Characterization of Coke on a Pt-Re/γ-Al2O3 Re-Forming Catalyst: Experimental and Theoretical Study

The characterization of coke on spent catalysts is key to understanding deactivation mechanisms in hydrocarbon transformations. In this paper we report the comprehensive characterization (using laser Raman spectroscopy, C-13 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 C-13 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 pi* peak. With increasing amounts of carbon on the catalyst there is also a concomitant decrease in the sigma* 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. The data indicate that a good model for the deposited coke is that of highly defected, medium-sized rafts with a short-range polycydic aromatic structure which have a variety of points of contact with the alumina surface, in particular with the 0 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 -> pi* transition of the carbon atoms bound to the oxygen of a phenoxide-like species bound to the alumina surface. 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.