Identifying reaction intermediates and catalytic active sites through in situ characterization techniques.

This tutorial review centers on recent advances and applications of experimental techniques that help characterize surface species and catalyst structures under in situ conditions. We start by reviewing recent applications of IR spectroscopy of working catalysis, emphasizing newer approaches such as Sum Frequency Generation and Polarization Modulation-infrared reflection absorption spectroscopy. This is followed by a section on solid-state NMR spectroscopy for the detection of surface species and reaction intermediates. These two techniques provide information mainly about the concentration and identity of the prevalent surface species. The following sections center on methods that provide structural and chemical information about the catalyst surface. The increasingly important role of high-pressure X-ray photoelectron spectroscopy in catalyst characterization is evident from the new and interesting information obtained on supported catalysts as presented in recent reports. X-Ray absorption spectroscopy (XANES and EXAFS) is used increasingly under reaction conditions to great advantage, although is inherently limited to systems where the bulk of the species in the sample are surface species. However, the ability of X-rays to penetrate the sample has been used cleverly by a number of groups to understand how changing reaction conditions change the structure and composition of surface atoms on supported catalyst.