Synthesis, Characterization, Electronic Structure, and Photocatalytic Behavior of CuGaO₂ and CuGa_1–<i>x</i>Fe_<i>x</i>O₂ (<i>x</i> = 0.05, 0.10, 0.15, 0.20) Delafossites

The photochemical reduction of CO₂ to chemicals, such as CO and CH₄, is a promising carbon management approach that can generate revenue from chemical sales to help offset the costs associated with the use of carbon-management technologies. Delafossite materials of the general stoichiometry ABO₂ are a new class of photocatalysts being considered for this application. Symmetry breaking in these materials, by chemical substitution, modifies the band structure of the solid, which enhances optical transitions at the fundamental gap and can therefore be used to engineer the photocatalytic performance of delafossites by adjusting the alignment of band edges with chemical redox potentials and enhancing the optical activity associated with the production of photoexcited charge carriers. The photochemical activity of CuGaO₂ and CuGa_1–<i>x</i>Fe_<i>x</i>O₂ (<i>x</i> = 0.05, 0.10, 0.15, 0.20) for the reduction of CO₂ has been studied. Our results show that the CuGaO₂ materials investigated have an optical gap at ∼3.7 eV in agreement with previous literature reports. An optical feature is also observed at ∼2.6 eV, which is not as commonly reported due to a weak absorption cross section. Alloying at the B-site with Fe to form CuGa_1–<i>x</i>Fe_<i>x</i>O₂ (<i>x</i> = 0.05, 0.10, 0.15, 0.20) creates new features in the visible and near-infrared region of the optical spectra for the substituted materials. Electronic density of states calculations indicate that B-site alloying with Fe creates new midgap states caused by O atoms associated with Fe substitution sites; increased Fe concentration contributes to broadening of these midgap states. The strain caused by Fe incorporation breaks the symmetry of the crystal structure giving rise to the new optical transitions noted experimentally. The photoreduction of CO₂ in the presence of H₂O vapor using CuGaO₂ and CuGa_1–<i>x</i>Fe_<i>x</i>O₂ produces CO with little evidence for other products such as H₂ or hydrocarbons. The impact of Fe alloying with Ga on the band structure and photochemical activity of this delafossite system is discussed

Active Sites and Structure–Activity Relationships of Copper-Based Catalysts for Carbon Dioxide Hydrogenation to Methanol

Active sites and structure–activity relationships for methanol synthesis from a stoichiometric mixture of CO₂ and H₂ were investigated for a series of coprecipitated Cu-based catalysts with temperature-programmed reduction (TPR), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and N₂O decomposition. Experiments in a reaction chamber attached to an XPS instrument show that metallic Cu exists on the surface of both reduced and spent catalysts and there is no evidence of monovalent Cu⁺ species. This finding provides reassurance regarding the active oxidation state of Cu in methanol synthesis catalysts because it is observed with 6 compositions possessing different metal oxide additives, Cu particle sizes, and varying degrees of ZnO crystallinity. Smaller Cu particles demonstrate larger turnover frequencies (TOF) for methanol formation, confirming the structure sensitivity of this reaction. No correlation between TOF and lattice strain in Cu crystallites is observed suggesting this structural parameter is not responsible for the activity. Moreover, changes in the observed rates may be ascribed to relative distribution of different Cu facets as more open and low-index surfaces are present on the catalysts containing small Cu particles and amorphous or well-dispersed ZnO. In general, the activity of these systems results from large Cu surface area, high Cu dispersion, and synergistic interactions between Cu and metal oxide support components, illustrating that these are key parameters for developing fundamental mechanistic insight into the performance of Cu-based methanol synthesis catalysts

Selective Electrocatalytic Reduction of CO2 into CO at Small, Thiol-Capped Au/Cu Nanoparticles

The electrochemical CO reduction reaction (CO RR) is a promising approach for converting fossil fuel emissions into environmentally sustainable chemicals and fuels. The ability to control the surface structure of CO RR nanocatalysts provides an opportunity to tune product selectivity. Bimetallic gold-copper catalysts have been identified as emerging electrocatalyst candidates, but Cu incorporation typically lowers product selectivity compared with pure Au. Here we show sustained CO selectivity and activity up to 49% Cu content in small (\u3c2 \u3enm), thiol-capped Au/Cu nanoparticles (NPs). Bimetallic NPs containing 49% Cu selectivity converted CO into CO with 100 ± 6% CO Faradaic efficiency and average mass activity of ∼500 mA/mg during a 12 h electrolysis experiment at -0.8 V vs RHE. Au/Cu NPs synthesized without thiol ligands selectively produced H , whereas larger (\u3e10 nm), thermally dethiolated Au/Cu NPs produced a wider product distribution including H , CO, and C H . Density functional theory (DFT) modeling of CO RR and H evolution at realistic, thiol-capped Au/Cu NP structures indicated that copper-thiol surface structures sustained CO selectivity by stabilizing key∗CO intermediates while making∗H binding less favorable. Calculations also predicted that removing a significant fraction of the thiol ligands would increase∗CO binding strength such that desorption of CO product molecules could become the most thermodynamically challenging step. This result, coupled with increased∗H stability on dethiolated nanoclusters, points to decreased CO RR selectivity for small, ligand-free catalysts, which is in line with experimental observations from our group and others. Our results demonstrate that thiol-ligand surface structures can sustain the CO selectivity of bimetallic Au/Cu NPs and reduce precious metal requirements for CO RR. 2 2 2 2 2 2 2 4 2 2 2

Highly Active and Stable Carbon Nanosheets Supported Iron Oxide for Fischer-Tropsch to Olefins Synthesis

Light olefins production utilizes the energy intensive process of steam cracking. Fischer-Tropsch to olefins (FTO) synthesis potentially offers a more sustainable alternative. Here we show a promising FTO catalyst comprised of iron oxide nanoparticles supported on carbon nanosheets (CNS) fabricated from the carbonization of potassium citrate, which incorporates well dispersed K-promoter throughout the CNS support. This catalyst exhibits, to the best of our knowledge, the highest iron time yield of 1790–1990 μmol /g ⋅ s reported in the literature, 41 % light olefins selectivity, and over 100 hours stable activity, making it one of the best performing FTO catalysts. Detailed characterization illustrates that the CNS support facilitates iron oxide reduction to metallic iron, leading to efficient transformation to the active iron carbide phase during FTO reaction. Since K is a commonly used promoter, our K-promoted CNS support potentially has broad utility beyond the FTO reactions demonstrated in the current study. CO F