Kinetics of hydrogen desorption from silica-supported rhodium

Abstract

Aiff°(H3+) is off by 3 kcal/mol (which is more than likely), this would bring the value up to AHf°298(H02) = 3.4 ± 2 kcal/mol, which is in good agreement with most data.A mean value °298( 02) = 3.5 kcal/mol is obtained by considering most of the data except those from Kochubei and Moin (which was ruled out) and from Khachatryan et al. (which has a large uncertainty). An upper limit of 4.5 is adopted from Foner and Hudson's measurements and Heneghan and Benson's measurements. A lower limit of 3.0 is assigned since three studies15•17•21 show that the value of AHf°(H02) has to be greater than 3.0 kcal/mol. Therefore, we recommend AHf°298(H02) = 3.5^5°kcal/mol as the best value.Acknowledgment. This work has been supported by the U.S. Army Research Office for scientific research under grant No. DAAG29-82-K-0043 and the National Science Foundation under grant No. CHE-79-26623. Registry No. Hydroperoxyl radical, 3170-83-0.

Chemical and Nanostructural Aspects of the Preparation and Characterisation of Ceria and Ceria-Based Mixed Oxide-Supported Metal Catalysts

“…This observation, which certainly complicates the interpretation of the diagrams, is not, however, unexpected in view of the effect of the temperature of evacuation on the magnetic susceptibility of different M/Ce0 2 catalysts on which hydrogen has been preadsorbed (71,204,217,218). All these results clearly show that, in the presence of a dispersed noble metal, the hydrogen desorption from ceria starts at fairly low temperatures, being significant at 373 K. Since the TPD peaks due to H 2 desorption from supported noble metal phases typically occur below 473 K (63,141,381), some overlapping between both, the direct desorption reaction from the metal and the backspillover process, seems to be unavoidable. A remarkable difference between the trace A 2 (T^j,,: 473 K) and traces B 2 and B 3 (Tram: 773 K) in Figure 4.7 is the position of the main peak.…”

Distinct roles of copper in bimetallic copper–rhodium three-way catalysts deposited on redox supports

“…The first steps of the NO reduction with rhodium is assumed to be the NO adsorption followed by its dissociation [39], which is inhibited in presence of oxygen [18]. The N2O formation is due to the reaction of adsorbed NO (not yet dissociated) with adsorbed atomic nitrogen [40][41][42] or with another adsorbed NO species [43]. Whatever the mechanism, the decrease of the N2O formation can be ascribed to better properties for NO dissociation when using reducible supports.…”

Temperature‐programmed desorption: Multisite and subsurface diffusion models

“…This is shown by simulating two-peak TPD spectra with a multisite model and a subsurface diffusion model, and comparing how the spectra change as these parameters are varied. The multisite model presented here assumes that the two peaks are produced by desorption from two distinct adsorption sites, and is similar to the model described by Chin and Bell (1983). The subsurface diffusion model assumes that the surface contains only one type of adsorption site, and that the high-temperature peak is caused by subsurface diffusion of the adsorbate.…”

“…For reasonable carrier gas flow rates, we also can neglect accumulation of the adsorbate in the reactor (Cvetanovic andAmenomiya, 1967, 1972;Falconer and Schwarz, 1983). This assumption was not made by Chin and Bell (1983) in their multisite model. We also assume that the…”