ELECTROANALYTICAL CHEMISTRY: NEW RESEARCH No part of this digital document may be reproduced, stored in a retrieval system or transmitted in any form or by any means. The publisher has taken reasonable care in the preparation of this digital document, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained herein. This digital document is sold with the clear understanding that the publisher is not engaged in rendering legal, medical or any other professional services.
ELECTROANALYTICAL CHEMISTRY: NEW RESEARCH
GRAHAM M. SMITHE Editor
Nova Science Publishers, Inc. New York
Copyright © 2008 by Nova Science Publishers, Inc.
All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material. Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS.
Library of Congress Cataloging-in-Publication Data Electroanalytical chemistry : new research / Graham M. Smithe (editor). p. cm. ISBN 978-1-60741-857-3 (E-Book) 1. Electrochemical analysis--Research. I. Smithe, Graham M. QD115.E5112 2008 543'.4--dc22 2007052758
Published by Nova Science Publishers, Inc.
New York
CONTENTS Preface
vii
Expert Commentary: Electrodes Based on Metallophthalocyanines Integrated with Carbon Nanotubes: Potential Hybrids for Enhancing Electron Transport Kenneth I. Ozoemena Chapter 1
Chapter 2
Chapter 3
Chapter 4
Index
Clathrate Hydrate Crystallization for Clean Energy and Environmental Technologies Peter Englezos, John Ripmeester and Robin Susilo
1
9
Corrosion Research Frontiers. Atmospheric Corrosion in Tropical Climate. On the Concept of Time of Wetness and Its Interaction with Contaminants Deposition F. Corvo, T. Pérez, Y. Martin, J. Reyes, L.R. Dzib, J.A. González and A. Castañeda
61
The Application of D-Statistics Based Tests of Randomness, Independence, and Trend to Electrochemical Observations Thomas Z. Fahidy
93
Self-Assembly Assisted Polypolymerization (SAAP): A Novel Approach to Prepare Multiblock Copolymers with a Controllable Chain Sequence and Block Length Liangzhi Hong, Fangming Zhu, Guangzhao Zhang, To Ngai and Chi Wu
109
123
PREFACE Electrochemistry can be broadly defined as the study of charge-transfer phenomena. As such, the field of electrochemistry includes a wide range of different chemical and physical phenomena. These areas include (but are not limited to): battery chemistry, photosynthesis, ion-selective electrodes, coulometry, and many biochemical processes. Although wide ranging, electrochemistry has found many practical applications in analytical measurements. The field of electroanalytical chemistry is the field of electrochemistry that utilizes the relationship between chemical phenomena which involve charge transfer (e.g. redox reactions, ion separation, etc.) and the electrical properties that accompany these phenomena for some analytical determination. This new book presents the latest research in this field. Expert Commentary - Carbon nanotubes (CNTs), notably single-walled (SWCNTs) or multi-walled (MWCNTs), have continued to attract immense research interests in electroanalytical chemistry because of their ability to exhibit unusual but excellent electrical conductivity and mechanical properties. They also possess modifiable sidewalls and openends, making them suitable for use as new materials for constructing efficient electrocatalysts and electrochemical sensors. Transition metal metallophthalocyanine (MPc) complexes have also proved themselves as powerful redox-active materials for modifying electrode surfaces for use as sensors. My group has been engaged in the rational design and integration of CNTs with certain transition MPc complexes, and exploring their potential applications in the fabrication of electrochemical responsive CNT-MPc based electrodes. This short commentary gives insights into the recent developments in this emerging research area as well as future trends. Chapter 1 - Clathrate or gas hydrates are non-stoichiometric crystalline materials formed by the inclusion of certain molecules into a framework of hydrogen-bonded water molecules under suitable temperature and pressure conditions. The resulting host-guest networks consist of cavities formed by water molecules enclosing the guest molecules. Typical guests include light hydrocarbon gases, carbon dioxide, hydrogen sulfide, hydrogen and nitrogen. The basic cavity formed by water molecules through hydrogen bonding is the pentagonal dodecahedron (512). This cavity is common to the three best known hydrate structures (I, II and H). A unit cell of structure I hydrate has 46 water molecules forming two dodecahedral (512) and six tetrakaidecahedral (51262) cavities. A unit cell of structure II has 136 water molecules forming 16 (512) and eight hexakaidecahedral (51264) cavities. The Structure H hydrate unit cell has 34 water molecules, three 512 cavities, two different dodecahedral cavities which have threesquare faces, six-pentagonal faces and three-hexagonal faces (435663) and a larger
viii
Graham M. Smithe
iscosahedral cavity with twelve pentagonal faces and six hexagonal faces (51268). It should be noted that unlike structures I and II which may form with a single guest species, structure H requires the presence of a small guest (like methane) and a large molecule guest substance (LMGS) like neohexane at ordinary pressures. Gas hydrates were first reported at the beginning of the 19th century, and until the 1930s they remained a scientific curiosity. At that time it was realized that hydrates were more likely to be the causative agent in blocking pipelines than ice. Today, gas hydrate control continues to be a problem in the oil and gas industry. In the 1960’s it was realized that natural gas hydrates are present in the geo-sphere with worldwide reserves estimated at 10,000 to 40,000 trillion cubic meters (TCM). Considerable efforts are underway to refine global estimates and to develop technology and exploit this resource. On the other hand these hydrates may decompose as a result of global warming or seafloor instability and release the methane gas. There is speculation that a runaway greenhouse effect could result, with some evidence for changes of such magnitude in the global paleoclimate (15,000 and 55 million years ago). Application of clathrate hydrate crystallization offers the possibility of the development of innovative technologies for natural gas storage and transportation, hydrogen storage and gas separation with applications for carbon dioxide capture from flue gas (CO2/N2/O2) or fuel gas (CO2/H2) mixtures. Clathrate hydrates have become an important research area spanning a variety of disciplines. It is of continuing great interest to chemical engineers who have played a key role in past developments. This report discusses areas where chemical engineers can advance the knowledge frontier. Chapter 2 - Atmospheric corrosion is the most extended type of corrosion in the World. Over the years, several papers have been published in this subject; however, most of the research has been made in non-tropical countries and under outdoor conditions. Results of outdoor and indoor corrosion rate and corrosion aggressivity in tropical corrosion test stations of Cuba and Mexico are reported. Time of wetness (TOW), considered as the time during which the corrosion process occurs, is an important parameter to study the atmospheric corrosion of metals. According to ISO-9223 standard, TOW is approximately the time when relative humidity exceeds 80% and temperature is higher than 0oC. No upper limit for temperature is established. In tropical climates, when temperature reaches values over 25oC, evaporation of water plays an important role and the possibility to establish an upper limit respecting temperature should be analyzed. The concept of TOW assumes the presence on the metallic surface of a water layer; however, there are recent reports about the formation of water microdrops during the initial periods of atmospheric corrosion, showing that the idea of the presence of thin uniform water layers is not completely in agreement with the real situation in some cases (particularly indoor exposures). Most of the research carried out to study the initial stages of atmospheric corrosion have been made on a clean surface without corrosion products; however, the metal is very often covered by thin or thick corrosion products after a given exposure time and these products usually act as retarders of the corrosion process. In the Cuban Isle, the influence of chloride ions is very significant in determining the corrosion rate. In the coastal territory of the Mexican Gulf, particularly at Campeche, the deposition of Chloride ions is lower. No previous reports have been made about the interaction between chloride deposition rate and rain. The influence of rain seems to be
Preface
ix
important in determining the acceleration rate of chloride ions on metals due to its washing effect. To consider the influence of the interaction chloride deposition rate–rain regime could be useful to improve the prognosis of corrosion aggressivity. The predominant wind direction corresponding to geographic sites result in an important parameter for chloride deposition and their influence on surface wetness. The calculation of Time of Wetness established in ISO 9223 should be revised based on new results obtained in outdoor and indoor conditions in tropical humid marine climate. Some proposals are made to improve the estimation of TOW, taking into account changes in its nature depending on outdoor or indoor exposure, linear relationship between time and TOW, the effect of rain, and the role of contaminants and air temperature. Chapter 3 - Randomness, independence and trend (upward, or downward) are fundamental concepts in a statistical analysis of observations. Distribution-free observations, or observations with unknown probability distributions, require specific nonparametric techniques, such as tests based on Spearman’s D – type statistics (i.e. D, D*, D**, Dk ) whose application to various electrochemical data sets is herein described. The numerical illustrations include surface phenomena, technology, production time-horizons, corrosion inhibition and standard cell characteristics. The subject matter also demonstrates cross fertilization of two major disciplines. Chapter 4 - Block copolymers have attracted much attention because of their novel properties and various promising potential applications. However, it is still difficult, if not impossible, to prepare multiblock copolymers with a controllable chain sequence and block length even though a variety of synthetic methods, such as anionic and controlled free radical living polymerization have been advanced. In recent years, we have proposed and developed a novel method of using the self-assembly of A-B-A triblock copolymers in a solvent which is selectively good for the two A-blocks. Such self-assembly concentrates and exposes the active groups attached on the two A-block ends so that they can be coupled together to form a long multiblock copolymer chain with its sequence and block length controlled by the initial triblock copolymer. In this review, we first illustrate how the SAAP concept was developed and exemplified in some real copolymer systems. Furthermore, we compare the coupling efficiency with and without the self-assembly, and demonstrate that SAAP provides an elegant way to prepare long multiblock copolymers.
In: Electroanalytical Chemistry: New Research Editor: G. M. Smithe
ISBN: 978-1-60456-347-4 © 2008 Nova Science Publishers, Inc.
Expert Commentary
ELECTRODES BASED ON METALLOPHTHALOCYANINES INTEGRATED WITH CARBON NANOTUBES: POTENTIAL HYBRIDS FOR ENHANCING ELECTRON TRANSPORT Kenneth I. Ozoemena* Department of Chemistry, University of Pretoria, Pretoria 0002, South Africa.
Abstract Carbon nanotubes (CNTs), notably single-walled (SWCNTs) or multi-walled (MWCNTs), have continued to attract immense research interests in electroanalytical chemistry because of their ability to exhibit unusual but excellent electrical conductivity and mechanical properties. They also possess modifiable sidewalls and open-ends, making them suitable for use as new materials for constructing efficient electrocatalysts and electrochemical sensors. Transition metal metallophthalocyanine (MPc) complexes have also proved themselves as powerful redox-active materials for modifying electrode surfaces for use as sensors. My group has been engaged in the rational design and integration of CNTs with certain transition MPc complexes, and exploring their potential applications in the fabrication of electrochemical responsive CNT-MPc based electrodes. This short commentary gives insights into the recent developments in this emerging research area as well as future trends.
1. Introduction Phthalocyanine complexes are organic macrocycles with 18 π-electrons, structurally resembling the naturally-occuring porphyrins complexes [1-3]. Electrodes modified with transition metal (notably Fe, Co, Mn, Ni) phthalocyanine (MPc, Fig.1) complexes have continued to generate immense research interests because of their well-established electrocatalytic properties [3-6]. *
E-mail address:
[email protected], Tel.: +27-12-420-2515; Fax: +27-12-420-4687 (Corresponding author)
2
Kenneth I. Ozoemena
R
R
Peripheral position N N
N M
N N
Non-peripheral position N
N N
R
R
Figure 1. Molecular structures of metallophthalocyanine (MPc) complex. MPc complexes may be substituted at the peripheral or non-peripheral positions as shown. M = metal ion, and R = substituent containing terminal functional groups such as –NH2 and –OH.
Carbon nanotubes (CNTs), notably single-walled (SWCNTs) or multi-walled (MWCNTs), have been receiving some attention amongst electroanalysts as electrode modifiers that are capable of enhancing electrochemical response [7-10]. Interestingly, both CNT and MPc contain π-electrons, thus facilitating their integration via π-π integration. Recent reports have indicated that MPc integrated with CNTs enhance electrochemical responses of MPc-based electrodes toward the detection of certain important analytes such as the hydrolysis products of V-type nerve agents [11-13], herbicide asulam [14] and mercaptoethanol and nitric oxide [15]. It may be predicted that one of the future implications of these reports is that many previous works on MPc-modified electrodes are likely to be revisited by many researchers on MPc-based electrodes or surfaces for sensing, catalysis, fuel cell, and many other potential applications. Thus, there is an urgent need to start building knowledge in this emerging field of CNT-MPc based electrodes. This short commentary gives insights into the recent developments, specifically touching on the main fabrication strategies (self-assembly, electrodecoration and drop-coating) currently being explored for integrating these two redox-active species onto electrode surfaces, their impacts on the heterogeneous electron transport properties, and future trends.
2. Electrode Modification Strategies CNTs are insoluble in any solvent, thus prior to use they are treated in strong acids to introduce oxygen-containing moieties (mainly –COOH group) to make them soluble [16]. The following strategies are currently employed.
Electrodes Based on Metallophthalocyanines Integrated with Carbon Nanotubes
3
2.1. Abrasive Adhesion (or Drop-Coating) of CNTs Preceding MPc Coating In this method, about 5 µL of the CNT DMF solution (1mg / 1ml DMF) is first placed onto the electrode and the solvent allowed to dry off in air or at mild oven temperature (ca. 80 o C). The same process is repeated with DMF solution of MPc solution (1 mM). The morphology of the films depends on the concentration of the casting solution, rate of solvent evaporation, nature of the solvent and roughness of the electrode surface. Important substrates for this are the pyrolytic graphites (highly oriented, basal or edge planes) because of the inherent ability of these electrodes to interact with CNTs via π-π interactions [7,17,18]. Adhesion of CNT onto glassy carbon electrode (GCE) is difficult and fraught with problems such as irreproducibility [19]. Abrasive immobilization is preferred for the MWCNT than the more expensive SWCNT. The drop-coating is possible when the CNT is pretreated in harsh acid conditions to introduce COOH, OH functionalities.
2.2. CNT Coating Preceding Electro-Decoration This method involves electrochemical deposition of the MPc onto CNT-modified electrode surface by repetitive cycling in a concentrated MPc solution (1 mM) within a specific potential window. The first cyclic voltammetric scan is usually similar to subsequent scans, indicating the formation of monomeric species only. Ozoemena et al [11] found that on certain occasions, as reported recently [11] during the electro-deposition of CoTAPc onto a basal plane pyrolytic graphite electrode (BPPGE) pre-modified with SWCNT, both cathodic and anodic waves may decrease continually and then stabilizes at a certain scan (a process known as ‘electrochemical adsorption’ or simply called ‘electrosorption’). Another form of electro-decoration is electropolymerization of the MPc complex, especially the MTAPc complex such as the NiTAPc onto CNT-modified electrodes [13]. The beauty of this technique is that the thickness and morphology of the resulting MPc polymeric film may be easily controlled by manipulating the deposition voltage, the number of cycle scans and concentration of the MPc solution.
2.3. Self-Assembly (Chemisorption) Process Self-assembly is a foremost strategy for forming highly stable, well-ordered ultra thin ‘self-assembled monolayer’ (SAM) films of redox-active species onto coinage metal surfaces [20,21]. The use of SAMs for sensing and catalytic purposes is well documented. It involves strong and irreversible chemisorption of the MPc complexes onto coinage metal based electrodes, notably gold. Gold surfaces are preferred for forming thiol-derivatised SAMs because of the well established specific and strong interaction of sulfur atoms with gold. From past experiences in the formation of MPc-based SAMs [22-31], we (my research group and collaborators) have now begun to exploit the SAM strategy in the integration of CNTs with transition MPc complexes [32,33] (Figure 1). Amino-substituted metallophthalocyanine complexes (MPc, Figure 2) can be covalently linked to acid-treated CNTs via the formation of amide bond (if R = amino group [32]) or ester bond (if R = hydroxyl group [33]).
4
Kenneth I. Ozoemena R
R N
Substituted MPc moiety
N
N
N
M
N
N
N N
R HO
R
O OH O
O
O
Self-assembled carboxylated SWCNT
O
S
O O NH
O NH S
NH
NH
Self-assembled cysteamine S
S
Figure 2. Carton showing the integration of a substituted metallophthalocyanine (MPc) complex onto a self-assembled single-walled carbon nanotube. The substitutent R could either be terminal hydroxyl (– OH) group forming an ester bond, or amino (–NH2) group forming an amide bond.
3. Impact of CNTs on Heterogeneous Electron Transport We have employed electrochemical impedance spectroscopy using common redox probe, [Fe(CN)6]4-/[Fe(CN)6]3- to interrogate the influence of CNTs on the electron transport behaviour of the MPc complexes [11-14,32,33]. Whatever the strategy used in forming the CNT-MPc based electrode, the apparent electron transfer rate constants (kapp) is highest for the MPc-CNT constructs (Bare-CNT-MPc) compared to either the bare electrode or the electrode modified with either the CNT (Bare-CNT) or MPc (Bare-MPc). Reports so far have also established that these MPc-CNT constructs (Bare-CNT-MPc) improve the electrochemical response of analytes, following the trend: Bare electrode < Bare-CNT < Bare-MPc < Bare-CNT-MPc. However, it is important to caution at this juncture that this trend may not be generalized for every analyte. This again calls for continued need for intense research in this field. The electron transfer mechanism for the CNT-MPc modified electrode may be represented as shown in Figure 3, where the immobilized MPc and CNT act as electrocatalyst and electron conducting species, respectively.
Electrodes Based on Metallophthalocyanines Integrated with Carbon Nanotubes
Analyte (Re d uc e d )
An aly te (Oxidiz e d )
So lu e t y l An a
MPc (o xidiz e d )
5
MPc (re d uc e d )
t io n
r Hy b c P -M CNT
id
Ele ctro de
eFigure 3. Proposed schematic representation of oxidative electrocatalysis at an electrode modified with CNT-MPc hybrid. In this case, the surface-confined MPc and CNT are hypothesized to act as electrocatalyst and electron conducting species, respectively.
The electron transport occurring in CNT-MPc SAM based electrodes is thought to proceed via four main steps: (i) electron-tunneling from SWCNT to gold, (ii) electron transport occurring within SWCNTs, (iii) electron-tunneling from the MPc ring and/or central metal, and (iv) heterogeneous electron transfer between the central metal of the MPc and analyte. The ‘cutting’ process of harsh acid-treatment adopted for introducing carboxylic moiety onto SWCNTs may generate local traps for charge transport, however, the high conductivity of the immobilized SWCNTs is sufficient enough to render SWCNTs as efficient conductive nanowires rather than charge traps [34,35].
4. Future Trends and Conclusion Research on the use of CNT-MPc based electrode in electroanalytical chemistry is still in its infancy. Without doubt, there is an enormous potential for using CNT-MPc-based electrodes for applications in areas such as environmental, industrial, food, pharmaceutical, clinical, and biomedical fields. Few studies have only been attempted with MPc complexes with Co, Fe and Ni as the central metals, meaning that there are many open doors for research on these and many other MPc complexes as redox mediators for the development of electrochemical sensors. Given the many advantages of electrochemical techniques (especially sensitivity to redox-active analytes, and amenability to automation,
6
Kenneth I. Ozoemena
miniaturization and remote operation) over other analytical techniques such as the spectroscopic and chromatographic methods, I envision that (i) important research works on screen-printed electrodes for one-shot analysis, ultramicroelectrodes, lab-on-chips, microband arrays, electrochemical sensors integrated with scanning electron microscopy and atomic force microscopy, etc, are likely to dominate the development of electrochemical sensors in the future; (ii) the prevalence of new types of diseases such as the drug-resistance tuberculosis, and HIV and AIDS will continue to heighten the need for rapid and sensitive onsite or point-of-care analysis; and (iii) the use of CNT-MPc composite (paste-based) electrodes, especially with the less expensive MWCNTs than the more expensive SWCNT. The main advantage of paste-based electrodes lies in their ease of regenerating the electrode surface. For example, in situations where the electrode surface becomes irrevocably contaminated or fouled new surface could easily be regenerated by polishing on clean aluminum paper.
References [1] Phthalocyanines: Properties and Applications, Lever, A.P.B.; Leznoff, C.C., Ed.; VCH Publishers: New York, 1989, 1993, 1996, Vol.1–4. [2] McKeown, N.B. Phthalocyanine Materials: Synthesis, Structure and Function, Cambridge University Press: Cambridge, 1998. [3] The Porphyrin Handbook, Kadish, K.M.; Smith K.M.; Guilard, R., Eds.; Academic Press: Boston, 1999, Vol.1-10; and 2003, Vol. 11–20. [4] Ozoemena, K.I.; Nyokong, T. In Encyclopedia of Sensors, Grimes, C. A.; Dickey, E.C. Pishko, M.V., Eds.; American Scientific Publishers: California, 2006, Vol.3, Chapter E, pp.157 – 200 [5] Nyokong, T. Coord. Chem. Rev. 2007, 251, 1707 [6] Vasudevan, P.; Phougat, N. Shukla, A.K. Appl. Organomet. Chem. 1996, 10, 591. [7] Banks, C.E.; Moore, R.R.; Davies, T.J.; Compton, R.G. Chem. Commun. 2004, 1804 [8] Jurkschat, K.; Xiaobo, J.; Crossley, A.; Compton, R.G. Analyst 2007,132, 21. [9] Banks, C.E.; Crossley, A.; Salter, C.; Wilkins, S.J.; Compton, R.G. Angew. Chem., Intl. Ed. 2006, 45, 2533. [10] Valcárcel, M.; Cárdenas, S.; Simonet, B.M. Anal. Chem. 2007, 79, 4788. [11] Ozoemena, K.I.; Pillay, J.; Nyokong, T. Electrochem. Commun. 2006, 8, 1391. [12] Pillay, J.; Ozoemena, K.I. Electrochim. Acta 2007, 52, 3630. [13] Pillay, J.; Ozoemena, K.I. Chem. Phys. Lett. 2007, 441, 72-77. [14] Siswana, M.; Ozoemena, K.I.; Nyokong, T. Electrochim. Acta 2006, 52, 114 [15] Silva, J. F.; Griveau, S.; Richard, C.; Zagal, J.H.; Bedioui, F. Electrochem. Commun. 2007, 9, 1629 [16] Liu, J.; Rinzler, A.G.; Dai, H.; Hanfer, J.H.; Bradley, R.K.; Boul, P.J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C.B.; Macias, F.R.; Shon, Y.S.; Lee, T.R.; Colbert, D.T.; Smalley, R.E. Science 1998, 280, 1253. [17] Moore, R.R.; Banks, C.E.; Compton, R.G. Anal. Chem. 2004, 76, 2677. [18] Wildgoose, G.G.; Leventis, H.G.; Streeter, I.; Lawrence, N.S.; Wilkins, S.J.; Jiang, L.; Jones, T.G.J.; Compton, R.G. ChemPhysChem. 2004, 5, 669. [19] Salimi, A.; Hallaj, R. Talanta 2005, 66, 967
Electrodes Based on Metallophthalocyanines Integrated with Carbon Nanotubes
7
[20] Finklea, H.O. Electrochemistry of Organised Monolayers of Thiols and Related molecules on Electrodes. In Electroanalytical Chemistry, Bard, A.J.; Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, pp.109-335. [21] Finklea, H.O. Self-assembled monolayers on Electrodes. In Encyclopedia of Analytical Chemistry: Applications, Theory and Instrumentations, Meyers, R.A. Ed.; John Wiley & Sons: Chichester, 2000; Vol. 11, pp.10090-101000 [22] Ozoemena, K.; Westbroek, P.; Nyokong, T. Electrochem. Commun. 2001, 3, 529. [23] Ozoemena, K.I.; Zhao, Z.X.; Nyokong, T. Electrochem. Commun. 2005, 7, 679. [24] Ozoemena, K.; Nyokong, T. Electrochim. Acta, 2002, 47, 4035. [25] Ozoemena, K.; Westbroek, P.; Nyokong, T. Electroanalysis, 2003, 15, 1762. [26] Ozoemena, K.; Nyokong, T. Talanta 2005, 67, 162. [27] Ozoemena, K.; Nyokong, T. J. Electroanal. Chem. 2005, 579, 283. [28] Ozoemena, K.I.; Nyokong, T. Electrochim. Acta 2006, 51, 5131 [29] Mashazi, P.N.; Ozoemena, K.I.; Maree, D.M.; Nyokong, T. Electrochim. Acta 2006, 51, 3489. [30] Mashazi, P.N.; Ozoemena, K.I.; Nyokong, T. Electrochim. Acta 2006, 52, 177. [31] Agboola, B.; Westbroek, P.; Ozoemena, K.I.; Nyokong, T. Electrochem. Commun. 2006, 9, 310 [32] Ozoemena, K.I.; Nyokong, T.; Nkosi, D.; Chambrier, I.; Cook, M. J. Electrochim. Acta 2007, 52, 4132 [33] Ozoemena, K.I; Nkosi, D. Electrochim. Acta 2008, 53, 2782. [34] Sheeney-Haj-Ichia, L.; Basnar, B.; Willner, I. Angew. Chem. Int. Ed. 2005, 44 78. [35] P. Diao, Z. Liu, J. Phys. Chem. B. 2005, 109, 20906.
In: Electroanalytical Chemistry: New Research Editor: G. M. Smithe
ISBN: 978-1-60456-347-4 © 2008 Nova Science Publishers, Inc.
Chapter 1
CLATHRATE HYDRATE CRYSTALLIZATION FOR CLEAN ENERGY AND ENVIRONMENTAL TECHNOLOGIES Peter Englezos1, John Ripmeester2 and Robin Susilo1,2 1.
Department of Chemical & Biological Engineering, University of British Columbia, Vancouver, BC, Canada 2. Steacie Institute for Molecular Sciences, National Research Council Canada, Ottawa, ON, Canada
Abstract Clathrate or gas hydrates are non-stoichiometric crystalline materials formed by the inclusion of certain molecules into a framework of hydrogen-bonded water molecules under suitable temperature and pressure conditions. The resulting host-guest networks consist of cavities formed by water molecules enclosing the guest molecules. Typical guests include light hydrocarbon gases, carbon dioxide, hydrogen sulfide, hydrogen and nitrogen. The basic cavity formed by water molecules through hydrogen bonding is the pentagonal dodecahedron (512). This cavity is common to the three best known hydrate structures (I, II and H). A unit cell of structure I hydrate has 46 water molecules forming two dodecahedral (512) and six tetrakaidecahedral (51262) cavities. A unit cell of structure II has 136 water molecules forming 16 (512) and eight hexakaidecahedral (51264) cavities. The Structure H hydrate unit cell has 34 water molecules, three 512 cavities, two different dodecahedral cavities which have threesquare faces, six-pentagonal faces and three-hexagonal faces (435663) and a larger iscosahedral cavity with twelve pentagonal faces and six hexagonal faces (51268). It should be noted that unlike structures I and II which may form with a single guest species, structure H requires the presence of a small guest (like methane) and a large molecule guest substance (LMGS) like neohexane at ordinary pressures. Gas hydrates were first reported at the beginning of the 19th century, and until the 1930s they remained a scientific curiosity. At that time it was realized that hydrates were more likely to be the causative agent in blocking pipelines than ice. Today, gas hydrate control continues to be a problem in the oil and gas industry. In the 1960’s it was realized that natural gas hydrates are present in the geo-sphere with worldwide reserves estimated at 10,000 to 40,000 trillion cubic meters (TCM). Considerable efforts are underway to refine global estimates and to develop technology and exploit this resource. On the other hand these hydrates may
10
Peter Englezos, John Ripmeester and Robin Susilo decompose as a result of global warming or seafloor instability and release the methane gas. There is speculation that a runaway greenhouse effect could result, with some evidence for changes of such magnitude in the global paleoclimate (15,000 and 55 million years ago). Application of clathrate hydrate crystallization offers the possibility of the development of innovative technologies for natural gas storage and transportation, hydrogen storage and gas separation with applications for carbon dioxide capture from flue gas (CO2/N2/O2) or fuel gas (CO2/H2) mixtures. Clathrate hydrates have become an important research area spanning a variety of disciplines. It is of continuing great interest to chemical engineers who have played a key role in past developments. This report discusses areas where chemical engineers can advance the knowledge frontier.
Introduction It is well known that when sufficient amounts of water and a hydrate-forming substance are brought into contact under appropriate temperature and pressure conditions a crystalline solid known as a gas or clathrate hydrate forms (van der Waals and Platteeuw, 1959; Davidson, 1973; Englezos, 1993; Sloan, 1998, Ripmeester, 2000; Koh, 2002; Sloan, 2003a; 2003b; 2004a; 2004b; 2005; Englezos and Lee, 2005; Chatti et al. 2005; Bishnoi and Clark, 2006). Gas hydrates were reported as early as 1810 (Davy, 1811). Following Sir Humphrey Davy’s report of aqueous solutions of chlorine that remained solid at temperature above 0°C (Davy, 1811) and Faraday’s confirmation in 1823, gas hydrates were a steady object of scientific curiosity for more than 100 years. Research efforts became more focused from the mid 1930’s on due to the suggestion that the unwanted solid material in gas transmission pipelines was in fact gas hydrate rather than ice, frequently forming plugs at temperatures above the icepoint (Hammerschmidt, 1934). Since then, extensive experimental and computational studies have been carried out in order to identify the equilibrium formation conditions for various hydrate forming systems. This research was driven by the need to establish methods to prevent the occurrence of hydrate of hydrate plugs in oil and gas pipelines. Hydrate research on flow assurance is still carried out especially with oil and gas exploration into deeper water and remote offshore areas. Despite the initial negative impression on gas hydrates, recently it has been realized that gas hydrates possess important roles towards energy and environmental issues as well, mainly due to its potential for gas holding capacity and separation purposes. Methane, and other natural gas components, trapped in ice-like lattices known as ‘gas hydrates’ or ‘clathrate hydrates’ have been found to exist naturally in the earth, especially offshore on the continental margins and under the permafrost in the Arctic (Makogon et al., 1972; 1987; Suess et al., 1999; Kvenvolden 1988; 1999; 2000; Reeburgh, 2003; Buffet and Archer, 2004; Klauda and Sandler, 2005). The exact quantity of natural gas in hydrate form is not known accurately, but is significant and considered to be a huge potential unconventional energy source for the future, with the latest estimates some 5 – 20 % of the global carbon budget. Unfortunately technologies for gas production do not exist as yet due to complications involved. Besides the economic aspect that has to be taken into consideration, the impacts on the environment, ecology, and geological stability are also important. The decomposition of natural hydrate may destabilize and alter the earth’s geological features (Glasby, 2003; Sultan et al., 2004a; 2004b) causing geohazards such as landslides, earthquakes, and even tsunamis if the mass movements are under water. On the other hand, current global warming may initiate the decomposition of natural hydrates (Hatzikiriakos and
Clathrate Hydrate Crystallization for Clean Energy and Environmental Technologies 11 Englezos, 1993; Kvenvolden, 2002). The first production test of methane hydrate was conducted in the Mackenzie Delta in the Canadian Arctic. Two test wells were drilled where samples cores were collected to understand its characteristic and evaluate the possibility for successful gas production (Bybee, 2004). Thermal and pressure stimulations were tested to measure both input conditions and reservoir responses that enabled the calibration and refinement of reservoir simulation models (Collet, 2005). Moreover, hydrates were also suggested to exist in extraterrestrial space especially on the outer planets and satellites, eg. Mars, Saturn, Uranus and Neptune (Delsemme and Swings, 1952; Delsemme and Wenger, 1970; Lunine and Stevenson, 1985; Koh, 2002; Osegovic and Max, 2005; Tobie et al., 2006; Machida et al., 2006; Hand et al., 2006). Titan, the largest moon of the Saturn is believed to have 100 km thickness of high pressure methane hydrate within its ice mantle (Loveday et al., 2001). Gas hydrate also offers opportunities to develop innovative technologies for gas storage and separation (Gudmundsson et al., 1994; 2000; Mori, Y. H., 2003; Englezos and Lee, 2005) as well as cool energy storage (Inaba, 2000; Tanasawa and Takao, 2002). The applications include storing and transporting natural gas (Khokkar et al., 1998; Seo and Lee, 2003; Mori, 2003; Thomas and Dawe, 2003; Lee et al., 2005b; Tsuji et al., 2005a; 2005b; 2004; Javanmardi et al., 2005; Abdalla and Abdullatef, 2005), hydrogen storage for the hydrogen economy of the future (Mao et al., 2002; Patchkovskii and Tse, 2003; Lee et al., 2005a; Schuth, 2005 Strobel et al., 2006; Hester et al., 2006; Hu and Ruckenstein, 2006), sequestration of carbon dioxide with in situ methane hydrate decomposition (Lee et al. 2003; Park et al., 2006a; House et al., 2006), separation of carbon dioxide from the flue gas (Seo et al., 2005a; Yoon et al., 2006; Park et al., 2006b), seawater desalination (Parker, 1942; Barduhn et al., 1962; Javanmardi and Moshfeghian, 2003; Rautenbach and Pennings, 1973) and refrigeration (Tomlison, 1982; Ternes, 1984; Mori and Mori, 1989a,b; Bi et al., 2006). For hydrate technology to be industrially applicable, several challenges need to be addressed such as: high gas storage capacity, efficient gas separation, fast phase transformations, and mild pressure-temperature conditions during processing, storage, and transportation. Research on gas hydrates involves engineers and scientists. Chemical engineers in particular can make significant contributions towards the commercialization of gas hydratebased technologies. Therefore, the objective of this chapter is to review developments and highlight areas of opportunity for the involvement of chemical engineers. The structure of the chapter is as follows. First clathrate hydrates are described with emphasis on their structures and the fundamentals of thermodynamic and kinetic properties. Subsequently, the clean energy technologies under development are discussed. Here, chemical engineers can play a crucial role in the successful scale-up of the various existing concepts. These include natural gas storage and transport, hydrogen storage, and carbon dioxide capture and sequestration. Flow assurance in the hydrocarbon production and transportation industry is the only industrial-scale area where continuous research activities both at fundamental and practical levels are ongoing. However the potentials of hydrates for other practical applications have not been exploited mostly because the technology is relatively new and not established. Finally the recovery of natural gas from natural deposits is briefly discussed and the interested reader is directed to a number of recent references.
12
Peter Englezos, John Ripmeester and Robin Susilo
Clathrate Hydrates Gas hydrates are true inclusion compounds and hence also are known as clathrates (Davidson, 1973). They are non-stoichiometric crystalline solids with the physical appearance of ice, and consist of water molecules that serve as the host material and guest molecules trapped in the host. Natural gas components such as: methane, ethane, propane, carbon dioxide, and hydrogen sulfide are typical guest molecules, although any hydrophobic molecule that fits the host cavities can be the guest. The water molecules are connected through hydrogen bonds forming cages/cavities that completely encage individual guest molecules. The crystal lattice consisting of empty cages is not stable thermodynamically, and there is a minimum guest content required to give a stable lattice. Hence, guest molecules must have the correct sizes and geometry in order to fit into the different cages. Normally there is a single molecule per cage except in the case of high pressure hydrates of atoms or small molecules (N2, H2, CH4, rare gases). There are no specific or directional interactions between the host and guest molecules with the weak van der Waals forces providing the key interaction. Thus the guest molecule is free to rotate and translate inside the cage. The crystal lattices have limited flexibility to accommodate guests of different size so that the crystal lattice parameters have well defined limits. Further discussion of structures, kinetics and thermodynamic properties are discussed in the next section.
Structures Hydrate cages are arranged and packed in different configurations, with three distinct crystal structures commonly encountered today: cubic structure I (sI) (Müller and vStackelberg, 1952; McMullan and Jeffrey, 1965), cubic structure II (sII) (vStackelberg and Müller, 1951; Mak and McMullan, 1965), and hexagonal structure H (sH) (Ripmeester et al., 1987). The cubic structures have two types of cavities (small/S and large/L) but the hexagonal Table 1. Hydrate structures and cage properties Structure Crystal system Space group Lattice parameters Number of cage Cage identification Ideal unit cell formula Cage radius
Coordination number
I Cubic Pm3n
II Cubic Fd3m
a = 12A
a = 17.3A
2
2
Small/S (512) Large/L (51262)
Small/S (512) Large/L (51264)
2S.6L.46H2O
16S.8L.136H2O
rS = 3.95A rL = 4.33A
rS = 3.91A rL = 4.73A
S = 20 L = 24
S = 20 L = 28
H Hexagonal P6/mmm a = 12.2 A c = 10A 3 Small/S (512) Medium/M (435663) Large/L (51268) 3S.2M.1L.34H2O rS = 3.91A rM = 4.06A rL = 5.71A S = 20 M = 20 L = 36
Clathrate Hydrate Crystallization for Clean Energy and Environmental Technologies 13 one has three cavities (small/S, medium/M and large/L). Experience has shown that the large cavities in all common hydrate structures usually are filled completely by a guest molecule. For the cubic structures, the small cavity may be empty and this depends on molecular size, so that a single guest species is enough to stabilize sI and sII hydrates. However a small “help guest” is required to maintain sH hydrate so that two guest species are required. Generally there is only one guest molecule in a cage. Few exceptions were reported at high pressures where smaller molecules like nitrogen (Kuhs et al., 1997), hydrogen (Mao et al., 2002), rare gases (Loveday et al., 2003a; Alavi et al., 2005; 2006b), and methane (Loveday et al., 2001; 2003a) may fill and stabilize the large cage of sII and sH with multiple guest occupancies of a cage. Small (S)
Medium (M)
Large (L)
-
512
Structure I (sI)
51262
-
512
Structure/Formula
2S.6L.46H2O
Structure II (sII)
51264
16S.8L.136H2O
Structure H (sH)
512
435663
51268
3S.2M.1L.34H2O
Figure 1. Hydrate cages.
The small cage is a polyhedron made of twelve-pentagonal faces (pentagonal dodecahedron/512). It is the basic cage that is commonly present in all hydrate structures. However, since lattice stability is derived from the filling of the largest cage in the system, their presence is key, however, they cannot be packed together to fill three-dimensional space. For the cubic structures the basic cages are a polyhedron made of twelve-pentagonal faces with two-hexagonal faces (51262) for sI hydrate and four-hexagonal faces (51264) for sII hydrate. The pentagonal dodecahedra then provide the space-filling building blocks. A unit cell of sI hydrates consist of 2-small cages, 6-large cages and 46 water molecules whereas sII hydrates consist of 16-small cages, 8-large cages and 136 water molecules. The hexagonal sH hydrate has 3-small cages, 2-medium cages, 1-large cage and 34 water molecules. The large cavity has twelve-pentagonal faces with eight-hexagonal faces (51268). Six-hexagonal faces
14
Peter Englezos, John Ripmeester and Robin Susilo
are located on the equatorial plane and linked to another large cage via the medium cages (435663). The other two-hexagonal faces are located at the polar plane and connected to another large cage. The small cage (512) fills the space remaining in the unit cell. The cages are shown in Figure 1 and their properties are summarized in Table 1. sI and sH hydrate both can be derived from the stacking of layers of pentagonal dodecahedra. The space between layers stacked in different ways is taken up by the other cages in the crystal lattice. The stable crystal structure formed depends on the guest molecule(s) present. Guest molecules that fill the void space of the cages efficiently are generally preferred. As expected, see Table 1, the size of all of the dodecahedral cages in the hydrate structures is quite similar although the cage symmetry is different in the three structures. However the large cage size increases on going from sI, to sII and sH. Hence the stable hydrate structure formed is governed by the size of the largest molecule(s) in a particular hydrate. Guest molecules with a van der Waals diameter less than ~9.7A have been reported as hydrate formers. Structure I hydrate is generally formed by smaller molecules (4.2A
