Chinese Journal of Catalysis

Table of Contents for VOL.37 No.10

2016, 37 (10): 0-0.

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Preface to the Special Issue on Gold Catalysis

Masatake Haruta, 黄家辉

2016, 37 (10): 1579-1579. DOI: 10.1016/S1872-2067(16)62544-5

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Single atom gold catalysts for low-temperature CO oxidation

Botao Qiao, Jin Xia Liang, Aiqin Wang, Jingyue Liu, Tao Zhang

2016, 37 (10): 1580-1587. DOI: 10.1016/S1872-2067(16)62529-9

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Low-temperature CO oxidation is important for both fundamental studies and practical applica-tions. Supported gold catalysts are generally regarded as the most active catalysts for low-temperature CO oxidation. The active sites are traditionally believed to be Au nanoclusters or nanoparticles in the size range of 0.5-5 nm. Only in the last few years have single-atom Au catalysts been proved to be active for CO oxidation. Recent advances in both experimental and theoretical studies on single-atom Au catalysts unambiguously demonstrated that when dispersed on suitable oxide supports the Au single atoms can be extremely active for CO oxidation. In this mini-review, recent advances in the development of Au single-atom catalysts are discussed, with the aim of illustrating their unique catalytic features during CO oxidation.

Advances in polymer-stabilized Au nano-cluster catalysis: Interplay of theoretical calculations and experiments

Hiroaki Koga, Kohei Sakata, Yoshinori Ato, Akihide Hayashi, Kohei Tada, Mitsutaka Okumur

2016, 37 (10): 1588-1593. DOI: 10.1016/S1872-2067(16)62463-4

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Polymer-stabilized Au nano clusters (NCs) with mean diameters of 2-10 nm exhibit unique catalytic properties. Several studies have shown that the key factors affecting the catalytic activity of polymer-stabilized Au NCs are control of the Au NC size, appropriate selection of polymers and optimization of the reaction conditions. This is because polymer-stabilized Au NCs exhibit a clear size effect in several catalytic reactions, and the catalytic activity differs with the type of polymer used and the reaction conditions. In order to elucidate the reason underlying the catalytic activity of the polymer-stabilized Au NCs, much attention is being devoted to the interplay of theoretical calculations and experiments in catalysis by polymer stabilized Au NCs. The present article mainly summarizes our progress in understanding this interplay in polymer-stabilized Au NC catalysis.

Gold-containing metal nanoparticles for catalytic hydrogen generation from liquid chemical hydrides

Xinchun Yang, Qiang Xu

2016, 37 (10): 1594-1599. DOI: 10.1016/S1872-2067(16)62547-0

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Liquid chemical hydrides, which store hydrogen in the form of chemical bonds, are considered one of the most promising classes of hydrogen storage materials. Their application depends heavily on the development of efficient catalytic systems. Gold-containing metal nanoparticles have exhibited excellent catalytic performance for hydrogen generation from liquid chemical hydrides. The present mini-review focuses on recent developments in hydrogen generation from liquid chemical hydrides using gold-nanoparticle and gold-containing heterometallic nanoparticle catalysts.

Vinyl chloride monomer production catalysed by gold: A review

Catherine J. Davies, Peter J. Miedziak, Gemma L. Brett, Graham J. Hutchings

2016, 37 (10): 1600-1607. DOI: 10.1016/S1872-2067(16)62482-8

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In this review we discuss the history of research into the use of gold for the acetylene hydrochlorination reaction, and describe the recent developments which have led to its commercialisation. We discuss the use of different precursors and the addition to gold of a secondary metal as methods which attempt to improve these catalysts, and consider the nature of the active gold species. The vast majority of poly vinyl chloride (PVC) produced globally still uses a mercuric chloride as a catalyst, despite the environmental problems associated with it. Due to the agreement by the Chinese government to remove mercury usage in the PVC industry over the course of the next few years there is an obvious need to find a replacement catalyst; the potential use of gold for this process has been well known for several decades and to date gold seems to be the best candidate for this, primarily due to its superior selectivity when compared to other metals.

Theoretical investigations on CO oxidation reaction catalyzed by gold nanoparticles

Keju Sun

2016, 37 (10): 1608-1618. DOI: 10.1016/S1872-2067(16)62476-2

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It is crucial to understand the mechanism of low temperature CO oxidation reaction catalyzed by gold nanoparticles so as to find out the origin of the high catalytic reactivity and extend the industrialization applications of nano gold catalysts. In this work, some theoretical works on CO adsorption, O₂ adsorption, atomic oxygen adsorption, formation of surface gold oxide films, reaction mechanisms of CO oxidation involving O₂ reaction with CO and O₂ dissociation before reacting with CO on gold surfaces and Au/metal oxide were summarized, and the influences of coordination number, charge transfer and relativity of gold on CO oxidation reaction were briefly reviewed. It was found that CO reaction mechanism depended on the systems with or without oxide and the strong relativistic effects might play an important role in CO oxidation reaction on gold catalysts. In particular, the relativistic effects are related to the unique behav-iors of CO adsorption, O adsorption, O₂ activation on gold surfaces, effects of coordination number and the wide gap between the chemical inertness of bulk gold and high catalytic activity of nano gold. The present work helps us to understand the CO oxidation reaction mechanism on gold catalysts and the influence of relativistic effects on gold catalysis.

Anisotropic gold nanoparticles: Preparation and applications in catalysis

Peter Priecel, Hammed Adekunle Salami, Romen Herrera Padilla, Ziyi Zhong, Jose Antonio Lopez-Sanchez

2016, 37 (10): 1619-1650. DOI: 10.1016/S1872-2067(16)62475-0

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Despite the high amount of scientific work dedicated to the gold nanoparticles in catalysis, most of the research has been performed utilising supported nanoparticles obtained by traditional impregnation of gold salts onto a support, co-precipitation or deposition-precipitation methods which do not benefit from the recent advances in nanotechnologies. Only more recently, gold catalyst scientists have been exploiting the potential of preforming the metal nanoparticles in a colloidal suspension before immobilisation with great results in terms of catalytic activity and the morphology control of mono- and bimetallic catalysts. On the other hand, the last decade has seen the emergence of more advanced control in gold metal nanoparticle synthesis, resulting in a variety of anisotropic gold nanoparticles with easily accessible new morphologies that offer control over the coordination of surface atoms and the optical properties of the nanoparticles (tunable plasmon band) with immense relevance for catalysis. Such morphologies include nanorods, nanostars, nanoflowers, dendritic nanostructures or polyhedral nanoparticles to mention a few. In addition to highlighting newly developed methods and properties of anisotropic gold nanoparticles, in this review we examine the emerging literature that clearly indicates the often superior catalytic performance and amazing potential of these nanoparticles to transform the field of heterogeneous catalysis by gold by offering potentially higher catalytic performance, control over exposed active sites, robustness and tunability for thermal-, electro- and photocatalysis.

Correlation between catalytic activity of supported gold catalysts for carbon monoxide oxidation and metal-oxygen binding energy of the support metal oxides

Takashi Fujita, Masanori Horikawa, Takashi Takei, Toru Murayama, Masatake Haruta

2016, 37 (10): 1651-1655. DOI: 10.1016/S1872-2067(16)62521-4

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The effect of a wide variety of metal oxide (MO_x) supports has been discussed for CO oxidation on nanoparticulate gold catalysts. By using typical co-precipitation and deposition-precipitation methods and under identical calcination conditions, supported gold catalysts were prepared on a wide variety of MO_x supports, and the temperature for 50% conversion was measured to qualitatively evaluate the catalytic activities of these simple MO_x and supported Au catalysts. Furthermore, the difference in these temperatures for the simple MO_x compared to the supported Au catalysts is plotted against the metal-oxygen binding energies of the support MO_x. A clear volcano-like correlation between the temperature difference and the metal-oxygen binding energies is observed. This correlation suggests that the use of MO_x with appropriate metal-oxygen binding energies (300-500 kJ/atom O) greatly improves the catalytic activity of MO_x by the deposition of Au NPs.

Halogen adsorbates on polymer-stabilized gold clusters: Mass spectrometric detection and effects on catalysis

Ryo Ishida, Setsuka Arii, Wataru Kurashige, Seiji Yamazoe, Kiichirou Koyasu, Yuichi Negishi, Tatsuya Tsukuda

2016, 37 (10): 1656-1661. DOI: 10.1016/S1872-2067(16)62501-9

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The mass spectrometry of gold clusters stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP) revealed the presence of Cl adsorbates derived from synthetic precursors, mainly on the Au₃₄ and Au₄₃ clusters. Changes in the amount of Cl adsorbates on the Au clusters did not affect the catalytic properties for the aerobic oxidation of benzyl alcohol, suggesting that the Cl atoms were only weakly bound to the Au clusters. In contrast, the replacement of Cl with Br on the Au₃₄ and Au₄₃ clusters significantly suppressed activity, without any influence on the electronic structure. This result indicated that the Br atoms were strongly bound to the Au clusters and sterically blocked their active sites. The substantial reduction of the catalytic activity by the Br adsorbates suggested that the Au₃₄ and Au₄₃ clusters made a major contribution to the catalytic activity of the Au:PVP.

Lewis acid-driven reaction pathways in synergistic cooperative catalysis over gold/palladium bimetallic nanoparticles for hydrogen autotransfer reaction between amide and alcohol

Hiroyuki Miyamura, Satoshi Isshiki, Hyemin Min, Shū Kobayashi

2016, 37 (10): 1662-1668. DOI: 10.1016/S1872-2067(16)62483-X

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Metal nanoparticle catalysts, especially gold and its bimetallic nanoparticle catalysts, have been widely used in organic transformations as powerful and green catalysts. The concept of employing two distinct catalysts in one reaction system, such as in cooperative and synergistic catalysis, is a powerful strategy in homogeneous catalysis. However, the adaption of such a strategy to metal nanoparticle catalysis is still under development. Recently, we have found that cooperative catalytic systems of gold/palladium bimetallic nanoparticles and Lewis acid can be used for the N-alkylation of primary amides through hydrogen autotransfer reaction between amide and alcohol. Herein, the results of a detailed investigation into the effects of Lewis acids on this hydrogen autotransfer reaction are reported. It was found that the choice of Lewis acid affected not only the reaction pathway leading to the desired product, but also other reaction pathways that produced several intermediates and by-products. Weak Lewis acids, such as alkaline-earth metal triflates, were found to be optimal for the desired N-alkylation of amides.

Highly selective supported gold catalyst for CO-driven reduction of furfural in aqueous media

Jing Dong, Mingming Zhu, Gaoshuo Zhang, Yongmei Liu, Yong Cao, Su Liu, Yangdong Wang

2016, 37 (10): 1669-1675. DOI: 10.1016/S1872-2067(16)62458-0

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The reductive transformation of furfural (FAL) into furfuryl alcohol (FOL) is an attractive route for the use of renewable bio-sources but it suffers from the heavy use of H₂. We describe here a highly efficient reduction protocol for converting aqueous FAL to FOL. A single phase rutile TiO₂ support with a gold catalyst (Au/TiO₂-R) that used CO/H₂O as the hydrogen source catalyze this reduction efficiently under mild conditions. By eliminating the consumption of fossil fuel-derived H₂, our process has the benefit afforded by using CO as a convenient and cost competitive reducing reagent.

Mechanism and active sites of CO oxidation over single-crystal Au surfaces and a Au/TiO₂(110) model surface

Tadahiro Fujitani, Isao Nakamura

2016, 37 (10): 1676-1683. DOI: 10.1016/S1872-2067(16)62516-0

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We describe the reaction mechanism and active sites for CO oxidation over a Au/TiO₂(110) model surface and Au single-crystal surfaces, along with the role of H₂O, on a molecular scale. At low temperature (<320 K), H₂O played an essential role in promoting CO oxidation, and the active site for CO oxidation was the perimeter of the interface between the gold nanoparticles and the TiO₂ support (Au^δ+-O^δ--Ti). We believe that the O-O bond was activated by the formation of OOH, which was produced directly from O₂ and H₂O at the perimeter of the interface between the gold nanoparticles and the TiO₂ support, and consequently OOH reacted with CO to form CO₂. This reaction mechanism explains the dependence of the CO₂ formation rate on O₂ pressure at 300 K. In contrast, at high temperature (>320 K), low-coordinated gold atoms built up on the surface as a result of surface reconstruction due to exposure to CO. The low-coordinated gold atoms adsorbed O₂, which then dissociated and oxidized CO on the metallic gold surface.

Formation and removal of active oxygen species for the non-catalytic CO oxidation on Au/TiO₂ catalysts

Daniel Widmann, R. Jürgen Behm

2016, 37 (10): 1684-1693. DOI: 10.1016/S1872-2067(16)62452-X

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Applying quantitative temporal analysis of products reactor measurements, we studied the reactive removal of active oxygen present on Au/TiO₂ catalysts after calcination at elevated temperatures (400℃) by CO pulses and its replenishment by O₂ pulses at 80℃, focusing on the nature of the active oxygen species. In contrast to previous studies, which mainly focused on and clarified the nature of the active oxygen species for the catalytic CO oxidation, which is reversibly formed and replenished under typical reaction conditions, this study demonstrates that directly after calcination an additional oxygen species is present. This species is also active for the CO oxidation, but it is not or only very little formed under typical reaction conditions. Implications of these results on the mechanistic understanding of the CO oxidation on Au/TiO₂, in particular on the role of different active oxygen species, will be discussed.

Preparation of gold nanoparticles supported on Nb₂O₅ by deposition precipitation and deposition reduction methods and their catalytic activity for CO oxidation

Toru Murayama, Masatake Haruta

2016, 37 (10): 1694-1701. DOI: 10.1016/S1872-2067(16)62508-1

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Nanoparticulate gold catalysts supported on niobium oxides (Nb₂O₅) were prepared by different deposition methods. The deposition precipitation (DP) method, DP method with urea, deposition reduction (DR) method and one-pot method were used to prepare a 1 wt% Au/Nb₂O₅ catalyst. Layered-type Nb₂O₅ synthesized by a hydrothermal method (Nb₂O₅(HT)) was the most suitable as a support among various types of Nb₂O₅ including commercially available Nb₂O₅ samples. It appeared that the large BET surface area of Nb₂O₅(HT) enabled the dispersion of gold as nanoparticles (NPs). Gold NPs with a mean diameter of about 5 nm were deposited by both the DP method and DR method on Nb₂O₅(HT) under an optimized condition. The temperature for 50% CO conversion for Au/Nb₂O₅(HT) prepared by the DR method was 73℃. Without deposition of gold, Nb₂O₅(HT) showed no catalytic activity for CO oxidation even at 250℃. Therefore, the enhancement of the activity by deposition of gold was remarkable. This simple Au/Nb₂O₅ catalyst will expand the types of gold catalysts to acidic supports, giving rise to new applications.

Support effect of zinc tin oxide on gold catalyst for CO oxidation reaction

Wei Li, Linying Du, Chunjiang Jia, Rui Si

2016, 37 (10): 1702-1711. DOI: 10.1016/S1872-2067(16)62468-3

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Nanostructured gold catalyst supported on metal oxide is highly active for the CO oxidation reaction. In this work, a new type of oxide support, zinc tin oxide, has been used to deposit 0.7 wt% Au via a deposition-precipitation method. The textural properties of Zn₂SnO₄ support have been tuned by varying the molar ratio between base (N₂H₄·H₂O) and metal ion (Zn²⁺) to be 4/1, 8/1 and 16/1. The catalytic tests for CO oxidation reaction revealed that the reactivity on Au-Zn₂SnO₄ with N₂H₄·H₂O/Zn²⁺=8/1 was the highest, while the reactivity on Au-Zn₂SnO₄ with N₂H₄·H₂O/Zn²⁺=16/1 was almost identical to that of the pure support. Both fresh and used catalysts have been characterized by multiple techniques including nitrogen adsorption-desorption, X-ray diffraction, transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray adsorption fine structure, and temperature-programmed reduction by hydrogen. These demonstrated that the textural properties, especially pore volume and pore size distribution, of Zn₂SnO₄ play crucial roles in the averaged size of gold nanoparticles, and thus determine the catalytic activity of Au-Zn₂SnO₄ for CO oxidation.

An examination of active sites on Au-Ag bimetallic catalysts based on CO oxidation over Au/Ag₂O and a comparison to Ag-contaminated Au powder

Yasuo Iizuka, Yasuhiro Hiragi, Hikaru Yakushiji, Takumi Miura

2016, 37 (10): 1712-1720. DOI: 10.1016/S1872-2067(16)62541-X

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There are two theories regarding the origin of the remarkable synergistic effect observed in Au-Ag bimetallic catalysts when applied to various oxidative reactions. One is based on the importance of the contact interfaces between AgO_x regions and the surface of the bulk Au as active working sites, while the other holds that charge transfer from Ag to Au in a surface Au-Ag alloy causes the catalytic activity. One key point in examining these theories and determining the origin of the synergy involves determining whether or not Ag exists as an oxide or as a metallic alloy on the Au surface. To confirm that enhanced activity results from contact between Ag₂O and Au nanoparticles (NPs), a comparative study of catalytic CO oxidation over Au/Ag₂O and Ag₂O was performed in the present work, using a closed recirculation reaction system. A reaction mixture consisting of a stoichiometric composition of CO and O₂ (CO/O₂=2/1) was supplied to both catalysts and the resulting pressure decrease rates were tracked, from which the amounts of gas consumed as well as the quantity of CO₂ produced were determined. The steady state reactions of both Au/Ag₂O and Ag₂O did not lead to any meaningful difference in the rate of pressure decrease during the oxidation. The pressure decrease over both catalysts was attributed to the reduction of surface lattice O on Ag₂O by CO. The results obtained for Au/Ag₂O are in good agreement with previous data resulting from the use of Ag-contaminated Au powder (Ag/Au-b) having an oxidized surfaces. This finding suggests that the perimeters between AgO_x zones and the bulk Au surface may not function as active sites during CO oxidation. A review of previous results obtained with Ag/Au-b specimens having so-called steady state surfaces indicates that AgO_x species in such materials are reduced to the 0 state to form a Ag-Au alloy that provides the active sites.

Enhanced catalytic activities and selectivities in preferential oxidation of CO over ceria-promoted Au/Al₂O₃ catalysts

Yu-Xin Miao, Jing Wang, Wen-Cui Li

2016, 37 (10): 1721-1728. DOI: 10.1016/S1872-2067(16)62469-5

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The preferential oxidation of CO (CO-PROX) is a hot topic because of its importance in pro-ton-exchange membrane fuel cells (PEMFCs). Au catalysts are highly active in CO oxidation. However, their activities still need to be improved at the PEMFC operating temperatures of 80-120℃. In the present study, Au nanoparticles of average size 2.6 nm supported on ceria-modified Al₂O₃ were synthesized and characterized using powder X-ray diffraction, nitrogen physisorption, transmission electron and scanning transmission electron microscopies, temperature-programmed hydrogen reduction (H₂-TPR), Raman spectroscopy, and in situ diffuse-reflectance infrared Fourier-transform spectroscopy. Highly dispersed Au nanoparticles and strong structures formed by Au-support interactions were the main active species on the ceria surface. The Raman and H₂-TPR results show that the improved catalytic performance of the Au catalysts can be attributed to enhanced strong metal-support interactions and the reducibility caused by ceria doping. The formation of oxygen vacancies on the catalysts increased their activities in CO-PROX. The synthesized Au catalysts gave excellent catalytic performances with high CO conversions (>97%) and CO₂ selectivities (>50%) in the temperature range 80-150℃.

Gold stabilized on various oxide supports catalyzing formaldehyde oxidation at room temperature

Bingbing Chen, Xiaobing Zhu, Yidi Wang, Limei Yu, Chuan Shi

2016, 37 (10): 1729-1737. DOI: 10.1016/S1872-2067(16)62470-1

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Gold stabilized on reducible oxide (CeO₂ and FeO_x) and irreducible oxide (γ-Al₂O₃, SiO₂, and HZSM-5) were prepared by deposition precipitation method and tested for catalytic oxidation of formaldehyde (HCHO) at room temperature under high GHSV of 600000 ml/(g·s). Au/γ-Al₂O₃ catalyst showed distinctive catalytic performance, presenting the highest initial HCHO conversion and stability. Correlating the reaction rate with Au particle size, there is a linear relationship, suggesting that the smaller Au particle size with higher dispersion possesses high reactivity for HCHO oxidation. All the catalysts deactivated at high GHSV (600000 ml/(g·s)), but in a quite different rate. Reducible oxide (CeO₂ and FeO_x) could stabilize gold through O linkage and therefore exhibits a better stability for HCHO oxidation reaction. However, the aggregation of gold particles occurred over Au/SiO₂ and Au/HZSM-5 catalysts, which result in the fast deactivation. Therefore, our results suggest that the reducibility of the supports for Au catalysis has no direct influence on the activity, but affects the catalytic stability.

Oxidation of formic acid on stepped Au(997) surface

Zong-Fang Wu, Zhi-Quan Jiang, Yue-Kang Jin, Feng Xiong, Guang-Hui Sun, Wei-Xin Huang

2016, 37 (10): 1738-1746. DOI: 10.1016/S1872-2067(16)62467-1

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The adsorption and reaction of formic acid (HCOOH) on clean and atomic oxygen-covered Au(997) surfaces were studied by temperature-programmed desorption/reaction spectroscopy (TPRS) and X-ray photoelectron spectroscopy (XPS). At 105 K, HCOOH molecularly adsorbs on clean Au(997) and interacts more strongly with low-coordinated Au atoms at (111) step sites than with those at (111) terrace sites. On an atomic oxygen-covered Au(997) surface, HCOOH reacts with oxygen atoms to form HCOO and OH at 105 K. Upon subsequent heating, surface reactions occur among adsorbed HCOO, OH, and atomic oxygen and produce CO₂, H₂O, and HCOOH between 250 and 400 K. The Au(111) steps bind surface adsorbates more strongly than the Au(111) terraces and exhibit larger barriers for HCOO(a) oxidation reactions. The surface reactions also depend on the relative coverages of co-existing surface species. Our results elucidate the elementary surface reactions between formic acid and oxygen adatoms on Au surfaces and highlight the effects of the coordination number of the Au atoms on the Au catalysis.

Catalytic cracking of light diesel over Au/ZSM-5 catalyst for increasing propylene production

Caixia Qi, Yunxia Wang, Xiaotao Ding, Huijuan Su

2016, 37 (10): 1747-1755. DOI: 10.1016/S1872-2067(16)62499-3

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The catalytic cracking of light diesel oil (235-337℃) over gold-modified ZSM-5 was investigated in a small confined fluidized bed at 460℃ and ambient pressure. Different Au/ZSM-5 catalysts were prepared by a modified deposition-precipitation method by changing the preparation procedure and the amount of gold loading and were characterized by X-ray diffraction, N₂ adsorption-desorption, temperature-programmed desorption of NH₃, transmission electron microscopy and inductively coupled plasma spectrometer. It was found that a small amount of gold had a positive effect on the catalytic cracking of light diesel oil and increased propylene production at a relatively low temperature. The maintenance of the ZSM-5 MFI structure, pore size distribution and the density of weak and strong acid sites of the Au/ZSM-5 catalysts depended on the preparation parameters and the Au loading. Simultaneous enhancement of the micro-activity and propylene production relies on a synergy between the pore size distribution and the relative intensity of the weak and strong acid sites. A significant improvement in the micro-activity index with an increase of 4.5 units and in the propylene selectivity with an increase of 23.2 units was obtained over the Au/ZSM-5 catalyst with an actual Au loading of 0.17 wt%.

Selective reductive coupling of nitro aliphatic compounds with aldehydes in hydrogen using gold catalyst

Larisha Cisneros, Pedro Serna, Avelino Corma

2016, 37 (10): 1756-1763. DOI: 10.1016/S1872-2067(16)62493-2

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Nitrones were synthesized in good yields directly from nitro aliphatic compounds, aldehydes, and H₂ using highly dispersed gold nanoparticles on titania. The high selectivity for nitrone synthesis contrasts with the platinum supported on carbon and corresponds to an increase from roughly 50% to 90%. The catalytic performance is tuned by precise control of the struc-ture of the active sites, the characteristics of the support and reaction conditions.

Hydrogen auto-transfer under aerobic oxidative conditions: Efficient synthesis of saturated ketones by aerobic C-C cross-coupling of primary and secondary alcohols catalyzed by a Au₆Pd/resin catalyst

Maoxiang Zhou, Leilei Zhang, Jeffrey T. Miller, Xiaofeng Yang, Xiaoyan Liu, Aiqin Wang, Tao Zhang

2016, 37 (10): 1764-1770. DOI: 10.1016/S1872-2067(16)62511-1

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Au and Au-containing bimetallic nanoparticles are promising catalysts for the green synthesis of fine chemicals. Here, we used a Au₆Pd/resin catalyst for the aerobic C-C cross-coupling of primary and secondary alcohols to produce higher ketones under mild conditions. This is of importance to the construction of a C-C bond. Various substrates were used in the reaction system, and moderate to good yields were obtained. The catalysts can be reused at least five times without decrease of yield. The control experiment and XAFS characterization results showed that hydrogen auto-transfer occurred on metallic Pd sites even under oxidative conditions. On alloying with Au, the Pd sites became resistant to oxidation and readily abstracted the β-H of the alcohols and transferred the hydride to the C=C bond in the reaction intermediate to give the saturated product.

Gold-iridium catalysts for the hydrogenation of biomass derived products

Lorenzo Landenna, Alberto Villa, Rodolfo Zanella, Claudio Evangelisti, Laura Prati

2016, 37 (10): 1771-1775. DOI: 10.1016/S1872-2067(16)62512-3

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Au-Ir and Au-Ru on TiO₂ catalysts prepared by sequential deposition-precipitation technique were compared with the corresponding monometallics in the hydrogenation of levulinic acid to γ-valerolactone. Interestingly the addition of Au to Ir/TiO₂ showed a detrimental effect on the activity of Ir monometallic catalyst whereas a positive synergistic effect was shown in the case of Ru. Both catalysts were reduced under H₂ to increase the M⁰-Au⁰ interaction. From previous DFT calculations and catalytic test, we addressed the lower activity of Au-Ir/TiO₂ than that of Ir/TiO₂ to the interference of Au into the redox mechanism of Ir atoms.

Gold-doping of carbon-supported palladium improves reduction catalysis

Yu-Lun Fang, Kimberly N. Heck, Zhun Zhao, Lori A. Pretzer, Neng Guo, Tianpin Wu, Jeffrey T. Miller, Michael S. Wong

2016, 37 (10): 1776-1786. DOI: 10.1016/S1872-2067(16)62530-5

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Bimetallic palladium-gold (PdAu) catalysts have better catalytic performance than monometallic catalysts for many applications. PdAu catalysts with controlled nanostructures and enhanced activities have been extensively studied but their syntheses require multiple and occasionally complicated steps. In this work, we demonstrated that supported PdAu catalysts could be simply prepared by doping a supported Pd catalyst with gold through wet impregnation and calcination. Resulting PdAu-on-carbon (PdAu/C) catalysts were tested for the room-temperature, aqueous-phase hydrodechlorination of trichloroethene. The most active PdAu/C catalyst (Pd 1.0 wt%, Au 1.1 wt%, dried/air/H₂ process) had an initial turnover frequency (TOF) of 34.0×10^-2 mol_TCE mol_Pd^-1 s^-1, which was >15 times higher than monometallic Pd/C (Pd 1.0 wt%, initial TOF of 2.2×10^-2 mol_TCE mol_Pd^-1 s^-1). Through X-ray absorption spectroscopy, the gold kept Pd from oxidizing under calcination at 400℃. Probable nanostructure evolution pathways are proposed to explain the observed catalysis.

Thermally robust silica-enclosed Au₂₅ nanocluster and its catalysis

Haijun Chen, Chao Liu, Min Wang, Chaofeng Zhang, Gao Li, Feng Wang

2016, 37 (10): 1787-1793. DOI: 10.1016/S1872-2067(16)62478-6

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Well-defined gold nanoclusters with average size less than 2 nm have emerged as a new and novel catalyst. The gold nanocluster loaded on the oxide surface usually aggregates to larger particles at high temperature (>300℃), which is caused by the removal of the surface ligands. We herein present a novel method to prepare Au₂₅ cluster catalyst (~1.3 nm) with high thermal stability (up to 400℃). Au₂₅@SiO₂ is synthesized via a co-hydrolyzing reaction of Au₂₅[SC₃H₆Si(OCH₃)₃]₁₈ and tetraethyl orthosilicate, and then it is treated at different temperature (e.g., 200, 300, 400℃) in air to remove the organic ligands. Au₂₅@SiO₂ is well characterized by transmission electron microscopy, ultraviolet-visible spectroscopy and diffuse reflectance UV-vis spectroscopy. Further, the Au₂₅@SiO₂ catalysts are investigated in the hydrogenation of p-nitrophenol into p-aminophenol.

C-X(X=Cl, Br, I) bond dissociation energy as a descriptor for the redispersion of sintered Au/AC catalysts

Xinping Duan, Yan Yin, Xuelin Tian, Jinhuo Ke, Zhaojun Wen, Jianwei Zheng, Menglin Hu, Linmin Ye, Youzhu Yuan

2016, 37 (10): 1794-1803. DOI: 10.1016/S1872-2067(16)62500-7

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Disintegration or redispersion of supported sintered gold nanoparticles (Au NPs) in the presence of alkyl halide can give catalyst regeneration or redispersion of sintered Au catalysts. The selectivity of alkyl halides, temperature and size distributions were investigated to elucidate the redispersion of Au NPs during halide-induced decomposition. This study proved that the alkyl halide induced the redispersion of sintered Au NPs which depended on the R-X (X=I, Br, Cl) bond dissociation energy (BDE) and thus provided a simple descriptor for the regeneration of inactive supported Au catalysts. A correlation between the BDE of R-X and dispersion efficiency was established. The tendency for disintegration and redispersion followed the R-X BDE of the alkyl halide. Compared to alkyl chlorides and bromides, iodides were more efficient for redispersing sintered Au NPs. As a descriptor, the BDE of R-I played a crucial role in particle redispersion. These findings provided insights into the mechanism of organic halide-induced Au NP disintegration and the effect of the halide type on the redispersion of sintered catalysts.

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