An Environmental Scanning Electron Microscopy Study of Activated Charcoal Gasification Catalyzed by MoO 3 in Air and in Oxygen and by a Eutectic Alloy of MoO 3 and V 2 O 5 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 MoO 3 in air and oxygen atmospheres and by a eutectic mixture and alloy of MoO 3 and V 2 O 5 . In an air atmosphere, the melting of MoO 3 starts at 630 °C and the reaction occurs by preferential gasification on the edges of carbon particles. In an O 2 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 MoO 3 and V 2 O 5 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 (MoO 3 or V 2 O 5 ) 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 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 disper- sion. It has been shown that bimetallic catalysts and also eutectic alloys with low melting temperatures can be more effective than the corresponding single compo- nents. 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 mix- tures, probably due to the fact that the eutectic salt phases have lower melting points, and, therefore, a higher degree of dispersion resulting from higher mobil- ity 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 MoO 3 and V 2 O 5 . 6 Only TGA and in situ XRD results have been reported on a eutectic alloy of MoO 3 and V 2 O 5 and their physical † 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. (9) McKee, D. W.; Spiro, C.; Kosky, P.; Lambdy, E. Fuel 1985, 64, 805. (10) McKee, D. W. Carbon 1987, 25, 587. 554 Energy & Fuels 1998, 12, 554-562 S0887-0624(97)00174-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/16/1998