Chemistry of Methyl Formate with TiC(100): Comparison of Experiment with Density Functional Calculations

Abstract

Titanium carbide is an important tribological material used as an antiwear coating on mechanical components. The TiC(100) surface has demonstrated interesting low-temperature chemistry with a variety of adsorbates, including methyl formate, the simplest ester and a model for this chemical functionality found in many aerospace lubricants. In an effort to identify and characterize surface adsorption and reaction products, we have undertaken a comparison of theoretical results employing density functional methods with experimental results using high-resolution electron energy loss spectroscopy (HREELS) obtained on clean, single-crystal TiC surfaces. The DFT results presented in this work have enabled the identification of an elusive surface reaction product with a characteristic vibration of 1120 cm-1 as a formyl (CHO) species bridging between a surface carbon and a titanium site. This result and others have demonstrated the importance of the lattice carbidic carbon atom in the adsorption of the breakdown products of methyl formate. Specifically, any atomic adsorbate and every reaction product with an unsatisfied valence on a carbon atom showed a theoretical preference for bonding with a lattice carbon rather than the titanium. Alternatively, reaction fragments containing oxygen preferred bonding to surface titanium sites through the oxygen atom, with the exception of carbon monoxide. The calculations show that the breakdown of methyl formate to methoxy and formyl groups, or to surface formate and methyl groups, is exothermic, while further decomposition reactions are endothermic. These results provide a rationale for the experimental observation of these surface reaction products at temperatures ranging from 150 to 300 K.