Abstract
Room temperature adsorption of carbon dioxide (CO2) on monocrystalline CaO(001) thin films grown on a Mo(001) substrate was studied by infrared reflection-absorption spectroscopy (IRAS) and quantum chemical calculations. For comparison, CO2 adsorption was examined on poorly ordered, nanoparticulate CaO films prepared on Ru(0001). For both systems, CO2 readily adsorbs on the clean CaO surface. However, additional bands were observable on the CaO/Ru(0001) films compared with CaO/Mo(001), because the stricter IRAS surface selection rules do not apply to adsorption on the disordered thin films grown on Ru(0001). Spectral evolution with increasing exposure of the IRA bands suggested the presence of several adsorption sites which are consecutively populated by CO2. Density functional calculations showed that CO2 adsorption occurs as monodentate surface carbonate (CO32-) species at monatomic step sites and other low-coordinated sites, followed by formation of carbonates on terraces, which dominate at increasing CO2 exposure. To explain the coverage-dependent IRAS results, we propose CO2 surface islanding from the onset, most likely in the form of pairs and other chain-like species, which were calculated as thermodynamically favorable. The calculated adsorption energy for isolated CO2 on the terrace sites (184 ± 10 kJ mol-1) is larger than the adsorption energy obtained by temperature programmed desorption (∼120-140 kJ mol-1) and heat of adsorption taken from microcalorimetry measurements at low coverage (∼125 kJ mol-1). However, the calculated adsorption energies become less favorable when carbonate chains intersect on CaO terraces, forming kinks. Furthermore, our assignments of the initial stages of CO2 adsorption are consistent with the observed coverage effect on the CO2 adsorption energy measured by microcalorimetry and the IRAS results.
Highlights
Adsorption and reactivity studies on calcium oxide (CaO) were primarily performed on powders
For CO2 on MgO(001), it has been shown that electron transfer from subsurface Mo can affect the preferred binding of CO2 as surface carboxylate rather than carbonate.[26,27]. In this combined experimental and theoretical study, we report on CO2 adsorption on well-characterized CaO thin films grown on metal substrates, focusing on the initial stages of adsorption in the low coverage regime
Well-ordered CaO(001) films grown on a Mo(001) substrate were used as a model system to study initial stages of CO2 adsorption on CaO by infrared reflection–absorption spectroscopy (IRAS)
Summary
Adsorption and reactivity studies on CaO were primarily performed on powders. It was demonstrated that CO2 adsorption capacities depend upon particle size and surface area.[6,7] Complete adsorption resulted in formation of calcium carbonate (CaCO3). The reversibility of the carbonate formation falls off with increasing carbonation/calcination cycles, accompanied by rapid decay of CO2 adsorption.[8] many studies addressed the stability of CaO particulates, with the aim of improved catalyst regeneration.[3,4,9] CaCO3 formation was found to occur in two kinetic regimes,[6] whereby the initial reaction was fast and closely related to the partial pressure of CO2,10 and a slower second stage that was diffusioncontrolled.[11]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
