Insights into $CO_{2}$ activation on defective ZnS surfaces

TL;DR

The paper examines CO$_{2}$ activation on defective ZnS surfaces by combining Near Ambient Pressure XPS (NAP-XPS) and Density Functional Theory (DFT) calculations. The experiments show CO$_{2}$ adsorption becomes favorable at $573\, \mathrm{K}$ under a $CO_{2}$ atmosphere (up to $0.55\, \mathrm{mbar}$) on ZnS with Zn vacancies, leaving a persistent $CO_{2}^{\delta -}$ fingerprint even after evacuation, while mixed-gas experiments reveal carbonate-like $CO_{3}^{-}$ species can form in the presence of oxygen or CO. The DFT results reveal that $CO_{2}$ binds weakly to pristine ZnS ($E_{ads}\approx -0.07$ eV) but binds more strongly at a surface vacancy ($E_{ads}\approx -0.18$ eV), and that $O_{2}$ adsorption is endothermic on pristine ZnS ($E_{ads}\approx +0.27$ eV) but exothermic on defected ZnS ($E_{ads}\approx -0.41$ eV) with a tilted O–O bond indicating dissociative tendency. In mixed environments, CO can enhance activation via a carbonate-like pathway, while $CO_{2}$-derived intermediates can form without full dissociation, suggesting defect-driven routes to CO$_{2}$ utilization and potential methanol synthesis with hydrogen. These findings identify Zn vacancy defects as key active sites for CO$_{2}$ capture/activation on ZnS and provide design principles for ZnS-based catalysts.

Abstract

In this work, we investigate $CO_{2}$ activation on ZnS using Near Ambient-Pressure X-ray photoelectron spectroscopy measurements (NAP-XPS) and density functional theory calculations (DFT). Our NAP-XPS experiments reveal that $CO_{2}$ adsorbs onto a defective ZnS surface upon heating above $473 \ K$ in a $CO_{2}$ atmosphere (up to $0.55 \ mbar$). The $CO_{2}$ adsorption fingerprint is detectable even after cooling to room temperature under ultra-high vacuum. Our DFT calculations suggest that $CO_{2}$ adsorption is energetically favorable on ZnS surfaces containing zinc vacancies, highlighting defect sites as key adsorption centers. Additionally, oxygen adsorption on a defective ZnS surface is exothermic, in contrast to the endothermic behavior observed on a defect-free surface. These findings contribute to a deeper understanding of defect-driven surface reactivity and may inform ZnS-based catalyst's design for $CO_{2}$ capture and reutilization.

Figures (6)

• Figure 1: NAP-XPS operando analysis of (a) Zn 2p and (b) C 1s spectra as a function of the heating temperature; (c) Relative atomic concentration of the C 1s components as a function of the temperature. The error bar is indicated in black.
• Figure 2: (a) UHV XPS measurements of C 1s components after NAP experiments at 0.10 mbar (top panel) and 0.55 mbar (bottom panel) of $CO_{2}$ ; (b) C 1s components relative concentration as a function of the pressure of $CO_{2}$ used in the experiments. In each NAP experiment, the sample was heated at 573 K. The relative concentration is with respect to the C 1s spectra collected under UHV conditions after the reaction. The error bar is indicated in black.
• Figure 3: Zn 2p (a) C 1s (b) and (c) O 1s components before (bottom panel), under (middle panel) and after (top panel) NAP-XPS experiments with a mixed environment of $CO$ and $CO_{2}$ (0.1 mbar of each gas). The spectra before and after reaction were collected at room temperature under UHV conditions, while the spectra under reaction were acquired with the sample heated at 573 K.
• Figure 4: Zn 2p (a) C 1s (b) and (c) O 1s components before (bottom panel), under (middle panel), and after (top panel) NAP-XPS experiments with a mixed environment of $O_{2}$ and $CO_{2}$. The spectra before and after reaction were collected at room temperature under UHV conditions, while the spectra under reaction were acquired with the sample heated at 573 K.
• Figure 5: DFT modeling of ZnS pristine (a),(b), and doped (c),(d) before (left panel) and after (right panel) interacting with $CO_{2}$. The unlabeled blue marks are hydrogen species used to passivate the bottom layers.