Influence of Step Defects on the H₂S Splitting on Copper Surfaces from First-Principles Microkinetic Modeling

An atomic level insight into the chemistry of hydrogen sulfide (H₂S) splitting by metallic copper (Cu) is a necessary prerequisite for understanding sulfur poisoning mechanism of Cu-based water–gas shift (WGS) catalysts. In the present work, we have combined periodic density functional theory predictions and a detailed microkinetic modeling of the H₂S dissociation on the stepped-defect (211), (311), and regular (111) faces of Cu to define the effect of step defects upon the reaction. The results indicate that on each surface examined, the dissociative adsorption facilely leads to the formation of element sulfur (S) via a stepwise H–S bond cleavage mechanism, with the initial molecular adsorption of H₂S preferred as the rate-limiting step. It has also been pointed out that the SH disproportionation reaction does not open an alternative path for surface atomic sulfur production under the studied reaction condition. These surfaces are all predicted to be significantly covered by the S species after sufficient exposure to a realistic environment containing only several ppm of H₂S. Furthermore, it is confirmed that (i) the full decomposition process is structure sensitive, and (ii) the driving force behind the step-enhanced activity of Cu toward this reaction arises not from kinetic but from thermodynamic factors. More importantly, our calculations have demonstrated that the H₂S tolerance of Cu steps (and other defects) for the WGS reaction is worsened by a factor of approximately 10³ as compared to a perfectly regular surface. Because these deficient sites are known as the most active sites of Cu-based shift catalysts in the absence of sulfur-containing species, it appears to be impossible to improve their activity without a dramatic loss of sulfur resistance through simply tuning catalyst surface morphology

Large Scale Two-Dimensional Flux-Closure Domain Arrays in Oxide Multilayers and Their Controlled Growth

Ferroelectric flux-closures are very promising in high-density storage and other nanoscale electronic devices. To make the data bits addressable, the nanoscale flux-closures are required to be periodic via a controlled growth. Although flux-closure quadrant arrays with 180° domain walls perpendicular to the interfaces (V-closure) have been observed in strained ferroelectric PbTiO₃ films, the flux-closure quadrants therein are rather asymmetric. In this work, we report not only a periodic array of the symmetric flux-closure quadrants with 180° domain walls parallel to the interfaces (H-closure) but also a large scale alternative stacking of the V- and H-closure arrays in PbTiO₃/SrTiO₃ multilayers. On the basis of a combination of aberration-corrected scanning transmission electron microscopic imaging and phase field modeling, we establish the phase diagram in the layer-by-layer two-dimensional arrays versus the thickness ratio of adjacent PbTiO₃ films, in which energy competitions play dominant roles. The manipulation of these flux-closures may stimulate the design and development of novel nanoscale ferroelectric devices with exotic properties