✍ Scribed by M.R. Prairie; J.G. Highfield; A. Renken
No coin nor oath required. For personal study only.
The advantage of coupling a diffuse-reflectance infrared cell with a continuous-feed recycle reactor for in situ kinetic and mechanistic studies is illustrated in the study of CO, methanation over Ru/TiO,. From transient and steady-state experiments, metal-adsorbed CO is identified as a major reaction intermediate and at 120°C occupies approximately 40% of the available ruthenium surface, while methane formation proceeds at a turnover frequency of 2.8 x lo-' s-l. In contrast, CO methanation is totally hindered under the same conditions. A simple kinetic model is proposed to account for the main trends observed. For CO, methanation, the temperature insensitivity of adsorbed CO coverage is interpreted on the basis of a generalized (two-stage) intermediate supply/consumption mechanism involving the reverse water-gas shift reaction.
In situ infrared spectroscopy has been used for many years in the study of catalysis for identifying adsorbed intermediates and their relationship to the gas phase for an enormous variety of reaction, adsorption, and desorption systems. Until recently, such experiments have been carried out in the transmission mode where a small continuous reactor (i.e., IR cell) contains a very thin, pressed wafer of the catalyst through which IR radiation is directed. In this arrangement, the reaction mixture enters the IR cell and is somehow directed around the catalyst wafer in a single-pass operation. This configuration often leads to internal and external transport limitations and completely uncharacterized gas mixing (Prairie et al., 1988;Kaul and Wolf, 1985). Furthermore, reliable information on how the gas composition relates to the adsorbate distribution seen by IR can only he obtained in the extreme of very low conversions, i.e., differential operation. To overcome some of these problems, Vannice et al. (1979) reported an elegant cell design in which the flowing gas mixture is forced through the pores of the pressed wafer, thereby greatly reducing the problem of diffusion and allowing the wafer to be realistically modeled as an ideal differential or integral reactor. However, this design places limitations on allowable total flow rates to avoid breaking the delicate catalyst wafer.
Many of the problems which plague the transmission arrangement can be avoided altogether by employing Diffuse-Reflectance Spectroscopy (DRS) which was recently introduced by Hamadeh et al.
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