(3do) Size Defined Catalysis: Tuning the Catalytic Properties by Selectively Designing Atomically-Precise Catalysts

Supported Au catalysts have been studied extensively for CO oxidation. These catalysts are known to catalyze this reaction even at sub-ambient temperatures. Numerous critical factors have been examined on these catalysts for CO oxidation including the size of Au, pretreatment, oxidation state of Au during the reaction, and the effects of support. However, the key factors that are believed to convert the “inert gold” into a highly active catalyst are (i) the size of Au particles, and (ii) the nature of support. While recent literature demonstrates catalytic activity of gold nanoparticles <2 nm, the next stage in fine tuning this catalysis process is to develop gold clusters prepared with atomic precision. Such atomically precise gold catalysts hitherto have not been investigated for CO oxidation.

 To that end, we have recently demonstrated the preparation of atomically-tailored Au clusters consisting of 38-atoms (Au₃₈) [1] and also synthesized Au clusters with 25-atoms (Au₂₅) from a previously reported method [2]. We prepared atomically-precise Au₃₈/TiO₂ catalysts and have attempted various heat pretreatments to remove the ligand while retaining the size of the cluster. Furthermore, we examined these Au/ TiO₂ catalysts for catalyzing CO oxidation using in situ FTIR and synchrotron radiation-based EXAFS, and monitored the surface species while investigating the activity of these catalysts with time on stream. We noticed the presence of certain carbonates and bicarbonates (from DRIFTS) and thiolate species from EXAFS as residual sulfides (after a reductive pretreatment to activate the catalyst) which interact with Au apparently at the Au-support interface. Interestingly, we observed that these thiolates congregate at the Au-Ti interface and are quickly oxidized by O₂ under oxidative conditions to form sulfonyl (Au-SO₂R) linkages between the Ai and TiO₂ surface [3]. These linkages act as oxygen radical transfer points from activated O₂ from TiO₂ surface to the Au cluster. So despite being a poison, we have shown that these thiolates appear to act as radical transfer agents. 

Another aspect of my research is to determine mechanistic pathways on Au₃₈/TiO₂ catalyst during CO oxidation by conducting real-time in-situ studies using synchrotron-radiation based X-ray absorption spectroscopy (XAS), operando IR (DRIFTS), and usage of isotopes. We have investigated CO oxidation on our catalysts using above techniques, and by carefully conducting planned experiments, we have also shown that CO oxidation takes place on our catalyst via a Mars van Kreven (redox) mechanism which involves the supply of lattice oxygen from TiO₂ support at the Au-TiO₂ interface. We support our experimental observations using theoretical methods, particularly using DFT, where we have studies CO oxidation on Au38 clusters and Au/TiO2 1-D nanorods.