Application of Nitrous Oxide as Probe Molecule for Surface Heterogeneity Identification
Ninth International Symposium “Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids” (ISSHAC-9)
17-23 Jul 2015
|| Melgunov Maksim Sergeevich
, Ayupov Artem Borisovich
Boreskov Institute of Catalysis SB RAS
Surface heterogeneity (SH) is the key factor for adsorbents and catalysts. The modern surface science techniques give qualitative description of SH, and usually there is a lack of information on concentration of surface active sites or some other quantitative measure of SH. One of the possible evaluations of the SH is the calculation of adsorption energy distributions (AEDs) from the adsorption data . However, traditional adsorbates are usually not suitable for many materials, for example, for amorphous surfaces due to low specificity to different surface sites. E.g., traditional nitrogen at 77K does not “feel” the differences between the surface sites in mesoporous element-silicates known as efficient catalysts for partial oxidation of bulky organic substrates. Other surface science methods, such as DR-UVis, IR spectroscopy, XPS, Raman, etc., give information of the state of the elements in the silicate environment. However, no quantitative knowledge on surface concentration of the elements in various states can be obtained so far. Thus, there is an actual task for elaboration of a technique suitable for simultaneous qualitative and quantitative characterization of SH. One of the possible ways to overcome the mentioned difficulties is the utilization of nitrous oxide (N2O). N2O adsorption isotherms at 195 K (dry ice-isopropanol cooling bath) can be measured by means of traditional adsorption instruments equipped with low pressure (lower than 1 torr) accessories. The cooling bath can last for 2-3 days allowing long experiments. As an example, we present here AEDs for Ti-SBA-3 titanosilicates using N2O as surface probing molecule. AEDs are calculated as a difference between AEDs for Ti-SBA-3 and AED for Ti-free SBA-3 material. One can observe the decrease of the surface concentration of sites with adsorption energy ~3,7 kJ/mol, that can be attributed to the decrease of the surface silanol groups, and the increase of the surface concentration of the sites with adsorption energy of ~4,5 and ~6,7 kJ/mol, that can be attributed to Ti species at two different states. In our contribution we discuss other examples of application of N2O as an adsorbate for characterization of not only SH, but also porous structure for mesoporous MOFs, traditional mesoporous adsorbents and catalyst supports.