In situ ultraviolet Raman spectroscopy of the reduction of chromia on alumina catalysts

A fluidized bed in situ cell is used to examine the chromia on alumina catalyst surface. The loading is varied from 1 to 20% chromium oxide on alumina. The structure of the surface is monitored as a function of temperature, reduction, and hydration. This is the first study to use ultraviolet excitation to study a reduced chromium catalyst surface. Reduction of the sample in hydrogen gas leads to the formation of noncrystalline Cr(3+) species. Addition of potassium to the surface reduces the size of the chromia clusters.     

Displacement of hexanol by the hexanoic acid overoxidation product in alcohol oxidation on a model supported palladium nanoparticle catalyst

This work characterizes the adsorption, structure, and binding mechanism of oxygenated organic species from cyclohexane solution at the liquid/solid interface of optically flat alumina-supported palladium nanoparticle surfaces prepared by atomic layer deposition (ALD). The surface-specific nonlinear optical vibrational spectroscopy, sum-frequency generation (SFG), was used as a probe for adsorption and interfacial molecular structure. 1-Hexanoic acid is an overoxidation product and possible catalyst poison for the aerobic heterogeneous oxidation of 1-hexanol at the liquid/solid interface of Pd/Al(2)O(3) catalysts. Single component and competitive adsorption experiments show that 1-hexanoic acid adsorbs to both ALD-prepared alumina surfaces and alumina surfaces with palladium nanoparticles, that were also prepared by ALD, more strongly than does 1-hexanol. Furthermore, 1-hexanoic acid adsorbs with conformational order on ALD-prepared alumina surfaces, but on surfaces with palladium particles the adsorbates exhibit relative disorder at low surface coverage and become more ordered, on average, at higher surface coverage. Although significant differences in binding constant were not observed between surfaces with and without palladium nanoparticles, the palladium particles play an apparent role in controlling adsorbate structures. The disordered adsorption of 1-hexanoic acid most likely occurs on the alumina support, and probably results from modification of binding sites on the alumina, adjacent to the particles. In addition to providing insight on the possibility of catalyst poisoning by the overoxidation product and characterizing changes in its structure that result in only small adsorption energy changes, this work represents a step toward using surface science techniques that bridge the complexity gap between fundamental studies and realistic catalyst models.     

The surface reconstructions of the (100) crystal faces of iridium,platinum and gold: I. Experimental observations and possible structural models

The structures of the reconstructed Ir(100), Pt(100) and Au(100) surfaces have been investigated. Low energy electron diffraction (LEED) patterns are analyzed and LEED intensity versus energy data are measured. A variety of structures is observed by LEED: Ir(100) exhibits a relatively simple (1 × 5) pattern; Pt(100) shows a series of closely related patterns, a typical representative of which has a $\text{(}\text{14}\text{1}\text{?1}\text{5}\text{)}$ structure; Au(100) usually exhibits a c(26 × 68) pattern, often inaccurately described in the literature as a (20 × 5) pattern. The reconstruction of Au(111) is also considered for comparison. Various plausible structural models are discussed, while laser simulation is used to lessen the number of these models. The analysis is completed in a companion paper where LEED intensity calculations are reported to determine the atomic locations.     

Imaging of atomic layer deposited (ALD) tungsten monolayers on alpha-TiO2(110) by X-ray standing wave Fourier inversion

A single atomic layer of tungsten grown by atomic layer deposition (ALD) on a single-crystal rutile TiO2(110) support is studied by the X-ray standing wave (XSW) technique. The surface structural and chemical properties were also examined using atomic force microscopy, X-ray photoelectron spectroscopy, and low-energy electron diffraction. The XSW measured set of hkl Fourier components for the W atomic distribution function are summed together to produce a model-independent 3D map of the W atoms relative to the rutile lattice. The 3D atomic image shows surface tungsten atoms equally occupying the two nonequivalent Ti sites with a slight outward displacement. This corresponds to the atop and bridge sites with respect to the underlying lattice oxygen atoms. These XSW measurements clearly show that ALD conformal layers can be highly coherent with respect to the substrate lattice.     

Structure of Mg2.56V1.12W0.88O8 and vibrational raman spectra of Mg2.5VWO8 and Mg2.5VMoO8

Mg(2.56)V(1.12)W(0.88)O(8) crystals were grown from a MgO/V(2)O(5)/WO(3) melt. X-ray single-crystal diffraction studies revealed that it is orthorhombic with space group Pnma, a = 5.0658(5) A, b = 10.333(1) A, c = 17.421(2) A, Z = 6, and is isostructural with Mg(2.5)VMoO(8). Raman spectra are reported, and the assignment of the Raman bands is made by comparing the metal-oxygen vibrations of VO(4)/WO(4) tetrahedra in Mg(2.5)VWO(8) with the metal-oxygen vibrations of VO(4)/MoO(4) tetrahedra in Mg(2.5)VMoO(8). The stretching vibrations appearing at 1016 and 1035 cm(-)(1) are assigned to Mo=O and W=O double bonds, respectively, associated with the Mg(2+) cation vacancies.