Optimizing Active Sites for High CO Selectivity during CO2 Hydrogenation over Supported Nickel Catalysts.

Controlling the selectivity of CO2 hydrogenation catalysts is a fundamental challenge. In this study, the selectivity of supported Ni catalysts prepared by the traditional impregnation method was found to change after a first CO2 hydrogenation reaction cycle from 100 to 800 °C. The usually high CH4 formation was suppressed leading to full selectivity toward CO. This behavior was also observed after the catalyst was treated under methane or propane atmospheres at elevated temperatures. In situ spectroscopic studies revealed that the accumulation of carbon species on the catalyst surface at high temperatures leads to a nickel carbide-like phase. The catalyst regains its high selectivity to CH4 production after carbon depletion from the surface of the Ni particles by oxidation. However, the selectivity readily shifts back toward CO formation after exposing the catalysts to a new temperature-programmed CO2 hydrogenation cycle. The fraction of weakly adsorbed CO species increases on the carbide-like surface when compared to a clean nickel surface, explaining the higher selectivity to CO. This easy protocol of changing the surface of a common Ni catalyst to gain selectivity represents an important step for the commercial use of CO2 hydrogenation to CO processes toward high-added-value products.

Controlling Reaction Selectivity over Hybrid Plasmonic Nanocatalysts.

The localized surface plasmon resonance (LSPR) excitation in plasmonic nanoparticles has been used to accelerate several catalytic transformations under visible-light irradiation. In order to fully harness the potential of plasmonic catalysis, multimetallic nanoparticles containing a plasmonic and a catalytic component, where LSPR-excited energetic charge carriers and the intrinsic catalytic active sites work synergistically, have raised increased attention. Despite several exciting studies observing rate enhancements, controlling reaction selectivity remains very challenging. Here, by employing multimetallic nanoparticles combining Au, Ag, and Pt in an Au@Ag@Pt core-shell and an Au@AgPt nanorattle architectures, we demonstrate that reaction selectivity of a sequential reaction can be controlled under visible light illumination. The control of the reaction selectivity in plasmonic catalysis was demonstrated for the hydrogenation of phenylacetylene as a model transformation. We have found that the localized interaction between the triple bond in phenylacetylene and the Pt nanoparticle surface enables selective hydrogenation of the triple bond (relative to the double bond in styrene) under visible light illumination. Atomistic calculations show that the enhanced selectivity toward the partial hydrogenation product is driven by distinct adsorption configurations and charge delocalization of the reactant and the reaction intermediate at the catalyst surface. We believe these results will contribute to the use of plasmonic catalysis to drive and control a wealth of selective molecular transformations under ecofriendly conditions and visible light illumination.

Reaction Pathway Dependence in Plasmonic Catalysis: Hydrogenation as a Model Molecular Transformation.

The localized surface plasmon resonance (LSPR) excitation in plasmonic nanoparticles can enhance or mediate chemical transformations. Increased reaction rates for several reactions have been reported due to this phenomenon; however, the fundamental understanding of mechanisms and factors that affect activities remains limited. Here, by investigating hydrogenation reactions as a model transformation and employing different reducing agents, H2 and NaBH4 , which led to different hydrogenation reaction pathways, we observed that plasmonic excitation of Au nanoparticle catalysts can lead to negative effects over the activities. The underlying physical reason was explored using density functional theory calculations. We observed that positive versus negative effects on the plasmonic catalytic activity is reaction-pathway dependent. These results shed important insights on our current understanding of plasmonic catalysis, demonstrating reaction pathways must be taken into account for the design of plasmonic nanocatalysts.

Bioinspired-Metalloporphyrin Magnetic Nanocomposite as a Reusable Catalyst for Synthesis of Diastereomeric (-)-Isopulegol Epoxide: Anticancer Activity Against Human Osteosarcoma Cells (MG-63).

In the present study, we developed a green epoxidation approach for the synthesis of the diastereomers of (-)-isopulegol benzyl ether epoxide using molecular oxygen as the oxidant and a hybrid manganese(III)-porphyrin magnetic reusable nanocomposite as the catalyst. High activity, selectivity, and stability were obtained, with up to four recycling cycles without the loss of activity and selectivity for epoxide. The anticancer effect of the newly synthesized isopulegol epoxide diastereomers was evaluated on a human osteosarcoma cell line (MG-63); both diastereomers showed similar in vitro potency. The measured IC50 values were significantly lower than those reported for other monoterpene analogues, rendering these epoxide isomers as promising anti-tumor agents against low prognosis osteosarcoma.

Economically attractive route for the preparation of high quality magnetic nanoparticles by the thermal decomposition of iron(III) acetylacetonate.

The thermal decomposition (TD) methods are among the most successful in obtaining magnetic nanoparticles with a high degree of control of size and narrow particle size distribution. Here we investigated the TD of iron(III) acetylacetonate in the presence of oleic acid, oleylamine, and a series of alcohols in order to disclose their role and also investigate economically attractive alternatives for the synthesis of iron oxide nanoparticles without compromising their size and shape control. We have found that some affordable and reasonably less priced alcohols, such as 1,2-octanediol and cyclohexanol, may replace the commonly used and expensive 1,2-hexadecanediol, providing an economically attractive route for the synthesis of high quality magnetic nanoparticles. The relative cost for the preparation of Fe3O4 NPs is reduced to only 21% and 9% of the original cost when using 1,2-octanediol and cyclohexanol, respectively.

Insights into the active surface species formed on Ta2O5 nanotubes in the catalytic oxidation of CO.

Freestanding Ta2O5 nanotubes were prepared by an anodizing method. As-anodized amorphous nanotubes were calcined at high temperature to obtain a crystalline phase. All materials were studied by means of BET analysis, XRD, TEM, SEM, XPS, and FTIR and were evaluated in the catalytic oxidation of CO. An XPS study confirmed the formation of different tantalum surface species after high temperature treatment of amorphous Ta2O5 nanotubes. Calcination at 800 °C generated Ta(4+) while higher temperature (1000 °C) treatment led to the formation of Ta(3+) species. These materials also showed significant differences in catalytic activity. Higher activity was observed for samples calcined at 800 °C than at 1000 °C, suggesting that Ta(4+) species are active sites for CO oxidation.

Strategies to improve hydrogen activation on gold catalysts.

Catalytic reactions involving molecular hydrogen are at the heart of many transformations in the chemical industry. Classically, hydrogenations are carried out on Pd, Pt, Ru or Ni catalysts. However, the use of supported Au catalysts has garnered attention in recent years owing to their exceptional selectivity in hydrogenation reactions. This is despite the limited understanding of the physicochemical aspects of hydrogen activation and reaction on Au surfaces. A rational design of new improved catalysts relies on making better use of the hydrogenating properties of Au. This Review analyses the strategies utilized to improve hydrogen-Au interactions, from addressing the importance of the Au particle size to exploring alternative mechanisms for H2 dissociation on Au cations and Au-ligand interfaces. These insights hold the potential to drive future applications of Au catalysis.

Solvent-Free Aerobic Oxidative Cleavage of Methyl Oleate to Biobased Aldehydes over Mechanochemically Synthesized Supported AgAu Nanoparticles.

The performance of mechanochemically synthesized supported bimetallic AgAu nanoalloy catalysts was evaluated in the oxidative cleavage of methyl oleate, a commonly available unsaturated bio-derived raw material. An extensive screening of supports (SiO2 , C, ZrO2 , Al2 O3 ), metallic ratios (Ag : Au), reaction times, temperatures, and use of solvents was carried out. The performance was optimized towards productivity and selectivity for the primary cleavage products (aldehydes and oxoesters). The optimal conditions were achieved in the absence of solvent, using Ag8 Au92 /SiO2 as catalyst, at 80 °C, reaction time of 1 h, substrate to catalyst=555 and 10 bar of molecular oxygen. A strong support effect was observed: the selectivity to aldehydes was best with silica as support, and to esters was best using zirconia. This shows not only that mechanochemical preparation of bimetallic catalysts is a powerful tool to generate useful catalyst compositions, but also that a safe, green, solventless synthesis of bio-derived products can be achieved by aerobic oxidative cleavage.

Design of a dinuclear nickel(II) bioinspired hydrolase to bind covalently to silica surfaces: synthesis, magnetism, and reactivity studies.

Presented herein is the design of a dinuclear Ni(II) synthetic hydrolase [Ni(2)(HBPPAMFF)(µ-OAc)(2)(H(2)O)]BPh(4) (1) (H(2)BPPAMFF = 2-[(N-benzyl-N-2-pyridylmethylamine)]-4-methyl-6-[N-(2-pyridylmethyl)aminomethyl)])-4-methyl-6-formylphenol) to be covalently attached to silica surfaces, while maintaining its catalytic activity. An aldehyde-containing ligand (H(2)BPPAMFF) provides a reactive functional group that can serve as a cross-linking group to bind the complex to an organoalkoxysilane and later to the silica surfaces or directly to amino-modified surfaces. The dinuclear Ni(II) complex covalently attached to the silica surfaces was fully characterized by different techniques. The catalytic turnover number (k(cat)) of the immobilized Ni(II)Ni(II) catalyst in the hydrolysis of 2,4-bis(dinitrophenyl)phosphate is comparable to the homogeneous reaction; however, the catalyst interaction with the support enhanced the substrate to complex association constant, and consequently, the catalytic efficiency (E = k(cat)/K(M)) and the supported catalyst can be reused for subsequent diester hydrolysis reactions.

On the stabilization of gold nanoparticles over silica-based magnetic supports modified with organosilanes.

The immobilization of gold nanoparticles (Au NPs) on silica is made possible by the functionalization of the silica surfaces with organosilanes. Au NPs could only be stabilized and firmly attached to silica-support surfaces that were previously modified with amino groups. Au NPs could not be stabilized on bare silica surfaces and most of the NPs were then found in the solution. The metal-support interactions before and after the Au NP formation, observed by X-ray absorption fine structure spectroscopy (XAFS), indicate a stronger interaction of gold(III) ions with amino-modified silica surfaces than with the silanol groups in bare silica. An amino-modified, silica-based, magnetic support was used to prepare an active Au NP catalyst for the chemoselective oxidation of alcohols, a reaction of great interest for the fine chemical industry.

Enhancement of hematoporphyrin IX potential for photodynamic therapy by entrapment in silica nanospheres.

The entrapment of hematoporphyrin IX (Hp IX) in silica by means of a microemulsion resulted in silica spheres of 33 ± 6 nm. The small size, narrow size distribution and lack of aggregation maintain Hp IX silica nanospheres stable in aqueous solutions for long periods and permit a detailed study of the entrapped drug by different techniques. Hp IX entrapped in the silica matrix is accessed by oxygen and upon irradiation generates singlet oxygen which diffuses very efficiently to the outside solution. The Hp IX entrapped in the silica matrix is also reached by iron(II) ions, which causes quenching of the porphyrin fluorescence emission. The silica matrix also provides extra protection to the photosensitizer against interaction with BSA and ascorbic acid, which are known to cause suppression of singlet oxygen generation by the Hp IX free in solution. Therefore, the incorporation of Hp IX molecules into silica nanospheres increased the potential of the photosensitizer to perform photodynamic therapy.

Ionic liquids as recycling solvents for the synthesis of magnetic nanoparticles.

This work describes an easy synthesis (one pot) of MFe(2)O(4) (M = Co, Fe, Mn, and Ni) magnetic nanoparticles MNPs by the thermal decomposition of Fe(Acac)(3)/M(Acac)(2) by using BMI·NTf(2) (1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) or BMI·PF(6) (1-n-butyl-3-methylimidazolium hexafluorophosphate) ionic liquids (ILs) as recycling solvents and oleylamine as the reducing and surface modifier agent. The effects of reaction temperature and reaction time on the features of the magnetic nanomaterials (size and magnetic properties) were investigated. The growth of the MNPs is easily controlled in the IL by adjusting the reaction temperature and time, as inferred from Fe(3)O(4) MNPs obtained at 150 °C, 200 °C and 250 °C with mean diameters of 8, 10 and 15 nm, respectively. However, the thermal decomposition of Fe(Acac)(3) performed in a conventional high boiling point solvent (diphenyl ether, bp 259 °C), under a similar Fe to oleylamine molar ratio used in the IL synthesis, does not follow the same growth mechanism and rendered only smaller NPs of 5 nm mean diameter. All MNPs are covered by at least one monolayer of oleylamine making them readily dispersible in non-polar solvents. Besides the influence on the nanoparticles growth, which is important for the preparation of highly crystalline MNPs, the IL was easily recycled and has been used in at least 20 successive syntheses.

Riboflavin Surface Modification of Poly(vinyl chloride) for Light-Triggered Control of Bacterial Biofilm and Virus Inactivation.

Poly(vinyl chloride) (PVC) is the most used biomedical polymer worldwide. PVC is a stable and chemically inert polymer. However, microorganisms can colonize PVC producing biomedical device-associated infections. While surface modifications of PVC can help improve the antimicrobial and antiviral properties, the chemically inert nature of PVC makes those modifications challenging and potentially toxic. In this work, we modified the PVC surface using a derivative riboflavin molecule that was chemically tethered to a plasma-treated PVC surface. Upon a low dosage of blue light, the riboflavin tethered to the PVC surface became photochemically activated, allowing for Pseudomonas aeruginosa bacterial biofilm and lentiviral in situ eradication.

Heterodinuclear Fe(III)Zn(II)-bioinspired complex supported on 3-aminopropyl silica. Efficient hydrolysis of phosphate diester bonds.

Presented herein is the synthesis and characterization of a new Fe(III)Zn(II) complex containing a Fe(III)-bound phenolate with a carbonyl functional group, which was anchored to 3-aminopropyl-functionalized silica as the solid support. The catalytic efficiency of the immobilized catalyst in the hydrolysis of 2,4-bis(dinitrophenyl)phosphate is comparable to the homogeneous reaction, and the supported catalyst can be reused for subsequent diester hydrolysis reactions.

Nanoparticle platform to modulate reaction mechanism of phenothiazine photosensitizers.

Herein, we report on the synthesis of photosensitizing nanoparticles in which the generation of different oxidizing species, i.e., singlet oxygen (1O2) or radicals, was modulated. Sol gel and surface chemistry were used to obtain nanoparticles with specific ratios of dimer to monomer species of phenothiazine photosensitizers (PSs). Due to competition between the reactions involving electron transfer within dimer species and energy transfer from monomer triplets to oxygen, the efficiency of 1O2 generation could be controlled. Nanoparticles with an excess of dimer have an 1O2 generation efficiency (S(delta)) of 0.01 while those without dimer have a S(delta) value of 0.4. Furthermore, we demonstrate that the PS properties of the nanoparticles are not subjected to interference from the external medium as is commonly the case for free PSs, i.e., PS ground and triplet states are not reduced by NADH and ascorbate, respectively, and singlet excited states are less suppressed by bromide. The modulated 1O2 generation and the PS protection from external interferences make this nanoparticle platform a promising tool to aid in performing mechanistic studies in biological systems. Also, it offers potential application in technological areas in which photo-induced processes take place.

Ligand-assisted preparation of palladium supported nanoparticles: a step toward size control.

Supported nanoparticles (SNPs) with narrow size distribution were prepared by H(2) reduction of Pd(2+) previously bound to ligand-modified silica surfaces. Interestingly, the size of the Pd(0) SNPs was tuned by the ligand grafted on the support surface. Amino- and ethylenediamino-functionalized supports formed Pd(0) SNPs of ca. 6 and 1 nm, respectively. The catalytic properties of both Pd(0) SNPs were investigated.

The role and fate of capping ligands in colloidally prepared metal nanoparticle catalysts.

Metal nanoparticles have received intense scientific attention in the field of catalysis. Precise engineering of nanomaterials' size, shape and surface composition, including adsorbed capping ligands, is of utmost importance to control activity and selectivity, and distinguish colloidally prepared metal nanoparticle catalysts from traditional heterogeneous catalysts. The interface between the material and the reaction medium is where the key interactions occur; therefore, catalysis occurs under the influence of capping ligands. In this Perspective review, we focus on the choice of capping ligands (or stabilizing agents), and their role and fate in different steps from preparation to catalysis. Evaluating the influence of the ligands on the catalytic response is not trivial, but the literature provides examples where the ligands adsorbed on the nanoparticle surface dramatically change the activity and selectivity for a particular reaction, while acting either as a dynamic shell or a passivation coating. Steric and electronic effects resulting from the presence of adsorbed ligands have been proposed to influence the catalytic properties. Attempts to remove the capping ligands are discussed, even though they are not always successful or even necessary. Finally, we outline our personal understanding and perspectives on the use of ligands or functionalized supports to tune the activity and selectivity of supported metal nanoparticles.

Tuning the Catalytic Activity and Selectivity of Pd Nanoparticles Using Ligand-Modified Supports and Surfaces.

The organic moiety plays an essential role in the design of homogeneous catalysts, where the ligands are used to tune the catalytic activity, selectivity, and stability of the transition metal centers. The impact of ligands on the catalytic performance of metal nanoparticle catalysts is still less understood. Here, we prepared supported nanoparticle (NP) catalysts by the immobilization of preformed Pd NPs on the ligand-modified silica surfaces bearing amine, ethylenediamine, and diethylenetriamine groups. After excluding any size effect, we were able to study the influence of the ligands grafted on the support surface on the catalytic activity of the supported nanoparticles. Higher activity was observed for the Pd NPs supported on propylamine-functionalized support, whereas the presence of ethylenediamine and diethylenetriamine groups was detrimental to the activity. Upon the addition of excess of these amine ligands as surface modifiers, the hydrogenation of alkene to alkane was fully suppressed and, therefore, we were able to tune Pd selectivity. The selective hydrogenation of alkynes into alkenes, although a considerable challenge on the traditional palladium catalysts, was achieved here for a range of alkynes by combining Pd NPs and amine ligands.