The morphological evolution of the MoO3(010) surface during reactions in methanol-air mixtures

Atomic force microscopy has been used to characterize the evolution of MoO3 (010) growth sand cleavage surfaces during reactions with air-N-2-MeOH mixt ures between 200 and 300 degrees C. At reaction temperatures greater than o r equal to 250 degrees C, closed step loops, bounded by undercoordinated su rface Mo sites, nucleate and grow as the (010) surface volatilizes in a nea rly layer-by-layer fashion. The density and crystallography of the new surf ace sites depend on the composition of the gas mixture in the reactor. In a ir-rich environments, the step loops have a rectangular shape and are elong ated along [001]. At any instant, only the uppermost one or two (010) layer s are affected by the process. As the concentration of Oz in the feed is re duced, the shape evolves to elongated triangles oriented along [100] and th e loops exist on multiple layers simultaneously. During reactions, the obse rved linear step edge densities can increase from 1x10(3)/cm to greater tha n 1.5 x 10(5)/cm. The potential influence of these undercoordinated surface Mo sites on the reactivity of particles with different aspect ratios is di scussed. (C) 1998 Academic Press.