Looking on Heterogeneous Catalytic Systems from Different Perspectives: Multitechnique Approaches as a New Challenge for In Situ Studies

Abstract

Adapting physico-chemical characterization techniques for in situ studies of real heterogeneous catalysts during conditioning, reaction, and deactivation processes is a research topic of growing importance. Up to now, in most studies only one single in situ technique is applied. However, new trends are focused on using a portfolio of several in situ methods to obtain comprehensive information on structure-reactivity relationships. This review discusses such application examples with the aim to point out the particular benefits that arise from multitechnique approaches for understanding processes like formation of active sites, interaction of those sites with reactant molecules, and catalyst deactivation. Except for Fourier-transform infrared spectroscopy and quasi-in situ X-ray photoelectron spectroscopy, the examples deal mainly with the combined use of bulk techniques such as electron paramagnetic resonance, ultraviolet/visible diffuse reflection spectroscopy, laser-Raman spectroscopy, temperature-programmed spectroscopy, X-ray diffraction, and X-ray absorption spectroscopy. However, although not being surface-sensitive per se, these techniques provide results relevant for the catalyzed reaction. Illustrative case studies comprise the investigation of (i) vanadia-based catalysts during selective oxidation and ammoxidation of toluene and during methanol synthesis, (ii) supported chromium oxides during octane aromatization, (iii) zeolite crystallization and catalysis, and (iv) a Cu/ZnO catalyst during methanol synthesis.