Douglas P. Hoffmann

EXAFS Smoothline Determination Using Splines: Finding the Optimal Smoothline and Fast Computation

The feasibility of the PC IMSL cubic spline smoothing routine CSSMH (ICSSCU in the mainframe edition) for numerical determination of the EXAFS smoothline (μo) is discussed. Examination of the cubic spline algorithm employed suggests that a relatively simple coding modification results in a considerably more efficient algorithm for use in EXAFS. The rudimentatary code already exists in the literature. This allows incorporation of the modified code into current programs without the large memory overhead necessary when IMSL is being used. The degree of flexibility is determined by a parameter which can have values in the range 0 to 1. The EXAFS smoothline requires that the algorithm be used at one extreme end of this range of flexibility. At the other end of this range the routine can be used for interpolation. A suggested path for finding the optimal smoothline, based on the use of coarse and fine smoothing parameters, is discussed.

Spatially Resolved ESCA Using Hadamard Masks

Use of Hadamard-based movable masks is demonstrated to be an effective method for obtaining spatially resolved information from x-ray photoelectron spectroscopy (XPS or ESCA). The concept of such imaging in electron spectroscopy is discussed in conjunction with the results obtained from a relatively simple mask system which was attached to a modified AEI ES200 photoelectron spectrometer. Resolution on the order of 1 mm2 was obtained with this configuration. The ultimate spatial resolution, on the order of microns, depends on engineering limitations concerned primarily with mask design and fabrication. The advantage of such a system stems from its inherent multiplexing nature, the requirements for only a single-channel detector, and the associated possibility of retro-fitting currently existing instrumentation.

Characterization of boron-modified Co/Al2O3 catalysts by EXAFS spectroscopy

EXAFS spectroscopy has been used to examine the effect of boron on the structure of Co/Al2O3 catalysts. The EXAFS results from unmodified Co/Al2O3 catalysts indicated that Co3O4 was formed at Co loadings of 1.5 wt % Co and higher. The lowest-loading (0.7 wt % Co) catalyst showed evidence of both tetrahedral and octahedral Co surface species. Modification of the Co/Al2O3 catalysts with 3 wt % B completely suppressed the formation of Co3O4 for Co loadings of 6 wt % Co and lower. In the absence of Co3O4, the Co species formed on the 3 wt % boron-modified catalysts were in a highly disordered state with large first shell Co-O coordinations and large Co-O nearest-neighbor distances. Results from a constant Co loading series (3 wt % Co) indicated that Co3O4 formation decreased as the boron loading increased. The distribution of supported Co species determined by EXAFS was compared with previous characterization by ESCA and gravimetric measurements; good agreement was found. A two-phase procedure was used to analyze the EXAFS data from the Co/B/Al2O3 catalysts. This two-phase analysis considered both the fraction of Co EXAFS from bulk Co oxide (Co3O4) and the remaining fraction from Co surface species. This allowed the amount of Co3O4 to be determined quantitatively and enabled structural information to be obtained from the Co surface phase.

Quantitative Characterization of Fe/Al2O3 Catalysts. Part I: Oxidic Precursors

The structure of a series of Fe/Al2O3 catalysts containing from 1 to 24 weight percent iron has been examined by XRD, EXAFS, Mössbauer, ESCA, and gravimetric analysis. Two iron phases, Fe2O3 and Fe+3 in solid solution in the alumina support, are characterized as a function of iron loading. The combination of ESCA H2 titration and gravimetric analysis allows both quantitative and qualitative measurement of the two species. The amount of Fe2O3 phase increases with increasing iron loading for Fe1 to Fe24, whereas the amount of Fe+3 solid-solution phase increases with increasing iron loading for Fe1 to Fe6 and then levels off to approximately 2.2 wt % Fe for Fe8 to Fe24. The particle size of the Fe2O3 phase, determined from ESCA measurements, increases with increasing iron loading.

Quantitative Characterization of Fe/Al2O3 Catalysts. Part II: Reduction, Sulfidation, and CO/Hydrogenation Activity

ESCA, Mössbauer spectroscopy, XRD, and CO chemisorption were used to study the reduction and sulfidation reactions of a series of 1 to 24 wt % Fe/Al2O3 catalysts. The speciation and particle size of the active phase were correlated with CO hydrogenation activity data. Two phases were previously identified in all oxidic catalysts: Fe2O3 and Fe+3 in solid solution with the alumina support. The Fe2O3 phase was found to reduce to Fe0 and sulfide to Fe1-xS. For the reduced and sulfided catalysts, Mössbauer was able to identify two iron species which were detected as a single solid solution species in the oxidic catalysts. The two species were found to differ by their location in the alumina support. One species is incorporated within the alumina matrix [Fe+2(A)] and the other species [Fe+2(B)] is present at the alumina surface. Both ESCA and CO chemisorption indicate that the Fe particle size increases with increasing iron loading. The turnover frequency (TOF) for CO hydrogenation appears to be a function of the extent of reduction and particle size of the metallic iron phase.

Application of cross-correlation analysis to EXAFS: Investigation of the cobalt phase on boron modified cobalt/alumina catalysts

Boron modified Co/Al2O3 catalysts, previously characterized by ESCA, are examined by EXAFS. A method of data analysis based on cross-correlation of the χ(E) spectra is used to determine the relative amounts of Co3O4 and cobalt surface phase present on the catalysts. The results are compared with those obtained previously by ESCA. The EXAFS analysis agrees well with the ESCA results. No gas-phase reactions were required to elucidate the relative proportion of cobalt species by EXAFS as is necessary for the ESCA analysis.

EXAFS characterization of Ti/Al2O3 supports and Co/Ti/Al2O3 catalysts

The structure of Ti/Al2O3 supports (0–14 wt% Ti) and Co/Ti/Al2O3 catalysts (3 wt% Co) was examined by EXAFS. The results indicated that the Ti was present primarily as a highly dispersed surface phase. The Ti EXAFS results indicated that the Ti species were octahedrally coordinated. Evidence of Ti—Ti interactions was found for all loadings (2–14 wt% Ti) suggesting that the Ti surface species are present as small clusters of TiO2.The Co EXAFS results showed evidence for several structurally different Co surface phases as a function of Ti loading. Evidence of a Co species interacting with the Ti surface phase was observed for the 3% Co/2% Ti-3%Co/6%Ti catalysts. At the highest loadings studied, 3%Co/8%Ti and 3%Co/14%Ti, evidence was found for a CoTiO3-like phase.