Structure-catalytic function relationship of SiO2-immobilized mononuclear Cu complexes: an EPR study

Quantitative In Situ Monitoring of Cu-Atom Release by Cu2O Nanocatalysts under Photocatalytic CO2 Reduction Conditions: New Insights into the Photocorrosion Mechanism

Cu₂O is among the most promising photocatalysts for CO₂ reduction, however its photocorrosion remains a standalone challenge. Herein, we present an in situ study of the release of Cu ions from Cu₂O nanocatalysts under photocatalytic conditions in the presence of HCO₃ as a catalytic substrate in H₂O. The Cu-oxide nanomaterials were produced by Flame Spray Pyrolysis (FSP) technology. Using Electron Paramagnetic Resonance (EPR) spectroscopy in tandem with analytical Anodic Stripping Voltammetry (ASV), we monitored in situ the Cu²⁺ atom release from the Cu₂O nanoparticles in comparison with CuO nanoparticles under photocatalytic conditions. Our quantitative, kinetic data show that light has detrimental effect on the photocorrosion of Cu₂O and ensuing Cu²⁺ ion release in the H₂O solution, up to 15.7% of its mass. EPR reveals that HCO₃ acts as a ligand of the Cu²⁺ ions, promoting the liberation of {HCO₃-Cu} complexes in solution from Cu₂O, up to 27% of its mass. HCO₃ alone exerted a marginal effect. XRD data show that under prolonged irradiation, part of Cu²⁺ ions can reprecipitate on the Cu₂O surface, creating a passivating CuO layer that stabilizes the Cu₂O from further photocorrosion. Including isopropanol as a hole scavenger has a drastic effect on the photocorrosion of Cu₂O nanoparticles and suppresses the release of Cu²⁺ ions to the solution. Methodwise, the present data exemplify that EPR and ASV can be useful tools to help quantitatively understand the solid-solution interface photocorrosion phenomena for Cu₂O.

Decoupling the Chemical and Mechanical Strain Effect on Steering the CO2 Activation over CeO2-Based Oxides: An Experimental and DFT Approach

Doped ceria-based metal oxides are widely used as supports and stand-alone catalysts in reactions where CO2 is involved. Thus, it is important to understand how to tailor their CO2 adsorption behavior. In this work, steering the CO2 activation behavior of Ce-La-Cu-O ternary oxide surfaces through the combined effect of chemical and mechanical strain was thoroughly examined using both experimental and ab initio modeling approaches. Doping with aliovalent metal cations (La3+ or La3+/Cu2+) and post-synthetic ball milling were considered as the origin of the chemical and mechanical strain of CeO2, respectively. Experimentally, microwave-assisted reflux-prepared Ce-La-Cu-O ternary oxides were imposed into mechanical forces to tune the structure, redox ability, defects, and CO2 surface adsorption properties; the latter were used as key descriptors. The purpose was to decouple the combined effect of the chemical strain (εC) and mechanical strain (εM) on the modification of the Ce-La-Cu-O surface reactivity toward CO2 activation. During the ab initio calculations, the stability (energy of formation, EOvf) of different configurations of oxygen vacant sites (Ov) was assessed under biaxial tensile strain (ε > 0) and compressive strain (ε < 0), whereas the CO2-philicity of the surface was assessed at different levels of the imposed mechanical strain. The EOvf values were found to decrease with increasing tensile strain. The Ce-La-Cu-O(111) surface exhibited the lowest EOvf values for the single subsurface sites, implying that Ov may occur spontaneously upon Cu addition. The mobility of the surface and bulk oxygen anions in the lattice contributing to the Ov population was measured using 16O/18O transient isothermal isotopic exchange experiments; the maximum in the dynamic rate of 16O18O formation, Rmax(16O18O), was 13.1 and 8.5 μmol g-1 s-1 for pristine (chemically strained) and dry ball-milled (chemically and mechanically strained) oxides, respectively. The CO2 activation pathway (redox vs associative) was experimentally probed using in situ diffuse reflectance infrared Fourier transform spectroscopy. It was demonstrated that the mechanical strain increased up to 6 times the CO2 adsorption sites, though reducing their thermal stability. This result supports the mechanical actuation of the "carbonate"-bound species; the latter was in agreement with the density functional theory (DFT)-calculated C-O bond lengths and O-C-O angles. Ab initio studies shed light on the CO2 adsorption energy (Eads), suggesting a covalent bonding which is enhanced in the presence of doping and under tensile strain. Bader charge analysis probed the adsorbate/surface charge distribution and illustrated that CO2 interacts with the dual sites (acidic and basic ones) on the surface, leading to the formation of bidentate carbonate species. Density of states (DOS) studies revealed a significant Eg drop in the presence of double Ov and compressive strain, a finding with design implications in covalent type of interactions. To bridge this study with industrially important catalytic applications, Ni-supported catalysts were prepared using pristine and ball-milled oxides and evaluated for the dry reforming of methane reaction. Ball milling was found to induce modification of the metal-support interface and Ni catalyst reducibility, thus leading to an increase in the CH4 and CO2 conversions. This study opens new possibilities to manipulate the CO2 activation for a portfolio of heterogeneous reactions.

Model-Based Nanoengineered Pharmacokinetics of Iron-Doped Copper Oxide for Nanomedical Applications

The progress in nanomedicine (NM) using nanoparticles (NPs) is mainly based on drug carriers for the delivery of classical chemotherapeutics. As low NM delivery rates limit therapeutic efficacy, an entirely different approach was investigated. A homologous series of engineered CuO NPs was designed for dual purposes (carrier and drug) with a direct chemical composition-biological functionality relationship. Model-based dissolution kinetics of CuO NPs in the cellular interior at post-exposure conditions were controlled through Fe-doping for intra/extra cellular Cu2+ and biological outcome. Through controlled ion release and reactions taking place in the cellular interior, tumors could be treated selectively, in vitro and in vivo. Locally administered NPs enabled tumor cells apoptosis and stimulated systemic anti-cancer immune responses. We clearly show therapeutic effects without tumor cells relapse post-treatment with 6 % Fe-doped CuO NPs combined with myeloid-derived suppressor cell silencing.

Tuning the Catalytic Properties of Copper-Promoted Nanoceria via a Hydrothermal Method

Copper-cerium mixed oxide catalysts have gained ground over the years in the field of heterogeneous catalysis and especially in CO oxidation reaction due to their remarkable performance. In this study, a series of highly active, atomically dispersed copper-ceria nanocatalysts were synthesized via appropriate tuning of a novel hydrothermal method. Various physicochemical techniques including electron paramagnetic resonance (EPR) spectroscopy, X-ray diffraction (XRD), N2 adsorption, scanning electron microscopy (SEM), Raman spectroscopy, and ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) were employed in the characterization of the synthesized materials, while all the catalysts were evaluated in the CO oxidation reaction. Moreover, discussion of the employed mechanism during hydrothermal route was provided. The observed catalytic activity in CO oxidation reaction was strongly dependent on the nanostructured morphology, oxygen vacancy concentration, and nature of atomically dispersed Cu2+ clusters.

Syntheses and characterization of monobasic tridentate Cu(ii) Schiff-base complexes for efficient oxidation of 3,5-di-tert-butylcatechol and oxidative bromination of organic substrates

Enhanced bifunctional catalytic activities are shown by monobasic tridentate Cu(ii) Schiff-base complexes.

Advances in click chemistry for silica-based material construction

Click chemistry is undoubtedly the most powerful 1,3-dipolar cycloaddition reaction in organic synthesis.

A functionalized phosphonate-rich organosilica layered hybrid material (PSLM) fabricated through a mild process for heavy metal uptake

A phosphonate-rich organosilica layered hybrid material (PSLM) made of 3-(trihydroxysilyl)propyl methylphosphonate, monosodium salt, as the single silica source, has been obtained from its aqueous solution through a xerogel process and mild thermal aging. The method is simple, affording bulk quantities of powdered PSLM in a single-step. The hybrid is stable in water and possesses a high content of phosphonate groups fixed on the solid matrix. In addition, PSLM shows good thermal stability, which exceeds 300°C in air. The material was characterized using SEM, TEM, XRD, FT-IR and TGA techniques. Potentiometric titrations show that PSLM bears high-surface density of phosphonate groups (3 mmol g(-1)). As a result, the material displays high metal uptake capacity for heavy metal ions such as Cu(2+) (2.72mmolg(-1)), Pb(2+) (1.67 mmol g(-1)) and Cd(2+) (1.00 mmol g(-1)) at neutral pH values e.g. the pH of natural waters. Detailed theoretical modeling using a Surface Complexation Model combined with Electron Paramagnetic Resonance (EPR) spectroscopy shows that the surface distribution of surface bound Cu(2+) ions is rather homogeneous e.g. copper-binding phosphonate sites are arranged in average distances 5-8Å.

Influence of anionic co-ligands on the structural diversity and catecholase activity of copper(ii) complexes with 2-methoxy-6-(8-iminoquinolinylmethyl)phenol

Pseudohalides induced structural diversity and catecholase activity of copper(ii) complexes are reported. The NCO-bound compound is the first example of a mononuclear square planar complex exhibiting catecholase activity.

Discrete complexes immobilized onto click-SBA-15 silica: controllable loadings and the impact of surface coverage on catalysis

Azidopropyl functionalized mesoporous silica SBA-15 were prepared with variable azide loadings of 0.03-0.7 mmol g(-1) (~2-50% of maximal surface coverage) through a direct synthesis, co-condensation approach. These materials are functionalized selectively with ethynylated organic moieties through a copper-catalyzed azide alkyne cycloaddition (CuAAC) or "click" reaction. Specific loading within a material can be regulated by either the azide loading or limiting the alkyne reagent relative to the azide loading. The immobilization of ferrocene, pyrene, tris(pyridylmethyl)amine (TPA), and iron porphyrin (FeTPP) demonstrates the robust nature and reproducibility of this two-step synthetic attachment strategy. Loading-sensitive pyrene fluorescence correlates with a theoretically random surface distribution, rather than a uniform one; site-isolation of tethered moieties ~15 Å in length occurs at loadings less than 0.02 mmol g(-1). The effect of surface loading on reactivity is observed in the oxygenation of SBA-15-[Cu(I)(TPA)]. SBA-15-[Mn(II)(TPA)]-catalyzed epoxidation exhibits a systematic dependence on surface loading. A comparison of homogeneous, site-isolated and site-dense complexes provides insight into catalyst speciation and ligand activity.

The Catalytic Function of Nonheme Iron (III) Complex for Hydrocarbon Oxidation

A detailed catalytic study of LFe(III)Cl (where L = 3-{2-[2-(3-hydroxy-1,3-diphenyl-allylideneamino)-ethylamino]-ethylimino}-1,3-diphenyl-propen-1-ol) for hydrocarbon oxidation was carried out, focusing on the role of solvent, atmospheric dioxygen, and oxidant on catalytic efficiency. The data showed that LFe(III)Cl catalyst was efficient in homogeneous hydrocarbon oxidations providing significant yields. Moreover, tert-BuOOH provided comparable oxidation yields with H(2)O(2), slightly favoring the formation of alcohols and ketones versus epoxides. Dioxygen intervened in the catalytic reaction, influencing the nature of oxidation products. The polarity of solvent strongly influenced the reaction rates and the nature of oxidation products. A mechanistic model is postulated assuming that LFe(III)Cl functions via the formation of iron-hydroperoxo-species, followed by a radical-based mechanistic path.

Mechanism of heavy metal uptake by a hybrid MCM-41 material: surface complexation and EPR spectroscopic study

A novel hybrid MCM-41-based material was synthesized by incorporation of AEDTC [N-(2-aminoethyl)dithiocarbamate] in the MCM-41 pores. The derived MCM-41 x AEDTC material possesses high AEDTC loading 35% [w:w], and a well-defined array of regular mesopores with a specific surface area of 632 m(2)/g. Heavy metal, Cd, Pb, Cu, and Zn, uptake was studied in detail at physiological pH values 6-8, by a combination of analytical and electron paramagnetic resonance (EPR) spectroscopic techniques. The analytical data show a significant improvement, i.e., 200-500%, for Pb, Cu, and Zn uptake by the MCM-41 x AEDTC hybrid vs the unmodified MCM-41. In contrast, Cd shows an exceptional behavior: (a) Cd uptake by MCM-41 x AEDTC is very low. (b) Competitive metal uptake experiments reveal that Cd ions cause a characteristic inhibition of Cu or Pb uptake by the MCM-41 x AEDTC while Cd binding itself always remained low. The present findings are analyzed by a combination of surface complexation modeling and EPR spectroscopy. Accordingly, in the MCM-41 x AEDTC the sulfur atoms of AEDTC provide strong binding sites for metal binding, with a stoichiometry [S(AEDTC)]:[Metal] = 1:1. Cd inhibits accessibility of Cu or Pb ions in the AEDTC sites.

Organically nanoporous silica gel based on carbon paste electrode for potentiometric detection of trace Cr(III)

A new ion-selective electrode (ISE) for the detection of trace chromium(III) was designed by using 2-acetylpyridine and nanoporous silica gel (APNSG)-functionalized carbon paste electrode (CPE). The presence of APNSG acted as not only a paste binder, but also a reactive material. With 7.5 wt% APNSG proportions, the developed electrode exhibited wide dynamic range of 1.0 x 10(-8) to 1.0 x 10(-3) M toward Cr(III) with a detection limit of 8.0 x 10(-9) M and a Nernstian slope of 19.8 +/- 0.2 mV decade(-1). The as-prepared electrode displayed rapid response (approximately 55 s), long-time stability, and high sensitivity. Moreover, the potentiometric responses could be carried out with wide pH range of 1.5-5.0. In addition, the content of Cr(III) in food samples, e.g. coffee and tea leaves, has been assayed by the developed electrode, atomic absorption spectrophotometer (AAS) and atomic emission spectrometer (ICP-AES), respectively, and consistent results were obtained. Importantly, the response mechanism of the proposed electrode was investigated by using AC impedance and UV-vis spectroscopy.

Controlled loadings in a mesoporous material: click-on silica

Hybrid mesoporous SBA-15 silicas were synthesized directly with variable alkylazide loading representing 2-50% surface coverage. These hybrid silica materials retain the favorable physical attributes of the parent SBA-15 materials and allow efficient covalent attachment of ethynylated organic moieties through a copper catalyzed 3 + 2 Huisgen cycloaddition reaction. Three distinctly different examples are provided demonstrating the efficiency and robust nature of this attachment synthetic strategy. The direct syntheses provide predefined loadings of randomly distributed organics within the materials, from site-dense to site-isolated. Such control over loadings along with simply implemented analytic procedures should facilitate the translation of homogeneous chemistries to heterogeneous supports.