Spectroscopy, infrared acid site concentration

Figure 11 Measured Brensted acid site density as a function of the nominal concentration from ammonium ion exchange, (o) infrared spectroscopy ( ) 2-propanol dehydration ) TPD of ammonia. Line represents perfect agreement. ...

The adsorption of molecular probes, followed by infrared or NMR spectroscopy and thermal desorption studies, is the most commonly adopted way to study basic sites. Carbon dioxide is frequently used in infrared studies, particularly of cationic zeolites with added alkali metals.Chloroform is also suitable, since the interaction with the chlorine atom and subsequently the C-Cl stretching frequency is a measure of the basic strength. NMR studies of basic zeolites have concentrated on the use of C containing probes such as methyl iodide and chloroform. Addition of methyl iodide results in its decomposition and the formation of methoxy groups at framework oxygens. The shift of the methyl carbon is expected to be related to the basicity of the framework, that is the tendency of the framework oxygens to donate electrons - C MAS NMR of methoxy groups prepared in this way shows a clear distinction between basic zeolites, such as Cs-X, and acidic zeolites, such as H-ZSM-5. 

The study of the dynamics of N isotope transfer under adsorption-desorption equilibrium (NO -1- O2 + He) revealed two types of NOx complexes, and their concentrations and formation rates (depending on NO and O2 concentrations) were estimated. According to in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) data, these complexes are assigned to nitrite-nitrate (1520 cm" ) and N02 species (2130 cm" ). Note that nitrite-nitrates and N02 differ clearly in the rates of their formation. Under the reaction conditions, the concentrations of both active species drop considerably. Therefore, two parallel reaction pathways were proposed that involve both active complexes. The rates of NOx complexes interaction with methane were also calculated, and the reaction with participation of N02 species was shown to proceed about 2.5 times faster than that of nitrite-nitrate. The N02 species was determined to form at the interface between CoO clusters and acid OH groups in zeolite (or at the paired Co -OH sites). This finding agrees well with in situ DRIFTS data that indicates that the N02 formation correlates with a drop in the acid OH group band intensity. 

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