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Li, Senzi; Li, Anwu; Krishnamoorthy, Sundaram; Iglesia, Enrique

doi:10.1023/a:1013284217689

Cited by 293 publications

(220 citation statements)

References 17 publications

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“…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. Clearly, the structure and composition of near-surface layers are more likely to influence catalytic turnovers than the structure of an inaccessible bulk; the easier access by reactants to near-surface regions also lead these regions to reach steady-state compositions and structures much more rapidly than particle cores, which are also probed by X-ray photons, but which are accessible only via bulk diffusion of oxygen and carbon.…”

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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“…These compositions were found to give the best catalytic performance in the Fischer-Tropsch synthesis. 21 All samples used in spectroscopic measurements were diluted to 10 wt % Fe using graphite powder (Alfa AESAR, 99.9995%, S g < 1 m 2 /g), pressed at 10 MPa, sieved to retain 180-250 µm particles, and placed within a quartz capillary cell. 22 The inertness of graphite was tested by flowing H 2 over Fe oxides mixed with graphite up to 873 K without any detectable formation of FeC x .…”

Section: Methodsmentioning

confidence: 99%

“…Cu does not influence the selectivity; the effects of K on FTS selectivity will be discussed in a subsequent paper. 21 Clearly, K and Cu lead to the formation of FeC x structures with higher specific surface area and smaller size. A consequence of their smaller size is their more complete carburization, which is not the direct cause of the higher FTS rates observed in the presence of K and/or Cu.…”

Section: Effects Of K and Cu On Catalyst Structure And On Fischer-tromentioning

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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“…In the latter protocols, the acidic solutions containing Cu and Ru complexes, unstable species in basic media, were added to Fe-Zn oxide and K 2 CO 3 particles on the surface of Fe-Zn oxide. The neutralization reaction of acidic species and K 2 CO 3 would then result from the addition of metal nitrate solutions, suggesting that these reactions caused the rapid precipitation of Cu(NO 3 ) 2 and Ru(NO)(NO 3 ) x (OH) y complexes to form large hydroxide crystallites, while K 2 CO 3 is converted to KNO 3 . Consequently, the particles of precipitated Cu and Ru hydroxides on the surface of Fe-Zn oxides would become larger and more heterogeneous on the surface during rapid precipitation compared to those promoted without neutralization.…”

Section: Effect Of Promoters On Fischer-tropsch Synthesis Rate and Sementioning

confidence: 99%

“…Several iron phases, such as Fe 3 O 4 and Fe 3 C, have been postulated in the literature to be the active sites for hydrocarbon formation [14][15][16]. It seems that the formation of Fe 3 O 4 and Fe carbide phases on Fe-Zn oxide and Fe-Zn-K 6 without Cu promoter took place slowly during the initial period (2 h), probably because of the Zn and K components inhibit the reduction of Fe 2 O 3 [2]. After 15 h on stream, Fe-Zn oxide showed the highest CH 4 formation rate on Fe-Zn oxide.…”

Section: Catalytic Behavior On Iron Based Catalysts During the Activamentioning

confidence: 99%

Design, Synthesis, and Mechanistic Evaluation of Iron-Based Catalysis for Synthesis Gas Conversion to Fuels and Chemicals

Iglesia¹,

Ishikawa²,

Ojeda³

et al. 2007

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Cited by 293 publications

References 17 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

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

Design, Synthesis, and Mechanistic Evaluation of Iron-Based Catalysis for Synthesis Gas Conversion to Fuels and Chemicals

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