Surface carbon formation

Figure 6.5 Structure-activity link between growth of surface carbon, formation of butadiene (BD) and formal oxidation state of vanadium (reaction conditions 0.4 mbar n-butane and 723 K).

Mechanism of Surface Carbon Formation During the Pyrolysis of Benzene in the Presence of Hydrogen... 

Cu foils were selected as inert surfaces for these experiments since they do not present significant catalytic activity (3). The rate of surface carbon formation on pure Cu polycrystalline foils was measured with the aid of a microbalance tubular flow reactor. The principles of this equipment were described in a previous paper (4). 

Figure 6.3 A comparison of experimentally observed OCVs and those predicted by the model and equilibrium calculations with and with out surface carbon formation.

In this chapter a detailed CFD study of the chemical and electrochemical processes in an internally reforming anode supported SOFC button cell was carried out. Detailed models for chemistry, electrochemistry and porous media transport have been implemented into the commercial CFD code FLUENT with the help of used defined functions (UDF). Simulation results were compared with experimentally reported data. The comparisons lead to the conclusion that precise calculation of surface carbon formation is critical for the accurate prediction of OCVs for hydrocarbon fuels with very low H2O content, and that Nemst equation may not be valid for the calculation of OCV for a fuel composition such as the one considered here. Anodic overpotentials showed remarkable difference from expected behavior. 

From the energy profiles, we are also able to tell that once the bent CO2 species occurs at the surfaces, it may readily leave the surface as a CO2 molecule, since this process has no obvious barrier. Thus, if we take the states with weakly adsorbed CO at the two surfaces as the ISs, the overall barriers of gas-phase CO2 formation at these two surfaces are just those for the formation of surface bent CO2. However, it should also be noted that once the bent CO2 species occurs at the surfaces, carbonate formation can compete with the CO2 desorption. In particular, we determined that, at Ce02(l 10), the reaction between adsorbed CO2 and a nearby lattice O3,. also has no barrier, and the CO2 at Ce02(l 11) is able to react with a nearby lattice 4, with a barrier of 0.35 eV. 

After reduction and surface characterization, the iron sample was moved to the reactor and brought to the reaction conditions (7 atm, 3 1 H2 C0, 540 K). Once the reactor temperature, gas flow and pressure were stabilized ( 10 min.) the catalytic activity and selectivity were monitored by on-line gas chromatography. As previously reported, the iron powder exhibited an induction period in which the catalytic activity increased with time. The catalyst reached steady state activity after approximately 4 hours on line. This induction period is believed to be the result of a competition for surface carbon between bulk carbide formation and hydrocarbon synthesis.(6,9) Steady state synthesis is reached only after the surface region of the catalyst is fully carbided. 

As the metal particle size decreases the filament diameter should also decrease. It has been shown that the surface energy of thirmer filaments is larger and hence the filaments are less stable (11,17-18). Also the proportion of the Ni(l 11) planes, which readily cause carbon formation, is lower in smaller Ni particles (19). Therefore, even though the reasons are diverse, in practice the carbon filament formation ceases with catalysts containing smaller Ni particles. Consequently, well dispersed Ni catalysts prepared by deposition precipitation of Ni (average metal particle size below 2-3 nm) were stable for 50 hours on stream and exhibited no filamentous coke [16]. 

Figure 6.2 VEEL spectra when a mixture of CO and 02 was coadsorbed at a Cu(l 10)-Cs surface (ctCs — 3.5 x 1014 cm-2) at 80 K and the adlayer warmed to 298 K. Note the formation of surface carbonate (cf. Figure 6.9). (Reproduced from Ref. 6).

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