Electrochemical Preparation And High Performance Of Platinum And Palladium Nanocatalysts With High-Index Facets

Platinum-group metal nanomaterials are widely used as catalysts applied in fuel cells, petrochemical industry and other important fields. The key scientific and technological issue of platinum catalysts is to further improve their activity, stability and utilization efficiency. Fundamental studies of single-crystal model catalysts have demonstrated that high-index planes of platinum-group metals exhibit generally much higher catalytic activity and stability than those of the low-index planes, such as {111}, {100}, and even {110}, because the high-index planes have a high density of atomic steps, ledges, and kinks, which usually serve as active sites for breaking chemical bonds. Thus, the shape-controlled synthesis of platinum-group metal nanocrystals bounded by high-index facets is a promising route for enhancing their catalytic activity and stability. It is, however, rather challenging to synthesize nanocrystals that are enclosed by high-index facets because of their high surface energy. Only low-index facets, such as {111} and {100}, bounded nanocrystals of platinum-group metals have been obtained by conventional synthesis methods.In this thesis, we have developed an electrochemical method to control the surface structure and growth of metal nanocrystals, and prepared successfully nanocatalysts of platinum, palladium bounded by high-index facets. The study has illustrated the synthesized nanocatalysts exhibit high activity and stability. The main results are as following:1. Tetrahexahedral (THH) Pt nanocrystals have been firstly prepared by the developed electrochemical method. The surface of the THH Pt nanocrystals is identified as {730} and {520} high-index facets by using HRTEM and SEM. The THH Pt nanocrystals exhibit 2～4 times higher catalytic activity than that of commercial Pt/C catalysts towards electrocatalytic oxidation of formic acid and ethanol in terms of current density. The THH Pt nanocrystals show also high chemical and thermal stability, and can maintain their shape and facets up to 800℃. It has confirmed that the key reason for the growth of THH Pt nanocrystals consists in that the high-index facets of Pt possess a higher stability than low-index facets under periodic oxidation-reduction conditions generated by square-wave potential. These above results were published in Science (2007, 316: 732-735), and were evaluated as "a breakthrough in the synthesis of nanoscale catalysts" (Science, 2007, 316: 699-700). Applying the same method, we have also prepared successfully THH, trapezohedral and concaved hexaoctahedral Pd nanocrystals and concaved hexaoctahedral Pt nanocrystals.2. Five-fold twinned Pt nanorods bounded by high-index facets of {hk0} were prepared through the developed electrochemical method. Unlike the shape of common five-fold twinned nanorod, the tips of the Pt nanorods are of decagon-based pyramidal shape, and the side surfaces contain a series of concavo-convex subfacets. We have observed for the first time the split of five-fold twinned nanorods at the twined boundaries.3. Five-fold twinned Pd nanorods were prepared by square-wave potential electrodeposition. It has been demonstrated that the shape and surface structure of Pd nanorods can be altered by varying the lower (E₁) and upper (E_u) limit of the square-wave potential. Pd nanorods grown at relatively low E_u show two ends of pentagonal pyramid shape bounded by high-index facets of {hkk}; while nanorods grown at high E_u give two ends of decagon-based pyramidal shape bounded by high-index facets of {hk0}. It has shown that the Pd nanorods exhibit higher catalytic activity than commercial platinum black catalysts towards the electro-oxidation of ethanol in alkaline solution.4. Pt nanothorns were prepared by square-wave potential electrodeposition. The Pt nanothorns exhibit about two times catalytic activity as high as mat of commercial Pt black towards the electro-oxidation of ammonia. The Pt nanothorns have a very high surface enhanced Raman scattering (SERS) activity. An enhancement factor of 2000 for adsorbed pyridine was measured, which is about one order of magnitude larger than that obtained from electrochemically roughened platinum electrodes. The Pt nanothorns also show a high infrared enhancement: IR band intensity (peak height) of CO adsorbed on Pt nanothorns is as high as 3%, yielding an enhancement factor of 17.By developing the electrochemical method in this thesis to control surface structure and growth of metal nanocatalysts, we have prepared successfully not only platinum and palladium tetrahexahedral nanocatalysts, but also other nanocrystals of various shape enclosed by different high-index facets. The current study has enriched the contents of surface structure controlled growth of metal nanocrystals and has deepened the understanding of the growth habits of metal nanocrystals. The as-prepared platinum-group nanoctalysts enclosed by high-index facets exhibit high activity and stability, which have opened an exciting avenue to improve the performance of metal nanocatalysts by means of controlling surface atomic arrangement, and made a big progress in the design and preparation of practical catalysts guided by knowledge gained from fundamental studies of single-crystal model electrocatalysts.