Minimum sulfur content of Co-Mo series low-temperature catalysts

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This post was last edited by shanypr on 2011-10-10 08:10. Minimum sulfur content in Co-Mo-based low-temperature shift catalysts 1. The concept of anti-sulfurization: Since the active components of cobalt-molybdenum-based shift catalysts are Co-Mo sulfides, a certain amount of H2S is required to maintain their sulfide structure. To date, our understanding of the sulfided structure of cobalt-molybdenum catalysts, particularly that of their active centers, is not yet thorough or comprehensive. It is generally believed that the active components of such catalysts exist primarily in the form of MoS2. The equilibrium H2S concentration can be calculated by reacting MoS2 with H2O; under certain vapor pressures, the following reaction occurs: MoS2 + 2H2O = MoO2 + 2H2S. Therefore, a certain amount of H2S must be present in the reaction gas. When the H2S concentration is below its equilibrium level, the aforementioned reaction proceeds in the forward direction, resulting in desulfurization. Due to differences in the sources of thermodynamic data for the reactants and products, the lowest H2S concentrations calculated using thermodynamic methods vary considerably, and these values do not correspond to actual operating conditions. Therefore, investigating the minimum hydrogen sulfide concentration is helpful for the development of this catalyst and also aids in determining the reasons for its deactivation. 2 Adsorption of H2S by catalysts. It is well known that copper-based catalysts used for methanol synthesis are highly sensitive to the S content in the feed gas; poisoning occurs as early as when S1×10－6 is present. Medium-temperature iron-chromium catalyst: Fe3O4 + 3H2S + H2 → 3FeS + 4H2O. At 350°C and a gas-to-vapor ratio of 0.7, thermodynamic calculations indicate that the minimum H2S concentration (on a dry basis) is 401×10‑6 (609 mg/Nm3); however, for certain pure Fe-Cr medium-temperature catalysts, their activity decreases when the H2S concentration reaches 100×10‑6, leading to the formation of FeS. Iron catalyst for ammonia synthesis: Fe + H2S → FeS + H2. Thermodynamic calculations indicate that the minimum H2S concentration required is 40×10‑6, whereas in actual operating conditions poisoning occurs when H2S exceeds 1×10‑6. Why is there such a large difference between the actual operating conditions and the theoretical values? The reason is that H2S possesses unshared electron pairs (the sulfur element can achieve a valence of +6), which allow it to form coordinate bonds with the shared electrons of the transition metals in the catalyst’s active components, thereby adhering strongly to the surface of the catalyst. As a result, the H2S concentration on the surface is **higher than that in the gas phase; many scholars have conducted research on this topic, and all of them have confirmed this fact. Similarly, since the Mo in MoS2 has 2 more electrons available for sharing (the Mo element can be in a +6 valence state), H2S will also form covalent bonds with MoS2 and adsorb on the surface of the catalyst, resulting in a higher H2S concentration on the surface than in the gas phase of the reactor bed. Since MoS2 has a much weaker ability to donate shared electrons compared to Cu, Fe, M, and Fe3O4, its adsorption capacity for H2S is also lower. Through experiments and calculations, we found that factors such as the proportion of active components in the catalyst, the method of preparation, the state of cobalt and molybdenum before sulfidation, the type of additives used, and the sulfidation process all result in significant variations in H2S adsorption capacity, with the adsorption coefficient ranging from 1.5 to 4. This is similar to how different Fe-Cr-based catalysts exhibit varying H2S adsorption capacities due to differences in their preparation methods, which in turn affects their sulfur resistance. The study also found that the greater the ability of the catalyst surface to adsorb H2S, the stronger its resistance to low sulfur levels, yet its activity decreases; this is likely because the adsorbed H2S covers the active surface. Therefore, the H2S adsorption capacity of the catalysts used in different conversion processes (medium-low conversion or full low conversion) should also vary.