title: Initial stages of CO2adsorption on CaO: a combined experimental and computational study.
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, CO2adsorption was examined on poorly ordered, nanoparticulate CaO films prepared on Ru(0001). For both systems, CO2readily 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 CO2adsorption 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 CO2exposure. To explain the coverage-dependent IRAS results, we propose CO2surface 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 CO2on 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 CO2adsorption are consistent with the observed coverage effect on the CO2adsorption energy measured by microcalorimetry and the IRAS results.
authors.affiliation: Institut für Chemie, Humboldt-Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany. js@chemie.hu-berlin.de.
authors.forename: Brian H
authors.identifier: http://orcid.org/0000-0003-3581-391X
authors.initials: BH
authors.lastname: Solis
authors.affiliation: Abteilung Chemische Physik, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany. shaikhutdinov@fhi-berlin.mpg.de.
authors.forename: Yi
authors.initials: Y
authors.lastname: Cui
authors.affiliation: Abteilung Chemische Physik, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany. shaikhutdinov@fhi-berlin.mpg.de.
authors.forename: Xuefei
authors.initials: X
authors.lastname: Weng
authors.affiliation: Abteilung Chemische Physik, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany. shaikhutdinov@fhi-berlin.mpg.de.
authors.forename: Jan
authors.initials: J
authors.lastname: Seifert
authors.affiliation: Abteilung Chemische Physik, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany. shaikhutdinov@fhi-berlin.mpg.de and Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Max-Eyth-Str. 2, 24118 Kiel, Germany.
authors.forename: Swetlana
authors.initials: S
authors.lastname: Schauermann
authors.affiliation: Institut für Chemie, Humboldt-Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany. js@chemie.hu-berlin.de.
authors.forename: Joachim
authors.initials: J
authors.lastname: Sauer
authors.affiliation: Abteilung Chemische Physik, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany. shaikhutdinov@fhi-berlin.mpg.de.
authors.forename: Shamil
authors.initials: S
authors.lastname: Shaikhutdinov
authors.affiliation: Abteilung Chemische Physik, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany. shaikhutdinov@fhi-berlin.mpg.de.
authors.forename: Hans-Joachim
authors.initials: HJ
authors.lastname: Freund