Adsorption Properties of Hydrogen and Carbon Dioxide in Prussian Blue Analogues M₃[Co(CN)₆]₂, M = Co, Zn

H2 and CO2 adsorption were studied in dehydrated Prussian blue analogues M3[Co(CN)6]2 (M = Co, Zn) using volumetric isotherm measurements. Both materials adsorbed 1.2−1.3 wt % of H2 at 77 K and 760 Torr with isosteric heats of adsorption ranging from 5.9 to 6.8 kJ/mol. High-pressure H2 isotherms at 77 K showed that Co3[Co(CN)6]2 started to saturate well above 6 atm with a saturation coverage of ∼1.9 wt %. These materials adsorbed approximately 17.6−19.7 wt % of CO2 at 253 K and 760 Torr with isosteric heats of adsorption of ∼25−28 kJ/mol. The CO2 saturation coverages from high-pressure isotherms at 263 K and 15 atm were ∼27.4−29.0 wt %. The displacement of CO2 by H2 in these compounds was investigated with Fourier transform infrared spectroscopy (FTIR). The FTIR experiments showed that CO2 physisorption at cryogenic temperatures produced an infrared peak at 2335 cm-1. Co-adsorption experiments revealed that H2 was able to displace preadsorbed CO2 if the PH2/PCO2 ratio was well above 100. The infrared results from the co-adsorption experiments also showed that H2 and CO2 competed for adsorption in the same pores under these conditions

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

FT-IR Study of CO₂ Adsorption in a Dynamic Copper(II) Benzoate−Pyrazine Host with CO₂−CO₂ Interactions in the Adsorbed State

A detailed correlation is presented between the in situ Fourier transform-infrared (FT-IR) spectra of adsorbed CO2, CuBzPyz host bands, and CO2 adsorption sites using previously reported crystal structures of CO2-loaded CuBzPyz and CO2 adsorption isotherms. Through the analysis of both in situ attenuated total reflectance FT-IR spectra taken at several points on the high pressure isotherm and in situ transmission FT-IR spectra acquired at low pressures and cryogenic temperatures, we provide additional insight into the pore-filling mechanism of CO2 on the structurally dynamic CuBzPyz host. The FT-IR spectrum of adsorbed CO2 shows distinct ν2 and ν3 spectral features that can be attributed to known CO2 adsorption sites observed in the reported crystal structure of the CO2-saturated phase of CuBzPyz. The availability of detailed high quality CO2-loaded structural data for CuBzPyz makes this system a case study for associating infrared spectral features with CO2 adsorption sites and should prove valuable for future interpretations of CO2 host−guest and guest−guest interactions when X-ray quality structural data is unavailable

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

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

Mechanism for the Dynamic Adsorption of CO₂ and CH₄ in a Flexible Linear Chain Coordination Polymer as Determined from In Situ Infrared Spectroscopy

Adsorption−desorption cycles for CO2 and CH4 on the one-dimensional coordination polymer, catena-bis(dibenzoylmethanato)-(4,4′-bipyridyl)nickel(II), “Ni-DBM-BPY”, showed pronounced step-shape isotherms, where minimal gas adsorption was detected below a threshold pressure and rapid gas uptake was observed above this threshold. Desorption isotherms from the saturated state displayed significant hysteresis from the adsorption isotherm path. Such behavior is rare in one-dimensional coordination polymers that lack a robust framework with permanent porosity. This step-shape adsorption behavior for CO2 was shown by in situ FT-IR measurements to be the result of a structural phase transition in the Ni-DBM-BPY host which arises from a change in conformation of the DBM ligands. After the structural transition, the adsorption spectrum of the adsorbed CO2 changed significantly due to an enhanced CO2 interaction with the host. A similar mechanism can be inferred for CH4 from the isotherm shape, but the host structural phase transitions could not be observed directly with CH4 uptake, since the threshold conditions were outside the temperature and pressure limits of the instrument. These reported results highlight the importance of in situ FT-IR measurements for determining gas adsorption mechanisms in flexible porous coordination polymers

Hydrogen Storage Properties of Metal Nitroprussides M[Fe(CN)₅NO], (M = Co, Ni)

The volumetric hydrogen adsorption isotherms of two isostructural dehydrated cubic metal nitroprussides M[Fe(CN)5NO] (M = Co2+, Ni2+) have been measured up to a pressure of 760 Torr at 77 and 87 K. These materials are among the most efficient H2 sorbents based on porous coordination polymers reported to date. The H2 uptake in both materials is ∼1.6 wt % at 77 K and 760 torr. These H2 capacities match those reported recently in the structurally related M3[Co(CN)6]2 compounds and are approximately 25% higher than those reported for Zn4O(1,4-benzenedicarboxylate)3 under the same conditions of temperature and pressure. The isosteric heats of H2 adsorption calculated from the 77 and 87 K isotherms for both materials were found to vary from ∼7.5 kJ/mol at 0.40 wt % coverage to ∼5.5 kJ/mol at 1.2 wt % coverage. The N2 BET surface areas were determined to be 634 m2/g and 523 m2/g for M = Ni and M = Co, respectively

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

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