AVS2011 Session SS-ThM: Oxide Surface Structure & Reactivity

The reactions of methyl (CH₃) and methylene (CH₂) surface species formed from the dissociation of halogenated methanes have been studied with temperature programmed desorption over Cr₂O₃(0001). Methyl fragments undergo a rate-limiting dehydrogenation to methylene and produce methane (CH₄) and ethylene (CH₂=CH₂) as the primary products. Comparison to earlier results for the Cr₂O₃(1012) surface show that methyl dehydrogenation is a structure-insensitive reaction, with the barrier to dehydrogenation being insensitive to the surface cation coordination number and/or local Cr–O site pair geometry. The ethylene formed from the dehydrogenation of CH₃ is associated with the surface diffusion of methylene species and subsequent coupling (C-C bond formation) reactions.

Separate studies of the reaction of methylene show that surface diffusion and coupling to ethylene can occur at lower temperatures than required for CH₃ dehydrogenation. The diffusion-limited coupling of CH₂ appears to be a structure-sensitive reaction, with the barrier to surface diffusion about 25 kJ/mol higher on Cr₂O₃(0001) than the (1012) surface.

The ubiquity of the interface between water and oxide materials has prompted a tremendous activity to determine the adsorption mechanisms at the microscopic scale on crystalline surfaces of simple oxides. Among these, magnesium oxide MgO offers the advantage of having various morphologies of good crystalline quality, involving cleaved crystal surfaces, high surface area samples and supported films, all of these dominated by the low index (100) orientation. On bulk MgO, the fivefold coordinated atoms of the basal (100) surface do not dissociate isolated H₂O molecules. Conversely, H₂O is easily dissociated at low-coordinated sites such as steps and kinks. A puzzling case is the adsorption of H₂O on metal-supported MgO films in the submonolayer range of which coverage by OH groups has been estimated to 60 to 70% of a monolayer [1,2]. Similar OH coverages were obtained by aging freshly prepared films in the residual atmosphere of the vacuum chamber [1], although the observation was contradicted by the finding of a more modest effect that cast doubt on OH being the cause of aging [2]. The extraordinary uptake of OH groups was attributed to the peculiarities of the electronic properties of the thin supported MgO films [3]. However, density functional theory hardly supports this view. Little changes in both the electronic structure and the capacity to dissociate H₂O are predicted for monolayer-thick MgO(100) islands that, at variance with experiment, are only expected to dissociate H₂O molecules along their borders [4].

In an attempt to solve the discrepancies between experiments and between experiment and simulation, MgO films of different stoichiometry were grown on Ag(100) by reactive deposition of Mg in an O₂ partial pressure, prior to being exposed to H₂O vapor and/or aged in vacuum [5]. Films were observed by scanning tunneling microscopy (STM) and water uptake was analyzed by x-ray photoemission spectroscopy (XPS). The stoichiometry and, consequently, the chemical activity towards hydroxylation of the MgO(100) films, was shown to strongly depend on the O₂ pressure during the film growth. Oxygen-deficient films undergo dramatic oxygen uptake either by exposure to H₂O or by aging in vacuum. Conversely, on stoichiometric MgO islands, XPS analysis and STM images are consistent with the prediction that H₂O only dissociates at the island edges.

[1] S. Altieri et al., Phys. Rev. B. 76 (2007) 205413.

[2] L. Savio et al., J. Phys. Chem. B 108 (2004) 7771.

[3]S. Altieri et al., Thin Solid Films 400 (2001) 9.

[4] A. M. Ferrari et al., Phys. Chem. Chem. Phys. 9 (2007) 2350.

Noble metals supported on oxide surfaces are of interest due to their catalytic activity and their sintering resistance is a subject of concern in terms of their long-term stability under reaction conditions. The heat of adsorption of silver on Fe₃O₄(111) grown on Pt(111) was measured using adsorption microcalorimetry, and its growth morphology was measured with low-energy ion scattering spectroscopy (LEIS) and Auger electron spectroscopy (AES). The initial sticking was 0.96 rising to 0.99 after 1 monolayer of Ag deposited. The LEIS and AES data were fitted to a hemispherical cap model for the Ag nanoparticles, which were found to grow with a particle density of 4 x 10¹² particles per cm². The initial heat of adsorption was found to be 220 kJ/mol, rising to 285 kJ/mol after 1 monolayer of deposited Ag. From these data, we extracted the energy of metal atoms versus the Ag particle size to which they attach on Fe₃O₄(111), and compare it to results on other single crystalline oxide surfaces.

Metal oxide spinels are an important class of materials in both ceramics technology and materials science. Although the prototypical ternary metal oxide spinel, magnesium aluminate spinel (MgAl₂O₄), is widely used, for instance, as a membrane in solid oxide fuel cells and in heterogeneous catalysis, either as a support for active metal nanoclusters or as a catalyst in its own right [1], many of the intricate details of its surface structure still remain unresolved. The atomic-scale surface characterization of similar spinel-type metal oxides has generally been challenging, due to the insulating nature of these materials, a property which limits the use of many standard surface science techniques.

Using a combination of non-contact atomic force microscopy (NC-AFM) and surface X-ray diffraction (SXRD), coupled together with density functional theory (DFT) structure calculations and NC-AFM simulations based on DFT, we have unraveled the complex structure of the polar MgAl₂O₄ (100) surface. Surprisingly, we find that the surface is terminated by an Al and O-rich structure, with a thermodynamically favored amount of Al atoms interchanged with Mg. These cation antisites, which are low-density defects in the bulk, may be a key element in determining the substrate properties of MgAl₂O₄, among others its basicity and the likelihood for formation of OH-groups, which are believed to be anchoring sites for metallic nanoclusters, such as Ni, commonly used in steam reforming catalysts.

[1] J. R. Rostrup-Nielsen, J. Sehested, and J. K. Nørskov, Adv. Catal. 47, 65 (2002).

We have used low energy electron microscopy (LEEM) to study in real time the growth of iron oxides on the fully oxidized and partially reduced (001) and (111) surfaces of yttria‑stabilized zirconia (YSZ). The FeO_x-YSZ system is currently used as a working material for solar thermochemical splitting of H₂O and CO₂ [1], but little fundamental information is available concerning the structure and composition of the mixed oxides and their surfaces. Upon Fe deposition in ~10^-6 Torr of O₂ background pressure, iron oxides grow on the surface. Low energy electron diffraction (LEED) patterns, dark field LEEM imaging, and LEEM I‑V measurements show that the composition and morphology of the surface oxide is very diverse, and depends on the substrate stoichiometry, crystallographic orientation, as well as on the deposition conditions and film thickness.

This work was supported by the LDRD program at Sandia National Laboratories, in the form of a Grand Challenge project entitled Reimagining Liquid Transportation Fuels: Sunshine to Petrol. Sandia is a multi‑program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy’s NNSA under contract DE‑AC0494AL85000.

[1] Diver, R.B., Miller, J.E., Allendorf, M.D., Siegel, N.P., Hogan, R.E., “Solar thermochemical water‑splitting ferrite‑cycle heat engines”, Journal of Solar Energy Engineering, 130 (2008) 041001