George D. Meitzner

Selective isomerization of alkanes on supported tungsten oxide acids

Abstract Tungsten oxide species form strong acid sites on ZrO 2 supports. After calcination at 1000–1100 K and promotion with Pt, these solids catalyze C 7+ alkane isomerization at 400–500 K with much higher selectivity than sulfated oxides or zeolitic acids at similar turnover rates. Alkane isomerization proceeds via biomolecular reactions involving hydrogen transfer from alkanes or H 2 , which cause the desorption of isomeric carbocations before β-scission occurs. On Pt/SO x -ZrO 2 , carbocation desorption is slow, leading to long surface residence times and extensive cracking. On Pt/WO x -ZrO 2 , carbocation desorption is rapid and surface isomerization steps limit n-heptane isomerization turnover rates. Saturation coverage by WO x surface species inhibits ZrO 2 sintering and its tetragonal to monoclinic structural transformation. High isomerization turnover rates appear to require the presence of WO x clusters on ZrO 2 surfaces. X-ray absorption at the W-L 1 and W-L III edges suggests the predominant presence of distorted octahedral species, even after dehydration at 673 K, in all WO x -ZrO 2 samples calcined at 1073 K. Tetrahedral species, which lead to a strong pre-edge feature in the W-L 1 absorption edge, are not detectable in these samples. UV-visible spectra suggest an increase in WO x domain size with increasing loading. These distorted octahedral WO x domains on ZrO 2 differ markedly in structure, reduction rates, and alkane isomerization turnover rates and selectivities from tetrahedral WO x species on Al 2 O 3 .

Structural analysis of unpromoted Fe-based Fischer–Tropsch catalysts using X-ray absorption spectroscopy

The structure of unpromoted precipitated Fe catalysts was determined by Mossbauer emission and X-ray absorption spectroscopies after use in the Fischer–Tropsch synthesis (FTS) reaction in well-mixed autoclave reactors for various periods of time. X-ray absorption near-edge spectroscopy (XANES), extended X-ray absorption fine structure (EXAFS) analysis, and Mossbauer spectroscopy showed consistent trends in the structural evolution of these catalysts during reaction. The nearly complete formation of Fe carbides during initial activation in CO was followed by their gradual re-oxidation to form Fe3O4 with increasing time-on-stream. Fe3O4 became the only detectable Fe compound after 450 h. The observed correlation between FTS rates and Fe carbide concentration, and the unexpected re-oxidation of the catalysts as CO conversion decreased, suggest that the deactivation of Fe catalysts in FTS reactions parallels the conversion of Fe carbides to Fe3O4. It appears that the CO activation steps responsible for replenishing carbidic surface species and for removing chemisorbed oxygen are selectively inhibited by deactivation of surface sites, leading to the oxidation of Fe carbide even in the presence of a reducing reactant mixture.

Spectroscopic and chemical characterization of active and inactive Cu species in NO decomposition catalysts based on Cu-ZSM5

The number and type of Cu2+ species present on O2-treated Cu-ZSM5 catalysts (Si/Al = 13.1–14.6) with varying Cu/Al ratios (0.12–0.60) were measured using temperature-programmed reduction in H2 or CO and desorption of O2 with He as the carrier. The effluent stream was monitored using mass spectrometry and the structure and oxidation state was determined in parallel by X-ray absorption spectroscopy. Isolated Cu2+ monomers and oxygen-bridged Cu2+ dimers interacting with Al–Al next nearest neighbor pairs were the predominant Cu species on these catalysts. The fraction of Cu present as dimers increased from 0.46 to 0.78 as Cu/Al ratios increased from 0.12 to 0.60, as expected from the decreasing average Cu–Cu distance with increasing Cu content. In contrast, monomers reached a plateau of ∼0.15 Cu2+/Al, suggesting that only some Al–Al pairs can interact with small Cu2+ monomer structures, while a much larger fraction can bind with larger oxygen-bridged Cu2+ dimers. The measured distribution of Cu dimers and monomers is consistent with the number and bond distances of Al–Al pairs for the Si/Al ratio in these ZSM5 samples. The distributions of Cu species obtained from the amount of CO2 formed (from CO), the amount of H2O formed (from H2), and the amount of CO adsorbed after reduction in CO are in excellent agreement. The number of oxygen atoms removed as O2 was significantly smaller than that removed with H2 or CO, suggesting that only proximate Cu dimers autoreduce via recombinative desorption steps. NO decomposition turnover rates (normalized per Cu dimer) were nearly independent of Cu content, except at the lowest Cu/Al ratio, consistent with the involvement of Cu dimers as the active Cu species in NO decomposition redox cycles on Cu-ZSM5. Multiple O2 and CO2 peaks during desorption and reduction in CO suggest the presence of Cu dimers with varying oxygen binding energy and reactivity. The Cu dimers initially formed at low Cu/Al contents during exchange are less reducible, consistent with their lower NO decomposition turnover rates.

Identification of sulfate in natural carbonates by x-ray absorption spectroscopy

Abstract We have analyzed sulfur K-edge X-ray absorption near-edge structures (XANES) and demonstrated, with a high degree of certainty, that sulfur is present in a variety of biogenic and diagenetic carbonates as sulfate. The sulfate is clearly not in the form of gypsum or anhydrite inclusions, and there is good evidence that the sulfate substitutes for carbonate. This finding validates systematic investigations of sulfate distribution in modern and ancient carbonate minerals. Further, it documents that carbonate sedimentation is a sink for marine sulfate, accounting for perhaps 15% of the annual sulfur sedimentary depositional flux.

Kinetic, infrared, and X-ray absorption studies of adsorption, desorption, and reactions of thiophene on H-ZSM5 and Co/H-ZSM5

Temperature programmed desorption and infrared and X-ray absorption near-edge spectroscopies were used during adsorption and reactions of thiophene in order to probe adsorbed intermediates and catalytic structures responsible for thiophene reactions with propane or H2 on H-ZSM5 and Co/H-ZSM5. Infrared spectra showed that thiophene interacts with acidic OH groups in H-ZSM5 via hydrogen bonding at ambient temperature. No additional bands were detected on Co/H-ZSM5, suggesting the absence of specific interactions with Co cations. During adsorption at ambient temperatures, infrared bands assigned to CH2 groups near CC bonds or S-atoms and to S–H species were detected and H-ZSM5 and Co/H-ZSM5 acquired colors typical of thiophene oligomers. Slightly above ambient temperatures, benzene and H2S formed from pre-adsorbed thiophene. These results indicate that hydrogen-bonded thiophene undergoes ring opening or oligomerization near ambient temperature on acidic OH groups in H-ZSM5. Some of the adsorbed thiophene (20–50%) interacts weakly with channel walls or with residual Na cations and desorbs unreacted. The remaining adsorbed thiophene desorbs as H2S, aromatic hydrocarbons, and organosulfur compounds, such as methylthiophene and benzothiophene, or forms irreversibly adsorbed unsaturated organic deposits. In situ infrared studies during thiophene and thiophene–propane reactions at 773 K on H-ZSM5 and Co/H-ZSM5 showed that surface coverages of thiophene-derived intermediates were low on acidic OH groups and Co cations. Co K-edge X-ray absorption near-edge spectra measured during these reactions confirmed that Co2+ cations do not reduce or sulfide; their local environment, however, changes slightly, apparently because of interactions of strongly adsorbed species with Co cations. Sulfur K-edge X-ray absorption spectra detected small amounts of organosulfur species, but no inorganic sulfides, after thiophene, thiophene–H2, and thiophene–propane reactions, consistent with the observed stability of exchanged cations against reduction and sulfidation. S∶Al ratios were less than 0.04 at. on all samples; these amounts represent less than 1% of the S-atoms removed from thiophene as H2S during catalytic propane–thiophene reactions.

Experimental aspects of X-Ray absorption spectroscopy

Abstract X-ray absorption spectroscopy (XAS), including X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine-structure spectroscopy (EXAFS), provides physical and chemical information on almost any element regardless of matrix or conditions. Experimental factors that are critical to successful measurements, including general beamline configuration and energy resolution, in situ cell design, detector gases and common artifacts are discussed. This review is intended to assist during experimental setup and data collection.

Fischer-Tropsch synthesis catalysts based on Fe oxide precursors modified by Cu and K: structure and site requirements

The reduction, carburization, and catalytic properties of Fischer-Trops ch synthesis (FTS) catalysts based on Fe-Cu were examined using kinetic and spectroscopic methods at reaction conditions. Fe2O3 precursors reduce to Fe3O4 and then carburize to form a mixture of Fe2.5C and Fe3C in both CO and H2/CO mixtures at 540-720 K. Oxygen removal initially occurs without FTS reaction as Fe2O3 forms inactive O-deficient Fe2O3 species during initial contact with synthesis gas at 523 K. FTS reactions start to occur as Fe 3O4 forms and then rapidly converts to FeCx. The onset of FTS activity requires only the conversion of surface layers to an active structure, which consists of FeCx with steady-state surface coverages of oxygen and carbon vacancies formed in CO dissociation and O-removal steps during FTS. The gradual conversion of bulk Fe3O4 to FeCx influences FTS rates and selectivity weakly, suggesting that the catalytic properties of these surface layers are largely independent of the presence of an oxide or carbide core. The presence of Cu and K increases the rate and the extent of Fe 3O4 carburization during reaction and the Fischer-Tropsch synthesis rates, apparently by decreasing the size of the carbide crystallites formed during reaction.

The location, structure, and role of MoOx and MoCy species in Mo/H-ZSM5 catalysts for methane aromatization

The location, structure, and role of MoO x and MoC y species in solid-state exchanged Mo/H-ZSM5 catalysts were probed by infrared, UV-visible, and X-ray absorption (XAS) spectroscopy, and by time-resolved mass spectrometric measurements of the initial products of methane reactions. UV-vis spectra of MoO 3 /H-ZSM5 mixtures showed a shift in edge to higher energies during treatment in air at 773 K, showing the dispersion of MoO 3 crystallites as smaller domains. The intensity of the OH infrared band at 3610 cm −1 decreased only above 773 K, indicating that Mo +6 exchange occurs only after MoO 3 dispersion on external zeolite surfaces. The amount of H 2 O evolved during exchange and the intensity decrease for the OH band give similar values for the residual OH density and these values indicate that each Mo +6 replaces one H + . Analysis of XAS showed that Mo species exist as (Mo 2 O 5 ) +2 dimers interacting with two exchange sites in all samples with Mo/Al F ratios In situ XAS detected the reduction and carburization of these dimers during CH 4 reactions at 930–973 K, concurrently with an increase in methane aromatization rates. Infrared OH bands became weaker during this process, indicating that reduction-carburization processes do not remove Mo species from exchange sites to re-form OH groups. The rate of hydrocarbon formation increased as O-atoms (O/Mo=2.5) are removed from (Mo 2 O 5 ) +2 to form MoC y species detected by XAS, which remain isolated and retain bonding to framework oxygens.

Discrimination of sulfate from sulfide in carbonates by electron probe microanalysis

Using standards and samples previously analyzed by X-ray Absorption Spectroscopy (XAS), and additional materials, we have demonstrated that the valence of sulfur in a carbonate thin section can be speciated reasonably well with an electron microprobe. The wavelength of sulfur K-α radiation varies slightly and systematically with valence; this can be detected as a shift in the peak position (Bragg angle of the crystal) in the wavelength dispersive spectrometer (WDS). In a series of model compounds, sulfate (S6+) and sulfite (S4+) were easily distinguished from sulfur (S80), disulfide (S22−), and sulfide (S2−). Further distinction within these two groups depends on the S concentration and instrument sensitivity; beam damage to the sample is a limiting factor. Microprobe analysis of carbonate samples previously studied by XAS yielded S speciation in the sulfate-sulfite grouping. This is consistent with the earlier, high-resolution, XAS finding of sulfate substituting for carbonate. Speciation of sulfur using the microprobe permits spatially resolved analysis of the S intimately associated with the carbonate material. The technique is extending our assessment of the form of S in a variety of marine bio-carbonates and reservoir dolomites. Determination of sulfur valence also constrains Eh conditions in the solution at the time of mineral formation or diagenesis.