UDC 66.097.3 : 543.456 BIBLID: 1450–7188 (2004) 35, 67-77
APTEFF, 35, 1-280 (2004)
APPLICATION OF SCANNING ELECTRON MICROSCOPY IN CATALYSIS Gizela A. Lomić, Erne E. Kiš, Goran C. Bošković and Radmila P. Marinković-Nedučin A short survey of various information obtained by scanning electron microscopy (SEM) in the investigation of heterogeneous catalysts and nano-structured materials have been presented. The capabilities of SEM analysis and its application in testing catalysts in different fields of heterogeneous catalysis are illustrated. The results encompass the proper way of catalyst preparation, the mechanism of catalyst active sites formation, catalysts changes and catalyst degradation during their application in different chemical processes. Presented SEM pictures have been taken on a SEM JOEL ISM 35 over 25 years of studies in the field of heterogeneous catalysis. KEYWORDS: Scanning microscopy; heterogeneous catalysts; nanostructured catalysts; texture; morphology; active phase formation; catalyst degradation INTRODUCTION Most industrial catalysts are multicomponent systems. This fact is making catalyst characterization very difficult, since it is necessary to know the chemical content and structure on a nano-scale level, oxidation states of active centers and their environment. The research in the filed of catalysis is also specific because it is necessary to have a large and easily accessible surface area, due to the fact that the actual catalytic act takes place in the thin surface layer. Most often this is achieved by reducing the size of catalyst particles. Therefore, for industrial catalysts, small, very often nano-sized, crystallites are widely used, i.e. they represent the active catalytic substance. It should be emphasized that very often catalysts or catalyst supports were required to have nanostructured material properties, like in the case of sponge-like structure catalysts (1). This is the reason why only the use of various sophisticated instruments can yield successful investigation.
Dr. Gizela A. Lomić, Prof., Dr. Erne E. Kiš, Prof., Dr. Goran C. Bošković, Prof., Dr. Radmila P. MarinkovićNedučin, Prof., University of Novi Sad, Faculty of Technology, 21000 Novi Sad, Bulevar Cara Lazara 1, Serbia and Montenegro
This paper shows that the application of SEM can be effective and specifically applicable in various areas of heterogeneous catalysis, especially in the case when usual investigation methods failed. SEM results, combined with results of various spectroscopic, diffraction, adsorption and other microscopic methods contribute to the explanation of catalyst active structure formation, creation of different phases and catalyst texture, as well as the causes of catalyst aging. All presented SEM results have been collected in the course of our research in the field of heterogeneous catalysis. In this paper we give the most interesting examples concerning: – the choice of catalyst material, – the influence of catalyst preparation on catalyst properties, – activation and active phase formation – structure/texture and catalyst morphology during the catalyst processing, application and deactivation.
EXPERIMENTAL The SEM-studies were perforemed on JEOL-JSM-35 instrument (JEOL USA, Inc., Peabody, Mass., USA) at an emission current of 1500 × 10-6 A and operating at 25 keV. All samples of heterogeneous and nano-structured catalysts were examined with gold coating except for the Pt-Rh commercial catalyst samples, as they were conductive.
RESULTS AND DISCUSSION Choice of raw materials for the synthesis of pillared clay catalysts Clay is one of the most commonly used and cheapest minerals in the world, which can be used as catalysts, catalyst supports, adsorbents, etc. However, very often, clays have to be improved by using various methods. The texture can be changed by pillaring process, where by intercalation of different ions, called pillars, the interlayer spacing is changed, changing thus the accessibility of active centers. Smectite clays are already pillared due to their low charge density and their swelling ability. Ion-exchange processes involving the exchangable cations of the clay initiate the production of pillared clays (PILC). Upon pillaring and calcination, the resulting materials contain metal oxide pillars that support the clay sheets, thereby exposing the internal surfaces of the clay layers. The size of these oligomers appears to control the size of the pore openings in the PILC. As a general feature, montmorillonite from the various deposits, shows very thin irregular lamellae of different sizes, which are partly folded (2). The montmorillonite crystal is actually a particular type of domain formed by the alignment of individual alumina-silicate layers. The clay particles are in a face-to-face arrangement (3). We present our study of the effect of pillaring, performed by intercalation of Al3+ and Fe3+ cations on clays of montmorillonite type. Fig.1 shows the parallel alignment of individual alumina-silicate lamellae in montmorillonite (Fig. 1a). 68
Fig. 1. SEM micrographs of montmorillonite (a) and the AlFe-PILC (b), (×5000)
After pillaring process and calcination the AlFe-patches formed kept the layers apart and the image of the layers becomes thicker (Fig. 1b), which indicates successful pillaring process. This was confirmed by successful removal of toxic organic compounds from water solutions (4, 5). A consequence of pillaring process is the increase of surface area (111.8/162.7 m2/g) and interlayer spacing (1.53/1.80 nm) in pillared clay system compared to the original one. At lower phenol content in water solutions (0.5 mg phenol/100 cm3) Al/Fe pillared clay catalyst reaches the activity of common homogeneous iron catalyst in the presence of UV light . This finding is very important since the strong bonding of iron to aluminum in the pillars hinders the iron leaching and consequently environment contamination by iron, which cannot be avoided with homogeneous iron catalyst. However, at higher phenol content in water solutions (5.0 mg phenol/100 cm3) the activity of pillared clay catalyst is significantly lower than the activity of the homogeneous iron catalyst . Influence of support origin on catalyst activity In supported metal catalysts, starting material for the support preparation, i.e. support origin, can have explicit and implicit influence on active center formation. The Fe/MgO catalyst for synthesis of higher hydrocarbons by hydrocondensation of carbon monoxide, shows different activity depending on MgO origin as the result of different degree of metal-support ineraction (MSI). Therefore, further morphological investigations were carried out with the aim of elucidating the effect of MgO origin. By SEM analysis it was determined ( Fig.2), that: – the Fe/MgO catalyst with the MgO originated from magnesium hydroxide has poorly ordered structure (Fig. 2a); – the Fe/MgO catalyst with the MgO originated from a basic magnesium hydroxycarbonate has plate-like structure (Fig. 2b); 69
the Fe/MgO catalyst with the MgO originated from magnesium oxalate has clearly formed cubic structure (Fig. 2c).
All results are in line with the well-known “memory effect”, which is in this case demonstrated as catalyst morphology following the morphology of the MgO precursor (6,7,8).
Fig. 2. SEM micrographs of Fe/MgO catalyst from magnesium hydroxide (a), from basic magnesium hydroxycarbonate (b) and from magnesium oxalate (c) (×480)
The morphology of MgO catalyst support influences the catalyst activity. The catalyst with well-ordered crystal structure, as is the catalyst derived from magnesium oxalate, has a good selectivity toward lower olefins . Further investigation showed that the origin of magnesium oxalate, used as a precursor for MgO preparation, can influence the Fe/MgO catalyst activity. The catalyst whose MgO support was produced from the oxalate synthetised from magnesium nitrate, shows higher activity comparing to the catalyst with Mg oxalate produced from magnesium acetate. SEM pictures of the Fe/MgO catalyst sample with the same chemical composition obtained from the support precursor of nitrate origin (Fig. 3a) and acetate origin (Fig. 3b) show visible differences in ordering of MgO (9).
Fig. 3. SEM micrographs of the Fe/MgO catalysts, support is originated from magnesium nitrate (a) and from magnesium acetate (b) (×4800)
These seemingly small differences in catalyst support origin caused different extent of iron reduction and formation of different types of iron carbide. Different extent of iron reduction and the presence of different iron carbide types significantly influence the catalysts activity and selectivity. The activity of catalyst produced from the support precursor of nitrate origin is an order of magnitude higher than that found for catalyst produced from the support precursor of acetate origin . Catalysts on stream. Replication of catalyst nanostructure on the products architecture Recent studies have pointed out that morphology of polyolefine particles is determined by the size, shape and porosity of the particles of the applied Ziegler-Natta catalysts (10). The structure of TiCl3 Ziegler-Natta catalyst, with particles diameter of 2-12 nm, determine primary polymer particles, whose size vary from 15-200 nm, and is determined by the catalyst porosity. The TiCl3 Ziegler-Natta catalysts with different morphology under identical conditions yield the polymer particles of different shape regularity and pore size distribution (11). The comparison of microscopic properties of the polymer and the catalyst gave us an explanation of this phenomenon, which is of interest for remote control of polymerization processes (Fig. 4). The Ziegler Natta catalysts were investigated by optical microscopy (OM) because of their chemical instability.
Fig. 4. OM micrographs of catalysts A (a) and B (b) (×100, ×400), SEM micrographs of polypropylene, produced by catalyst A (c) and by catalyst B (d) (×540, ×1000)
The distinct analogy in structures and substructures illustrates the mechanism of replication of the catalyst nano-structure on the architecture of the polypropylene particles. It can be seen that when a micro-spherical catalyst (C) is used (Fig. 4b) the obtained polypropylene particles have regular structure (Fig. 4d), and when irregularly shaped catalysts (A) is used (Fig. 4a) the polypropylene particles had an irregular structure (Fig. 4c) (12 ). Catalyst deactivation Causes of catalyst activity decrease are manyfold and are a consequence of an array of physico-chemical changes in aging process, up to part or complete, temporary or permanent deactivation of active centers due to poisoning. In elucidation of these unwanted phenomena SEM is successful as a complementary method to different complex spectroscopic, adsorption and other methods. Because of the practical importance and various causes of this phenomenon, some types of deactivation are going to be represented. Catalyst deactivation by mechanical blocking of active sites. The Pt-Rh catalyst gauze is used for ammonia oxidation in nitrous acid processing. The catalyst activity and life significantly decrease in the presence of small amounts of Au, Mn, Ni, Cu, Ca, Mg, Ba, Al, Fe, Cl and S. Decline in activity can happen when active surface is covered with various alloys and noble metal oxides (13). Deactivation can take place during the short process of catalyst activation too, but it seldom happens. The activation process represents restructuring of catalyst gauze surface and forming of sponge-like active platinum. The cause of the loss of activaty during this process in the ammonia oxidation plant can be explained using scanning electron microscopy which gives practical importance to this investigation (Fig. 5).
Fig. 5. SEM micrographs of new Pt-Rh catalyst gauze (a), the used Pt-Rh catalyst gauze (b) and structure sponge-like active platinum (c) (×360, ×360, ×1000)
On the surface of unused catalyst gauze no foreigin substances can be detected, except for some nodules and markings, which were formed during the wire drawing in catalyst manufacturing (Fig. 5a). On the deactivated gauze some non-homogeneous crystallites are visible, mostly at the crossover points (Fig. 4b) (14). The X-ray fluorescence spectrometry (RFS) and electron microprobe analysis of detected accumultion of Fe, Ni, S as well 72
as small amounts of Ca and Mg. These are known as poisons for the particular catalyst, which explains the loss of catalyst activity. They mechanically block the active centers and therefore yield the retardation of the activation process, i.e a decrease in sponge-like nanostructured platinum formation take place. Small amounts of active structures, i.e. of spongelike platinum, are insufficient to achieve the desired catalyst activity (Fig 5c) (15). Catalyst deactivation by the loss of active component. Hydrodesulphurization, hydrodenitriphication and hydrodeoxidation of petroleum and their fractions are imposed by new severe environmental regulations. Besides there exist regulations for catalysts protection in processing of the heavier petroleum fractions and residue processing. These fractions are characterised by high concentration of catalyst poisons that inevitably lead to deactivation of the catalysts. Co(Ni)O-MoO3/Al2O3 is standard industrial catalyst for hydrodesuphurization and hydrodenitriphication. The activation of these catalysts is performed in H2S atmosphere, where a complex MoSn active phase is formed. By scanning tunneling microscope (STM) (16) a finely ordered nano-sized structure of that mentioned phase was determined. Further results elucidate the main reason for NiO-MoO3/Al2O3. catalyst deactivation, noticed after the regeneration process in an industrial middle distillate hydroprocessing plant. In such cases, detection of the causes in activity decline is important for deciding if the catalyst should be regenerated or discharged or disposed. The results of XRD analysis indicate restructuring of the original octahedral structure of the active phase and reveal the formation of a lower active crystalline molybdate. This collapse of active structure that occurs at the hot spots of the catalyst bed during regeneration process under oxidation conditions can result in evaporation of MoO3, i.e. in the loss of the active component. SEM analysis undoubtedly confirmed the presence of MoO3 crystals obtained by condensation of the vapour of this phase in the reactor nozzle (Fig. 6) (16). This phenomenon was later simulated in the catalyst model system in the presence of 25% MoO3 on alumina support. In Fig. 7 a SEM micrograph of the catalyst model system for hydrodesulphurisation is shown, after treatment at 1000 0C. Laboratory simulation of catalyst aging confirmed the already mentioned loss of active component due to molybdenum oxide crystals formation at the laboratory reactor outlet. The shape and morphology of these crystals are identical to MoO3 crystals detected in the industrial reactor nozzle, except for that they are somewhat smaller. Catalyst deactivation by irreversible binding of active component. For industrial hydrogen production from gaseous and liquid hydrocarbons Ni/Al2O3 steam-reforming catalysts are used. One of the causes of deactivation of these catalysts is the active component binding with support in the NiAl2O4 spinel. The mechanism and kinetics of spinel formation is determined by the type and origin of Al2O3. The rate of NiAl2O4 formation on α-Al2O3 is by one or two orders of magnitude lower than the spinel formation on the thermodynamically unstable γ-Al2O3, or on the different aluminum oxyhydroxides. Similar observation was made during the formation of other spinels on appropriate supports (18). During the catalyst preparation metallic ions (Ni, Cu) diffused less to deeper layers of the compact α-Al2O3 support, which is not the case with γ-Al2O3 and other aluminum oxyhydroxides (Fig. 8a and 8b). The homogeneity of metal distribution and its contact with αAl2O3, γ-Al2O3 and aluminum oxyhydroxide particles are different. These differences result in different rate of irreversible binding of active metal component into the corresponding spinel compounds. 73
Fig. 6. SEM micrograph of sample in the reactor nozzle after HDS catalyst regeneration (×600)
Fig. 7. SEM micrograph of hydrodesulphurization catalyst model system (×600)
Fig. 8. SEM micrographs of γ-A l2O3.particles (a) and of α-Al2O3 particles (b), (×4800)
The activation of alumina supported nickel catalyst is difficult when the formation of NiAl2O4 has taken place, and in severe cases temperatures above 900°C may be required for complete reduction (19). CONCLUSION The presented SEM pictures illustrated a wide application of this microscopic method in the field of heterogeneous catalysis. SEM investigations, combined with spectroscopic, diffraction and other microscopic methods, are very succesful in testing all the phases of preparation, activation, regeneration and deactivation of heterogeneous catalysts. It should be emphasized that many of crucial problems of heterogeneous catalysis can be elucidated and resolved by using scanning electron microscopy.
ACKNOWLEDGMENT This work was supported by the Ministry of Science, Technologies and Development of the Republic of Serbia, Project: 1368 75
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ПРИМЕНА СКЕНИНГ ЕЛЕКТРОНСКЕ МИКРОСКОПИЈЕ У КАТАЛИЗИ Гизела А. Ломић, Ерне Е. Киш, Горан Ц. Бошковић и Радмила П. Маринковић-Недучин У раду је дат кратак преглед података добијених скенинг електронском микроскопијом при испитивању хетерогених катализатора и наноструктурних материјала. На овај начин су илустроване погодности и низ могућности примене ове методе у различитим областима хетерогене катализе, што је драгоцено с обзиром да се најчешће карактеризација и дефинисање структуре и текстуре ових система ради на нивоу наноструктуре, те су уобичајене методе некад немоћне. Информације добијене овом методом доприносе дефинисању и разјашњењу низа проблема који се јављају у овој области катализе, као што су најпогодији начин припреме катализатора, механизам формирања активних центара, промене и деградација катализатора током примене у различитим хемијским процесима. Предочене слике скенинг електронске микроскопије резултат су више од 25 године рада у области хетерогене катализе а биле су добијене на уређају SEM JOEL ISM 35. Received 20 January 2004 Accepted 17 May 2004