Linkage Isomerism of CO Adsorbed on Alkali Halides

Abstract

Variable-temperature FTIR spectra of CO adsorbed on NaCl and KBr films are shown to reveal linkage isomerism, that is, formation of C- and O-bonded adsorption complexes with the same cationic sites. Not all of the sites can be involved in that, only the three- or four-coordinated cations located on kinks, edges, or steps of microcrystals. For NaCl, these adsorption complexes account for the high-frequency (HF) C-O stretching bands at 2175 and 2160 cm(-1). Coresponding LF bands of O-bonded CO were found at 2115 and 2124 cm(-1). In the spectrum of KBr, the HF band is poorly resolved, and only one LF band can be clearly seen, near 2124 cm(-1). The value of the isomerization enthalpy for complexes that account for the bands at 2160 and 2124 cm(-1) of CO adsorbed on NaCl, estimated from the experiment, is 4.0 ± 0.2 kJ/mol. Quantum mechanical calculations by DFT methods applied to NaCl model clusters interacting with the CO molecule confirm the existence of two potential wells with adsorption and isomerization energies close to the experimentally measured values, and enable us to estimate the height of the potential barrier between the two adsorption states.

Bridging the Pressure and Materials Gap in Heterogeneous Catalysis: A Combined UHV, In Situ, and Operando Study Using Infrared Spectroscopy

“…The situation is improved by recording reference spectra for KBr powders 37 (Figure 1b), a material where under these conditions (295 K, 1 bar CO) the amount of CO adsorbed on the surface is negligibly small. 38 We find that in the difference spectra obtained by first normalizing the spectra in the frequency regime between 2080 and 2040 cm −1 and then subtracting the KBr reference data from the ceria data, vibrational bands arising from CO adsorbed on ceria surfaces can be clearly identified (Figure 2). After the subtraction scheme, the difference DRIFT spectrum clearly shows two peaks centered at 2157 and 2175 cm −1 with a FWHM value of around 13 cm −1 .…”

“…Although we know from the corresponding UHV studies that under these conditions CO species adsorbed at the surface of ceria nanoparticles must be present, in the raw data these adsorbate vibrations are almost completely obscured by the intense gas-phase signals. The situation is improved by recording reference spectra for KBr powders (Figure b), a material where under these conditions (295 K, 1 bar CO) the amount of CO adsorbed on the surface is negligibly small …”

The influence of the local environment on the electron accepting ability of the cation: induced Lewis acidity

“…Confirmation of these ideas was the work [21] on the adsorption of CO on a sputtered NaCl film. As expected, the replacement of the oxygen environment of the cation with single-charged halogen anions leads to the appearance of an absorption band in the spectrum, whose behavior allows it to be considered as belonging to O-bound molecules, and the low intensity indicates insignificant number of sites on which this form of adsorption is realized.…”