(6v) Site-Specific Techniques for Identification of Active Sites of Supported Transition Metal Oxide and Late Transition Metal Catalysts

Identification and characterization of catalytic active sites are the prerequisites for an atomic level understanding of the catalytic mechanism and rational design of high-performance heterogeneous catalysts. Supported transition metal oxide and late transition metal catalysts represent two groups of most studied and widely used catalysts for many industrial applications. One common feature of these catalysts is the structural diversity of their surface species. In many cases, only a limited portion of the surface species determines overall catalytic performances. For instance, it has been well known that only less than 1-2% of the total metal atoms in supported molybdena catalysts contribute to olefin metathesis reactions. Identifying the structure of such a small portion of active species is an extremely challenging task. Most techniques employed by researchers either provide statistically limited information such as microscopy, or sample-averaged information such as X-ray absorption spectroscopy. From this point of view, site-specific techniques that are capable of providing statistically sufficient information on site identification and quantification as well as the activity evaluation of specific sites are highly desired.

Here we show our recent efforts in developing site-specific spectroscopy to identify active sites in several heterogeneous catalytic reactions.  In the first case, we combine UV resonance Raman spectroscopy and electron microscopy measurements to unambiguously identify the monomeric nature of the active sites for MoO₃/SiO₂ catalysts in olefin metathesis. In another case, we show that infrared spectroscopy, using CO as a probe molecule, can distinguish and quantify supported Pt single atoms and nanoparticles. Further, we observe that only the CO molecules adsorbed on Pt nanoparticles can react at low temperatures upon O₂ or H₂O exposure, showing that the active sites in CO oxidation and water-gas shift reactions are present on the nanoparticles but not on single atoms. The atomic-level understanding of the active site structure will advance the rational design of high-performance heterogeneous catalysts.