The structural evolution of the molybdenum oxide (010) surface during reduction and oxidation reactions.

摘要

Scanning probe microscopy (SPM) has been used to study the surface structure of selected binary molybdenum oxides and the structural evolution of the MoO{dollar}sb3{dollar}(010) surface during gas phase reduction and oxidation reactions. These observations have been used to develop an improved understanding of the relationship between the structure and reactivity of MoO{dollar}sb3.{dollar} During redox reactions above {dollar}350spcirc{dollar}C, H{dollar}sb3{dollar}O catalyzes the volatilization of MoO{dollar}sb3{dollar} where strain is localized at the (010) surface. Under oxidizing conditions, volatilization is confined to step edges and dislocation/surface intersections. During reduction, the crystallographic shear (CS) planes that nucleate at the (010) surface provide an abundant source of strain and volatilization leads to the formation of surface pits bounded by {dollar}{lcub}{dollar}h0l{dollar}{rcub}.{dollar} When these pitted surfaces are oxidized above {dollar}350spcirc{dollar}C, the pits grow and their bounding edges eventually reorient to {dollar}langle 001rangle{dollar} and {dollar}langle 100rangle.{dollar} As a result, thermal treatments can be used to tailor the morphology of the MoO{dollar}sb3{dollar}(010) surface.; During reactions with N{dollar}sb2{dollar}-alcohol mixtures {dollar}(200 le {lcub}rm T{rcub} le 400spcirc{dollar}C), MoO{dollar}sb3{dollar} intercalates H liberated from the alcohol during chemisorption. Protonation leads to the topotactic formation of acicular {dollar}{lcub}rm H{rcub}sb{lcub}x{rcub}{lcub}rm MoO{rcub}sb3 (0.23 le x le 0.4){dollar} precipitates along the {dollar}langle 203rangle{dollar} axis of the (010) surface. During reactions with N{dollar}sb2{dollar}-methanol mixtures at {dollar}300spcirc{dollar}C, H{dollar}sb{lcub}x{rcub}{dollar}MoO{dollar}sb3{dollar} precipitates preferentially at {dollar}{lcub}{dollar}h0l{dollar}{rcub}{dollar} planes that bound the (010) surface. When the density of {dollar}{lcub}{dollar}h0l{dollar}{rcub}{dollar} sites is increased (by introducing pits) more hydrogen is intercalated during the reaction. These results lead to the conclusion that methanol chemisorption is favored at the undercoordinated Mo afforded by {dollar}{lcub}{dollar}h0l{dollar}{rcub}{dollar} surfaces.; During the partial oxidation of methanol in air-N{dollar}sb2{dollar}-MeOH mixtures {dollar}rm(T ge 250spcirc C),{dollar} closed step loops, bounded by undercoordinated surface Mo, nucleate and grow as the MoO{dollar}sb3{dollar}(010) surface volatilizes in a nearly layer-by-layer fashion. The density and crystallography of the new sites depend on the composition of the reactor feed. In air rich environments, rectangular step loops elongated along {dollar}langle 001rangle{dollar} exist on the top-most one or two {dollar}{lcub}{dollar}010{dollar}{rcub}{dollar} layers of the crystal. As the concentration of O{dollar}sb2{dollar} in the feed is reduced, the step loop shape evolves to triangles elongated along {dollar}langle 100rangle{dollar} and the loops exist on multiple layers simultaneously. During these reactions, the density of undercoordinated surface Mo sites can increase from {dollar}rm 1 times 10sp{lcub}10{rcub}/cmsp2{dollar} to more than {dollar}rm 1.5 times 10sp{lcub}12{rcub}/cmsp2.{dollar}