Surface Chemistry of Trimethylaluminum on Pd(111) and Pt(111)

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• 作者：Amir Gharachorlou ; Michael D. Detwiler ; Lukas Mayr ; Xiang-Kui Gu ; Jeffrey Greeley ; Ronald G. Reifenberger ; W. Nicholas Delgass ; Fabio H. Ribeiro ; Dmitry Y. Zemlyanov
• 刊名：Journal of Physical Chemistry C
• 出版年：2015
• 出版时间：August 20, 2015
• 年：2015
• 卷：119
• 期：33
• 页码：19059-19072
• 全文大小：805K
• ISSN：1932-7455

文摘

The behavior of trimethylaluminum (TMA) was investigated on the surfaces of Pt(111) and Pd(111) single crystals. TMA was found to dissociatively adsorb on both surfaces between 300鈥?73 K. Surfaces species observed by high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS) after TMA adsorption at 300 K included Al-CH₃ and CH_x,ads (x = 1, 2, or 3) on Pt(111), and ethylidyne (CCH₃), CH_x,ads (x = 1, 2, or 3), and metallic Al on Pd(111). Density functional theory (DFT) calculations predicted methylaluminum (MA, Al-CH₃) to be the most kinetically favorable TMA decomposition product on (111) terraces of both surfaces, however, HREELS signatures for Al-CH₃ were detected only on Pt(111), whereas ethylidyne was observed on Pd(111). XPS demonstrated higher amounts of carbonaceous species on Pt(111) than on Pd(111). DFT calculations showed that further dissociation of MA to metallic aluminum and methyl groups to be more kinetically favorable on step sites of both metals. In our proposed reaction mechanism, MA migrates to and dissociates at Pd(111) steps at 300 K forming adsorbed methyl groups and metallic Al. Some methyl groups dehydrogenate and recombine forming ethylidyne. Metallic Al or ejected Pd atoms from steps diffuse across Pd(111) terraces until coalescing into irregularly shaped islands on terraces or steps, as observed by scanning tunneling microscopy (STM). Upon heating above 300 K, the Pd鈥揂l alloy diffuses into the Pd bulk. On Pt(111), a high coverage of carbon-containing species following TMA adsorption at 300 K prevented MA diffusion and dissociation at steps, as evidenced by isolated clusters of MA in STM images. Heating above 300 K resulted in MA dissociation, but no Pt鈥揂l alloy formation was observed. We conclude that the differing abilities of Pd and Pt to hydrogenate carbonaceous species plays a key role in MA dissociation and alloy formation, and therefore, the adsorption and dissociation chemistry of TMA depends on properties of the metal substrate surface and determines thin film morphology and composition.