Active Centers in Heterogeneous Catalysts: Formation and Study with in situ Methods
UK Russia seminar on catalysis (Британско-российский семинар по катализу)
20-21 Nov 2017
|| Bukhtiyarov Valerii Ivanovich
Boreskov Institute of Catalysis SB RAS
In the recent years a widespread currency in heterogeneous catalysis has been gained by the fundamental approach to the development of the new catalytic systems, which differs from the traditional empirical strategy in that the molecular design (controlled synthesis) of the active component is preceded by an in-depth study of the mechanisms of the catalytic reactions and structure of the active centers. Evidently the efficiency of this approach depends essentially on the level of development of the physical methods for studying the solid, including, first of all, the surface sensitive methods underlying such field of research as Surface Science.
More than thirty years of use of the physical methods of the surface study significantly advanced the understanding of such phenomena as chemisorption, interaction in the adsorption layers, the chemical reactions on the surface of the solid. However, information registered in the conditions of Surface Science experiments (P < 10-4 Pa) may have little in common with the real surface of a working catalyst (P > 102 Pa). To solve this problem the researchers lately undertake the numerous attempts to develop some new or modernize the existing methods to study of the catalyst surface in order to conduct the experiments at a pressure of real catalysis. The methods able to work at high pressures list polarization-modulation infrared reflection absorption spectroscopy (PM IRAS) and sum frequency generation spectroscopy (SFG), scanning tunnelling microscopy (STM) and atomic force microscopy (AFM), and some others. A special place in this list belongs to the methods of absorption of the X-rays (XANES and EXAFS) as well as X-ray photoelectronic spectroscopy (XPS), which attracts a special attention of the researchers due to its universality.
The given lecture presents a review of the studies of the structure of active centers of the supported metal catalysts immediately in the course of the catalytic reaction, i.e. in situ. The reaction of epoxidation of ethylene and oxidation of hydrocarbons are used to demonstrate how the reaction medium changes the chemical and phase state of the active component, and finally forms new adsorption species. The results in this work show that the nature of the active centers performing the catalytic reaction can be determined only when the characterization of the catalyst surface is accompanied with the testing of the catalytic properties (activity and selectivity) by the methods of mass-spectrometry or gas chromatography.
The author thanks RFBR for the financial support (project 17-03-01378A).