Catalysts hierarchical approach

Grunwaldt J-D, Wagner JB, Dunin-Borkowski RE. Imaging Catalysts at Work A Hierarchical Approach from the Macro- to the Meso- and Nano-scale. ChemCatChem. 2013 5, DOI 10.1002/cctc.201200356. 

In view of catalytic potential applications, there is a need for a convenient means of characterization of the porosity of new catalyst materials in order to quickly target the potential industrial catalytic applications of the studied catalysts. The use of model test reactions is a characterization tool of first choice, since this method has been very successful with zeolites where it precisely reflects shape-selectivity effects imposed by the porous structure of tested materials. Adsorption of probe molecules is another attractive approach. Both types of approaches will be presented in this work. The methodology developed in this work on zeolites Beta, USY and silica-alumina may be appropriate for determination of accessible mesoporosity in other types of dealuminated zeolites as well as in hierarchical materials presenting combinations of various types of pores. 

The above approach of integrating analytically (under certain assumptions) across the porous wall the species balances to obtain local soot consumption rates can be extended for the case of more reactions occurring in the porous wall. In the presence of a precious metal catalyst, the hydrocarbons and the carbon monoxide of the exhaust gases are also oxidized. It can be assumed that all the reactions in the porous wall occur hierarchically (according to their... 

The inherent limitations of the use of zeolites as catalysts, i.e. their small pore sizes and long diffusion paths, have been addressed extensively. Corma reviewed the area of mesopore-containing microporous oxides,[67] with emphasis on extra-large pore zeolites and pillared-layered clay-type structures. Here we present a brief overview of different approaches to overcoming the limitations regarding the accessibility of catalytic sites in microporous oxide catalysts. In the first part, structures with hierarchical pore architectures, i.e. containing both microporous and mesoporous domains, are discussed. This is followed by a section on the modification of mesoporous host materials with nanometre-sized catalytically active metal oxide particles. 

Due to the frequently observed chemical memory of a working catalyst, reproducible synthesis of the active mass with respect to all synthetic steps is a basic requirement. Moreover, an integrated approach requires the consideration of a catalyst as a hierarchical system taking into account mass transport and thermal conduction properties, as well as mechanical stability in the early stages of the development of synthetic concepts closing the cycle of rational catalyst design. 

To introduce additional functionality and to allow for a wider variety of structural features, branching of the primary CNF/CNT with secondary, usually thirmer ones, has been performed [113,114]. Primary as well as secondary carbon nanostructures were grown in CVD processes. After electrodepositing iron as growth catalyst and subsequent CNT growth, secondary CNTs were grown in a similar fashion to yield hierarchically structured layers. The principles of such an approach are visualized in Figure 10.22. It was shown with impedance spectroscopy and other techniques, that the electrochemical properties of such composites are superior to those of unbranched CNF. 

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