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

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 alumina 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 usingmore » 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
Authors:
ORCiD logo [1] ;  [2] ;  [3] ;  [3] ;  [4] ;  [5] ;  [5] ;  [2] ;  [3] ;  [2]
  1. SLAC National Accelerator Lab., Menlo Park, CA (United States)
  2. Univ. of Washington, Seattle, WA (United States). Dept. of Physics
  3. Honeywell UOP, Des Plaines, IL (United States)
  4. Iowa State Univ., Ames, IA (United States). Dept. of Chemistry
  5. National Inst. of Standards and Technology (NIST), Gaithersburg, MD (United States). Materials Measurement Lab.
Publication Date:
Grant/Contract Number:
FG02-03ER15476; AC02-76SF00515; AC02-98CH10886; AC02-05CH11231
Type:
Accepted Manuscript
Journal Name:
ACS Catalysis
Additional Journal Information:
Journal Volume: 7; Journal Issue: 2; Journal ID: ISSN 2155-5435
Publisher:
American Chemical Society (ACS)
Research Org:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 13C MAS NMR; carbon NEXAFS; catalyst deactivation; coke characterization; DFT calculations; Pt-Re/alumina; Raman; TPO
OSTI Identifier:
1347923
Alternate Identifier(s):
OSTI ID: 1347926; OSTI ID: 1360785

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-Al2O3 Re-Forming Catalyst: Experimental and Theoretical Study. United States: N. p., 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-Al2O3 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.. 2017. "Characterization of Coke on a Pt-Re/γ-Al2O3 Re-Forming Catalyst: Experimental and Theoretical Study". United States. doi:10.1021/acscatal.6b02785. https://www.osti.gov/servlets/purl/1347923.
@article{osti_1347923,
title = {Characterization of Coke on a Pt-Re/γ-Al2O3 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 = {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 alumina 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.},
doi = {10.1021/acscatal.6b02785},
journal = {ACS Catalysis},
number = 2,
volume = 7,
place = {United States},
year = {2017},
month = {1}
}