(219b) Theoretical Studies and Experimental Investigations on the Removal of Trace Metal Contaminants from Syngas Using Monometallic and Bimetallic Adsorbents

To utilize the syngas derived from the gasification or pyrolysis of carbonaceous feedstocks for the production of valuable chemicals, the syngas has to be free from various trace contaminants such as Hg, As, Se, P, Cd, and Pb. These elements get released during gasification or pyrolysis in their elemental form, hydride form (AsH₃, H₂Se, PH₃), or sulfide form (PbS). Since they are volatile at temperatures above 400 °C and are potential poisons to the catalyst(s) employed downstream for syngas processing, they need to be eliminated. Various commercial carbon and sulfur-based adsorbents are used for syngas cleanup, but are inefficient for the removal of trace elemental contaminants at high temperatures. Hou et al. (2014) have reported that the strongest adsorption of one such contaminant viz. gas-phase elemental mercury (Hg⁰) occurs on Pd-based adsorbents and on a few bimetallic adsorbents. In the present work, the adsorption of various gas-phase contaminants, viz., Hg⁰, AsH₃, H₂Se, PH₃, Cd, Pb, and PbS is studied on monometallic and novel bimetallic surfaces. Insights are obtained into the dissociation of AsH₃, H₂Se, and PH₃ on metals and into the segregation behaviour of Hg, As, Se, P, Cd, and Pb in alloys. Further, the most promising monometallic adsorbents are synthesized and their efficacy for Hg⁰ adsorption for different operating conditions is given.

Methodology

First-principles density functional theory (DFT) calculations are performed to calculate the adsorption energy and to evaluate the most stable geometric configurations for the adsorption of gas-phase Hg⁰, AsH₃, H₂Se, PH₃, Cd, Pb, and PbS on various metal surfaces. The calculations are carried out using the Vienna ab initio simulation package (VASP) and by employing the Perdew-Burke-Ernzerhof (PBE), optPBE-vdW, optB88-vdW, and PBE-D3 functionals of the generalized gradient approximation along with an energy cut off of 400 eV. The reaction energy pathway for AsH₃ dissociation on various metals is studied using the CI-NEB method. AsH₃ dissociation on the (1 1 1) surface of various metals is considered to occur by the sequence of the elementary reactions described by Eqs. (R1)-(R4), where the subscript ‘gas’ represents the gas-phase species and the subscript ‘ads’ represents the adsorbed surface species.

AsH_{3 (gas)} → AsH_{3 (ads)} (R1)

AsH_{3 (ads)} → AsH_{2 (ads)} + H _(ads) (R2)

AsH_{2 (ads)} → AsH _(ads) + H _(ads) (R3)

AsH _(ads) → As _(ads) + H _(ads) (R4)

Similarly, calculations are performed considering the step-wise abstraction of H atoms from H₂Se and PH₃ as well and the reaction energy barriers and the reaction energies for the elementary reactions are calculated.

The Pt-based monometallic adsorbents shortlisted from DFT are prepared using the incipient wetness method using doped aluminas (SIRAL-40, SASOL), metal oxides (Ceria, zirconia, titania) and silica-based molecular sieve (SBA-15) as support materials.

Results and Discussion

DFT calculations show that the adsorption of Hg⁰, Cd, Pb, and PbS is the most favourable on the hollow sites of most metal surfaces, whereas the adsorption of hydrides AsH₃, H₂Se, and PH₃ is favourable on the top sites of most metals considered in this study. The binding energy calculations indicate that the adsorption of gas-phase Hg, AsH₃, H₂Se, PH₃, Cd, Pb, and PbS is exothermic on various metals. The calculations reveal that Rh, Ru, Pd, Pt, and Ir are the most promising metals for the removal of these contaminants from gas streams.

AsH₃ dissociation on various metals is studied and the reaction energy diagram for AsH₃ dissociation is shown in Figure 1. The analysis of the calculated reaction energies and activation barriers for the catalytic dissociation of AsH₃ on various metal surfaces shows that its dissociation is more facile on Rh, Ru, Pd, Ir, and Pt as compared to Cu, Ag, and Au. Further, the dissociation of AsH₃ on Ir, Pt, and Pd is found to occur without the formation of the intermediate adsorbed species AsH. The dissociation on these metal surfaces occurs in two steps with the first dissociation step given by Eq.(1) and the second dissociation step as AsH_{2 (ads)} → As _(ads) + 2 H _(ads). Similar results are observed for the dissociation of H₂Se and PH₃ on various metals. Further, segregation energy calculations predict the tendency of elements Hg, As, Se, P, Cd, and Pb to segregate towards the surface of Ru, Rh, Pt, Ir, Pd, Cu, Ag, and Au after alloy formation. Furthermore, the formation energy calculations indicate the possibility of the formation of alloys between metals Ag, Au, Cu, Ir, Pd, Pt, Rh, and Ru and elements Hg, As, Se, P, Cd, and Pb. The calculations of the partial density of states for different alloys containing Hg, As, Se, P, Cd, and Pb provide information on the alterations in the electronic properties of the alloy surface and the effect of these contaminants on specific orbitals.

DFT calculations are performed to evaluate the binding of the trace contaminants on 56 different compositions of bimetallic adsorbents. The binding energy of AsH₃ on the (1 1 1) plane of various bimetallic surfaces is given in Figure 2. It is found that bimetallic adsorbents with an overlayer of Ru, Rh, Ir, or Pt on Ag or Au exhibit a higher affinity for AsH₃ as compared to the monometallic adsorbents. Similar results are obtained for the adsorption of Hg, H₂Se, PH₃, Cd, and Pb. The adsorption of these contaminants on the overlayered bimetallics is stronger than that on monometallic surfaces due to the ligand and strain effects. Bimetallic adsorbents comprising of a single overlayer of Rh, Ru, Pd, Pt, and Ir on Ag, Au, or Cu are the novel and cost-effective adsorbents identified for the removal of gas-phase contaminants Hg, AsH₃, H₂Se, PH₃, Cd, and Pb from syngas.

The d-band center, d-band width, and upper edge of the d-band for various monometallic and bimetallic surfaces are calculated and correlated to the adsorption energies of various contaminants . The increase in adsorption energy of Cd on various monometallic and bimetallic surfaces is correlated to the upward shift in the d-band center (Figure 3). Similar results are obtained for Hg, AsH₃, H₂Se, PH₃, and Pb adsorption on various monometallic and bimetallic surfaces. Thus, the adsorption of these contaminants on various surfaces is found to be consistent with the d-band model.

The initial Hg⁰ adsorption efficiency and the adsorption capacity of the lab-synthesized Pt adsorbents Pt/SBA-15, Pt/S-40 (SIRAL-40), Pt/ZrO₂, Pt/TiO₂, and Pt/CeO₂ are evaluated. The adsorption efficiency of these Pt-based adsorbents at 270 °C for various compositions of syngas is shown in Figure 4. The Hg⁰ adsorption efficiency of Pt/SBA-15 is more than 95% in a N₂ atmosphere. Under these conditions, the adsorption efficiency of Pt/SBA-15 is the highest followed by Pt/S-40, Pt/ZrO₂, Pt/TiO₂, and Pt/CeO₂. The adsorption efficiency over Pt/S-40, Pt/ZrO₂, Pt/TiO₂, and Pt/CeO₂ reduces significantly in the presence of syngas components H₂ and H₂O thus showing their inhibiting effect towards Hg⁰ adsorption. In contrast, the adsorption efficiency of Pt/SBA-15 is retained above 90% in the presence of H₂ and H₂O. Characterization results from XRD and H₂-TPR techniques indicate the presence of Pt nanoparticles in the elemental state on Pt/SBA-15, Pt/S-40, and Pt/TiO₂, and the presence of Pt nanoparticles in the oxidized form on Pt/ZrO₂ and Pt/CeO₂. Various other characterization techniques such as UV-vis spectroscopy, BET surface area analysis, Field emission-SEM, and TEM are used to study the physical and chemical nature of the adsorbents, and their correlation with the adsorption activity, which would be described in the full manuscript. The bimetallic adsorbents identified by DFT will be prepared and the effect of operating conditions will also be discussed in the full manuscript.

References

[1] Hou, W., Zhou, J., Yu, C., You, S., Gao, X., Luo, Z. (2014). Pd/Al₂O₃ Sorbents for Elemental Mercury Capture at High Temperatures in Syngas. Ind. Eng. Chem. Res. 53, 9909-9914.