Raman spectroscopy carbonaceous deposits

Carbon-containing deposits may accumulate on surfaces during reactions with hydrocarbons. Details of the formation and nature of such species have been described by Stair (2007). Raman spectroscopy is a tool that is well suited to the investigation of carbonaceous deposits. An early investigation of such carbon-containing deposits was reported by Brown et al. (Brown et al., 1977) in 1977. If sufficient oxygen is present in a hydrocarbon environment, carbon deposition will be minimized or will not occur, and the catalysts may remain fully oxidized. 

The average oxidation state of a metal in a catalyst during reaction was found to be related to the presence of carbonaceous deposits on the surface. As the feed for propane ODH was depleted in O2, the catalyst was readily reduced (Mul et al., 2003) and amorphous carbon-containing deposits formed. This behavior was corroborated by UV-vis DRS (Mul et al., 2003 Puurunen and Weckhuysen, 2002) and by combination of UV-vis DRS and Raman spectroscopy (Kuba and Knozinger, 2002 Nijhuis et al., 2003). 

The transition from amorphous carbon-containing deposits to graphite-like species and finally to graphitic carbon typically proceeds via polyaromatic heterocycles (Guisnet and Magnoux, 2001), which are not easily detected by conventional Raman spectroscopy because of fluorescence problems (Chua and Stair, 2003 Li and Stair, 1996). The use of UV excitation provides a powerful means to circumvent fluorescence problems and tackle the identification of the carbonaceous deposits (Chua and Stair, 2003). This subject was discussed in detail by Stair (2007). Polyaromatic deposits were burned off very quickly upon restoration of oxidizing conditions (Boulova et al., 2001 Mul et al., 2003 Puurunen and Weckhuysen, 2002 Puurunen et al., 2001). 

The formation of hydrocarbons from methanol catalyzed by zeolite H-MFI has been investigated extensively 60,61). As with many hydrocarbon conversions, the catalytic activity of the methanol-to-hydrocarbons reaction decreases over time as a result of the buildup of high-molecular-weight carbonaceous deposits (coke). UV Raman spectroscopy was employed to characterize the accumulation and chemical nature of deposited hydrocarbons as a function of time and reaction temperature with both methanol and dimethyl ether as reactants and with zeolite MFI of various Si/Al atomic ratios as catalysts the first account of this work reported results for a zeolite MFI with low acid content (Si/Al = 90) (62). Both polyolefin and a cyclopentadienyl species were observed as intermediates during the formation of polyaromatic retained hydrocarbons. These observations strongly confirm the mechanism of coke formation proposed by Schulz and Wei (63) involving aromatic formation via a five-membered ring... 

As already discussed, carbon deposition is a significant problem for SOFC anodes operated on carbonaceous fuels. Deposits fill the open microstructure of the anode leading to a reduction in performance over time from restricted fuel access as well as metal dusting corrosion and catastrophic failure via cracking and delamination. The group of Rob Walker, previously based Maryland and now in Montana, has greatly advanced the used of in-situ Raman spectroscopy, particularly for the investigation of carbon deposition [113, 147, 160-162]. 

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