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Comment on "The nature of cationic adsorption sites in alkaline zeolites--single, dual and multiple cation sites" by P. Nachtigall, M. R. Delgado, D. Nachtigallova and C. O. Arean, Phys. Chem. Chem. Phys., 2012, 14, 1552.

The article ‘The nature of cationic adsorption sites in alkaline zeolites—single, dual and multiple cation sites’ by P. Nachtigall, M. R. Delgado, D. Nachtigallova and C. O. Arean that was recently published in this journal aims at clarifying the different types of adsorption sites that can occur by using CO as a probe molecule. The experimental IR data reviewed have, for the most part, been already published using variable temperature IR (VTIR) as an innovative technique. Additionally, VTIR spectra of CO adsorption on Faujasite NaY zeolites are reported for the first time. While the experimental conditions of their previous work are mostly detailed in the corresponding cited articles, those of NaY are poorly reported. This lack of clarity limits the reader’s understanding and makes it especially difficult to reproduce these experiments independently in the laboratory. Some of the questionable points in the article (details that need clarifying) are discussed below. It is reported in section 3.1 of the paper that a ‘thin selfsupported wafer’ is used for experiments on Na-FER, Na-ZSM5, NaY and Na-A zeolites. The corresponding IR spectra recorded after CO adsorptions are displayed in Fig. 2. The exhibited absorbance values are all lower than 0.4 units, except for the NaY zeolite which reaches a value of nearly 1.4. Because this striking difference in absorbance values is never underlined nor discussed by the authors, the notion of a ‘thin self-supported wafer’ is strongly questionable for NaY in the presented work. Many parameters could explain this difference in absorbance values for NaY. For instance, the intensity of the bands can be related to the number of cations involved in the adsorption of CO. The more cations there are, the more intense the IR band will be. It is well known that the number of cations is directly connected to the Si/Al ratio. However, this rule is approximate as some of these cationic sites are not accessible to the CO probe. In the case of NaY only SII sites are accessible to the CO probe. As a result, the obvious increase of the intensity of the IR bands for the NaY zeolite can not be directly justified with this rule. A careful inspection of spectra reported in Fig. 2 shows that another key factor is the temperature considered for CO adsorption. Indeed, the IR spectra of NaY were recorded between 155 and 222 K, a temperature range lower than those of the other zeolites presented in Fig. 2 (210–280 K). As a result, the adsorption of CO in the case of NaY is thermodynamically favored compared to other zeolites, which explains the increased intensity of the IR bands. It is highly questionable why the authors did not consider the CO adsorption on NaY with the same close temperature range they did for other zeolites. The increase of IR bands presented in Fig. 2 must also be connected with the equilibrium pressure of CO as an influencing factor. It is well known that the IR bands’ intensity generally increases with pressure and in fact, corresponds to higher CO doses being introduced and thus to a higher surface coverage. Unfortunately, the influence of CO pressure on band intensity can not be highlighted in their study since it would have required the same temperature range conditions for comparison. Therefore, from the spectra presented in Fig. 2 and the VTIRmethod (as the authors applied it) one can not differentiate temperature and pressure as independent influences. This would have required operating under either isotherm or isobar conditions. As discussed above, the impact of these three factors (Si/Al ratio, temperature and pressure conditions) have not been adequately described in their paper and should be clarified. The second point is with regards the calculation of the surface coverage (termed y) and its connection to the A and AM values in the case of the NaY zeolite. As noted by the authors, when considering a Langmuir-type adsorption process, the following equations can be deduced: