Editorial

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Open AccessEditorial

Special Issue: Coinage Metal (Copper, Silver, and Gold) Catalysis

by

Sónia Alexandra Correia Carabineiro

Molecules 2016, 21(6), 746; https://doi.org/10.3390/molecules21060746 - 08 Jun 2016

Cited by 9 | Viewed by 3806

Abstract

The subject of catalysis by coinage metals (copper, silver, and gold) comes up increasingly day-by-day. This Special Issue aims to cover the numerous aspects of the use of these metals as catalysts for several reactions. It deals with synthesis and characterization of copper, [...] Read more.

The subject of catalysis by coinage metals (copper, silver, and gold) comes up increasingly day-by-day. This Special Issue aims to cover the numerous aspects of the use of these metals as catalysts for several reactions. It deals with synthesis and characterization of copper, silver and gold based catalysis, their characterization and use, both for heterogeneous and homogeneous catalysis, and some of their potential applications. Full article

(This article belongs to the Special Issue Coinage Metal (Copper, Silver, and Gold) Catalysis)

Research

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Open AccessArticle

Gold Incorporated Mesoporous Silica Thin Film Model Surface as a Robust SERS and Catalytically Active Substrate

by

Anandakumari Chandrasekharan Sunil Sekhar

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Chathakudath Prabhakaran Vinod

Molecules 2016, 21(5), 667; https://doi.org/10.3390/molecules21050667 - 20 May 2016

Cited by 6 | Viewed by 5845

Abstract

Ultra-small gold nanoparticles incorporated in mesoporous silica thin films with accessible pore channels perpendicular to the substrate are prepared by a modified sol-gel method. The simple and easy spin coating technique is applied here to make homogeneous thin films. The surface characterization using [...] Read more.

Ultra-small gold nanoparticles incorporated in mesoporous silica thin films with accessible pore channels perpendicular to the substrate are prepared by a modified sol-gel method. The simple and easy spin coating technique is applied here to make homogeneous thin films. The surface characterization using FESEM shows crack-free films with a perpendicular pore arrangement. The applicability of these thin films as catalysts as well as a robust SERS active substrate for model catalysis study is tested. Compared to bare silica film our gold incorporated silica, GSM-23F gave an enhancement factor of 10³ for RhB with a laser source 633 nm. The reduction reaction of p-nitrophenol with sodium borohydride from our thin films shows a decrease in peak intensity corresponding to –NO₂ group as time proceeds, confirming the catalytic activity. Such model surfaces can potentially bridge the material gap between a real catalytic system and surface science studies. Full article

(This article belongs to the Special Issue Coinage Metal (Copper, Silver, and Gold) Catalysis)

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Open AccessArticle

Ethyl Acetate Abatement on Copper Catalysts Supported on Ceria Doped with Rare Earth Oxides

by

Sónia Alexandra Correia Carabineiro

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Michalis Konsolakis

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George Emmanouil-Nontas Marnellos

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Muhammad Faizan Asad

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Olívia Salomé Gonçalves Pinto Soares

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Pedro Bandeira Tavares

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Manuel Fernando Ribeiro Pereira

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José Joaquim de Melo Órfão

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José Luís Figueiredo

Molecules 2016, 21(5), 644; https://doi.org/10.3390/molecules21050644 - 17 May 2016

Cited by 31 | Viewed by 6941

Abstract

Different lanthanide (Ln)-doped cerium oxides (Ce_0.5Ln_0.5O_1.75, where Ln: Gd, La, Pr, Nd, Sm) were loaded with Cu (20 wt. %) and used as catalysts for the oxidation of ethyl acetate (EtOAc), a common volatile organic compound (VOC). [...] Read more.

Different lanthanide (Ln)-doped cerium oxides (Ce_0.5Ln_0.5O_1.75, where Ln: Gd, La, Pr, Nd, Sm) were loaded with Cu (20 wt. %) and used as catalysts for the oxidation of ethyl acetate (EtOAc), a common volatile organic compound (VOC). For comparison, both Cu-free (Ce-Ln) and supported Cu (Cu/Ce-Ln) samples were characterized by N₂ adsorption at −196 °C, scanning/transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and temperature programmed reduction in H₂. The following activity sequence, in terms of EtOAc conversion, was found for bare supports: CeO₂ ≈ Ce_0.5Pr_0.5O_1.75 > Ce_0.5Sm_0.5O_1.75 > Ce_0.5Gd_0.5O_1.75 > Ce_0.5Nd_0.5O_1.75 > Ce_0.5La_0.5O_1.75. Cu addition improved the catalytic performance, without affecting the activity order. The best catalytic performance was obtained for Cu/CeO₂ and Cu/Ce_0.5Pr_0.5O_1.75 samples, both achieving complete EtOAc conversion below ca. 290 °C. A strong correlation was revealed between the catalytic performance and the redox properties of the samples, in terms of reducibility and lattice oxygen availability. Νo particular correlation between the VOC oxidation performance and textural characteristics was found. The obtained results can be explained in terms of a Mars-van Krevelen type redox mechanism involving the participation of weakly bound (easily reduced) lattice oxygen and its consequent replenishment by gas phase oxygen. Full article

(This article belongs to the Special Issue Coinage Metal (Copper, Silver, and Gold) Catalysis)

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Open AccessArticle

Identification of Subnanometric Ag Species, Their Interaction with Supports and Role in Catalytic CO Oxidation

by

Yulia Kotolevich

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Ekaterina Kolobova

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Evgeniy Khramov

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Jesús Efren Cabrera Ortega

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Mario H. Farías

,

Yan Zubavichus

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Rodolfo Zanella

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Josué D. Mota-Morales

,

Alexey Pestryakov

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Nina Bogdanchikova

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Vicente Cortés Corberán

Molecules 2016, 21(4), 532; https://doi.org/10.3390/molecules21040532 - 22 Apr 2016

Cited by 13 | Viewed by 4875

Abstract

The nature and size of the real active species of nanoparticulated metal supported catalysts is still an unresolved question. The technique of choice to measure particle sizes at the nanoscale, HRTEM, has a practical limit of 1 nm. This work is aimed to [...] Read more.

The nature and size of the real active species of nanoparticulated metal supported catalysts is still an unresolved question. The technique of choice to measure particle sizes at the nanoscale, HRTEM, has a practical limit of 1 nm. This work is aimed to identify the catalytic role of subnanometer species and methods to detect and characterize them. In this frame, we investigated the sensitivity to redox pretreatments of Ag/Fe/TiO₂, Ag/Mg/TiO₂ and Ag/Ce/TiO₂ catalysts in CO oxidation. The joint application of HRTEM, SR-XRD, DRS, XPS, EXAFS and XANES methods indicated that most of the silver in all samples is in the form of Ag species with size <1 nm. The differences in catalytic properties and sensitivity to pretreatments, observed for the studied Ag catalysts, could not be explained taking into account only the Ag particles whose size distribution is measured by HRTEM, but may be explained by the presence of the subnanometer Ag species, undetectable by HRTEM, and their interaction with supports. This result highlights their role as active species and the need to take them into account to understand integrally the catalysis by supported nanometals. Full article

(This article belongs to the Special Issue Coinage Metal (Copper, Silver, and Gold) Catalysis)

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Open AccessArticle

Causes of Activation and Deactivation of Modified Nanogold Catalysts during Prolonged Storage and Redox Treatments

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Vicente Cortés Corberán

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Alexey Pestryakov

Molecules 2016, 21(4), 486; https://doi.org/10.3390/molecules21040486 - 13 Apr 2016

Cited by 17 | Viewed by 5720

Abstract

The catalytic properties of modified Au/TiO₂ catalysts for low-temperature CO oxidation are affected by deactivation and reactivation after long-term storage and by redox treatments. The effect of these phenomena on the catalysts was studied by HRTEM, BET, SEM, FTIR CO, XPS and [...] Read more.

The catalytic properties of modified Au/TiO₂ catalysts for low-temperature CO oxidation are affected by deactivation and reactivation after long-term storage and by redox treatments. The effect of these phenomena on the catalysts was studied by HRTEM, BET, SEM, FTIR CO, XPS and H₂ TPR methods. The main cause for the deactivation and reactivation of catalytic properties is the variation in the electronic state of the supported gold, mainly, the proportion of singly charged ions Au⁺. The most active samples are those with the highest proportion of singly charged gold ions, while catalysts with a high content of trivalent gold ions are inactive at low-temperatures. Active states of gold, resistant to changes caused by the reaction process and storage conditions, can be stabilized by modification of the titanium oxide support with transition metals oxides. The catalyst modified with lanthanum oxide shows the highest stability and activity. Full article

(This article belongs to the Special Issue Coinage Metal (Copper, Silver, and Gold) Catalysis)

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Open AccessArticle

On the High Sensitivity of the Electronic States of 1 nm Gold Particles to Pretreatments and Modifiers

by

Oxana Martynyuk

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Yulia Kotolevich

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Rodrigo Vélez

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Jesus Efren Cabrera Ortega

,

Hugo Tiznado

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Trino Zepeda Partida

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Josué D. Mota-Morales

,

Alexey Pestryakov

and

Nina Bogdanchikova

Molecules 2016, 21(4), 432; https://doi.org/10.3390/molecules21040432 - 31 Mar 2016

Cited by 8 | Viewed by 5849

Abstract

In this paper, the effect of modifiers and pretreatments on the electronic states of 1 nm gold nanoparticles (AuNPs) supported on silica was systematically studied. AuNPs deposited on silica (particle size of 2–4 nm) modified with Ce, La and Fe oxides, were studied [...] Read more.

In this paper, the effect of modifiers and pretreatments on the electronic states of 1 nm gold nanoparticles (AuNPs) supported on silica was systematically studied. AuNPs deposited on silica (particle size of 2–4 nm) modified with Ce, La and Fe oxides, were studied by FTIR of adsorbed CO after different redox treatments at 100, 300 and 500 °C. This study was conducted at room temperature to allow detecting the electronic states of gold, which is more likely involved in CO oxidation at the same temperature. AuNP size distribution was measured by HRTEM. It is shown that the electronic state of gold species (Au_n^δ−, Au⁰, Au_n^δ+, Au⁺) in 1 nm AuNPs is sensitive to the modifier as well as to the temperatures of redox pretreatments. Supports modified with the same additives but containing larger AuNPs (~3, 4, 5, and 7 nm) were also studied. They showed that Au⁰ remains stable irrespective of additives and redox pretreatments, indicating no significant effect of such treatments on the electronic properties of larger AuNPs. Samples with a predominant AuNP size of 2 nm are an intermediate case between these two groups of materials. Full article

(This article belongs to the Special Issue Coinage Metal (Copper, Silver, and Gold) Catalysis)

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Open AccessArticle

Aroylhydrazone Cu(II) Complexes in keto Form: Structural Characterization and Catalytic Activity towards Cyclohexane Oxidation

by

Manas Sutradhar

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Elisabete C. B. A. Alegria

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M. Fátima C. Guedes da Silva

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Luísa M. D. R. S. Martins

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Armando J. L. Pombeiro

Molecules 2016, 21(4), 425; https://doi.org/10.3390/molecules21040425 - 29 Mar 2016

Cited by 26 | Viewed by 5935

Abstract

The reaction of the Schiff base (3,5-di-tert-butyl-2-hydroxybenzylidene)-2-hydroxybenzohydrazide (H₃L) with a copper(II) salt of a base of a strong acid, i.e., nitrate, chloride or sulphate, yielded the mononuclear complexes [Cu(H₂L)(NO₃)(H₂O)] (1), [Cu(H₂ [...] Read more.

The reaction of the Schiff base (3,5-di-tert-butyl-2-hydroxybenzylidene)-2-hydroxybenzohydrazide (H₃L) with a copper(II) salt of a base of a strong acid, i.e., nitrate, chloride or sulphate, yielded the mononuclear complexes [Cu(H₂L)(NO₃)(H₂O)] (1), [Cu(H₂L)Cl]·2MeOH (2) and the binuclear complex [{Cu(H₂L)}₂(µ-SO₄)]·2MeOH (3), respectively, with H₂L⁻ in the keto form. Compounds 1–3 were characterized by elemental analysis, Infrared (IR) spectroscopy, Electrospray Ionisation Mass Spectrometry (ESI-MS) and single crystal X-ray crystallography. All compounds act as efficient catalysts towards the peroxidative oxidation of cyclohexane to cyclohexyl hydroperoxide, cyclohexanol and cyclohexanone, under mild conditions. In the presence of an acid promoter, overall yields (based on the alkane) up to 25% and a turnover number (TON) of 250 (TOF of 42 h⁻¹) after 6 h, were achieved. Full article

(This article belongs to the Special Issue Coinage Metal (Copper, Silver, and Gold) Catalysis)

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Open AccessFeature PaperArticle

Clean Transformation of Ethanol to Useful Chemicals. The Behavior of a Gold-Modified Silicalite Catalyst

by

Ermelinda Falletta

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Michele Rossi

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Joaquim Henrique Teles

and

Cristina Della Pina

Molecules 2016, 21(3), 379; https://doi.org/10.3390/molecules21030379 - 19 Mar 2016

Cited by 4 | Viewed by 6207

Abstract

Upon addition of gold to silicalite-1 pellets (a MFI-type zeolite), the vapor phase oxidation of ethanol could be addressed to acetaldehyde or acetic acid formation. By optimizing the catalyst composition and reaction conditions, the conversion of ethanol could be tuned to acetaldehyde with [...] Read more.

Upon addition of gold to silicalite-1 pellets (a MFI-type zeolite), the vapor phase oxidation of ethanol could be addressed to acetaldehyde or acetic acid formation. By optimizing the catalyst composition and reaction conditions, the conversion of ethanol could be tuned to acetaldehyde with 97% selectivity at 71% conversion or to acetic acid with 78% selectivity at total conversion. Considering that unloaded silicalite-1 was found to catalyze the dehydration of ethanol to diethylether or ethene, a green approach for the integrated production of four important chemicals is herein presented. This is based on renewable ethanol as a reagent and a modular catalytic process. Full article

(This article belongs to the Special Issue Coinage Metal (Copper, Silver, and Gold) Catalysis)

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Open AccessArticle

Synthesis of Isoxazole and 1,2,3-Triazole Isoindole Derivatives via Silver- and Copper-Catalyzed 1,3-Dipolar Cycloaddition Reaction

by

Mohamed Mehdi Rammah

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Wafa Gati

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Hasan Mtiraoui

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Mohamed El Baker Rammah

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and

Molecules 2016, 21(3), 307; https://doi.org/10.3390/molecules21030307 - 04 Mar 2016

Cited by 11 | Viewed by 6785

Abstract

The CuI- or Ag₂CO₃-catalyzed [3+2] cycloaddition of propargyl-substituted dihydroisoindolin-1-one (3) with arylnitrile oxides 1a–d (Ar = Ph, p-MeC₆H₄, p-MeOC₆H₄, p-ClC₆H₄) produces in good yields [...] Read more.

The CuI- or Ag₂CO₃-catalyzed [3+2] cycloaddition of propargyl-substituted dihydroisoindolin-1-one (3) with arylnitrile oxides 1a–d (Ar = Ph, p-MeC₆H₄, p-MeOC₆H₄, p-ClC₆H₄) produces in good yields novel 3,5-disubstituted isoxazoles 4 of the ethyl-2-benzyl-3-oxo-1-((3-arylisoxazol-5yl)methyl)-2,3-dihydro-1H-isoindole-1-carboxylate type. With aryl azides 2a–d (Ar = Ph, p-MeC₆H₄, p-OMeC₆H₄, p-ClC₆H₄), a series of 1,4-disubstituted 1,2,3-triazoles 6 (ethyl-2-benzyl-3-oxo-1-((1-aryl-1H-1,2,3-triazol-4-yl)methyl)-2,3-dihydro-1H-isoindole-1-carboxylates) was obtained. The reactions proceed in a regioselective manner affording exclusively racemic adducts 4 and 6. Compared to the uncatalyzed cycloaddition, the yields are significantly improved in the presence of CuI as catalyst, without alteration of the selectivity. The regio- and stereochemistry of the cycloadducts has been corroborated by an X-ray diffraction study of 4a, and in the case of 6a by XH-correlation and HMBC spectra. Full article

(This article belongs to the Special Issue Coinage Metal (Copper, Silver, and Gold) Catalysis)

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Open AccessArticle

Au-Based Catalysts: Electrochemical Characterization for Structural Insights

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Molecules 2016, 21(3), 261; https://doi.org/10.3390/molecules21030261 - 25 Feb 2016

Cited by 10 | Viewed by 4550

Abstract

Au-based catalysts are widely used in important processes because of their peculiar characteristics. The catalyst performance depends strongly on the nature and structure of the metal nanoparticles, especially in the case of bimetallic catalysts where synergistic effects between the two metals can be [...] Read more.

Au-based catalysts are widely used in important processes because of their peculiar characteristics. The catalyst performance depends strongly on the nature and structure of the metal nanoparticles, especially in the case of bimetallic catalysts where synergistic effects between the two metals can be occasionally seen. In this paper, it is shown that electrochemical characterisation (cyclovoltammetry CV and electrochemical impedance spectroscopy EIS) of AuPd systems can be used to determine the presence of an electronic interaction between the two metals, thus providing a strong support in the determination of the nature of the synergy between Au and Pd in the liquid phase oxidation of alcohols. However, it seems likely that the strong difference in the catalytic behavior between the single metals and the bimetallic system is connected not only to the redox behaviour, but also to the energetic balance between the different elementary steps of the reaction. Full article

(This article belongs to the Special Issue Coinage Metal (Copper, Silver, and Gold) Catalysis)

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