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TEM and in-situ EM study of the dispersion of MoO3 on SiO2
Ultramicroscopy 29 (1989) 233-246 North-Holland, Amsterdam
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T E M AND IN-SITU EM STUDY OF THE DISPERSION OF MoO 3 ON SiO 2
A. DATTA and J.R. REGALBUTO Department of Chemical Engineerin~ University of Illinois at Chicago, Chicago, Illinois 60680, USA
and
C.W. ALLEN Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA Received at Editorial Office October-November 1988; presented at Conference May 1988
Molybdenum trioxide supported on nonporous silica spheres and planar silica substrates were used with TEM and in-situ EM (environmental cell and heating stage), respectively, to study the spreading behavior of the hexagonal and orthorhombic forms of MoO 3 on SiO2. At temperatures below 500 o C, supported orthorhombic crystallites are stable, while supported hexagonal crystaUites, formed by an initial calcination of the precursor at 300 o C, spread drastically and in doing so produce a novel, multilayered and amorphous silica-supported morphology for MoO 3. The mechanism of spreading appears to be the formation of mobile species on hexagonal crystallites, and subsequent migration across the SiO2 surface.
1. Introduction
Supported molybdenum or its trioxide has a wide variety of uses in heterogeneous catalysis, for catalytic reactions such as hydrodesulfurization, hydrogenation and dehydrogenation, reforming, and hydrocracking, to name a few. The dispersion and morphology of the trioxide, often a precursor phase of the final catalyst, have much to do with final catalytic activity. A number of trioxide forms exist over various support materials. At low loadings, the material is generally completely dispersed and of a monolayer type while at higher loadings, bulk crystallites form [1]. Some supports such as alumina and titania exhibit stronger interactions with MoO3 such that intermetallic oxide phases form [2]. Over SiO2, however, only the monolayer and bulk crystalline (orthorhombic) forms have been reported; these formed with an initial calcination of the impregnated precursor always greater than 450 ° C. At this high initial calcination temperature, surface loadings less than I atom M o / n m 2 [1] yield the well dispersed catalyst,
whose morphology is only slightly altered by subsequent treatments [3], while loadings greater than this value result in bulk crystallites. There appears to be no way to redisperse bulk, orthorhombic crystallites of MoO 3 on SiO3 once they form [4], short of extremely high calcination temperatures, at which trioxide losses occur due to volatilization [5]. In recent characterization of MoO3/SiO 2 catalysts with X-ray diffraction (XRD) among other methods [6], it has been shown that the dispersion of high-loading MoO 3 (4 atoms M o / n m 2) can be controlled through the initial formation of the little-studied hexagonal crystalline phase. A summary of XRD results from this study is shown in fig. 1. The supported hexagonal phase, formed by an initial 300 °C air calcination, is shown in pattern a and is compared to the orthorhombic phase formed by a 500 °C calcination (pattern b). A bulk crystalline (poorly dispersed) orthorhombic phase (pattern c) was formed by a 500 ° C calcination of the poorly dispersed hexagonal phase, while continued calcination at 300°C
0304-3991/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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A. Datta et al. / T E M and in-situ E M study of dispersion of M o O 3 on SiO 2
Ortho (021)
(040)
(110)
(020)
d
Hex
5; 5 ;
(loo)
(210)
;,"
" d ' ;; ' ~;
;6"2; " 2 ; ~ i DgG~,
;~ ~ ;
",
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Fig. 1. X R D characterization of calcined M o O 3 / S i O 2 [6]: (a) 300 o C, 2 h; (b) 500 o C, 2 h; (c) 300 o C, 2 h and 500 o C, 2 h; (d) 50 o C, 2 h and 300 o C, 4 h; (e) 300 ° C, 6 h; (f) 300 o C, 6 h and 500 ° C, 2 h; (g) 300 ° C, 6 h a n d 500 ° C, 6 h.
caused the hexagonal phase to become mostly amorphous (pattern e) and a calcination of this dispersed phase at 500 °C produced a dispersed orthorhombic phase (pattern f). The dispersed form (actually, mostly amorphous and some orthorhombic crystallites) slowly sintered to bulk orthorhombic crystallites (pattern g) during continued calcination at 500 ° C. The high-loading, well dispersed sample represents a new morphology of silica-supported MoO3: that of an amorphous, multilayer form. It is the purpose of the present study to verify the existence of this MoO3/SiO2 form and to explore the mechanism of its formation, by direct visualization of morphology with electron microscopy. Electron microscopy has become more and more popular for catalyst characterization as various techniques have been developed specifically for this application. Two such techniques involve the use of model catalyst supports to eliminate the complexity of images of porous, high-surface-area support materials. Simple nonporous spherical or cuboidal particles permit edge-on viewing of sup-
ported material and have been used for the characterization of Rh on SiO2 [7] and Ru on SiO2 and MgO [8], for example. Thin, planar films provide a clear two-dimensional projection of supported materials; this type of sample has been used recently to study Rh/SiO 2 [9], Pt/WO3/SiO 2 [10], MoO3-MoS2/A1203111], and Pt/AI203 [12]. Typically in these types of studies, the same region of sample is followed through a series of pretreatment steps (which themselves are performed outside of the microscope), so that morphological changes induced by the pretreatments can be unambiguously observed. An additional degree of simulation is achieved when samples can be treated in-situ, using a heated stage and an environmental cell to maintain a relatively high pressure of a reactant gas [13]. This technique has been used, for example, to characterize the interaction of Pd with amorphous carbon in 02 and CO 2 atmospheres [14]. In the present study, nonporous SiO2 spheres approximately 2000 ,~ in diameter have been used as model supports, with conventional transmission electron microscopy (TEM). Micrographs of the same sample areas containing hexagonal or orthorhombic crystalhtes were taken, before and after a second calcination step performed outside of the microscope. To further understand the mechanism of MoO 3 spreading and phase transformation, insitu EM with planar SiO2 substrates were used. The molybdena containing phase was monitored from its initial state as an amorphous dried ammonium molybdate precursor, through various crystallization and dispersion processes induced by different heat and gas treatments.
2. Experiment
2.1. Spherical supported catalysts for TEM Nonporous silica spheres of an average diameter of 2300 A were prepared by a standard technique [15]. These were impregnated to incipient wetness with ammonium(IV) molybdate (Aldrich) for a bulk loading of 4 wt%, or a surface loading of 16 atoms M o / n m 2. Samples were then dried at room temperature under vacuum, for about 12 h.
A. Datta et al. / T E M and m-situ EM study of dispersion of MoO3 on SiO 2
The dried samples were calcined at 300 or 500 ° C in air for 2 h to produce large hexagonal or orthorhombic crystallites, respectively. Powdered samples were dispersed in deionized water and were deposited onto holey carbon substrates (Formvar backing removed), backed by 400 mesh London finder grids (Earnest Fullam). Grids were removed from the microscope sample holder and placed in a quartz clamping device during subsequent treatments performed in a conventional furnace. A JEOL 100-CX microscope with analytical pole-piece and standard single-tilt stage was operated at 100 kV to obtain all micrographs.
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tamination was minimized by limiting beam exposure at all, but especially low, temperatures, but had to be taken into consideration in subsequent discussion.
3. Results 3.1. T E M with spherical S i O 2
The distribution of MoO 3 on the nonporous SiO2 spheres was quite inhomogeneous on a microscopic scale. Most spheres were entirely bare while others, such as that shown in fig. 2A, had very large crystallites attached to them. The par-
2.2. P l a n a r supported samples f o r in-situ E M
To produce in-situ EM samples, solutions of 10 -3 M ammonium molybdate(IV) tetrahydrate were spread in a thin film over planar SiO 2 substrates (Formvar backing removed) on 400 mesh gold grids (Ladd). The wetted film evaporated at ambient conditions within 10 rain. Dried samples were placed directly in the microscope sample holder. The modified K r a t o s / A E I high voltage electron microscope (HVEM) at Argonne National Laboratory's H V E M - T a n d e m Ion Accelerator Facility [13] was operated at 120 kV to achieve adequate contrast of often amorphous samples. A single-tilt heating stage was used in conjunction with the environmental cell. Most imaging was performed with a 50 /~m objective aperture. A slowly flowing stream of one to two torr of gas atmosphere (02, N2, or H2) was used; temperatures as high as 6 0 0 ° C could be achieved with these pressures. The accuracy of temperature measurement in the environmental cell was poor and was estimated to be within ± 40 o C. Sample contamination was a constant problem in the in-situ studies. When viewing sample areas at ambient temperature, contamination layers became visible in a matter of minutes. The effects of contamination were believed present at temperatures even up to about 300 ° C. Particles exposed to the beam in extended amounts did not spread while others away from the region of beam exposure did. Con-
Fig. 2. TEM with hexagonal M o O 2 on spherical SiO2: (A) before and (B) after a second 300 o C, 4 h air calcination; (B) is slightly astigmatic.
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ticular phase shown in this figure is the hexagonal, formed after a 2 h 300 ° C air calcination of the precursor. Here a cluster of SiO 2 and MoO 3 is supported over a hole in the carbon substrate. One small SiO 2 sphere had adhered to a large (2050 ,~ diameter) SiO 2 sphere. Given the dimensions of the MoO 3 crystallites and the surface area of the SiO 2 spheres, the local surface loading was approximately 18 atoms M o / n m 2, which i s about the same as the bulk concentration. After an additional 4 h calcination at 3 0 0 ° C (fig. 2B), the M o O 3 crystallites had all but disappeared, presumably covering a large fraction of the SiO z surface. It appears that some may have even spread over to the edge of the holey carbon film. Small particles seen on the lower-left-hand edge of the
Fig. 4. TEM with orthorhombic MoO3 on spherical SiO3: (A) before and (B) after a second 500 o C, 4 h air calcination.
Fig. 3. TEM with hexagonal MoO3 on spherical SiO2: (A) before and (B) after a second 300 o C, 4 h air calcination.
sphere also appeared to disintegrate during the calcination. An amorphous covering appeared on the small SiO 2 sphere and necking effects (arrowed) were present at the contact area between it and the larger SiO 2 sphere. The diameter of the small sphere increased by about 45 ,~. These observations indicate the presence of thick noncrystalline multilayers of material, presumed to be M o O 3. Several other hexagonal crystallites, formed again by the 300 o C, 2 h air calcination, are shown in fig. 3A. The carbon film is seen at left. Tilting the sample revealed that the bottom, central portion of the long, thick M o O 3 needle was in contact with the lower SiO2 sphere. Shown in fig. 3B at very nearly the same orientation is the same re-
A. Datta et al. / T E M and in-situ E M study of dispersion of MoO~ on SiO 2
gion after a second 4 h, 300 ° C air calcination. It is clearly seen that dissolution occurred where the crystallite was in contact with the SiO 2 surface. Channels are faintly visible on the SiO 2 surface. But also the shape of the needle was more rounded in fig. 3B and a thick (40 ,~) overlayer appeared on it (arrowed). In contrast, the behavior of an orthorhombic needle adhering to a SiO 2 sphere, similar in size and appearance to the hexagonal needle, is illustrated in fig. 4. This needle had been formed by a 500 ° C, 2 h air calcination. After an additional 500°C, 4 h calcination the crystallite remained virtually unchanged. The morphology of the SiO 2 surface, containing numerous very small particles, was also not significantly altered. 3.2. ln-situ E M with planar SiO 2
In-situ studies were performed with the environmental cell to further establish the mechanism of spreading of hexagonal MoO 3 and the hexagonal-to-orthorhombic phase transition. The experiment illustrated in fig. 5 was conducted at an initial indicated temperature of 380 o C. Experiments performed at an indicated temperature of 300 o C exhibited the same type of behavior, but at a much slower rate. The 3 8 0 ° C experiment illustrates the most extensive morphological changes of the hexagonal phase recorded in one experiment. In any case, because of the poor accuracy of the temperature measurement, the phases present were always identified crystallographically. Only qualitative trends of each phase's behavior with respect to temperature can be stated. In the region of fig. 5A, it is again seen that the distribution of MoO 3 is very inhomogeneous on a microscopic scale. Two large amorphous particles of MoO 3 precursor appear on the SiO 2 film a short distance away from a grid bar (upper-lefthand corner and on left-hand side of subsequent micrographs). In this experiment, much care was taken to minimize the effects of localized beam heating. The beam was greatly defocussed except when micrographs were taken. However, particle number 2 was extensively exposed to the electron beam at ambient temperature during alignment and focussing. This particle did not subsequently
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spread, probably due to an initial contamination coating (as mentioned in section 2). After 20 rain at 380 o C (fig. 5B), however, large, generally round, crystallites developed concentrically around partitle 1. F r o m their much lower contrast these crystallites are also seen to be very fiat. Such a morphology will be referred to hereafter as a " t h i n disk" morphology. After 60 rain a far greater number of crystallites had been produced and spread outward from the original particle. The size of these new, mostly circular and fiat, crystallites was inversely proportional to distance away from the original particle. Interestingly, spreading occurred in a preferred direction, away from the grid bar. An electron diffraction pattern taken during this segment of the experiment is shown in fig. 6C. Assignments are made for hexagonal spotty rings; the (100) spots, ordinarily of relatively weak intensity, are presumed lost in the central spot. In this pattern the (200) and (300) rings exhibit much higher intensity than for a randomly oriented sample. Preferred diffraction by (100) planes implies that the thin disks are preferentially oriented with basal planes parallel to the substrate. Over the next ten minutes, many significant changes occurred (fig. 5D, 70 rain). First, the original thick particle itself assumed the thin disk morphology, and as it did the nearest larger particles became even larger as their number density decreased. Apparently this change occurred at least partly through the coalescence of whole disks. The smaller more distant particles also increased slightly in diameter. Symmetrical circular spots of low contrast were observed near the center of many of the medium sized and smaller crystallites, such as the arrowed crystallite in fig. 5D. At 90 min (fig. 5E), crystallites again were of smaller size and higher number density. Many whole crystallites divided, as the pear-shaped crystallite in the center of fig. 5E was in the process of doing. Another small but interesting feature was that some of the low-contrast spots were now darkened circles. The light-spotted crystallite in fig. 5D is pointed out again in figure 5E as an example; this contrast may be due to diffraction or to changes in thickness. The last mlcrograph of fig. 5 (F) was taken
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Fig. 5. In-situ EM with planar SiO2: (A) prior to 380 o C calcination in 2 Tort 02; (B) after 20 rain; (C) after 60 rain; (D) after 70 rain; (E) after 90 rain; (F) 10 rain after a temperature increase to 500 o C.
after the temperature was increased to an indicated value of 500 ° C in an attempt to induce the hexagonal-orthorhombic phase change. After 10 min at 500 o C this change had not occurred. How-
ever, very rapid spreading from the hexagonal crystaUites took place as most dissolved completely. A thick surface film became discernable from the lighter, exposed patches of the SiO2
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Fig. 6. In-situ EM with planar SiO2. Conversion of bulk hexagonal thin disks (A) to the orthorhombic phase (B) caused by localized beam heating. Typical diffraction patterns of (C) hexagonal and (D) orthorhombic crystallites.
substrate (arrowed). The exposed SiO2 patches are more clearly visible in fig. 6A (again arrowed), an enlarged and high-contrast image from the same negative. The phase change to the orthorhombic phase was induced by temporarily focussing the electron beam on this area to increase local temperature. The transformation of all crystaUites occurred simultaneously in the few seconds the beam was highly focussed. While these rapid changes occurred in the bulk crystaUites, it is interesting to note that the thick surface film remained essentially unchanged, with the exception of some bare patches and perhaps even holes in the substrate which appeared where the thin disks had been.
The same exposed SiO2 areas are again indicated in fig. 6B. The diffraction pattern from this region, with the (020), (110) and (040) rings of the orthorhombic phase indexed, is shown in fig. 6d. The phase transformation was also studied under conditions in which temperature was increased just after the hexagonal phase had formed but before it had appreciably spread as above. Data for this case are given in two figures. In fig. 7, micrographs of initial amorphous and final crystalline phases are shown. Events between these two states occurred very rapidly; to adequately document them the experiment was videotaped. Photographs excerpted from this videotape (which
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f Fig. 7. In-situ EM with planar SiO2. Same sample area featuring (A) amorphous precursor before and (B) crystalline material after an initial crystallization to the hexagonal phase immediately followed by a 443°C calcination. Significant localized beam heating occurred.
unfortunately are of much poorer resolution than the videotape itself) are given in fig. 8. Fig. 8 then represents the transition between figs. 7A and 7B. In fig. 7A an in_homogeneous distribution of the precursor, present mainly in two large deposits, is seen. During subsequent videotaping and photographing, the electron beam was focussed on the upper deposit ( # 1). The beam intensity was much higher than in the experiment of fig. 5, and was never defocussed. Calcination was begun at 358 ° C. Figs. 8A and 8B record the crystallization of material within the original confines of the deposit, which took place in approximately 30 rain. Thereafter, the indicated temperature was increased to 443 ° C. Within several minutes the bulk crystallites had widened and perhaps thinned somewhat and small crystallites appeared throughout the entire region (fig. 7C). At 443°C and with the additional beam heating the crystallites grew to be elongated orthorhombic particles. Their growth rate was quite rapid; the arrowed crystaUites in fig. 8D grew about 0.3 #m in less than 1 rain (fig. 8E), or at a rate of 60 ,~/s (their growth was readily observable in the real-time
video recording). In figs. 8D and 8E most crystallites appear to be needles, but in fact they are the polyhedral platelets seen in fig. 7B, oriented edgeon to the substrate at this point. This was discovered when, following the videotaped sequence, temperature was increased by about 5 0 ° C and many of the "needles" were filmed as they "fell over". Fig. 7B was made at this point. As temperature was increased again it was seen that virtually every large crystallite was a polyhedral thin platelet. Fig. 7B reveals the scale of the morphological changes which had occurred in this experiment. Quite a large amount of mass had accumulated in a very large area surrounding the original precursor deposit. (Magnification in this image is low enough that the grid bars appear in the upper and lower left hand corners.)However, only a few small crystallites marked the presence of deposit # 2. Presumably the balance of the material originally in deposit # 2 had migrated toward particle # 1 and comprised many of the platelets. Other sources of MoO 3 could have been the large crystallites seen near the grid bars (arrowed).
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Fig. 9. In-situ EM with planar SiO2. Amorphous precursor before (A) and orthorhombic crystaUites after 50 min calcination at 550 ° C (B).
A typical region of sample before and after an immediate calcination at 500 ° C is shown in fig. 9. In this experiment, beam exposure was kept to a very low level. Here again, the initial precursor dispersion is very inhomogeneous (fig. 9a). A crease and parallel scrape marks are seen in the SiO2 substrate. Migration occurred as the sample was calcined, but had ceased after about 5 min. At this time MoO 3 was mainly present as large (500-2000 A) orthorhombic crystallites, which are in the same size range as those formed over spherical SiO 2 (figs. 2-4). Also present in the scratched area (but not in the smooth substrate in the upper and lower left of the figure) were small patches of thick films. Fig. 9B was taken after 50 min calcination; the only change which occurred after the initial crystallization period was the growth of several elongated polyhedral crystallites as seen,
for example, in the upper right corner of the figure. Several other experiments were carded out in which the oxygen atmosphere was replaced by 1-2 Torr of hydrogen or nitrogen after the precursor's crystallization to the hexagonal phase had been completed in oxygen. In both of these atmospheres, one inert and the other reducing, the same qualitative spreading behavior was observed in roughly the same ranges of temperature. At this point, then, it does not appear that atmosphere significantly influences spreading behavior.
4. Discussion
A summary of the results from TEM studies of MoO 3 supported on spherical S i O 2 is as follows: (1) Hexagonal crystallites, even of very large
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size ( > 800 A), disintegrated in many cases almost completely over SiO 2 upon calcination in air at 300 o C. (2) Thick (multilayered) amorphous overlayers of M o O 3 w e r e present both on SiO 2 and on MoO 3 itself, as a result of this dissolution. (3) Disintegration appeared to occur primarily where the hexagonal crystallites were in contact with a SiO2 surface. (4) Orthorhombic crystallites, formed by an immediate 500°C calcination, were completely stable to continued calcination at 500 ° C. Controlled atmosphere EM with planar SiO 2 substrates revealed that: (5) In the absence of contamination, spreading hexagonal MoO3 crystallites exhibited a "thin disk" morphology; these reversibly agglomerated and divided as whole crystallites as well as lost and gained material from unresolved, possibly monomeric, migrating species. (6) In the absence of severe localized beam heating, spreading of the thin disks occurred in a preferred direction away from a nearby grid bar. (7) Many of the thin disks were marked by circular spots of lower, and in some cases higher, contrast than the surrounding disk. (8) At higher temperatures, but before the hexagonal to orthorhombic transition had occurred, most hexagonal crystallites disintegrated completely as a thick surface film was resolved. This film appeared largely stable even as the transformation of remaining bulk crystallites was induced by localized beam heating. (9) When the temperature of a sample containing poorly dispersed hexagonal M o O 3 w a s rapidly increased, large, thin polyhedral orthorhombic platelets oriented edge-on to the substrate accumulated in a region of localized beam heating. The original particles in the area appeared not to have spread. (10) Also in the absence of contamination, a precursor immediately calcined at 500°C displayed some mobility upon formation of mainly large orthorhombic crystallites, but thereafter showed no spreading. In fact, a slight sintering was observed. The previous characterization of high-Mo-loading, high-surface-area MoO3/SiO 2 samples [6] is
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strongly supported by the present characterization of spherical and planar supported samples by electron microscopy. Spreading of the hexagonal phase (fig. 1, a ---, e) was seen at indicated temperatures of 300 to 500°C (figs. 2, 3, and 5), and occurred faster as temperature increased. The formation of a well dispersed orthorhombic sample (actually, mostly thick amorphous overlayers plus some orthorhombic crystallites) from a well dispersed hexagonal sample (fig. 1, e ~ f) is represented by fig. 6, in which the conversion of bulk crystallites occurs while the thick surface film remains generally unaffected. The creation of a poorly dispersed orthorhombic phase from a poorly dispersed hexagonal phase (fig. 1, a ~ c) is illustrated by figs. 7 and 8. Finally, the stability of the poorly dispersed orthorhombic phase to continued calcination (fig. 1, b ~ d) is witnessed in figs. 4 and 9, and the slow sintering of the orthorhombic phase from amorphous overlayers (fig. 1, f ~ g) may be evidenced by several regions in fig. 9 (upper right polyhedral crystallites). The findings regarding the stability of the orthorhombic phase on SiO 2 are in complete agreement with other studies of this system [4]. Several significant morphological observations can be made from these EM results. First, the direct imaging of thick layers of amorphous MoO 3 has confirmed the existence of this novel silicasupported morphology. As the thickness of an MoO 3 monomer is about 5 ~, [16], the 22 increase of the small SiO 2 sphere in fig. 2B corresponds to 4-5 layers of MoO 3. The overlayer on the MoO 3 needle in fig. 3B is about 40 A thick and represents even a greater number of layers. A comparison of the hexagonal and orthorhombic crystallites formed by mobile species on the planar silica support (figs. 5 and 7) is quite interesting. Both types of crystallites exhibit a preferred, thin crystal habit. In the case of the hexagonal phase the "thin disks" are oriented with their basal planes parallel to the substrate, while the thin polyhedral orthorhombic crystallites are oriented perpendicular to the substrate. These edge-oriented crystallites would appear to be a larger scale analog of edge-oriented MoS2 crystallites observed on an alumina film [11]. Another difference is that the hexagonal thin disks
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reversibly grew and shrank. In fig. 5, the thin disks were decreasing in diameter before the conversion of the original, thick particle (figs. 5A-5C) to the thin disk morphology (fig. 5D). It is thought that associated with this conversion was a relatively rapid release of mass; and the temporary increase in concentration of migrating species caused the disks to grow (fig. 5D). Thereafter, they shrank again as the additional mass dissipated. Thin disk crystallites were also seen to grow together and divide as whole crystallites; their formation thus appears to be reversible. On the other hand, when orthorhombic crystallites formed, as in figs. 6 through 9, they did so irreversibly. This corresponds to the known metastability of unsupported, bulk hexagonal MoO 3, and its irreversible transformation to the orthorhombic bulk phase [171. In general, it appears that below about 500 ° C, MoO 3 present on SiO 2 initially as hexagonal crystallites is thermodynamically stabilized by forruing a dispersed, amorphous surface film, while MoO 2 present initially as orthorhombic crystallites is thermodynamically stable as the bulk crystallites. The amorphous multilayers formed from the hexagonal phase, stable at moderate temperatures, may represent a kinetic hindrance to the formation of bulk orthorhombic crystallites at or above 500 ° C. The stability of amorphous MoO3 layers appears to increase as the layers become deeper. Thick, visible overlayers formed from the highly dispersed hexagonal phase (figs. 5F and 6A) do not sinter even as existing bulk hexagonal crystallites convert to the orthorhombic phase. On the other hand, if the temperature of a poorly dispersed hexagonal sample is suddenly increased (fig. 8C), entirely new orthorhombic crystallites form (figs. 8D-8F), which must be fed by a thin, undetectable layer of mobile species. (Similar types of mobile, thin, unresolved surface films have been reported in EM studies of other systems [18,19].) X R D results have shown that eventually even the thicker multilayers sinter somewhat (ref. [6], and fig. 1, f ~ g), again indicating the thermodynamic stability of large orthorhombic crystallites. (Preliminary experiments to this point have shown that the orthorhombic phase itself spreads, but at a much higher temperature than those
reported here.) In summary, hexagonal MoO 3 would appear to have a higher surface free energy than SiO2, while the orthorhombic phase would have a value lower than SiO 2. Only one value of surface free energy has been reported for MoO 3, 65 e r g / c m 2 at 795 K [20]. Using a temperature correction of - 0 . 1 e r g / c m 2 • o C as has been done with other oxides [19], the surface free energy of MoO 3 at 298 K would be 115 e r g / c m 2. That of SiO 2 at 298 K is much higher, 605 e r g / c m 2 [20], indicating the thermodynamic potential for MoO 3 to spread on SiO 2. Several key features regarding the mechanism of the formation of mobile species and their migration are indicated by the present results. Amorphous overlayers appeared not only on SiO 2, but also on crystalline MoO 3 itself (fig. 3B). As overlayers formed on the needle connected to the SiO 2 sphere, its thickness decreased and it became rounded. Even the other crystallites in the figure, which are not in direct contact with SiO 2, appear pitted and overlayered in some areas. These observations suggest that material from the crystalrite surfaces became loosened from the oxide lattice even away from the SiO 2 surface. However, once formed, mobile MoO 3 species would have the SiO 2 surface to escape to. Perhaps the central region of the needle disintegrated most rapidly because it was here that the mobile species could be removed, in turn permitting more material to be removed from the nearby lattice. The role of the SiO 2 surface would not be to assist in the formation of mobile species, but rather to provide them a thermodynamically permissible place to go. That the SiO 2 surface is not needed for the formation of mobile MoO 3 is supported by recent work in which overlayers on Pt were detected in samples of unsupported MoO 3 [21]. At this point, the question arises as to how the mobile species are transported to and across SiO:. That the SiO2 surface promoted the rate of dissolution of the oxide lattice (fig. 3) suggests that the spreading mechanism is surface diffusion of the mobile species. Were a volatilization mechanism in effect the MoO 3 needle would be evenly deteriorated rather than preferentially at the MoO3-SiO 2 interface. The temperatures employed here were well below those at which volatilization
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problems have been reported for MoO 3 [5], further discounting a vapor phase transport mechanism. For the hexagonal phase, both of the surface diffusion growth mechanisms for supported crystallites, the crystallite growth model and the atomic (or perhaps MoO 3 monomer in this case) migration model [22], appear operative and reversible in fig. 5. Orthorhombic crystaUites were observed to exhibit irreversible growth only by atomic migration (figs. 7 and 8) at temperatures below 500 ° C. The theory of the formation of mobile species from hexagonal crystallites and their spreading by surface diffusion can be tested in view of the in-situ EM experiments. In doing so, the effects of contamination and localized surface heating by the electron beam must be considered. As was explained in the experimental section, contamination was a problem in areas exposed to a relatively focussed electron beam, while at lower temperatures (< 300°C). It was always observed that particles of precursor extensively exposed to the beam did not spread even while other particles in relatively unexposed regions did (particle 2, fig. 5A, and particle 1, figs. 7 and 8). The crystallites formed from particle 1 in figs. 7 and 8 did not spread, even though the bulk appeared to undergo a phase change (figs. 8B and 8C). These results must be caused by a layer of surface contamination which prevented the amorphous species from forming on the crystallite surfaces, or at least blocked the diffusion of amorphous species. The observed contamination effect supports the theory of mobile species production from bulk crystalrites. The effects of localized beam heating also support a surface diffusion mechanism. Crystallites were consistently observed to accumulate in areas under a relatively focussed, high-intensity beam. Figs. 7 and 8 are good examples of this phenomenon; it occurred in several other experiments not illustrated here. This net influx of material is likely due to enhanced surface diffusion arising from either electro-transport [23], diffusion in the electric field associated with local charging of the SiO2 film, or from thermo-transport [23], diffusion in a temperature gradient. Thermo-transport could
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also be responsible for the directional migration of material away from the grid bar seen in fig. 5. In a region heated by a very spread beam, which is the case here, the conducting grid bars would be good heat sinks. In both cases, then, with the focussed or spread beam, diffusion occurs in the direction of increasing temperature. No such behavior has been documented to date regarding either of these phenomena on ceramic surfaces on which molecular species are migrating. A final observation in support of a surface diffusion mechanism is that the spreading behavior occurred independently of atmosphere. The predominant chemical interactions must be those between the trioxide and the support.
5. Conclusions
Transmission and controlled-atmosphere electron microscopy studies have confirmed earlier XRD findings that the dispersion of silica-supported MoO3 can be controlled through the initial formation of a hexagonal phase. Whereas the stable morphology of the orthorhombic phase is that of large crystallites, hexagonal crystallites tend to disperse even to the extent of existifig as amorphous multilayers. This form represents a novel morphology for SiO2 supported MoO3 catalysts. Other morphological findings are that over a p l a n a r SiO 2 substrate, the hexagonal phase spreads as thin disks oriented with basal planes parallel to the surface. These reversibly increase and decrease in diameter as dictated by the surrounding surface concentration of mobile MoO 3. Immediate calcination of the precursor at high temperatures resulted in large, irregularly shaped orthorhombic crystallites, while thin, very large, polyhedral orthorhombic crystallites oriented edge-on to the substrate could be produced by calcination of poorly dispersed hexagonal crystallites. The low-temperature mechanism of spreading of MoO3 appears to involve the formation of amorphous MoO 3 at the outer surfaces of hexagonal crystallites, and their subsequent diffusion across the SiO2 surface.
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Acknowledgements The support of Amoco Corporation and The Illinois Department of Commerce and Community Affairs (Illitech Program) is greatly appreciated, as is the technical support of the staffs of the HVEM-Tandem Facility at Argonne National Laboratory and the Electron Microscopy Facility at the University of Illinois at Chicago.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
R. Thomas et al., J. Catalysis 84 (1983) 275. D.S. Zingg, J. Phys. Chem. 84 (1980) 2989. L. Rodrigo et al., J. Catalysis 105 (1987) 175. S.R. Stampfl et al., J. Catalysis 105 (1987) 445. H.S. Ghandhi, H.C. Yao and H.K. Stepien, Am. Chem. Soc. Symp. Ser. 178 (1982) 143. A. Datta and J.R. Regalbuto, in preparation. S. Chakraborti, A.K. Datye and N.L. Long, J. Catalysis 108 (1987) 444. A.K. Datye, A.D. Logan and N.J. Long, J. Catalysis 109 (1988) 76. C. Lee and L.D. Schmidt, J. Catalysis 101 (1986) 123.
[10] J.R. Regalbuto, C.W. Allen and E.E. Wolf, J. Catalysis 108 (1987) 304. [11] T.F. Hayden and J.A. Dumesic, J. Catalysis 103 (1987) 366. [12] I. Sushumna and E. Ruchenstein, J. Catalysis 109 (1988) 433. [13] C.W. Allen, E.A. Ryan and S.T. Ockers, in: Proc. 46th Annual EMSA Meeting, Milwaukee, WI, 1988, Ed. G.W. Bailey (San Francisco Press, San Francisco, 1988) p. 806. [14] M. Boudart et al., Appl. Catalysis 11 (1984) 117. [15] D.G. Huizenga and D.M. Smith, AIChE J. 32 (1986) 1. [16] J. Sonnemans and P. Mars, J. Catalysis 31 (1973) 209. [17] JCPDS Powder Diffraction File, No. 21-569. [18] E. Ruchenstein and I. Sushumna, J. Catalysis 97 (1986) 1. [19] E. Ruchenstein, Am. Chem. Soc. Symp. Ser. 298 (1986) 152. [20] S.H. Overbury, P.A. Bertrand and G.A. Somorjai, Chem. Rev. 75 (1975) 547. [21] J.-G. Kim and J.R. Regalbuto, in preparation. [22] E. Ruchenstein and D.B. Dadyburjor, Rev. Chem. Eng. 1 (1983) 251. [23] R.E. Hummel, in: Electro- and Thermo-Transport in Metals and Alloys, Eds. R.E. Hummel and H.B. Huntington (Metallurgical Society, 1977) p. 93. [24] T. Henenkamp, in: Electro- and Thermo-Transport in Metals and Alloys, Eds. R.E. Hummel and H.B. Huntington (Metallurgical Society, 1977) p. 68.
233
T E M AND IN-SITU EM STUDY OF THE DISPERSION OF MoO 3 ON SiO 2
A. DATTA and J.R. REGALBUTO Department of Chemical Engineerin~ University of Illinois at Chicago, Chicago, Illinois 60680, USA
and
C.W. ALLEN Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA Received at Editorial Office October-November 1988; presented at Conference May 1988
Molybdenum trioxide supported on nonporous silica spheres and planar silica substrates were used with TEM and in-situ EM (environmental cell and heating stage), respectively, to study the spreading behavior of the hexagonal and orthorhombic forms of MoO 3 on SiO2. At temperatures below 500 o C, supported orthorhombic crystallites are stable, while supported hexagonal crystaUites, formed by an initial calcination of the precursor at 300 o C, spread drastically and in doing so produce a novel, multilayered and amorphous silica-supported morphology for MoO 3. The mechanism of spreading appears to be the formation of mobile species on hexagonal crystallites, and subsequent migration across the SiO2 surface.
1. Introduction
Supported molybdenum or its trioxide has a wide variety of uses in heterogeneous catalysis, for catalytic reactions such as hydrodesulfurization, hydrogenation and dehydrogenation, reforming, and hydrocracking, to name a few. The dispersion and morphology of the trioxide, often a precursor phase of the final catalyst, have much to do with final catalytic activity. A number of trioxide forms exist over various support materials. At low loadings, the material is generally completely dispersed and of a monolayer type while at higher loadings, bulk crystallites form [1]. Some supports such as alumina and titania exhibit stronger interactions with MoO3 such that intermetallic oxide phases form [2]. Over SiO2, however, only the monolayer and bulk crystalline (orthorhombic) forms have been reported; these formed with an initial calcination of the impregnated precursor always greater than 450 ° C. At this high initial calcination temperature, surface loadings less than I atom M o / n m 2 [1] yield the well dispersed catalyst,
whose morphology is only slightly altered by subsequent treatments [3], while loadings greater than this value result in bulk crystallites. There appears to be no way to redisperse bulk, orthorhombic crystallites of MoO 3 on SiO3 once they form [4], short of extremely high calcination temperatures, at which trioxide losses occur due to volatilization [5]. In recent characterization of MoO3/SiO 2 catalysts with X-ray diffraction (XRD) among other methods [6], it has been shown that the dispersion of high-loading MoO 3 (4 atoms M o / n m 2) can be controlled through the initial formation of the little-studied hexagonal crystalline phase. A summary of XRD results from this study is shown in fig. 1. The supported hexagonal phase, formed by an initial 300 °C air calcination, is shown in pattern a and is compared to the orthorhombic phase formed by a 500 °C calcination (pattern b). A bulk crystalline (poorly dispersed) orthorhombic phase (pattern c) was formed by a 500 ° C calcination of the poorly dispersed hexagonal phase, while continued calcination at 300°C
0304-3991/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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Ortho (021)
(040)
(110)
(020)
d
Hex
5; 5 ;
(loo)
(210)
;,"
" d ' ;; ' ~;
;6"2; " 2 ; ~ i DgG~,
;~ ~ ;
",
20
Fig. 1. X R D characterization of calcined M o O 3 / S i O 2 [6]: (a) 300 o C, 2 h; (b) 500 o C, 2 h; (c) 300 o C, 2 h and 500 o C, 2 h; (d) 50 o C, 2 h and 300 o C, 4 h; (e) 300 ° C, 6 h; (f) 300 o C, 6 h and 500 ° C, 2 h; (g) 300 ° C, 6 h a n d 500 ° C, 6 h.
caused the hexagonal phase to become mostly amorphous (pattern e) and a calcination of this dispersed phase at 500 °C produced a dispersed orthorhombic phase (pattern f). The dispersed form (actually, mostly amorphous and some orthorhombic crystallites) slowly sintered to bulk orthorhombic crystallites (pattern g) during continued calcination at 500 ° C. The high-loading, well dispersed sample represents a new morphology of silica-supported MoO3: that of an amorphous, multilayer form. It is the purpose of the present study to verify the existence of this MoO3/SiO2 form and to explore the mechanism of its formation, by direct visualization of morphology with electron microscopy. Electron microscopy has become more and more popular for catalyst characterization as various techniques have been developed specifically for this application. Two such techniques involve the use of model catalyst supports to eliminate the complexity of images of porous, high-surface-area support materials. Simple nonporous spherical or cuboidal particles permit edge-on viewing of sup-
ported material and have been used for the characterization of Rh on SiO2 [7] and Ru on SiO2 and MgO [8], for example. Thin, planar films provide a clear two-dimensional projection of supported materials; this type of sample has been used recently to study Rh/SiO 2 [9], Pt/WO3/SiO 2 [10], MoO3-MoS2/A1203111], and Pt/AI203 [12]. Typically in these types of studies, the same region of sample is followed through a series of pretreatment steps (which themselves are performed outside of the microscope), so that morphological changes induced by the pretreatments can be unambiguously observed. An additional degree of simulation is achieved when samples can be treated in-situ, using a heated stage and an environmental cell to maintain a relatively high pressure of a reactant gas [13]. This technique has been used, for example, to characterize the interaction of Pd with amorphous carbon in 02 and CO 2 atmospheres [14]. In the present study, nonporous SiO2 spheres approximately 2000 ,~ in diameter have been used as model supports, with conventional transmission electron microscopy (TEM). Micrographs of the same sample areas containing hexagonal or orthorhombic crystalhtes were taken, before and after a second calcination step performed outside of the microscope. To further understand the mechanism of MoO 3 spreading and phase transformation, insitu EM with planar SiO2 substrates were used. The molybdena containing phase was monitored from its initial state as an amorphous dried ammonium molybdate precursor, through various crystallization and dispersion processes induced by different heat and gas treatments.
2. Experiment
2.1. Spherical supported catalysts for TEM Nonporous silica spheres of an average diameter of 2300 A were prepared by a standard technique [15]. These were impregnated to incipient wetness with ammonium(IV) molybdate (Aldrich) for a bulk loading of 4 wt%, or a surface loading of 16 atoms M o / n m 2. Samples were then dried at room temperature under vacuum, for about 12 h.
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The dried samples were calcined at 300 or 500 ° C in air for 2 h to produce large hexagonal or orthorhombic crystallites, respectively. Powdered samples were dispersed in deionized water and were deposited onto holey carbon substrates (Formvar backing removed), backed by 400 mesh London finder grids (Earnest Fullam). Grids were removed from the microscope sample holder and placed in a quartz clamping device during subsequent treatments performed in a conventional furnace. A JEOL 100-CX microscope with analytical pole-piece and standard single-tilt stage was operated at 100 kV to obtain all micrographs.
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tamination was minimized by limiting beam exposure at all, but especially low, temperatures, but had to be taken into consideration in subsequent discussion.
3. Results 3.1. T E M with spherical S i O 2
The distribution of MoO 3 on the nonporous SiO2 spheres was quite inhomogeneous on a microscopic scale. Most spheres were entirely bare while others, such as that shown in fig. 2A, had very large crystallites attached to them. The par-
2.2. P l a n a r supported samples f o r in-situ E M
To produce in-situ EM samples, solutions of 10 -3 M ammonium molybdate(IV) tetrahydrate were spread in a thin film over planar SiO 2 substrates (Formvar backing removed) on 400 mesh gold grids (Ladd). The wetted film evaporated at ambient conditions within 10 rain. Dried samples were placed directly in the microscope sample holder. The modified K r a t o s / A E I high voltage electron microscope (HVEM) at Argonne National Laboratory's H V E M - T a n d e m Ion Accelerator Facility [13] was operated at 120 kV to achieve adequate contrast of often amorphous samples. A single-tilt heating stage was used in conjunction with the environmental cell. Most imaging was performed with a 50 /~m objective aperture. A slowly flowing stream of one to two torr of gas atmosphere (02, N2, or H2) was used; temperatures as high as 6 0 0 ° C could be achieved with these pressures. The accuracy of temperature measurement in the environmental cell was poor and was estimated to be within ± 40 o C. Sample contamination was a constant problem in the in-situ studies. When viewing sample areas at ambient temperature, contamination layers became visible in a matter of minutes. The effects of contamination were believed present at temperatures even up to about 300 ° C. Particles exposed to the beam in extended amounts did not spread while others away from the region of beam exposure did. Con-
Fig. 2. TEM with hexagonal M o O 2 on spherical SiO2: (A) before and (B) after a second 300 o C, 4 h air calcination; (B) is slightly astigmatic.
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ticular phase shown in this figure is the hexagonal, formed after a 2 h 300 ° C air calcination of the precursor. Here a cluster of SiO 2 and MoO 3 is supported over a hole in the carbon substrate. One small SiO 2 sphere had adhered to a large (2050 ,~ diameter) SiO 2 sphere. Given the dimensions of the MoO 3 crystallites and the surface area of the SiO 2 spheres, the local surface loading was approximately 18 atoms M o / n m 2, which i s about the same as the bulk concentration. After an additional 4 h calcination at 3 0 0 ° C (fig. 2B), the M o O 3 crystallites had all but disappeared, presumably covering a large fraction of the SiO z surface. It appears that some may have even spread over to the edge of the holey carbon film. Small particles seen on the lower-left-hand edge of the
Fig. 4. TEM with orthorhombic MoO3 on spherical SiO3: (A) before and (B) after a second 500 o C, 4 h air calcination.
Fig. 3. TEM with hexagonal MoO3 on spherical SiO2: (A) before and (B) after a second 300 o C, 4 h air calcination.
sphere also appeared to disintegrate during the calcination. An amorphous covering appeared on the small SiO 2 sphere and necking effects (arrowed) were present at the contact area between it and the larger SiO 2 sphere. The diameter of the small sphere increased by about 45 ,~. These observations indicate the presence of thick noncrystalline multilayers of material, presumed to be M o O 3. Several other hexagonal crystallites, formed again by the 300 o C, 2 h air calcination, are shown in fig. 3A. The carbon film is seen at left. Tilting the sample revealed that the bottom, central portion of the long, thick M o O 3 needle was in contact with the lower SiO2 sphere. Shown in fig. 3B at very nearly the same orientation is the same re-
A. Datta et al. / T E M and in-situ E M study of dispersion of MoO~ on SiO 2
gion after a second 4 h, 300 ° C air calcination. It is clearly seen that dissolution occurred where the crystallite was in contact with the SiO 2 surface. Channels are faintly visible on the SiO 2 surface. But also the shape of the needle was more rounded in fig. 3B and a thick (40 ,~) overlayer appeared on it (arrowed). In contrast, the behavior of an orthorhombic needle adhering to a SiO 2 sphere, similar in size and appearance to the hexagonal needle, is illustrated in fig. 4. This needle had been formed by a 500 ° C, 2 h air calcination. After an additional 500°C, 4 h calcination the crystallite remained virtually unchanged. The morphology of the SiO 2 surface, containing numerous very small particles, was also not significantly altered. 3.2. ln-situ E M with planar SiO 2
In-situ studies were performed with the environmental cell to further establish the mechanism of spreading of hexagonal MoO 3 and the hexagonal-to-orthorhombic phase transition. The experiment illustrated in fig. 5 was conducted at an initial indicated temperature of 380 o C. Experiments performed at an indicated temperature of 300 o C exhibited the same type of behavior, but at a much slower rate. The 3 8 0 ° C experiment illustrates the most extensive morphological changes of the hexagonal phase recorded in one experiment. In any case, because of the poor accuracy of the temperature measurement, the phases present were always identified crystallographically. Only qualitative trends of each phase's behavior with respect to temperature can be stated. In the region of fig. 5A, it is again seen that the distribution of MoO 3 is very inhomogeneous on a microscopic scale. Two large amorphous particles of MoO 3 precursor appear on the SiO 2 film a short distance away from a grid bar (upper-lefthand corner and on left-hand side of subsequent micrographs). In this experiment, much care was taken to minimize the effects of localized beam heating. The beam was greatly defocussed except when micrographs were taken. However, particle number 2 was extensively exposed to the electron beam at ambient temperature during alignment and focussing. This particle did not subsequently
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spread, probably due to an initial contamination coating (as mentioned in section 2). After 20 rain at 380 o C (fig. 5B), however, large, generally round, crystallites developed concentrically around partitle 1. F r o m their much lower contrast these crystallites are also seen to be very fiat. Such a morphology will be referred to hereafter as a " t h i n disk" morphology. After 60 rain a far greater number of crystallites had been produced and spread outward from the original particle. The size of these new, mostly circular and fiat, crystallites was inversely proportional to distance away from the original particle. Interestingly, spreading occurred in a preferred direction, away from the grid bar. An electron diffraction pattern taken during this segment of the experiment is shown in fig. 6C. Assignments are made for hexagonal spotty rings; the (100) spots, ordinarily of relatively weak intensity, are presumed lost in the central spot. In this pattern the (200) and (300) rings exhibit much higher intensity than for a randomly oriented sample. Preferred diffraction by (100) planes implies that the thin disks are preferentially oriented with basal planes parallel to the substrate. Over the next ten minutes, many significant changes occurred (fig. 5D, 70 rain). First, the original thick particle itself assumed the thin disk morphology, and as it did the nearest larger particles became even larger as their number density decreased. Apparently this change occurred at least partly through the coalescence of whole disks. The smaller more distant particles also increased slightly in diameter. Symmetrical circular spots of low contrast were observed near the center of many of the medium sized and smaller crystallites, such as the arrowed crystallite in fig. 5D. At 90 min (fig. 5E), crystallites again were of smaller size and higher number density. Many whole crystallites divided, as the pear-shaped crystallite in the center of fig. 5E was in the process of doing. Another small but interesting feature was that some of the low-contrast spots were now darkened circles. The light-spotted crystallite in fig. 5D is pointed out again in figure 5E as an example; this contrast may be due to diffraction or to changes in thickness. The last mlcrograph of fig. 5 (F) was taken
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Fig. 5. In-situ EM with planar SiO2: (A) prior to 380 o C calcination in 2 Tort 02; (B) after 20 rain; (C) after 60 rain; (D) after 70 rain; (E) after 90 rain; (F) 10 rain after a temperature increase to 500 o C.
after the temperature was increased to an indicated value of 500 ° C in an attempt to induce the hexagonal-orthorhombic phase change. After 10 min at 500 o C this change had not occurred. How-
ever, very rapid spreading from the hexagonal crystaUites took place as most dissolved completely. A thick surface film became discernable from the lighter, exposed patches of the SiO2
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Fig. 6. In-situ EM with planar SiO2. Conversion of bulk hexagonal thin disks (A) to the orthorhombic phase (B) caused by localized beam heating. Typical diffraction patterns of (C) hexagonal and (D) orthorhombic crystallites.
substrate (arrowed). The exposed SiO2 patches are more clearly visible in fig. 6A (again arrowed), an enlarged and high-contrast image from the same negative. The phase change to the orthorhombic phase was induced by temporarily focussing the electron beam on this area to increase local temperature. The transformation of all crystaUites occurred simultaneously in the few seconds the beam was highly focussed. While these rapid changes occurred in the bulk crystaUites, it is interesting to note that the thick surface film remained essentially unchanged, with the exception of some bare patches and perhaps even holes in the substrate which appeared where the thin disks had been.
The same exposed SiO2 areas are again indicated in fig. 6B. The diffraction pattern from this region, with the (020), (110) and (040) rings of the orthorhombic phase indexed, is shown in fig. 6d. The phase transformation was also studied under conditions in which temperature was increased just after the hexagonal phase had formed but before it had appreciably spread as above. Data for this case are given in two figures. In fig. 7, micrographs of initial amorphous and final crystalline phases are shown. Events between these two states occurred very rapidly; to adequately document them the experiment was videotaped. Photographs excerpted from this videotape (which
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2
f Fig. 7. In-situ EM with planar SiO2. Same sample area featuring (A) amorphous precursor before and (B) crystalline material after an initial crystallization to the hexagonal phase immediately followed by a 443°C calcination. Significant localized beam heating occurred.
unfortunately are of much poorer resolution than the videotape itself) are given in fig. 8. Fig. 8 then represents the transition between figs. 7A and 7B. In fig. 7A an in_homogeneous distribution of the precursor, present mainly in two large deposits, is seen. During subsequent videotaping and photographing, the electron beam was focussed on the upper deposit ( # 1). The beam intensity was much higher than in the experiment of fig. 5, and was never defocussed. Calcination was begun at 358 ° C. Figs. 8A and 8B record the crystallization of material within the original confines of the deposit, which took place in approximately 30 rain. Thereafter, the indicated temperature was increased to 443 ° C. Within several minutes the bulk crystallites had widened and perhaps thinned somewhat and small crystallites appeared throughout the entire region (fig. 7C). At 443°C and with the additional beam heating the crystallites grew to be elongated orthorhombic particles. Their growth rate was quite rapid; the arrowed crystaUites in fig. 8D grew about 0.3 #m in less than 1 rain (fig. 8E), or at a rate of 60 ,~/s (their growth was readily observable in the real-time
video recording). In figs. 8D and 8E most crystallites appear to be needles, but in fact they are the polyhedral platelets seen in fig. 7B, oriented edgeon to the substrate at this point. This was discovered when, following the videotaped sequence, temperature was increased by about 5 0 ° C and many of the "needles" were filmed as they "fell over". Fig. 7B was made at this point. As temperature was increased again it was seen that virtually every large crystallite was a polyhedral thin platelet. Fig. 7B reveals the scale of the morphological changes which had occurred in this experiment. Quite a large amount of mass had accumulated in a very large area surrounding the original precursor deposit. (Magnification in this image is low enough that the grid bars appear in the upper and lower left hand corners.)However, only a few small crystallites marked the presence of deposit # 2. Presumably the balance of the material originally in deposit # 2 had migrated toward particle # 1 and comprised many of the platelets. Other sources of MoO 3 could have been the large crystallites seen near the grid bars (arrowed).
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r<
o
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Fig. 9. In-situ EM with planar SiO2. Amorphous precursor before (A) and orthorhombic crystaUites after 50 min calcination at 550 ° C (B).
A typical region of sample before and after an immediate calcination at 500 ° C is shown in fig. 9. In this experiment, beam exposure was kept to a very low level. Here again, the initial precursor dispersion is very inhomogeneous (fig. 9a). A crease and parallel scrape marks are seen in the SiO2 substrate. Migration occurred as the sample was calcined, but had ceased after about 5 min. At this time MoO 3 was mainly present as large (500-2000 A) orthorhombic crystallites, which are in the same size range as those formed over spherical SiO 2 (figs. 2-4). Also present in the scratched area (but not in the smooth substrate in the upper and lower left of the figure) were small patches of thick films. Fig. 9B was taken after 50 min calcination; the only change which occurred after the initial crystallization period was the growth of several elongated polyhedral crystallites as seen,
for example, in the upper right corner of the figure. Several other experiments were carded out in which the oxygen atmosphere was replaced by 1-2 Torr of hydrogen or nitrogen after the precursor's crystallization to the hexagonal phase had been completed in oxygen. In both of these atmospheres, one inert and the other reducing, the same qualitative spreading behavior was observed in roughly the same ranges of temperature. At this point, then, it does not appear that atmosphere significantly influences spreading behavior.
4. Discussion
A summary of the results from TEM studies of MoO 3 supported on spherical S i O 2 is as follows: (1) Hexagonal crystallites, even of very large
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size ( > 800 A), disintegrated in many cases almost completely over SiO 2 upon calcination in air at 300 o C. (2) Thick (multilayered) amorphous overlayers of M o O 3 w e r e present both on SiO 2 and on MoO 3 itself, as a result of this dissolution. (3) Disintegration appeared to occur primarily where the hexagonal crystallites were in contact with a SiO2 surface. (4) Orthorhombic crystallites, formed by an immediate 500°C calcination, were completely stable to continued calcination at 500 ° C. Controlled atmosphere EM with planar SiO 2 substrates revealed that: (5) In the absence of contamination, spreading hexagonal MoO3 crystallites exhibited a "thin disk" morphology; these reversibly agglomerated and divided as whole crystallites as well as lost and gained material from unresolved, possibly monomeric, migrating species. (6) In the absence of severe localized beam heating, spreading of the thin disks occurred in a preferred direction away from a nearby grid bar. (7) Many of the thin disks were marked by circular spots of lower, and in some cases higher, contrast than the surrounding disk. (8) At higher temperatures, but before the hexagonal to orthorhombic transition had occurred, most hexagonal crystallites disintegrated completely as a thick surface film was resolved. This film appeared largely stable even as the transformation of remaining bulk crystallites was induced by localized beam heating. (9) When the temperature of a sample containing poorly dispersed hexagonal M o O 3 w a s rapidly increased, large, thin polyhedral orthorhombic platelets oriented edge-on to the substrate accumulated in a region of localized beam heating. The original particles in the area appeared not to have spread. (10) Also in the absence of contamination, a precursor immediately calcined at 500°C displayed some mobility upon formation of mainly large orthorhombic crystallites, but thereafter showed no spreading. In fact, a slight sintering was observed. The previous characterization of high-Mo-loading, high-surface-area MoO3/SiO 2 samples [6] is
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strongly supported by the present characterization of spherical and planar supported samples by electron microscopy. Spreading of the hexagonal phase (fig. 1, a ---, e) was seen at indicated temperatures of 300 to 500°C (figs. 2, 3, and 5), and occurred faster as temperature increased. The formation of a well dispersed orthorhombic sample (actually, mostly thick amorphous overlayers plus some orthorhombic crystallites) from a well dispersed hexagonal sample (fig. 1, e ~ f) is represented by fig. 6, in which the conversion of bulk crystallites occurs while the thick surface film remains generally unaffected. The creation of a poorly dispersed orthorhombic phase from a poorly dispersed hexagonal phase (fig. 1, a ~ c) is illustrated by figs. 7 and 8. Finally, the stability of the poorly dispersed orthorhombic phase to continued calcination (fig. 1, b ~ d) is witnessed in figs. 4 and 9, and the slow sintering of the orthorhombic phase from amorphous overlayers (fig. 1, f ~ g) may be evidenced by several regions in fig. 9 (upper right polyhedral crystallites). The findings regarding the stability of the orthorhombic phase on SiO 2 are in complete agreement with other studies of this system [4]. Several significant morphological observations can be made from these EM results. First, the direct imaging of thick layers of amorphous MoO 3 has confirmed the existence of this novel silicasupported morphology. As the thickness of an MoO 3 monomer is about 5 ~, [16], the 22 increase of the small SiO 2 sphere in fig. 2B corresponds to 4-5 layers of MoO 3. The overlayer on the MoO 3 needle in fig. 3B is about 40 A thick and represents even a greater number of layers. A comparison of the hexagonal and orthorhombic crystallites formed by mobile species on the planar silica support (figs. 5 and 7) is quite interesting. Both types of crystallites exhibit a preferred, thin crystal habit. In the case of the hexagonal phase the "thin disks" are oriented with their basal planes parallel to the substrate, while the thin polyhedral orthorhombic crystallites are oriented perpendicular to the substrate. These edge-oriented crystallites would appear to be a larger scale analog of edge-oriented MoS2 crystallites observed on an alumina film [11]. Another difference is that the hexagonal thin disks
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reversibly grew and shrank. In fig. 5, the thin disks were decreasing in diameter before the conversion of the original, thick particle (figs. 5A-5C) to the thin disk morphology (fig. 5D). It is thought that associated with this conversion was a relatively rapid release of mass; and the temporary increase in concentration of migrating species caused the disks to grow (fig. 5D). Thereafter, they shrank again as the additional mass dissipated. Thin disk crystallites were also seen to grow together and divide as whole crystallites; their formation thus appears to be reversible. On the other hand, when orthorhombic crystallites formed, as in figs. 6 through 9, they did so irreversibly. This corresponds to the known metastability of unsupported, bulk hexagonal MoO 3, and its irreversible transformation to the orthorhombic bulk phase [171. In general, it appears that below about 500 ° C, MoO 3 present on SiO 2 initially as hexagonal crystallites is thermodynamically stabilized by forruing a dispersed, amorphous surface film, while MoO 2 present initially as orthorhombic crystallites is thermodynamically stable as the bulk crystallites. The amorphous multilayers formed from the hexagonal phase, stable at moderate temperatures, may represent a kinetic hindrance to the formation of bulk orthorhombic crystallites at or above 500 ° C. The stability of amorphous MoO3 layers appears to increase as the layers become deeper. Thick, visible overlayers formed from the highly dispersed hexagonal phase (figs. 5F and 6A) do not sinter even as existing bulk hexagonal crystallites convert to the orthorhombic phase. On the other hand, if the temperature of a poorly dispersed hexagonal sample is suddenly increased (fig. 8C), entirely new orthorhombic crystallites form (figs. 8D-8F), which must be fed by a thin, undetectable layer of mobile species. (Similar types of mobile, thin, unresolved surface films have been reported in EM studies of other systems [18,19].) X R D results have shown that eventually even the thicker multilayers sinter somewhat (ref. [6], and fig. 1, f ~ g), again indicating the thermodynamic stability of large orthorhombic crystallites. (Preliminary experiments to this point have shown that the orthorhombic phase itself spreads, but at a much higher temperature than those
reported here.) In summary, hexagonal MoO 3 would appear to have a higher surface free energy than SiO2, while the orthorhombic phase would have a value lower than SiO 2. Only one value of surface free energy has been reported for MoO 3, 65 e r g / c m 2 at 795 K [20]. Using a temperature correction of - 0 . 1 e r g / c m 2 • o C as has been done with other oxides [19], the surface free energy of MoO 3 at 298 K would be 115 e r g / c m 2. That of SiO 2 at 298 K is much higher, 605 e r g / c m 2 [20], indicating the thermodynamic potential for MoO 3 to spread on SiO 2. Several key features regarding the mechanism of the formation of mobile species and their migration are indicated by the present results. Amorphous overlayers appeared not only on SiO 2, but also on crystalline MoO 3 itself (fig. 3B). As overlayers formed on the needle connected to the SiO 2 sphere, its thickness decreased and it became rounded. Even the other crystallites in the figure, which are not in direct contact with SiO 2, appear pitted and overlayered in some areas. These observations suggest that material from the crystalrite surfaces became loosened from the oxide lattice even away from the SiO 2 surface. However, once formed, mobile MoO 3 species would have the SiO 2 surface to escape to. Perhaps the central region of the needle disintegrated most rapidly because it was here that the mobile species could be removed, in turn permitting more material to be removed from the nearby lattice. The role of the SiO 2 surface would not be to assist in the formation of mobile species, but rather to provide them a thermodynamically permissible place to go. That the SiO 2 surface is not needed for the formation of mobile MoO 3 is supported by recent work in which overlayers on Pt were detected in samples of unsupported MoO 3 [21]. At this point, the question arises as to how the mobile species are transported to and across SiO:. That the SiO2 surface promoted the rate of dissolution of the oxide lattice (fig. 3) suggests that the spreading mechanism is surface diffusion of the mobile species. Were a volatilization mechanism in effect the MoO 3 needle would be evenly deteriorated rather than preferentially at the MoO3-SiO 2 interface. The temperatures employed here were well below those at which volatilization
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problems have been reported for MoO 3 [5], further discounting a vapor phase transport mechanism. For the hexagonal phase, both of the surface diffusion growth mechanisms for supported crystallites, the crystallite growth model and the atomic (or perhaps MoO 3 monomer in this case) migration model [22], appear operative and reversible in fig. 5. Orthorhombic crystaUites were observed to exhibit irreversible growth only by atomic migration (figs. 7 and 8) at temperatures below 500 ° C. The theory of the formation of mobile species from hexagonal crystallites and their spreading by surface diffusion can be tested in view of the in-situ EM experiments. In doing so, the effects of contamination and localized surface heating by the electron beam must be considered. As was explained in the experimental section, contamination was a problem in areas exposed to a relatively focussed electron beam, while at lower temperatures (< 300°C). It was always observed that particles of precursor extensively exposed to the beam did not spread even while other particles in relatively unexposed regions did (particle 2, fig. 5A, and particle 1, figs. 7 and 8). The crystallites formed from particle 1 in figs. 7 and 8 did not spread, even though the bulk appeared to undergo a phase change (figs. 8B and 8C). These results must be caused by a layer of surface contamination which prevented the amorphous species from forming on the crystallite surfaces, or at least blocked the diffusion of amorphous species. The observed contamination effect supports the theory of mobile species production from bulk crystalrites. The effects of localized beam heating also support a surface diffusion mechanism. Crystallites were consistently observed to accumulate in areas under a relatively focussed, high-intensity beam. Figs. 7 and 8 are good examples of this phenomenon; it occurred in several other experiments not illustrated here. This net influx of material is likely due to enhanced surface diffusion arising from either electro-transport [23], diffusion in the electric field associated with local charging of the SiO2 film, or from thermo-transport [23], diffusion in a temperature gradient. Thermo-transport could
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also be responsible for the directional migration of material away from the grid bar seen in fig. 5. In a region heated by a very spread beam, which is the case here, the conducting grid bars would be good heat sinks. In both cases, then, with the focussed or spread beam, diffusion occurs in the direction of increasing temperature. No such behavior has been documented to date regarding either of these phenomena on ceramic surfaces on which molecular species are migrating. A final observation in support of a surface diffusion mechanism is that the spreading behavior occurred independently of atmosphere. The predominant chemical interactions must be those between the trioxide and the support.
5. Conclusions
Transmission and controlled-atmosphere electron microscopy studies have confirmed earlier XRD findings that the dispersion of silica-supported MoO3 can be controlled through the initial formation of a hexagonal phase. Whereas the stable morphology of the orthorhombic phase is that of large crystallites, hexagonal crystallites tend to disperse even to the extent of existifig as amorphous multilayers. This form represents a novel morphology for SiO2 supported MoO3 catalysts. Other morphological findings are that over a p l a n a r SiO 2 substrate, the hexagonal phase spreads as thin disks oriented with basal planes parallel to the surface. These reversibly increase and decrease in diameter as dictated by the surrounding surface concentration of mobile MoO 3. Immediate calcination of the precursor at high temperatures resulted in large, irregularly shaped orthorhombic crystallites, while thin, very large, polyhedral orthorhombic crystallites oriented edge-on to the substrate could be produced by calcination of poorly dispersed hexagonal crystallites. The low-temperature mechanism of spreading of MoO3 appears to involve the formation of amorphous MoO 3 at the outer surfaces of hexagonal crystallites, and their subsequent diffusion across the SiO2 surface.
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Acknowledgements The support of Amoco Corporation and The Illinois Department of Commerce and Community Affairs (Illitech Program) is greatly appreciated, as is the technical support of the staffs of the HVEM-Tandem Facility at Argonne National Laboratory and the Electron Microscopy Facility at the University of Illinois at Chicago.
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