Sulfur poisoning temperature

Conditions of hydrogenation also determine the composition of the product. The rate of reaction is increased by increases in temperature, pressure, agitation, and catalyst concentration. Selectivity is increased by increasing temperature and negatively affected by increases in pressure, agitation, and catalyst. Double-bond isomerization is enhanced by a temperature increase but decreased with increasing pressure, agitation, and catalyst. Trans isomers may also be favored by use of reused (deactivated) catalyst or sulfur-poisoned catalyst. 

Figure 9. Relative rate of CO hydrogenation as a function of copper coverage on a Ru(OOOl) catalyst Reaction temperature 575K. Results for sulfur poisoning from Figure 7 have been replotted for comparison.

However, now there is still the uncertainty as to the relative increase in anode/ electrolyte interfacial resistance under different current densities. Primdahl and Mogensen [39] found that the relative increase in anode interfacial resistance due to sulfur poisoning is independent of temperature and cell current density (up to 100 mA/cm2) when the anode was subject to 35 ppm H2S at 1000°C. Whether this is also the case when the cell temperature is lower (i.e., at 750°C), the H2S concentration is lower (i.e., 1 ppm), and the current density is higher (i.e., up to 1 A/cm2) is not clear at the current stage. 

A small amount of sulfur in the fuel dramatically degrades the performance of Ni-YSZ anodes due to the adsorption of sulfur on Ni surfaces. The extent of sulfur poisoning, as measured by the relative increase in cell resistance, always increases with H2S concentration in the fuel, but decreases with cell operating temperature and cell current density. Sulfur poisoning of Ni-based anode is generally more reversible as the cell temperature increases and as H2S concentration or exposure time is reduced. 

Sulfur poisons catalytic sites in the fuel cell also. The effect is aggravated when there are nickel or iron-containing components including catalysts that are sensitive to sulfur and noble metal catalysts, such as found in low temperature cell electrodes. Sulfur tolerances are described in the specific fuel cell sections of this handbook." In summary, the sulfur tolerances of the cells of interest, by percent volume in the cleaned and altered fuel reformate gas to the fuel cells from published data, are ... 

Catalyst formulations for ATR fuel processors mainly depend on the fuel and the operating temperature. ATR catalysts are required to be active simultaneously for hydrocarbon oxidation and SR reactions, be robust at high temperatures for extended periods and be resistant to sulfur poisoning and carbon deposition, especially in the catalytic zone that runs oxygen limited [33]. Moreover, they must be resistant to intermittent operation and cycles, especially in start-up and shut-down steps. 

Thermodynamics. At higher temperatures, Ni and other metal catalysts are believed to be less vulnerable to sulfur poisoning, since their sulfides are thermodynamically less stable. Figure 14 shows the change in AG with temperature for the following reaction. 

On nickel catalysts, which have been extensively studied (12-14), McCarty and Wise (14) showed that in a temperature range from 373 to 873 K, sulfur coverage of nearly half a monolayer can be reached with H2S partial pressures as low as 1-10 ppb. Such results, indicating that the equilibrium H2S <= H2(g) + S(a) is totally displaced toward the right side, show the very high affinity of sulfur for transition metals and thus the difficulties in avoiding sulfur poisoning in metallic catalysis. 

The deleterious effect of water vapor was speculated to be due to its inhibition of carbon formation freeing the metal surface for interaction by H2S. Thus, sulfur poisoning of nickel at high temperature (above 673 K) may be more representative of a carbon-fouled surface, whereas at low temperatures it may be more characteristic of the clean metal surface. Again, this needs to be confirmed by direct measurements of carbon and sulfur adsorption. For Ni/Al203 and Ni/ZrOz the extent of sulfur deactivation was about fiftyfold at 673 K at 523 K the extent of deactivation was about 1000-fold. However, for Raney Ni the extent of sulfur deactivation was tenfold higher at 673 K than at 523 K this difference in behavior also needs confirmation and explanation. 

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