Catalysts

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Open AccessEditorial

New Trends in Gold Catalysts

by Leonarda F. Liotta

Catalysts 2014, 4(3), 299-304; https://doi.org/10.3390/catal4030299 - 16 Jul 2014

Cited by 8 | Viewed by 6349

Abstract

Gold is an element that has fascinated mankind for millennia. The catalytic properties of gold have been a source of debate, due to its complete chemical inertness when in a bulk form, while it can oxidize CO at temperatures as low as ~200 [...] Read more.

Gold is an element that has fascinated mankind for millennia. The catalytic properties of gold have been a source of debate, due to its complete chemical inertness when in a bulk form, while it can oxidize CO at temperatures as low as ~200 K when in a nanocrystalline state, as discovered by Haruta in the late 1980s [1]. Since then, extensive activity in both applied and fundamental research on gold has been initiated. The importance of the catalysis by gold represents one of the fasted growing fields in science and is proven by the promising applications in several fields, such as green chemistry and environmental catalysis, in the synthesis of single-walled carbon nanotubes, as modifiers of Ni catalysts for methane steam and dry reforming reactions and in biological and electrochemistry applications. The range of reactions catalyzed by gold, as well as the suitability of different supports and the influence of the preparation conditions have been widely explored and optimized in applied research [2]. Gold catalysts appeared to be very different from the other noble metal-based catalysts, due to their marked dependence on the preparation method, which is crucial for the genesis of the catalytic activity. Several methods, including deposition-precipitation, chemical vapor deposition and cation adsorption, have been applied for the preparation of gold catalysts over reducible oxides, like TiO₂. Among these methods, deposition-precipitation has been the most frequently employed method for Au loading, and it involves the use of tetrachloroauric (III) acid as a precursor. On the other hand, the number of articles dealing with Au-loaded acidic supports is smaller than that on basic supports, possibly because the deposition of [AuCl₄]⁻ or [AuOH_xCl₄_−x]⁻ species on acidic supports is difficult, due to their very low point of zero charge. Despite this challenge, several groups have reported the use of acidic zeolites as supports for gold. Zeolites are promising supports for Au stabilization, because of the presence of ion-exchange sites, such as NH⁴⁺, that can be substituted by Au⁺ ions through the elimination of NH₄Cl [3]. Moreover, zeolites, due to their high thermal stability, the presence of a large surface area and micropores, may hinder Au sintering. [...] Full article

(This article belongs to the Special Issue New Trends in Gold Catalysts)

Research

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333 KiB

Open AccessArticle

Influence of Gold on Hydrotalcite-like Compound Catalysts for Toluene and CO Total Oxidation

by Eric Genty, Renaud Cousin, Sylvie Capelle and Stéphane Siffert

Catalysts 2013, 3(4), 966-977; https://doi.org/10.3390/catal3040966 - 12 Dec 2013

Cited by 14 | Viewed by 8628

Abstract

X₆Al₂HT500 hydrotalcites, where X represents Mg, Fe, Cu or Zn were synthetized and investigated before and after gold deposition for toluene and CO total oxidation reactions. The samples have been characterized by specific areas, XRD measurements and Temperature Programmed [...] Read more.

X₆Al₂HT500 hydrotalcites, where X represents Mg, Fe, Cu or Zn were synthetized and investigated before and after gold deposition for toluene and CO total oxidation reactions. The samples have been characterized by specific areas, XRD measurements and Temperature Programmed Reduction. Concerning the toluene total oxidation, the best activity was obtained with Au/Cu₆Al₂HT500 catalyst with T₅₀ at 260 °C. However, catalytic behavior of Au/X₆Al₂HT500 sample in both reactions depends mainly on the nature of the support. Full article

(This article belongs to the Special Issue New Trends in Gold Catalysts)

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4520 KiB

Open AccessArticle

Investigation on the Stability of Supported Gold Nanoparticles

by Michela Signoretto, Federica Menegazzo, Valentina Trevisan, Francesco Pinna, Maela Manzoli and Flora Boccuzzi

Catalysts 2013, 3(3), 656-670; https://doi.org/10.3390/catal3030656 - 21 Aug 2013

Cited by 12 | Viewed by 8615

Abstract

The procedures leading to the preservation of catalytic performances of Au/ZrO₂ samples have been investigated. The three potential causes of deactivation, namely the particle growth by sintering of gold nanoparticles, the metal leaching and the formation of un-reactive species which inhibit the [...] Read more.

The procedures leading to the preservation of catalytic performances of Au/ZrO₂ samples have been investigated. The three potential causes of deactivation, namely the particle growth by sintering of gold nanoparticles, the metal leaching and the formation of un-reactive species which inhibit the reaction, have been evaluated. In particular, this paper deals with the stability of gold nanoparticles: (1) under storage conditions; (2) with time on stream for a gas phase reaction (LT-WGSR); (3) with time on stream for a liquid phase reaction (furfural oxidative esterification). Full article

(This article belongs to the Special Issue New Trends in Gold Catalysts)

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1231 KiB

Open AccessArticle

Fabrication and Catalytic Activity of Thermally Stable Gold Nanoparticles on Ultrastable Y (USY) Zeolites

by Takashi Sanada, Chika Murakami, Kinga Góra-Marek, Keiko Iida, Naonobu Katada and Kazu Okumura

Catalysts 2013, 3(3), 599-613; https://doi.org/10.3390/catal3030599 - 9 Jul 2013

Cited by 12 | Viewed by 8616

Abstract

Au was deposited on ultrastable Y (USY) zeolites using an ion-exchange method. Up to 5.5 wt% Au was introduced into the NH₄-form of USY zeolites. In contrast, deposition of Au hardly took place on the H- and Na-forms of Y-type zeolites, [...] Read more.

Au was deposited on ultrastable Y (USY) zeolites using an ion-exchange method. Up to 5.5 wt% Au was introduced into the NH₄-form of USY zeolites. In contrast, deposition of Au hardly took place on the H- and Na-forms of Y-type zeolites, NH₄-forms of mordenite, and ZSM-5. Treatment of the Au-loaded USY zeolite in a H₂ atmosphere, afforded Au⁰ nanoparticles. These particles were thermally stable even at 973 K, where their mean particle diameter was 3.7 nm. In contrast, highly aggregated Au particles were observed after thermal treatment at temperatures lower than 523 K, followed by storage in air for a month. The resulting particle sizes were in good correlation with the IR band intensity of the adsorbed CO and the catalytic activity of Au in the aerobic oxidation of benzyl alcohol. The Au nanoparticles showed highest activity when the Au/USY zeolite was thermally treated at 673–973 K. A negligible deactivation was observed after repeating the reaction at least 12 times. In the case of Au/TiO₂ catalyst prepared by the deposition-precipitation method, the highest activity was observed at 573 K, which was lower than the temperature used for the Au/USY zeolites. This study demonstrated the potential use of the NH₄-form of USY zeolites for supporting Au. Full article

(This article belongs to the Special Issue New Trends in Gold Catalysts)

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1439 KiB

Open AccessArticle

Ni-Based Catalysts for Low Temperature Methane Steam Reforming: Recent Results on Ni-Au and Comparison with Other Bi-Metallic Systems

by Hongjing Wu, Valeria La Parola, Giuseppe Pantaleo, Fabrizio Puleo, Anna M. Venezia and Leonarda F. Liotta

Catalysts 2013, 3(2), 563-583; https://doi.org/10.3390/catal3020563 - 5 Jun 2013

Cited by 141 | Viewed by 18254

Abstract

Steam reforming of light hydrocarbons provides a promising method for hydrogen production. Ni-based catalysts are so far the best and the most commonly used catalysts for steam reforming because of their acceptably high activity and significantly lower cost in comparison with alternative precious [...] Read more.

Steam reforming of light hydrocarbons provides a promising method for hydrogen production. Ni-based catalysts are so far the best and the most commonly used catalysts for steam reforming because of their acceptably high activity and significantly lower cost in comparison with alternative precious metal-based catalysts. However, nickel catalysts are susceptible to deactivation from the deposition of carbon, even when operating at steam-to-carbon ratios predicted to be thermodynamically outside of the carbon-forming regime. Reactivity and deactivation by carbon formation can be tuned by modifying Ni surfaces with a second metal, such as Au through alloy formation. In the present review, we summarize the very recent progress in the design, synthesis, and characterization of supported bimetallic Ni-based catalysts for steam reforming. The progress in the modification of Ni with noble metals (such as Au and Ag) is discussed in terms of preparation, characterization and pretreatment methods. Moreover, the comparison with the effects of other metals (such as Sn, Cu, Co, Mo, Fe, Gd and B) is addressed. The differences of catalytic activity, thermal stability and carbon species between bimetallic and monometallic Ni-based catalysts are also briefly shown. Full article

(This article belongs to the Special Issue New Trends in Gold Catalysts)

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5075 KiB

Open AccessArticle

Electron Transfer at Gold Nanostar Assemblies: A Study of Shape Stability and Surface Density Influence

by Mariana Chirea

Catalysts 2013, 3(1), 288-309; https://doi.org/10.3390/catal3010288 - 12 Mar 2013

Cited by 26 | Viewed by 11845

Abstract

Gold nanostars of ~70 nm tip to tip distances were synthesized by a seed mediated method and covalently self-assembled on 1,5-pentanedithiol modified electrodes. Electron transfer kinetics at the AuNS/dithiol modified electrodes were studied as a function of AuNS surface density which was varied [...] Read more.

Gold nanostars of ~70 nm tip to tip distances were synthesized by a seed mediated method and covalently self-assembled on 1,5-pentanedithiol modified electrodes. Electron transfer kinetics at the AuNS/dithiol modified electrodes were studied as a function of AuNS surface density which was varied by increasing their self-assembly time from 8 h, 16 h, 24 h to 32 h. Excellent electrocatalytic properties of AuNSs were observed toward electrochemistry of [Fe(CN)₆]^4−/3− redox couple. The apparent heterogeneous electron transfer constant, k_et, has progressively increased with the surface density of AuNSs bonded to the electrodes from 0.65 × 10⁻⁵ cm s⁻¹ (8 h), 1.47 × 10⁻⁵ cm s⁻¹ (16 h), 3.95 × 10⁻⁵ cm s⁻¹ (24 h) to an excellent 85.0 × 10⁻⁵cm s⁻¹ (32 h). Electrochemical charging of nanostars was confirmed, for the first time, by 79 times increase of double layer capacitance, C_dl, from 0.34 µF (8 h) to 27 µF (32 h). The electrochemical charging of AuNSs had also a strong influence on the electron tunneling process through the 1,5PDT molecules being more efficient at dense layers of AuNSs. The tunneling parameter, β, has decreased from 1.13 Å⁻¹ (16 h) to 0.50 Å⁻¹ (32 h). The AuNSs were chemically stable toward [Fe(CN)₆]^4−/3− showing no change in shape after electrochemical measurements. Full article

(This article belongs to the Special Issue New Trends in Gold Catalysts)

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1868 KiB

Open AccessArticle

Defect-Driven Restructuring of TiO₂ Surface and Modified Reactivity Toward Deposited Gold Atoms

by Kenneth Park, Vincent Meunier, Minghu Pan and Ward Plummer

Catalysts 2013, 3(1), 276-287; https://doi.org/10.3390/catal3010276 - 8 Mar 2013

Cited by 5 | Viewed by 7012

Abstract

A partially reduced TiO2 surface exhibits increasingly complex nature when forming various defects, whose stoichiometry, structure and properties are markedly different from those of bulk TiO2. Using scanning tunneling microscopy and density functional theory, we investigate different types of surface defects formed by [...] Read more.

A partially reduced TiO2 surface exhibits increasingly complex nature when forming various defects, whose stoichiometry, structure and properties are markedly different from those of bulk TiO2. Using scanning tunneling microscopy and density functional theory, we investigate different types of surface defects formed by Ti interstitials on TiO2 (110) and their reactivity toward deposited gold atoms. Sub-stoichiometric strands greatly enhance bonding of Au by transferring the excess charges from the reduced Ti3+ onto the strands. Thus the sub-stoichiometric strands behave as strong electron donor sites toward reactants. On the contrary, fully stoichiometric nanoclusters provide increased Au bonding through its 1-coordinated oxygen, which acts as a strong electron acceptor site. Specific interactions between Au and defects as well as the implication of electron donor/acceptor complexes for catalytic reactions are discussed. Full article

(This article belongs to the Special Issue New Trends in Gold Catalysts)

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Review

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530 KiB

Open AccessReview

Gold Nanoparticles as the Catalyst of Single-Walled Carbon Nanotube Synthesis

by Yoshikazu Homma

Catalysts 2014, 4(1), 38-48; https://doi.org/10.3390/catal4010038 - 5 Mar 2014

Cited by 20 | Viewed by 8803

Abstract

Gold nanoparticles have been proven to act as efficient catalysts for chemical reactions, such as oxidation and hydrogen production. In this review we focus on a different aspect of the catalysis of gold nanoparticles; single-walled carbon nanotube (SWCNT) synthesis. This is not a [...] Read more.

Gold nanoparticles have been proven to act as efficient catalysts for chemical reactions, such as oxidation and hydrogen production. In this review we focus on a different aspect of the catalysis of gold nanoparticles; single-walled carbon nanotube (SWCNT) synthesis. This is not a traditional meaning of catalytic reaction, but SWCNTs cannot be synthesized without nanoparticles. Previously, gold was considered as unsuitable metal species as the catalyst of SWCNT synthesis. However, gold nanoparticles with diameters smaller than 5 nm were found to effectively produce SWCNTs. We discuss the catalysis of gold and related metals for SWCNT synthesis in comparison with conventional catalysts, such as iron, cobalt, and nickel. Full article

(This article belongs to the Special Issue New Trends in Gold Catalysts)

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2603 KiB

Open AccessReview

Gold Nanoparticle-Biological Molecule Interactions and Catalysis

by Jonathan G. Heddle

Catalysts 2013, 3(3), 683-708; https://doi.org/10.3390/catal3030683 - 3 Sep 2013

Cited by 32 | Viewed by 13417

Abstract

This review gives a brief summary of the field of gold nanoparticle interactions with biological molecules, particularly those with possible catalytic relevance. Gold nanoparticles are well known as catalysts in organic chemistry but much is unknown regarding their potential as catalysts of reactions [...] Read more.

This review gives a brief summary of the field of gold nanoparticle interactions with biological molecules, particularly those with possible catalytic relevance. Gold nanoparticles are well known as catalysts in organic chemistry but much is unknown regarding their potential as catalysts of reactions involving biological molecules such as protein and nucleic acids. Biological molecules may be the substrate for catalysis or, if they are the ligand coating the gold particle, may be the catalyst itself. In other cases biological molecules may form a template upon which gold nanoparticles can be precisely arrayed. As relatively little is currently known about the catalytic capabilities of gold nanoparticles in this area, this review will consider templating in general (including, but not restricted to, those which result in structures having potential as catalysts) before going on to consider firstly catalysis by the gold nanoparticle itself followed by catalysis by ligands attached to gold nanoparticles, all considered with a focus on biological molecules. Full article

(This article belongs to the Special Issue New Trends in Gold Catalysts)

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