Substrate-Dependent Reactivity of Water on Metal Carbide Surfaces

Didziulis, Stephen V.; Frantz, Peter; Perry, Scott S.; Elbjeirami, Oussama; Imaduddin, S.; Merrill, Philip B.

doi:10.1021/jp9923668

Cited by 46 publications

(61 citation statements)

References 38 publications

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“…Region 1 has a very similar spectral shape to the carbides presented in Figure 42, with the only difference that the shoulder between -0.5 and 0 V disappearing after oxidation. This is consistent with the evolution towards WO 3 Figure 52b) relative to the 50/50 surface oxidized at a lower temperature (green areas in Figure 50b). Some fraction of the carbide was protected from oxidation by graphite until being exposed by graphite Figure 53 shows the STS map of a representative region.…”

Section: Oxidation Of Codeposited Samplessupporting

confidence: 88%

“…The spectra in region 1 have transitioned to a state with an LDOS much closer to that observed for WO 3 . This transition seems to evolve gradually with the negative polarity (filled states) side of the dI/dV curve approaching the x-axis while the empty states remain relatively static.…”

Section: Section 45 Oxidation Of Annealed C60 On W/mgo(100)supporting

confidence: 58%

“…It is possible that this variation could be due to the thickness of the WO 3 however. Figure 55 shows a representative region of the surface with a mobile cluster on the left and a group of islands.…”

Section: Oxidation Of Codeposited Samplesmentioning

confidence: 99%

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Synthesis and Oxidation of non-Stoichiometric Tungsten Carbide Studied by Scanning Tunneling Microscopy/Spectroscopy

McClimon¹

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Tungsten Carbide (WC) is a promising catalyst material for applications where cost is paramount, such as microbial fuel cells. Unfortunately, oxidation depresses performance which is a critical problem with respect to long term performance. In this work, non-stoichiometric carbon-rich WC is investigated to infer whether an excess of carbon near the surface can reduce or be utilized to repair oxidation of the active catalytic surface. The carbides are synthesized via a physical vapor deposition of metallic tungsten and C 60 , which allows fine control over the composition of the resulting film. The films are studied predominantly via Scanning Tunneling Microscopy/Spectroscopy (STM/STS).Prior to synthesizing the carbide, a number of RT depositions are performed onto graphite substrates to understand the interactions between the C 60 and W prior to thermallyinduced destruction of the C 60 cage and formation of the carbides. These experiments showed a surprising lack of interaction between small W clusters and adjacent C 60 molecules. This result contrasts with literature of analogous experiments showing significant charge transfer from W to C 60 and strong interaction as a result. For W deposited on top of C 60 layers, it is found that isolated W atoms diffuse easily into the interstices of the C 60 matrix and that for high coverages, interactions between the W and C 60 induce a significant reduction in the C 60 bandgap. These interactions can also stabilize very small islands of C 60 on graphite that otherwise require hundreds of molecules before a stationary C 60 island can form.Intermediate annealing at 400-500C of C 60 on epitaxial W/MgO(100) produces novel nanostructures with a very similar size and shape to C 60 but with metallic conductivity, a unique atomic-scale stripe pattern on their surface, occasional mobility under the influence of the STM iv tip, and show signs of evolving towards WO 3 under oxidation. These structures increase in diameter and slowly dissolve into the W surface under increasing annealing temperatures.Carbide thin films are synthesized by codeposition onto MgO substrates held above 600˚C. These thin films are determined to be predominantly metastable, cubic WC 1-x phase. At a C/W ratio of 60/40 and above, carbon is observed to surface segregate and form graphite on the surface of the film. At a ratio of 60/40 the surface is heavily populated with graphene, rather than graphite.Oxidation of the films at elevated temperatures in UHV shows that morphological changes, even at atomic resolution, were absent. STS shows that signs of oxidation overspread the surface immediately but that WO 3 nucleates well-defined islands which grow in size with harsher oxidation conditions. For the graphite covered surfaces, graphite appears to protect the underlying carbide from oxidation, and between 300 and 400˚C, O 2 begins to etch the graphite.The underlying carbide appears to oxidize similarly to films without a graphitized surface.Overall, the WC films oxidize much less rapidly than W cluste...

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Section: Oxidation Of Codeposited Samplessupporting

confidence: 88%

Section: Section 45 Oxidation Of Annealed C60 On W/mgo(100)supporting

confidence: 58%

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Synthesis and Oxidation of non-Stoichiometric Tungsten Carbide Studied by Scanning Tunneling Microscopy/Spectroscopy

McClimon¹

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“…6 The formaldehyde species discussed above is calculated to have an adsorption energy 4 kJ/mol more stable than that of CO. Given that the molecular methyl formate desorbs from the surface at temperatures between 120 and 200 K and CO desorption peaks at approximately 150 K, one would expect that formaldehyde would desorb well below room temperature, implying that the observed evolution of formaldehyde at higher temperatures is reaction limited.…”

Section: Results and Analysismentioning

confidence: 99%

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

Didziulis

Kim

2007

J. Phys. Chem. C

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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...

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“…Recently, it was reported that these properties were related to crystallographic textures and/or orientations of TiC films [13][14][15][16]. Therefore, much interest has been focused on crystallographic orientation and epitaxial growth of TiC.…”

Section: Introductionmentioning

confidence: 99%

Carbonizing Processes of Titanium Thin Films by Carbon-Implantation

Nishida

Wang

Yamamura

et al. 2005

e-J. Surf. Sci. Nanotechnol.

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Observations of the crystallographic structural changes of deposited Ti films (100 nm thick) during Cimplantation were made by using transmission electron microscope (TEM) for the purpose to elucidate the carbonizing processes of Ti films. The Ti films were deposited by an electron beam heating method onto thermally cleaned NaCl (001) surfaces held at room temperature (RT). Both the 200 kV conventional TEM and the 400 kV analytical high resolution TEM combined with ion accelerators were used. The deposited Ti films consisted of mainly (03 · 5)-oriented hcp-Ti and (110)-oriented CaF2 type TiHx (x ∼ 1.5). Carbon ions (C + ) with 26 keV were implanted into the Ti films held at RT in the 400 kV TEM. The H atoms which constituted TiHx were released from the as-deposited Ti films during the early stage of C-implantation. In the C-implanted Ti film (C/Ti=0.54), there coexisted (001)-and (110)-oriented NaCl-type TiCy and a small amount of hcp-Ti. The results of TEM observation indicated that (001)-and (110)-oriented TiCy were epitaxially formed by the transformation of (03 · 5)-oriented hcp-Ti and (110)-oriented TiHx, respectively. In the cases, the hcp-Ti sublattice is transformed into fcc-Ti sublattice of TiCy with inheritance of square atomic arrangement of hcp-Ti. The hcp-fcc transformation was discussed with the results of a self-consistent charge discrete variational (DV)-Xα molecular orbital (MO) calculation.

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Substrate-Dependent Reactivity of Water on Metal Carbide Surfaces

Cited by 46 publications

References 38 publications

Synthesis and Oxidation of non-Stoichiometric Tungsten Carbide Studied by Scanning Tunneling Microscopy/Spectroscopy

Synthesis and Oxidation of non-Stoichiometric Tungsten Carbide Studied by Scanning Tunneling Microscopy/Spectroscopy

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

Carbonizing Processes of Titanium Thin Films by Carbon-Implantation

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