Insights into Copper Sulfide Formation from Cu and S K edge XAS and DFT studies

Abstract

An understanding of the fundamentals of the reaction between CuO with trace amounts of H 2 S to form CuS products is critical for the optimal utilization of this process in sulfur removal applications. Unfortunately, CuS is a complex material, featuring various Cu 2-x S compounds (with 0 ≤ x ≤ 1), distorted crystal phases, and varying electronic structures and coordination environments of Cu and S ions. In this work, we combine ex situ and in situ X-ray absorption spectroscopy (XAS) at S and Cu K edges, fixed bed sorption experiments, DFT simulations, and other characterization techniques to speciate the CuS products formed at different temperatures (298−383 K) and from CuO sorbents with different crystallite sizes (2.8−40 nm). The results of our analysis identify the formation of a distorted CuS layer at the surface of CuO crystals with disulfide groups with shorter Cu−S bonds and higher delocalization of the positive charge of the Cu center into (S 1− ) 2 . This distorted CuS layer dominates the XAS signal at lower temperatures (298−323 K) and at the initial stages of sulfidation at higher temperatures (353 and 383 K) where conversion is low (<40%). First-principles atomistic simulations confirm the thermodynamic favorability of the formation of surface (S 1− ) 2 on both CuO (111) and (1̅ 11) surfaces, providing further support for our experimental observations. Furthermore, these simulations reveal that the presence of disulfide bonds stabilized surface hydroxyl groups, leading to lower Gibbs Free Energies of their surface migration.

Lanthanum induced lattice strain improves hydrogen sulfide capacities of copper oxide adsorbents

“…The phase and strain identification from XRD analysis support the conclusion of La incorporation into CuO. As mentioned above, the La-containing samples exhibit higher removal capacities than the CuO sample despite possessing larger CuO crystallites, and according to previous work (both by the authors 23,25 and others 22 ), the increase in capacity results from strain induced by lattice expansion. Thus, the addition of La into CuO disrupts the lattice and leads to the increase in H 2 S capacity.…”

“…In summary, the laboratory‐based characterization showed that CuO‐La‐2, compared to CuO‐La‐1, features larger crystals but has a higher inclusion of La throughout the CuO phase. These results suggest that the improvement in sulfur uptake by CuO‐La‐2, compared to CuO‐La‐1, is related to a higher concentration of La dopants which induce lattice expansion and strain, which have been previously identified as a means to increase H 2 S capacity of CuO adsorbents 22,23,25 …”

“…Another important spectral feature that speciates the type of copper sulfide present are the pre‐edge peaks at 2469.9 eV (c) and 2470.7 eV (b) in Figure 3. These features correspond to S 2− 1s ➔ Cu 2+ 3d 9 and S 2 2− 1s ➔ Cu 2+ 3d 9 transitions, respectively 25,41 . Figure 3 shows that the S K‐edge spectra of both spent La‐CuO samples and CuS reference contain both pre‐edge features, while peak (b) is absent in the spent CuO‐1 spectra.…”

“…The sulfidation of copper oxide is associated with a unit-cell volume expansion (60%) that disrupts lattice structure of the remaining CuO phase at the CuO-CuS interface. 25 This disruption is likely a critical factor for the overall extent of sulfidation, and explains, in part, the apparent lack of correlation between capacity and textural properties. [22][23] F I G U R E 1 STEM images of CuO-La-1 (a-b) and CuO-La-2 (d-e).…”

“…These results suggest that the improvement in sulfur uptake by CuO-La-2, compared to CuO-La-1, is related to a higher concentration of La dopants which induce lattice expansion and strain, which have been previously identified as a means to increase H 2 S capacity of CuO adsorbents. 22,23,25…”