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The dynamic behavior of the ethanol adsorption on g-alumina were investigated at 180 and 2008C by the transient-response method coupled with FT±IR data of the catalyst surface. The existence of three adsorbates was demonstrated: a reacting species which is the precursor for the formation of the gas-phase ethene; an inhibiting species responsible for the low steadystate reaction rate; and a spectator species accumulating on the catalyst surface. Their infrared spectra indicate an ethoxidelike structure for the three adsorbates. Their C±H stretching bands can be depicted by four Lorentzians whose parameters indicate different surface environments. The surface concentration of the reacting species was determined on the basis of the transient ethene response. A value of 0.77AE0.07 mol/kg cat was found at 1808C. The surface concentration of the spectator species was determined by ex-situ thermogravimetric experiments. A value of 0.13AE0.01 mol/kg cat was found at 1808C. The most likely structure of this species corresponded to an ethanol molecule coordinated in a bidentate manner on Al 3 cations, with stabilisation of the alcoholic-hydroxyl group via lateral hydrogen bonding with an adjacent surface hydroxyl. # 1998 Elsevier Science B.V. All rights reserved.

Periodic operation was applied to the CO2 methanation reaction at 383 K on 2% Ru/TiO*. A continuous feed recycle reactor combined with a diffuse reflectance infrared cell and a mass spectrometer allowed to follow simultaneously the gas phase CO, and CH4 as well as the adsorbed species (CO), and (formate),. Experiments consisting of periodic variations of CO2 in the hydrogen feed showed response curves with maxima/minima shifted in time in the sequence CO,, (formate) a + (CO) =--f CH,. Similar delays measured for the (CO) a formation and hydrogenation indicate that both of these processes are limiting the overall reaction rate. A kinetic model was proposed and verified under periodic conditions. The main experimental trends, which are pronounced time lags between CO;?, (CO) d and CH4, could be described satisfactorily.

Abstract--Transient experiments were applied to the study of the adsorbed CO intermediate, (CO),,, formed during the CO2 methanation reaction on a 2% Ru/TiO2 catalyst at 383 K.Step-up experiments showed that the (CO),, formation steps are inhibited by H20 and enhanced by H 2.Step-down experiments showed that the (CO),, hydrogenation is not influenced by the partial pressure of water. Based on the fact that water inhibits the overall CO 2 methanation, it is deduced that the rate limiting process in the overall reaction is (CO),, formation.

The oxidation of CO on silica-supported hematite (Fe 2 O 3 ) was studied by the step-response method in a tubular fixed-bed reactor, at temperatures ranging between 270 and 350°C. The oxidation process appeared to proceed through two stages. Firstly, oxygen atoms adsorbed on the surface of hematite react with gas phase CO according to an Eley-Rideal mechanism. Once that adsorbed oxygen has been consumed to some extent, surface oxygen from the lattice of iron oxide is removed in a second stage involving CO adsorption and CO reactive desorption steps, thus generating surface oxygen vacancies. Further reduction of hematite proceeds through diffusion of subsurface oxygen into surface oxygen vacancies. On this basis, a kinetic model was developed, which quantitatively describes the transient behavior of the oxidation process. The activation energies and pre-exponential factors of the rate constants and characteristic subsurface oxygen diffusion time could be determined.