IN SITU AND OPERANDO MOLECULAR STRUCTURE-PERFORMANCE RELATIONSHIPS

Miguel A. Bañares
Institute for Catalysis, CSIC; Catalytic Spectroscopy Laboratory

     Full text: Not available
     Last modified: February 20, 2006

Abstract
The performance at the surface of materials is of paramount relevance for many applications: in catalysis, energy, health, sensors, and corrosion-resistant materials among others. In addition, the surface of the materials plays a critical role in environmental chemical processes. The understanding of the structure-property relationships at the molecular level provides tools for the development of catalysts or other functional materials. This is a fundamental scientific challenge with direct industrial, energy-related and environmental impact.

Materials research has currently emphasized nanotechnology. As the size reduces, the surface area increases and becomes more and more important. Surface phenomena, in particular surface chemistry, are an integral part of any research on nanomaterials. It is imperative to increase the fundamental knowledge and understanding of the chemistry occurring at surfaces and interfaces of working nano-materials and the factors that tune it.

Surface science in catalysis and other surface phenomena may break the process into elementary steps. Many studies have been carried out on clean single-crystal surfaces in high vacuum conditions. The relevance of these studies to understand real processes may be under debate. The surface chemistry of the adsorbate is expected to be the same, regardless of the conditions of the experiment. Furthermore, higher pressure allows for the steady-state formation of weakly adsorbed intermediates in significant concentrations.

Real nano-materials with large surfaces are much more complex than the model single crystals. To single out the active site under real working conditions of the catalyst is an enormous task. It requires a combination of techniques and the development or adaptation of techniques, which allow measurements under catalytic conditions (high temperatures and high pressures). This is the field of in situ spectroscopy. We have recently developed a new methodology that combines the determination of catalyst activity/selectivity and its molecular structure in a single experiment (1). We have named this methodology “operando” (Latin for “working”), which underlines the simultaneous characterization of both catalyst molecular structure and activity/selectivity.

There is a need to force studies to get as close as possible to real process conditions. Thus, it is critical to determine the structure of the materials while they are “working”. Experimental techniques have been developed that allows for the study of the surface properties of materials under realistic conditions of operation in the laboratory or on industrial scale. With these in situ spectroscopic techniques the different surface sites and surface species of a material under working conditions can be identified and related to its surface properties. Among them, Raman spectroscopy is an extremely powerful catalyst characterization technique that provides fundamental information about catalytic molecular structures and surface reaction intermediates. Raman is particularly suited since it is sensitive to surface amorphous species present at the surface or nano-scaled crystalline phases; these do not generate diffraction pattern below 4 nm, which hampers the use of in situ XRD. This few nm sizes are critical to induce new reactive properties (3). Most importantly, Raman spectroscopy can provide this information under industrial reaction conditions (temperature, pressure, etc) (3). For instance, operando Raman-GC studies have uncovered the role of different catalytic sites for ammoxidation of propane to acrylonitrile (1,2). Fundamental information about the interaction between Sb and V allows the development of highly efficient catalysts for propane ammoxidation to acrylonitrile.

The presentation will show the application of operando Raman studies to environmental processes for remediation of contaminants (dimethyl formamide, ammonia, etc) and it will also discuss the extension of the operando Raman spectroscopy methodology to the combinatorial evaluation of catalysts. The combinatorial operando Raman screening of catalysts will not only identify novel catalyst compositions that are efficient for the targeted reaction product, but will also identify the specific catalyst functionalities that are required for highly active and selective catalysts. Raman screening will dramatically minimize the number of combinatorial experiments that will be needed to discover novel catalysts.

References

1. “Operando Raman study of alumina-supported Sb-V-O catalyst during propane ammoxidation to acrylonitrile with on line activity measurement”, M. O. Guerrero-Pérez and M. A. Bañares, Chem. Commun. 12, 1292 (2002).
2. “Operando Methodology: combination of in situ spectroscopy and simultaneous activity measurements under catalytic reaction conditions”, Miguel A. Bañares, Catal. Today 100, 71 (2005) (SPECIAL ISSUE NUMBER 100)
3. “In situ and Operando Raman Spectroscopy for the Structural Characterization of Operating Catalysts” M. A. Bañares, G. Mestl, Adv. Catal. (2006) in preparation


Acknowledgements

The supports of the Spanish Ministry of Education and Science (MAT2002-04000-C02-01 and CTQ2005-02802/PPQ), NATO Collaborative Linkage Grant (ESP.NR.NRCLG 981857), and
Polish-Spanish Joint Research Project (2003-PL0021) are gratefully acknowledged.