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Title: Characterization of Coke on a Pt-Re/γ-Al 2 O 3 Re-Forming Catalyst: Experimental and Theoretical Study

Authors:
ORCiD logo [1];  [2];  [3];  [3];  [4];  [5];  [5];  [2];  [3];  [2]
  1. SSRL, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States
  2. Department of Physics, University of Washington, Seattle, Washington 98195, United States
  3. Honeywell UOP, Des Plaines, Illinois 60017, United States
  4. Department of Chemistry, Iowa State University, Ames, Iowa 50011, United States
  5. Materials Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
Publication Date:
Research Org.:
Brookhaven National Laboratory (BNL), Upton, NY (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1409532
Report Number(s):
BNL-114584-2017-JA¿¿¿
Journal ID: ISSN 2155-5435
DOE Contract Number:
SC0012704
Resource Type:
Journal Article
Resource Relation:
Journal Name: ACS Catalysis; Journal Volume: 7; Journal Issue: 2
Country of Publication:
United States
Language:
English

Citation Formats

Bare, Simon R., Vila, F. D., Charochak, Meghan E., Prabhakar, Sesh, Bradley, William J., Jaye, Cherno, Fischer, Daniel A., Hayashi, S. T., Bradley, Steven A., and Rehr, J. J. Characterization of Coke on a Pt-Re/γ-Al 2 O 3 Re-Forming Catalyst: Experimental and Theoretical Study. United States: N. p., 2017. Web. doi:10.1021/acscatal.6b02785.
Bare, Simon R., Vila, F. D., Charochak, Meghan E., Prabhakar, Sesh, Bradley, William J., Jaye, Cherno, Fischer, Daniel A., Hayashi, S. T., Bradley, Steven A., & Rehr, J. J. Characterization of Coke on a Pt-Re/γ-Al 2 O 3 Re-Forming Catalyst: Experimental and Theoretical Study. United States. doi:10.1021/acscatal.6b02785.
Bare, Simon R., Vila, F. D., Charochak, Meghan E., Prabhakar, Sesh, Bradley, William J., Jaye, Cherno, Fischer, Daniel A., Hayashi, S. T., Bradley, Steven A., and Rehr, J. J. Tue . "Characterization of Coke on a Pt-Re/γ-Al 2 O 3 Re-Forming Catalyst: Experimental and Theoretical Study". United States. doi:10.1021/acscatal.6b02785.
@article{osti_1409532,
title = {Characterization of Coke on a Pt-Re/γ-Al 2 O 3 Re-Forming Catalyst: Experimental and Theoretical Study},
author = {Bare, Simon R. and Vila, F. D. and Charochak, Meghan E. and Prabhakar, Sesh and Bradley, William J. and Jaye, Cherno and Fischer, Daniel A. and Hayashi, S. T. and Bradley, Steven A. and Rehr, J. J.},
abstractNote = {},
doi = {10.1021/acscatal.6b02785},
journal = {ACS Catalysis},
number = 2,
volume = 7,
place = {United States},
year = {Tue Jan 24 00:00:00 EST 2017},
month = {Tue Jan 24 00:00:00 EST 2017}
}
  • The characterization of coke on spent catalysts is key to understanding deactivation mechanisms in hydrocarbon transformations. Here, we report the comprehensive characterization (using laser Raman spectroscopy, 13C MAS NMR, temperature-programmed oxidation, XPS, and carbon K-edge NEXAFS) of coke on a series of spent Pt-Re re-forming catalysts as a function of time on stream and position in the catalytic bed. Laser Raman spectroscopy is shown to be rather insensitive to the carbon species present, while 13C MAS NMR finds that the carbon is present primarily as aromatic carbon. The TPO data are consistent with the coke being present on the aluminamore » support and not to a large extent covering the metallic Pt-Re nanoclusters, but the data do suggest the presence of more than one type of coke present. The carbon K-edge NEXAFS data, however, clearly differentiate the types of coke species present. In the more coked samples the features ascribed to graphite become more pronounced, together with an increase in the aromaticity, as judged by the intensity of the π* peak. With increasing amounts of carbon on the catalyst there is also a concomitant decrease in the σ* C–H peak, indicating that the carbon is becoming less hydrogenated. Furthermore, by using a linear combination of C NEXAFS spectra for n-hexane, benzene, and broadened highly oriented pyrolytic graphite (HOPG), we estimate the compositional change on the coke species, verifying the aliphatic to aromatic conversion. The data indicate that a good model for the deposited coke is that of highly defected, medium-sized rafts with a short-range polycyclic aromatic structure which have a variety of points of contact with the alumina surface, in particular with the O atoms. In agreement with the NMR, there is evidence for the C–O functionality from the presence of a shoulder in the C NEXAFS spectra that is ascribed, as a result of DFT calculations, to a 1s → π* transition of the carbon atoms bound to the oxygen of a phenoxide-like species bound to the alumina surface. Finally, these data confirm earlier Soxhlet extraction studies and show that extraction process did not substantially change the character of the coke from what it was while still in contact with the catalyst surface.« less
  • The characterization of coke on spent catalysts is key to understanding deactivation mechanisms in hydrocarbon transformations. Here, we report the comprehensive characterization (using laser Raman spectroscopy, 13C MAS NMR, temperature-programmed oxidation, XPS, and carbon K-edge NEXAFS) of coke on a series of spent Pt-Re re-forming catalysts as a function of time on stream and position in the catalytic bed. Laser Raman spectroscopy is shown to be rather insensitive to the carbon species present, while 13C MAS NMR finds that the carbon is present primarily as aromatic carbon. The TPO data are consistent with the coke being present on the aluminamore » support and not to a large extent covering the metallic Pt-Re nanoclusters, but the data do suggest the presence of more than one type of coke present. The carbon K-edge NEXAFS data, however, clearly differentiate the types of coke species present. In the more coked samples the features ascribed to graphite become more pronounced, together with an increase in the aromaticity, as judged by the intensity of the π* peak. With increasing amounts of carbon on the catalyst there is also a concomitant decrease in the σ* C–H peak, indicating that the carbon is becoming less hydrogenated. By using a linear combination of C NEXAFS spectra for n-hexane, benzene, and broadened highly oriented pyrolytic graphite (HOPG), we estimate the compositional change on the coke species, verifying the aliphatic to aromatic conversion. Furthermore, the data indicate that a good model for the deposited coke is that of highly defected, medium-sized rafts with a short-range polycyclic aromatic structure which have a variety of points of contact with the alumina surface, in particular with the O atoms. In agreement with the NMR, there is evidence for the C–O functionality from the presence of a shoulder in the C NEXAFS spectra that is ascribed, as a result of DFT calculations, to a 1s → π* transition of the carbon atoms bound to the oxygen of a phenoxide-like species bound to the alumina surface. Finally, these data confirm earlier Soxhlet extraction studies and show that extraction process did not substantially change the character of the coke from what it was while still in contact with the catalyst surface.« less
  • Studies of coke and product profiles along catalyst beds of Pt/Al{sub 2}O{sub 3} and Pt-Re/Al{sub 2}O{sub 3} during n-heptane reforming have provided insight into catalyst deactivation and reaction mechanisms. The addition of rhenium to a Pt catalyst changes coke and product profiles similar to those observed with an increase in pressure. Sulfur modifies these profiles in the opposite direction of rhenium. It affects the catalyst in a similar manner to a decrease in reactor pressure: a reduction in hydrogenolysis activity, an increase in dehydrogenation and coke-make, and a shift of the maximum in the coke profile toward the bottom ofmore » the reactor. Due to a slow stripping of sulfur during reforming reactions, an increasing sulfur concentration along the bed is observed at the end of several-days run. As a consequence, the coke profile does not match exactly with the C5-ring naphthene concentration profile, as was previously found on both unsulfided Pt and Pt-Re catalysts. The observed increase in coke-make and dehydrogenation activity, when sulfur is added to the Pt-Re catalyst, is probably due to a decrease in the hydrogen surface fugacity produced through electronic modifications of Pt by adsorbed sulfur atoms. 23 refs., 8 figs., 3 tabs.« less
  • Catalysts supported on [gamma]-Al[sub 2]O[sub 3] were prepared from [Re[sub 2]Pt(CO)[sub 12]], and from Pt (NH[sub 3])[sub 4](NO[sub 3])[sub 2] and NH[sub 4]ReO[sub 4]. The former samples were characterized by infrared and X-ray photoelectron spectroscopies (XPS) and by temperature-programmed reduction (TPR); the latter were characterized by TPR. [Re[sub 2]Pt(CO)[sub 12]] was initially chemisorbed on the [gamma]-Al[sub 2]O[sub 3] surface. Upon treatment in H[sub 2] at about 150[degrees]C, the cluster fragmented and formed rhenium subcarbonyls, and at about 400[degrees]C the sample was decarbonylated. Adsorption of CO and of NO as probe molecules gave evidence of metallic Pt, but there was nomore » evidence of adsorption on Re. The XPS data indicating the Re binding energies give evidence of the presence of low-valent cationic Re in the sample after the treatment at 400[degrees]C in H[sub 2]. In contrast, when a mixture of samples of Re on [gamma]-Al[sub 2]O[sub 3] and Pt on [gamma]-Al[sub 2]O[sub 3] prepared from [H[sub 3]Re[sub 3](CO)[sub 12]] and [(CH[sub 3])[sub 2]Pt(COD)], respectively, was treated under equivalent conditions, the Re was present in a high-valent cationic form (Re[sup 7+]), and Pt was metallic. It is concluded that Pt facilitated the reduction of Re and that Pt was likely near the rhenium in the sample prepared from [Re[sub 2]Pt(CO)[sub 12]]. The TPR data are consistent with the foregoing results. The TPR data characterizing the samples prepared from the metal salts show that the degree of hydroxylation of the [gamma]-Al[sub 2]O[sub 3] support significantly influenced the reduction of the Re and the Pt, but these data are not sufficient to determine the interactions between the two metals. 50 refs., 5 figs., 5 tabs.« less