An Environmental Scanning Electron Microscopy Study of Activated

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Energy & Fuels 1998, 12, 554-562

An Environmental Scanning Electron Microscopy Study of Activated Charcoal Gasification Catalyzed by MoO3 in Air and in Oxygen and by a Eutectic Alloy of MoO3 and V2O5 in Air I. F. Silva Departamento de Quimica, Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, 2825 Monte de Caparica, Portugal

M. Klimkiewicz† and S. Eser*,‡ Materials Research Laboratory and Fuel Science Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received September 9, 1997

An environmental scanning electron microscope (ESEM) was used to study the gasification of an activated carbon catalyzed by MoO3 in air and oxygen atmospheres and by a eutectic mixture and alloy of MoO3 and V2O5. In an air atmosphere, the melting of MoO3 starts at 630 °C and the reaction occurs by preferential gasification on the edges of carbon particles. In an O2 atmosphere, however, homogeneous gasification is observed in all directions. This difference can be explained by more extensive oxidation of the carbon surface in an oxygen atmosphere and the more effective spreading of the catalysts on charcoal surfaces. The ESEM experiments showed that some particles of the eutectic alloy of MoO3 and V2O5 had higher melting points than those of the single oxides. The analysis of alloy particles by EDS and by electron microprobe indicated that large areas of single oxides are segregated and encapsulated by the other oxide in some alloy particles. It is proposed that the encapsulation of either oxide (MoO3 or V2O5) inhibits the initial contact of the catalyst with the carbon surface and leads to different reactions than those which take place on carbon surfaces. A lower activity of the “eutectic” alloy, compared to that of the binary mixture with the eutectic composition, is, thus, ascribed to the heterogeneous composition of alloy particles which can give rise to the formation of different oxide phases with higher melting points than those of the single oxides.

Introduction Several metal oxides are known to be good catalysts for the oxidation reactions of solid carbons in oxidizing atmospheres.1-8 The mechanisms associated with the catalytic activity of metal oxides are still debated. Among the important factors cited for affecting the catalytic activity of the metal oxides are the mobility of the catalyst particles on carbon surfaces and the wetting of the carbon surfaces by the catalysts.1-3,5 Studies on the catalytic influence of molybdenum trioxide during the oxidation of graphite showed that melting of the oxide is followed by rapid formation of pits on the †

Materials Research Laboratory. Fuel Science Program. (1) McKee, D. Carbon 1970, 8, 623. (2) Baker, R. T. K.; Harris, P.; Kemper, D.; Waite, R. Carbon 1974, 12, 179. (3) Yang, R.; Wong, C. J. Catal. 1984, 85, 154. (4) Moreno-Castilla, C.; Carrasco-Marin, F.; Rivera-Utrilla, J. Fuel 1990, 69, 354. (5) Hayden, T.; Dumesic, J.; Sherwood, R.; Baker, R. T. K. J. Catal. 1987, 105, 299. (6) Silva, I. F.; Palma, C.; Klimkiewicz, M.; Eser, S. Carbon, in press. (7) Silva, I. F.; Lobo, L.; McKee, D. W. J. Catal. 1997, 170, 54. (8) Pan, Z.; Yang, R J. Catal. 1991, 130, 161. ‡

graphite surface in addition to channeling in some areas with the observed mobility of the catalyst droplets.1-3,5 Melting of the catalyst’s phases improves wetting of the carbon substrate and leads to better catalyst dispersion. It has been shown that bimetallic catalysts and also eutectic alloys with low melting temperatures can be more effective than the corresponding single components. McKee et al.9,10 observed that the catalytic activity of the eutectic salts was much greater than that of the equivalent composition of the single oxide mixtures, probably due to the fact that the eutectic salt phases have lower melting points, and, therefore, a higher degree of dispersion resulting from higher mobility on carbon surfaces. We have reported results from the TGA, in situ XRD, and environmental scanning electron microscopy (ESEM) studies on catalytic gasification of an activated charcoal in an air atmosphere in the presence of MoO3 and V2O5.6 Only TGA and in situ XRD results have been reported on a eutectic alloy of MoO3 and V2O5 and their physical (9) McKee, D. W.; Spiro, C.; Kosky, P.; Lambdy, E. Fuel 1985, 64, 805. (10) McKee, D. W. Carbon 1987, 25, 587.

S0887-0624(97)00174-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/16/1998

Activated Charcoal Gasification over MoO3

Energy & Fuels, Vol. 12, No. 3, 1998 555

Figure 1. (a) ESEM micrograph of the activated carbon with MoO3 catalyst particles (A) and (B) at room temperature (t ) 0 min). (b) ESEM micrograph of the onset of melting of catalyst particles MoO3 (A, B) at 630 °C (t ) 35 min). (c) ESEM micrograph at a higher magnification of activated carbon and MoO3 (A) catalyst particle at 630 °C (t ) 37 min). (d) ESEM micrograph of complete melting of the catalyst particle at 630 °C after holding for 6 min (t ) 41 min).

mixture with the same eutectic composition.6 These results showed that the eutectic alloy had a lower activity than the physical mixture, and, in some XRD experiments with the eutectic alloy, a much higher V2O5/MoO3 ratio was observed than that expected for the eutectic alloy with the presence of other phases. These findings suggested that either the alloy sample was not homogeneous or it was not stable under gasification conditions.6 In this paper, we report observations from an ESEM study of the activated charcoal gasification catalyzed by the eutectic alloy of MoO3 and V2O5 and their binary mixture with the same composition. Electron microprobe and energy-dispersive X-ray spectroscopy (EDS) analyses of the eutectic alloy sample were also carried out to characterize the alloy particles. Additional ESEM experiments were conducted to study the catalytic effect of MoO3 on gasification of an activated carbon in air and oxygen atmospheres to compare with the results reported earlier on the MoO3 catalysis of charcoal only in an air atmosphere.6,7 Earlier ESEM experiments in conjunction with in situ XRD data suggested that the

reduction of MoO3 to MoO2 did not lead to catalyst spreading in an air atmosphere. Experimental Section An activated charcoal sample, BDH33033, which was derived from a lignocellulosic material, was used in the ESEM experiments. The charcoal was the same material as that used in the previous study.6 It has a N2 BET surface area of 1230 m2/g, and calculated micropore and total pore volumes of 0.27 and 0.40 cm3/g, respectively, with an average pore diameter of 1.3 nm.6 For experiments with MoO3, a physical mixture of 3.3 wt % MoO3 was prepared by mixing the powdered charcoal with the molybdenum oxide. The ESEM experiments were conducted in an air and oxygen atmosphere at a constant pressure of 2.2 Torr in each case and temperatures up to 660 °C in oxygen and 800 °C in air with samples placed in alumina crucibles. The eutectic alloy was prepared at 700 °C by fusion of a finely ground mixture consisting of MoO3 (54.3 wt %) and V2O5 (45.7 wt %) which corresponds to the eutectic melting point of 618 °C.1 After fusion the alloy was slowly cooled in air. For ESEM experiments, the alloy particles were mixed with the powdered activated charcoal to give an initial catalyst

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Figure 2. (a) ESEM micrograph of activated carbon with MoO3 catalyst particles (A) and (B) at 430 °C in oxygen (t ) 0 min). (b) ESEM micrograph of beginning of melting of MoO3 particles (A, B) at 630 °C, (t ) 8 min). (c) ESEM micrograph of complete melting of particle A at 630 °C (t ) 9 min). (d) ESEM micrograph at higher magnification of area E showing the MoO3 particle (A) melted and the activated carbon particle partially gasified at 630 °C, (t ) 13 min). (e) ESEM micrograph showing edge gasification (F) and pitting (G) of the carbon particle where the catalyst particle A melted at 630 °C (t ) 14 min). (f) ESEM micrograph showing further gasification by edge (F) and pitting (G) at 630 °C (t ) 16 min). (g) ESEM micrograph of carbon particle X and melted catalyst particle B, showing the gasification of particle X by edge and pitting at 630 °C (t ) 22 min). (h) ESEM micrograph at a higher magnification showing more clearly pitting and edge gasification on particle X (t ) 24 min). (i) ESEM micrograph showing further gasification of particle X by edge gasification and pitting (t ) 27 min). concentration of 6 wt %. A physical mixture of MoO3 (54.3 wt %) and V2O5 (45.7 wt %) with the eutectic composition was also prepared and added to the carbon to have an initial concentration of 6 wt % catalyst mixture. The ESEM experiments on the eutectic mixture and alloy of MoO3 and V2O5 were conducted in an air atmosphere at a constant pressure 2.2 Torr and elevated temperatures up to 665 °C depending on the catalyst studied. An ElectroScan E-3 environmental scanning electron microscope was used in the gasification experiments to observe and record the behavior of activated charcoal and catalyst particles. Both photomicrography and videotaping techniques were used to record the observations. The details of the experimental procedure were reported elsewhere.6,11 An ISI-DS-130 SEM equipped with Kevex 8000 EDS (energy dispersive spectrometer) and a Cameca SX-50 electron micro(11) Gergova, K.; Eser, S.; Schobert, H. H.; Klimkiewicz, M.; Brown, P. Fuel 1995, 74, 1042.

probe were used to determine the composition and homogeneity of the alloy particles, using metal Mo and V as standards.

Results and Discussion The results from ESEM experiments on MoO3 and eutectic mixture and alloy of MoO3 and V2O5 are presented in separate sections. Multiple experiments were carried out with each catalyst system. The results presented here and illustrated by sequences of micrographs are all consistent and repeatable. Charcoal Gasification Catalyzed by MoO3 in Air and Oxygen Atmospheres. A time sequence of ESEM micrographs in Figure 1a-d shows the melting of an MoO3 particle (marked A) near a charcoal particle upon (12) Yang, R.; Wong, C. J. Catal. 1984, 85, 154.


Figure 3. (a) SEM micrograph of the mixture of MoO3 and V2O5 powders (A, MoO3 particle; and B, V2O5 particles one either side). (b) X-ray map for Mo for the same field shown in Figure 3a. (c) X-ray map for V for the same field shown in Figure 3a.

heating in air. The melting of the catalyst particle A started at 630 °C and was completed within 6 min. The micrographs 1c and 1d show that, following the melting process, the carbon particle shrinks by gasification of


the side edge which is in contact with the catalyst particle. This sequence resembles the multilayer edge gasification reactions seen during catalytic gasification of graphite studied by controlled atmosphere transmission electron microscopy and scanning tunneling microscopy.2,3,5 A similar preferential gasification on the side edge of the apparently isotropic charcoal particle observed in the previous study was attributed to a possible structural anisotropy of this highly porous material, present in a smaller scale than the microscopic resolution.6 Charcoal particles which were not in contact with catalyst particles did not show any visible change at 630 °C for approximately an hour. In situ XRD experiments conducted in air at 1 atm showed that MoO3 is reduced to MoO2 above 600 °C in the presence of the activated charcoal.6 It appears that the melting of MoO3 is associated with a phase transformation to MoO2 which is followed by rapid gasification of the carbon, as was suggested by McKee1 based on the results from TGA experiments on MoO3 and a mixture of MoO3 with graphite. Parallel ESEM experiments of carbon gasification with MoO3 were also carried out in an oxygen atmosphere at the same pressure as that used with the experiments in the air atmosphere. The ESEM micrograph in Figure 2a shows two MoO3 particles (A and B) which start to melt at 630 °C (Figure 2b), the same temperature at which MoO3 was observed to melt in an air atmosphere.6 It appears, however, that the melting of the MoO3 particle is faster than that observed in air, as shown by the micrograph in Figure 2c which was taken only one minute after taking the micrograph in Figure 2b. The sequence of micrographs in Figure 2d-f shows the shrinkage of a clearly visible carbon particle marked E which was in contact with the catalyst particle A before it started to melt, as shown in Figure 2c. In addition to rapid edge gasification (in the area marked F) similar to that observed in the air atmosphere, the particle E shows gasification in all directions which leads to surface roughening and pitting, as seen clearly in area G. These observations indicate that, in an oxygen atmosphere, gasification takes place in all directions, as opposed to the dominant edge gasification reactions observed in an air atmosphere. The time sequence of micrographs in Figure 2g-i shows the gasification of carbon particle X which was in contact with the catalyst particle B before it melted as shown Figure 2c. Similar to the behavior of the charcoal particle E, both edge gasification and pitting of the carbon particle X are observed in the micrographs of Figure 2g-i. The streaks seen on the transparent part of the gasifier particle X can be attributed to the woody structure of the carbon precursor. These streaks may represent less reactive parts of the charcoal structure. The behavior of charcoal particles E and X was not an isolated incident, but common to many other particles observed during the ESEM experiments. It appears from these observations that gasification of the charcoal sample in an oxygen atmosphere takes place in all directions, as different from the apparent anisotropic gasification in an air atmosphere.6 The isotropic gasification observed in the oxygen atmosphere can be


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Figure 4. (a) ESEM micrograph showing a large MoO3 particle surrounded by small V2O5 particles on carbon (T ) 330 °C, t ) 0 min). (b) ESEM micrograph of the same area, showing complete melting of V2O5 particles and the onset of melting of MoO3 particle (T ) 595 °C, t ) 15 min). (c) ESEM micrograph showing the complete melting of MoO3 particle (T ) 630 °C, t ) 18 min). (d) ESEM micrograph of the same area showing fast edge gasification of the carbon particle which was in contact with the catalyst mixture (T ) 630 °C, t ) 20 min). (e) ESEM micrograph of the same carbon particle showing edge gasification and pitting with catalyst droplets (T ) 630 °C, t ) 23 min). (f) ESEM micrograph of the same particle showing further gasification and catalyst droplets (T ) 630 °C, t ) 27 min).

attributed to more effective catalyst dispersion and enhanced oxygen diffusion on the carbon surface, as suggested by Pan and Yang.8 To understand the

different gasification behavior in air and oxygen atmosphere one has to consider the major steps involved in MoO3 catalysis of carbon gasification. It has been



clearly established1,6,7 that reduction of MoO3 to MoO2 and reoxidation of MoO2 to MoO3 are important steps involved in catalytic carbon gasification, as represented below by the reactions 1 and 2, respectively.

MoO3 + C f MoO2 + CO

(1)

MoO2 + 1/2O2 f MoO3

(2)

Both reactions are thermodynamically favored with calculated standard free energy changes (∆G°) of -81 and -99 kJ/mol for reactions 1 and 2, respectively.7 In situ XRD, ESEM, and TGA experiments showed that the melting of the catalyst particles on charcoal surfaces was coincident with the reduction of MoO3 to MoO2.6,7 It can seen that the reduction reaction does not depend directly on oxygen pressure. This may explain the close correspondence observed between the results obtained by in situ XRD and ESEM experiments conducted under substantially different pressures, 1 atm versus 2.2 Torr, respectively. As mentioned above, the ESEM experiments conducted in air and oxygen atmospheres showed no significant difference in the temperature when catalyst particles started to melt, but the melting of the particles was much faster in the oxygen atmosphere. These observations and the thermodynamic data on reduction and oxidation reactions strongly suggest that the uniform particle gasification observed in all directions in an oxygen atmosphere can be explained by more effective spreading of molten catalysts on charcoal surfaces. It can be considered that gasification in an oxygen atmosphere in ESEM would produce a more extensively oxidized carbon surfaces which would facilitate a better wetting of the charcoal surfaces by the molten catalysts and their more effective spreading on the oxidized surfaces of charcoal particles. Charcoal Gasification Catalyzed by a Eutectic Mixture and Alloy of MoO3 and V2O5. Figure 3a shows a secondary electron SEM image of a sample taken randomly from the eutectic mixture of MoO3 and V2O5 with the corresponding X-ray maps for Mo (Figure 3b) and V (Figure 3c). Most MoO3 particles have slab like shapes (e.g., particle marked A in Figure 3, a and b) whereas most V2O5 particles tend to have more rounded shapes (e.g., particles on both sides of the label B in Figure 3, a and c) and comparatively small particle size, as seen in the micrograph and the corresponding X-ray maps. These differences in particle shape help identify the MoO3 and V2O5 particles in the ESEM experiments. All the fields of the eutectic mixture sample observed under SEM showed the same features as those exemplified in Figure 3. Figure 4 shows micrographs obtained from the ESEM experiments on the activated carbon heated with the catalyst mixture in an alumina crucible. Figure 4a shows a large MoO3 particle in the center of the micrograph surrounded by small particles of V2O5. Figure 4b shows that all the V2O5 particles melted at 595 °C around a needlelike carbon particle. A complete melting of the MoO3 particle occurred upon further heating to 630 °C, as shown in Figure 4c. These melting temperatures for MoO3 and V2O5 are the same as those observed in the ESEM experiments with the corresponding single oxides.6 After holding 9 min, the needlelike carbon particle which was in contact with the

Figure 5. (a) SEM micrograph of the MoO3 and V2O5 alloy. (b) X-ray map for Mo for the same field shown in Figure 5a. (c) X-ray map for V for the same field shown in Figure 5a.

catalyst particles showed rapid gasification (Figure 4df) by edge recession with the appearance of catalyst droplets (Figure 4, e and f). This activity of MoO3 has been reported also by other researchers.1-3,6 A comparison of micrographs in Figure 4d-f also shows that both edge gasification and pitting had occurred.


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Table 1. Electron Microprobe Analysis of Selected Eutectic Alloy Particles in Different Positions particle

position

V2O5 (wt %)

MoO3 (wt %)

1 1 1 2 2 2 3 3 4 4 5 5

1 2 3 1 2 3 1 2 1 2 1 2

46.5 46.6 87.9 44.6 46.6 43.9 46.8 45.9 47.9 45.6 2.8 2.8

53.1 52.7 10.8 53.6 51.2 53.5 52.4 53.7 49.9 47.2 92.7 95.2

Figure 5a shows a secondary electron image of a sample taken randomly from the eutectic alloy of MoO3 and V2O5 with the corresponding X-ray maps for Mo (Figure 5b) and V (Figure 5c). It can be seen from the micrograph and the X-ray maps that the alloy particles are larger than those seen in the catalyst mixture (Figure 3a) and that the composition of the alloy particles is not uniform. For example, the large particle marked A in Figure 5a-c contains an area which is enriched in V2O5 with a small concentration of MoO3 present, if any, as seen in the X-ray maps. In contrast, most of the particles B and C appear to be much richer in MoO3 than in V2O5; the particle D, on the other hand, appears to be richer in V2O5 than in MoO3. There are also particles in the alloy sample, such as particle E, which show a uniform distribution of MoO3 and V2O5 which is consistent with the eutectic alloy composition. These qualitative observations, which were repeatable with multiple samples, indicate that not all the particles in the alloy samples have a uniform composition which corresponds to the eutectic composition; some alloy particles contain inclusions with high concentrations of one oxide or the other. For a more quantitative analysis of the alloy particles, electron microprobe was used. Table 1 shows the composition of different positions in five different alloy particles, selected using the backscattered electron image. For most cases a good material balance was obtained with the weight percentages of the two oxides in a given position adding close to 100%. The mass deficiency observed in a few cases can be attributed to surface roughness. Particles 2, 3, and 4 in Table 1 represent the alloy particles with rather uniform composition in different regions which correspond very closely to the eutectic composition (54.3 wt % MoO3 and 45.7 wt % V2O5). Particles 1 and 5, on the other hand, contain regions which have very different compositions compared to the eutectic composition. The position 3 in particle 1, for example, shows a V2O5 inclusion in the particle, whereas particle 5 has regions which are very rich in MoO3. Figure 6 shows a backscattered electron micrograph of particle 1 in which the interior dark region is the V2O5 rich inclusion represented by position 3 of particle 1 shown in Table 1. The size of the V2O5 inclusion in the particle shown in Figure 6 is comparable to the size of the catalyst particles observed in the ESEM experiments. The results from EDS and electron microprobe analyses of the alloy samples indicate that the catalyst particles are hetero-

Figure 6. A backscattered electron micrograph of the alloy particle 1 in Table 1.

geneous at the same scale as the size of the particles used in the ESEM experiments. Figure 7 shows the ESEM micrographs obtained during the experiments with the eutectic alloy. The micrographs in Figure 7a,b show three catalyst particles A, B, and E. Upon heating to 630 °C, particle B shows a structural change with the formation of fibrous extensions (Figure 7c). The particles A and E, on the other hand, do not show any visible change. With further temperature increase to 655 °C, particle B appears to melt, again with slight change in the appearance of particles A and E (Figure 7d). The increase in temperature from 600 °C (Figure 7b) to 655 °C (Figure 7d) causes substantial edge gasification of the carbon particle C. These observations, combined with the results from EDS and electron microprobe analyses, suggest that some particles such as A and E did not have the eutectic composition. The particles with the eutectic composition, such as particle B, on the other hand, melted and spread on the carbon surface and catalyzed the gasification reactions. The micrographs in Figure 7, e and f, show the melting of the particles A and E when the temperature was increased to 665 °C, followed by rapid edge gasification reactions on carbon particles C and D. In addition to edge gasification, the micrographs in Figure 7, g and h, show vigorous pitting action in regions F and G, and Figure 7i shows a complete gasification of particle C and areas F and G at 665 °C. In order to follow the behavior of the eutectic alloy particles upon heating, ESEM experiments were conducted with the alloy particles heated in an alumina crucible without the presence of carbon. The micrographs in Figure 8 show that most of the alloy particles (Figure 8a) go through a transformation with the formation of fibrous structures (Figure 8b) at 600 °C, similar to that observed with the particle B at a slightly higher temperature on the carbon surface, and melted and spread on the alumina crucible by 650 °C (Figure 8c). Some particles, however, remained solid at 650 °C (Figure 8c), probably because of a different phase transformation from those which led to the melting and spreading of the other particles. This behavior is similar to that of the particles A and E on the carbon



Figure 7. (a) ESEM micrograph showing catalyst particles A, B, and E (T ) 575 °C, t ) 0 min). (b) ESEM micrograph at a higher magnification of catalyst particles A, B, and E (T ) 600 °C, t ) 3 min). (c) ESEM micrograph of particles A, B, and E showing edge gasification of the surrounding carbon particles (T ) 630 °C, t ) 9 min). (d) ESEM micrograph showing the melting of particle B with further edge gasification of the surrounding carbon particles (T ) 655 °C, t ) 15 min). (e) ESEM micrograph showing the catalyst particles (A and E) and edge gasification of carbon particles C and D (T ) 665 °C, t ) 23 min). (f) ESEM micrograph showing the melting of particles A and E and edge gasification of carbon particles C and D (T ) 665 °C, t ) 36 min). (g) ESEM micrograph of the same region showing further edge gasification of particles C and D and pitting action on regions F and G (T ) 665 °C, t ) 45 min). (h) ESEM micrograph showing further pitting in areas F and G (T ) 665 °C, t ) 46 min). (i) ESEM micrograph showing complete gasification of particle C and areas F and G (T ) 665 °C, t ) 53 min).

surfaces, which did not melt until 665 °C. These observations agree with the EDS and electron microprobe data, indicating that not all the particles in the eutectic alloy sample had the eutectic composition, or some particles went through different transformations to form phases with higher melting points than those of the eutectic alloy and the individual components of the alloy. As mentioned before, some in situ XRD experiments with the eutectic alloy sample on carbon surfaces showed a much higher V2O5/MoO3 ratio than that of the eutectic composition and the presence of several phases apart from MoO3 and V2O5, including lower oxides produced by reduction of V2O5.6 These observations are

consistent with the expected behavior of a catalyst particle which contains a V2O5 inclusion similar to that in particle 1 shown in Table 1 and Figure 6. Such a particle would exhibit a high melting point, because it does not have the eutectic composition. Further, the V2O5 inclusion would not have any contact, at least initially, with the carbon surface, which was shown to be very important for the initial reduction of the oxides, the important first step for catalytic activity controlled by melting and spreading of the particles.6,7 On the basis of EDS and electron microprobe analysis and ESEM experiments on the eutectic mixture and the eutectic alloy, heterogeneous composition of the alloy particles can explain the relatively low activity of the


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advantage of having a eutectic mixture which has a lower melting point than those of the individual components.

Conclusions The ESEM experiments on charcoal gasification catalyzed by MoO3,V2O5, and their eutectic alloy clearly showed the important effect of the initial interaction between the catalyst particles and the carbon surface. It was seen that MoO3 spreads more readily on the carbon surface in an oxygen atmosphere than in an air atmosphere under comparable conditions. This observation was explained by the more effective reduction, and subsequent melting and spreading of the catalyst particles on more extensively oxidized carbon surfaces in an oxygen atmosphere. As a result of more effective dispersion of the catalysts on the carbon surface, the gasification reactions proceed in all directions on a charcoal particle in an oxygen atmosphere, in contrast to the reactions on the particle edges only, as observed in an air atmosphere. The EDS and electron microprobe analysis of the “eutectic” alloy samples showed that some catalyst particles did not have the eutectic composition and contained large inclusions of almost pure single oxides. The size of these inclusions, or microstructural heterogeneity in the alloy particles, is comparable to the size of the catalyst particles used in the ESEM experiments. Along with the EDS and electron microprobe data, the ESEM gasification experiments with the eutectic mixture and the eutectic alloy of MoO3 and V2O5 suggest that the heterogeneous composition of the alloy particles is responsible for the relatively low activity of the eutectic alloy in charcoal gasification compared to that of the eutectic mixture of the powders. Encapsulation of either oxide in a catalyst particle would inhibit its initial contact with the carbon surface, and, thus, inhibit the reduction of the metal oxide which was shown to be the critically important first step in the reduction/ oxidation cycle which controls the activity of MoO3 and V2O5 catalysts in carbon gasification.6,7 Both sets of experiments with MoO3 and with the eutectic alloy of MoO3 and V2O5 demonstrated the critical importance of the interaction of the catalyst particles with the carbon surfaces as influenced by the reaction atmosphere and the compositional heterogeneity of the catalyst particles.

Figure 8. (a) ESEM micrograph of the eutectic alloy sample at room temperature (T ) 25 °C, t ) 0 min). (b) ESEM micrograph showing a structural transformation of the eutectic alloy at 600 °C (t ) 17 min). (c) ESEM micrograph showing complete melting of most particles with the presence of some solid particles at 650 °C (t ) 25 min).

eutectic alloy in charcoal gasification compared to that of the eutectic mixture of the powders.6 The heterogeneous composition of the alloy particles negates the

Acknowledgment. The authors gratefully acknowledge the Luso American Foundation and NATO Invotan for the financial support and C. Palma for sample preparation. Mr. Mark Angelone of the Materials Characterization Laboratory at Penn State University provided the electron microprobe data. We extend our gratitude to Dr. Douglas McKee for many useful discussions and suggestions on the manuscript. We also thank the anonymous reviewers for reading the manuscript very carefully with constructive criticism. EF970174A

An Environmental Scanning Electron Microscopy Study of Activated

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