Heterogeneous Gold Catalysts and Catalysis: Rsc: Volume 18 (Catalysis Series) - Hardcover

Hardcover

ISBN 10: 184973917X ISBN 13: 9781849739177

Publisher: Royal Society of Chemistry, 2014

Synopsis

Presenting a comprehensive review of this rapidly-evolving field, this book provides postgraduates and newcomers to the field with a thorough background to the use of gold in catalysis.

About this title

About the Author

Zhen Ma was born in Shanghai in November of 1976. He enrolled as a chemistry student at Fudan University in 1994. In route to earning a BS in chemistry, Zhen carried out research on catalysis of mesoporous silica-supported heteropolyacids under the direction of Prof. Zi Gao. After graduating in 1998, Zhen became an MSc student in the same university to study the preparation and environmental catalysis (Freon decomposition) of novel solid strong acids under Prof. Zi Gao. There he earned an MS degree in chemistry in 2001. Subsequently, he moved to University of California, Riverside and studied in situ spectroscopic characterization of liquid-solid interfaces of catalysis relevance with Prof. Francisco Zaera, and earned a PhD degree in chemistry in 2006. He then moved to Oak Ridge National Laboratory as a postdoc and studied environmental catalysis (CO oxidation and water-gas shift) using gold catalysts with Dr. Sheng Dai. He stared as an Associate Professor at Fudan University in December of 2009. Currently, his research interest is still on environmental catalysis by gold catalysts and other nanomaterials.

From the Back Cover

Once considered an inert element, gold has recently gained attention as a catalyst. With hundreds of papers being published each year, this book presents a comprehensive review of this rapidly-evolving field, with contributions by leading experts across the globe. Going through the chapters citing the primary literature, the reader will gain a thorough background to the use of gold in catalysis, as well as the latest methods for the preparation of gold catalysts. Other chapters demonstrate the characterisation and modelling of gold-catalysed reactions, with consideration given to both the fundamentals and commercial applications of this emerging group of catalysts.

Written to be accessible by postgraduates and newcomers to the field, this book will also benefit experienced researchers and therefore be an essential reference in the laboratory.

Excerpt. © Reprinted by permission. All rights reserved.

Heterogeneous Gold Catalysts and Catalysis

By Zhen Ma, Sheng Dai

The Royal Society of Chemistry

Copyright © 2014 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-1-84973-917-7

Contents

Chapter 1 Stabilizing Gold Nanoparticles by Solid Supports Zhen Ma and Sheng Dai, 1, 
Chapter 2 Well-defined Gold Nanoclusters for Catalytic Applications Gao Li and Rongchao Jin, 27, 
Chapter 3 Nanocrystal Superlattices: Novel Model Materials for Study of Catalysis Yijin Kang, Vojislav R. Stamenkovic and Christopher B. Murray, 47, 
Chapter 4 Preferential Oxidation of Carbon Monoxide in Hydrogen Stream over Gold Catalysts Yu-Wen Chen and Natarajan Sasirekha, 73, 
Chapter 5 Twenty Years of Golden Future in the Water Gas Shift Reaction T. Ramírez Reina, M. González Castaño, S. Palma, S. Ivanova and J. A. Odriozola, 111, 
Chapter 6 Gold Catalysis in the Complete Oxidation or Decomposition of Small Molecule Pollutants Chunyan Ma and Zhengping Hao, 140, 
Chapter 7 Superficial Modification of Semiconductors with Gold Nanoparticles for Photocatalytic Applications Juan Carlos Durán-Álvarez, Rodolfo Zanella and Socorro Oros-Ruíz, 155, 
Chapter 8 Advances in Catalytic Reactions by Gold-based Catalysts Through the Radical Chain Mechanism Xue-Jing Yang, Like Ouyang, Jing Xu and Yi-Fan Han, 229, 
Chapter 9 Heterogeneous Gold Catalysts for Selective Oxidation Reactions Poernomo Gunawan, Rong Xu and Ziyi Zhong, 288, 
Chapter 10 Selective Oxidation of Biomass-Derived Secondary Alcohols Olga A. Simakova and Dmitry Yu. Murzin, 401, 
Chapter 11 Harnessing the Selective Catalytic Action of Supported Gold in Hydrogenation Applications Xiaodong Wang, Maoshuai Li and Mark A. Keane, 424, 
Chapter 12 Metal-Support Interaction Effects on Gold Catalysts over Reducible Oxides Hongjing Wu and Leonarda F. Liotta, 462, 
Chapter 13 Role of Oxygen Vacancies in Gold Oxidation Catalysis O. H. Laguna, M. I. Domínguez, F. Romero-Sarria, J. A. Odriozola and M. A. Centeno, 489, 
Chapter 14 Infrared Spectroscopic Insights into the Role of the Support in Heterogeneous Gold Catalysis Zili Wu and Steven H. Overbury, 512, 
Chapter 15 Model Catalysts for Au Catalysis: From Single Crystals to Supported Nanoparticles Weixin Huang, 533, 
Subject Index, 575, 

CHAPTER 1

Stabilizing Gold Nanoparticles by Solid Supports

ZHEN MA AND SHENG DAI

1.1 Introduction

Catalysis by nanostructured materials has attracted tremendous interest recently. Nanostructured catalysts may have interesting catalytic properties associated with their small sizes and geometric/electronic structures. In particular, Haruta and co-workers found that gold nanoparticles finely dispersed on some metal oxide supports have excellent activities in low-temperature CO oxidation. This finding has been followed by thousands of studies on supported gold catalysts and their catalytic applications in environmental catalysis and chemical synthesis.

Gold nanoparticles may be synthesized via a traditional colloidal chemistry approach, in which AuCl4- ions are reduced by sodium citrate, tetrakis(hydroxymethyl)phosphonium chloride (THPC) or sodium borohydride (NaBH4), and the formed gold nanoparticles can be stabilized by polyvinylalcohol (PVA), polyvinylpyrrolidone (PVP), polydiallyldimethylammonium chloride (PDDA), or cetyltrimethylammonium bromide (CTAB). These gold nanoparticles (colloids) can be used either directly as catalysts in liquid phase or deposited onto solid supports.

Alternatively, gold nanoparticles can be formed on solid supports by loading a gold precursor (usually a gold salt or complex) onto solid supports followed by reduction or calcination. During the reduction or calcination process, the gold cations are reduced into gold atoms that aggregate into gold nanoparticles. The extent of agglomeration during this stage depends on many factors such as the ambient, temperature, and duration of the process as well as the nature of solid supports.

Supported metal catalysts are usually composed of metal nanoparticles and solid supports. Solid supports may provide a platform for dispersing and stabilizing gold nanoparticles so as to expose more surface gold atoms to the reactants, thus increasing catalytic activity. They may tune the oxidation state of gold by charge transferring or by mediating the reducing degree of gold precursors upon calcination or reduction. Some supports may undergo phase transformation or structural collapse under high temperatures, thus aggravating the sintering of gold nanoparticles on these supports or leading to the encapsulation of gold nanoparticles by these supports. Figure 1.1(c) shows a schematic diagram illustrating the phase transformation of a support at high temperatures. Besides the facets related to metal–support interactions mentioned above, solid supports may participate in catalysis by adsorbing and activating reactants as well as supplying active oxygen. They may also, of course, strongly adsorb some reaction intermediates or products, leading to catalyst deactivation.

Although solid supports can disperse gold nanoparticles, the sintering of gold nanoparticles at elevated temperatures is often inevitable because of their low melting points and high surface free energies. Figure 1.1(a) and Figure 1.1(b) show two models (crystalline migration and atom migration) proposed for the sintering of metal nanoparticles on supports. In the first model, entire metal crystallites migrate, collide, and coalesce on the support surface. In the second model, metal atoms migrate from one crystallite to another via the surface or the gas phase, making big crystallite bigger and small crystallites smaller. The phase transformation or structural collapse of supports under elevated temperatures, as shown in Figure 1.1(c), may exacerbate the sintering or encapsulation of gold nanoparticles.

Because the catalytic activities of supported gold catalysts often decrease sharply as the gold nanoparticle agglomerate under elevated temperatures but a high temperatures is often encountered during the calcination, operation, and regeneration of catalysts, it is necessary to enhance the thermal stability of supported gold nanoparticles. This can be achieved by improving synthesis details, e.g., thoroughly washing away residual chloride that may facilitate sintering. However, a more versatile way is to tune the structural environment surrounding gold nanoparticles, e.g., by strengthening the metal-support interaction and by designing sturdy inorganic shells that encapsulate gold nanoparticles. These strategies rely on synthesis and modification of catalytic materials.

Most of the publications relevant to gold catalysis deal with the conventional synthesis, characterization, and applications of supported gold nanoparticles, as well as the elucidation of the nature of active sites and reaction mechanisms. Only a small portion of publications have addressed the thermal stability and stabilization of gold nanoparticles on solid supports. The Dai group at the US Oak Ridge National Laboratory has been interested in designing new-structured gold nanocatalysts with enhanced properties, including catalytic activity, stability on stream, and thermal stability. It is known from these studies that catalytic performance and thermal stability of supported gold catalysts depend critically on their composition and catalyst structure. Below we first summarize some recent advances in the stabilization of gold nanoparticles by solid supports, and then furnish our perspectives on future development.

1.2 Catalysts with Complex Interfacial Structures

1.2.1 General Considerations

Supported gold catalysts are usually prepared by loading gold onto supports (e.g., TiO2, ZrO2, SiO2, Al2O3, Fe2O3, and CeO2) via impregnation, deposition–precipitation, co-precipitation, and colloidal deposition. When supported gold catalysts are prepared by impregnation, deposition–precipitation, or co-precipitation methods, the gold precursors (gold cations) reduce to metallic gold atoms that migrate and aggregate into gold nanoparticles upon reduction or calcination. When gold catalysts are prepared by loading preformed gold nanoparticles (colloids) onto supports, it is not necessary to reduce the metallic gold nanoparticles again, but a calcination or pretreatment step may still be needed to remove organic capping agents that may influence catalytic activity and to enhance the metal–support interaction.

Regardless of preparation methods, the formed supported gold catalysts usually have simple metal–support interfaces (e.g., Au-TiO2). The sintering of gold nanoparticles on these neat metal oxide supports is a common problem under elevated temperatures. Attempts have been made to build up more complex interfaces for enhancing the thermal stability. For instance, Figure 1.2 illustrates the structural feature of Au/TiO2/SiO2 (i.e., gold nanoparticles supported on TiO2-modified SiO2 support), highlighting the additive–support (TiO2-SiO2) and metal–additive (Au-TiO2) interfaces in addition to the metal–support (Au-SiO2) interface. The presence of additional interfaces or complex structures may mitigate the sintering of gold nanoparticles due to the enhanced metal–support interaction.

1.2.2 Pre-modification of Supports before Loading Gold

Au/SiO2 catalysts are usually not very active for CO oxidation and gold nanoparticles on SiO2 can sinter easily, unless the preparation method is carefully chosen. To enhance the thermal stability, one idea is to modify the SiO2 support by another metal oxide before loading gold. For instance, Tai and co-workers developed Au/TiO2/SiO2 catalysts. SiO2 wet-gel was prepared by the hydrolysis of Si(OCH3)4 in the presence of NH4OH, and was subsequently soaked in a toluene solution of Ti(iso-OC3H7)4. Dodecanethiol-capped gold nanoparticles (2.1 nm) were then deposited onto TiO2/SiO2. For comparison, a TiO2 support was prepared using Ti(iso-OC3 H7)4 as the precursor, and was used to load gold nanoparticles. As shown in Figure 1.3, gold nanoparticles in Au/TiO2/SiO2 were still small (average diameter 2.2 nm) after calcination in air at 400 °C, whereas those in Au/TiO2 grew obviously (average diameter 4.0 nm). Although the authors did not show the sintering behavior of gold nanoparticles on a neat SiO2 support, this study nicely showed that gold nanoparticles exhibit high thermal stability on TiO2-modified SiO2 gel.

In another work, Yan et al. developed Au/TiO2 /mesoporous SiO2. The mesoporous SiO2 (SBA-15) surface was functionalized by amorphous TiO2 via a surface–sol-gel method, using Ti(OC4H9)4 as the precursor. Gold was then loaded onto the support via a deposition-precipitation method. For comparison, Au/P25 TiO2 was also prepared. Here the 'P25 TiO2' refers to a commercial TiO2 furnished by Degussa. Although the as-synthesized Au/P25 TiO2 showed high activity in CO oxidation when the reaction temperature was below -20 °C, the Au/P25 TiO2 calcined at 300 °C was much less active due to the aggregation of gold nanoparticles. For comparison, the activities of the as-synthesized and 300 °C calcined Au/TiO2/mesoporous SiO2 were similar, due to the preservation of small gold nanoparticles at 300 °C. The authors additionally showed that it was difficult to load gold onto mesoporous SiO2via deposition–precipitation due to the low isoelectric point of SiO2, and the obtained gold nanoparticles were usually big.

Above, we have highlighted two examples for the pre-modification of SiO2 supports by TiO2 before loading gold. The presence of TiO2 species not only stabilizes gold nanoparticles on the modified supports, but also increases the isoelectric point of supports (note that the isoelectric point of TiO2 is higher than that of SiO2), thus increasing the gold loading when the deposition–precipitation method is used to load gold. In addition, the additional Au-TiO2 interface leads to high activity in CO oxidation. SiO2 supports can also be modified by other metal oxides (e.g., CoOx, ZnO, CeO2, CuO) to increase the dispersion of gold nanoparticles on supports. The role of these metal oxide additives is similar to that of the TiO2 additive mentioned above.

Au/TiO2 is the most studied gold catalyst. If prepared properly, it should be active for low-temperature CO oxidation, but the sintering of gold nanoparticles on TiO2 is still a problem. Yan et al. developed a new catalyst, Au/Al2O3/ TiO2, for CO oxidation. First, the P25 TiO2 support was modified by amorphous Al2O3via surface–sol-gel processing of Al(sec-OC4H9)3 followed by controlled hydrolysis. Gold was then loaded onto Al2O3/TiO2via deposition–precipitation using HAuCl4 as the precursor. Interestingly, the gold nanoparticles on the Al2O3/TiO2 support showed excellent thermal stability upon aging at 500 °C, whereas gold nanoparticles on neat TiO2 sintered significantly.

Ma et al. subsequently prepared Au/MxOy/TiO2 catalysts. In the preparation, the surface–sol-gel method was not used. Instead, a traditional impregnation method was used to load metal nitrates onto TiO2. The metal nitrates/TiO2 were calcined to form MxOy/TiO2 supports, and gold was loaded via deposition–precipitation. The use of impregnation instead of the surface–sol-gel method to functionalize TiO2 support was based on several considerations. First, it was thought that a catalyst (e.g., Au/Al2O3/ TiO2) prepared involving decomposing a metal nitrate (e.g., Al(NO3)3) on TiO2 support followed by loading gold should exhibit a similar performance compared with its counterpart prepared using a surface–sol-gel method. Second, the nitrate decomposition method is less demanding and therefore suitable for large-scale preparation. Third, the synthesis via the surface–solgel method is constrained by the availability, expensiveness, and storage stability of metal alkoxide precursors.

It was found that Au/MxOy/TiO2 (M = Ca, Ni, Zn, Ga, Y, Zr, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb) retained significant activity in CO oxidation even after thermal treatment at 500 °C. This was explained by the enhanced thermal stability of gold nanoparticles caused by the surface modification of TiO2 support by certain metal oxides, as demonstrated by X-ray diffraction (XRD) and transmission electron microscopy (TEM) data. In addition, it was speculated that the presence of metal oxide additives with redox properties adjacent to gold nanoparticles may change the oxidation state of gold and the redox property of the support. More experiments should be performed to better understand the promotional effects of the metal oxide additives on TiO2 support.

The empirical observations mentioned above have led to further fundamental research. Liu and co-workers shed light on the stabilizing effect of the amorphous Al2O3 by means of density functional theory (DFT) calculations. Figure 1.4 shows the models proposed for the local structures of a gold atom or a two-layer gold strip on an Al2O3/TiO2 surface. These models were used for their theoretical calculations. The authors found that the binding of gold on Al2O3/TiO2 was much stronger than that on TiO2, and the stronger binding was valid for some other metals (i.e., Ag, Cu, Pt, Pd, Ir) on Al2O3/TiO2 compared with these metals on TiO2. This finding explained the enhanced thermal stability of gold nanoparticles on Al2O3/ TiO2 versus TiO2. A further idea would be to extend this DFT approach to the Au/MxOy/ TiO2 system to obtain deeper insights.

The pre-modification strategy is not limited to the modification of oxide supports by another metal oxide. For instance, TiO2 was treated by an aqueous H3PO4 solution before loading gold. That treatment was found to enhance the thermal stability of gold nanoparticles. A similar stabilization effect was found with Au/H3PO4-Al2O3 and Ag/H3PO4-TiO2. However, the presence of phosphorus species also suppressed the catalytic activities. In addition, it is not clear why the H3PO4 treatment can help stabilize gold or silver nanoparticles.

1.2.3 Post-modification of Supported Gold Catalysts

Supported gold catalysts are usually prepared by loading gold onto neat metal oxide supports such as TiO2. However, few attempts have been made to additionally modify supported gold catalysts when gold was already loaded onto solid supports. Ma et al. developed PO43 —/Au/TiO2 catalysts. In the synthesis, Au/TiO2 was prepared by deposition–precipitation, reduced at 150 °C in H2–Ar, and soaked in a diluted H3 PO4 solution, followed by washing and drying. This treatment can stabilize gold nanoparticles on TiO2, as evidenced by the fact that PO43-/Au/TiO2 calcined at 500 °C still had small gold nanoparticles and appreciable activity in CO oxidation at room temperature. However, overloaded phosphate ions may lead to low catalytic activity due to the blockage of active sites. In addition, it is not clear why the H3PO4 treatment can stabilize gold nanoparticles.

(Continues...)

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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2014
• Language: English
• ISBN 10: 184973917X
• ISBN 13: 9781849739177
• Binding: Hardcover
• Number of pages: 633
• Editor: Zhen Ma