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

Abstract

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, 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. 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. 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.

Preserved in a Shell: High‐Performance Graphene‐Confined Ruthenium Nanoparticles in Acetylene Hydrochlorination

“…Temperatures exceeding 1073 K lead to a gradual degradation of the graphene layer, as visualized with HRTEM (Figure S25) and further corroborated by temperature‐programmed oxidation experiments, showing a significantly decreased low‐temperature CO evolution (ca. 450–520 K) for Ru/NC‐g‐1173 in comparison to Ru/NC‐g (Figure S26). Since temperatures exceeding the initial carbonization temperature of NC‐g (1073 K) lead to a steady decrease in the total nitrogen‐content, we speculate that the nature of the graphene layer is characterized by a significant contribution of nitrogen functionalities, as has been recently demonstrated for the graphitization of nitrogen‐containing precursors .…”

3‐D X‐ray Nanotomography Reveals Different Carbon Deposition Mechanisms in a Single Catalyst Particle

“…Various analytical methods have been used in the past to study carbon deposits in solid catalysts providing bulk information on both the species and origin of the coke accumulated during catalyst operation. Studies on industrial catalysts are less common; coke in industrial reforming, hydrotreating, or cracking catalysts was studied using solid-state carbon magic angle spinning nuclear magnetic resonance ( 13 C-MAS-NMR), [3,[6][7][8][9][10][11] supercritical fluid extraction (SFE), [3,7] electron paramagnetic resonance (EPR), [12] near-edge X-ray absorption fine structure (NEXAFS), [11,13] X-ray photoelectron spectroscopy (XPS), [7,11] X-ray diffraction (XRD), [14] matrix-assisted laser/desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS), [6] temperature-programmed hydrogenation (TPH) and oxidation (TPO), [11,15] Raman spectroscopy, [11,14] UV-vis microspectroscopy, [16] proton-induced X-ray emission (PIXE), and nuclear reaction analysis (NRA). [17] These techniques often rely on coke-containing samples from which the catalyst was leached (e. g., by dissolution in hydrofluoric acid [4] ) and provide either bulk information or 2-D data at a spatial resolution that is too low to study the relation of catalyst structure and composition on the one hand and coke on the other hand.…”

Quantitative imaging of carbon in heterogeneous refining catalysts

“…When spatially resolved analysis is required, several techniques can be used, such as Raman spectroscopy, 9 X-ray absorption spectroscopy (XANES), 10 electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopies. 11 These techniques provide molecular or functional group characterization. However, they are not well adapted for a carbon-poisoning study, as no elemental information can be obtained.…”