Scanning Probe Microscopy of Cleaved Molybdates: a-MoO3(010), Mo18O52(100), Mo8O23(010), and h-Mo4O11(100)

volves determining the population and distribution of these defects. Scanning probe microscopy was used to examine the cleaved surfaces of four binary molybdates: a-MoO3(010), Mo18O52(100), Scanning probe microscopy (SPM), a term we use here to Mo8O23(010), and h-Mo4O11(100). The Mo18O52(100) and refer collectively to scanning tunneling microscopy (STM) Mo8O23(010) surfaces were imaged in air and vacuum using and atomic force microscopy (AFM), can potentially be scanning tunneling microscopy (STM). The contrast associated used to identify atomic-scale defects on substoichiometric with two types of surface/crystallographic shear (CS) plane molybdate surfaces. It is, however, necessary to first estabintersections has been unambiguously identified; shear normal lish precedents that can be used for image interpretation; to the surface creates a line of vertical relief 1.5 Å high and this is the objective of the present paper. Herein, we deshear in the surface plane creates a line of dark contrast. The scribe the contrast that is observed in STM and AFM contrast from the surface/CS plane intersection arises, in part, images of four stoichiometric molybdenum oxide binary from local variations in the electronic properties. These signaphases: a-MoO3(010), Mo18O52(100), Mo8O23(010), and htures are easily distinguished from features on the fully oxidized Mo4O11(100). We take the (010) surface of the most oxia-MoO3(010) surface. STM images of h-Mo4O11(100) reveal a dized phase as the ‘‘ideal’’ surface structure, a cornersurface terminated by tetrahedral groups. In each case, we find sharing arrangement of MoO6 octahedra similar to a h100j that the atomic-scale contrast can be interpreted based on the layer of the ReO3 structure. The next two phases, Mo18O52 arrangement of surface polyhedra that is expected to result from and Mo8O23 , contain CS planes that intersect the surface cleavage of the longest, weakest bonds.  1996 Academic Press, Inc. in two distinct ways. In the first structure, there is a component of the shear normal to the surface plane so that in addition to changing the coordination environment of the INTRODUCTION atoms on the CS plane, the shear creates some surface Molybdenum oxides and mixed metal molybdates are relief. In the second structure, the shear lies entirely in the used as catalysts for a number of partial oxidation reactions surface plane and there is no topographic relief. The final (1). Studies designed to quantify the relative reactivity of phase, h-Mo4O11 , is distinguished from the other three different MoO3 facets have led to the conclusion that the because its surface is terminated entirely by tetrahedral surface properties of this compound are surface structure MoO4 groups instead of octahedral MoO6 groups. The sensitive (2–7). Surface structure determinations, which remainder of this section contains a more detailed compariappear necessary to identify the mechanism of this strucson of the relevant crystal structures and a brief summary ture-sensitivity, are complicated by the fact that stoichiomof previous studies of the molybdenum oxide surfaces. etry compensating structural defects are integral components of the active material’s surface; when an adsorbed (a) Structural Chemistry of the Molybdenum Oxides hydrocarbon is oxidized, the catalyst itself is reduced and The structural chemistry of the molybdenum oxides was a defect must be created on the surface. In the near stoidescribed several decades ago in a series of papers by chiometric region, MoO3 reduction is compensated by Khilborg (10–15) and the relevant structural parameters oxygen vacancies. Further from stoichiometry, however, for the phases of interest are summarized in Table 1. These transmission electron microscopy has shown that crystallostructures can be thought of as relatives of the ReO3 (D09) graphic shear (CS) planes are the compensating defect structure, which is a three dimensional corner-sharing octa(8, 9). Thus, characterization of the relevant surfaces inhedral network. The fully oxidized phase, a-MoO3 , has the layered structure illustrated in Fig. 1 (11). Each layer is composed of two corner-sharing octahedral nets that 1 To whom correspondence should be addressed.