_<i>n</i>=2]) at Cu:Im ratios of 1:0, 1:10, and 1:50. The superhyperfine pattern originates from two strongly coordinating N³ imino nitrogens of the Im ring. Angular selective ¹⁴N ENDOR analysis revealed the ^N<i>A</i> tensor of [34.8, 43.5, 34.0] MHz, with e²<i>qQ</i>/<i>h</i> = 2.2 MHz and η = 0.2 for N³. The hyperfine and quadrupole values for the remote N¹ amine nitrogens (from HYSCORE) were found to be [1.5, 1.4, 2.5] MHz with e²<i>qQ</i>/<i>h</i> = 1.4 MHz and η = 0.9. ¹H ENDOR also revealed three sets of ^H<i>A</i> tensors corresponding to the nearly equivalent H²/H⁴ protons in addition to the H⁵ and H¹ protons of the Im ring. The spin Hamiltonian parameters for the geometry optimized structures of [Cu­(acac)₂Im_<i>n</i>=2], including <i>cis</i>-mixed plane, <i>trans</i>-axial, and <i>trans</i>-equatorial, were calculated. The best agreement between theory and experiment indicated the preferred coordination is <i>trans</i>-equatorial [Cu­(acac)₂Im_<i>n</i>=2]. A number of other Im derivatives were also investigated. 4(5)-methyl-imidazole forms a [Cu­(acac)₂(Im-<b>3</b>)_<i>n</i>=2] <i>trans</i>-equatorial adduct, whereas the bulkier 2-methyl-imidazole (Im-<b>2</b>) and benzimidazole (Im-<b>4</b>) form the [Cu­(acac)₂(Im-<b>2,4</b>)_<i>n</i>=1] monoadduct only. Our data therefore show that subtle changes in the substrate structure lead to controllable changes in coordination behavior, which could in turn lead to rational design of complexes for use in catalysis, imaging, and medicine

Improving the Selectivity of Photocatalytic NO<i>_x</i> Abatement through Improved O₂ Reduction Pathways Using Ti_0.909W_0.091O₂N_<i>x</i> Semiconductor Nanoparticles: From Characterization to Photocatalytic Performance

In this paper, we provide detailed insight into the electronic–crystallographic–structural relationship for Ti_0.909W_0.091O₂N_<i>x</i> semiconductor nanoparticles, explaining the mutual electronic and magnetic influence of the photoinduced stable N- and W-based paramagnetic centers, their involvement in the photoinduced charge-carrier trapping, and their role in improving the nitrate selectivity of the photocatalytic oxidation of NO_<i>x</i> to nitrates. In particular, reduced tungsten species in various crystallographic environments within the anatase host lattice were observed as playing a fundamental role in the storage and stabilization of photogenerated electrons. Here, we show how these reduced centers can catalyze multielectron transfer events without the need for rare and expensive platinum-group metals (PGMs). This allows for the versatile and elegant configuration of redox potentials. As a result, electron-transfer processes that are kinetically inaccessible with metal oxides such as TiO₂ can now be accessed, enabling dramatic improvements in reaction selectivity. The photocatalytic abatement of NO_<i>x</i> toward nontoxic products is exemplified here and is shown to pivot on multiple routes for molecular oxygen reduction. The same rationale can furthermore be applied to other photocatalytic processes. The observations described in this work could open new and exciting avenues in semiconductor photocatalysis for environmental remediation technologies in which the optimization of molecular oxygen reduction, together with the pollutant species to be oxidized, becomes a central element of the catalyst design without relying on the use of rare and expensive PGMs