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

Hardcover

ISBN 10: 0854042040 ISBN 13: 9780854042043

Publisher: Royal Society of Chemistry, 1996

Synopsis

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.

About this title

About the Author

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.

Excerpt. © Reprinted by permission. All rights reserved.

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 results obtained from the series of the above examinations are considered as follows. ZnO in the Cu-ZnO catalysts is reduced to ZnOx at a relatively high temperature and ZnOx migrate onto the Cu surface and partially dissolve into the Cu particle forming a Cu-Zn alloy. Then the surface of the alloy is mildly reoxidized by C02 under the reaction conditions required for hydrogenation of C02 to form the catalytically active sites stabilizing the cu+ species.

Burch et al. have reported that the addition of ZnO to a Cu/Si02 catalyst generated an increase of activity for methanol synthesis. Fujitani et al.1a have confirmed that when a mechanical mixture of Cu/SiO2 and ZnO/SiO2 was reduced, the lattice constant for Cu increased and the presence of Zn in a Cu particle was also observed by XRD and TEM-EDX. Figure 3 illustrates the proposed synergy model for the mechanical mixture of Cu/SiO2 and ZnO/SiO2.

2.3 Relation Between Oxygen Coverage and Activity of Methanol Synthesis.- Chinchen et al. have examined the oxygen coverage of the Cu surface by the reactive frontal chromatography (RFC) technique. From the results (Figure 4) that the oxygen coverage of the post-reaction catalysts increased in proportion to the CO2/CO ratio of feed gas, it is suggested that the activity of the methanol synthesis is related to the degree of oxygen coverage.

Fujitani et al. have also measured the coverage of oxygen by using the RFC method as well as the specific activities for methanol synthesis from CO2 over Cu-based catalysts which include various metal oxides. As shown in Figure 5, there was an excellent correlation between the specific activity (this means the production rate per unit surface area of Cu) and the oxygen coverage (Θ). The activity increased linearly with oxygen coverage at Θ <0.16 and then decreased at Θ >0.18. It is believed that the presence of oxygen on the Cu surface formed during the reaction is indicative of the presence of Cu+ species bound to the surface oxygen. Therefore, the results demonstrate that the active component is not only Cu+ but also Cu0 and the ratio of Cu+ to Cu0 controls the activity of methanol synthesis.

It is noted that the specific relationship was obtained for various catalysts containing different metal oxides. This provided the evidence that the effect of added metal oxide is ascribed to the difference in the amount of Cu+ stabilized by both surface oxygen and foreign elements. It is worthwhile to note that for all Cu-based catalysts specific activities can be ranked by the coverage of oxygen, in other words, can be plotted on one curved line.

A known theory states that the active sites of the Cu-ZnO catalysts for methanol synthesis are metallic Cu which is not dependent on any synergy effect with metal oxides or supports. According to their reports, at the first step of the reaction scheme, the CO2 component adsorbs on the partially oxidized Cu as a symmetric carbonate, and then this is hydrogenated to a formate species. Also it is claimed that the role of CO is to keep the Cu in a highly reduced state; therefore, the oxygen coverage of the Cu is a function of the CO2/CO ratio. In this thesis, the electronic nature of the active site during the reaction seems to be a little indefinite.

On the other hand, from much research work and aforementioned studies, the Cu+ species stabilized by a certain structure must be the active center for methanol synthesis. The structure stabilizing the active site is considered to exist due to the contiguity between Cu and ZnO. Relatively small clusters of ZnO on the Cu surface and/or small clusters of Cu on the ZnO surface must make up the specific chemical and physical structure, such as amorphous Cu, two-dimensional layer, small Cu clusters, partially oxidized alloy, lattice defect structure, etc., which cannot significantly be formed at the contact points between large agglomerates.

Either way, many research groups have admitted that the role of support such as ZnO is to make the Cu component highly dispersed. Therefore, it is most important how the dispersed Cu catalyst which effectively generates the catalytically active site for methanol synthesis as described above is achieved. To obtain the excellent catalyst, the conditions for the selective preparation of the desired precursor and the mechanism of precursor formation will be discussed in the next section.

3 Selective Formation of Precursors and Their Structures

In general, coprecipitated Cu-ZnO catalysts for methanol synthesis and water-gas shift (WGS) reactions are prepared through the following steps:

raw salts -> intermediates -> precursors -> oxide catalysts -> activated catalysts (precipitation) (calcination) (reduction)

Precursors obtained from the precipitation from both solutions of raw metal salts and alkali are composed of basic carbonates of copper and zinc such as malachite (MA) Cu2CO3 (OH)2, aurichalcite (AU) (Zn,Cu)5(CO3)2(OH)6, and hydrozincite (HZ) Zn5(CO3)2(OH)6. For the ratio of Cu to Zn in industrial catalysts, the precursor would not exist as monophasic. Even if it seems to be monophasic, the atomic ratio of Cu to Zn in the double salts would be continuously variable. Since amorphous intermediates of the precursor are also known, the real phase cannot be well characterized by XRD measurements. Of course the structure and distribution of precursors are also very sensitive to the precipitation conditions (temperature, rate, pH, etc.) as well as precipitation agents. Consequently, the characteristics of precursors are not determined only by the starting composition.

To realize the active sites that have the structure described above, well-dispersed copper metal and/or fine interdispersion of copper and zinc oxide are essential. Therefore, identifying the precursor(s) that produce the desired active site is important, but the knowledge of how to produce them selectively on an industrial scale from the above scheme is critical in the development of an improved catalyst. To obtain the desired precursor(s) the following factors must be well understood: the relation between preparation conditions and the distribution of precursors, the factor of controlling these reactions, and the mechanism that is able to explain the whole reaction system.

3.1 Addition Rates and Obtained Precursors.- Fujita et al. have studied the influence of feed rates on the distribution of resultant precursors in the course of reverse precipitation, in which a mixed solution (1.0 M) of copper and zinc nitrates was added to the solution of sodium bicarbonate (1.2 M). At the molar ratio Cu/Zn=30/70, using method A (slow feed rate 0.56 cm3/min) formed no AU; however, only AU was produced at faster rates: both B (medium feed rate 2.5 cm3/min) and C (fast feed rate 33 cm3/min).

The measurement of XRD, differential thermal analysis (DTA), and FTIR spectroscopy has revealed that the initial precipitates produced by method A were zincian-MA and sodium zinc carbonate (SZC) Na2Zn3(CO3)4, and that SZC was gradually converted to copper-HZ. In the method B, SZC was also observed but no copper-containing species were observed. However, the presence of copper in the precipitates was confirmed by ultraviolet/visible (UVNIS) spectroscopy and chemical analysis. These observations indicate that the copper-containing species were amorphous and SZC decreased with time and AU was formed as a result.

As for the pure zinc precipitate from zinc nitrate, HZ was formed via SZC regardless of the rate of addition. In contrast, the structure of the pure copper precipitate was markedly varied according to the precipitation method. MA was observed at various stages of preparation by slow addition, but the initial precipitates were amorphous when prepared by the method B.

3.2 Effect of Concentration of NaHCO3 Solution on the Distribution of Precursors. - A large composition change of precursors was obtained against the Cu/Zn ratio, as illustrated in Figure 6, from the three different concentration of NaHCO3 solutions (0.1, 0.6 and 1.2 M) which contained the same molar amount of NaHCO3 as the above experiments. Using 0.6 and 1.2 M NaHCO3 solutions, MA was formed in a wide range of Cu/Zn ratios, even at the ratio of Cu/Zn=5/5, only MA was produced with 1.2 M alkaline solution. The proportion of MA decreased with decreasing concentration of alkali. No MA but only AU was produced with 0.1 M NaHCO3 solution at the ratio of Cu/Zn=5/5. When precipitating with zinc nitrate, only HZ was made with all NaHCO3 solutions.

Consequently, it is obvious that even when the ratio Cu/Zn is constant, the composition of precursors can be altered widely depending on the conditions of the catalyst preparation.

3.3 Nature of Initial Intermediates and Precursors.- It is well known that during the coprecipitation of Cu-ZnO catalysts the color of the precipitates changes from blue to greenish blue after a certain period, and carbon dioxide evolution is often observed. Little progress has been made in the study on the mechanism that can interpret the above phenomena.

For the precipitation from copper nitrate in the above experiments, Fujita et al. have also reported that intermediates or precursors were altered depending on the feed rate. MA was formed by slow addition but, while using fast or medium feeding rate, the initial precipitates are amorphous. These amorphous precipitates were found to have hydroxyl and carbonate groups by FTIR measurement and they decomposed to CuO at elevated temperatures, producing CO2 and H2O. These observations indicate that the amorphous precipitates were copper hydroxycarbonates (CHC) expressed as Cu2(CO3)x(OH)4-2x, where x is from 0 to 1. On the basis of thermogravimetric analysis (TGA) and gas analysis, the x-value for CHC was estimated to be 0.27 in the first 4 minutes of the preparation of precipitates. The x-value increased with time and finally approached 1.0 for MA species. Hence, MA was formed through anion exchange of OH- in CHC with CO32- in the solid solution.

(Continues...)

Excerpted from Catalysis Volume 12 by James J. Spivey. Copyright © 1996 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 1996
• Language: English
• ISBN 10: 0854042040
• ISBN 13: 9780854042043
• Binding: Hardcover
• Number of pages: 354