The Coking of a Solid Catalyst Rationalized with Combined Raman and Fluorescence Lifetime Microscopy.

The formation of carbon deposits is a major deactivation pathway for solid catalysts. Studying coking on industrially relevant catalysts is, however, often challenging due to the sample heterogeneity. That is especially true for zeolite-containing catalysts where fluorescence often hampers their characterization with Raman spectroscopy. We turned this disadvantage into an advantage and combined Raman and fluorescence (lifetime) microscopy to study the coking behavior of an equilibrium catalyst material used for fluid catalytic cracking of hydrocarbons. The results presented illustrate that this approach can yield new insights in the physicochemical processes occurring within zeolite-containing catalyst particles during their coking process. Ex situ analyses of single catalyst particles revealed considerable intra-sample heterogeneities. The sample-averaged Raman spectra showed a higher degree of graphitization when the sample was exposed to more hexane, while the sample-averaged fluorescence lifetime showed no significant trend. Simultaneous in situ Raman and fluorescence (lifetime) microscopy, used to follow the coking of single particles, gave more insights in the changing fluorescence dynamics. The rise and decline of the average fluorescence lifetime suggested the prolonged presence of smaller coke species that are quenched more and more by adjacent larger polyaromatics acting as Förster-resonance-energy-transfer acceptors.

Operando time-gated Raman spectroscopy of solid catalysts.

Operando Raman spectroscopy is a powerful analytical tool to provide new insights in the working and deactivation principles of solid catalysts. Intense fluorescence can obscure Raman spectra to the extent that they become uninterpretable. Time-gated Raman spectroscopy, based on pulsed excitation and time-gated detection, suppresses background fluorescence based on its slower time dynamics compared to Raman scattering. In this work, we demonstrate and quantify the benefit of time gating for operando Raman spectroscopy, using the propane dehydrogenation reaction over Pt-Sn-based catalyst materials as a case study. Experimental time-gated Raman spectroscopy data are fitted to a time-trace model that is used to optimize time gating for the maximum signal-to-background-noise ratio. Time-gated Raman spectra of a spent propane dehydrogenation catalyst material show lower background fluorescence compared to the time-integrated Raman spectra counterparts. Simultaneous operando time-gated and time-integrated Raman spectroscopy experiments demonstrate the benefit of time gating to obtain more distinct Raman features, especially in the early coking stages where spectra are dominated by background fluorescence.

Effect of ingested nutrients on the release of thrittene into the human circulation.

Thrittene is a recently described peptide with a sequence homologous with somatostatin-28 ((1-13)) but is produced independent of the preprosomatostatin gene. It is localized in epithelial cells in stomach and gut mucosal crypts and in neuronal cell bodies in the myenteric plexus and enteric axons. It is also present in human plasma. The aim of this study was to determine whether the release of thrittene into the circulation was affected by the ingestion of nutrients and, if so, whether the pattern of release was distinct from the closely related peptide somatostatin-28 (S-28). Thrittene was indirectly measured in human plasma by an RIA using antiserum F4. F4 interacts with the Asn(5)-Pro(6) region shared by S-28 and thrittene. The contribution of S-28 to F4 immunoreactivity (F4-IR) was determined using a specific two-site assay, and this measure was subtracted from the total F4-IR to give an estimate of thrittene levels. Plasma for assay was taken from healthy men on 4 separate days before and after intake of: a mixed meal (715 kcal), and meals containing primarily fat (25 g; 225 kcal), carbohydrate (100 g; 454 kcal), and protein (22 g; 100 kcal). After the mixed meal, both S-28 and thrittene rose by 50-100% within 30 min and gradually declined by 4 h. These increments were mimicked after ingestion of fat. By contrast, thrittene levels increased after carbohydrate but not protein intake, whereas S-28 concentrations rose after protein but not carbohydrate ingestion. These findings indicate that thrittene is secreted into the mammalian circulation during food intake and raise the possibility that thrittene may play a role in nutrient disposition. The dichotomy in responses of thrittene and S-28 to ingestion of the major macronutrients suggests that they are secreted from different gastrointestinal cells.

Thrittene, homologous with somatostatin-28((1-13)), is a novel peptide in mammalian gut and circulation.

Preprosomatostatin is a gene expressed ubiquitously among vertebrates, and at least two duplications of this gene have occurred during evolution. Somatostatin-28 (S-28) and somatostatin-14 (S-14), C-terminal products of prosomatostatin (ProS), are differentially expressed in mammalian neurons, D cells, and enterocytes. One pathway for the generation of S-14 entails the excision of Arg13-Lys14 in S-28, leading to equivalent amounts of S-28((1-12)). Using an antiserum (F-4), directed to the N-terminal region of S-28 that does not react with S-28((1-12)), we detected a peptide, in addition to S-28 and ProS, that was present in human plasma and in the intestinal tract of rats and monkeys. This F-4 reacting peptide was purified from monkey ileum; and its amino acid sequence, molecular mass, and chromatographic characteristics conformed to those of S-28((1-13)), a peptide not described heretofore. When extracts of the small intestine were measured by RIA, there was a discordance in the ratio of peptides reacting with F-4 and those containing the C terminus of ProS, suggesting sites of synthesis for S-28((1-13)) distinct from those for S-14 and S-28. This was supported by immunocytochemistry, wherein F-4 reactivity was localized in gastrointestinal (GI) endocrine cells and a widespread plexus of neurons within the wall of the distal gut while immunoreactivity to C-terminal domains of S-14 and S-28 in these neurons was absent. Further, F-4 immunoreactivity persisted in similar GI endocrine cells and myenteric neurons in mice with a targeted deletion of the preprosomatostatin gene. We believe that these data suggest a novel peptide produced in the mammalian gut, homologous with the 13 residues of the proximal region of S-28 but not derived from the ProS gene. Pending characterization of the gene from which this peptide is derived, its distribution, and function, we have designated this peptide as thrittene. Its localization in both GI endocrine cells and gut neurons suggests that thrittene may function as both a hormone and neurotransmitter.