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. IntroductionTitanium carbide (TiC) is an industrially important hard coating material that improves the performance of steel components in mechanical systems by resisting wear, limiting adhesive interactions with tribological counterfaces, and presumably decreasing the chemical breakdown of lubricants. 1,2 In previous work, the interfacial chemical properties of TiC have been probed on a fundamental level through the adsorption of several small molecules with clean TiC(100) surfaces under ultrahigh vacuum conditions. In these studies, this surface has shown significant chemical reactivity at low temperatures. Such reactivity is consistent with the view of transition metal carbidic phases having important catalytic properties. 3,4 For example, monolayer methanol, 5 water, 6,7 and oxygen 8 are dissociatively adsorbed at 150 K, forming products that are identifiable with surface spectroscopies. Such reactivity, however, can be less desirable for tribological interfaces if the breakdown of lubricant species results. The reaction of esters with carbides is of particular interest due to the use of ester-based lubricants in aerospace applications. 9 Work detailing the reaction of the simplest ester, methyl formate, on TiC at 150 K and higher has been recently published. 10 That work showed predominantly...

Trends in the Surface and Catalytic Chemistry of Transition-Metal Ceramics in the Deoxygenation of a Woody Biomass Pyrolysis Model Compound

“…The results of Kim et al for the dissociation of methyl formate over TiC(100) using HREELS showed that oxygen-terminated methoxyl groups bound preferentially to Ti sites and carbon-terminated methyl groups to carbon sites. 77 These results suggest that the nonmetal surface atoms of ceramics do play a direct role in surface reactions and may present special chemistry not accessible when using TMs.…”

ChemInform Abstract: Reaction Pathways of Biomass‐Derived Oxygenates over Metals and Carbides: From Model Surfaces to Supported Catalysts

“…FeAl(1 00)T PD, [146] XPS [146] methanol FeAl(110)T PD, [147] UPS, [147] XPS [147] methanol TiAl(1 00)T PD, [146] XPS [146] methanol TiAl TPD, [147] UPS, [147] XPS [147] methanol Co/Mo(110)T PD, [148] HREELS [148] methanol FeS 2 (1 00)T PD, [149] AES, [149] XPS, [149] HREELS, [149] PAX [149] methyl acetate Ni(111)T PD, [113] RAIRS [113] methyl formate W(1 00)T PRS [150] methyl formate C/W(1 00)T PRS [150] methyl formate TiC( 100)X PS, [151] HREELS, [151,152] TPD, [151] DFT [152] methyl formate VC (1 00)X PS, [151] HREELS, [151] TPD [151] methyl formate Ni(111)T PD, [113] RAIRS [113] phenol Pt(111)T PD, [153] HREELS, [153] XPS [153] propanal WC DFT, [154] TPD, [154] HREELS [154] propanal Mo 2 CD FT, [99] TPD [99] vinyl acetate Pd(111)R AIRS, [155] TPD, [155] DFT [155] 1-propanol Pt(111)D FT [156]…”