Visible Light Photoreduction of CO₂ Using CdSe/Pt/TiO₂ Heterostructured Catalysts

A series of CdSe quantum dot (QD)-sensitized TiO₂ heterostructures have been synthesized, characterized, and tested for the photocatalytic reduction of CO₂ in the presence of H₂O. Our results show that these heterostructured materials are capable of catalyzing the photoreduction of CO₂ using visible light illumination (λ > 420 nm) only. The effect of removing surfactant caps from the CdSe QDs by annealing and using a hydrazine chemical treatment have also been investigated. The photocatalytic reduction process is followed using infrared spectroscopy to probe the gas-phase reactants and gas chromatography to detect the products. Gas chromatographic analysis shows that the primary reaction product is CH₄, with CH₃OH, H₂, and CO observed as secondary products. Typical yields of the gas-phase products after visible light illumination (λ > 420 nm) were 48 ppm g⁻¹ h⁻¹ of CH₄, 3.3 ppm g⁻¹ h⁻¹ of CH₃OH (vapor), and trace amounts of CO and H₂

Selective Electrocatalytic Activity of Ligand Stabilized Copper Oxide Nanoparticles

Ligand stabilization can influence the surface chemistry of Cu oxide nanoparticles (NPs) and provide unique product distributions for electrocatalytic methanol (MeOH) oxidation and CO₂ reduction reactions. Oleic acid (OA) stabilized Cu₂O and CuO NPs promote the MeOH oxidation reaction with 88% and 99.97% selective HCOH formation, respectively. Alternatively, CO₂ is the only reaction product detected for bulk Cu oxides and Cu oxide NPs with no ligands or weakly interacting ligands. We also demonstrate that OA stabilized Cu oxide NPs can reduce CO₂ into CO with a ∼1.7-fold increase in CO/H₂ production ratios compared to bulk Cu oxides. The OA stabilized Cu oxide NPs also show 7.6 and 9.1-fold increases in CO/H₂ production ratios compared to weakly stabilized and nonstabilized Cu oxide NPs, respectively. Our data illustrates that the presence and type of surface ligand can substantially influence the catalytic product selectivity of Cu oxide NPs

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

A Quantum Alloy: The Ligand-Protected Au_{25–<i>x</i>}Ag_<i>x</i>(SR)₁₈ Cluster

Recent synthetic advances have produced very small (sub-2 nm), ligand-protected mixed-metal clusters. Realization of such clusters allows the investigation of fundamental questions: (1) Will heteroatoms occupy specific sites within the cluster? (2) How will the inclusion of heteroatoms affect the electronic structure and chemical properties of the cluster? (3) How will these very small mixed-metal systems differ from larger, more traditional alloy materials? In this report we provide experimental and computational characterization of the ligand-protected mixed-metal Au_{25–<i>x</i>}Ag_<i>x</i>(SC₂H₄Ph)₁₈ cluster (abbreviated as Au_{25–<i>x</i>}Ag_<i>x</i>, where <i>x</i> = 0–5 Ag atoms) compared with the unsubstituted Au₂₅(SC₂H₄Ph)₁₈ cluster (abbreviated as Au₂₅). Density functional theory analysis has predicted that Ag heteroatoms will preferentially occupy sites on the surface of the cluster core. X-ray photoelectron spectroscopy revealed Au–Ag state mixing and charge redistribution within the Au_{25–<i>x</i>}Ag_<i>x</i> cluster. Optical spectroscopy and nonaqueous electrochemistry indicate that Ag heteroatoms increased the cluster lowest unoccupied molecular orbital (LUMO) energy, introduced new features in the Au_{25–<i>x</i>}Ag_<i>x</i> absorbance spectrum, and rendered some optical transitions forbidden. In situ spectroelectrochemical experiments revealed charge-dependent Au_{25–<i>x</i>}Ag_<i>x</i> optical properties and oxidative photoluminescence quenching. Finally, O₂ adsorption studies have shown Au_{25–<i>x</i>}Ag_<i>x</i> clusters can participate in photomediated charge-transfer events. These results illustrate that traditional alloy concepts like metal-centered state mixing and internal charge redistribution also occur in very small mixed-metal clusters. However, resolution of specific heteroatom locations and their impact on the cluster’s quantized electronic structure will require a combination of computational modeling, optical spectroscopy, and nonaqueous electrochemistry

Experimental and Computational Investigation of Au₂₅ Clusters and CO₂: A Unique Interaction and Enhanced Electrocatalytic Activity

Atomically precise, inherently charged Au₂₅ clusters are an exciting prospect for promoting catalytically challenging reactions, and we have studied the interaction between CO₂ and Au₂₅. Experimental results indicate a reversible Au₂₅–CO₂ interaction that produced spectroscopic and electrochemical changes similar to those seen with cluster oxidation. Density functional theory (DFT) modeling indicates these changes stem from a CO₂-induced redistribution of charge within the cluster. Identification of this spontaneous coupling led to the application of Au₂₅ as a catalyst for the electrochemical reduction of CO₂ in aqueous media. Au₂₅ promoted the CO₂ → CO reaction within 90 mV of the formal potential (thermodynamic limit), representing an approximate 200–300 mV improvement over larger Au nanoparticles and bulk Au. Peak CO₂ conversion occurred at −1 V (vs RHE) with approximately 100% efficiency and a rate 7–700 times higher than that for larger Au catalysts and 10–100 times higher than those for current state-of-the-art processes

Photomediated Oxidation of Atomically Precise Au₂₅(SC₂H₄Ph)₁₈^– Nanoclusters

The anionic charge of atomically precise Au₂₅(SC₂H₄Ph)₁₈^– nanoclusters (abbreviated as Au₂₅^–) is thought to facilitate the adsorption and activation of molecular species. We used optical spectroscopy, nonaqueous electrochemistry, and density functional theory to study the interaction between Au₂₅^– and O₂. Surprisingly, the oxidation of Au₂₅^– by O₂ was not a spontaneous process. Rather, Au₂₅^––O₂ charge transfer was found to be a photomediated process dependent on the relative energies of the Au₂₅^– LUMO and the O₂ electron-accepting level. Photomediated charge transfer was not restricted to one particular electron accepting molecule or solvent system, and this phenomenon likely extends to other Au₂₅^––adsorbate systems with appropriate electron donor–acceptor energy levels. These findings underscore the significant and sometimes overlooked way that photophysical processes can influence the chemistry of ligand-protected clusters. In a broader sense, the identification of photochemical pathways may help develop new cluster-adsorbate models and expand the range of catalytic reactions available to these materials

Screening Hofmann Compounds as CO₂ Sorbents: Nontraditional Synthetic Route to Over 40 Different Pore-Functionalized and Flexible Pillared Cyanonickelates

A simple reaction scheme based on the heterogeneous intercalation of pillaring ligands (HIPLs) provides a convenient method for systematically tuning pore size, pore functionality, and network flexibility in an extended series of pillared cyanonickelates (PICNICs), commonly referred to as Hofmann compounds. The versatility of the approach is demonstrated through the preparation of over 40 different PICNICs containing pillar ligands ranging from ∼4 to ∼15 Å in length and modified with a wide range of functional groups, including fluoro, aldehyde, alkylamine, alkyl, aryl, trifluoromethyl, ester, nitro, ether, and nonmetalated 4,4′-bipyrimidine. The HIPL method involves reaction of a suspension of preformed polymeric sheets of powdered anhydrous nickel cyanide with an appropriate pillar ligand in refluxing organic solvent, resulting in the conversion of the planar [Ni₂(CN)₄]_<i>n</i> networks into polycrystalline three-dimensional porous frameworks containing the organic pillar ligand. Preliminary investigations indicate that the HIPL reaction is also amenable to forming Co­(L)­Ni­(CN)₄, Fe­(L)­Ni­(CN)₄, and Fe­(L)­Pd­(CN)₄ networks. The materials show variable adsorption behavior for CO₂ depending on the pillar length and pillar functionalization. Several compounds show structurally flexible behavior during the adsorption and desorption of CO₂. Interestingly, the newly discovered flexible compounds include two flexible Fe­(L)­Ni­(CN)₄ derivatives that are structurally related to previously reported porous spin-crossover compounds. The preparations of 20 pillar ligands based on ring-functionalized 4,4′-dipyridyls, 1,4-bis­(4-pyridyl)­benzenes, and <i>N</i>-(4-pyridyl)­isonicotinamides are also described

Inverting Transient Absorption Data to Determine Transfer Rates in Quantum Dot–TiO₂ Heterostructures

Transient absorption spectroscopy is a powerful technique for understanding charge carrier dynamics and recombination pathways. Analyzing the results is not trivial due to nonexponential relaxation dynamics away from equilibrium, leading to a disparity in reported charge-transfer rates. An inversion analysis technique is presented that transforms transient signals back into their original rate equation. The technique is demonstrated on two CdSe/TiO₂ heterostructures with different surface states. Auger recombination is identified at higher carrier densities and radiative recombination at lower carrier densities. The heterostructure with additional surface traps exhibits increased trap-state Auger recombination at high carrier densities and changes to radiative recombination at low carrier densities due to a Shockley–Read–Hall process. Carrier-dependent electron-transfer rates are determined and compared to common methods that only capture the magnitude of the charge transfer at specific carrier densities. The presented transient absorption analysis provides direct understanding of the recombination mechanisms with minimal additional analysis or with presumption of decay mechanisms

Growth of Single- and Bilayer ZnO on Au(111) and Interaction with Copper

The stoichiometric single- and bilayer ZnO(0001) have been prepared by reactive deposition of Zn on Au(111) and studied in detail with X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory calculations. Both single- and bilayer ZnO(0001) adopt a planar, graphite-like structure similar to freestanding ZnO(0001) due to the weak van der Waals interactions dominating their adhesion with the Au(111) substrate. At higher temperature, the single-layer ZnO(0001) converts gradually to bilayer ZnO(0001) due to the twice stronger interaction between two ZnO layers than the interfacial adhesion of ZnO with Au substrate. It is found that Cu atoms on the surface of bilayer ZnO(0001) are mobile with a diffusion barrier of 0.31 eV and likely to agglomerate and form nanosized particles at low coverages; while Cu atoms tend to penetrate a single layer of ZnO(0001) with a barrier of 0.10 eV, resulting in a Cu free surface

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