(9e) Adsorption of Trace Metal Contaminants from Coal-Derived Syngas on Metal Surfaces and Novel Bimetallic Adsorbents

To produce methanol from syngas obtained from the gasification of coal, the syngas needs to be free from trace metal contaminants such as elemental gas-phase mercury (Hg⁰), which can poison the methanol synthesis catalyst. The temperature at the gasifier outlet and those required for the synthesis of methanol are high (>200 °C). However, most of the commercial adsorbents are inefficient for Hg⁰ removal from syngas at high temperatures and in the presence of water vapor. Previous studies have shown that the strongest Hg⁰ adsorption occurs on expensive Pd-based adsorbents, and the efficiency of these adsorbents drastically decreases with an increase in temperature. In this work, novel bimetallic adsorbents are identified for the removal of Hg⁰ at high temperatures, and insights into the segregation behavior of alloys and the poisoning effect of Hg⁰ is obtained. Further, the most promising monometallic and bimetallic adsorbents are synthesized and their efficacy for Hg⁰ adsorption at high temperatures is shown.

DFT calculations are coupled with Gibbs free energy calculations to evaluate the feasibility of Hg⁰ adsorption on two different types of bimetallic adsorbents, i.e., homogenous alloys and overlayers, for Hg⁰ removal at temperatures between 50 to 200 °C. Hg⁰ binding energy and Gibbs free energy calculations are performed on 56 different compositions of bimetallic adsorbents. For most homogeneous alloys and over-layered adsorbents, Hg⁰ adsorption is stronger than that on the individual component metals due to the ligand and strain effects. Novel bimetallic adsorbents comprising of Rh, Ru, or Pd overlayers on Ag and Au exhibit a high Hg⁰ adsorption strength, and the binding is even stronger than that on the best reported material, i.e., monometallic Pd. The effect of increasing overlayers of Pd, Pt, Rh, Ir, and Ru is evaluated, and it is found that even a single overlayer of these metals on Ag and Au increases the binding strength significantly. The d-band center, d-band width, and upper edge of the d-band for various monometallic and bimetallic surfaces are calculated. An increase in the Hg⁰ binding strength is correlated to the upward shift in the d-band center (Figure 1 (a)). Further, segregation energy calculations predict the tendency of Hg to segregate towards the surface of amalgams and disturb the perfect planar geometry of the Pd, Pt, Rh, Ru, Ir, Cu, Ag, and Au surfaces to form a non-crystalline Hg-rich amalgam surface. An analysis of the binding of various adsorbates (H, O, N, and S) shows that the adsorption becomes significantly weaker on various sites in close proximity to pre-adsorbed Hg. Moreover, for certain adsorbates, adsorbents and site locations, the adsorption does not take place on the proximal sites. The partial density of states is analyzed for various amalgam surfaces, which explains the poisoning effect of Hg⁰ on metallic catalysts.

The bimetallic adsorbents shortlisted from DFT and Gibbs free energy calculations are prepared by the galvanic displacement method, whereas the monometallic adsorbents are prepared using the incipient wetness method. The initial Hg⁰ adsorption efficiency of various lab-synthesized monometallic and bimetallic adsorbents supported on doped aluminas (SIRAL-40, SASOL) and mesoporous SBA-15 is shown in Figure 1 (b). Ag supported on SIRAL-40 (S-40) is efficient only at low temperatures (less than 150 °C). However, the bimetallic adsorbents Pd-Ag/S-40 and Rh-Ag/S-40 exhibit a significantly high initial adsorption efficiency (>96%) at 300 °C. Moreover, these adsorbents are active for Hg⁰ adsorption even at 350 °C. To the best of our knowledge, any material capable of adsorbing Hg⁰ at such high temperatures is not reported in the literature, thus marking a breakthrough in warm syngas cleanup. More bimetallic adsorbents identified by DFT will be prepared and the effect of operating conditions will be described in the full manuscript.