Analysis of mixtures of compounds of copper using K-edge X-ray absorption spectroscopy

Meitzner, G.; Huang, Erxiong

doi:10.1007/bf00321692

Cited by 18 publications

(15 citation statements)

References 2 publications

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“…The spectra of samples were described as linear combinations of the near-edge spectra of the three standard compounds identified by principal component analysis. 15 The detailed analysis procedure has been described elsewhere 3 . Figure 2 shows the change in the concentrations of the Fe 2 O 4 continued to convert to FeC x , FTS reaction rates remained nearly constant, even as the structures changed markedly, suggesting that only a fraction of the Fe species, probably those near surfaces, are involved in FTS reactions and that they form rapidly during initial contact with synthesis gas.…”

Section: Resultsmentioning

confidence: 99%

Spectroscopic and Transient Kinetic Studies of Site Requirements in Iron-Catalyzed Fischer−Tropsch Synthesis

et al. 2001

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The structure, reduction/carburization, and catalytic performance of K-and Cu-promoted Fe 2 O 3 during initial contact with synthesis gas were examined by combining kinetic analysis of the initial stages of FischerTropsch synthesis (FTS) with X-ray absorption spectroscopy. Oxygen removal initially occurs without FTS reactions as Fe 2 O 3 is reduced to inactive oxygen-deficient Fe 2 O 3 species. Hydrocarbon synthesis reactions become detectable only as Fe 3 O 4 forms and rapidly converts to FeC x . FTS reactions require only the incipient conversion of the surface layers to a dynamic and active surface phase, which consists of FeC x with steadystate surface coverages of vacancies, formed via oxygen and carbon removal during the formation of monomers, CO 2 , and H 2 O. Such surfaces tend to respond to changes in the contacting gas phase within turnover times by changing the relative surface concentration of carbon, oxygen, CO, and hydrogen. The catalytic behavior of these dynamic surfaces is largely independent of the carbide or oxide nature of the particle cores. These findings, combined with the rapid formation and interconversion of Fe 3 O 4 and FeC x within characteristic FTS turnover times, make the definite assignment of FTS activity to either phase neither appropriate nor kinetically rigorous. The presence of K and Cu increases FTS rates, and the steady-state extent of carburization by providing nucleation sites for the formation of smaller FeC x crystallites. These smaller active domains lead, in turn, to shorter diffusion paths and to a larger number of sites for CO adsorption/dissociation and for FTS reactions. These structural promotion effects of K and Cu were consistent with reaction rates, X-ray absorption spectra, and site density measured on promoted and unpromoted catalysts as a function of time in contact with synthesis gas.

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Section: Resultsmentioning

confidence: 99%

Spectroscopic and Transient Kinetic Studies of Site Requirements in Iron-Catalyzed Fischer−Tropsch Synthesis

et al. 2001

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“…Near-edge spectra were analyzed by fitting linear combinations of spectra from Cu metal, CuO, Cu(OH) 2 , and Cu 2 O standards from −20 to +60 eV energies with respect to the edge [32]. The standard spectra were allowed to shift in energy and were adjusted in amplitude, and added together to find the best fit to the sample spectra.…”

Section: Resultsmentioning

confidence: 99%

New insights into methanol synthesis catalysts from X-ray absorption spectroscopy

Meitzner¹,

Iglesia

1999

Catalysis Today

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X-ray absorption spectroscopic studies from several different groups provide a consistent structural picture of methanol synthesis catalysts. Copper metal is the principal Cu species detected in all in-situ XAS studies. A small amount of Cu(II) persisted in many samples reduced below 600 K, but it appears to be catalytically irrelevant. Cu(I) was not detected in any of the in-situ XAS studies. The Zn structure did not change in response to chemical treatments in any of the ZnO-containing methanol synthesis catalysts.We conclude that the slow approach to steady-state rate of methanol synthesis from CO/H 2 mixtures cannot result from changes in the Cu metal component of Cu/SiO 2 catalysts. By eliminating this possibility, we provide indirect evidence for the proposal that initial induction periods reflect slow changes in the surface of the SiO 2 or ZnO component in the catalysts. A bifunctional mechanism involving the formation of formate from CO on support hydroxyl groups would increase methanol synthesis rates as OH groups are formed by hydrolysis of Si-O bonds using the H 2 O formed in slow methanation side reactions. The bifunctional mechanism is not required for the synthesis of methanol from CO 2 -containing mixtures, because formate can form on Cu directly from CO 2 and H 2 .

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“…23 Linear fits to the pre-edge region (6.900-7.100 keV) were subtracted from the spectra, and the spectra were normalized by a sixth-order polynomial fit of the post-edge region (7.240-8.120 keV). Principal component analysis 16 and linear combination methods 17 were used to calculate the relative abundance of the various Fe phases using the near-edge spectral region between 7.090 and 7.240 keV for each sample and for the Fe reference compounds.…”

Section: Methodsmentioning

confidence: 99%

“…The X-ray absorption near-edge spectrum (XANES) is sensitive to the oxidation state and to the symmetry of the absorber atom. 15 Comparisons between these features and those in compounds with known local structure can be used to quantify the fraction of each absorber present within each type of local coordination environment using principal component analysis (PCA) 16 and linear combination 17 methods. The region beyond the absorption edge contains oscillations in absorbance, which arise from the scattering of the emitted photoelectrons by the atoms surrounding the absorber.…”

Section: Introductionmentioning

confidence: 99%

Structure and Site Evolution of Iron Oxide Catalyst Precursors during the Fischer−Tropsch Synthesis

2001

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The contacting of Fe oxide precursors with synthesis gas (H 2 /CO mixtures) leads to structural and chemical changes and to the formation of the active sites required for the Fischer-Tropsch synthesis (FTS). The local structure and oxidation state of the starting Fe 2 O 3 precursors promoted by Cu and/or K were probed using in situ X-ray absorption spectroscopy during these processes. The activation of these precursors occurs via reduction to Fe 3 O 4 followed by carburization to form FeC x . FTS reaction rates increased markedly during the initial stages of carburization, suggesting that the conversion of near-surface layers of Fe 3 O 4 to FeC x is sufficient for the formation of the required active sites. Thus, bulk structural probes, and any ex situ techniques without concurrent measurements of the products evolved during activation and FTS, can lead to misleading structurefunction relations. The initial rate of carburization and the extent of carburization and the FTS rate at steadystate were higher for Fe 2 O 3 precursors containing either Cu or K. These effects were stronger when both promoters were present. K and Cu provide CO and H 2 activation sites, which lead to the nucleation of multiple carbide regions on Fe oxide surfaces. The larger number of nucleation sites leads to higher initial carburization rates and to smaller FeC x crystallites. These smaller crystallites, in turn, provide higher active surface areas, shorter bulk diffusion distances, more complete carburization of Fe 2 O 3 precursors, and higher steady-state FTS rates. These effects of K and Cu on the number of available FeC x sites were confirmed by titrating surface sites using CO after FTS reactions. It appears that K and Cu act predominantly as structural promoters, which increase the surface area of the active FeC x phases. Chemical promotion of catalytic rates by these additives appears not to be required to explain the observed FTS rate enhancements in the presence of K or Cu. These structural promotion effects of Cu and K account for the apparent but non-causal correlation between bulk FeC x contents and FTS rates at steady-state, even though the carburization of only near-surface regions in bulk Fe 3 O 4 is sufficient to achieve steady-state FTS turnover rates.

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Analysis of mixtures of compounds of copper using K-edge X-ray absorption spectroscopy

Cited by 18 publications

References 2 publications

Spectroscopic and Transient Kinetic Studies of Site Requirements in Iron-Catalyzed Fischer−Tropsch Synthesis

Spectroscopic and Transient Kinetic Studies of Site Requirements in Iron-Catalyzed Fischer−Tropsch Synthesis

New insights into methanol synthesis catalysts from X-ray absorption spectroscopy

Structure and Site Evolution of Iron Oxide Catalyst Precursors during the Fischer−Tropsch Synthesis

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