- Email: [email protected]
Recent advances in nanocatalysis research
Current Opinion in Solid State and Materials Science 6 (2002) 401–406
Recent advances in nanocatalysis research Pratibha L. Gai a,b , *, Ryan Roper c , Mark G. White c a
Central Research and Development, DuPont, Experimental Station, Wilmington, DE 19880 -0356, USA b Department of Materials Science, Eng., University of Delaware, Newark, DE, USA c School of Chemical Engineering, Georgia Institute of Technology, Atlanta, GA 30332 -0100, USA Received 19 August 2002; accepted 27 August 2002
Abstract Recent developments in nanocatalysis research are reviewed. They demonstrate the important role of nanocatalysts in the advances of catalytic sciences and technology. Special emphasis is given to the synthesis and characterization of nanosized, supported metal and metal oxide structures. 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Nanotechnology is a hybrid science combining chemistry and engineering. Research and development in this field attempts to manipulate nanoscale structures and then integrate these into larger material components and systems. The possibilities are almost endless and nanotechnology is predicted to have a more profound effect on future society than did automobiles, airplanes, televisions and computers of the 20th century. Dr. Richard Smalley, Nobel laureate, and discoverer of buckminsterfullerene, predicts ‘Nanotechnology will reverse the damage caused by the Industrial Revolution.’ More than 15 government organizations and agencies, nine professional societies and seven official science and engineering academic institutions are now fully involved with the newly-formed National Nanotechnology Initiative in the USA. This 497 million dollar initiative has been made a top priority by the President of the United States and is expected to place the US at the forefront of nanotechnology research [*1]. Countries in Europe and Asia are also setting up nanotechnology initiatives. Some of the major initial advances are expected to come in areas
*Corresponding author. Tel.: 11-302-695-9203; fax: 11-302-6951664. E-mail addresses: [email protected] (P.L. Gai), [email protected] (M.G. White).
of increasing rates of reaction using nanocatalysts in catalytic processes and the integration of molecular electronic function with advanced silicon technology. Scientists believe that breakthroughs in nanotechnology for some areas can be expected in as little as 5 years from now and for some at most, a few decades away. For now, much of nanotechnology is still a vision; and if and when accomplished, it just might be our greatest achievement, completely changing all aspects of our lives [*2]. It is well known that the catalytic activity of supported metal particle catalysts is strongly dependent on the size and shape of the particles. Nanoparticle catalysts are highly active since most of the particle surfaces can be available to catalysis. Novel structural tools are key to understanding their nanostructures. In conventional preparations of catalysts including coprecipitation or filling of the support with an aqueous solution of metal particle precursor, it is difficult to control the catalyst particle size and shape. Recently, novel synthesis methods for nanocatalysts and nanophase materials and novel nanostructural methods have been reported in the literature. Included in these reports are the syntheses of mixed metal oxide nanostructures showing the following morphologies: spheres, wires, ropes tubes, and paintbrushes. These syntheses may be with and without a solvent. Many of the nanocatalysts have found applications in ammonia synthesis, environmental protection, photocatalysis, waste removal, fiber and mechanical industries. Highlights of this current research are reviewed in the following paragraphs.
1359-0286 / 02 / $ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S1359-0286( 02 )00109-2
P.L. Gai et al. / Current Opinion in Solid State and Materials Science 6 (2002) 401–406
402
Nomenclature ETEM: STEM: HRTEM: LVSEM: ESEM:
Environmental transmission electron microscopy scanning transmission electron microscopy high resolution TEM Low voltage scanning electron microscopy environmental SEM
2. Supported metal and oxide nanocatalysts and novel methods for determining nanostructures Ruthenium is known to have one of the highest catalytic activities for ammonia synthesis. Miyazaki et al. [*3] have successfully prepared Ru nanoparticles (up to 6.3% by wt.) by the reduction of RuCl 3 in ethylene glycol. The nanoparticles, about 5 nanometers (nm) in size as shown by transmission electron microscopy (TEM), have been supported on g-alumina substrates. The nanocatalysts have shown very high activity for ammonia synthesis. Ru-based nanophase systems are shown to be efficient catalysts, especially in the catalytic processes for the removal of toxic SO 2 in oxygen-free exhaust gases from coal fired power plants, combustion engines, boilers. Ishiguro et al. [4] have used nanophase Ru catalysts supported on titania and derived from Ru salts and Ru cluster complexes for the efficient reduction of SO 2 . Several novel nanocatalysts have been developed for nitrile hydrogenation reactions in the liquid phase for the fiber industry [*5,*6]. Gai et al. [*5] have employed Ru based cluster catalysts (a few % by wt.) with promoters on titania supports synthesized by a single step sol–gel process for the efficient hydrogenation of nitriles in the liquid phase. The nanocatalyst sizes are of the order of 1 nm as shown in Fig. 1. Using in situ wet-ETEM [*6] (described below) the selective hydrogenation of adiponitrile (ADN) over Ru based nanoclusters
Fig. 1. HRTEM of cobalt promoted Ru nanocatalysts (e.g. arrowed) on titania.
with transition metal promoters on titania supports has been demonstrated in situ, in the solution phase. The parallel reaction chemistry shows the heterogeneous catalyst to be a very efficient hydrogenation catalyst. Recent developments for determining the nanocatalyst nanostructure under operating environments have been striking. A powerful in situ atomic resolution-environmental TEM (atomic resolution-ETEM) method has been pioneered by Gai et al. [*7–*10] which has demonstrated atomic resolution under gas pressures (mbars) and elevated temperatures, enabling direct, real-time probing of catalytic processes at the atomic level. This novel method provides information on the dynamic structural (both the real and reciprocal space) and chemical evolution of catalyst surfaces during catalytic processes [*8–*10], as well as live reaction modes of operation under gas environments and temperatures, which can not be obtained readily by other methods. The in situ development was highlighted by the American Chemical Society’s Chem. and Eng. News [*11a,b]. The design of the novel atomic resolution-ETEM instrument developed by Gai et al. [*8–*10] has now been adopted by commercial TEM manufacturers and later versions of this instrument have been installed in other laboratories. Other researchers have reproduced in situ atomic resolution-ETEM data of Gai et al. [8–10]: for example, methanol catalysis studies have been carried out using commercial instruments [12] and studies of Ziegler– Natta catalysts have been reported with a related instrument [13]. The in situ ETEM method has been advanced recently for direct probing of solid catalysts under liquid environments (hereafter referred to as wet-ETEM) on the molecular scale, while the catalyst is immersed in liquids [*6]. These in situ nanostructural methods are providing a better fundamental understanding of dynamic catalytic reactions. The use of high angle annular dark field-scanning TEM (HAADF-STEM) to determine the 3D-structure of supported metal nanocatalysts at very high spatial resolution of ,1 nm has been elegantly demonstrated for Pd–Ru nanocatalysts supported on mesoporous silica by Midgley and Thomas et al. [*14] and Z-Contrast tomography for 3-D catalyst nanostructural analysis based on Rutherford Scattering [15]. In this method, high angle scattering intensity of electrons from thin samples follows the Z 2 dependence. It is achieved by tilting the sample to a series of different and finely spaced angles of 2-D projection. The use of the HAADF-STEM signal removes the complexity
P.L. Gai et al. / Current Opinion in Solid State and Materials Science 6 (2002) 401–406
of conventional bright field scattering in HRTEM and the associated diffraction complications, and the images are directly interpretable. Advanced FE STEM / TEM, the so-called 2-2-2 200 kV field emission STEM / TEM has been developed by Boyes et al. [16], combining atomic resolution imaging with atomic level chemical and crystallographic analyses with 2 ˚ (0.2 nm) resolution in each of the TEM, STEM and A chemical analyses modes (hence 2-2-2), and providing new opportunities in nanocatalysis. The development of novel ultrahigh-resolution, low-voltage SEM (LVSEM) operating at a few keV, is a very powerful surface tool for both the structure and chemical analyses of nanocatalyst surfaces [*17]. Energy filtered-TEM (EFTEM), surface profile imaging of solid catalyst surfaces in a HRTEM and environmental-SEM are also playing an important role in nanocatalysis research. Clusters of metal atoms formed in gas phase have been size selected and deposited on MgO and titania films by Heiz et al. [18]. This is a significant development in the demonstration of size-selected clusters on oxide surfaces. The development of novel methods for the neutralization of chlorinated waste is crucial because of the need to eliminate the toxicity and environmental hazards associated with the waste. Recently, nanocrystalline alkaline earth metal oxides have attracted considerable attention as effective absorbents for toxic substances such as NO 2 , SO 2 and HCl. MgO as an efficient dehydrologenation catalyst has been described by Mishakov et al. [19]. The nanocrystalline catalysts have been prepared by dissolving clean Mg metal in methanol and the methoxide has been used to form a hydroxide gel which is then heated in vacuum at 500 8C to form nanometer sized MgO particles. Antimony-doped (Sb)–SnO 2 nanocatalysts are of interest in the oxidation of propylene to acrolein, the oxidative dehydrogenation of butanes to 1–3 butadiene and selective oxidation of olefins. In the selective oxidation reactions, promoter distributions and electronic structures of the catalysts are shown to play a key role in the reactions [20]. Recently, electron energy loss spectroscopy (EELS) combined with Z-contrast and HRTEM imaging have been used to show that the particle size and the crystallinity in Sb-doped SnO 2 catalysts increase with the calcination temperature [21]. The EEL spectra are used to infer the oxidation states of Sb calcined at different temperatures. Gold is known to be catalytically inactive; however, it has been recently found that ultrafine gold particles exhibit a very high activity in many catalytic reactions. Au nanocatalysts with the (diam.) size range of 2.4 to 10.6 nm on titania supports have been effectively used for the CO oxidation by Boccuzzi et al. [22]. The effect of calcination temperature on the catalytic activity has been examined using TEM and FTIR methods. Nanocatalysts are also finding applications in fuel cell technology [23]. Hexairidium carbonyl clusters, [Ir 6 (CO) 16 ], in the micropores of faujasite zeolite have been eluciadated by HRTEM. These
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catalysts are of interest in hydrocarbon oxidation catalysis [24]. Nanorods of gold have been synthesized by economical wet chemical methods [25] which may be of interest in preparing other nanosystems. Some interesting properties of metals confined in time and nanometer space of different shapes have been reviewed recently [*26]. Titania based nanocatalysts are finding increasing applications in photocatalysis. Photocatalytic reactions are of considerable interest because of their applicability to the treatment of pollutants and wastes and the utilization of solar energy. TiO 2 has a high oxidizing power that is capable of oxidizing organic carbonaceous substances to CO 2 in the presence of water and oxygen. TEM has been used to study particulate structures in TiO 2 (P25) and B-promoted / TiO 2 catalysts in the photocatalytic production of hydrogen from water [27]. Modification of TiO 2 with boron has been found to be highly active for the reaction. Morphology of the standard photocatalytic material (Degussa P-25) consisting of titania photocatalysts containing both the anatase and rutile phases (with the ratio 3:1) has been examined by TEM by Ohno et al. [28]. The studies show that the phases form a physical mixture of agglomerates of anatase with sizes of |85 nm and rutile of |25 nm. The mixed phase catalyst is found to be very efficient for the photocatalytic oxidation of naphthalene. Heterogenous photocatalytic oxidation (PCO) of organic pollutants is important for air decontamination. Titania (Degussa P25) is shown to be an efficient photocatalyst in the decomposition of formic acid [29].
3. Silicon-based systems Silica is the metal oxide of choice for a number of adsorbent and catalyst applications. As an adsorbent, silica is used to remove water from gas streams and as a partitioning agent in chromatography. Silica is also used as a support for metals and metal oxides. The cracking of petroleum into gasoline is one of the largest uses of silica, when combined with alumina, as a catalyst. Recent work has focused upon the synthesis of silicon as a nanostructure.
4. Modeling of Si nanotubes Fagan et al. [*30] discussed the modeling of silicon nanotubes in an attempt to understand the principles relevant to the formation of this hypothetical structure and to compare the properties of these structures to those actually observed for carbon nanostructures. They showed that simple molecular modeling tools could be used to explain why the synthesis of silicon metal nanotubes has not been observed.
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5. Si nanostructures from hydrothermal synthesis The synthesis of silica-based nanostructures, known as zeolites and molecular sieves, has been reported in the literature for many years. Structural elucidation of microporous and mesoporous zeolite catalysts and molecular sieves by HRTEM and related methods has been reviewed recently [*31]. We report here some recent literature citations that are relevant to the newest nanostructure: ropes, nanotubes and paintbrushes.
5.1. Nanoropes Moyakawa et al. [32] report that ‘nanoropes’ may be synthesized by merely extending the reaction time for the MCM-41 synthesis at conditions similar to those normally used to make the MCM-41 product. Changes in the morphology from spherical to rod-shaped particles occurred when the synthesis time was increased past 48 h under pH basic conditions. The d-spacing of the (100) peak increases with increasing synthesis time; the surface area decreases and the pore sizes increase as well as the wall thicknesses. These results are consistent with a further condensation of the silica. 29 Si MAS-NMR data suggest that the silica has reacted further to from more of the fully condensed Q 4 silica units.
5.2. Nanotubes MCM-41 preparations [*33] using an acid as part of the synthesis can be hydrothermally treated after synthesis with an aqueous ammonia solution to improve the order and stabilitly of the nanostructures. The key factors in this post-synthesis treatment are temperature, ammonia concentration, and aging time. The post-synthesis aging described here dramatically improves the stability of the calcined solids on exposure to boiling water for 6 h. The authors claim that the ammonia hydrothermal treatment is effective in improving the hexagonal order of the mesostructures. The pore-size distribution is narrowed by the ammonia treatment and the average pores size is shifted to larger sizes as a result of the treatment. Transmission electron microscopy clearly shows the presence of nanotubes and ropes in these samples. Apparently, the treatment with ammonia did not destroy these features that were present in the untreated sample.
and each bundle showing a characteristic width of about 30–40 nm and a length of 200 nm. These structures are formed in mild alkaline conditions at temperatures near 80 8C and they are not observed at temperatures higher than 90 8C.
6. Si nanostructures from high-temperature synthesis The synthesis of metal oxides by the flame hydrolysis of liquid metal halides has been reported in the literature for many years. The Cabot Corporation [35] has marketed nanosized particles of silicon having surface areas from 200 to 300 m 2 / g. Transmission electron micrographs of these solids show a particle that is roughly spherical. White et al. [35,36,*37,*38] recently reported a synthesis technique that produces nanospheres and nanowires with quite regular shape. The metal and its associated oxide are heated to 1300 8C in argon and the resulting products are harvested from colder parts of the synthesis reactor. One remarkable aspect of this technique is that no liquid solvent is used and no off-gases are produced. We anticipate that this technique could be used to lower the costs of making some of the current catalysts. The nanospheres were spherical and demonstrated narrow, size distribution between 40 and 45 nm in diameter (Fig. 2) This high-temperature technique can also be used to produce silica-supported CuO [36] and silica-supported SnO x [*37]. These supported catalysts show high dispersions of the reactive metal oxide with particle sizes of the supported metal as small as 3 nm on a 40 nm silica particle [*38]. Nanotubes and nanofiber arrays [39] may be synthesized
5.3. Paintbrushes Bundles of silica nanotubes [34] have been reported to form nanostructures known as paintbrushes using sol–gel synthesis. These authors used cetylpyridinium bromide as the template under mild, alkali conditions (pH59.5, 80 8C, for 3 days). Transmission electron micrographs showed single nanotubes growing on the (001) surfaces of silica that collected to form the ‘paintbrush’ feature. While each nanotube is |5 nm in diameter, the tubes form bundles,
Fig. 2. TEM of silica nanospheres.
P.L. Gai et al. / Current Opinion in Solid State and Materials Science 6 (2002) 401–406
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Fig. 3. Nanotubes and nanofiber arrays.
using the same technique but by altering the temperature of the reactor and flow rate of inert gas through the device (Fig. 3). The lengths of these tubes / wires are a few microns and the diameters are 70–80 nm. We have characterized the silica nanospheres and nanotubes [40] for the phenol hydroxylation reaction to show that the reactivity per unit surface of each nanostructure is about the same.
7. Conclusions This review highlights the roles of nanocatalysts and novel nanostructural methods for a variety of catalytic reactions.
References Papers of particular interest have been highlighted as: * of special interest.
[*1] [*2] [*3] [4] [*5]
[*6]
[7] [*8]
[*9]
[*10] [11] [12]
www.nano.gov www.howstuffworks.com / nanotechnology.htm Iyazaki A, Balint I, Aika K, Nakano Y. J Catal 2001;204:364. Ishiguro A, Liu Y, Nakajima T, Wakatsuki Y. J Catal 2002;206:159. Gai PL, Kourtakis K, Ziemecki S. In situ real time environmental high resolution microscopy of nanometer size novel xerogel catalysts for hydrogenation reactions in nylon 6,6. Microscopy and Microanalysis (Springer) 2000;6:335. Gai PL, editor, Microscopy and Microanalysis, Development of wet-ETEM for liquid-catalyst reactions, Vol. 8, Cambridge University Press, 2002, p. 21. Gai PL, Kourtakis K. Solid State defect mechanism in VPO. Science 1995;267:661. Boyes ED, Gai PL. Environmental high resolution electron microscopy and applications in chemical science. Ultramicroscopy 1997;67:219. (a) Gai PL. Direct probing of gas molecule–solid catalyst interactions on the atomic scale. Adv Mater 1998;10:1259. (b) Gai PL. Acta Cryst. 1997;B53:346. Gai PL. Environmental high resolution electron microscopy of gas-catalyst reactions. Top Catal 1999;8:97. (a) Haggin J. Chem Eng News (ACS) 1995;73:39; (b) Jacoby M. C&E News 2002;80:26. (a) Hansen TW et al. Proceedings of the 12th Euro Congress on EM
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[13] [*14]
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P.L. Gai et al. / Current Opinion in Solid State and Materials Science 6 (2002) 401–406 (Publ Czech Electron Microscopy Soc) 2000;2:537; (b) Hansen TW et al. Science 2001;294:1508. Oleshko V, Crozier P, Cantrell R, Westwood AJ. Electron Micr 2002;51:S27. Midgely PA, Weyland M, Thomas JM, Johnson BFG. A technique in 3-dimensional nanostructural analysis based on Rutherford scattering, Chem Commun 2001:907. Pennycook S, Rafferty B, Nellist PD. Z-contrast imaging in an aberration-corrected STEM. Microscopy and Microanalysis 2000;6:343. Boyes ED, Ringnalda J, van der Stam MAJ, Fliervoet TF, Van Cappellen E. A 2-2-2 200 keV field emission STEM / TEM system. Microscopy and Microanalysis (ProcMSA) 2001;7:232. Boyes ED. High resolution and low voltage SEM imaging and chemical microanalysis. Adv Mater 2000;10:1277. Heiz U. Surf Sci 2001. Mishakov IV, Bedilo AF, Richards RM, Chesnokov VV, Volodin AM, Zaikovskii VI, Buyanov RA, Klabunde KJ. J Catal 2002;206:40. Boyes ED, Gai PL, Warwick C. Nature 1985;313:666. Browning et al. J Catal 2001;198:120. Boccuzzi F, Chiorino A, Manzoli M, Lu P, Akita T, Ichikawa S, Haruta M. J Catal 2001;202:256. Stroud R. et al. Microscopy and Microanalysis 2000:6. Tesche B, Kawi S, Gates BC. Catal Lett 2000;70:57–60. Gai PL, Harmer MA. Nano Lett 2002;2:771. El-Sayed M. Some interesting properties of metals. Acc Chem Res 2001;34:257–64. Moon SC, Mametsuka H, Tabata S, Suzuki E. Photocatalytic production of H2 from TiO 2 and B / TiO 2 . Catal Today 2000;58:125.
[28] Ohno T, Sarukawa K, Tokieda K, Matsumura M. J Catal 2001;203:82. [29] Lee GD, Falconer JL. Catal Lett 2000;70:145. [*30] Fagan SB, Mota R, Baierle RJ, Paiva G, da Silva AJR, Fazzio A. Stability investigation and thermal behavior of a hypothetical silicon nanotube. J Mol Struct (Theochem) 2001;539:101. [*31] Thomas JM, Terasaki O, Gai PL, Zhou W, Gonzalez-Calbet J. Acc Chem Res 2001. [32] Mokaya R. Hydrothermally-induced morphological transformation of mesoporous MCM-41 silica. Micropor Mesopor Mat 2001;44– 45:119. [*33] Lin HP, Mou CY, Liu SB, Tang CY, Lin CY. Post-synthesis treatment of acid-made mesoporous silica materials by ammonia hydrothermal process. Micropor Mesopor Mat 2001;44–45:129. [34] Yuan Z, Zhou W. A novel morphology of mesoporous molecular sieve MCM-41. Chem Phys Lett 2001;333:427. [35] Cabot Corporation sales literature. [36] Gole JL, Stout JD, Rauch WL, Wang ZL. Direct synthesis of silicon nanowires, silica nanospheres, and wire-like nanosphere agglomerates. Appl Phys Lett 2000;76:2346. [*37] Gole JL, White MG. Nanocatalysis: selective conversion of ethanol to acetaldehyde using mono-atomically dispersed copper on silica nanospheres. J Catal 2001;204:249. [*38] Gole JL, Wang ZL. SnO x nanocrystallites supported by silica nanostructures. Nano Lett 2001;1:449. [39] Gole JL, Wang ZL, Dai ZR, Stout J, Gao RP, White MG. Silica based nanospheres, nanowires, nanosubstrates, nanotubes, and nanofiber arrays, Invited paper, Progr Coll & Polym Sci, in press. [40] Erickson A, Jacob A, Gole JL, White MG. Nanosolids for the hydroxylation of phenol, in preparation.
Recent advances in nanocatalysis research Pratibha L. Gai a,b , *, Ryan Roper c , Mark G. White c a
Central Research and Development, DuPont, Experimental Station, Wilmington, DE 19880 -0356, USA b Department of Materials Science, Eng., University of Delaware, Newark, DE, USA c School of Chemical Engineering, Georgia Institute of Technology, Atlanta, GA 30332 -0100, USA Received 19 August 2002; accepted 27 August 2002
Abstract Recent developments in nanocatalysis research are reviewed. They demonstrate the important role of nanocatalysts in the advances of catalytic sciences and technology. Special emphasis is given to the synthesis and characterization of nanosized, supported metal and metal oxide structures. 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Nanotechnology is a hybrid science combining chemistry and engineering. Research and development in this field attempts to manipulate nanoscale structures and then integrate these into larger material components and systems. The possibilities are almost endless and nanotechnology is predicted to have a more profound effect on future society than did automobiles, airplanes, televisions and computers of the 20th century. Dr. Richard Smalley, Nobel laureate, and discoverer of buckminsterfullerene, predicts ‘Nanotechnology will reverse the damage caused by the Industrial Revolution.’ More than 15 government organizations and agencies, nine professional societies and seven official science and engineering academic institutions are now fully involved with the newly-formed National Nanotechnology Initiative in the USA. This 497 million dollar initiative has been made a top priority by the President of the United States and is expected to place the US at the forefront of nanotechnology research [*1]. Countries in Europe and Asia are also setting up nanotechnology initiatives. Some of the major initial advances are expected to come in areas
*Corresponding author. Tel.: 11-302-695-9203; fax: 11-302-6951664. E-mail addresses: [email protected] (P.L. Gai), [email protected] (M.G. White).
of increasing rates of reaction using nanocatalysts in catalytic processes and the integration of molecular electronic function with advanced silicon technology. Scientists believe that breakthroughs in nanotechnology for some areas can be expected in as little as 5 years from now and for some at most, a few decades away. For now, much of nanotechnology is still a vision; and if and when accomplished, it just might be our greatest achievement, completely changing all aspects of our lives [*2]. It is well known that the catalytic activity of supported metal particle catalysts is strongly dependent on the size and shape of the particles. Nanoparticle catalysts are highly active since most of the particle surfaces can be available to catalysis. Novel structural tools are key to understanding their nanostructures. In conventional preparations of catalysts including coprecipitation or filling of the support with an aqueous solution of metal particle precursor, it is difficult to control the catalyst particle size and shape. Recently, novel synthesis methods for nanocatalysts and nanophase materials and novel nanostructural methods have been reported in the literature. Included in these reports are the syntheses of mixed metal oxide nanostructures showing the following morphologies: spheres, wires, ropes tubes, and paintbrushes. These syntheses may be with and without a solvent. Many of the nanocatalysts have found applications in ammonia synthesis, environmental protection, photocatalysis, waste removal, fiber and mechanical industries. Highlights of this current research are reviewed in the following paragraphs.
1359-0286 / 02 / $ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S1359-0286( 02 )00109-2
P.L. Gai et al. / Current Opinion in Solid State and Materials Science 6 (2002) 401–406
402
Nomenclature ETEM: STEM: HRTEM: LVSEM: ESEM:
Environmental transmission electron microscopy scanning transmission electron microscopy high resolution TEM Low voltage scanning electron microscopy environmental SEM
2. Supported metal and oxide nanocatalysts and novel methods for determining nanostructures Ruthenium is known to have one of the highest catalytic activities for ammonia synthesis. Miyazaki et al. [*3] have successfully prepared Ru nanoparticles (up to 6.3% by wt.) by the reduction of RuCl 3 in ethylene glycol. The nanoparticles, about 5 nanometers (nm) in size as shown by transmission electron microscopy (TEM), have been supported on g-alumina substrates. The nanocatalysts have shown very high activity for ammonia synthesis. Ru-based nanophase systems are shown to be efficient catalysts, especially in the catalytic processes for the removal of toxic SO 2 in oxygen-free exhaust gases from coal fired power plants, combustion engines, boilers. Ishiguro et al. [4] have used nanophase Ru catalysts supported on titania and derived from Ru salts and Ru cluster complexes for the efficient reduction of SO 2 . Several novel nanocatalysts have been developed for nitrile hydrogenation reactions in the liquid phase for the fiber industry [*5,*6]. Gai et al. [*5] have employed Ru based cluster catalysts (a few % by wt.) with promoters on titania supports synthesized by a single step sol–gel process for the efficient hydrogenation of nitriles in the liquid phase. The nanocatalyst sizes are of the order of 1 nm as shown in Fig. 1. Using in situ wet-ETEM [*6] (described below) the selective hydrogenation of adiponitrile (ADN) over Ru based nanoclusters
Fig. 1. HRTEM of cobalt promoted Ru nanocatalysts (e.g. arrowed) on titania.
with transition metal promoters on titania supports has been demonstrated in situ, in the solution phase. The parallel reaction chemistry shows the heterogeneous catalyst to be a very efficient hydrogenation catalyst. Recent developments for determining the nanocatalyst nanostructure under operating environments have been striking. A powerful in situ atomic resolution-environmental TEM (atomic resolution-ETEM) method has been pioneered by Gai et al. [*7–*10] which has demonstrated atomic resolution under gas pressures (mbars) and elevated temperatures, enabling direct, real-time probing of catalytic processes at the atomic level. This novel method provides information on the dynamic structural (both the real and reciprocal space) and chemical evolution of catalyst surfaces during catalytic processes [*8–*10], as well as live reaction modes of operation under gas environments and temperatures, which can not be obtained readily by other methods. The in situ development was highlighted by the American Chemical Society’s Chem. and Eng. News [*11a,b]. The design of the novel atomic resolution-ETEM instrument developed by Gai et al. [*8–*10] has now been adopted by commercial TEM manufacturers and later versions of this instrument have been installed in other laboratories. Other researchers have reproduced in situ atomic resolution-ETEM data of Gai et al. [8–10]: for example, methanol catalysis studies have been carried out using commercial instruments [12] and studies of Ziegler– Natta catalysts have been reported with a related instrument [13]. The in situ ETEM method has been advanced recently for direct probing of solid catalysts under liquid environments (hereafter referred to as wet-ETEM) on the molecular scale, while the catalyst is immersed in liquids [*6]. These in situ nanostructural methods are providing a better fundamental understanding of dynamic catalytic reactions. The use of high angle annular dark field-scanning TEM (HAADF-STEM) to determine the 3D-structure of supported metal nanocatalysts at very high spatial resolution of ,1 nm has been elegantly demonstrated for Pd–Ru nanocatalysts supported on mesoporous silica by Midgley and Thomas et al. [*14] and Z-Contrast tomography for 3-D catalyst nanostructural analysis based on Rutherford Scattering [15]. In this method, high angle scattering intensity of electrons from thin samples follows the Z 2 dependence. It is achieved by tilting the sample to a series of different and finely spaced angles of 2-D projection. The use of the HAADF-STEM signal removes the complexity
P.L. Gai et al. / Current Opinion in Solid State and Materials Science 6 (2002) 401–406
of conventional bright field scattering in HRTEM and the associated diffraction complications, and the images are directly interpretable. Advanced FE STEM / TEM, the so-called 2-2-2 200 kV field emission STEM / TEM has been developed by Boyes et al. [16], combining atomic resolution imaging with atomic level chemical and crystallographic analyses with 2 ˚ (0.2 nm) resolution in each of the TEM, STEM and A chemical analyses modes (hence 2-2-2), and providing new opportunities in nanocatalysis. The development of novel ultrahigh-resolution, low-voltage SEM (LVSEM) operating at a few keV, is a very powerful surface tool for both the structure and chemical analyses of nanocatalyst surfaces [*17]. Energy filtered-TEM (EFTEM), surface profile imaging of solid catalyst surfaces in a HRTEM and environmental-SEM are also playing an important role in nanocatalysis research. Clusters of metal atoms formed in gas phase have been size selected and deposited on MgO and titania films by Heiz et al. [18]. This is a significant development in the demonstration of size-selected clusters on oxide surfaces. The development of novel methods for the neutralization of chlorinated waste is crucial because of the need to eliminate the toxicity and environmental hazards associated with the waste. Recently, nanocrystalline alkaline earth metal oxides have attracted considerable attention as effective absorbents for toxic substances such as NO 2 , SO 2 and HCl. MgO as an efficient dehydrologenation catalyst has been described by Mishakov et al. [19]. The nanocrystalline catalysts have been prepared by dissolving clean Mg metal in methanol and the methoxide has been used to form a hydroxide gel which is then heated in vacuum at 500 8C to form nanometer sized MgO particles. Antimony-doped (Sb)–SnO 2 nanocatalysts are of interest in the oxidation of propylene to acrolein, the oxidative dehydrogenation of butanes to 1–3 butadiene and selective oxidation of olefins. In the selective oxidation reactions, promoter distributions and electronic structures of the catalysts are shown to play a key role in the reactions [20]. Recently, electron energy loss spectroscopy (EELS) combined with Z-contrast and HRTEM imaging have been used to show that the particle size and the crystallinity in Sb-doped SnO 2 catalysts increase with the calcination temperature [21]. The EEL spectra are used to infer the oxidation states of Sb calcined at different temperatures. Gold is known to be catalytically inactive; however, it has been recently found that ultrafine gold particles exhibit a very high activity in many catalytic reactions. Au nanocatalysts with the (diam.) size range of 2.4 to 10.6 nm on titania supports have been effectively used for the CO oxidation by Boccuzzi et al. [22]. The effect of calcination temperature on the catalytic activity has been examined using TEM and FTIR methods. Nanocatalysts are also finding applications in fuel cell technology [23]. Hexairidium carbonyl clusters, [Ir 6 (CO) 16 ], in the micropores of faujasite zeolite have been eluciadated by HRTEM. These
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catalysts are of interest in hydrocarbon oxidation catalysis [24]. Nanorods of gold have been synthesized by economical wet chemical methods [25] which may be of interest in preparing other nanosystems. Some interesting properties of metals confined in time and nanometer space of different shapes have been reviewed recently [*26]. Titania based nanocatalysts are finding increasing applications in photocatalysis. Photocatalytic reactions are of considerable interest because of their applicability to the treatment of pollutants and wastes and the utilization of solar energy. TiO 2 has a high oxidizing power that is capable of oxidizing organic carbonaceous substances to CO 2 in the presence of water and oxygen. TEM has been used to study particulate structures in TiO 2 (P25) and B-promoted / TiO 2 catalysts in the photocatalytic production of hydrogen from water [27]. Modification of TiO 2 with boron has been found to be highly active for the reaction. Morphology of the standard photocatalytic material (Degussa P-25) consisting of titania photocatalysts containing both the anatase and rutile phases (with the ratio 3:1) has been examined by TEM by Ohno et al. [28]. The studies show that the phases form a physical mixture of agglomerates of anatase with sizes of |85 nm and rutile of |25 nm. The mixed phase catalyst is found to be very efficient for the photocatalytic oxidation of naphthalene. Heterogenous photocatalytic oxidation (PCO) of organic pollutants is important for air decontamination. Titania (Degussa P25) is shown to be an efficient photocatalyst in the decomposition of formic acid [29].
3. Silicon-based systems Silica is the metal oxide of choice for a number of adsorbent and catalyst applications. As an adsorbent, silica is used to remove water from gas streams and as a partitioning agent in chromatography. Silica is also used as a support for metals and metal oxides. The cracking of petroleum into gasoline is one of the largest uses of silica, when combined with alumina, as a catalyst. Recent work has focused upon the synthesis of silicon as a nanostructure.
4. Modeling of Si nanotubes Fagan et al. [*30] discussed the modeling of silicon nanotubes in an attempt to understand the principles relevant to the formation of this hypothetical structure and to compare the properties of these structures to those actually observed for carbon nanostructures. They showed that simple molecular modeling tools could be used to explain why the synthesis of silicon metal nanotubes has not been observed.
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5. Si nanostructures from hydrothermal synthesis The synthesis of silica-based nanostructures, known as zeolites and molecular sieves, has been reported in the literature for many years. Structural elucidation of microporous and mesoporous zeolite catalysts and molecular sieves by HRTEM and related methods has been reviewed recently [*31]. We report here some recent literature citations that are relevant to the newest nanostructure: ropes, nanotubes and paintbrushes.
5.1. Nanoropes Moyakawa et al. [32] report that ‘nanoropes’ may be synthesized by merely extending the reaction time for the MCM-41 synthesis at conditions similar to those normally used to make the MCM-41 product. Changes in the morphology from spherical to rod-shaped particles occurred when the synthesis time was increased past 48 h under pH basic conditions. The d-spacing of the (100) peak increases with increasing synthesis time; the surface area decreases and the pore sizes increase as well as the wall thicknesses. These results are consistent with a further condensation of the silica. 29 Si MAS-NMR data suggest that the silica has reacted further to from more of the fully condensed Q 4 silica units.
5.2. Nanotubes MCM-41 preparations [*33] using an acid as part of the synthesis can be hydrothermally treated after synthesis with an aqueous ammonia solution to improve the order and stabilitly of the nanostructures. The key factors in this post-synthesis treatment are temperature, ammonia concentration, and aging time. The post-synthesis aging described here dramatically improves the stability of the calcined solids on exposure to boiling water for 6 h. The authors claim that the ammonia hydrothermal treatment is effective in improving the hexagonal order of the mesostructures. The pore-size distribution is narrowed by the ammonia treatment and the average pores size is shifted to larger sizes as a result of the treatment. Transmission electron microscopy clearly shows the presence of nanotubes and ropes in these samples. Apparently, the treatment with ammonia did not destroy these features that were present in the untreated sample.
and each bundle showing a characteristic width of about 30–40 nm and a length of 200 nm. These structures are formed in mild alkaline conditions at temperatures near 80 8C and they are not observed at temperatures higher than 90 8C.
6. Si nanostructures from high-temperature synthesis The synthesis of metal oxides by the flame hydrolysis of liquid metal halides has been reported in the literature for many years. The Cabot Corporation [35] has marketed nanosized particles of silicon having surface areas from 200 to 300 m 2 / g. Transmission electron micrographs of these solids show a particle that is roughly spherical. White et al. [35,36,*37,*38] recently reported a synthesis technique that produces nanospheres and nanowires with quite regular shape. The metal and its associated oxide are heated to 1300 8C in argon and the resulting products are harvested from colder parts of the synthesis reactor. One remarkable aspect of this technique is that no liquid solvent is used and no off-gases are produced. We anticipate that this technique could be used to lower the costs of making some of the current catalysts. The nanospheres were spherical and demonstrated narrow, size distribution between 40 and 45 nm in diameter (Fig. 2) This high-temperature technique can also be used to produce silica-supported CuO [36] and silica-supported SnO x [*37]. These supported catalysts show high dispersions of the reactive metal oxide with particle sizes of the supported metal as small as 3 nm on a 40 nm silica particle [*38]. Nanotubes and nanofiber arrays [39] may be synthesized
5.3. Paintbrushes Bundles of silica nanotubes [34] have been reported to form nanostructures known as paintbrushes using sol–gel synthesis. These authors used cetylpyridinium bromide as the template under mild, alkali conditions (pH59.5, 80 8C, for 3 days). Transmission electron micrographs showed single nanotubes growing on the (001) surfaces of silica that collected to form the ‘paintbrush’ feature. While each nanotube is |5 nm in diameter, the tubes form bundles,
Fig. 2. TEM of silica nanospheres.
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Fig. 3. Nanotubes and nanofiber arrays.
using the same technique but by altering the temperature of the reactor and flow rate of inert gas through the device (Fig. 3). The lengths of these tubes / wires are a few microns and the diameters are 70–80 nm. We have characterized the silica nanospheres and nanotubes [40] for the phenol hydroxylation reaction to show that the reactivity per unit surface of each nanostructure is about the same.
7. Conclusions This review highlights the roles of nanocatalysts and novel nanostructural methods for a variety of catalytic reactions.
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