Abstract
A series of microstructured, supported platinum (Pt) catalyst films (supported on single-crystal yttria-stabilized zirconia) and an appropriate Pt catalyst reference system (supported on single-crystal alumina) were fabricated using pulsed laser deposition and ion-beam etching. The thin films exhibit area-specific lengths of the three-phase boundary (length of three-phase boundary between the Pt, support, and gas phase divided by the superficial area of the sample) that vary over 4 orders of magnitude from 4.5 × 102 to 4.9 × 106 m m–2, equivalent to structural length scales of 0.2 μm to approximately 9000 μm. The catalyst films have been characterized using X-ray diffraction, atomic force microscopy, high-resolution scanning electron microscopy, and catalytic activity tests employing the carbon monoxide oxidation reaction. When Pt is supported on yttria-stabilized zirconia, the reaction rate clearly depends upon the area-specific length of the three-phase boundary, l(tpb). A similar relationship is not observed when Pt is supported on alumina. We suggest that the presence of the three-phase boundary provides an extra channel of oxygen supply to the Pt through diffusion in or on the yttria-stabilized zirconia support coupled with surface diffusion across the Pt.
Highlights
In heterogeneous catalysis the reaction mixture comes into contact with the surface of a catalyst, which often consists of supported particles of the active component dispersed and immobilized on a high-surface-area carrier
The oxide support can interact with the active phase through geometrical effects, electronic effects,5a,6 or the creation of special interfacial sites,5a,7 or even as a source or sink of intermediate species that may diffuse over the catalyst.5b,8 The surface diffusion of intermediate species, promoters, etc. can affect sites even micrometers away from the interface.8b from a structural point of view heterogeneous catalysts represent a rather complex situation
In broad agreement with ref 8, that for the YSZ-supported samples the presence of the three-phase boundary facilitates an additional channel of oxygen supply to the Pt surface via a diffusional process which has a diffusional length scale on the order of 10 μm or less
Summary
In heterogeneous catalysis the reaction mixture comes into contact with the surface of a catalyst, which often consists of supported particles of the active component dispersed and immobilized on a high-surface-area carrier. For the patterned YSZ-supported samples an increasing l(tpb) (decreasing Pt length scale) appears to be correlated with higher rates of CO2 production. If the structural length scale of the Pt films is much greater than the diffusional length scale of any diffusing intermediate species supplied from the support, we would expect any modification of coverage to be confined to a region close to the tpb; the diffusional length scale depends upon the ratio of the surface diffusion coefficient of the diffusing species to the equivalent first-order rate constant for consumption of the diffusing species through the reaction. Dependence of the rate of CO2 production (rCO2) normalized by the length of the three-phase boundary (rCO2′) versus l(tpb) at 250, 270, and 290 °C for the patterned Pt/YSZ samples (1PtYSZ, 4PtYSZ, and 16PtYSZ). Diffusional process which has a diffusional length scale on the order of 10 μm or less
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