Platinum cluster size has a significant influence on the activity, selectivity, and stability as well as the reaction mechanism during propane dehydrogenation (PDH). Wellcontrolled platinum catalysts of different cluster sizes are prepared by a seed growth method and supported on calcined hydrotalcite. The Pt catalysts show strong structure-sensitive behavior both in the C−H bond activation of propane and in the C−C bond activation to yield ethylene, methane, and coke. The Pt clusters of small cluster sizes, with (211) dominating on the surface, have a lower dehydrogenation energy barrier and thus higher activity. However, large Pt clusters with Pt(111) dominating result in a weakened binding strength of propylene and an increased energy barrier for the activation of C−H bonds in propylene, which leads to higher selectivity toward propylene by lowering the possibility of deep dehydrogenation. Kinetic analysis illustrates that the reaction order in hydrogen decreases and activation energy increases with an increasing Pt cluster size. Combined with density functional theory calculations and isotope effect experiments, it gives strong evidence of the change in reaction mechanism with Pt cluster size. It suggests that on small Pt clusters that are mostly surrounded by undercoordinated surface sites, the first C−H bond activation is likely to be the rate-determining step, while the second C−H bond activation is kinetically relevant on large Pt particles with terrace sites dominating.

Certain commercial aluminum alloys can become electrochemically activated by heat-treatment as a result of enrichment of the trace element Pb at the surface. For a better understanding of the nature of activation, Pb enrichment resulting from annealing for 1-24 h at 300-600°C in air, followed by quenching in water, was investigated on an AlPb binary model alloy, by use of electrochemical polarization, electron optical techniques, and glow discharge optical emission spectroscopy. Most of the enriched Pb was found to be near the oxide film-aluminum matrix interface, probably in solid solution with aluminum. The surface concentration reached an apparent saturation level of 0.8 wt % at 600°C, up from 20 ppm in the bulk. In addition, segregated metallic Pb particles were detected at an increasing density and size with increasing time of annealing at 600°C. However, segregation of Pb particles did not have an appreciable effect on activation. It is suggested, therefore, that the electrochemical activation is related to reduced passivity of the overlying oxide by Pb enriched in solid solution at the metal surface and ensuing pitting potential depression in the combined presence of aggressive chloride ions in the test solution.Electrochemical activation of various commercial aluminum alloys resulting from high-temperature heat-treatment has been a subject of attention for the past several years 1-7 because of its importance in galvanic and filiform corrosion. The phenomenon is observed by heat-treatment at temperatures above 350°C. It is characterized by deep corrosion potential transients with characteristic arrests in slightly acidified chloride solutions, starting from highly negative potentials in relation to the usual pitting or corrosion potential of about −0.75 V SCE . 8 In addition, anodic polarization in neutral chloride solutions gives high anodic current output with oxidation peaks corresponding to the potential arrests. [4][5][6]9 Recent work on commercial AA8000 and 3000 series alloys related the cause of activation to the enrichment of a metallic near surface layer by lead, which was present in the material as a trace element only at the parts per million level. 5,6 This layer, which existed in the metal phase right under the metal-oxide interface, was a fraction of a micrometer thick on the average on these rolled materials, but the thickness varied significantly locally from nearly zero to about 1 m. We refer to this layer as the subsurface layer throughout this paper.In continuing work by use of model binary AlPb alloys, the concentration of enriched Pb in the subsurface layer was found to reach the order of 1 wt % independent of the bulk Pb content in the test range of 5-50 ppm, as revealed by quantitative glow discharge-optical emission spectroscopy ͑GD-OES͒. The maximum enrichment seemed to correspond to the metal-oxide interface. Mechanical polishing or caustic etching appeared to remove most of the lead from the surface. However, the etched surface of the commercial alloys was enriched again with le...

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The segregation of Pb on model binary AlPb alloys, containing 20 and 50 ppm Pb, as a result of heat-treatment in air at 600°C and its influence on electrochemical properties have been studied. Enrichment of metallic Pb, concentrated toward the oxide side of the oxide-metal interface, was confirmed by X-ray photoelectron spectroscopy. Transmission electron microscopy revealed a nearly continuous nanometer-scale Pb film at the oxide-metal interface. Significant anodic activation of the AlPb alloy surface in relation to pure Al in chloride media is attributed to the Pb film destabilizing the thermal oxide. The degree of activation was limited by the surface coverage of the film, and discrete Pb particles in the oxide did not contribute to the activation. After initiation at certain grain boundaries and discrete sites on grain bodies, corrosion in the active state spread nearly two-dimensionally as the Pb film on the corroded sites was destroyed as a result of corrosion, and corroded sites repassivated. The formation of the ␥-Al 2 O 3 thermal oxide during heat-treatment was thus crucial in the formation and existence of the Pb film wetting the metal surface.Electrochemical activation of certain commercial and model aluminum alloys resulting from high-temperature heat-treatment has been the object of significant attention recently because of its significance in filiform and galvanic corrosion, as reviewed in Ref. 1. The activation was attributed to the enrichment of trace element Pb at the surface as a result of high-temperature heat-treatment. It was characterized by a significant shift in the pitting potential in the negative direction relative to the well-known pitting potential of pure aluminum in chloride solution. In addition, a high anodic current output was observed at potentials as low as −0.95 V SCE , where aluminum is normally expected to be passive.The degree of activation was suggested to be limited by the solid solubility of enriched lead in an aluminum surface sublayer with a fraction of a micrometer thickness. However, the existence and exact position of the postulated Pb layer could not be proven. Although the solid solubility of Pb in bulk aluminum is reported to be 0.2 wt % at the monotectic temperature 659°C, 2 the solid solution Pb concentration in the sublayer was reported to attain a level of about 1 wt % for specimens heat-treated at 600°C and water quenched. 1 The surface was passivated after the active sublayer was corroded either as a result of free exposure in an acidified chloride solution or by potentiostatic polarization in neutral chloride solution at potentials between −0.95 and −0.75 V SCE . The amount of aluminum corroded before surface repassivation corresponded well with the average thickness of the Pb enriched sublayer determined by glow discharge optical emission spectroscopy ͑GDOES͒.Increasing the annealing time by several hours, followed by water quenching, resulted in significant segregation of Pb at the surface. However, the electrochemical activation was nearly identical for...