Simulation of the behavior of supported metal catalysts in real reaction atmospheres by means of model catalysts

Abstract

Model catalysts formed of small crystallites of Ni, Co, or Fe supported on thin, electron transparent films of nonporous alumina were heated in chemical atmospheres having compositions in the range encountered in the steam-reforming reaction. Electron microscopy and electron diffraction were used to investigate the physical and chemical changes that occurred. For a better understanding of the phenomena involved, the effect of the mixture is compared to the effects caused by the individual components and by their combinations. It is shown that, depending upon the nature of the catalyst as well as upon the composition of the mixture, sintering or/and coking can occur much more rapidly in mixtures than in single-component atmospheres. This suggests that a cooperative action of the reaction components is the cause of the early deactivation of the steam-reforming catalysts. Various phenomena such as carbon deposition (as films or patches on the surface of the catalyst and on the substrate or as filaments), deformation of the crystallites, severe sintering, and permanent loss of metal to the gas stream were observed. Explanations are provided for the behavior of the Ni Al 2O 3 , Co Al 2O 3 , and Fe Al 2O 3 catalysts, in various atmospheres, by considering the competition between carbon deposition and its gasification, as well as the strength of the interactions between crystallites and substrate. Two kinds of filaments were observed: carbonaceous filaments and metal oxide filaments which probably contain some carbon. A thermodynamic condition for the formation of filamentous carbon is suggested. If the sum of the interfacial free energies between carbon and crystallite, and carbon and substrate is smaller than that between crystallite and substrate, then carbon could penetrate between crystallites and substrate and filaments would be generated. Indeed, carbonaceous filaments were identified only when the catalyst particles were present as metals. In such cases, the interfacial free energy between the metal crystallite and alumina is much greater than that between the oxidized metal and alumina and the above thermodynamic condition is more likely to be satisfied. Permanent loss of active metal to the gas stream occurred, either because of the disintegration of the crystallites caused by carbon precipitation inside the particles, and/or possibly because of the formation of volatile Carbonyls.