Probing the Low-Temperature Water鈥揋as Shift Activity of Alkali-Promoted Platinum Catalysts Stabilized on Carbon Supports
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- 作者：Branko Zugic ; Shiran Zhang ; David C. Bell ; Franklin (Feng) Tao ; Maria Flytzani-Stephanopoulos
- 刊名：Journal of the American Chemical Society
- 出版年：2014
- 出版时间：February 26, 2014
- 年：2014
- 卷：136
- 期：8
- 页码：3238-3245
- 全文大小：532K
- 年卷期：v.136,no.8(February 26, 2014)
- ISSN：1520-5126
文摘
We report on the direct promotional effect of sodium on the water鈥揼as shift activity of platinum supported on oxygen-free multiwalled carbon nanotubes. Whereas the Na-free Pt catalysts are shown to be completely inactive, the addition of sodium is found to improve the water鈥揼as shift activity to levels comparable to those obtained with highly active Pt catalysts on metal oxide supports. The structure and morphology of the catalyst surface was followed using aberration-corrected HAADF-STEM, which showed that atomically dispersed platinum species are stabilized by the addition of sodium. In situ atmospheric-pressure X-ray photoelectron spectroscopy (AP-XPS) experiments demonstrated that oxidized platinum Pt鈥揙Hx contributions in the Pt 4f signal are higher in the presence of sodium, providing evidence for a previously reported active-site structure of the form Pt鈥揘ax鈥揙y鈥?OH)z. Pt remained oxidized in all redox experiments, even when a H2-rich gas mixture was used, but the extent of its oxidation followed the oxidation potential of the gas. These findings offer new insights into the nature of the active platinum-based site for the water鈥揼as shift reaction. A strong inhibitory effect of hydrogen was observed on the reaction kinetics, effectively raising the apparent activation energy from 70 卤 5 kJ/mol (in product-free gas) to 105 卤 7 kJ/mol (in full reformate gas). Increased hydrogen uptake was observed on these materials when both Pt and Na were present on the catalyst, suggesting that hydrogen desorption might limit the water鈥揼as shift reaction rate under such conditions.