In Situ Generation of Radical Coke and the Role of Coke-Catalyst Contact on Coke Oxidation

A thermogravimetric analyzer (TGA) equipped for flowing hydrocarbon gases allowed in situ deposition of coke on catalyst and support samples with excellent coke-catalyst contact. The coke deposition on the catalysts and supports, which occurs via a gas phase radical mechanism, depends on the reaction time, temperature, hydrocarbon concentration, and sample external surface area but not on the chemical composition of the support under the conditions used. The coke samples, including in situ generated samples and an industrial coke sample, are characterized quantitatively by both deconvolution of Raman spectra and temperature-programmed oxidation (TPO) analyses. Thermal aging of coke is shown to be effective in increasing the hardness of the coke samples. Ceria dispersed on α-alumina, used as a model catalyst for coke oxidation, allows coke oxidation at lower temperatures. Using these catalysts, coke deposited in situ is shown to oxidize similarly to ground (tight contact conditions) coked catalyst samples, suggesting that in situ coke deposition in the TGA can be used to generate samples with realistic coke-catalyst contacting, as might be found in an industrial reactor or catalyst bed. In situ coking is also observed to be reproducible and reliable as compared to loose and tight contact methodologies

Formation and Oxidation/Gasification of Carbonaceous Deposits: A Review

A wide variety of hydrocarbon processes, catalytic or noncatalytic, involve the formation of carbon deposits, either on catalysts or on reactor (or engine/exhaust) surfaces. Therefore, researchers have developed a large array of catalysts to aid the combustion of these deposits. Recently, the mechanism of catalytic carbon oxidation and/or gasification has been the focus of research in an attempt to design better catalysts for carbon removal. With this approach, understanding the mechanism of formation of different types of carbon deposits is desired. Efforts undertaken for studying oxidation or gasification of various forms of carbon deposits are discussed in this review, along with the techniques used to study the mechanism of oxidation/gasification. The kinetics of catalyzed and noncatalytic carbon oxidation are described in detail. The effect of reactive gases such as NO_<i>x</i>, water vapor, CO₂, and SO₂ on the gasification behavior of carbon deposits is also discussed. Reaction rates of oxidation/gasification of carbon under different operating conditions have been calculated, allowing for a comprehensive overview of carbon removal reactivity

Mechanistic Insight into the Synthesis of Higher Alcohols from Syngas: The Role of K Promotion on MoS₂ Catalysts

Operando infrared spectroscopy in combination with a kinetic study is used to elucidate the role of potassium on the conversion of carbon monoxide over K-promoted MoS₂ catalysts. More specifically, the initial break-in transient has been studied in detail. Stabilization of reaction intermediates, and effect of promoter on the intrinsic properties of MoS₂ are discussed. Adsorbed alkoxy species were found to play an important intermediate role in the syngas to alcohol route, and it was found that potassium stabilizes these species. Moreover, the electronic properties of MoS₂ change upon promotion, thereby allowing for a relatively easier activation of the CO molecule and a reduced hydrogenation activity toward alkanes