| Summary: | 博士 === 元智大學 === 化學工程與材料科學學系 === 105 === Part I
This study adopts a modified atomic layer deposition (ALD) process to prepare size-controlled Platinum (Pt) nanoparticles over the surface of carbon black, showing superior catalytic activity toward ethanol oxidation. Two types of ALD precursors, (methylcyclopentadienyl) trimethyl platinum (MeCpPtMe3) and oxygen (O2), were used to grow Pt deposits at 250°C. For 30 ALD cycles, the pulse period of MeCpPtMe3 serves as a key factor in controlling the particle size and the weight loading of Pt deposits. The Pt growth rates over the carbon support can be attributed to the surface coverage of Pt-O* sites, diffusion rate of MeCpPtMe3, and lateral interaction between each active site. Since the MeCpPtMe3 dose strongly affects the Pt particle size and the deposit density, the growth of ALD-Pt can be taken into account as diffusion-limiting. Due to its surface-catalyzed reaction steps, the small-sized ALD-Pt catalysts offer better catalytic activity, CO tolerance, and long-term stability as compared with the large-sized ones. On the basis of the results, the modified ALD technique exhibits a great potential for tuning the Pt particle size and weight loading onto the carbon support for fuel cell application.
Part II
The present work adopts an ALD technique to synthesize highly-crystalline Pt nanoparticles onto carbon powders, offering superior catalytic activity toward methanol oxidation within the temperature range of 25-55°C. Uniformly-dispersed Pt nanoparticles with an ultralow loading are coated over the carbon supports, served as catalyst materials for methanol electro-oxidation. Experimental results reveal that ALD-Pt catalyst offers not only an improved catalytic activity toward methanol oxidation but also superior CO tolerance, as compared to commercial Pt one. The decreased current ratio for direct to indirect pathway with an increase in temperature is found, referring to the kinetic limitations for the formation and oxidation of Pt-(CO)ads sites at high temperatures. Followed up Arrhenius-type behavior, small apparent activation energies (i.e., ca. 30.3 and 41.7 kJ mol-1) of ALD-Pt catalyst can be achieved for dehydrogenation of methanol (direct pathway) and oxidation of adsorbed CO species (indirect pathway) in methanol oxidation reaction. The low potential barrier on ALD-Pt catalyst is attributed to small particle size (i.e., average particle size of 2.1 nm) and oxidized Pt surface (i.e., native Pt-O* active sites) that efficiently enhance the catalytic activity and CO tolerance, respectively. As a result, this study examines the influence of temperature on catalytic activity and anti-poisoning performance on the ALD-Pt catalyst, in which the surface chemistry and structural motif is more efficient at electrochemically oxidizing methanol and improving the CO tolerance.
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