A Multi-Technique Study of $CO_2$ Adsorption on $Fe_3$$O_4$ Magnetite

TL;DR

This study combines experimental techniques to analyze how CO2 molecules adsorb on Fe3O4 surfaces, revealing specific binding sites, coverage effects, and desorption behaviors relevant for understanding surface chemistry of metal oxides.

Contribution

It provides a detailed multi-technique analysis of CO2 adsorption on Fe3O4(001), identifying binding sites, coverage-dependent phenomena, and desorption characteristics, which were not previously characterized in such detail.

Findings

CO2 binds strongly at defect sites and Fe3+ sites on Fe3O4.

Adsorption leads to monolayer compression approaching CO2 ice density.

Second layer desorbs at lower temperature than multilayers, indicating metastability.

Abstract

The adsorption of $CO_2$ on the $Fe_3$$O_4$(001)-($\sqrt{2}$ $\times$ $\sqrt{2}$)R45{\deg} surface was studied experimentally using temperature programmed desorption (TPD), electron spectroscopies (UPS and XPS), and scanning tunneling microscopy (STM). $CO_2$ binds most strongly at defects related to Fe2+ including antiphase domain boundaries in the surface reconstruction and above incorporated Fe interstitials. On the pristine surface, $CO_2$ adsorbs molecularly at fivefold-coordinated Fe3+ sites with a binding energy of 0.4 eV. Above a coverage of 4 molecules per ($\sqrt{2}$ $\times$ $\sqrt{2}$)R45{\deg} unit cell, further adsorption results in a compression of the first monolayer up to a density approaching that of a $CO_2$ ice layer. Surprisingly, desorption of the second monolayer occurs at a lower temperature ($\approx$ 84 K) than $CO_2$ multilayers ($\approx$ 88 K), suggestive of a metastable phase or diffusion-limited island growth. The paper also discusses design considerations for a vacuum system optimized to study the surface chemistry of metal oxide single crystals, including the calibration and characterisation of a molecular beam source for quantitative TPD measurements.