The Partial Oxidation of Methanol by MoO 3 (010) Surfaces with Controlled Defect Distributions
- PDF / 3,019,398 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 38 Downloads / 176 Views
Figure 1. Polyhedral representation of the MoO3 structure. MoO3 has a primitive orthorhombic cell (Pbnm) and its lattice parameters are a=3.963 A, b=13.856 A, c=3.6966 A.
[100]~
V [010] [1
]
[001] ý[001]
53 Mat. Res. Soc. Symp. Proc. Vol. 497 01998 Materials Research Society
broken for each Mo atom and the average Mo coordination number along the step must be 5. In this case, the average might be made up of equal numbers of 4- and 6-coordinate Mo. Experimentally, it is possible to produce undercoordinated sites with reducing treatments at 400 'C in H 2/N2 mixtures [5]. Under these conditions, voids, bounded by step loops, form on the surface. We have used atomic force microscopy (AFM) to characterize both the freshly cleaved surface and surfaces containing voids. In this paper, we describe the evolution of these two characteristic surfaces during reactions in MeOH/N2 mixtures. In each case, HxMoO3 precipitates are observed to form as a product of the reaction, but the distribution of the precipitates differs. The mechanisms by which the undercoordinated Mo at the edges of the voids might affect the transfer of protons from the alcohol to MoO 3 are discussed. EXPERIMENTAL Single crystals of MoO 3 were grown by chemical vapor transport in sealed, evacuated quartz ampoules [6]. The surface reactions were performed in a quartz tube with flowing gas at I atm. A magnetic transfer rod (consisting of a permanent magnet sealed within a quartz rod) was used to move the samples into and out of the hot zone of the furnace. The single crystals were mounted on a steel disc by spot welding a thin strip of Ta foil across the basal facet. The steel disc was attached to the transfer rod by chromel wire spring clips. After cleaving the crystal and sealing the reaction tube, the system was purged by alternately evacuating and backfilling with the reactant gas mixture three times. The samples were then transferred to the hot zone of the furnace and reacted for a pre-determined time at temperature. To create surface voids with undercoordinated Mo atoms, as-received 10%H 2 /N2 (forming gas) with a nominal H2 0 concentration of 20 ppm was used to reduce the samples at 400'C in a 350 cc/min flow of the gas. Samples were then transferred to the cool zone of the reaction tube and allowed to sit under a flow of forming gas for approximately 30 min while the hot zone was cooled to the desired temperature for the MeOH reaction. Subsequently, the gas composition was changed to a MeOHIN 2 mixture and the sample was transferred back to the hot zone for reaction under a 200 cc/min flow. The N2 (Prepurified, Matheson) carrier gas was dried with CaSO 4 (Drierite) and a liquid N2 trap prior to being saturated in a methanol (99.9+%, Aldrich) bubbler. Following reaction, the MoO 3 samples were cooled to room temperature under a flow of the reactant gas mixture and then transferred to an AFM housed in a glove box with an atmosphere of continuously-purified Ar (02 and H20 < 5 ppm). In most cases, the samples were transferred directly to the glove bo
Data Loading...
