Catalysis: Volume 12 (Specialist Periodical Reports, Band 12) - Hardcover

Hardcover

ISBN 10: 0854042040 ISBN 13: 9780854042043

Verlag: Royal Society of Chemistry, 1996

Inhaltsangabe

There is an increasing challenge for chemical industry and research institutions to find cost-efficient and environmentally sound methods of converting natural resources into fuels chemicals and energy. Catalysts are essential to these processes and the Catalysis Specialist Periodical Report series serves to highlight major developments in this area. This series provides systematic and detailed reviews of topics of interest to scientists and engineers in the catalysis field. The coverage includes all major areas of heterogeneous and homogeneous catalysis and also specific applications of catalysis such as NOx control kinetics and experimental techniques such as microcalorimetry. Each chapter is compiled by recognised experts within their specialist fields and provides a summary of the current literature. This series will be of interest to all those in academia and industry who need an up-to-date critical analysis and summary of catalysis research and applications. Catalysis will be of interest to anyone working in academia and industry that needs an up-to-date critical analysis and summary of catalysis research and applications. Specialist Periodical Reports provide systematic and detailed review coverage in major areas of chemical research. Compiled by teams of leading experts in their specialist fields, this series is designed to help the chemistry community keep current with the latest developments in their field. Each volume in the series is published either annually or biennially and is a superb reference point for researchers. www.rsc.org/spr

Über diesen Titel

Über die Autorin bzw. den Autor

Professor Spivey is the McLaurin Shivers Professor of Chemical Engineering at Louisiana State University and Director of the DOE Energy Frontier Research Center at LSU. Professor Spivey's research interests include the application of the principles of heterogeneous catalysis to catalytic combustion, control of sulfur and nitrogen oxides from combustion processes, acid/base catalysis (e.g., for condensation reactions), hydrocarbon synthesis, and the study of catalyst deactivation.

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Catalysis Volume 12

A Review of Recent Literature

By James J. Spivey

The Royal Society of Chemistry

Copyright © 1996 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-0-85404-204-3

Contents

Chapter 1 Structure of Active Sites of Cu-ZnO Catalysts and Selective Formation of Relevant Precursors 1 By Toshia Matsuhisa, 1, 
Chapter 2 Property and Catalysis of Aluminophosphate-based Molecular Sieves By Tatsumi Ishihara and Yusaku Takita, 21, 
Chapter 3 Fischer-Tropsch Synthesis. Mechanism Studies Using Isotopes By Ajoy Raje and Burtron H. Davis, 52, 
Chapter 4 Catalyst Materials for High-temperature Combustion By Koichi Eguchi, 132, 
Chapter 5 Vapor Phase Condensation Reactions Using Formaldehyde or Methanol By M. Ai, 152, 
Chapter 6 Electrochemical Activation of Catalysis: In Situ Controlled Promotion of Catalyst Surfaces By Constantinos G. Vayenas and Stylianos G. Neophytides, 199, 
Chapter 7 Electrosorption Studies in Electrocatalysis By Gyorgy Hordnyi, 254, 
Chapter 8 Applications of Electron Spin Resonance Spectroscopy to Catalysis in Direct Coal Liquefaction By Mohindar S. Seehra and Manjula M. Ibrahim, 302, 
Chapter 9 Theoretical Study of Some Conditions' Effect on Catalytic Reaction By Shigemi Mandai, 321, 

CHAPTER 1

Structure of Active Sites of Cu-ZnO Catalysts and Selective Formation of Relevant Precursors BY TOSHIO MATSUHISA

1 Introduction

In 1933 the first synthetic methanol was industrially produced in Japan at Hikoshima in Shimonoseki, which is where this review is being composed. The Hikoshima plant produced the feedstock gas from coal gasification and the catalyst consisted of Zn0-Cr203 which was developed by Japanese researchers. The plant had a capacity of only 5 metric tons per day of methanol.

Today methanol has become a very important feedstock for the production of many chemicals. Use as a clean fuel has increased and methanol is used in the production of the popular oxygenated fuel additive, MTBE. It has also been postulated that methanol could be a carrier of energy for safe transportation between remote countries. Furthermore, to prevent a greenhouse effect caused by C02 generated from the tremendous oxidation reactions on the earth, methanol synthesis from C02 is regarded as one of the potential solutions to decrease C02 by the reaction with hydrogen which is produced by electrolysis of water, for example. Due to the increasing demand for methanol, many researchers are involved in the development of more active methanol synthesis catalysts.

When desulfurized feedstocks became available for methanol synthesis, the highly active Cu-ZnO-Al2O3 catalysts replaced the low activity, poison-resistant ZnO-Cr2O3 catalysts. It is well known that the Cu-based catalyst system must demonstrate significant synergy with other components to achieve high methanol synthesis activity, and therefore much research has been focused to clarify the origin of the high activity which then can be used to establish guidelines to develop a new improved catalyst. The true nature of catalytic activity often originates from the complex effect among multicomponents. The Cu-ZnO catalyst has been the typical example for the elucidation of mechanism of catalytic behavior.

There have already been many reports and reviews on this field, so the aim of this short review is not to cover all the subjects on the methanol synthesis and its catalyst but to summarize the recent reports on the structure of the active sites and the formation mechanism of precursors. This review additionally addresses the possibility of improving the catalyst performance based on the recent progress.

2 Nature of Active Sites

The mechanism from which the activity of methanol synthesis on the Cu-ZnO catalysts originates is still the object of considerable controversy. The main subjects of controversy can be summarized in the two questions:

1. Are the active sites metallic or monovalent Cu species?

2. What is the role of metal oxides especially ZnO?

2.1 Structure of Active Sites. - Klier and others have claimed that the active phase is a Cu+ species dissolved in ZnO. Estimating the amount of dissolved Cu+ reflected irreversible chemisorption of CO in proportion to the dissolved Cu+. The existence of Cu+ in the active state is verified by means of Auger electron spectroscopy (AES), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), but it is also pointed out that the Cu+ concentration depends upon the total content of Cu in the catalyst.

Okamoto et al. have studied the chemical state of the Cu species in a H2-reduced catalyst surface using X-ray photoelectronic spectroscopy (XPS). In high Cu content catalyst (>25 wt% CuO) the predominant Cu species were well-dispersed metal particles whereas in low Cu content catalysts (<10 wt% CuO) the Cu was found to be distributed in a two-dimensional epitaxial Cu+-Cu0 layer over ZnO.

Tohji et al. have observed the temperature dependence of the coordination number (N) of Cu-ZnO catalysts reduced by hydrogen (30 mol% Cu, coprecipitation and impregnation) by means of in situ EXAFS. The results are shown in Table 1.

Below 408 K, the N of Cu-0 bond was about 1 and the N of Cu-Cu bond was 5 to 9 which was smaller than 12 of bulk Cu. Over 423 K, no Cu-0 bond was observed and the N in Cu-Cu bond increased to the same N as bulk Cu. It is also found that the peak intensity of Cu-Cu bond was reversibly altered during subjecting the catalyst to a heating/cooling cycle below 473 K.

The observations from the data can be modeled as shown in Figure 1 which indicate that a quasi-two-dimensional layer epitaxially developed over the ZnO exists at low temperature and it transforms into small Cu metal clusters at higher temperature. These small clusters carry the catalytically active sites, and their average size was estimated to be 20 to 30 from the coordination numbers. The uncrystallized clusters are expected to be very mobile and easily transformed by temperature changes.

From the above studies it is concluded that, especially in low Cu content catalysts, an intimate contact between Cu and ZnO stabilizes the Cu+ active sites with some special structural contributions in the activated state.

2.2 The Role of ZnO.- Kanai et al. have measured the lattice constant of Cu for the Cu-ZnO catalysts used for C02 hydrogenation at 523 K after reduction at a different temperature. As shown in Figure 2, the results indicate that the lattice constant of Cu increases with increasing content of Zn at higher temperature. This observation was attributable to the formation of a Cu-Zn alloy where the average content of Zn in a Cu particle was estimated to be about 20 percent for the catalyst (Cu/ZnO=50/50 by weight) reduced at 723 K. The observation with transmission electron microscopy-energy-dispersion X-ray (TEM-EDX) also confirmed that Zn exists in Cu particles at more than ten different sites of the Cu/ZnO catalyst reduced at 723 K. The content of zinc in a Cu particle was estimated to be 16 to 18 percent by EDX, which is in good agreement with the value obtained by X-ray diffraction (XRD). By means of CO temperature-programmed desorption (TPD) and Fourier transform infrared (FTIR) measurements, it is observed that the catalyst reduced at 723 K had no ability for CO adsorption, but when this catalyst was oxidized by N20 or under methanol synthesis conditions from C02 and H2, the ability of CO adsorption was reacquired.

These...

Details zur Ausgabe

• Verlag: Royal Society of Chemistry
• Erscheinungsdatum: 1996
• Sprache: Englisch
• ISBN 10: 0854042040
• ISBN 13: 9780854042043
• Einband: Gebundene Ausgabe
• Anzahl der Seiten: 352
• Herausgeber: Spivey James J
• Kontakt zum Hersteller: Nicht verfügbar
• Verantwortliche Person: gpsr@libri.de; gpsr@libri.de; ; Friedensallee 273; Hamburg; 22763; Deutschland