Reassignment of the Vibrational Spectra of Carbonates, Formates, and Related Surface Species on Ceria: A Combined Density Functional and Infrared Spectroscopy Investigation

Abstract

Using a combination of state-of-the-art computational modeling and Fourier transform infrared (FTIR) spectroscopy study of the surface species formed during interaction of CO2 or CO with activated (stoichiometric), reduced, and hydroxylated ceria, CeO2, we assigned various experimentally observed vibrational modes to individual types of surface species. We considered carbonates CO3 2–, formates HCO2 –, and hydrogen carbonates CO2(OH)− bound in various ways to the surface of a ceria nanoparticle. Since the structure of the surface carbonate species is particularly versatile, we introduced a notation of different types of such species and computationally determined the regions where the characteristic vibrational frequencies of each type of species can be found. The complementary FTIR measurements of the surface species produced under different conditions revealed the actual experimental vibrational peaks and allowed estimation of the accuracy of the computational method to reproduce the frequencies of different vibrational modes. Thus, combining computed and experimental data we suggest a sound, partly new assignment of the vibrational bands in the complex IR spectra of surface (hydrogen)carbonate and formate species on ceria. The proposed reassignment of the vibrational peaks enables reliable detection of the surface species on ceria surface using vibrational spectroscopy. This is critical for the meaningful analysis of the reactivity of these species and the clarification of the mechanisms of the rich variety of surface processes on ceria.

Surface sites on Pt–CeO2mixed oxide catalysts probed by CO adsorption: a synchrotron radiation photoelectron spectroscopy study

“…23,37,42,43 A tentative assignment can be given based on the DFT study by Vayssilov et al 28 According to this work, carbonates are formed on the CeO 2 nanoparticles predominantly in bidentate and tridentate configurations. The bidentate geometry, denoted by Vayssilov et al as (1.30), 28 is obtained by binding a CO 3 2À entity to three Ce ions via one singly-coordinated and one threefoldcoordinated oxygen atoms. Threefold-coordinated oxygen sites exist on {111} and {110} facets while {100} facets expose only twofold-coordinated oxygen sites.…”

“…Peak (A) could then be associated with one or both of the remaining tridentate geometries. The first species, denoted (1.2.1), 28 binds to two cerium cations via one twofold-coordinated and two singly-coordinated oxygen atoms. The second carbonate, denoted (1.3.1), 28 binds to three cerium cations via one threefoldcoordinated and two singly-coordinated oxygen atoms.…”

“…The first species, denoted (1.2.1), 28 binds to two cerium cations via one twofold-coordinated and two singly-coordinated oxygen atoms. The second carbonate, denoted (1.3.1), 28 binds to three cerium cations via one threefoldcoordinated and two singly-coordinated oxygen atoms. Albrecht et al 23 reported tridentate carbonate species with a binding energy of 290.2 eV on a flat CeO 2 (100) surface (exposing twofoldcoordinated oxygen sites).…”

“…[17][18][19][20][21][22][23][24][25][26] In such cases the identification of the species and their adsorption sites is often assisted by density functional calculations. [25][26][27][28][29] In particular, interaction of CO molecules with planar CeO 2 (111), CeO 2 (110), and CeO 2 (100) surfaces 27 as well as CeO 2 particles 28 has been modelled by density functional theory (DFT). It has been reported that CO reacts with CeO 2 (111) rather weakly and does not yield carbonates.…”

“…The differences between the tridentate carbonates are related to the coordination of their oxygen atoms to cerium cations. 28 In contrast to IR spectroscopy, high resolution synchrotron radiation photoelectron spectroscopy (SRPES) allows direct determination of the oxidation state of Pt as well as the sitespecific assignment of the adsorbed surface species. For instance, CO molecules adsorbed on different Pt surface sites can be identified by the binding energy of the corresponding features in the C 1s spectrum.…”