The paper addresses possible ambiguities in the determination of the state of platinum species by the stretching frequency of a CO probe, which is a common technique for characterization of platinum-containing catalytic systems. We present a comprehensive comparison of the available experimental data with our theoretical modeling (density functional) results of pertinent systems – platinum surfaces, nanoparticles and clusters as well as reduced or oxidized platinum moieties on a ceria support. Our results for CO adsorbed on-top on metallic Pt⁰, with C–O vibrational frequencies in the region 2018–2077 cm⁻¹, suggest that a decrease of the coordination number of the platinum atom, to which CO is bound, by one lowers the CO frequency by about 7 cm⁻¹. This trend corroborates the Kappers–van der Maas correlation derived from the analysis of the experimental stretching frequency of CO adsorbed on platinum-containing samples on different supports. We also analyzed the effect of the charge of platinum species on the CO frequency. Based on the calculated vibrational frequencies of CO in various model systems, we concluded that the actual state of the platinum species may be mistaken based only on the measured value of the C–O vibrational frequency due to overlapping regions of frequencies corresponding to different types of species. In order to identify the actual state of platinum species one has to combine this powerful technique with other approaches.

The amount and location of acid sites (strong Brønsted and weak silanols) in zeolites are crucial for their applications. In this work we revealed the enigma of the complex H-bonded silanol networks in pure silica MFI-type zeolites using ¹H solid-state NMR and IR spectroscopy combined with DFT calculations. The silanol sites have a determining role in phase selectivity during the zeolite synthesis and also play an imperative role in setting their acidity, stability, lifetime and hydrophobicity. The spectral signatures of silanol defects in pure silica zeolites are disclosed. Based on the experimental and theoretical observations we identified four types of silanols in pure silica zeolites which varied depending on the crystallite size: (i) isolated (free) silanols, not participating in hydrogen bonds either as proton-donors nor as proton-acceptors, (ii) proton-acceptor silanols, participating in hydrogen bonds only as proton-acceptors, (iii) proton-donor silanols, participating in weak hydrogen bonds and (iv) medium and strong hydrogen-bonding silanols, participating as proton-donors or simultaneously as proton-donors and proton-acceptors. The main factor determining the strength of the hydrogen bond of a specific silanol group is the possibility of its proton to approach closer to the oxygen center of the proton accepting silanol. This suggested that the flexibility of the zeolite fragment at which the proton-donating and proton-accepting silanols are bound is the main factor determining the formation and strength of hydrogen bonds and their corresponding spectral features.