Morphology and etching processes on macroscopic metal catalysts

Abstract

Changes in the morphology of catalysts during their preparation and use are probably important to the stability of most catalyst systems, but, in spite of extensive investigations of these phenomena, the rates of these changes and the structures formed are typically characterized only qualitatively and no consensus exists as to the mechanisms of morphology changes. In this paper experiments on the morphology of macroscopic unsupported metal catalysts in unreactive gases (thermal etching) and in reactive gases (catalytic etching) are reviewed, and a number of mechanisms which may be responsible for etching are summarized. In both thermal and catalytic etching a variety of structures such as faceting to form specific crystal planes, grooving of grain boundaries, pit and hillock formation, undercutting, and crystallite growth are observed. All studies indicate strong dependences of morphological features and etching rates on temperature, pressure, gas composition, impurities, flow velocities, and time. Most studies have been for exothermic reactions on Group VIII transition metals, but even for similar reactions and similar metals, correlations are frequently elusive. Morphology changes have frequently been interpreted through equilibrium considerations, the driving force assumed to be the reduction in surface free energy which accompanies faceting. The mechanisms of mass transport in many systems are probably capillarity—induced surface and volume diffusion and evaporation-condensation processes. These considerations may be adequate to explain simple morphological features especially in thermal etching under certain simplified conditions. Additional complexities arise in catalytic etching, where the superposition of chemical reactions onto a process driven by capillarity can significantly alter mass transport. Moreover, the undercutting and crystallite growths, which occur when catalyst material is being continuously lost (e.g. in the form of volatile compounds), produces nonequilibrium processes requiring a detailed specification of the boundary layer at the catalyst surface.