Carbon evolution, activation energy

Adsorption and reaction of C2H4. Ethylene reversibly adsorbed on clean Au(lll) at 95 K. Desorption of ethylene in TPD from the clean Au(lll) surface was observed from approximately 100 - 250 K, with a large peak at 104 K. The reason for the broad desorption peak is uncertain. Outka and Madix (8) have shown that a broad range of desorption is typical of C2 hydrocarbons on Au(llO). They propose that these molecules are weakly bound and therefore occupy a variety of binding sites with different activation energies, consequently they desorb at different temperatures. We observed no H2 evolution from the surface up to temperatures of 700 K and AES showed no residual carbon on the surface after heating. Thus, C2H4 is reversibly, and most likely molecularly, adsorbed on clean Au(lll). 

Carbon dioxide and hydrogen also interact with the formation of surface formate. This was documented for ZnO by the IR investigation of Ueno et al. (117) and, less directly, by coadsorption-thermal decomposition study (84). Surface complex was formed from C02 with H2 at temperatures above 180°C, which decomposed at 300°C with the evolution of carbon monoxide and hydrogen at the ratio CO Hs 1 1. When carbon dioxide and hydrogen were adsorbed separately, the C02 and H2 desorption temperatures were different, indicating conclusively that a surface complex was formed from C02 and H2. A complex with the same decomposition temperature was obtained upon adsorption of formaldehyde and methanol. Based upon the observed stoichiometry of decomposition products and upon earlier reported IR spectra of C02 + H2 coadsorbates, this complex was identified as surface formate. Table XVI compares the thermal decomposition peak temperatures and activation energies, product composition, and surface... 

An analysis of the rate of CO, CO2 and H2O evolution during TPO of industrial and laboratory coked cracking catalysts has provided information on the mechanism and energetics of coke combustion. The mechanism has been deduced from previously reported studies on amorphous carbon oxidation [8], while rate parameters have been calculated from non-linear regression simulations of the TPO spectra. The rate of water vapour formation has not been analysed due to re-adsorption expected to affect the apparent kinetics. "Soft" and "hard" coke have been identified in the spectra, and oxidation activation energies of each compared. A further outcome of this work is the proposal that coke deposition on cracking catalysts proceeds from "soft" to "hard" coke via a series of dehydrogenation or dehydration steps. 

Enthalpies of dissociation may be determined from measurements of the variation of the equilibrium pressure of the gaseous product with reaction temperature [60], In the study of the kinetics of these reactions (Chapters 8 and 12) consideration must be given to the possible influence of the reverse reaction on the rate measurements [68], Kinetic parameters should be measured at very low pressures of HjO or CO2 in the reaction vessel [45], At higher pressures, equilibria may be established within the pores of the solid product. This is given as the explanation for the frequent observation that the value of the enthalpy of dissociation of a particular carbonate is close to the value of the activation energy measured for evolution of COj [45]. 

Table 78.5 The hydrogen evolution rates and activation energy for the dehydrogenation of various organic hydrides on carbon-supported Pt catalysts at 325 °C...

Raman spectroscopy has been successfully employed to follow the kinetics of reactions at HTR Kessler et al. [197] studied the decomposition of tertiary butyl peroxypivalate in solution, obtaining the same activation energy as those which have been obtained by other conventional methods. Brill et al. have recently published an interesting series of articles [198-200] on the vibrational spectroscopy [Fourier transform infrared (FTIR) and Raman] of hydrothermal reactions, covering the decomposition of species such as urea, ammonium carbonate, cyanamide, or dicyandiamide. All of these works demonstrate the ability of vibrational spectroscopic techniques to obtain detailed information about the composition and evolution of aqueous systems in extreme conditions,... 

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