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Title: Fischer-tropsch synthesis: temperature programmed EXAFS/XANES investiation of the influence of support type, cobalt loading, and nobel metal promoter addition to the reduction behavior of cobalt oxide particles.

Abstract

TPR-XANES/EXAFS carried out using a novel multi-sample holder provided key information for verifying the nature of the chemical transformations occurring during cobalt Fischer-Tropsch synthesis catalyst activation in hydrogen. In the past, assumptions had to be made regarding the nature of the cobalt species present along the trajectory of a standard TPR experiment. The new technique directly provided insight into (a) the nature of the reduction process of cobalt oxide species and (b) the resulting cobalt crystallite size, as a function of the strength of the catalyst support interaction with the cobalt oxide species. A two-step reduction process involving Co{sub 3}O{sub 4} to CoO and CoO to Co{sup 0} transformations over standard calcined catalysts was observed and quantified over all catalysts exhibiting both weak interactions (e.g., Co/SiO{sub 2}) and strong interactions (e.g., Co/Al{sub 2}O{sub 3}) with the support. Noble metal promoter (e.g., Pt) addition strongly improved the reducibility of cobalt oxide species, most likely via a H{sub 2} dissociation and spillover mechanism. Increasing cobalt loading, on the other hand, led to a measurable, but lesser, improvement on reducibility, due to the larger resulting particle size that resulted in less surface contact with the support. Higher reduction temperatures were needed to effectivelymore » reduce cobalt oxide particles deposited on strongly interacting surfaces in comparison with unsupported Co{sub 3}O{sub 4} or only weakly interacting supported cobalt catalyst. Nevertheless, despite lower extents of reduction, the smaller resulting Co particles on the more strongly interacting catalysts generally led to higher Co{sup 0} active site densities. The addition of the noble metal promoter to strongly interacting supported catalyst significantly decreased the temperature required to reduce the cobalt oxides to Co{sup 0} particles; this allows one to take advantage of the higher Co{sup 0} surface areas arising from the combination of a smaller average Co{sup 0} particle size and a higher extent of reduction.« less

Authors:
; ; ; ; ; ; ;
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
FE; National Aeronautic and Space Administration (NASA); Commonwealth of Kentucky
OSTI Identifier:
947529
Report Number(s):
ANL/CMT/JA-58434
Journal ID: ISSN 0926-860X; ACAGE4; TRN: US200905%%54
DOE Contract Number:
DE-AC02-06CH11357
Resource Type:
Journal Article
Resource Relation:
Journal Name: Appl. Catal. A; Journal Volume: 333; Journal Issue: 2007
Country of Publication:
United States
Language:
ENGLISH
Subject:
01 COAL, LIGNITE, AND PEAT; 08 HYDROGEN; CATALYST SUPPORTS; CATALYSTS; COBALT; COBALT OXIDES; DISSOCIATION; FISCHER-TROPSCH SYNTHESIS; HYDROGEN; PARTICLE SIZE; PROMOTERS; STRONG INTERACTIONS; SURFACE AREA; SYNTHESIS; TRANSFORMATIONS; WEAK INTERACTIONS

Citation Formats

Jacobs, G., Ji, Y., Davis, B. H., Cronauer, D., Kropf, A. J., Marshall, C. L., Chemical Engineering, and Univ. of Kentucky. Fischer-tropsch synthesis: temperature programmed EXAFS/XANES investiation of the influence of support type, cobalt loading, and nobel metal promoter addition to the reduction behavior of cobalt oxide particles.. United States: N. p., 2007. Web. doi:10.1016/j.apcata.2007.07.027.
Jacobs, G., Ji, Y., Davis, B. H., Cronauer, D., Kropf, A. J., Marshall, C. L., Chemical Engineering, & Univ. of Kentucky. Fischer-tropsch synthesis: temperature programmed EXAFS/XANES investiation of the influence of support type, cobalt loading, and nobel metal promoter addition to the reduction behavior of cobalt oxide particles.. United States. doi:10.1016/j.apcata.2007.07.027.
Jacobs, G., Ji, Y., Davis, B. H., Cronauer, D., Kropf, A. J., Marshall, C. L., Chemical Engineering, and Univ. of Kentucky. Mon . "Fischer-tropsch synthesis: temperature programmed EXAFS/XANES investiation of the influence of support type, cobalt loading, and nobel metal promoter addition to the reduction behavior of cobalt oxide particles.". United States. doi:10.1016/j.apcata.2007.07.027.
@article{osti_947529,
title = {Fischer-tropsch synthesis: temperature programmed EXAFS/XANES investiation of the influence of support type, cobalt loading, and nobel metal promoter addition to the reduction behavior of cobalt oxide particles.},
author = {Jacobs, G. and Ji, Y. and Davis, B. H. and Cronauer, D. and Kropf, A. J. and Marshall, C. L. and Chemical Engineering and Univ. of Kentucky},
abstractNote = {TPR-XANES/EXAFS carried out using a novel multi-sample holder provided key information for verifying the nature of the chemical transformations occurring during cobalt Fischer-Tropsch synthesis catalyst activation in hydrogen. In the past, assumptions had to be made regarding the nature of the cobalt species present along the trajectory of a standard TPR experiment. The new technique directly provided insight into (a) the nature of the reduction process of cobalt oxide species and (b) the resulting cobalt crystallite size, as a function of the strength of the catalyst support interaction with the cobalt oxide species. A two-step reduction process involving Co{sub 3}O{sub 4} to CoO and CoO to Co{sup 0} transformations over standard calcined catalysts was observed and quantified over all catalysts exhibiting both weak interactions (e.g., Co/SiO{sub 2}) and strong interactions (e.g., Co/Al{sub 2}O{sub 3}) with the support. Noble metal promoter (e.g., Pt) addition strongly improved the reducibility of cobalt oxide species, most likely via a H{sub 2} dissociation and spillover mechanism. Increasing cobalt loading, on the other hand, led to a measurable, but lesser, improvement on reducibility, due to the larger resulting particle size that resulted in less surface contact with the support. Higher reduction temperatures were needed to effectively reduce cobalt oxide particles deposited on strongly interacting surfaces in comparison with unsupported Co{sub 3}O{sub 4} or only weakly interacting supported cobalt catalyst. Nevertheless, despite lower extents of reduction, the smaller resulting Co particles on the more strongly interacting catalysts generally led to higher Co{sup 0} active site densities. The addition of the noble metal promoter to strongly interacting supported catalyst significantly decreased the temperature required to reduce the cobalt oxides to Co{sup 0} particles; this allows one to take advantage of the higher Co{sup 0} surface areas arising from the combination of a smaller average Co{sup 0} particle size and a higher extent of reduction.},
doi = {10.1016/j.apcata.2007.07.027},
journal = {Appl. Catal. A},
number = 2007,
volume = 333,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • Fe-based catalysts containing different amounts of Mn were tested for Fischer-Tropsch synthesis using a stirred tank reactor at 270 C, 1.21 MPa, and H{sub 2}:CO = 0.7. Catalyst activation by carburization with 10% CO/He was followed by Temperature Programmed Reduction/X-ray Absorption Spectroscopy (TPR-EXAFS/XANES) from room temperature to 300 C. {gamma}-Fe{sub 2}O{sub 3} was converted into iron carbides, whereas MnO{sub x} was reduced to oxygen deficient MnO. Mn hindered Fe carburization, such that the carburized catalyst displayed higher Fe{sub 3}O{sub 4} content than the catalyst without Mn. EXAFS fitting indicates that the carburized catalyst contained a mixture of Hgg carbide, Fe{submore » 3}O{sub 4}, and Mn oxides. Increasing Mn content led to higher CH{sub 4} and light product selectivities, and lower light olefin selectivities. Higher and stable conversions were obtained with a catalyst containing an almost equimolar Fe/Mn ratio relative to the catalyst without Mn. Selectivity trends are attributed to the higher WGS rates observed on the FeMn catalysts, consistent with the structural differences observed.« less
  • The promoting impact of alkali metals (i.e., Li, Na, K, Rb, Cs) on the carburization rate of Fe in Fe/Si catalysts was investigated by X-ray absorption spectroscopy. A multisample holder was used, allowing nearly simultaneous examination of the catalysts during activation in a CO/He mixture. With the white line intensity and shape as a fingerprint for oxidation state, TPR/XANES analysis enabled us to measure the relative composition of the different compounds as a function of the carburization time, temperature, and atomic number of the group 1 promoter. At the same time, TPR/EXAFS provided information on the changes in local atomicmore » structure that accompanied the oxidation state changes. The rate of carburization increased in the following order: unpromoted < Li < Na < K = Rb = Cs. After 10 h of treatment the samples containing K, Rb, and Cs were completely carburized, and residual quantities of iron oxides were detected in both unpromoted and Li-promoted samples. The EXAFS spectra after carburization could be fitted well by considering a model containing Hagg carbide and Fe{sub 3}O{sub 4}. After 10 h of CO/He treatment at 290 C, the main component observed was Hagg carbide. A model containing Hagg and {var_epsilon}-carbides, and Fe{sub 3}O{sub 4}, was also investigated. However, the r-factor was not significantly impacted by including {var_epsilon}-carbide in the fitting, and the resulting contribution of {var_epsilon}-carbide in each catalyst from the model was virtually negligible. Selectivity differences are thus not likely due to changes in the carbide distribution. Rather, the alkali promoter increases the CO dissociative adsorption rate, resulting in an increase in the surface coverage of dissociated CO and an inhibition in the olefin readsorption rate. This in turn results in higher olefin selectivities, in agreement with previous catalytic tests.« less
  • A 2% Ru-promoted 15% Co/Al2O3 catalyst was tested after reduction and after being subjected to oxidation-reduction cycles. The catalysts were characterized over four oxidation-reduction cycles by XANES/EXAFS, TPR, HRTEM, and EDS elemental mapping. The oxidation-reduction treatments were found to assist in sintering the metallic clusters to a larger size, and to promote mixing on at least the order of the nanoscale. The larger crystallites in closer proximity to the Ru promoter led to a more facile reduction of the cobalt crystallites. In addition, a catalyst exposed to two oxidation-reduction cycles resulted in slightly higher conversion, higher a-value product, slightly lowermore » methane selectivity, and greater stability over a reduced freshly calcined catalyst.« less
  • In this study, the catalytic activity and changes in the oxidation state during the Fischer Tropsch (FT) reaction was investigated on subnanometer size-selected cobalt clusters deposited on oxide (Al2O3, MgO) and carbon-based (ultrananocrystalline diamond UNCD) supports by temperature programmed reaction (TPRx) combined with in-situ grazing-incidence X-ray absorption characterization (GIXAS). The activity and selectivity of ultrasmall cobalt clusters exhibits a very strong dependence on cluster size and support. The evolution of the oxidation state of metal cluster during the reaction reveals that metal-support interaction plays a key role in the reaction.